Enhancing Fuel Properties and Energy Performance with Co-Hydrothermal Carbonization of Coffee Waste and Swine Manure

Enhancing Fuel Properties and Energy Performance with Co-Hydrothermal Carbonization of Coffee Waste and Swine Manure | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Enhancing Fuel Properties and Energy Performance with Co-Hydrothermal Carbonization of Coffee Waste and Swine Manure Kyung Jin Min, Doo Young Oh, Eunyoung Lee, Jin Hwan Lee, Do Yeon Kim, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6199561/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract This study evaluates the fuel properties and energy performance of hydrochar produced through co-hydrothermal carbonization (Co-HTC) of swine manure (SM) and coffee waste (CW). The effects of reaction temperature (160°C, 200°C, 240°C) and mixing ratio (SM:CW = 1:1, 3:1, 5:1) on combustion characteristics, energy densification ratio (EDR), and energy yield (EY) were analyzed. The results indicate that Co-HTC effectively enhances fuel quality by leveraging the synergistic interactions between SM and CW. At 240°C and a 1:1 mixing ratio, the produced hydrochar exhibited a combustion characteristic index (S) of 9.1 × 10⁻⁹ %²/min²·°C³, an EDR of 1.39, and an EY of 57%, demonstrating superior fuel performance. The high lignin content in CW promoted fixed carbon retention, while the elevated nitrogen content in SM contributed to potential agricultural applications. Additionally, Co-HTC effectively reduced O/C and H/C atomic ratios, resulting in enhanced energy density. Despite an increase in ash content with higher SM ratios, EY values remained high across all conditions, with a maximum of 61.8% at a 5:1 ratio, suggesting that energy retention was not significantly hindered. These findings confirm that Co-HTC is a viable approach to converting organic waste into high-energy-density solid fuel, offering a sustainable solution for bioenergy production and waste valorization. Co-Hydrothermal Carbonization Energy yield Energy densification ratio Combustion characteristic index Coffee Grounds Swine Manure Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 1. Introduction Efficient biomass management plays a crucial role in sustainable development, addressing both waste treatment and energy recovery. Improper disposal of organic waste contributes to greenhouse gas emissions, water contamination, and soil degradation, leading to significant environmental and public health concerns (Ferrentino et al., 2023 ; He et al., 2013 ; Libra et al., 2011 ). Conventional waste treatment methods such as landfilling and incineration have low efficiency and are not environmentally sustainable, driving increasing interest in biomass conversion technologies for energy recovery (Funke and Ziegler, 2010 ; Kim et al., 2017 ; Park et al., 2018 ; Sevilla and Fuertes, 2009 ). Hydrothermal carbonization (HTC) has emerged as an eco-friendly thermochemical process for converting biomass into high-energy-density solid fuels (Li et al., 2021 ; Lucian et al., 2018 ; Pauline and Joseph, 2020 ). Recent studies have compared batch and continuous HTC processes to optimize hydrochar production from various biomass sources, particularly swine manure (Ipiales et al., 2024 ). Sharma et al. ( 2020 ) reviewed HTC processes, highlighting key mechanisms, operating parameters, and applications in energy and environmental sectors. Ischia et al. ( 2024 ) further summarized recent advances, technological challenges, and future directions in HTC research. HTC is particularly advantageous as it eliminates the need for energy-intensive drying processes, making it cost-effective and environmentally friendly (Hoekman et al., 2011 ; Lee and Park, 2021 ; Lee et al., 2021 ). Conducted at moderate temperatures (180–250°C) under autogenous pressure, HTC facilitates dehydration and decarboxylation reactions, leading to increased carbon content and reduced oxygen levels in the biomass (Gascó et al., 2018 ; Massaya et al., 2021 ; Mumme et al., 2011 ; Park et al., 2021 ). The resulting hydrochar exhibits high energy density, reduced oxygen content, and improved combustion characteristics, making it suitable for long-term storage and handling (Zhao et al., 2021 ). Recent studies suggest that co-hydrothermal carbonization (Co-HTC) is a promising alternative to overcome the limitations of single-feedstock HTC (Ipiales et al., 2023 ). By co-processing multiple biomass sources, Co-HTC enhances fuel properties while mitigating individual feedstock drawbacks (Islam and Reza, 2023 ; Lang et al., 2018 ). Swine manure (SM) and coffee waste (CW) are two organic waste types that require immediate and effective management due to their environmental impacts (Xiong et al., 2019 ). SM contains high nitrogen and sulfur levels, posing risks of water contamination and eutrophication, while its high ash content can lower fuel quality (Gascó et al., 2018 ; Lang et al., 2022 ; Mumme et al., 2011 ). In contrast, CW is characterized by high fixed carbon content and low ash levels, making it a promising feedstock for high-quality hydrochar production (Hu et al., 2022 ; Kumar et al., 2021 ; Santana et al., 2020 ). Furthermore, recent work by Dang et al. ( 2023 ) demonstrated that applying tailored HTC conditions to spent coffee grounds can significantly enhance fuel properties and improve energy recovery potential. However, during HTC, the thermal degradation of lignin in CW can generate aromatic compounds and volatile organic compounds (VOCs), potentially increasing emissions and residual pollutants in the process water (Dang et al., 2023 ; Hoekman et al., 2011 ; Mäkelä et al., 2015 ). Co-HTC has the potential to overcome these challenges by utilizing the complementary properties of SM and CW to improve fuel characteristics (Ipiales et al., 2023 ). The high carbon and lignin content in CW enhances energy density, while the nitrogen content in SM improves the potential use of hydrochar as a soil amendment (Devnath et al., 2024 ; Khosravi et al., 2024 ; Islam and Reza, 2023 ; Xiong et al., 2024 ; Zhao et al., 2021 ). Additionally, Co-HTC has been shown to reduce ash content and improve combustion behavior, thereby enhancing hydrochar quality (Gascó et al., 2018 ; Lang et al., 2019 ; Mumme et al., 2011 ; Sevilla and Fuertes, 2009 ). This study aims to evaluate the fuel properties and thermal behavior of hydrochar produced through Co-HTC of SM and CW. To achieve this, (1) the effects of reaction temperature on hydrochar yield and fuel characteristics were analyzed, and (2) the influence of mixing ratio (1:1, 3:1, 5:1) and reaction temperature (160°C, 200°C, 240°C) on fuel performance was systematically investigated. Ultimately, the synergistic effects of Co-HTC were assessed to identify the optimal process conditions for producing high-quality solid fuel. The findings of this study provide insights into the potential for developing high-efficiency biofuels from organic waste, contributing to sustainable waste management and advancements in energy conversion technologies. 2. Materials and methods 2.1. Materials In this study, SM and CW were used as feedstocks for the Co-HTC process. SM was collected from a livestock farm in Gyeonggi Province, South Korea, while CW was obtained from a commercial coffee shop in Seoul, South Korea. Prior to HTC experiments, both feedstocks were dried at 105°C for 24 hours and then ground to a particle size of ≤ 2 mm to ensure homogeneity. The proximate and ultimate compositions of SM and CW are summarized in Table 1 . SM exhibited a high ash content (29.8%) and relatively low fixed carbon content (8.8%), which may limit its direct application as a solid fuel due to reduced combustion efficiency and increased pollutant emissions (Khan et al., 2009 ). Additionally, the ratio of volatile matter to combustible carbon (0.87) was significantly high, indicating a lower combustion efficiency and higher emissions when used as a standalone solid fuel. Table 1 Proximate and ultimate analysis of SM and CW Property Unit Swine Manure (SM) Coffee Grounds Waste (CW) Proximate Analysis Moisture wt.% 96.4 61.9 Volatile matter wt.% 61.4 78.2 Fixed carbon wt.% 8.8 20.6 Ash wt.% 29.8 1.3 Ultimate Analysis Carbon (C) wt.% 40.0 51.0 Hydrogen (H) wt.% 6.0 6.6 Oxygen (O) wt.% 18.7 38.5 Nitrogen (N) wt.% 3.9 1.9 Sulphur (S) wt.% 1.7 0.7 High Heating value (HHV) MJ/kg 18.8 22.3 Lignin (%) wt.% 3.9 28.2 Cellulose (%) wt.% 2.7 16.7 Hemicellulose (%) wt.% 5.8 27.2 Carbohydrate (%) wt.% 21.1 77.3 Protein (%) wt.% 27.5 14.1 Fat (%) wt.% 19.9 7.2 On the other hand, CW had a significantly lower ash content (1.3%), and a higher fixed carbon content (20.6%) compared to SM. Although CW exhibited a higher volatile matter content (78.4%) than SM (61.4%), its volatile-to-combustible carbon ratio (0.79) was also high, posing challenges for its direct use as a solid fuel. The ultimate analysis revealed that CW contained higher carbon (51.0%) and oxygen (38.5%) contents, whereas SM exhibited lower carbon (40.0%) and oxygen (18.7%) contents. Generally, fuels with lower H/C and O/C ratios tend to experience reduced energy loss, smoke, and water vapor formation during combustion, making them more suitable for solid fuel applications (Liu et al., 2013 ). However, CW exhibited higher hydrogen and oxygen contents, resulting in H/C and O/C ratios that did not reach those of lignite or fossil fuels. Additionally, SM had relatively higher nitrogen (3.9%) and sulfur (1.7%) contents than CW, suggesting potential applications as an agricultural soil amendment. 2.2 Experimental apparatus In this study, HTC experiments were conducted using a custom-designed high-temperature and high-pressure reaction system, ensuring stable operation under extreme conditions. The experimental setup consisted of a reactor, heating system, pressure monitoring and control system, and cooling system, each optimized for efficient control and stable operation of the HTC process (Fig. 1 ). The HTC reactor was made of stainless steel (SUS 316) with a total internal volume of 1,000 mL. To enhance corrosion resistance, an anti-corrosion coating was applied to the internal surface. The reactor was designed to withstand a maximum operating temperature of 400°C and a maximum pressure of 150 bar, allowing for stable operation even under high-pressure conditions generated during the HTC process. The internal temperature of the reactor was monitored in real-time using a K-type thermocouple, and a PID (Proportional-Integral-Derivative) control system was employed to maintain temperature fluctuations within ± 5°C. The electric heating system provided a maximum heating rate of 10°C/min and was automatically regulated to maintain the set temperature for a predefined reaction duration. During the reaction, internal pressure was continuously monitored using a high-pressure gauge. To prevent overpressure conditions, the system was equipped with a safety valve that automatically activated when the maximum allowable pressure was exceeded. For rapid cooling after the reaction, an external cooling water circulation system was utilized. The cooling process was facilitated by a cooling coil attached to the reactor, which allowed efficient temperature reduction immediately after the reaction, thereby preventing excessive prolongation of the reaction. 2.3 Experimental conditions HTC experiments were conducted at 160°C, 200°C, 240°C, 280°C, and 320°C for individual feedstocks (SM and CW). For Co-HTC, SM and CW were mixed in 1:1, 3:1, and 5:1 mass ratio before being subjected to the HTC process. In all experiments, the total feedstock loading was maintained at 80 g, and based on previous experimental results, distilled water was added to maintain a fixed moisture content of 80%. The reaction time was set to 1 hour after reaching the target temperature. To ensure an oxygen-free environment, N₂ purging was performed at a flow rate of 400 mL/min for 10 minutes before heating. 2.4 Experimental methods This study evaluated the fuel properties of hydrochar produced from SM and CW via Co-HTC through various analytical methods. All analytical procedures were conducted in accordance with the Standard Methods for the Examination of Water and Wastewater, 24th edition (2023) to ensure reliability and reproducibility (APHA et al., 2023). (1) Elemental analysis Elemental composition analysis was conducted following ASTM D5373 and ISO 17247 standards. Approximately 1 g of the sample was combusted at high temperatures, and the generated gases were analyzed using a Flash 2000 Elemental Analyzer (Thermo Fisher Scientific, USA) to determine carbon (C), hydrogen (H), nitrogen (N), and sulfur (S) contents. (2) HHV measurement The HHV was determined according to ASTM D5865 by completely combusting 1 g of the sample in an oxygen-rich atmosphere. A Parr Model 1341 Plain Jacket Calorimeter (Parr Instrument Company, USA) was used to measure the thermal energy released during combustion. (3) Proximate analysis Proximate analysis was conducted according to ASTM D7582 to determine the moisture, volatile matter, fixed carbon, and ash content of the samples. Moisture content was measured by drying the sample at 105°C and recording the weight loss. Volatile matter was determined by heating the sample to 950°C under an inert atmosphere and measuring the weight reduction. Ash content was measured by combusting the sample at 650°C and weighing the remaining residue. Fixed carbon content was calculated as the remaining fraction after moisture, volatile matter, and ash were accounted for. (4) Thermogravimetric analysis (TGA) Thermal stability and decomposition characteristics of the hydrochar samples were evaluated using a TG 209 F3 thermogravimetric analyzer (Netzsch, Germany) following ASTM E1131. TGA and derivative thermogravimetric (DTG) analysis were performed to investigate combustion behavior (Cai et al., 2016 ; Yi et al., 2013 ). Approximately 5 ± 0.2 mg of the sample was subjected to a temperature range of 30–900°C at a heating rate of 20°C/min under an airflow rate of 20 mL/min. (5) Protein content analysis Protein content was determined following Standard Methods 4500-Norg B. 0.5–1 g of the sample was digested using a mixture of potassium sulfate and copper sulfate (9:1 wt%) with 10 mL of sulfuric acid for up to 90 minutes. The nitrogen content was quantified using a Foss KjeltecTM 8400 analyzer (Foss, Hillerød, Denmark) by titration with 0.1N hydrochloric acid solution. (6) Lipid content analysis Lipid content was determined according to ASTM D7060. 2–3 g of the sample was dried for 2 hours, then lipids were extracted using ether at 80°C for 8 hours. The remaining lipid mass was quantified after ether recovery using A2000 and XT 15 analyzers (Ankom Technology, Macedon, NY, USA). (7) Carbohydrate content calculation Carbohydrate content was calculated using elemental analysis results based on the following equation Carbohydrate (%) = 100 – Moisture (%) – Protein (%) – Fat (%) – Ash (%) (1) (8) Fiber analysis Fiber analysis was conducted following Standard Methods 973.18 and ASTM D1103. Ankom A2000 was used to determine lignin, cellulose, and hemicellulose contents in SM and CW. The samples were ground to less than 1 mm, and 72% sulfuric acid (H₂SO₄) pretreatment was applied to determine lignin content. Acid Detergent Fiber (ADF) and Neutral Detergent Fiber (NDF) methods were used to calculate cellulose and hemicellulose contents, respectively. (9) Hydrochar yield, energy density, and combustion characteristic index After each HTC experiment, the produced hydrochar was separated, dried, and weighed to determine hydrochar yield(HY) using Eq. (2). The energy densification ratio (EDR) was determined using Eq. (3), while the energy yield (EY) was calculated based on the measured HHV of each sample using Eq. (4) (Poomsawat and Poomsawat, 2021 ; Zhang et al., 2016 ). HY (%) = (M h /M b ) × 100 (2) EDR = HHV h /HHV b (3) EY (%) = EDR × HY (%) (4) where M h is the mass of dried hydrochar (g), M b is the initial mass of biomass (g), HHV h is the higher heating value of hydrochar (MJ/kg), and HHV b ​is the higher heating value of raw biomass (MJ/kg). The combustion characteristic index (S) is a key parameter for evaluating fuel ignition ease, burning velocity, and burnout temperature. It is calculated using Eq. (5), which quantifies overall combustion reactivity (Poomsawat and Poomsawat, 2021 ). S = [(dw/dt) max × (dw/dt) mean ] / (T i 2 × T f ) (5) where (dw/dt) max is the maximum mass loss rate (wt%/min), (dw/dt) mean is the average mass loss rate (wt%/min), T i​ is the ignition temperature (°C), and T f is the burnout temperature (°C). 3. Results and discussion 3.1 Hydrochar yield of SM and CW HTC is an efficient thermochemical process for converting biomass into high-energy-density solid fuel. The yield and characteristics of hydrochar significantly depend on the reaction temperature. In this study, hydrochar yield was analyzed at different HTC temperatures for SM and CW. The reaction temperature had a significant effect on hydrochar yield (Fig. 2 ). For SM, the hydrochar yield increased from 37.8% at 160°C to 50.8% at 320°C. Similarly, for CW, the yield increased from 22.2% at 160°C to 48.4% at 320°C. This increase can be attributed to the removal of volatile matter with increasing HTC temperature, leading to an increase in the relative carbon content of the hydrochar. These results align with previous studies, confirming the relationship between HTC temperature and hydrochar yield (Hoekman et al., 2011 ; Libra et al., 2011 ). The mass recovery of SM and CW increased with rising HTC temperatures. At 160°C, the mass recovery rates were 50.8% for SM and 48.4% for CW. As HTC temperature increased, the loss of volatile matter also increased, but the relative proportion of solid residue became more significant. CW exhibited a more pronounced decrease in mass recovery due to its higher volatile matter content. The decomposition of hemicellulose, cellulose, and lignin during HTC varies across different temperature ranges. Hemicellulose begins to decompose at approximately 180–200°C and is largely degraded by 260°C, releasing acetic acid, furfural, and hydroxymethylfurfural, which contribute to reduced oxygen content and enhanced fuel quality in hydrochar (Hoekman et al., 2011 ; Libra et al., 2011 ). Cellulose starts to decompose around 240°C, with major decomposition occurring between 280°C and 320°C. This process generates hydroxymethylfurfural, levoglucosan, and other volatile compounds, which contribute to an increase in fixed carbon content and energy density (Funke & Ziegler, 2010 ; Sevilla & Fuertes, 2009 ; Yang et al., 2007 ). Lignin decomposition starts around 280°C and continues gradually up to temperatures above 500°C, leading to the formation of aromatic compounds. The thermal stability of lignin contributes to the increase in fixed carbon content and the reduction of oxygen content in hydrochar, thereby improving its long-term fuel stability (Gascó et al., 2018 ; He et al., 2013 ). Furthermore, Wang et al. ( 2021 ) demonstrated that delignification significantly influences the thermal degradation reactivity of hemicellulose and cellulose in wood cell walls, providing useful insights into the thermal behavior of lignocellulosic biomass during thermochemical processing. The higher mass recovery of CW compared to SM at increasing HTC temperatures can be attributed to two key factors. First, CW contains a higher proportion of lignin and fixed carbon than SM. As HTC temperature increases, lignin undergoes transformation into stable aromatic compounds, thereby increasing the fixed carbon content of hydrochar (Sevilla & Fuertes, 2009 ). In contrast, SM has a relatively high ash content, leading to an increase in residual ash proportion as temperature rises, which limits fuel conversion efficiency (Gascó et al., 2018 ). Second, the lignin-cellulose network structure of CW enhances hydrochar formation. During HTC, the thermal stability and aromatic structure of lignin facilitate hydrochar retention (Libra et al., 2011 ). At temperatures above 280°C, CW's mass recovery increases significantly due to lignin's thermal stability. Since lignin decomposes over a broad temperature range (280–500°C), the formation of stable aromatic carbon structures results in an increased proportion of residual carbon, contributing to higher mass recovery (He et al., 2013 ). TGA results indicate that SM and CW exhibit slight mass loss below 100°C due to the removal of free and bound moisture (Fig. 3 ). This initial mass loss, caused by the evaporation of residual water in biomass, is commonly observed in HTC preheating stages (Jang et al., 2020 ; Lee et al., 2022 ). Between 160°C and 280°C, CW exhibited a more rapid mass loss compared to SM, primarily due to the decomposition of hemicellulose (180–260°C) and cellulose (240–320°C) (Apaydın Varol and Mutlu, 2023 ; Lee et al., 2022 ). CW, having a higher cellulose and lipid content, released more volatile matter within this temperature range. In contrast, SM exhibited a slower decomposition rate, with a portion of organic components persisting even at 240°C. Above 280°C, CW continued to exhibit gradual mass loss, although the rate of decomposition slowed. This trend is attributed to the thermal stability of lignin, which decomposes gradually between 280°C and 500°C, leading to the formation of stable aromatic structures (Cho et al., 2020 ). Meanwhile, SM demonstrated an increase in ash content above 280°C due to the presence of mineral components that remain stable at high temperatures. This phenomenon suggests that SM's conversion to hydrochar is limited by its higher inorganic content, which increases the proportion of non-combustible material rather than fixed carbon. CW was evaluated as having superior fuel properties compared to SM due to its higher fixed carbon content and lower ash content. At 320°C, CW exhibited optimal fuel characteristics, making it a favorable feedstock for high-quality solid fuel production. Conversely, SM, due to its high ash content, showed limited improvement in fuel quality with increasing HTC temperature, but its potential application as a carbon storage material remains significant. 3.2 Elemental composition and fuel characteristics The elemental composition and fuel characteristics of hydrochar provide critical insights into the thermochemical transformation of biomass during HTC. In this study, C, H, O, N, and S contents, as well as the H/C and O/C atomic ratios and HHV of hydrochar produced from SM and CW at various reaction temperatures, were analyzed. As shown in Fig. 4 , the carbon content of hydrochar exhibited a consistent increasing trend with rising HTC temperature. The carbon content of SM increased from 45.0% at 160°C to 54.2% at 320°C, while that of CW increased from 56.2% at 160°C to 76.5% at 320°C, indicating that CW had a higher carbon retention than SM. The greater increase in CW’s carbon content can be attributed to its high lignin content, which facilitates the formation of aromatic structures during HTC, leading to an increase in carbon density (Sevilla & Fuertes, 2009 ). In contrast, the carbonization of SM was relatively limited due to its high ash content, which increased with rising temperature and restricted fuel quality enhancement (Gascó et al., 2018 ). Meanwhile, the hydrogen and oxygen contents showed a decreasing trend with increasing HTC temperature. This reduction is mainly attributed to dehydration and decarboxylation reactions during HTC, which facilitate biomass carbonization by removing oxygen and hydrogen from the solid phase (Hoekman et al., 2011 ). The oxygen content of SM decreased significantly from 21.0% at 160°C to 4.4% at 320°C, while CW’s oxygen content dropped from 32.8–3.5% over the same temperature range. The enhanced oxygen removal with increasing HTC temperature led to an increase in fixed carbon content, resulting in improved fuel quality (Funke & Ziegler, 2010 ). As oxygen and hydrogen were progressively eliminated during HTC, the fixed carbon content increased, leading to an enhancement in HHV. The HHV of SM increased from 21.2 MJ/kg at 160°C to 25.1 MJ/kg at 320°C, while CW exhibited a more substantial increase from 23.7 MJ/kg to 32.9 MJ/kg. The greater improvement in HHV for CW is primarily attributed to its high lignin content, which promotes efficient carbonization at elevated temperatures. Lignin-rich biomass generally retains a higher proportion of fixed carbon during HTC, resulting in the production of high-energy-density fuel, aligning with previous findings (Funke & Ziegler, 2010 ). The Van Krevelen diagram in Fig. 5 illustrates the reduction in the H/C and O/C atomic ratios of SM and CW with increasing reaction temperature. As temperature increased, the H/C ratio of SM declined from 1.65 to 1.39, while CW’s H/C ratio decreased from 1.62 to 1.15. Similarly, the O/C ratio exhibited a significant decrease, with SM declining from 0.35 to 0.06 and CW from 0.44 to 0.03. This trend indicates that HTC effectively facilitates dehydration and deoxygenation reactions, leading to a reduction in atomic ratios (Reza et al., 2014 ). Notably, CW exhibited a sharper decrease in O/C ratio with increasing HTC temperature, ultimately attaining a carbon structure comparable to anthracite coal (Hoekman et al., 2011 ). In contrast, SM exhibited a relatively gradual reduction in O/C ratio, which can be attributed to its high ash content, limiting the extent of fuel quality improvement during HTC (Mumme et al., 2011 ). In summary, CW underwent more effective carbonization than SM during HTC, resulting in a higher energy-density fuel. The increase in HTC temperature led to an enhancement in the carbon content of hydrochar and a reduction in oxygen content, which significantly improved fuel quality. However, the presence of high ash content in SM restricted fuel quality improvement despite increasing HTC temperatures. The significant reduction in H/C and O/C atomic ratios of CW indicates that its hydrochar closely resembles anthracite coal, thereby improving its potential as a high-quality solid fuel. Optimizing HTC conditions can further maximize biomass conversion into high-energy fuel. 3.3 Effects of Co-HTC temperature and mixing ratio on hydrochar fuel properties The mixing ratio and reaction temperature in Co-HTC play a critical role in determining hydrochar fuel characteristics. Co-HTC is recognized as a promising technique for overcoming the limitations of individual biomass feedstocks and enhancing fuel quality through biomass interactions. This study evaluated the fuel characteristics of hydrochar produced at different mixing ratios (1:1, 3:1, 5:1) and reaction temperatures (160°C, 200°C, 240°C). The yield of hydrochar in HTC is influenced by the decomposition rate of biomass, the extent of volatile matter loss, and the efficiency of the carbonization reaction. In Co-HTC, biomass interactions further affect pyrolysis behavior and hydrochar formation process (Hoekman et al., 2011 ). At a 1:1 mixing ratio, hydrochar yield decreased from 58.4% at 160°C to 48.6% at 240°C (Fig. 6 ). The decline in yield was more pronounced at higher CW proportions (1:1 and 3:1), likely due to the high volatile content in CW, which was lost during HTC. In contrast, hydrochar yield was relatively stable in the 5:1 ratio, indicating that SM’s high ash content hindered complete carbonization (Gascó et al., 2018 ). While hydrochar yield generally decreases with increasing HTC temperature, this behavior is closely related to biomass composition and the complex chemical reactions occurring during HTC. The high lignin content of CW results in increased carbon density at elevated temperatures, but the removal of volatile fractions leads to greater mass loss. Conversely, SM’s high ash content leads to higher yield retention, primarily due to the residual presence of inorganic components rather than increased carbonization. To maximize hydrochar productivity in HTC, it is essential to consider biomass composition and thermochemical transformations while optimizing the mixing ratio. Excessive CW proportions may lead to higher volatile loss, while an overabundance of SM may result in increased ash content, reducing carbonization efficiency. Furthermore, hydrochar’s carbon content increased while oxygen content decreased with rising HTC temperature and CW proportion. This trend highlights the occurrence of dehydration, decarboxylation, and deoxygenation reactions in HTC, which contribute to fuel quality improvement (Hoekman et al., 2011 ; Wang et al., 2020 ). At a 1:1 mixing ratio, the carbon content increased from 56.4% at 160°C to 64.2% at 240°C. In contrast, the 5:1 mixing ratio showed a smaller increase from 47.5% at 160°C to 58.4% at 240°C, indicating that higher SM proportions restricted carbonization efficiency. CW’s high lignin content facilitated effective oxygen removal, whereas SM’s high ash content limited fuel quality improvement (Sevilla & Fuertes, 2009 ). These results suggest that Co-HTC can mitigate the limitations of individual biomass feedstocks. While CW alone retains high carbon, it has low nitrogen content, which may lead to nutrient losses during combustion. In contrast, SM’s high nitrogen content enhances its potential as a soil amendment (Mumme et al., 2011 ). Biomass subjected to HTC undergoes an increase in carbon density while simultaneously experiencing a reduction in oxygen and hydrogen content, leading to a decrease in the O/C and H/C atomic ratios (Funke & Ziegler, 2010 ). At a 1:1 mixing ratio, the O/C ratio decreased from 0.27 at 160°C to 0.18 at 240°C, while the H/C ratio declined from 1.65 to 1.34 (Fig. 7 ). The pronounced decrease in the O/C and H/C ratios at a higher CW proportion (1:1) is attributed to the high lignin content in CW, which facilitates the transformation into aromatic structures during HTC, effectively removing oxygen and hydrogen (Sevilla & Fuertes, 2009 ). These changes are primarily governed by dehydration, decarboxylation, and deoxygenation reactions occurring during HTC. Studies have shown that HTC conditions significantly influence carbonization efficiency and energy densification, particularly when applied to SM in different operational settings (Ipiales et al., 2024 ), further supporting the observed reduction in O/C and H/C ratios as biomass undergoes progressive conversion into aromatic and carbonaceous structures. This atomic ratio variation plays a critical role in determining fuel properties, as a lower O/C and H/C ratio is directly linked to an increase in energy density (Hoekman et al., 2011 ). At a 1:1 mixing ratio, the HHV increased from 24.4 MJ/kg at 160°C to 27.9 MJ/kg at 240°C. The higher CW proportion contributed to a substantial increase in HHV, primarily due to the enhanced formation of aromatic carbon structures. These results indicate that HTC promotes the removal of oxygen while increasing carbon density, thereby enhancing the energy density of the fuel (Funke & Ziegler, 2010 ). TGA and DTG analysis (Fig. 8 ) demonstrated that increasing HTC temperature significantly affected volatile matter release and thermal decomposition behavior. At a 1:1 mixing ratio, volatile matter loss occurred gradually at 160°C, whereas at 240°C, it became more pronounced, indicating substantial mass reduction. CW-rich hydrochars exhibited greater volatile release, attributed to the high lignin content in CW, which undergoes gradual thermal degradation over a broad temperature range (Sevilla & Fuertes, 2009 ). DTG curves revealed two distinct peaks for CW-rich hydrochars, corresponding to volatile release and fixed carbon formation. The high lignin content in CW contributed to a broader, slower degradation process, which enhanced combustion stability by sustaining char formation over an extended temperature range. Conversely, SM-rich hydrochars decomposed more rapidly, leading to higher volatile losses and reduced carbon retention. These observations are consistent with previous studies on biomass pyrolysis kinetics, which highlight lignin's role in delaying thermal degradation and promoting residual carbon formation (El-Sayed et al., 2024 ). In contrast, at a 5:1 mixing ratio, where the SM content was predominant, the thermal decomposition process was more gradual. The increased ash content in SM likely hindered thermal degradation, limiting overall mass loss despite the rise in HTC temperature (Gascó et al., 2018 ). By adjusting the mixing ratio and reaction temperature in the Co-HTC process, the fuel quality of biomass-derived hydrochar can be significantly improved. A higher CW proportion led to a marked increase in carbon density and HHV, highlighting the effectiveness of lignin-rich biomass in fuel property enhancement. In contrast, higher SM proportions resulted in increased ash content, which restricted fuel quality improvement but suggested potential applications for agricultural utilization rather than combustion-based energy production. With increasing HTC temperature, the continuous decline in the O/C and H/C ratios indicated that the hydrochar structure progressively transitioned to a composition resembling that of coal. The optimal fuel properties were achieved at a 1:1 mixing ratio and 240°C, demonstrating that this combination offers the best balance between fuel quality enhancement and hydrochar productivity. 3.4 Fuel performance evaluation The fuel performance of hydrochar produced through HTC is a critical factor in determining combustion characteristics and energy efficiency. In HTC-treated hydrochar, as temperature increases, volatile matter decreases while fixed carbon content increases, directly impacting combustion reactivity and energy density. This study evaluated fuel performance based on S, EDR, and EY. The S index is a crucial indicator of ignition ease, combustion rate, and burnout temperature. As shown in Table 2 , the 1:1 mixing ratio exhibited the highest S value at 160°C (39.0 × 10⁻⁹ %²/min²·°C³), while the lowest S value was observed at 240°C (9.1 × 10⁻⁹ %²/min²·°C³). This decline suggests that as HTC progresses, volatile matter is removed, while fixed carbon increases, leading to a reduction in combustion reactivity. In contrast, the 3:1 and 5:1 mixing ratios showed the highest S values at 200°C, which subsequently decreased at 240°C. In particular, the 5:1 mixing ratio demonstrated a higher ignition temperature (Ti) and lower combustion rate, indicating decreased reactivity. This behavior is primarily attributed to the high ash content in SM, which acts as an inert component, lowering combustion efficiency (Gascó et al., 2018 ). Consequently, at a 1:1 ratio, 160°C provided the best combustion performance, whereas at higher SM ratios, 200°C yielded more favorable combustion characteristics. Table 2 Combustion characteristic index of hydrochar produced under various HTC conditions Temperature (℃) SM:CW T i (°C) T f (°C) (dw/dt) max (%/min) (dw/dt) mean (%/min) S×10 − 9 (% 2 /min 2 ·°C 3 ) 160 1:1 165 550 -2.7628 -0.2115 39.0 3:1 175 575 -1.8244 -0.2029 21.0 5:1 175 610 -1.2486 -0.1854 12.4 200 1:1 175 580 -1.5487 -0.2052 17.9 3:1 180 560 -5.1611 -0.2142 60.9 5:1 180 575 -4.6929 -0.2082 52.4 240 1:1 185 545 -0.6955 -0.2439 9.1 3:1 185 515 -2.3273 -0.2509 33.1 5:1 185 570 -0.9396 -0.2060 9.9 HTC reduces volatile matter, leading to mass loss; however, it enhances carbon density, which improves HHV and overall fuel quality. Figure 9 illustrates the EDR variations under different HTC conditions. At 160°C, the EDR for the 1:1 ratio was 1.19, increasing to 1.36 at 240°C. The 3:1 ratio showed an EDR increase from 1.01 to 1.36, indicating significant energy densification. The 5:1 ratio exhibited a smaller increase in EDR from 1.02 to 1.38, due to the higher ash content in SM, which limits carbon densification. The increase in reaction temperature facilitated a higher EDR and EY, a trend consistent with previous comparative studies of HTC and low-temperature pyrolysis, where process conditions played a critical role in optimizing solid biofuel properties (Wang et al., 2019 ). The rise in EDR results from oxygen removal through dehydration and decarboxylation reactions during HTC, which enhances carbon concentration (Funke & Ziegler, 2010 ). However, higher SM content (5:1) introduces more ash, limiting energy densification. This suggests that fuel efficiency may be negatively affected by excessive inorganic content in SM. Meanwhile, EY (%) quantifies the proportion of energy retained in hydrochar compared to raw biomass. For the 1:1 ratio, EY increased from 45.2% at 160°C to 57% at 240°C. For the 3:1 ratio, EY increased from 34.8–60.3%, demonstrating a significant rise. The 5:1 ratio showed an EY increase from 34.9–61.8%, similar to the 3:1 ratio, despite its higher ash content. In all conditions, EY improved with rising temperature, confirming that HTC effectively enhances carbon retention. Notably, at 240°C, CW’s high lignin content facilitated fixed carbon formation, significantly increasing EY. This progressive increase in fixed carbon content with HTC not only enhances energy density but also improves combustion stability, particularly at higher temperatures (Hoekman et al., 2011 ; Yao et al., 2024 )." As the reaction temperature increased, the S index decreased, while both EDR and EY showed an increasing trend. This suggests that HTC improves fuel quality by removing volatile matter and increasing carbon density, but at the same time, it reduces ignition ease and combustion reactivity. The 1:1 mixing ratio at 240°C emerged as the optimal condition for maximizing EDR and EY. For the 5:1 ratio, ash accumulation resulted in a significant reduction in the S index, while EDR improvement remained limited. This confirms that Co-HTC with a higher CW proportion is more effective in enhancing carbonization efficiency and improving fuel properties. Optimizing HTC temperature and mixing ratio is crucial for achieving both improved combustion characteristics and energy density. 4. Conclusion This study assessed the fuel properties and energy performance of hydrochar produced from SM and CW via Co-HTC at various temperatures (160–240°C) and mixing ratios (1:1, 3:1, 5:1). The findings demonstrated that HTC significantly altered the elemental composition, heating value, and combustion reactivity of the biomass, enhancing its viability as a solid fuel. The optimal fuel quality improvement was observed at 240°C, particularly at the 1:1 mixing ratio, where C content increased to 64.2%, O/C and H/C atomic ratios declined, and HHV improved to 27.9 MJ/kg. However, as HTC temperature increased, the S decreased, indicating reduced ignition ease and burnout efficiency. Conversely, EDR and EY exhibited a consistent upward trend, with the highest EY values recorded at 57% (1:1), 60.3% (3:1), and 61.8% (5:1) at 240°C. This suggests that while higher CW content promoted carbon retention and fuel densification, higher SM content retained more energy despite its elevated ash content, which restricted further energy densification. These results indicate that Co-HTC effectively improves carbon density and energy retention, although ash accumulation in SM constrains combustion performance. The findings underscore the need to balance HTC conditions to optimize both carbonization efficiency and energy recovery, with the 1:1 and 3:1 mixing ratios at 240°C emerging as the most favorable conditions. This study confirms that Co-HTC is a viable strategy for converting organic waste into high-energy-density solid fuel, offering a sustainable approach to bioenergy production and waste valorization. Future research should explore process optimization strategies to further enhance combustion efficiency and mitigate ash-related challenges in hydrochar applications. Declarations Acknowledgments This paper was supported by Konkuk University Researcher Fund in 2024 and National Research Foundation grant funded by the Korea government (MSIT) (RS-2023-00219272, RS-202400338631). Funding This is not applicable. Authors’ Contributions Kyung Jin Min: Formal analysis, Data curation, Writing-original draft, Doo Young Oh: Formal analysis, Methodology, Eunyoung Lee: Data curation, Visualization, Jin Hwan Lee: Investigation, Do Yeon Kim: Investigation, Ki Young Park: Conceptualization, Supervision, Writing-review & editing Ethical Approval This is not applicable. Consent to Participate This is not applicable. Consent to Publish This is not applicable. Competing Interests The authors declare no competing interests. Data Availability Statement Data will be made available on reasonable request. References American Public Health Association (APHA), American Water Works Association (AWWA), & Water Environment Federation (WEF), 2023. Standard Methods for the Examination of Water and Wastewater (24th ed.). APHA Press, Washington, DC. Apaydın Varol, E., Mutlu, Ü., 2023. TGA-FTIR analysis of biomass samples based on the thermal decomposition behavior of hemicellulose, cellulose, and lignin. Energies 16(9), 3674. https://doi.org/10.3390/en16093674 Cai, J., Li, B., Chen, C., Wang, J., Zhao, M., Zhang, K., 2016. Hydrothermal carbonization of tobacco stalk for fuel application. Bioresour. Technol. 220, 305-311. https://doi.org/10.1016/j.biortech.2016.08.098 Cho, C.H., Lee, S.O., Kang, D.K., Hong, S.H., Kang, C.K., Lee, J.W., 2020. Preparation and characterization of lignin/chlorinated polyvinyl chloride blended fibers for low-cost carbon fibers. J. Korean Soc. Dyers Finish. 32(2), 111-119. https://doi.org/10.5764/TCF.2020.32.2.111 Dang, C.H., Farru, G., Glaser, C., Fischer, M.G., Libra, J.A., 2023. Enhancing the fuel properties of spent coffee grounds through hydrothermal carbonization: Output prediction and post-treatment approaches. Sustainability 16(1), 338. https://doi.org/10.3390/su16010338 Devnath, B., Khanal, S., Shah, A., Reza, T., 2024. Influence of hydrothermal carbonization (HTC) temperature on hydrochar and process liquid for poultry, swine, and dairy manure. Environ. 11(7), 150. https://doi.org/10.3390/environments11070150 El-Sayed, S.A., Khass, T.M., Mostafa, M.E., 2024. Thermal degradation behaviour and chemical kinetic characteristics of biomass pyrolysis using TG/DTG/DTA techniques. Biomass Convers. Biorefin. 14(15), 17779-17803. https://doi.org/10.1007/s13399-023-03926-2 Ferrentino, R., Sacchi, G., Scrinzi, D., Andreottola, G., Fiori, L., 2023. Valorization of swine manure for a circular approach through hydrothermal carbonization. Biomass Bioenergy 168, 106689. https://doi.org/10.1016/j.biombioe.2022.106689 Funke, A., Ziegler, F., 2010. Hydrothermal carbonization of biomass: A summary and discussion of chemical mechanisms for process engineering. Biofuels Bioprod. Biorefin. 4(2), 160–177. https://doi.org/10.1002/bbb.198 Gascó, G., Paz-Ferreiro, J., Álvarez, M.L., Saa, A., Méndez, A., 2018. Biochars and hydrochars prepared by pyrolysis and hydrothermal carbonisation of pig manure. Waste Manag. 79, 395–403. https://doi.org/10.1016/j.wasman.2018.08.015 He, C., Giannis, A., Wang, J.Y., 2013. Conversion of sewage sludge to clean solid fuel using hydrothermal carbonization: Hydrochar fuel characteristics and combustion behavior. Appl. Energy 111, 257–266. https://doi.org/10.1016/j.apenergy.2013.04.084 Hoekman, S.K., Broch, A., Robbins, C., 2011. Hydrothermal carbonization (HTC) of lignocellulosic biomass. Energy Fuels 25(4), 1802–1810. https://doi.org/10.1021/ef101745n Hu, Y., Gallant, R., Salaudeen, S., Farooque, A.A., He, S., 2022. Hydrothermal carbonization of spent coffee grounds for producing solid fuel. Sustainability 14(14), 8818. https://doi.org/10.3390/su14148818 Ipiales, R.P., Mohedano, A.F., Diaz-Portuondo, E., Diaz, E., De la Rubia, M.A., 2023. Co-hydrothermal carbonization of swine manure and lignocellulosic waste: A new strategy for the integral valorization of biomass wastes. Waste Manag. 169, 267-275. https://doi.org/10.1016/j.wasman.2023.07.018 Ipiales, R. P., Sarrion, A., Diaz, E., de la Rubia, M. A., Diaz-Portuondo, E., Coronella, C. J., & Mohedano, A. F., 2024. Swine manure management by hydrothermal carbonization: Comparative study of batch and continuous operation. Environ. Res. 245, 118062. https://doi.org/10.1016/j.envres.2023.118062 Ischia, G., Berge, N. D., Bae, S., Marzban, N., Román, S., Farru, G., Fiori, L., 2024. Advances in research and technology of hydrothermal carbonization: Achievements and future directions. Agronomy 14(5), 955. https://doi.org/10.3390/agronomy14050955 Islam, M. T., Reza, M. T., 2023. Evaluation of fuel and combustion properties of hydrochar derived from co-hydrothermal carbonization of biomass and plastic. Biomass Bioenergy 172, 106750. https://doi.org/10.1016/j.biombioe.2023.106750 Jang, E. S., Song, E. H., Yoon, J. H., Kim, Y. M., 2020. Study on the pyrolysis kinetics of solid fuel from livestock manure. J. Ind. Chem. 31(4), 443-451. https://doi.org/10.14478/ace.2020.1047 Khan, A. A., de Jong, W., Jansens, P. J., Spliethoff, H., 2009. Biomass combustion in fluidized bed boilers: Potential problems and remedies. Fuel Process. Technol. 90(1), 21-50. https://doi.org/10.1016/j.fuproc.2008.07.012 Khosravi, A., Yuan, Y., Liu, Q., Zheng, H., Hashemi, M., Tang, Y., Xing, B., 2024. Hydrochars as slow-release phosphorus fertilizers for enhancing corn and soybean growth in an agricultural soil. Carbon Res. 3(1), 7. https://doi.org/10.1007/s44246-023-00086-w Kim, D., Park, S., Park, K. Y., 2017. Upgrading the fuel properties of sludge and low-rank coal mixed fuel through hydrothermal carbonization. Energy 141, 598-602. https://doi.org/10.1016/j.energy.2017.09.113 Kumar, N., Weldon, R., Lynam, J. G., 2021. Hydrothermal carbonization of coffee silverskins. Biocatal. Agric. Biotechnol. 36, 102145. https://doi.org/10.1016/j.bcab.2021.102145 Lang, Q., Liu, Z., Li, Y., Xu, J., Li, J., Liu, B., Sun, Q., 2022. Combustion characteristics, kinetic and thermodynamic analyses of hydrochars derived from hydrothermal carbonization of cattle manure. J. Environ. Chem. Eng. 10(1), 106938. https://doi.org/10.1016/j.jece.2021.106938 Lang, Q., Zhang, B., Liu, Z., Chen, Z., Xia, Y., Li, D., Gai, C., 2019. Co-hydrothermal carbonization of corn stalk and swine manure: Combustion behavior of hydrochar by thermogravimetric analysis. Bioresour. Technol. 271, 75-83. https://doi.org/10.1016/j.biortech.2018.09.100 Lang, Q., Zhao, Q., Li, W., 2018. Co-hydrothermal carbonization of lignocellulosic biomass and swine manure: Hydrochar properties and heavy metal transformation behavior. Bioresour. Technol. 249, 464–470. https://doi.org/10.1016/j.biortech.2018.06.084 Lee, E. J., Jung, S. A., Jung, Y. J., Hyung, S. J., Lee, J. W., 2022. Chemical and structural properties of dry and wet torrefied biomass. Trends Agric. Life Sci. 60, 17-25. https://doi.org/10.29335/tals.2022.60.17 Lee, J., Park, K. Y., 2021. Conversion of heavy metal-containing biowaste from phytoremediation site to value-added solid fuel through hydrothermal carbonization. Environ. Pollut. 269, 116127. https://doi.org/10.1016/j.envpol.2020.116127 Lee, J., Cho, S., Kim, D., Ryu, J., Lee, K., Chung, H., Park, K. Y., 2021. Conversion of slaughterhouse wastes to solid fuel using hydrothermal carbonization. Energies 14(6), 1768. https://doi.org/10.3390/en14061768 Li, Q., Lin, H., Zhang, S., Yuan, X., Gholizadeh, M., Wang, Y., Hu, X., 2021. Co-hydrothermal carbonization of swine manure and cellulose: Influence of mutual interaction of intermediates on properties of the products. Sci. Total Environ. 791, 148134. https://doi.org/10.1016/j.scitotenv.2021.148134 Libra, J. A., Ro, K. S., Kammann, C., Funke, A., Berge, N. D., Neubauer, Y., Emmerich, K. H., 2011. Hydrothermal carbonization of biomass: A summary and discussion of chemical mechanisms for process engineering. Biofuels Bioprod. Biorefin. 5(5), 547–552. https://doi.org/10.1002/bbb.198 Liu, Z., Quek, A., Hoekman, S. K., Balasubramanian, R., 2013. Production of solid biochar fuel from waste biomass by hydrothermal carbonization. Fuel 103, 943-949. https://doi.org/10.1016/j.fuel.2012.07.069 Lucian, M., Volpe, M., Gao, L., Piro, G., Goldfarb, J. L., Fiori, L., 2018. Impact of hydrothermal carbonization conditions on the formation of hydrochars and secondary chars from the organic fraction of municipal solid waste. Fuel 233, 257-268. https://doi.org/10.1016/j.fuel.2018.06.060 Mäkelä, M., Benavente, V., Fullana, A., 2015. Hydrothermal carbonization of lignocellulosic biomass: Influence of process parameters on hydrochar properties. Appl. Energy 155, 576-584. https://doi.org/10.1016/j.apenergy.2015.06.022 Massaya, J., Pickens, G., Mills-Lamptey, B., Chuck, C. J., 2021. Enhanced hydrothermal carbonization of spent coffee grounds for the efficient production of solid fuel with lower nitrogen content. Energy Fuels 35(11), 9462-9473. https://doi.org/10.1021/acs.energyfuels.1c00870 Mumme, J., Eckervogt, L., Pielert, J., Diakité, M., Rupp, F., Kern, J., 2011. Hydrothermal carbonization of anaerobically digested maize silage. Bioresour. Technol. 102(19), 9255–9260. https://doi.org/10.1016/j.biortech.2011.06.099 Park, J. E., Lee, G. B., Jeong, C. J., Kim, H., Kim, C. G., 2021. Determination of relationship between higher heating value and atomic ratio of hydrogen to carbon in spent coffee grounds by hydrothermal carbonization. Energies 14(20), 6551. https://doi.org/10.3390/en14206551 Park, K. Y., Lee, K., Kim, D., 2018. Characterized hydrochar of algal biomass for producing solid fuel through hydrothermal carbonization. Bioresour. Technol. 258, 119-124. https://doi.org/10.1016/j.biortech.2018.03.003 Pauline, A. L., Joseph, K., 2020. Hydrothermal carbonization of organic wastes to carbonaceous solid fuel–A review of mechanisms and process parameters. Fuel 279, 118472. https://doi.org/10.1016/j.fuel.2020.11847 Poomsawat, S., Poomsawat, W., 2021. Analysis of hydrochar fuel characterization and combustion behavior derived from aquatic biomass via hydrothermal carbonization process. Case Stud. Therm. Eng. 27, 101255. https://doi.org/10.1016/j.csite.2021.101255 Reza, M. T., Becker, G., Sachsenheimer, K., Mumme, J., 2014. Hydrothermal carbonization (HTC): Near infrared spectroscopy and partial least squares regression for hydrochar process optimization. Appl. Energy 135, 379–385. https://doi.org/10.1016/j.biortech.2014.03.008 Santana, M. S., Santana, W. M., Lira, T. S., 2020. Hydrochar production from defective coffee beans by hydrothermal carbonization. Bioresour. Technol. 300, 122653. https://doi.org/10.1016/j.biortech.2019.122653 Sevilla, M., Fuertes, A. B., 2009. The production of carbon materials by hydrothermal carbonization of cellulose. Carbon 47(9), 2281–2289. https://doi.org/10.1016/j.carbon.2009.04.026 Sharma, R., Jasrotia, K., Singh, N., Ghosh, P., Srivastava, S., Sharma, N. R., Kumar, A., 2020. A comprehensive review on hydrothermal carbonization of biomass and its applications. Chem. Afr. 3, 1-19. https://doi.org/10.1007/s42250-019-00098-3 Wang, C., Fan, Y., Hornung, U., Zhu, W., Dahmen, N., 2020. Char and tar formation during hydrothermal treatment of sewage sludge in subcritical and supercritical water: Effect of organic matter composition and experiments with model compounds. J. Clean. Prod. 242, 118586. https://doi.org/10.1016/j.jclepro.2019.118586 Wang, J., Minami, E., Asmadi, M., Kawamoto, H., 2021. Effect of delignification on thermal degradation reactivities of hemicellulose and cellulose in wood cell walls. J. Wood Sci. 67, 1-11. https://doi.org/10.1186/s10086-021-01952-0 Wang, Y., Qiu, L., Zhu, M., Sun, G., Zhang, T., Kang, K., 2019. Comparative evaluation of hydrothermal carbonization and low-temperature pyrolysis of Eucommia ulmoides Oliver for the production of solid biofuel. Sci. Rep. 9(1), 5535. https://doi.org/10.1038/s41598-019-38849-4 Xiong, J. B., Pan, Z. Q., Xiao, X. F., Huang, H. J., Lai, F. Y., Wang, J. X., Chen, S. W., 2019. Study on the hydrothermal carbonization of swine manure: The effect of process parameters on the yield/properties of hydrochar and process water. J. Anal. Appl. Pyrolysis 144, 104692. https://doi.org/10.1016/j.jaap.2019.104692 Xiong, W., Luo, Y., Shangguan, W., Deng, Y., Li, R., Song, D., Xiao, R., 2024. Co-hydrothermal carbonization of lignocellulosic biomass and swine manure: Optimal parameters for enhanced nutrient reclamation, carbon sequestration, and heavy metals passivation. Waste Manage. 190, 174-185. https://doi.org/10.1016/j.wasman.2024.09.019 Yang, H., Yan, R., Chen, H., Lee, D. H., Zheng, C., 2007. Characteristics of hemicellulose, cellulose, and lignin pyrolysis. Fuel 86(12-13), 1781-1788. https://doi.org/10.1016/j.fuel.2006.12.013 Yao, Y., Xia, P., Zhao, L., Yang, C., Huang, R., Wang, K., 2024. Effects of temperature and duration on the combustion characteristics of hydrochar prepared from traditional Chinese medicine residues using hydrothermal carbonization process. Int. J. Energy Res. 2024(1), 3595643. https://doi.org/10.1155/er/3595643 Yi, Q., Qi, F., Cheng, G., Zhang, Y., Xiao, B., Hu, Z., Xu, S., 2013. Thermogravimetric analysis of co-combustion of biomass and biochar. J. Therm. Anal. Calorim. 112, 1475-1479. https://doi.org/10.1007/s10973-012-2744-1 Zhao, P., Lin, C., Li, Y., Wu, S., Cui, X., Ding, Y., Liu, F., Guo, Q., 2021. Value-added products from pyrolysis of hydrochar derived from polyvinyl chloride and alkali coal. J. Clean. Prod. 329, 129769. https://doi.org/10.1016/j.jclepro.2021.129769 Zhang, S., Chen, T., Li, W., Dong, Q., Xiong, Y., 2016. Physicochemical properties and combustion behavior of duckweed during wet torrefaction. Bioresour. Technol. 218, 1157-1162. https://doi.org/10.1016/j.biortech.2016.07.086 Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6199561","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":447418202,"identity":"63375c00-2959-43a4-a8ec-890cc7c354a7","order_by":0,"name":"Kyung Jin Min","email":"","orcid":"","institution":"Konkuk University","correspondingAuthor":false,"prefix":"","firstName":"Kyung","middleName":"Jin","lastName":"Min","suffix":""},{"id":447418203,"identity":"000629c6-7299-4d7c-8444-73b5693a0af6","order_by":1,"name":"Doo Young Oh","email":"","orcid":"","institution":"Konkuk University","correspondingAuthor":false,"prefix":"","firstName":"Doo","middleName":"Young","lastName":"Oh","suffix":""},{"id":447418204,"identity":"d8271f5f-ac04-4920-b60e-6b83f1ce0d6e","order_by":2,"name":"Eunyoung Lee","email":"","orcid":"","institution":"Konkuk University","correspondingAuthor":false,"prefix":"","firstName":"Eunyoung","middleName":"","lastName":"Lee","suffix":""},{"id":447418205,"identity":"39befeb3-a043-48c5-9efb-366cad5343da","order_by":3,"name":"Jin Hwan Lee","email":"","orcid":"","institution":"Konkuk University","correspondingAuthor":false,"prefix":"","firstName":"Jin","middleName":"Hwan","lastName":"Lee","suffix":""},{"id":447418206,"identity":"56f02180-7058-48ed-873d-08ab0acd0cce","order_by":4,"name":"Do Yeon Kim","email":"","orcid":"","institution":"Konkuk University","correspondingAuthor":false,"prefix":"","firstName":"Do","middleName":"Yeon","lastName":"Kim","suffix":""},{"id":447418207,"identity":"6063ff6e-5605-42da-b9f2-44028c7e144b","order_by":5,"name":"Ki Young Park","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA2klEQVRIiWNgGAWjYBACCTB5wMaAgYGNgbEBxOEhTksa6VoOk6BFckbuwc8VZ84b80u3pUnOYLCTZ+A5+wCvFmmJvGTJMzdum0nOOXZMcgNDsmEDb7sBXi1yEjkGkg0fbtsY3Ehvk3zAwJzAwM+G32FALcY/Gz6cg2mpJ6xFWiLHTLLhxgEzgxtpIIcdTmDgbcOvRbLnXZplw5lkY8kZacmWMwyOG7bxHMOvReJ47uGbDcfsDPsl0gxv9lRUy/PzpOHXwiCQg8wDRw8hwH+GoJJRMApGwSgY6QAArVJCEM13e9cAAAAASUVORK5CYII=","orcid":"","institution":"Konkuk University","correspondingAuthor":true,"prefix":"","firstName":"Ki","middleName":"Young","lastName":"Park","suffix":""}],"badges":[],"createdAt":"2025-03-11 02:47:07","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6199561/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6199561/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":81573596,"identity":"def03ea3-1e9b-4ca1-ba81-20e393c81384","added_by":"auto","created_at":"2025-04-28 16:59:06","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":51433,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of the hydrothermal carbonization system.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-6199561/v1/d97b08cb46db6dae04737327.png"},{"id":81573827,"identity":"7ff418e2-ba3d-4fc6-b925-68dfb755ca03","added_by":"auto","created_at":"2025-04-28 17:07:06","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":238246,"visible":true,"origin":"","legend":"\u003cp\u003eFixed carbon and ash composition of hydrochar produced from SM and CW at different temperatures.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-6199561/v1/cf71cacd70158dbc51f18c96.png"},{"id":81574643,"identity":"df473854-0287-4c8d-9aeb-58686b18140e","added_by":"auto","created_at":"2025-04-28 17:15:06","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":237219,"visible":true,"origin":"","legend":"\u003cp\u003eTGA curves of hydrochar produced from SM and CW.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-6199561/v1/8c212f98acbca6f1675cd302.png"},{"id":81574645,"identity":"a0c4cee2-af4d-4dd3-a490-d831488ceeef","added_by":"auto","created_at":"2025-04-28 17:15:06","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":360962,"visible":true,"origin":"","legend":"\u003cp\u003eElemental composition (wt%) and HHV changes of hydrochar produced from SM and CW at different temperatures.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-6199561/v1/7fc0c8965cf3ecde6a4de83f.png"},{"id":81573828,"identity":"c085f389-f08f-4958-9eef-1c88a10aaca3","added_by":"auto","created_at":"2025-04-28 17:07:06","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":257248,"visible":true,"origin":"","legend":"\u003cp\u003eVan krevelen diagram of hydrochar produced from SM and CW at different temperatures.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-6199561/v1/b500ad4b502dd2d5cf49b0a0.png"},{"id":81573830,"identity":"9de7a58f-fb1b-40af-bd5f-cd9e44811dde","added_by":"auto","created_at":"2025-04-28 17:07:06","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":360999,"visible":true,"origin":"","legend":"\u003cp\u003eElemental composition (C, H, O, N, S) and yield of hydrochar produced from Co-HTC at different mixing ratios and temperatures\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-6199561/v1/c7b86020bc037c15b2465d12.png"},{"id":81573617,"identity":"9f045d69-1475-451e-8596-9564914c7aa3","added_by":"auto","created_at":"2025-04-28 16:59:06","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":287165,"visible":true,"origin":"","legend":"\u003cp\u003eVan krevelen diagram and HHV variation of hydrochar as a function of mixing ratio and reaction temperature.\u003c/p\u003e","description":"","filename":"image7.png","url":"https://assets-eu.researchsquare.com/files/rs-6199561/v1/de63db1f14e47f2b1777e158.png"},{"id":81574644,"identity":"68c96833-4d60-4626-a5fe-2a81f96d018e","added_by":"auto","created_at":"2025-04-28 17:15:06","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":614850,"visible":true,"origin":"","legend":"\u003cp\u003eTGA and DTG analysis of hydrochar derived from Co-HTC at different reaction conditions; a) 160℃, b) 200℃, c) 240℃.\u003c/p\u003e","description":"","filename":"image8.png","url":"https://assets-eu.researchsquare.com/files/rs-6199561/v1/ca5d3439cd1765cbd903a5e4.png"},{"id":81574994,"identity":"a3c35e77-dee5-4735-a71d-9e71a2dbb116","added_by":"auto","created_at":"2025-04-28 17:23:06","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":214352,"visible":true,"origin":"","legend":"\u003cp\u003eEnergy yield and energy densification ratio of hydrochar produced under fifferent HTC conditions.\u003c/p\u003e","description":"","filename":"image9.png","url":"https://assets-eu.researchsquare.com/files/rs-6199561/v1/96b22c8cc93e24551e7fcde5.png"},{"id":89115510,"identity":"fe137abe-1b15-485e-8369-b9877596f6be","added_by":"auto","created_at":"2025-08-14 22:16:39","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3429939,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6199561/v1/aeaa6eb6-3ba7-450e-9263-0c19c30b0567.pdf"}],"financialInterests":"","formattedTitle":"Enhancing Fuel Properties and Energy Performance with Co-Hydrothermal Carbonization of Coffee Waste and Swine Manure","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eEfficient biomass management plays a crucial role in sustainable development, addressing both waste treatment and energy recovery. Improper disposal of organic waste contributes to greenhouse gas emissions, water contamination, and soil degradation, leading to significant environmental and public health concerns (Ferrentino et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; He et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Libra et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Conventional waste treatment methods such as landfilling and incineration have low efficiency and are not environmentally sustainable, driving increasing interest in biomass conversion technologies for energy recovery (Funke and Ziegler, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Kim et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Park et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Sevilla and Fuertes, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2009\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eHydrothermal carbonization (HTC) has emerged as an eco-friendly thermochemical process for converting biomass into high-energy-density solid fuels (Li et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Lucian et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Pauline and Joseph, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Recent studies have compared batch and continuous HTC processes to optimize hydrochar production from various biomass sources, particularly swine manure (Ipiales et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Sharma et al. (\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) reviewed HTC processes, highlighting key mechanisms, operating parameters, and applications in energy and environmental sectors. Ischia et al. (\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2024\u003c/span\u003e) further summarized recent advances, technological challenges, and future directions in HTC research. HTC is particularly advantageous as it eliminates the need for energy-intensive drying processes, making it cost-effective and environmentally friendly (Hoekman et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Lee and Park, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Lee et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Conducted at moderate temperatures (180\u0026ndash;250\u0026deg;C) under autogenous pressure, HTC facilitates dehydration and decarboxylation reactions, leading to increased carbon content and reduced oxygen levels in the biomass (Gasc\u0026oacute; et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Massaya et al., \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Mumme et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Park et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). The resulting hydrochar exhibits high energy density, reduced oxygen content, and improved combustion characteristics, making it suitable for long-term storage and handling (Zhao et al., \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eRecent studies suggest that co-hydrothermal carbonization (Co-HTC) is a promising alternative to overcome the limitations of single-feedstock HTC (Ipiales et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). By co-processing multiple biomass sources, Co-HTC enhances fuel properties while mitigating individual feedstock drawbacks (Islam and Reza, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Lang et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eSwine manure (SM) and coffee waste (CW) are two organic waste types that require immediate and effective management due to their environmental impacts (Xiong et al., \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). SM contains high nitrogen and sulfur levels, posing risks of water contamination and eutrophication, while its high ash content can lower fuel quality (Gasc\u0026oacute; et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Lang et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Mumme et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). In contrast, CW is characterized by high fixed carbon content and low ash levels, making it a promising feedstock for high-quality hydrochar production (Hu et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Kumar et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Santana et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Furthermore, recent work by Dang et al. (\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) demonstrated that applying tailored HTC conditions to spent coffee grounds can significantly enhance fuel properties and improve energy recovery potential. However, during HTC, the thermal degradation of lignin in CW can generate aromatic compounds and volatile organic compounds (VOCs), potentially increasing emissions and residual pollutants in the process water (Dang et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Hoekman et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; M\u0026auml;kel\u0026auml; et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2015\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eCo-HTC has the potential to overcome these challenges by utilizing the complementary properties of SM and CW to improve fuel characteristics (Ipiales et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). The high carbon and lignin content in CW enhances energy density, while the nitrogen content in SM improves the potential use of hydrochar as a soil amendment (Devnath et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Khosravi et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Islam and Reza, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Xiong et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Zhao et al., \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Additionally, Co-HTC has been shown to reduce ash content and improve combustion behavior, thereby enhancing hydrochar quality (Gasc\u0026oacute; et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Lang et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Mumme et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Sevilla and Fuertes, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2009\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThis study aims to evaluate the fuel properties and thermal behavior of hydrochar produced through Co-HTC of SM and CW. To achieve this, (1) the effects of reaction temperature on hydrochar yield and fuel characteristics were analyzed, and (2) the influence of mixing ratio (1:1, 3:1, 5:1) and reaction temperature (160\u0026deg;C, 200\u0026deg;C, 240\u0026deg;C) on fuel performance was systematically investigated. Ultimately, the synergistic effects of Co-HTC were assessed to identify the optimal process conditions for producing high-quality solid fuel. The findings of this study provide insights into the potential for developing high-efficiency biofuels from organic waste, contributing to sustainable waste management and advancements in energy conversion technologies.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Materials\u003c/h2\u003e \u003cp\u003eIn this study, SM and CW were used as feedstocks for the Co-HTC process. SM was collected from a livestock farm in Gyeonggi Province, South Korea, while CW was obtained from a commercial coffee shop in Seoul, South Korea. Prior to HTC experiments, both feedstocks were dried at 105\u0026deg;C for 24 hours and then ground to a particle size of \u0026le;\u0026thinsp;2 mm to ensure homogeneity.\u003c/p\u003e \u003cp\u003eThe proximate and ultimate compositions of SM and CW are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. SM exhibited a high ash content (29.8%) and relatively low fixed carbon content (8.8%), which may limit its direct application as a solid fuel due to reduced combustion efficiency and increased pollutant emissions (Khan et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). Additionally, the ratio of volatile matter to combustible carbon (0.87) was significantly high, indicating a lower combustion efficiency and higher emissions when used as a standalone solid fuel.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eProximate and ultimate analysis of SM and CW\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eProperty\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eUnit\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSwine Manure (SM)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCoffee Grounds Waste (CW)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"4\" nameend=\"c4\" namest=\"c1\"\u003e \u003cp\u003eProximate Analysis\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMoisture\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ewt.%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e96.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e61.9\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eVolatile matter\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ewt.%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e61.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e78.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFixed carbon\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ewt.%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e8.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e20.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAsh\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ewt.%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e29.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"4\" nameend=\"c4\" namest=\"c1\"\u003e \u003cp\u003eUltimate Analysis\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCarbon (C)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ewt.%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e40.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e51.0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHydrogen (H)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ewt.%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e6.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eOxygen (O)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ewt.%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e18.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e38.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNitrogen (N)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ewt.%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.9\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSulphur (S)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ewt.%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHigh Heating value (HHV)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMJ/kg\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e18.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e22.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLignin (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ewt.%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e28.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCellulose (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ewt.%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e16.7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHemicellulose (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ewt.%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e27.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCarbohydrate (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ewt.%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e21.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e77.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eProtein (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ewt.%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e27.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e14.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFat (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ewt.%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e19.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e7.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eOn the other hand, CW had a significantly lower ash content (1.3%), and a higher fixed carbon content (20.6%) compared to SM. Although CW exhibited a higher volatile matter content (78.4%) than SM (61.4%), its volatile-to-combustible carbon ratio (0.79) was also high, posing challenges for its direct use as a solid fuel.\u003c/p\u003e \u003cp\u003eThe ultimate analysis revealed that CW contained higher carbon (51.0%) and oxygen (38.5%) contents, whereas SM exhibited lower carbon (40.0%) and oxygen (18.7%) contents. Generally, fuels with lower H/C and O/C ratios tend to experience reduced energy loss, smoke, and water vapor formation during combustion, making them more suitable for solid fuel applications (Liu et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). However, CW exhibited higher hydrogen and oxygen contents, resulting in H/C and O/C ratios that did not reach those of lignite or fossil fuels. Additionally, SM had relatively higher nitrogen (3.9%) and sulfur (1.7%) contents than CW, suggesting potential applications as an agricultural soil amendment.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Experimental apparatus\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eIn this study, HTC experiments were conducted using a custom-designed high-temperature and high-pressure reaction system, ensuring stable operation under extreme conditions. The experimental setup consisted of a reactor, heating system, pressure monitoring and control system, and cooling system, each optimized for efficient control and stable operation of the HTC process (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe HTC reactor was made of stainless steel (SUS 316) with a total internal volume of 1,000 mL. To enhance corrosion resistance, an anti-corrosion coating was applied to the internal surface. The reactor was designed to withstand a maximum operating temperature of 400\u0026deg;C and a maximum pressure of 150 bar, allowing for stable operation even under high-pressure conditions generated during the HTC process.\u003c/p\u003e \u003cp\u003eThe internal temperature of the reactor was monitored in real-time using a K-type thermocouple, and a PID (Proportional-Integral-Derivative) control system was employed to maintain temperature fluctuations within \u0026plusmn;\u0026thinsp;5\u0026deg;C. The electric heating system provided a maximum heating rate of 10\u0026deg;C/min and was automatically regulated to maintain the set temperature for a predefined reaction duration.\u003c/p\u003e \u003cp\u003eDuring the reaction, internal pressure was continuously monitored using a high-pressure gauge. To prevent overpressure conditions, the system was equipped with a safety valve that automatically activated when the maximum allowable pressure was exceeded.\u003c/p\u003e \u003cp\u003eFor rapid cooling after the reaction, an external cooling water circulation system was utilized. The cooling process was facilitated by a cooling coil attached to the reactor, which allowed efficient temperature reduction immediately after the reaction, thereby preventing excessive prolongation of the reaction.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Experimental conditions\u003c/h2\u003e \u003cp\u003eHTC experiments were conducted at 160\u0026deg;C, 200\u0026deg;C, 240\u0026deg;C, 280\u0026deg;C, and 320\u0026deg;C for individual feedstocks (SM and CW). For Co-HTC, SM and CW were mixed in 1:1, 3:1, and 5:1 mass ratio before being subjected to the HTC process.\u003c/p\u003e \u003cp\u003eIn all experiments, the total feedstock loading was maintained at 80 g, and based on previous experimental results, distilled water was added to maintain a fixed moisture content of 80%. The reaction time was set to 1 hour after reaching the target temperature. To ensure an oxygen-free environment, N₂ purging was performed at a flow rate of 400 mL/min for 10 minutes before heating.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Experimental methods\u003c/h2\u003e \u003cp\u003eThis study evaluated the fuel properties of hydrochar produced from SM and CW via Co-HTC through various analytical methods. All analytical procedures were conducted in accordance with the Standard Methods for the Examination of Water and Wastewater, 24th edition (2023) to ensure reliability and reproducibility (APHA et al., 2023).\u003c/p\u003e \u003cp\u003e(1) Elemental analysis\u003c/p\u003e \u003cp\u003eElemental composition analysis was conducted following ASTM D5373 and ISO 17247 standards. Approximately 1 g of the sample was combusted at high temperatures, and the generated gases were analyzed using a Flash 2000 Elemental Analyzer (Thermo Fisher Scientific, USA) to determine carbon (C), hydrogen (H), nitrogen (N), and sulfur (S) contents.\u003c/p\u003e \u003cp\u003e(2) HHV measurement\u003c/p\u003e \u003cp\u003eThe HHV was determined according to ASTM D5865 by completely combusting 1 g of the sample in an oxygen-rich atmosphere. A Parr Model 1341 Plain Jacket Calorimeter (Parr Instrument Company, USA) was used to measure the thermal energy released during combustion.\u003c/p\u003e \u003cp\u003e(3) Proximate analysis\u003c/p\u003e \u003cp\u003eProximate analysis was conducted according to ASTM D7582 to determine the moisture, volatile matter, fixed carbon, and ash content of the samples. Moisture content was measured by drying the sample at 105\u0026deg;C and recording the weight loss. Volatile matter was determined by heating the sample to 950\u0026deg;C under an inert atmosphere and measuring the weight reduction. Ash content was measured by combusting the sample at 650\u0026deg;C and weighing the remaining residue. Fixed carbon content was calculated as the remaining fraction after moisture, volatile matter, and ash were accounted for.\u003c/p\u003e \u003cp\u003e(4) Thermogravimetric analysis (TGA)\u003c/p\u003e \u003cp\u003eThermal stability and decomposition characteristics of the hydrochar samples were evaluated using a TG 209 F3 thermogravimetric analyzer (Netzsch, Germany) following ASTM E1131. TGA and derivative thermogravimetric (DTG) analysis were performed to investigate combustion behavior (Cai et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Yi et al., \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Approximately 5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2 mg of the sample was subjected to a temperature range of 30\u0026ndash;900\u0026deg;C at a heating rate of 20\u0026deg;C/min under an airflow rate of 20 mL/min.\u003c/p\u003e \u003cp\u003e(5) Protein content analysis\u003c/p\u003e \u003cp\u003eProtein content was determined following Standard Methods 4500-Norg B. 0.5\u0026ndash;1 g of the sample was digested using a mixture of potassium sulfate and copper sulfate (9:1 wt%) with 10 mL of sulfuric acid for up to 90 minutes. The nitrogen content was quantified using a Foss KjeltecTM 8400 analyzer (Foss, Hiller\u0026oslash;d, Denmark) by titration with 0.1N hydrochloric acid solution.\u003c/p\u003e \u003cp\u003e(6) Lipid content analysis\u003c/p\u003e \u003cp\u003eLipid content was determined according to ASTM D7060. 2\u0026ndash;3 g of the sample was dried for 2 hours, then lipids were extracted using ether at 80\u0026deg;C for 8 hours. The remaining lipid mass was quantified after ether recovery using A2000 and XT 15 analyzers (Ankom Technology, Macedon, NY, USA).\u003c/p\u003e \u003cp\u003e(7) Carbohydrate content calculation\u003c/p\u003e \u003cp\u003eCarbohydrate content was calculated using elemental analysis results based on the following equation\u003c/p\u003e \u003cp\u003eCarbohydrate (%)\u0026thinsp;=\u0026thinsp;100 \u0026ndash; Moisture (%) \u0026ndash; Protein (%) \u0026ndash; Fat (%) \u0026ndash; Ash (%) (1)\u003c/p\u003e \u003cp\u003e(8) Fiber analysis\u003c/p\u003e \u003cp\u003eFiber analysis was conducted following Standard Methods 973.18 and ASTM D1103. Ankom A2000 was used to determine lignin, cellulose, and hemicellulose contents in SM and CW. The samples were ground to less than 1 mm, and 72% sulfuric acid (H₂SO₄) pretreatment was applied to determine lignin content. Acid Detergent Fiber (ADF) and Neutral Detergent Fiber (NDF) methods were used to calculate cellulose and hemicellulose contents, respectively.\u003c/p\u003e \u003cp\u003e(9) Hydrochar yield, energy density, and combustion characteristic index\u003c/p\u003e \u003cp\u003eAfter each HTC experiment, the produced hydrochar was separated, dried, and weighed to determine hydrochar yield(HY) using Eq.\u0026nbsp;(2). The energy densification ratio (EDR) was determined using Eq.\u0026nbsp;(3), while the energy yield (EY) was calculated based on the measured HHV of each sample using Eq.\u0026nbsp;(4) (Poomsawat and Poomsawat, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Zhang et al., \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2016\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eHY (%) = (M\u003csub\u003eh\u003c/sub\u003e/M\u003csub\u003eb\u003c/sub\u003e) \u0026times; 100 (2)\u003c/p\u003e \u003cp\u003eEDR\u0026thinsp;=\u0026thinsp;HHV\u003csub\u003eh\u003c/sub\u003e/HHV\u003csub\u003eb\u003c/sub\u003e (3)\u003c/p\u003e \u003cp\u003eEY (%)\u0026thinsp;=\u0026thinsp;EDR \u0026times; HY (%) (4)\u003c/p\u003e \u003cp\u003ewhere M\u003csub\u003eh\u003c/sub\u003e is the mass of dried hydrochar (g), M\u003csub\u003eb\u003c/sub\u003e is the initial mass of biomass (g), HHV\u003csub\u003eh\u003c/sub\u003e is the higher heating value of hydrochar (MJ/kg), and HHV\u003csub\u003eb\u003c/sub\u003e ​is the higher heating value of raw biomass (MJ/kg).\u003c/p\u003e \u003cp\u003eThe combustion characteristic index (S) is a key parameter for evaluating fuel ignition ease, burning velocity, and burnout temperature. It is calculated using Eq.\u0026nbsp;(5), which quantifies overall combustion reactivity (Poomsawat and Poomsawat, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eS = [(dw/dt)\u003csub\u003emax\u003c/sub\u003e \u0026times; (dw/dt)\u003csub\u003emean\u003c/sub\u003e] / (T\u003csub\u003ei\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e\u0026times; T\u003csub\u003ef\u003c/sub\u003e) (5)\u003c/p\u003e \u003cp\u003ewhere (dw/dt)\u003csub\u003emax\u003c/sub\u003e is the maximum mass loss rate (wt%/min), (dw/dt)\u003csub\u003emean\u003c/sub\u003e is the average mass loss rate (wt%/min), T\u003csub\u003ei​\u003c/sub\u003e is the ignition temperature (\u0026deg;C), and T\u003csub\u003ef\u003c/sub\u003e is the burnout temperature (\u0026deg;C).\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Hydrochar yield of SM and CW\u003c/h2\u003e \u003cp\u003eHTC is an efficient thermochemical process for converting biomass into high-energy-density solid fuel. The yield and characteristics of hydrochar significantly depend on the reaction temperature. In this study, hydrochar yield was analyzed at different HTC temperatures for SM and CW.\u003c/p\u003e \u003cp\u003eThe reaction temperature had a significant effect on hydrochar yield (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). For SM, the hydrochar yield increased from 37.8% at 160\u0026deg;C to 50.8% at 320\u0026deg;C. Similarly, for CW, the yield increased from 22.2% at 160\u0026deg;C to 48.4% at 320\u0026deg;C. This increase can be attributed to the removal of volatile matter with increasing HTC temperature, leading to an increase in the relative carbon content of the hydrochar. These results align with previous studies, confirming the relationship between HTC temperature and hydrochar yield (Hoekman et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Libra et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2011\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe mass recovery of SM and CW increased with rising HTC temperatures. At 160\u0026deg;C, the mass recovery rates were 50.8% for SM and 48.4% for CW. As HTC temperature increased, the loss of volatile matter also increased, but the relative proportion of solid residue became more significant. CW exhibited a more pronounced decrease in mass recovery due to its higher volatile matter content.\u003c/p\u003e \u003cp\u003eThe decomposition of hemicellulose, cellulose, and lignin during HTC varies across different temperature ranges. Hemicellulose begins to decompose at approximately 180\u0026ndash;200\u0026deg;C and is largely degraded by 260\u0026deg;C, releasing acetic acid, furfural, and hydroxymethylfurfural, which contribute to reduced oxygen content and enhanced fuel quality in hydrochar (Hoekman et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Libra et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Cellulose starts to decompose around 240\u0026deg;C, with major decomposition occurring between 280\u0026deg;C and 320\u0026deg;C. This process generates hydroxymethylfurfural, levoglucosan, and other volatile compounds, which contribute to an increase in fixed carbon content and energy density (Funke \u0026amp; Ziegler, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Sevilla \u0026amp; Fuertes, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Yang et al., \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). Lignin decomposition starts around 280\u0026deg;C and continues gradually up to temperatures above 500\u0026deg;C, leading to the formation of aromatic compounds. The thermal stability of lignin contributes to the increase in fixed carbon content and the reduction of oxygen content in hydrochar, thereby improving its long-term fuel stability (Gasc\u0026oacute; et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; He et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Furthermore, Wang et al. (\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2021\u003c/span\u003e) demonstrated that delignification significantly influences the thermal degradation reactivity of hemicellulose and cellulose in wood cell walls, providing useful insights into the thermal behavior of lignocellulosic biomass during thermochemical processing.\u003c/p\u003e \u003cp\u003eThe higher mass recovery of CW compared to SM at increasing HTC temperatures can be attributed to two key factors. First, CW contains a higher proportion of lignin and fixed carbon than SM. As HTC temperature increases, lignin undergoes transformation into stable aromatic compounds, thereby increasing the fixed carbon content of hydrochar (Sevilla \u0026amp; Fuertes, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). In contrast, SM has a relatively high ash content, leading to an increase in residual ash proportion as temperature rises, which limits fuel conversion efficiency (Gasc\u0026oacute; et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Second, the lignin-cellulose network structure of CW enhances hydrochar formation. During HTC, the thermal stability and aromatic structure of lignin facilitate hydrochar retention (Libra et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). At temperatures above 280\u0026deg;C, CW's mass recovery increases significantly due to lignin's thermal stability. Since lignin decomposes over a broad temperature range (280\u0026ndash;500\u0026deg;C), the formation of stable aromatic carbon structures results in an increased proportion of residual carbon, contributing to higher mass recovery (He et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2013\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eTGA results indicate that SM and CW exhibit slight mass loss below 100\u0026deg;C due to the removal of free and bound moisture (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). This initial mass loss, caused by the evaporation of residual water in biomass, is commonly observed in HTC preheating stages (Jang et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Lee et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Between 160\u0026deg;C and 280\u0026deg;C, CW exhibited a more rapid mass loss compared to SM, primarily due to the decomposition of hemicellulose (180\u0026ndash;260\u0026deg;C) and cellulose (240\u0026ndash;320\u0026deg;C) (Apaydın Varol and Mutlu, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Lee et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). CW, having a higher cellulose and lipid content, released more volatile matter within this temperature range. In contrast, SM exhibited a slower decomposition rate, with a portion of organic components persisting even at 240\u0026deg;C.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAbove 280\u0026deg;C, CW continued to exhibit gradual mass loss, although the rate of decomposition slowed. This trend is attributed to the thermal stability of lignin, which decomposes gradually between 280\u0026deg;C and 500\u0026deg;C, leading to the formation of stable aromatic structures (Cho et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Meanwhile, SM demonstrated an increase in ash content above 280\u0026deg;C due to the presence of mineral components that remain stable at high temperatures. This phenomenon suggests that SM's conversion to hydrochar is limited by its higher inorganic content, which increases the proportion of non-combustible material rather than fixed carbon.\u003c/p\u003e \u003cp\u003eCW was evaluated as having superior fuel properties compared to SM due to its higher fixed carbon content and lower ash content. At 320\u0026deg;C, CW exhibited optimal fuel characteristics, making it a favorable feedstock for high-quality solid fuel production. Conversely, SM, due to its high ash content, showed limited improvement in fuel quality with increasing HTC temperature, but its potential application as a carbon storage material remains significant.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Elemental composition and fuel characteristics\u003c/h2\u003e \u003cp\u003eThe elemental composition and fuel characteristics of hydrochar provide critical insights into the thermochemical transformation of biomass during HTC. In this study, C, H, O, N, and S contents, as well as the H/C and O/C atomic ratios and HHV of hydrochar produced from SM and CW at various reaction temperatures, were analyzed.\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, the carbon content of hydrochar exhibited a consistent increasing trend with rising HTC temperature. The carbon content of SM increased from 45.0% at 160\u0026deg;C to 54.2% at 320\u0026deg;C, while that of CW increased from 56.2% at 160\u0026deg;C to 76.5% at 320\u0026deg;C, indicating that CW had a higher carbon retention than SM. The greater increase in CW\u0026rsquo;s carbon content can be attributed to its high lignin content, which facilitates the formation of aromatic structures during HTC, leading to an increase in carbon density (Sevilla \u0026amp; Fuertes, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). In contrast, the carbonization of SM was relatively limited due to its high ash content, which increased with rising temperature and restricted fuel quality enhancement (Gasc\u0026oacute; et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eMeanwhile, the hydrogen and oxygen contents showed a decreasing trend with increasing HTC temperature. This reduction is mainly attributed to dehydration and decarboxylation reactions during HTC, which facilitate biomass carbonization by removing oxygen and hydrogen from the solid phase (Hoekman et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). The oxygen content of SM decreased significantly from 21.0% at 160\u0026deg;C to 4.4% at 320\u0026deg;C, while CW\u0026rsquo;s oxygen content dropped from 32.8\u0026ndash;3.5% over the same temperature range. The enhanced oxygen removal with increasing HTC temperature led to an increase in fixed carbon content, resulting in improved fuel quality (Funke \u0026amp; Ziegler, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2010\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eAs oxygen and hydrogen were progressively eliminated during HTC, the fixed carbon content increased, leading to an enhancement in HHV. The HHV of SM increased from 21.2 MJ/kg at 160\u0026deg;C to 25.1 MJ/kg at 320\u0026deg;C, while CW exhibited a more substantial increase from 23.7 MJ/kg to 32.9 MJ/kg. The greater improvement in HHV for CW is primarily attributed to its high lignin content, which promotes efficient carbonization at elevated temperatures. Lignin-rich biomass generally retains a higher proportion of fixed carbon during HTC, resulting in the production of high-energy-density fuel, aligning with previous findings (Funke \u0026amp; Ziegler, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2010\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe Van Krevelen diagram in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e illustrates the reduction in the H/C and O/C atomic ratios of SM and CW with increasing reaction temperature. As temperature increased, the H/C ratio of SM declined from 1.65 to 1.39, while CW\u0026rsquo;s H/C ratio decreased from 1.62 to 1.15. Similarly, the O/C ratio exhibited a significant decrease, with SM declining from 0.35 to 0.06 and CW from 0.44 to 0.03. This trend indicates that HTC effectively facilitates dehydration and deoxygenation reactions, leading to a reduction in atomic ratios (Reza et al., \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Notably, CW exhibited a sharper decrease in O/C ratio with increasing HTC temperature, ultimately attaining a carbon structure comparable to anthracite coal (Hoekman et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). In contrast, SM exhibited a relatively gradual reduction in O/C ratio, which can be attributed to its high ash content, limiting the extent of fuel quality improvement during HTC (Mumme et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2011\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn summary, CW underwent more effective carbonization than SM during HTC, resulting in a higher energy-density fuel. The increase in HTC temperature led to an enhancement in the carbon content of hydrochar and a reduction in oxygen content, which significantly improved fuel quality. However, the presence of high ash content in SM restricted fuel quality improvement despite increasing HTC temperatures. The significant reduction in H/C and O/C atomic ratios of CW indicates that its hydrochar closely resembles anthracite coal, thereby improving its potential as a high-quality solid fuel. Optimizing HTC conditions can further maximize biomass conversion into high-energy fuel.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Effects of Co-HTC temperature and mixing ratio on hydrochar fuel properties\u003c/h2\u003e \u003cp\u003eThe mixing ratio and reaction temperature in Co-HTC play a critical role in determining hydrochar fuel characteristics. Co-HTC is recognized as a promising technique for overcoming the limitations of individual biomass feedstocks and enhancing fuel quality through biomass interactions. This study evaluated the fuel characteristics of hydrochar produced at different mixing ratios (1:1, 3:1, 5:1) and reaction temperatures (160\u0026deg;C, 200\u0026deg;C, 240\u0026deg;C).\u003c/p\u003e \u003cp\u003eThe yield of hydrochar in HTC is influenced by the decomposition rate of biomass, the extent of volatile matter loss, and the efficiency of the carbonization reaction. In Co-HTC, biomass interactions further affect pyrolysis behavior and hydrochar formation process (Hoekman et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). At a 1:1 mixing ratio, hydrochar yield decreased from 58.4% at 160\u0026deg;C to 48.6% at 240\u0026deg;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). The decline in yield was more pronounced at higher CW proportions (1:1 and 3:1), likely due to the high volatile content in CW, which was lost during HTC. In contrast, hydrochar yield was relatively stable in the 5:1 ratio, indicating that SM\u0026rsquo;s high ash content hindered complete carbonization (Gasc\u0026oacute; et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). While hydrochar yield generally decreases with increasing HTC temperature, this behavior is closely related to biomass composition and the complex chemical reactions occurring during HTC. The high lignin content of CW results in increased carbon density at elevated temperatures, but the removal of volatile fractions leads to greater mass loss. Conversely, SM\u0026rsquo;s high ash content leads to higher yield retention, primarily due to the residual presence of inorganic components rather than increased carbonization.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo maximize hydrochar productivity in HTC, it is essential to consider biomass composition and thermochemical transformations while optimizing the mixing ratio. Excessive CW proportions may lead to higher volatile loss, while an overabundance of SM may result in increased ash content, reducing carbonization efficiency.\u003c/p\u003e \u003cp\u003eFurthermore, hydrochar\u0026rsquo;s carbon content increased while oxygen content decreased with rising HTC temperature and CW proportion. This trend highlights the occurrence of dehydration, decarboxylation, and deoxygenation reactions in HTC, which contribute to fuel quality improvement (Hoekman et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Wang et al., \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). At a 1:1 mixing ratio, the carbon content increased from 56.4% at 160\u0026deg;C to 64.2% at 240\u0026deg;C. In contrast, the 5:1 mixing ratio showed a smaller increase from 47.5% at 160\u0026deg;C to 58.4% at 240\u0026deg;C, indicating that higher SM proportions restricted carbonization efficiency. CW\u0026rsquo;s high lignin content facilitated effective oxygen removal, whereas SM\u0026rsquo;s high ash content limited fuel quality improvement (Sevilla \u0026amp; Fuertes, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). These results suggest that Co-HTC can mitigate the limitations of individual biomass feedstocks. While CW alone retains high carbon, it has low nitrogen content, which may lead to nutrient losses during combustion. In contrast, SM\u0026rsquo;s high nitrogen content enhances its potential as a soil amendment (Mumme et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2011\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eBiomass subjected to HTC undergoes an increase in carbon density while simultaneously experiencing a reduction in oxygen and hydrogen content, leading to a decrease in the O/C and H/C atomic ratios (Funke \u0026amp; Ziegler, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). At a 1:1 mixing ratio, the O/C ratio decreased from 0.27 at 160\u0026deg;C to 0.18 at 240\u0026deg;C, while the H/C ratio declined from 1.65 to 1.34 (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). The pronounced decrease in the O/C and H/C ratios at a higher CW proportion (1:1) is attributed to the high lignin content in CW, which facilitates the transformation into aromatic structures during HTC, effectively removing oxygen and hydrogen (Sevilla \u0026amp; Fuertes, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). These changes are primarily governed by dehydration, decarboxylation, and deoxygenation reactions occurring during HTC. Studies have shown that HTC conditions significantly influence carbonization efficiency and energy densification, particularly when applied to SM in different operational settings (Ipiales et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), further supporting the observed reduction in O/C and H/C ratios as biomass undergoes progressive conversion into aromatic and carbonaceous structures.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThis atomic ratio variation plays a critical role in determining fuel properties, as a lower O/C and H/C ratio is directly linked to an increase in energy density (Hoekman et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). At a 1:1 mixing ratio, the HHV increased from 24.4 MJ/kg at 160\u0026deg;C to 27.9 MJ/kg at 240\u0026deg;C. The higher CW proportion contributed to a substantial increase in HHV, primarily due to the enhanced formation of aromatic carbon structures. These results indicate that HTC promotes the removal of oxygen while increasing carbon density, thereby enhancing the energy density of the fuel (Funke \u0026amp; Ziegler, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2010\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eTGA and DTG analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e) demonstrated that increasing HTC temperature significantly affected volatile matter release and thermal decomposition behavior. At a 1:1 mixing ratio, volatile matter loss occurred gradually at 160\u0026deg;C, whereas at 240\u0026deg;C, it became more pronounced, indicating substantial mass reduction. CW-rich hydrochars exhibited greater volatile release, attributed to the high lignin content in CW, which undergoes gradual thermal degradation over a broad temperature range (Sevilla \u0026amp; Fuertes, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2009\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eDTG curves revealed two distinct peaks for CW-rich hydrochars, corresponding to volatile release and fixed carbon formation. The high lignin content in CW contributed to a broader, slower degradation process, which enhanced combustion stability by sustaining char formation over an extended temperature range. Conversely, SM-rich hydrochars decomposed more rapidly, leading to higher volatile losses and reduced carbon retention. These observations are consistent with previous studies on biomass pyrolysis kinetics, which highlight lignin's role in delaying thermal degradation and promoting residual carbon formation (El-Sayed et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn contrast, at a 5:1 mixing ratio, where the SM content was predominant, the thermal decomposition process was more gradual. The increased ash content in SM likely hindered thermal degradation, limiting overall mass loss despite the rise in HTC temperature (Gasc\u0026oacute; et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eBy adjusting the mixing ratio and reaction temperature in the Co-HTC process, the fuel quality of biomass-derived hydrochar can be significantly improved. A higher CW proportion led to a marked increase in carbon density and HHV, highlighting the effectiveness of lignin-rich biomass in fuel property enhancement. In contrast, higher SM proportions resulted in increased ash content, which restricted fuel quality improvement but suggested potential applications for agricultural utilization rather than combustion-based energy production.\u003c/p\u003e \u003cp\u003eWith increasing HTC temperature, the continuous decline in the O/C and H/C ratios indicated that the hydrochar structure progressively transitioned to a composition resembling that of coal. The optimal fuel properties were achieved at a 1:1 mixing ratio and 240\u0026deg;C, demonstrating that this combination offers the best balance between fuel quality enhancement and hydrochar productivity.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Fuel performance evaluation\u003c/h2\u003e \u003cp\u003eThe fuel performance of hydrochar produced through HTC is a critical factor in determining combustion characteristics and energy efficiency. In HTC-treated hydrochar, as temperature increases, volatile matter decreases while fixed carbon content increases, directly impacting combustion reactivity and energy density. This study evaluated fuel performance based on S, EDR, and EY.\u003c/p\u003e \u003cp\u003eThe S index is a crucial indicator of ignition ease, combustion rate, and burnout temperature. As shown in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, the 1:1 mixing ratio exhibited the highest S value at 160\u0026deg;C (39.0 \u0026times; 10⁻⁹ %\u0026sup2;/min\u0026sup2;\u0026middot;\u0026deg;C\u0026sup3;), while the lowest S value was observed at 240\u0026deg;C (9.1 \u0026times; 10⁻⁹ %\u0026sup2;/min\u0026sup2;\u0026middot;\u0026deg;C\u0026sup3;). This decline suggests that as HTC progresses, volatile matter is removed, while fixed carbon increases, leading to a reduction in combustion reactivity. In contrast, the 3:1 and 5:1 mixing ratios showed the highest S values at 200\u0026deg;C, which subsequently decreased at 240\u0026deg;C. In particular, the 5:1 mixing ratio demonstrated a higher ignition temperature (Ti) and lower combustion rate, indicating decreased reactivity. This behavior is primarily attributed to the high ash content in SM, which acts as an inert component, lowering combustion efficiency (Gasc\u0026oacute; et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Consequently, at a 1:1 ratio, 160\u0026deg;C provided the best combustion performance, whereas at higher SM ratios, 200\u0026deg;C yielded more favorable combustion characteristics.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eCombustion characteristic index of hydrochar produced under various HTC conditions\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTemperature (℃)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSM:CW\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eT\u003csub\u003ei\u003c/sub\u003e\u003c/p\u003e \u003cp\u003e(\u0026deg;C)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eT\u003csub\u003ef\u003c/sub\u003e\u003c/p\u003e \u003cp\u003e(\u0026deg;C)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e(dw/dt)\u003csub\u003emax\u003c/sub\u003e\u003c/p\u003e \u003cp\u003e(%/min)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e(dw/dt)\u003csub\u003emean\u003c/sub\u003e\u003c/p\u003e \u003cp\u003e(%/min)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eS\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;9\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e(%\u003csup\u003e2\u003c/sup\u003e/min\u003csup\u003e2\u003c/sup\u003e\u0026middot;\u0026deg;C\u003csup\u003e3\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e160\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1:1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e165\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e550\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-2.7628\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e-0.2115\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e39.0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3:1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e175\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e575\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-1.8244\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e-0.2029\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e21.0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5:1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e175\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e610\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-1.2486\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e-0.1854\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e12.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e200\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1:1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e175\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e580\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-1.5487\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e-0.2052\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e17.9\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3:1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e180\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e560\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-5.1611\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e-0.2142\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e60.9\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5:1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e180\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e575\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-4.6929\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e-0.2082\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e52.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e240\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1:1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e185\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e545\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-0.6955\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e-0.2439\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e9.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3:1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e185\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e515\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-2.3273\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e-0.2509\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e33.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5:1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e185\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e570\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-0.9396\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e-0.2060\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e9.9\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eHTC reduces volatile matter, leading to mass loss; however, it enhances carbon density, which improves HHV and overall fuel quality. Figure\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e illustrates the EDR variations under different HTC conditions. At 160\u0026deg;C, the EDR for the 1:1 ratio was 1.19, increasing to 1.36 at 240\u0026deg;C. The 3:1 ratio showed an EDR increase from 1.01 to 1.36, indicating significant energy densification. The 5:1 ratio exhibited a smaller increase in EDR from 1.02 to 1.38, due to the higher ash content in SM, which limits carbon densification. The increase in reaction temperature facilitated a higher EDR and EY, a trend consistent with previous comparative studies of HTC and low-temperature pyrolysis, where process conditions played a critical role in optimizing solid biofuel properties (Wang et al., \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). The rise in EDR results from oxygen removal through dehydration and decarboxylation reactions during HTC, which enhances carbon concentration (Funke \u0026amp; Ziegler, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). However, higher SM content (5:1) introduces more ash, limiting energy densification. This suggests that fuel efficiency may be negatively affected by excessive inorganic content in SM.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eMeanwhile, EY (%) quantifies the proportion of energy retained in hydrochar compared to raw biomass. For the 1:1 ratio, EY increased from 45.2% at 160\u0026deg;C to 57% at 240\u0026deg;C. For the 3:1 ratio, EY increased from 34.8\u0026ndash;60.3%, demonstrating a significant rise. The 5:1 ratio showed an EY increase from 34.9\u0026ndash;61.8%, similar to the 3:1 ratio, despite its higher ash content. In all conditions, EY improved with rising temperature, confirming that HTC effectively enhances carbon retention. Notably, at 240\u0026deg;C, CW\u0026rsquo;s high lignin content facilitated fixed carbon formation, significantly increasing EY. This progressive increase in fixed carbon content with HTC not only enhances energy density but also improves combustion stability, particularly at higher temperatures (Hoekman et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Yao et al., \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\"\u003c/p\u003e \u003cp\u003eAs the reaction temperature increased, the S index decreased, while both EDR and EY showed an increasing trend. This suggests that HTC improves fuel quality by removing volatile matter and increasing carbon density, but at the same time, it reduces ignition ease and combustion reactivity. The 1:1 mixing ratio at 240\u0026deg;C emerged as the optimal condition for maximizing EDR and EY. For the 5:1 ratio, ash accumulation resulted in a significant reduction in the S index, while EDR improvement remained limited. This confirms that Co-HTC with a higher CW proportion is more effective in enhancing carbonization efficiency and improving fuel properties. Optimizing HTC temperature and mixing ratio is crucial for achieving both improved combustion characteristics and energy density.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eThis study assessed the fuel properties and energy performance of hydrochar produced from SM and CW via Co-HTC at various temperatures (160\u0026ndash;240\u0026deg;C) and mixing ratios (1:1, 3:1, 5:1). The findings demonstrated that HTC significantly altered the elemental composition, heating value, and combustion reactivity of the biomass, enhancing its viability as a solid fuel. The optimal fuel quality improvement was observed at 240\u0026deg;C, particularly at the 1:1 mixing ratio, where C content increased to 64.2%, O/C and H/C atomic ratios declined, and HHV improved to 27.9 MJ/kg. However, as HTC temperature increased, the S decreased, indicating reduced ignition ease and burnout efficiency. Conversely, EDR and EY exhibited a consistent upward trend, with the highest EY values recorded at 57% (1:1), 60.3% (3:1), and 61.8% (5:1) at 240\u0026deg;C. This suggests that while higher CW content promoted carbon retention and fuel densification, higher SM content retained more energy despite its elevated ash content, which restricted further energy densification. These results indicate that Co-HTC effectively improves carbon density and energy retention, although ash accumulation in SM constrains combustion performance. The findings underscore the need to balance HTC conditions to optimize both carbonization efficiency and energy recovery, with the 1:1 and 3:1 mixing ratios at 240\u0026deg;C emerging as the most favorable conditions. This study confirms that Co-HTC is a viable strategy for converting organic waste into high-energy-density solid fuel, offering a sustainable approach to bioenergy production and waste valorization. Future research should explore process optimization strategies to further enhance combustion efficiency and mitigate ash-related challenges in hydrochar applications.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis paper was supported by Konkuk University Researcher Fund in 2024 and National Research Foundation grant funded by the Korea government (MSIT) (RS-2023-00219272, RS-202400338631).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis is not applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors’ Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eKyung Jin Min: Formal analysis, Data curation, Writing-original draft, Doo Young Oh: Formal analysis, Methodology, Eunyoung Lee: Data curation, Visualization, Jin Hwan Lee: Investigation, Do Yeon Kim: Investigation, Ki Young Park: Conceptualization, Supervision, Writing-review \u0026amp; editing\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical Approval\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis is not applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to Participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis is not applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to Publish\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis is not applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability Statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eData will be made available on reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAmerican Public Health Association (APHA), American Water Works Association (AWWA), \u0026amp; Water Environment Federation (WEF), 2023. Standard Methods for the Examination of Water and Wastewater (24th ed.). APHA Press, Washington, DC.\u003c/li\u003e\n\u003cli\u003eApaydın Varol, E., Mutlu, \u0026Uuml;., 2023. TGA-FTIR analysis of biomass samples based on the thermal decomposition behavior of hemicellulose, cellulose, and lignin. Energies 16(9), 3674. https://doi.org/10.3390/en16093674\u003c/li\u003e\n\u003cli\u003eCai, J., Li, B., Chen, C., Wang, J., Zhao, M., Zhang, K., 2016. Hydrothermal carbonization of tobacco stalk for fuel application. Bioresour. Technol. 220, 305-311. https://doi.org/10.1016/j.biortech.2016.08.098\u003c/li\u003e\n\u003cli\u003eCho, C.H., Lee, S.O., Kang, D.K., Hong, S.H., Kang, C.K., Lee, J.W., 2020. Preparation and characterization of lignin/chlorinated polyvinyl chloride blended fibers for low-cost carbon fibers. J. Korean Soc. Dyers Finish. 32(2), 111-119. https://doi.org/10.5764/TCF.2020.32.2.111\u003c/li\u003e\n\u003cli\u003eDang, C.H., Farru, G., Glaser, C., Fischer, M.G., Libra, J.A., 2023. Enhancing the fuel properties of spent coffee grounds through hydrothermal carbonization: Output prediction and post-treatment approaches. Sustainability 16(1), 338. https://doi.org/10.3390/su16010338\u003c/li\u003e\n\u003cli\u003eDevnath, B., Khanal, S., Shah, A., Reza, T., 2024. Influence of hydrothermal carbonization (HTC) temperature on hydrochar and process liquid for poultry, swine, and dairy manure. Environ. 11(7), 150. https://doi.org/10.3390/environments11070150\u003c/li\u003e\n\u003cli\u003eEl-Sayed, S.A., Khass, T.M., Mostafa, M.E., 2024. Thermal degradation behaviour and chemical kinetic characteristics of biomass pyrolysis using TG/DTG/DTA techniques. Biomass Convers. Biorefin. 14(15), 17779-17803. https://doi.org/10.1007/s13399-023-03926-2\u003c/li\u003e\n\u003cli\u003eFerrentino, R., Sacchi, G., Scrinzi, D., Andreottola, G., Fiori, L., 2023. Valorization of swine manure for a circular approach through hydrothermal carbonization. Biomass Bioenergy 168, 106689. https://doi.org/10.1016/j.biombioe.2022.106689\u003c/li\u003e\n\u003cli\u003eFunke, A., Ziegler, F., 2010. Hydrothermal carbonization of biomass: A summary and discussion of chemical mechanisms for process engineering. Biofuels Bioprod. Biorefin. 4(2), 160\u0026ndash;177. https://doi.org/10.1002/bbb.198\u003c/li\u003e\n\u003cli\u003eGasc\u0026oacute;, G., Paz-Ferreiro, J., \u0026Aacute;lvarez, M.L., Saa, A., M\u0026eacute;ndez, A., 2018. Biochars and hydrochars prepared by pyrolysis and hydrothermal carbonisation of pig manure. Waste Manag. 79, 395\u0026ndash;403. https://doi.org/10.1016/j.wasman.2018.08.015\u003c/li\u003e\n\u003cli\u003eHe, C., Giannis, A., Wang, J.Y., 2013. Conversion of sewage sludge to clean solid fuel using hydrothermal carbonization: Hydrochar fuel characteristics and combustion behavior. Appl. Energy 111, 257\u0026ndash;266. https://doi.org/10.1016/j.apenergy.2013.04.084\u003c/li\u003e\n\u003cli\u003eHoekman, S.K., Broch, A., Robbins, C., 2011. Hydrothermal carbonization (HTC) of lignocellulosic biomass. Energy Fuels 25(4), 1802\u0026ndash;1810. https://doi.org/10.1021/ef101745n\u003c/li\u003e\n\u003cli\u003eHu, Y., Gallant, R., Salaudeen, S., Farooque, A.A., He, S., 2022. Hydrothermal carbonization of spent coffee grounds for producing solid fuel. Sustainability 14(14), 8818. https://doi.org/10.3390/su14148818\u003c/li\u003e\n\u003cli\u003eIpiales, R.P., Mohedano, A.F., Diaz-Portuondo, E., Diaz, E., De la Rubia, M.A., 2023. Co-hydrothermal carbonization of swine manure and lignocellulosic waste: A new strategy for the integral valorization of biomass wastes. Waste Manag. 169, 267-275. https://doi.org/10.1016/j.wasman.2023.07.018\u003c/li\u003e\n\u003cli\u003eIpiales, R. P., Sarrion, A., Diaz, E., de la Rubia, M. A., Diaz-Portuondo, E., Coronella, C. J., \u0026amp; Mohedano, A. F., 2024. Swine manure management by hydrothermal carbonization: Comparative study of batch and continuous operation. Environ. Res. 245, 118062. https://doi.org/10.1016/j.envres.2023.118062\u003c/li\u003e\n\u003cli\u003eIschia, G., Berge, N. D., Bae, S., Marzban, N., Rom\u0026aacute;n, S., Farru, G., Fiori, L., 2024. Advances in research and technology of hydrothermal carbonization: Achievements and future directions. Agronomy 14(5), 955. https://doi.org/10.3390/agronomy14050955\u003c/li\u003e\n\u003cli\u003eIslam, M. T., Reza, M. T., 2023. Evaluation of fuel and combustion properties of hydrochar derived from co-hydrothermal carbonization of biomass and plastic. Biomass Bioenergy 172, 106750. https://doi.org/10.1016/j.biombioe.2023.106750\u003c/li\u003e\n\u003cli\u003eJang, E. S., Song, E. H., Yoon, J. H., Kim, Y. M., 2020. Study on the pyrolysis kinetics of solid fuel from livestock manure. J. Ind. Chem. 31(4), 443-451. https://doi.org/10.14478/ace.2020.1047\u003c/li\u003e\n\u003cli\u003eKhan, A. A., de Jong, W., Jansens, P. J., Spliethoff, H., 2009. Biomass combustion in fluidized bed boilers: Potential problems and remedies. Fuel Process. Technol. 90(1), 21-50. https://doi.org/10.1016/j.fuproc.2008.07.012\u003c/li\u003e\n\u003cli\u003eKhosravi, A., Yuan, Y., Liu, Q., Zheng, H., Hashemi, M., Tang, Y., Xing, B., 2024. Hydrochars as slow-release phosphorus fertilizers for enhancing corn and soybean growth in an agricultural soil. Carbon Res. 3(1), 7. https://doi.org/10.1007/s44246-023-00086-w\u003c/li\u003e\n\u003cli\u003eKim, D., Park, S., Park, K. Y., 2017. Upgrading the fuel properties of sludge and low-rank coal mixed fuel through hydrothermal carbonization. Energy 141, 598-602. https://doi.org/10.1016/j.energy.2017.09.113\u003c/li\u003e\n\u003cli\u003eKumar, N., Weldon, R., Lynam, J. G., 2021. Hydrothermal carbonization of coffee silverskins. Biocatal. Agric. Biotechnol. 36, 102145. https://doi.org/10.1016/j.bcab.2021.102145\u003c/li\u003e\n\u003cli\u003eLang, Q., Liu, Z., Li, Y., Xu, J., Li, J., Liu, B., Sun, Q., 2022. Combustion characteristics, kinetic and thermodynamic analyses of hydrochars derived from hydrothermal carbonization of cattle manure. J. Environ. Chem. Eng. 10(1), 106938. https://doi.org/10.1016/j.jece.2021.106938\u003c/li\u003e\n\u003cli\u003eLang, Q., Zhang, B., Liu, Z., Chen, Z., Xia, Y., Li, D., Gai, C., 2019. Co-hydrothermal carbonization of corn stalk and swine manure: Combustion behavior of hydrochar by thermogravimetric analysis. Bioresour. Technol. 271, 75-83. https://doi.org/10.1016/j.biortech.2018.09.100\u003c/li\u003e\n\u003cli\u003eLang, Q., Zhao, Q., Li, W., 2018. Co-hydrothermal carbonization of lignocellulosic biomass and swine manure: Hydrochar properties and heavy metal transformation behavior. Bioresour. Technol. 249, 464\u0026ndash;470. https://doi.org/10.1016/j.biortech.2018.06.084\u003c/li\u003e\n\u003cli\u003eLee, E. J., Jung, S. A., Jung, Y. J., Hyung, S. J., Lee, J. W., 2022. Chemical and structural properties of dry and wet torrefied biomass. Trends Agric. Life Sci. 60, 17-25. https://doi.org/10.29335/tals.2022.60.17\u003c/li\u003e\n\u003cli\u003eLee, J., Park, K. Y., 2021. Conversion of heavy metal-containing biowaste from phytoremediation site to value-added solid fuel through hydrothermal carbonization. Environ. Pollut. 269, 116127. https://doi.org/10.1016/j.envpol.2020.116127\u003c/li\u003e\n\u003cli\u003eLee, J., Cho, S., Kim, D., Ryu, J., Lee, K., Chung, H., Park, K. Y., 2021. Conversion of slaughterhouse wastes to solid fuel using hydrothermal carbonization. Energies 14(6), 1768. https://doi.org/10.3390/en14061768\u003c/li\u003e\n\u003cli\u003eLi, Q., Lin, H., Zhang, S., Yuan, X., Gholizadeh, M., Wang, Y., Hu, X., 2021. Co-hydrothermal carbonization of swine manure and cellulose: Influence of mutual interaction of intermediates on properties of the products. Sci. Total Environ. 791, 148134. https://doi.org/10.1016/j.scitotenv.2021.148134\u003c/li\u003e\n\u003cli\u003eLibra, J. A., Ro, K. S., Kammann, C., Funke, A., Berge, N. D., Neubauer, Y., Emmerich, K. H., 2011. Hydrothermal carbonization of biomass: A summary and discussion of chemical mechanisms for process engineering. Biofuels Bioprod. Biorefin. 5(5), 547\u0026ndash;552. https://doi.org/10.1002/bbb.198\u003c/li\u003e\n\u003cli\u003eLiu, Z., Quek, A., Hoekman, S. K., Balasubramanian, R., 2013. Production of solid biochar fuel from waste biomass by hydrothermal carbonization. Fuel 103, 943-949. https://doi.org/10.1016/j.fuel.2012.07.069\u003c/li\u003e\n\u003cli\u003eLucian, M., Volpe, M., Gao, L., Piro, G., Goldfarb, J. L., Fiori, L., 2018. Impact of hydrothermal carbonization conditions on the formation of hydrochars and secondary chars from the organic fraction of municipal solid waste. Fuel 233, 257-268. https://doi.org/10.1016/j.fuel.2018.06.060\u003c/li\u003e\n\u003cli\u003eM\u0026auml;kel\u0026auml;, M., Benavente, V., Fullana, A., 2015. Hydrothermal carbonization of lignocellulosic biomass: Influence of process parameters on hydrochar properties. Appl. Energy 155, 576-584. https://doi.org/10.1016/j.apenergy.2015.06.022\u003c/li\u003e\n\u003cli\u003eMassaya, J., Pickens, G., Mills-Lamptey, B., Chuck, C. J., 2021. Enhanced hydrothermal carbonization of spent coffee grounds for the efficient production of solid fuel with lower nitrogen content. Energy Fuels 35(11), 9462-9473. https://doi.org/10.1021/acs.energyfuels.1c00870\u003c/li\u003e\n\u003cli\u003eMumme, J., Eckervogt, L., Pielert, J., Diakit\u0026eacute;, M., Rupp, F., Kern, J., 2011. Hydrothermal carbonization of anaerobically digested maize silage. Bioresour. Technol. 102(19), 9255\u0026ndash;9260. https://doi.org/10.1016/j.biortech.2011.06.099\u003c/li\u003e\n\u003cli\u003ePark, J. E., Lee, G. B., Jeong, C. J., Kim, H., Kim, C. G., 2021. Determination of relationship between higher heating value and atomic ratio of hydrogen to carbon in spent coffee grounds by hydrothermal carbonization. Energies 14(20), 6551. https://doi.org/10.3390/en14206551\u003c/li\u003e\n\u003cli\u003ePark, K. Y., Lee, K., Kim, D., 2018. Characterized hydrochar of algal biomass for producing solid fuel through hydrothermal carbonization. Bioresour. Technol. 258, 119-124. https://doi.org/10.1016/j.biortech.2018.03.003\u003c/li\u003e\n\u003cli\u003ePauline, A. L., Joseph, K., 2020. Hydrothermal carbonization of organic wastes to carbonaceous solid fuel\u0026ndash;A review of mechanisms and process parameters. Fuel 279, 118472. https://doi.org/10.1016/j.fuel.2020.11847\u003c/li\u003e\n\u003cli\u003ePoomsawat, S., Poomsawat, W., 2021. Analysis of hydrochar fuel characterization and combustion behavior derived from aquatic biomass via hydrothermal carbonization process. Case Stud. Therm. Eng. 27, 101255. https://doi.org/10.1016/j.csite.2021.101255\u003c/li\u003e\n\u003cli\u003eReza, M. T., Becker, G., Sachsenheimer, K., Mumme, J., 2014. Hydrothermal carbonization (HTC): Near infrared spectroscopy and partial least squares regression for hydrochar process optimization. Appl. Energy 135, 379\u0026ndash;385. https://doi.org/10.1016/j.biortech.2014.03.008\u003c/li\u003e\n\u003cli\u003eSantana, M. S., Santana, W. M., Lira, T. S., 2020. Hydrochar production from defective coffee beans by hydrothermal carbonization. Bioresour. Technol. 300, 122653. https://doi.org/10.1016/j.biortech.2019.122653\u003c/li\u003e\n\u003cli\u003eSevilla, M., Fuertes, A. B., 2009. The production of carbon materials by hydrothermal carbonization of cellulose. Carbon 47(9), 2281\u0026ndash;2289. https://doi.org/10.1016/j.carbon.2009.04.026\u003c/li\u003e\n\u003cli\u003eSharma, R., Jasrotia, K., Singh, N., Ghosh, P., Srivastava, S., Sharma, N. R., Kumar, A., 2020. A comprehensive review on hydrothermal carbonization of biomass and its applications. Chem. Afr. 3, 1-19. https://doi.org/10.1007/s42250-019-00098-3\u003c/li\u003e\n\u003cli\u003eWang, C., Fan, Y., Hornung, U., Zhu, W., Dahmen, N., 2020. Char and tar formation during hydrothermal treatment of sewage sludge in subcritical and supercritical water: Effect of organic matter composition and experiments with model compounds. J. Clean. Prod. 242, 118586. https://doi.org/10.1016/j.jclepro.2019.118586\u003c/li\u003e\n\u003cli\u003eWang, J., Minami, E., Asmadi, M., Kawamoto, H., 2021. Effect of delignification on thermal degradation reactivities of hemicellulose and cellulose in wood cell walls. J. Wood Sci. 67, 1-11. https://doi.org/10.1186/s10086-021-01952-0\u003c/li\u003e\n\u003cli\u003eWang, Y., Qiu, L., Zhu, M., Sun, G., Zhang, T., Kang, K., 2019. Comparative evaluation of hydrothermal carbonization and low-temperature pyrolysis of Eucommia ulmoides Oliver for the production of solid biofuel. Sci. Rep. 9(1), 5535. https://doi.org/10.1038/s41598-019-38849-4\u003c/li\u003e\n\u003cli\u003eXiong, J. B., Pan, Z. Q., Xiao, X. F., Huang, H. J., Lai, F. Y., Wang, J. X., Chen, S. W., 2019. Study on the hydrothermal carbonization of swine manure: The effect of process parameters on the yield/properties of hydrochar and process water. J. Anal. Appl. Pyrolysis 144, 104692. https://doi.org/10.1016/j.jaap.2019.104692\u003c/li\u003e\n\u003cli\u003eXiong, W., Luo, Y., Shangguan, W., Deng, Y., Li, R., Song, D., Xiao, R., 2024. Co-hydrothermal carbonization of lignocellulosic biomass and swine manure: Optimal parameters for enhanced nutrient reclamation, carbon sequestration, and heavy metals passivation. Waste Manage. 190, 174-185. https://doi.org/10.1016/j.wasman.2024.09.019\u003c/li\u003e\n\u003cli\u003eYang, H., Yan, R., Chen, H., Lee, D. H., Zheng, C., 2007. Characteristics of hemicellulose, cellulose, and lignin pyrolysis. Fuel 86(12-13), 1781-1788. https://doi.org/10.1016/j.fuel.2006.12.013\u003c/li\u003e\n\u003cli\u003eYao, Y., Xia, P., Zhao, L., Yang, C., Huang, R., Wang, K., 2024. Effects of temperature and duration on the combustion characteristics of hydrochar prepared from traditional Chinese medicine residues using hydrothermal carbonization process. Int. J. Energy Res. 2024(1), 3595643. https://doi.org/10.1155/er/3595643\u003c/li\u003e\n\u003cli\u003eYi, Q., Qi, F., Cheng, G., Zhang, Y., Xiao, B., Hu, Z., Xu, S., 2013. Thermogravimetric analysis of co-combustion of biomass and biochar. J. Therm. Anal. Calorim. 112, 1475-1479. https://doi.org/10.1007/s10973-012-2744-1\u003c/li\u003e\n\u003cli\u003eZhao, P., Lin, C., Li, Y., Wu, S., Cui, X., Ding, Y., Liu, F., Guo, Q., 2021. Value-added products from pyrolysis of hydrochar derived from polyvinyl chloride and alkali coal. J. Clean. Prod. 329, 129769. https://doi.org/10.1016/j.jclepro.2021.129769\u003c/li\u003e\n\u003cli\u003eZhang, S., Chen, T., Li, W., Dong, Q., Xiong, Y., 2016. Physicochemical properties and combustion behavior of duckweed during wet torrefaction. Bioresour. Technol. 218, 1157-1162. https://doi.org/10.1016/j.biortech.2016.07.086\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Co-Hydrothermal Carbonization, Energy yield, Energy densification ratio, Combustion characteristic index, Coffee Grounds, Swine Manure","lastPublishedDoi":"10.21203/rs.3.rs-6199561/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6199561/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study evaluates the fuel properties and energy performance of hydrochar produced through co-hydrothermal carbonization (Co-HTC) of swine manure (SM) and coffee waste (CW). The effects of reaction temperature (160\u0026deg;C, 200\u0026deg;C, 240\u0026deg;C) and mixing ratio (SM:CW\u0026thinsp;=\u0026thinsp;1:1, 3:1, 5:1) on combustion characteristics, energy densification ratio (EDR), and energy yield (EY) were analyzed. The results indicate that Co-HTC effectively enhances fuel quality by leveraging the synergistic interactions between SM and CW. At 240\u0026deg;C and a 1:1 mixing ratio, the produced hydrochar exhibited a combustion characteristic index (S) of 9.1 \u0026times; 10⁻⁹ %\u0026sup2;/min\u0026sup2;\u0026middot;\u0026deg;C\u0026sup3;, an EDR of 1.39, and an EY of 57%, demonstrating superior fuel performance. The high lignin content in CW promoted fixed carbon retention, while the elevated nitrogen content in SM contributed to potential agricultural applications. Additionally, Co-HTC effectively reduced O/C and H/C atomic ratios, resulting in enhanced energy density. Despite an increase in ash content with higher SM ratios, EY values remained high across all conditions, with a maximum of 61.8% at a 5:1 ratio, suggesting that energy retention was not significantly hindered. These findings confirm that Co-HTC is a viable approach to converting organic waste into high-energy-density solid fuel, offering a sustainable solution for bioenergy production and waste valorization.\u003c/p\u003e","manuscriptTitle":"Enhancing Fuel Properties and Energy Performance with Co-Hydrothermal Carbonization of Coffee Waste and Swine Manure","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-28 16:59:01","doi":"10.21203/rs.3.rs-6199561/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"504ef3d0-6250-4e65-8581-fcd6d2f7c264","owner":[],"postedDate":"April 28th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-08-14T22:08:31+00:00","versionOfRecord":[],"versionCreatedAt":"2025-04-28 16:59:01","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6199561","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6199561","identity":"rs-6199561","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}