Fates of nutrient elements and heavy metals during thermal conversion of cattle slurry-derived anaerobic digestates

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Lane, Olli Sippula, Jorma Jokiniemi, Mikko Heimonen, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-3972486/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 30 Dec, 2024 Read the published version in Bioresources and Bioprocessing → Version 1 posted 5 You are reading this latest preprint version Abstract Thermal processes are emerging as promising solutions to recovering phosphorus and other nutrient elements from anaerobic digestates. The feasibility of nutrient element recovery depends largely on the fates of nutrient elements and heavy metals during thermal processing. This study assesses the partitioning of macronutrients (N, P, K, Na, Ca and Mg) and heavy metals (Zn, Cu, and Mn) between condensed and gaseous phases during thermal conversion of cattle slurry digestates in gas atmospheres of pyrolysis, combustion, and gasification processes. This study also assesses the chemical forms of macronutrients retained in combustion ashes. The partitioning of elements between condensed and gaseous phases was quantified by mass balances based on elemental analyses of char and ash residues. The char and ash residues were prepared in a fixed-bed, batch reactor at temperatures within the range 800–1000°C. Powder X-ray diffraction was used to identify the chemical forms of macronutrient elements in combustion ashes. Volatilisation of P was low (< 20%) when the digestates were heated in inert and oxidising atmospheres, whereas a reducing atmosphere volatilized P to a major extent (~ 60% at 1000°C). Oxidising atmospheres increased volatilisation of N but suppressed volatilisation of K, Na, and Zn. Volatilisation of the following elements was low (< 30%) in all investigated operating conditions: Ca, Mg, Mn, and Cu. The combustion ashes contained both high concentrations of P (around 7 w/w%) and acceptable concentrations of regulated heavy metals (Cu, and Zn) for application on agricultural and forest soils in Finland. Phosphorous was retained in the combustion ashes in the form of whitlockite. This form of P is expected to be available to plants when the ashes are added to soil. Ash utilization combustion gasification pyrolysis phosphorous recovery volatilization biochar Figures Figure 1 Figure 2 Figure 3 Figure 4 1. Introduction The most commonly used process to convert biomass wastes into biogas, anaerobic digestion, generates large quantities of a by-product called “anaerobic digestate”. This material is produced in the form of a slurry, which predominately consists of water, undigested organic material, and inorganic minerals. Until now it has been a common practice to apply anaerobic digestates to soils since it is often rich in phosphorous, nitrogen and other essential plant macronutrients (Lukehurst et al., 2010 ; Möller & Müller, 2012 ). However, there is increasing concern over the environmental impacts of this practice. Anaerobic digestates that have been derived from animal excrements generally contain a few percent N (Fontaine et al., 2019; Sørensen et al., 2011 ). Application of anaerobic digestates to soils can cause nutrient leaching into nearby water bodies and high emissions of nitrogenous pollutants, particularly nitrous oxide and ammonia, into the atmosphere (Nkoa, 2014 ). Nitrous oxide is a strong greenhouse gas that causes depletion of ozone in the atmosphere (IPCC, 2022). Emissions of ammonia have adverse impacts on air quality and can contribute to eutrophication of surface waters (Nkoa, 2014 ). Since anaerobic digestates tend to contain high concentrations of P and N in chemical forms that are readily leached (Güngör & Karthikeyan, 2005 ; Svoboda et al., 2013 ), rapid nutrient run-off from fields that have been treated with anaerobic digestates have been reported (Nkoa, 2014 ). Anaerobic digestates can also contain various pathogens, e.g., Salmonella spp. , and pharmaceuticals which threaten the health of crops, livestock, and humans (Bagge et al., 2005 ; Li et al., 2018 ). Direct application of anaerobic digestates to soils has potential to cause significant damage to the environment, particularly in areas where regional animal production has led to a considerable surplus of manure or where anaerobic digestates are produced in large quantities (Potter et al. 2010 ). This is motivating research into more sustainable utilisation options for anaerobic digestates. Anaerobic digestates can potentially be used as feedstocks for thermal processes that produce one or more of the following products: fertilisers, heat, power, fuels, and chemicals. Technologies based on thermal processes, particularly combustion, pyrolysis, and gasification, have long been used to extract value from biomass and are well-established with large-scale installations in operation worldwide. Thermal processes can remove nitrogen from, and destroy harmful pathogens and pharmaceuticals contained within, anaerobic digestates (Leppälahti & Koljonen, 1995 ; Seeker, 1990 ; Winter et al., 1999 ). Furthermore, thermal processes result in a substantial reduction in total solids volume. Valuable inorganic nutrient elements that already exist in anaerobic digestates in high concentrations, e.g., P and K, have potential to become substantially enriched and better stabilised in the ash (or char) residues following thermal processing. The potential to utilise these residues for production of mineral fertilisers and biochar soil ameliorants has been investigated by previous researchers (e.g. Thygesen & Johnsen, 2012 ; Catenacci et al., 2022 ; Basinas et al., 2023 ). Despite this, the application of anaerobic digestate-derived chars and ashes to soils is seldom practiced at large scale. Recent studies have compared the potential of different thermal processes, including combustion, gasification, and pyrolysis, for nutrient recovery from sewage sludges (e.g., Thomsen et al., 2017 ; Zhu et al., 2022 ; Hannl et al., 2024 ). These studies have shown that the type of thermal process and operating conditions impacts the recovery, chemical speciation, and bioavailability of the nutrients. On the other hand, limited work has been done to compare the potential of thermal processes for nutrient recovery from cattle slurry digestates. Detailed knowledge of the fates of valuable nutrient elements under different thermal processing conditions of anaerobic digestates is needed for the development of effective thermal recovery processes. In addition, there is a need to study the fates of heavy metal contaminants, particularly Zn, Cu, and Mn, during the thermal processing of cattle slurry digestates, since these elements can prohibit the use of ash (or char) residues as a fertiliser or soil amendment if they become too concentrated in the target ash (or char) fraction. In general, solid ash and char residues vary greatly in their physical and chemical properties depending on where and how they are collected in thermal processes (Dahl et al., 2009 ; Lind et al., 1999 ). The partitioning of nutrient elements and heavy metals into different ash streams, e.g., bottom ash, fly ash, and air pollution control residues, depends largely on the extent to which they volatilise from the biomass bed (Clarke & Sloss, 1992 ; Nzihou & Stanmore, 2013 ; Obernberger et al., 1997 ; Kortelainen et al., 2015 ). This in turn depends on the chemical composition of the biomass feedstock and on operating parameters, especially temperature and gas atmosphere (Lane et al., 2015a ; Lane et al., 2015b ; van Lith et al., 2008 ). Current knowledge of the volatile behaviour of nutrient elements and heavy metals during thermal processing of biomass is largely rooted in studies based on conversion of feedstocks which typically have low concentrations of P, e.g., coal and wood (e.g., Bläsing & Müller, 2010 ; Lind et al., 1999 ; van Lith et al., 2008 ). Several recent studies (e.g., Hedayati et al., 2021 ; Ren & Li, 2015 ; Skoglund et al., 2014 ) have shown that the ash-forming constituents of these feedstocks follow different transformation pathways to the ash-forming constituents of biomass feedstocks that contain high concentrations of P. For example, the alkali metals (K and Na), which typically form sulphates, chlorides, silicates, and carbonates during combustion of low-P feedstocks, preferentially form phosphates during the combustion of P-rich feedstocks (Grimm et al., 2012 ). Reported differences in ash behaviour such as this, have been attributed to the high affinity of P for base metal cations (Boström et al., 2012 ) and can be expected to significantly impact on the partitioning of nutrient elements and heavy metals within thermal processes. Despite recent advancements in scientific understanding on ash formation with P-rich fuels (Hedayati et al., 2021 ; Falk et al., 2020 ; Nordin et al., 2020 ; Häggström et al., 2023 ; Lidman et al., 2023) it is still difficult to accurately predict the extent of volatilisation of macronutrient elements and heavy metals during thermal conversion of cattle slurry digestates based only on existing literature. Further work is needed to better understand how key operating parameters impact the volatility of macronutrient elements and the volatility of heavy metals, that may contaminate the ash (or char) residues produced in thermal processes and therefore make challenging their utilisation. Furthermore, there is a clear need to study the chemical speciation of the macronutrients in the formed ash fractions to design a process that could be used to efficiently recycle nutrients from anaerobic digestates. Therefore, the primary objective of this study was to assess the influence of operating conditions, particularly gas atmosphere and temperature, on the volatility of macronutrient elements (P, N, K, Ca, Mg, and Na) and heavy metals (Zn, Cu, and Mn) during thermal conversion of cattle slurry-derived anaerobic digestates. A second objective was to identify the chemical forms of macronutrient elements (P, and K) that are retained in the formed combustion ashes. Finally, the implications of the results for utilising ash and char residues for soil improvement are discussed. 2. Methods 2.1 Anaerobic digestates Two samples of anaerobic digestate were used in this work. Both samples were produced at a farm-scale biogas plant, located at the Maaninka Research station (63° 8'34.05"N, 27°19'7.23"E), about 40 km away from the city of Kuopio in the Northern Savonia region of Finland. The biogas plant uses cattle excrement, including faeces and urine, as feedstock and is operated by the Natural Resources Institute Finland. One sample, referred to as “AD-2018”, was produced during the summer of 2018. The other sample, referred to as “AD-2019”, was produced the following year, during the summer of 2019. Both samples were provided in 1 L air-tight plastic containers in the form of slurries. The moisture contents of the as-received slurries, determined by oven-drying 50 g test portions for 24 hours at 105°C, were both within the range 94–95% w/w. Water was removed from the slurries in two stages. In the first stage, the slurries were centrifuged for 5 minutes at 5000 rpm. Solids were separated from the supernatant by decanting. This first stage of water removal simulates mechanical dewatering processes in which water-soluble inorganic elements are partly removed from the solids. Centrifugation reduced the moisture contents of the slurries to values within the range 88–89% w/w. The remaining moisture was completely evaporated from the dewatered slurries by oven-drying (48 hours, 105°C). Inorganic elements contained within the remaining moisture precipitate onto the digestates during oven-drying. The dried digestates were homogenised by milling with a laboratory knife mill equipped with a 1 mm screen. The homogenised samples of digestate were used for the release experiments and composition analyses. 2.2 Release Experiments Nutrient element and heavy metal release experiments were carried out by heating small samples of the anaerobic digestates (0.5–0.7 g) in a laboratory-scale, fixed-bed tube reactor. The reactor setup features: a sample insertion probe for fast heating and quenching of samples in controlled gas atmospheres; a sample holder with porous walls to promote contact between biomass particles and gaseous reactants; a product gas dilution system for safe venting of flammable gas mixtures containing hydrogen; and a Fourier-transform infrared gas analyser (FTIR) for monitoring the concentrations of evolved product gases. Schematic diagrams and a detailed description of the reactor setup have been reported previously (Lane et al., 2020c ). Reaction progress was monitored in select tests by measuring the concentrations of gaseous compounds, including CO 2 , CO, CH 4 , C 2 H 6 , C 3 H 8 , NO, N 2 O, HCN, NH 3 , SO 2 , and H 2 O, in the product gases and by analysing the ash (or char) residues for total carbon. Samples were heated in a range of environments relevant to pyrolysis, combustion, and gasification processes (see Table S1 in the supplementary material for the complete experimental matrix). Combustion tests were carried out at two reaction temperatures, 800°C and 1000°C, in an atmosphere consisting of 1.5% O 2 and 98.5% N 2 . The investigated combustion temperatures span (approximately) the range of operating temperatures used in commercial fluidised-bed biomass combustion and gasification technologies. Operating temperatures that are significantly lower than 800°C, typically result in unacceptably low reaction rates. On the other hand, operating temperatures that are significantly higher than 1000°C, typically result in problematic levels of ash melting. A low concentration of oxygen was used for the combustion experiments to prevent temperature overshoot caused by ignition of evolved volatiles during the devolatilisation phase of combustion. Samples were heated for total durations of 60 and 25 minutes at combustion temperatures of 800°C and 1000°C, respectively. These heating times were sufficiently long for complete (> 99.5%) burnout of carbon in the samples. Pyrolysis and gasification tests were conducted at a consistent temperature (1000°C) to enable the assessment of the impact of gas atmosphere on macronutrient and heavy metal volatility. The use of a high reaction temperature promotes complete destruction of pathogenic organisms in sludges and, in the case of pyrolysis processes, promotes formation of chars with beneficial properties for soil amelioration (Nicholas et al., 2023 ; Tomczyk et al., 2020 ). In the pyrolysis tests, samples were heated in a pure N 2 atmosphere for a total duration of 20 minutes. Gasification tests were carried out in two gas atmospheres, pure CO 2 and 10% H 2 in N 2 . Samples were heated for a total duration of 120 minutes in all gasification tests. At the end of each reaction, the quenched ash (or char) residues were removed from the reactor, weighed to the nearest 0.01 mg, crushed to a fine powder with an agate mortar and pestle, and then analysed to determine the concentrations of macronutrient elements (P, K, Na, Mg, Ca, and N) and heavy metals (Zn, Cu, and Mn) in the residues. The release of these elements from the anaerobic digestates to the gas phase was then calculated by mass balance according to Eq. 1 ; $$\:{R}_{i}\left(\%\right)=\left[1-\left(\frac{{W}_{r}}{{W}_{AD}}\right)\left(\frac{{C}_{r,i}}{{C}_{AD,i}}\right)\right]\times\:100$$ 1 where R i is the release of element i in weight percent, W r and W AD are the weights (mg) of the residue and anaerobic digestate feedstock respectively, and C r,i and C AD,i are the concentrations (mg kg − 1 , dry basis) of element i in the residue and anaerobic digestate, respectively. 2.3 Composition analyses A range of analytical techniques was used to measure element concentrations in the anaerobic digestates and in the thermal conversion residues, i.e., chars and ashes. Detailed descriptions of the techniques and evaluations of their accuracies when applied to various ashes have been reported previously (Lane et al., 2020a , c ). The accuracies of the techniques when applied to biomass were verified by analysing a biomass standard reference material BCR 129 (see Table S2 ) and performing replicate analyses on the anaerobic digestates. The concentrations of C, N, H, and S were determined by oxidation of small samples (1.5 mg) in a micro elemental analyser. Water-soluble chlorine was determined by ion chromatography following leaching of chloride from samples (100 mg) with hot water. The concentrations of 24 metals and metalloids (Al, As, Bi, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Ni, P, Pb, Sb, Si, Sn, Ti, Tl, V, and Zn) were determined using solution-based inductively coupled plasma mass-spectrometry following hot, pressurised digestion of samples (50–100 mg) in concentrated acids (HNO 3 , HF, and H 3 BO 3 ). Method blanks and certified reference materials, BCR 129 (hay powder), BCR 176R (municipal solid waste incineration fly ash), and NIST 1648a (urban dust), were interspersed with analysis batches for quality control. The standard ash contents of the anaerobic digestates were determined by measuring sample weight loss following two consecutive stages of combustion in air (250°C for 1 h and then 550°C for 2 h) in a muffle furnace. The used method is a modified version of international standard ISO 18122:2015. The modified version uses smaller test portions (0.3 g) than specified in the standard method. Combustion ashes prepared at 800°C and 1000°C from AD-2018 were analysed for crystalline phases using powder X-ray diffraction (XRD). Details of the instrument setup, data acquisition parameters, analysis software, and diffraction databases have been reported previously (Lane et al., 2020c ). 3. Results and Discussion The chemical compositions of the two anaerobic digestates, AD-2018 and AD-2019 are presented in Table 1 . Elemental composition and ash content variations between the two samples are minor. The main ash-forming elements in the samples are K, Si, Ca, P, Cl, Mg, S, Na, Fe, and Al (in descending order of concentration). The samples contain the following heavy metals in significant concentrations: Mn (299 and 333 mg kg − 1 ), Zn (223 and 298 mg kg − 1 ) and Cu (55 and 68 mg kg − 1 ). Most of the Cu and Zn in the digestates presumably originated from cattle feed, which contained around 15 mg kg − 1 of Cu and 76 mg kg − 1 of Zn (both values on a dry basis). These metals are added to livestock feeds to improve cattle health and immunity (Goselink & Jongbloed, 2012 ). Other heavy metals (and metalloids), with the exception of Ti, are present in the digestates in relatively low concentrations (< 10 mg kg − 1 ). The volatilities of these elements were not considered in this study. Table 1 Elemental compositions and ash contents of the anaerobic digestates. AD-2018 AD-2019 Cᵃ (% w/w) 41.9 ± 0.4 41.8 ± 0.4 Hᵃ (% w/w) 5.26 ± 0.03 5.27 ± 0.03 Nᵃ (% w/w) 2.14 ± 0.05 2.22 ± 0.05 Sᵃ (% w/w) 0.44 ± 0.1 0.38 ± 0.08 Clᵇ (% w/w) 1.11 ± 0.03 1.01 ± 0.03 Kᶜ (% w/w) 2.9 ± 0.2 3.4 ± 0.2 Siᶜ (% w/w) 2.3 ± 0.3 1.4 ± 0.2 Caᶜ (% w/w) 1.5 ± 0.1 1.7 ± 0.1 Pᶜ (% w/w) 1.11 ± 0.05 1.20 ± 0.05 Mgᶜ (% w/w) 0.92 ± 0.04 1.03 ± 0.04 Naᶜ (% w/w) 0.49 ± 0.04 0.42 ± 0.04 Feᶜ (% w/w) 0.201 ± 0.004 0.145 ± 0.003 Alᶜ (% w/w) 0.135 ± 0.005 0.087 ± 0.003 Mnᶜ (mg kg − 1 ) 299 ± 6 333 ± 7 Znᶜ (mg kg − 1 ) 223 ± 33 298 ± 43 Tiᶜ (mg kg − 1 ) 107 ± 2 86 ± 2 Cuᶜ (mg kg − 1 ) 55 ± 3 68 ± 4 Niᶜ (mg kg − 1 ) 10 ± 1 10 ± 1 Crᶜ (mg kg − 1 ) 9.5 ± 1 6 ± 0.6 Mo c (mg kg-1) 4.2 ± 1 3.5 ± 0.8 Vᶜ (mg kg − 1 ) 3.2 ± 0.4 1.81 ± 0.2 Coᶜ (mg kg − 1 ) 2.2 ± 0.3 1.5 ± 0.2 Pbᶜ (mg kg − 1 ) 1.5 ± 0.2 1.1 ± 0.2 Snᶜ (mg kg − 1 ) 1.6 ± 0.4 < 1 Biᶜ (mg kg − 1 ) < 1 1.5 Cdᶜ (mg kg − 1 ) < 1 < 1 Sbᶜ (mg kg − 1 ) < 1 < 1 Asᶜ (mg kg − 1 ) < 1 < 1 Tlᶜ (mg kg − 1 ) < 1 < 1 ashᵈ (% w/w) 19.8 ± 0.2 20.0 ± 0.2 a = determined by combustion in a micro-elemental analyser b = determined by IC following hot water extraction c = determined by ICP-MS following pressurised acid digestion d = ash content determined at 550°C The yields of ash (or char) following thermal conversion of AD-2018 in different operating conditions are presented in Table 2 . Also presented in Table 2 are the measured concentrations of macronutrient elements and heavy metals in the ashes and pyrolysis char. Ash/char yields and ash/char composition data for AD-2019 is presented in the supplementary material (see Table S3). Ash yields following combustion in 1.5% O 2 / 98.5% N 2 and CO 2 -gasification were between 15 and 20% lower than the standard ash contents. This result is attributed to the low combustion temperature (550°C) used to determine the standard ash contents, which can result in reduced decomposition of carbonates (Mlonka-Mędrala et al., 2020 ) and reduced release of volatile inorganic species such as Cl (Johansen et al., 2011 ). Table 2 Char/ash yields following thermal conversion of AD-2018 at different operating conditions, and concentrations of macronutrient elements and heavy metals in the char and ash residues. temperature reactant gas composition yield of ash or char concentration in char or ash residue (°C) (%) P (%) K (%) Na (%) Ca (%) Mg (%) C (%) N (%) Zn (mg kg − 1 ) Cu (mg kg − 1 ) Mn (mg kg − 1 ) 800 1.5% O 2 / bal. N 2 16.9 ± 1.7 6.8 ± 0.3 11.7 ± 0.7 2.7 ± 0.1 9.2 ± 1.0 5.7 ± 0.4 0.13 ± 0.01 < 0.05 1260 ± 106 323 ± 25 1871 ± 129 1000 1.5% O 2 / bal. N 2 16.3 ± 1.6 6.8 ± 0.3 11.4 ± 0.7 2.6 ± 0.1 8.9 ± 1.0 5.7 ± 0.4 < 0.1 < 0.05 793 ± 67 321 ± 25 1889 ± 130 1000 N 2 33.0 ± 3.3 3.1 ± 0.1 4.9 ± 0.3 0.6 ± 0.03 4.3 ± 0.5 2.6 ± 0.2 49.1 ± 4.8 1.02 ± 0.05 29 ± 2.4 162 ± 13 870 ± 60 1000 CO 2 16.3 ± 1.6 7.6 ± 0.3 12.2 ± 0.8 2.8 ± 0.1 9.6 ± 1.0 6.0 ± 0.4 0.13 ± 0.01 < 0.05 776 ± 65 281 ± 22 1867 ± 129 1000 10% H 2 / bal. N 2 28.1 ± 2.8 1.7 ± 0.1 3.2 ± 0.2 0.2 ± 0.01 4.7 ± 0.5 2.8 ± 0.2 48.6 ± 4.7 0.38 ± 0.02 28 ± 2.4 155 ± 12 1026 ± 71 3.1. Release of macronutrient elements The fractional release of macronutrient elements to the gas phase during combustion in 1.5% O 2 / 98.5% N 2 is presented in Fig. 1 . Results are presented for both samples of anaerobic digestate at two combustion temperatures, 800°C and 1000°C. Differences in release results between the two anaerobic digestate samples are minor. The impact of different gas atmospheres, pure N 2 , 1.5% O 2 in N 2 , pure CO 2 , and 10% H 2 in N 2 , on the release of these elements at 1000°C is shown in Fig. 2 . Low negative release values are shown for some tests. The low negative release values are attributed to analysis errors. The errors in the release values were evaluated by propagation of the errors in the elemental concentration measurements and ash (or char) yield measurements which were based on two standard deviations of the measured quantities. Errors were found to diminish with increasing release. Phosphorous was completely retained in the ashes following combustion (both temperatures) and following CO 2 -gasification at 1000°C. This agrees with previous studies that indicate only minor volatilization of P during combustion of sewage sludges (Falk et al., 2020 ; Häggström et al., 2023 ). In contrast, a moderate amount of P (~ 8– 20%) was released to the gas phase when the anaerobic digestates were pyrolyzed in pure N 2 at 1000°C. The fractional release of P was even greater (~ 58– 64%) following conversion of the anaerobic digestates in the reducing atmosphere consisting of 10% H 2 in N 2 . This indicates that reducing conditions promote volatilisation of P from anaerobic digestates. The analysis of carbonaceous emissions during N 2 -pyrolysis of AD-2018 (see Figure S1 ) showed significant concentrations of CO and CH 4 in the product gases. Peak concentrations of CO and CH 4 during pyrolysis were ~ 3% v/v and ~ 0.4% v/v respectively. Significant concentrations of H 2 (not measured) are also expected in the pyrolysis product gases. Thus, the release of P was clearly associated with reducing conditions during conversion of the anaerobic digestates. The extent of release of the alkali metals, K and Na, varied greatly (32–71% for K and 6–96% for Na) over the range of investigated operating conditions. In combustion conditions, K was more volatile than Na. The release of K during combustion in 1.5% O 2 / 98.5% N 2 ranged from 32–33% at 800°C and ranged from 37–43% at 1000°C. The release of Na during combustion in the same atmosphere ranged from 9–15% at 800°C and ranged from 16–27% at 1000°C. The extent of release of K and Na was greater following pyrolysis in pure N 2 and was greater again following thermal conversion in the reducing atmosphere consisting of 10% H 2 in N 2 . The release of K and Na at 1000°C was lowest when the anaerobic digestates were heated in CO 2 . The impact of the different gas atmospheres on volatility was greater for Na than for K. In general, the release of K and Na during thermal conversion of biomass is largely influenced by the formation of alkali metal containing silicates and phosphates that retain the alkali metals in coarse ash fraction (Sippula et al., 2017 ; Hedayati et al., 2021 ). Calcium and magnesium had either low positive or low negative release values in all investigated operating conditions. This indicates that Ca and Mg are either not released at all or are released in only minor quantities. The low negative values are a result of experimental uncertainties. Nitrogen was completely released (> 99.5%) to the gas phase during thermal conversion of the anaerobic digestates in the two atmospheres which contain oxygen (1.5% O 2 in N 2 and pure CO 2 ). On the other hand, thermal conversion of the anaerobic digestates at 1000°C in the pure N 2 atmosphere and in the atmosphere consisting of 10% H 2 in N 2 resulted in incomplete release of N (84–87% release in pure N 2 and 93–95% release in 10% H 2 / 90% N 2 ). Measurements of nitrogenous compounds in the emissions of anaerobic digestate combustion (see Figure S2 ) showed high concentrations of NO and N 2 O. Emissions of oxides of nitrogen (NO x ) in practical biomass-fired systems are caused mainly by oxidation of biomass-derived nitrogen rather than reactions involving atmospheric N 2 (Glarborg et al., 2003 ). One or more control measure, e.g., air staging and selective catalytic reduction (SCR) of NO x , will most likely be needed to reduce emissions of NO x to acceptable levels during the conversion of anaerobic digestates derived from cattle slurries in industrial-scale installations. The XRD spectra for the combustion ashes prepared from AD-2018 are presented in Fig. 3. Two major crystalline phases were identified in the combustion ash prepared at 800°C: merwinite (Ca 3 Mg(SiO 4 ) 2 ) and whitlockite (Ca 9 (K,Mg,Fe)(PO 4 ) 6 (PO 3 OH)). Three major crystalline phases were identified in the combustion ash prepared at 1000°C: diopside (CaMgSi 2 O 6 ), tricalcium aluminate (Ca 3 Al 12 O 6 ), and whitlockite (Ca 9 (K,Mg,Fe)(PO 4 ) 6 (PO 3 OH)). Possible minor phases in the combustion ash prepared at 800°C (not shown in Fig. 3) include: potassium magnesium phosphate (KMgPO 4 ), polyhalite (K 2 Ca 2 Mg(SO 4 ) 4 ·2H 2 O), and antigorite (Mg 3 (Si 2 O 5 )(OH) 4 ). Polyhalite was also identified as a possible minor phase in the combustion ash prepared at 1000°C. The XRD analyses show that a substantial portion of P in the anaerobic digestates forms whitlockite during combustion at both 800°C and 1000°C. A large range of phosphates, which contain varying proportions of Ca, Mg, and K, have been reported by previous investigators in combustion ashes derived from other types of P-rich biomass, such as sewage sludge, meat bone meal, and cereal grains (Li et al., 2013 ; Lindström et al., 2007 ; Öhman et al., 2003 ; Skoglund et al., 2014 ). The formation of phosphates with high molar ratios of alkaline earth metals to K, such as whitlockite, can be considered advantageous with respect to the operation of industrial thermal processes, since these phosphates generally melt at higher temperatures than K-rich phosphates and consequently, tend to alleviate rather than exacerbate ash-related operational issues such as fouling, deposition, and, in the case of fluidised-bed reactor technologies, bed agglomeration (Lindström et al., 2007 ; Skoglund et al., 2014 ; Steenari et al., 2009 ). The presence of alkali metals in a whitlockite mineral structure has also been suggested to be positive regarding the plant bioavailability of phosphorus in ashes (Herzel et al., 2020 ; Falk et al., 2020 ). 3.2. Release of heavy metals The fractional release of Zn, Cu, and Mn to the gas phase during thermal conversion of AD-2018 is presented in Fig. 4 for different operating conditions. The volatility of Zn varied greatly over the range of investigated operating conditions and was largely impacted by both temperature and gas atmosphere. Zinc had low volatility during combustion at 800°C but was partly released to the gas phase during combustion and CO 2 -gasification at 1000°C (~ 40–45%). The release of Zn was greatest (~ 96–97%) when the anaerobic digestate was heated in the reducing gas (10% H 2 / 90% N 2 ) and inert gas (pure N 2 ) atmospheres. The high release of Zn during pyrolysis in pure N 2 is attributed to local reducing conditions caused by formation of CO from the decomposition of carbon. The increased volatility of zinc in reducing conditions is consistent with thermodynamic equilibrium analyses of zinc speciation during combustion of biomass in fuel-rich and fuel-lean conditions (Elled et al., 2008 ; Sørum et al., 2003 ). Reducing gases, including CO and H 2 , reduce solid compounds of zinc to atomic zinc gas whereas oxidising atmospheres promote formation of non-volatile silicates and aluminates of Zn (Lane et al., 2020b ; Sørum et al., 2003 ; Sinclair, 2005 ). The release of Mn was at most minor (< 20%) in all investigated operating conditions. Small fractions of Cu were released during CO 2 -gasification (~ 16% released) and during thermal conversion in the atmosphere consisting of 10% H 2 in N 2 (~ 20% released). The release of Cu was at most minor (< 20%) in the pyrolysis and combustion atmospheres (pure N 2 and 1.5% O 2 in N 2 ). 3.3. Implications for application of cattle slurry-derived ash and char residues onto soils The inorganic constituents of biomass are separated into different ash (or char) fractions in industrial-scale thermal processes. Ashes that are separated from the product gases downstream of the reacting bed of biomass are generally enriched in volatile elements. These ashes are collectively referred to as fly ashes. Ash fractions that are collected below the reacting bed of biomass are enriched in non-volatile elements. These ashes are generally referred to as bottom ashes, or bed ashes in the case of fluidised-bed reactor technologies. The partitioning of P in cattle slurry-derived digestates between bottom ashes and fly ashes will likely depend largely on the composition of the gas phase in contact with the reacting bed of anaerobic digestate. Given that the volatility of P in oxidising and inert gas atmospheres is low, it is expected that P will largely report to bottom ashes in commercial combustion processes. Previous investigators (e.g., Skoglund et al., 2014 ) have shown P to report mainly to coarse ash fractions during the combustion of biomass derived from municipal sewage sludges in laboratory-scale fluidised-bed reactors. However, the presence of local reducing zones in combustion processes, e.g., at the entrance of grate-fired combustors, may cause at least partial volatilisation of P and reduced recovery of P in bottom ashes. Nordin et al. ( 2020 ) compared the extent of transfer of P to bottom ashes during the combustion of municipal sewage sludge in two reactor types: an 8 MW th grate-fired boiler and a fixed-bed, laboratory-scale furnace with excess supply of oxygen to the sample. The authors reported lower transfer of P to bottom ashes in the grate-fired boiler where reducing zones are encountered. In gasification processes, which typically have high concentrations of gas phase reductants, it is expected that P will volatilise to a significant extent and condense on fly ash fractions. The laboratory-prepared combustion ashes were approximately 6-fold more concentrated in P than the dried anaerobic digestates and contained around 7% P w/w. Similar concentrations of P could be expected in the bottom ashes produced in industrial-scale combustors. Phosphorous concentrations in the laboratory-prepared combustion ashes are comparable with concentrations of P in low-grade phosphate rock ore. The global average grade of phosphate rock ore has been estimated to be around 10% P w/w, or 22.5% P 2 O 5 (Van Kauwenbergh, 2010 ), however, economic grades can be as low as 2.5% P w/w. According to the XRD analyses, a major part of phosphorus was present in the combustion ashes as mixed cation whitlockite minerals. These minerals generally have low phosphorus solubilities. Nevertheless, they have been reported to have suitable properties as slow-release fertilisers (Kumpiene et al., 2016 ; Mackay et al., 2017 ). Recent studies show that co-combustion or co-gasification of sludges with alkali metal-rich biomass causes an increase in the alkali metal content of formed whitlockite minerals (Herzel et al., 2020 ; Nordin et al., 2020 ). It is understood that this increases the bioavailability of P in ashes (Stemann et al., 2015 ; Herzel et al., 2020 ). Furthermore, the addition of dry biomass to moist anaerobic digestates increases the heating value of the combustion feedstock. Therefore, co-combustion of mixtures of moist anaerobic digestates with dry, alkali metal-rich biomass could potentially be used as a strategy to improve combustion performance and the fertiliser properties of the bottom ashes. Another potential strategy to increase the bioavailability of P in ashes is to treat the ashes in a thermochemical process with addition of a sodium salt. This strategy has been shown to convert whitlockite minerals to a mineral (buchwaldite) that makes P more bioavailable (Stemann et al., 2015 ). Potassium is partially released to the gas phase in thermal processes and is expected to report to both bottom ash and fly ash in significant quantities. Higher temperatures and reducing gas atmospheres favour the release of K to the gas phase and are expected to increase the yield of K in fly ashes. Zinc and copper are both essential micronutrients for plants (Alloway, 2013 ). The addition of controlled quantities of these elements to soils can potentially be beneficial if the soil is deficient in these elements. However, high concentrations of Zn and Cu can have toxic effects on plants and on soil dwelling organisms (Alloway, 2013 ; Nagajyoti et al., 2010 ; Reichman, 2002 ). For this reason, the upper limits of the concentrations of Cu and Zn in biomass ashes for application onto agricultural and forest soils are limited by EU-level and national regulations (see Table 3 for limit values). Bottom ashes produced from combustion of cattle slurry-derived anaerobic digestates are expected to contain acceptable levels of Cu and Zn for application on agricultural and forest soils in Finland. Table 3 Upper limits of Cu and Zn in biomass ash for application on agricultural and forest soils in European countries. Limit values are compared with global averages in topsoil and concentrations measured in the anaerobic digestate derived combustion ashes. Cu (mg kg − 1 , dry basis) Zn (mg kg − 1 , dry basis) Global average in topsoil (2013) 14 62 Anaerobic digestate derived combustion ashes Combustion at 800 ºC 323 ± 25 1260 ± 106 Combustion at 1000 ºC 321 ± 25 793 ± 67 Upper limit for ash application on agricultural soils EU regulation 2019/1009 Sweden Finland 600 600 600 1,500 800 1,500 Upper limit for ash application on forest soils Finland 700 4,500 Sweden 400 7,000 Fly ashes produced from thermal conversion of anaerobic digestates derived from cattle slurry have potential to contain high concentrations of the macronutrients, P and K, provided that the anaerobic digestates are exposed to reducing gaseous environments during thermal conversion. Such environments were found to promote significant volatilisation of these two elements. While fly ashes have potential to contain high concentrations of both P and K they will almost certainly be contaminated with high levels of Cl. It is expected that fly ashes will contain prohibitive levels of Cl given the high concentrations of Cl in the anaerobic digestates (1.0 and 1.1% w/w) and the high volatility of Cl at temperatures encountered in thermal processes (Björkman & Strömberg, 1997 ; Johansen et al., 2011 ; Lane et al., 2015a ). High concentrations of Cl in fly ashes are expected to prevent direct application of the fly ashes to soils due to the risk of increasing soil salinity. Nitrogen was substantially released from the anaerobic digestates during both combustion and gasification. The nitrogen released in these processes forms gaseous species that are expensive to capture in industrial-scale installations, including N 2 , NO, N 2 O, NH 3 , and HCN (Glarborg et al., 2003 ; Leppälahti & Koljonen, 1995 ). Given the high contents of N in the anaerobic digestates (2.1 and 2.2% w/w) it is recommended that practical installations be designed and operated in a way that promotes high conversion of fuel-N to N 2 and minimal conversion of fuel-N to deleterious nitrogenous products, particularly NO and N 2 O, e.g. by employing air staging, selective noncatalytic reduction, or selective catalytic reduction. A small fraction of N (~ 13–16%) was retained in the solid char residues following pyrolysis of the anaerobic digestates at 1000°C. While the application of biomass derived chars to soils can significantly reduce emissions of NO and N 2 O from soils, this is not universally true for all biochar-soil combinations (Van Zwieten et al., 2009 ; Zheng et al., 2012 ). Additional research is needed to assess the potential of pyrolysis to mitigate emissions of NO and N 2 O. Conclusions Phosphorous had low volatility in all atmospheres except the reducing atmosphere consisting of 10% H 2 in N 2 . Phosphorous was retained in the combustion ashes as mixed cation whitlockite minerals. The volatilities of K, Na, and Zn were significantly impacted by temperature and were lowest in the oxygen-containing atmospheres (1.5% O 2 in N 2 and pure CO 2 ). Copper, manganese, calcium, and magnesium had low volatilities in all investigated operating conditions. Bottom ashes produced from combustion of cattle slurry digestates are expected to meet legislative requirements for application onto agricultural and forest soils in Finland. Co-combustion of cattle slurry digestates with dry, alkali metal-rich agricultural residues is a potential strategy to improve both combustion performance and the bioavailability of phosphorous in the ashes. Further research is recommended to assess the benefits and limitations of this strategy. Declarations Ethics approval and consent to participate Not Applicable Consent for publication Not Applicable Availability of data and materials The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request. Competing interests The authors declare that they have no competing interests. Funding The work presented in this paper was financially supported by the Research Council of Finland grant to NJS (grant # 311970) and the Finnish Ministry of Agriculture and Forestry (VN/28562/2020-MMM-2). Authors' contributions DL - idea conception, data collection, manuscript preparation, OS - idea conception, supervision, manuscript editing, NJS - idea conception, manuscript editing, aquiring funding. All other authors provided valuable comments and edited the manuscript. 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Lane","email":"","orcid":"","institution":"Commonwealth Scientific and Industrial Research Organisation","correspondingAuthor":false,"prefix":"","firstName":"Daniel","middleName":"J.","lastName":"Lane","suffix":""},{"id":378587866,"identity":"eb557af2-72fc-42d1-87a9-6f763b95543d","order_by":1,"name":"Olli Sippula","email":"","orcid":"","institution":"University of Eastern Finland - Kuopio Campus: Ita-Suomen yliopisto - Kuopion kampus","correspondingAuthor":false,"prefix":"","firstName":"Olli","middleName":"","lastName":"Sippula","suffix":""},{"id":378587867,"identity":"c6c17353-9995-41bf-b5bc-fa1df4a20e5c","order_by":2,"name":"Jorma Jokiniemi","email":"","orcid":"","institution":"University of Eastern Finland - Kuopio Campus: Ita-Suomen yliopisto - Kuopion kampus","correspondingAuthor":false,"prefix":"","firstName":"Jorma","middleName":"","lastName":"Jokiniemi","suffix":""},{"id":378587868,"identity":"b61f9c4c-3a92-441b-9032-ed69fe446c3e","order_by":3,"name":"Mikko Heimonen","email":"","orcid":"","institution":"University of Eastern Finland - Kuopio Campus: Ita-Suomen yliopisto - Kuopion kampus","correspondingAuthor":false,"prefix":"","firstName":"Mikko","middleName":"","lastName":"Heimonen","suffix":""},{"id":378587869,"identity":"c6dcaf7b-2b7c-4d3b-bb90-baf456ba7669","order_by":4,"name":"Niko M. Kinnunen","email":"","orcid":"","institution":"University of Eastern Finland - Kuopio Campus: Ita-Suomen yliopisto - Kuopion kampus","correspondingAuthor":false,"prefix":"","firstName":"Niko","middleName":"M.","lastName":"Kinnunen","suffix":""},{"id":378587870,"identity":"163342d9-977f-4de8-ae27-bdd7a243cfa1","order_by":5,"name":"Perttu Virkajärvi","email":"","orcid":"","institution":"Natural Resources Institute Finland: Luonnonvarakeskus","correspondingAuthor":false,"prefix":"","firstName":"Perttu","middleName":"","lastName":"Virkajärvi","suffix":""},{"id":378587871,"identity":"73d6605a-56cd-4f48-a7a4-a42e3a3a70f2","order_by":6,"name":"Narasinha J. Shurpali","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABEElEQVRIie3PvUoDQRDA8ZGF3WbitScH+gorgQTxI6+SI3B2IvgCgcDZCHmBoK8Qe4sJA9pE0ka4IkHYKkXsTggkezmIFneJZcD9F8sW89sPAJdrb9PQtKtc7w+FGBAQgOr8lUghm2uCvP2eXwRQ54SKR+vqffqV3iY3XluZz8pLciIVzhmHF4CqmJw9XFcD1ObOJ6xX0ZjTWFT6jOMIUJT8giIIQHPYJpQBEh/kZM7QKCMjI75TS55ImYw0YoETS5blt4wj6aMlfYJaRkJLwD6MthAjzzPyzFg76pFpxULqQW/YwlIyisRHuuDw8e3e+DNKLrseTyez16tj9KjYbMrP/JnCHfObdh3scrlc/7EV5rdfTkLNOHMAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0003-1052-4396","institution":"Natural Resources Institute Finland: Luonnonvarakeskus","correspondingAuthor":true,"prefix":"","firstName":"Narasinha","middleName":"J.","lastName":"Shurpali","suffix":""}],"badges":[],"createdAt":"2024-02-20 10:25:30","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-3972486/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-3972486/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1186/s40643-024-00828-7","type":"published","date":"2024-12-30T15:57:40+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":70309519,"identity":"f13cc943-bb22-47b9-b87a-323144105d77","added_by":"auto","created_at":"2024-12-02 04:00:07","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":294730,"visible":true,"origin":"","legend":"\u003cp\u003eFractional release of macronutrient elements to the gas phase during combustion of the anaerobic digestates at 800 °C and 1000 °C.\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-3972486/v1/f64cced31703de778bc3acee.jpeg"},{"id":70309517,"identity":"c28d57db-ad2c-4307-adda-816a6cbaf2e7","added_by":"auto","created_at":"2024-12-02 04:00:07","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":625316,"visible":true,"origin":"","legend":"\u003cp\u003eFractional release of macronutrient elements to the gas phase during thermal conversion of anaerobic digestates at 1000 °C in different gas atmospheres.\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-3972486/v1/14b44a0246514320bbc8a4da.jpeg"},{"id":70309518,"identity":"93bb1c78-cda8-4174-aa23-20e3138f2f89","added_by":"auto","created_at":"2024-12-02 04:00:07","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":129735,"visible":true,"origin":"","legend":"\u003cp\u003eXRD spectra for combustion ashes prepared at 800 °C (top panel) and 1000 °C (bottom panel).\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-3972486/v1/54ddedf9bd4b007e4acc35bf.png"},{"id":70309521,"identity":"e8700b3c-418e-4c22-bb8d-7e86db1a6aca","added_by":"auto","created_at":"2024-12-02 04:00:07","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":228464,"visible":true,"origin":"","legend":"\u003cp\u003eFractional release of heavy metals to the gas phase during thermal conversion of AD-2018. Subplot (a) shows the impact of combustion temperature and subplot (b) shows the impact of gas atmosphere at 1000 °C.\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-3972486/v1/a20049790bba641aeb340b1f.jpeg"},{"id":73093986,"identity":"dd7b731c-d1bf-4ee7-9e82-357f0f4f20b2","added_by":"auto","created_at":"2025-01-06 16:22:59","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2133358,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3972486/v1/b70f8bc0-5755-4ddf-bfa7-e2bba36576f9.pdf"},{"id":70309520,"identity":"6c60760c-5be7-4aaa-9101-8b667a6477dc","added_by":"auto","created_at":"2024-12-02 04:00:07","extension":"docx","order_by":6,"title":"","display":"","copyAsset":false,"role":"supplement","size":82145,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryInfoV404112024clean.docx","url":"https://assets-eu.researchsquare.com/files/rs-3972486/v1/46d2d78f600cfac392cb4ccf.docx"},{"id":70309522,"identity":"a11b5449-21c5-492c-bf1a-a0f7e28ebc62","added_by":"auto","created_at":"2024-12-02 04:00:07","extension":"pdf","order_by":7,"title":"","display":"","copyAsset":false,"role":"supplement","size":48934,"visible":true,"origin":"","legend":"","description":"","filename":"revisedgraphicalabstractLane8November.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3972486/v1/4c3ca8f4336a242c4b23d2fb.pdf"}],"financialInterests":"","formattedTitle":"Fates of nutrient elements and heavy metals during thermal conversion of cattle slurry-derived anaerobic digestates","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe most commonly used process to convert biomass wastes into biogas, anaerobic digestion, generates large quantities of a by-product called \u0026ldquo;anaerobic digestate\u0026rdquo;. This material is produced in the form of a slurry, which predominately consists of water, undigested organic material, and inorganic minerals. Until now it has been a common practice to apply anaerobic digestates to soils since it is often rich in phosphorous, nitrogen and other essential plant macronutrients (Lukehurst et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; M\u0026ouml;ller \u0026amp; M\u0026uuml;ller, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). However, there is increasing concern over the environmental impacts of this practice.\u003c/p\u003e \u003cp\u003eAnaerobic digestates that have been derived from animal excrements generally contain a few percent N (Fontaine et al., 2019; S\u0026oslash;rensen et al., \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Application of anaerobic digestates to soils can cause nutrient leaching into nearby water bodies and high emissions of nitrogenous pollutants, particularly nitrous oxide and ammonia, into the atmosphere (Nkoa, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Nitrous oxide is a strong greenhouse gas that causes depletion of ozone in the atmosphere (IPCC, 2022). Emissions of ammonia have adverse impacts on air quality and can contribute to eutrophication of surface waters (Nkoa, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Since anaerobic digestates tend to contain high concentrations of P and N in chemical forms that are readily leached (G\u0026uuml;ng\u0026ouml;r \u0026amp; Karthikeyan, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Svoboda et al., \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2013\u003c/span\u003e), rapid nutrient run-off from fields that have been treated with anaerobic digestates have been reported (Nkoa, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Anaerobic digestates can also contain various pathogens, e.g., \u003cem\u003eSalmonella spp.\u003c/em\u003e, and pharmaceuticals which threaten the health of crops, livestock, and humans (Bagge et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Li et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Direct application of anaerobic digestates to soils has potential to cause significant damage to the environment, particularly in areas where regional animal production has led to a considerable surplus of manure or where anaerobic digestates are produced in large quantities (Potter et al. \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). This is motivating research into more sustainable utilisation options for anaerobic digestates.\u003c/p\u003e \u003cp\u003eAnaerobic digestates can potentially be used as feedstocks for thermal processes that produce one or more of the following products: fertilisers, heat, power, fuels, and chemicals. Technologies based on thermal processes, particularly combustion, pyrolysis, and gasification, have long been used to extract value from biomass and are well-established with large-scale installations in operation worldwide. Thermal processes can remove nitrogen from, and destroy harmful pathogens and pharmaceuticals contained within, anaerobic digestates (Lepp\u0026auml;lahti \u0026amp; Koljonen, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e1995\u003c/span\u003e; Seeker, \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e1990\u003c/span\u003e; Winter et al., \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e1999\u003c/span\u003e). Furthermore, thermal processes result in a substantial reduction in total solids volume. Valuable inorganic nutrient elements that already exist in anaerobic digestates in high concentrations, e.g., P and K, have potential to become substantially enriched and better stabilised in the ash (or char) residues following thermal processing. The potential to utilise these residues for production of mineral fertilisers and biochar soil ameliorants has been investigated by previous researchers (e.g. Thygesen \u0026amp; Johnsen, \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Catenacci et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Basinas et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Despite this, the application of anaerobic digestate-derived chars and ashes to soils is seldom practiced at large scale.\u003c/p\u003e \u003cp\u003eRecent studies have compared the potential of different thermal processes, including combustion, gasification, and pyrolysis, for nutrient recovery from sewage sludges (e.g., Thomsen et al., \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Zhu et al., \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Hannl et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). These studies have shown that the type of thermal process and operating conditions impacts the recovery, chemical speciation, and bioavailability of the nutrients. On the other hand, limited work has been done to compare the potential of thermal processes for nutrient recovery from cattle slurry digestates.\u003c/p\u003e \u003cp\u003eDetailed knowledge of the fates of valuable nutrient elements under different thermal processing conditions of anaerobic digestates is needed for the development of effective thermal recovery processes. In addition, there is a need to study the fates of heavy metal contaminants, particularly Zn, Cu, and Mn, during the thermal processing of cattle slurry digestates, since these elements can prohibit the use of ash (or char) residues as a fertiliser or soil amendment if they become too concentrated in the target ash (or char) fraction. In general, solid ash and char residues vary greatly in their physical and chemical properties depending on where and how they are collected in thermal processes (Dahl et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Lind et al., \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e1999\u003c/span\u003e). The partitioning of nutrient elements and heavy metals into different ash streams, e.g., bottom ash, fly ash, and air pollution control residues, depends largely on the extent to which they volatilise from the biomass bed (Clarke \u0026amp; Sloss, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Nzihou \u0026amp; Stanmore, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Obernberger et al., \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Kortelainen et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). This in turn depends on the chemical composition of the biomass feedstock and on operating parameters, especially temperature and gas atmosphere (Lane et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2015a\u003c/span\u003e; Lane et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2015b\u003c/span\u003e; van Lith et al., \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2008\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eCurrent knowledge of the volatile behaviour of nutrient elements and heavy metals during thermal processing of biomass is largely rooted in studies based on conversion of feedstocks which typically have low concentrations of P, e.g., coal and wood (e.g., Bl\u0026auml;sing \u0026amp; M\u0026uuml;ller, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Lind et al., \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; van Lith et al., \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Several recent studies (e.g., Hedayati et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Ren \u0026amp; Li, \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Skoglund et al., \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2014\u003c/span\u003e) have shown that the ash-forming constituents of these feedstocks follow different transformation pathways to the ash-forming constituents of biomass feedstocks that contain high concentrations of P. For example, the alkali metals (K and Na), which typically form sulphates, chlorides, silicates, and carbonates during combustion of low-P feedstocks, preferentially form phosphates during the combustion of P-rich feedstocks (Grimm et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Reported differences in ash behaviour such as this, have been attributed to the high affinity of P for base metal cations (Bostr\u0026ouml;m et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2012\u003c/span\u003e) and can be expected to significantly impact on the partitioning of nutrient elements and heavy metals within thermal processes.\u003c/p\u003e \u003cp\u003eDespite recent advancements in scientific understanding on ash formation with P-rich fuels (Hedayati et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Falk et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Nordin et al., \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; H\u0026auml;ggstr\u0026ouml;m et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Lidman et al., 2023) it is still difficult to accurately predict the extent of volatilisation of macronutrient elements and heavy metals during thermal conversion of cattle slurry digestates based only on existing literature. Further work is needed to better understand how key operating parameters impact the volatility of macronutrient elements and the volatility of heavy metals, that may contaminate the ash (or char) residues produced in thermal processes and therefore make challenging their utilisation. Furthermore, there is a clear need to study the chemical speciation of the macronutrients in the formed ash fractions to design a process that could be used to efficiently recycle nutrients from anaerobic digestates. Therefore, the primary objective of this study was to assess the influence of operating conditions, particularly gas atmosphere and temperature, on the volatility of macronutrient elements (P, N, K, Ca, Mg, and Na) and heavy metals (Zn, Cu, and Mn) during thermal conversion of cattle slurry-derived anaerobic digestates. A second objective was to identify the chemical forms of macronutrient elements (P, and K) that are retained in the formed combustion ashes. Finally, the implications of the results for utilising ash and char residues for soil improvement are discussed.\u003c/p\u003e"},{"header":"2. Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Anaerobic digestates\u003c/h2\u003e \u003cp\u003eTwo samples of anaerobic digestate were used in this work. Both samples were produced at a farm-scale biogas plant, located at the Maaninka Research station (63\u0026deg; 8'34.05\"N, 27\u0026deg;19'7.23\"E), about 40 km away from the city of Kuopio in the Northern Savonia region of Finland. The biogas plant uses cattle excrement, including faeces and urine, as feedstock and is operated by the Natural Resources Institute Finland. One sample, referred to as \u0026ldquo;AD-2018\u0026rdquo;, was produced during the summer of 2018. The other sample, referred to as \u0026ldquo;AD-2019\u0026rdquo;, was produced the following year, during the summer of 2019. Both samples were provided in 1 L air-tight plastic containers in the form of slurries. The moisture contents of the as-received slurries, determined by oven-drying 50 g test portions for 24 hours at 105\u0026deg;C, were both within the range 94\u0026ndash;95% w/w.\u003c/p\u003e \u003cp\u003eWater was removed from the slurries in two stages. In the first stage, the slurries were centrifuged for 5 minutes at 5000 rpm. Solids were separated from the supernatant by decanting. This first stage of water removal simulates mechanical dewatering processes in which water-soluble inorganic elements are partly removed from the solids. Centrifugation reduced the moisture contents of the slurries to values within the range 88\u0026ndash;89% w/w. The remaining moisture was completely evaporated from the dewatered slurries by oven-drying (48 hours, 105\u0026deg;C). Inorganic elements contained within the remaining moisture precipitate onto the digestates during oven-drying. The dried digestates were homogenised by milling with a laboratory knife mill equipped with a 1 mm screen. The homogenised samples of digestate were used for the release experiments and composition analyses.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Release Experiments\u003c/h2\u003e \u003cp\u003eNutrient element and heavy metal release experiments were carried out by heating small samples of the anaerobic digestates (0.5\u0026ndash;0.7 g) in a laboratory-scale, fixed-bed tube reactor. The reactor setup features: a sample insertion probe for fast heating and quenching of samples in controlled gas atmospheres; a sample holder with porous walls to promote contact between biomass particles and gaseous reactants; a product gas dilution system for safe venting of flammable gas mixtures containing hydrogen; and a Fourier-transform infrared gas analyser (FTIR) for monitoring the concentrations of evolved product gases. Schematic diagrams and a detailed description of the reactor setup have been reported previously (Lane et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2020c\u003c/span\u003e). Reaction progress was monitored in select tests by measuring the concentrations of gaseous compounds, including CO\u003csub\u003e2\u003c/sub\u003e, CO, CH\u003csub\u003e4\u003c/sub\u003e, C\u003csub\u003e2\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e, C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e, NO, N\u003csub\u003e2\u003c/sub\u003eO, HCN, NH\u003csub\u003e3\u003c/sub\u003e, SO\u003csub\u003e2\u003c/sub\u003e, and H\u003csub\u003e2\u003c/sub\u003eO, in the product gases and by analysing the ash (or char) residues for total carbon.\u003c/p\u003e \u003cp\u003eSamples were heated in a range of environments relevant to pyrolysis, combustion, and gasification processes (see Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e in the supplementary material for the complete experimental matrix). Combustion tests were carried out at two reaction temperatures, 800\u0026deg;C and 1000\u0026deg;C, in an atmosphere consisting of 1.5% O\u003csub\u003e2\u003c/sub\u003e and 98.5% N\u003csub\u003e2\u003c/sub\u003e. The investigated combustion temperatures span (approximately) the range of operating temperatures used in commercial fluidised-bed biomass combustion and gasification technologies. Operating temperatures that are significantly lower than 800\u0026deg;C, typically result in unacceptably low reaction rates. On the other hand, operating temperatures that are significantly higher than 1000\u0026deg;C, typically result in problematic levels of ash melting. A low concentration of oxygen was used for the combustion experiments to prevent temperature overshoot caused by ignition of evolved volatiles during the devolatilisation phase of combustion. Samples were heated for total durations of 60 and 25 minutes at combustion temperatures of 800\u0026deg;C and 1000\u0026deg;C, respectively. These heating times were sufficiently long for complete (\u0026gt;\u0026thinsp;99.5%) burnout of carbon in the samples.\u003c/p\u003e \u003cp\u003ePyrolysis and gasification tests were conducted at a consistent temperature (1000\u0026deg;C) to enable the assessment of the impact of gas atmosphere on macronutrient and heavy metal volatility. The use of a high reaction temperature promotes complete destruction of pathogenic organisms in sludges and, in the case of pyrolysis processes, promotes formation of chars with beneficial properties for soil amelioration (Nicholas et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Tomczyk et al., \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). In the pyrolysis tests, samples were heated in a pure N\u003csub\u003e2\u003c/sub\u003e atmosphere for a total duration of 20 minutes. Gasification tests were carried out in two gas atmospheres, pure CO\u003csub\u003e2\u003c/sub\u003e and 10% H\u003csub\u003e2\u003c/sub\u003e in N\u003csub\u003e2\u003c/sub\u003e. Samples were heated for a total duration of 120 minutes in all gasification tests. At the end of each reaction, the quenched ash (or char) residues were removed from the reactor, weighed to the nearest 0.01 mg, crushed to a fine powder with an agate mortar and pestle, and then analysed to determine the concentrations of macronutrient elements (P, K, Na, Mg, Ca, and N) and heavy metals (Zn, Cu, and Mn) in the residues. The release of these elements from the anaerobic digestates to the gas phase was then calculated by mass balance according to Eq.\u0026nbsp;\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e;\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:{R}_{i}\\left(\\%\\right)=\\left[1-\\left(\\frac{{W}_{r}}{{W}_{AD}}\\right)\\left(\\frac{{C}_{r,i}}{{C}_{AD,i}}\\right)\\right]\\times\\:100$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere R\u003csub\u003e\u003cem\u003ei\u003c/em\u003e\u003c/sub\u003e is the release of element \u003cem\u003ei\u003c/em\u003e in weight percent, W\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e and W\u003csub\u003e\u003cem\u003eAD\u003c/em\u003e\u003c/sub\u003e are the weights (mg) of the residue and anaerobic digestate feedstock respectively, and C\u003csub\u003e\u003cem\u003er,i\u003c/em\u003e\u003c/sub\u003e and C\u003csub\u003e\u003cem\u003eAD,i\u003c/em\u003e\u003c/sub\u003e are the concentrations (mg kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, dry basis) of element \u003cem\u003ei\u003c/em\u003e in the residue and anaerobic digestate, respectively.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Composition analyses\u003c/h2\u003e \u003cp\u003eA range of analytical techniques was used to measure element concentrations in the anaerobic digestates and in the thermal conversion residues, i.e., chars and ashes. Detailed descriptions of the techniques and evaluations of their accuracies when applied to various ashes have been reported previously (Lane et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2020a\u003c/span\u003e,\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003ec\u003c/span\u003e). The accuracies of the techniques when applied to biomass were verified by analysing a biomass standard reference material BCR 129 (see Table \u003cspan refid=\"MOESM2\" class=\"InternalRef\"\u003eS2\u003c/span\u003e) and performing replicate analyses on the anaerobic digestates. The concentrations of C, N, H, and S were determined by oxidation of small samples (1.5 mg) in a micro elemental analyser. Water-soluble chlorine was determined by ion chromatography following leaching of chloride from samples (100 mg) with hot water. The concentrations of 24 metals and metalloids (Al, As, Bi, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Ni, P, Pb, Sb, Si, Sn, Ti, Tl, V, and Zn) were determined using solution-based inductively coupled plasma mass-spectrometry following hot, pressurised digestion of samples (50\u0026ndash;100 mg) in concentrated acids (HNO\u003csub\u003e3\u003c/sub\u003e, HF, and H\u003csub\u003e3\u003c/sub\u003eBO\u003csub\u003e3\u003c/sub\u003e). Method blanks and certified reference materials, BCR 129 (hay powder), BCR 176R (municipal solid waste incineration fly ash), and NIST 1648a (urban dust), were interspersed with analysis batches for quality control. The standard ash contents of the anaerobic digestates were determined by measuring sample weight loss following two consecutive stages of combustion in air (250\u0026deg;C for 1 h and then 550\u0026deg;C for 2 h) in a muffle furnace. The used method is a modified version of international standard ISO 18122:2015. The modified version uses smaller test portions (0.3 g) than specified in the standard method. Combustion ashes prepared at 800\u0026deg;C and 1000\u0026deg;C from AD-2018 were analysed for crystalline phases using powder X-ray diffraction (XRD). Details of the instrument setup, data acquisition parameters, analysis software, and diffraction databases have been reported previously (Lane et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2020c\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cp\u003eThe chemical compositions of the two anaerobic digestates, AD-2018 and AD-2019 are presented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Elemental composition and ash content variations between the two samples are minor. The main ash-forming elements in the samples are K, Si, Ca, P, Cl, Mg, S, Na, Fe, and Al (in descending order of concentration). The samples contain the following heavy metals in significant concentrations: Mn (299 and 333 mg kg\u003csup\u003e− 1\u003c/sup\u003e), Zn (223 and 298 mg kg\u003csup\u003e− 1\u003c/sup\u003e) and Cu (55 and 68 mg kg\u003csup\u003e− 1\u003c/sup\u003e). Most of the Cu and Zn in the digestates presumably originated from cattle feed, which contained around 15 mg kg\u003csup\u003e− 1\u003c/sup\u003e of Cu and 76 mg kg\u003csup\u003e− 1\u003c/sup\u003e of Zn (both values on a dry basis). These metals are added to livestock feeds to improve cattle health and immunity (Goselink \u0026amp; Jongbloed, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Other heavy metals (and metalloids), with the exception of Ti, are present in the digestates in relatively low concentrations (\u0026lt; 10 mg kg\u003csup\u003e− 1\u003c/sup\u003e). The volatilities of these elements were not considered in this study.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\u003cdiv class=\"gridtable\"\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\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\u003eElemental compositions and ash contents of the anaerobic digestates.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e\u003ccolgroup cols=\"3\"\u003e\u003c/colgroup\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAD-2018\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAD-2019\u003c/p\u003e \u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCᵃ (% w/w)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e41.9 ± 0.4\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e41.8 ± 0.4\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHᵃ (% w/w)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5.26 ± 0.03\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5.27 ± 0.03\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNᵃ (% w/w)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2.14 ± 0.05\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.22 ± 0.05\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSᵃ (% w/w)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.44 ± 0.1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.38 ± 0.08\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eClᵇ (% w/w)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.11 ± 0.03\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.01 ± 0.03\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eKᶜ (% w/w)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2.9 ± 0.2\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3.4 ± 0.2\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSiᶜ (% w/w)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2.3 ± 0.3\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.4 ± 0.2\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCaᶜ (% w/w)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.5 ± 0.1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.7 ± 0.1\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePᶜ (% w/w)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.11 ± 0.05\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.20 ± 0.05\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMgᶜ (% w/w)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.92 ± 0.04\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.03 ± 0.04\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNaᶜ (% w/w)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.49 ± 0.04\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.42 ± 0.04\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFeᶜ (% w/w)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.201 ± 0.004\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.145 ± 0.003\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAlᶜ (% w/w)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.135 ± 0.005\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.087 ± 0.003\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMnᶜ (mg kg\u003csup\u003e− 1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e299 ± 6\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e333 ± 7\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eZnᶜ (mg kg\u003csup\u003e− 1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e223 ± 33\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e298 ± 43\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTiᶜ (mg kg\u003csup\u003e− 1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e107 ± 2\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e86 ± 2\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCuᶜ (mg kg\u003csup\u003e− 1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e55 ± 3\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e68 ± 4\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNiᶜ (mg kg\u003csup\u003e− 1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e10 ± 1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10 ± 1\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCrᶜ (mg kg\u003csup\u003e− 1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e9.5 ± 1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e6 ± 0.6\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMo\u003csup\u003ec\u003c/sup\u003e (mg kg-1)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e4.2 ± 1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3.5 ± 0.8\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eVᶜ (mg kg\u003csup\u003e− 1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3.2 ± 0.4\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.81 ± 0.2\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCoᶜ (mg kg\u003csup\u003e− 1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2.2 ± 0.3\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.5 ± 0.2\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePbᶜ (mg kg\u003csup\u003e− 1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.5 ± 0.2\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.1 ± 0.2\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSnᶜ (mg kg\u003csup\u003e− 1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.6 ± 0.4\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt; 1\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBiᶜ (mg kg\u003csup\u003e− 1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026lt; 1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.5\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCdᶜ (mg kg\u003csup\u003e− 1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026lt; 1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt; 1\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSbᶜ (mg kg\u003csup\u003e− 1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026lt; 1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt; 1\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAsᶜ (mg kg\u003csup\u003e− 1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026lt; 1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt; 1\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTlᶜ (mg kg\u003csup\u003e− 1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026lt; 1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt; 1\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eashᵈ (% w/w)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e19.8 ± 0.2\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e20.0 ± 0.2\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003ctfoot\u003e\u003ctr\u003e\u003ctd colspan=\"3\"\u003ea = determined by combustion in a micro-elemental analyser\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd colspan=\"3\"\u003eb = determined by IC following hot water extraction\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd colspan=\"3\"\u003ec = determined by ICP-MS following pressurised acid digestion\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd colspan=\"3\"\u003ed = ash content determined at 550°C\u003c/td\u003e\u003c/tr\u003e\u003c/tfoot\u003e\u003c/table\u003e\u003c/div\u003e \u003cp\u003e\u003c/p\u003e \u003cp\u003eThe yields of ash (or char) following thermal conversion of AD-2018 in different operating conditions are presented in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Also presented in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e are the measured concentrations of macronutrient elements and heavy metals in the ashes and pyrolysis char. Ash/char yields and ash/char composition data for AD-2019 is presented in the supplementary material (see Table S3). Ash yields following combustion in 1.5% O\u003csub\u003e2\u003c/sub\u003e / 98.5% N\u003csub\u003e2\u003c/sub\u003e and CO\u003csub\u003e2\u003c/sub\u003e-gasification were between 15 and 20% lower than the standard ash contents. This result is attributed to the low combustion temperature (550°C) used to determine the standard ash contents, which can result in reduced decomposition of carbonates (Mlonka-Mędrala et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) and reduced release of volatile inorganic species such as Cl (Johansen et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2011\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\u003cdiv class=\"gridtable\"\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\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c11\" colnum=\"11\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c12\" colnum=\"12\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c13\" colnum=\"13\"\u003e\u003c/div\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\u003eChar/ash yields following thermal conversion of AD-2018 at different operating conditions, and concentrations of macronutrient elements and heavy metals in the char and ash residues.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e\u003ccolgroup cols=\"13\"\u003e\u003c/colgroup\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\u003ereactant gas composition\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eyield of ash or char\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colspan=\"10\" nameend=\"c13\" namest=\"c4\"\u003e \u003cp\u003econcentration in char or ash residue\u003c/p\u003e \u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e(°C)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e(%)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eP (%)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eK (%)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eNa (%)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eCa (%)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eMg (%)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003eC (%)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003eN (%)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003eZn (mg kg\u003csup\u003e− 1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003eCu (mg kg\u003csup\u003e− 1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c13\"\u003e \u003cp\u003eMn (mg kg\u003csup\u003e− 1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e800\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.5% O\u003csub\u003e2\u003c/sub\u003e / bal. N\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e16.9 ± 1.7\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6.8 ± 0.3\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e11.7 ± 0.7\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2.7 ± 0.1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e9.2 ± 1.0\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e5.7 ± 0.4\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.13 ± 0.01\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e\u0026lt; 0.05\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003e1260 ± 106\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003e323 ± 25\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c13\"\u003e \u003cp\u003e1871 ± 129\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1000\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.5% O\u003csub\u003e2\u003c/sub\u003e / bal. N\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e16.3 ± 1.6\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6.8 ± 0.3\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e11.4 ± 0.7\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2.6 ± 0.1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e8.9 ± 1.0\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e5.7 ± 0.4\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e\u0026lt; 0.1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e\u0026lt; 0.05\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003e793 ± 67\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003e321 ± 25\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c13\"\u003e \u003cp\u003e1889 ± 130\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1000\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eN\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e33.0 ± 3.3\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3.1 ± 0.1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e4.9 ± 0.3\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.6 ± 0.03\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e4.3 ± 0.5\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e2.6 ± 0.2\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e49.1 ± 4.8\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e1.02 ± 0.05\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003e29 ± 2.4\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003e162 ± 13\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c13\"\u003e \u003cp\u003e870 ± 60\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1000\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e16.3 ± 1.6\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e7.6 ± 0.3\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e12.2 ± 0.8\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2.8 ± 0.1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e9.6 ± 1.0\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e6.0 ± 0.4\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.13 ± 0.01\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e\u0026lt; 0.05\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003e776 ± 65\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003e281 ± 22\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c13\"\u003e \u003cp\u003e1867 ± 129\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1000\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e10% H\u003csub\u003e2\u003c/sub\u003e / bal. N\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e28.1 ± 2.8\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.7 ± 0.1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e3.2 ± 0.2\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.2 ± 0.01\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e4.7 ± 0.5\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e2.8 ± 0.2\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e48.6 ± 4.7\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e0.38 ± 0.02\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003e28 ± 2.4\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003e155 ± 12\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c13\"\u003e \u003cp\u003e1026 ± 71\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/table\u003e\u003c/div\u003e \u003cp\u003e\u003c/p\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Release of macronutrient elements\u003c/h2\u003e \u003cp\u003eThe fractional release of macronutrient elements to the gas phase during combustion in 1.5% O\u003csub\u003e2\u003c/sub\u003e / 98.5% N\u003csub\u003e2\u003c/sub\u003e is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Results are presented for both samples of anaerobic digestate at two combustion temperatures, 800°C and 1000°C. Differences in release results between the two anaerobic digestate samples are minor. The impact of different gas atmospheres, pure N\u003csub\u003e2\u003c/sub\u003e, 1.5% O\u003csub\u003e2\u003c/sub\u003e in N\u003csub\u003e2\u003c/sub\u003e, pure CO\u003csub\u003e2\u003c/sub\u003e, and 10% H\u003csub\u003e2\u003c/sub\u003e in N\u003csub\u003e2\u003c/sub\u003e, on the release of these elements at 1000°C is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Low negative release values are shown for some tests. The low negative release values are attributed to analysis errors. The errors in the release values were evaluated by propagation of the errors in the elemental concentration measurements and ash (or char) yield measurements which were based on two standard deviations of the measured quantities. Errors were found to diminish with increasing release.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003ePhosphorous was completely retained in the ashes following combustion (both temperatures) and following CO\u003csub\u003e2\u003c/sub\u003e-gasification at 1000°C. This agrees with previous studies that indicate only minor volatilization of P during combustion of sewage sludges (Falk et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Häggström et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). In contrast, a moderate amount of P (~ 8– 20%) was released to the gas phase when the anaerobic digestates were pyrolyzed in pure N\u003csub\u003e2\u003c/sub\u003e at 1000°C. The fractional release of P was even greater (~ 58– 64%) following conversion of the anaerobic digestates in the reducing atmosphere consisting of 10% H\u003csub\u003e2\u003c/sub\u003e in N\u003csub\u003e2\u003c/sub\u003e. This indicates that reducing conditions promote volatilisation of P from anaerobic digestates. The analysis of carbonaceous emissions during N\u003csub\u003e2\u003c/sub\u003e-pyrolysis of AD-2018 (see Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e) showed significant concentrations of CO and CH\u003csub\u003e4\u003c/sub\u003e in the product gases. Peak concentrations of CO and CH\u003csub\u003e4\u003c/sub\u003e during pyrolysis were ~ 3% v/v and ~ 0.4% v/v respectively. Significant concentrations of H\u003csub\u003e2\u003c/sub\u003e (not measured) are also expected in the pyrolysis product gases. Thus, the release of P was clearly associated with reducing conditions during conversion of the anaerobic digestates.\u003c/p\u003e \u003cp\u003eThe extent of release of the alkali metals, K and Na, varied greatly (32–71% for K and 6–96% for Na) over the range of investigated operating conditions. In combustion conditions, K was more volatile than Na. The release of K during combustion in 1.5% O\u003csub\u003e2\u003c/sub\u003e / 98.5% N\u003csub\u003e2\u003c/sub\u003e ranged from 32–33% at 800°C and ranged from 37–43% at 1000°C. The release of Na during combustion in the same atmosphere ranged from 9–15% at 800°C and ranged from 16–27% at 1000°C. The extent of release of K and Na was greater following pyrolysis in pure N\u003csub\u003e2\u003c/sub\u003e and was greater again following thermal conversion in the reducing atmosphere consisting of 10% H\u003csub\u003e2\u003c/sub\u003e in N\u003csub\u003e2\u003c/sub\u003e. The release of K and Na at 1000°C was lowest when the anaerobic digestates were heated in CO\u003csub\u003e2\u003c/sub\u003e. The impact of the different gas atmospheres on volatility was greater for Na than for K. In general, the release of K and Na during thermal conversion of biomass is largely influenced by the formation of alkali metal containing silicates and phosphates that retain the alkali metals in coarse ash fraction (Sippula et al., \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Hedayati et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eCalcium and magnesium had either low positive or low negative release values in all investigated operating conditions. This indicates that Ca and Mg are either not released at all or are released in only minor quantities. The low negative values are a result of experimental uncertainties.\u003c/p\u003e \u003cp\u003eNitrogen was completely released (\u0026gt; 99.5%) to the gas phase during thermal conversion of the anaerobic digestates in the two atmospheres which contain oxygen (1.5% O\u003csub\u003e2\u003c/sub\u003e in N\u003csub\u003e2\u003c/sub\u003e and pure CO\u003csub\u003e2\u003c/sub\u003e). On the other hand, thermal conversion of the anaerobic digestates at 1000°C in the pure N\u003csub\u003e2\u003c/sub\u003e atmosphere and in the atmosphere consisting of 10% H\u003csub\u003e2\u003c/sub\u003e in N\u003csub\u003e2\u003c/sub\u003e resulted in incomplete release of N (84–87% release in pure N\u003csub\u003e2\u003c/sub\u003e and 93–95% release in 10% H\u003csub\u003e2\u003c/sub\u003e / 90% N\u003csub\u003e2\u003c/sub\u003e). Measurements of nitrogenous compounds in the emissions of anaerobic digestate combustion (see Figure \u003cspan refid=\"MOESM2\" class=\"InternalRef\"\u003eS2\u003c/span\u003e) showed high concentrations of NO and N\u003csub\u003e2\u003c/sub\u003eO. Emissions of oxides of nitrogen (NO\u003csub\u003ex\u003c/sub\u003e) in practical biomass-fired systems are caused mainly by oxidation of biomass-derived nitrogen rather than reactions involving atmospheric N\u003csub\u003e2\u003c/sub\u003e (Glarborg et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). One or more control measure, e.g., air staging and selective catalytic reduction (SCR) of NO\u003csub\u003ex\u003c/sub\u003e, will most likely be needed to reduce emissions of NO\u003csub\u003ex\u003c/sub\u003e to acceptable levels during the conversion of anaerobic digestates derived from cattle slurries in industrial-scale installations.\u003c/p\u003e \u003cp\u003eThe XRD spectra for the combustion ashes prepared from AD-2018 are presented in Fig.\u0026nbsp;3. Two major crystalline phases were identified in the combustion ash prepared at 800°C: merwinite (Ca\u003csub\u003e3\u003c/sub\u003eMg(SiO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e) and whitlockite (Ca\u003csub\u003e9\u003c/sub\u003e(K,Mg,Fe)(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e6\u003c/sub\u003e(PO\u003csub\u003e3\u003c/sub\u003eOH)). Three major crystalline phases were identified in the combustion ash prepared at 1000°C: diopside (CaMgSi\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e6\u003c/sub\u003e), tricalcium aluminate (Ca\u003csub\u003e3\u003c/sub\u003eAl\u003csub\u003e12\u003c/sub\u003eO\u003csub\u003e6\u003c/sub\u003e), and whitlockite (Ca\u003csub\u003e9\u003c/sub\u003e(K,Mg,Fe)(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e6\u003c/sub\u003e(PO\u003csub\u003e3\u003c/sub\u003eOH)). Possible minor phases in the combustion ash prepared at 800°C (not shown in Fig.\u0026nbsp;3) include: potassium magnesium phosphate (KMgPO\u003csub\u003e4\u003c/sub\u003e), polyhalite (K\u003csub\u003e2\u003c/sub\u003eCa\u003csub\u003e2\u003c/sub\u003eMg(SO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e4\u003c/sub\u003e·2H\u003csub\u003e2\u003c/sub\u003eO), and antigorite (Mg\u003csub\u003e3\u003c/sub\u003e(Si\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e)(OH)\u003csub\u003e4\u003c/sub\u003e). Polyhalite was also identified as a possible minor phase in the combustion ash prepared at 1000°C. The XRD analyses show that a substantial portion of P in the anaerobic digestates forms whitlockite during combustion at both 800°C and 1000°C. A large range of phosphates, which contain varying proportions of Ca, Mg, and K, have been reported by previous investigators in combustion ashes derived from other types of P-rich biomass, such as sewage sludge, meat bone meal, and cereal grains (Li et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Lindström et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Öhman et al., \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Skoglund et al., \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). The formation of phosphates with high molar ratios of alkaline earth metals to K, such as whitlockite, can be considered advantageous with respect to the operation of industrial thermal processes, since these phosphates generally melt at higher temperatures than K-rich phosphates and consequently, tend to alleviate rather than exacerbate ash-related operational issues such as fouling, deposition, and, in the case of fluidised-bed reactor technologies, bed agglomeration (Lindström et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Skoglund et al., \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Steenari et al., \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). The presence of alkali metals in a whitlockite mineral structure has also been suggested to be positive regarding the plant bioavailability of phosphorus in ashes (Herzel et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Falk et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Release of heavy metals\u003c/h2\u003e \u003cp\u003eThe fractional release of Zn, Cu, and Mn to the gas phase during thermal conversion of AD-2018 is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e for different operating conditions. The volatility of Zn varied greatly over the range of investigated operating conditions and was largely impacted by both temperature and gas atmosphere. Zinc had low volatility during combustion at 800°C but was partly released to the gas phase during combustion and CO\u003csub\u003e2\u003c/sub\u003e-gasification at 1000°C (~ 40–45%). The release of Zn was greatest (~ 96–97%) when the anaerobic digestate was heated in the reducing gas (10% H\u003csub\u003e2\u003c/sub\u003e / 90% N\u003csub\u003e2\u003c/sub\u003e) and inert gas (pure N\u003csub\u003e2\u003c/sub\u003e) atmospheres. The high release of Zn during pyrolysis in pure N\u003csub\u003e2\u003c/sub\u003e is attributed to local reducing conditions caused by formation of CO from the decomposition of carbon. The increased volatility of zinc in reducing conditions is consistent with thermodynamic equilibrium analyses of zinc speciation during combustion of biomass in fuel-rich and fuel-lean conditions (Elled et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Sørum et al., \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). Reducing gases, including CO and H\u003csub\u003e2\u003c/sub\u003e, reduce solid compounds of zinc to atomic zinc gas whereas oxidising atmospheres promote formation of non-volatile silicates and aluminates of Zn (Lane et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2020b\u003c/span\u003e; Sørum et al., \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Sinclair, \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2005\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe release of Mn was at most minor (\u0026lt; 20%) in all investigated operating conditions. Small fractions of Cu were released during CO\u003csub\u003e2\u003c/sub\u003e-gasification (~ 16% released) and during thermal conversion in the atmosphere consisting of 10% H\u003csub\u003e2\u003c/sub\u003e in N\u003csub\u003e2\u003c/sub\u003e (~ 20% released). The release of Cu was at most minor (\u0026lt; 20%) in the pyrolysis and combustion atmospheres (pure N\u003csub\u003e2\u003c/sub\u003e and 1.5% O\u003csub\u003e2\u003c/sub\u003e in N\u003csub\u003e2\u003c/sub\u003e).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.3. Implications for application of cattle slurry-derived ash and char residues onto soils\u003c/h2\u003e \u003cp\u003eThe inorganic constituents of biomass are separated into different ash (or char) fractions in industrial-scale thermal processes. Ashes that are separated from the product gases downstream of the reacting bed of biomass are generally enriched in volatile elements. These ashes are collectively referred to as fly ashes. Ash fractions that are collected below the reacting bed of biomass are enriched in non-volatile elements. These ashes are generally referred to as bottom ashes, or bed ashes in the case of fluidised-bed reactor technologies.\u003c/p\u003e \u003cp\u003eThe partitioning of P in cattle slurry-derived digestates between bottom ashes and fly ashes will likely depend largely on the composition of the gas phase in contact with the reacting bed of anaerobic digestate. Given that the volatility of P in oxidising and inert gas atmospheres is low, it is expected that P will largely report to bottom ashes in commercial combustion processes. Previous investigators (e.g., Skoglund et al., \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2014\u003c/span\u003e) have shown P to report mainly to coarse ash fractions during the combustion of biomass derived from municipal sewage sludges in laboratory-scale fluidised-bed reactors. However, the presence of local reducing zones in combustion processes, e.g., at the entrance of grate-fired combustors, may cause at least partial volatilisation of P and reduced recovery of P in bottom ashes. Nordin et al. (\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) compared the extent of transfer of P to bottom ashes during the combustion of municipal sewage sludge in two reactor types: an 8 MW\u003csub\u003eth\u003c/sub\u003e grate-fired boiler and a fixed-bed, laboratory-scale furnace with excess supply of oxygen to the sample. The authors reported lower transfer of P to bottom ashes in the grate-fired boiler where reducing zones are encountered. In gasification processes, which typically have high concentrations of gas phase reductants, it is expected that P will volatilise to a significant extent and condense on fly ash fractions.\u003c/p\u003e \u003cp\u003eThe laboratory-prepared combustion ashes were approximately 6-fold more concentrated in P than the dried anaerobic digestates and contained around 7% P w/w. Similar concentrations of P could be expected in the bottom ashes produced in industrial-scale combustors. Phosphorous concentrations in the laboratory-prepared combustion ashes are comparable with concentrations of P in low-grade phosphate rock ore. The global average grade of phosphate rock ore has been estimated to be around 10% P w/w, or 22.5% P\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e (Van Kauwenbergh, \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2010\u003c/span\u003e), however, economic grades can be as low as 2.5% P w/w. According to the XRD analyses, a major part of phosphorus was present in the combustion ashes as mixed cation whitlockite minerals. These minerals generally have low phosphorus solubilities. Nevertheless, they have been reported to have suitable properties as slow-release fertilisers (Kumpiene et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Mackay et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Recent studies show that co-combustion or co-gasification of sludges with alkali metal-rich biomass causes an increase in the alkali metal content of formed whitlockite minerals (Herzel et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Nordin et al., \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). It is understood that this increases the bioavailability of P in ashes (Stemann et al., \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Herzel et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Furthermore, the addition of dry biomass to moist anaerobic digestates increases the heating value of the combustion feedstock. Therefore, co-combustion of mixtures of moist anaerobic digestates with dry, alkali metal-rich biomass could potentially be used as a strategy to improve combustion performance and the fertiliser properties of the bottom ashes. Another potential strategy to increase the bioavailability of P in ashes is to treat the ashes in a thermochemical process with addition of a sodium salt. This strategy has been shown to convert whitlockite minerals to a mineral (buchwaldite) that makes P more bioavailable (Stemann et al., \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2015\u003c/span\u003e).\u003c/p\u003e \u003cp\u003ePotassium is partially released to the gas phase in thermal processes and is expected to report to both bottom ash and fly ash in significant quantities. Higher temperatures and reducing gas atmospheres favour the release of K to the gas phase and are expected to increase the yield of K in fly ashes.\u003c/p\u003e \u003cp\u003eZinc and copper are both essential micronutrients for plants (Alloway, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). The addition of controlled quantities of these elements to soils can potentially be beneficial if the soil is deficient in these elements. However, high concentrations of Zn and Cu can have toxic effects on plants and on soil dwelling organisms (Alloway, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Nagajyoti et al., \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Reichman, \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). For this reason, the upper limits of the concentrations of Cu and Zn in biomass ashes for application onto agricultural and forest soils are limited by EU-level and national regulations (see Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e for limit values). Bottom ashes produced from combustion of cattle slurry-derived anaerobic digestates are expected to contain acceptable levels of Cu and Zn for application on agricultural and forest soils in Finland.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\u003cdiv class=\"gridtable\"\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\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eUpper limits of Cu and Zn in biomass ash for application on agricultural and forest soils in European countries. Limit values are compared with global averages in topsoil and concentrations measured in the anaerobic digestate derived combustion ashes.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e\u003ccolgroup cols=\"3\"\u003e\u003c/colgroup\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCu (mg kg\u003csup\u003e− 1\u003c/sup\u003e, dry basis)\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eZn (mg kg\u003csup\u003e− 1\u003c/sup\u003e, dry basis)\u003c/p\u003e \u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGlobal average in topsoil (2013)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e14\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e62\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colspan=\"3\" nameend=\"c3\" namest=\"c1\"\u003e \u003cp\u003e\u003cb\u003eAnaerobic digestate derived combustion ashes\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCombustion at 800 ºC\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e323 ± 25\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1260 ± 106\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCombustion at 1000 ºC\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e321 ± 25\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e793 ± 67\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colspan=\"3\" nameend=\"c3\" namest=\"c1\"\u003e \u003cp\u003e\u003cb\u003eUpper limit for ash application on agricultural soils\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eEU regulation 2019/1009\u003c/p\u003e \u003cp\u003eSweden\u003c/p\u003e \u003cp\u003eFinland\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e600\u003c/p\u003e \u003cp\u003e600\u003c/p\u003e \u003cp\u003e600\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1,500\u003c/p\u003e \u003cp\u003e800\u003c/p\u003e \u003cp\u003e1,500\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colspan=\"3\" nameend=\"c3\" namest=\"c1\"\u003e \u003cp\u003e\u003cb\u003eUpper limit for ash application on forest soils\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFinland\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e700\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4,500\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSweden\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e400\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e7,000\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/table\u003e\u003c/div\u003e \u003cp\u003e\u003c/p\u003e \u003cp\u003eFly ashes produced from thermal conversion of anaerobic digestates derived from cattle slurry have potential to contain high concentrations of the macronutrients, P and K, provided that the anaerobic digestates are exposed to reducing gaseous environments during thermal conversion. Such environments were found to promote significant volatilisation of these two elements. While fly ashes have potential to contain high concentrations of both P and K they will almost certainly be contaminated with high levels of Cl. It is expected that fly ashes will contain prohibitive levels of Cl given the high concentrations of Cl in the anaerobic digestates (1.0 and 1.1% w/w) and the high volatility of Cl at temperatures encountered in thermal processes (Björkman \u0026amp; Strömberg, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Johansen et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Lane et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2015a\u003c/span\u003e). High concentrations of Cl in fly ashes are expected to prevent direct application of the fly ashes to soils due to the risk of increasing soil salinity.\u003c/p\u003e \u003cp\u003eNitrogen was substantially released from the anaerobic digestates during both combustion and gasification. The nitrogen released in these processes forms gaseous species that are expensive to capture in industrial-scale installations, including N\u003csub\u003e2\u003c/sub\u003e, NO, N\u003csub\u003e2\u003c/sub\u003eO, NH\u003csub\u003e3\u003c/sub\u003e, and HCN (Glarborg et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Leppälahti \u0026amp; Koljonen, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e1995\u003c/span\u003e). Given the high contents of N in the anaerobic digestates (2.1 and 2.2% w/w) it is recommended that practical installations be designed and operated in a way that promotes high conversion of fuel-N to N\u003csub\u003e2\u003c/sub\u003e and minimal conversion of fuel-N to deleterious nitrogenous products, particularly NO and N\u003csub\u003e2\u003c/sub\u003eO, e.g. by employing air staging, selective noncatalytic reduction, or selective catalytic reduction.\u003c/p\u003e \u003cp\u003eA small fraction of N (~ 13–16%) was retained in the solid char residues following pyrolysis of the anaerobic digestates at 1000°C. While the application of biomass derived chars to soils can significantly reduce emissions of NO and N\u003csub\u003e2\u003c/sub\u003eO from soils, this is not universally true for all biochar-soil combinations (Van Zwieten et al., \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Zheng et al., \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Additional research is needed to assess the potential of pyrolysis to mitigate emissions of NO and N\u003csub\u003e2\u003c/sub\u003eO.\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003ePhosphorous had low volatility in all atmospheres except the reducing atmosphere consisting of 10% H\u003csub\u003e2\u003c/sub\u003e in N\u003csub\u003e2\u003c/sub\u003e. Phosphorous was retained in the combustion ashes as mixed cation whitlockite minerals. The volatilities of K, Na, and Zn were significantly impacted by temperature and were lowest in the oxygen-containing atmospheres (1.5% O\u003csub\u003e2\u003c/sub\u003e in N\u003csub\u003e2\u003c/sub\u003e and pure CO\u003csub\u003e2\u003c/sub\u003e). Copper, manganese, calcium, and magnesium had low volatilities in all investigated operating conditions. Bottom ashes produced from combustion of cattle slurry digestates are expected to meet legislative requirements for application onto agricultural and forest soils in Finland. Co-combustion of cattle slurry digestates with dry, alkali metal-rich agricultural residues is a potential strategy to improve both combustion performance and the bioavailability of phosphorous in the ashes. Further research is recommended to assess the benefits and limitations of this strategy.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot Applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot Applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe work presented in this paper was financially supported by the Research Council of Finland grant to NJS (grant # 311970) and the Finnish Ministry of Agriculture and Forestry (VN/28562/2020-MMM-2).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026apos; contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eDL - idea conception, data collection, manuscript preparation, OS - idea conception, supervision, manuscript editing, NJS \u0026nbsp;- idea conception, manuscript editing, aquiring funding. All other authors provided valuable comments and edited the manuscript.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors acknowledge the financial support from the above mentioned funding sources. We thank the field staff of the Maaninka Research station of the Natural Resources Institute Finland for making the anaerobic digestate samples available for the study.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAlloway, B. J. (ed.) 2013. \u003cem\u003eHeavy metals in soils: trace metals and metalloids in soils and their bioavailability\u003c/em\u003e. Springer Science \u0026amp; Business Media, Dordrecht, the Netherlands.\u003c/li\u003e\n\u003cli\u003eBagge, E., Sahlstr\u0026ouml;m, L., Albihn, A. 2005. 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Thermal treatment of sewage sludge: A comparative review of the conversion principle, recovery methods and bioavailability-predicting of phosphorus. \u003cem\u003eChemosphere,\u003c/em\u003e \u003cstrong\u003e291\u003c/strong\u003e, 133053.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"bioresources-and-bioprocessing","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"biob","sideBox":"Learn more about [Bioresources and Bioprocessing](http://bioresourcesbioprocessing.springeropen.com)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/biob/default.aspx","title":"Bioresources and Bioprocessing","twitterHandle":"@SpringerOpen","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Ash utilization, combustion, gasification, pyrolysis, phosphorous recovery, volatilization, biochar","lastPublishedDoi":"10.21203/rs.3.rs-3972486/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3972486/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThermal processes are emerging as promising solutions to recovering phosphorus and other nutrient elements from anaerobic digestates. The feasibility of nutrient element recovery depends largely on the fates of nutrient elements and heavy metals during thermal processing. This study assesses the partitioning of macronutrients (N, P, K, Na, Ca and Mg) and heavy metals (Zn, Cu, and Mn) between condensed and gaseous phases during thermal conversion of cattle slurry digestates in gas atmospheres of pyrolysis, combustion, and gasification processes. This study also assesses the chemical forms of macronutrients retained in combustion ashes. The partitioning of elements between condensed and gaseous phases was quantified by mass balances based on elemental analyses of char and ash residues. The char and ash residues were prepared in a fixed-bed, batch reactor at temperatures within the range 800\u0026ndash;1000\u0026deg;C. Powder X-ray diffraction was used to identify the chemical forms of macronutrient elements in combustion ashes. Volatilisation of P was low (\u0026lt;\u0026thinsp;20%) when the digestates were heated in inert and oxidising atmospheres, whereas a reducing atmosphere volatilized P to a major extent (~\u0026thinsp;60% at 1000\u0026deg;C). Oxidising atmospheres increased volatilisation of N but suppressed volatilisation of K, Na, and Zn. Volatilisation of the following elements was low (\u0026lt;\u0026thinsp;30%) in all investigated operating conditions: Ca, Mg, Mn, and Cu. The combustion ashes contained both high concentrations of P (around 7 w/w%) and acceptable concentrations of regulated heavy metals (Cu, and Zn) for application on agricultural and forest soils in Finland. Phosphorous was retained in the combustion ashes in the form of whitlockite. This form of P is expected to be available to plants when the ashes are added to soil.\u003c/p\u003e","manuscriptTitle":"Fates of nutrient elements and heavy metals during thermal conversion of cattle slurry-derived anaerobic digestates","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-12-02 04:00:02","doi":"10.21203/rs.3.rs-3972486/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Accept","date":"2024-11-23T08:16:55+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2024-11-15T12:54:44+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-11-15T12:53:26+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-11-10T23:21:14+00:00","index":"","fulltext":""},{"type":"submitted","content":"Bioresources and Bioprocessing","date":"2024-11-09T00:28:48+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"bioresources-and-bioprocessing","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"biob","sideBox":"Learn more about [Bioresources and Bioprocessing](http://bioresourcesbioprocessing.springeropen.com)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/biob/default.aspx","title":"Bioresources and Bioprocessing","twitterHandle":"@SpringerOpen","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"f3a8f4eb-f56d-4282-b991-c670c52608f9","owner":[],"postedDate":"December 2nd, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-01-06T16:21:10+00:00","versionOfRecord":{"articleIdentity":"rs-3972486","link":"https://doi.org/10.1186/s40643-024-00828-7","journal":{"identity":"bioresources-and-bioprocessing","isVorOnly":false,"title":"Bioresources and Bioprocessing"},"publishedOn":"2024-12-30 15:57:40","publishedOnDateReadable":"December 30th, 2024"},"versionCreatedAt":"2024-12-02 04:00:02","video":"","vorDoi":"10.1186/s40643-024-00828-7","vorDoiUrl":"https://doi.org/10.1186/s40643-024-00828-7","workflowStages":[]},"version":"v1","identity":"rs-3972486","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3972486","identity":"rs-3972486","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}