Biochars prepared from biogas residues: Temperature is a crucial factor that determines their physicochemical properties

Abstract The quantity of biogas residues is continually increasing and causing a great threat to the environmental protection and sustainable development of bio-natural gas engineering. This study concentrated on evaluating the feasibility of biochar production using various biogas residues. The feedstock of the anaerobic digestion was sewage sludge (SR), spent mushroom compost (MR), wheat straw (STR) and distillers’ grains (DL). Biogas residue collected from the above biogas plants was applied to prepare biochars at different temperatures (300°C, 500°C and 700°C). The results showed that the biochar yield ranged from 45.13–66.42% at 300°C, 34.13–51.50% at 500°C, and 30.33–47.06% at 700°C. The C content in STR- and DL-biochar increased under higher temperatures; this was due to the loss of –OH functional surface groups as a result of dehydration and the decomposition of nitrogen-functional groups. With increasing temperature, the P and K contents, pH, specific surface area (SSA) and pore volume increased. The functional groups of the STR-biochar decreased significantly, and the aromatic compounds were decomposed. The biochar (except SR-biochar) structures were more stable under high temperature. The morphological structure of DL-biochar at 700°C was the most stable. The SSA at 700°C increased 11.98- to 33.43-fold compared with that at 300°C. The total pore volume at 700°C increased 2.22 to 10.25 times that at 300°C. Overall, based on the principal component analysis, the biochar derived from wheat straw biogas residue exhibited the best performance regarding its physicochemical properties.