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

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. Thus, four biogas residues (sewage sludge, SR; spent mushroom compost, MR; wheat straw, STR; distillers’ grains, DL) were collected to prepare biochars at different temperatures (300 °C, 500 °C, and 700 °C) in this study. The functional groups, crystal structure, and physical and chemical properties of biochars were characterized to evaluate the suitability of preparing biochar from different biogas residues. The results showed that the biochar yield ranged from 45.13 to 66.42% at 300 °C, 34.13 to 51.50% at 500 °C, and 30.33% to 47.06% at 700 °C. The carbon (C) content in STR and DL biochar increased under higher temperatures. Further, with increasing temperature, the P and K contents, pH, Brunauer–Emmett–Teller surface area (S BET ), and total pore volume (TPV) 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 S BET of four biochars at 700 °C increased 11.98- to 33.43-fold compared with that at 300 °C. And the TPV of four biochars at 700 °C increased 2.22 to 10.25 times that at 300 °C. STRB700 has higher S BET , because C–H bond stretching in aliphatic formation and C = C stretching in hemicelluloses in STRB relatively decomposed under high temperature (300 °C and 500 °C), especially under 700 °C. The high temperature is favourable to the formation of smaller microporous.