Pyrolysis of the anaerobic digestion solid by-product: Characterization of digestate decomposition and screening of the biochar use as soil amendment and as additive in anaerobic digestion

Abstract

An industrial digestate was tested under pyrolysis to validate the influence of various temperatures on the potential of waste to provide biochars capable to improve soil productivity and enhance the efficiency of anaerobic digestion. The thermochemical conversion of digestate and the related kinetics were also examined by thermogravimetry and compared to those of biomass.Digestate provided greater proportions of biochar (41.3–66.5 %wt.) than biomass (15.5–48.6 %wt.) at all temperatures, although cellulose and hemicellulose were less thermally stable than in the untreated biomass as indicated by the low activation energies of 150–180 kJ/mol and 83.1–88.2 kJ/mol, respectively compared to those of the polysaccharides in biomass. The behaviour of digestate during conversion and the quantity of generated biochar were both unaffected by the heating rate. Biochar yields however decreased from 55 to 41.3 %wt. in the isothermal tests with the increment of pyrolysis temperature from 300 to 700 °C. The soil amelioration ability of digestate was almost unaffected by pyrolysis at temperatures below 400 °C. Biochar produced at 500 °C possessed the greatest potential to improve soil fertility in the long-term due to its high alkalinity (pH 11.46) and modest cation exchange capacity (CEC) (72.2 cmol kg−1). The high-temperature biochar with slightly higher pH (11.51) and significantly larger surface area (116.1 m2/g) was the optimal choice for soil amelioration. Compared to the other biochars, the one produced at the highest temperature was also the best anaerobic digestion agent because of the higher alkalinity (pH 11.51), better minerals content (P, K, Ca, Al) and larger than 100 m2/g surface area, all properties that are desirable for the improvement of stability, methane productivity and mitigation of CO2, H2S and free-NH3 in a digestion system.