Solid tailings after supercritical CO2 extraction of lignocellulosic biomass as a source of quality biochar for energetic use and as soil improvement

Lignocellulosic biomass is a rich source of bioactive compounds that are extracted industrially from different parts of the plant. The extraction process generates residues that can contain from 75% to 95% of the raw material depending on the species. Among the many potential products of post-extraction residue’ processing there is biochar. The research objective was: i) to evaluate the parameters of biochars derived from post-extraction bark, wood and bark and wood of four lignocellulosic species; and ii) to discuss the parameters in the context of biochar functionality as an energy carrier and soil improver. The residues were subjected to pyrolysis at the three temperatures 170, 270, and 370°C, which correspond to the initiation of carbonization, and two biochars that differ in the decomposition rates of hemicelluloses, cellulose and lignin. On average, biochars had a high energy value owing to the increased content of the total and fixed carbon and calorific value by 77.0-78.4% DM, 64.6-66.7% DM and 25.8-30.1 MJ kg–1, respectively. The higher quantity of ash after processing of bark residues than wood residues implicates a lower energy value but at the same time the ash obtained is a better source of mineral compounds in the context of soil fertilization. Concerning the use of biochar as a soil improver, the biochars demonstrated lower H/C and O/C molar ratios, that indicate raised stability and resistance to the geochemical decomposition in soil. It was proven that the bark-based biochars had much higher concentrations of micro- and macronutrients as well as a higher pH, while processed wood fractions resulted in higher concentrations of total carbon and fixed carbon in biochar. The research results suggest that lignocellulose biomass extraction residues can serve as a valuable input material for production of biochar.