Production and Characterization of Biochar from Almond Shells

Biomass from specialty crops, including almonds, walnuts, and numerous others, serves as an important resource for energy and materials as agricultural systems evolve towards greater sustainability and circularity in management and operations. Biochar was produced from almond shells in a laboratory furnace at temperatures between 300 and 750 °C for residence times of 30 and 90 min with moisture contents of 5% to 15% wet basis. Response surface methodology was used to optimize the biochar yield. Feedstock and product temperatures were continuously monitored throughout the experiments. In addition, larger batches of biochar were also produced in a fixed-bed pilot-scale pyrolyzer. The yield of biochar was determined as a weight fraction of the amount of oven-dry almond shells used in each experiment. Physical and chemical characteristics of biochars were evaluated. Pyrolysis temperature and time were found to be the significant parameters affecting the biochar yield, with second-order regression models derived to fit yield results. As anticipated, highest biochar yields (65%) were obtained at a pyrolysis temperature of 300 °C and a pyrolysis time of 30 min due to the limited volatilization at this short residence at low temperature affecting torrefaction of the feedstock. The average biochar yield from the fixed-bed pilot-scale experiments was 39.5% and more closely aligned with the fixed carbon from standard proximate analyses. Higher pyrolysis temperatures resulted in higher C:N ratio and pH with the highest C:N ratio of 19:1 and pH of 10.0 obtained at a pyrolysis temperature of 750 °C for 90 min. Particle density increased with the increase of pyrolysis temperature. Results of this study can aid in predicting biochar yields from almond shells under different pyrolysis conditions and determining the amount of biochar required for different applications.