Effect of Pyrolysis Temperature and Feedstock Sources on Physicochemical Characteristics of Biochar

Converting feedstock into biochar is a popular approach to overcome the disposal problem, yet the role of waste type and pyrolysis temperature on biochar properties is not understood well. In this study, biochars were produced from various feedstock such as tea waste, apple wood, wheat straw and walnut shell at 300, 400, 500 and 600°C with 1-hour residence time. The results showed that increase in pyrolysis temperature significantly decreased biochar mass yield. The maximum and minimum mass yields were observed in walnut shell at 300˚C and apple-wood-derived biochars at 600˚C by 69 and 20%, respectively. The produced biochar had pH range between 5.3 to 9.7, and its pH value and ash content increased significantly with increasing pyrolysis temperature, except for walnut shell. Total concentrations of P, Ca, K, Na, Fe, Zn, Cu, and Mn and available concentrations of K, Ca, Mg, and P increased with pyrolysis temperature increasing in all samples, except at walnut shell-derived biochar. According to CHN analysis, by increasing pyrolysis temperature, the total carbon concentration increased but total nitrogen and hydrogen concentrations decreased. The pH value decreased with time until 72 hours, beyond which a near steady-state condition was attained. Relationships between pH and CaCO3-eq content of biochars were close and linear. The FT-IR spectra showed that aromatic C increased by increment in heating. Also, by increasing pyrolysis temperature, the mean pore diameter decreased but micropores volume increased and led to increase in the specific surface area of biochars. The results of this study suggest that biochars produced at 300 and 400˚C may have potential as fertilizer in calcareous soils because of low pH and EC, with high mass yield.