Activation kinetics of biochars from peanut shells, cashew nut shells, and millet stalks under isothermal conditions in CO2 atmosphere

This study investigates the reactivity kinetics of biochar from biomass. The biochars were obtained by pyrolyzing peanut shells (PNS_800), cashew nut shells (CNS_800), and millet stalks (MS_800) at 800 °C in a fixed bed reactor. The chemical composition of the biochar samples shows that silicon (Si), potassium (K), and magnesium (Mg) are the major elements in the biochar of peanut shells (PNS_800) while potassium and magnesium are the major elements in the biochar of cashew nut shells (CNS_800) and millet stalks (MS_800). The biochars were activated in a CO₂ (200 Nml/min) atmosphere at temperatures 1123 K, 1173 K, and 1223 K under atmospheric pressure. The random pore model (RPM) and a modified random pore model (MRPM) were used to correlate the reactivity profiles versus carbon conversion and to determine the kinetic parameters. It was observed that biochar reactivity increases as the temperature increases, attaining at least three times at 1173 K than those corresponding to 1123 K. Furthermore, the increase in reactivity is more pronounced with the biochar MS_800. It was observed that the RPM model cannot follow the kinetic of the experimental reactivity of all biochar samples. However, a better fitting of the reactivity is obtained when using the MRPM model. The activation energies (E_a) are distributed in the range of 98.11–148.46 kJ/mol while the pre-exponential factors (k₀) are in the range of 19.31–249.53 s^-1. It was observed that for the MRPM model, the lower activation energy and the lower pre-exponential factor were obtained by the biochar CNS_800. However, E_a et k₀ are well evaluated for PNS_800 and MS_800 with a coefficient of determination of 99.76%. The proposed modified random pore model could be used to describe the reactivity of biochar from biomass as well as the reactivity of coal.