Effect of wet torrefaction on the physicochemical characteristics and gasification behavior of biochar

Abstract

Wet torrefaction (WT) is an effective biomass upgrading technology that has recently received widespread attention. However, the effects of WT on the physicochemical properties and gasification behaviors of biochar remain unclear, which limits the practical application of WT biomass as a fuel for gasification. In this study, hydrochar was prepared from corn stalk (CS) and poplar sawdust (PS) by a batch reactor. Then, biochar was prepared by rapid pyrolysis of hydrochar in a fixed-bed reactor. The physicochemical structure and gasification characteristics of fuel were studied. It was found that the AAEMs content in WT biochar was significantly reduced. The contents of K and Mg in biochar were reduced by more than 90 % and the contents of Ca were reduced by more than 75 % compared with the raw biochar. Below 220 °C, the graphitization degree of biochar increased with WT temperature. The specific surface area of CS char increased after WT, while the PS char decreased. The gasification reactivity of biochar decreased dramatically with the increase of WT temperature, and the time required for complete gasification increased obviously. The random pore model (RPM) has a satisfactory fitting on biochar gasification behavior. Therefore, RPM can be used to predict the biochar gasification behavior for other gasification temperatures. Activation energy of WT biochar is increased and the gasification reactivity is decreased compared with the raw biochar. The gasification activation energy of WT biochar for CS and PS were 225.80–285.59 kJ/mol and 146.39–264.25 kJ/mol, respectively. The research on the gasification kinetics of WT biochar can provide very important information for the reasonable design and operation of the gasifier.