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

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 degrees 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 gasifi-cation 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.

Automated summary

Wet torrefaction (WT) is a promising technology for biomass upgrading, but its effects on the properties and gasification behaviors of biochar are not well understood. This study investigated the physical and chemical structure of biochar produced from WT biomass and its gasification characteristics. The results showed that WT reduced the content of alkaline earth metal ions in biochar and also lowered the content of K and Mg by more than 90%, and of Ca by more than 75%. The graphitization degree of biochar increased with increasing WT temperature below 220°C. The specific surface area of biochar from corn stalk increased after WT, while that from poplar sawdust decreased. The gasification reactivity of biochar decreased significantly with higher WT temperatures, and the time needed for complete gasification increased accordingly. The random pore model (RPM) provided a good fit to the gasification behavior of biochar and can be used to predict the gasification behavior at different temperatures. The activation energy of WT biochar increased, resulting in decreased gasification reactivity. The activation energies for the gasification of CS and PS biochar were in the ranges of 225.80-285.59 kJ/mol and 146.39-264.25 kJ/mol, respectively. This study provides important information for the design and operation of gasifiers.