Effect of temperature on behavior and mechanism of biochar gasification in the mixed CO2 and H2O atmosphere

Abstract

The effects of temperature (750–1300 °C) and gasifying agents (CO2, H2O, and their mixture) on the gasification characteristics of biomass char (biochar) were experimentally studied using a batch-type vertical tube furnace. It was found that gasification behaviors in the atmospheres of CO2, H2O, and their mixture could be classified into two regimes: chemical reaction control and diffusion control. In addition, a mechanism transition phenomenon from additive mechanism to competitive mechanism was observed in the chemical reaction control regime. The syngas results indicated that in the competitive mechanism, both the char-CO2 reaction and char-H2O reaction were inhibited. Based on these results, an assumption of char active sites was proposed to explain the mechanism transition. It appears that the transition is mainly caused by insufficient char active sites, resulting in CO2 and H2O molecules competing for the limited shared active sites. Furthermore, it was observed that in the competitive mechanism, there is no obvious priority for accessing the shared active sites when both CO2 and H2O are present.