Effects of inherent ash particles on the powdered biochar structure evolution in H2O and CO2 gasification

As the use of powdered feedstocks in efficient combustion/gasification processes increases, the impact of inherent ash becomes more significant. This study investigates the effects of inherent ash particles, with their catalytic actions, on the external and internal structural evolution of powdered biochar during H2O and CO2 gasification. The external structure, including char surface morphology and ash particle distribution on the surface, was analyzed using SEM-EDX and binarization. Three catalytic kinetic models-the catalytic volume reaction model (CVRM), catalytic shrinking core model (CSCM), and catalytic random pore model (CRPM)- were applied to evaluate internal structure evolution. The results show that char fragments from H2O gasification exhibit a pronounced pore structure with an eroded and fractured carbon matrix. Ash particles are compact and spherical, embedded in surface grooves and eroded pore edges. Model fitting suggests that the internal structure evolution follows the pattern described by CRPM. These findings suggest that H2O molecules penetrate the pores, promote fragmentation, and expose more active surface area and catalytic ash particles, thereby accelerating the reaction rate. In contrast, char fragments from CO2 gasification show smooth surfaces and regular edges, with ash particles loosely adhering to the pores and exhibiting minimal catalytic activity. The internal structure evolution in CO2 gasification is well described by the CSCM. These external and internal structure evolutions indicate that the char-CO2 reaction mainly occurs on the char surface, with ash particles gradually exposed but lacking significant catalytic effect. Based on these findings, two distinct char physical structure evolution forms under the influence of inherent ash particles were proposed for H2O and CO2 gasification.