Comparative investigation on steam gasification performances of special herbaceous and woody biomass in a two-stage gasification system

This study compares the continuous steam gasification performances of herbaceous and woody biomass (i.e., Giant Miscanthus (GM) and pruned apple tree (PAT)) in a compact two-stage gasification system integrating an auger-type pyrolyzer with a bubbling fluidized-bed gasifier in order to clarify how feedstock type and steam supply influence the operational stability and gasification performance under pure-steam conditions. A steam-to-biomass (S/B) ratio range of 1.0–2.5 was selected for the steam gasification, and it is found that the performance of GM gasification maintained stable over a broader S/B window than that of PAT, indicating a higher tolerance to steam-rich operation for this special herbaceous biomass. For both feedstocks, excessive steam input led to a pronounced deterioration in carbon conversion efficiency as well as cold gas efficiency due to a decrease in bed temperature and a reduction in effective gas residence time, demonstrating that the gasification performance under high S/B conditions could be constrained primarily by thermal and hydrodynamic limitations. The observed gasification performance differences between GM and PAT were attributed to their distinct devolatilization behaviors, char reactivities and alkali and alkali earth contents, which collectively influenced steam-assisted reactions and thermal stability within the gasifier. Overall, GM achieved gasification performance comparable to PAT within an appropriate thermally sustainable S/B window in the present small-scale staged gasification system, supporting the rational utilization of locally available biomass resources under region-specific conditions.