Simulation of air-steam gasification of woody biomass in a bubbling fluidized bed using Aspen Plus: A comprehensive model including pyrolysis, hydrodynamics and tar production

Abstract

A comprehensive model was developed to simulate gasification of pine sawdust in the presence of both air and steam. The proposed model improved upon the premise of an existing ASPEN PLUS-based biomass gasification model. These enhancements include the addition of a temperature-dependent pyrolysis model, an updated hydrodynamic model, more extensive gasification kinetics and the inclusion of tar formation and reaction kinetics. ASPEN PLUS was similarly used to simulate this process; however, a more extensive FORTRAN subroutine was applied to appropriately model the complexities of a Bubbling Fluidized Bed (“BFB”) gasifier. To confirm validity, the accuracy of the model's predictions was compared with actual experimental results. In addition, the relative accuracy of the comprehensive model was compared to the original base-model to see if any improvement had been made.Results show that the model predicts H2, CO, CO2, and CH4 composition with reasonable accuracy in varying temperature, steam-to-biomass, and equivalence ratio conditions. Mean error between predicted and experimental results is calculated to range from 6.1% to 37.6%. Highest relative accuracy was obtained in CO composition prediction while the results with the least accuracy were for CH4 and CO2 estimation at changing steam-to-biomass ratios and equivalence ratios. When compared to the original model, the comprehensive model predictions of H2 and CO molar fractions are more accurate than those of CO2 and CH4. For CO2 and CH4, the original model predicted with comparable or better accuracy when varying steam-to-biomass ratio and equivalence ratios but the comprehensive model performed better at varying temperatures.