An Aspen Plus Kinetic Model for the Gasification of Biomass in a Downdraft Gasifier

Abstract

Gasification is a useful technology to recover energy from renewable biomass by producing a versatile syngas which can be converted into useful chemicals or fuels, or used directly for energy generation. The quality and composition of the syngas is highly dependent on the biomass feedstock, design parameters and process conditions, such as temperature, gasifying agent and Equivalence Ratio (ER). Downdraft gasifiers are considered to be a good option for low tar syngas production. In this work, a kinetic model for a downdraft gasifier is assembled and incorporated into a flowsheet using Aspen Plus with the aim of performing detailed process analysis. The model is organised according to the assumption that in a downdraft gasifier pyrolysis, oxidation and reduction occur almost as separate consecutive processes, with the pyrolysis considered as an instantaneously occurring process while oxidation and reduction are governed by chemical kinetics. The model has been validated against experimental data for different conditions of ER ranging from 0.2 to 0.35. The results show an overall agreement of the main species, with slight discrepancies in the prediction of CH₄, which is over-predicted at lower ERs and under predicted at ER 0.345. This has an effect on the calculated Lower Heating Value (LHV) of the syngas which is generally higher than the experimental value. A set of sensitivity analyses were performed to investigate the impact of the value of the Char Reactivity Factor (CRF) on the composition of the producer gas and the kinetic parameters used in the model on the production of CH₄. Sensitivity analyses show that a CRF of 14 gives the best prediction of the syngas composition and that the kinetics of the reactions in the reduction zone do not have a large impact on the final levels of methane in the syngas. More important is the sensitivity to variation of the kinetic parameters in the oxidation stage. By doubling the rate of oxidation of CH₄ in the oxidation zone, the final levels of CH₄ in the syngas are reduced by almost 20%.