Optimization of Biomass Circulating Fluidized Bed Gasifier for Synthesis Applications using Simulation and Response Surface Methodology

Abstract Biomass gasification is a complex process that uses heat and catalysts to convert biomass into syngas, which can be applied in Fischer-Tropsch (FT) synthesis to produce liquid fuels. As FT requires specific syngas compositions, this work performs an optimization of the operating conditions of a pressurized circulating fluidized bed (CFB) gasifier for such purposes. The CFB operated with sugarcane bagasse as a feedstock and steam as a gasifying agent and was simulated in Aspen Plus v8.6 based on Gibbs free energy minimization with restricted equilibrium. The optimization followed the response surface methodology (RSM), consisting of central composite designs performed in Statistica 7.0 at a 99.0 % confidence level to obtain simplified mathematical models between input factors (temperature, T; steam-to-biomass ratio, S/B; and moisture content, MC) and output factors (cold gas efficiency, CGE; H2/CO ratio; gasifier heat duty, Qgasifier; higher heating value, HHV; and H2, CO, CO2, and CH4 contents). The models were used to generate four optimized cases of pressurized CFBs that returned H2/CO ratios of 2.15, required for FT reactors. The optimized CFB operates at 916 °C, 15 bar, S/B of 1.12, and 10 wt.% moisture, generating a syngas stream of low CO2 and CH4 contents, which may reduce the syngas cleaning and conditioning costs downstream the gasifier. The results agree with optimizations using other feedstocks and sets of objectives, showing the validity of the models.