Kinetics of H₂S removal using alkaline residue as in-bed sorbent in fluidized bed gasification of biomass and wastes

Abstract

The use of alkaline residue from the acetylene industry (carbide slag, CS) as in-bed sorbent to remove H₂S from syngas from biomass/waste fluidized bed (FB) gasification was investigated. Measurements were conducted in a laboratory FB applying differential conversion technique, tracking the reaction using both gas measurements and solid analysis. Apparent reaction kinetics was obtained, without external mass transfer limitations, for different gas mixtures containing CO₂, H₂O, CO, H₂ and CH₄, having relatively low H₂S concentration (500–1000 ppm). The addition of CO₂ and H₂O to an N₂-H₂-H₂S mixture showed that the reaction rate was reduced in the presence of these species. Varying the H₂ concentration between 7 and 14% in simple (N₂-H₂-H₂S) reactive gas mixtures did not affect the results, while both CO and H₂ potentially affected the reaction rate if also CO₂ was present in the gas. Other operating parameters, such as sorbent particle size, sulfidation- and calcination temperature also affected the CaO sulfidation rate, which increased with temperature between 800 and 900 °C and was enhanced by higher calcination temperature. Specific surface area and pore size distribution of the calcined CS was measured before and after the sulfidation reaction, showing that CS maintained a large fraction of its original pore volume even after most of the CaO had been converted into CaS. The sulfidation rate of calcined limestone was also measured for comparison showing that the reaction rate using CS was slightly lower, but similar to that using limestone, showing that CS is a promising alternative to conventional calcium-based sorbents for high temperature H₂S removal, contributing to promote the circular economy, while avoiding the current stacking of CS at the acetylene production sites.