Comparison of dolomite and lime as sorbents for in-situ H2S removal with respect to gasification parameters in biomass gasification

This work presents an experimental study on the properties of lime, limestone, calcined and uncalcined dolomite regarding their application as in-situ desulfurization sorbents in biomass gasification. Several parameters have been varied consecutively in order to distinguish the differences between the CaO-based sorbents and elucidate the influence of gasification conditions on the steady state desulfurization results. The parameters include gasification temperature, residence time, steam content of the synthesis gas, and particle size of the sorbents. The conversion of sorbents has been determined to evaluate the necessary amount of sorbent required for the reduction of the residual H2S content to a steady state equilibrium value. Results show that the variation of temperature in the range of 700–825 °C has no influence on the residual H2S content. The variation of steam content from 30 to 42 vol% in the syngas showed a rather significant change of residual H2S for lime increasing from 330 to 786 ppmv and a less considerable rise of 410–578 ppmv H2S for dolomite under steady state conditions. For the particle size, it was shown that a lower discharge rate of larger particles in the range of 250–500 μm was dominant as an influencing factor as compared to better pore diffusion in smaller particles of 100–200 μm in diameter. A clear difference has been observed for the conversion of CaO to CaS in fully calcined dolomite as compared to the other sorbents tested. The conversion is clearly higher in lime, limestone and dolomite with an average of 35% as compared to fully calcined dolomite with an average of 15%.