Proposal for a Regenerative High-Temperature Process for Coal Gas Cleanup with Calcined Limestone

This paper deals with a new regenerative, high-temperature process for coal gas desulfurization using calcined limestone (CaO) as a sorbent. The process includes three steps. First H 2 S reacts with CaO toward CaS (sulfidation), which is then partly oxidized with SO 2 in a second step to obtain CaSO 4 and elemental sulfur. Finally, in a third step (regeneration) the solid mixture of CaS and CaSO 4 remaining after the oxidation step is decomposed to produce CaO and SO 2 which are both reused in the first and second steps, respectively. The proposed process thus yields elemental sulfur as the final product. Several sulfidation−oxidation−regeneration cycles (up to nine) were carried out in a thermogravimetric analyzer (TGA) using various calcined limestones. The results are promising, showing a constant absorption capacity of 30−50% from two cycles on. On the basis of kinetic experiments and thermodynamic considerations, the optimum temperatures were determined at 815−845 °C for the desulfurization of Shell coal gas, 600−700 °C for oxidation, and 1100−1200 °C for regeneration. Furthermore, the applicability of calcined limestone for nonregenerative HCl removal was examined. The reaction between CaO and HCl was found to proceed fast enough to enable practical application. The limestone utilization was found to vary from 8% at 200 °C to 80% at 600 °C.