Activated Carbons for Syngas Desulfurization: Evaluating Approaches for Enhancing Low-Temperature H₂S Oxidation Rate

Abstract

Its relatively low cost and high surface area makes activated carbon an ideal adsorbent candidate for H2S removal. However, physical adsorption of H2S is not very effective; therefore, methods to facilitate reactive H2S oxidation on carbons are of interest. The performance of H2S removal of non-impregnated, impregnated, and doped activated carbon in low-temperature syngas was evaluated in fixed-bed breakthrough tests. The importance of oxygen content and relative humidity was established for reactive H2S removal. Impregnates especially improved the adsorption rate compared to non-impregnated carbons. Non-impregnated carbons could however retain a high capture capacity with sufficient contact time. In a relative performance test, the best performance was achieved by doped activated carbon, 320 mg g−1. Ammonia in syngas was found to significantly improve the adsorption rate of non-impregnated activated carbon. A small quantity of ammonia was consumed by the carbon bed, suggesting that ammonia is a reactant. Finally, to validate ammonia-enhanced desulfurization, bench-scale experiments were performed in biomass-based gasification syngas. The results show that when the ammonia concentration in syngas was in the tens of ppm range, 40–160 ppm H2S oxidation proceeded rapidly. Ammonia-enhanced oxidation allows utilization of cheaper non-impregnated activated carbons by in situ improvement of the adsorption kinetics. Ammonia enhancement is therefore established as a viable method for achieving high-capacity H2S removal with unmodified activated carbons.

Keywords

• syngas desulfurization
• activated carbon
• hydrogen sulfide
• H₂S oxidation
• ammonia