Interaction mechanism among CO, H2S and CuO oxygen carrier in chemical looping combustion: A density functional theory calculation study

Abstract

When using coal-derived syngas or coal as fuel in chemical looping combustion (CLC), CO as a representative pyrolysis/gasification product and H2S as the main sulfurous gas coexist in fuel reactor. Either CO or H2S can absorb on the surface of CuO (the active component of Cu-based oxygen carriers), and reactions will occur among them. In this study, density functional theory (DFT) calculations are conducted to investigate the interaction among H2S, CO, and CuO, including: the reaction between CO and H2S over CuO particle, the influence of CO on the H2S dissociation and further reaction process, and the impact of H2S dissociation products on CO oxidation. Firstly, the co-adsorption results suggest that H2S might directly react with CO to produce COS via the Eley–Rideal mechanism, while CO prefers to react with HS* or S* via the Langmuir–Hinshelwood mechanism. This means that the reaction mechanisms between CO and H2S will change as the H2S dissociation proceeds, which has already been forecasted by the co-adsorption energies and verified by all of potential Eley–Rideal and Langmuir–Hinshelwood reaction pathways. Then, the influence of CO on the H2S dissociation process is examined, and it is noted that the presence of CO greatly limits the dissociation of H2S due to the increased energy barrier of the rate-determining dehydrogenation step. Furthermore, the impact of H2S dissociation products on CO oxidation by CuO is also investigated. The presence of H2S and S* significantly supresses the CO oxidation activity, while the presence of HS* slightly promotes the CO oxidation activity. Finally, the complete interaction mechanisms among H2S, CO, and CuO are concluded. It should be noted that COS will be inevitably produced via the Langmuir–Hinshelwood reaction between surface S* and CO*, which is prior to H2O generation and subsequent sulfidation reaction.