Thermodynamic Investigation of Carbon Deposition and Sulfur Evolution in Chemical Looping Combustion with Syngas

Chemical looping combustion (CLC) with syngas, a synthesized gas mixture of CO, H2, CO2, H2O(g), N2, and H2S, was investigated using thermodynamic simulation, with focus on carbon deposition and sulfur evolution in CLC. Five metal oxides, such as NiO, CuO, Fe2O3, Mn3O4, and CoO, were selected as oxygen carriers for CLC application. Different influencing factors on the formation of carbon deposits were investigated, including pressure, fuel reactor (FR) temperature, oxygen excess number Φ (denoting the availability of lattice oxygen in the oxygen carrier to the fuel), and fuel gas composition. Higher temperature and larger oxygen excess number Φ inhibited the formation of carbon deposits while the pressurized condition caused the opposite. The increase of H2O(g) and CO2 fraction in syngas reduced carbon deposition while, in contrast, a larger H2S occurrence in syngas led to more carbon deposits to be formed. A sensitivity analysis to the different factors revealed that carbon deposition was mainly determined by the FR temperature and the oxygen carriers provided while other factors played a minor role. In addition, the predominant C-bearing species and their distributions at different temperatures were thermodynamically investigated. At low FR temperature and oxygen-deficient condition (i.e., oxygen excess number Φ < 1), the predominant carbon species as solid deposits were mainly elemental carbon or carbonates for NiO, CuO, Fe2O3, and CoO while MnC2 and MnCO3 were the main species for Mn3O4. In terms of the evolution of sulfur in CLC with syngas containing a basic composition of CO, N2, H2, and H2S, the low pressure, high temperature, and adequate lattice oxygen would make more sulfur species form in the gas phase. After that, CO2 and H2O(g) were introduced into the syngas, and they were found to possibly serve as additional oxidizers to convert H2S into SO2. The oxidation function of CO2 was slightly stronger than that of steam. Again, the evolution and distribution of various sulfur species was studied. For four metal oxides (NiO, Fe2O3, Mn3O4, and CoO), the most possible solid sulfur compounds were Ni3S2, Fe0.84S, MnSO4, and Co0.89S, respectively. But for CuO, at Φ < 1, Cu2S was the main solid sulfur compound while at Φ > 1 CuSO4 and Cu2SO4 dominated.