Simulation of sulfur distribution in chemical looping combustion (CLC) using CaSO4 as oxygen carrier

Compared to metal oxides, the sulfates (such as CaSO4), if adopted as oxygen carriers in chemical looping combustion (CLC), present several advantages such as low cost, easy availability and superior oxygen transfer capacity, whilst the SO2 emission and sulfur deposit in the process could be a big concern. In this study, thermodynamic simulations were conducted to investigate the sulfur distribution in a CLC system using CaSO4 as oxygen carrier with focuses on three aspects, i.e., the reduction reaction of CaSO4 with syngas in the fuel reactor (FR), the most possible reaction pathways in FR at different conditions as well as the effect of various factors (temperature, total pressure and oxygen provided in FR) on the distribution of sulfur species. Several findings were attained: (i) On the main products and reaction pathways: at low FR temperature of 100 to 400 degrees C, the main sulfur species and carbon deposit were H2S and CaCO3, respectively, which were produced via the methanation of CO with H-2 coupled with the shifting reaction of CO with H2O(g) and the ensuing thermochemical sulfate reduction (TSR). Then at 400 similar to 915 degrees C, CaS and CO2 were the main products through the reduction reaction of CaSO4 with H-2 or CO, and both products increased with FR temperature. Furthermore, at FR temperature larger than 915 degrees C, due to the initiation of the side reaction, i.e., the chemical interaction between CaS and CaSO4, the percentage of CaS declined; in contrary, the percentages of CaO, H-2 and CO increased, possibly due to the consumption of part of CaSO4 in the side reaction and thus not enough lattice oxygen available. (ii) On the influencing factors: the optimized reaction condition for the operation of FR in CLC system was suggested based on the simulation results. The initiation of chemical interaction between CaSO4 and CaS occurred at around 915 degrees C and pressurized condition led to the slight decrease in the percentage of CaS due to the prolongation of TSR. To ensure a full reduction of CaSO4 to CaS without accompanying side reactions, the optimized condition for. FR was around 900 degrees C, atmospheric condition and carefully controlled Phi(fr) (lattice oxygen excess number) around unity. Overall, this study indicated the most suitable conditions of using CaSO4 as an oxygen carrier in CLC, with the highest reactivity and minimized SO2 emissions.