Migration of sulfur in in-situ gasification chemical looping combustion of Beisu coal with iron- and copper-based oxygen carriers

Chemical looping combustion (CLC) is an energy conversion technology with high efficiency and inherent separation of CO2. The existence of sulfur in coal may affect the CO2 purity and the performance of oxygen carrier due to the interactions between sulfur contaminants and oxygen carrier. The migration of sulfur in Beisu coal during the in-situ gasification chemical looping combustion (iG-CLC) process using two oxygen carriers (iron ore and CuO/SiO2) was investigated respectively. The thermodynamic analysis results showed the formation of metal sulfides was thermodynamically favored at low temperatures and low oxygen excess coefficients, while they were obviously inhibited and the production of SO2 was significantly promoted with an increase in temperature and oxygen excess coefficient. Moreover, part of sulfur was captured and fixed in the forms of alkali/alkaline earth metal sulfate due to the high amount of alkali/alkaline earth metal oxides in the coal ash or/and oxygen carrier. The experimental results showed that the sulfur in coal mainly released in the form of SO2, and the sulfur conversion efficiency (XS) in the reduction stage were 51.04% and 48.24% when using iron ore and CuO/SiO2 respectively. The existence of metal sulfides was observed in the reduced oxygen carriers. The values of XS in the reoxidation process reached 3.80% and 7.64% when using iron ore and CuO/SiO2 respectively. The residue and accumulation of sulfur were also found on the surfaces of two oxygen carriers.