Investigation into sulfur release in reductive decomposition of calcium sulfate oxygen carrier by hydrogen and carbon monoxide

Chemical looping combustion (CLC) is a promising technology with the inherent property of separating CO2 from flue gas. For calcium sulfate (CaSO4) oxygen carrier, the inhibition of the produced sulfurous gases in the reduction of CaSO4, including sulfur dioxide (SO2), hydrogen sulfide (H2S) and carbonyl sulfide (COS), is the key for a CLC system. In this paper, the sensitivities of reacting temperature, oxygen ratio number (defined in this paper) and the mole fraction of both carbon monoxide (CO) and hydrogen(H2) in the syngas to the sum of the amounts of released SO2, H2S and COS are discussed respectively. Thermo-gravimetric analysis (TGA) tests demonstrated that the amount of the produced sulfurous gases is greatly dependent on the partial pressure of H2 or CO in the reduction of CaSO4. When the partial pressure of H2 or CO is higher than 40 kPa, the production of sulfurous gases, indicating the deterioration of the recyclability of CaSO4, can be prevented completely even if the reacting temperature is as high as 1000 °C. A new kind of CaSO4/CaCO3 oxygen carrier is prepared using a mechanical mixing method. The addition of CaCO3 nanoparticles largely improves the recyclability of the oxygen carrier in comparison with the fresh CaSO4 oxygen carrier, without CaCO3 nanoparticles, in a multi-cycle TGA test.