Phase diagram of CaSO₄ reductive decomposition by H₂ and CO

Abstract

The CaSO₄ reductive decomposition is an interesting issue in both the reduction zone of fluidized bed combustors (FBCs) for coal combustion and the fuel reactor of chemical-looping combustion (CLC) system. Under CO or H₂ atmosphere, CaSO₄ is reduced to CaS and CaO, together with the releases of gas sulfides, which causes environmental pollutions. To lessen sulfur release, it is important to figure out the chemical stability of CaSO₄ reductive decomposition. Thus, the chemical stability of CaSO₄/CaS/CaO under CO or H₂ atmosphere was studied in consideration of SO₂, COS and H₂S emissions. The results show that regions I (VI), II (V), and III (IV) are the stability fields of CaSO₄, CaS and CaO, respectively. The range for CaO stability is increasing with reaction temperature and partial pressures of CO₂ and H₂O. Within the reaction temperature range of 800 and 1,000 °C, when the CaSO₄-CO-H₂ reaction system reaches the triple equilibrium point, the main gas sulfur released is SO₂, followed by H₂S, while COS generation is much smaller. In a real reaction system, when the values of real P _H2/P _H2O (P _CO /P _CO2), P _SO2, P _H2S (P _COS ), P _H2O (P _CO2) and T fall into Region I (IV), or II (V), the final product should be CaSO₄ or CaS, and the sulfur release from CaSO₄ reduction can be controlled.