Study On The Migration And Transformation Of Sulfur In Inorganic Sulfur-bearing Compounds During Chemical-looping Combustion Process

Coal occupies the main position in China’s energy consumption structure.Traditional process of coal-burning can produce amounts of greenhouse gases and pollutants,including SO_x and NO_x,etc.The emission of greenhouse gases and pollutants seriously pollutes the environment and endangers human existence.Chemical-looping combustion（CLC）is a novel technology of indirect non-flame combustion with the advantage of inherent CO₂ separation.Although this technology can avoid generation of NO_x,it can not inhibit the release of sulfurous gases.The occurrence of sulfur in coal is complex.In order to understand the transformation behavior of inorganic sulfur in coal,three typical inorganic sulfur-containing compounds（FeS₂,CaSO₄ and FeSO₄）were selected to study the behavior of migration and transformation of sulfur during the process of CLC process.Three inorganic sulfur-containing compounds were respectively loaded in activated carbon（AC）,and AC-FeS₂,AC-CaSO₄ and AC-FeSO₄ were used as the research samples.The thermodynamic simulation of these three samples in the process of coal gasification was carried by using HSC chemistry 5.0,and the experiments of coal gasification under temperature-programmed and constant-temperature conditions were carried out respectively.The simulation results showed that,temperature and the ratio of water-to-carbon（S/C）can affect the existence form and distribution of sulfur in the reaction system.When the temperature was low,sulfur in FeS₂ and FeSO₄ was converted into H₂S and iron sulfides（mainly FeS）,and the fraction of H₂S increased with the increase of temperature.While sulfur in CaSO₄ was mainly converted into H₂S and SO₂,and high temperature promotes the formation of SO₂.Sulfur in the FeSO₄ existed almost exclusively in the form of H₂S and the variation of S/C had little effect on the sulfur distribution of FeSO₄.However,there was a significant effect of S/C on the sulfur distribution of FeS₂ and CaSO₄,and the part of sulfur will exist in the forms of FeS and CaS respectively at low S/C values.The gasification experimental results showed that,the initial decomposition temperatures of inorganic sulfur-containing compounds（FeS₂,FeSO₄ and CaSO₄）were 400℃,500℃and 650℃.At950℃,H₂S was the main sulfur-containing component,and the fraction of sulfur in FeS₂,CaSO₄ and FeSO₄ converted to H₂S within 20.5 min was 67.47%,43.02%and 78.09%,respectively.In order to analyze the interactions between oxygen carriers and sulfurous gases during CLC,the reactions between major sulfurous gases（H₂S and CS₂）and common oxygen-carrying active metal oxides（Fe₂O₃,CuO and NiO）were thermodynamically analyzed.It was found that the reactions between the sulfurous gases and the three oxygen carriers to form metal sulfides were thermodynamically feasible.Moreover,in order to investigate the distribution of sulfur in three sulfur-containing compounds during the process of CLC,thermodynamic simulation studies were conducted for the reduction stage of CLC based on Fe-,Cu-and Ni-based oxygen carriers.Simulation results showed that,for Fe-and Ni-based oxygen carriers,when at low temperatures,H₂S was the main sulfur-bearing component and the fraction of H₂S decreased gradually with the increase of temperature and oxygen excess coefficient（R）.Whereas for Cu-based oxygen carrier,no H₂S was found in the reaction system.When using Cu-and Ni-based oxygen carriers,especially,they were more easily vulcanized to form sulfides（e.g.Cu₂S,Ni₃S₂）at low temperature or low oxygen excess coefficient.However,sulfurous gases had a weak vulcanization effect on Fe-based oxygen carrier.In addition,compared with the other two sulfur-containing compounds,CaSO₄ had a higher stability,and the part of CaSO₄ was not decomposed at 650-950℃.The experiments of CLC of the three sulfur-containing compounds loaded in activated carbon when using different oxygen carriers were carried out in a small fluidized bed reactor.Results showed that SO₂ was the main sulfur-containing gas in both the reduction stage and the oxidation.The fraction of sulfur converted to sulfur-containing gases in the reduction stage(X_S,g,red)decreased in the order of Fe->Cu->Ni-based oxygen carrier.This maybe because Ni-and Cu-based oxygen carriers were easier to react with sulfur to form metal sulfides.Taking FeS₂ as an example,X_S,g,red,g,red based on Fe-,Cu-and Ni-based oxygen carriers were90.38%,82.22%and 78.00%respectively.The composition of oxygen carrier after the reduction stage was analyzed by X-ray diffractometer（XRD）.The presence of Cu₂S and N₃S₂was detected in the Cu-and Ni-based oxygen carriers,respectively.The distribution of sulfurous gases during the five cycles remained stable.However,the EDX analysis of the elements on the surface of oxygen carrier showed that some sulfur accumulated and remained on the surface of oxygen carrier.Therefore,to avoid the negative influence of sulfur in coal on the process of CLC,it is necessary to do further studies on the accumulation and removal of sulfur in multi-cycle and long-term operation.