Investigation On The Evolution Mechanism Of Sulfur In Coal-fueled Chemical Looping Combustion With Partial Oxygen Uncoupling

Coal-based fossil fuels not only meet the increasing energy demand of human beings,but also lead to a large amount of CO₂ emissions,resulting in global warming and other problems.Controlling and reducing CO₂ emissions from coal combustion is essential.In the CO₂ capture,utilization and storage（CCUS）technology,pre-combustion,post-combustion capture and oxy-fuel combustion all need high cost of carbon capture,which is not conducive to the economic operation of the system.Therefore,new combustion technology with lower carbon capture cost has attracted more and more attention.As an innovative combustion technology,chemical looping combustion（CLC）could achieve internal separation of CO₂,cascade utilization of energy,effective emission reduction of NO_x and other pollutants.Oxygen carrier is an important component and research basis of CLC technology.Considering the good sintering resistance but low reaction activity of Fe₂O₃-based oxygen carrier,and the highly reactive CuO-based oxygen carrier is limited by sintering at high temperatures,the two kinds of metal oxygen carriers mentioned above were innovatively combined,and the chemical looping combustion with partial oxygen uncoupling technology（CLPOU）with CuFe₂O₄ as oxygen carrier was proposed.In the coal-fueled CLC process,the new technology is expected to achieve the effective transfer of active oxygen and the full conversion of coal,which has great application potential and development prospect.In the coal-fueled chemical looping combustion,sulfur in coal has great harms to the reactivity of oxygen carrier,safe operation of system,post-treatment of CO₂ and environment.Therefore,it has become a hot and difficult point to explore the migration rule and evolution mechanism of sulfur in coal-fueled chemical looping combustion technology and realize the directional transformation and efficient in-situ removal of sulfur.Aiming at the above research hotspot and difficulty,in this work,typical high sulfur coals rich in different sulfur forms were selected for CLPOU technology with CuFe2O4 as oxygen carrier,the evolution of different sulfur forms and the mechanism of their interreactions with CuFe2O4 were investigated.The high purity CuFe₂O₄ combined oxygen carrier with partial oxygen uncoupling characteristic was synthesized by sol-gel combustion method（SGCS）.The oxygen releasing performance of the self-prepared CuFe₂O₄ oxygen carrier and its CLPOU reaction performance with a typical lignite were studied in a high-temperature fixed-bed reactor,and the oxygen release kinetic calculation of CuFe₂O₄ oxygen carrier was carried out.In the CLPOU experiment with lignite as the model fuel,the CuFe₂O₄ oxygen carrier showed excellent reaction performance.Increasing reduction temperature,oxygen carrier/fuel ratio and adding CO₂ as gasification medium could effectively promote the reaction between coal char and CuFe₂O₄ oxygen carrier.The result of multiple-cycle experiments indicated that CuFe2O4 oxygen carrier has good cycle stability.Without adding CuFe2O4 oxygen carrier,the evolution and product distribution of gaseous and solid sulfur were studied during the process of temperature programmed pyrolysis and isothermal pyrolysis of typical high sulfur coals.The sulfur content of pyrolysis solid-phase coal char increased in the presence of basic minerals and carbon matrix.Under isothermal condition,with the increase of pyrolysis temperature,due to the lack of internal hydrogen involved in coal,inorganic sulfur migrated to organic sulfur,and the release of H2S,COS and SO₂ showed a downward trend,while the release of CS₂ increased with temperature,indicating that CS2 may be derived from the direct gas-solid reaction of pyrite with CH4 and the reaction between pyrite and carbon matrix at high temperature.Compared with 100%N₂,the C—S bond involved in coal was more likely to undergo selective cleavage in the trace O₂or CO₂ atmosphere,thus promoting the release of gaseous sulfur（especially COS and SO₂）during pyrolysis.After adding quantitative CuFe₂O₄,during the CLPOU process of typical high sulfur coals and CuFe₂O₄ oxygen carrier,no matter in temperature programmed experiment or isothermal experiment,the gaseous sulfur at the reactor outlet was only SO₂,and the three kinds of high sulfur coal with different sulfur forms demonstrated different gaseous sulfur emission curves.The SO₂ emission peak at low temperature mainly derived from the interaction between CuFe2O4 and inorganic pyrite or pyrolysis gaseous sulfur products with inferior thermal stability of organic sulfur.The SO₂ emission peak at high temperature mainly derived from the oxidation of residual pyrite,stable organic sulfur in coal or some metal sulfides by CuFe₂O₄.By increasing reduction temperature and oxygen carrier addition,the oxidation of pyrite sulfur or organic sulfur was promoted,resulting in a significant increase of SO₂ emission,while sulfate mainly released SO₂ through thermal decomposition,and the increase of oxygen carrier addition ratio have no significant impact.Adding CO₂ as gasification medium could restrain SO2 generation to a certain extent.In order to further realize the effective control and directional transformation of gaseous SO2 during the CLPOU process,the effects of various Ca-based sulfur fixatives and different addition ratios on the release of carbon and sulfur were investigated.It was found that the addition of nano CaCO₃ could effectively reduce the emission of gaseous sulfur components and generate the desulfurized slag CaSO4.On this basis,for the sake of exploring an efficient way to utilize the extra oxygen in the desulfurized slag CaSO4,the CaSO4-Mn3O4 mixed oxygen carrier with core-shell structure were prepared by modification.The reaction characteristics of CaSO4-based mixed oxygen carrier were preliminarily researched by TG-FTIR,and during the reaction of CaSO4-based mixed oxygen carrier with lignite,the full utilization of lattice oxygen,the effective capture of CO2 and the effective inhibition and fixation of SO₂ released by CaSO₄ side reactions could be realized through the unique oxygen transfer characteristics and utilization way between reduced MnO and CaSO₄ matrix.To sum up,this work has investigated the evolution of different sulfur components and the mechanism of their interactions with CuFe₂O₄ oxygen carrier during the coal-fueled chemical looping combustion with partial oxygen uncoupling process,and the directional transformation conditions of sulfur components and the efficient utilization way of the extra oxygen in the desulfurized slag CaSO₄ were explored to realize the online synergistic control of the carbon and sulfur components,which has certain academic research value and reference significance to understand the coal-fueled chemical looping combustion technology and promote its application and promotion.