Construction Of Coal Pyrolysis-Chemical Looping Gasification Coupled System And Study Of Oxygen Carrier

Chemical looping gasification,as a novel coal utilization technology,can realize the non-contact gasification of coal and air by circulation of the oxygen carrier particles between air reactor and gasification reactor,which is an effective way to realize clean and efficient use of coal.A new coal utilization system which was called coal pyrolysis-chemical looping gasification coupled system and based on coal-based chemical looping gasification technology was proposed.In this coupled system,the coal was first pyrolyzed in pyrolysis unit and then the char obtained from pyrolysis was used as the solid fuel of chemical looping gasification unit,and the oxygen carrier was used as the heat carrier of the pyrolysis unit.Thus,this coupled system could realize the cascade utilization of coal.The feasibility of the coupled system was studied.Moreover,in order to prepare oxygen carriers that meet the reaction requirements,the effects of active components,carriers,preparation methods and active additives on the gasification performance,cycle life and oxygen release characteristics of oxygen carriers were also studied.Related research works are as follows:(1)The influences of thermodynamic parameters on the performance of coal pyrolysis-chemical looping gasification coupled system were analyzed by using Aspen Plus,and the process conditions of key reaction units were also optimized.Moreover,the coupled system was assessed synthetically based on the mass balance,heat balance and economic analysis.The results of mass balance showed that at the same feeding rate(100 kg/h),the phenol wastewater discharge of the coupled system was much lower than that of the coal pyrolysis system(8.98 kg/h).These indicated that the discharge of phenol wastewater could be reduced greatly by using the phenol wastewater produced in coal pyrolysis and water vapor produced in coal drying as the gasification agent of gasification unit.The results of heat balance showed that the energy utilization of the coupled system was 88.2%,which was 1.4%higher than the coal-based chemical looping gasification system and merely 4.4%lower than the coal pyrolysis system.In addition,the economic analysis results showed that when the annual treatment amount of coal was lm tonnes,the payback period of the project was 10 years and had good profitability and return on investment.Hence,this coupled system is an economic and feasible production technology of syngas and tar.(2)The effects of steam concentrations,gasification modes and the kinds of metal oxide oxygen carriers on the composition of gas products,the selectivity of syngas and the carbon conversion of fuel were studied by a fixed bed reactor.It was found that the yield of syngas reached the maximum when the steam concentration of carrier gas was 15 vol.%.Moreover,the results for fixed bed gasification of coal-quartz sand and coal-oxygen carriers showed that the addition of oxygen carrier helped to improve the carbon conversion of fuel and the selectivity of syngas.And the interaction between oxygen carrier and coal char was beneficial to complete conversion of solid fuel.The yields of syngas during mixed gasification of coal with four metal oxide oxygen carriers respectively were concluded as:Mn/SiO2>Co/SiO2>Fe/SiO2>Ni/SiO2.(3)By using iron oxide as active component,the effects of five metal promoters including Ba,Ca,Co,Ni and Cu on the gasification performance of Fe-based composite oxygen carriers were studied.It was found that the metal promoter improved the gasification performance of oxygen carrier by forming solid solution with Fe.When the five Fe-based composite oxygen carriers reacted with coal,Fe2O3-NiO@Al2O3 could not only obtain high carbon conversion of fuel,but also had relatively high syngas selectivity and the syngas selectivities for mixed and stratified gasification were 81.01%and 88.28%,respectively.It should be noted that,for Fe2O3-CuO@Al2O3,the gasification mode had less influence on the yield of gas product and carbon conversion of fuel due to the oxygen uncoupling effect.Moreover,there was obvious CO2 release peak when the gasification reaction temperature was below 800?.However,when the other four Fe-based composite oxygen carriers reacted with coal,the release peaks of H2,CO and CO2 appeared above 800?.(4)The effect of adding amount of active additive on the gasification performance,cycle performance and curie point of Fe-based composite oxygen carrier was studied by using iron oxide as active component and Ni as active additive.When the loading amount of NiO was 20 wt.%,the Fe-based composite oxygen carrier could obtain higher carbon conversion and syngas selectivity when reacted with coal.And the oxygen carrier still had porous structure after regeneration,showing good thermal stability.Moreover,the test results of cycle stability of oxygen carrier also showed that the carbon conversion of fuel and syngas selectivity did not change significantly with the increase of cycling times and the oxygen carrier showed good cycle performance.Among the five Fe-based oxygen carriers,except for Fe-based oxygen carrier without adding NiO,the curie points of the other four Fe-Ni composite oxygen carriers before and after gasification were highur than reaction temperature(550?)of pyrolysis unit in the coupled system,which met the requirement of magnetic separation of oxygen carrier and char.