Characteristics And Mechanism Of Coal Char Pressurized Chemical Looping Gasification

Chemical looping gasification of coal,which is uses oxygen carriers to provide heat and lattice oxygen in a fuel reactor,coal is oxidized into H2,CO,CH4.Chemical looping gasification is the most promising coal gasification technology which has high energy efficiency and little environmental pollution.Compared to the oxygen carrier reduction,the reaction rate of coal gasification is slow,which is the main rate control step of coal chemical chain gasification.Coal gasification can be divided into two steps,coal pyrolysis into coal char and char gasification.With few surface functional groups and low reactivity of char,it is particularly important to improve the gasification reaction rate of char.Pressure is one of the important factors affecting the reaction rate of coal gasification.Fe2O3/Bentonite,which was prepared by mechanical mixing method,was used as oxygen carrier in the coal char chemical looping gasification under pressure in a pressurized fixed bed reactor.The pressure was divided into the total pressure of the system(0.46-0.80MPa)and the partial pressure of steam(0.32-0.56MPa).When discussed the influence of total pressure,the partial pressure of steam was fixed,when discussed the influence of partial pressure,the total pressure was fixed.The effects of pressure on the carbon structure of char,functional group structure of char and the pore structure of char were analyzed by means of Raman,FTIR and N2 adsorption/desorption.The mechanism of gasification reaction of char under pressure chemical looping gasification was discussed with kinetic equation.The results indicate that,increasing the total pressure and increasing the partial pressure of steam are both beneficial to improving the reaction rate of coal char chemical looping gasification.The total pressure increased by 72.9%,the gasification reaction rate of coal char increased by 88%;the partial pressure of steam increased by 25%,and the gasification reaction rate of coal char increased by 11%,with the increase of partial pressure of steam,the active sites on the surface of coal char decreased,and the gasification reaction rate of coal char decreased.This is because the process of coal char gasification under high pressure can be described by gas-solid reaction model(SCM),the coal gasification reaction occurs firstly on the surface of coal char,and the chemical looping gasification of coal char is mainly controlled by chemical reaction.When the total pressure is 0.8 MPa and the partial pressure is 0.4 MPa,the activation energy is 99.957 kJ/mol,which is lower than the chemical looping gasification of 0.1 MPa iron-based oxygen carrier.By analyzing the surface functional groups,the increase of total pressure is favorable to the oxidation of coal char,the increase of partial pressure of steam is favorable to the fracture of carbon bond of aromatic ring,and the gasification activity of coal char is improved.By analyzing the carbon structure,the reaction degree and graphitization degree of coal char increase with the increase of system total pressure,and the reactivity of coal char decreases first and then increases,with the increase of partial pressure of steam,the graphitization degree of coal char decreases and the reactivity increases.Through N2 adsorption/desorption test,it is found that increasing the total pressure and the partial pressure is beneficial to increasing the reactivity and specific surface area of coal char,the total pressure of system is increased by 39%,and the specific surface area of coal char is increased by 42%at the same reaction time under different pressures;the specific surface area of coal char was increased by 57%at the same reaction time under different pressures while the partial pressure of steam increased by 50%.With the increase of pressure,the structure of coal char changes,which is the reason for the increase of reaction rate of chemical looping gasification of coal char.The experimental results show that it is feasible to increase the gasification reaction rate of coal char by increasing the pressure.