The Study Of Single Char Particle Conversion Characterictics And Conversion Rate Model Under Chemical Looping Combustion

Chemical Looping Combustion（CLC）due to its inherent CO₂ capture features with low energy consumption has received the widespread attention.With the development of this noval technology,the researchers began to apply solid fuel（such as coal and biomass）to CLC,that led to the in-situ Gasification Chemical Looping Combustion（i G-CLC）and Chemical looping with Oxygen Uncoupling（CLOU）.In the fuel reactor,the gasification/oxidation of char particle occurs after the devolatilization process.The char conversion process is the key to the final CO₂ capture efficiency.Therefore,it is urgent to study the char conversion characteristics and develop conversion rate model of char particle.Firstly,the single particle model was used to study the conversion characteristics of char in the mixed atmosphere of CO₂/H₂O.The simulation was carried out under different reaction temperatures,particle sizes and fluidized medium compositions.The results show that the char gasification was mainly controlled by chemical reaction kinetics,and the reaction temperature had a great influence on the char conversion characteristics.Then,the char conversion rate was analyzed,and found that the competition effect between CO₂ and H₂O gasification for active site is not obvious,the agglomeration of gasification product（CO）within the particle inhibit the char conversion rate under mixed atomosphere compare to the separate gasification.On this basis,one reaction rate model of char convert under mixed gasification is:r_gasi=r_{H2 O}+b×r_CO2,and the value of b is determined as 0.6382.Then,the conversion characteristics of char particle under low O₂ concentration and high concentration of CO₂/H₂O were studied（typical CLOU atomsphere）.Firstly,the effect of the gasification reactions of char oxidation reaction was investigated.The results show that under the CLOU atmosphere,gasification reaction does not significantly reduce the char particle reaction temperature and affect the external diffusion of O₂.Based on the detailed analysis,it can be concluded that the oxidation occurs only at the surface,while gasification reactions occur inside the char particle.Further,the contributions of gasification reactions to the char conversion was calculated via a formula proposed in this paper.The results show that the contribution of H₂O gasification reaction is between 2-18%,and the contribution of CO₂ gasification reaction is less than 3%.The contribution depends on the reaction temperature and particle size.And with the increase of O₂ concentration or the decrease of gasification agent concentration,temperature and char size,the contribution of the gasification reactions will decrease significantly.Basis on the above,this paper further developed the char particle conversion rate model under CLOU atmosphere.According to previous studies,the oxidation reaction will inhibit the gasification reaction,so conversion rate can be expressed as:r_mix=φ×r_gasi+r_oxid,the physical meaning ofφis the proportion of the gasification reaction area to the grain volume,which can be linearly expressed as:φ=a+cη.According to the study in the thirdly chapter of this paper,the conversion rate under mixed gasification atmosphere can be expressed as:r_gasi=r_H2O+r_{CO 2}.Thus,the char conversion rate model under CLOU atmosphere can be obtained as follows:r_mix=（a+cη）×(r_H2O+b×r_CO2)+r_{oxi d}.Furthermore,the variables in the formula are obtained by fitting the results of the single particle simulation.Then the char conversion rates calculated by this model are compared with those obtained by single particle simulations,and the relative errors are always less than 5% in the char particle conversion process,which well validated our kinetic model for accurate prediction of char conversion rate in CLC.