Reaction Kinetics At The Particle And Gas-solid Interface Scales In Chemical Looping Combustion Of Coal With Oxygen Uncoupling

As a novel fossil fuel utilization technology,chemical looping combustion,which has the characteristic of CO₂ inherent separation,is one of the most promising technologies to deal with the global warming issue.In terms of the energy structure in China,the development of chemical looping combustion of coal needs to be made a high priority.As a method to utilize solid fuel in chemical looping combustion,chemical looping with oxygen uncoupling(CLOU)can make the conversion rate of solid fuel closely match the conversion rate of oxygen carrier,which makes this technology have more practical application prospects.In CLOU of coal,reactions taken place in the fuel reactor are very complicated,including,at least,O₂ release from oxygen carrier decomposition,pyrolysis/oxidation/gasification of coal,heterogeneous reactions between pyrolysis/gasification products and oxygen carrier,and homogeneous reactions between pyrolysis/gasification products and lean O₂.So far,the effects of these chemical reactions on the conversion processes of solid fuel and oxygen carriers as well as the interaction between the reactions have not been well understood.For example,most researchers mainly focus on the combustion of coal char and ignore the gasification of it;most researchers mainly focus on the oxygen release from oxygen carriers and ignore the heterogeneous reactions between pyrolysis/gasification products and oxygen carriers.But in fact,on the one hand,high concentrations carbon dioxide and(or)steam exist in CLOU reactor.the carbon dioxide and(or)steam may influence the combustion process of coal char,and in addition the gasification may also directly contribute to the conversion of coal char;on the other hand,there is a certain concentration of reducing gas in the CLOU reactor,which is produced by coal pyrolysis and coal char gasification.The reducing gases may react directly with the oxygen carriers and(or)promote the oxygen uncoupling process by consuming the gasous oxygen released by the oxygen carriers.In terms of CFD simulation,the processes occurring at the particle scale can not be directly resolved by CFD simulation,so it is necessary to establish the corresponding sub models.However,due to the limitation of experimental method and accuracy,it is difficult to distinguish the processes at the particle scale directly from the experimental results.Particle-resolved simulation has been widely used in researches at particle scale because it can consider the detailed reaction kinetics as well as the heat and mass transfer inside and outside the particles.Therefore,in CLOU of coal,it is a very promising research direction to study the heat and mass transfer and chemical reactions on the particle scale and then establish the corresponding CFD sub-model by particle-resolved simulations.Consequently,the following work has been carried out in this paper.Firstly,particle-resolved simulation of coal char particles under typical CLOU conditions(low oxygen concentration and high carbon dioxide concentration atmosphere at?950?)was carried out.The contributions of gasification reaction and oxidation reaction in the process of coal char conversion were analyzed.The interaction mechanism between the gasification reaction and oxidation reaction is clarified.The results show that the conversion of coal char is mainly contributed by oxidation reaction,but the contribution of gasification reaction can not be simply ignored.The oxidation reaction happens in the external layer of the particle and inhibits the gasification reaction from the perspective of reaction mechanism.Therefore,the gasification reaction only happens in the inner core of the particle,rather than happens within the whole particle.The conversion rate of coal char particle under CLOU conditions can be modeled by r_mix=?r_gasi+r_oxid.Secondly,based on the conclusions of particle-resolved simulations,by modifying the conventional intrinsic model on coal char combustion,an intrinsic model for conversion rate of coal char particle in chemical looping with oxygen uncoupling conditions was proposed.The differences between the proposed model and the conventional models,which either considered the oxidation only or considered the oxidation and gasification contributions independently,were also analyzed.The results showed that the contribution of gasification reaction in coal char conversion increases with the decrease of oxygen concentration,particle temperature and particle size.The factor,?,has the same change tendency.Then,the oxygen uncoupling rate of CuO sample was measured on a self-built thermogravimetric analyzer.The oxygen uncoupling kinetics of CuO samples was analyzed by a multi-scale reaction kinetics analysis model.The effect of particle morphology on the conversion rate of CuO oxygen carrier was then briefly analyzed.It is found that the transfer of the dominant defects in the ash layer is not the rate limiting step of the conversion process of CuO grain.The gas diffusion in crucible,gas diffusion in sample layer,polydispersity of grain size and sintering of samples should be considered when describing the relationship between conversion rate and conversion of the sample.There is an optimal grain size(245 nm)which makes the overall conversion rate of the particles the fastest.The intrinsic activation energy is determined as 281.23 kJ/mol.Finally,the particle-resolved simulations were carried out for the CuO oxygen carrier particles.The competition between direct gas-solid reduction and oxygen uncoupling under CLOU condition was studied.The contributions of the two reactions in conversion of oxygen carrier was quantitatively analyzed and the interaction between them was also analyzed.It is found that whether the conversion process of particles was dominated by direct gas-solid reduction or by oxygen uncoupling depended on the temperature and hydrogen concentration conditions.The exothermic gas-solid reduction increased the particle temperature and further facilitated the oxygen uncoupling rate,which was determined to be the predominant interaction for CuO conversion.