Transformation Behaviors Of MoO₃ As Gaseous Oxygen Carrier And Reaction Characteristics Of Coal In Chemical Looping Gasification

Conventional coal gasification technologies are often suffered with parasitic energy penalty and additional capital expenses because of oxygen separation preparation,which also caused an amount of CO₂ emissions.Chemical-looping gasification?CLG?of coal to yield syngas is a clean and effective coal gasification technology which can realize the cascade utilization of energy and avoid the use of air separation unit due to lattice oxygen replaces the oxygen,therefore,CLG has the potential to reduce CO₂.In this study,a novel concept of coal gasification using chemical-looping technology with molybdenum trioxide?MoO₃?as a gaseous oxygen carrier?CLG?was proposed.MoO₃ as an oxygen carrier has potential to increase the reaction rate of coal and MoO₃,as well as to reduce the difficulty of oxygen carrier-ash separation in the CLG process due to MoO₃'s volatility.In this study,thermodynamic analysis of the reaction between coal and MoO₃ were investigated by HSC Chemistry 6.0 based on the Gibbs free energy minimization principle.The effects of key parameters such as the mass ratio of MoO₃/C?R?and temperature on the transformation behavior of MoO₃ were investigated in a fixed bed reactor.In addition,the effects of the above key parameters on the gasification reactivity of different degree of coalification coal in complex atmosphere were also investigated.At last,the hydrogen production capacity and cyclic stability of MoO₃ were studied.The main results and conclusions are as follows:1.MoO₃ can be used as an oxygen carrier in chemical looping gasification?CLG?by analyzing the Ellingham diagram.According to the comparison of thermodynamic parameters of the reactions that the system may be involved,MoO₃ had an excellent oxidization ability to carbon than to syngas,so MoO₃ was more suitable for chemical looping gasification?CLG?.2.The key parameters of the mass ratio of MoO₃ to graphite?R?and the temperature had an important influence on the transformation behavior of MoO₃.When T>1273 K,Mo was generated,when T<1273 K,MoO₂ were the main products.In order to save the amount of MoO₃,the temperature should keep at higher 1273 K.When MoO₃ was insufficient?R<5?,the side reaction2Mo+C?Mo₂ C will happen which can lead to ineffective carbon conversion.When R=5,it was suitable for carbon conversion,no side reactions occur.When MoO₃ was excess?R>5?,the side reaction2MoO₃+Mo?3MoO₂ will occur which can lead to ineffective MoO₃ conversion.In order to avoid side reactions,R=5 was suitable.3.The carbon conversion efficiency of gas coal,fat coal and coke coal were46.80%,33.91%and 29.54%respectively in absence of H₂O.With the increase of F,the effective carbon conversion increases gradually and the carbon conversion efficiency increased to 55.00%,46.40%and 40.00%when sufficient H₂O was added.Therefore,the reactivity of the three kinds of bituminous coals were successively:gas coal>fat coal>coke coal,which was positively correlated with the volatile content.4.By analyzing the evolution of H₂ in CLG process,it was concluded that the main reasons for the change of H₂ were as follows:Mo+2H ₂O?MoO₂+2H₂.Based on the above analysis,the hydrogen production capacity and cycling performance of MoO₃ oxygen carriers were also investigated,and it was found that the water conversion efficiency could reach94.84%at 1373 K.Through the study of cyclic properties,it was found that there was no sintering phenomenon of MoO₃ oxygen carrier and it had excellent cyclic stability.