Experimental Study Of Coal Chemical Looping Combustion Using Copper-iron Oxygen Carrier With Oxygen Uncoupling

The prime energy consumption is still coal in future determined by the characterization of energy in China.However,the utilization of coal brings large emissions of greenhouse gas,CO₂,leading to the aggravation of global warming and then some eco-environmental problems.Chemical looping combustion?CLC?of coal is a significant method of solving the conflict between energy utilization and greenhouse gas emission:CLC can capture CO₂ without extra energy consumption during fossil fuels utilization process reducing the emission of CO₂.But CLC is limited by various aspects such as low fuel conversion efficiency and high bed material circulation.Oxygen carrier is a key factor to break the limitation of CLC.Copper-iron Oxygen carrier possesses the characterizations of low cost,high reactivity,anti-sintering,environment benign,oxygen uncoupling and being exothermic reacted with reduction gas,which is important for pushing the limitations and development of CLC.This paper comprehensively investigated the properties of copper-iron oxygen carrier in redox reaction,the performance of CLC for coal based copper-iron oxygen carrier and transfer behavior of fuel-nitrogen.Firstly,according to the calculation results of HSC software,fuel conversion characteristics were analyzed based on copper-iron oxygen carrier.The existence of copper based oxygen carrier could increase the reaction equilibrium constant of the reactions between copper-iron oxygen carrier and reduction gases,and then the requirement of oxygen carrier decreased for fuel complete conversion.At high temperature,solid reaction occurred between CuO and Fe₂O₃ in copper-iron oxygen carrier producing CuFe₂O₄.Copper-iron oxygen carrier could also release oxygen and the starting temperature for oxygen uncoupling decreased compared to CuO.The results of TPR shown that the reactivity of CuFe₂O₄ was higher than CuO or Fe₂O₃ indicating the synergistic effect existed between copper and iron in CuFe₂O₄.In addition,the interaction effects were investigated on a fixed bed.It exhibited that CuO could enhance the oxygen carrier reactivity in favor of fuel conversion especially solid fuel,and the melting of CuO can be inhibited for the existence of iron based oxygen carrier leading to high reactivity.Adding of Al₂O₃ or SiO₂ improved the cyclic stability of copper-iron oxygen carrier.Controlling the calcination temperature,time and cooling way played a part in controlling the reactivity of oxygen carrier.Secondly,in order to ensuring the successful operation of oxygen carrier in CLC reactor,oxygen carrier must be particles with a certain size and then could be fluidized.Therefore,copper-iron oxygen carrier particles were prepared by impregnation and mechanical mixing based Al₂O₃/SiO₂support,and investigated in a batch fluidized bed.It shown that:?1?the reactivity of oxygen carrier prepared by impregnation was stable but the stability decreased with increasing CuO content;?2?the oxygen carrier prepared by mechanical mixing presented high reactivity in fuel conversion but its mechanical strength was low leading to low lifetime;?3?copper-iron oxygen carrier had a positive effect on pyrolysis and gasification of coal,and the enhancement was determined by the ratio of oxygen carrier to fuel in weight and reaction temperature;?4?steam had significant acceleration on fuel conversion but the effect of value in ratio of steam to coal was little;?5?the analysis of reaction kinetics shown that mass transfer controlled the deuteric fuel conversion rate and stochastic nucleation model could describe the characterization of reaction kinetics.Thirdly,for purpose of enhancing fuel conversion by increasing the ratio of oxygen carrier to fuel,iron ore was used as support to prepare copper-iron oxygen carrier.In a batch fluidized bed,the performance of copper-iron oxygen carrier prepared by using hematite as support was studied in redox reaction and coal conversion.The results showed that:?1?when hematite was used as support,the lattice oxygen in hematite was easier to be obtained by reduction gas improving the oxygen carrier in reactivity;?2?both oxygen carriers prepared by impregnation and mechanical mixing were same in reaction reactivity for reduction gas;?3?the copper-hematite oxygen carrier had a stable reaction reactivity for the uniformly distribution of copper on the surface of hematite particle and the formation of CuFe₂O₄;?4?copper-hematite oxygen carrier had positive effects on the release of volatile,and the conversion of fixed carbon and reduction gas,which was mainly determined by the reaction temperature;?5?the conversion of coal based on copper-hematite was also conformed to stochastic nucleation model.Fourthly,since the properties of oxygen uncoupling and high reactivity for copper-iron oxygen carrier influenced the emission of NO during CLC process,the characterization of NO emission with using copper-iron oxygen carrier was investigated in fixed bed and batch fluidized bed.And the results showed that CuO leaded to more fuel-N being converted into NO,which was enhanced by the oxygen uncoupling characteristic.As zhundong coal used this paper contained much alkali metal,the effects of alkali metal existence forms were studied on fuel conversion and generation of NO.The results exhibited that water soluble alkali metal inhibited the conversion of fuel but accelerated the generation of NO.At last,the performance of CLC with coal based copper-iron oxygen carrier was investigated on a continuous fluidized bed.Since fuel reactor was usually bubbling fluidized bed,the forming of gas bubble inhibited the gas-solid contact leading to the escaping of reduction gas and then the loss of energy.As a result,this paper designed a new CLC reactor:internal was installed in reactor to improve the gas-solid contact and then the reduction gas conversion.Cold test was implemented and the results shown that stable operation could be obtained without gas leakage occurred between air reactor and fuel reactor.Then,the performance of CLC for coal was investigated with using copper-iron oxygen carrier in this reactor and it shown that the reduction gas could be converted effectively and the fuel conversion depended on the temperature in fuel reactor.