Synergetic Effects And Reaction Regulation Mechanisms Of Low-cost Composite Oxygen Carriers In Coal Chemical Looping Process

As a novel way for the low-carbon utilization of fossil fuels,chemical looping combustion（CLC）aims to achieve full oxidation of fossil fuels into CO₂ and H₂O,while chemical looping gasification（CLG）is to obtain synthesis gas（CO,H₂,etc.）by partial oxidation of hydrocarbon fuels.With regard to above two processes,the low-cost as well as high-performance oxygen carrier（OC）holds the bottleneck that restricts the large-scale industrial application of both processes.In this work,the key scientific issues such as the synergetic effects and reaction regulation mechanisms of composite OCs during coal-derived chemical looping process were investigated,which includes the followings:1)The blending ratio of copper ore to hematite（or red mud）in composite OC was determined based on the fact that the thermal neutrality was achieved during the reduction stage,and then the cyclic stability and reduction kinetics of OCs were studied.The heat flow curves indicated that the Cu20Fe80@C OC can achieve auto-thermal balance when reacting with fuel gas.Meanwhile,the copper ore/red mud composite OC with thermal neutrality was determined as Cu10.9Red89.1@C,holding the same Cu O/Fe₂O₃ ratio as that of Cu20Fe80@C.50 cyclic redox tests showed that composite OCs（i.e.Cu20Fe80@C and Cu10.9Red89.1@C）exhibited good reactivity as well as cyclic stability.The reduction kinetics analysis at low temperature revealed that the reactions of all OCs with H₂ can be described by the shrinking core model.2)The combustion performance of composite OCs during in-situ gasification chemical looping combustion of coal/char were examined in a batch fluidized bed reactor.The results indicated that,regardless of the oxygen to fuel ratios and fuel types,composite OCs（i.e.Cu20Fe80@C and Cu10.9Red89.1@C）obtained obviously higher CO₂ yield or combustion efficiency than that of red mud.Meanwhile,the synergetic contribution of Cu10.9Red89.1@C on coal/char conversion was observed,which was attributed to the gaseous oxygen release from uncoupling contents of Cu O（Cu Fe₂O₄）in the copper ore at the early carbon conversion stage and alkali metal catalysis at the later stage of carbon conversion.Moreover,it was found that the oxygen transfer rate peak value attained with coal was much greater than that with coal char,but it was opposite case in terms of the total oxygen transfer amount.3)The synergetic effects between copper ore and hematite（or red mud）were investigated using either fuel gas or coal char as fuel.As compared with the Cu20Fe80@C,Cu10.9Red89.1@C exhibited stronger synergetic reactivity during in-situ gasification chemical looping combustion of coal char,and the synergetic contribution can be mainly reflected in three aspects:（i）the presence of copper ore improving the reducibility of hematite or red mud;（ii）the gaseous oxygen released from copper ore accelerating the char gasification step;（iii）the alkali metal contained in red mud catalyzing the char conversion.4)The regulation mechanisms of low-cost OCs in coal-fed CLG process were explored.Coal/char CLG tests indicated that the gasification rate of solid fuel was closely related to both lattice oxygen donation capacity and the alkali metal content in OC,while the reduced OCs catalyzed the conversion of char gasification gases towards the H₂-rich products.In addition,the reduced Cu20Fe80@C exhibited highest catalytic activity towards the water-gas shift reaction,followed by the reduced Fe100@C and reduced red mud.Regarding the H₂-rich production by steam–iron reactions,the Cu20Fe80@C OC showed a clear advantage over red mud once both OCs experienced deep reduction.5)The low-cost OCs prepared on a large scale by hydroforming were evaluated under CLC and CLG modes in a semi-continuous fluidized bed reactor.The CLC results showed that the copper ore/red mud composite OC still exhibited superior combustion reactivity toward lignite under semi-continuous operation.The CLG results indicated that the copper ore/hematite composite OC exhibited best CLG performance through comprehensive consideration of average syngas yield and average carbon capture efficiency.