Study On The Chemical Looping Combustion Performance Of Red Mud-based Oxygen Carriers

As a new combustion technology that has attracted much attention in recent years,chemical looping combustion has the advantages of internal CO₂ separation,energy gradient utilization and low NO_x emission,which is of great significance for achieving the goal of"carbon neutrality"in China,where coal is the main source of electricity.The development of inexpensive and easily available,highly reactive and stable oxygen carriers is the key to the development and industrial application of chemical looping combustion technology.As a bulk solid waste,the resource utilization of red mud has significant economic and ecological benefits.The high content of iron oxides in red mud gives it the potential to be used as a cheap chemical looping oxygen carrier.This work investigated the feasibility of using two red mud（XARM and ZZRM）,large-scale industrial solid wastes,as the oxygen carrier（OC）candidate for chemical looping combustion（CLC）.Based on the reactivity and cycling stability of RM,highly reactive components including Ni O and Cu O were used to ehance the performance of RM.Detailed characterizations on the combustion reactivity,cycling stability,compositions and mophorlogy evolutions as well as reduction kinetics for the raw and modified RM were performed and compared.The main findings of the work were summarized as follows.Both XARM and ZZRM show the capacity of redox reactions during CLC cycling with gaseous fuel（CO/N₂）,thus can be used as potential OC for CLC reactions.However,the performance of the RM remains to be enhanced.After modifications of the RM with Ni O and Cu O,the oxygen transport capacity（OTC）and reactivity of XARM are obviously imporved.Nevertheless,the modified XARM are sintered more severely thus worsening its cycling stability.Comparatively,the Cu O modified XARM shows higher reactivity and OTC,as well as stronger sintering resistance.Both Ni O and Cu O are effective modifers for ZZRM as the decorated ZZRM exhibits better CLC perfomance compared to the raw sample,i.e.,higher reactivity,OTC and cycling stability.Kinetic studies on the reduction of the red mud-based OC show that the reduction of raw and Ni O-modified XARM follows a 3D dispersion（spherical）model.The reduction of Cu O-modified XARM,raw ZZRM and Cu O-modified ZZRM obey nucleation and growth（n=1）model,while that of Ni O-modified ZZRM follows a 3D dispersion（cylindrical）model.CLC tests with coal char as solid fuel show that the carbon conversion（X）of XARM is 78%,while it is improved to 84%and 90%after modifying XARM with Ni O and Cu O,respectively.Meanwhile,the corresponding CO₂ selectivity(（4）was enhanced from 84%to 90%and 100%,respectively.The X and(4 of CLC with ZZRM is only 76%and 40%,respectively,showing that the reactions occrred are gasification rather than combustion.after modification with Ni O and Cu O,its X increases to 84%and 85%,meanwhile its(4 increases to 60%and 97%,respectively.These results indicate that the modifications are effective in enhancing the performance of ZZRM during CLC reactions,especially with Cu O.By comparison,XARM and the modified XARM are superior to ZZRM and its decorated samples,especially by using Cu O as the additional components.Both RM show slight agglomerations,sinters and deteriorating stability during CLC.After modification with Ni O,the resultant samples show higher tendency of agglomeration,while the Cu O-modification exhibit only slight sintering,thus enhancing both reactivity and cycling stability of the decorated samples.