Preparation And Performance Of Composite Oxygen Carriers In Coal Chemical Looping Combustion

Coal chemical looping combustion technology is a new coal-based carbon capture technology,which has important practical significance for slowing down the greenhouse effect and improving the natural environment.Compared with the existing CO₂ capture technology,coal chemical looping combustion technology has the characteristics of high system efficiency and low capture cost.During this process,the oxygen carrier not only provides oxygen source,but also circulates between the air reactor and the fuel reactor as a heat carrier.In addition,the oxygen carrier can also be used as a catalyst for tar cracking,coal gasification and other reactions.Therefore,how to develop kind of oxygen carrier with high performance has always been the focus and difficulty for coal chemical looping combustion technology.Currently,iron-based oxygen carriers have been widely concerned because of their low price,abundant resources and friendly environment,and they are considered to be one of the promising industrial oxygen carriers.However,iron oxide as an oxygen carrier can only convert from Fe₂O₃to Fe₃O₄,making its oxygen utilization rate low.Moreover,it is easy to sinter at high temperature.Its performance can be improved by preparing bimetallic composite oxygen carriers,doping alkali metals or adding inert components,but there are still some disadvantages no matter which improving measures are employed.For example,the preparation of composite oxygen carrier with double active components can improve the oxygen utilization rate of oxygen carrier,but its stability needs to be improved;the reactivity of oxygen carrier can be improved by alkali metal doping,but alkali metal is easy to lose;the addition of inert components can improve the sintering resistance of iron-based oxygen carrier,but the reaction between inert components and iron oxide will occur at a certain temperature,which will reduce the reactivity of the carrier,and there are also problems such as uneven distribution of active components.Considering the characteristics of spinel structure materials,such as high melting point,high mechanical strength at high temperature,high chemical inertness and good thermal shock resistance,if the active components and inert materials can be integrated into a crystal structure,the above problems can be effectively solved.In this work,NiFeAlO₄ oxygen carrier with spinel structure is prepared in order to improve the activity of oxygen carrier by using the synergistic effect of bimetals and enhance the stability of oxygen carrier by using Al³⁺ion.Secondly,NiFeAlO₄ oxygen carrier was doped with alkali metals,and the catalytic effect of alkali metals was used to further improve the reactivity of oxygen carrier.The effects of the preparation method,oxygen carrier dosage and composition of oxygen carriers on the structure,reaction activity and cycle performance of oxygen carriers were studied.The main conclusions are described as below:1.The NiFeAlO₄ oxygen carrier was successfully synthesized by co-precipitation method and sol-gel method.It was found that compared with the oxygen carrier prepared by co-precipitation method,the NiFeAlO₄ oxygen carrier prepared by sol-gel method had better reactivity performance,and its structure stability was good.2.The mass ratio between oxygen carrier and coal has an important effect on the coal conversion behavior in chemical looping combustion.With the increasing dosage of NiFeAlO₄-SG oxygen carrier,the carbon conversion rate gradually increased.When the mass ratio of oxygen carrier to coal was 20:1,the carbon conversion rate was 86.7%,which was much higher than that of coal pyrolysis（34.0%）,and the CO₂ concentration was 93.5%.The above phenomenon is closely related to the extent of coal combustion.With the gradual increase of mass ratio between the oxygen carrier and coal,the coal is gradually transformed from incomplete combustion to complete combustion.This is because NiFeAlO₄-SG oxygen carrier can not only promote the conversion of volatile to gas,but also produce more CO₂ gas by solid reaction with coke.3.The addition of alkali metals could effectively promote the conversion of coal,reduce the reaction temperature between oxygen carrier and coal,and increase the reaction rate between oxygen carrier and coal.It was found that when the doping ratio of alkali metal K was 5%and the precursor was carbonate,5%K₂CO₃-OC-SG oxygen carrier prepared by sol-gel method showed superior reaction performance.When the mass ratio of 5%K₂CO₃-OC-SG oxygen carrier to coal was 20:1,the carbon conversion rate reached 99.0%,while the CO₂concentration was only 85.6%.This was because the doping of alkali metal K could promote the conversion of coal,due to the promotion of alkali metal on gasification reaction,i.e.C（s）+CO₂（g）→2CO（g）reaction,more CO gas was generated,which eventually lead to the decrease of CO₂ concentration.4.After studying the circulation performance of oxygen carrier,it is found that:After 10 cycles,the reaction performance of NiFeAlO₄-SG oxygen carrier decreased significantly,which was mainly due to the obvious particle aggregation of NiFeAlO₄-SG oxygen carrier during the cycle,and the diffraction peak of NiO appeared in the crystal structure after 10 cycles.In contrast,5%K₂CO₃-OC-SG oxygen carrier remained stable reaction performance after 10 cycles and its crystal structure has not changed.Compared with NiFeAlO₄-SG oxygen carrier,5%K₂CO₃-OC-SG oxygen carrier had better cycle stability,which was mainly due to the improvement of the reaction activity of oxygen carrier and the inhibition of the aggregation of oxygen carrier by the doping of alkali metal K.