High-performance Oxygen Carriers For Chemical Looping Processes And The Relevant Reaction Kinetics Study

Chemical looping,in which a kind of recyclable solid intermediate is used to transfer substances and energy,splits one reaction into two or more subreactions that take place in isolated steps or spaces,can eventually achieve in situ separation of reaction products.During the past 30 years,application of the chemical looping concept has undergone intensive development,which was no longer limited to the application of chemical looping combustion(CLC).Allowing for the unique staged reaction mode and in situ products separation characteristic of chemical looping,more researchers are trying to adopt the chemical looping concept to modify traditional chemical engineering processes,expecting to obtain the desired products in a more economic and efficient way.However,in terms of the current development status,high-performance as well as economically feasible oxygen carrier remains the bottleneck that limits the successful industrialization of chemical looping processes.In this work,high-performance oxygen carrier for chemical looping processes and the relevant reaction kinetics involved were studied,which mainly includes the followings:Firstly,the sintering issue of copper ore at high temperatures was addressed by cement decoration.Through TG tests,the optimum cement adding ratio was attained as 20 wt.%,based on the comprehensive consideration of reactivity,economy and anti-sintering property of the oxygen carrier.When compared with the raw copper ore oxygen carrier,cement decoration can significantly enhance the high-temperature cyclic stability of copper ore.Meanwhile,as the anti-sintering property of copper ore was greatly improved after cement addition,obvious reaction performance enhancement was observed in chemical looping with oxygen uncoupling(CLOU)of lignite coal.Thermal mechanical analysis indicated a high glass transition temperature of 1141.6? for the Cu-C-20 oxygen carier(copper ore with 20 wt.%of cement addition),which was much higher than that of CuO/CuAl2O4(998.2?)and CuO@TiO2-Al2O3(996.6?),validating the high anti-sintering property of Cu-C-20.Moreover,the bed inventory in fuel reactor and raw material cost of the Cu-C-20 oxygen carrier when being applied to the actual CLC process was calculated.The results indicated that the Cu-C-20 showed superior operating economy over synthetic Cu-based oxygen cariers.Secondly,based on the optimized cement adding ratio(20 wt.%),cement was further used as binding media for the preparation of mixed copper ore/iron ore oxygen carrier.According to the TG results,when the copper ore/iron ore mixing ratio was 20:80,high-performance oxygen carrier particle can be attained,i.e.,Cu20Fe80@C.Moreover,when synthesis gas was used as fuel gas,thermal neutral can be achieved with the Cu20Fe80@C oxygen carrier in fuel reactor.Subsequently,further investigation of the Cu20Fe80@C oxygen carrier with coal in iG-CLC process was conducted via batch fluidized bed tests,and the effects of steam concentration,oxygen carrier to fuel ratio,coal type as well as cycle number were examined.As observed,in the iG-CLC process with lignite coal,the steam concentration and oxygen carrier to fuel ratio were not the higher the better,which should be systematically considered basing on the fuel conversion efficiency,operating economy and the dimension of the reactor.In addition,the Cu20Fe80@C oxygen carrier did not show sufficiently high conversion efficiency towards anthracite coal,indicating that it is not suitable for the iG-CLC process of high-rank coal.XRD characterization of the cycled(30 times)Cu20Fe80@C showed that the cement content in the oxygen carrier mainly existed as gehlenite phase(Ca2Al2SiO7),and no obvious chemical interaction between the cement and active phase was observed,which is suitable for large scale application.Then,the chemical looping concept was applied to n-hexane oxidative cracking for light olefins production,using the Fe-substituted barium hexaaluminate as oxygen carrier.Generally,the activity of the Fe-substituted barium hexaaluminates(BaFexAl12-xO19,x=1,2,3,4,6)towards n-hexane showed an increasing trend with the increase of Fe-substitution level.However,all the base BaFexAl12-xO19 samples were easily to over-oxidize n-hexane cracking products,leading to the formation of large amounts of CO2/CO(COx).Na2WO4 promotion was used to tune the selectivity of the base BaFe6Al16O19 oxygen carrier(with the highest activity),and the effect of Na2WO4 promotion ratio was examined.It was found that when the Na2WO4 promotion ratio was 20 wt.%(20-NaW/BaFe6),the olefin selectivity in chemical looping oxidative cracking of n-hexane could be improved to 83.%(at 700? and 9000 h-1),which was very comparable to the olefin selectivity attained in n-hexane thermal cracking process,i.e.,85.2%.Meanwhile,due to the high selective H2 combustion property of the 20-NaW/BaFe6 oxygen carrier,the n-hexane conversion achieved in chemical looping oxidative cracking process was over twice as high as in thermal cracking process(26.0%vs.12.8%).These results verified the feasibility of n-hexane oxidative cracking via chemical looping,as well as the superiority of the chemical looping approach over thermal cracking way.XPS characterization was used to analyze the near-surface elements distribution of the BaFe6Al16O19 oxygen carrier before and after Na2WO4 promotion.As indicated,Ba suppression at the near-surface of the oxygen carrier was the main reason for the improved olefin selectivity of the 20-NaW/BaFe6.Finally,using a typical Cu-based oxygen carrier,i.e.,CuO@TiO2-Al2o3,as the research medium,reaction kinetics under the chemical looping environment were studied,including the oxygen release and uptake reaction kinetics of the Cuo@TiO2-Al2O3 oxygen carrier in CLOU,as well as the reduction kinetics with H2,CO and CH4 in CLC process.Moreover,the reaction kinetics of coal chars in chemical looping combustion processes were revealed.The oxygen release and oxygen uptake reactions of the CuO@TiO2-Al2O3 oxygen carier can be described by the nucleation and nuclei growth model and chemical reaction model,respectively.The reduction of CuO@TiO2-Al2O3 with H2 and CO were both subjected to the shrinking core model,while the reaction with CH4 can be described by the nucleation and nuclei growth model.According to the reaction kinetics analysis,the kinetic parameters of the aforementioned reactions were attained.Investigation on the char-CO2 gasification and char-O2 combustion reaction kinetics of three different coal chars in analogous CLOU atmosphere indicated that:in CLOU condition,the char-CO2 reaction rate can exceed over the char-O2 reaction rate,especially for the low rank lignite coal.With the increase of coal char rank,the contribution of char-CO2 gasification reaction to the overall char conversion tended to decrease in relatively to the char-O2 combustion reaction.For the anthracite coal char,the high-concentration CO2 even suppressed the char-O2 combustion reaction,leading to a decreased char conversion rate in O2/CO2 atmosphere in comparison to O2/N2.Basing on the experimental results and several assumptions,the char conversion rate equations attained in separate char-O2 reaction(rcomb)and char-CO2 reaction(rgasi)were used to predict the char conversion rate in mixed O2/CO2 atmosphere(rmix),i.e.,rmix=?·rcomb+rgasi.Here.? was defined as the influencing factor of the char-CO2 reaction on char-O2 reaction.As compared with the experimental results,the char conversion rate in O2/CO2 atmosphere predicted by the proposed model equation showed relatively high reliability.