Simulation And Optimization Of Chemical Looping Processes Based On Oxygen Carrier/Fuel Staged Conversion

The concept of chemical looping is to use oxygen carriers to decouple specific traditional reactions such as combustion and gasification into two-stage sub-reactions carried out in different atmospheres.On the one hand,the oxygen transfer characteristics of oxygen carriers give chemical looping the advantage of carbon dioxide inherent separation,making it a recognized sustainable carbon capture technology;on the other hand,the staged reaction strategy provides chemical looping with advantages of high energy efficiency and multi-product separation,providing more application potential for chemical looping.In this paper,two different chemical looping staged conversion processes were proposed for the staged oxidation of oxygen carriers and the staged utilization of fuels,respectively,and the simulation optimization and life cycle assessment of the two processes were carried out.The main contents are as follows:Firstly,this paper established a chemical looping methane steam reforming process（SO-CLSMR）based on oxygen carrier two-stage oxidation of Ca-Fe composite oxygen carriers with H₂/CO ratio controllable syngas and pure hydrogen.The process includes reforming reactor,splitting reactor,and air reactor.The self-heating of the system can be achieved by adjusting the two-stage oxidation distribution ratio of the oxygen carrier.The simulation results show that when the reduction temperature is 850℃,the pressure is 1 bar,and the OC/CH₄ molar ratio is 0.5,the methane conversion reaches the maximum value of 99.8%,the syngas yield reaches 2.50 kmol/kmol CH₄,and the H₂/CO ratio is close to 2.When the two-stage oxidation distribution ratioδ=0 and the splitting temperature is in the range of 500～900℃,the maximum hydrogen production of the splitting reactor reaches 14.9 kmol/h;whenδ≥0.69,the system can achieve self-heating.The process has exergy efficiency,hydrogen efficiency and effective thermal efficiency of 78.12%,94.52%and 90.59%,respectively,which are 13.11%,34.07%,and 27.95%higher than the chemical looping methane steam reforming.Secondly,this paper constructed a biomass chemical looping pyrolysis process based on biomass pyrolysis-gasification two-stage conversion process aiming to biomass featuring high volatiles and high oxygen content by reduced Ca-Fe composite oxygen carrier to regulate the oxygen content and composition of biomass pyrolysis products.The process includes three reactors:pyrolysis reactor,gasification reactor,and air reactor.The in situ upgrading of bio-oil can be achieved by adjusting the reduced oxygen carrier-biomass ratio.Taking corn stover as an example,the proposed biomass chemical looping pyrolysis process was simulated and optimized.The results showed that when the reduced oxygen carrier-biomass ratio was 0.35,the mass content of oxygen elements in bio-oil decreased from 32.66%to 18.50%,and the deoxygenation effect was significant.Under the optimized conditions,the yield of bio-oil reached 54.98wt.%,the carbon conversion rate of semichar reached 100%,and the flow rate of syngas reached 1.08 kmol/h.The biomass chemical looping pyrolysis process has the system exergy efficiency of 84.45%and the product exergy efficiency of 64.98%,which is46.11%,14.83%and 16.59%,8.19%higher than the conventional biomass gasification process and biomass pyrolysis process,respectively.When the combustion distribution ratio of the pyrolysis gas increases to 0.75,the entire system can achieve self-heating.Finally,taking corn stover as an example,the proposed biomass chemical looping pyrolysis process fuel-staged conversion was evaluated with the collection,transportation,pretreatment,and the chemical looping staged conversion process of biomass as system boundary.The evaluation results showed that the acidification potential,global warming potential,human toxicity potential,photochemical oxidation potential and aerosol of the process are 0.0046,0.0075,0.030,0.00011,and 0.00024,respectively,which are 69.33%,58.33%,31.82%,15.38%,and 14.29%lower than the direct biomass pyrolysis process.This process has less environmental impact than the direct biomass pyrolysis process.