Study Of Coal Chemical Looping Hydrogen Generation With Composite Oxygen Carrier

Greenhouse gases cause global warming,among which CO₂ emission is mainly from fossil fuels.Hydrogen energy as a clean energy has attracted great attention.Steam methane reforming（SMR）currently used for large-scale H₂ production is energy intensive and has high CO₂ emissions.China has abundant and extensive coal resources,so it is particularly important to explore how to use coal to produce hydrogen cleanly and efficiently.Chemical looping hydrogen generation（CLHG）,based on chemical looping combustion（CLC）and steam-iron processes for hydrogen production,can simultaneously capture carbon and produce high purity H₂.In this paper,the cheap iron-based oxygen carrier was selected to explore the reaction performance of bituminous coal in the process of coal chemical looping hydrogen production from two aspects of the modification of oxygen carrier and the optimization of the reaction process,and the deep reduction and carbon deposition of Fe₂O₃ were analyzed.This paper focuses on the following three aspects:（1）The reaction characteristics of Meihuajing bituminous coal chemical looping hydrogen generation from with Fe₂O₃/Al₂O₃ composite oxygen carrier were investigated in a fixed bed.The optimal operating conditions were determined as follows:reduction temperature 900℃,steam oxidation temperature 900℃,steam flow rate 0.1 L/min.Under these experimental conditions,the carbon conversion of coal CLHG is 76.05%,the hydrogen yield is 1.52 L/g,and the hydrogen concentration is 87.1%.By comparing the performance of Fe₂O₃ and Fe₃O₄ oxygen carriers in the six cycles,the carbon conversion and hydrogen production in the three-reactor hydrogen production with Fe₂O₃ as oxygen carrier only slightly decreased during the cycle,while the hydrogen production in the two-reactor hydrogen production with Fe₃O₄ significantly decreased.However,the hydrogen production in the first experiment reached 1.68 L/g,which was12%higher than that of Fe₂O₃.（2）Mg was added to Fe₂O₃/Al₂O₃ oxygen carrier by impregnation method.Coal chemical looping hydrogen generation was carried out in a fixed bed,and the mechanism of action of Mg was deeply analyzed.The XRD results show that the characteristic peak of Mg Al₂O₄ is enhanced and the characteristic peak of Fe Al₂O₄ is gradually disappeared when the content of Mg increases from 1%to 26.5%,indicating that Mg weakens the interaction between Fe and Al.SEM shows that the oxygen carrier particles decrease after adding Mg,and the sintering resistance is excellent.Compared with the experiments of different Coal/OC mass ratio,the carbon conversion and hydrogen production were the highest when the mass ratio was 0.5/15.Among the oxygen carriers with different Mg contents,Fe40Mg20Al40 had the best reaction performance,and the carbon conversion and hydrogen production were 81.75%and 1.7182 L/g,which increased by 10.2%and 58.5%compared with Fe40Al60,respectively.After 10 cycles,the surface of Fe40Mg20Al40 is only slightly sintered,and the carbon conversion and hydrogen production are both above 78%and 1.52 L/g,showing good cycling performance.（3）By optimizing the reaction path process,it is proposed to enhanced coal chemical looping hydrogen generation,deep secondary reduction,secondary hydrogen production,only one air oxidation,and solve the problem that the reduction degree of Fe based oxygen carrier is not deep in the CLHG process.By comparing the CLHG experiments of enhanced Coal with different mass ratios of Coal/OC,the hydrogen yield was the highest when the mass ratio was 1/15,which was 1.74 L/g.By comparing the outlet gas of the third reactor with that of the enhanced CLHG,and using XRD analysis,it is verified that the oxygen carrier in the enhanced CLHG process is reduced to Fe O/Fe more,and the residual carbon is further reacted in the steam oxidation stage,making the hydrogen production higher than that of the third reactor CLHG by 18.4%.Compared with the hydrogen production performance of the two reactors,three reactors and the three processes of enhanced CLHG in the six cycles,the carbon conversion rate of enhanced CLHG was similar to that of the three reactors,and hydrogen production was always higher than that of the latter.The maximum single hydrogen production was 1.76L/g,and the cumulative hydrogen production in the cycle was 9.54 L,showing better hydrogen production capacity.