Characteristics Investigation Of Coal Chemical Looping Gasification Using Iron-manganese Based Oxygen Carrier

Coal gasification technology is one of the key technologies for clean conversion and utilization of coal.Traditional coal gasification technology needs air separation unit,which not only increases the costs of investment and operation,but also reduces energy efficiency in the whole gasification process.Chemical looping gasification technology uses oxygen carrier instead of expensive air separation unit to provide oxygen source.It decouples the gasification process into two-step reactions,which not only improves the efficiency of gasification through cascade utilization of energy,but also reduces pollutant emissions.The development of oxygen carriers with high reactivity and good mechanical property is one of the key issues in chemical looping gasification technology.In this paper,iron-manganese composite oxygen carrier with oxygen decoupling characteristics was prepared by sol-gel method.The characteristics of the chemical looping gasification reaction between iron-manganese composite oxygen carrier and coal under different operating conditions were systematically investigated,and the interaction mechanism between oxygen carrier and coal in gasification process was further explored.Furthermore,the oxygen transfer mechanism and evolution rule of oxygen carrier were elaborated.X-ray diffraction(XRD),thermogravimetry(TG),temperature programmed reduction(H₂-TPR)and specific surface area test(BET)were used to characterize the physical and chemical properties of the iron-manganese composite oxygen carriers.The results showed that the iron-manganese composite oxygen carriers presented the characteristics of oxygen decoupling,and there is a synergistic effect between the active components.Generally,oxygen carriers are doped with inert carriers can improve its reactivity and stability.The iron-manganese oxygen carrier doped with alumina has a larger surface area(6.19 m²/g)and a smaller grain size(35.02 nm).The iron element could be reduced to Fe⁰,while the manganese element was reduced to Mn O during the reduction process.Coal chemical looping gasification experiments using iron-manganese oxygen carrier under different operation conditions were carried out in a fixed bed to obtain gasification reaction characteristics and rules.Oxygen carrier M25F75with a molar ratio of iron to manganese of 3 has the maximum syngas yield and carbon conversion rate;As the increased ratio of oxygen carrier to coal,the carbon conversion rate gradually increases,the selectivity of syngas,the hydrogen/carbon ratio,and the heating value of gas product showed a downward trend.Considering the factors of economic cost and gas yield comprehensively,the ratio of oxygen carrier to coal equals to 1 is selected as the condition for the following experimental investigation;the temperature increase promotes the deep conversion of coal,and the syngas yield,carbon conversion rate and heating value gradually increase and become stable with the temperature increase to 1000?,the thermodynamic equilibrium is reached at 900?;The introduction of steam can effectively adjust the hydrogen to carbon ratio in the product from 1 to 2.4.The syngas yield,carbon conversion rate and heating value show a trend of increasing firstly,and then slowing down with the increase of steam.When the amount of steam is 0.045 ml/min,the syngas yield(0.054 mol/g)and heating value(12.8 MJ/Nm³)reach their peak values.As increased doping amount of Al₂O₃,the syngas yield,syngas selectivity,carbon conversion rate,heating value and hydrogen to carbon ratio increases first and then decreases.The oxygen carrier doping with 30%Al₂O₃ has the highest syngas yield of 0.017 mol/g and syngas selectivity of 82.17%;the oxygen carrier showed good cycle stability in20-cycle experiments.Methane,toluene and char were selected as the model compounds of coal pyrolysis products.By studying the gasification reaction characteristics between the oxygen carrier and the model compound,the functional mechanism of the oxygen carrier in the multi-phase reaction process was revealed.In the process,it acts as both oxygen source and catalyst.The oxygen carrier has oxidation and catalytic cracking effects on coal pyrolysis gas,and redox and catalytic cracking reactions take place between oxygen carrier and tar.In addition,oxygen carrier provides oxygen source for the gasification of char.The dual functions of oxygen carrier with oxygen and catalysis can effectively improve the conversion rate of coal and the syngas yield.The oxygen carriers with different reduction degree were characterized by XRD and XPS to reveal the oxygen transport mechanism and phase evolution law of the oxygen carrier lattice.The lattice oxygen in the oxygen carrier is gradually released during the reaction.The consumption of surface oxygen would lead to the concentration difference of lattice oxygen inside and outside of the oxygen carriers.Driven by the different concentration of lattice oxygen,lattice oxygen migrated from body phase to surface phase and converted into adsorbed oxygen;The phase evolution law of oxygen carrier undergoes a stepwise reduction process from iron-manganese metal oxide to iron-manganese solid solution and finally to iron and manganese oxide.