Chemical Looping Gasification Of Biomass Coupled With CO₂ Splitting With Fe-Based Mixed Oxygen Carriers

With the economic and social development as well as the extensive utilization of fossil energy,the greenhouse effect caused by CO₂emissions is becoming increasingly serious.Additionally,China has become increasingly dependent on fossil energy imports,which makes great challenges to the national energy security.Biomass resource is clean and renewable resource with various sources and carbon neutral characteristics.High-quality syngas is achieved by using lattice oxygen from oxygen carriers instead of molecular oxygen via Chemical Looping Gasification（CLG）of biomass,which reveals advantages of cheap oxygen source and avoiding dilution of inert gas.High purity CO has been prepared directly by the reaction of CO₂ and reduced oxygen carrier without high energy consumption and specific catalyst.Under this background,this paper proposes CLG of biomass coupled CO₂ splitting reaction to prepare high-quality syngas and CO,realizing the efficient and clean conversion of biomass and resource utilization of greenhouse gas CO₂.Thermodynamic simulation was performed to analyze reaction equilibrium limit and adjust rule of the CLG coupled with CO₂ splitting reaction.Oxygen carriers were prepared by introduction exogenous active metal components on the cheap iron ore.Reaction mechanism and the influence of key parameters on the CLG coupled with CO₂ splitting reaction were investigated.Also,the reaction kinetics characteristics and lattice oxygen transfer mechanism of oxygen carriers were revealed.the main content is as follows:（1）Thermodynamic analysis of reaction process;（2）Synthesis and reaction evaluation of Fe-based mixed oxygen carries;（3）Chemical looing reaction coupled CO₂ reaction process and influencing factors;（4）Experimental investigation on CLG of biomass coupled CO₂ splitting;（5）Reaction kinetics and lattice oxygen migration mechanism of oxygen carries.The main contents and results are summarized as follows:（1）The thermodynamic simulation process proved that the reaction system can achieve high-quality syngas from biomass and split CO₂ to CO with graded conversion of biomass char.The biomass carbon conversion efficiency in CLG was up to 96.6%,and the CO₂ conversion in CO₂ splitting was up to 93.1%.High temperature can strengthen the reaction process,but also increase the thermal load of system.Reaction pressure showed positive effects on reaction rate and equilibrium time,which is beneficial to the coupling with power generation system.However,it had an inhibitory effect on the equilibrium conversion efficiency.The ratio of biomass to oxygen carrier B/O played a key role in the regulation of reaction products.The reaction system can realize self-heating operation.（2）Oxygen carriers were prepared by introduction exogenous active metal components on the cheap iron ore matrix.The exogenous metal reacted with Fe₂O₃ in ore to form mixed oxide/solid solution such as CaFe₂O₅,NiFe₂O₄ and Ce Fe O₃ to improve the crystal structure of oxygen carrier,increase oxygen vacancy concentration and strengthen lattice oxygen transfer.The reactivity order of exogenous metal was NiO>Co₂O₃>CeO₂>CaO.Oxygen carrier modified by NiO indicated good reaction performance in chemical looping reaction.The H₂/CO of gas product 0.72 and CH₄ conversion 55.86%were achieved by using this oxygen carrier.While,oxygen carriers modified by CeO₂,CaO exhibt higher CO₂ conversion efficiency in CO₂ splitting stage,which were53.74%and 52.78%,respectively.The oxygen carrier can recover its lattice oxygen at high temperature with air and the compostion of CO₂ in exhaust gas was less,which indicated that graded conversion of carbon was realized in this reaction process.（3）In the early stage of chemical looping reaction,the oxidation reaction of lattice oxygen and dry reforming of CH₄ were main reactions,while the dry reforming of CH₄ promoted the increase of H₂/CO in gas product.The H₂/CO maximum value could be obtained at 1/5 of the reaction process.The supply of lattice oxygen decreased in the later stage of reaction,which leaded to the cracking and reforming reaction of CH₄as the main reactions.CO₂ splitting was restricted by oxygen vacancy and gas solid mass transfer,whose reaction process presented a steady slowing trend.Exogenous ions can enhance the lattice oxygen transfer of oxygen carrier,improve H₂/CO of gas product,reduce the content of impurities in pyrolysis gas and improve the conversion efficiency of CO₂.The maximum H₂/CO of gas products in chemical looping process was 3times that of initial value by using CaO modified oxygen carrier.Also,the maximum instantaneous CO yield and CO₂conversion in CO₂ splitting reaction were 2.47 mmol/min/g oxygen carrier and 86.13%.Reaction temperatures,exogenous metal content were positively correlated with instantaneous yield of CO,while the WHSV was negatively correlated with CO₂ conversion.The reactivity of oxygen carriers remained stable in15 cycles.The reaction paths of the two oxygen carriers are as follows:Ca₂Fe₂O₅→CaO/Fe→Fe₃O₄/CaFe₃O₅→Ca₂Fe₂O₅/CaFe₂O/Fe₂O₃/CeO₂/Ce Fe O₃→Ce₂O₃/Fe→Ce Fe O₃/CeO₂/Fe₃O₄→Fe₂O₃/CeO₂。（4）Syngas and CO produced from CLG coupled with CO₂ with Fe based oxygen carriers.Exogenous metals can enhance the reaction activity of oxygen carrier.In CLG reaction,NiO modified oxygen carrier showed the highest carbon conversion efficiency of 74.18%and syngas heating value of 13.32 MJ/m³,corresponding to CO₂ conversion efficiency 64.09%in CO₂ splitting.Increasing temperature can enhance the reaction process.The carbon conversion efficiency of CLG increased from 50.84%to 75.07%,and CO₂splitting conversion increased from45.25%to 63.90%with the rising temperature from 750℃to 950℃.The increase of B/O also promoted the CO₂ splitting reaction.The oxygen carrier was reduced to Fe/Ni alloy in CLG,and then oxidized to Fe₃O₄/Ni in the CO₂ splitting stage.In 20 cycle reactions,the reactivity of oxygen carrier was stable,the carbon conversion efficiency was maintained at67.52%,and the CO₂ splitting rate was 73.50%.（5）The activation energy of CLG reaction corresponded to four reaction stages of water separation,biomass pyrolysis,gas reforming and the reaction of biomass char and lattice oxygen.At the stage of water separation,the reaction activation energy was lower,which was 21.86k J/mol whenαwas 0.1.The reaction activation energy increased during biomass pyrolysis,while the maximum activation energy of biomass char reaction with oxygen carrier was displayed due to the limitation of solid-solid mass transfer and reduced reaction interface.This leads to maximum energy barrier needed to be overcome.Whenαwas0.9,the reaction activation energy was 373.78 k J/mol.In the CO₂ splitting process,the activation energy which increased from 6.77 k J/mol to 29.04k J/mol indicated a steady increase trend due to the decrease of oxygen vacancy and the increase of mass transfer resistance inside the lattice.In the chemical looping reaction,lattice oxygen O^2-was first dissociated as chemisorption oxygen O^2-/O^-and transferred from the bulk phase to reaction surface,where it converted into physically adsorbed oxygen O₂and reacted with the fuel.The transfer mechanism was:O^2–e_{（lattice）}（？）→O₂^–/O^–e_{（chemical）}（？）→O^e_2（phy）（？）→O_2（g）.Metal ions Fe³⁺/Fe²⁺and Ni²⁺in oxygen carries were reduced to the lower metal elements in chemical looping reaction.The dissolution rate of the Ni-Fe bond is lower than that of the Ni-O bond.The migration of lattice oxygen in CO₂ splitting reaction showed an opposite trend and the valence of Fe element increased correspondingly,while the valence of Ni element was basically stable.This study exhibits that the CLG of biomass coupled CO₂ splitting can achieve high quality syngas and recycle CO₂ to CO with low cost oxygen source,complementary energy and carbon graded conversion.The experimental result provides a new way for exploring and developing new efficient biomass syngas production technology and resource utilization of greenhouse gas CO₂.