Experimental Study Of Biomass Chemical Looping Catalytic Gasification Using Iron-based Perovskite Oxygen Carrier

As a renewable resource with a long history,biomass is rich in energy storage and is considered to be a carbon neutral fuel.Biomass chemical looping gasification（BCLG）is a novel gasification technology which can utilize biomass energy efficiently and cleanly.It uses the lattice oxygen in the oxygen carrier to complete the partial oxidation of biomass raw materials,where oxygen carrier is the key issue to the whole process.The iron-based perovskite oxygen carrier has broad application prospects in the BCLG due to its good thermal stability,catalytic activity and adjustable crystal structure.The present work optimized the preparation conditions of iron-based perovskite oxygen carriers,and systematically studied the performance of BCLG based on iron-based perovskite oxygen carriers.And the characteristics of tar formation,the migration and conversion behavior of nitrogen in fuel during the process were also investigated.The preparation of high-purity iron-based perovskite oxygen carriers with a single perovskite phase is the basis for the research on the performance of BCLG.The glycine auto-combustion method and the citric acid-nitrate sol-gel method were used for the chemical synthesis of iron-based perovskite oxygen carriers.And parameters including the p H value of the precursor solution,the amount of citric acid,the heat treatment temperature and the calcination method were controlled to optimize the preparation process of traditional citric acid-nitrate sol-gel method.Both the auto-combustion method and the sol-gel method could be used to prepare La Fe O₃（LF）oxygen carriers with a single perovskite phase.It indicated that LF had a stable crystal structure and was less sensitive to preparation conditions.The synthesis of Ba Fe O₃（BF）has more stringent requirements on the preparation conditions,and the optimized sol-gel method is more suitable for preparing BF oxygen carriers containing almost no impurities.When the p H of precursor was 8,the evaporation temperature was 80°C,the drying temperature was 120°C,the pre-combustion temperature was300°C,and the calcination temperature was 750°C,the perovskite oxygen carrier calcined in the muffle furnace had the best crystal structure.The thermodynamic software HSC Chemistry 6.0 was used to analyze and calculate the BCLG performance of sawdust with LF as oxygen carrier,and BCLG experiments were conducted in a fixed bed reactor.The influence of oxygen carrier to biomass mass ratio（O/B）,gasification temperature and steam concentration on the gasification performance was investigated.The feasibility of applying iron-based perovskite oxygen carrier in BCLG process was verified via both theoretical analysis and experimental research.Theoretical analysis results showed that LF had a low reactivity with synthesis gas and a low CO₂ production,indicating that it was suitable for the BCLG process.Experimental results showed that compared with hematite,LF was more appropriate for CLG of sawdust.When the steam concentration was 40.4%and the gasification temperature was 900°C,highest gasification efficiency was obtained and the synthesis gas had the highest calorific value.However,the cyclic performance of LF was poor.After 5 redox cycles,the gasification efficiency dropped by more than50%.Thus,its activity needed to be further improved.In view of the relatively low activity and poor cyclic reaction performance of LF,Ba and Ni were used to dope on the A site and B site in the framework of LF to improve its reaction and catalytic reactivity.Through X-Ray diffraction,H₂-temperature programmed reduction,X-ray photoelectron spectra and other characterization methods,the influence of Ba and Ni doping amount on the crystal structure and reaction performance of La_1-xBa_xFe O₃（x=0,0.3,0.5,0.7,0.9,1.0）and La Ni_yFe_1-yO₃（y=0,0.1,0.3,0.5,0.7,1.0）were investigated.Gasification experiments of algae with iron-based perovskite oxygen carrier were carried out in the fixed bed reactor.It was found that the substitution of La by Ba on the A-site of LF increased the ratio of Fe⁴⁺and chemically adsorbed oxygen on the surface of the oxygen carrier,and significantly improved the CLG performance of microalgae.Among them,La_0.3Ba_0.7Fe O₃（L3B7F）exhibited the best reactivity.Compared with undoped LF,L3B7F oxygen carrier had higher reactivity and cycle stability after 10 cycles of experiments.The active Ni component was embedded into the bulk phase of LF by doping on the B-site,thus improving the activity of oxygen carrier.Among several La Ni_yFe_1-yO₃ oxygen carriers,La Ni_0.5Fe_0.5O₃（LN5F5）had both good catalytic gasification performance and regeneration ability.After 10 cycles of experiments,slight sintering occurred on the surface of the oxygen carrier,resulting in a decrease in activity.The characteristics of tar formation during CLG of microalgae based on iron-based perovskite oxygen carrier were explored in a fixed bed reactor comined with GC-MS method.Results showed that the type and yield of tar during CLG process was effectively reduced,compared with the direct gasification of microalgae.Tar compounds with the molecular weight between 150 and 200 were significantly reduced by using hematite and LN5F5 as oxygen carriers,while LF and L3B7F could effectively reduce the generation of macromolecular tar with molecular weight larger than 200.Among them,L3B7F,as a hydrogenation catalyst,could open the benzene ring structure of macromolecular aromatic hydrocarbons,making it easier to decompose into small molecules.The increase in gasification temperature was conducive to the decomposition of macromolecular tar and the conversion to small molecular substances.This may be due to the fact that high temperature would accelerate the oxygen diffusion rate and electron migration rate,and improve the reducibility and catalytic activity of the perovskite oxygen carrier.The incease in the amount of oxygen carrier made more surface active oxygen participate in the oxidative decomposition of tar.In the meantime,the specific surface area of the tar catalytic cracking reaction also increased,which was beneficial to the conversion of macromolecular tar to small molecular organic compounds.The characteristics of nitrogen migration and transformation during microalgae CLG process with iron-based perovskite as oxygen carriers were investigated in a fixed bed reactor by means of an infrared gas analyzer.Results showed that NH₃ is the main nitrogen-containing gas released during the pyrolysis stage of microalgae.Perovskite could promote the release of NH₃ and HCN,while hematite tended to oxidize the precursors to NO.NO could be stored in the form of nitrate and nitrite on the surface of perovskite oxygen carrier at low temperature.The nitrate and nitrite decomposed at high temperature,and nitrogen element was released in the form of N₂,NO and N₂O,which reduced NO_x during gasification process.NO was the main NO_x product in the microalgae CLG process.With the increase of the gasification temperature,the NO_x emission increased first and then decreased.The decrease of NO_x emission at high temperature may be attributed to the selective catalytic reduction of NO_x by the reducing gas on the surface of the perovskite.The inceased oxygen carrier provided more lattice oxygen,larger reaction area and more catalytically active sites,promoting the conversion of NH₃,HCN and NO to NO₂.The increase of steam concentration promoted the production of NO_xduring the gasification process,and the production of NO₂was more sensitive to changes in steam concentration.