Investigation Of Biomass Chemical Looping Gasification For Hydrogen-Rich Syngas Production With Low Tar Containing

Biomass gasification is a crucial technology in the biomass thermal-chemical conversion technologies.However,on one hand,biomass is a macromolecular structure,a large amount of the tar was produced during gasification,and it needed a device to eliminate the tar.On the other hand,biomass usually contains much moisture.In the traditional process,the fuel needs to be dried first and then uses generated steam as a gasifying agent,and the device is complicated,resulting in the lack of economic efficiency.Aiming at these problems,this thesis proposed using biomass self-moisture as a gasifying agent,and using oxygen carrier（OC）to in-situ eliminate the tar for hydrogen-rich syngas production with low tar containing.The diffusion of the biomass self-moisture during the reactions was studied.The tar conversion on the OC was investigated.The phase transfer of the iron-based OC was understanded.The coupling dual bed reactor was setup,and the feasibility of the hydrogen-rich syngas production with low tar containing was verified.The basis for the future industry application of the biomass chemical looping gasification process was provided.The effect of the self-moisture in the biomass during gasification was first investigated in the paper.The concept of biomass self-moisture chemical looping gasification was proposed.The difference between the biomass self-moisture and the steam in the reactions was compared.The effect of the OC for biomass self-moisture gasification and the enhancement of the OC for the fuel conversion was studied.The results showed that effects of the same amount of the self-moisture and the steam were different.The reason was the different directions of the diffusion.The diffusion of the moisture content was from the inside out,while the diffusion of the steam was from the outside in.The result would make a difference on the reforming reactions and improved the reacting conditions when the temperature increasing.The use of the self-moisture as a gasifying agent was beneficial for the reduction of the energy and the increase of the efficiency.The gas yield could improve 14.11%,and the gas yield was promoted 36.47%.Naphthalene was used as a tar model compound,and the effects of the metal and the lattice oxygen in the OC for the naphthalene conversion were studied,respectively.The different stoichiometric ratios were analyzed.The relationship between the OC and the tar for the conversion route was established.The results showed that there was no significant difference for the naphthalene conversion for different OCs（CuO,NiO and Fe₂O₃）.When the OCs were on the metal phases,the conversion was at the range of 40-50%,but when the OCs were on the oxidized phases,the conversion was over 90%.When the amount of the lattice oxygen decreasing,the tar oxidation became difficult,leading to the decrease of CO₂.The reactions between the naphthalene and the OC went through three stages:completed oxidation,partial oxidation,and metal adsorption.From the results of thermodynamic analysis,the products had no CO and H₂ when the lattice oxygen being sufficient（stoichiometric ratio>80）.The products of CO₂ and H₂O decreasing rapidly when the lattice oxygen being scant（stoichiometric ratio<20）,and this caused the insufficient reactions.The investigation between biomass and iron-based OC was conducted on a self-designed thermogravimetric fixed-bed reactor.The phase transfer of the iron-based OC during the process was identified,and the control of the OC extent was obtained.The results showed that the reactions first went through a biomass-pyrolysis stage and then a partial-char-oxidation stage during the whole experimental process.The extent of OC reduction followed the order of iron（III）oxide,red mud,and natural hematite.The reaction rate of iron（III）oxide was the fastest,while the rate of the natural hematite was the slowest.Two kep points of CO/CO₂=0.97,2.61 were identified,and the phase transfer was Fe^8/3+→Fe²⁺and Fe²⁺→Fe⁰.Three different types of OC could all reduced to the metal Fe-phase,but the conditions of sintering and agglomeration were different.The coupling dual bed reactor was designed,and the object of hydrogen-rich syngas production with low tar containing was achieved using the iron-based OC for reduction/oxidation on the reaction temperature from 790oC to 880oC and the steam-to-biomass mass ratio（S/B）from 1.0kg/kg to 2.0 kg/kg.The hydrogen production was controlled by six main chemical reactions.The energy efficiency of the system was evaluated.The results showed that when increasing reaction temperature,ΔG of the water-gas shift reaction and the iron-steam reaction approached zero,and leading to a weaker hydrogen production.In the fuel reactor（FR）,the intensity of self-moisture gasification could reach 135.64 kg/（m² h）.The gas composition from the outlet was mainly CO₂,and the proportion could be at range of 60%-80%.In the gasifier,the H₂ composition could reach at 71.97%in the syngas.From the result of the energy efficiency of the whole system,the LHV of the syngas was controlled to promote 13.34%,and the cold gas efficiency was 60.57%of the maximum.The types of the tar were mainly aromatic hydrocarbons,containing benzene ring or naphthalene ring.The tar yield was at the range of 5-13 g/Nm³.Comparing with other technologies,the advantage of the coupling dual bed reactor was producing high H₂/CO syngas on a relatively lower temperature and lower S/B.The H₂/CO could reach 3.47 mol/mol.