Study On The Oxidation Mechanism Of Biomass Multi-Stage Oxygen Gasification Process

The development and utilization of renewable energy is a useful solution for solving the energy and environment problems.Biomass is an important renewable energy and has been payed attention widely.Biomass gasification is kind of commercial-potential technology for its low cost,strong adaptability to raw materials and diversiform utilization of syngas.However,high tar content in syngas is the obstacle for its commercialization and the development of low-tar,high-efficiency gasification technology is a hot research area.Focus on the existing problem of two stage gasifier in the world,this paper proposed multi-stage oxygen gasification process,which controlled biomass to be converted into syngas by reasonable distribution of oxygen during the whole gasification process.In order to investigate the characteristics of biomass oxidative pyrolysis,the TG-DSC-MS method was used to investigate the pyrolysis of cellulose,hemicellulose,lignin and rice straw under inert and oxidative atmospheres.The mass loss,products release and reaction heat properties were analyzed.The results show that oxygen promoted pyrolysis reaction rate and bring reaction into lower temperature zone.The solid residues of cellulose and hemicellulose are much under oxidative conditions,while the change is much less for lignin.Lignin char forms the skeleton of biomass char and oxidative pyrolysis benefits the formation of char pore.Cellulose inert pyrolysis is an endothermic process and gradually turns to be exothermic with oxygen concentration increase.The turning point is about 3%O₂.The heat needed for biomass pyrolysis self-maintain is mainly from oxidative of cellulose and hemicellulose.At the same time,oxygen addition promotes the yields of CO,CO₂,H₂ and CH₄.DRIFT results indicate that oxygen promotes the evolution of functional groups:Aliphatic groups fall off to form methane and other hydrocarbons;the aromaticity is induced.The gasification reactivity of rice straw char derived under oxidative condition is improved.In order to reveal the reaction mechanism of biomass oxidative pyrolysis,the DFT?density functional theory?was used to build and compare the reaction paths of inert and oxidative pyrolysis of cellulose.The reactants,transient states,intermediate and products of each paths were optimized.Frequency calculations were conducted to calculate the potential energy of coordinates.The results show that cellulose inert pyrolysis is a net endothermic process which is caused by breaking of glucosidic bond,ring open and the formation of small compounds.The physical adsorption of oxygen with cellulose releases some part of heat.The oxidative pyrolysis of cellulose has two paths:one paths is initiation by thermal cracking of glucosidic bond,then the chemical adsorption of oxygen with intermediates and further degradation.Another path is the direct chemical adsorption of oxygen with cellulose to form complex and further hydrogen abstraction.The chemisorption process is strong exothermic process which provided the energy for biomass pyrolysis——the so called“in-situ heat release induced pyrolysis”mechanism.The energy barrier is much smaller for direct hydrogen abstract reaction which means this process has priority under lower temperature.The oxidative pyrolysis will form oxygen-containing functional group,such as lactone and carboxyl groups,which benefit the formation of CO₂.In order to investigate homogenous conversion of biomass pyrolysis tar,a tube reactor was used to investigate the homogenous reaction of model tar compounds?guaiacol,anisole,furfural and toluene?under different conditions.The results indicate that oxygenic compounds have higher reactivity.Guaiacol first degrades by the cracking of O-CH₃ bond and further undergoes decarbonylation to form small compounds.But the polymerization reactions accompany this process to form PAHs.Compared with anisole,the naphthalene yield of guaiacol is much less due to the double-phenoxy structure and the formation of cyclopentadienyl is inhibited.Non-aromatic,such as furfural could form aromatic compounds via C3/C4 paths.The addition of external oxygen could convert tar into small compounds and the formation of refractory compounds,such as benzene,naphthalene is greatly inhibited.Reasonable oxygen amount could realize high-efficiency tar reduction with sacrifice of combustible gases.In order to solve the high PAHs yield in partial oxidation zone,biomass tar homogenous conversion mechanism and effect factors are investigated.The detailed reaction mechanism was used to simulate different tar model compounds homogenous reaction and the key reaction paths were build.The results show that thermal cracking could reduce tar but leads to serious polymerization to form PAHs and soot controlled by hydrogen shift mechanism.The addition of oxygen to aromatic ring could destroy its stability by electron donating.Therefore,oxygenic compounds,such as phenols is more reactive.The addition of oxygen could bring in reactive radical,such as OH,O,HO₂,etc.which benefit tar crack.Partial oxidation inhibits the polymerization of cyclopentadienyl via RSR mechanism.ROP?rate of production?results show that oxygen is consumed in the initial 1 second and oxygen doesn't react with tar directly which react with the initial tar radicals to form oxygenic intermediates.These intermediates play key roles in tar reduction.Calculation shows that staged oxygen combustion could achieve lower tar yields.In order to develop a complete mechanism model to simulate primary tar conversion,the RMG?reaction mechanism generation?method coupled with quantum chemistry calculation was used.The model results agree well with experimental results both for model tar compounds and real tar conversion.This model could be used to simulate biomass tar homogenous tar conversion in gasification process.In order to investigate the effect of oxygen on char bed reduction process,a bench scaled fixed bed reactor was used.The effects of temperature,amount of oxygen,with/without char bed on tar reduction were analyzed.The results show that the combination of oxygen and char bed has great effect for tar reduction,both for PAHs and non-PAHs,such as toluene.At 700-800?,great carbon deposition on char pore appears under inert atmosphere.The addition of oxygen could inhibit carbon deposition by partial oxidation of tar.At the same time,oxidation reaction could increase char bed temperature to promote the carbon conversion efficiency.