Characteristics Of Cottonstalk Low-temperature Deoxygenation Pretreatment Coupled With Mg-based Additives And Its Effect On Fast Pyrolysis

Converting abundant biomass resource into high-value solid biochar,liquid bio-oil and biogas can solve many of the nation's energy problems.However,the high oxygen content,high moisture content,degradation during storage,and low energy density of raw biomass are critical challenges in using agricultural residues as feedstock.Especially the high content of oxygen,not only caused low heating value but also adversely affect the following thermo-chemical conversion process.Therefore,it is necessary to carry out deoxidation pretreatment of biomass.However,related researches have shown that deoxidation effect of torrefaction pretreatment is limited and the energy consumption of low-temperature pyrolysis pretreatment is high.So,this study proposed to add Mg-additives in the process of low-temperature deoxidation pretreatment?200-350??to strengthen the deoxidation effect and further improve the properties of biomass.In addition,the low-temperature deoxidation pretreatment process is combined with fast pyrolysis or catalytic fast pyrolysis to study the effect of pretreatment temperature,type of Mg-additives,the mass ratio between additives and biomass on the product of pyrolysis.Firstly,the pyrolysis characteristics of cottonstalk was studied by using thermogravimetric Analysis?TGA?and pyrolysis kinetic method.The results shown that the presence of Mg-additives and increased of mass ratio caused the initial pyrolysis temperature increased,pyrolysis weight loss rate decreased.Additive S₂ and S₃ can reduce the pyrolysis activation energy of cottonstalk,and the pyrolysis activation energy of cottonstalk was decreased with the increase of mass ratio.Low-temperature deoxidation pretreatment experiment was conducted in a tube furnace to study the effect of temperature,type of Mg-based additives and the mass ratio between additive and biomass on the product distribuction,physical and chemical properties of solid product and release features of liquid and gaseous products.The results showed that,the oxygen content,hydrogen content and solid product yield were decreased with the increase of pretreatment temperature,while the carbon content,heating value and yield of solid and gaseous products were increased.The introduction of Mg-additives can further promote the removal of oxygen,enhancement of heat value,gaseous products yield and relatively reduce the liquid product yield when pretreatment temperatureis higher than 260?.The deoxidation effect of three additives is enhanced in the order of S₃?S₁?S₂,the additive S₂ can reduce the loss of hydrogen element while promoting the removal of oxygen,but the increase of the mass ratio between additives and cotton stalk can not further reduce the oxygen content of cotton stalk.In addition,the presence of Mg-based additives can also promote the precipitation of oxygen-containing small molecules of gaseous products such as CO₂ and CO during the low-temperature deoxidation process.Additive S₂ can dramatically decreased the relative content of acids and increased the relative content of phenols and ketones.Then,the fast pyrolysis and catalytic fast pyrolysis experiment were carried out in the Py-GC/MS to investigate the effect of pretreatment temperature and type of additives on the characteristics of products.And found that elevated of pretreatment temperature lead to the relative contents of aromatic hydrocarbons,hydrocarbons,phenols,ketones in the fast pyrolysis bio-oil increased,while the aldehydes,alcohols,furan and sugar decreased,the relative content of acid compounds significantly reduced.The introduction of Mg-based additives during the low-temperature deoxidation pretreatment process can play a role of catalyst in the fast pyrolysis process,thus improved the relative content of aromatic hydrocarbons and hydrocarbons,inhibited the formation of acids and phenols and promoted the conversion to ketones,among which the effect of additive S₂ was the most obvious.HZSM-5 catalyst can realize the directional transformation of fast pyrolysis products to aromatic hydrocarbons.The presence of Mg-based additives can further promote this orientation transformation process in a certain temperature range,even if without thermal pretreatment process addition of additive S₁,S₂ and S₃ make the aromatics content of pyrolysis bio-oil up to 84.37%,87.58%and 85.47%respectively.However,it should noted that if the pretreatment temperature is too high?above 320??,the relative content of aromatics will be decreased.Therefore,this study suggestes that the optimum temperature of low-temperature deoxidation pretreatment under the action of Mg-based additives was 260-290?.Finally,the influence mechanism of pretreatment temperature and the mass ratio between additives and biomass on the quality of pyrolysis product were deeply studied by a fixed bed bench,and found that with the elevation of pretreatment temperature,the yield of solid products continuously increased after fast pyrolysis,the yield of liquid products continuously decreased,while the yield of gaseous product increased first and then decreased.The addition of Mg-based additives inhibited the degree of increasement of solid product yield and promoted the formation of gaseous product,simultaneously,the increase of the mass ratio bentween additives and cotton stalk can enhance these effects.The volume fraction of CO in the gasous product produced by fast pyrolysis was significantly decreased with the increase of the pretreatment temperature,and the volume fraction of CO₂,H₂ and CH₄ increased;the presence of additive S₂ and the increase of mass ratio between additives and cottonstalk can further promoted the formation of H₂.The low-temperature deoxidation pretreatment under Mg-additives coupled with fast pyrolysis can effectively promoted the relative content of aromatic hydrocarbons in bio-oil and reduce the relative content of other typical oxygen-containing compounds other than ketones.