Study Of Deoxygenation Mechanism Of Gas-pressurized Torrefaction And Fuel Properties Of Torrefied Biomass

Biomass is the fourth largest energy after coal,oil and natural gas.However,its utilization is seriously restricted by the high moisture content,low energy density and poor grindability.All of these properties are attributed to the rich oxygen-containing functional groups and high oxygen content of biomass.Torrefaction is one of the most promising pretreatment technologies to upgrade biomass,but its deoxygenation efficiency is insufficient.Recently,a novel gas-pressurized torrefaction（GP torrefaction）was proposed in our previous work,which can efficiently remove the oxygen of biomass.However,the deoxygenation mechanism of GP torrefaction remain unclear,which is important for the control and optimization of this method.The fuel properties of the GP torrefied biomass are also unknown,which can guide its application direction.To solve the above issues,rice straw（RS）was used as raw material to study the effects of volatile partial pressure(P_vol)and total pressure(P_tot)on GP torrefaction.The results show that P_vol was the root cause for the deep deoxygenation of biomass.The optimal P_vol was about 1.30 MPa,and P_vol increase promoted the decomposition of cellulose in biomass.Then,cellulose was used as a model compound to explore the conversion mechanism of biomass in GP torrefaction.Combined with the chemical structure evolution of RS,the deoxygenation mechanism of GP torrefaction,self-promoted deoxygenation process of biomass at low temperature,was proposed.It can be divided into three stages:volatile formation stage,self-promoted dehydration stage and aromatization stage.Finally,the bulk density,energy density,water absorption,grindability and combustion characteristics of raw biomass,AP and GP torrefied biomasses were analyzed and compared.The results show that the energy density of biomass was improved by 56%via GP torrefaction,and its hydrophobicity,grindability and ignition temperature of biomass were enhanced significantly,so as the fuel quality of biomass was significantly enhanced and closed to lignite.