Study Of Pressurized Torrefaction Of Biomass And The Thermal Conversion Of Solid Products

Biomass waste is the fourth largest energy resource after coal,petroleum,and natural gas.However,its thermochemical conversion is restricted by its high oxygen content,high water content and low calorific value.Therefore,it is essential to deoxygenate and upgrade biomass before utilization.In our previous study,we proposed a pressurized torrefaction pretreatment method,by which we found that gas pressure torrefaction pretreatment could increase the yields of gas products and liquid and suppress the tar formation during biomass gasification effectively.Based on the founding,we further investigates of the effect of gas pressure on biomass torrefaction and the application of its solid product,which are as follows:Rice straw and wood chips were chosen as the raw material for traditional torrefaction and gas pressure torrefaction.It was found that gas pressure torrefaction had much better effect on biomass upgrading than traditional torrefaction.It is supposed that originated from the fact that the biomass had secondly reacted with the gaseous products in the closed system,and higher pressure accelerated related reactions.Among the pressure range we chose,which was from 1.7MPa to 5.0MPa,1.7MPa resulted in better products.In specific under this condition,the volatile matter of the char from rice straw was reduced from 75.2%to 53.8%,and O/C and H/C ratios were reduced from 0.69 and 1.28 to 0.22 and 0.96,respectively.Moreover,the high calorific value（HHV）of the char increased from 15.9MJ/kg to 28.5MJ/kg,which was close to lignite.And the energy loss during the process was only 21%.As for the sawdust char,the volatile matter of the char decreased from 76.4%to 46.8%,the O/C and H/C ratios decreased from 0.62 and 1.56 to 0.21 and 1.31,respectively.The HHV of the sawdust char increased from 19.12MJ/kg to 29.26MJ/kg and the energy loss during the baking process is only 18%.The combustion and pyrolysis characteristics of pressurized torrefaction char were studied systematically as well.It was found that the ignition temperature of biomass increased significantly after pressurized torrefaction and was further increased with the increment of pressure:the ignition temperature of RS and BD increased by 36-66℃and 106-120℃,respectively.The spontaneous combustion tendency of carbon material can be presented by the ignition temperature to some extent,so it could be supposed that the torrefied biomass had less spontaneous combustion tendency and was beneficial for storage and transportation.Based on the pyrolysis characteristics of RS and BD.It was found that the gas-torrefactied solid product had higher char yield（55%）,and its carbon content was as high as 81.3%.Also more CH₄ and H₂ could be found in pyrolysis gas,and the calorific value of the gas product was as high as 12.2MJ./Nm³ to 15.3MJ/Nm³,which could be used as a combustible gas.Although bio-oil from RS and BD had low yields,the content of aliphatic hydrocarbons was high in RS-based pyrolysis bio-oil,and the content of phenolic substances is high in sawdust pyrolysis bio-oil,both bio-oil had the potential to produce phenolic compounds.So it could be concluded that biomass gas pressure torrefaction has obvious advantages over traditional torrefaction,and it is a highly-efficient deoxygenation and upgrading method for biomass utilization.