Effect Of Torrefaction Pretreatment On Reactivity And Gas Production Characteristics Of Biomass Gasification

Biomass is the only renewable carbon source with storability and carbon neutrality.Promoting the use of biomass energy is an effective way to reduce the consumption of fossil energy and restrain greenhouse gas emissions.Torrefaction is an effective technology for improving biomass quality.However,the effects of different torrefaction process on the gasification reactivity of biochar have not been quantitatively evaluated.The effects of dry torrefaction and wet torrefaction on biomass gasification were studied systematically in this study.Dry and wet torrefied biomass samples were prepared by a fixed bed reactor and batch reactor.A self-built fixed bed steam reaction device was used to investigate the effects of gasification time,gasification temperature,and gasification agent flow rate on the total gas yield,individual gas component yield and gas component volume content of torrefied biomass.The biomass samples were quickly pyrolyzed to produce biochar.The effects of dry and wet torrefaction on the microstructure and gasification reactivity of biochar were explored.Thermogravimetric analyzer(TG)was used to explore the effect of torrefaction on the reactivity of gasification,and the kinetic parameters were calculated.When the gasification temperature is 900℃ and ratio of water vapor flow to biomass mass(S/M)is 1.5,the gasification gas of the original corn stalk CS and dry torrefied corn stalk DCS240 was completely released within 45 min,and the H2 and CO yields of the wet torrefied corn stalk WCS240 were still higher than 0.2 mmol/min after 45 min of gasification.The gas yield of H2 reached the maximum at the 10th min.CH4 mainly appeared in the first 10 min of gasification reaction.With the increase of gasification temperature,the total gas yield of gasification increases.The gasification temperature had a greater effect on the wet torrefied biomass.At low temperatures(700℃ and 800℃),the wet torrefied biomass was not fully gasified.At 1000℃ isothermal gasification,the wet torrefied biomass was completely gasified,and the gas produced reached the maximum of 62.03 mmol/g.With the increase of S/M,the total gas yield of biomass gasification increases,and the yield and concentration of H2 increase.When the S/M is 2,the total gas yield of CS,DCS240 and WCS240 was 38.15,48.65,and 41.14 mmol/g,respectively,and the concentration of H2 was 38.97%,42.69%,and 45.61%,respectively.Therefore,dry torrefied biomass is suitable for gasification at 700-1000℃ and S/M is 0.5-2,while wet torrefied biomass is suitable for gasification at gasification temperature higher than 900℃ and S/M higher than 1.5.At the same torrefaction temperature,the influence of wet torrefaction on biomass H/C and O/C is higher than that of dry torrefaction.The H/C and O/C of torrefied biomass at 240℃ are close to that of lignite.The results indicate that the content of potassium in biochar varies from 15.47%to 16.92%after dry torrefication,while wet torrefication can effectively remove potassium in biochar,and the removal efficiency can reach more than 90%.X-Ray Diffraction results show that The number of aromatic layers(LC)and the mean number of aromatic layers Nmean)in the microcrystalline structure unit of dry torrefied biochar vary from 0 to 1.58%compared with the raw samples,and the increased range of Lc and Nmean values of wet torrefied biochar is 6.20%-20%compared with the raw samples.The crystallinity of wet torrefied biochar is significantly increased.Scanning electron microscopy showed that dry torrefied biomass coke had obvious pores on its surface,while wet torrefied biochar had dense surface.The specific surface area(SSA)of dry torrefied biochar was larger than that of wet torrefied biochar.The SSA of dry torrefied CS and PS biochar were 1.33 and 85.94 times that of wet torrefied biochar,respectively.TG isothermal gasification showed that dry torrefaction had little effect on the gasification reactivity of biochar,while wet torrefaction significantly reduced the reactivity of biochar gasification.At the gasification temperature of 900-1000℃,the reaction rate K of raw biochar of corn straw and poplar sawdust ranges from 0.38 to 1.11 min-1.The K values of dry torrefied char ranged from 0.30 to 1.21 min-1,and those of wet torrefied char ranged from 0.005 to 0.20 min-1.The Modified random pore model model had a good fit for both dry and wet torrefied biochar gasification reaction behavior.The activation energy E of dry torrefied biochar ranged from 99.89 to 149.84 kJ/mol,and those of wet torrefied biochar ranged from 205.74 to 267.51 kJ/mol.In this paper,the applicable conditions of dry torrefied and wet torrefied biomass gasification,the change of biochar gasification reactivity and the kinetic parameters of gasification reaction were proved through experimental research,which provided a theoretical reference for the practical application of torrefied biomass.