Process And Mechanism Of Thermochemical Conversion Of Torrefied Biomass For Hydrogen-rich Syngas Production

Biomass energy are abundant and cheap,which has attracted the attention of researchers due to its advantages such as clean,renewable and carbon neutral.However,its shortcomings such as low energy density,high moisture and oxygen content and moisture absorption limit its large-scale application.As a pretreatment,torrefaction can effectively overcome the above disadvantages as well as improve the physicochemical properties of biomass.In this paper,the effect of torrefaction on the physicochemical properties of biomass was investigated through a small torrefaction experimental equipment.The raw materials used in the experiment were rice husks and wood chips.（1）Compared with the raw materials,the maximum increase in fixed carbon content of torrefied rice husk and sawdust were 41.08%and 67.47%;the maximum increase in C element content were 8.53%and 12.35%;the content of volatile matter and O element decreased significantly,and the content of H element also decreased slightly;O/C and H/C also showed a decreasing trend.（2）The higher heating value increased,with the largest increase of11.06%（rice husk）and 15.88%（pine sawdust）.（3）The hemicellulose was decomposed a lot because of torrefaction.When temperature reached 300℃,the hemicellulose content in the torrefied rice husk and sawdust was only 2.41%and 1.06%,with a decrease of more than 90%;the relative content of lignin increased significantly,with the largest increase of 119%（rice husk）and 208%（pine sawdust）;the content of cellulose increased first and then decreased.The reasons for the decrease in O/C and H/C as well as the increase of higher heating value due to torrefaction were analyzed from the perspective of component changes.（4）After torrefaction,the internal fiber structure was destroyed and the grindability was improved.With the deepening of torrefaction,the particles below 100 mesh increased significantly after rice husk was ground.The oxygen-containing functional groups such as-OH and C-O were reduced,and the hydrophobicity was increased.Compared with the raw materials,the hydrophobicity of rice husk and pine sawdust torrefied at 250℃for 30min were increased by28.08%and 25.66%,respectively.The results showed that torrefaction improved the quality of biofuel.Steam gasification experiments were carried out using raw and torrefied biomass to investigate the effect of torrefaction on hydrogen production by gasification.Using medium torrefied biomass（rice husk and pine sawdust torrefied at 250°C for 30 minutes）,the optimal gasification temperature（900°C）and steam volume(0.23 g·（min-g biomass）^-1)were determined.The volume fraction of H₂in the syngas(α_H2)and the rate of hydrogen production（μ_H）were maximum under this condition,which meant the best condition of hydrogen production was achieved.Compared with raw biomass,α_H2,μ_H and gas production rate（η_v）were increased in different degrees during torrefied biomass gasification,with the maximum increases inη_v of 38.29%（rice husk）and 57.18%（wood chips）,respectively.Carbon conversion rate during torrefied biomass gasification increased significantly,which may be related to the fact that torrefaction enhanced the pyrolytic properties of the biomass.With the increase of torrefaction temperature,the lower heating value of syngas gradually decreased,which was related to the decrease of CH₄ and C₂H_m（C₂H₆,C₂H₄,C₂H₂）content in syngas.Except carbon conversion rate,the effect of extending torrefaction residence time on the gasification was similar to the rise in torrefaction temperature.The change in carbon conversion rate was not regular during the gasification of biomass torrefied at different residence times.At 250°C,compared to the biomass torrefied for 10min,gasification of those which were torrefied for 120min had a betterη_vandμ_H.The increase ofη_v of rice husk and sawdust was 11.66%and 9.66%;the increase ofμ_H was12.90%and 13.13%,respectively.This showed that extending residence time can improve the hydrogen production effect of torrefied biomass gasification to a certain extent when the heat source temperature was low.The catalytic gasification experiments of torrefied biomass were carried out using Ni（NO₃）₂·6H₂O as catalyst.Using medium torrefied biomass,the optimal catalyst to torrefied biomass mass ratio（C/B）was determined.When C/B=0.3,α_H2 was maximum among of conditions;meanwhile,theη_v andμ_H of catalytic gasification of rice husk showed a decreasing trend;the increase ofη_v andμ_Hof catalytic gasification of pine sawdust became slower significantly.So,the optimal C/B was determined to be 0.3.Then,the effect of catalysts on biomass torrefied at different temperatures was explored.The addition of catalyst increasedα_H2,η_v,andμ_H to different degrees,and the increase rates were recorded asΔα_H2,Δη_v,andΔμ_H.With the increase of torrefaction temperature,the increase ratio ofα_H2 in syngas from catalytic gasification of rice husk(Δα_H2-TRH)totally showed a decreasing trend;the increase ratio ofα_H2 in syngas from catalytic gasification of pine sawdust(Δα_H2-TPS)decreased between 200℃-250℃as well as 280℃-300℃and increased between 250℃-280℃.It was speculated that this trend was related to the decrease of hemicellulose content and the increase of the sum of lignin and cellulose content.The decrease of hemicellulose content led to the decrease of Ni²⁺adsorption,while the increase of the sum of lignin and cellulose content had an opposite effect.For torrefied biomass,Δη_vandΔμ_H had a similar trend toΔα_H2.Extending the residence time had little improvement in the catalytic gasification of torrefied biomass.In addition,if the syngas was used as a fuel,it was not appropriate to use torrefaction pretreatment and nickel catalyst at the same time.