| Agricultural and forestry waste is characterized by their high oxygen content,high moisture content,low calorific value,hygroscopic nature,and low bulk density,which make it necessary to pretreat the agricultural and forestry waste before the thermo-chemical utilization.Torrefaction is a biomass pretreatment technology with mild conditions and simple operation.The fuel properties of the biomass could be improved by torrefaction,which makes it more proper for thermal chemical conversion utilization.However,the deoxygenation and upgrade performance of biomass under mild conditions（200-300℃）of torrefaction are limited.It is necessary to optimize or improve the traditional torrefaction method.A gas-pressurized torrefaction（GP）method was proposed in our previous study to enhance deoxygenation during torrefaction.And the effectiveness and feasibility of the method and the gasification characteristics of the biomass torrefied by the GP were preliminarily studied and verified.In order to improve the research on GP torrefaction methods and promote the development and industrial application of GP torrefaction,the research was conducted in present work based on three main parts of the GP torrefaction method（the reaction process,the characteristics of the GP torrefied biomass,and the efficient utilization method of GP torrefied biomass）.The key scientific issues involved in this work are:（1）The influence of pressure on GP torrefied biomass properties needs to be studied.（2）The reaction mechanism of GP torrefaction is still unknown.（3）The kinetics study of the GP torrefaction process is not clear.（4）Appropriate subsequent thermal chemical conversion based on the characteristics of GP torrefied biomass need to be explored.To solve the above key scientific issues,in-depth researches were conducted in this study:（1）The yield variation and elemental composition,structural properties,and thermal decomposition behavior,combustion properties of the torrefied biomass were investigated in detail.The effect of pressure on the properties of torrefied biomass was obtained.The oxygen content in GP torrefied rice straw was as low as 22.78%,and the HHV（higher heating value）was 27.35 MJ/kg.Efficient decomposition of cellulose and removal of oxygen in biomass was achieved at a mild condition of 1.3 MPa and 250℃.The effect of deoxygenation and quality upgrading would be further enhanced with the increase of pressure.（2）A semi-closed（SC）torrefaction was conducted.The characteristic of the torrefied biomass of traditional（AP）torrefaction,SC torrefaction and GP torrefaction were compared to quantify the role of secondary reactions and pressure.The contribution index of secondary reaction and pressure on key parameters of torrefied biomass was obtained.The pressure enhanced the upgrading of biomass at around 200℃.The contribution index of pressure on mass loss,HHV enhancement,oxygen removal,and cellulose reduction of the GP torrefaction at 200℃was as high as 63.69%,81.86%,63.87%and 93.26%,respectively.At250℃,the contribution index of pressure on mass loss,HHV enhancement,oxygen removal,and cellulose reduction was in the range of 30%-60%,while the contribution index of the secondary reaction was in the range of 40-70%.The advantage of GP torrefaction was caused by the synergistic effect of pressure and secondary reactions.（3）Based on the systematic experiments and the preliminary mechanism analysis,the reaction models of AP torrefaction,SC torrefaction,and GP torrefaction processes were established.The modeling results fitted well with the experimental results.The quality of the proposed models is as high as 97.05%,99.08%and 98.96%,respectively.Significant gas to liquid conversion occurred during the GP torrefaction process with a reaction rate constant of 0.0952 min-1.The secondary reaction mainly contributed significantly to the conversion of liquid to solid.The conversion of solid to gas and liquid to gas was enhanced by the secondary reaction.The pressure leads to a significant conversion of gas to liquid in addition to enhancing the reaction rate of the conversion of liquid to gas.（4）The reaction mechanism of the GP torrefaction process was proposed and modeled through an in-depth analysis of the changes in the yield,fuel properties,chemical structure and thermal decomposition properties of the torrefied biomass at different time points during the GP torrefaction.During the GP torrefaction,oxygen in the biomass was mainly removed through deoxygenation and depolymerization of hemicellulose and cellulose,while most of the carbon remained in biomass.Volatiles and solid intermediates were produced at the beginning of torrefaction.The volatiles decomposition or reaction with intermediates,which further contributes to the deoxygenation of the biomass as well as to the transfer of the carbon from the volatiles to the torrefied biomass.In addition,the intermediates are further polymerized and aromatized to form the final GP torrefied biomass.Secondary reactions mainly occur during the secondary decomposition of volatiles and the interaction between volatiles and intermediates,while the effect of pressure promotes the aromatization of intermediates in addition to the primary decomposition and secondary reactions of biomass.（5）The thermal chemical utilization of co-gasification of GP torrefied biomass with coal was proposed based on the characteristics of torrefied biomass.The synergistic effect of co-gasification,syngas yield and composition were explored.In addition,a kinetic study of the co-gasification process was conducted.It was confirmed that the GP torrefaction can enhance the synergistic effects during the co-gasification process,increase the carbon conversion of coal by 6.12%.The kinetic analysis showed that the GP torrefaction could significantly promote the reactivity of the co-gasification process,reduce the activation energy and improve the frequency factor of co-gasification.This study further confirms the feasibility of GP torrefaction from the perspective of theoretical foundations and practical application based on the investigation of the mechanism of GP torrefaction and the thermal utilization of GP torrefied biomass.This work laied the foundation for the development of the GP torrefaction method. |
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