Effects Of Different Torrefaction Methods On Combustion Process And Char Characteristics Of Biomass

Torrefaction is a effective pretreatment methods which serves to improve the properties of biomass in relation to direct firing or co-firing with coal for electric generation.This implies,in turn,that the high heating value,grindability,biodegradability,volumetric energy density and ignition characteristics increase with the torrefaction severity,which reduce the cost of transportation,storage and boiler operation.Apart from traditional torrefaction,the newly emerging technologies such as wet torrefaction,oxidative torrefaction and pressurized torrefaction have recently attracted great interest from academic groups and economic feasibility.Study of the influencing factors on bio-fuel characteristics and combustion process under industrial conditions will be of great importance for biomass electricity generation in the future.This study is a continuation of previous works on the influence of torrefaction on wood particle combustion,which aims to analyze the effects of reaction conditions such as flue gas temperature,oxygen concentration and gas velocity on the combustion characteristics of raw and traditionally torrefied wood in a single particle combustor（SPC）.The reactor is hereafter fixed at a definite combustion condition（i.e.1226±26℃,3.0±0.1vol%O₂,3m/s）,focusing on the effects of different torrefaction methods on the high-temperature combustion process and its char characteristics.By studying the combustion characteristics of wood particle with varying wet-torrefied severity,linear regressions were performed to fit ignition,devolatilization and char burnout time with correlation coefficients（R²）no less than 0.961,0.995 and 0.988,respectively.Furthermore,owing to the diminished AAEMs during the wet torrefaction,accumulated char yield decreases steadily from 8.2%to 2.8%and the T_max of char is shifted from 489℃ to 642℃,with the rise of torrefaction severity,which in turn contributed to the increased char burnout time.Taking into account the fuel properties,combustion behavior and high temperature char characteristics of oxidatively torrefied biomass,it is interesting to note that increased torrefaction oxygen content results in a higher carbon content and fix-carbon content,more cross-linked aromatic structure as well as less volatile content remained in the residue torrefied solid,which may affect the volatile release and enhance the secondary reactions in the subsequent pyrolysis and thereby lead to a denser and heavier char particle with similar reactivity.By using the least squares method,the SPC char yield（i.e.high heating rate char）and fixed carbon（i.e.slow heating rate char yield）can be described by a linear regression equation（R²=0.85 and 0.83）as a function of the decreased mass yield,respectively,indicating that the torrefaction severity is a good indicator of the changes in subsequent pyrolysis char yield.Finally,the chemical structural features of SPC char produced from pressurized-torrefied solid were examined with Raman and XPS spectroscopy,which were further correlated with char characteristics.The overall results indicate that the relative proportion of larger（>6 rings）aromatic ring systems and smaller ones and cross-linking density increase with the higher char yield.However,the surface groups were investigated by XPS technique in relation to C-containing and O-containing functional groups,which have no clear relationship with char yield and char reactivity,respectively.