Research On Upgrading Biomass Fuels Via Pressurized Torrefaction

Biomass features low sulfur and nitrogen contents,highly volatile matter,and carbon-neutral characteristics.Its large-scale application as an energy source plays an important role in promoting the realization of "carboncrest and carbon neutralization" in China.However,utilization of raw biomass as power fuel presents several limitations,such as low energy density,poor grindability and strong water absorption.Torrefaction is considered as an effective way to upgrading biomass fuels.Gas pressurized torrefaction and wet torrefaction have become a new research hotspot because of their low torrefaction temperature and superior fuel properties of torrefied biomass.This study based on the pressurized torrefaction,the upgrading effects of dry torrefaction at atmospheric pressure and gas-pressurized torrefaction on biomass fuel were compared under same torrefaction temperature and same mass yield.Then the effects of pressurized torrefaction on biomass fuel properties and the thermal conversion behavior were investigated.In addition,a combined pretreatment was proposed according to the characteristics of wet torrefaction.And the effects of different washing agents on the removal of alkali metals in biomass were compared.Then the thermal conversion characteristics of biomass after wet torrefaction and combined pretreatment were explored.Poplar sawdust(PS)and corn stalk(CS)were torrefied in a horizontal quartz tube reactor and batch autoclave reactor at atmospheric pressure(AP)and 5 MPa gas pressure(GP),respectively.The effects of AP and GP torrefaction on the physicochemical properties and combustion characteristics of biomass were studied.The result showed that the presence of gas pressure during torrefaction affects the physicochemical and combustion characteristics significantly,compared with AP torrefaction,the GP torrefied biomass has higher energy density.the oxygen-containing functional group and crystallinity decreased after GP torrefaction,and the surface morphology notably changed.Compared with PS and PSAP,the maximum weight loss rate of PSGP decreases slightly,and the temperature corresponding to the peak value changes within a small range.However,the maximum weight loss rate of CSGP has dropped significantly,and the corresponding peak temperature appears at 400℃,which is nearly 100℃ higher than CS and CSAP.Compared with CS,the combustion characteristics of CSAP have been improved.While CSGP decomposes a large amount of volatiles during the torrefaction process.so the ignition stability index is reduced to 0.3534×104.while the ignition stability index and comprehensive combustion characteristics of PSGP and PSAP increased by 0.3%.22.9%,20.5%.and 26.2%compared with its raw form.respectively.The average activation energy values of PSGP and CSGP are 101.2 and 76 kJ/mol.respectively.which are 24.98%and 47.80%lower than the original samples.Corn stalks were torrefied under atmospheric pressure and gas pressurized condition at different temperatures.Results showed that with the increase of temperature,the deoxidation efficiency and energy density of corn stalk increased for both AP and GP torrefaction.At the same temperature,deoxidation efficiency of GP torrefaction was much higher than that of AP torrefaction.At the same mass yield,the required temperature of GP torrefaction was about 40℃ lower than that of AP torrefaction.The energy yield,carbon yield,deoxidation efficiency and energy density of GP torrefied biomass are 1.125,1.142,1.539 and 1.131 times that of AP torrefied biomass,respectively,while the equilibrium water content is only 73.9%that of AP torrefaction.It can be seen that GP torrefaction has a better performance than AP torrefaction on biomass upgrading.In the structure of carbon,the graphitization degree of the GP torrefied CS was higher,and the proportion of macrocyclic aromatic hydrocarbons increased.In the fixed bed reactor pyrolysis experiments,it was found that the content of H2 and CH4 in the pyrolysis gas increased after GP torrefaction,250-GP reached the maximum value of 14.06%and 40.02%.The relative content of phenols in the pyrolysis liquid products increased,up to 51.11%,and the content of furans and acids decreased.Aqueous phase product from wet torrefaction gets a better performance than water in the removing of AAMEs and ash,and the removal efficiency increased with the increase of torrefaction temperature.The highest removal efficiency of K,Ca,Na,Mg and ash reached 95.00%,60.38%,100.00%.73.08%and 51.30%in the sample of L260CS,respectively.The liquid and gas yield increased as the AAEMs removed in the washing process,the contents of phenols and sugars increased while the acids decreased,and the CH4 in the gas products increased significantly.On the TGA curve,the Tp shifted to the high temperature region,and the wf decreased due to the decrease of AAEMs.With the increase of torrefaction temperature,the content of phenols in the liquid phase products increases while the content of acids decreases,and the content of sugars increased first and then decreased,reached the maximum value of 4.23%after torrefied at 180℃.The CO content in the pyrolysis gas product decreased while the CH4 content increased.Compared samples with only wet torrefaction.the combined pretreatment can further increase the yield of liquid and gas products,reducing the yield of char,and significantly increasing the content of phenols in the liquid products.