Study On Hydrothermal Pretreatment Of Biomass And Pyrolysis Characteristics Of Solid Products

As the only carbon-based renewable energy,biomass can be converted into useful fuels or high value-added chemicals by thermochemical methods,and it is considered to be the most likely alternative to traditional fossil energy sources.However,the current biomass utilization of thermal conversion method to produce fuel or chemicals still has many problems.For example,because the biomass raw material contains the characteristics of complex structure,high water content,high oxygen content,low calorific value and small bulk density,it makes the bio-oil lower fuel quality,more complex composition and more difficult to separate.Therefore,it is of great significance to enhance the fuel properties of biomass,improve the quality of bio-oil and increase the degree of product enrichment to take a certain pretreatment process before using biomass directly in thermochemical devices.In this paper,the effects of hydrothermal pretreatment on the physical and chemical properties of several typical biomass and the effect of hydrothermal pretreatment on the pyrolysis of biomass were investigated,which provided ideas for the subsequent and efficient utilization of biomass.The effects of different biomass types,different reaction parameters(reaction temperature,holding time and solid-liquid ratio)on the formation of gas,liquid and solid products under low temperature and hydrothermal conditions were investigated.The study found that hydrothermal pretreatment increased the C content of raw materials,reduced the O content,improved the calorific value and had a significant de-ashing effect.Under different conditions,the primary liquid product was furfural,and there were a few kinds of phenols.The gas products were mainly CO2 and the hydrothermal pretreatment of biomass was mainly composed of hemicellulose degradation and accompanied by a small amount of lignin.When the holding time was 120 min,the degree of hydrothermal degradation of biomass was deepened,the secondary cracking product was obviously increased,and the reaction temperature had amost significant effect on the yield of solid product.Then the physical and chemical properties of corn stalk and sawdust before and after hydrothermal pretreatment were tested and analyzed.It was found that the removal of alkali and alkaline earth metal elements in corn stalk and wood chips was significant,and the content of alkali and alkaline earth metal elements decreased by more than 96% at 180 ? hydrothermal temperature.The crystal structure of cellulose was well preserved in the temperature range of 160-220 ?,and the relative crystallinity increased with the increase of temperature.Hydrothermal treatment can increase the specific surface area of corn stalk and sawdust,pore volume,but the degree of change is small.At the same time,it was found that microwave torrefaction had the greatest advantage in improving the chemical composition and improving the fuel characteristics of cotton stalk among three different torrefaction pretreatments.Wet torrefaction(ie,hydrothermal pretreatment)had the most significant effect of de-ashing and the most moderate reaction conditions.While conventional torrefaction can achieve the highest energy yield at the same mass yieldFinally,The pyrolysis kinetics and product properties from rapid pyrolysis of corn stalk and wood chips before and after hydrothermal pretreatment were studied.It was found that hydrothermal pretreatment enhanced the activation energy of pyrolysis reaction of biomass.The maximum weight loss rate increased with the increase of the temperature of the hydrothermal pretreatment process,reached the highest point at 180 ?,then decreased,and the temperature of the maximum weight loss rate increased.The thermogravimetric peaks changed narrower and narrower.The degree of degradation of hemicellulose was enhanced with the increase of hydrothermal temperature.The yield of pyrolysis oil increased first and then decreases with the increase of hydrothermal pretreatment temperature and reaches the maximum at 180 ?.The pyrolysis gas product was mainly composed of CO,CO2,H2 and CH4,in which the relative content of CO was the largest,and the distribution of the rapid pyrolysis liquid product varied with the hydrothermal pretreatment temperature,including aldehydes,ketones,acids,phenols,dehydrated sugars and its derivatives,etc.,With the increase of hydrothermal pretreatment temperature,acetic acid was not detected at 200 ?,and the relative content of dehydrated sugar and its derivatives gradually increased.For corn stalk and wood chips,the maximum content of dehydrated sugar and its derivatives was 51.26% and 72.8% at 220 ? and 220 ?,respectively.The main dehydrated sugars and derivatives included ?-D-allose,levoglucan and 3,4-altrosan.It can be seen that the hydrothermal pretreatment has a significant effect on the regulation of biomass pyrolysis products and the enrichment of dehydrated sugar and its derivatives,which is possible for grading use of biomass.