Study On The Mechanism Of Biomass Low-temperature Deoxygenation And Its Effect On Pyrolysis

Biomass is the only renewable energy that can be converted into liquid fuels and the only renewable source as sustainable carbon carrier to substitute for fossil fuel,which has good compatibility with fossil fuels.However,the high moisture content,high oxygen content,low energy density,poor grindability and fluidity of biomass limited its further application.Low-temperature deoxygenation pretreatment can remove the moisture and oxygen and improve the energy density and grindability.Given this,deoxidization characteristics and its influence on biomass pyrolysis was studied,and then in-depth studied the mechanism of low-temperature pre-treatment process,especially focus on the deoxygenation mechanism.After a great deal of experimental research and theoretical analysis,the study can provide the theory instruction and the reliable scientific reference for the high efficient pretreatment and quality utilization of biomass.Firstly,the characteristics of low temperature deoxidization process and the precipitation behavior of the biomass components and natural products were studied by thermogravimetric analysis,fixed-bed low-temperature deoxidization device combined with portable infrared analyzer.The kinetic mechanism of biomass deoxidization at low temperature and the mechanism of oxygen-containing gas evolution were studied by means of thermal analysis kinetics.It was found that in the process of low temperature deoxidization,hemicellulose showed a losing weight(228~317 °C),O was precipitated in the form of H2 O and CO2,the deoxidation temperature of cellulose was higher(314~390 °C),the precipitation form of O was mainly CO and CO2,while lignin had less lossing weight during low temperature deoxidation,O was mainly precipitated as H2 O,corn stalk had similar reaction zone(220~385 °C)with cotton stalk,the precipitation of O was H2 O,CO and CO2 and other forms;during the low temperature deoxidation process,the reaction order of three components was 1,and for corn stalk and cotton stalk the reaction order was 2,the reaction activation of cellulose,hemicellulose,lignin,corn stalk and cotton stalk were 238.99,77.19,28.42,109.27,120.91 kJ·mol-1,respectively.Then the evolution process of biomass microstructure during low temperature deoxidation was studied by Fourier transform infrared spectroscopy(FTIR)combined with generalized two-dimensional correlation spectroscopy(2D-COS),and the mechanism of low temperature deoxidation of biomass was revealed.It was found that the removal of C-O functional groups and OH from the hemicellulose was initiated at 230 °C in the process of low temperature deoxidation,and the reaction was completed at 320 °C.After 290 °C,the OH and C-O functional groups disappeared and a large amount of C = C and C = O functional groups were generated in cellulose deoxidation.The lignin has high OH,C-O and C = C functional groups.With the increase of treatment temperature,the oxygen-containing functional groups of lignin were slowly cleaved and removed.The removal of OH was mainly occurred at 290 °C in cotton stalks,while the decarbonylation occurred in corn stalks.At the same time,the effects of low temperature deoxidation on the physico-chemical properties of the solid products of the fixed-bed low-temperature deoxidization system were studied by element,industry and water absorption analysis.It was found that the main deoxygenation temperature of hemicellulose was 230~260 ?C,and 64.48% oxygen was removed,for cellulose the range was 290~320 °C,46.32% oxygen was removed.The deoxidation effect of lignin was not obvious,only 14.80% oxygen was removed at 390 °C.The main deoxidization interval of corn straw and cotton stalk were 260-390 °C and 260~350 °C,respectively,the removal of oxygen were 65.49% and 55.10%.With the increase of deoxygenation temperature,the trend of rising calorific value was also obvious,the caloric values of cellulose,hemicellulose,lignin,cornstalk and cotton stalk were increased by 47.09%,58.71%,10.58%,31.88% and 36.39% respectively after 390 °C treatment.The low temperature deoxidization treatment decreased the water absorption and was conducive to the storage and transportation of biomass.Then,the rapid pyrolysis and liquefaction characteristics of deoxidized biomass were studied by fast cracking analyzer,GC-MS and fixed-bed rapid pyrolysis reaction platform.The effects of low-temperature deoxidation on the composition of bio-oil were mainly analyzed.The results showed that lower temperature deoxidization(200~290 °C)only reduced the yield of biomass pyrolysis oil,higher temperature deoxidization(290~390 °C)also reduced the type and yield of oxygenated organic in pyrolysis oil,but improved the yield and selectivity of aromatics.Low-temperature deoxidization treatment is beneficial to reduce the coking amount of the catalyst,remove more oxygen-containing organic compounds,and the more the catalyst is used,the more the pyrolysis oil component becomes,the stronger of the aromatic selectivity.Finally,a pilot-scale rotary kiln reactor was built for low-temperature deoxidation which were based on the previous research results.Simulated low-temperature deoxidation experiment under inert atmosphere and the flue gas atmosphere were carried out.The physical and chemical properties of the solid products were investigated.Water absorption,and combustion properties of the samples were analyzed.The results show that the temperature is the main factor affecting the deoxidation characteristics at low temperature range from 200 °C to 260 °C.When the temperature is higher than 260 °C,the atmosphere become the main factor.Low-temperature deoxidation reduced the energy consumption and water absorption of the biomass,and the effect was more obvious under the simulated atmosphere.The product had similar combustion properties of lignite under low temperature deoxidization in flue gas atmosphere,but the treatment temperature should not exceed 260 °C.