Study On Hydrothermal Conversion Characteristics Of Biomass In CO₂ Atmosphere

Due to the rapid growth of the world economy,energy demand is constantly increasing.Fossil fuels such as oil,coal,and natural gas are not renewable,and the concentrated using of which in a short period of time produces large amounts of gases such as CO₂,NO_x,SO₂,and CH₄ emitted into the atmosphere,causing serious environmental pollution,leading to global warming and other phenomena,threatening the survival of human.There are many advantages of biomass,such as huge reserves,renewable,environmentally friendly,etc.In particular,it can achieve zero CO₂emissions and is considered to be a suitable substitute for fossil fuels.Among various biomass utilization technologies,hydrothermal treatment is widely concerned because the raw materials do not need to be dried,the reaction conditions are mild,the energy consumption is low,and the use of water as a solvent will not cause environmental pollution.In this paper,corn stalk is selected as raw material.By changing the reaction temperature and pressure,N₂ atmosphere is used as a control group to explore the effect of CO₂ as a reaction atmosphere on the process of biomass hydrothermal conversion,so as to find a new way of CO₂ utilization.First,the corn stalk was hydrothermally treated under two atmospheres of CO₂and N₂,and the effect of hydrothermal temperature（210℃,240℃,270℃,300℃,330℃）on the yields and characteristics of the tri-state products was studied.The results showed that the CO₂ atmosphere inhibited the repolymerization and char formation during the hydrothermal process,and the relative crystallinity of cellulose in the hydrochar was higher than that in the N₂ atmosphere,indicating that the hydrolysis of cellulose was suppressed.At the same time,more energy in biomass is transferred to the liquid product in the CO₂ atmosphere.The relative content of furfural in the liquid product at 210°C exceeds 50%.At 240°C,the yield of ketones is the highest,and high temperature is beneficial to the enrichment of phenolic products.Secondly,the hydrothermal experiments under different initial CO₂ pressures（1.5MPa,2.3MPa,3MPa,3.7MPa）and using acid-washed corn stalks were performed.Combined with the experimental results of model compounds（xylan and cellulose）,it was found that with the increase of pressure,more CO₂ was dissolved in water,which promoted the hydrolysis of hemicellulose and produced more gas products,but at the same time suppressed the hydrolysis of cellulose,which may be because the minerals in biomass react with CO₂,thereby weakening their catalytic effect.When the initial pressure increased to 3.7MPa,CO₂ showed the promotion of cellulose hydrolysis.In addition,CO₂ is conducive to the depolymerization of lignin to produce more phenolic compounds,and may be combined with lignin derivatives,thereby preventing lignin repolymerization.Since the CO₂ atmosphere promotes the hydrolysis of hemicellulose and lignin,it is possible to obtain a hydrochar rich in cellulose and in absence of alkali and alkaline earth metals,which favors the subsequent high-value utilization.Finally,the thermogravimetric analyzer and PY-GC/MS were used to study the pyrolysis weight loss process and pyrolysis products of the hdrochars obtained under different hydrothermal conditions.The pyrolysis of hydrochar is divided into three stages:drying,cellulose decomposition and lignin decomposition.With the increase of hydrothermal temperature to 270℃,the cellulose decomposition peak on the thermogravimetric DTG curve almost disappeared,indicating that the hydrochar no longer contains cellulose,which is consistent with the change of furan products detected by PY-GC/MS.With the increase of hydrothermal temperature and initial pressure,the activation energy of hydrochar pyrolysis continues to decrease,which is mainly due to the suppression of lignin repolymerization in the hydrothermal process.The pyrolysis of low-temperature hydrochar can produce sugars,and the pyrolysis of high-temperature hydrochar can generate more hydrocarbons.In particular,the pyrolysis of hydrochar obtained from acid-washed corn stalks and treated in the CO₂atmosphere can produce higher yields of anhydrosugars,which is a new method of biomass utilization.