Mechanism Research On Pyrolysis And Ignition Of Holocellulose Model Compounds

Biomass accounts for a large proportion of the combustibles in forest fires and building fires.Therefore,biomass combustion has attracted extensive attention in the fire field.To understand,predict and prevent combustion in essence,it is necessary to study its chemical reaction mechanism in detail.In the process of fire spread,the unburned area is heated and biomass pyrolysis occurs.After that,the released combustible gas is oxidized.Ignition occurs when the concentration of OH radical accumulates to a certain extent.The pyrolysis and ignition of biomass persist in the combustion process.Biomass is mainly composed of hemicellulose,cellulose,and lignin.Holocellulose is a general term for hemicellulose and cellulose.Its pyrolysis and ignition characteristics can partly reflect the behavior and law of the biomass combustion process.However,due to its complex composition and structure,it is very difficult to explore the reaction mechanism directly through experimental analysis.The commonly used methods are used to study its monomers,dimers,oligomers,and other components.At present,the research on the pyrolysis and ignition mechanism of holocellulose is not systematic and comprehensive.Based on this,this paper mainly carries out the following three aspects:(1)In this study,arabinose was selected as the model compound of hemicellulose.The pyrolysis experiments were conducted by combined thermogravimetry-Fourier infrared spectroscopy-gas chromatography-mass spectrometry(TG-FTIR-GC-MS)technique.The evolution of functional groups was observed.Experimental results indicate that the main gas products are H2O,CO2,and alkanes and the main macromolecular products are furans.In addition,density functional theory(DFT)calculations were utilized to reveal the formation mechanisms and pathways of the major products(soot composition)from arabinose pyrolysis.Initially,compared with ring-condensation and dehydration reactions,arabinose prefers to undergo a ring-opening reaction to form acyclic D-arabinose,which is the primary source of each product.Furfural is the most abundant furan derivative.The ring-condensation first of arabinose and the isomerization first of D-arabinose are the favorable paths to form it in terms of energy.The C1-C2 bond scission of D-arabinose is more favorable to form furan and formic acid.The dehydration mechanism first of arabinose is the dominant path to generate 2,7-dioxabicyclo[2.2.1]heptan-5-one.The formation of acetic acid is kinetically and thermodynamically favorable.Once it is formed,it is easy to decompose into CO2 and CH4.(2)Glucose,cellobiose,and cellulose were selected as the research objects,and the corresponding relationship between their mass loss and volatiles during pyrolysis was revealed by the combined TG-FTIR-GC-MS technique.The pyrolysis processes of glucose and cellobiose are similar,and both have two obvious weight loss peaks.Cellulose has only one obvious weight loss peak due to the influence of chain length and hydrogen bond.In the severe weight loss stage,cellulose has the same kinds of pyrolysis products as cellobiose and glucose,but it forms more H2O,CO2,alkanes,and carbonyl compounds.According to the detection results of GC-MS,glucose was selected as the model compound of cellulose to carry out DFT calculation.The possible paths of glucose decomposition to form various main products(toxic and harmful substances in fires)were proposed,and the competitiveness of each product formation path was analyzed.Based on the concerted reaction mechanism,glucose is easier to pyrolyze to furan,followed by furfural and levoglucosenone,and finally 1,4:3,6-dianhydro-α-D-glucopyranose.Based on the free radical reaction mechanism,glucose is more easily pyrolyzed to 1-(2-furanyl)-ethanone,followed by 5-methyl-2-furancarboxaldehyde,and finally 2-methyl-furan.(3)Based on the above pyrolysis experimental results,the main products can be classified into two categories:hydrocarbons(alkanes,aromatic hydrocarbons,etc.)and oxygenated compounds(furan derivatives,sugar derivatives,etc.).The potential energy surfaces of butyl+O2 and heptyl+O2(hydrocarbons)were systematically studied by ab initio calculation.Based on Rice-Ramsperger-Kassel-Marcus/Master Equation(RRKM/ME)theory,the rate constants were solved.It is found that the intramolecular H-transfer of RO2 directly affects the accumulation of OH radical concentration during the ignition process.In terms of energy,the intramolecular H-transfer reaction of RO2 through the transition states of six-membered and seven-membered rings is the most favorable.In addition,the rate of intramolecular H-transfer reaction of RO2 and the number of isomers formed increase with the increase of the chain length of linear alkanes.Next,the calculated rate constants of butyl and O2 were verified by the modified JSR model.The simulation results of C4 products were in good agreement with the experimental data.Finally,based on the reaction mechanism of n-butane and n-heptane,the low-temperature oxidation kinetic model of furan derivatives(oxygen-containing compounds)was constructed.The results indicate that low-temperature oxidation is a chain reaction process,and the formation of peroxides is very important.In addition,it is also found that furan derivatives can form soot precursors.During the low-temperature oxidation of 2-methyl-furan and furfural,C4 species play a major role in the formation of intermediates(ie,soot precursors)to generate polycyclic aromatic hydrocarbons.Once the cyclopentadienyl radical is formed,its recombination can directly yield naphthalene without any additional intermediates.