Thermal Transport And Release Mechanism Of Volatile Organic Components During Biomass Pyrolysis

In recent years,the preparation of biochar by pyrolysis biomass and the application of biochar in removing pollutants from the environment has been a broad concern.However,various volatile organic compounds(VOCs)were released during pyrolysis,which was rarely discussed.Therefore,in this experiment,16 VOCs were selected as the VOCs released during the pyrolysis of tobacco,and the mechanisms of the release of the 16 VOCs by thermal migration were analyzed from kinetic and thermodynamic aspects.The release mechanism of VOCs from biomass pyrolysis was investigated.In this study,the release kinetics and release thermodynamics of VOCs were investigated to investigate the mechanism of release by heat migration.In the experimental part,two VOCs were selected from each of the eight categories:acids,ketones,esters,phenols,alcohols,aldehydes,heterocycles,and lactones,to explore the migration release mechanism.The migration-release kinetics and thermodynamics of these 16 VOCs were investigated under two heating methods:central and circumferential.The central heating mode was set at five temperature bands of 250℃,300℃,320℃,350℃ and 380℃,and the circumferential heating mode was set at five temperature bands of 200℃,220℃,230℃,250℃ and 280℃.The release kinetics of the 16 VOCs were fitted to both central and circumferential heating methods.The fitted models included a zero-level kinetic model,a pseudo-primary kinetic model,and a pseudo-secondary kinetic model.phenethyl ester,eugenol,L-menthol,linalool,p-anisaldehyde,2,3,5-trimethylpyrazine,2-methylpyrazine,γ-decalactone,and γundecalactone were more consistent with the pseudo primary release kinetic model,and guaiacol was more consistent with the pseudo secondary release kinetic model.In the circumferential heating mode,β-viologenone,phenethyl acetate,eugenol,guaiacol,γdecalactone,and γ-undecalactone were more consistent with the zero-level release kinetic model,and valeric acid,3-methylvaleric acid,menthone,benzyl acetate,L-menthol,linalool,p-anisaldehyde,phenylethylaldehyde,2,3,5-trimethylpyrazine,and 2methylpyrazine were more consistent with the pseudo-level release kinetic model.The cumulative release of 16 VOCs at each temperature band was compared.In the central heating mode,the release of acids was significantly higher than other classes at each temperature band as the number of samples increased.In the circumferential heating mode,the release of alcohols was initially highest with increasing temperature,followed by the highest release of acids.The activation energy Ea was obtained by fitting the Arrhenius equation to the kinetic fitted release rate k values of 16 VOCs under central and circumferential heating.The ease of release of two VOCs of the same class under both heating methods was analyzed and compared,among which valeric acid(circumferential),3-methylvaleric acid(circumferential),β-viologenone(central),menthone(circumferential),phenethyl acetate(center),eugenol(circumferential),L-menthol(circumferential),linalool(circumferential),p-anisaldehyde(circumferential),and phenylethyl aldehyde(circumferential)were more easily released.Benzyl acetate,guaiacol,2,3,5-trimethylpyrazine,2-methylpyrazine,ydecalactone,and y-undecalactone could not be compared because the activation energies could not be calculated.The order of release of the two VOCs of the same category under two heating methods was determined separately,in which,under the central heating method,valeric acid of acids,β-violone of ketones,phenethyl acetate of esters,linalool of alcohols,phenylethylaldehyde of aldehydes and 2,3,5-trimethylpyrazine of heterocyclics were released preferentially.Under the circumferential heating method,3-methylvaleric acid of acids,menthone of ketones,phenol of 3-methylpentanoic acid for acids,menthone for ketones,eugenol for phenols,linalool for alcohols,phenylethylaldehyde for aldehydes and y-decalactone for lactones were preferentially released under circumferential heating.The Gibbs free energy change ΔG was calculated for all 16 VOCs under both heating methods,indicating that VOC release is a non-spontaneous process.The entropy change ΔS was less than 0,indicating a tendency to maintain the equilibrium inside and outside the system during the release,except for the enthalpy change ΔH for menthone and phenylethylaldehyde with circumferential heating,which was less than 0.All of them were greater than 0,indicating that the release of most VOCs requires heat absorption.