Experimental Study On Cellulose Pyrolysis Mechanism Based On Wire-mesh Reactor And Py-GC/MS

Biomass energy is an important development direction of clean energy in the future.Developing biomass pyrolysis technology for bio-oil has strategic significance to our country.Cellulose is the most abundant content in biomass and its pyrolysis characteristics reflect pyrolysis mechanism of biomass to a great extent.As a result,revealing cellulose pyrolysis mechanism is helpful to improve the quality of bio-oil directly and effectively,which could reduce subsequent costs in refinery.However,there are still a lot of controversy about its pyrolysis mechanism at present,such as the composition of active cellulose,cleavage mode of the glycosidic bond,model compound for the molecular dynamic simulation of cellulose pyrolysis reaction,and the effect of cellulose reducing end on its pyrolysis characteristics is still ambiguous.Therefore,based on the wire-mesh reactor and Py-GC/MS etc.,related research was conducted to study the pyrolysis mechanism of cellulose from four aspects:active cellulose composition,pyrolysis kinetics,model compound,the effect of cellulose reducing ends on its pyrolysis process.First of all,formation characteristics of active cellulose from fast pyrolysis of cellulose were investigated in the wire-mesh reactor which minimized the secondary reaction during pyrolysis,and the effect of free radical inhibitor on the yield of anhydro-sugars was investigated as well.It was found that the cellulose chain begun to depolymerize to form active cellulose by random cleavage at about 240 ?,and main composition of active cellulose was anhydro-sugars with various degrees of polymerization?DPs?.Through the analysis of elemental composition and surface functional groups of original cellulose and cellulose chars obtained at different pyrolysis temperatures,it was found that obvious intramolecular dehydration occurred during fast pyrolysis of cellulose in the wire-mesh reactor.The bio-oil from fast pyrolysis of cellulose pyrolysis contained anhydro-sugars?levoglucosan,cellobiosan,cellotriosan,furan isomer of levoglucosan?,furans?furfural,5-hydroxymethyl furfural?,partially decomposed sugar-ring-containing oligomers?PDSRCOs?,among which the compound with a relative molecular weight of 222 was supposed to the product from reverse aldolisation reaction of cellulose sugar ring.Addition of free radical inhibitor to cellulose significantly reduced the yield of levoglucosan,suggesting the depolymerization of cellulose chain was via free radical reaction.Secondly,weight loss and gaseous products formation characterisitcs were investigated by employing thermogravimetric and thermogravimetric-mass spectrometry to reveal cellulose pyrolysis behaviour under slow heating rate.The results has shown that slow heating rate was more conducive to the formation of char,and with the rising of heating rate,apparent activation energy and pre-exponential factor were lower.Light oxygenates generated from pyrolysis of cellulose were mainly composed of H₂O,CO,CO₂,CH₄.Pre-treatment of cellulose under low temperature?250 ?,30-60 min?promoted intermolecular dehydration.Based on the experimental results,a new model of cellulose pyrolysis is presented in this paper,and the kinetic parameters of pyrolysis in the new model are obtained by numerical calculation.Thirdly,product distributions from Py-GC/MS pyrolysis of three kinds of active cellulose component?cellobiosan,cellotriosan,cellotetraosan?and four kinds of cellulose model compounds?glucose,cellobiose,cellotriose,a-cyclodextrin?reported previously were compared with that from cellulose pyrolysis to choose the most suitable model compound.The experimental results showed that cellotriosan was the most suitable model compound of cellulose,followed by a-cyclodextrin,while reducing sugar like glucose,cellobiose,cellotriose was not suitable.Based on the molecular dynamic simulation of cellotriosan pyrolysis,it was found that pyrolysis process of cellotriosan could be divided into three stages:low-temperature dehydration,depolymerization of glycosidic bond and cracking of glucose unit,cracking of levoglusoan unit.Little amount of hydroxyl radicals was formed during low-temperature dehydration stage,and precursors of levoglucosan and many small molecules were produced during the second stage,while the levoglusoan unit was cracked into small molecules during the third stage.Finally,the effect of cellulose reducing ends on the pyrolysis characteristics was carried out.Although reducing end content in cellulose was relatively low,it maked cellulose decompose at lower temperature and facilitated the formation of char,light oxygenates and furans.Reducing the content of the reducing-ends could improve the thermal stability of cellulose and increase the yield of levoglucosan.After thorough analysis of the experimental results,it seemed that the influence of reducing ends on cellulose pyrolysis was because acids produced by reducing ends decomposition during pyrolysis entered into the liquid active cellulose and subsequently catalyzed the decomposition of anhydro-sugars generated by cellulose depolymerization.