The Influence Of Boric Acid On Pyrolysis Behavior Of Wood

Biomass is the only renewable carbon resource that can be converted to replace conventional liquid petroleum fuels and chemicals.Thermochemical conversion technology can maximize the conversion of biomass into liquid fuel or chemical raw materials.The product has high energy density,high added value,and is easy to store and transport.Its essence is the pyrolysis reaction of biomass macromolecule.It is very important to study the pyrolysis characteristics of biomass,especially the composition,distribution and formation mechanism of biomass pyrolysis products under the action of catalyst,which is vital to realize the directional and controllable pyrolysis of biomass.Boric acid(BA),as a traditional flame retardant for wood materials,exhibits excellent flame retardant and smoke suppression capabilities.Most studies attribute it to the physical obstacle role of boron oxide in the molten glass state,while its chemical catalysis in the pyrolysis of woody biomass is rarely mentioned.In fact,boric acid,as an acid catalyst,has been proved to be an efficient catalyst in catalyzing sugar dehydration,alcohol-aldehyde condensation and other reactions.Therefore,the study on the pyrolysis behavior of woody biomass under the action of boric acid is not only beneficial to the exploration of the flame retardant mechanism of boric acid,but also can provide a theoretical basis for the realization of the controlled pyrolysis of woody biomass.In this paper,the effects of different pyrolysis conditions on the pyrolysis behavior of wood and its main components were studied.The effects of different pyrolysis temperature,catalytic method and the amount of boric acid and borax on the composition and distribution of pyrolysis products of wood flour(WF)with boric acid were studied by TG-DTG?Py-GC/MS and small fixed-bed pyrolyzer.The samples were characterized by FTIR,SEM,EDX and XED.Furthermore,glucose,xylose and chlorogenic acid were used as model compounds of cellulose,hemicellulose and lignin,respectively.The mechanism of boric acid on the main components of wood flour was further explored.The main results are as follows:Boric acid successfully infiltrated into wood cell cavity and deposited on the cell wall evenly via impregnation treatment.The addition of boric acid to some extent changed the surface morphology of wood fiber,reduced the crystallinity of cellulose,and formed B-O-C bond with wood.Boric acid changed the pyrolysis behavior of wood flour and significantly improved its thermal stability.The pyrolysis products of wood flour and its three components were complex and the content of each product was low,which is not easy to be separated and utilized.The composition and distribution of pyrolysis products of the BA-impregnated wood flour were obviously changed.Boric acid treatment had more influence on the distribution and composition of wood pyrolysis products than borax treatment.With the increase of the amount of boric acid,the pyrolysis products tended to be simple,and the content of acetic acid gradually increased.When the mass ratio of BA/WF was 2,the relative content of acetic acid was as high as 91.28%.In the presence of boric acid,the relative content of acetic acid and furfural in cellulose pyrolysis products was greatly improved.The peak area of acetic acid was 4.2 times that of untreated cellulose,and the relative content of furfural was up to 30%.Boric acid treatment increased the relative content of furfural in xylan and xylose pyrolysis products by 60.61%and 93.61%,respectively.Boric acid promoted the decomposition of lignin to phenol and acetic acid,and the peak area of phenol was about 6.2 times that of untreated lignin pyrolysis alone.The study of model compounds showed that boric acid,as a solid acid,can catalyze the decomposition of cellulose into monosaccharides,which further undergo dehydration and cyclization to produce small molecular compounds such as acetic acid,formic acid,and furfural.Boric acid promoted the depolymerization,ring-opening and dehydration reactions of xylan to produce a large amount of furfural and acetic acid.Boric acid catalyzed the pyrolysis of lignin,which aggravated the breakage of the phenylpropane side chain and the ether bond and C-C bond connecting the lignin structural unit,and maked lignin easier to produce phenol and acetic acid.The effect of boric acid on the pyrolysis behavior of woody biomass was obviously temperature dependent.When the temperature was lower than 300?,boric acid could increase the thermal decomposition temperature of biomass and improved its thermal stability,thereby playing a flame retardant effect.At temperatures higher than 350?,boric acid showed excellent chemical catalysis,which greatly simplified the composition of biomass pyrolysis products and maked acetic acid the main liquid product.It provided a promising method for the production of acetic acid from woody biomass pyrolysis.