TG-FTIR Study on Corn Straw Pyrolysis-influence of Minerals

Available online at www.sciencedirect.comSCIENCEdoinnor.CHEM. RES. CHINESE U.2006 ,22( 4),524- -532TG-FTIR Study on Corn Straw Pyrolysis. influence of Minerals'YANG Chang-yan'2** ,LU Xue-song' ,LIN Wei-gang' . ,YANG Xue-ming' and YAO Jian-zhong'1. Multiphase Reaction Lab , Institute of Process Engineering , Chinese Academy of Sciences ,Beijing 100080 ,P. R. China ;2. Graduate School of the Chinese Academy of Sciences , Bejing 100039 ,P. R. ChinaReceived April 5 , 2005In order to study the effect of minerals on biomass pyrolysis , experiments on pyrolysis of corn straw with differentpretreatment methods were performed by using a thermogravimetric analyze( TGA ) coupled with a Fourier transforminfrared( FTIR ) spectrometer. The pretreatment methods included water washing and acid washing. The experimentalresults show that acid washing can remove almost all K* and 78% of Cat+ , while water washing only removes most ofK+. 'The existence of K + and Ca2+ obviously favors the formation of compounds containing carbonyI groups and CO2 ,but it will decrease the yields of compounds containing C- -0- -C groups. However , the formation of H2O , CO andCH4 are slightly affected by the removal of inorganic ions. With regard to the structure of the metal ions-adsorbed cel-lulose characterized by IR analysis , it can be considered that there is an”ion force" between metal ions and cellulos-ic biomass. The results of thermal kinetic analysis show that this force can make the reaction activation energy of thebiomass pyrolysis decrease. A new mechanism is proposed for explaining the efct of inorganic ions on cellulose py-rolysis.Keywords Biomass ; TG-FTIR ; Pretreatment ; Minerals ; Pyrolysis mechanismArticle ID 1005 -9040( 2006 )-04-524-09Introductioncellulose pyrolysis , and phenols of lignin pyrolyis7I.Recently , concern about emission of greenhouseThe pyrolysis behavior of lignocellulosic biomassgases from combustion of fossil fuels has prompted a re-and cellulose is significantly influenced by ash andnewable interest in the combustion of biofuels due toexisted inorganic ions，Inorganic ions favor ththeir CO2 neutrality 121. Pyrolysis of biomass( celluloseformation of gas , char , and hydroxyacetaldehyde butand lignocellulose materials , etc. ) is of great interestthey cause the total liquid yield to decrease and inhibitin the production of chemicals and fuel5'3- 6].the formation of levoglucosan.There have been many mechanisms proposed forBiomass is comprised of hemicellulose , cellulose,lignin ,a lttle of extract and ash. These componentsthe pyrolysis of cellulose or lignocellulosic bioresult in different pyrolysis products because of differentmass'[9,12 13]to interpret the above phenomena , of whichmechanisms of thermal decomposition. The volatiletwo are important. One is Shafizadeh's mechanism'products of cellulose pyrolysis are mainly comprised ofin which levoglucosan is first produced in cellulose pylevoglucosan，hydroxyacetaldehyde ，acetol ( Schemerolysis , yielding glucose , and then glucose undergoes a1 ) ,etc. Acetic acid and furfural are products of hemi-C2-C3 cleavage to form hydroxylacetaldehyde( Glycol-aldehyde ) ; the other is Richard' s mechanism that the0、HO-HOOHhydroxyacetaldehyde formation must involve the diver-sion of reaction, channels prior to the formation of levo-LevoglucosanHydroxyacetaldehydeAcetolgluc中国煤化工Scheme 1 Structures of levoglucosan , hydroxyacetal-TY H. CN M H Gaim at the pyrolysis ofdehyde and acetol.biomass , and the pyrolysis mechanisms focus on the* Supported by the National High Technology Research and Development Program of China( No. 2003AA514023 ) and the Na-tional Basic Research Program of Chind( No. 2004CB719700 ).* *百商数据rrespondence should be addressed. E-mail : eyyang@ home. ipe. ac. cn .No. 4YANG Chang-yan et al.525formation paths of pyrolysis products , especially forExperimentalthose of levogancosan and hydroxylacetaldehyde. How-1 Materialsever , the mechanism of metal ions acting on biomassCorn straw was pretreated by washing with roompyrolysis is not clear. Little information is available re-temperature water ,60 C water and a0. 5% nitric acidgarding the relationship between the formation of CO2 ,solution. All the washing processes were performed byH20 and that of the primary pyrolytic products such asmeans of soaking and churning each sample of 1 g inlevoglucosan , hydroxyacetaldehyde and acids etc. inthe above washings( 100 mL for each washing ) forinterpreting the pyrolytic mechanism of cellulose loaded12 h. After washing , the corn straw samples were fil-with inorganic ions.tered ,and then dried at 105 C for 10 h. The acid-In order to clarify the relationship between thewashed corn straw sample was washed with distilled wa-yield of pyrolysis liquid and the existence of inorganicter until it was neutral , then filtered and dried at 105ons , and to understand the mechanism of the effect olC for 10 h. Before the experiments , the pretreatedinorganic ions on biomass pyrolysis , we investigated thecorn straw samples were finely ground into powder( lesseffect of pretreatment methods including water washingthan 100 meshes ) in order to minimize the heat-transferand acid washing on corn straw pyrolysis by using a resis-tance and decrease the influence of vapor-solidthermogravimetric analyzer coupled with a Fourierand secondary reactions. The properties of the materialstransform infrared analyzer.are listed in Tables 1 and 2.Table 1 Chemical compositions and proximate analyses of the samples with different pretreatment methodsComponent analysi( % , mass fraction )Proximate analysis( % , mass fraction )Corn straw sampleHono-celluloseLigninExtract *MoistureV olatile matterAshOriginal74. 9013.412.343.5870.585.77Room temperature water-washed7.4713.231.883.2777.134.1560 C aler-washed77.4314. 011.7278.283.810.5% HNO3 -washed79. 2512. 88.2.902.3780. 592. 60* Benzene-ethanol extractable( 2: 1 , volume ratio ).Table 2 Ultimate and K , Ca analyses of the samples with different pretreatment methodsUltimate analysis( % , mass fraction )Metal elementary analysis( % , mass fraction )Corm straw sampleCHN0K+Ca2+45.85.30.642.30. 630. 7044. 65.245.30.010. 5760 C Water-washed45. 15.40.445.20. 0345. 65.50.745. 50. 0020. 152 Carbon , Hydrogen and Nitrogen Contentsments were conducted in a region of 600- 4000 cm-.The contents of carbon , hydrogen and nitrogen in4 TG-FTIR Experimental Setupthe air-dried samples were determined by using an EAIThe TG-FTIR system was a computer-controlledCE440 elemental analyzer.facility( shown in Fig. 1 ) consisting of a Netzsch STAThe oxygen content was calculated as the differ-449C TG-DTA/ DSC thermogravimetric analyzer cou-ence between the total content and the sum of carbon ,pled with a Bruker Equindx 55 FTIR spectrometer andhydrogen and nitrogen. The Ca and K contents wereobtained by using a Dionex DX500 ICP ion chromato-graph and a Perkin-Elmer Aanalyst 200 spectrometer.3 FTIR Spectra of Solid Samples4The FTIR spectra of the samples were determined5with a Bruker Equindx 55 Fourier transform infraredanalyzer.中国煤化工_7Each of the dried samples was mixed with IRMHCNMHG工grade KBr( Aldrich ) and milled ，respectively. The↑Ventconcentration of each of the samples was 1 % ( massFig.1 TG-FTIR experimental system.fraction ) in the potassium bromide disk for IR analy-1. Helium cylinder ;2. mass flowmeter ;3. netzsch STA 449C ther-sis. The background spectrum of pure KBr was sub-mogravimetric analyzer ;4. crueible and sample ; 5. heat pipeline ;tracted frO万内数据mple' s spectra. The FTIR experi-6. gas cell ;7. bruker equindx 55 FTIR spectrometer ;8. computer.526CHEM. RES. CHINESE U.Vol. 22was employed to investigate the pyrolysis of corn straw.sample , the content of Ca2+ in the hot water-washedA helium sweep gas flow of 60 mL/ min( from the heli-sample is higher which may be caused by the dissolvingum gas cylinder ) was used to take the evolved volatileof saccharide and pectinti in corn straw that willgases from the TGA directly to the gas cell which wasmake the viscosity of the water solution increase andheated up to 200 C for IR analysis. Each FTIR spec-thus suppress the lixiviation of Ca2 +trum was obtained by scanning 32 times in 30 s. In allFig. 2 shows the FTIR spectra of the originalthe experiments , the samples were heated from 30 toroom temperature water-washed ，hot water-washed900 C at a heating rate of 40 C/min. The amount ofand 0.5%( in mass fraction ) HNO;-washed corn strawsample would not affect the thermogravimetric charac-samples , respectively. For all the samples , there is noteristics , but would influence the intensity of IR ab-change in the bands related to the pyronose ring stretc-sorption. Therefore , in order to reduce the influence tohing( 1060 ,1109 ,and 1163 cm-1 " .18] of cellulose ,IR spectra , the mass of each sample was( 8. 71士indicating that the rings are unmodified. The bands re-0.02 ) mg. Each experiment shows a very good repro-lated to the C- -H vibrations( 1430 , 1379 , 1315 , andducibility with respect to the mass loss and the absor-900 cm-' ) "] of the samples are not changed by acidbance of TG-FTIR spectrum of the sample.washing and water washing.Results and DiscussionChemical Composition and IR Analysis of CornStrawaThe chemical compositions and the proximateanalysis results of all the corn straw samples are shownwnbin Table 1. The contents of potassium( K+ ) and calci-um( Ca2+ ) , and the elemental analysis results of thesamples are listed in Table 2. Owing to the remo-val of1200 1600 2000 2400 2800 3200 3600 4000ash , the relative contents of cellulose and hemicellu-D/em-1lose increase by water washing. Because one kind ofFig.2 The FTIR spectra( 600- 4000 cm-1 ) of all thelignins of biomass is soluble in acid( named as acid-untreated and treated corn straw.soluble lignin ) , the relative content of lignin decreasesa. Original corm straw ;b. 0. 5%( mass fraction ) HNO3and the relative contents of cellulose and extract in-acid washed corn straw ic. 60 C hot water washed corncrease by acid washing. The chemical compositionsstraw ;d. room temperature water washed corm straw.and the proximate analysis results of all the samples areIn the processes of acid washing and water wash-similar but the contents of K* , Ca2+ and ash. Siliconing , the metal ions chemically bound to the carboxyl( Si ), an inert matter during the pyrolysis4] , is notgroups of uronic acid in biomass'are mostly re-taken into account here.moved , which results in a slight increase of the absor-The different pretreatment methods can removebance of the C= O vibrations( 1710 cm-' ) related theash and inorganic ions to different degrees. The remo-to carboxyl groups of uronic acid in the hemicelluloseval of ash can reach 55% by acid washing , and rangeof the pretreated samples. The hydroxyl groups of poly-from 25% to 35% by water washing. The removal ofsaccharides and lignin in the samples seem to be al-K* is over 95% by both water washing and acid wash-tered slightly by water washing and acid washing.ing , but the removal of Ca2+ only attains to 78% by2 Thermogravimetric Analysis of Corn Strawacid washing and almost zero by water washing. TheSamplesmetal ions are chemically bound to the carboxyl , hy-Fig. 3 shows the curves of thermogravimetric ana-droxyl groups of polysaccharides and lignin present inlysis( TGA ) and differential thermogravimetry( DTG )biomass 1516].for the original , room temperature water-washed ,60 9CAs the affinity of Ca2+ toward biomass is higherwate中国煤化工acid-washed corn strawthan that of K+[151 , the removal of Ca2+ is more diff-sam:YHCNMHG°C/ min , respectively.cult than that of K* . An acidic condition is favorableFrom the' IGA and DIU curves ，the pyrolysisto the removal of metal ion. Therefore , the percentagesprocess with temperature increasing can be divided intoof removal of metal ions by acid washing are higherfour stages : drying( 0- -150 9C ) , transition( 150- -200than those_ by water washing. Compared with the con-9C ) ,pyrolysis( 200- -600 C and carbonization( 600-tent of Ca5整气he room temperature water-washed900 C ). The thermal decomposition of all the washedNo. 4YANG Chang-yan et al.527100(A)mass fraction ) than the original corn straw sample80( 70% ). The contents of the residual char are differ-ent , which are 18. 6% for the acid-washed corn strawsample ,18. 7% for the 60 C water-washed corn straw40-sample ,19. 4% for the room temperature water-washedcorn straw sample , and 24. 6% for the original corn20 tstraw sample. This indicates that the removal of ash ,K+ and Ca2+ is in favor of devolatilization. The maxi-100200 300 400 500 600 700 800 900t/Cmum rate of thermal degradation increases after wash-5[(B)ing and its corresponding temperature is shifted from350 C to 360- -380 C due to the washing. It must beemphasized that the temperature with the maximum rate5-of thermal degradation by acid washing is lower thanthat by water washing , which may result from the de--25 tcrease in polymerization degree and crystallinity , andthe structure loosening of the straw by acid washing-35Hpretreatment 21 22].100 200 300 400 500600 700 800 9003 FTIR Analysis of the Evolved GasesFig. 3 The thermogravimetric analysis( A ) and dif-To understand and clarify the effect of pretreat-ferential thermogravimetric analysis( B ) char-ment on corn straw pyrolysis ，the evolved gases fromacteristics of original and pretreated corthe pyrolysis were investigated in this work , includingstraw samples at a heating rate of 40 C/ min.H2O ,CO , CO2 , CH4 , vapor of aldehydes , vapor ofOriginal straw ;-- --- room temperature washedcarboxylic acids and vapor compounds containingHNO3 washed straw.C-0- -C groups( for example , levoglucosan , etc. ).corn straw samples starts at approximate 220 C exceptBand assignments' 23- 27 J for the spectra of these evolvedthat of the original corn straw sample which starts atgases from corn straw pyrolysis are summarized in Ta-200 C.ble 3. These evolved gases are comprised of a numberIn the pyrolysis stage , the washed corn straw sam-of compounds with oxygen-containing functionalles have much higher volatile contents( 77%- -79% ,groups.Table 3 Infrared absorption band assignments for the compounds with oxygen-containingfunctional groups from pyrolysis of all corn straw samplesBand assignmentW avenumber/ cm-Wavenumber/ cm 1H203745 , 1508C= X( Aldehydes ,etc. )1700CH,3014 , 1306C- -0- -a( Ethers ,etc. )1183CO2181 ,2116Levoglucosan1183 1131 ,10502359 ,2342 , 669Glycoaldehyde860C=0( Aeids )1745To study the evolution characteristics of the aboveof H2O. The evolution curves of CO take on a broadgas products , the characteristic absorptions of them arepeak from 200 to 600 C in the pyrolysis stage and theused to describe the evolving process. The plots offurther depolymerization forms a peak at about 600-changes in absorbance values of the IR spectra of the800 C in the carbonization stage' 28291. The CO2 evo-identifed pyrolysis gases as a function of temperaturelution curves take on a much higher peak at aboutare shown in Fig. 4.340-中国煤化工-tage and a small peak atThere are two peaks at about 80-120 C andabouConization stages.340- 380 C in the evolution curves of H20( 3745TYHC N M H G the evolution curves ofcm ). The former corresponds to the evaporation ofCH4 that covers the pyrolysis and carbonization stages.water adsorbed in the samples , and the latter corre-The evolution curves of carboxylic acids ，aldehydessponds to the volatile water from pyrolysis. Theand compounds containing C- -0- -C groups show onlyevolution婀努数据of CO and CO2 is different from thata peak at about 340- -380 C in the pyrolysis stage.528CHEM. RES. CHINESE U.Vol. 220.10「0. 0303745 cm-12181 cm-10. 080. 025H2OCO0. 0200. 060. 0150. 040. 010 |2 0.020. 0050.00↓0. 000 6888.100200 300 400 500 600 700 800 900100 200 300 400 500 600 700 800 900l/C1/C3014 cm- -10.32359 cm-10.04CH4CO20.030.20. 020.10.010. 00.00k100200300400500 600 700 800 900100200300400500600 700 800 900t/C0.181183 cm-11745 cm-1; 0.152Compounds containingCarboxylic acids &0. 09C- -0- -C groupaldehydes专0.03wwwww0.0Fig. 4 The evolving characteristics of pyrolysis gases for all the corn straw samples at a heating rate of 40 C/ min.-一Original straw ; .... room temperature washed straw ; ....60 C water washed straw ; .... 5% NHO3 washed straw.Except for those of CO and CH4 , the curves of the e-can be used to represent the yields of evolved gases re-volved gases shift to higher temperatures by waterspectively.washing and acid washing.In the pyrolyis stage , the yields of acids and al-In the pyrolysis stage ，the removal of K * anddehydes increase by 17% - -22% by water washing,Cat+ affects much slightly on the evolution of H2O , CObut decrease with 26% by acid washing. The yield ofand CH4. However , the evolutions of CO2 , carboxylicCO2 decreases with 40% by water washing and byacids , aldehydes and compounds containing C- -0- -C89% with acid washing. However , the yields of com-groups are obviously affected by the pretreatment. Dur-pounds containing C- -0- -C groups including levoglu-ing the pyrolysis stage ranging from 200 to 600 C , the .cosan , etc. , increase by 15% with water washing andintegrated areas of the IR absorption peaks for severalby 41% with acid washing. The characteristic of theevolved gases such as H2O ,CO2，carboxylic acids andCO2 formation is similar to those of acids and alde-compounds containing C- -O- -C groups are shown inhydes , but contrary to those of compounds containingTable 4. Because the amounts of samples were theC- -0-C groups.same as in the TG-FTIR experiments , the integratedFrom the above results , it can be seen that theareas of the IR absorption peaks of the evolved gasesformation and changes of CO2 , compounds containingTable 4 Integrated areas of theR absorption p中国煤化工fromstraw pyrolysis in a range from 200 to 6CarborTYHCNMHGSampleH20C-0-C groupOriginal corn straw16. 1433. 26.55. 020. 91Room temperature water-washed cormn strw11. 6138. 8433.0221. 3160 C Water-washed corm straw13.4840. 4735.8124. 020.5% HN西方数据corm straw12. 62.24. 716.2129. 46No. 4YANG Chang-yan et al.529C= 0 groups and compounds containing C- -0- -C .(A)(B).0groups are related to the existence of metal ions.4 Mechanism of K* , Ca2+ Action on Biomass Py-QHyoHrolysisCe -OHCs 004.1 Adsorption Structure of lonsAs mentioned in section 3 , metal ions are chemi-cally bound to carboxyl and hydroxyl groups ,etc. , inaKbiomass. W hether metal ions exist in biomass originallyor they are adsorbed to biomass later is not importantthis is because the styles of action between metal ionsFig.6 The adsorption styles of Ca2+ to cellulose.and biomass are similar 30 . Therefore , the structure ofed. This results in an increase in the band lengths and5% Ca2+ -adsorbed cellulose was investigated in thisa decrease in the band energies. Therefore , metalwork by immerging pure cellulose( from Sigma ) into aions-loaded cellulose displays a thermal instability andCaCl2 solution. The absorbances of the C- -O asymme-the pyronose rings tend to break down during thermaltric stretching vibration( 1060 ,1109 ,and 1163 cm -1 )decomposition' 36The effect is called' ion force'and those of the C- -C stretching vibration( 1379 andwhich also exists in coal pyrolysis 37,38.750 cm -1 ) of pure cellulose decrease with the adsorp-4.2 Analysis of Pyrolysis Kineticstion of Ca2+ to biomass , which is confirmed by the IRIn order to a better understand the effect of ionspectra of the original and acid-washed corn straw sam-force' of metal ions on the pyrolysis of biomass , theples as shown in Fig. 5. This illuminates that the struc-kinetics of biomass pyrolysis was studied. .ture of the pyronose rings of cellulose is affected by theWith respect to biomass pyrolysis ，there are dif-adsorption of Ca+. This should be the case in the ad-ferent kinetic reaction models' 39 1. The Coats-Redfernsorption of metal ions to biomass。[31]method 400 has been widely used because it is simple0.0.21 106018and easy to use with data obtained from a thermogravi-0.11637metric analyzer. Therefore , in this work , the Coats-0. oLRedfern method was applied to calculating the kinetic500 1000 1500 2000 2500 3000 3500 4000D/cm- -1parameters of biomass pyrolysis.; 0.2In a pyrolysis reaction of solid materials , the rateof decomposition can be expressed as :i 0.1是0da= k(ax)= Aexp( - 2)( a)E(1 )0/cm-1where a represents the decomposed fraction at tempera-(C)ture T[ a=( mo-m )( mo-m。)x100% ];h is therate constant ;A is a pre-exponential factor and E is theactivation energy of the reaction ;β represents dT/dt ;R and T have their usual thermodynamic meanings./cm- 'Fig.5Comparison between the solid-IR spectrum ofThrough the rearrangement and integration , the follow-cellulose and that of cellulose after the adsorp-ing equations can be deduced :tion of Ca2*.( A ) Cellulose with5% Ca2+ ;( B ) pure cellulose ;( C )In[=-(1二()]=CaCl2.1n[(1 - 2RT)E- RT- (forn=1) (2)From the above analysis ,it can be concluded thatIβEthere is an interaction between adsorbed metal ions and「-1n(1-a)-"]cellulose，oligo- saccharide or polysaccharide in bio-中国煤化工)于mass. The adsorption of metal ions , such as Ca2+ , toMHCNMHG (forn≠1) (3)cellulose is described in Fig. 632-351.L BE\E K7Owing to the inducement of the absorbed metalThe analysis results of the pyrolysis kinetics areions , the electron cloud of the pyronose rings of cellu-listed in Table 5.lose is attracted , so the band lengths and band energiesIt can be seen that the activation energies of theof C- -C瓶方数据- -C in the pyronose rings are affect-pyrolysis reactions after the pretreatment increase from530CHEM. RES. CHINESE U.Vol. 2251. 68 to 69.35- -74.64 kJ/mol , and the pyrolysisof metal ions makes the reaction activation energy of bi-temperature increases correspondingly by water washingomass pyrolysis decrease , which also exists in the py-and acid washing. This indicates that the' ion force'rolysis of metal ion-loaded cellulose.Table 5 Kinetic parameters for the pyrolysis of the original and pretreated corn straw samplesTemperature ofActivation energyPre-exponentialSampleRelative cofficient ,Rweight loss ,1/CE/(kJ. mol-|)factor ,A/s-1Original corn straw249- -38251. 68172. 090. 997.Room temperature water-washed corn straw276- 41269. 354575. 970.99860 C Water-washed corm straw283- -41273. 079881.390. 5% HNO;-washed com straw289- 41874. 6410259. 850.9994.3 Mechanism of K* , Ca2+ Action on Pyrolysis4.2 ,it can be seen that the pyrolysis of cellulose , aAs stated before , there is a main controversy overmajor chemical composition in biomass , is strongly in-Shafizadeh's mechanism and Richards' mechanism onfluenced by the existence of metal ions , especially al-the route of the formation of pyrolysis products : levo-kali and alkali earth metal ions , and produces a lot ofglucosan and glycoaldehyde. It is considered in the for-primary pyrolysis products : CO2 ，hydroxyacetalde-mer that hydroxyacetaldehyde is originated from thehyde , other compounds containing C= 0 groups , etc.C2- -Cz cleavage of the intermediate ，levoglu-However , none of the previous mechanisms can inter-cosant9AI],while it is thought in the latter thatpret this phenomenon reasonably.hydroxyacetaldehyde comes from the fragmentation ofFrom the results of the experiments , we proposeC2-C3 , C,-C。, etc. in the pyronose rings , andthat the formation of levoglucosan and hydroxyacetalde-levoglucosan is not an intermediate of the formation ofhyde should be a result of a parallel reaction to the py-hydroxyacetaldehyde. However , the formation of levo-rolysis of cellulose. The detailed mechanisms of cellu-glucosan cannot be explained with the ltter27 A2].lose pyrolysis with and without metal ions are shown inFrom the analyses in section 3 and section 4. 1-Scheme 2 and Scheme 3 , respectively.CH2OHCH2OorHO QHK+, Ca2+ ect as catalystsoHO OHChz+HO OFCHr -O-a+ CO2 + HC-CH: +HzC-C-CH3 + Compounds containing C一O group etc.0HOHCH2-O .HO/ QHScheme 2 Mechanism of cellulose pyrolysis with metal ions.The ion force' affects the pyronose rings of cel-of metal ionsE 431.lulose and probably causes the homolysis of the pyro-Without the action of ion force' , namely , with-nose rings that produces compounds containing C= Oout metal ions , the depolymerization by transglycosida-groups , acids and CO2 during thermal decomposition :tion is an important and primary reaction in pure cellu-the scission of C- -C2 and C,- -03 bonds in pyronoselose中国煤化Irms a primary pyrolysisrings and that of β-1 4 glycosidic bonds produce CO2 ;prodCNMHher products containingthe C,- C4 and C- -C, scission forms hydroxyacetalde-C- (CHgroups， suc ds celubilose1os[44hyde ; C3-C. and C;-0, scission forms acetol( 1-For the pyrolysis of impure cellulose , cellulosichydrozy-2-propanone ) , etc. Acetol is also a primarymonomers bound with metal ions undergo a homolyticpyrolysis product and has the same characteristic of for-cleavage to form hydroxyacetaldehyde , acids and CO2mation as硬密ydroxyacetaldehyde due to the effectetc. ; however , other cellulosic monomers without bind-No. 4YANG Chang-yan et al.531CH2OHoCH2O-HtsQHXoHHNo catalysts such as salts etc.tQ, QH,OHQHCH2- oth0Y QHXOHHO'OH- J1=1125CHz-OCH2- 0HQ+o QH+ Other compounds containing C- -0- C groupHO' QHOFScheme 3 Mechanism of cellulose pyrolysis without metal ions.ing with metal ions undergo a depolymerization reactionallel and competitive reactions : one is the formation ofof transglycosidation to form levoglucosan , etc.compounds containing C = 0 groups , such as acidsDuring the pyrolysis stage ranging from 200 toaldehydes CO2 ,etc. ,and the other is the formation of600 C ,the metal ions are generally not volatile andcompounds containing C- -0- -C groups , such as cel-they can form metal-organic matters in the char. Thelubilose , levoglucosan , glucose , ethers , etc. Metalion force' action of K+ is similar to that of Ca2+ , butions act as a catalyst that favors the formation of com-it is relatively weak.pounds containing carbonyl groups and CO2. ThisThe compositions of pyrolysis products of othermechanism accords with the data of the pyrolysis of thechemical , such as hemicellulose and lignin , are pronepretreated corn straw samples.to be affected by metal ions. With the addition of metalConclusionsions , the fragmentation of molecules is enhanced in the(1 ) As the affinity of Ca2+ toward biomass ispyrolysis of hemicellulose 451 , while the demethoxyla-higher than that of K* , the removal of Ca2+ is moretions enhances and the formaldehyde formation is sup-difficult than that of K* .pressed in the pyrolsis of lignin pyrolysis 4t .( 2 ) The TG-FTIR experimental results with theFinally a detailed conceptual mechanism is pro-water-washed and acid-washed corn straw samples showposed for biomass pyrolysis with different contents ofthat the yields of char from the corn straw pyrolysis dis-metal ions as shown in Scheme 4. It includes two par-play a change order in agreement with the following or-Compoundsder : original straw > room temperature water-washedcontaining C = 0 groupHomolytic cleavagestraw > hot water-washed straw > acid-washed straw. InCO2.high K, Ca .Hydroxyacetaldehydethe pyrolysis of biomass without metal ions , compoundsring scissionAcetic acidand opening,Formic acidcontaining C- -0- -C groups are the main productsAcidswhile the yield of incondensable gases is less than thatAldehydesfor biomass with metal ions , and the yield of pyrolysist.SlowBiomassliquidis is higher. With metal ions , compounds contai-pyrolysisning C = 0 groups are the main products while theLevoglucosanGlucoseyield of incondensable gas is more , and the yield of py-TransglycosidationOther hexosanroly中国煤化工yield is almost unaffect-EthersLowornoK,Caetc.edYHCN MH Ghe CO and H20 yieldsdepolymerizatonare onlysnghtly altected. However , the CO2 yield isglycoside scission Compounds containingc-0-C groupobviously affected by washing , especially nitric acidScheme 4 A proposed conceptual mechanism for bio-washing.mass pyrolysis with different contents of met-( 3 ) The results of IR analysis of Ca2+ -loaded cel-万疖敞据lulose and thermal kinetic analysis show that there is an532CHEM. RES. CHINESE U.Vol. 22effect called ion force' in metal ion-loaded biomass.tural and Food Chemistry ,2004 ,52( 4) ,839In a pyrolysis process , the action of ion force' results[21 ] MartinezJ. M. , Reguant J. , Montero M. A. ,et al. ,Industrial& Engineering Chemistry Research ,1997 ,36 3 ) ,688in a homolytic cleavage and opening of pyronose rings[ 22 ] Basch A. , Lewin M. ,Journal of Polymer Science-Polymer Chem-in cellulose ，which produces hydroxyacetaldehyde ,istry Edition ,1973 ,11( 12 ),3071acetol , CO2 ,etc. ; whereas the reaction of transglyco-[23 ] SoaresS. , Ricardo N. M. 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