Kinetics study on biomass pyrolysis for fuel gas production

http :/www. zju. edu. cn/jzus; E-mail : jzus@ zju. edu. cnISSN 1009 - 3095 Journal of Zhejiang University SCIENCE V.4 ,No.4 ,P .441 - 447 July - Aug. , 2003Kinetics study on biomass pyrolysis for fuel gas production'CHEN Guan-y(陈冠益y' , FANG Meng-xianf 方梦祥y , ANDRIES,J! ,LUO Zhong-yan(骆仲泱尸, SPLIETHOFF ,H.' ,CEN Ke-f(岑可法尸( ' Section of Thermal Power Engineering , Department of Mecharical Engineering and MarineTechnology , De[fi University of Technology , the Netherlands )( 2 Clean Energy and Environment Engineering Key Lab of MOE , Institute for Thermal PowerEngineering，Zhejiang Universily ，Hangzhou 310027 , China )E-mail : tG. Chen @ wbmt . tudelft . nl , 归mxfang@ cmee. zju. edu. cnReceived Nov .32002 ; revision accepted Jan.5 2003Abstract : Kinetic knowledge is of great importance in achieving good control of the pyrolysis and gasificationprocess and optimising system design. An overall kinetic pyrolysis scheme is therefore addressed here. The ki-netic modelling incorporates the following basic steps : the degradation of the virgin biomass materials into pri-mary products( tar , gas and semi-char ), the decomposition of primary tar into secondary products and thecontinuous interaction between primary gas and char. The last step is disregarded completely by models in theliterature. Analysis and comparison of predicted results from different kinetic schemes and experimental dataon our fixed bed pyrolyser yielded very positive evidence to support our kinetic scheme .Key words : Biomass ，Pyrolysis , Kinetic study ，Gas productionDocument code : ACLC number : TK223INTRODUCTIONfuel cell for the production of decentralised elec-tricity for local consumers or as supplement foSince biomass is an abundant , inexpensive grid electricity ,our focus is on biomass pyrolysisand renewable resource , its conversion to syn- for fuel gas production.thetic fuels and chemical products appears at-There are three obstacles to direct applica-tractive. Among all kinds of biomass conversion tion of the above -mentioned kinetic models into .routes , pyrolysis is being given increasing atten- our case. First , every model has its specific ex-tion since the process is simple and may be opti- perimental conditions，especially at low tempera-mised to produce different proportion of products ture and very low mass flow of volatiles ; second ,for different end-users. In addition , pyrolysis is these models are based on very short residencethe first step of gasification. A good understand- time of the volatile phase in the reactor ; third，ing of pyrolysis mechanism is therefore a basic biomass material itself is always case- dependentprerequisite for controling the gasification pro-because of the complicated and varying composi-cess .tion. Therefore a characteristic kinetic model isBiomass pyrolysis has been investigated ex- necessary to be developed for our application .tensively in the past decades and many kineticThe objective of the present work is to deve-models for biomass ( mostly using wood ) pyrolys- lop, an jinnovative_ kinetic pyrolysis approach ,is have been developed. High temperature pyrol- i. e中国煤化工is( OKP ) approach ,ysis of biomass for fuel gas production , however，baseMHC NMH Gsis of pine sawdustis rarely reported in publications. Considering wastes at high temperature and long residencefuel gas has very flexible applications in gas tur-time of the volatile phase in the reactor for fuelbine , gas engine , industrial furnace , and even gas production.Prijed?京数振3409000 Suppoted by the Naional Key Basie Reearcr Pogram( 973 )of Chima442CHEN Guanyi , FANG Mengxiang et al.The second category can predict gas , tar andLITERATURE REVIEWchar yields , but not the decomposition of the pri-mary products , and so , cannot be extended tosystems different from the one which they werePhysically，bulk biomass pyrolysis can bederived from. The suitable temperature for thisviewed as a four-stage process , depending on thetype of model , for example ，is limited to theparticle temperature ,i.e.( a )below 100C , norange of 350 9C一400 °C ( Thurner et al.significant pyrolysis occurs ;( b) 100 °C - 1301981 ). Compared with the first two categories ,9C , most moisture evaporates ;( c )130 C -450the third category is more suitable for kinetic de9C , primary pyrolysis occurs ;( d ) above 450 C，scription of biomass pyrolysis , since it includescontinuous devolatilization starts ( Rav eendran etthe transformation of the primary degradation ofal: ,1995; Chen,1998). In fact this. zonation the virgin materials and secondary decompositionscheme does not represent real pyrolysis behav- of the primary products. This is the real case oc-iour which involves large scale overlapping ofcurring in pyrolysis of biomass under temperaturedifferent stages. Furthemmore ，char-gasification of higher than 500C . Unfortunately this kind ofreaction could occur following the fourth stage at model was set up on the basis of rapid quenchingtemperature higher than 800C.of the primary pyrolysis products and thus the in-Due to the above- mentioned complexity asso-teractions among primary products are neglected .ciated with the pyrolysis process and the varying The fourth category can satisfacorily give pre-composition of biomass materials,a large num- dicted results on the char yield of large particleber of complex reactions are involved in the py- biomass pyrolysis, but , for the case of smallrolysis process and therefore it is impossible to particles of feedstock , its precision is doubtful .obtain a widely accepted biomass pyrolysis mod- Moreover , this model cannot predict tar or gasel. For simplification，cllulose ,a main compo- yield. The last category developed for scrap tiresition of biomass , was first paid attention to bypyrolysis appears interesting , while only weight-many researchers( Antal , 1995 ; Broido et al.，loss rate can be predicted. In sum , all these1975 ; Varhegyi et al. ，1994 ; Zaror et al.，models disregard the interactions such as char-1988 ; Curtis et al. ，1988 ). However ，even gasification reaction among primary products.very good cellulose pyrolysis models will still en- Char-gasification can be significant at high tem-counter serious difficulties when used directly for perature ，in particular for rice straw pyrolysisbiomass pyrolysis. For this reason , different ki- due to high alkaline and alkaline earth metalsnetic models for biomass pyrolysis were subse- originating in its composition. Alkaline metalsquently proposed. In general，biomass( and sol- always favour the char- gasification by the cata-id wastes ) pyrolysis kinetic models ( cellulose lytic route below :models included ) can be divided into five cate-C+ CO2一→2CO( Reaction 1 )gories :C+ H2O-→CO+ H2.( Reaction 2 )( a) one-step，global model( Pyle et al. ,1984 ; Samolado et al. , 1991 )In addition , Klose and Wiest( 1999 ) prop-( b ) one-step , multi- reaction model( Thurn- osed a kinetic model , which included two-steper et al. , 1981 ; Koufopanos et al. , 1989 )reaction , but the second step involved two paral-( c ) two-step , semi-global model ( Blasi，lel decompositions of primary char and tar re-1993 ; Font et al. 1990 )spectively .( d ) chain growth model ( Ahuja ,1996 )( e ) multi-step , semiglobal model( Leung，KIN中国煤化工1998 )The first category is a very simple correlationTHCNMHGmodel and can be applied to predict well the Kinetic characteristics of experimental conditionsweight-loss curve and therefore is welcomed byIn our experimental conditions characterizedmany researchers interested in the weight-change by high temperature and long residence time ofduring biomass. pyrolysis. The proportion of gas volatiles in the reactor for fuel gas productionand tar yiedy李hot given by this type of model. ( without inert carrier-gas ), it was found thatKinetics study on biomass pyrolysis for fuel gas production443when a certain level of temperature was reached，associated with our experimental conditions. Un-sawdust particles always immediately released der our experimental conditions aimed at maxi-volatiles in gaseous phase which was always in mum production of medium calorific value gas ,the state of gas , and in the tar phase which step 2 and step 3 exert significant influences oncould be condensed into the liquid phase. The the final products formed , and thus attention istar phase started to decompose into gases at high paid to them. Generally speaking ，the OKPtemperature of 500 9C and above and even into model describes in detail the significant second-carbon-containing deposition with the long resi- ary cracking of the primary pyrolysis product ofdence time. The char-gasification was perceiv- tar and interaction between semi-char and gas-ably based on the comparison of measured char eous phase which always exist irrespective of re-yield from two tests( one at very short residence action temperature and residence time of the vol-time and the other for long residence time ). Ob- atile phase .The adsorption ，deposition ，andviously those models mentioned above do not char-gas reaction reported by Ahuja et al .completely reflect the physical phenomena asso- ( 1996 ) , cracking and repolymerization reactionsciated with the whole process of pyrolysis in our revealed by Anthony and Howard( 1976 )are in-fixed bed reactor. Therefore the following steps corporated in this model.have to be applied : Step 1，the degradation ofKinetic equations are described by the fol-the virgin biomass materials into primary prod- lowing ordinary differential equations .ucts( tar , gas and semi-char ); Step 2 , the de-，dWacomposition of the primary tar into secondary=-( hqW"} + h2W?+ h3Wy )(1)products( tar , char and gas ); Step 3，the ac-companying interaction( i.e. reaction 1 and 2 ) dC1between primary gas and semi-char. Step 3 is dt= hi3W"s一hsC1(2 )normally very weak under experimental condi-dC21tions applicable for previous models and thusdt= 8h4C1(3)usually ignored by previous researchers. Howev-er , at high temperature and long residence time，dC22particularly with the help of the alkaline compo-= hsTa(4) .nent inherent in biomass , this reaction could bedG21very influential on the final product distribution= δhsG(5)and so cannot be neglected. The definition oftar，gas and char in the kinetic scheme is : for d G22tar ，only hydro- carbon compounds ( C3 += k6Ta( 6)above )，not including gaseous phase of steam ;dGforgas,allgases(CO,CO2,CH4,H2)whichlt= k2W? - h4Gi(7 )can' t decompose under normal pyrolysis condi-tion，and including gaseous phase of steam ; for dTnlchar，only solid residue ，mostly of carbon .=kqW!-(hs+K。+h-)Ta(8)However for the experimental determination ofd T。2the final pyrolysis product in the fixed bed , the= k-T。( 9)H2O is collected in liquid phase prior to the gasmeasurement and therefore only included in theWhere , Wa is degradable biomass weight , Ci istar yield. For the correspondence between the primary char weigh( kg), C21，C2 are second-kinetic definition and experimental determination ar中国煤化工s primary gas weightof pyrolysis products , the vapour is counted only ( kgMHCN MH Gary gas weight( kg),in gaseous phase .Ta1 Is prumary tar weight( kg ) , Ta2 is secondaryOKP model schemetar weight( kg ) ,δ is deposition coefficient repr-The basic principle of the OKP model prop-esenting the contacting situation of the surfaceosed by u isshown in literature( Chen ,1998). Cand G1 ;hj=A,exp( - E;/RT ); the initialThe kinefoOne incorporates three basic steps conditions are :at time t=0, Ws= Wo ,C1=444CHEN Guanyi , FANG Mengxiang et al.C21 = C2= G21= G22= Ta1= Ta2=0the iteration of numerical calculation the instan-The residual weight at time t can be defined taneous yield of gas and tar is necessary ，andby :therefore the assumption on this point is : instan-M= W.+ C1+ C2+ C22( 10)taneous gas yield is proportional to the final gayield by a magnitude of_ relative instantaneousThe final tar( T。) yield and gas( G ) yield carbon conversion rate( X; ) which is a dimen-can be derived from :sionless ratio derived from the experimental dataobtained in a laboratory-made isothermal appara-T。= Ta+ To2tus and fixed bed pyrolyser together. The tar canG= G1+ G21+ G22be obtained by difference. The instantaneouscarbon conversion rate is defined as :The solution methodW。- W7;Under the isothermal condition( for our fixedX; =Wo-W∞bed , isothermal assumption is applicable ) ,i.e.T= To , rate constant h;( h - hy ) defined byX;Xi=文。Arrhenius' s law is a constant. The reaction or-der( n1 ,n2 ,n3 )is set tobe 1.0 ,thus the set ofHere X; is the instantaneous carbon conver-ordinary diferential equations together with the sion determined in the isothermal apparatus , x。initial Eq. ( 10 ) can be solved by numericalis the final carbon conversion obtained in themethods ( Chen ,1998 ).The seven kinetic parameters plus a physicalfixed bed pyrolyser .parameter are estimated by fitting Eqs. 1 - 9 toEXPERIMENTAL SECTIONthe weight-loss curve derived from an isothermalapparatus. The best fitting value is obtained byminimizing the sum of square function by meansIsothermal experiments were carried out inof a hybrid method combining features from the an isothermal apparatus which was developed inNewton- Raphson，steep- descent algorithm. It our laboratory. In fact ，it is an up-down fur-should be noted that the initial values of h1一hynace , with an instantaneous mass-change mea-are very crucial in order to achieve rapid conver- surement. The main element of this device is agence. The initial valueof k ,h2 ,hi3 and ks ,hi6 tubular ceramic reactor with 60 mm inner diame-:an be derived from literature ( Thurner andter and 500 mm in height. The tubular reactor isMann ,1981 ;Blasi ,1993 ), however ,it is impos- inserted vertically into an electrically heated tu-sible to evaluate the initial values of h4 ,h7 frombular furnace. The sample material was placedprevious publications. For simplification of cal-in a stainless-steel mesh basket hung on a ther-culation , kj is assumed to be the same value asmocouple which is connecting to the low part ofthat of h6. Full details of obtaining the initialthe weight- loss analyzer. The weight- loss signalvalues of hs are shown in publication ( Chen ,of the sample thus can be detected via theweight-loss analyzer at any time at a given oper-1998 ). Compared with the initial values of hi -ating temperature .h们7 , the initial gas yield and tar yield appearThe fixed-bed reactor was made of stainlessmore important as they both can influence the steel with inner diameter of 80 mm and horizon-calculation process and also dominate the final tal height of 750 mm( Fig.1 ). The reactor waspyrolysis product distribution. However ，no re- divided into a pyrolysis zone at the bottom sec-ported experience with respect to them can be tion中国煤化工”。at the upper seefound in kinetic models available. Moreover , tar tione , biomass particlesand gas yield cannot be easily obtained from the wereMYHCNMHGitiontoformthepri-isothermal apparatus. In this work , the tar yield mary products and were also subjected to reac-and gas yield were determined experimentally in tion 1 and 2. In the cracking zone , only tar wasour fixed-bed pyrolyser at temperature of 9009C .decomposed there to form secondary products.Obviously the, experimental yield means the final The outlet of the pyrolysis reactor was connectedyield , not Ktaneous yield. However , during to a condenser. The volatile phase flew into theKinetics study on biomass pyrolysis for fuel gas production445condenser , then into a spherical dryer ,and then and tar yield are of priority. The used biomassinto a water tank and a gas vent. The connecting material was pine sawdust with the following ap-tube before the condenser is always well- insulat- proximate analysis data( wt. % ): M , 12.27 ;ed to prevent tarry components from condensing. A ,0.83 ; V , 70.55 ; FC , 16.35 ; with heatingvalue of 18064 kJ/kg. The calculated kinetic pa-rameters of pine sawdust calculated are shown in3-1n11Table 1.! 13Table 1 Kinetic data calculated for pine sawdust pyrol-ysis at high temperature凸2(70|FrequencyApparentDeposition14Reactionfactor( A; )activationcofficientenergy( E)Fig.1 Fixed bed reactorh110.75.95E+61. thermometer ; 2. temperature controller ; 3. eletrical92.51.35E+5furnace ; 4. pyrolysis reactor ;5 . thermocouple ; 6. Chessel .recorder ;7. computer ;8. condenser ;9.U-tube ; 10. reg-115.02.50E +6.ulator ; 11. sample port ; 12. gas tank ; 13. U-tube ; 14.h.84.52.00E+51.15regulator130.21.38E +5The determination of hold time at a certainh,90.75.05E +6reaction condition is determined by ensuring thath75.05E+6no further condensation occur in the cold traps .At the end of the experiment , the pyrolysis resi-Fig.2 shows the comparison of predicteddue weighed to determine yields ，and the gasweight-loss of sawdust particles against the re-yield is usually measured based on the variationsults obtained experimentally in the isothermalof water tank volume and calibrated by the valuefrom the volumetric flow-meter. The obvious fea-tures of our fix-bed reactor are : high operating100temperature ( 800°C - 950°C ) and long resi-90◆experimental datadence time of volatiles( > 8s ).80theoretical data7(MODEL VALIDATION AND DISCUSSIONSS(40The possible variation in the ash content of3(various sample materials can lead to erroneous2(interpretation of the experimental data ，so the10biomass weight and residual weight reported inthis paper were corrected as follows05200 250M = Residual weight-Ash weightTime (s)W = Biomass weight- Ash weightFig.2Weight-loss predicted against experimentalAs mentioned above , kinetic data were ob-results in the isothermal apparatus for sawdust pyrol-tained by fitting data with weight-loss from theysis at 900Cisothermal apparatus based on the estimation of中国煤化工instantaneous gas and tar yields. The estimation app:CNMH(be seen OKP modelof gas and tar yield involved in the kinetics of still:RYHight-change occurringfixed bed reactor and of the isothermal appara- in the isothermal apparatus although isothermaltus , and therefore the kinetic data are expectedapparatus doesn' t obviously involve in the sec-to meet the operating conditions achieved in the ondary reaction and interactions of primary prod-isothermal 2rPagatus where only weight-loss is ucts. The good prediction is due to the fact thatconcerned命效描fixed bed where the gas yieldthe solution of OKP model is still by ftting cal-446CHEN Guanyi , FANG Mengxiang et al.culated data with data from the isothermal appa- the experimental data obtained in the isothermalratus .conditions .The relationship between final gas yield and( c ) Dimensionless pyrolysis rate K( = | h,tar yield experimentally obtained in the fixed-bed - 的。 1/k。) is introduced. .pyrolyser at 900C and the theoretically calculat-The kinetic models can be evaluated by ref-ed are shown in Table 2 , showing good agree-erring to K obtained above，which actually repr-ment of predicted results with experimental data.esents the deviation between data experimentallyBased on the balance of total mass , the theoreti-obtained in our fixed bed at high temperature ofcally calculated char was somewhat lower than900C and results theoretically predicted accord-that experimentally obtained. The differenceing to different kinetic schemes. Table 3 showscould be attributed to the overestimation of char-the comparison of different K. As can be seengasification reaction as low alkali content is asso-from this Table , OKP model minimizes the devi-ciated with pine sawdust .ation. However other models , compared withTable 2 Comparison of gas yield and tar yield for saw-OKP model , are simpler and lead to a simpledust pyrolysis at 900Ccomputational process , and furthermore they ap-pear very practical at low or middle heating rateExperimentalTheoretical DataItem .Data( wt.% )( wt.% )( non- isothermal ) of thermalgrav imetric apparatusor of some tubular reactors for oil- like produc-Gas yield72.674.7tion，therefore they are still recommended forTar yield10.29.8different specific cases .Table 3Calculated K values using different kineticEVALUATION OF THE KINETIC MODELSmodels for prediction of fixed-bed pyrolysis ofpine sawdustOKPFontLeungThurmerAhujaMost kinetic models that developed weremodelet al. and Wang and Mannet al.based on different pyrolysis mechanisms ，and<11%< 17%<21%< 38%<15%found applicable to particular experimental con-ditions. There is no comprehensive evaluation onthose kinetic models as there is no standard forsuch evaluation. A standard is proposed here CONCLUSIONSbased on the following items :( a )A common total pyrolysis rate hp ( kg .The biomass pyrolysis kinetic model whichs-) is used , which_ is defined by the equationincorporates the primary decomposition of the(11 ). The value of h, is determined by the ki-original biomass materials , the continuous de-netic representation of weight-time relationship ,composition of the primary products , and the in-based on the range of temperature under isother-teractions among the compositions of primarymal conditions .products has been developed here and is found to( b)hp can be described as :be accurate predicting the pyrolysis behaviour ofw'( t )dt =- h。△t∞(11)pine sawdust in the fixed-bed reactor at hightemperature. Compared with other kinetic mode-ls , OKP model predicts better for sawdust parti-and can be approximately expressed as : .clesnwlveicin the fivod hed at 900C. The de-中国煤化工;viatiI and predicted py-'Ot; =-的。△tx( 12) roly!MYHC N M H Gricuous for the appli-cations of different models. However , it is be-Where OW: = weight-loss during pyrolysis in lieved that the deviation could be more seriousa specific time zone△t;( t;-1 ti );△t∞=t∞-when different kinetic models are used for theto ,is the total pyrolysis time ; he is defined by prediction of the gas yield from biomass pyrolysisthe equatib数据) and its value is determined by in the fixed bed at high temperature .Kinetics study on biomass pyrolysis for fuel gas production447Curtis ,L.J. and Miller ,D.J. , 1988. Transport model withradiative heat transfer for rapid cellulose pyrolysis .ACKNOWLEDGMENTSInd. Eng. Chem. Res. ,27 :1775- 1783.Font, R. , Marcilla , A. and Verdu, E.，1990. Kineticmodel of biomass. Ind. Eng. Chem. Res. ,29 :1846The contribution to the experiments and- 1957.valuable discussions by Dr. Chunjiang Yu isKlose, w. and Wiest , W. , 1999. 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