Modeling pyrolysis of charring material in fire Modeling pyrolysis of charring material in fire

Modeling pyrolysis of charring material in fire

  • 期刊名字:科学通报
  • 文件大小:241kb
  • 论文作者:YANG Lizhong,Chen Xiaojun,ZHOU
  • 作者单位:State Key Laboratory of Fire Science
  • 更新时间:2020-07-08
  • 下载次数:
论文简介

NOTESModeling pyrolysis ofwood is a combination of the external flux u lth the fIluxfrom the lame. The rate of heat release rises 10 a maxi-charring material in firemum rapidly, then a char layer builds up gradally as thepyrolysis front moves inward. Its rate of heat releaseYANG Lizhong, CHEN Xiaojun, ZHOU Xiaodong,11 Aeventually reaches a more or less steady valuc. At the tem-WANG Qing'an & FAN Weichengperature above 300C the char layer begins lto break downState Key Laboratory of Fire Science. University of Science and Tech-rapidly. The char layer shrinks under the same conditionnolog) of China. Hefei 230026. Chinaand pressure gradients are set up within the materials.Correspondence should bhe adressed to Yang L.izhong (e-mail: zxd@Small cracks appear on the surface. and these cracks allowustc.edu.cn)volatiles to escape more easily. The cracks graduallyAbstract A modifled model of pyrolysis for charringwiden as the char layer deepens.materials in fire has been proposed in this note. In tbis modelBecause of its importance and complexity of pyroly-some special factors which sbow the ellet on pyrolysis aresis of charring materials, there is a subsantial volume ofconsidered, i.e. heat loss by convection and radiation causedwork regarding the ignition. pyrolysis. buming. and char-by surface temperature rise and shrinkage of char surfacering behavior of wood and cellulosic materials experi-are considered, Experimental device is designed specially formentally and thereically' ". A thorough review on thisvalidating the reliability of the model. Effects of density ofwork has been described by other researchers' " and it ismaterials and heat radiation on pyrolysis of materials havealso been investigated.not the intent of this work. Here we only present themKeywords: charring materials, pyrolysis, fire, thermal radiation,very briefly.model.Kanury described ignition of solid miaterials underradiation conditins'4. Roberts. Simms. and SuubergCombustion is a complex interplay of chemical reac-studied the effect of kinetics on wood pyrolysish- . Sev-tion. heat, and mass transfer, and combustion of solid ma-eral models for the burning rate of solid maternals, bothterials becomes more complex because of the existingcharring and noncharring, have been developed. Examplessolid phase. Main combustible materials appearing ininclude the studies hy Kung8] Wichman et al!9. andbuilding fire are wood and other cellulosic materials, thusBrehob et al.l0l. These models range lfrom simple treat-study on their combustion characteristics during fire playsments of the ignition and burning process using pure heatan important role. Maybe it is the key to study ignition ofconduction models to the use of complex chemical kinet-solid materials and fire spreading of solid materials.ics for the pyrolysis of a charring materials. However.The pyolysis behavior of solid matenials can be di-each model has is retrictions.vided into two types: noncharring and charing". Non-In this nole, a modified one-dimensional pyrolysischarring materials such as PMMA bum away completely,model. which considers uransient conduction. gas convec-leaving no residue. and can be modeled by the theorytion, endothermic effect of process. nonlinear boundarysimilar to fammable liquids. In contrast, charing materi-condition and shrinkage of the slab. is pruposed. Theals leave relaively significant amounts of residue whenmode! is also compared with the experimenta! data ob-they bum. The pyrolysis of charing materials such astained from our research on white pine.wood is a complex interplay of chemistry, heal, and masstransfer. Charring materials must be modeled in terms ofa1 Theorypyrolysis front penetrating into the materials with an in-The model proposed in the note considers some fac-creasing surface temperature but without a well-definedtors that have a strong influence on pyrolysis of solid ma-steady state. The char layer forms an increasing thermalterials, such as transient heat conduction inside solid ma-resistance between the exposed surface and the pyrolysisterials, the effect of gas convection on heat transfer andfront. resuling in a continuously decreasing rate of heatthermal effect of pyrolysis process. Moreover, thermalrelease after the frst peak.properties of materials are permitted to vary with tem-Considering that a“thick" sample of wood with itsperature.surface is uniformly exposed t0 a constant extemal heat( i ) Assumptions. It is a complex process forflux. So the heal and mass transfer through the sample canwood pyrolysis and combustion. Thus it is very difcult tobe considered one-dimensional. Ignition can Occur at sur-consider all factors affecting the process in numericalface temperature anywhere from 200C up to 400C. Alsimulation. Specific assumptions are needed l0 establishthe instant of ignition. the heat fiux to the surface of themathematical model.|1 Jnssens. M.. Experimental study on pyrolysis of wood. Ph. D. Thesis UIniverity of Gen1. Belgium.1991.Chinese Science Blletin Vol. 47 No. 5 March 2002中国煤化工425MHCNMHGNOTES小The volatiles do not accumulate within the charwhere n is lhe reaction index. E. thc reacuon itint encrgylayer. but they can be produced and leave the surface ofand A the pre exponential factor.solid Immediately. Actually. most volatiles exit immedi-Based on the above lwo equations. we ohtainalely. others penetrate into the inner of solid;121 Moisture content of solid is neglected. Actually.dp,13)waler mainly affects the mass loss at the early stage ofdrPw- PR7heating and postpones surface temperature risc;The gas mass conservation equaton Is5 given as(3) Chemical reactions between volatile and charlayer as well as that between char and air are ignored,Mg_ a双(4)(4) Materials properties are assumed to be linear)x dr°with the range of the considered temperature.(ii) Controling equation. The suface and innerwhere Mg is the mass of volatiles.The energy conservation equation is given hytemperature of wood will increase at exlermal heat flux.When temperature reaches the vaporization temperature,d(p.h). df, oTi, d(Mgh,1-0盟.15)water begins to vapor. Most water vapor produced exits01from the upper surface. lttle penetrates into the surround-ing. After all water exits. surface temperature of woodwhere P,P; =ρh+ph.h=。CgdT. subscrlpt g心rises and the pyrolysis Trate begins to increase and a pyro-fers to gas, Q is the reaction heat of the pyroly is proccss.ly sis reaction zone is formed. Along with propagation ofthe reaction zone. different wood layers decompose si-mulaneously at different temperatures because of theLCc.orT h.=6Cne7. h-=C(C,JT.6)temperature field of the inner wood and different productsCombining eq. (4) with eq (5). we obtaincould be formed. Features of the model and the physics ofthe thermal decomposition of wood are porayed in fig. 1.0T_ 8(. 8Td" d1 dx(" d!+M:Cq,Radialive↑Gas out(Convecion &+eth+- hR __. e7, heat fluxreradiationPn-P. P。-P」("har layerThe boundary condition isAcrsc layer=q'-h(T、 -Tπ)-E0(7- 181dx.x=0The initial condition isVirgin laycrT=T. ρ=Pi Mg =0 it=()1191Fnge. 1. Schemaic model for dcmositionnMaterials properies are assumed (0 be linear func-tions with lempcrature. i.c.The densit! of decormposed wood is given ask=k"+k (T-Ty).101_PPu.:P:+Pr.()qρ≈°+°p(T-T小.where p. is the density of decomposed wood, Pr thewhere k" is the thermal conductivity at constant lempera-density of char. ρ。the density of decomposing woodture. k' and c。 are linear coefficients with temperature.c° is the themmal capacity at constant temperature.and P。the density of the virgin wood.Three unknown paramelers. i.e. p../I. Mg(x.I)Arhenius equation is chosen 10 describe he reactionand T(x.1) can be solved according to eqs. 131 (41 andrale(7)._p、(-E(li) Modifications on lhe formner model. Mainn.=-一可= p%"Aexp:i RT2)modifications of the model are as follows 1山1 (ienerally.426Chines中国煤化工。March 2002MYHCNMHGNOTESequal thermial fTux density was applied und the heat loss Crank-Nicolson method is used to solvc the心 dimen~\ia convcction and radiation caused by the rise of lhesionless equations. Numerical modcling decripuon canwoou surface temperature were nol considered for the be also found in Kung 's paper'”.existed models. The heat loss is considered in the model2 Description of experimentspresented in the note. which is described in ey. (8). (2)Aclually. the wood surface shrinks along with the woodA device to study the charaterstics of fire ill andecomposition. This situation is considered in the model.early stage has been developed by Stalce Key I aboralory1iv1 Dimensionless analysis and solution. The fol- of Fire Science (SKLFS). Here we only give d simplc de-lowing dimensionless variables are selected to consolidatescription of the device. A more cxtcnsive and completethe variablesdescription of the device is given in ref. [12]|. The deviceis made of stainless steel. Its outeris 1.5mX 1.5 mX20 m.P。=P。== PsAclually, there exists a specimen holder (550 mm X.550Psmm) on the rack. The device can mainly be used to meas-ure parameters such as f]ame temperature. rlarme image_工0元=RTToarca, smoke components. smoke temperature. >noke flux.weight loss of sample, ignition temperature 01 simplc andignition time.The distance between he sample and the reciangleradialor may change from 300 lo 800 mm. The sanple isignited using the electrical radiator. Power of the radiatoris 0)-110 kW and can be adjustcd continuously. The crosssection of the radiator is 450 mm X 450 mim. Sumples arckwplaccd on the holder and ignitcd by electrical radiator. A=一i=-Cx。Pnl: k-一Q=Cp.Iminimum of two runs is carried out for each mterial ateach heat flux. If the runs are significantly different. athird run is madc.ACe.P.r _ MgCp,!kw(1-P.)kn3 Results and discussionsSubstituting the dimensionless variables in the aboveAs described above, lhe surface temperaturc and theequavums. i.e. eqs. (3), (4) and eqs. (7)-(9).mass loss rate of samples are the two key parameters dur-ing the thermal decomposition of solid materials. Lsually.(12the surface lemperature of the samnple is the controllingdfactor to dctermine whether Inaterials can be ignited or notwhen there is not anly pilol ignition source. The mass lIossdMg_師,rate is related with the combustion rate and the heal心13)lease ralte is calculated based on the mas loss rate".Q()=m0(1> X1)H:行_ p,c。.行1页where H。is combustion heat of volatiles. X11 is thecombustion eficiency (<1.0).Fig. 2 gives expcrimenial and theoretical values olthe mass loss ratc of white pine under therm.l raudiation.MCnof_ 107(14)ig.了is a comparison of theoretical value und TG cx-perimcnts.k五kδAs shown in fig. 2. the calculation \ilue is litlehigher than the expcrimcntal one at the early viage. A、a= 4"l川w_1)_O(;*-1). (15)whole. they match well. These results have shown thal (hedx τ=nkuTomodel prescnted in the note is reasonable. Bul ds shown infig. 3. ltte discrepancy exists between thcorctical andT=1.页=1. M:=0 (F=0).(161experimental values. This may be due 1) dilerent envi-Chinese Sclence Blltin Vol. 47 No.5 March 2002中国煤化工427MHCNMHGNOTESronment between TG experimcnts and pyrolysis of solid1.0materials at real fire.09-一1000kgm)8..700hgm400 kg'm)7-0.9{一Experment value0.610.8-.... Calculation value自0.507-)46-0.3-05-02- 50 kW!m'.1.0 400 80012001600200“240050 kWm'Heating tmneis0.2Fig. 4. Efeet of densty on decomposition 150 kWmi.01-0 300 600 900 120015001800Hcating time/s8001700-Fig. 2 Comparison beween experiments and deomposition.600 |.....500-1.1].4001.01- 1001 kgm'.... Calculation0.91.... 700 kgem 'TG experiment08吴200400 kgm'710040 kWmi0400800 1200160021024000.4Heating time/s)3}2Fig. 5. Effect of densily on surtace tenperature rise 140 kWmi 1.0.1]0.0100 200 300 400 500 600~ 700.0丁Temperature/°C30 kW:miFig. 3. Comparnson between calculated values and TG experiments.08-0.71一500knm"Figs. 4- -7 give some calculation results based on the民06-model. Figs. 4 and 5 indicate the dependence of decom-i 05-position and the surface temperature on the density of04+solid matcrials at the fixed thermal radiation, respectively.700 kg/m'As shown in these two figures, the mass loss increases.2-with the density of materials decreasing. however, theσ T 60000200 300 3600variation of surface temperature shows a litle complexity.Healng time/sIt increases with the decrease of the density first, thendecreases.Fig. 6. Effer of extermal termnal fux on decompostuon.Figs. 6 and 7 show the dependence of the mass lossand the surface temperature of materials on extemal ther-velopment and the latter alfects fire occurrence. The big-mal radiation. As can be seen from these two figures, theger the heat release rale of malerials and the shorter themass loss and the surface tcmperature of solid materialsignition time. the more dangerous the materials.increase with the increase of thermal radiation.Considering the following equation'"”:Generally, the fire isk of materials can be asessed(T, -T户using the following Wo parameters: the heat release rate(18)and the ignition time. The former is related to the fire de-(g“)428Chines中国煤化工。March 2002.MYHCNMHGNOTES7001fre risk of materials. Actually, other parameters, such as65010001thermal condoctivity should also be considered.SIO(ii) The extemal heat flux also shows a large efiet4501on the fire risk of materials. Generally speaking. with the40013501increase of lhe extemal thermal flux. the mas loss rate300 Iand the heat release rale also increase, the ugnition time250-30 kW/m'becomes shorter, thus the fire risk of materials shows the200米.. 40 kW/mi'tendency of increase.一- 50 kW/m'700 kgm'Acknowledgements This work was spponed by the Nanonal Nanural600 12008002003000300Science Foundaion ofChina (Grant Nos. 59936140 and 500602)210Heating timne/sReferencesFig 7. Efe ofexernal hermal flux on srtice emperaure.1. Speapornt, M.1. Quinber.1. G. Pedicion the bumng ot wodwhere q" is the net heat obtained, kpc the thermalusing an intgral model. Combustion and Flame.200. 123: 308.2. Anderson, a W.. A buming rate model for caring matenals,ineria of materials. T, the surface ignition temperature,Gaitherburg, MD Naltional Istute of Standards and Icnolngy.1。 the ignition time and To the environmental tempera-3. Kanuny. A. M. SFPE Handtook of Hire Prorection Engineeng.2nd ed. Soriety of Fire Prorection Engncer. Boston, MA. 199ture.2-190.2- -204.q'=q'-qG.(19)4. Kanury, A. M. Rale of chamng combustion m a fire Procedingsof the 14th Symposium (nt) on Combustion. Pensylvania Suatewhere 4" is themal flux. and q the critical thermalUnieriy. The Combustion Istitute, Pisburgh, 1922 615.flux needed to ignit the materials.5. Roberts, A. F. A review of kinetics data for the pyrolysis of moodAs shown in fig. 6, the bigger the external thermnaland relatcd substances. Combustion and Fame. 1970. 14: 261.fux and the mass lss, the bigger the heat loss rate; as canSimms. D. L. Darnage 1o cllulosic solids bhy thermusl radiation.be seen from fig. 7. the bigger the extemal thermal fux,Combustion and Flame.1962. 6: 303.the shorter the ignition time.7. Suberg. E. M.. Milosvbjevic, 1.. Llly, W. D. Behavor of char.Summarizing the above analysis, we can draw aring malerials in simulated fire eovronments, Gaitersburg. MD:conclusion. With the increase of materials density and theNational lstitute of Suandards and Tetnolog. 194.645extema! thermal f0ux. the mass loss and heat release rate. Kung, H. C. A mathemalncal model of wood prolyb. Comhus-also increase. the ignition time becomes shorter, thus thetion and Flame, 1972. 18:185.fire risk of materials shows a tendency of an increase. The9. Wichmun, I. s.. Aureya, A. A sipied model for the prolysso[result matches well with the experiens!'s.chaning maieriaks, Combssion and Flame.1987. 68x: 231Brehob, E. G, Kulkami, A. K.. Tme -dependent mass loss rateIRemarksbehavior of wall aterials under extemal radiarion. Fire and Ma-A modified model has been presented in the note.terials, 1991. 17: 249.Some important factors t0 afect thermal decompositionOhleliller, T. J. Kashiwagi. T. Wemer. K. Wood gasificaton aare considered in the model, ie. the heat loss by convec-fire level heat flux. Combustion and Flame. 1987. 69: 155.tion and radiation caused by the surface temperature rise12. Yang Lichong. Deng Zhua, Fan Weicheng er al. Experimentaland shrinkage of the char layer are considered and any nsudy on caraterstis a early stagc of fire-- Measurement andcan be applied in Arhenius equation. The mass loss rateanalysis on eartly development of flame image artra. 」Fire Sc-of thermnal decomposition of white pine is calculatedences, 2001.19(3): 190.based on the model. The resuts show a good agreement13. Drysdale, D.. An Inrnducto to Frte Dyramcs. Lundon: Jnnwith the experimental ones. In adition, efects of the ma-Wiley & Soas, 1987.terials density and the extemal thermal fux on decompo-4. Tewaron, A. Famability parameters of materials Ignton.sition have been studied theretically using the modelcombustion. and fire prpagation, J. Fire Sciences. 1994,. 124;:329.under the large size sample condition. Main conclusionsare as follows:I5. Yang Lizhong. Deng Zhihua. Chen Xiagun. Experimcntal stdy1 i)The density of malerials shows a large efcte onon fire charceristics of matcrials. Fire Safety S.rence (in (hInese), 2000. 9(4): 32.the fire risk, However, it needs to be noted that thisconclusion has only considered the effect of density on(eceived Octoter 18. 2001)Chinese Science Blletin Vol.47 No.5 March 2002中国煤化工429MYHCNMH G

论文截图
版权:如无特殊注明,文章转载自网络,侵权请联系cnmhg168#163.com删除!文件均为网友上传,仅供研究和学习使用,务必24小时内删除。