Study on pyrolysis and gasification of wood in MSW

ISSN 101- 0742Journal of Ensironmental Sciences Vol. 18, No. 2, pp. 407--415, 2006CNI1- 2629/XArticle ID: 1001 -0742(2006)02-0407-09CLC number: X705 Document code:AStudy on pyrolysis and gasification of wood in MSWNI Ming-jiang, XIAO Gang, CHI Yong", YAN Jian-hua, MIAO Qi, ZHU Wen-li, CEN Ke-fa(Institute for Themal Power Finginccring, State Key Laboratory of Clean Energy Utiliation, Zhejang University, Hangzhou 310027, China. E-muil:chiyong@zju.edu.cn)efficiency, it was planed that all the main components in MSW and somc typical kinds of MSW were pyrolyzed/gasificd to proposc ancxpcrt systcm for raw MSW. In this paper, wood, which was a prevalent component in MSW, was pyrolyzed and gasified influidized-bed reactors at difcrent apparent excess air ratios (EARs), lemperatures and fluidizing velocities. For pyrolysis, with tem-increasing from 400C to 700"C , the yicld of pyrolysis char decreased while that of pyrolysis gas increased (in this paperrespectively from 28% to 20% and from 10% to 35%), and when temperature was 500C，the yicld of pyrolysis tar reached the highest,up to 38% in this paper. It was the oplimum for gasification when temperature was 600C and apparent EAR was 0.4. Under theexperimental conditins of this paper, gasification eficiency achieved 73%, lower heat valuc(LHV) reached 5800 kJ/(Nm) and yicldof syngas was 2.01 Nm?/kg. Lower fluidizing velocity was useful to upgrade gasification eficiency and LHV of syngas for woodgasifcation. Based on the results, the reactive courses and mechanism were analyzed respectively for wood pyrolysis and gasification,Keywords: wood; pyrolysis; gasification; municipal solid wastc(MSW)Introductionbustionand melting zonc (Osada et al, 2000; OsadaNowadays, the quantity of municipal solid wasteand Osada， 2002). (6) The Korea Institutc of(MSW) has increased evidently, especially inMachinery and Materals (KIMM) proccss: il consistsdeveloping countries. For example, in China, theof a stoker furmace used as a pyrolysiser/gasifier anddisposcd volume of MSW was about 137 X10* l inmelting furmace modified from glass melting furnace2002 (Complied by National Bureau of Statistics of(Kim, 2004). The above pyrolysis/gasification andChina, 2003). With the requirement of environmentmelting technologies are suitable for the treatment ofprotection, pyrolysis/gasification and melting techno-MSW with high calorific value in developed countries.logy is proposed to be one of the most efficient meansIn order to develop MSW pyrolysis/gasificationto dispose MSW with less pollutant emissions,and melting technology for changeful MSW inespecially including PCDD/Fs and heavy-metalsdeveloping countries, it was planed that all the mainwhich are strongly concermed by public.components in MSW and some typical kinds of MSWBy far, some technologies have been proposedwere pyrolyzed/gasified to proposc an cxpcrt systcm,and industrialized: (1) Siemens/KWU Schwel-Brennwhich could forecast the suitable tcchnology anctechnology: it consists of a rotating pyrolysis dnumopcrating conditions for MSW disposal therc. The .indirectly hcatcd by recirculation fluc gas and a highmass percentage of woody garbage in MSW is abouttemperature combustionand melting furmace(Malkow,4%- 12%， as showed in Table I (Patumsawad and2004; Kawai and Taguchi, 2000). (2) ThermoselectCiffe, 2002; Plaza et al,, 1996; Sukrut et al, 2002;process: it consists of a rotating degasification channelNakamura et al., 1996; Mastro and Mistretta, 2004).indirectly heated by circulation of hot medium and anAs the predominant ingredient, wood was selected tooxygen-blown gasification and melting furmacebe pyrolyzed/gasified in fluidized-bed reactors. And(Malkow, 2004; Calaminus and Stahlberg, 1998). (3)the characteristics of pyrolysis and asification wouldVon Roll RCP technology: it consists of anbe analyzed and introduced as one of importantoxygen-blown grate-type gasification chamber, anparamcters into the cxpert system.oxygen-blown combustion and melting furmace and anActually, wood pyrolysis/gasification is involvedoxygcn-blown CFB burner (Malkow， 2004). (4)in many other technologies, like MSW gasificationFluidized bed gasification and swirl-flow meltingand co-combustion (Larson et al, 1996; Scott andprocess: it consists of a fluidized-bed gasifier, and aStanltheim, 1998), woodswirl-flow combustionand melting furmace (Malkow, gasi中国煤化Ines or engine(Kimn,2004; Suzuki and Takahashi, 2002; Tame an200MYHC N M H Gul., 2002; BridgwaterTaniguchi, 2002). (5) MSW direct melting system: itet al, 2002; Babu, 1995; Chen et al, 2003; Drift etis a rich-oxygen-air blown fumace which consists ofal, 2001), wood gasifcation by steam/air/CO2(Lucasthrce parts, drying zone, reaction zone and com-et al, 2004; Tsai et al, 2001), biomass IGCC(Babu,哆推n: The Natinal Natural Science Foundaion of CainNo.59836210); Coresponding auhor.408NI Mingjiang et al.Vol.181995; Chen et al., 2003), biomass fuel cell (Babu,gasification and melting technology was much1995; Kirill et ul., 1998; Larson el al, 1996),different, because if the temperature of pyrolysis/co-gasification of biomass and other kinds ofgasification is below 400C , most of MSW can not befeedstock (Drift et al, 2001; Pinto el al, 2002),pyrolyzed/gasified completely, or if the tempcrature isbiomass pyrolyis/fast pyrolysis/hydropyrolysis(Collotover 700C , much of low-mclting point metals wouldet al, 999; Stenseng et al, 2001; Cetin et al, 2004),volatilized, which could hardly be moved out frombiomass combustion/co-combustion(Moghtaderi etcxhausted gas. In addition, the content of tar in syngasal, 2004), biomass carbonization(Savova et al., 2001;into combustion and melting fumacc did not need toKlose and Schinkel, 2002), other similar or relativebe strictly controlled like that in biomass IGCC or fueltechnologies (Schmicder et al, 2000; Pindoria et al,cell. After all, in many sides these studies could be1998; Wu et al, 2002). But MSW pyrolysis/used as reference for our research.Table 1 Physical characteristics of MSW in developing countries and developed countries, %CountryPaperWoody garbage Textile Plasic and uber Kitchen garbage Metals GlassOtherChinaBeiing5.435.752.9710.3751.83 .1.0017.22Shanghai4.5811.608.5962.090.892.887.0Dongguan6.447.8319.2831.216.103.37 1.02Thailand1323039Argenlina11.67.45s.44.10.9 10.2USA.3S1742:Japan36.324.165.8619.4417.333.96 8.76 7.81taly27.274.384.0013.96. 31.712.708.33.641 Materials and methodsselected for fecdstock of experiments. The sized ofstock was treated into about 5 mmX5 mm X5 mm for1.1 Characteristics of woodpyrolysis/gasification experiments. And the proximateWood is the most common components in MSW,and ultimate analysis was reported in Tablc 2.and in this study, one-off chopstick of wood wasIn this study, apparent EAR was defined as: theTable 2 Proximate and utimate analysis of woodProximate analysisUJtinate analysisM,%A.%V,%FC,%Q, kJkgC.%H,%N,%S. %O,%8.422.0076.4213.16 .1728743.73C40.96Nores: M. moisture; A. ash; V. volatile; FC. fixed carhon; Q. bumb calorife valucvolume of air supplied for 1 kg feedstock1.2 Experimental set-upreaction/theoretical air volume for complete com-The main cxperimental works were carried out inbustion of I kg feedstock. In order to correspondingtwo fluidized beds on lab scale and an auiliarywith other definitions in other papcrs, the apparentexperiment was fulillel in a thermogravimetricEAR and real EAR arc listed in Table 3. Real EARanalyzer before pyrolysis and gasification. Thewas defined as: total oxygen quantity for 1 kgexperiments were carried out at 400- -700C , whichfccdstock reaction(including both the supplied air andwas consistent with the above mentioncd MsWthe oxygen in wood iself/theorctical air volume fopyrolyis/gasification and melting technologies.complete combustion of 1 kg feedstock (including theThe fluidized-bed pyrolysis reaclor was equippedoxygcn in wood itself); air/stock ratio (Nm'/kg) waswith a steel tube of inside diameter of 30 mm and totaldefined as: the volume air provided for 1 kg feedstocknlqccd in an electricalreaction/l kg feedstock reaction.furm中国煤化工maintaining theinitil:YHCN M H Grolysis reactor, asTable3 Kxcess uir ratio at diferent definitionshowea In rig.l. N2 was actea as fluidicd mediumApparent EAR. w/w0.and the fluidizing velocity was about 0.33 and 0.48Nm/s. Feedstock was fed by batch from the top of theReal EAR, w/w.410.S60.710.85reactor where equipped with a valve. As soon as theAirstock nalio Nm'kg 0.751.492.262.99stock was fcd into the reactor, the valve was closedNo.2Sudy on pyrolyis and gifcation of wood in MSW4092 Results and discussion112.1 Thermogravimetric analysis of woodThe thermogravimetric analysis was made in suchprocedure: al first, fccdstock was put into the analyzer2(向and the beginning temperature of furnace was kept at25C，then, the furnace was heated at 100C/min, andwhen the temperature achieved 100C it was kept for1 min to eliminate moisture in feedstock, afterwardsthe furnace was heated at 100 "C/min until the tem-peralure rcached 400C, 500C,600C，700C, at last,the temperature was kept until the total time o6experiments reached 700 s to make sure the reactionFig! listuion of luidized-bed pyrolysis reactorwas complcted.. nirgn-sppiere, 2. flow .meter. 3. fuidiod-bcod pyrolysisThe thermogravimetric analysis of wood in airN;reacor, 4. temperalue contollerel and heater, s. lar clletorr 6. gasis shown in Fig3. I1 was confirmned that wood couldbag; 7. vacuum fan; 8. gas mecce; 9 fiter; 10. cooler; I1. feder ;be combusted (in air) or pyrolyzed (in N) completely12. couling pipeover 500C. It was noticed by comparing Fig.3 thatquickly, gaseous product of pyrolysis entercd into apyrolysis was included in combustion, and pyrolysiscooler, condensate was collected in vessels, ancoursc was from 180 s 1o 290 s when the temperatureuncondensable gas was cllcted in a gas-bag. Afterwas about from 240'C to 400°C , Babu has a similarpyrolysis had been going on for 2 min that was longresult (1995). And afer that pyrolysis almost stoppedenough for reaction, the fluidized bed was clcancd upand combustion of pyrolysis char predominated fromand solid product of pyrolysis was collected.290 s to 500 s when the temperature was about fromThe fluidized-bed gasifer was cquipped with a400C to 500 C, 600 C, 700 C, until the reaction wassteel tube of inside diameter of 30 mm and total heightcompleted. It could be estimated that the volatile fromof 560 mm, which was placed in an clectrical fumacefeedstock was involved in combustion or othersame to pyrolysis reactor, as showed in Fig.2. Air wassecondary-reactions, but it could hardly showed inacted as fluidized medium and the fuidizing velocitythermogravimetric analysis (Zainal et al. 2001;was about 0.33 or 0.48 Nm/s. Feedstock was fcdMitchel, 1995).continuously, and the speed of feeding was adjusted tocontrol apparent excess air ratio (EAR) in the gasifierfrom 0.2 to 0.8. In experiments, solid and condensable00 iproducts were cllcted by filter and uncondensablegas was collected in gas-bag.The products of pyrolysis or gasification weredetected in proximatc and ultimatc analycr, gaschromatogram analyzer and so on.40400C .1020元500e--600个0一.有700，01002003004005006007001008060_0C 500C 600七中国煤化工_12MHC NMHG 700Fig2 lustration of luidied bed gasifier1. fan; 2. fowmer, 3. gas meter, 4. bffer; 5. fuidied bed gsifer; 6.Fig.3 Thermogravimetric analysis of wood in airta) and in N:(b)lemperalure conroller and heater; 7. coling piper 8. feeder; 9. syngascl片数糖.1. gashag.12 vacwumfin410NI Mingjiang et alVol.l82.2 Experiments of wood pyrolysis in a fluidi-it seemed no distinct difference caused by fluidizingzed-bed reactorvelocity. So, only analysis of pyrolysis in 0.33 m/s wasThe experiments of pyrolysis were made in thediscussed in this part.different fuidizing velocities (0.33 and 0.48 m/s), and10-Pyrolysis charb00-Pyralysis tarPyrolysis char》0 - _▲- Pyrolysis gas. - Steam io pyrolysis gasPytolysis tarB0 -情70Pyralysis gasTotal of pyrolysis productsSteam in pyrolysis gas. 70--+s0 F403020 t2(▲0←S00500600700Pyrolysis temperaturc,心Pyrolysis teroperatur, CFig. 4 Mass balance(a) and energy balacc(b) of wood pyolysis products at dfferent tempcratures2.2.1 Mass and energy percentage of pyrolysiskg feedstock/that of 1 kg feedstock X 100%.productsWith temperature increasing, the yield ofPyrolysis experiments of wood were done atpyrolysis gas inercased largely from 10% to 35%，various temperatures in a fluidized-bed reactor, andwhile the yield of pyrolysis char decreased tardilythe analysis of mass and energy percentage offrom 28% to 20%. The most suitable temperature forproducts were showed in Fig 4. Mass percentage is thepyrolysis tar producing was about 500"C , and underweight of product from 1 kg feedstock/1 kg feedstockthis condition the yield was up to 38%. The tolal massX 100%。Energy percentage is heat of product from 1percentage of products was 92% - - -95% .Table4 Proximate and ultimate analysis of char from difcrent pyrolysis temperaturesT'emperature,TProximale analysisUitimatc analysisM, %A.% .V,%FC, %Q,kJikgC.%H,%0%N,%S,%4006.507.2428.0254.932174263.882.7516.190.090.046.977.8322.0663.152203566.272.7717.620.300.127.358.7812.0871.792374472.531.6910.310.350.135.7510.096.7677.402796880.161.284.150.20Note: M, A, v, FC and Q are the samne as Table2It seemed that cnergy percentage followed thesimilar rule of that of mass percentage. Because theexperiments were done in lab scale, much of energywas lost. It was worthy to notice the steam in masspercentage analysis compared with the moisture inVVfecdstock. It could be estimated that much of H in400C500C600C700Cwood was transformed into H2O to fulfil heat inpyrolysis course. If wood pyrolysis was carried out inig.S Tar of wood from different pyrolysis termperaturesindustrializcd plant, relative heat loss would beTable 5Characteristics of tar from fluidized-bed pyrolysisdecreased and mass percentage of H would bereactor at different temperatureincreased in prolysis gas or tar rather than steam.中国煤化工2.2.2 Analysis of pyrolysis charMaHCNMHGi36.01 36.69With pyrolysis temperature increasing, the8606 7361....characterstics trend of pyrolysis char was clear asshowed in Table 4. Quantity of heat and C% of2.2.3 Analysis of pyrolysis tarpyrolysis char increased while volatilc percentIt was very different in appcarance of pyrolysis tardecrcased with temperature increasing.from fluidized-bed reactor at various pyrolysisNo.2Sudy on pyrolysis and gasifcation of wood in MSWtemperatures, as showed in Fig5. The pyrolysis tarabsorb the moisture in it, however maybe there waswas more liquid and lcss fuscous as temperaturestill some error in measurement. Quantity of heat ofincreased.wet pyrolysis tar was also tested, and showed in TableThe pyrolysis tar was very hydrophilic and5. With temperature increasing, percentage ofthermo-active, so it was difficult to mcasurc themoisture increased, while HHV of wet pyrolysis tarmoisturc of it. In this paper, silica gel was used tdecreased.. -1CO2 - Hz0.30 b; 20000-●-CH4 -+- C2H-+- Yield60卜-A-CO -x- C2Hs- LHV1800016000 ξ50卜息寸0.2014000 212000ot.0.15 (1000020-8000;上0.106000f40004050000700.05400600 700T, Cr, tFig.6 Percentage of componenls(a) and LIV and yield(b) in wood pyrolysis gas at dferent temperatures2.2.4 Analysis of pyrolysis gasreaction of wood was almost completed at 400CThe percentage and yield of main components(N2(Chen el al., 2003). And during 400- -700C, the mainfree) were analyzed in Fig.6. CO2 and CO were thesccondary reactions were carried out betweendominant components in pyrolysis gas. At 500C，pyrolysis gas and pyrolysis tar, in which steam mightthere was a decrease for many components inonly act as a product or an activator rather than apyrolysis gas, which was possibly caused by the yieldreactant.increase of pyrolysis tar, and the same reason for theReaction course of wood pyrolysis was analyzeddeercasc of lower heat value(LHV) of gas at 500C. At .as follows:700C，tar re-pyrolysis could not provide as muchWood .CO+ CO2+ H.O+heavy pyrolysis tarenergy for gas as before and caused a lttle decrease of+ pyrolysis char(1)LHV, but it must be declared that yicld of pyrolysisHeavy pyrolysis tar - -. Co+CO2+ H2gas and the total heat still increased.+ meddle- molecule tar(2)Table 6 Yield of components in wood pyrolysts gas fromMeddle-molecule tar - CO+ CH4+ C.H.+CH,.+ light tar3)fuidized beds with N: during different temperatures, x10- Nm/kgLight tar-- - CO+ CO+ H+lighter tar(4)Temp. range,CCO: CO H;CH, CH。 CH,(1) In 25- 400C te mperature rangc, the main25- -40031.32 28.27 0 3.91 0.34 2.34possible reaction was pyrol ysis reaction of wood, as400- 50035.4 997 10.07 1.230.110.50Rec.(1). The amount of pyrolysis gas was small, of500- -600-<6.5 42.00 0.61 20.86 1.28 15.09 .which about 50% was CO, about 40% was CO and no600- -70016.46 29.67 8.68 1.20 0.32 1.38H2 was produced. Pyrolysis tar was dense and fuscous,which was called heavy pyrolysis tar in this paper.2.2.5 Analysis of pyrolysis course(2) In 400 500"C temperature range, there wereBased on the analysis above, it could be taken astwo main Rec.(1) and (2). The yield of pyrolysis tarhypotheses for analysis of pyrolysis course of wood:and gas both incrcascd. Rec. (1) was nearly finished(1) the data abovc was rcasonable and the error couldduring this temperature range. On the other hand, duebe endurable; (2) the reaction course of pyrolysisto final temperature increased, speed of woodcould be divided in terms of temperature range; (3)pyrolysis_ was_ enhanced and lcss of tar wasduring each divided temperature range, products ofdecc |中国煤化工atalysis(Chenel al.,reaction would not be ffected by final temperature.200; produced. Rec.(2)The analysis of pyrolysis course was carricd outwasYHcNMHGrolysistarwasless.and the data was listed in Table 6.fuscous, H2 was produccd and the yield of pyrolysisBy thermogravimetric analysis of wood, massgas was increased during this temperature range.percentage analysis and proximate/ultimate analysis of(3) In 500- 600C tcmpcrature range, the concei-pyrolysis char, it could be bclieved that pyrolysisvable main was Rec. (3). It was very different from.412NI Ming-jiang et al.Vol.18Rec. (2), because more other gaseous componentswere produced during this temperature range.(4) In 600- 700C temperature range, the8000 Iconceivable main reaction was Rec.(4), and therc arcless energy 1ransformed from tar to gas than Rec.(3).6000(5) Actually, Rec.(1) (4) might exist during allpyrolysis temperature ranges, one of them might be .4000predominant at one tcmpcralurc rangc.20002.3 Wood gasification in fluidized bedsIn this scction, it was discussed and compared600that the experiments of wood gasification in lower。0.0.4fluidizing velocity (0.33 m/s) and higher fluidizingEecsaipo 0.8.6”400心faliovelocity (0.48 m/s). At first, the experiments of woodgasification in lower velocity were analyzed to give aclear impression, and then, the differcnces caused bydiferent velocities were brought forward and3.0十analyzed.2.52.3.1 Energy products from fluidizedbed gasifier2.0Because O2 was takcn into gasifcation reaction1.5 tand steam was very dificult to collect completely,1.0三0.5nly encrgy percentagc was takcn into account, as00shown in Figs.7 and 8. Char and tar could hardly be0.020.500 。separated, which commingled in syngas and were0.600 心0.:collected together in a filter, so they were analyediriotogether as one kind of products. Gasificationefficiency is the total heat of syngas from 1 kgrig9 LVI(a) and yield(b) of syngas o[ wood at diferet EARsslock/that of 1 kg stock X 100%; yield of syngas is theand temperturestotal volume of syngas from 1 kg stock/1 kg stock.When temperature was 600C and apparent EAR70 twas 0.4, the gasification efficicncy of wood was thehighest (about 73%), and under this condition the省s0percentagc of char and 1ar was about 12%.车402.3.2Analysis of syngasg30The LHV and yicld of syngas were anaiyzed日20(Fig. 9). With temperature increasing or apparent EAR700decrcasing, LHV of syngas increased; while yield of0.2syngas was greatly influented by apparcnt EAR but500 心bardly dcpendent of temperature.mtio0.8'400 Nm"lkgtar, char and syngas.EARCCHCH6CIHILReferences:0.151:210-1Babu s P, 1995. Thermal gasification of biomass technology216112127:developments: end of task report lor 1992 to 1994 [J] Biomass2347215776-15and Bioenergy.9: 271- -285.Bridgwater A V, Tot A J, Brummer J G.2002. 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