A Preliminary Study of the Plasma Pyrolysis of Waste Tyres

第3卷第1期过程工程学报VoL3 No.12003年2月The Chinese Jourmal of Process EngineeringFeb. 2003A Preliminary Study of the Plasma Pyrolysis of W aste TyresTANG Lan (唐兰)，HUANG Hai-tao (黄海涛),ZHAO Zeng-li (赵增立),WU Chuang- zhi (吴创之)(Guangzhou Institute of Energy Conversion,Chinese Academy of Sciences, Guangzhou, Guangdong 510070, China)Abstract: Thermal plasma pyrolysis of waste tyres for recovering energy was performed in a nitrogenplasma reactor. The main gaseous products were identified by chromatography as H, CO, CH, C2H2 andso on. From a series of experiments, the effects of the process parameters of thermal plasma pyrolysiswere investigated. Under our experimental conditions with steam injection, the total contents of H2 andCO reached up to 38.3% in the gas product, C2H2 up to 4%, and the maximum calorific value of thepyrolysis gas was 8.96 MJ/m'. The results indicate that plasma-assisted thermal decomposition of wastetyre particles may be a useful way for recovering energy and useful chemicals.Key words: thermal plasma; pyrolysis; waste tyres; waste recyclingCLC No: TQ352Document Code: AArticle ID: 1009 -606X(2003)01-0086-05INTRODUCTIONMotor vehicle tyres are nondurable goods, neither fusible nor soluble under normal conditionsand cannot be remoulded into other shapes without serious degradation. Common treatments suchas land illing or direct incineration (without recovery of value) not only waste the resources butalso increase the area of land with pollution. The increasing production of tyres has causedconsiderable disposal problem. To explore the real potential of waste tyres for possible material andenergy recovery, an efficient strategy should be found.Thermal treatment and resource recovery seem to be among the most advanced and significantdisposal methods. Pyrolysis has the potential of transforming solid wastes into useful recyclableproducts, but conventionally leads to a wide spectrum of pyrolysis products which are difficult to beseparated and utilized. Recently, attention has been paid to plasma -assisted pyrolysisl[1-3]. Thepyrolysis gas, having an appreciable calorific value, can be used for combustion; similar viewpointhas been reported by other authors; Chang et al!" used old tyres as the raw material for productionof syngas by thermal plasma pyrolysis, and the combustible heat value of the produced gas was 4~7MJ/m'. Compared with the conventional pyrolysis, plasma pyrolysis has a number of uniqueadvantages. For example, it provides a circumstance of high temperature and high energy forreaction, the reaction sample is heated up to a high temperature rapidly, and some reactions will takeplace which would not appear in conventional pyrolysis. It is an appropriate method for polymerpyrolysis. The main objective of the research work中国煤化工study the conversionand to characterize the gaseous product in N2 plasma:MYHCNMHG.Received date: 2002-08- 13, Accepted date: 2002- 10- 14Biography: Tang Lan (1977-), female, native of Guiyang city, Guizhou Province, majoring in heat energy engineering.1期TANG Lan, et al: A Preliminary Study of the Plasma Pyrolysis of W aste Tyres372 EXPERIMENTALThe center of the experimental setup is the plasma reactor with a maximum electric powerinput of 55 kV.A. A schematic of the reactor is shown in Fig.1. It consists of two main parts: thearc-discharge plasma generator (typical operating conditions are listed in Table 1) and the reactionchamber (1 000 mm in height and 50 mm in inner diameter), in addition to some accessories. Thecore temperature of the plasma torch at the entrance of the reaction cavity was calculated fromenthalpy of the working gas (nitrogen) to be in the range of 2500~3 500 K. Running water was usedto cool the system. The waste tyre sample was provided by Guangzhou Resource Recycling Company,the particle size of the sample used in this study was 50~80 mesh. The proximate, ultimate analysesand calorific value are listed in Table 2. Gas product samples were withdrawn through the samplingline, collected by rubber bags and analyzed in a GC-20B-1 gas chromatography system (Shimadzu,Japan). Solid residues were saved in the ash tank. Gaseous product yield was determined in eachexperiment by measuring the volume and calculating the weight of gas products. The solid residueyield and the fly ash yield were determined by mass balance.1. Nitrogen gas cylinder2. Control system of plasma generator3. Plasma reaction chamber4. Control system of screw feeder5. Feed hopper6. Gas sampling system7. Ash tank8. Cyclone separatorer ~uoCC amplingbag 9. Filter十十10. Water scrubber11. Cooling system12. Observational portto GC smpling bag.t tothe air13. Water steam generator10Fig.1 Schematic of the plasma reactorcooling waterTable 1 Typical operating conditions of the plasma reactorParameterRangeOptimumPlasma generator voltage (V)220-250220Plasma generator current (A)120-250160Plasma generator power (kVA)26.4-62.535.2Working gas flow rate (m/h)Working gas pressure (MPa)0.4Carrier gas (N2) flow rate (m/h)Sample powder feed ratle (g/min)20-100Water steam pressure (MPa)0.4 .Table 2 Ultimate and proximate analyses of feedstockMaterialProximate analysis (%,∞)中国煤化工__M.”V2 FC” A4(J/kg)Tyre particles0.93 63.67 26.43 8.97HCNMHG743730Note: Subscript denote averages on the dry basis; 1) Moisture; 2) Volatile; 3) Fixed carbon; 4) Ash; 5) Heat value.88过程工程学报3卷3 RESULTS AND DISCUSSION3.1 Product DistributionUnlike conventional pyrolysis, which has gaseous, liquid and solid products, waste tyreparticles are transformed only into gaseous and solid products in the process of thermal plasmapyrolysis. The key process parameters likely to affect the yields of product are power input, feedrate and steam injection, as illustrated in Figs.2 to 4.It can be seen from Figs.2 and 3 that due to the heat transfer and system energy, the yield of gasincreases as the input power is increased. From 30.8 to 48.4 kV.A, the yield of the gas increasesfrom 39.98% to 45.00%，higher input power does not decrease solid yield in the process. It is .suggested that the tyre decomposition at about 35.2 kV.A is completed. On the contrary, whenincreasing the feed rate, the yield of gas decreases obviously.30 r)0 rI Charyield30 fChar yieldWith steam injection 80 g/min|70 FI Gas yield30 t口Without steam injectionFeed rate: 80 g/min，0F70 EInput power 35.2kV A.50 Fwithout steam injectionwithout ste am injection. feed rate 80 g/min60 F603 50一40 t30十20 t20 I0-10|30.8 35.2 39.6 44 48.440810Char yield Gas yieldInput power (kV. A)Feed rate (g/min)ProductFig.2 Effect of input power on gasFig.3 Effect of feed rate on gasFig.4 Comparison of product yieldsand char yieldsof the experiment with orwithout water steam injectionWater steam is injected into the plasma reaction chamber together with the tyre particles inorder to improve product quality and get syngas so that the range of application can be extended andthe process be more economical. The comparison of gas and char yields corresponding to theprocess with or without water steam injection is given in Fig.4. With water steam injection, the yieldof the gas increases from 0.1 86 m2/min (42%, w) to 0.226 m/min (77%, w) at the same input powerand feed rate level (increased by 80%).3.2 Gas Composition and Calorific ValueThe gas composition from plasma pyrolysis of tyres is very different from that of conventionalpyrolysis. The conventional pyrolysis of tyres at 700°C gives(%)4: CH4 20.6, CH4 8.9, CH6 8.1,CzH 4.5, CjHg 3.2, CHg 16, C.H1o 3.8, CO 10.4, CO2 11.4. However, it can be seen from Table 3that the main gaseous products of plasma pyrolysis中国煤化工hy are H, CO, CH,C2H, O2, CO2, CH6, CH4 and other hydrocarbons..MYHCNMHG'of the advantages ofplasma pyrolysis; on a N-free basis, the concentration of Hh can reach up to about 57%. H2 is aclean gas fuel which can be used in several applications. In this study, it is desirable to get more gas1期TANG Lan, et al: A Preliminary Study of the Plasma Pyrolysis of Waste Tyres39yield and higher gaseous calorific value. On the basis of the experimental data summarized in Table3, the effects of plasma pyrolysis conditions on gas composition and gas calorific value arediscussed as follows.Table3 Gas composition and calorific value from plasma pyrolysisFeed rate Power inputGas composition (%, φ, dry basis)Solid conversion Gas calonific value(g/min)__ (kVA)_ H，CH___ CO_ CH_ CH C.H.+ unknown(%)(MJ/m)44.0435.28.750.713.07 0.284.5760.65.314.21.02 3.21 0.543.925.5542.2 .7.56F, 96.418.38 0.85 3.92 0.382.765.740.98 .7.34F, 122.518.541.013.27 0.543.65.30.987.9478.0630.812.070.71 2.75 0. 252.5739.686.76P275.3639.615.23 0.6 4.2 0.21.576.244.36.0186.6415.770.674.13 0.251.546.3443.16.4880.0448.416.15 0.69 4.25 0.271.425.83456.12S___ 75__35.224.12_09814.17 0.411.75778.96Note: Gas composition is balanced with N2, F-F. = tests with different feed rates, P-P = tests with different input power,S, = test with addition of steam3.2.1 Influence of power inputAs seen in Table 3, by increasing the input power from 30.8 to 39.6 kV.A, H2 concentration isincreased from 12.07% to 15.23%, CO concentration is increased from 2.75% to 4.2%, and the solidconversion is increased from 39.68% to 44.30%; but from 39.6 to 48.4 kV.A, only minor influenceof power input on the component of the gas was observed, and the efficiency did not increasefurther also. Concerning gas calorific value, one can see that the calorific value of the pyrolytic gasincreases firstly to a maximum value of about 7.56 MJ/m3 with increasing input power from 30.8 to35.2 kV.A, but it afterwards decreases, which suggests that the high calorific value gas such as CH,CH ulteriorly decomposed into H2 and elemental carbon with the elevation of the input power.Further increase of input power might increase the energy density in the system, but the maximumtemperature achieved in the discharge zone increases only slightly because of increased loss ofdischarge power as radiationl5l. It can be suggested that within the range of this study, the powerinput is not the key parameter influencing the pyrolysis, so we can get the approximate result usinglower power input with the otherwise same conditions.3.2.2 Effect of feed rateThe second important parameter with influence on conversion was detected during theinvestigation of the change of particle feed rate into the plasma jet. Sample feed rates were variedover the range of 40~ 120 g/min. From Table 3 we can see the increase of H2 and CO concentrationdue to the increase of feed rate. The same trends are found about the calorific value of the gas, butthe solid conversion is decreased from 60.60% to 30.98%. So we suppose that increasing the feedrate may decrease the amount of energy available for heating each particle and likely decrease theaverage temperature of particles; increasing the feed中国煤化Ince on the efficiencyof heat transfer. These two factors lead to decreasedYHCNM H Gand increased carbonresidues.90过程工程学报3卷3.2.3 Influence of steam injectionBy water steam injection, oxygen content is increased in the system, and the carbon conversioncan be increased. There are some chemical reactions occurring during plasma gasification wheninjecting water steam:C+H2O→CO + H2.Table 3 shows a trend of large increase of the amounts of H2 and CO and a slight decrease ofC2H2 concentration. The summation of H, CO concentrations reaches up to about 38.3% due tosteam injection reaction; without water steam injection, the concentration of H is about 15% andthere is scarcely CO. Water steam injection can elevate carbon conversion up to 77%. The gasproduct from plasma pyrolysis with steam can be utilized as syngas after separation of other gaseousproducts; also the calorific value of the pyrolytic gas has a large elevation to 8.96 MJ/m'. Thismeasure counteracts the disadvantage of limited application of gaseous product of plasma pyrolysisand favors practical applications.4 CONCLUSIONSExperiments on plasma pyrolysis of tyre particles show that:(1) Increasing the input power can increase the pyrolysis conversion and the yield of thegaseous product, but the calorific value of the pyrolytic gas would firstly increase then decrease.(2) Increasing the feed rate can increase the yield of the gaseous product and the gas calorificvalue, but a large feed rate would lead to a lower conversion.(3) Water steam injection is the main parameter influencing the gaseous products distributionand the gas quality. By addition of steam, the concentration of syngas can reach up to 38.3% and thecalorific value is 8.96 MJ/m3.REFERENCES:[1] Chang JS, GuB W, Looy P C, et al. Thermal Plasma Pyrolysis of Used Old Tyres for Production of Syngas [J]. J. Enviro. Sci.Health, 1996, A31(7): 1781-1799.[2] Guddeti R R, Knight R, Grossmann E O. Depolymerization of Polypropylene in an Induction-coupled Plasma []. Ind. Eng.Chem. Res, 2000, 39(5): 1171-1176.[3] Zhao Z, Huang H, Wu C, et al. Biomass Pyrolysis in an Argon/hydrogen Plasma Reactor [J. Chem. Eng. Technol. (Eng. LifeSci. Section), 2001, 2411): 197-199.[4] De Marco Rodriguez I, Laresgoiti M F, Cabrero M A, et al. Prolysis of Scrap Tyres ]. Fuel Process. Tehnol., 2001, 72: 9 -22.[5] Wei D Y C, Farouk B, Apelian D. Melting Metal Power Particles in an Inductively Coupled R.F. Plasma Torch [D]. Metall.Trans. B, 1988, 19B: 213-226.中国煤化工MHCNMHG