Low Temperature Dechlorination of Densified Refuse Derived Fuel in Pyrolysis

Mar.2005J. China Univ. of Mining &Tech (English Edition)VoL.15 No.1Low Temperature Dechlorination of DensifiedRefuse Derived Fuel in PyrolysisLI Wei' , XIE Qiang1 School of Chemical Engineering and Technology, China University of Mining & Technology,.Xuzhou, Jiangsu 221008, China;"School of Chemical and Environmental Engineering, China University of Mining & Technology; Bejing 100083, ChinaAbstract: Study on behavior of chlorine contained in oval-shaped densified refuse derived fuel (d-RDF) preparedfrom municipal solid waste in pyrolysis was carried out by means of temperature-programmed electrical furmace, andthe gas evolving from pyrolysis was investigated by FTIR. De-HCl rate was calculated by determining the emissionfraction of HCI in the fue gas and the fraction of Cl left in the pyrolysis residue. The results show that CI in the d-RDFreleases primarily in the form of HCl during the pyrolysis, and the initial releasing temperature of HCI enhances withthe increase of heating rate. Meanwhile, the higher the end temperature of pyrolysis, the more the CI released. De-HClrate is about 70% when the end temperature of pyrolysis is around 600"C. Besides, mechanism of Cl release is dis-cussed.Key words: municipal solid waste; refuse derived fuel; pyrolysis, dechlorination; HCI; FTIRCLC number: X 7051 Introductionposed as an altermative technology to exploit the en-In recent years the amount of municipal solidergy in MSW at no risk of formation of toxic diox-insl!-3!.waste (MSW) increases rapidly. With the depletion ofSo far, the characteristics of pyrolysis and com-suitable sites for MSW landiling and the enhance-ment of public attention to environment and ecology,bustion of pulverized main combustible componentsmore and more MSW is treated by waste-to-energyof MSW or RDF have been widely investigated bytechnology (for instance incineration) other than bymeans of thermal gravimetric analysis (TGA) andits direct lanfilling, and its thermal energy is recov-differential thermal analysis (DTG) 146), and theered for power generation. However, corrosion on theevolving emissions from MSW or RDF TGA processsurface of the heat exchanger in the boiler caused bywere determined and studied (7-8]. However, it need toacidic flue gases such as HCl limits the. power gen-be further validated that whether the results obtainederation efficiency to a considerable low level. Mea-from TGA experiments using tiny amount of pulver-nwhile, the existence of HCl may also stimulate theized samples (<10 20 mg in general), epecially theformation of highly toxic substances such as dioxinsresults about behavior of chlorine during pyrolysiswhich consequently cause serious environmentalare the same as that under industrial conditions usingpollution. Preparation of densified refuse derived fueldensified RDF pellets (>5 g in general, 25- 45 mm in(RDF) from MSW by crushing, drying, screening andsize).briquetting, followed by RDF pyrolysis at low tem-tainedMH中国煤化工of chorine co-perature with the aim to dechlorination, and gasifica-C N M H Guse derived fueltion of char derived from RDF pyrolysis were pro-(d-RDF) which prepared from municipal solid wasteReceived 25 October 2004; accepted 20 January 2005Project supported by Foundation of Education Department of Jiangsu Government for Commercialization of High Technology Developed by UniversitiesCore万弓数据l: +86-516 3884079, E-mail adress vidylee@e 163 comJ. China Univ. of Mining & Tech. (English Edition)VoL.I5 No.1in pyrolysis was carried out by means of tempera-energy-containing substance, such as carbonaceousture-programmed electrical furnace, and the gasshale, coal refuse, as well as MSW and RDF 12.evolving from pyrolysis was investigated by FTIRThus in this study, composition and properties ofwith the aim to provide practical data for high effi-RDF were analyzed and tested by the coal analysiscient and clean energy utilization of MSW.methods. The results are shown in Table 2.2 Experimental2.1 Preparation of samplesThe composition of MSW varies greatly fromregions and seasons. On the basis of survey andmodification of characteristics of MSW in China 9, a .formulation for an artificial RDF was proposed, asshowed in Table 1. The major advantages with anFig. 1 Densified refuse derived fuel (RDF- -5)artificial RDF in study include that the compositionof RDF is very homogeneous, the content of ele-Table 2 Characterization of Coal and RDFments is well known and the mixture can easilyM AVPCCHO*NSClQm 1(kJ.kg^)changed as the MSW content is changed. Thus thecoal 2.3 9.4 38.1 61.9 78.75.0 14.5 1.2 0.629910.5artificial RDF used in experimental study makes theRDF 4.2 41.3 33.7 20.9 59.87.6 27.4 0.3 0.9 3.910470.5result to be of general significance.*by differenceTable 1 Formulation for artifcial refuse derived fuelPaper (0.28 %)waste newspaper, book, packing paper2.2 Pyrolysis of RDFCellulose (7.49 %) flax, cotton clothThe schematic diagram of the experimental sys-organic Wood (1.25 %)Sawdusttem, which mainly consists of a temperature-pro-Plastic and rubberPVC, foamed plastic, weaved bag, plasticgrammed electric furnace for pyrolysis and a Niolet(33.74 %) _bottle, waste tyreNexus470 Fourier Transform Infrared SpectrometerMetal ( 1.756 %)iron and oxideCeramics and glassbottle, waste glass(FTIR), and both of them were jointed by TG-FTIRinorganic (5.49 %)interface, is shown in Fig. 2.Ash and otherssoil, gravel, coal briquete slag,. saw-(50.00 %)dust<10 mmMaterials in Table 1 were collected. All compo-nents were air dried and shredded into particles withsize less than 3mm, adding coal powder (<1mm)西西from Xuzhou Qishan Coal Mine which account fo20 per cent of total weight, then mixed well- propor-Fig.2 Schematic diagram of experimental system for pyro-lysis and dechlorinationtionally. The characteristics of the coal are shown in1. N2; 2. buffer bttl; 3. flow meter; 4. ectric furnace; 5. d-RDF sample;Table 2. A hydraulic machine (NYL2000) and special6. temperature coller, 7. beated tube;mold l1o) were used to prepare densified oval -shaped8. TG-FTIR iterface; 9. computer; 10. FTIR; 11. HCI sampling bte;12. induced draft fanRDF, as showed in Fig. 1. Acording to the classifica-tion of American Society for Testing and Materials中国煤化工d in desicator at 50(ASTM)", this RDF can be classified as densifiedCMHCNMHurately before runs.refuse derived fuel (d-RDF or RDF- -5). So far therePyrolysls expucuis WeIe cauried out at differenthave been no established standards for the analysisheating rates -5 C/min, 10 C/min and 20 C/min.and test of RDF in China. However, standards forAnd the end temperature at each heating rate is 400coal analysis are often adapted in studies of solidC, 500 C and 600"C, respectively. Pure nitrogen万万数据LIWei et al.Low Temperature Dechorination of Densified Refuse Derived Fuel in Pyrolysiswas used as an inert purge gas, both to prevent theEach RDF sample was a coarse mixture of vari-presence of air in the pyrolysis zone and to remove ety of large grains, as a result its chemical composi-gaseous and condensable products forming duringtion was always heterogeneous, and the exactpyrolysis, which is designed to minimize secondarychemical composition in each sample may differinteractions with the hot solid residue. A constantfrom the data in the ultimate analysis which results inflow rate of 300 cm/min (at 25 C and 101 N/m2) ofgreat errors in the chlorine mass balance calculation.the purge gas was fed in the furnace. Evolving gasAn improved mass balancing calculation method wasfrom pyrolysis passed through TG-FTIR interfaceadopted in this paper. Assuming that almost all chlo-and was heated to 200 C to minimize the condensa-rine components in the RDF were completely trans-tion of high boiling point gases in the transfer tubeformed into inorganic chlorine (water soluble) duringand then entered the gas cell of FTIR with KBr win-the experiment, and the loss of Cl was small enoughdow where were analyzed qualitatively. The FTIRto be ignored, the total amount of chlorine (q) in eachwas set to scan 32 times per second, and the scansample is approximately equal to the sum of chlorineprecision was 4 cm~'(qg) obtained from gas analysis and the chlorine (qs)Then the evolving gas passed through the HClin the residue obtained from ash analysis. Thus thesampling bottles with diluent NaOH solution (0.1reality of the data was not deteriorated by the fluc-mol/L). The absorbent was collected and then ana-tuation of the chlorine contents in the sample and thelyzed quantitatively by an improved titration ofvalue is more close to the real chlorine in each sam-AgNO3 method [14. The ash or residual char wasple.collected after each run, and then washed withion-free water to dissolve the Cr' ions. These cr'3 Results and Discussionsions representing the quantity of CI captured by the3.1 Form of Cl releasing from pyrolysisash or char were also determined by the method de-Typical 3D FTIR spectra of evolving gas fromscribed above.pyrolysis of RDF are shown in Fig. 3. It can be seen2.33 Calculation of de-Clthat a lot of organic and inorganic species such asThe mass balance of chlorine in the wholeHCl (2600 -3000 cm-) l15), water, organic acids andprocess can be presented as following:alcohols are released and can be identified in theq=qg +q, +l.(1)spectra. HCI released from 230 C, and the intensitywhere q is the total amount of CI in each sample,of the peak (HC1) increased with the increase ofwhich was calculated by the ulimate analysis; 9g istemperature. No other peaks of Cl-containing com-the amount of Cl emitted as HCI gas; qs is thepounds can be found in the spectra, as a result it canamount of water soluble CI captured by the residue; Ibe concluded that Cl contained in the RDF are re-is the amount of Cl lost in sample collecting proce-leased mainly in the form of HCI.dure, including gas leaking, formation of water-un-The organic components containing Cl insolvable organic Cl and so on.MSW include paper, plants, fibers and some plasticThe de-HCl capability in RDF during pyrolysis(esp. PVC), chemical composition of which can becan be evaluated by measuring the emission fractionexpressed in a chemical formula as CH,OzSiNClk.of Cl as HCI in flue gas, which is defined asIts pyrolysis reactions can be described as follow-9ging: .E=-(2)q中国煤化工H+CO+H20where E is the ration of evolving amount of CI in theform of HCI to the total CI amount of RDF sample,THCNMHGwhich is also simply defined as the de-HCl effi-The gaseous products of RDF are quite complex,ciency.while the products are in more stable form. Stability.J. China Uniy. of Mining & Tech. (English Edition)Vol.15 No.1of substances is relevant to the bond energy of somenon-equable heat transfer, and the pores of RDFsingle bonds, and the larger the bond energy of singlewas poorly developed, so they started to decom-bonds, the more stable the substances, and vice versa.pose at quite high temperature.Bond energy of some single bonds is shown in Table3.3 De-HCI efficiency3. It can be seen that HCl, H2O and other substanceDe-HCl efficiency E under different pyrolysisare most stable ones containing the same elements,conditions are shown in Fig. 4. It can be seen that Ei.e. the products of pyrolysis of CxHyO2SiNjClk are decreased when heating rate increased in the rangemainly in such forms.less than 350 C, which could be explained thatmore energy was accumulated at slow heating ratethat caused more compounds decomposed and then:35more Cl released in the form of HCl. E increased as: 30temperature got higher under all three heating con-0.ditions. In the higher temperature range (400- -600C), Cl releasing under fast heating rate increased:120001000rapidly because the substance in the surface of RDFWavenumbers/ cm-pellet was almost react out and the temperature gra-Fig.3 Typical 3D FTIR spectra of evolving gas from pyro-dient in the RDF pellet decreased, as a result mo-lysis of RDF (heating rate: 10 C/min)lecular bonds ruptured in a short time under fastTable 3_ Bond energy of some single bonds_ kJ/molheating rate and Cl atom parted from the organicSingle bond__ Bond energy_ Single bond__ Bond energycompounds, which combined with H atom and re-C- Cl243HC- CH3347leased in the form of HCl.H-CI|CH,CH2CH2- OHCH- H435HO- -H46480-C- CHCH234C1- PCl2326+s C/min70+ -i0 C /minCI -CHyCH2CH2CI-N19CH-CH3CH 020560H-NH2_38CH SCh50-3.2 Influencing factors on Cl release资40It also can be found that the higher initialtemperature of release of Cl occurred at a faster20-heating rate. Initial temperature 200 C, 230 C and10300350400450500550600270 C was corresponding to heating rates oft/C5 C/ min, 10 C/min and 20 C/min, respectively.Fig. 4 De-HCl rate under different pyrolysis conditionsIt indicated that slow heating rate favored to theAt 600 C (end temperature of pyrolysis), Erelease of CI at low temperature. Decompositionunder different heating rates were all approximatereactions of organic compounds under slow heating70 %. Incomplete release of Cl in RDF may berate occurred at lower temperature because long' caused by the following reasons:heating time and equable heat transfer were benefit1) The molcular structure of some organicto accumulate energy for the organic compounds incompounds may not be destroyed during pyrolysisthe RDF to decompose. In addition, slow heatingor polvmerization reactions of the products of firstrate promoted the growth of the interior pores ofde中国煤化工me time, cl com-RDF, which resulted in gas including HCl releasedbineYHC N M H Gich resulted in theat relatively low temperature. When heated at fastincomplete release of CI.rate, the organic compounds could hardly accumu-2) RDF contains certain amount of inorganiclate enough energy in the short time for the sake ofCl (for instance NaC1), which will release at veryLIWei etal.Low Temperature Dechlorination of Densified Refuse Derived Fuel in Pyrolysis5high temperature7.the aim to dechlorination and gasification of char3) Some metal oxides contained in RDF, suchderived from RDF pyrolysis had suggested an alter-as CaO, Fe2O3, can react with gaseous HCl and formnative technology to exploit the energy in MSW at nocorresponding metal chloride, especially at highrisk of formation of toxic dioxins. Meanwhile studytemperature, i.e. these metal oxides contained mighton behavior of releasing of Cl during pyrolysis ofplay an important role in capturing Cl in the soliddensified RDF pellets is the key step to industrializeresidue [18].technology.3.4 Dechlorination of small amount of pulverizedIn this paper, RDF with well representative andRDF and coarse densified RDF: a compari-technical performance was prepared from the pri-mary components of MSW and exploited in pyrolysisXIE Qiang et al 16 19 studied the pyrolysisexperiments to investigate the behavior of chlorine incharacteristics of pulverized MSW or RDF samplesMSW/RDF under a condition similar to industrialusing TG-FTIR, and found that Cl in them releasedprocess. Results showed that Cl contained in RDFall in the form of HCl at almost the same initial tem-released mainly in the form of gaseous HCl duringperature, and the amount of releasing HCl increasedpyrolysis, the decomposition of organic compoundswith the increase of temperature. However, de-HClcontaining Cl occurred at 200 270 C, and the initialduring pyrolysis of densified RDF demanded longerreleasing temperature of HCl increased with the in-time and kept releasing even at 600 C, which dif-crease of heating rate. Meanwhile, de-HCl efficiencyfered obviously from that organic Cl released com-was approximate 70 % when end temperature of py-pletely in some temperature range (260- -500 C) inrolysis was 600 C, which may be explained by thatthe TG experiments with small amount of pulverizedthe incomplete decomposition of compounds con-RDF. The difference may be caused by the followingtaining Cl, RDF containing inorganic Cl and metalreasons: the samples used in TG experiments wereoxide contained in RDF may play an important rolevery small both in quantity and in size, which reactedin capturing the Cl in the solid residue. Comparedquickly for fast heat transfer. However, RDF pelletswith the results obtained from the experiments usingused in this paper were prepared by high pressuremall amount of pulverized RDF samples, Cl con-briquetting technology, densified and stable, hencetained in densified RDF pellets released in the samethe de-volatilization took place layer by layer as theform of HCl during pyrolysis at almost the same ini-local temperature reached the thermal destructiontial temperature of release of CI, and the amount oftemperature and it took longer time.HCI released got more as temperature rose. 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