Energy & Fuels 1997, 11, 1061-1072
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Pyrolysis of Furniture and Tire Wastes in a Flaming Pyrolyzer Minimizes Discharges to the Environment Claro I. Sainz-Diaz,*,† David R. Kelly,‡ Chistopher S. Avenell,§ and Anthony G. Griffiths§ Estacion Experimental del Zaidin (C.S.I.C.), Prof. Albareda, 1. 18008-Granada, Spain, Department of Chemistry, University of Wales, Cardiff, CF1 3TB, Cardiff, U.K., and Mechanical Engineering and Energy Studies, School of Engineering, University of Wales, Cardiff, CF2 1YF, Cardiff, U.K. Received January 3, 1997X
Wood furniture waste and scrap tires were pyrolyzed in a pilot scale batch flaming pyrolyzer. The effect of temperature, fuel/air ratio, and reaction times on the temperature distribution, gas and char pyrolysis yields, oxygen levels, SO2 and NOx emissions, and pyrolysis gas composition were studied. Low emission levels of NOx , SO2, and heavy metals were observed in the pyrolysis of wastes with high content of nitrogen (chipboard), sulfur, or heavy metals (scrap tires), respectively. The main components of the pyrolysis gas were acetylene, methane, and carbon monoxide. Gas chromatographic, GC-MS, and FT-IR studies of the heavy hydrocarbons fraction of pyrolysis gas were consistent with each other and showed the presence of alcohols, carboxylic derivatives, heterocyclic and phenolic compounds in furniture waste pyrolysate, and aromatic compounds in tires pyrolysate. Kovats indices for GC-MS retention times were calculated for a series of organic compounds of environmental interest. Organic compounds in the pyrolysate were identified from mass spectra and by comparison of retention times with authentic standards or published Kovats indices. The heating value of the pyrolysis gas from furniture waste and scrap tire was 8.7 and 5.6 MJ/m3, respectively.
Introduction The environmental problems of the disposal of diverse industrial and domestic solid wastes has grown considerably in recent years. At the same time dwindling stocks of fossil fuels in some regions and particular environmental problems with the emissions from the combustion of these fuels have led to research into the potential of using solid waste either as a supplementary supply of energy or for conversion to an alternative fuel.1 Currently, the most widely used method of waste exploitation is landfilling in many developing countries,2 but this has important hazard risks, such as leakage of toxic chemicals, uncontrolled emissions of gases, accidental fires, long breakdown times, etc. Direct incineration and biodigestion are strong alternatives. However, such processes are somewhat restricted in the types of waste material that can be utilized along with operational restrictions inherent in the process that lead to limits on their performance.3 Biomass is one of the main groups of solid wastes. It is mainly generated by the agricultural, farming, forestry, furniture, food, and paper-making industries. This biomass has no highly toxic constituents. However, if poorly processed, it can
still be a significant pollutant especially in landfill and uncontrolled incineration processes. The consumption of biomass by open burning is known to produce products of partial combustion, some of which are known carcinogens. Biomass has an important role as a combustible product, being the main fuel in developing countries (Figure 1).1,4,5 One difficult group of wastes is that from the furniture industry. Although the main waste is generated before the painting process, many raw components are wood composites, containing ureaformaldehyde, phenol-formaldehyde, and isocyanates resins. In 1990, wood composite production reached an estimated total of about 125 million m3 worldwide. Over 40% of this volume involved plywood and OSB (oriented strandboard) products, which contain about 700 kt of phenol-formaldehyde resin solids as the primary adhesive binder.6,7 Urea-formaldehyde resin is the only one used in the U.K. for the manufacture of chipboard. It is also used in most MDF (medium-density fiberboard) and is sometimes combined with melamine.8 The incineration of these wood composites can cause severe pollution problems.9 Another important environmental problem is heavy metal emissions from waste incinera-
* To whom correspondence should be addressed. † Estacion Experimental del Zaidin (C.S.I.C.). ‡ Department of Chemistry, University of Wales. § School of Engineering, University of Wales. X Abstract published in Advance ACS Abstracts, August 15, 1997. (1) Syred, N. Waste Energy Utilization Technology; School of Engineering, University of Wales: Cardiff, U.K., 1993. (2) Nels, C. H. In Pyrolysis and Gasification; Ferrero, G. L., Maniatis, K., Buekens, A., Bridgwater, A. V., Eds.; Elsevier: London, 1989; pp 379-386. (3) Beenackers, A. A. C. M.; Bridgwater, A. V. In Pyrolysis and Gasification; Ferrero, G. L., Maniatis, K., Buekens, A. Bridgwater, A. V., Eds.; Elsevier: London, 1989; pp 129-155.
(4) Arshad, A. S. Ph.D. Thesis, Department of Mechanical Engineering, University of Wales, Cardiff, U.K., 1993. (5) Avenell, C. S. Ph.D. Thesis, Department of Mechanical Engineering, University of Wales, Cardiff, U.K, 1997. (6) Tiedeman, G. T.; Isaacson, R. L.; Sellers, T. For. Prod. J. 1994, 44 (3), 73-75. (7) Gardner, D. J.; Sellers, T. For. Prod. J. 1986, 36 (5), 61-67. (8) Dinwoodie, J. M.; Higgins, J. A.; Paxton, B. H.; Robson, D. J. Wood Sci. Technol. 1991, 25 , 383-96. (9) Littorin, M.; Truedsson, L.; Welinder, H.; Skarping, G.; Martensson, V.; Sjoholm, A.-G. Scand. J. Work, Environ. Health 1994, 20, 216-22.
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Figure 1. Biomass in the developing countries’ energy resources.
Figure 2. Main destinations of tire wastes in the U.K. (1992).
tion and in domestic and inefficient biomass combustion processes, zinc being the most abundant heavy metal in these emissions. In 1991 the heavy metals emissions in Austria were about 1400 t/yr, of which zinc emissions were 235 t/yr.10 Non-natural industrial waste, such as scrap tires, is another important waste in this group. The production of this waste is increasing rapidly worldwide, especially in developing countries. In EC, the U.S.A., and Japan the production of tire waste was 4.5 Mt/yr (1989).11 Landfilling is the main treatment (Figure 2),12 and since they are nonbiodegradable, they are likely to continue to be a problem for many years to come. This situation has further stimulated research into improving current systems together with exploration of other possible approaches. One such method is pyrolysis, the thermal degradation of organic compounds in a low oxygen environment. This leads to volatilization of compounds followed by thermal scission reactions that produce a range of products in all three phases. Pyrolysis can be an endothermic process. However, the quantity of gas generated is in most cases more than sufficient to support the process if recycled for combustion to heat the waste material.13 One of the advantages of this process is the flexible nature of the reaction.14,15 Pyrolysis can have some advantages over incineration, such as reduced levels of NOx, SO2, and heavy metals, owing to the lower operating temperatures, lower oxygen concentration, and much lower air flow rate.1,16 (10) Winiwarter, W.; Schneider, M. The emission inventory: application and improvement. Proceedings of the A&WMA/EPA Conference, Raleigh, NC, 1994. (11) Sainz-Diaz, C. I. Pyrolysis of solid wastes. Report 1927 (Euroflam Programme); U.W.C.C.: Cardiff, U.K., 1994. (12) Leigh Environmental Ltd. Report; Birmingham, U.K., 1993. (13) Bridgwater, A. V. Biomass for Energy and Industry; Grassi, G., Gosse, G., dos Santos, G., Eds.; Elsevier: London, 1989; Vol. I, pp 2489-2496. (14) Avenell, C. S.; Griffiths, A. J.; Syred, N. Proceedings of IEE Clean Power 2000 Conference; IEE: London, 1993. (15) Esnouf, C.; Francois, O.; Churin, E. Biomass for Energy and Industry; Grassi, G., Gosse, G., dos Santos, G., Eds.; Elsevier: London, 1989; Vol. I, pp 2482-2488. (16) Heanley, C. R. Proceedings of Energy Technology Support Unit Workshop; R. E. E. Consultants: Birmingham, U.K., 1990.
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In the past 20 years many pyrolysis studies have been reported17,18 on a diverse range of materials, such as coals,19 lignocellulosics products,20,21 tires,22 and wastes.23 However, these studies were mainly done for research purposes on a benchtop scale apparatus, which relied on electrical resistance or induction heating.24,25 Further modifications have been applied to use fast, ultra-, and vacuum pyrolysis at this scale.13,26 Some studies of pyrolysis at the pilot or industrial scale have been published.3,27,28 Most of these are very interesting but require high investment costs, which can only be justified for specific materials or compounds. Some aspects of a prototype pyrolyzer with hot air pipes as an indirect heating system have been studied in Cardiff, but the scaling-up costs of this system were significantly high for its application in solid wastes processing.29,30 Recently, a low cost flaming pyrolyzer has been developed at pilot scale to produce fuel-gas and char from solid wastes.31 The flaming pyrolysis process differs from conventional pyrolysis by heating the waste directly with a gas-fired burner running substoichiometrically, thereby eliminating the thermal losses of the heat exchange media in indirect firing. SO2 and NOx emissions are low, and the pyrolysis gas (whose main component is acetylene) has an acceptable calorific value. In this work, this study has been extended to other solid wastes, such as plywood and chipboard wastes. One of the aims of this work was to undertake a more exhaustive analysis of the emissions of this flaming pyrolysis processing for different wastes. Experimental Section Raw Materials. In the present work, wood furniture wastes, chipboard and plywood wastes, and scrap tires have been selected as solid wastes. The wood furniture wastes were obtained from a furniture factory in the locality. The waste consisted of blocks, 10 cm × 20 cm × 3 cm, that were not pretreated or painted. In general the proportion of wood composite pieces was less than 10% except in the specific case (17) Piccaglia, R.; Galletti, G. C.; Traldi, P. J. High Resolut. Chromatogr. 1990, 13, 52-55. (18) Greenwood, P. F.; Zhang, E.; Vastola, F. J.; Hatcher, P. G. Anal. Chem. 1993, 65, 1937-1946. (19) Suuberg, E. M.; Unger, P. E.; Lilly, W. D. Fuel 1985, 64, 956962. (20) Jegers, H. E.; Klein, M. T. Ind. Eng. Chem. Process. Des. Dev. 1985, 24, 173-183. (21) Rodriguez-Maroto, J. M.; Garcia, F.; Rodriguez-Mirasol, J.; Cordero, T.; Suau, R.; Rodriguez, J. J. J. Environ. Sci. Health 1993, A28 (3), 651-662. (22) Williams, P. T.; Besler, S.; Taylor, D. T. Fuel 1990, 69, 14741482. (23) Garcia, A. N.; Font, R.; Marcilla, A. J. Anal. Appl. Pyrol. 1992, 23, 99-119. (24) Caballero, J. A.; Font, R.; Marcilla, A.; Garcia, A. N. J. Anal. Appl. Pyrolysis 1993, 27, 221-244. (25) Edye, L. A.; Richards, G. N. Environ. Sci. Technol. 1991, 25 (6), 1133-1137. (26) Scott, D. S.; Piskorz, J.; Radlein, D.; Czernik, S. In Pyrolysis and Gasification; Ferrero, G. L., Maniatis, K., Buekens, A. Bridgwater, A. V., Eds.; Elsevier: London, 1989; pp 201-208. (27) Avenell, C. S.; Griffiths, A. J.; Syred, N. Proceedings of ASEAN Conference on combustion of solids and treatment of products; ASEAN: London, 1995. (28) Herguido, J.; Corella, J.; Gonzalez-Saiz, J. Ind. Eng. Chem. Res. 1992, 31, 1274-1282. (29) Thorndyke, S. J. The Green Machine Waste Disposal Project. Report No. 46-10439; Ontario Research Foundation: Ontario, Canada, 1987. (30) Deleted in proof. (31) Avenell, C.; Sainz-Diaz, C. I.; Griffiths, A. J. Fuel 1996, 75, 1167-1174.
Pyrolysis of Furniture and Tire Wastes where only chipboard and plywood furniture wastes were used as feedstock. In this case, an equal mixture of both materials was used. The scrap tires were taken from a truck tires waste processing factory Leigh Environmental Ltd. in Birmingham. This waste was free of steel and granulated to 8 mm average size, having a calorific value of 37 800 KJ/kg. The composition of these scrap tires was mainly a polymer (50-65%), carbon black (25-40%), and some organic compounds as additives. The polymer was a mixture of copolymer of styrene/butadiene (5%) and natural rubber (polyisoprene) (95%). The sulfur content (1.5%) was one of the highest within the scrap tires.22 The heavy metal content was very low, except zinc, which was present in a significant amount in this raw material (1% as free form and 1-3% as zinc oxide). This tire material has been selected because it represents one of the potentially highest pollutant sources of sulfur and zinc. Chemical Analysis. A TestoTherm-33 portable gas analyzer was used to analyze the O2, CO, CO2, NO, NO2, and SO2 contents of the pyrogas. This analyzer was calibrated with standard gases. Filters for particles and liquids, which are between the probe and the analyzer, were changed in each run. For these pyrolysis studies, the actual carbon monoxide levels exceeded the CO limit of the analyzer. When the CO levels were higher or near of this limit (4%), the data were obtained by gas chromatography. A 104 Pye-Unicam gas chromatograph (GC) with thermal conductivity detector (TCD) and two packed columns (silica gel 60/80, 1.83 m × 3 mm i.d., and molecular sieves 5 Å 80/ 100, 1.83 m × 3 mm i.d.) was used to analyze hydrogen, carbon monoxide, methane, and other light hydrocarbons with helium as a carrier gas. Heavier organic compounds were detected by means of a Perkin-Elmer gas chromatograph with flame ionization detector (FID) and a capillary column (BPX5, 25 m × 0.32 mm × 0.25 µm) with nitrogen as a carrier gas. The gas chromatography linked to mass spectrometry (GCMS) was performed on a TRIO-1 mass spectrometer with an EI+ source at 70 eV and scanning 29-650 mass units in 0.9 s with 0.1 s interscan time. The GC was fitted with a fusedsilica capillary column DB-17 (30 m × 0.25 mm i.d.) with a stationary phase consisting of 50% phenyl and 50% methylsilicone. Hence, it is a moderately polar column that is recommended for separating aromatics.32 An injector port temperature of 300 °C and an oven temperature program (T1 ) 30 °C/5 min, rate ) 7 °C/min, T2 ) 250 °C/40 min) were used. The pyrolysate solution (1 µL) or a dilution in methylene chloride was injected with a split ratio of 50:1 and a solvent delay of 4 min. Helium carrier gas was maintained at a head pressure of 8 psi. All peaks in the chromatograms of the pyrolysate were checked for homogeneity in single-ion chromatography of at least three high-abundance ions. To identify the sample components, high-quality products were used as standards. Naphthalene was used as an internal standard on all runs and gave reproducible retention times of (0.03 min. Since the mass spectrometer only acquires one spectrum per second, the ultimate resolution possible is 0.017 min. Retention times reported are the averages of at least duplicate runs. Homologous series were analyzed in single runs and retention times confirmed by regression analysis. FT-IR analysis was performed with a Mattson 3000 FT-IR spectrometer in a KBr demountable cell. FT-IR spectra were recorded at 2 cm-1 resolution by accumulating 256 scans with the IR beam transmitted through the sample and by subtracting 256 background scans accumulated in the absence of sample. The CO2 and moisture content were purged from the sample and detector chambers of the spectrometer by using nitrogen gas. The zinc content was analyzed by atomic absorption spectroscopy (AAS) with a Video 22 Instrumentation Laboratory spectrometer at a wavelength of 213.9 nm. The extraction of the zinc from the scrap tires matrix (raw (32) Yancey, J. A. J. Chromatogr. Sci. 1994, 32, 403-413.
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Figure 3. Batch flaming pyrolyzer (i × j × k ) 132 cm × 122 cm × 114 cm): (a) windows (×9); (b) inlet burner’s port with 10 cm i.d.; (c) exhaust ports with 10 cm i.d.; (d) provision for different burner arrangement. material and pyrolysis chars) was achieved in a HPA (highpressure asher) autoclave by digestion in an aqueous solution of HCl and HNO3 at 300 °C and 140 bar.33 The temperature was raised from 25 to 300 °C in 30 min and held at 300 °C for 150 min with a cooling time of 1 h. Each sample was repeated 4 times, averaging the corresponding zinc content values. Elemental analysis was performed using a C, H, and N Technicon or a Perkin-Elmer 240B autoanalyzer with an accuracy of (0.05%. Pyrolysis Procedure. The pyrolytic processes have been studied in a batch fixed-bed flaming pyrolyzer at pilot scale, which has been described elsewhere (Figure 3).31 This pyrolyzer is an experimental prototype for preliminary studies and corresponds to one section of a continuous flaming pyrolyzer, which is being developed at Cardiff.11,29,30 The reactor can process up to 130 L of feedstock in a simple run with a significant reaction zone of 1061 L. This is intended to increase the level of scission reactions by thermal degradation following the initial volatilization process. Only one gas burner was used with variable thermal input up to a maximum power of 200 kW. The burner is a simple premix type with the addition of a cowl to promote a stable flame. Natural gas from the North Sea (94.4% of methane, calorific value ) 38.62 MJ/m3) was used as the burner fuel introduced through a failsafe system prior to being metered by a rotameter up to a maximum flow of 350 L min-1. All experiments were run using a fuel excess of 10% or 50% from the stoichiometric air/ fuel ratio. Taking into account that the stoichiometric ratio of methane to air is 1/9.5 v/v, a 10% fuel excess means a 1.1/ 9.5 v/v fuel/air ratio and a 50% excess means a ratio of 1.5/ 9.5. Two temperature levels were studied 450-500 and 600700 °C. The feedstock (25 kg for wood furniture waste and 20 kg for scrap tire in each run) was introduced and spread in the tray to give a layer 100 mm thick. Air is supplied by a centrifugal fan driven by a 10 KW, 30 A electric motor at 2800 rev min-1. The air flow rate is metered by a series of calibrated flow meters with a maximum capacity of 3760 L min-1. A preset pressure sensor is installed in the air pipe and connected with the gas solenoid valves, ensuring that the gas flow is shut off in the event of the centrifugal blower failing. The tray was charged with raw material, with the reactor in a cool state. During the warmup phase the minimum flow of air and gas was used to warm the (33) Anton Paar Company, Graz, Austria.
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Table 1. Elemental Analysis of Raw Materials and Pyrolysis Chars (%) material
carbon
hydrogen
nitrogen
oxygena
ash
woodb plywood chipboard chipboard charc plywood charc wood char-1d wood char-2e scrap tyref tyres charg,h
43.70 48.79 40.36 68.14 63.15 61.95 73.09 78.82 84.60
5.30 6.30 6.23 1.96 1.91 4.79 2.82 8.40 0.86
0.20 0.30 4.55 2.93
50 56 48 25 33 32 22
