Gasification of plastic waste as waste-to-energy or waste-to-syngas recovery route

Anke Brems, Raf Dewil, Jan Baeyens, Rui Zhang

College of Life Science and Technology, The Key Laboratory of Bioprocess of Beijing, Beijing University of Chemical Technology, Beijing, China.
Department of Chemical Engineering, Chemical and Biochemical Process Technology and Control Section, Katholieke Universiteit Leuven, Heverlee, Belgium.

DOI: 10.4236/ns.2013.56086                

Abstract

The disposal of plastic solid waste (PSW) has become a major worldwide environmental problem. New sustainable processes have emerged, i.e. either advanced mechanical recycling of PSW as virgin or second grade plastic feedstock, or thermal treatments to recycle the waste as virgin monomer, as synthetic fuel gas, or as heat source (incineration with energy recovery). These processes avoid land filling, where the non-biodegradable plastics remain a lasting environmental burden. Within the thermal treatments, gasification and pyrolysis gain increased interest. Gasification has been widely studied and applied for biomass and coal, with results reported and published in literature. The application to the treatment of PSW is less documented. Gasification is commonly operated at high temperatures (> 600℃ to 800℃) in an air-lean environment (or oxygen-deficient in some applications): the air factor is generally between 20% and 40% of the amount of air needed for the combustion of the PSW. Gasification produces mostly a gas phase and a solid residue (char and ashes). The use of air introduces N2 in the product gases, thus considerably reducing the calorific value of the syngas, because of the dilution. The paper will review the existing literature data on PSW gasification, both as the result of laboratory and pilot-scale research. Processes developed in the past will be illustrated. Recently, the use of a sequential gasification and combustion system (at very high temperatures) has been applied to various plastic-containing wastes, with atmospheric emissions shown to be invariably below the legal limits. Operating results and conditions will be reviewed in the paper, and completed with recent own lab-scale experimental results. These results demonstrate that gasification of PSW can be considered as a first order reaction, with values of the activation energy in the order of 187 to 289 kJ/mol as a function of the PSW nature.

Keywords

Plastic Waste; Re-Use; Gasification; Syngas; Char; Sustainability; Reaction Rate

1. INTRODUCTION: PSW AND ITS TREATMENT OPTIONS

Plastics are light-weight, durable, and versatile, allowing their incorporation into a diverse range of applications. In recent years, the environmental, social and economic impact of plastics has been the topic of the political agenda, with a focus on sustainable production and the decoupling of adverse environmental effects from waste generation. The disposal of waste plastics has become a major worldwide environmental problem. The USA, Europe and Japan generate about 55 million tons of post consumer plastic waste [1]. These waste products were previously dumped in landfill sites, a non-sustainable and environmentally questionable option. The number of landfill sites and their capacity are moreover decreasing rapidly and in most countries the legislation on landfills is becoming increasingly stringent.

New sustainable processes have emerged, i.e. 1) the advanced mechanical recycling of post-consumer plastic waste as virgin or second grade plastic feedstock; and 2) thermal treatments to recycle the waste as virgin monomer, as synthetic fuel gas, as hydrocarbon feedstock, or as a heat source (incineration with energy recovery). These processes avoid land filling, where the non-biodegradable plastics remain a lasting environmental burden. Plastic solid waste (PSW) treatment can be divided in four methods, as illustrated in Figure 1, each individual method providing a unique set of advantages making it particularly suited and beneficial to a specific location, application or product requirement [2,3]. The purpose of recycling is to minimize the consumption of finite natural resources, and this is especially relevant in the case of plastics which account for the use of 4% - 8% of the global oil production [3]: re-using plastics is the required course of action, with the additional benefit of reducing emissions associated with plastic production [3]. The most appropriate recovery method is chosen considering the environmental, economic and social impact of a particular technique. Figure 1 illustrates the position of each recycling method within the production chain.

2. THERMO-CHEMICAL RECYCLING OF PSW

Thermo-chemical recycling refers to advanced technology processes which convert plastic materials into smaller molecules, usually liquids or gases, which are suitable for use as a feedstock for the production of new petrochemicals and plastics [1]. Products have moreover been proven to be useful as fuel. The technology behind its success is the de-polymerisation process that can result in a very profitable and sustainable industrial scheme, providing a high product yield and a minimum residual waste. Processes of pyrolysis, gasification, hydrogen-nation, and steam/catalytic cracking have been previously reported upon [1].

Recently, much attention has been paid to chemical recycling (mainly pyrolysis, gasification and catalytic degradation) as a method of producing various hydrocarbon fractions from PSW. By their nature, a number of polymers are advantageous for such treatment. Thermolysis is the treatment of PSW in the presence of heat at controlled temperatures and under a controlled environment. Thermolysis processes can be divided into pyrolysis (thermal cracking in an inert atmosphere), gasification (in the sub-stoichiometric presence of air usually leading to CO and CO₂ production) and hydrogenation (hydrocracking) [4].

Pyrolysis can be successfully applied to Polyethylene teraphthalate (PET), polystyrene (PS), polymethylmetacrylate (PMMA) and certain polyamides such as nylon, efficiently depolymerising them into constitutive monomers [5-7]. Polyolefins, and in particular polyethylene (PE), has been targeted as a potential pyrolysis feedstock for fuel (gasoline) production, or to produce waxes as feedstock for synthetic lubricants, albeit with a limited success.

The development of value added recycling technologies is highly desirable as it would increase the economic incentive to recycle polymers. Several methods for thermochemical recycling are presently in use, such as direct gasification, and degradation by liquefaction [8]. Various degradation methods for obtaining petrochemicals are presently under investigation, and conditions suitable for pyrolysis and gasification are being researched extensively [9].

Catalytic cracking and reforming facilitate the selective degradation of waste plastics. The use of solid catalysts such as silica-alumina, ZSM-5 or zeolites, effect tively converts polyolefins into liquid fuel, giving lighter fractions as compared to thermal cracking.

Gasification has recently been attracting increased attention as thermo-chemical recycling technique. Its main advantage is the possibility of treating heterogeneous and

Figure 1. Schematic of recycling methods and their position within the processing line (Adapted from [2]).

contaminated polymers with limited use of pre-treatment, whilst the production of syngas creates different applications in synthesis reactions or energy utilisation. Gasification has been widely studied and applied for biomass and coal, with results reported and published in literature. The application for the treatment of plastic solid waste is less documented, although the number of publications increases exponentially.

Figure 2 shows different thermolysis schemes, main technologies and their main products obtained, as initially presented by Mastellone [10].

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	Baeyens, J., Brems, A. and Dewil, R., (2010) Recovery and recycling of post-consumer waste materials—Part 2. Target wastes (glass beverage bottles, plastics, scrap metal and steel cans, end-of-life tyres, batteries and house hold hazardous waste). International Journal of Sustain able Engineering, 3, 232-245. doi:10.1080/19397038.2010.507885
[2]	Al-Salem, S.M., Lettieri, P. and Baeyens, J. (2009) Recycling and recovery routes of plastic solid waste (PSW): A review. Waste Management, 29, 2625-2643. doi:10.1016/j.wasman.2009.06.004
[3]	Al-Salem, S.M., Lettieri, P. and Baeyens, J., (2010) The valorization of plastic solid waste (PSW) by primary to quaternary routes: From re-use to energy and chemicals. Progress in Energy and Combustion Science, 36, 103-129. doi:10.1016/j.pecs.2009.09.001
[4]	Ahrenfeldt, J. (2007) Characterisation of biomass producer gas as fuel for stationary gas engines in combined heat and power production. Ph.D. Thesis, Technical University of Denmark, Lyngby.
[5]	Yoshioka, T., Gause, G., Eger, C., Kaminsky, W. and Okuwaki, A. (2004) Pyrolysis of polyethylene terephthalate in a fluidised bed plant. Polymer Degradation and Stability, 86, 499-504. doi:10.1016/j.polymdegradstab.2004.06.001
[6]	Smolders, K. and Baeyens, J. (2004) Thermal degradation of PMMA in fluidised beds. Waste Management, 24, 849-857. doi:10.1016/j.wasman.2004.06.002
[7]	Brems, A., Baeyens, J., Beerlandt, J. and Dewil, R. (2011) Thermogravimetric pyrolysis of waste polyethylene-terephthalate and polystyrene: A critical assessment of kinetics modeling. Resources, Conservation and Recycling, 55, 772-781. doi:10.1016/j.resconrec.2011.03.003
[8]	Steiner, C., Kameda, O., Oshita, T. and Sato, T. (2002) EBARA’s fluidized bed gasification: Atmospheric 2 × 225 t/d for shredding residues recycling and two-stage pressurized 30 t/d for ammonia synthesis from waste plastics. Proceedings of Second International Symposium on Feedstock Recycle of Plastics and Other Innovative Plastics Recycling Techniques, Ostend, 8-11 September 2002.
[9]	Aguado, J., Serrano, D.P., Miguel, G.S., Escola, J.M. and Rodriguez, J.M. (2007) Catalytic activity of zeolitic and mesostructured catalysts in the cracking of pure and waste polyolefins. Journal of Analytical and Applied Pyrolysis, 78, 153-161. doi:10.1016/j.jaap.2006.06.004
[10]	Mastellone, M.L. (1999) Thermal treatments of plastic wastes by means of fluidised bed reactors. Ph.D. Thesis, Second University of Naples, Naples.
[11]	Arena, U. and Mastellone, M.L., (2006) Fluidized bed pyrolysis of plastic wastes. In: Scheirs, J. and Kaminsky, W., Eds., Feedstock Recycling and Pyrolysis of Plastic Wastes: Converting Waste Plastics into Diesel and Other Fuels, John Wiley & Sons, Chichester. doi:10.1002/0470021543.ch16
[12]	Scheirs, J. (1998) Polymer recycling. Wiley, New York.
[13]	Vermeulen, I., Van Caneghem, J., Block, C., Baeyens, J. and Vandecasteele, C. (2011) Automotive shredder residue (ASR): Reviewing its productions from end-of-life vehicles (ELVs) and its recycling, energy and chemicals valorization. Journal of Hazardous Materials, 190, 8-27. doi:10.1016/j.jhazmat.2011.02.088
[14]	Wallmann, P.H., Thorsness, C.B. and Winter, J.D. (1998) Hydrogen production from wastes. Energy, 23, 271-278. doi:10.1016/S0360-5442(97)00089-3
[15]	Pinto, F., Franco, C., Andre, R.N., Miranda, M., Gulyurtlu, I. and Cabrita, I. (2002) Co-gasification study of biomass mixed with plastic wastes, Fuel, 81, 291-297. doi:10.1016/S0016-2361(01)00164-8
[16]	VTT (2004) Power production from waste and biomass IV. Proceedings of the VTT Symposium, Finland, 8-10 April 2002.
[17]	Buekens, A.G. (1978) Resource recovery and waste treatment in Japan. Resource Recovery and Conservation, 3, 275-306. doi:10.1016/0304-3967(78)90011-2
[18]	Hasegawa, M., Fukuda, X. and Kunii, D. (1974) Gasification of solid waste in a fluidized bed with circulating sand. Conservation and Recycling, 3, 143-153. doi:10.1016/0361-3658(79)90004-3
[19]	Borgianni, C., Filippis, P.D., Pochetti, F. and Paolucci, M. (2002) Gasification process of wastes containing PVC. Fuel, 14, 1872-1833.
[20]	Xiao, G., Jin, B., Zhou, H., Zhong, Z. and Zhang, M. (2007) Air gasification of polypropylene plastic waste in fluidized bed gasifier. Energy Conversion and Management, 48, 778-786. doi:10.1016/j.enconman.2006.09.004
[21]	Matsunami, J., Yoshida, S., Yokota, O., Neuzka, M., Tsuji, M. and Tamaura, Y. (1999) Gasification of waste tyre and plastic (PET) by solar thermochemical process for solar energy utilization. Solar Energy, 65, 21-23. doi:10.1016/S0038-092X(98)00085-1
[22]	Pinto, F., Franco, C., Andre, R.N., Tavares, C., Dias, M. and Gulyurtlu, I. (2003) Effect of experimental conditions on co-gasification of coal, biomass and plastic wastes with air/steam mixture in a fluidized bed system. Fuel, 82, 1967-1976. doi:10.1016/S0016-2361(03)00160-1
[23]	Slapak, M.J.P., Kasteren, J.M.N.V. and Drinkenburg, A.A.H. (2000) Design of a process forsteam gasification of PVC waste. Resources, Conservation and Recycling, 30, 81-93. doi:10.1016/S0921-3449(00)00047-1
[24]	Xiao, G., Ni, M., Chi, Y., Jin, B., Xiao, R., Zhong, Z. and Huang, Y. (2009) Gasification characteristics of MSW and ANN prediction model. Waste Management, 29, 240-244. doi:10.1016/j.wasman.2008.02.022
[25]	Aznar, M.P., Caballero, M.A., Sancho, J.A. and Francs, E. (2006) Plastic waste elimination by co-gasification with coal and biomass in fluidized bed with air in pilot plant. Fuel Processing Technology, 87, 409-420. doi:10.1016/j.fuproc.2005.09.006
[26]	Cozzani, V., Nicolella, C., Rovatti, M. and Tognotti, L., (1997) Influence of gas phase reactions on the product yields obtained in the pyrolysis of polyethylene. Industrial and Engineering Chemistry Research, 36, 342-348. doi:10.1021/ie950779z
[27]	Stiles, H.N. and Kandiyoti, R. (1989) Secondary reactions of flash pyrolysis tars measured in a fluidized bed pyroly sis reactor with some novel design features. Fuel, 86, 275-282. doi:10.1016/0016-2361(89)90087-2
[28]	Zolezzi, M., Nicolella, C., Ferrara, S., Iacobucci, C. and Rovatti, M. (2004) Conventional and fast pyrolysis of automotive shredder residues (ASR). Waste Management, 24, 691-699. doi:10.1016/j.wasman.2003.12.005
[29]	Miscolczi, N., Bartha, L., Deák, G. and Jóver, B. (2004) Thermal degradation of municipal solid waste for production of fuel-like hydrocarbons. Polymer Degradation and Stability, 86, 357-366.
[30]	Ciliz, N.K., Ekinci, E. and Snape, C.E. (2004) Pyrolysis of virgin and waste polyethylene and its mixtures with waste polyethylene and polystyrene. Waste Management, 2, 173-181. doi:10.1016/j.wasman.2003.06.002
[31]	Ponzio, A., Kalisz, S. and Blasiak, W. (2006) Effect of operating conditions on tar and gas composition in high temperature air/steam gasification (HTAG) of plastic containing waste. Fuel Processing Technology, 3, 223-233. doi:10.1016/j.fuproc.2005.08.002
[32]	Franco, C., Pinto, F., Gulyurtlu, I. and Cabrita, I. (2003) The study of reactions influencing the biomass gasification process. Fuel, 82, 835-842. doi:10.1016/S0016-2361(02)00313-7
[33]	Marquez-Montesinos, F., Cordero, T., Rodriguez-Mirasol, J. and Rodriguez, J.J. (2002) CO2 and steam gasification of grapefruit skin char. Fuel, 81, 423-429. doi:10.1016/S0016-2361(01)00174-0
[34]	Zanzi, R., Sjostrom, K. and Bjornbom, E. (1996) Rapid high-temperature pyrolysis of biomass in a free fall reactor. Fuel, 75, 545-550. doi:10.1016/0016-2361(95)00304-5
[35]	Zanzi, R., Sjostrom, K. and Bjornbom, E. (2002) Rapid pyrolysis of agricultural residues at high temperature. Biomass and Bioenergy, 23, 357-366. doi:10.1016/S0961-9534(02)00061-2
[36]	Narvaez, A., Orio A., Aznar, M.P. and Corella, J. (1996) Biomass gasification with air in an atmospheric bubbling fluidized bed: effect of six operational parameters. Indus trial and Engineering Chemistry Research, 35, 2110 2120. doi:10.1021/ie9507540
[37]	Harder, M.K. and Forton, O.T. (2007) A critical review of developments in the pyrolysis of automotive shredder residue. Journal of Analytical and Applied Pyrolysis, 79, 387-394. doi:10.1016/j.jaap.2006.12.015
[38]	Mancini, G., Tamma, R. and Viotti, P. (2010) Thermal process of fluff: Preliminary test on a full scale treatment plant. Waste Management, 30, 1670-1682. doi:10.1016/j.wasman.2010.01.037
[39]	Vigano, F., Consonni, S., Grosso, M. and Rigamonti, L., (2010) Material and energy recovery from automotive shredder residue (ASR) via sequential gasification and combustion. Waste Management, 30, 145-153. doi:10.1016/j.wasman.2009.06.009
[40]	Cho, S.J., Jung, H.Y., Seo, Y.C. and Kim, W.H. (2010) Studies on gasification and melting characteristics of automotive shredder residue. Environmental Engineering Science, 27, 577-586. doi:10.1089/ees.2009.0389
[41]	Lin, K.S., Chowdhury, S. and Wang, Z.P. (2010) Catalytic gasification of automotive shredder residues with hydro gen generation. Journal of Power Sources, 195, 6016 6023. doi:10.1016/j.jpowsour.2010.03.084
[42]	Donaj, P., Yang, W., Blasiak, W. and Forsgren, C. (2010) Recycling of automotive shredder residue with a micro wave pyrolysis combined with high temperature steam gasification, Journal of Hazardous Materials, 182, 80-89. doi:10.1016/j.jhazmat.2010.05.140
[43]	Donaj, P., Blasiak, W., Yang, W. and Forsgren, C. (2011) Conversion of microwave pyrolysed ASR’s char using high temperature agents. Journal of Hazardous Materials, 185, 472-481. doi:10.1016/j.jhazmat.2010.09.056
[44]	WGT (2002) Waste gas technology energy from waste. http://www.wgtuk.com/ukindex.html
[45]	Weissman, R. (1997) Recycling of mixed plastic waste by the Texaco gasification process. In: Hoyle, W. and Karsa, D.R., Eds., Chemical Aspects of Plastics Recycling, The Royal Society of Chemistry Information Services, Cam bridge.
[46]	Croezen, H. and Sas, H. (1997) Evaluation of the Texaco gasification process for treatment of mixed household waste. Final Report of Phase 1 and 2, CE, Delft, The Netherlands.
[47]	Tukker, A., de Groot, H., Simons, L. and Wiegersma, S. (1999) Chemical recycling of plastic waste: PVC and other resins. European Commission, DG III, Final Report, STB-99-55 Final, Delft, The Netherlands.