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Energy & Fuels 2006, 20, 1536-1549
Fast Pyrolysis of Halogenated Plastics Recovered from Waste Computers William J. Hall and Paul T. Williams* Energy and Resources Research Institute, UniVersity of Leeds, Leeds, LS2 9JT, United Kingdom ReceiVed February 27, 2006. ReVised Manuscript ReceiVed May 16, 2006
The disposal of waste computers is an issue that is gaining increasing interest around the world. In this paper, results from the fast pyrolysis in a fluidized bed reactor of three different waste computer monitor casings composed of mainly acrylonitrile-butadiene-styrene (ABS) copolymer and two different waste computer body casings composed of mostly poly(vinyl chloride) (PVC) type polymers are presented. Preliminary characterization of the waste plastics was investigated using coupled thermogravimetric analysis-Fourier transform infrared spectrometry (TGA-FT-IR). The results showed that the plastics decomposed in two stages. For the ABS-containing monitor casings, aromatic and aliphatic material were released in the first and second stages. The PVC-containing computer body casing samples showed a first-stage evolution of HCl and a second stage evolution of aromatic and aliphatic material and further HCl. In addition, each of the five plastics was fast-pyrolyzed in a laboratory-scale fluidized bed reactor at 500 °C. The fluidized bed pyrolysis led to the conversion of most of the plastics to pyrolysis oil, although the two PVC computer body cases produced large quantities of HCl. The pyrolysis oils were characterized by GC-MS and it was found that they were chemically very heterogeneous and contained a wide range of aliphatic, aromatic, halogenated, oxygenated, and nitrogenated compounds.
Introduction The European Union disposed of around 0.73 million tons of plastic derived from waste electrical and electronic equipment (WEEE) in 2002; this figure is expected to increase by 20% in 2007.1 Included in the category of waste electrical and electronic equipment are refrigerators, washing machines, personal computers, printers, mobile phones, radios, television sets, electrical drills, sewing machines, etc. Around 30% of WEEE plastics contain fire retardants,2 which are mostly based on polybrominated aromatic compounds.3 Approximately 50% of waste WEEE plastics are high-impact polystyrenes, with the next largest fraction being acrylonitrile-butadiene-styrene (ABS) copolymer;4 PVC is also present in WEEE plastics.5 Recent EU legislation, in the form of the Waste Electrical and Electronic Equipment Directive,6 seeks to encourage the recycling and recovery of this waste stream. More than 90% of such waste is currently landfilled,6 as it represents a difficult waste to recycle. The EU Directive calls for, among other things, * Corresponding author. Tel: 0113 3432504. E-mail:
[email protected]. (1) Waste Management World; International Solid Waste Association: Copenhagen, Denmark, Nov/Dec 2002; pp 20-30. (2) Tange, L. Technical Workshop on Flame Retardant Plastics, Tokyo; Bromine Science and Environmental Forum: Brussels, Belgium, 2002. (3) An Introduction to Brominated Flame Retardants; Bromine Science and Environmental Forum: Brussels, Belgium 2000. (4) Tange, L. Technical Workshop Report on Sustainable Management for Plastics with Bromine, Tokyo, Nov 19, 1999; Bromine Science and Environmental Forum: Brussels, Belgium, 1999. (5) Vehlow, J.; Bergfeldt, B.; Hunsinger, H.; Jay, K.; Mark, F. E.; Tange, L.; Drohmann, D.; Frisch, H. Recycling of Bromine from Plastics Containing Brominated Flame Retardants in State-of-the-Art Combustion Facilities; Bromine Science and Environmental Forum: Brussels, Belgium, 2003. (6) European Commission, COM (2000-0158), 347 2000, Final, 2000/ 0158 (COD), Proposal for a Directive of the European Parliament and of the Council on Waste Electrical and Electronic Equipment. Official J. Eur, Communities Dec. 19, 2000, C 365.
the recovery of the major plastic components of WEEE. A significant proportion of WEEE is the plastic outer casings from computer bodies and computer monitors. Recycling of plastic waste generally involves low-level processing such as granulation or pelletization followed by melt or partial melt and extrusion to form the end product. The end products also tend to be for low-grade use such as dustbin sacks, pipes, backyard furniture, plastic lumber, and plastic/wood composites. The low-grade uses for mixed-plastic recycled materials has led to research in alternative processes for plastics recycling. For example, there has been research interest in the pyrolysis of waste plastic, in which the plastic is heated in an inert atmosphere to produce mainly a gas and an oil/wax product; for some plastics, a char product is also formed. The oil/wax can be used as a fuel or chemical feedstock, and the gas has sufficient calorific value to be used as a process fuel. The thermal recycling of waste plastics by pyrolysis represents an environmentally attractive route for recycling, by preserving valuable petroleum resources and diverting nondegradable waste away from waste landfill disposal. Although a large number of papers have been written regarding the pyrolysis of plastics,7-10 most have used model polymers and only a few reports use actual waste plastics. The pyrolysis of real-world plastic wastes can differ from model polymers because the polymer might already be slightly degraded and will be contaminated with dirt from its working environment; also, it is difficult to grind mixed-plastic waste (7) McNeill, I. C.; Memetea, L.; Cole, W. J. Polym. Degrad. Stab. 1995, 49, 181-191. (8) Brebu, M.; Uddin, M. A.; Muto, A.; Sakata, Y.; Vasile, C. Energy Fuels 2000, 14, 920-928. (9) Uemura, Y.; Azeura, M.; Ohzuno, Y.; Hatate, Y. J. Chem. Eng. Jpn. 2001, 34, 1293-1299. (10) Williams, P. T.; Williams, E. A. J. Inst. Energy 1998, 71, 81-93.
10.1021/ef060088n CCC: $33.50 © 2006 American Chemical Society Published on Web 07/01/2006
Pyrolysis of Halogenated Plastics from Waster Computers
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Table 1. Characteristics of the Waste Plastics waste plastic sample
country of origin
date
main material
computer monitor back cover computer monitor back cover computer monitor back cover computer body casing computer body casing
Taiwan Indonesia Taiwan
1994 1995 1991 1997 1997
ABS copolymer ABS copolymer ABS copolymer PVC PVC
into a uniform feedstock composition. The fillers used in realworld plastic products can also have an effect on the pyrolysis of the polymer, meaning that individual plastic samples will decompose in a different manner.11,12 Additionally, there has been little research into the pyrolysis of fire-retardant acrylonitrile-butadiene-styrene, which makes up a significant proportion of WEEE plastics. In this work, the fast pyrolysis of three different waste computer monitor casings composed of mainly acrylonitrilebutadiene-styrene (ABS) and two different waste computer body casings composed of mostly poly(vinyl chloride) (PVC) has been investigated. Coupled thermogravimetric analysisFourier transform infrared spectrometry was used for initial characterization of the pyrolysis of the five plastics, but the main body of the work was carried out using a 1m × 120 mm diameter fluidized bed reactor that was preheated to 500 °C. All five plastics contained different concentrations of halogens and metals and will therefore give an insight into the effect the different composition of plastic computer and monitor cases has on the product composition from fast pyrolysis. The oil and gas products were characterized in detail; in particular, the potential of the product oil as a potential chemical feedstock or petroleum-substitute fuel was investigated. Materials and Methods Materials. A large selection of plastic computer bodies and monitor cases were screened for the presence of halogens using scanning electron microscopy with energy-dispersive X-ray analysis (SEM-EDX). The system used was a Cambridge Scanning Co. Camscan Series III SEM coupled with EDX and had fully computer-based data handling and imaging. The cases that were found to contain halogens were ground to
