Energy & Fuels 2006, 20, 2557-2563
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Simultaneous Combustion of Waste Plastics with Coal for Pulverized Coal Injection Application Sushil Gupta,* Veena Sahajwalla, and Jacob Wood CooperatiVe Research Centre for Coal in Sustainable DeVelopment, School of Materials Science and Engineering, The UniVersity of New South Wales, Sydney, NSW 2052, Australia ReceiVed June 14, 2006. ReVised Manuscript ReceiVed July 31, 2006
A bench-scale study was conducted to investigate the effect of simultaneous cofiring of waste plastic with coal on the combustion behavior of coals for PCI (pulverized coal injection) application in a blast furnace. Two Australian coals, premixed with low- and high-density polyethylene, were combusted in a drop tube furnace at 1473 K under a range of combustion conditions. In all the tested conditions, most of the coal blends including up to 30% plastic indicated similar or marginally higher combustion efficiency compared to those of the constituent coals even though plastics were not completely combusted. In a size range up to 600 µm, the combustion efficiency of coal and polyethylene blends was found be independent of the particle size of plastic used. Both linear low-density polyethylene (LLDPE) and high-density polyethylene (HDPE) are shown to display similar influence on the combustion efficiency of coal blends. The effect of plastic appeared to display greater improvement on the combustion efficiency of low volatile coal compared to that of a high volatile coal blend. The study further suggested that the effect of oxygen levels of the injected air in improving the combustion efficiency of a coal-plastic blend could be more effective under fuel rich conditions. The study demonstrates that waste plastic can be successfully coinjected with PCI without having any adverse effect on the combustion efficiency particularly under the tested conditions.
Introduction In the last fifty years, the plastic industry has seen tremendous growth such that plastic products have become a critical part of our lifestyle. For example, plastic production in Japan has reached around 15 million tons per year resulting in about 9 million tons per year of related waste, 50% of which is associated with municipal solid waste.1 In Australia, more than one million tons of plastic is consumed each year.2 There is an increasing problem with its disposal due to a small percentage of recycling.2 Waste plastic can be recycled in a number ways, namely, material, chemical, or energy recycling. In Australia, only 13.2% of plastic is recycled for material recovery while the rest of it is disposed either through land-filling or burning in incinerators.2 Plastic materials do not degrade readily and can leach toxic elements in land while conventional burning is often associated with hazardous emissions such as dioxins. For sustainable development, sound environmental and economically feasible solutions are urgently required to dispose the plastic waste. Worldwide, the steel industry is facing severe pressures to minimize its impact on the environment by improving the efficiency of energy and resource utilization particularly to reduce the carbon intensity of the blast furnace process. One of the major worldwide strategies of energy management of a blast furnace is the reduction of fuel/coke consumption.3 Plastic injection into the tuyeres of blast furnaces is expected to reduce * Corresponding author. Tel.: + 61-2-93854433. Fax: 61-2-93855956. E-mail:
[email protected]. (1) Asanuma, M.; Ariyama, T.; Sato, M.; Murai, R.; Nonka, T. ISIJ Int. 2000, 44 (3), 244-251. (2) National Plastics Recycling Main SurVey Report 3071-04; PACIA Association, Nolan-ITU Pty Ltd: Australia, September, 2003. (3) Ziebik, A.; Stanek, W. Energy 2001, 26, 1119-1173.
the associated CO2 emissions because plastics embody two critical properties. Their combustion energy is at least as high as the pulverized coal normally injected, and their higher ratio of hydrogen to carbon means less CO2 as a combustion product.4 The steel industries in Japan and Europe are using this opportunity to improve the economic and environmental performance of blast furnaces.1,5,6 Plastic injection technology was developed by Bremen Steel Works, Germany, which is currently injecting more than 50 000 tons of plastic each year3 while JFE steelworks (Japan) has pioneered the technology of plastic injection by establishing the first integrated system for injecting waste plastic.6,7 Currently, JFE is also injecting a similar annual tonnage of waste plastic including poly(vinyl chloride) (PVC).6,7 In 1996, the Pohang Iron and Steel Company in Korea initiated plastic injection in blast furnaces but later discontinued this due to economic and combustibility issues.4 Recently, it has been proposed that simultaneous injection of waste plastic with coal in an innovative oxygen blast furnace could save up to 25% of carbon emissions.8 The Australian steel industry consumes more than 4 milllion tons of coke each year in blast furnaces; therefore, plastic injection technology has the potential to provide substantial coke savings. Undoubtedly, the availability of waste plastic and the processing cost are the most critical (4) Kim, D.; Shin, S.; Sohn, S.; Choi, J.; Ban, B. J. Hazard. Mater. 2000, 94 (3), 213-222. (5) Janz, J.; Weiss, W. Third European Ironmaking Congress, Ghent, Belgium, Sept 16-18, 1996; pp 114-119. (6) Goto, K.; Murai, R.; Murao, A.; Sato, M.; Asanuma, M.; Ariyama, T. International Blast Furnace Lower Zone Symposium, Wollongong, AIMM, Illawara, Australia, Sept 25-27, 2005; p 1-1. (7) Wakimoto, K. 60th Ironmaking Conference, Baltimore, USA, Mar 25-28, 2001; iss: Warrendale, PA, 2001; pp 473-483. (8) Murai, R.; Sato, M.; Ariyama, T. Proceedings of the Workshop on Science and Technology of InnoVatiVe Ironmaking for aiming at Energy Half Consumption, Tokyo, Japan, Nov 27-28, 2003; pp 205-209.
10.1021/ef060271g CCC: $33.50 © 2006 American Chemical Society Published on Web 09/02/2006
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requirements for successful utilization of waste plastic in a blast furnace. Outside Australia, often there are substantial subsidies available for plastic waste utilization, which has led to the development of special equipment for plastics injection. However, for Australia and other countries where such subsidies do not exist, to be economically viable, processed wastes will need to be mixed with coal so that existing coal injection facilities can be used. Once injected, the combustion performance of plastics or coal/plastics mixtures is important as these could adversely influence the blast furnace operation. Plastic waste can be broadly grouped into two categories, depending on the source of collection including municipal solid waste (MSW) and industrial waste which contain polyethylene (PE), polystyrene (PS), polypropylene (PP), and poly(vinyl chloride) (PVC) as the major constituents.9 Chlorinated polymers such as PVC are not preferred for direct injection in a blast furnace due to the well-known corrosive effects in off-gas systems. Pulverized coal injection (PCI) combustion has been extensively studied; however, limited studies have been conducted regarding the influence of factors affecting the combustion performance of coal when mixed with plastic.1,4,5,7,10-12 These studies are often conducted in specially designed facilities or hot models simulating raceway conditions of a blast furnace. One of the most important features of these facilities is their ability to simulate combustion conditions for short residence times of milliseconds as well as different raceway locations. Several approaches are used for the assessment of coal burnout or combustion efficiency. In some studies, the combustion efficiency was estimated on the basis amount of CO2 generated, while others used visual analysis of the tuyere flame. In addition to special furnaces, bench-scale reactors such as a DTF (drop tube furnace) or TGA (thermogravimetric analysis) have also been used to compare the combustion reactivity of the injected fuels. The bench-scale reactors are used to study the effect of coal properties in terms of the degree of carbon conversion or weight loss under controlled combustion conditions such as heating rates and temperature.13-19 In a DTF, the combustion efficiency is expressed in terms of coal burnout as estimated on the basis of ash conservation. The residence time of coal particles in a drop tube furnace is often on the order of seconds, being much higher than that often experienced in an industrial blast furnace raceway. Therefore, these are often used to compare the effect of fuel properties rather than complete simulation of raceway conditions. Effect of Plastic Size. Combustion/gasification behavior of fuels is influenced by chemical and physical properties of fuels including molecular structure, particle size, density, and com(9) Kato, K.; Nomura, S.; Uematsu, H. ISIJ Int. 2002, 42, S10-S13. (10) Vamvuka, D.; Schwanekamp, G.; Gudenau, H. W. Fuel 1996, 75 (9), 1145-1150. (11) Morgan, D. J.; Kosters, M.; Haas, J.; van de Kamp, W. L. EC Joule III Program- Clean Coal Technology Report No. JOF3-CT95-001013-Kamp-IFRF, 1999. (12) Babich, A.; Gudenau, H. W.; Senk, D.; Formoso, A.; Menendez, J. L.; Kochura, V. International Blast Furnace Lower Zone Symposium, Wollongong, AIMM, Illawara, Australia, 2005; pp 16-1. (13) Sahajwalla, V. Gupta, S. Wood, J.; Saha-Chaudhary, N. AustraliaJapan Iron & Steelmaking Symposium, Sydney, Austraila, 22-23 July 2004. (14) Courtemanche, B.; Levendis, Y. A. Fuel 1998, 77 (3), 183-196. (15) Henrich, E.; Burkle, S.; Meza-Renken, Z. I.; Rumple, S. J. Anal. Appl. Pyrolysis 1999, 49, 221-241. (16) Sorum, L.; Gronli, M. G.; Hustad, J. E. Fuel 2001, 80, 1217-1227. (17) Pinto, F.; Franco, C.; Andre, R. N.; Miranda, M.; Gulyurtlu, I.; Cabrita, I. Fuel 2002, 81, 291-297. (18) Garcia, A. N.; Esperanza, M. M.; Font, R. J. Anal. Appl. Pyrolysis 2003, 68-69, 577-598. (19) Lu, L.; Sahajwalla, V.; Harris, D. Metall. Mater. Trans. B 2001, 32, 811-820.
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bustion environment such as gas composition and temperature. A PCI study reported that the effect of particle size of coal on combustion efficiency is also dependent on oxygen stoichiometry and coal rank such that the combustion efficiency of high rank coal was improved with an increasing oxygen to carbon (O/C) ratio up to 2.5.10 Coarser particles of high rank coal provided higher combustion efficiency under fuel lean conditions (O/C > 2), while smaller particles ( 4) combustion efficiency of finer particles (
