Energy Fuels 2011, 25, 108–118 Published on Web 12/08/2010
: DOI:10.1021/ef101246q
Cocombustion of Pulverized Coal with Waste Plastic and Tire Rubber Powders S. Singh, W. Nimmo,* M. Tayyeb Javed,† and Paul T. Williams Energy and Resources Research Institute, School of Process, Environmental and Materials Engineering, the University of Leeds, LS2 9JT, U.K. †On leave from the Pakistan Institute of Engineering and Applied Sciences, P.O. Nilore Islamabad, Pakistan. Received September 15, 2010. Revised Manuscript Received October 19, 2010
The generation of scrap tires in the world is approximately 1 billion per year and is set to increase in the foreseeable future as the number of cars on the roads increases. In this paper we present results from a pilot scale study on the cofiring of fine tire rubber (FTR) powder and plastic (high-density polyethylene, HDPE) with pulverized coal (PC) with a view to an application in power generation as an alternative to biomass, particularly their role in the combustion behavior of PC. We have performed experiments with a South African (SAf) coal and a South American (SAm) coal of different composition for a range of cofiring levels, up to 25%, and several configurations of combustion air distribution (air-staging). The fuels were fired through the burner and particular regard was paid to the effect on CO2, NOx, SO2, and carbon burnout. The results show that coal quality has a significant role during cofiring. In present investigations when HDPE was cofired with the SAm coal, a significant reduction in NOx was observed. Lower NOx emissions are achieved due to the different combustion behavior of the fuels. That is, modifying the primary combustion zone environment makes it more difficult for the intermediate HCN and NH3 species to oxidize to NOx due to the reduced O2 concentration at the point of fuel-N release in the flame. The influence of the tire-N content is thought to be small in the flames studied. We have seen around 20% NOx reductions when cofiring tire with the SAf coal for a 20% tire/coal cofiring ratio but insignificant NOx reductions when cofiring tire with SAm coal under the same conditions, indicating similar combustion characteristics and flame dynamics. In all cases, good burnout was achieved with less than 8% carbon in ash. The reduction of NO by the cofiring of tire-coal and plastic-coal fuel blends was further compared to historical literature involving the cofiring of biomass. It is noted that the ignition behavior of fuels of tire and plastic may differ to that of biomass; however, the overall reduction in NO is still observed in both cases. Historical air-staged experiments involving the cofiring of biomass were not able to provide similarly high reductions of NO to the staged cofiring of tire-coal and plastic-coal fuel blends. It is thought that the particle size of the biomass fired is a significant factor that is influencing the NO reduction efficiencies within a staged flame.
or whole tire; crumbing for application as soft surfaces or floor coverings, and environmental engineering applications. Products manufactured from recovered rubber crumb will ultimately require disposal or recycling at the end of their useful life. The use of high-density polyethylene (HDPE) is also widespread in our daily lives. Its use range from carrying bags and bottles to buckets. During the year 2007, the global HDPE market reached a volume of more than 30 million tons. Worldwide demand for HDPE is primarily influenced by rapidly growing industries, especially those of packing and construction.5 Most of the products goes to recycle bins after there first use and that shows a big potential for its better use in cofiring with the fossil fuel and coal in particular. We have chosen powdered HDPE as model material since pulverized waste plastic is not readily available. Fossil fuels are still major energy sources for electricity generation. Worldwide about 41% of electricity is generated by coal combustion. To assist in the sustainability of coal combustion, cocombustion is a valid practical option.6-8
Introduction The generation of scrap tires in the world is approximated as 1 billion per year and is set to increase in the foreseeable future as car and truck transportation continues to expand throughout the world.1,2 In Europe, the recent EC Waste Landfill Directive3 (1999) set a deadline for the banning of whole and shredded tires from landfill sites by 2006 creating opportunities for development of new ideas for tire disposal.4 Scrap tires are classified as a waste, meaning that they are no longer required for their original purpose but this does not mean that the material is completely without use for other applications. Tire rubber cannot be recycled for use in tire manufacture due to the chemical changes that occur during the rubber vulcanization process. Current alternatives in the U.K. involve energy recovery in cement kilns burning chipped *To whom correspondence should be addressed. Telephone: 0113 3432513. Fax: 0113 467310. E-mail:
[email protected]. (1) Rubber Manufacturers Association. http://www.rma.org/. (2) Nimmo, W.; Gibbs, B. M.; Williams, P. T. Combustion of coal with waste. Joint Meeting of the Coal Research Forum, Environment Division and the Royal Society of Chemistry, Energy Section, University of Nottingham, Nottingham, U. K., September 15, 2009. (3) The U.K. Environment Agency. http://www.environmentagency.gov.uk. (4) Opportunities and Barriers to Scrap Tyre Recycling, AEA Technology Report, 1994. r 2010 American Chemical Society
(5) Ceresana Research. Market Study: Polyethylene-HDPE, http:// www.ceresana.com/en/market-studies/plastics/polyethylene-hdpe/. (6) IEA. Key Word Energy Statics; International Energy Agency: Paris, France, 2008; p 24, 51, 57. (7) Task 32, IEA. Technical Status of Biomass Co-firing; 2009; p 4, 7, 10. (8) Munir, S.; Nimmo, W.; Gibbs, B. M. Energy Fuels 2010, 24, 2146– 2153.
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Energy Fuels 2011, 25, 108–118
: DOI:10.1021/ef101246q
Singh et al.
plastic (HDPE), a second hopper/screw system was employed, whereby the tire and HDPE were fed onto the vibrating tray of the primary coal feeder to allow a premixing of the fuels before entering the feed line to the burner. It was found that unstable operation could be a problem if the fuels were not allowed to mix sufficiently (see Figure 1b). Under these conditions (position A), the feed in the transport pipe work tended to form coal-rich and coal-lean variation in the fuel mix to the burner which caused pressure fluctuations in the furnace as the much more volatile tire fuel burned more rapidly than the coal. This was a greater problem with the South African coal and was solved by increasing the residence time of the tire and HDPE on the primary feed vibrating tray (position B). This had a correcting effect on the fluctuations in the fuel mixing caused by the periodic cofuel feed as it dropped off the screws onto the tray by evening out the distribution within the mix. In practice, the tire formed a slowly spreading line of fuel on top of the coal feed as it traveled along the tray which combined as it entered the entrainment system. Secondary air was supplied with a degree of swirl from an air box in the burner for flame stability. Operation under air-staging conditions was achieved by reducing the primary zone stoichiometry and introducing overfire air (tertiary air) after about 1 s for further burnout of the fuel. All air flows were metered by rotameter with corrections for pressure applied where appropriate. Levels of airstaging up to 40% were applied to the current study. Coals for the tests were South African (SAf) and South American (SAm) and supplied by E.On (Nottingham, U.K.).10 The tire for the tests was supplied by SRC Ltd (Cheshire, U.K.).11 High-density polyethylene (HDPE) plastic at a size of
