98/01995 Coprocessing of polymeric waste with coal: Reaction of polyethylene and coal model compounds : De Witt, M. J. and Broadbelt, L. J. Prepr. Pap. Am. Chemical Soc., Div. Fuel Chem., 1997, 42, (1), 38–42

Abstract

Environmental and economical concerns over diminishing landfill space and the growing abundance of mixed plastic waste mandate development of viable strategies for recovering highvalued resouIces from waste polymers. Co-processing of waste polymer mixtures with coal allows for the simultaneous conversion of coal and plastics into high-valued fuels. However, there is limited information about the underlying reaction pathways, kinetics, and mechanisms controlling coal liquefaction in the presence of polymeric materials. A series of model compound experiments has been conducted, providing a starting point for unraveling the complex, underlying chemishy. Neat pyrolysis studies of model compounds of polyethylene and coal were conducted in batch reactors. Tetradecane (C,,H,) was used as a polyethylene mimic, and 4-(naphthylmethyl)bibenzyl was used as a coal model compound. Reaction temperatures were 420 and 500'C, and batch reaction times ranged from 5-150 minutes. Derailed product analysis using gas chromatography and mass spectrometry enabled the reactant conversion and product selectivities to be determined. Reaction of single components and binary mixtures allowed the kinetic coupling between feedstocks to be examined. INTRODUCTION Recently, concerns over the inadequacy of current treatment and disposal methods for mixed plastic wastes have driven the exploration of new strategies for viable plastics resource recovery. The emphasis of the recovery is to obtain high-valued, useful products from the waste polymers. Post-consumer waste plastics are a major contributor to the municipal solid waste (MSW) sueam, constituting approxhnately 11% by weight and 21% by volume of waste in landfillsl. Over 40% of the landfills in the United States were closed in the past decade, and it is estimated that over half of the remaining ones will be full by the end of the century>. This poses a significant dilemma since there appears to be no immediate decrease in the usage of plastic products; in fact, due to their versatility, the usage will most likely increase. The current motivation for the recovery of plastics is due to government mandates, rather than to industrial initiatives. Some states, such as California, Oregon, and Wisconsin, have passed laws which specify that p h t i c bottles must be manufacnued from a minimum of 25% recycled plastics. Germany has dictated that over 80% of all plastic packaging must be recycled by methods other than combustion by 19963-5. Conventional plastics recycling technologies encounter a number of difficulties which range from costly separation to removal of impurities and contaminants. A consequence of these problems is that products manufactured from recycled polymers a~ of lower quality and higher cost (approximately 10% higher for high-density polyethylene (HDPE)) than those from the corresponding virgin polymet'. As a result, in the United States, only a b u t 4% of 30 million tons of total plastics produced each year is recycled6. Coprocessing of polymeric waste with other materials may provide potential solutions to the deficiencies of c m n t resowe recovery processes, including unfavorable process economics. By incovorating polymeric waste as a minor feed into an existing process, variations in plastic supply and composition could be mediated and as a result, allow for continuous operation. One option for coprocessing is to react polymeric waste with coal under direct liquefaction condition~2~~8. Coprocessing of polymeric waste with coal provides for simultaneous conversion of both feedstocks into high-valued fuels and chemicals. EXPERIMENTAL In order to obtain information about underlying reaction pathways, kinetics, and mechanisms without the complicating effects of the macrostructure, experiments were performed using model compounds for both coal and polyethylene, a voluminous component of mixed plastic waste. To mimic the structure of coal, 4-(naphthylmethyl)biben~l (NBBM) was used. NBBM contains both condensed and isolated aromatic species connected by short alkyl chains. An added feature is that it contains five different aromatic-aliphatic or aliphatic-aliphatic carboncarbon bonds. Successful predictions of the relevant primary products for real systems using NBBM confirmed the adequacy of this model compound, and thus, it will be employed in this study9-12. The structure of NBBM is depicted in Figure 1. Although numerous hydrocarbons may serve as appropriate model compounds for high density and low density polyethylene, teuadecane, C,,H,,, was chosen as an appropriate compromise in reactant size. Figure 1: Structure of coal model compound 4-(naphthylmethyl)bibenzyl.