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1. IntroductionEach year the US alone produces 250 million tons of municipal solid waste (MSW). Among it, over 50% of the non-recyclable MSW ends up in landfill sites [1]. The landfilled MSW takes away valuable land and creates numerous potential environmental problems. In fact, the discarded MSW represents a tremendous energy source. Waste-to-energy (WTE) technologies can mitigate negative impacts of MSW and provide sustainable energy from low-cost feedstock. Examples of these technologies include incineration, gasification, anaerobic digestion, and pyrolysis [2] and [3]. Pyrolysis depolymerizes dry feedstock under an oxygen free environment. When the pyrolysis temperature is moderately high (450–550 °C) [4], the volatiles arise from pyrolysis process can be condensed to become liquid product, called pyrolysis-oil [5]. Unlike other technologies that produce heat or gases, pyrolysis-oil is transportable liquid and can be upgraded to transportation fuels or other platform chemicals [6]. Another advantage of the pyrolysis process is that it has low requirements for the feedstock type and reactor design, thus technology is relatively easy to scale up.While MSW consists of many different types of materials, biomass and plastics make up a majority of the composition [1]. When biomass is pyrolyzed alone, it produces a number of oxygenated products, such as sugars, aldehydes, ketones, acids and phenols. The presence of oxygen in the pyrolysis-oil (resulting from an abundance in the biomass feedstock) lowers the heating value and also causes thermal instability and corrosiveness [7]. On the other hand, plastic wastes are rich in hydrogen and contain much less oxygen than biomass. High density polyethylene (HDPE), the most commonly used plastic for example, has virtually no oxygen. Thus, compared to pyrolyzing biomass alone, co-pyrolyzing biomass and waste plastics increases carbon and hydrogen contents in the feedstock and could be beneficial in improving the quality of pyrolysis-oil. As a result, higher quality pyrolysis-oil could potentially reduce the costs associated with catalytic hydro-deoxygenation, which is required to process it into hydrocarbon fuels
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