COMBUSTION BEHAVIOR AND CHARACTERISTICS OF THE HYDROCARBONS RESULTED FROM THERMAL CRACKING OF SELECTED POLYOLEFIN BASED PLASTICS

The growing global challenge associated with the management of waste plastics has motivated extensive scientific research into their conversion into value-added hydrocarbon fuels. Processes such as thermal cracking and catalytic pyrolysis have been developed at both  aboratory and industrial scales to produce waste plastic pyrolyzed oil (WPO). While the production methodologies and fuel properties have been extensively studied, relatively few investigations have explored the combustion behavior and emission characteristics of WPO in internal combustion engines. This study addresses this critical gap by comprehensively evaluating the production, characterization, and combustion behavior of WPO derived from waste polyolefins (PE - Polyethylene, PP Polypropylene, and PS -  Polystyrene).  The research was conducted in three stages. In the first stage, thermal cracking experiments were carried out using a small-scale plant developed by Rohan et. al. (in his MPhil research), achieving high conversion rates (up to 97%) by optimizing process parameters such as reactor temperature and pressure. Set temperatures of 475 °C for PE, 425 °C for PP, 400 °C for PS, and 500 °C for PE-PP mixtures yielded high product recovery rates with efficient reaction times. The resulting fuels were primarily composed of light and middle distillates, indicating their suitability for combustion applications. An extensive analysis of the physical and chemical properties of the derived fuels was conducted in the second stage. Distillation curves, densities, viscosities, flash points, and calorific values were determined. Fuels exhibited high energy content (41.5 - 43.6 MJ/kg), low pour points, and significant light distillate fractions (35% - 60%), making them comparable to commercial gasoline and diesel. However, fuels derived from PE showed higher viscosities, necessitating blending or further refinement for optimal engine use. In the third stage, combustion behavior and emission characteristics were evaluated in both compression ignition (CI) and spark ignition (SI) engines. In CI engines, WPO blends (up to 50% by volume) with diesel demonstrated stable operation, favorable brake thermal efficiency (BTE), and lower exhaust gas temperatures compared to diesel. Blends beyond 50% resulted in engine instability, including speed fluctuations and stalling. Brake specific fuel consumption (BSFC) remained comparable to diesel for blends up to 50%, with slight increases at higher blend ratios. Emission analysis showed a reduction in nitrogen oxides (NOₓ) emissions with increasing WPO content, although carbon monoxide (CO) emissions increased at higher blend ratios. In SI engines, blends of up to 60% WPO (derived from PP) with gasoline maintained stable engine performance, comparable brake power, and consistent BSFC and BTE across a range of loads. However, blends with 70% WPO showed some instability, particularly at lower oads, indicating a threshold for practical application. Emission results revealed that CO and unburned hydrocarbon (UHC) emissions increased slightly at low loads but improved significantly at higher loads, with reductions in CO and CO₂ emissions observed for high load operation. Overall, this study confirms that hydrocarbon fuels derived from waste plastics, particularly PP and PE, possess strong potential as sustainable alternatives to conventional fossil fuels or use in CI and SI engines. Blending WPO with diesel and gasoline up to 50% and 60% respectively enables effective utilization without major engine modifications, offering a practical route for waste valorization and contributing to environmental sustainability. Future research should focus on optimizing fuel properties, refining blending strategies, and adjusting engine parameters to further enhance performance, reduce emissions, and expand the operational range for higher WPO content.

KEY WORDS 
Waste Polyolefins (PE, PP, PS), Pyrolysis, Thermal Cracking, Waste Plastic Oil (WPO), Fuel Composition, Distillation Characteristics, Calorific Value, Flash Point, Pour Point, Density, Viscosity, Combustion Behavior, Compression Ignition (CI) Engine, Spark Ignition (SI) Engine, Engine Performance, Emission Characteristics, Alternative Fuels, Sustainable Approach.