Thermal Depolymerization of Scrap Tires Into Liquid Fuels: Upgradation and Utilization In Diesel Engine

Abstract

Conversion of scrap tire into fuel oils has attracted commercial attention since revenue can be generated from inexpensive and abundant feedstock while easing waste management issues. Globally, 1.5 billion scrap tires are generated every year. Environmental accumulation of tire waste is a global problem, and one way to control the problem is to convert them into fuels and specialty chemicals. There are various approaches for recycling scrap tires, such as re-treading, reclaiming useful products for playgrounds, open incineration, pyrolysis, gasification, and illegal dumping. Illegal dumping often provides a site for breeding mosquitoes, rodents, and larvae formation. Open burning releases a thick black plume of smoke with 1,3-butadiene, nitrogen, carbon, and sulfur oxides with the release of hazardous polyaromatic hydrocarbons. Out of the approaches mentioned above to recycle tires, pyrolysis is an interesting energy recovery process due to the formation of solid (carbon black) and steel wires (in the case of the tire), pyro-gas, along oil products. Production of crude tire pyrolysis oil from scrap tires is a promising approach by thermal depolymerization at an oxygen starved atmosphere and a temperature of 400-600 oC. The primary objective of present study is to refine CTPO by the principle of selective adsorption and preferential solubility using cost-effective adsorbent and solvent and utilization as a fuel in a single-cylinder diesel engine. A field study was conducted in a 10-ton rotating autoclave reactor to optimize scrap tire pyrolysis parameters (400 oC, 10 oC/min, 0.2 bar, 4 rpm), and investigate the existing problems in the industry with a special focus on applying CTPO in diesel engines. Crude tire pyrolysis oil (CTPO) is a dark brown to black colored syrupy liquid with C6-C24 organic compounds with various classes such as paraffin, olefins, terpenes, aromatics, nitrogen, and sulfur-containing compounds, oxygen-containing compounds. The major challenge for utilizing CTPO in engine or furnace is the inferior fuel properties such as low heat content, low flash point, high acidity, low cetane index, creaming or phase separation in storage tanks, pungent smell due to the presence of dibenzothiophenes and mercaptans. However, thermal distillation is widely used as an upgradation technology implemented in most of the small scale tire pyrolysis units. Distillation needs huge capital investment and energy, making the process less attractive and unsuitable for the long-term run. In the present study, a straightforward, robust, inexpensive, and scalable up-gradation strategy for refining CTPO by preferential solubility and selective adsorption to utilize single-cylinder direct-injected stationary engines is formulated. A limited study has been attempted for the up- gradation of CTPO using adsorbents and solvents. The present study also envisages extensive characterization of CTPO, StTPO and diesel to comprehend the fuel chemistry in terms of physical, thermal, and chemical analysis through various analytical techniques. GC×GC TOF- MS analysis showed that sulfur, benzene derivatives, naphthalene’s and polyaromatic hydrocarbons were lowered by 48.86%, 25.68%, 43.69%, and 27.79%, respectively. The batch scale process's oil yield is improved by 95% compared to the laboratory scale upgradation strategy. Experimental results found that StTPO40 is a binary optimal blend in terms of performance, combustion, and emissions. The emissions from StTPOxx were significantly improved after upgradation by silica gel as adsorbent and petroleum ether as a diluent. Furthermore, ethyl levulinate, a potential bio-diluent with high oxygenate, was also utilized as an additive to StTPOxx blends to scrutinize performance, combustion, and emissions of single- cylinder, direct-injected stationary diesel engine, which is another novelty of the present study. The emission components are significantly dropped down after the upgradation of CTPO, but the performance was slightly lowered after the refining process. The nitrous oxide emission from StTPO40 and StTPO40EL10 was significantly reduced by 43.09% and 44.54%, respectively. Heat release from StTPOxx and StTPOxxEL10 were higher than diesel due to the high amount of polyaromatics hydrocarbons, naphthalenes, and benzene derivatives. StTPO40EL10 is a ternary optimal blend in terms of performance, combustion, and emission, with EL as a potential diesel additive. It can be concluded that the StTPOxx and StTPOxxEL10 can be fully utilized in a diesel engine without any modifications and operational failures. In short, the lower blend percentage of StTPO40EL10 and StTPO40 can be used as an alternative fuel for a single-cylinder direct- injected diesel engine. In contrast, the higher blend percentage (StTPO60EL10, StTPO80EL10, StTPO90EL10, StTPO60, StTPO80, and StTPO100) can be utilized in boilers, furnaces, burners and marine engines.