In the twenty-first century, considerable efforts have been made to convert biomass and plastic waste into value-added products. Conversion of biomass and waste plastic to fuels and chemicals holds promise to fulfill the global agenda of pursuing a carbon-neutral and waste-free human society. Biomass encompasses a wide variety of natural organic materials, both plant-based and animal-based, that can serve as potential sources for chemical production and energy fuels. The potentials of biomass as a sustainable energy source we discussed in our earlier article here.
Plastics are mostly petroleum derived synthetic polymers and extensively utilised in daily life for products such as packaging, household items, electronics, and clothing contributed to the advancement of mankind. Some of the highly used polymers such as low-density polyethylene, high density polyethylene, polypropylene, polyvinyl chloride, polystyrene and polyethylene terephthalate are produced globally and could be the source for fuel production. Plastics are typically non-biodegradable and persist in landfills for extended periods and pose substantial environmental concerns. Global annual plastic production exceeds 300 million tons, with little decline anticipated. By 2015, approximately 6,300 million metric tons of plastic had accumulated, and without effective management measures, an estimated 1,200 million metric tons of plastic could end up in landfills by 2050. According to an American Chemical Society Journal article, recycling plastic can effectively address plastic waste management issues while producing valuable products. If worldwide plastic waste could be recycled using efficient chemical recycling technology, it could save about 3.5 billion barrels of oil annually, equivalent to USD 176 billion.
Plastic disposal is a major concern for humankind and is primarily recycled in different ways, namely primary (closed loop), secondary (mechanical), tertiary (chemical recycling), and quaternary (energy recovery by incineration), as shown in Figure 1. Primary recycling reintroduces the used material into the same material, while secondary recycling converts it into new plastic products through mechanical processes. Tertiary or chemical recycling produces small molecules from waste plastics, which can be a potential feedstock for fuels, chemicals, and new polymers. Globally, many chemical processes exist, including gasification, hydrocracking, pyrolysis, hydrolysis, and methanolysis etc. Quaternary recycling, or energy recovery, involves the incineration of waste plastics and the recovery of energy through the generation of heat and/or electricity.
Figure 1: Plastic waste disposal methods In the twenty-first century, considerable efforts have been made to convert biomass and plastic waste into value-added products. However, a closer look at these materials reveals compositional and structural similarities, such as those between a piece of wood and a disposable mask. Researchers have developed thermochemical methods for both biomass and plastics to convert them into hydrocarbons, particularly for petrol, jet fuel, diesel, and lubricants, depending on their carbon-chain length and n/iso-paraffin ratios. These fuels can be produced by treating various plastic wastes as feedstock materials using pyrolysis or hydrocracking techniques. Figure 2 shows a generalised scheme for producing fuels from waste plastic. In the first step, waste plastic is depolymerized by pyrolysis or hydrocracking to obtain a raw liquid fuel, which is then further refined to produce the desired fuel. In pyrolysis, the feedstock is treated in an inert atmosphere (e.g., nitrogen gas, N2) in the presence of catalysts or thermally. Pyrolysis has been established as a proven technology for the oil and gas industries. Hydrocracking uses hydrogen pressure (3-10 MPa) at moderate temperatures (150-400 °C) to treat plastic waste with a catalyst in the presence of hydrogen, producing high-quality petrol. The pyrolysis of high-density polyethylene (HDPE) using ultrastable-Y-zeolite as a catalyst produces hydrocarbons in the gasoline range (C4-C12).
Hydrocarbons with boiling ranges of 35–190 °C can be used as motor gasoline, those between 190 and 290 °C are suitable for diesel #1, and those ranging from 290 to 340 °C are used as diesel #2. Potential feedstock for fuel production include PE, PP, polystyrene (PS), PET, and PVC, which have a common calorific value for fuel oils of 20900 Btu/lb. Both cracking and pyrolysis break the polymer chains into useful low molecular weight compounds by heating in the absence of oxygen. Recently, researchers showed that plastic waste could be converted into ultra-low sulfur diesel (ULSD). ULSD, also known as clean diesel, contains 15 ppm or less sulfur and is currently produced from crude oil.
Figure 2: Fuel production from waste plastic Sustainable aviation fuels (SAFs) which are essentially a kerosene presently derived from petroleum are considered to be a major hurdle to achieving net zero CO2 emissions in aviation. India's government has set a goal of 1% SAF in jet fuel for international flights by 2027, with this percentage doubling to 2% by 2028. To achieve these targets, the country will require approximately 140 million liters of SAF annually. In an effort to replace fossil-derived aviation fuels, hydroprocessed esters and fatty acids (HEFA) produced from greases, oils, and fats are expected to be the initial dominant pathway for producing synthetic paraffinic kerosene (SPK). During this time, waste, agricultural and forest residues as well as alcohol pathways will make only a modest contribution. (https://pib.gov.in/PressReleaseIframePage.aspx?PRID=1925417)
A minimum of 8.4 vol. proportion of aromatics that can be included without stranding the remaining paraffins fraction in existing aircraft and other infrastructure. Swelling of elastomer O-Rings in fuel systems is primarily due to aromatics. Consistent swelling is critical to ensure the seal does not lose its integrity and leak fuel. Crude oil-derived aviation fuels contain an average of 8-20% aromatics. While SPK is a less aromatic content (less than 0.5 vol %) hydrocarbon fuel made mainly from iso-paraffins and paraffins. Due to this limitation and others (e.g., density), SPK is typically limited to 50% by volume in a petroleum-derived Jet A blend. The blending limit: a limit to greenhouse gas (GHG) savings in flight. To increase the blend volume of SPK, and thereby help reduce GHG emissions, aromatic molecules can be incorporated from renewable resources including both biomass or waste streams such as plastics.
According to a recent article published by a group of researchers from Illinois Sustainable Technology Center, University of Illinois at Urbana-Champaign and United States Department of Agriculture-Agricultural Research Service (USDA-ARS) in American Chemical Society journal ACS Sustainable Chemistry and Engineering, Ethylbenzene could be used as a fuel additive in synthetic paraffinic kerosene which is used as a non-petroleum based Jet Fuel. Polystyrene is utilized in consumer/institutional goods, packaging, electrical and electronic goods, furniture, building/construction, transportation as well as industrial/machinery. Most of the polystyrene becomes waste, forming a source which can be converted to aromatics.
Figure 3: Polystyrene conversion to ethylbenzene Figure 3 shows the ethylbenzene production where pyrolysis of Polystyrene beads at elevate temperature (375-400 oC) produces a styrene rich liquid which further hydrogenated (50 bar H2 for 1h) to give ethylbenzene. Distillation of the produced liquid results in 90 % purified polystyrene. The addition of ethylbenzene (16.5 % v/v) suggested to be advantageous to maintain the ASTM specifications of Jet fuel. This study provides a pathway to address the twin issues of plastic pollution and SPK property improvement.
