Design, Construction and Performance Evaluation of a Low-Cost Pilot-Scale Semi-Batch Pyrolysis Plant for Converting Polystyrene and Low-Density Polyethylene Waste into Fuel Oil

Every year, humanity produces hundreds of millions of tonnes of plastic. While some of this material finds its way into recycling streams, a vast amount ends up in landfills or scattered in the environment, where it persists for centuries. One promising way to deal with this waste is to turn it back into fuel. This process, called pyrolysis, involves heating plastic in a sealed container without oxygen. Without oxygen to burn, the plastic does not turn into smoke and ash; instead, it breaks down into liquid hydrocarbons that can be used as fuel oil, along with some gas and solid residue. While scientists have studied this in small laboratory settings for years, building a working machine that is affordable enough for developing regions and testing it with real, dirty street waste remains a significant challenge.

In a recent study, a team of researchers from Nigeria and Finland set out to build such a machine and see how it performed. They designed and constructed a pilot-scale plant capable of processing 35 kilograms of plastic at a time. The entire system was built from locally sourced materials, including galvanized steel for the main cooking pot, a furnace lined with a mixture of clay and sand to hold the heat, and a condenser made from stainless steel tubing to cool the vapors back into liquid. The researchers tested two very different types of plastic waste collected from the streets of Owerri, Nigeria: polystyrene, the rigid foam used in food containers, and low-density polyethylene, the flexible plastic used in carrier bags. They did not wash or chemically clean the plastic; they used it exactly as they found it, complete with labels, inks, and dirt, to see how real-world conditions would affect the outcome.

When they heated the polystyrene, the results were strikingly efficient. The machine converted 72 percent of the plastic weight into liquid oil, and the process left no solid residue behind. This happens because polystyrene tends to unzip itself into its original building blocks when heated, turning almost entirely into vapor that can be captured as liquid. However, the team noticed that a large portion of the material, about 28 percent, escaped as gas or fine mist that the condenser could not catch. This suggests the cooling system was overwhelmed by the speed at which the polystyrene turned into vapor. The oil produced was thick and dark, with a density higher than standard diesel fuel, and it had a flash point of 50 degrees Celsius, just slightly below the safety minimum for commercial diesel.

The experience with the flexible carrier bags, known as low-density polyethylene, was quite different. This plastic is much tougher to break down. The researchers had to heat it longer and use more energy to get it to decompose. In this run, the machine produced less oil relative to the amount of plastic put in—only 56.9 percent—and it left behind a significant amount of solid, waxy residue weighing 800 grams. This residue indicates that not all of the plastic chains broke apart completely. However, the oil from this plastic was lighter and cleaner in terms of sulfur content, though it still contained more sulfur than clean diesel fuel. The researchers calculated that breaking down the polyethylene required about 2.5 times more heat energy than the polystyrene, reflecting the greater thermal stability of the long, branched chains in the carrier bags.

A critical finding emerged when the researchers analyzed the sulfur content of the oils. The oil from the polystyrene contained a surprisingly high amount of sulfur, far more than the plastic itself should have. Since the plastic polymer contains no sulfur, the researchers traced this contamination to the unwashed street waste, which included adhesive labels, printing inks, and other materials stuck to the plastic. This highlights a major hurdle: if the plastic is not sorted and cleaned before processing, the resulting fuel will be contaminated and require further treatment. The oil from the polyethylene also had elevated sulfur, but at a much lower level, likely coming from additives within the plastic or mixed-in waste.

The study concludes that building a low-cost plant to turn plastic waste into fuel is technically feasible, even with simple materials and unsorted feedstock. The machine successfully produced liquid fuel that met the viscosity requirements for diesel engines, meaning it flows correctly through fuel lines. However, the researchers identified clear areas for improvement. The condenser, which cools the vapors, needs to be larger or more efficient to capture the fast-moving gases from polystyrene. The reactor itself, made of galvanized steel, is not suitable for long-term use at high temperatures because the zinc coating can melt and release fumes; future versions should use stainless steel or carbon steel. Furthermore, the high sulfur levels in the oil confirm that sorting and washing the plastic before it enters the machine is essential for producing a high-quality fuel. The work demonstrates that while the technology works, the quality of the final fuel depends heavily on how carefully the waste is prepared and how well the machine is designed to handle the specific chemistry of different plastics.