Concerns surrounding the world’s current dependence on fossil fuels and their negative environmental impacts have instigated research into alternative, renewable and sustainable energy sources. With exponential population and economic growth there is a concomitant increase in plastic usage. With space becoming a premium commodity and environmental protection a necessity, landfilling of the majority of the world’s waste is no longer feasible. Thus, research is being carried out into waste-to-energy processes and refuse derived fuels. Of these processes, pyrolysis is increasingly being used to turn waste plastic into diesel. This article will explore how pyrolysis works and if it can live up to the expectations.
2 The Plastic Age
Plastic is formally defined as ‘any one of a large group of materials consisting wholly or in part of combinations of carbon with oxygen, hydrogen, nitrogen, and other organic or inorganic elements which, while solid in the finished state, at some stage in its manufacture is made liquid, and thus capable of being formed into various shapes, most usually through the application, either singly or together, of heat and pressure.’
Less formally, plastics are an inexpensive and durable group of materials which have the desirable property of being moldable into a limitless variety of products that find use in a vast range of applications. Due to the diversity in application and their convenience in use, the production of plastics has increased markedly over the last 60 years with a concomitant increase in waste generation, albeit with a time lag dictated by the product lifetime. However, current levels of plastic usage and disposal generates several environmental problems. Approximately 4% of world’s oil (irreplaceable) production is used as feedstock for plastics and a further 3–4% is expended to provide energy for their manufacture. A major portion of plastic produced each year (up to 50%) is used to make single-use disposable items of packaging or other short-lived products that are discarded within a year of manufacture (Hopewell, Dvorak, and Kosior 2009). Compounding this is the short lag-time from manufacture to discard of this application of plastic.
Unfortunately, the bulk of plastics are known not to be biodegradable, and will remain in landfills or the environment for at least decades, and probably for centuries. Even plastics known to be degradable may persist for a considerable time depending on local physical environmental factors, such as levels of ultraviolet light exposure, oxygen and temperature.
Therefore, the combined impact of the large quantity of the short usage lifespan applications and the high durability and longevity of the polymers means substantial quantities of discarded plastics are accumulating as debris in landfills, natural habitats and oceans worldwide. There is growing concern about the potential hazards and the environmental impacts of landfills. Increased waste means that more, larger, unsightly landfills are required bringing with them noise, smell and pollution. There are also long-term risks of contamination of soils and groundwater by some additives and breakdown byproducts in plastics. As a result, the general public is becoming less accepting of current waste disposal techniques and there is increasing pressure on government and industry to find alternatives to dumping and incineration.
3 Reduce, Reuse, Recycle – Regenerate?
Recycling is one of the most important actions currently available to reduce these impacts and represents one of the most dynamic areas in the plastics industry today. Recycling not only provides opportunities to reduce oil usage, carbon dioxide emissions and the quantities of waste requiring disposal, but additional, useful products can be derived from the plastics. Recycling is typically grouped into 4 main categories: primary (mechanical reprocessing into a product with equivalent properties), secondary (mechanical reprocessing into products requiring lower properties), tertiary (recovery of chemical constituents) and quaternary (recovery of energy).
Only approximately 20% of waste plastics can be effectively recycled by primary and secondary mechanical recycling technologies. Beyond this, the plastics become increasingly mixed and contaminated with impurities such as soil, metal foils, labels and food remnants. Plastics, as high molecular weight substances, cannot be purified by physical processes like distillation, extraction, or crystallization. They can only be recycled by gasification or pyrolysis of their macromolecules into smaller fragments. Pyrolysis (Greek: pyro “fire” and lysis “separating”) is a tertiary recycling technique suitable for converting plastic waste into oil products, combustible gas, fuels or monomers using a thermal decomposition process under oxygen-free conditions. The pyrolysis products can then be refined (following a catalytic upgrading process) by conventional petrochemical separation processes to yield higher-value commodities. Moreover, it allows the treatment of mixed, unwashed plastic wastes. Further, there are no toxic or environmentally harmful emissions or by-products. Pyrolysis thus has great potential for recycling heterogeneous plastic waste that cannot be economically physically separated and classed. The production of gasoline, kerosene and diesel from waste plastics is an emerging technological solution to the vast amount of plastics that cannot be economically recovered by conventional recycling.
In short, the key advantages of pyrolysis which differentiate it from other recycling systems mean that this process:
is capable of handling a feed consisting of mixed plastics that cannot be efficiently separated or recycled by alternative means;
is completely tolerant of contaminated and soiled plastics
enables recycling of laminated plastic films and multilayer packaging systems, including systems which include aluminium foil layers which cannot be removed by preprocessing systems
4 Pyrolysis: The Magic Wand?
Pyrolysis produces three phases of fuel products, solid (char), liquid (pyrolysis/fuel oil) and gas in different ratios depending on process conditions and process type. Once the process conditions are optimised, and when coupled with catalytic cracking, pyrolysis of plastic waste can generate liquid fuel yields of up to 80% with the resulting product resembling diesel fuel, kerosene, gasoline or other useful hydrocarbon liquids . Sharma et al. 2014 demonstrated using high density polyethylene grocery bags as a feedstock produced 74% fuel oil, 9% gas and 17% solid char.
There are two main types of pyrolysis: conventional (or slow) and fast pyrolysis. Vacuum pyrolysis is a hybrid between the two combining the low heating rates used in conventional pyrolysis with the very short volatiles residence times in fast pyrolytic set ups. Each has different process conditions and set ups and are used to achieve different end products. Vacuum pyrolysis and conventional pyrolysis are very similar in process set up, differing only in the process atmosphere and residence times, which are much shorter for vacuum conditions. The reduced residence times decreases the likelihood of undesirable secondary reactions whereby the primary pyrolysis products are further degraded. Thus, vacuum pyrolysis produces more oil and less char and gas than conventional pyrolysis, making it particularly suitable for plastic to diesel operations.
The pyrolysis product yield and composition are controlled not only by the temperature, but also by the residence time. As a general rule of thumb, the higher the pyrolysis temperature, the higher the yield of non-condensable gaseous products and the lower the yield of liquid fuels such as diesel. The optimum temperature range for the production of diesel products from waste plastics is 390–425◦C.
Following pyrolysis in the thermal cracking reactor, which performs the thermal decomposition of the plastic, the gas stream is purified of hydrochloric acid derived from polyvinyl chloride (PVC) feedstock (through a scrubber) and subsequently reformed by contacting with a catalyst in a reactor (Figure 1). The condensate resulting from this reaction is routed to a fractionating tower, where the desired fuels can be separated. The carbon residue precipitated in the thermal cracking reactor is discharged out of the process continuously by using a centrifugal separator. There are a wide variety of reactor designs operating commercially on this principle, known as catalytic upgrading, with operating parameters optimised based on the feedstock composition and the desired output fuel.
Once optimised, the plastic-derived diesel produced using this technique is suitable as a direct replacement for conventional diesel. By and large, in studies comparing the two, compression ignition engines required no modification to run on the plastic-derived diesel and exhibited increased thermal efficiency, and a reduction in smoke (Mani, Nagarajan, and Sampath 2011). Any viscosity variations of the plastic-derived diesel can be offset typically using Diethyl Ether to achieve air-fuel combustion ratios similar to that of conventional diesel, which has been shown to improve the cetane rating beyond that of conventionally derived diesel (Devaraj, Robinson, and Ganapathi 2015). It should however be noted that the performance of the waste plastic derived diesel is always tied to the composition of the feedstock.
5 Challenges and Conclusion
While shown to be technically feasible, the potential for conversion of waste plastic to diesel cannot be concluded without examining the challenges, the biggest of these being the need for a consistent feedstock. The quality and physical properties of the diesel resulting from any pyrolysis process is heavily dependent on the feedstock and the operating parameters. These parameters can be fine-tuned to compensate for changes in the feedstock composition, but this takes time and knowledge of the feedstock and the process. This makes it difficult to do in remote installations. Therefore, coupled with any plastic-to-diesel installation, a waste sorting and characterisation facility would be required for efficient diesel production.
Further challenges arise when benchmarked against renewable energy sources such as solar voltaic installations, primarily on a cost per energy basis. However, these analyses fail to include the beneficial aspects of using plastic as a feedstock rather than being sent to landfill or for conventional recycling. What has been shown by numerous studies is that pyrolysis does present a desirable solution when compared with traditional recycling, incineration and landfilling of waste plastics. Pyrolysis therefore represents an powerful additional tool for integrated waste management schemes, and with time and additional research can certainly provide an economic solution for waste plastic in modern society.
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