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| Fig. 1: Plastic Recycling Via Pyrolysis. (Image Source: D. LaJoie) |
It is estimated that the human race generates approximately 400 million metric tons of plastic waste each year. [1] To make matters worse, a projected 9% of all plastic produced has ever been recycled. Another 80% ends up in our natural environment or occupies landfills, and the remaining plastic waste ends up being burned or incinerated, further contributing to the worlds total carbon emissions and air pollution. [2] Specifically in the United States, which is one of the largest contributors of plastic waste by weight and per capita, households generated an estimated 51 million tons of plastic waste in 2021 and recycled a measly 2.4 million tons. [3,4] To combat this global issue, the plastic recycling process known as pyrolysis is going to be analyzed.
Pyrolysis, a thermochemical conversion technique that involves superheating plastic polymers (around 450-800°C) in the absence of oxygen to break them down into their monomer components as well as into small hydrocarbon chains, is an important step in making closed-loop recycling possible in the petrochemical industry. [5] Although it can be conducted on mechanically separated plastic waste, a unique advantage of the pyrolysis technology is its ability to use various different types of plastic as a single feedstock, eliminating the usual and tedious step of sorting. After pyrolysis is conducted, the resulting product stream is then separated into light hydrocarbon vapors, liquid oil and fuel components, as well as heavier solid products like wax or char. Controllable process conditions like temperature, residence time, reactor type, and the use of a catalyst in the reactor all affect the end product yield distribution. [5]
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| Fig. 2: Overview of Pyrolysis Process. (Image source: D. LaJoie) |
For example, one study, which analyzed the pyrolysis of a mixture of different plastics in a batch reactor at 500°C with a catalyst present, yielded 50.67% light hydrocarbon gas, 43.40% liquid oil, and 5.93% solid char. [5] A different study, which performed pyrolysis on mechanically sorted polypropylene plastic in a fluidized bed reactor at 510°C with a residence time of 7 seconds and no catalyst present, yielded 6.9% light hydrocarbon gas, 35.4% liquid oil, and 57.5% solid wax. [6] With close to 100% total conversion rate of the plastic waste into new usable products in both scenarios, the efficiency and potential benefits of the pyrolytic process are clear.
The pyrolysis of waste plastics may be more environmentally conscious than alternative options. Compared to incineration, which can only convert plastic into heat and electricity, pyrolysis transforms waste into new products in the form of gases, liquids, and solids to be repurposed for an array of applications. Some of these applications include becoming components of gasoline, diesel, feedstock for new plastic production, and even fuel for the pyrolysis process itself. In one pyrolysis plant for mixed polyolefins, the light hydrocarbon gas it produced was co-combusted internally to provide the thermal and electrical energy requirements for the pyrolysis reaction, which was calculated to be 1.71 MJ/kg plastic converted. [7]
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| Fig. 3: Global Polymer Flows in 2016 (MM metric tons/yr). (Image Source: D. LaJoie, after Hundertmark et al. [19]) |
Furthermore, life cycle assessments of multiple different plastic pyrolysis processes found that they consistently have lower greenhouse gas emissions when compared to incineration. [8] One study showed that pyrolysis of mixed plastic waste emits 50% less carbon dioxide than incineration of mixed plastic waste. [9] Similar findings showed that pyrolysis, when combined with mechanical recycling of plastic packaging waste, resulted in a reduction of the carbon footprint of plastic landfilling and incineration by 67% and 76%, respectively. [10]
Additionally, the incineration of plastic can cause numerous adverse environmental issues including the formation of dioxins, fly ash, and other toxins. [7] Also of note, pyrolysis helps reduce the leakage of waste plastic to the ocean and reduces the negative impacts on marine ecosystems. [11,12] Even with all these influential factors present, a very small percentage of plastic waste is being taken advantage of via pyrolysis. As an example, in Europe, approximately over 5 million tons of plastic waste is currently recycled mechanically while only around 50,000 tons is chemically recycled through methods such as pyrolysis. [13]
As for profitability, there are many confounding factors that contribute to the economic makeup of a plastic pyrolysis plant. The total capacity, or amount of raw plastic feed a plant can process, is among the most vital values to consider. One study found that a slow pyrolysis plant for mixed polyolefins needed a minimum capacity of 70 kt/year to be economically feasible. [14] In addition to capacity, the economics of pyrolysis are very sensitive to the price per kilogram of the plastic feedstock used, as well as any capital costs associated with a given plant. In one process plant which prioritized the production of naphtha, a high assumed feedstock cost of $0.60/kg drove the minimum selling price (MSP) of the pyro-naphtha product up to $2.18/kg in order to turn a profit. As a comparison, this MSP is approximately 4.3 times higher than that of fossil-naphtha. In this case, economic feasibility is immensely dependent on the price of plastic feed. The associated contribution of the feedstock cost to the MSP of $1.50/kg was three times larger than that of the actual pyrolysis process cost contribution of $0.50/kg. [15]
Because of this, a wide range of feedstock costs have been assumed within pyrolysis literature, with some studies assuming no cost and others assuming costs up to $0.60/kg. [7] If a plant had no associated feedstock costs and obtained its plastic waste for free, pyrolysis of the material into heavy and light oil was found to be profitable at capacities of 10.5 to 35 kt/year. However, plastic waste obtained at a mere $88/t (approximately $0.088/kg) was only profitable at the 35 kt/year capacity. [16] One plant found a way to offset the associated high cost of feedstocks by separating out value-added chemicals like ethylene, propylene, and aromatics from the pyrolysis products, which resulted in a maximum calculated feedstock cost of $460/t being profitable. [17]
Due to economics of scaling, larger process plants are generally considered to be more profitable than those with smaller capacities. [16] In fact, one analysis found that scaling up their pyrolysis process from 0.8 kt/year to 840 kt/year significantly increased the profitability of the process, with the highest capacity producing revenue with a positive net present value within year one of operation. [18] Other technoeconomic studies have reported minimum plant capacities for economic feasibility of 70 kt/year and 35 kt/year, respectively. [14,16]
Pyrolysis is an alternative method of recycling that repurposes plastic waste into new, useable compounds and offers multiple environmental benefits over other current plastic waste disposal methods. Although many benefits are observed, this technology is severely underutilized in today's society due to economic constraints. With further growth and expansion of plant capacity and the overall use of pyrolysis, the world's plastic waste crisis may be able to be minimized in an efficient and eco-friendly manner.
© Dominic LaJoie. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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