By Jeffrey Seay, PhD, PE
A common concern that is often voiced with respect to Plastic Derived Fuel Oil (PDFO) is that although it does remove plastic from the ecosystem, when the fuel is burned to generate energy, it releases carbon dioxide, which is of course a greenhouse gas. This concern is valid. Converting plastic waste into fuel oil does remove plastic from the environment but also contributes to releasing carbon dioxide into the atmosphere just like every other liquid fuel. However, determining whether the net impacts of this process are positive or negative requires a more finely tuned examination.
First, liquid fuels are the primary source of transportation energy globally. Despite the recent advances in electric vehicles, the internal combustion engine still reigns supreme in the transportation sector. Therefore, the question we must address is whether PDFO is better or worse for the environment than traditional liquid transportation fuels. To properly assess the environmental impacts of this process, we must consider the overall lifecycle impacts of traditional fuels, versus PDFO.
For the purpose of comparison, traditional liquid fuel begins its lifecycle in an oil field. From there it is pumped out of the ground, transported to a refinery, distilled into its various fractions, transported to a distributor, then to the fuel station, where it is finally pumped into a vehicle. Each step of this life cycle requires an expenditure of energy and thus results in greenhouse gas emissions. These lifecycle emissions are referred to a “Well to Tank” emissions. Of course, the exact emissions depend upon many factors like the location of the oil field and how far the intermediate products have been transported. That said the Well to Tank emissions has been reported by the Air Resources Board of California to be 20.43 gCO2/MJ of fuel for Ultra Low Sulfur Diesel refined in California. This is a reasonable number to use as a point of comparison.
PDFO begins its lifecycle at the point a post-consumer use plastic item is discarded. It isn’t reasonable to include the production of the original item in the assessment, since the original item wasn’t produced for the purpose of making fuel. We call this process Trash to Tank. At this point, the Life Cycle consists of collecting the post-consumer plastic, transporting it to a facility for converting it to PDFO, transporting the PDFO to a fuel station, where, like the traditional liquid fuel, is pumped into a fuel tank for use. This is where the calculations get complicated. The energy source used to convert the plastic into PDFO matters, the distance the plastic is transported matters and the efficiency of the process matter, and the type of plastic used as raw material for PDFO matters.
To estimate the lifecycle carbon dioxide emissions for PDFO, we must first calculate the energy required to convert plastic into PDFO via pyrolysis. If we consider polyolefin plastics only, this energy ranges from 9,468 kJ/kg for High Density Polyethylene, to 9,097 kJ/kg for Low Density Polyethylene to 5,434 kJ/kg for Polypropylene. If the energy needed to drive this pyrolysis process comes from electricity produced from natural gas, the US Energy Information Administration estimates that 0.91 lb of CO2 (0.417 kg) are released per kWh of electricity. This means that 0.116 g of CO2 are released for every kJ of electricity consumed. This number varies, depending on the source of electricity. It will be significantly higher if coal is used, significantly lower if a renewable energy source like wind, solar or hydroelectric are used. The process efficiency also matters. The energy that must be put into the process will be higher if there are significant losses due to thermal inefficiencies. All this is to say that the carbon dioxide emissions for converting plastic waste into PDFO vary greatly. Using electricity produced by natural gas, and assuming 100% efficiency, the carbon dioxide emissions range from 25.73 g/kJ for Low Density Polyethylene to 24.42 g/kJ for High Density Polyethylene to 15.73 g/kJ for Polypropylene. These are in the same range as the reported average well to tank emissions for traditional liquid fuels. Of course, this is before considering any energy put into sorting and transporting the plastic before pyrolysis or transporting the PDFO after pyrolysis. So, what does this mean for the environmental impacts?
Unfortunately, the final answer as to whether converting post-consumer plastic waste into PDFO is better or worse for the environment than traditional liquid transportation fuel is complicated. There are certainly plausible scenarios where it is better in terms of carbon dioxide emissions and others where it isn’t. Of course, this is without considering the benefits removing post-consumer plastic waste from the ecosystem, which are significant. Engineers for Sustainable Energy Solutions is focused on low-cost, low-tech solutions that are still efficient, safe and effective. Using solar power to convert plastic waste into PDFO in a developing world setting where the plastic is collected and sorted locally by hand and converted to PDFO for local use is a net benefit to the environment in terms of carbon dioxide emissions. In fact, in this scenario, the Trash to Tank carbon dioxide emissions are nearly zero. Other scenarios require more study as each one is unique. As with most things, there are no easy answers. The bottom line though is that converting post-consumer polyolefin plastic into PDFO can be done in a manner that is environmentally responsible, and therefore continued research in this area should be encouraged.
For more information on this topic, please refer to:
Joshi, C. and J. Seay (2020): “Total Generation & Combustion Emissions of Plastic Derived Fuels: A Trash to Tank Approach”, Environmental Progress and Sustainable Energy, Vol. 39, Issue 5. DOI:10.1002/ep.13151.