The Problem with Plastics and Fuel Decarbonization Policies: Could Nonbiodegradable Plastics Be Better for the Environment in the Short Term than Biodegradable Plastics?

by Shashi Menon (EcoEngineers) ... The latter material eventually emits carbon — contributing to higher levels of greenhouse gases (or GHGs) in the atmosphere — whereas nonbiodegradable plastics can sequester carbon for longer periods of time. Is this actually true? And how does the science back it all up?

A Basic Life-Cycle Analysis of Biofuels

Let’s first briefly revisit the idea of recycling carbon. The core concept is that plants absorb carbon from the air during photosynthesis. When we convert that carbon to a biofuel and eventually rerelease it into the air through combustion, it’s not adding to the overall inventory levels of carbon in the atmosphere.

The CO2 absorbed by the corn plant in the above example could have been emitted by a nearby coal-fired power plant, but we accept the carbon as biogenic. We will use this definition of biogenic throughout this article.2

Are Plastics Biogenic?

But what about the carbon in used petroleum-based products (such as plastics, tires, asphalt, lubricants, industrial gases, and a multitude of chemicals) that are reprocessed to make fuel after their first useful life is over? Should the process of recycling these carbon sources be viewed the same as biogenic carbon? Should government policies to reduce carbon emissions give fuels made from recycled petroleum products “biogenic” status, allowing the emissions from their combustion to be zero?

It depends on whether the used petroleum product degrades on land (biodegradable) and releases the carbon it contains in the form of carbon dioxide or methane into the atmosphere, or whether it keeps the carbon locked up in its synthetic molecules (nonbiodegradable), effectively sequestering it. Of course, this does not take into account the issue of plastic product pollution — especially in marine habitats — and the environmental harms to ecology, marine life, and aesthetics. In this article, though, we are solely going to focus on atmospheric GHG emission levels.

We need to simultaneously reduce the CO2 inventory levels in the atmosphere and prevent the levels from becoming worse than they are today. Every molecule of new fossil carbon released into the atmosphere in this context should be counted because it is adding to the inventory level. We don’t want plastics to biodegrade and emit carbon molecules into the atmosphere; we want them to maintain their chemical structure and keep the carbon chains intact.

But everything degrades over time. So the question is this: How quickly will a material biologically degrade and emit CO2?

A recent study from the University of California, Santa Barbara has attempted to establish the degradation half-lives of commonly used plastic products, and the results are enlightening. Not all plastics are created equal, and they have different degradation rates.

According to this research, the rate of degradation depends on the type of plastic; the surface area; and the exposure to oxygen, sunlight, and water. For example, a PVC pipe buried in land has a degradation half-life of 5,000 years! There is absolutely no reason to convert this pipe into a fuel. On the other hand, a thin plastic bag buried in a landfill has a degradation half-life of 4.6 years. That means in 20 years, 95% of the bag will have decomposed into residual biomass, CO2, and CH4.

If something doesn’t degrade for 100 or 500 years, then for all practical purposes, we can say it is sequestered. Fossil carbon that is earthbound and will stay earthbound for 100 years or more is not contributing to the problem of high atmospheric GHG levels. This means all nonbiodegradable petroleum-based products could be buried in a landfill and left there for a long period of time without incident. We could even incentivize the use of atmospheric recycled carbon and make more of those plastic products that will ultimately be buried.

Assuming the carbon in petrochemicals and plastics is emitted into the atmosphere and then absorbed by a plant, the carbon becomes biogenic, and its combustion is effectively carbon neutral. A used plastic bag degrading in a landfill can produce landfill gas, which is also treated as biogenic. However, if the bag itself is converted to a fuel, then it is not always treated as biogenic. This is a key limitation in policy, and it is necessary to refine this approach to develop better fuel decarbonization regulations.

Converting plastics and chemicals made from petroleum into “renewable” fuels is an alternate end-of-life disposal strategy. It prevents the use of a freshly extracted unit of petroleum-based carbon to make an equivalent unit of energy. If the plastic or chemical is nonbiodegradable, then the emission from its combustion is effectively the same as that from the petroleum-based fuel it is substituting — and possibly even higher with the added processing for the reuse. However, if it is biodegradable (within a reasonable time frame), it prevents any emissions associated with its baseline disposal procedure.

Moreover, the plastic bag in the landfill will degrade within 20 years and emit carbon into the atmosphere. The total life cycle carbon intensity of the plastic bag should include the emissions from the gathering, manufacturing, distribution, and use of the raw material plus the emissions factor associated with its degradation rate and atmospheric CO2/CH4 formation.

A plastic or chemical derived from petroleum that is biodegradable should be given the same benefits for its reuse as fuel: The carbon intensity of the resulting fuel should include the avoided emissions from its baseline disposal process, and its combustion should be given the same treatment as that of biogenic (i.e., zero Scope 1 impact). It should not matter whether the carbon from a petroleum source is repurposed to fuel before or after it is disposed, emitted into the atmosphere, and reabsorbed by a plant.

The problem with plastic products is not limited to GHG emissions. It also includes irresponsible disposal and littering on a massive scale — especially in marine habitats. Acknowledging the absence of GHG emissions due to extremely slow biodegradation rates of certain plastics should not be seen as a license to litter. Moreover, the production of plastics is energy-intensive and results in a positive GHG emission into the atmosphere. We should prevent this as much as possible in order to strive for a net-zero economy. READ MORE