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Waste plastic could outstrip coal as a viable energy source

Waste plastic could outstrip coal as a viable energy source
Neil Stacey, James Fox, Klaus Moller, Diane Hildebrandt, David Glasser, and Celestin Sempuga
By Neil Stacey, James Fox, Klaus Moller, Diane Hildebrandt, David Glasser, and Celestin Sempuga
10 May 2022
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Through thermochemical conversion, we can eliminate this toxic pollutant and make it profitable.

Microplastic particles have been found everywhere on earth, from the peak of a remote Antarctic mountain to the deepest part of the ocean. Every place on Earth with breathable air has micro-plastic particles in that air. Even more alarming, these particles are everywhere in our bodies, in all our organs and even circulating freely in our blood.

We do not yet know the extent of the health problems this causes, but evidence is emerging of links between micro-plastic exposure and common health problems. And this is after a comparatively brief exposure — just ten years ago, the levels of microplastics in the environment were half of what they are now. The long-term effects of prolonged exposure to current levels are guaranteed to be far worse and everyone alive today is at risk. Those yet to be born will be at greater risk still — every successive child who is born is born into the highest levels of microplastic pollution that have ever existed because the concentration of microplastics moves in only one direction — upward.

Even if plastic pollution was somehow to be halted at present levels, the health effects would continue to worsen as the duration of exposure increases.

And halting plastic pollution is all but impossible. Even if all production of plastics was halted overnight, environmental levels of microplastic particles would continue to climb.

Eight billion tons of plastic … and counting


Over 8 billion tons of plastic have been manufactured to date and only a small fraction of that has entered the environment so far. Most of the rest is either still in use or in landfills. Recent research has shown that landfills continuously leach microplastic particles into groundwater, and deep-seated landfill fires have been found to be more commonplace than we thought, resulting in low-temperature partial combustion of plastic, which produces highly toxic by-products. Over the much longer term, no landfill is safe from extreme weather, fires, animal incursions, seismic activity, or the effects of conflicts.

Plastics are remarkably recalcitrant molecules with lifespans of centuries or more, a time frame long enough to see entire nations rise and fall. There is scarcely a spot on Earth that has not been the site of a war at some point in the last 500 years, and there is little reason to believe that the next 500 years will be different.

In short, every molecule of plastic that is manufactured is very likely to find its way into the environment, unless it is intentionally converted into something else entirely. The plastic that has entered the environment so far is merely the tip of the iceberg. Even if every plastic factory were shut down tonight, the potential would still exist for the amount of microplastic in the environment to increase by an order of magnitude, with untold consequences for the health and quality of life of all species on Earth.

Nor is it useful to use waste plastic to make roads, for instance: incorporating the plastic into road aggregate makes it inaccessible to further processing. This means it will inevitably become microplastic. To tackle the waste plastic crisis we must look beyond the obvious short-term effects of pollution and focus on the ultimate fate of polymer molecules, which means avoiding temporary measures that seem promising on the surface.

The difficulty of resolving this crisis is greatly exacerbated by global crisis fatigue. Public attention and government funding are finite resources, and both are largely committed to climate change, with little leftover for new, emerging issues. This means that the problem of waste plastic accumulation must be solved without relying on social and political pressure, and without detracting from the more prominent cause of combating climate change.

If we are to secure for ourselves such luxuries as breathable air and drinkable water, we must not only eliminate plastic as quickly as we are producing it; we must go faster, to work through the enormous backlog of plastic we have already created. We must do this in the absence of massive popular support, which in practice means that we have to do it in a way that turns a profit, and we must also do it without causing more CO2 emissions.

Fortunately, this is achievable with minor tweaks to existing technology. South African scientists comparing processes for converting waste plastic into energy or fuels have found that its potential exceeds that of coal, in terms both of energy yield per ton and of CO2 emissions.

Those researchers (who include some of the authors of this piece) use a methodology referred to as process targeting, which uses fundamental thermodynamics, rather than current performance, to compare alternative chemical pathways.

Read the paper that explains the process here.

This approach allows for valid comparisons between new technologies and existing processes which are already operating close to their limits of performance. In that paper, which is reasonably corroborated by parallel research, we only considered flowsheets comprised of well-established unit operations and reactions with well-known chemistry, and at reasonable temperatures and pressures. This means that the targets are within reach with current technology and equipment, though a fair bit of investment in process development and optimisation would be needed.

Waste plastic can generate revenue


This is an exciting finding because it means that waste plastic of suitable types can be permanently eliminated in a way which generates enough revenue to make it more valuable than coal. This means that there is, in principle, an economic incentive to replace coal extraction with waste plastic processes, with a net reduction in CO2 production. Good news all around, at least as far as the comparison with coal is concerned.

One pitfall, however, is that on those same measures of CO2 emissions and yield per ton, plastic waste performs worse than natural gas.

One could argue that the other costs of natural gas extraction and distribution, along with the urgent need to eliminate plastic, far outweigh that difference in CO2 emissions. But unfortunately, we inhabit a world where metrics are often prized above the things they were intended to measure and so, elevated CO2 emissions may be considered intolerable even if the overall effect on the environment is beneficial.

In order to meet our requirements, then, we have to ensure that these plastics-using processes specifically displace coal-using processes. Broadly speaking, the largest backlogs of plastic accumulation are in first-world countries where gas has largely phased out coal. But many developing nations still use coal in large quantities, whether for electricity production or metallurgical processes. In order to displace coal with plastic at a large enough scale, then, we would need the secure transport of suitable waste plastics from developed countries to developing ones, to displace coal where it is still being used. But international regulations mostly prohibit this.

Hazardous wastes


The movement of hazardous wastes, which plastic is broadly considered to be, is governed by the Basel Convention. This international treaty has been in force since 1992 and has been ratified by 199 parties, with just one notable exception. The United States has signed but not ratified the convention, but tends to adhere to its precepts. So the Basel Convention stands as one of the most universal and effective sources of international law in force today.

In 2019 the Convention’s Annex IX was amended to regulate waste plastic, with a short list of exceptions. That amendment states that waste plastic is considered non-hazardous if it is comprised of a mix of Polypropylene (PP), Polyethylene (PE), and Polyethylene terephthalate (PET) and it is “destined for separate recycling of each material and almost free from contamination with other materials.”

The use of the word “recycling” excludes any processes that permanently eliminate plastic waste. Even if that semantic issue were worked around in some way, however, the Convention would still prohibit the most profitable applications for waste plastic as an energy source because of the ban on mixing.
While mechanical recycling does not tolerate mixes of plastics, thermochemical conversion can handle just about any mix of suitable plastics, removing the need for costly, inconvenient sorting processes.

Mixtures of biomass and plastic are another promising feedstock prohibited under the current rules. Plastic packaging contaminated with food is the single most common type of household waste, and cleaning plastic packaging of those residues is a water-intensive process, and so a great deal of plastic ends up discarded rather than being processed because the costs of cleaning exceed the benefits.

With thermochemical processes, this step is unnecessary; biomass is itself a viable energy material for thermochemical conversion, so its presence simply adds to the amount of energy available. What’s more, recent studies have shown that the combination of plastic and biomass improves performance beyond the sum of what can be achieved using the two feedstocks separately. So, excluding mixtures of plastics, and combinations of plastic and biomass, severely hampers the economic and environmental benefits of using plastic as an energy source.

Recycling doesn’t eliminate plastic


In short, the Basel Convention currently prevents the transport of most of the usable feedstocks for these processes and imposes severe and unnecessary separation constraints on the handful of remaining feedstocks.

The reason for this is that the regulations were formulated solely with recycling in mind. Recycling, of course, is integral to any plan to limit waste plastic production because it reduces the demand for new plastic and, consequently, the rate of plastic manufacture, but it is far from a complete solution. Recycling does not permanently eliminate polymer molecules, and each cycle of recycling shortens the constituent polymer chains of which plastic is comprised, bringing it closer to its eventual fate of becoming a swarm of circulating microparticles. In other words, recycling extends the lifespan of plastic but does not change where it ultimately ends up.

It is more important for international law to regulate the final fate of waste plastic molecules when they exit this circular-ish economy, as they all eventually must. This can best be achieved by acknowledging waste plastic as a potentially valuable energy source and regulating it accordingly. This means ensuring that viable feedstocks can be easily and economically traded to the nations who can benefit most from their use, which will also most benefit the environment. It would also be helpful if regulation were to ensure that plastic feedstocks are not contaminated by materials unsuitable or problematic for these processes, such as Poly-vinyl chloride (PVC), which produces some chlorine compounds that are rather hard on equipment and pipelines and therefore require additional unit operations to handle.

Some of the process pathways available can still handle considerable quantities of PVC so it’s not necessarily a fatal contaminant, but we do need to design processes around a reasonably consistent feedstock, so some well-crafted regulation would be a tremendous help. Mostly, though, we just need international laws that don’t prohibit the best solutions to humankind’s problems while withholding a valuable energy source from developing nations.

Plastic is a threat on the scale of climate change


The infiltration of micro-plastics into the environment — and our bodies — is a scary and permanent threat, comparable to climate change.

CO2 is, at least, not actually toxic. Even at the highest concentrations, it could conceivably reach the atmosphere, it is tolerated by all species breathing it. It is also merrily metabolised by the most abundant life forms on the planet, which means that its concentration is self-limiting, to some degree, because the increases in biomass accumulation triggered by CO2 will mitigate its levels — higher levels of CO2 stimulate faster plant growth, which in turn accelerates the removal of CO2 from the atmosphere.

CO2 is not intrinsically harmful to the environment; rather, it changes the environment. By definition, we will be maladapted to those changes because we are well-adapted to the current state of the environment. Animals and plants have less capacity to adapt than we do, so they will, again by definition, be worse affected than we are. The problem with climate change is simply that its pace of change exceeds the rate at which we, and the other species we share our atmosphere with, are able to comfortably adapt.

Plastic, on the other hand, is just unambiguously horrible. It isn’t metabolised at a meaningful rate by anything at all and so its levels are only limited by the rate that we can produce it.
While it is true that some micro-organisms have been found to eat plastic under limited circumstances, this happens very, very slowly.

A similar situation arose once before in the Earth’s history. When tree-like plants first evolved the ability to produce lignin, no organisms were able to metabolise it, making the biomass of those plants somewhat like plastic in that they persisted more or less indefinitely. This led to a period in geological history referred to as the Carboniferous, in which indigestible plant material piled up to such an extent that the coal it eventually formed is still around today. Micro-organisms did eventually evolve the ability to break down lignin and metabolise the products, bringing an end to the Carboniferous — but the bad news is that it took 60 million years.

We need to deal with waste plastic a bit more rapidly than that. DM

Neil Stacey and Diane Hildebrandt are researchers at the University of the Witwatersrand. James Fox, David Glasser and Celestin Sempuga are at the University of South Africa. Klaus Moller is at the University of Cape Town.

First published by GroundUp.



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