People hate plastics, or at least often say they do. In fact they are happy to have their lives saved by single use medical plastics, and they reward the convenience of plastics at the cash register whenever they’re offered, unless somebody gets in the way with regulations, but they still claim to hate them.
Why the hate? Because plastics are cheap and durable, they invariably become a waste problem. And because we’ve largely socialized the cost of waste management, there isn’t a direct economic incentive- other than where that is imposed by regulation- to reduce waste plastic generation. So most of us- this household included- seems to be positively drowning in single use plastic waste. We dutifully chuck it all into the recycling bin and haul it out to the curb every 2 weeks, but we are also aware that less than 10% of waste plastic ends up actually being recycled. The rest is often combusted (particularly in Europe) or landfilled, because it is collected in a form that is too dirty and mixed to make mechanical recycling feasible.
One person’s waste is often seen as another person’s resource. And so the notion of what beneficial purpose we can put this mixed, dirty plastic waste “resource” to, has plagued the imaginations of countless people over the years.
Conversion to Energy or Fuels
Plastic is a fossil origin resource, with very few exceptions. So no, you can’t take mixed municipal solid waste, or even “refuse derived fuel” (RDF) from sorted MSW, and convert it to other fuels by pyrolysis or gasification and get away with calling those fuels “biofuels” or “green fuels”. Had you left the plastic alone, and simply not burned it, but rather landfilled it, its fossil origin CO2 would have remained out of the atmosphere for literally tens of thousands of years. And no, “air-filling” is not a green alternative to landfilling. If you want my opinions on the inherent greenwash that is “waste to energy or fuels”, you can read them in my article here. I’ve recently re-read it and I stand by everything it says.
https://www.linkedin.com/pulse/waste-energy-fuels-great-greenwashing-machine-paul-martin
Circular Plastics- New Monomers From Post Consumer Plastic Waste
If you’ve been reading my work for an appreciable period, you likely know already that I view the whole notion of a “circular economy” as standing squarely against the 2nd law of thermodynamics- and in that battle, thermodynamics always wins. Rather than the foolhardy notion of “circular economy”, we should always be talking about optimal recycle.
(my modification of the popular circular economy cartoon, which gets things totally wrong and hence needed fixing)
The optimal recycle rate is rarely zero and is NEVER 100%, for 2nd law reasons. As we try to reduce the amount of matter waste from any process to 100%, the 2nd law looms large and eats an ever-increasing amount of energy. As our energy sources become cleaner as we decarbonize our energy system, the optimal amount of recycle- the amount which generates the lowest net environmental harm- will increase for all processes. But it will never be 100%.
The real question this piece must answer though, is this: is it possible to develop a process to take post-consumer plastics and to chemically disassemble them into fresh monomers suitable for making new plastics
?
The TL&DR answer to that question is yes- but it’s very, very difficult. It is much easier to pretend that you’re doing that, and to end up with most of the mass being used for other purposes, particularly as fuels, with most of the mass therefore ending up as CO2 in the atmosphere. And that’s inarguably worse than simply landfilling the plastic and being done with it!
Why Fresh Monomers?
Well, because the reason we can’t recycle this shit is that it is a mixture of plastics generally- or even when it can be sorted into a pure stream of just one kind of plastic- PET water or pop bottles for example- the plastic still contains dyes or pigments, additives, fillers and other materials that render the mechanical recycled plastic product unsuitable for certain uses. That’s not a problem as my other piece mentions: we should have no more problem with PET bottle waste being used to make carpet fibre, than we have with copper wire being recycled into copper pipe and tubing rather than back into copper wire.
That said, if we could de-polymerize the material into its monomers, purify those monomers to the (sometimes extreme) levels of purity that is required to make polymers of the necessary quality, and then blend those with fresh monomer to make up for losses, we could have true circularity for those materials. On paper, or in a spreadsheet, at least.
Types of Polymers
The problem, and the size and quantity of devils associated with making fresh monomers out of post consumer waste plastic, varies greatly with the sort of polymer we’re talking about.
Polyolefin Thermoplastics
Far and away the greatest mass of plastic, and hence of waste plastic, is thermoplastics, particularly polyethylene (PE) and polypropylene (PP), with polystyrene (PS) and PVC coming up close behind. These polymers are all made from olefin monomers, i.e. monomer chemicals containing a carbon-carbon double bond called a “vinyl group”, with either catalysts or free radical initiators used to cause them to polymerize (to link up into long chains, a process which releases a lot of heat).
Pyrolysis
Of these major polymers, only PS depolymerizes to a significant fraction of its monomer when you heat it up in the absence of oxygen (a process called pyrolysis). Even the yield of PS back to styrene is quite low in the best case. There is no catalyst capable of selectively reversing the process of polymerization back to pure monomers, and there are good chemical reasons to suspect that no such catalyst will ever be developed.
Pyrolysis is reached for because it is “easy”, conceptually at least, and it yields a pyrolysis oil which can be sent to a petroleum refinery. Done very carefully, small markets for short polymers otherwise made from fresh monomer, can be supplied by pyrolysis of waste plastics. However, because those markets are small, most producers try to produce a generic pyrolysis oil- and of that oil, only a tiny fraction of that oil will end up being made back into olefin monomers again- perhaps 10% at best. The rest will end up in fuels, and the fossil CO2 in that waste plastic will end up unnecessarily in the atmosphere. Pyrolysis is disastrous when any quantity of PVC is found in the waste stream- the yield to very dangerous halo-organic molecules such as chlorinated dioxins and furans can be significant and cannot be entirely eliminated, and the best case product is hydrogen chloride which must be scrubbed out of the product vapour. There are also additives in all of these polymers which can produce hazardous chemicals in the gas, liquid and solid (char/inorganics) fractions coming out of the pyrolysis process.
So to call pyrolysis “chemical recycling” is, frankly, a massive greenwash.
You could increase yields to monomers by feeding pre-treated pyrolysis oil to a dedicated fluid catalytic cracking unit, but the unit economics would be poor at best.
Gasification to Syngas, and Methanol to Olefins
The other approach used for these materials is gasification, where waste materials are heated- often by partial combustion, often with steam as a co-reagent- to the point where large molecules can no longer exist. The product is, compared to what you get when you feed natural gas to a steam reformer, quite a dirty synthesis gas containing CO2, CO, hydrogen, methane, nitrogen (if air is used as a combustion feed), water vapour, and of course HCl, HBr and HF if the waste isn’t completely free of halogens. It is possible, with knowledge, effort and considerable investment, to clean up this gas stream to produce a feed suitable for methanol production, but there will be a significant amount of fossil origin CO2 that will need to either be disposed of via CCS or expensively back-reacted with green hydrogen to produce more CO and water using the water-gas shift reaction. Neither option is very appealing economically.
While methanol is itself not a monomer, about 30% of world methanol production is already being converted to olefins including ethylene and propylene in places like China and India.
Gasification to syngas, syngas to methanol, and then methanol to olefins- wow, that’s a lot of steps! But this process can, if you can afford to do it correctly (which is an open question!), result in considerably higher yields of waste polyolefins to fresh olefin monomers than can be achieved by pyrolysis. However those many steps, each with energetic losses, GHG emissions and capital and other operating costs to contend with, can make the economics challenging. Waste materials also are bulky and expensive to ship, so plants tend to be small in scale, complicating matters further. The unit economics of post consumer waste plastic back to olefins of monomer purity via gasification and methanol to olefins (MTO) are at present undemonstrated.
The added complication is the desire to use methanol as a fuel, particularly for shipping. It will become obvious that people starting with mixed municipal solid waste or waste-derived solid fuel, will want to count some of their resulting syngas and methanol as having come from biomass, and hence to be suitable for use as a fuel, while the rest might only be acceptable if it can be economically back-converted to polymers. The opportunities for fraud and the need for stringent regulatory control is therefore evident.
And no, Fischer Tropsch is not a suitable consumer of the resulting syngas, either. FT stands for “fundamentally terrible”, or a ruder but more accurate word starting with F may also be substituted here.
So far therefore, chemical recycling of polyolefin thermoplastics is at best a concept, and more often than that, if pyrolysis is involved, it can be a borderline environmental fraud- effectively a waste to energy process masquerading as waste to chemicals or plastics. This makes it hard for legitimate project proponents who are truly interested in decarbonization and environmental solutions, to differentiate themselves from the enviro-alchemists who attempt to convince us that waste to anything is better than landfilling. It’s important to realize that it ain’t necessarily so!
Thermosets
Thermosets are a class of polymers which involve crosslinks between the polymer chains. Crosslinking provides beneficial mechanical properties to polymers such as rubber, coatings, epoxies and vinylester resins which are used to make composites such as glass- and carbon-fibre reinforced plastics etc. However, crosslinking processes are even harder to reverse than polymerization itself. So far, nobody has good processes for recycling any post consumer thermoset plastic back to its constituent monomers that I’m aware of. I’m constantly hearing people announce that they’ve found beneficial ways to recycle materials such as tires or wind turbine blades, but they usually mean recovering energy or similar pyrolysis oils to what we’ve previously mentioned, and reinforcing materials like glass fibre or carbon black. That is a long way from true chemical recycling, and in my view there isn’t a good prospect for that to change any time soon. Redesign of the source polymers might yield some hope, but if crosslinks can be easily reversed, environmental durability of the material will almost certainly suffer.
While the potential for re-use or repurposing of post consumer thermosets is very real- witness the wonders which are done with rubber crumb from tires, and the creative solutions for the re-use of sections of fatigued wind turbine blades- most thermosets are destined for landfill. And that frankly shouldn’t concern us too much, because they are inert in landfill and their environmental impact ends after they are properly collected and disposed of.
Condensation Polymers
A whole class of polymers, some of them thermoplastics, others thermosets, are so-called condensation polymers. The main examples include polyesters like polyethylene terephthalate (PET), polyamides (Nylon) and polyurethanes, and natural examples such as cellulose. These polymers involve either two different monomers, or bifunctional monomers with different reactive groups at each end. These groups react with one another to form chains, with the elimination of a small molecule in the process (generally but not always water).
Polyesters and polyamides involve the reaction of an organic acid group with an alcohol or an amine respectively, with water being the byproduct. It is therefore possible, at least in theory, under the right chemical conditions, to hydrolyze (break with water) these polymer molecules to re-form the monomers. Regenerating isocyanates from polyurethanes is, in contrast, not something worth considering. Over millennia, hydrolysis may even happen, at least in theory, on contact with subsurface water- therefore unlike polyolefins which are absolutely stable in landfill with high certainty, there may be some arguable environmental benefit to not landfilling certain condensation polymers. It is also possible to carry out alcoholysis, by substituting an alcohol like methanol or ethylene glycol for water, which can give certain advantages.
Condensation polymers require very high purity monomers. Any quantity of a monomer with a damaged or missing 2nd reactive group becomes a chain terminator, limiting the molecular weight and hence badly affecting the mechanical properties of the polymer. Even once polymers are broken apart by hydrolysis or alcoholysis, purification to pure monomer can be expected to be a tricky problem.
While it’s clear that when mechanical recycling is possible (as it is readily with PET), it is the more environmentally sensible approach, it certainly is technically feasible to achieve even quite high yields of fresh monomer from post-consumer condensation polymers. However, a good 3rd party lifecycle analysis study is essential to understand whether or not the option of chemical recycling is of any value at all, even if the company developing the process thinks that they can make money doing it. The capital and energy and reagent intensity can all be very large.
Summary and Conclusions
While it is clear from even a perfunctory LCA that mechanical recycling, where feasible, is a better alternative, chemical recycling can be an option for condensation polymers- but only where a good, disinterested 3rd party LCA shows a real environmental benefit relative to landfilling (an option often ignored in LCAs). The main problem polymers, PE and PP however, can only be meaningfully recycled by a multistep process involving gasification, methanol synthesis and conversion to olefins, which has not yet been economically demonstrated. And thermosets are largely a disposal problem, not one of feasible recycling, until we invent reversible crosslinks that are environmentally durable.