Energy engineer Dr Andrew Rollinson sets out the case as to why pyrolysis and ‘plastic to fuels’ is not a sustainable solution to the plastics problem.
Compounded by the breakdown of the global recycling market, it seems that most governments and local authorities throughout the world are grasping at straws to find a quick fix for the plastic waste problem. Plastic is piling up on land and threatening the biosphere through its contamination of the oceans. At the same time most governments exhibit a morbid fear of doing anything which could oppose continuous economic growth.
Wonder technology fixes such as pyrolysis for ‘plastic to fuels’ (1) and green energy from waste (EfW) are therefore offered up as the future solution. For, if such machines were capable of simply and sustainably converting plastic into fuel or energy, then citizens may feel encouraged to buy more and waste more, liberated from guilt with the knowledge that anything they saw and wanted could be purchased.
But this premise is inherently flawed. Pyrolysis of plastic can never be sustainable. In a recent academic journal, I detail why the concept is thermodynamically unproven, practically implausible, and environmentally unsound (2).
Pyrolysis occurs when solid organic matter is heated, resulting in the release of gases, oils, and char, hence the word’s etymological root of “loosening or change by fire”. It is an old technology, formerly applied by heating up wood to produce substances such as methanol, acetone, and creosote, prior to petrochemical refining routes. When wood is slowly pyrolysed the char is called ‘charcoal’; when coal is pyrolysed the char is called ‘coke’; and with plastics there is little or no char produced at all.
R. Fludd, Tractatus Secundi Pars VII De Motu. in libros quatuor divisa, pp. 433-468 (Oppenheim: de Bry, 1618).
The modern notion is to pyrolyse plastic (and other municipal refuse) into a gas or oil which is then useable as a commodity, invariably a “fuel”, in its own right. This conveniently ignores the fact that pyrolysis is an energy consuming process: more energy has to be put in to treat the waste than can actually be recovered. It can never be sustainable.
And what of the fuel from these ill-conceived schemes? All pyrolysis EfW or ‘plastic to fuels’ products must be combusted to liberate energy, thus releasing the same quantity of carbon dioxide than if the plastic had been incinerated directly. The product’s existence has merely been an intermediary stage in the combustion of fossil fuels.
But the idea is even more imprudent. There are substantial flaws with the pyrolysis of plastics concept. It has been tacit knowledge for almost one hundred years that this type of waste is practically incompatible with these technologies (3). Also, heavy metals and dioxins become concentrated in the resulting products making then unsuitable as fuels, because when combusted they are released to the environment.
Despite this many governments continue to waste millions deceiving the public in pursuit of an ‘innovation’ that holds the sustainable answer. They ignore the above-mentioned scientific antecedents, and a wake of commercial failures (4).
Academic research has also been drawn in, attracted by the competition for financial rewards. With a prevalence in many countries for grant funding which links industry, innovation, and academic research, ethical hazards have been created and these have yielded poisoned fruit (2).
Many modern academic research articles present pyrolysis with positive connotations, appraising it in terms of “energy recovery” or “conversion” efficiencies. This is despite huge overall energy demands. In one study the concept was described as “high efficiency”, but results showed that the system operated with gross negative efficiencies, using between 5 and 87 times more energy than could be obtainable from the pyrolysis products.
Graphical abstract from: Rollinson, A., Oladejo, J.M. 2019. ‘Patented blunderings’, efficiency awareness, and self-sustainability claims in the pyrolysis energy from waste sector. Resources, Conservation and Recycling, 141, pp. 233-242.
Perhaps worst of all, some research groups have recently claimed that pyrolysis plants can be self-sustaining. By doing so they commit a blunder which exposes them to instant discredit, for they ignore the second law of thermodynamics. Such a folly is akin to the antiquated pursuit of perpetual motion.
Perpetual motion is impossible because it violates the laws of thermodynamics. These laws underpin all engineering, and indeed all universal interactions. The first law states that energy must be conserved – what goes in must come out. The second law states that whenever there is energy transfer some quantity must always be lost to a system’s surroundings (measured as “entropy”).
The inviolable nature of the second law was perhaps best explained by Arthur Eddington in his famous Gifford Lectures (5):
“There are other laws which we have strong reason to believe in, and we feel that a hypothesis which violates them is highly improbable; but the improbability is vague and does not confront us as a paralysing array of figures, whereas the chance against a breach of the second law can be stated in figures which are overwhelming.”
Once one fully understands these laws, the folly of such schemes, and the sophistry of corporate attempts to claim “sustainability”, becomes instantly clear. It is therefore essential to grasp this concept if ever humanity is to make a transition to a sustainable future.
Pyrolysis can never be a sustainable answer to the inconvenient truth of Big Plastic. This lies in the widespread implementation of strategies for “reduction” and “re-use”, along with a preference for creating products with in-built recyclability and/or which are built to last. The elephant in the room is capitalism (6) and the throwaway culture that the present version of this economic system has created – ever demanding new markets, more sales, more consumption, and more waste.
The full text of the paper can be accessed for free here in Resources, Conservation and Recycling until 23rd December 2018.
1. Phan, A. How we can turn plastic waste into green energy. The Conversation, 1st October 2018.
2. Rollinson, A., Oladejo, J.M. 2019. ‘Patented blunderings’, efficiency awareness, and self-sustainability claims in the pyrolysis energy from waste sector. Resources, Conservation and Recycling, 141, pp. 233-242.
3. Mavropoulos, A. 2012. History of Gasification of Municipal Solid Waste through the eyes of Mr Hakan Rylander (online), 19th April 2012.
4. Tangri, N., Wilson, M. 2017. Waste gasification and pyrolysis: high risk, low yield processes for waste management. A technology risk analysis (online).
5. Eddington, A.S., The Nature of the Physical World, 1927. Cambridge University Press: London. pp.68-71.
6. Pigott, A. Capitalism is killing the world’s wildlife populations, not ‘humanity’. The Conversation, 1st November 2018.
Doctor Andrew Rollinson specialises in small-scale biomass gasification research and is the author of Gasification: succeeding with small-scale systems published by Lowimpact.org.
J Socci said on February 28, 2019
The paper referenced by A. Rollinson that discusses whether prolysis plants can be self-sustaining refers to MSW as a feedstock. Here the author is referring to plastic wastes as a feedstock, which will have much higher heating values (more energy), therefore the energy balance will be completely different.
Andrew Rollinson said on February 28, 2019
Dear J Socci,
I am the author of both articles.
My journal paper covered pyrolysis of MSW (and sewage sludge – literally a MSW, though not often categorised as such). Plastic is a high component of MSW for most countries, so the reference to plastics here is relevant. Indeed, my journal paper specifically referred to the extra problems of plastic pyrolysis, namely: the amorphous nature of the plastic hydrocarbons meaning that usually zero char can be produced (relevant because char usually has the highest energy density of the three pyrolysis product phases), and that dioxins concentrate in the pyrolysis oils.
Pure plastic will have a higher energy density than raw MSW, but the fact is inescapable that to pyrolyse this material requires energy. In my paper I show that the energy produced in the pyrolysis gas and oil is less than the energy needed to pyrolyse it (enthalpy for pyrolysis) along with the entropic losses and auxilliary electricity associated with the concept of this type of plant.
Of course there was another theme to my journal paper, and this blog, which is the absence of satisfactory energy balances when these technologies are presented. Some don’t even mention the need for energy inputs at all while simultaneously claiming that their process is “sustainable”. I absolutely include “plastics to fuels” in that category.
Andrew
J Socci said on February 28, 2019
Why are you bothered about the production of char from plastics when the hydrocarbon gases are readily combustible to provide energy for the process? There are understandable issues surrounding the toxic byproducts generated by the pyrolysis of certain plastics, this can be negated through proper feedstock selection.
I don’t think you provide any tangible figures referencing energy balance values. A paper on the energy balance of a 1kg/h test rig (https://dx.doi.org/10.5541/ijot.5000147483) calculates that for beech wood all the energy losses for the process equates to only 6.5 % on HHV basis. Plastic generally has a much higher HHV than biomass, therefore it would be expected the combustion of the produced non-condensable gases would provide more than enough energy to drive the process. I do agree that there are some reasonable environmental considerations for the pyrolysis of plastics, but the pyrolysis of pure plastic waste and MSW is very different.
Andrew Rollinson said on February 28, 2019
I mentioned char because it is the most energy dense of the three pyrolysis products. Many of the proposed waste pyrolysis concepts claim to make use of char to heat the retort. It is just not feasible.
Do you have any evidence of a pyrolysis of waste plant where (as you say) “the hydrocarbon gases are readily combustible to provide energy for the process”. I’d like to see that evidence please of such an operational plant. It is no good saying “…would be expected…” and “…would provide…” Such claims constitute the crux of my paper.
Thanks for the link to the test rig fed with beech wood. I’ll read it over the weekend.
Andrew
Andrew Rollinson said on February 28, 2019
I’ve just had a quick look at that beech wood test rig paper on fast pyrolysis. Here is what Tony Bridgwater said about fast pyrolysis in 2012:
“The by-product gas only contains about 5% of the energy in the feed and this is not sufficient for pyrolysis” (Biomass and Bioenergy, 38, pp.68-94).
By the way, I’ve met Tony a couple of times. A nice man. And, I am friends with a couple of people in his group at Aston. We probably agree that pyrolysis has a niche and is an interesting technology. I just want there to be more transparency on its practical utility and energy demands, for many people are passing it off as being a sustainable solution to waste. It isn’t.
J Socci said on February 28, 2019
Yes, he is my supervisor! You now have another friend in his group at Aston! 🙂
I guess that paper is talking about fast pyrolysis of biomass, in which case the major product is organic liquid yield and the gas in mainly composed of CO and CO2. My point is when pyrolysing plastics, depending on process parameters, there will be a high yield of combustible hydrocarbon gases (alkanes and alkenes), as well as a liquid fraction. So I think to compare plastic pyrolysis to a perpetual motion machine is misleading.
Totally agree, and I think proponents of pyrolysis often overlook the high CO and CO2 yields, however, pyrolysis may be a better alternative to incineration which a large proportion of waste is subjected to currently.
Cheers
Joe
Mark F. Martino said on March 8, 2019
I’m only a mediocre engineer who hasn’t designed anything in years so I appreciate your indulgence with my musing and any suggestions.
While plastics cause a lot of trouble, they are not going away for a while and there is a lot out there. Even if the energy in is more than the energy out, there is a way to make plastic pyrolysis viable. If homes, apartments, and businesses had there own pyrolysis units, they could convert their waste into energy on site. The syngas could be used to generate electricity directly. This would save the energy used to transport and sort the waste. It also prevents the pollution from the vehicles transporting the waste. And maybe other waste could be processed.
Andrew Rollinson said on March 11, 2019
Dear Mark,
There are practical problems with domestic pyrolysis of waste/plastics machines. These relate to heat losses from the reactor during feeding, corrosion and erosion caused by the high ash and chlorine content, removing ash (and again having to maintain high internal temperature in the reactor while doing this), and the tarry nature of the pyrolysis gas which sticks to downstream components necessitating system shutdown for repeated cleaning. All these relate to pyrolysis on any scale.
The above are reasons why the concept of a domestic pyrolysis reactor is an impractical pipe dream. Yet, surprisingly, you will find people promoting that they have developed the concept…. and seeking naive investors for the gadget that is the answer to all domestic waste. Avoid these people.
Notwithstanding the practical aspects above, let me explain again about the energy balance: The practical aspects do impact on the energy balance because each time the reactor is opened to feed or to clean, the internal temperature cools down, this demands greater energy input and also dramatically changes the chemistry of the gas and oils that are produced. But, let us assume that you can discount them and that you have heated up your reactor containing plastic:
Assuming that you had heated your reactor up to 550C. This cost you 10 kWh of electricity. You then need to pyrolyse all the plastic, and this will cost you a further 3kWh of electricity, then the gas and oil is burned in an adjacent combustion chamber where it heats water and the water is used to turn a generator so that in total you can produce 5kWh of electricity. Overall you have disposed of your plastic at a cost to you of 8kWh. This could be electricity from the grid or your battery bank.
These are NOT renewable energy devices.
Yes, it seems ideal to be able to convert your domestic waste into energy in a kitchen appliance type gadget, but fundamental universal laws dictate that it is not possible. The idea is, as I have said, akin to the antiquated pursuit of perpetual motion.
Andrew
Andrew Rollinson said on March 11, 2019
Dear Mark,
I’ve just re-read your post, and I believe that I perhaps haven’t answered it:
Yes, IF the systems could be made to work, and IF one is willing to pay high energy costs for melting plastic at home, then it would obviate having to transport and sort the waste on a regional level, and would remove some vehicular pollution.
The corporate-sector waste contractors who have locked-in local authorities to supplying them with municipal waste for a term of 25 years would probably have something to say about it.
Mark F. Martino said on March 11, 2019
Dear Andrew,
Thank you for filling in details of my understanding of the pyrolysis of plastic, especially the numbers you included in your explanation. I thought I had found a pretty good approximation of a solution when I discovered the Dung Beetle project in Africa, which involves a portable pyrolysis unit. I can see how it has the problems you pointed out. So, even at a small, controlled scale, pyrolysis is not the way to dispose of plastic or other kinds of trash.
I appreciate the research and work you are doing and hope there is another solution beyond recycling and composting. Here in Kirkland, Washington, we’ve been recycling for decades, but there always seems to be massive amounts of plastic and other trash that cannot be recycled. Instead, it goes to a collection site about a half mile from my home. From there, trucks haul it to a railroad station where it is loaded onto railroad cars and taken to a landfill in Oregon. I watched this happen for many years. It really seemed to me that the local pyrolysis of the trash in small, discrete batches had to use less energy and cause less pollution than that. Thank you for showing me why we need to pursue other solutions.
Wally said on March 22, 2019
I wouldn’t even go as far as to say I know something about this topic. But I do have a question. Using tires and plastics to create a diesel fuel, although it requires a lot of energy to do. Wouldn’t it still help to solve a problem that has to be dealt with? Doing something with waste. Or is it only moving the problem to another area?
Andrew Rollinson said on March 26, 2019
Dear Wally,
True, it would “do something” with the waste. Some people attempt to pass this of as being sustainable, but it isn’t.
Zhao Zhang said on April 1, 2019
An impressive paper by Dr. A.
I resolutely oppose small pyrolysis systems in your home, your backyard or in the community as it maybe encouraged.
Small pyrolysis systems are often open systems without any tail gas treatment. They may bring problems of serve air pollution as the pyrolysis gases often contains PAHs, N-PAHs, HCN, CO, etc. In a open system, these gases can be easily leak to the air and cannot fully converted in the combustion systems.
When I saw the small pyrolysis systems making biochar in household, I was very sad.
These systems discharge a lot of dark smoke, but the farmers thought they are saving the world. When we talk about the carbon negative of Biochar, has anyone thought the emmission of Black carbon and brown carbon?
Many thanks to Dr. A for the critical review, which reveal the truth of this field.