Major chemical companies are backing pyrolysis plants for converting plastic waste into hydrocarbon feedstocks that can be turned into plastics again. They see it as a way to capture more plastics than they can with conventional mechanical recycling alone. But conducting pyrolysis at a significant scale will pose challenges. For instance, developers of pyrolysis processes have to tune their plants so they can convert a variety of polymers into products that petrochemical makers can use. Moreover, some plastics, such as polyvinyl chloride, can complicate the pyrolysis systems. How the technology works in the real world will affect the public&#;s perception of the plastics industry.
In September, Dow declared a milestone in its effort to mitigate the flow of plastic waste. The big chemical company and Mura Technology took the wraps off a project in Böhlen, Germany, to build a plant based on Mura&#;s supercritical steam process. The facility will convert mixed plastic waste into hydrocarbon liquids that Dow will load into its ethylene cracker at the site for conversion back into new plastics.
The plant will be the largest of its kind in Europe, diverting 120,000 metric tons (t) of waste per year from incinerators. It will be six times the size of Mura&#;s first plant, still under construction in Teesside, England. The partners hope to build additional facilities at Dow sites in Europe and the US for a total of 600,000 t of annual capacity.
&#;Böhlen is sort of a base case, and it will just get larger from there,&#; Oliver Borek, Mura&#;s chief commercial officer, said during a press conference. An executive from the engineering firm KBR, which is licensing the process beyond Dow and Mura, noted that his firm is already designing three plants in South Korea and one in Japan.
Petrochemical makers are fully behind the broad array of pyrolysis processes, like Mura&#;s, under development around the world. Nearly every large chemical company&#;Dow, BASF, Shell, ExxonMobil, LyondellBasell Industries, Sabic, Ineos, Braskem, and TotalEnergies, to name some&#;either has joined hands with a smaller firm developing a process or is creating its own.
These firms argue that pyrolysis can make up for the shortcomings of mechanical recycling, the familiar process of washing and repelletizing the plastics that consumers drop into blue bins. Only two polymers&#;the polyethylene terephthalate (PET) found in soda and water bottles and the high-density polyethylene in milk jugs and other such containers&#;are widely recycled at an appreciable scale. And it is difficult to get even these relatively homogeneous materials up to the contamination specifications needed for food-contact use. In all, mechanical recycling manages to capture only about 9% of plastics in the US, according to the US Environmental Protection Agency.
Recyclers can tackle a few more resins with depolymerization processes that break down polymers into their chemical precursors. For example, methanolysis can be used to recycle PET products like fibers and sheets that aren&#;t amenable to mechanical methods. And firms have been breaking down nylon using hydrolysis for many years.
We kind of joke sometimes that every day we need to make a birthday cake, but the ingredients keep changing all the time, and the birthday cake better be good and taste the same.
Eric Hartz, cofounder, Nexus Circular
But the bulk of the plastics we use&#;the candy wrappers, stand-up pouches, potato chip bags, protective packaging, single-use cups, frozen food bags, razors, toothpaste tubes, cotton swabs, and other objects of our daily lives&#;defy both mechanical recycling and depolymerization.
These items are constructed from multiple plastics that are nearly impossible to separate. Plus they are mostly made of polyolefins like polyethylene and polypropylene, which have strong carbon-carbon bonds that resist depolymerization. For these mixed plastics, pyrolysis is the industry&#;s only currently viable tool for recovering raw materials and making new polymers.
Credit: Honeywell UOP
But a pyrolysis reactor isn&#;t a magic box that can make the plastics industry&#;s waste problems vanish. The process is superficially simple: using high temperatures in the absence of oxygen to break down plastics into a mixture of smaller molecules known as pyrolysis oil. Yet converting the different kinds of plastics that can end up as waste into an uncontaminated feedstock&#;such as the C5&#;C12 paraffins that would be an ideal naphtha feedstock for an ethylene cracker&#;poses considerable challenges. Plastics companies will need to overcome these challenges if they are to debunk environmentalists&#; objections and meet their own goals for reducing waste and carbon emissions.
&#;We kind of joke sometimes that every day we need to make a birthday cake, but the ingredients keep changing all the time, and the birthday cake better be good and taste the same,&#; says Eric Hartz, cofounder and president of the pyrolysis firm Nexus Circular. &#;There&#;s a kind of art going on here when dealing with heterogeneous inputs as opposed to homogeneous. There&#;s not a perfect science to it about why some compounds behave the way they do in these environments.&#;
This industry-backed path to plastics circularity chemically breaks down plastics into their component parts so they can be made into new plastics.
Credit: Alex Tullo/Will Ludwig/C&EN
1. Pretreatment
The feedstock for pyrolysis plants is ideally made up of polyolefins such as polyethylene and polypropylene. Errant materials like oxygen-containing polyethylene terephthalate and chlorine-laden polyvinyl chloride are removed.
2. Pyrolysis
The plastics are heated to about 500 °C in the absence of oxygen. The longer molecules break into liquid fractions like naphtha and diesel, solid cuts like waxes, and lower-molecular-weight gases. In most plants, roughly 10% of the product is char, a by-product.
3. Landfill disposal
The char is hauled to the landfill or can be added to asphalt or concrete. Most plants burn the gases for heat.
4. Upgrading
For the output to be suitable for making new plastics, adsorbents and hydroprocessing may be needed to remove chlorine, nitrogen, and other pollutants. A hydrocracker, or similar unit, is sometimes needed to further break down large molecules.
5. Using waste
The naphtha is processed in an ethylene cracker to create ethylene and propylene, building blocks for more polyethylene and polypropylene.
One challenge of pyrolysis is the variability of the feedstock. The different polymers that are fed into a pyrolysis reactor break along different patterns. In particular, molecules with high degrees of branching crack more easily than linear ones.
According to a review paper by University of Minnesota Twin Cities bioproduct and biosystem engineer Roger Ruan and other scientists, polypropylene decomposes at 378&#;456 °C, while low-density polyethylene breaks apart at 437&#;486 °C, and high-density polyethylene at 452&#;489 °C. As a result, firms processing mixed plastic waste must select a temperature&#;normally over 500 °C&#;at which all the polymers they take in on a given day will break down.
However, temperature affects the composition of a pyrolysis unit&#;s output. Pyrolysis yields useful liquids, such as naphtha and diesel. But it also creates less-desirable waxes that might need to be broken down further. And pyrolysis makes lighter gases that are typically burned as fuel in the reactor. High temperatures and long reactor residence times might cut wax output and yield more naphtha, but they also create gases that have limited utility.
High temperatures can also lead to dehydrogenation, cyclization, aromatization, and Diels-Alder reactions, thereby creating more aromatics. &#;For fuels and so on, it&#;s fine,&#; Ruan says. &#;But sometimes we want naphtha feedstock for new plastics production; we don&#;t want a lot of aromatics.&#;
And feeding the wrong plastics into pyrolysis reactors creates inefficiency and can contaminate the output. PET contains oxygen and tends to form carbon dioxide, Ruan says. Polyvinyl chloride (PVC) yields chlorinated compounds. Additionally, some plastics have a lot of inorganic additives, such as carbon black, carbonate, and clay. They lead to the formation of char, which pyrolysis operators must dispose of as solid waste.
Environmentalists loathe pyrolysis. And a growing number of jurisdictions, such as California, don&#;t consider it recycling at all. One critic is Jan Dell, a chemical engineer who founded and heads the Last Beach Cleanup, an environmental organization. She has helped larger environmental groups, such as the Natural Resources Defense Council and Greenpeace, prepare reports on the practice. For presentations, Dell has compiled 16 pages of objections.
One of Dell&#;s primary complaints is that pyrolysis facilities can&#;t actually accept the mixed plastic waste they claim they can. The residual PVC, PET, and other materials in the stream gum up the process too much.
&#;There&#;s too many types,&#; Dell says. &#;There are too many additives. You can&#;t recycle them all together, and separating them out defies the second law of thermodynamics. It is just impossible to reorder&#;like Humpty Dumpty&#;all these plastics once they&#;ve been put into a curbside bin.&#;
Dell contends that Renewlogy, a Utah-based company that was developing a pyrolysis plant, folded for precisely this reason. Her bullet points even contain a photo from a Nexus Circular facility in Atlanta showing bales of relatively clean plastic film of the type used at warehouses&#;evidence, she says, that the company isn&#;t accepting much postconsumer mixed plastic waste.
It is just impossible to reorder&#;like Humpty Dumpty&#;all these plastics once they&#;ve been put into a curbside bin.
Jan Dell, founder, the Last Beach Cleanup
A second charge is that pyrolysis is really incineration, even though pyrolysis reactors operate in the absence of oxygen. &#;If you look at just the pyrolysis vessel itself, no, there&#;s no burning. I have to agree with that,&#; Dell says. &#;But here&#;s the deal: How do you heat that pyrolysis vessel to the 900 to 1,500 °F you need? You heat it by incinerating the gas that comes off of it.&#;
Dell points to the pyrolysis company Brightmark, which disclosed to the EPA that 70% of the output from a plant it is building in Ashley, Indiana, will be gases that it plans to use for energy or flare. Brightmark now says those figures were submitted in error. Such gases represent only about 18% of the output, the firm says, and it is submitting the updated figure to the EPA.
Another critique has to do with scale. Dell says that roughly 120,000 t per year of pyrolysis and other chemical recycling capacity is currently onstream in the US. This represents a minuscule fraction of the overall plastics production of about 56 million t in North America in , according to the American Chemistry Council. Just one new polyethylene plant has about 500,000 t of annual capacity.
To critics like Dell, pyrolysis is a greenwashing scheme meant to fool the public into thinking plastics are recycled more than they actually are. She points out that the industry, under similar pressure in the early s, built up a lot of recycling capacity, only to shutter it when the projects proved unworkable and public attention faded. The industry is now repeating this pattern, Dell says.
Industry executives say they are more committed than ever to recycling and are eager to practice pyrolysis at large scale. Their firms are building facilities that are bigger than before and are testing them in the real world. They are aware of the wrinkles in a pyrolysis-based recycling system and say they are determined to iron them out.
Manav Lahoti, global sustainability director for hydrocarbons at Dow, says experimentation will improve the systems over the long run.
&#;Sometimes you take this approach where there are successes and then there are failures and you singularly focus on the failures and say, &#;Oh, it&#;s just not working,&#;&#;&#; he says. &#;Is there a momentum with companies like us to actually create a solution for this challenge? The answer to that is yes. And then along the way, you will have some successes and you will have some failures.&#;
Credit: Nexus Circular
Brightmark is experiencing its share of obstacles. The plant that the company is starting up in Indiana, at a cost of $260 million, is designed to convert 100,000 t of mixed plastic waste per year into naphtha, diesel, and industrial waxes.
At the end of , Bob Powell, Brightmark&#;s CEO, said construction of the facility was 80% complete and ready to ramp up production in . But by April , the company had manufactured only about 2,000 t of product. Now Powell says it will run at full scale next year.
&#;The biggest challenge at this time has been COVID,&#; Powell says. The pandemic delayed the delivery of equipment and made it difficult to find enough labor to work at the plant. The company also suffered a fire in May that it calls a &#;minor setback&#; in an .
In another setback, Brightmark canceled plans to build a $680 million plant in Macon, Georgia, that would have been four times the size of the Ashley facility. The plant faced local opposition. And Brightmark was counting on about $500 million in bonds from the Macon-Bibb County Industrial Authority, but the deal stipulated that Brightmark get the Ashley plant going, and the county pulled its support after determining that the company hadn&#;t succeeded in Indiana.
Powell says Brightmark was treated unfairly. &#;These are questions that I think with a tour of the facility that folks could have had answers to pretty quickly,&#; he says.
Brightmark, an early mover in pyrolysis, has also had to contend with evolving market demands, according to Powell. When the company started building its facility in , the main concern was diverting plastics from the waste stream. Naphtha and diesel output was earmarked for the fuel market.
Link to Sihai Energy
&#;In the intervening 3 years, the demand for fully circular plastics has exponentially grown,&#; he says. Now Brightmark aims to sell naphtha to chemical companies as a plastics raw material and wants to see its diesel end up in chemical markets as well.
Because of this increased demand for recycled feedstock, Nexus Circular is getting a lot of attention from big petrochemical makers. The company&#;s plant in Atlanta has 13,000 t of annual capacity. About 80% of its output is what Hartz describes as a mix of naphtha, gasoline, diesel, and heavier waxes. The rest is gases that Nexus uses for heat.
Shell and Chevron Phillips Chemical are already using output from the Nexus plant in their US petrochemical crackers. Dow has agreed to take the output from a plant in Dallas that will be twice the size of the Atlanta plant. Nexus also aims to build a 30,000 t unit in Chicago that will supply Braskem, one of its investors.
Hartz says one thing that sets his company apart is that it can handle different forms of plastics, such as usually hard-to-recycle films, with limited presorting. Nor does it need to process its output with distillation or hydrotreating, which he says &#;adds tremendous costs&#; as well as environmental burden.
Hartz happily concedes Dell&#;s point that Nexus is persnickety about the waste plastics it consumes. To avoid sorting and postprocessing, the firm does focus on acquiring relatively homogeneous polyolefin streams, like pallet wrap from retailers. While these materials might not fit everyone&#;s idea of postconsumer plastics, Hartz says, they were still destined for the landfill.
Nexus pays a premium for such plastics versus mixed household waste. &#;They&#;re not free if you want the right quality,&#; Hartz says. &#;If you want garbage, then you&#;re going to have to set up a very, very expensive process before you can even use it.&#;
Many other companies are taking that approach and attempting to procure more mixed waste. &#;If you talk about true circularity going forward at scale, you are talking about mixed plastic waste; you are not talking about presorted polyolefins,&#; says Artem Vityuk, a global market manager at BASF. &#;You are really trying to expand the base of feedstock, and you need to be able to work with really contaminated feed.&#;
That means the output of pyrolysis units that consume a broad array of plastics must be upgraded to eliminate contamination. BASF recently introduced a new portfolio, called PuriCycle, of catalysts and adsorbents that eliminate such contaminants. The portfolio targets pyrolysis plants trying to meet customer specifications and petrochemical companies that want to clean feedstock coming from multiple sources.
Vityuk explains that contaminants such as halogens, oxygen, nitrogen, and metals are all found in the hydrocarbons coming out of pyrolysis plants. &#;That is what is in the plastics,&#; he says.
These contaminants can be nettlesome. An ethylene cracker might tolerate only 1 ppm of chlorine in its feed, so even one piece of PVC pipe in a pyrolysis reactor&#;s daily delivery can cause problems for a chemical company customer. BASF offers adsorbents to soak up the chlorine compounds. The product line also includes adsorbents and prehydrogenation catalysts marketed as being able to filter out particulate matter and eliminate the most reactive compouinds from the feedstock stream.
BASF also offers hydroprocessing catalysts similar to those that oil refineries use to displace sulfur with hydrogen. &#;We actually optimized the catalyst to make sure it&#;s suited for service in plastic pyrolysis oils,&#; Vityuk says. &#;It&#;s not a copy and paste from the refining area.&#; For example, rather than focusing on sulfur, the catalysts help remove nitrogen, which is in plastics such as nylon.
Steve Deutsch, a consultant with the Catalyst Group, says pyrolysis oil variability is an issue that the industry will need to tackle. While there are standards for ethylene cracker raw materials, &#;there&#;s nothing similar for pyrolysis oil,&#; he says. &#;The industry needs to evolve in such a way that it becomes more standardized.&#;
Petrochemical companies are starting to build infrastructure to process the products of pyrolysis plants. Shell is constructing upgraders at its chemical complexes in Moerdijk, the Netherlands, and Singapore to remove contaminants from pyrolysis oil sourced from third parties. Each will be capable of handling 50,000 t of oil per year.
&#;Due to the nascent stage of the chemical recycling industry, we can expect a large variation in the quality of the pyrolysis oil, whereby the upgrading step becomes integral in increasing usable quantities,&#; says Philip Turley, global general manager for plastics circularity at Shell.
Shell has already locked up supplies from various firms that might be able to supply the upgraders with pyrolysis oil. For instance, it has invested in BlueAlp, and the two plan to build 30,000 t per year of plastics pyrolysis capacity in the Netherlands. Shell also has an offtake agreement with Pryme, another European pyrolysis company.
Similarly, Dow is working with the engineering firm Topsoe to build a purification unit for pyrolysis oil at its complex in Terneuzen, the Netherlands. Like Shell, Dow says its unit is meant to purify and homogenize feedstocks that come from a variety of pyrolysis plants. &#;Some have a feedstock that still needs cleaning or processing before you can put it into a cracker. Some we can&#;t even look at,&#; Lahoti says, pointing to those with a high aromatic and naphthalene content.
&#;When you go from the start-up phase, which is where a lot of these companies are, you start thinking about how these technologies fit in the chemical industry,&#; Lahoti says. &#;And this is where a company like Dow allows some of these start-ups to be successful because we bring our technology experience, our processing experience, and we kind of help pull some of these feeds into our system.&#;
Lahoti says pyrolysis processes themselves are evolving to better suit the petrochemical industry. He sees &#;a transition away from conventional pyrolysis to different technologies, some of which were built on conventional pyrolysis as a foundation but have advanced to a point where it&#;s not just pyrolysis.&#;
For instance, Lahoti doesn&#;t consider Mura&#;s technology to be pyrolysis. The key development is supercritical steam, which transfers heat directly to the polymer particles. In ordinary pyrolysis, heat comes from the kiln and is transferred by poorly conductive plastic particles. &#;The bigger you make the kiln, the harder it gets for that heat to get in there,&#; Lahoti says.
Firms are also introducing catalysis into the pyrolysis reactors themselves. In addition to lowering the activation energy of the process, catalysts can tune the output to more desirable products. After pyrolysis, you&#;re left with some molecules with 40 or 50 carbons&#;too big to feed to a cracker directly, the Catalyst Group&#;s Deutsch says. &#;With catalytic pyrolysis, you can make that distribution both narrower and toward the lighter end.&#;
Since , LyondellBasell has been running a pilot plant in Ferrara, Italy, to test its catalytic pyrolysis technology, called MoReTec. While the company hasn&#;t said what catalysts it is testing, many pyrolysis research groups are working with zeolite catalysts, such as ZSM-5, that are commonly used in refining.
A sure sign that interest in pyrolysis is taking off is that large engineering companies are licensing technologies to third parties that want to get into the business. KBR is licensing Mura&#;s process. Lummus Technology is marketing a technology from New Hope Energy. And late last year, Honeywell UOP unveiled its own process, called UpCycle.
Kevin Quast, global business lead for Honeywell&#;s plastics circularity business, says Honeywell UOP&#;s reputation is a big help in the marketplace. &#;People like the UOP name,&#; he says. The firm &#;is very familiar with these types of technologies and is able to take something from pilot scale to a commercial scale.&#;
UOP bought the rights to a process that had been running in Europe for several years in the s. The company has been piloting and refining it for 2½ years.
Quast notes some key differences between UpCycle and other pyrolysis systems. For instance, Honeywell UOP has a pretreatment step in which it selects the right plastics for the system and melts them down before they go into the main reactor. What comes out is a light fraction, like naphtha and diesel, as well as a heavier cut. The heavier stream can be sent to a fluidized catalytic cracker to make propylene.
Honeywell UOP is forming two joint ventures, one in Spain with the infrastructure firm Sacyr and another in Texas with the recycler Avangard Innovative. Honeywell UOP is also licensing the process for plants in China and Turkey.
The scale of the Honeywell UOP plants&#;30,000 t per year&#;is a &#;sweet spot&#; for the amount of feedstock that can be gathered in a midsize city, Quast says. He questions whether some of the larger pyrolysis plants that have been announced will be able to acquire feedstocks economically.
Another sign that pyrolysis is hitting the big time is that one of the world&#;s largest oil companies, ExxonMobil, is making a push. Next month, the company will complete a pyrolysis facility at its petrochemical complex in Baytown, Texas, with the capacity to process 30,000 t of plastics per year.
&#;We&#;re actually processing the plastic waste directly in our own facility,&#; says Natalie Martinez, feed-to-value business manager at ExxonMobil. She declines to provide more detail about the equipment being used or the postprocessing involved.
Colocating plastics pyrolysis at a petrochemical complex allows ExxonMobil to use gases that stand-alone systems have to consume for fuel, Martinez says. &#;Everything that is coming out of the process is being utilized in an integrated facility,&#; she says.
ExxonMobil has a joint venture with the pyrolysis firm Agilyx, called Cyclyx International, that is dedicated to finding feedstock for the plant. ExxonMobil is exploring optical sorting and advanced analytics to manage the large amounts of material that will be headed there. &#;You can&#;t support that with hand sortation,&#; Martinez says.
If a large oil and chemical company like ExxonMobil can operate the process successfully at large scale, it will be the ultimate refutation of pyrolysis naysayers. The company aims to roll out the technology at plants around the world to reach a goal of recycling 500,000 tons of plastics annually by the end of .
&#;We know that scale, and being able to do this globally, is really going to be the key to ultimate success,&#; Martinez says. &#;It&#;s not just the technical aspects of processing plastic waste. We know that&#;s achievable and doable. It&#;s scale that will be meaningful and providing a solution to society.&#;
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Plastic pyrolysis is not widely used primarily due to environmental concerns and economic viability issues. The process, while promising in theory as a method to convert waste plastic into useful energy sources like oil and gas, faces significant challenges in practice.
Environmental Concerns: Conventional pyrolysis techniques emit toxic gases such as nitrogen oxides and sulfur dioxide, which pose serious environmental and health risks. These emissions occur when the process is not adequately controlled, highlighting the need for strict regulatory oversight and advanced technology to mitigate these harmful outputs. The release of these pollutants contradicts the goal of environmental sustainability, making the process less attractive despite its potential benefits in waste management and energy generation.
Economic Viability: The economic feasibility of plastic pyrolysis plants is highly variable and often marginal. The cost of production is heavily influenced by the availability and cost of local feedstock. While smaller, mobile units are considered more economically viable due to their flexibility and lower overheads, they still face challenges in terms of operational efficiency and product quality. The end products, particularly the oil and gas derived from pyrolysis, must meet certain standards to be marketable. However, the quality of these products is often compromised due to the difficulty in uniformly controlling temperature across the feedstock, especially in conventional pyrolysis setups. This inconsistency in product quality leads to lower market prices, making the process less economically attractive.
Regulatory and Infrastructure Challenges: In regions with stringent landfill policies, the adoption of pyrolysis might seem practical due to reduced tipping costs. However, the lack of infrastructure for centralized power production and the need for local power generation in remote areas add to the complexity and cost of setting up and operating pyrolysis plants. Additionally, regulatory requirements for the use of renewable carbons and the availability of carbon credits can influence the economic viability of pyrolysis, but these factors are not consistently favorable across all regions.
In summary, while plastic pyrolysis offers a potential solution to waste plastic management and energy generation, its widespread adoption is hindered by environmental concerns related to toxic emissions and economic challenges stemming from variable production costs and product quality issues. These factors necessitate further technological advancements and regulatory support to make plastic pyrolysis a more viable and sustainable option.
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