Chemical cycle: will it be a "panacea" for the disposal of waste plastics?

Zhu Xingxiang, president of the China Circular economy Association, has exposed the cruel truth of plastic recycling: only 9 per cent of the world's 9 billion tons of plastic products are recycled. Most plastics end up in landfills or incinerators. Compared with the recycling rate of waste paper and scrap steel, the recycling proportion of plastics in China is astonishingly low. All kinds of plastics in life are thrown into roadside trash bins and mixed with metal and waste paper, but most of the metal and waste paper can be recycled, and most plastic waste can only go to landfills. We are in urgent need of effective recycling methods to deal with waste plastics.

Recovery ratio of all kinds of commonly used Materials in China in 2017-2020

Among all kinds of commonly used plastics, the recovery proportion of PET is relatively high. This is because the recovery step of PET plastic is relatively simple, and the technology development is more mature: the recycling of PET plastic bottles only needs three steps: cleaning, drying and granulation. More importantly, PET plastic itself has good mechanical properties, light weight, durability, high strength and high recyclability.

Recovery ratio of various commonly used plastic materials

Where is the "good" of chemical cycle?

At present, there are two main methods of plastic recycling, mechanical recycling and chemical recycling.

The mechanical recycling of plastics refers to the use of physical and mechanical methods to grind the plastics, wash them slightly, and then make new objects.

The chemical recovery and reuse of plastics are related to gasification and high temperature decomposition. Through these chemical treatment processes, plastic polymers are broken down into small molecular monomers. These small molecular monomers can be used to produce new plastic polymer materials of the same kind, or to further produce different kinds of polymer materials. But in most cases, the chemical recycling of plastics simply burns waste plastics, which releases a lot of carbon into the air. The terms "pyrolysis", "solvent decomposition" and "depolymerization" are also used to refer to the different technologies of the method. No matter what the process is called, if the final product burns, it is plastic fuel.

Chemical recycling is a hot technology at present, and it is called the "savior" of plastic recycling. Compared with mechanical cycle, chemical cycle has certain advantages:

(1) Mechanical recycling of waste PET plastic to make new bottles requires high purity and transparent raw plastic materials, but chemical cycle can deal with low purity plastic raw materials, there is no such restriction.

(2) the pyrolysis chemical cycle equipment has a stronger ability to deal with low purity raw materials than the depolymerization facility. Pyrolysis of long polymer chains to short-chain hydrocarbons such as diesel oil and naphtha can be achieved under low oxygen conditions and temperatures above 400 ℃.

However, the pyrolysis chemical cycle equipment does not support the treatment of PET polymers because it contains oxygen. Therefore, PET plastic can only be recycled by mechanical treatment. The pyrolysis chemical cycle is most suitable for the treatment of the mixture of polyethylene and polypropylene.

At present, pyrolysis companies are mainly focused on fuel production, but more companies are starting to use this process to make naphtha, which can be transported to petrochemical plants and turned back into polyethylene and polypropylene.

Will chemical recycling be the perfect answer for plastic recycling?

Many studies have shown that the high temperature pyrolysis oil in plastic waste has high toxicity. Therefore, only a very small amount of high temperature cracking oil can enter the existing cracking process.

There are two ways to solve this problem, one is to purify and improve the quality of high temperature cracking oil until it meets the cracking standard, which is a process of extremely high energy consumption, carbon enrichment and low production capacity. The second solution is to dilute a small amount of pyrolysis oil with large amounts of fossil crude oil, which can effectively reduce toxicity, but also lead to low production of recycled plastics. In addition, it is not clear whether the emission data, energy requirements and quality requirements of high temperature cracking oil are still valid in the case of a high proportion of pyrolyzer input.

In fact, chemical cycle technology has a record of failure in economic investment. By 2017, core technologies known as pyrolysis and gasification had wasted at least $2 billion on cancelled or failed projects. After all, even if these projects do manage to produce some plastics, there is no market for them because the prices of raw petroleum-based plastics are at rock bottom and expensive energy-intensive technologies such as chemical recycling cannot compete.

The chemical cycle can still cause environmental pollution. The chemical cycle can be divided into two types: thermal decomposition (pyrolysis) and depolymerization into monomers. The essence of pyrolysis is to burn plastics and decompose them into small molecular organic compounds, so each step of the chemical cycle will release greenhouse gases and have an environmental impact.

In addition, plastic waste contains a lot of toxins (studies have shown that disposable food packaging may contain more than 100000 chemicals), and heating plastic releases toxic substances such as carbon monoxide, carbon dioxide and dioxins. These toxic substances will eventually enter the environment, such as air, water and so on. As a result, most "chemical recycling" is actually making pollutants rather than new plastics. Burning these fuels can cause not only climate problems, but also health problems.

At present, the chemical cycle lacks objective research and has no economic value. The research results of chemical cycle LCAs (life cycle assessment) can easily be misleading. LCAs is usually carried out in a narrow range within which the energy mix is carried out on specific waste categories and uses assumptions and other variables that would have provided very different results. However, the results have been widely spread without full disclosure, giving the illusion that decisive conclusions can be drawn from the study.

For a long time, the petrochemical industry has described the feasibility of chemical recovery in an optimistic way. However, current data show that the future of this industry is a far cry from the actual capabilities of the technology. At present, most of the existing research on chemical recovery is funded by industries with a vested interest in technological success, and there is a lack of independent reports or studies on the subject of chemical recovery.

How to solve the problem of chemical cycle

However, chlorine is only a small part of the toxic substances produced by the chemical cycle of plastics. Plastic waste itself contains a lot of toxins, while plastic pyrolysis releases another series of toxic substances. Because it is difficult to understand the mechanism of the interaction between mixed chemicals, it is almost impossible to completely remove toxic substances from chemically recycled plastics.

At present, the methods to improve the chemical cycle are very limited, which is not enough to change the fact that the chemical cycle has high energy consumption, high equipment requirements, and produces a large number of greenhouse gases and toxic substances. The research and evaluation of chemical cycle is funded by vested interests, and the results of the study are biased. Therefore, before a fair and comprehensive chemical cycle research report is released, we should carefully consider chemical cycle as an appropriate way to deal with waste plastics in terms of investment and policy guidance.