Posted on 18/10/2023 by Pierre Jaraud
The principle of plastic pyrolysis
The aim of this article is to explain how a plastic pyrolysis machine works, and why it’s so useful. It also explains the difficulties that Tri-Haut has faced in implementing pyrolysis as part of its plastic waste treatment solution.
Plastics pyrolysis uses the reverse process used in the plastics industry: its aim is to break up chains of molecules – known as polymers – by cracking, in order to return them to a state close to their initial state. This is how liquid and gaseous hydrocarbons are obtained at the end of the process.
Before starting the pyrolysis cycle, waste preparation is essential. Plastics of the PE and PP types are sorted out, as they are the only ones suitable for the process. This is followed by washing with water and drying to obtain the best possible quality products. Finally, the plastics are shredded to accelerate thermal degradation in the reactor.
Prolysis involves initiating oxygen-free combustion in the reactor (P). This is achieved by heating the reactor to a temperature close to 450°, as our pyrolysis is categorized as low-temperature. The plastic will rise in temperature in the reactor, until it reaches its melting point. After melting, the plastic will continue to rise in temperature and finally begin to vaporize. The plastic in the form of gaseous vapour passes through the pipes connecting the reactor to the condensers (C1). These pipes act as a cooling system, enabling liquefaction to take place. The liquid phase is then collected in the various condensers, depending on the temperature of the different products.
At the end of the pyrolysis cycle, two types of products are recovered:
- Fuel oil. In the various condensers, the liquid phase, also known as “pyrolysis oil”, is collected during the cycle. It is directly flammable, even if its quality requires pre-combustion or flue gas treatment.
- Gas. The gas phase has not fully condensed and can be recovered downstream of the plant. A storage system (gasometer) is usually used.
However, the process also produces two types of waste:
- Wax. This is the solidified form of pyrolysis oils, visually similar to a paste. It is not easily flammable, and its production is highly likely to clog the machine and lead to its breakdown.
- Coal. This is the ultimate waste product. In concrete terms, it’s a kind of burnt dust, which concentrates all the elements and additives in the plastic that can’t burn. This waste is very difficult to recycle.
Tri-Haut’s work on pyrolysis
Tri-Haut’s approach to plastic pyrolysis was initially empirical. Right from the year the association was founded, a team worked on the subject with the aim of building a pyrolysis plant in Nepal. To fully understand the stakes involved in such a project, we need to take a closer look at the conditions under which the machine would be set up.
The pyrolysis machine is intended to operate in Pangboche, a remote village 4000 m above sea level in the Everest valley. The machine will be operated by a local operator trained by the association, not an expert in industrial processes. Having these constraints in mind is important for understanding the Tri-Haut approach.
After deciding to abandon work on the incinerator (see related article), the first team turned their attention to pyrolysis for the added value it could extract from plastic waste. Time-pressed, they adopted an empirical approach, building a prototype in Kathmandu.
This prototype, with a capacity of 5 litres, was built in a factory with limited technical resources, compared with what is done in France, for example. The idea was to build the pyrolysis system on site, so as to be able to find the materials needed to repair it.
This first team carried out a battery of tests to study the influence of various parameters on product quality and quantity. These included: final heating temperature, type of plastic, quality of plastic preparation, etc. This test phase also highlighted a number of areas of concern and difficulty, such as leakage zones and conditions leading to machine clogging.
A year later, the second team was able to carry out further tests on the same prototype, with different objectives. The influence of the length of the cooling pipe and the types of plastics processed were studied.
However, it was the third team that decided to make pyrolysis its priority from the end of 2022, with the aim of building the final version in Nepal during 2024. To compensate for the lack of knowledge highlighted, and to complement the results obtained during the two years of experimentation, a theoretical approach was favored.
Over a period of several months, the Tri-Haut team synthesized the state of the art and made numerous contacts with professionals in the field, in order to increase its expertise in the subject. The actual dimensioning work began, and despite the numerous aids provided by experts in the field, the work proved tedious.
With hindsight, this can be explained in several ways:
- Pyrolysis is a relatively young and not yet mature technology. Its use in industry is mainly oriented towards biomass pyrolysis, so resources on plastic pyrolysis are rare.
- The team were novices in the field, and had to learn on the job on their own.
- The desire to focus development on low-tech added further constraints, which were not mastered by the companies in the sector who were asked to help with development. Indeed, Tri-Haut pyrolysis differs in many respects from the pyrolysis processes developed in industry.
Despite these difficulties, Tri-Haut’s work resulted in an almost complete dimensioning of the pyrolysis system. This meant that only a few details remained to be ironed out, and the equipment list could be sent to Nepal to prepare for the team’s arrival.
Reasons for stopping development
However, during an exchange with Earthwake’s R&D team and management, Tri-Haut was warned about the feasibility of its pyrolysis project.
A number of comments were made, which are summarized below:
- Despite all our efforts, the fuel obtained will be of poor quality, so that it cannot be burned in an engine, at the risk of clogging it and leading to its destruction. What’s more, burning it by any means releases toxic fumes that require extensive treatment at every point of combustion.
- The extraction of these pyrolysis oils remains dangerous, as the slightest imperfection in the process leads to the formation of gasoline vapors with a high flammability potential.
- Cleaning the tank at the end of the cycle to remove the carbon requires the use of tools. Even the slightest spark can cause an explosion, as dense, flammable gases stagnate at the bottom of the tank.
- It is not possible to ensure the absence of oxygen in the pipes. This poses risks of auto-ignition, particularly at the beginning and end of the cycle. The only way to inert the pyrolysis is by injecting nitrogen, but this solution is not feasible in the geographical context of the project.
- Pressurizing the pyrolysis gas requires very expensive ATEX compressors. If non-standard compressors are used, there is a risk of explosion. Gas at too low a pressure burns poorly and risks fouling the boiler.
In short, Tri-Haut was confronted with the risks posed by low-tech pyrolysis, which is far less safe than the industrial models that served as inspiration. These risks are not only limited to potential material damage, but can also endanger people’s health. With 10 years’ experience in the field, it would have been irresponsible and presumptuous to ignore Earthwake’s warnings. Not wishing to endanger the health of the sorting center’s staff or its members, Tri-Haut took the difficult decision to suspend the development of pyrolysis.
However, the work carried out is not lost, as it will be passed on to Madindra, a Nepalese engineer wishing to develop this technology in Kathmandu, in a more suitable context.