The pyrolysis

Posted on 10/07/2021 by Olivier Robelin

In our previous article, Valentin presented incineration, a technical waste management process for treating waste that cannot be recovered by other means of recycling or recovery.

Today, we are looking at a technology less known in our countries : pyrolysis.

Firstly, what is pyrolysis ?

It is in fact a combustion without oxygen which makes it possible not to create flames in the combustion chamber. It allows to transform solid matter into a liquid and gaseous state. It is interesting to process plastics by this technology because they are formed from petroleum, which can be reused after the process.

Here is a short video showing the process schematically :

In summary, the main phases of pyrolysis are as follows :

Pre-treatment of plastic : not all plastics obtain good yields with pyrolysis, so it is necessary to select them to obtain satisfactory oil. In our case, we will be able to process the majority of plastics apart from PET, which corresponds to plastic bottles.
Plastic shredding : for the pyrolysis process to go as well as possible, it is necessary to shred the waste to have small and uniform residues. In this way evaporation is simplified.
Combustion : once the plastic has been shredded, it can be placed in the combustion chamber. We are interested here in so-called “low temperature” pyrolysis, with combustion approaching 450°C (compared to 850°C for incineration). This temperature is sufficient for having a transition to the gaseous state. Then, this flow is directed to a condenser which makes it possible to extract a part in oil (about 80%), and a part in gas (about 15%). There remains about 5% of carbonaceous residues which we have not mentioned. They are extracted from the process before passing through the condenser.
Refining : depending on the use of the oil, it may be necessary to refine it to obtain the different fuels desired in the case of powering an engine, for example.

Credit to EarthWake

So now that you know the basic principles of pyrolysis, it is interesting to compare it to incineration because it has many advantages over incineration, particularly in the case of our project :

Technologically, it is a much simpler system, which falls into the low-tech family. It can therefore be set up much more easily with also simplified maintenance. In addition, it is possible to set up a unit with a low treatment capacity, unlike incineration.

Gas recovery allows it to be reinjected into the system to heat the combustion chamber, this allows self-supply. This is not the case for an incinerator that does not operate continuously. In Nepal, at altitude, it is complicated to transport the fuel.

The combustion temperature, which is much lower than incineration, facilitates the treatment of fumes. Indeed, they are not toxic and the filtration system is simplified. On the other hand, the heating power necessary to reach the optimum temperature is lower in the case of pyrolysis (450°C against 850°C).

The share of pyrolysis residue is less important and above all does not fall within the scope of hazardous waste, unlike incineration. This is partly due to the types of waste treated and there is therefore no particular treatment to be provided for these residues. However, the fact of treating only certain types of plastics does not make it possible to treat as much waste as with incineration…

Pyrolysis is therefore suitable for our project, namely a completely isolated region. However, it should be noted that this technology is still underdeveloped compared to incineration. This is due to the difficulty of developing it on an industrial scale. Indeed, waste treatment infrastructures include incinerators that process several tons of waste per hour, which is not currently possible with pyrolysis.

We also mentioned the problem of certain waste that can only be treated by incineration (PET, textiles, etc.). This share is quite low in our case, but in our Western countries, it is more important. It would then be necessary to put in place other means to be able to treat them.

Pyrolysis also requires extensive sorting beforehand. Given the volumes of waste that we produce in our countries, we should review our sorting methods and above all put in place greater means for their treatment, both in terms of infrastructure and awareness.

Finally, we always come back to the same observation, but the main solution is not to produce these wastes. So let’s start by reducing the amount of waste we produce !