Plastic waste can be extruded, melted and then molded, or transformed into molecules of industrial interest -
© Jantsarik / Shutterstock (via The Conversation)
Global plastics production reached 359 million tonnes in 2018, according to a study published by our partner The Conversation.
Recycling plastic waste will be a major environmental issue in the years to come.
The analysis of this phenomenon was led by Andrei Khodakov, research director at CNRS, Deizi Peron, post-doctoral fellow (both at the University of Lille) and Alan Barrios, doctoral student at the École Centrale de Lille.
For many years, human civilization has been accustomed to living according to a “make, take, throw” model.
A consequence of this way of life is the large-scale production of plastic waste.
Over the past five decades, global plastics production has grown steadily, reaching an aggregate annual production of 359 million tonnes in 2018;
it is estimated that these numbers will continue to increase in the years to come.
Because of this high production, one of the big challenges is the management of this waste.
In the past, due to high costs, plastics were not recycled.
However, with the implementation of new laws and regulations, it becomes more and more urgent to develop new efficient, non-polluting technologies that easily adapt to different types of plastics;
the objective being to recycle 50% of plastic waste in 2025 to 55% in 2030, against around 35% currently in France and Belgium.
One way to deal with this plastic residue is by incineration.
In this case, it is possible to take advantage of the energy potential of these plastics, which can thus generate electricity.
Plastic waste has a high calorific value.
Their combustion makes it possible to heat the water and generate steam.
The steam spins the turbine, whose mechanical energy is converted into electricity using an alternator.
However, this method has the drawback of emitting greenhouse gases, in particular carbon dioxide (CO2) and sometimes toxic products, such as acid gases (HCl, SO2, HF), dioxins, furans, heavy metals, polychlorinated compounds ...
How do we recycle plastic today?
Over the years, several recycling methods have been developed, such as “primary recycling”, where used plastics are recovered by “extrusion”, generating materials similar to the initial materials.
However, this type of process requires selective and separate plastic waste collection for each type of plastic: polyethylene, polypropylene, etc., which poses a problem of significant operating costs.
Most often, “sorted” plastic waste is indeed mixtures of different types of plastic materials.
“Secondary” mechanical recycling includes the collection, sorting and washing of waste.
Then the plastics are directly melted and molded into a new shape, or made into granules.
Secondary recycling is only possible when the plastic waste consists of simple polymers, because the more complex and contaminated the waste, the more difficult it is to sort and recycle by this technique.
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In addition to primary and secondary recycling, “tertiary recycling” is chemical recycling.
In this type of recycling, plastics are converted into smaller molecules, usually liquids or gases, such as pyrolysis oil or synthesis gas, which are commonly used as raw material to obtain new fuels (kerosene, dimethyl ether, gas oil) and chemicals (eg methanol, olefins, alcohols, fertilizers, insecticides, fungicides).
Among all recycling methods, "chemical recycling" has recently gained attention, especially pyrolysis, hydrocracking methods where plastics are mixed with petroleum products and simultaneously processed with them in existing refining units, as well as gasification.
Among these three chemical recycling methods, gasification is particularly interesting, because it has the great advantage of treating heterogeneous and contaminated polymers while requiring little pretreatment.
It also makes it possible to obtain "synthesis gas", a mixture of hydrogen and carbon monoxide, which is used in various applications as a gaseous fuel or chemical intermediate, for example, for the synthesis of liquid fuels and methanol.
New waste recovery methods for the industrial scale
With industrial players in the sector, we have identified plastic waste streams of interest for gasification, i.e. those composed of polyethylene, polypropylene, polystyrene waste, rigid and flexible polyurethane foams, multilayer packaging, or composites reinforced with carbon fibers or tungsten carbide, which have found applications in the aerospace, automotive and maritime industries.
The idea is to produce basic chemicals from plastic waste for reuse in industry.
Innovative technologies being developed as part of the PSYCHE project © A. Khodakov
Our European project Interreg Psyche focuses on the gasification of plastic waste into synthesis gas and then on the conversion of synthesis gas into light olefins.
A pilot gasifier based on the Vortex technology developed by the University of Ghent, is being dimensioned at the Catholic University of Louvain.
This gasifier is based on the rotary movement of gases and solid particles, which allows better mixing of reactants and better heat transfer compared to conventional gasification technologies.
The sizing of a reactor involves the calculation of its volume and the flow rate of the raw materials necessary to obtain the desired productivity.
Vortex reactor (a) and eggshell-type iron carbide nanoparticles in catalysts for the Fischer-Tropsch process (b) © A. Khodakov
Next, we want to produce, from synthesis gas, basic chemicals for industry, in this case “light olefins”: ethylene, propylene and butylene.
Olefins are essential synthetic blocks in the chemical industry, widely used in the synthesis of various products such as polymers, paints and solvents.
Traditionally, light olefins are obtained by pyrolysis, steam cracking or by fluid catalytic cracking of petroleum, but these processes generate a lot of by-products and they have a high cost.
This is why alternative routes for obtaining light olefins are being studied.
Develop new catalysts
The biggest challenge is the development of catalysts, substances which increase the speed of a chemical reaction without appearing to participate in this reaction, selective and stable when operating for several months.
These catalysts make it possible to obtain the high yield of light olefins and prevent the formation of by-products in a Fischer-Tropsch process which converts the synthesis gas resulting from the gasification into hydrocarbons.
Our Lille team develops high performance catalysts for the production of olefins from synthesis gas.
Synthesis gas from the gasification of plastics contains impurities that are harmful to catalysts.
Gas purification as part of the Psyche project is carried out by the Center for Technological Resources in Chemistry (CERTECH).
Synthesis of light olefins from synthesis gas "Syngas" by the Fischer-Tropsch reaction © A. Khodakov
Iron-based catalysts are generally the catalysts of choice for the synthesis of light olefins by the Fischer-Tropsch reaction, due to their low cost, high selectivities and flexibilities.
The performance of the iron nanoparticle catalyst can be improved by promoting small amounts of different elements called "promoters".
The promoter is generally considered to be an additive, which is itself inactive, but can improve the activity, selectivity and / or stability of a catalyst.
As part of the PSYCHE project, we have discovered new, highly efficient promoters for iron-based catalysts.
These promoters improve the productivity of catalysts and reduce the formation of reaction by-products.
They are based on metals commonly used for soldering such as bismuth, tin and antimony, which are mobile at the extremely high surface area and form eggshell-like nanoparticles.
With a fundamental understanding of the mechanism of the Fischer-Tropsch synthesis, the structure of the catalyst and the modeling of the reaction kinetics in collaboration with the University of Ghent, we have succeeded in increasing the yield of light olefins 10 times. .
The catalysts newly developed within the framework of this project exhibit increased stability against sintering and carbon deposition, which would allow their eventual use in the industrial synthesis of light olefins from synthesis gas generated by gasification of plastic waste.
The carbon efficiency of synthesis of light olefins from plastic waste by this new technology reaches 35-40%.
This technology offers a sustainable solution to the combustion of specific plastic waste.
The use of synthesis gas obtained from the gasification of plastic waste for the production of chemicals also creates reuse, which makes it possible to reduce the use of fossil raw materials.
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This analysis was written by Andrei Khodakov, research director at CNRS, Deizi Peron, post-doctoral fellow (both at the University of Lille) and Alan Barrios, doctoral student at École Centrale de Lille. The original article was published on The Conversation website.
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