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Déchets plastiques, plastic waste

Plastic waste: transforming a problem into energy

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The linear life cycle of plastics, too rarely recycled, exerts a heightened pressure on the environment. The process of gasification makes it possible to reduce this impact by transforming more waste – currently incinerated or left in landfill – into useful resources like hydrogen or biomethane. Javier Escudero, process engineering researcher at IMT Mines Albi, aims to perfect this approach to make it easier to implement locally.

Plastic waste is so abundant, it is practically like rain. While we know well that it is piling up in landfills and oceans, an American study published in the journal Science has also shown that plastic particles are even in the air we breathe. Despite increased levels of pollution, international plastic production continues its explosive growth. However, at the end of the chain, the recycling industry has never managed to keep up with consumption. In France, the average recycling rate for all plastics is 28%, a percentage mainly obtained from bottle recycling (54.5% of the total). The vast majority of these materials is therefore incinerated or sent to landfill.

In order to respond efficiently to the plastic crisis, other forms of reuse or recycling must be developed. “Gasification means we can transform this waste into useful energy vectors, while losing as little material as possible”, explains Javier Escudero, Process Engineering Researcher at IMT Mines Albi. It is an alternative that contributes to a circular economy approach.

Some plastics not so fantastic

Rigid plastics used for bottles are generally made from a single material, which makes it easier to recycle them. For plastic films, which represent 40% of waste deposits, this is not the case. They are made from a multilayer combination of various plastics, such as polyethylene, polyurethane, and so on, sometimes joined with other materials like cardboard. The complex configuration of chemicals makes recycling such packaging too expensive. This means that in recycling centers, these products are overwhelmingly used for solid recovered fuel (SRF) – non-hazardous waste used for energy production. They are incinerated to feed turbines and generate electricity.

Another kind of waste that is ineligible for recycling is packaging from chemical products (industrial and mass market), considered hazardous. Some of the toxic compounds (chorine, sulfur, metals, etc.) are removed from the surface by pre-washing. However, certain atoms are absorbed into the material and cannot be removed by prewashing. This is where the advantages of gasification come in. “It makes it possible to process all plastics – SRF and contaminated ones – with less prewashing beforehand, as well,” emphasizes Escudero.

Moreover, this process has greater capacity for recycling plastic waste than incineration, as it produces chemical compounds that can be reused by industry, The synthesis gases can be burnt to generate energy (heat, electricity) with better yield than combustion. They can also be reprocessed and stored in the form of gas to be used as fuel (biomethane, hydrogen). To achieve this, one of the challenges of research is to observe the influence of pollutants, and therefore the composition of plastics, on products obtained from gasification.

Transforming materials down to the last crumb

Ground-up waste is compacted in the form of pellets, all the same size, to facilitate their transformation into gas in the gasifier. But if you want to recycle as much waste as possible, you need to adapt the gasification operating parameters, depending on the types of plastics contained in the pellets. For example, processing at a low temperature will break the long chains of polymers in plastic films. The molecules are then broken up again in the next step, as is done in petrochemistry. This produces a wide variety of products: hydrogen, methane, acetylene, and heavier molecules as well.

Processing at a higher temperature will produce more synthesis gas. However, it also produces more aromatic molecules like benzene and naphthalene. These compounds have a very stable structure and are very difficult to break into useful molecules. They may turn into soot – solids that build up in pipes – representing a significant loss of materials. The objective of Escudero’s research into gasification is therefore to combine the advantages of these two methods of processing, to avoid solid residue forming while producing as much gas as possible.

To do so, the researcher and his team are mainly focusing on gas injection, which breaks the molecular bonds of the materials being processed. Where and at which point in the process should injection take place? In connection to what? How does the material react? These questions, and many others, must be answered to improve the process.  The gasifier at the Valthera technological platform, located at IMT Mines Albi and used for the tests, can process around 20 kilograms of material per hour. The process recycles not only the materials but also their energy. “Gasification reactions require energy to occur. This means that we use the energy stored in the materials to power their transformation,” explains the researcher.

Use less, convert more

Hydrogen and biomethane obtained through gasification directly power the goals of the French energy transition. Gasification therefore transforms materials made from fossil fuels into renewable energy. However, this process remains restricted to the context of research. “There are still many small aspects to study in designing gasifiers, to make them higher-performing and more mature for a certain amount of material. We are also going to concentrate on purifying synthesis gases with the aim of finding even cheaper solutions,” concludes Escudero. Gasification could supplement waste management channels at a local level. However, cost remains the greatest obstacle to small industrial actors adopting this method.

Anaïs Culot

hydrogène décarboné carbon-free hydrogen

Carbon-free hydrogen: how to go from gray to green?

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The industrial roll-out of hydrogen production only makes sense if it emits little or no carbon dioxide. Researchers at IMT schools are working on various alternatives to the use of fossil fuels, such as electrolysis and photocatalysis of water, plasma pyrolysis of methane, and pyrolysis and gasification of biomass.

Currently, the production of one ton of hydrogen results in 12 tons of CO2 emissions and 95% of the world’s hydrogen is produced from fossil resources. This is what we call gray hydrogen. A situation that is incompatible with the long-term roll-out of the hydrogen industry. Especially since, even if the CO2 emitted by current processes can be captured in a controlled environment, fossil resources will not be able to meet the government’s ambitions for this energy. It is therefore essential to develop other modes of “carbon-free hydrogen” production. Within the Carnot H2Mines network, researchers from the different IMT schools are working on processes that could turn the color palette of today’s hydrogen to green.

From blue to green

One process in line with the French government’s plan published last September is water electrolysis. This consists in separating an H2O molecule into hydrogen and oxygen using an electricity supply. This is a carbon-free solution, provided the electricity comes from a renewable source. But why turn an already clean energy into gas? “Hydrogen enables the storage of large amounts of energy over the long term, which batteries cannot do on a large scale to power an entire network,” explains Christian Beauger,  a researcher in materials science at Mines ParisTech. Gas therefore partly responds to the problem of intermittent renewable energies.

Researchers therefore want to improve the performance of electrolyzers in order to make them more competitive on the market. The goal is to find the best possible balance between yield, lifespan and reduced costs. Electrolyzers are made up of several electrochemical cells containing two electrodes and an electrolyte, as in the case of fuel cells. There are three main families: alkaline solutions with liquid electrolyte, polymer membrane technologies (PEM) and high-temperature systems based on ceramic solid oxide (SOC). Each presents its own problems.

At Mines ParisTech, Christian Beauger’s team is seeking to increase the lifespan of PEM electrolyzers by focusing on the materials used at the anode. “We are developing new catalyst supports in the form of metal oxide aerogels which must be electronically conductive and capable of resisting corrosion in a humid environment, at a temperature of 80°C and subjected to potentials often higher than 2 volts“, says the researcher. Another major problem also affects the materials: the cost of an electrolyzer. The catalyst present on PEM electrodes is iridium oxide, a compound that is too expensive to encourage widespread use of future high-power electrolyzers. For this reason, researchers are working on catalysts based on iridium oxide nanoparticles. This reduces the amount of material and thus the potential cost of the system.

Shedding light on photocatalysis

In the laboratory, an alternative using solar energy to break water molecules into hydrogen and oxygen is also being considered. This is photocatalysis. The semiconductors used can be immersed in water in powder form. Under the effect of the sun’s rays, the electron-hole pairs created provide the energy needed to dissociate the water molecules. However, the energy levels of these charge carriers must be controlled very precisely to be useful.

“We form defects in materials that introduce energy levels whose position must be compatible with the energy required for the process,” explains Christian Beauger. This ultra-precise work is delicate to carry out and determines the efficiency of photocatalysis. There is still a long way to go for photocatalysts, the most stable of which hardly exceed 1% in efficiency. But this method of hydrogen production should not be dismissed too quickly, as it is cheaper and easier to set up than a system combining a renewable energy source and an electrolyzer.

Turquoise hydrogen using methane pyrolysis

At Mines ParisTech, Laurent Fulcheri’s team, which specializes in plasma processes, is working on the production of hydrogen not from water, but from the pyrolysis of methane. This technique is still little known in France, but has been widely explored by our German and Russian neighbors. “This process requires electricity, as for the electrolysis of water, but its main advantage is that it requires about seven times less electricity than water electrolysis. It can therefore produce more hydrogen from the same amount of electricity,” he says.

In practice, researchers crack molecules of methane (formula CH4) at high temperature. “To do this, we use a gas in the plasma state to provide thermal energy to the system. It is the only alternative to provide energy at a temperature above 1,500°C without CO2 emissions and on an industrial scale,” says Laurent Fulcheri. The reaction thus generates two valuable products: hydrogen (25% by mass) and solid carbon black (75% by mass).  The latter is not to be confused with CO2 and is notably used in tire rubber, batteries, cables and pigments. The carbon is thus stored in the materials and can theoretically be recycled ad infinitum. “The production of one ton of carbon black by this method avoids the emission of 3 tons of CO2 compared to current methods”, adds the researcher.

This process has already proven itself across the Atlantic. Since 2012, researchers at Mines ParisTech have been collaborating with the American start-up Monolith Materials, which has developed a technology directly inspired by their work. Its location in Nebraska is not insignificant, as it gives it direct access to wind energy in the heart of the corn belt, a major agricultural area in the United States. The hydrogen produced is then transformed into ammonia to fertilize the surrounding corn farms.

Although the machine is working, the research of Laurent Fulcheri’s team, a major player in the start-up’s R&D, is far from over. “Hydrogen production is the simplest task, because the gas purification processes are fairly mature. On the other hand, the carbon black produced can have drastically different market values depending on its nano-structure. The objective is now to optimize our process in order to be able to generate the different qualities of carbon black that meet the demands of consumer industries,” says the researcher. Indeed, the future of this technology lies in the short-term valorization capacities of the two co-products.

Biomass processing: a local alternative

At IMT Mines Albi, Javier Escudero‘s team is working on thermochemical processes for the transformation of biomass by pyrolysis and gasification. Organic waste is heated to high temperatures in a reactor and converted into small molecules of synthesis gas. The hydrogen, carbon monoxide, methane and CO2 thus produced are captured and then recombined or separated. For example, the CO2 and hydrogen can be used to form synthetic methane for use in natural gas networks.

However, a scientific issue has yet to be solved: “The synthesis gas produced is always accompanied by inorganic molecules and large organic molecules called tars. Although their concentration is low, we still require an additional gas purification stage,” explains Javier Escudero. The result is an increase in processing costs that makes it more difficult to implement this solution on a small scale. The researcher is therefore working on several solutions. For example, the exploration of different catalyst materials that could accelerate certain reactions to separate molecules from waste, while eliminating tars.

This approach could be envisaged as a form of local energy recovery from waste. Indeed, these technologies would enable a small and medium-scale territorial network with reactor sizes adapted to those of the collection centers for green waste, non-recovered agricultural residues, etc. However, there is also a need to clarify the regulations governing this type of facility. “For the moment, the law is not clear on the environmental constraints imposed on such structures, which slows down their development and discourages some manufacturers from really investing in the method,” says the researcher.

There is no shortage of solutions for the production of carbon-free hydrogen. Nevertheless, the economic reality is that in order to be truly competitive, these processes will have to produce hydrogen cheaper than hydrogen from fossil fuels.

By Anaïs Culot

Hydrogen: transport and storage difficulties

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Does hydrogen hold the key to the great energy transition to come? France and other countries believe this to be the case, and have chosen to invest heavily in the sector. Such spending will be needed to solve the many issues raised by this energy carrier. One such issue is containers, since hydrogen tends to damage metallic materials. At Mines Saint-Étienne, Frédéric Christien and his teams are trying to answer these questions.

In early September, the French government announced a €7 billion plan to support the hydrogen sector through 2030. With this investment, France has joined a growing list of countries that are betting on this strategy: Japan, South Korea and the Netherlands, among others.

Nevertheless, harnessing this component poses major challenges across the supply chain. Researchers have long known that hydrogen can damage certain materials, starting with metals. “Over a century ago, scientists noticed that when metal is plunged into hydrochloric acid [from chlorine and hydrogen], not only is there a corrosive effect, but the material is embrittled,” explains Frédéric Christien, a researcher at Mines Saint-Étienne1. “This gave rise to numerous studies on the impact of hydrogen on materials. Today, there are standards for the use of metallic materials in the presence of hydrogen. However, new issues are constantly arising, since materials evolve on a regular basis.”

Recovering excess electricity produced but not consumed

For the last three years, the Mines Saint-Étienne researcher has been working on “power-to-gas” research. The goal of this new technology: recover excess electricity rather than losing it, by converting it to gaseous hydrogen through the process of water electrolysis.

Read more on I’MTech: What is hydrogen energy?

“Power-to-gas technology involves injecting the resulting hydrogen into the natural gas grid, in a small proportion, so that it can be used as fuel,” explains Frédéric Christien. For individuals, this does not change anything: they may continue to use their gas equipment as usual. But when it comes to transporting gas, such a change has significant repercussions. Hence the question posed to specialists about the durability of materials: what impact may hydrogen have on the steel that makes up the majority of the natural gas transmission network?

Localized deformation

In collaboration with CEA Grenoble (Atomic Energy Commission), the Mines Saint-Étienne researchers have spent three years working on a sample of pipe in order to study the effect of the gas on the material. It is a kind of steel used in the natural gas grid.

The researchers observed a damage mechanism, through the “localization of plastic deformation.” In concrete terms, they stretched the sample so as to replicate the mechanical stress that occurs in the field, due in particular to changes in pressure and temperature. Typically, such an operation results in lengthening the material in a diffuse and homogeneous way, up to a certain point. Here, however, under the effect of hydrogen, all the deformation is concentrated in one place, gradually embrittling the material in the same area, until it cracks. Under normal circumstances, a native oxide layer of the material prevents the hydrogen from penetrating inside the structure. But under the action of mechanical stress, the gas takes advantage of the crack to cause localized damage to the structure.

But it must be kept in mind that these findings correspond to laboratory tests. “We’re a long way from industrial situations, which remain complex,” says Frédéric Christien. “It’s obviously not the same scale. And, depending on where it’s located, the steel is not always the same – some have lining while others don’t and it’s the same thing for heat treatments.” Additional studies will therefore be needed to better understand the effect of hydrogen on the entire natural gas transport system.

The production conundrum

Academic research thus provides insights into the effects of hydrogen on metals under certain conditions. But can it go so far as to create a material that is completely insensitive to these effects? “At this point, finding such a dream material seems unrealistic,” says the Mines Saint-Étienne researcher. “But by tinkering with the microstructures or surface treatments, we can hope to significantly increase the durability of the metals used.”

While the hydrogen sector has big ambitions, it must first resolve a number of issues. Transport and storage safety is one such example, along with ongoing issues with optimizing production processes to make them more competitive. Without a robust and safe network, it will be difficult for hydrogen to emerge as the energy carrier of the future it hopes to be.

By Bastien Contreras.

1 Frédéric Christien is a researcher at the Georges Friedel Laboratory, a joint research unit between CNRS/Mines Saint-Étienne

Fuel cells in the hydrogen age

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Hydrogen-powered fuel cells are recognized as a clean technology because they do not emit carbon dioxide. As part of the energy transition aimed at transforming our modes of energy consumption and production, the fuel cell therefore has a role to play. This article will look at the technologies, applications and perspectives with Christian Beauger, a researcher specialized in material sciences at Mines ParisTech

How do fuel cells work?

Christian Beauger: Fuel cells produce electricity and heat from hydrogen and oxygen reacting at the heart of the system in electrochemical cells. At the anode, hydrogen is oxidized to protons and electrons —the source of the electric current— while at the cathode, oxygen reacts with the protons and electrons to form water. These two electrodes are separated by an electrolyte that is gas-tight and electronically insulated. As an ion conductor, it transfers the protons from the anode to the cathode. To build a fuel cell, several cells must be assembled (a stack). The nominal voltage depends on their number. Their size determines the maximum value of the current produced. The dimensions of the stack (size and number of cells) depend on what the device is to be used for.

Within the stack, the cells are separated by bipolar plates. Their role is to supply each cell with gas, to conduct electrons from one electrode to the other and to cool the system. A fuel cell produces as much electricity as heat, and the temperature of use is limited by the materials used, which vary according to the type of cell.

Finally, the system must be supplied with gas.  Since hydrogen does not exist naturally, it must be produced and stored in pressurized tanks. The oxygen used comes from the air, supplied to the fuel cell by means of a compressor.

What is the difference between a fuel cell and a battery?

CB: The main difference between the two is in their design. A battery is an all-in-one technology whose size depends both on the power and the desired autonomy (stored energy). On the contrary, in a fuel cell, the power and energy aspects are separated. The available energy depends on the amount of hydrogen on board, often stored in a tank. So a fuel cell has very varying levels of autonomy depending on the size of the tanks. The power is linked to the size of the stack. Recharging times are also very different. A hydrogen-powered vehicle can be refueled in minutes, compared to a battery that usually takes several hours to charge.

What are the different types of fuel cells?

CB: There are five major types that differ according to the nature of their electrolytes. Alkaline fuel cells use a liquid electrolyte and have an operating temperature of around 70°C. With my team, we are working on low-temperature fuel cells (80°C) whose electrolyte is a polymer membrane; these are called PEMFCs. PAFCs use phosphoric acid and operate between 150°C and 200°C. MCFCs have an electrolyte based on molten carbonates (600-700°C). Finally, those with the highest temperature, up to 1000 °C, use a solid oxide (SOFC), i.e. a ceramic electrolyte.

Their operating principle is the same, but they do not present the same problems. Temperature influences the choice of materials for each technology, but also the context in which they are used. For example, SOFCs take a long time to reach their operating temperature and therefore do not perform optimally at start-up. If an application requires a fast response, low-temperature technologies should be preferred. Overall, PEMFCs are the most developed.

What are the technical challenges facing fuel cell research?

CB: The objective is always to improve performance, i.e. conversion efficiency and lifespan, while reducing costs.

For the PEMFCs we are working on, the amount of platinum required for redox reactions should be reduced. Limiting the degradation of the catalyst support is another challenge. For this purpose, we are developing new supports based on carbon aerogels or doped metal oxides, with better corrosion resistance under operating conditions. They also provide a better supply of gas (hydrogen and especially air) to the electrodes. We have also recently initiated research on platinum-free catalysts in order to completely move away from this expensive material.

Another challenge is cooling. One option to make more efficient use of the heat produced or to reduce cooling constraints in the mobility sector is to be able to increase the operating temperature of PEMFCs. The Achilles heel here is the membrane. With this in mind, we are working on the development of new composite membranes.

The path for SOFCs is reversed. With a much higher operating temperature, there are fewer kinetic losses at the electrodes and therefore no need for expensive catalysts. On the other hand, the heavy constraints of thermomechanical compatibility limit the choice of SOFC constituent materials. The objective of the research is therefore to lower the operating temperature of SOFCs.

Where do we find these fuel cells today?

CB: PEMFCs are the most widespread and marketed, primarily in the mobility sector. The fuel cell vehicles offered by Hyundai or Toyota, for example, carry a fuel cell of about 120 kW. Electricity is generated on board by the fuel cell hybrid to a battery. The battery preserves the fuel cell during strong accelerations. Indeed, although it is capable of rapidly supplying the required energy, this driving phase accelerates the degradation of the core materials. Fuel cells can also be used as range extenders as originally developed by SYMBIO for Renault electric vehicles. In this case, hydrogen takes over when the battery weakens. The fuel cell can then recharge the battery or power the electric motor.

Another example of commercialization is micro-cogeneration, which makes it possible to use the electricity and heat produced by the fuel cell. In Japan, the Ene Farm program, launched in 2009, has enabled tens of thousands of residential cogeneration systems to be marketed, built using PEMFC or SOFC stacks with a power output of around 700 W.

You mentioned the deterioration of materials and the preservation of fuel cells in use: what about their lifespan?

CB: Lifespan is mainly impacted by the stability of the materials, especially those found in the electrodes or that make up the membrane. The highly oxidizing environment of the cathode can lead to the degradation of the electrodes and indirectly of the membranes. The carbon base of PEMFC electrodes has a particular tendency to oxidize at the cathode. The platinum on the surface can then come away, agglomerate, or migrate towards the membrane to the point of degrading it. Ultimately, the target for vehicles is 5,000 hours of operation and 50,000 hours for stationary applications. We must be at two-thirds of that goal now.

Read more on I’MTech Hydrogen: transport and storage difficulties

What are the prospects for fuel cells now that hydrogen is receiving investment support?

CB: Applications for mobility are still at the heart of the issue. Interest is shifting towards heavy vehicles (buses, trains, light aircraft, ships) for which batteries are insufficient. Alstom’s iLint hydrogen train is being tested in Germany. The aeronautics sector is also conducting tests on small aircraft, but hydrogen-powered wide-body aircraft are not for the immediate future. PEMFCs have the advantage of offering a wide range of power to meet the needs of the various uses from mobile applications (computer, telephone, etc.) to industry usage.

Finally, it is difficult to talk about fuel cells without talking about hydrogen production. It is also often talked about as a means of storing renewable energy. To do this, the reverse process to that used in the fuel cell is required: electrolysis. The water is dissociated into hydrogen and oxygen by applying a voltage between the two electrodes.

Overall, it should be remembered that fuel cell deployment only makes sense if the method of hydrogen production has a low carbon footprint. This is one of the major challenges facing the industry today.

Interview by Anaïs Culot.

For more information about fuel cells:

Turning exhaust gases into electricity, an innovative prototype