Coal-based power plant in Datteln, Germany.
There are four sites producing electricity from coal in France -
© Arnold Paul / Wikimedia, CC BY-SA
CO₂ can undergo chemical or biological conversion into value-added products and fuels, according to a study published by our partner The Conversation.
Several European teams are working in this direction as part of the Green Deal, which aims to reduce greenhouse gas emissions in Europe by at least 55% by 2030.
The analysis of this research was conducted by Andrei Khodakov, research director at the CNRS and Sara Navarro Jaén, researcher in Materials Science and Heterogeneous Catalysis (both at the University of Lille).
In the European Union, energy production and consumption are currently responsible for over 75% of greenhouse gas emissions.
The European Green Deal proposes to reduce greenhouse gas emissions in Europe by at least 55% by 2030, compared to 1990 levels, in order to achieve the goal of climate neutrality by 2050. "Climate Neutrality" is an initiative launched in 2015 by the UN to achieve the goal of a climate neutral world by the middle of the century, in accordance with the Paris Agreement.
Demonstration on the occasion of the “climate strike” in Lausanne (Switzerland), in April 2019 © Delia Giandeini / Unsplash
To achieve this climate neutrality, joint efforts of governments, businesses and institutions are needed.
Currently, in addition to limiting emissions, there are two strategies to deal with the constant increase in CO2 content in the biosphere: carbon capture and storage (in English,
Carbon Capture and Storage, CCS
) and and the use of carbon (in English,
Carbon Capture and Utilization, CCU
).
Storage (CCS) is based on the capture of CO2, including its separation, compression and transport, for permanent storage in a geological layer.
This involves capturing CO2 from power plants and industrial facilities (e.g. coal-fired power stations, aluminum plants), concentrating it, pressurizing it and then storing it underground in geological formations or to use it for enhanced oil recovery.
However, the technological and economic feasibility on a larger scale of this strategy has not been demonstrated.
Reuse (UCC) involves either the direct technological use of CO2, in soft drinks or fire extinguishers for example, or its chemical or biological conversion into value-added products and fuels.
Converting CO2 into fuels could be very interesting.
It should be noted that fuels are major sources of carbon dioxide emissions and that most of the CO2 emitted into the atmosphere comes from their combustion.
Schematic presentation of the European Interreg E2C project: from renewable electricity and carbon dioxide released by industrial facilities, carbon is extracted in order to manufacture either "recycled" carbon-based fuels or carbon molecules of interest to industry © interreg E2C / The Conversation
As part of the European project E2C (“Electrons to high value chemical products”) our team is involved in the direct conversion of CO2 into dimethyl ether (DME).
DME is an oxygenated fuel with the formula CH3-O-CH3.
Its high cetane number, zero sulfur content and low production of particles and nitrogen oxides make it a “cleaner” fuel than diesel.
Automobile manufacturers Shanghai Diesel Co, AB Volvo, Isuzu Trucks and Nissan Diesel are actively developing heavy-duty vehicles powered by DME.
DME produced from CO2 using renewable electricity would have a low carbon footprint and have very little impact on the climate.
Producing hydrogen by electrolysis with renewable energies
DME can be produced directly by catalysis from CO2 and H2.
For the process to be sustainable, the different ingredients must also be.
Dihydrogen can be produced by electrolysis of water by applying an electric current to extract the hydrogen and oxygen atoms from the water molecule.
This is where renewable electricity comes in.
But this is not enough: certain technological challenges must be met, such as the efficiency and durability of electrolysers, as well as their efficiency.
At present, the high cost of this "clean" hydrogen can present a challenge for the direct synthesis of dimethyl ether from CO2.
The fact that the prices of renewable electricity are falling helps to make this option financially feasible in the future.
The success of large-scale hydrogen production through water electrolysis depends on the availability of cheap renewable electricity and favorable political rules.
Our partners at the University of Exeter in the UK are working on reducing the cost of hydrogen by developing a range of new catalysts for electrolysers.
In addition, teams from Polytech Lille are developing new algorithmic methods to optimize the use of surplus electricity from renewable energies (when there is too much wind for example) to produce hydrogen. by electrolysis.
Converting atmospheric CO l'atmosphère into fuel
To convert CO2 directly to DME, there are two steps: in the first, the CO2 is hydrogenated to form methanol, while in the second, the methanol is dehydrated to form DME.
Step 1:
Hydrogenation of CO2: CO2 + 3H2 ↔ CH3OH + H2O
Step 2:
Dehydration of methanol: 2CH3OH ↔ CH3OCH3 + H2O
One of the missions of our Lille team is to hydrogenate the CO2 and then dehydrate the methanol at the same time in a single reactor and with a single catalyst.
For this, it is necessary to design “hybrid” catalysts, those which contain two catalytic functions for these two DME synthesis steps.
Catalysts based on copper nanoparticles have proven to be excellent systems for the first step, namely the conversion of CO2 to methanol.
However, their stability is limited due to the “sintering” of the nanoparticles during the reaction: catalysis is a surface phenomenon and sintering leads to the increase in the size of metal nanoparticles and to the reduction of the active surface of the catalyst.
Within the framework of the E2C project, we have developed new bimetallic hydrogenation nanocatalysts, based on palladium-promoted copper, which show improved selectivity and stability compared to conventional catalysts for the hydrogenation of CO2 to methanol.
To carry out the second step, the dehydration of methanol, acid catalysts such as zeolites, microporous crystalline aluminosilicates, were added to the catalyst formulation.
We have optimized their acidity in order to achieve maximum yield - that is, to increase the amount of DME produced per gram of CO2.
Principle of SEDMES technology © The Conversation
These new catalysts have made it possible to obtain DME yields close to the maximum values defined by thermodynamics.
In addition, the production of unwanted products such as carbon monoxide or hydrocarbons has been suppressed.
These results demonstrate the feasibility of applying such a process in practice at the laboratory scale.
In order to further increase the yield of DME and move to industrial scale, we are developing catalytic systems for the synthesis of “sorption-enhanced” dimethyl ether (in English, Sorption-Enhanced-DME-Synthesis, SEDMES).
This method makes it possible to modify the thermodynamic equilibrium state, according to the principle of Le Chatelier, and to obtain a conversion of carbon dioxide close to 100% with a selectivity of about 80% in DME, thanks to the adsorption of water formed during the reaction.
This process is currently being developed on an industrial scale at the partner ECN-TNO, in the Netherlands.
These collaborations between university laboratories and industrial centers are essential to accelerate the development of these new technologies.
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This analysis was written by Andrei Khodakov, research director at the CNRS and Sara Navarro Jaén, researcher in Materials Science and Heterogeneous Catalysis (both at the University of Lille).
The original article was published on The Conversation website.
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