2024-06-20 06:30:57
Since 2018, EDF and LRGP have been developing a new technology family using membrane processes to treat liquid and gaseous effluents from power plants. Objectives: to increase the energy efficiency and environmental performance of these treatments. Two new processes will soon be tested on power plants.
Since the commissioning of nuclear power plants in France, several families of technologies have been used to treat their liquid and gaseous effluents. These include thermal technology, which consists of heating water to degas or evaporate the water, or adsorption technology, which allows compounds to be retained to be eliminated on a solid. Since 2018, EDF and the LRGP1 (Reactions and Process Engineering Laboratory) have created a joint team called Mélusine within which they are developing a new technological family using membrane processes. Objectives: to divide by ten the energy consumed by the processes used to treat effluents from power plants, as well as to continue to improve their environmental performance.
A first process is developed to recycle boron using reverse osmosis, a technology with a high level of maturity, used for example to desalinate seawater. This chemical element is used to control the flow of neutrons at the heart of power plants. Two types of commercial membranes were tested, as explained by Éric Favre, professor of process engineering at the University of Lorraine and researcher at the LRGP: “We have developed a new architecture of a multi-stage membrane system and defined the right pressure and flow rates to achieve the boron separation performance of our specifications. Tests of the two membranes have given encouraging results and an international patent has been filed. Finally, we have selected a membrane and manufactured a prototype which will be tested on a plant in service in the coming months.”
To eliminate gases dissolved in water (xenon, krypton, nitrogen, dihydrogen, dioxygen, etc.), a second technology is developed to carry out degassing. It is based on the use of membrane contactors, in which a driving force is exerted by means of a moderate vacuum associated with a flush with nitrogen. This dual action ensures a dilution effect and maximizes the extraction rate. “Membrane contactors are widely used for example for deoxygenation, underlines Eric Favre. We adapted this process and managed to get to very low residual gas levels, 100,000 times lower than the dissolved gas content in the aqueous mixture at the system inlet.” The advantage of this technique is that it consumes much less energy than the reference thermal process. It is also very modular, compact and saves floor space and volumes in buildings right from the design stage of the installation. This technology has also been patented and is due to be tested on a full scale in power plants soon.
Transmembrane distillation implemented using fatal heat
Transmembrane distillation is a third solution currently under development. It involves using heat to eliminate volatile compounds, except that here there is no need to reach the boiling point of water to recover them by condensation, unlike the classic thermal route, such as evaporation. The water is heated to between 60 and 70 degrees and placed in contact with a hydrophobic membrane that only allows vapors to pass through. The compound can then be recovered in vapor form through the membrane if it is volatile, or in concentrated form in water if it is slightly or not volatile. “The advantage of this process is that it allows the recovery of the fatal heat of a power plant, usually little used, but which is sufficient to provide driving force for transmembrane distillation.analyzes Eric Favre. This is an emerging technique and there are almost no suppliers on an industrial scale. Our work is still at the laboratory scale and the next step will be to build a pilot.
A fourth process is under study and uses electrodeionization membranes in order to recover ionizable species in water, such as salts, metals, as well as ammonia in its dissolved form in NH4+. The driving force is here delivered using an electric current with the presence of ion-permeable membranes arranged between two solutions, and ion exchange resins, allowing a selective transfer of ions from one solution to the other under the action of the electric field.
“With these four membrane technologies, we have a toolbox from which we can choose the most relevant process to eliminate, or even recover, a compound present in power plant effluents.rejoices Éric Favre. For a year now, we have signed a new agreement with EDF for a period of 5 years and have expanded the scope of the Mélusine laboratory’s research work. We are also working on DAC (Direct Air Capture) type technologies with the aim of taking advantage of the waste heat from power plants to capture CO2 in the air and thus improve their carbon footprint. This is a subject on which we have already been working with EDF for around twenty years, well before the creation of the joint Mélusine laboratory.”
1 The LRGP is a joint unit of the CNRS and the University of Lorraine
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