Present in abundance in the Sun, hydrogen gas is nearly absent from our own atmosphere. So it has to be extracted from the molecules that contain it, such as water and organic compounds.
The principal field uses organic compounds mainly composed of hydrogen and carbon, such as natural gas, coal or biomass. It currently accounts for more than 90% of hydrogen production.
The second field produces hydrogen through the decomposition of water. It uses either an electric current for electrolysis or a series of chemical reactions for the thermochemical cycles which make it possible to "break" the molecules of water in order to obtain hydrogen.
Another field uses sunlight. This method of hydrogen production is still in the research stage: studies are being conducted in the laboratory to produce hydrogen through microscopic algae or bacteria.
From organic compounds
Extracting hydrogen from organic compounds requires breaking the solid bonds between carbon and hydrogen. This in turn requires the use of a series of high-temperature chemical reactions at high temperature, often in the presence of catalyzers.
One of the technologies used is reforming, which consists of producing a reaction between methane with water to obtain a synthesis gas that contains hydrogen. This is the main echnology used in the idustrial production of hydrogen. The sources are natural gas and biogas.
Future biogas, Spridlington, England
Gasification is another process for transforming a solid compound into a hydrogen-rich gas. The sources are mainly coal and biomass.
These hydrogen production technologies are CO2-emitting. Using renewable souces, including biomass and biogas (from biomass or waste products), or combining with technologies for capturing and storing CO2 can lead to improvements in the carbon footprint of production.
Reforming natural gas
Natural gas, essentially composed of methane (CH4), contains sulfur which must be removed during a hydrodesulfurization step. Once purified, the methane reacts with steam to form a synthesis gas containing carbon monoxide (CO) and hydrogen (H2). Then, the water vapor reacts with the carbon monoxide of the synthesis gas to form carbon dioxide (CO2) and more hydrogen. This blend, rich in CO2 and H2 , is then purified to obtain about 99.9% hydrogen.
Through the decomposition of water
Separating the components water molecules makes it possible to produce hydrogen. Water molecules (H2O) is composed of two hydrogen atoms (H) and one oxygen atom (O). To produce hydrogen, it is necessary to break the bonds of this molecule. Different processes exist using an electric current or a succession of chemical reactions.
Electrolysis of water
The electric current decomposes the water molecule into ions of hydroxide (OH)- at the cathode and into H+ protons at the anode. The protons accept electrons in an oxidation reaction thus forming hydrogen gas.
The electrolysis itself does not emit any CO2 . But in an overall calculation, the production of electricity must be taken into account. In the case where the electricity used is produced from sources that do not emit CO2 (renewable energies, etc.), hydrogen will be produced without releasing any greenhouse gas. This method is generally used to produce low volumes of hydrogen or near low-cost electric power sources (hydroelectricity). Currently, a lot of research is being done to improve the efficiency of electrolysis. One area of improvement is the high-temperature electrolysis of water vapor.
High-temperature steam electrolysis uses less energy because, since the reaction takes place between 700 ° C. and 1000 ° C., cell performance (kinetics) is vastly improved. This technology can, for example, be coupled with a set of solar mirrors which concentrate the Sun's rays to reach these very high temperatures, which would make it possible to produce hydrogen with virtually no greenhouse gases.
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The thermochemical cycles involve the phenomenon of decomposition of water into hydrogen and oxygen which takes place spontaneously at a very high temperature. This decomposition can be carried out at lower temperatures through cycles of chemical reactions.
This method of hydrogen production is in the research stage. It should make it possible to produce hydrogen by limiting greenhouse gas emissions.
Producing hydrogen using only sunlight and water may sound like the stuff of science fiction. But in fact, many microorganisms naturally produce hydrogen using light, in a process known as photosynthesis.
This is true of various single-celled green algae and some cyanobacteria, which have the advantage of producing hydrogen from solar energy using only water.
This method for producing hydrogen does not result in any direct greenhouse gas emissions. Today, it is in the stage of laboratory experimentation. Researchers are trying to understand, copy and optimize this natural process in the hope of possibly industrializing it.