Two types of technologies using microwaves would make it possible to have carbon-free hydrogen. One, still at the research stage, is based on redox reactions with water. The other, already marketable, “cracks” methane and produces solid carbon.
The small hydrogen molecule continues to turn heads. Promoted as an essential building block for the energy transition, it arouses multiple interests and the solutions are progressing, as France Hydrogène recently reminded us by publishing an overview of technologies across the entire value chain. However, the sector still needs to take steps to ensure carbon-free production in very large quantities. Because, despite the hundreds of millions of public money injected over the past four years, we have not yet achieved large-scale industrialization.
Advances in research
If water electrolysis remains the preferred production method for the transition, alternatives are being explored by researchers and companies, particularly in the area of microwaves. Thus, last July, Spanish researchers from ITACA and ITQ in Valencia published an article in Advanced Energy Materials reporting progress on the ability of certain materials to react with water under the effect of micro -waves to produce hydrogen.
The principle is as follows: subjected to electromagnetic radiation (2.45 GHz), the metal oxide based on Cerium and Gadolinium “expels” part of its oxygen atoms. After this first reduction step, the decline or cessation of microwaves leads to the vacant spaces in the atomic structure being replaced by oxygen from the water molecules. This results in reoxidation of the metal and production of hydrogen. The redox cycle can be repeated, alternately generating oxygen and hydrogen.
The researchers’ work consisted of better characterizing the power levels of the microwaves and the temperature of the material which allows the reaction, and their impact on the conductivity of the metal oxide. They tested several alternative materials by replacing Gadolinium (Gd) with Lanthanum (La), Yttrium (Y), Ytterbium (Yb), Erbium (Er) or Neodymium (Nd). The formula with Lanthanum proved to be the most productive of hydrogen, at an average level of 1.41 mL/g over twenty cycles, for a microwave power oscillating between approximately 20 and 45 W/g and a temperature between 100 and 400°C.
Potentially, this method of producing hydrogen would bring several advantages. Compared to other thermochemical processes, the temperature level is two to three times lower and the production of oxygen during the reduction reaction is much faster (less than a minute compared to around twenty minutes). Compared to water electrolysis, this process involves contactless electrification and avoids having to separate oxygen and hydrogen.
Microwave technology already accessible
However, this research work is still far from reaching a commercial stage. On the contrary, a company like Sairem has already developed a production system, accessible to manufacturers who have a local need for hydrogen. The principle is to carry out methane pyrolysis at very high temperature via microwaves (915 MHz and 2.45 GHz). In this case we speak of “turquoise” hydrogen. The machines offered by Sairem have a power of at least 100 kW and can be arranged to deliver several megawatts. The possibility of adjusting the power and therefore the enthalpy provided allows control of the reaction.
Three advantages are linked to this technology. Firstly, it preserves water resources, unlike electrolysis, the consumption of which is approximately 10 liters of water per kg of H2. It also consumes less energy than the latter, up to 7 times less. Second, it does not produce CO2unlike conventional methane reforming, because the carbon is recovered as compound CnHn (acetylene, ethylene or even carbon nanotubes) or solid (carbon black, graphene, etc.) which are all materials in high demand in industry. When biomethane is used, the process amounts to carbon capture. Third, Sairem believes that its technology is competitive, because it only takes 10 to 50 kWh to produce 1 kg of H2 and that the carbon co-products are economically recoverable.
While microwaves are already well used industrially for other applications of drying, heating (for example of ceramics), defrosting, etc., it could therefore provide an interesting complement to water electrolysis technology. . And finally, increase the supply of carbon-free hydrogen to make this aspect of the energy transition a reality.
