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Hydrogen from ‘reverse fuel’ cells

Upgrading biogas to natural gas—methane—requires hydrogen. However, this so-called methanization process generates heat as a by-product. And heat is required to produce hydrogen by means of electrolysis. Therefore, methanization ties in well with electrolytic production of hydrogen.

“The methanization process in which biogas is upgraded to methane gives off a lot of heat. So when methanization and electrolysis occur simultaneously, the heat from the methanization plant can be used to generate the steam needed in the electrolysis plant. Things fit together perfectly,” explains Søren Primdahl, General Manager in the New Business RD department at Haldor Topsøe.

Haldor Topsøe has now verified the technology at a demonstration plant linked to a biogas plant in Jutland. The plant upgrades 10 m3 of biogas per hour to ensure the gas is of at least the same quality as natural gas.

Similar to fuel cells

The electrolytic cells are produced by Haldor Topsøe in Lyngby, where the now divested subsidiary Topsoe Fuel Cell A/S, which produced fuel cells, was located. This is no coincidence. Initially, the square ceramic cells used for electrolysis look completely like solid fuel cells. A microscope is needed to see the differences.

“Electrolytic cells and fuel cells are almost identical. Previously, methane was poured over the fuel cells to generate electricity, but the opposite process is now applied. Now, we split water by means of electricity and combine it with a methanator to create the end-product methane,” says Søren Primdahl.

The similarity between electrolysis and fuel cells is underlined by their names containing the letters SOC, which stand for solid, oxide, and cells. Fuel cells furthermore contain the letter F for fuel, while electrolytic cells contain the letter E for electrolysis—i.e. SOEC cells.

The art of stacking cells

In connection with both SOFC and SOEC, the cells are based on patented technology developed by DTU Energy. Haldor Topsøe holds a licence to use the patents, based on which they have produced a series of patents.

“The cells are actually only one of several technologies. The cells must be stacked to achieve sufficient efficiency. We are stacking specialists, and we have also developed a number of other solutions for combining stacks as well as for the entire SOC system,” Søren Primdahl underlines, and continues:

“The biggest difference is that the energy balance with SOEC is different. In connection with fuel cells, high-quality heat is a great added bonus when producing power from methane. When using electrolysis, in which case the process is reversed, we can instead exploit waste heat generated in the SOEC stacks, but also by using steam for the electrolysis instead of liquid water.”

Inspiration for carbon monoxide plant

In addition to the methanization work, Haldor Topsøe has achieved a breakthrough in relation to a similar use of electrolysis. SOEC can also be used to produce CO (carbon monoxide—also called carbon monoxide) from CO2 and electricity.

“Carbon monoxide is applied in a variety of industrial processes, for example in connection with the production of medicine, electronics, and fine chemicals. The material can be delivered by special trucks, but it’s expensive, and authorization is required. It’s definitely easier to produce CO on-site where it’s intended to be used and to use it immediately,” says Søren Primdahl.

The plant, which has been named eCOs (‘e’ for electrolysis and ‘CO’ for carbon monoxide), is designed as modules. The modules can be combined into, for example, a plant capable of producing 96 m3 of CO per hour.

“We believe that the technology is viable for companies that need approx. 25 up to a few hundred m3 of CO per hour. If there’s a greater need, another type of plant would be preferred—which, by the way, Topsøe also supplies. So in that way, the electrolysis plant fits in well with our range of products,” says Søren Primdahl.

Source: DTU

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