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Bifurcation of hydrogen energy

Modern electricity generators based on fuel cells (FC) are incredibly efficient, demonstrating the electrical output at 60% and above. Using hydrogen gas as a fuel, due to the electrochemical oxidation reaction at the outlet of PE we obtain water. The perfect solution. Though questions remain.

Hydrogen production (for example, by hydroelectrolysis) is an energy-intensive process and requires another energy-consuming power plant. In addition, pure hydrogen is more difficult and dangerous to handle with than traditional hydrocarbon (liquid and gaseous) fuels. In addition, the development of an extensive specific (cryogenic or high-pressure) infrastructure for hydrogen production, storage and transportation requires not only huge investments, but also the similarly tectonic changes in economic and social relations in the field of energy consumption.

All of the above issues become serious constraints and obstacles, especially in stand-alone (autonomous) applications (i.e. those that are remote from the basic infrastructure of hydrogen production and transportation), such as mobile (portable or transport) means and low-power stationary installations.

There is a way out of this situation as well. This is the generation and simultaneous consumption of hydrogen in the place where it is needed. In this case, the fuel is some type of hydrocarbon fuel, from which hydrogen can be obtained relatively easily. This is no longer an ideal solution from the point of view of strict Green Deal followers — emissions will include carbon dioxide in addition to water. But with some trade-offs, such as the use of bio-fuels or the use of carbon dioxide conversion techniques, some progress can be made.

Promising fuel cells

According to a brief overview of the most modern fuel cells (FC) and the feasibility of their autonomous use, today the most promising are two FC types:

  • SOFC — solid oxide fuel cell
  • PEMFC — proton-exchange membrane fuel cell

However, given the current power limit (see figure below [1]), SOFC is probably the most important FC type for a autonomous power plant.

Fuel Cells and Hydrogen Production / Editors: T.E. Lipman, A.Z. Weber. Second Edition / Springer, 2018

It is claimed that emissions of all FC types do not contain carbon, however, this is true in the case of supply of pure hydrogen to FC.

Therefore, rejecting from consideration the storage of significant amounts of pure hydrogen, for the autonomous production of hydrogen the so-called reforming of hydrocarbon fuels (methane, kerosene, methanol, SAF, etc.) was introduced.

FCT — Fuel Cell Technologies — SOFC (http://www.fuelcelltoday.com/technologies/sofc)

Solid oxide fuel cells operate at very high temperatures, the highest among all types of fuel cells from about 800ºC to 1000 °C. They can have an efficiency of more than 60% when converting fuel into electricity. And if the heat produced is also used, their overall efficiency in converting fuel into energy can be over 80% [2].

SOFC uses a solid ceramic electrolyte, such as yttrium oxide-stabilized zirconium oxide, instead of liquid or membrane. The high operating temperature of FC means that the fuel can be reformed inside the fuel cell itself, eliminating the need for external reforming and allowing the use of units with a variety of hydrocarbon fuels. They are also relatively resistant to a small amount of sulfur in the fuel compared to other types of fuel cells, and therefore can be used with carbon gas.

Another advantage of high operating temperatures is that the reaction kinetics are improved, which eliminates the need for a metal catalyst.

However, high temperature has some disadvantages:

  • these fuel elements need more time to launch and reach operating temperature,
  • they must be made of durable, heat-resistant materials,
  • they must be shielded to prevent heat loss, and
  • exhaust gas heat recovery may be required.

CO2 utilization

An important feature of the SOFC-type FC is the absence of NOx and SOx in emissions. That is, this FC provides clean CO2 in the exhaust, which is a good prerequisite for technologies of carbon capturing in emissions.

Based on this idea, it is worth refer the rather innovative news that has just recently appeared in the media. This is “The conversion of carbon dioxide into oxygen and carbon”. The original news here [3].

The approaches to carbon dioxide conversion mentioned in this article can be considered as a basis for the technology of its utilization in exhaust gases. This process requires energy, which is why heat of the exhaust gases can be used for this purpose.

A by-product of this process is carbon, which can be used for production of carbon fiber materials for various purposes, the demand for which will continue to grow.

References

  1. Fuel Cells and Hydrogen Production / Editors: Timothy E. Lipman, Adam Z. Weber. A Volume in the Encyclopedia of Sustainability Science and Technology, Second Edition / Springer, 2018
  2. FCT — Fuel Cell Technologies — SOFC (http://www.fuelcelltoday.com/technologies/sofc)
  3. Liquid metal proven to be cheap and efficient CO2 converter | OCTOBER, 2021 | Phys.org (https://phys.org/news/2021-10-liquid-metal-proven-cheap-efficient.html)

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