Climate-friendly electricity from ammonia

Using hydrogen to generate electricity does not cause any climate-damaging emissions, but storing and transporting the gas poses technical challenges. With this in mind, Fraunhofer researchers are using ammonia as a starting material. Ammonia is cracked in a high-temperature fuel cell stack, and the hydrogen produced in this process is converted to electricity. The waste heat can also be utilised.

A team of researchers with Professor Laura Nousch from the Fraunhofer Institute for Ceramic Technologies and Systems IKTS in Dresden has developed a demonstrator based on a high-temperature fuel cell stack (solid oxide fuel cell, SOFC) that can use ammonia to generate electricity directly and with high efficiency. Electricity and heat are generated in a single compact system — without CO₂ emissions or other harmful by-products.

“Ammonia has been used in the chemical industry for decades, to produce fertilisers for example, so there are established and familiar processes of handling this substance,” Nousch said. “As a hydrogen carrier, ammonia offers high energy density, and at the same time it is relatively easy to store and transport. It is therefore an ideal starting material for climate-friendly generation of electricity and heat energy.”

In the process, ammonia (NH₃) is first conditioned and fed into the cracker, where it is heated to temperatures of 300°C or higher. In response, it breaks down into hydrogen (H₂) and nitrogen (N₂). When the process is completed, the nitrogen can simply be released together with water vapour as harmless exhaust gases. The hydrogen is fed into the high-temperature fuel cell. In a ceramic electrolyte, it flows over the anode, while air streams pass the cathode. Splitting the hydrogen releases electrons that move from the anode to the cathode, generating electricity.

Schematic of the principle behind an ammonia SOFC system. ©Fraunhofer IKTS.

In addition to water vapour, the electrochemical reaction also produces thermal energy and the afterburning also generates heat.

“The heat is used to maintain the high temperature inside the cracker and is also released as waste heat. The latter can then be used for purposes like heating buildings,” Nousch said.

When designing the system, the researchers at Fraunhofer IKTS drew on their decades of expertise in working with ceramic fuel cell stacks. The team was able to build a fuel cell demonstrator that handles the entire process of breaking ammonia down into hydrogen and subsequently generating electricity from it all in one device. The efficiency of this method, just like those based on natural gas, stands at 60%, but with the difference that ammonia SOFC systems are comparatively simple and robust in structure.

The system is suited to smaller industrial companies that want to generate electricity without carbon emissions, or for municipalities and local utility companies looking to supply green heat to their customers. Even large ships can be equipped with ecofriendly drives based on ammonia/hydrogen in this way.

“Targeted design and smart thermal management are combined with other modifications to aspects such as the power and the size of the fuel cell stacks,” Nousch said. “We are then able to devise customised solutions for climate-friendly generation of electricity and heat, especially for small and medium-sized enterprises.”

Top image caption: Demonstration system for carbon-free generation of electricity with ammonia in high-temperature fuel cells (SOFCs). ©Fraunhofer IKTS.