Hydrogen is widely seen as a vital component in efforts to decarbonize the world’s power supply. One example of this is a strategy being piloted by at least a couple of major gas turbine manufacturers, which involves storing “green hydrogen” produced through electrolysis using excess wind or solar power when renewable energy supplies exceed grid demand. Then, when the tables turn and demand exceeds renewable energy supplies, the carbon-free green hydrogen is burned in combustion turbines to provide sustainable clean energy to the grid. It’s not a perfectly efficient energy conversion, but it is a method that can be used essentially as a renewable energy storage mechanism, reducing demand for fossil fuels.
The movement of hydrogen is not so simple though. Today, hydrogen is transported from the point of production to the point of use via pipeline and over the road in cryogenic liquid tanker trucks or gaseous tube trailers. Because hydrogen has a relatively low volumetric energy density, its transportation, storage, and final delivery to the point of use comprise a significant cost and result in some of the energy inefficiencies associated with using it as an energy carrier. However, ammonia offers one possible solution for the hydrogen transport problem.
The chemical formula for ammonia is NH3. Like hydrogen, ammonia can be combusted in gas turbines and reciprocating engines. Unlike hydrogen, however, ammonia can be more easily transported and stored in liquid form, something fertilizer companies have been doing for decades.
“Hydrogen is really being looked at as a key means of transporting energy around the world and fueling the world in an environment where carbon emissions aren’t acceptable,” Erik Mayer, vice president of Clean Energy Solutions with CF Industries, said as a guest on The POWER Podcast. “We convert large quantities of hydrogen into ammonia, currently for the fertilizer market but ultimately that same ammonia molecule is being looked at as an efficient way of being able to move hydrogen molecules around the world, whether they’re sourced from natural gas or whether they’re sourced from electrolysis.”
Mayer said the advantage ammonia offers over hydrogen is that it is a liquid at moderately low temperatures and can be stored as liquid under relatively low pressure, similar to how liquefied petroleum gas (LPG) is stored. Concerning how the ammonia is used, Mayer said there are two possible ways: ammonia can be burned directly or it can be “cracked,” that is, decomposed over a catalyst, back to hydrogen. Because there are no carbon atoms in ammonia, there is no CO2 released when it is burned in either case.
“Some power generators, particularly in Japan, are looking at co-firing ammonia in coal-fired plants where they could replace say 20% of the coal they would burn with ammonia and essentially reduce the coal plant’s emissions by 20%,” said Mayer. “That technology is being proven in a pilot test that is relatively understood and known to work.”
At the same time, a number of organizations are working on combusting ammonia in gas turbine-type power applications. Mayer said Mitsubishi Heavy Industries expects to have a gas turbine that can be run on pure ammonia by 2025. There are still challenges, but Mayer said some of them can be overcome by cracking the ammonia.
“The technology to crack ammonia back to hydrogen exists today. It’s used in a few niche applications it just doesn’t exist on a large scale,” Mayer explained, noting that there are “a lot of very strong technology companies” that are working on scaling that up. “In a power application, you wouldn’t need to convert necessarily all of it back to hydrogen, but just enough to raise the heating value of the gas so that the gas turbine functions properly,” he said.
A downside of burning ammonia is that it produces relatively high NOx emissions. Mayer said those can be somewhat managed through combustion controls, but ultimately, there are proven technologies such as selective catalytic reduction (SCR) systems that can be used to keep NOx emissions within required limits.
One big application that CF Industries sees as a growth opportunity for ammonia is as a marine fuel. “The marine industry uses large quantities of bunker fuel to do these transoceanic voyages, and the amount of energy required makes it impossible for them to convert to something like batteries,” Mayer said. “Some of the larger marine engine manufacturers are planning to be able to inject ammonia in replacement of carbon-based fuels, almost to 100%, and they think that technology will be fully developed in the next couple of years.”
To hear the full interview, which includes additional talk about hydrogen and ammonia, as well as a discussion on other sustainability initiatives CF Industries is pursuing, including carbon capture projects at its Donaldsonville, Louisiana, and Yazoo City, Mississippi, facilities, listen to The POWER Podcast. Click on the SoundCloud player below to listen in your browser now or use the following links to reach the show page on your favorite podcast platform:
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—Aaron Larson is POWER’s executive editor (@AaronL_Power, @POWERmagazine).