NASA has used liquid hydrogen as a fuel since the 1970s. It has seemed just around the corner for earthbound, civilian use almost as long.

We can now create green hydrogen, which produces no waste except water. It’s expensive and hard to work with.

Green ammonia might help make green hydrogen more practical. But it’s not just around the corner.

Hydrogen is the most abundant element in the universe, but on earth anyway, it doesn’t exist as a pure gas. It occurs naturally only in molecules with other elements. Science has developed two ways to break it free.

The most common and least expensive is called steam reforming. It subjects natural gas to high heat to release the hydrogen. Along with various greenhouse gases. The other is called electrolysis. It uses an electric current in water to split the hydrogen from the oxygen. It costs a lot more and requires more energy, but if that energy comes from renewable sources, it produces green hydrogen.

President George W. Bush championed green hydrogen and devoted massive funding to it. The research encountered numerous technological and financial barriers. For example, hydrogen is such a small molecule that it easily escapes from any container or pipeline. It takes a lot of energy to liquify it. And liquid hydrogen embrittles steel containers.

Therefore, the Obama administration abandoned green hydrogen in favor of developing technologies that seemed more easily within reach.

What is ammonia?

Pure ammonia is a colorless gas at room temperature with a pungent smell. It is highly irritating and corrosive. It becomes a liquid either under compression or cooling it to about 33º C below zero (28º F below zero).

The ammonia you buy at the store is a solution of ammonia gas and water.

Ammonia is classified as an “extremely hazardous substance.” Any facility that uses large quantities of it must meet strict reporting requirements.

Almost 90% of the ammonia used in the US goes to make fertilizer. In industry, it is also a precursor for any compound that uses synthetic nitrogen. It’s antimicrobial. And, of course, it’s possible to buy it for household cleaning. Plenty of common household cleansers include ammonia.

Industry has long sold ammonia for its nitrogen content. It looks forward to adding sales for its hydrogen content.

Ammonia carries about 70% more energy than an equal volume of liquid hydrogen and three times as much energy as hydrogen gas. By weight, it has more than 20 times the specific energy of currently available lithium batteries.

Diesel fuel has twice the energy of ammonia, whether measured by volume or weight, but, of course, it has a terrible environmental impact. Phasing out fossil fuels means phasing out diesel.

Ammonia is the world’s second-most produced chemical. It is a good way to store energy. Storing ammonia can actually store more hydrogen than storing hydrogen gas.

In fact, it costs almost 30 times more to store hydrogen than ammonia. And ammonia is so much denser that a gallon of ammonia actually has half again as much hydrogen in it as a gallon of hydrogen!

Making ammonia

Current methods of ammonia production require dirty, energy-intensive processes. It begins by extracting natural gas from the ground, which releases a lot of methane. Then the gas goes through a steam reforming process to release the hydrogen, which releases carbon dioxide.

The resulting hydrogen is H₂, or a molecule of two hydrogen atoms. To make ammonia means combining it with nitrogen in the form of N₂ taken from the atmosphere.

But ammonia is NH₃. Making it requires splitting the nitrogen and hydrogen molecules and recombining the atoms.

The standard way of making ammonia, the so-called Haber-Bosch process, is now more than 100 years old. German chemist Fritz Haber discovered how to fix nitrogen from the air in 1905. Another chemist, Carl Bosch, soon developed the process at industrial scale.

It has operated on natural gas as a feedstock and used fossil fuels to supply the energy to the factory. Therefore, the Haber-Bosch process emits nearly two tons of carbon dioxide to make one ton of ammonia. Switching to renewable energy and water from electrolysis cleans it up considerably, but there’s still room to improve it.

For example, it’s possible to use green energy instead of burning fossil fuels, and add carbon capture and storage to the process, which produces so-called blue ammonia. It still uses natural gas as feedstock, and it still releases methane.

Making green ammonia

Green ammonia starts with green hydrogen. Then, it uses green energy to power the Haber-Bosch process. It’s an inefficient way to use green energy, but at least it causes no emissions.

Another way to make green ammonia entails using “reverse fuel cell” technology to make ammonia from renewable energy, air, and water without using electrolysis. So far, this technology works too slowly to produce ammonia at industrial scale. It might be the most promising path to making sustainable liquid fuels, but it will take a lot of research and money to make it work.

Using ammonia for energy

There are at least three ways to use ammonia as a fuel:

• Convert it back to N₂ and H₂. The hydrogen can either serve as a combustion fuel or produce electricity in a hydrogen fuel cell. Unfortunately, this method yields only about 19% of the energy required to make the ammonia in the first place.
• Use it directly as a combustion fuel. Ammonia burns only at very high temperatures. Properly controlled, it emits only nitrogen gas and water. Unfortunately, if it isn’t properly controlled, it emits a lot of nitrous oxide, another potent greenhouse gas. This method yields about 21% of the energy required to make the ammonia.
• Use it to run a high-temperature solid oxide fuel cell. This method returns about half of the energy needed to make the ammonia, but the current technology is very expensive and very slow.

Ammonia-fueled combustion engines have existed since the 1870s. They have been little used, however, because fossil fuels have been less expensive and easier to work with. Ammonia is more difficult to ignite and burns more slowly. An engine leak releases toxic chemicals. Even a well-running ammonia engine releases the greenhouse gas nitrogen oxide.

It is a mature, well-understood technology, though. Catalytic converters can solve some of the problems. Research is currently underway both to improve the technology further and to retrofit gas and diesel engines to run on ammonia.

Producing electricity from ammonia resembles a Rube Goldberg invention. First, use electricity to make hydrogen. Use that hydrogen to make ammonia. Transport and store the ammonia. Then use the ammonia to run a turbine to make electricity. The result? An output of only about 20-30% of the input. Even so, the advantages of easy storage and transportation of the ammonia may outweigh the disadvantages of such poor efficiency.

Roadblocks green ammonia must overcome

The International Energy Agency estimates that hydrogen-based fuels will have to supply nearly 30% of transportation fuels by 2050 for the world to achieve zero emissions by then.  Ammonia will be especially important for the shipping industry.

This post has been mostly concerned with ammonia as an energy storage medium and as a fuel. But, of course, it is also a major component of chemical fertilizer. The University of Minnesota is conducting a study of green ammonia. Michael Reese, the project’s director, estimates that using it for fertilizer, fuel, and heat could reduce agriculture’s carbon footprint by as much as 90% for certain crops.

Production of ammonia will have to increase 100-fold in coming decades to meet anticipated demand. But if the ammonia comes from the standard process, using it will only shift emissions from transportation and shipping to ammonia production.

Green ammonia is about four times as expensive as conventional ammonia. Much of the technology necessary to harness it is still experimental. So far, most green ammonia projects produce tens of thousands of tons, compared to hundreds of thousands of tons of typical factory output for conventional ammonia.

For green ammonia to reach its potential, it must become both less expensive to make and less expensive to use. The steep drop in the cost of wind and solar ensures that green hydrogen and green ammonia will eventually become cheaper than producing them with fossil fuels. To scale them up fast enough will require government subsidies for a while.

Besides scaling up production of green ammonia, the hydrogen economy will require some way to reconfigure engines to run on ammonia instead of current fossil fuels.

Some green ammonia projects already in the works

Governments and industry need to determine if green ammonia’s advantages are worth the effort and expense. So far, it appears, they are moving forward.

Saudi Arabia and Air Products, a US company, have announced a project intended to generate 1.2 million tons of green ammonia annually. That amount exceeds the typical conventional ammonia factory’s output.

In addition, Air Products intends to ship the ammonia to specialized plants around the world. These plants will “crack” the ammonia to produce enough green hydrogen to power up to 15,000 trucks and buses.

Incitec Pivot, Australia’s largest fertilizer supplier, makes about two million tons of ammonia every year. The high cost of gas as a feedstock almost led to the shutdown of its Gibson Island plant in Brisbane. Instead. Incitec Pivot and Fortescue Future Industries have begun to find a way to make the plant run on green hydrogen.

FFI plans to build an electrolysis plant there to make 50,000 tons of green hydrogen every year. That hydrogen will become feedstock for making green ammonia as a fertilizer ingredient. Beyond fertilizer, FFI uses green hydrogen to power haulage at its iron mines. It looks forward to using ammonia––perhaps from Gibson Island––to run its shipping fleet.

But for now, green ammonia and green hydrogen remain among very promising technologies that must still overcome many hurdles before they can fulfill their potential.

Related posts:

Can hydrogen fuel cells provide green electricity and storage?
Green hydrogen energy: when will it fulfill its promise?
Hydrogen as an alternative fuel
Using solar electrolysis for clean hydrogen energy storage

Sources:

Facts about ammonia / New York State Department of Health
From fertilizer to fuel: can ‘green’ ammonia be a climate fix? / Nicola Jones, Yale Environment 360. January 20, 2022
Green ammonia: The rocky pathway to a new clean fuel / Loz Blain, New Atlas. September 3, 2021
High gas prices lead to conversion of ammonia plant to green hydrogen / David Waterworth, CleanTechnica. December 30, 2021