From Fertilizer to Fuel: Can ‘Green’ Ammonia Be a Climate Fix?

by Nicola Jones (Yale Environment 360) Ammonia has been widely used as a fertilizer for the last century. Now, using renewable energy and a new method for making ammonia, researchers and entrepreneurs believe “green” ammonia can become a significant clean fuel source for generating electricity and powering ships.

The wind powers a chemical plant that makes ammonia, which can not only be spread as fertilizer under the turbines, but also fuels an experimental tractor, stores energy for a non-windy day, and — soon — will heat the barns that dry their grains. All without producing CO2.

“For deep decarbonization of agriculture, you switch to green ammonia,” says Michael Reese, director of the University of Minnesota project. The university’s studies have shown that using green ammonia (‘green’ in the sense that it is made with renewable energy) for fertilizer, fuel, and heat could drive down farming’s carbon footprint by as much as 90 percent for corn and small grain crops. “That’s transformative,” Reese says.

Advocates of this alternative, zero-carbon liquid fuel see green ammonia’s reach expanding far beyond farms. They predict a vast new market for green ammonia as a fuel, eventually outstripping the planet’s already enormous (and growing) demand for ammonia as fertilizer. A 2021 International Energy Agency report forecasts that to hit zero emissions by 2050, hydrogen-based fuels (including ammonia) should account for nearly 30 percent of transport fuels by 2050, up from basically zero today. That report predicts that cars will run on batteries and planes on biofuels, but ammonia will be vital for the shipping industry, which is currently responsible for 3 percent of global emissions and trying hard to whittle that down fast.

While batteries are efficient, they are best suited to storing smaller amounts of electricity for hours or days; a 2020 Oxford Institute of Energy Studies report concluded that for large-scale, long-term energy storage, liquid ammonia is hard to beat. Countries including Japan, Australia, the Netherlands, and the United Kingdom have national plans to use green ammonia to store (and export) their renewable energy surpluses.

Ammonia is toxic, and burning it can potentially produce an even more potent greenhouse gas than CO2.

Pure hydrogen (H₂) was once touted as the fuel of the future. But hydrogen has issues: as a liquid it needs cryogenic temperatures of around -250 degrees C; as a gas it needs to be stored at high pressure; in air, it’s explosive. Ammonia (NH3), on the other hand, is easy to store as a liquid and still packs a punch, with about half the energy density of traditional fossil fuels. Although ammonia is toxic, the world already has a vast system for making, storing, and transporting it.

The simplest way to slash emissions from ammonia production is to take natural gas out of the equation and instead make hydrogen by splitting water with electricity sourced from renewables. The rest of the Haber-Bosch process remains the same, powered by renewable electricity.

There are risks with such expansion, says Faria (Jimmy Faria, a chemical engineer at the University of Twente in the Netherlands), including accidental leakage of ammonia and even environmental pollution with highly-concentrated salt — a byproduct of the desalination needed to make all the water for green hydrogen.

To get green ammonia going fast enough and big enough, government policies will likely be needed to help subsidize green hydrogen and encourage economies of scale, says chemical engineer Prodromos Daoutidis, who works with Reese at the University of Minnesota. Until then, there’s a danger that industries like shipping that hope to use ammonia as fuel will end up using ‘dirty’ ammonia for power, simply shifting emissions from one industry (shipping) to another (ammonia production).

The idea is to produce ammonia directly in an electrochemical cell, without having to make hydrogen as an ingredient. This so-called “generation 3” technology has proven a hard chemistry nut to crack after many years of work.

Ammonia burns slower and is harder to ignite than fossil fuels; most ammonia engines need a dose of diesel or hydrogen to get them going. If engines leak unburned ammonia, that can be toxic. And ammonia engines tend to produce nitrogen oxide, also a potent greenhouse gas. There are catalytic converters that can solve this problem, though.

Major engine manufacturers, including German MAN Energy Solution and Swiss WinGD, are now developing ammonia-fueled engines and kits to retrofit old engines so they can run on ammonia, with first products expected to be on ships in 2024. READ MORE