A better way to make ammonia

Artificially produced ammonia is crucial for many applications, most importantly agriculture, where it is used to grow 40% of the world’s food. Traditionally, ammonia production generates hydrogen through steam reforming of natural gas. However, advances in water and solid oxide electrolysis using renewable electricity provide more environmentally friendly alternatives.

Pfromm and Aframehr examined a collection of alternative production methods to determine the energy cost to manufacture ammonia based on renewable electricity. They compared the energy cost of ammonia when hydrogen is produced using natural gas, conventional electrolysis of liquid water, and solid oxide electrolysis of steam.

Solid state electrolysis is promising because it requires substantially less electricity, and instead splits water into hydrogen and oxygen using ceramic membranes. The authors found that electricity costs were 20% lower than liquid phase electrolysis. However, this method also requires steam production, and the steam heating costs minimize the electricity savings.

“If you had heat at no cost, that would be great, but that’s not really usually available,” said author Peter Pfromm. “Solid oxide electrolysis probably won’t change the game of making hydrogen or ammonia.”

The authors did discover that producing hydrogen — and by extension, ammonia — with renewable energy was at least as cost-effective as producing it with natural gas. They hope that renewably produced ammonia can be used in multiple industries, such as fertilization in agriculture and as an internal combustion fuel in shipping and aviation, to lower carbon dioxide emissions.

“About 3% of all the carbon dioxide that we produce from fossil fuels is from making ammonia,” said Pfromm. “We can kill that off with renewable electricity.”

Source: “Green ammonia from air, water, and renewable electricity: energy costs using natural gas reforming, solid oxide electrolysis, liquid water electrolysis, chemical looping, or a Haber-Bosch loop,” by Peter H. Pfromm and Wrya Aframehr, Journal of Renewable and Sustainable Energy (2022). The article can be accessed at https://doi.org/10.1063/5.0101709 .