When considering the future, the best available roadmap is looking to the past. It is no secret that our world currently runs on oil and gas. According to the US Energy Information Administration, in 2020, the US received 68% of its consumed energy from the oil and gas sector. With rising public sentiment against the fossil fuel industry, why is it so hard to move on? Besides the obvious point that most of the US (and the world’s) energy infrastructure has been built around the industry for a century, two other aspects of hydrocarbons stick out as their best traits: energy density and transportability.
This gives rise to the question: what potential renewable energy infrastructure component owns these traits that can continue to support the global energy infrastructure, like oil? The answer may be green ammonia.
Green ammonia has been produced with no carbon emissions, either through the proven Haber-Bosch process utilizing green hydrogen or a novel electrolysis (currently in development). Anhydrous ammonia is already a prevalent commodity at present, but what sets it apart is the “green” aspect. Essentially, energy created at a “high time” in other validated renewable energy technologies (e.g. photovoltaic panels, wind turbines) can be stored in the form of other molecules, instead of stored in long-term storage batteries or worse – wasted. Despite its challenges, green ammonia as an energy carrier has great potential.
High Energy Density
Green ammonia has the highest energy density among non-carbonaceous energy carriers, almost double that of hydrogen. Simply due to ammonia’s molecular structure, there are three hydrogen atoms for the same volume that diatomic hydrogen has two hydrogen atoms. Using broad assumptions about intermolecular interactions, ammonia has roughly 150% volumetric hydrogen efficiency as hydrogen.
Energy Efficient Storage
Hydrogen is compressed at 80 times higher pressure than ammonia and cooled to 220 degrees less Celsius to achieve a comparable liquid state. Both of these processes (compression and refrigeration) translate to more energy usage at the initial liquefaction and continued liquefaction over transport, which detracts from its value.
A Validated Pipeline Already Exists
Today, liquid ammonia is moved around the world in long-distance pipelines, tanker trucks, and even tanker ships. This means a lower entry cost for scale-up.
Like Hydrogen, It Can Also Be Burned for Energy
Work is progressing on technology readiness for known technology, like internal combustion engines, to run on ammonia instead of oil products.
So we all agree to go full “green” ahead with the ammonia as an energy transport, right? Not so fast. As with any technology, green ammonia as a solution has its drawbacks.
Lifecycle Energy Efficiency
There is incrementally reduced energy efficiency by converting hydrogen into ammonia and back. While this doesn’t outright condemn its place as a puzzle piece in our future energy economy, it does provoke a pause to consider.
Environmental Impact of Spills
While many safeguards will be in place to mitigate the potential of a spill, the reality is there will most likely be a large aquatic release if tankers start traversing the oceans with liquid ammonia far more often. This will happen very infrequently, but the impacts will be more severe than other energy sources (e.g., hydrogen).
Compatibility with Hydrogen Fuel Cells
Polymer electrolyte membrane (PEM) fuel cells are sensitive to any ammonia in the hydrogen stream. If PEM fuel cells are a prominent technology in the future, this will require more energy and equipment to ultra-purify hydrogen after converting from ammonia.
The Combustion Has Potential Byproduct NOx Production Associated With It
The ideal combustion products of ammonia and oxygen present no qualms – diatomic nitrogen is 80% of the atmosphere and water is already being produced in the hydrocarbon combustion. But when nitrogen is present with oxygen at high temperatures, there will always be some amount of nitrogen oxides (NOx) produced, which is a greenhouse gas itself.
As with all new technologies, time will tell. But also, as with all other technologies, physical and chemical properties provide clues to the solution — and the limits to the proposed answers. The world is in the nascent stages of a large-scale energy transition, and perhaps one day the breaking news story will not be the rising or falling price of crude oil and gasoline, but the rising tide of green ammonia.
