Green Ammonia and the Energy Transition

Green ammonia is ammonia - chemical formula NH3 - that is made using only air and water as its raw ingredients, and whose manufacture is 100% powered by renewable energy.

Its most immediate purpose is to replace all the ammonia currently being produced using fossil fuels, mainly to make fertilisers. But has a far greater potential role in our zero carbon future as an energy dense fuel for power stations, ships, aircraft, and a host of industrial applications; and as a medium for the long term storage and long distance movement of renewable energy.

Reinventing the Global Energy Ecosystem

A sustainable Earth needs global greenhouse gas emissions to start reducing immediately, and reach zero by 2050. [1] The electrification of energy based on renewable generation - mainly from solar and wind - is key to achieving this goal, of a true zero carbon economy.

But while wind and solar are amply able to supply all the world’s energy, they can’t provide it on demand exactly where and when needed.

Solar PV won’t generate power at night, for example. There can be entire weeks of still, cloudy weather, known as Dunkelflaute (from the German) with minimal renewable power generation. And temperate countries depending mainly on solar power will experience great reductions in generation during winter months.

So we need ways of storing up renewable energy generated when and where it’s abundant, so we can use it to keep our lights on and economies moving when and where it’s scarce.

Pumped storage hydroelectric plants can certainly help, but they are mainly used to even out the gap between ever fluctuating supply and demand on electrical grids. To serve this purpose they often have relatively low storage capacity, but the ability to generate considerable power for a short time while firm plant is fired up, and so smooth over a major grid event like a nuclear power station going offline.

Batteries also have an important support role in smoothing out fluctuations in demand and supply, and in storing energy for hours or days to meet short term needs. But they’re far too expensive (for the energy they store) to see countries through weeks of Dunkelflaute, or to store summer energy through until winter. Nor do they have the capacity for long distance aviation, or long distance shipping.

For these tasks we need carbon free fuels that are easy and safe to store and use, cheap to produce, and can be quickly and efficiently turned back into electricity, or heat, or motive power, on demand.

Hydrogen?

The easiest zero carbon fuel to make is hydrogen, which can be generated from renewable electricity by electrolysing water. Oxygen is also produced and may have a saleable value. This hydrogen can be stored in tanks at high pressure, as metal hydrides at much lower pressure, or in salt caverns 500m or more underground. This last option is best suited to large scale hydrogen storage, however large deposits of salt are relatively scarce, and many suitable sites are already in use for storing hydrocarbons.

The problem with hydrogen is that it’s very light and its molecules fiercely independent. To achieve liquidity it needs to be cooled to -253C (20K) at ambient pressure, with a density of just 71 kg per cubic metre. Alternatively it may be compressed to a supercritical gas at 350 bar at ambient temperature, with an even lower density of 26 kg per cubic metre. The choice is therefore between very low temperature or very high pressure, with low density of fuel in either case.

Bulk storage of hydrogen is therefore a viable option where a very large volume of containment is available, in order to avoid the high energy costs of cooling or compression. This is possible in salt caverns, and the UK is fortunate in having numerous salt domes in its geology where gas storage caverns may be made (like this recently reported example at Portland Harbour on England’s south coast, offering a potential 320,000 m3 of storage capacity). But for a fuel to power the Green energy transition globally we need to look elsewhere.

Ammonia

Ammonia is hydrogen’s sister fuel - one nitrogen atom with three hydrogen atoms. As such ammonia gas contains, volume for volume, 50% more hydrogen than pure hydrogen gas.

Or to compare the two in their liquid phases: ammonia liquefies at ambient pressure at -33.3C, with a density of 665 kg per cubic metre; hydrogen must be cooled to -253C, with a density of 71 kg per cubic metre. Hence liquid ammonia contains, volume for volume, about 65% more hydrogen than liquid hydrogen itself.

Ammonia can also be liquefied at ambient pressure at 8 bar, compared to hydrogen’s 350 bar (to make a liquid-like supercritical gas). It can be made from hydrogen using the Haber Bosch process with a low energy overhead. It’s cheap to transport, and can be used as a fuel almost as easily as hydrogen. It’s also straightforward to break ammonia down into hydrogen and nitrogen at the point of use. All this makes it an effective way to store and transport hydrogen without having to resort to extreme low temperatures and / or high pressures. [2]

Using ammonia for energy is very similar to using hydrogen. Both can be burned in engines or used in fuel cells to make electricity. Both can be used as industrial fuels for heat, or in specialised applications like steel making. Some ‘dual fuel’ engines and fuel cells will even be able to use either hydrogen or ammonia.

So Green ammonia and hydrogen should not be seen as rival solutions for a zero carbon economy, but complementary. Ammonia will often thrive where hydrogen struggles to compete: in long distance shipping and aviation using fuel cells and electric motors, for example; and power grids can rely on globally traded ammonia to back up locally generated renewable energy. [3]

The additional cost of the Haber Bosch process makes Green ammonia more expensive to make than hydrogen. But because it’s so much easier and cheaper to store and transport, it can be made where the major cost - renewable electricity - is cheapest, and be shipped to distant markets. As a globally traded bulk fuel, Green ammonia will enjoy the price advantage.

Significant advances are also being made in raising the efficiency of the Haber Bosch process of ammonia synthesis, for example by using novel catalysts, which will lead to cost reductions. [4]

Boosting the prospects for both hydrogen and ammonia is the dramatically falling cost of renewable energy, especially solar. In 2021 the 600 MW Al Shuaibah 1 solar PV project in Saudi Arabia achieved a record low price of $0.01 per kWh. Prices have risen since due to increasing financing costs, but the technology itself keeps on getting cheaper. The investment required for the Al Shuaibah 1 and 2 projects, collectively 2.63 GW, came to $2.2 billion, or about $0.84 per Watt of solar capacity.

New technologies such as perovskite tandem cells will reduce solar generation costs even further. [5, 6] Efficiency increases in the electrolysis of water to hydrogen are also anticipated, producing significant reductions in the cost of green hydrogen, and hence Green ammonia.

Because fuel cells are much more efficient than engines burning fossil fuels, the cost of the actual usable energy delivered is likely to outcompete fossil fuels within a few years. 

Hydrogen and ammonia will take over many of the key roles of gas and oil respectively, but they will do more; they can turn iron ore into steel, replacing coal, and make green fertilisers – key to agriculture and feeding the world.

Ammonia has the energy density needed to make it a viable aircraft fuel. Fuel cells can convert it to electricity to power electric propellers (a very efficient form of propulsion). This would make air travel sustainable and clean, as well as fast

Figure 1.2: During WW2 in Belgium, petroleum fuels were short, and some buses were powered by a mixture of ammonia and coal gas.

Figure 1.1: Mitsubishi power commences development of world’s first Ammonia-fired 40MW class gas turbine system

Figure 1.3: Mitsubishi power commences development of world’s first Ammonia-fired 40mw class gas turbine system (placeholder caption)

Figure 1.4: Ammonia could power the next generation of airlines

Broad Linkages

The diagram below shows many of the key linkages in the Hydrogen / ammonia economy.  Whereas development in each of the blocks shown will contribute to the progress of decarbonisation, the dark red blocks highlight key areas where R&D will enable ammonia and hydrogen to outcompete the fossil alternatives.  The diagram shows how a limited number of developments in these key areas can transform swathes of the economy.

A Plan of Action

More detailed analysis of these high-priority R&D areas has shown that many of the advances needed are already in laboratories, or, as in the case of aviation for example, are a matter of engineering research and development rather than new science. The first objective must be to focus attention, and thus funding, on these key R&D targets.

In addition to research, to get costs competitive with fossil fuels, industrialists will have to build production plants to help them learn from experience and move down the cost curve. Financiers will have to become comfortable with the risks, and users will have to be able to rely on supply.

Bringing these different elements together in parallel, fast enough to save the climate, is a major challenge. A second objective will therefore be to help accelerate this development and deployment.

The transition to a hydrogen/ammonia economy is made easier because we already have global industries making ammonia and hydrogen. [7] That brings 100 years of experience in handling and transporting hydrogen and ammonia safely.

Nonetheless using hydrogen and ammonia as fuels at large scale may raise public concerns over safety. A third objective will therefore be to build a clear safety case and to stimulate the development of the procedures and regulations needed to deliver that safety and maintain public confidence and acceptance.

References

  1. Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield (eds.). IPCC, 2018: Summary for Policymakers. In: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty. (2018).

  2. The Royal Society. Ammonia: zero-carbon fertiliser, fuel and energy store. (2020).

  3. MAN Energy Solutions: an ammonia engine for the maritime sector. Ammonia Energy Associationhttps://www.ammoniaenergy.org/articles/man-energy-solutions-an-ammonia-engine-for-the-maritime-sector/.

  4. MacFarlane, D. R. et al. A Roadmap to the Ammonia Economy. Joule S2542435120301732 (2020) doi:10.1016/j.joule.2020.04.004.

  5. New tandem perovskite-silicon solar cell breaks efficiency record. New Atlashttps://newatlas.com/energy/tandem-perovskite-silicon-solar-cell-efficiency-record/ (2020).

  6. Cholteeva, Yoana. Record Breaking Solar perovskites. Power Technology https://www.power-technology.com/features/record-breaking-solar-perovskites/ (2020).

  7. Industry report sees multi-billion ton market for green ammonia. Ammonia Energy Association https://www.ammoniaenergy.org/articles/industry-report-sees-multi-billion-ton-market-for-green-ammonia/.