While there is global potential to generate renewable energy at costs already competitive with fossil fuels, a means of storing and transporting this energy at a very large scale is a roadblock to large-scale investment, development and deployment. Ammonia produced from renewables can be a viable liquid fuel replacement for many current-day uses of fossil fuels, including as a shipping bunker fuel; as a diesel substitute in transportation; as a replacement fuel in power turbines; and even as a potential jet fuel. The global transportation of ammonia by pipeline and bulk carrier is already a well-developed technology.

Researchers at Monash University in Australia are proposing a roadmap to renewable ammonia being produced in the future at a scale that is significant in terms of global fossil fuel use. This, they suggest in a paper in Joule, will emerge via three overlapping technology generations:

• Generation 1 is based on an expansion of current-day Haber-Bosch ammonia production using CO₂sequestration or offsets.

• Generation 2 moves the Haber-Bosch process to renewable sources of hydrogen.

• Generation 3 avoids the need for the Haber-Bosch process entirely by direct electrochemical conversion of N₂ to NH₃. One of the attractive features of generation 3 technology is that it can be implemented at any level of scale, from kW to GW, and in a highly distributed fashion.

Credit: Joule, MacFarlane et al.

Just over a century ago, the discoveries by Haber and Bosch made possible the industrial production of ammonia and ammonia-based fertilizers that today feed the world and are the source of most of our nitrogen-containing chemicals, materials, and pharmaceuticals. The process generates H₂ from natural gas or coal through steam reforming and combines it with N₂, which has been separated from air by a cryogenic process, to form NH₃.

The reaction between N₂ and H₂ requires temperatures in excess of 400 ˚C and pressures above 200 bar to be facile, and therefore the capital cost of plant and equipment is substantial. Ammonia production is currently responsible for ~1.0% of global greenhouse gas emissions (or about 1.4% of global CO₂ emissions); these values increase further if CO₂ emissions associated with natural gas extraction are included.

Over the last decade, momentum has been building to transform the Haber- Bosch (H-B) ammonia industry toward renewable sources of hydrogen, for example, from water electrolysis or solar thermal cycles. This goal has provided the background to the broader vision of ammonia becoming a transportable store of renewable energy that we will discuss in this paper. The challenge both for the existing industry, as well as a much-expanded renewable energy industry is, of course, economics.

—MacFarlane et al.

Generation 1. Gen 1 involves the use of carbon sequestration or offsets to bring the net carbon impact of the ammonia production to zero (blue ammonia). Carbon sequestration adds cost and plant complexity on top of the existing H-B technology. For this reason, the researchers said, it is likely to represent only a transitional solution, helping to establish a market for ammonia beyond the fertilizer and chemical industries.

Modern H-B plants produce ammonia at an energy cost of at least 8 MWh tonne^-1. Recognizing that the lower heating value (LHV) of ammonia is 5.2 MWh tonne^-1, this represents an energy efficiency of only 65%.

Generation 2. Gen 2 renewable ammonia is produced from H-B technology but employs renewable, rather than fossil-fuel-sourced, hydrogen. This has the advantage that existing H-B plants can be transitioned to this new hydrogen supply without major disruption or mothballing. Powered by fully renewable electricity derived from a 20 kW wind turbine, a Siemens demonstrator produces H₂, using a proton exchange membrane (PEM) water electrolyzer, to form around 30 kg NH₃ daily.

Gen 2 technology has significant long-term scope in terms of the ammonia economy, limited only by the substantial investment and long lead time required to establish new facilities.

Generation 3. Gen 3 technology is based on the electroreduction of N₂ to ammonia by direct or mediated means. The H-B process is no longer required; instead, the reaction is driven by electrochemical reduction and the H source is water.

There are several modes of this process being actively researched:

• eNRR in which an electrocatalyst enables direct electron and proton addition to the N₂ molecule; and

• indirect or mediated mechanisms in which a redox mediator such as Li⁺ is first reduced and then, via a series of reactions, ammonia is produced and the mediator is regenerated.

Ammonia economy roadmap showing current and projected contributions of current and Gen 1 (purple), Gen 2 (light blue), and Gen 3 (green) ammonia production technologies. Total NH₃ production is shown as a dotted orange curve. These projections are compared with the projected ammonia production for chemical industry, shipping fuel use (dashed dark blue, according to the predicted 3.6% annual increase over the next three decades), and global fuels consumption (solid gray, 2018 value taken from the BP Statistical Review of World Energy). Note the logarithmic ordinate scale. CRI is a commercial readiness index used by the Australian Renewable Energy Agency. Credit: Joule, MacFarlane et al.

Resources

• MacFarlane et al. (2020) “A Roadmap to the Ammonia Economy”, Joule doi: 10.1016/ j.joule.2020.04.004