Ammonia for Power: a literature review

“Ammonia for Power” is an open-access literature review that includes over 300 citations for recent and ongoing research in the use of ammonia in engines, fuel cells, and turbines, as well as providing references to decades of historical case studies and publications. The review, written by a consortium of ammonia energy experts from the University of Cardiff, University of Oxford, the UK’s Science and Technology Facilities Council, and Tsinghua University in China, can be found in the November 2018 edition of Progress in Energy and Combustion Science.

Its scope is broad, encompassing all power generation technologies, in stationary and mobile applications, for energy storage, transportation, and electricity markets as well as other industrial uses.

Ammonia, with its established transportation network and high flexibility, could provide a practical next generation system for energy transportation, storage and use for power generation. Therefore, this review highlights previous influential studies and ongoing research to use this chemical as a viable energy vector for power applications, emphasizing the challenges that each of the reviewed technologies faces before implementation and commercial deployment is achieved at a larger scale. The review covers technologies such as ammonia in cycles either for power or CO2 removal, fuel cells, reciprocating engines, gas turbines and propulsion technologies, with emphasis on the challenges of using the molecule and current understanding of the fundamental combustion patterns of ammonia blends.
Valera-Medina et al, Ammonia for power, November 2018

Ammonia for Power: Energy Storage

One of the main factors driving research in ammonia combustion is the need for large-scale energy storage. The ability to regenerate power from energy stored in ammonia’s chemical bonds will allow far greater penetration of intermittent renewable resources like wind and solar, enabling deep decarbonization of power grids and broader energy economies.

To date, a number of mechanical, electrical, thermal, and chemical approaches have been developed for storing electrical energy for utility-scale services … The only sufficiently flexible mechanism allowing large quantities of energy to be stored over long time periods at any location is chemical energy storage [7] …

The capital costs of ammonia energy storage are comparable to or better than those for compressed air and pumped hydro but without the attendant geological constraints, and substantially lower than other challenger technologies such as electric batteries [33] …

Estimates of the capital costs ($/kW) for ammonia energy storage (between 1350 and 1590 $/kW [29]) indicate it will be competitive compared to battery storage technologies such as Li-ion, NaS and VREDOX (between 850 and 3,660 $/kW [64]), but with the advantage of considerably cheaper (∼2(O)) capacity costs inherent in a liquid fuel.
Valera-Medina et al, Ammonia for power, November 2018

For those unfamiliar with the rapidly evolving energy storage sector, it is important to understand that ammonia does not compete against batteries for those short-term, small-scale applications that are now familiar from media coverage of battery-makers like Tesla and others. Today’s oil and gas systems, which include vast distribution assets, storage hubs, and strategic reserves, provide our economies with large-scale, seasonal energy storage in the form of chemical bonds. It is not plausible to replace these fossil assets with batteries because energy density and capital cost make batteries uncompetitive, by some orders of magnitude, at that scale. However, replacing these fossil energy storage assets with ammonia is cost competitive and provides the opportunity to decarbonize economies without compromising (more likely, increasing) energy security.

Ammonia for Power: Internal Combustion Engines

The review summarizes ongoing global investigations into the use of ammonia as a fuel for internal combustion engines, including systems that propose using ammonia as a solo fuel, a dual fuel, and a drop-in fuel, with details on ammonia emulsions and blends. It covers almost every end-use for power generation, from light-duty vehicles to diesel locomotives and industrial power.

The combustion of ammonia is challenging, due primarily to its low reactivity, but yields nitrogen gas and water, with a stoichiometric Air Fuel Ratio (AFR) of 6.06 by weight …

Analyses of the feasibility of ammonia as a sustainable fuel in internal combustion engines based on thermodynamic performance, system effectiveness, driving range, fuel tank compactness and cost of driving have also been performed [10], [11]. Not surprisingly, the studies concluded that to make ammonia a viable fuel in ICEs, ammonia needs to be mixed with other fuels as combustion promoters due to ammonia’s low flame speed and high resistance to auto-ignition … a dual-fuel approach was usually chosen to implement ammonia combustion in IC engines [179], [180] … [181] showed that ammonia fueled engines have low power losses, no more corrosion and no more lubricant consumption than conventional fuels …

It has been demonstrated [184], [185] that high performance can be achieved using ammonia/gasoline fueling, a three-way catalytic converter capable of cleaning emissions under stoichiometric and rich conditions over short and long distances [186]. Replacement of diesel with diesel/ammonia has also been attempted [187] showing promising results with modification to current diesel engines. Some of the results demonstrated that peak engine torque could be achieved by using different combinations of diesel and ammonia, with a monotonic CO2 reduction for the same torque output for systematic NH3 increase. Additionally, lower NOx emissions were measured for ammonia fuel mixes not exceeding 60% NH3 [188]. Combinations such as gasoline/ammonia and ethanol/ammonia [189], ammonium nitrate/ammonia [190] and even pure oxygen using 100% ammonia [191] have been also attempted, showing that these fuel mixtures can provide elevated power outputs under stable conditions, although mainly conditioned by the NOx emissions product of the combustion process.
Valera-Medina et al, Ammonia for power, November 2018

Perhaps the most interesting area of research is in the use of ammonia-hydrogen mixtures, which enable dual-fuel combustion despite requiring only one fuel tank.

Of particular interest is the use of hydrogen in the ammonia blends, as the molecule can be recovered through splitting of ammonia, with the previously stated improvements in combustion performance. Studies show that ammonia can be blended with hydrogen at levels as low as 5% H2 [202], still providing good power response. Higher doping ratios have also been deployed [203], showing, for example, that 10% hydrogen addition provided optimum efficiency and effective power.

Liquid ammonia contains 1.7 times as much hydrogen as liquid hydrogen itself [206] … an ammonia tank (1 MPa) contains 2.5 times as much energy as a hydrogen tank (at 70 MPa) by volume, i.e. a hydrogen tank of 770 L (350 kg) could be replaced by an ammonia tank of 315 L (172 kg).
Valera-Medina et al, Ammonia for power, November 2018

Ammonia for Power: Emissions

Emissions from ammonia combustion, specifically NOx and unburnt ammonia, have been an area of constant investigation for some decades, although the rate of research has clearly accelerated in recent years.

As for all combustion systems, emissions from ammonia combustion play a crucial role in deployment; thus dedicated research has focused in this area … Applications with flue gases exhaust treatment showed lower levels than legally required when a SCR catalyst was used to eliminate all NOx emissions …

All modern combustion vehicles are now required to operate SCR catalyst systems and/or fuel additive systems to reduce nitrogen oxides to N2 gas. Interestingly, these systems work through the addition of chemicals that decompose to ammonia, and ammonia then reduces the NOx within the gas flow …

Therefore, NOx emissions from ammonia fueled combustion devices may be mitigated in a similar fashion. However, it should be noted that for devices fueled by ammonia a ready reservoir of ammonia for NOx reduction will exist and therefore it may be possible to design ammonia fueled systems that do not require secondary exhaust clean-up or high cost catalyst systems to achieve emission free exhaust. Furthermore, ammonia combustion is often improved through preheating or partial decomposition of the ammonia prior to combustion and it may be possible to parasitically use the waste heat from exhaust systems to pre-heat or decompose ammonia while simultaneously removing NOx pollutants.

In summary, clearly further research developing internal combustion technologies fueled by ammonia is necessary. There is now increasing interest in these systems and considerable competition to produce the first commercially viable devices. However, as described previously, the challenge of reducing further NOx and unburned ammonia remains at the heart of this research and technological field.
Valera-Medina et al, Ammonia for power, November 2018

One sure-fire strategy for avoiding NOx emissions from using ammonia fuel is simply to replace the internal combustion engine with the fuel cell, which emits atmospheric nitrogen and water, and the paper reviews extensive progress on ammonia fueled fuel cells. While I won’t summarize that section here, I write separately on an older literature review that provides an excellent additional resource for understanding the technical potential and technological readiness of direct ammonia fuel cells, “Ammonia as a suitable fuel for fuel cells.”

“Ammonia for Power” highlights a more recent notion, however: hybrid systems. I first wrote about the ICE-FCEV hybrid concept following the US Department of Energy’s ARPA-E Summit in 2017 (The new generation of fuel cells: fast, furious, and flexible). In 2018, however, a new paper by Dincer and Ezzat, provides a fascinating and extensive comparative assessment of ammonia-fueled ICE and ICE-FCEV systems.

Recent studies [208] evaluate the implementation of hybrid systems. Developments show two systems using ammonia-hydrogen fuel for either an internal combustion engine or a combination of an ICE with PEM fuel cells to power a vehicle, Fig. 23. The study was performed with and without dissociation of ammonia. The results from the theoretical study show that the amount of exergy that is recovered using ammonia dissociation (i.e. hydrogen) are considerable, i.e. 16.4% and 13.1% for the ICE and the ICE-PEM systems respectively, consequence of the added hydrogen coming from the dissociation unit. Thus, the use of hydrogen from cracked ammonia is highly beneficial when compared to pure ammonia injection. It must be emphasized that integration of heat recovery for dissociation units played an important role in improving these systems, concept that can be expanded to other combustion technologies to increase efficiency while minimizing exergy destruction.
Valera-Medina et al, Ammonia for power, November 2018

Ammonia for Power: Gas turbines to micro-thrusters

The paper provides an extensive review of investigations into ammonia as a fuel for gas turbines, spanning projects in the USA starting in the 1960s, to large-scale experiments in Italy by Enel in the 1990s, to the recent deployment of ammonia in utility-scale (155MW) power generation in Japan.

However, it also reports a new area of research: the space propulsion micro-thruster:

Micro-thrusters fueled with ammonia, a revolutionary concept for propulsion of small space vehicles, has also received some interest especially in Russia, China and the USA. Blinov et al. [239], [240] presented some work in terms of the design features and performance of ammonia electrothermal micro-thrusters, showing that they can become a competitive, cost effective option due to their specific impulse increase ∼20%. Fatuev et al. [241] have also presented work on the development of ACETAM, a rocket fuel based on the fluidization of gaseous acetylene by a highly concentrated dilution in liquefied ammonia. The characteristics of the fuel show improvement and higher stability when compared to other blends at various operating pressures, nozzle expansion degrees and efficiency through various stages during space launching.
Valera-Medina et al, Ammonia for power, November 2018

Ammonia for Power: conclusion

Perhaps the most important point that the review article emphasizes is not technological but, rather, relates to market adoption. Alternative fuels are too often seen as being in competition with each other, rather than complementary to each other. However, the relationship between ammonia and hydrogen is unique: ammonia can be used either as a fuel or as a source of hydrogen fuel, enabling hydrogen fuel technologies like the PEM fuel cell.

In terms of energy density, liquid ammonia contains 15.6 MJ/L, which is 70% more than liquid hydrogen (9.1 MJ/L at cryogenic temperature) or almost three times more than compressed hydrogen (5.6 MJ/L at 70 MPa). In terms of driving range, a 60.6 L fuel tank of ammonia provides a driving range of 756 km, almost twice the range of the same volume of liquid hydrogen (417 km) and three times the range of the same volume of compressed hydrogen (254 km) [114] …

Ammonia still faces a long way before being entirely recognised as a fuel for power applications. Although technical barriers are overcome by continuous, high quality research combined with advanced innovation … one of the main reasons of this trend is the fierce competition between ammonia and other fuels. As stated, ammonia should not be regarded as a competitor to the hydrogen economy, but as an enabler. Therefore, ammonia can find its niche of application amongst some other fuels that are currently under research. In order to recognise these applications, it is necessary to know some specific characteristics of ammonia to allow comparison with other hydrogen sources employed in power systems …

Ammonia shows both low specific energy and laminar burning velocities combined with high auto-ignition temperatures and elevated ignition energies, making it more difficult to burn in its pure form. As a consequence, blends with hydrogen, i.e. which can be obtained relatively easy from cracking of the ammonia molecule, have been mostly attempted in more practical applications. Moreover, the use of ammonia with gases such as methane, i.e. that shares similar density, viscosity and heat capacity, makes relatively easy its implementation in co-firing applications.
Valera-Medina et al, Ammonia for power, November 2018

The full text of the literature review, and citations to all of the 315 sources, is available online at Ammonia for Power.