Ammonia as fuel
Ammonia is of interest in the context of global
decarbonisation not only because it can be used as a carrier of hydrogen, but
also because it can be a carbon-free fuel in its own right. Kobayashi et al. (2019)
note the difficulties of transporting and storing hydrogen, and that ammonia,
which is much easier to handle, “comprises 17.8% of hydrogen by mass and can be
produced from renewable hydrogen and nitrogen separated from air.” Since ammonia
has long been used “as a fertilizer, chemical raw material, and refrigerant”
there is ample experience in its use. The paper explores the successful use of
ammonia as fuel in gas turbines and in industrial furnaces, and discusses methods
of overcoming problems associated with its use, such as low combustibility and
the production of nitrogen oxides (NOx). The authors have a particular interest
in ammonia as a means of satisfying Japan’s need for imported carbon-free fuel,
following closure of all its nuclear power plants in 2011 after the Fukushima accident.
A technical review from Mitsubishi (2019) describes recent
research on carbon dioxide-free energy in the context of Japan’s Strategic
Innovation Promotion Program. CO2-free fuel will be needed “where it is
difficult to use renewable energy and for use in fields such as transportation
where CO2 capture and storage cannot be applied.” While hydrogen energy has in
the past been studied in Japan for reasons of energy security, recent interest
has been further motivated by “the purpose of preventing global warming”. Hydrogen
can be transported in liquefied form, as organic hydride, and as a component of
ammonia, which can be cracked to separate it into hydrogen and nitrogen using
catalysts, but “can be used directly as a fuel in high temperature fuel cells,
internal combustion engines” and gas turbines (David et al., 2016). The
Mitsubishi report notes that when hydrogen and ammonia are prepared from fossil
fuels such as oil, gas and coal, CO2 capture and storage is necessary to avoid
emission to the atmosphere. The production of hydrogen by electrolysis of water
therefore becomes attractive if it can be done cheaply enough, and hydrogen
made in this way and combined with atmospheric nitrogen using sustainable power
produces ‘green’ ammonia. Much research has focused on the direct use of
ammonia in gas turbines, reciprocating engines, boilers and industrial furnaces,
and in solid oxide fuel cells. Ammonia becomes a liquid at -33°C under
atmospheric pressure, or at ambient temperature under a pressure of 8.5 atmospheres,
whereas hydrogen requires a much lower temperature to
liquefy and consequently a large amount of energy is needed. Ammonia has
disadvantages such as its smell and toxicity, and its main use may therefore be
restricted to controlled areas “such as in power plants, factories and cargo
vessels.” The report describes ammonia production from natural gas at a number
of existing industrial facilities, along with CO2 capture and storage. It
points out that ammonia is at present more expensive than coal, liquefied
natural gas or crude oil on the basis of its calorific value, so that for it to
be widely used as CO2-free fuel, it “seems that some political incentive is
necessary in the early stages of introduction.”
“A Roadmap to the Ammonia Economy” is the title of a report on work supported by the Australian Research Council and the Australian Renewable Energy Agency (MacFarlane et al., 2020). Australia’s energy situation is very different from that of Japan. It is one of the arid regions in which “solar insolation is optimum”, with the potential to produce large amounts of renewable electrical energy which could be used to make hydrogen by electrolysis. Given an adequate supply of water, this might allow Australia to export hydrogen to neighbouring countries, perhaps in the form of ammonia. According to the report, ammonia “produced from renewables is widely seen as 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.” Three overlapping generations of production are foreseen; the first expands production using the present Haber-Bosch method, but with CO2 sequestration or offsets; the second moves the Haber-Bosch process “to renewable sources of hydrogen”, and the third uses direct electrochemical conversion of nitrogen to ammonia. The problem of water demand in ammonia production might be met through the use of seawater, either through desalination or directly through processes described in the report. Consideration is given to the capital costs of ammonia production; the safety of ammonia use (which is considered comparable with that of liquid natural gas); the cost of ammonia compared with liquid fossil fuels (considered comparable); its use in converted internal combustion engines, as a marine fuel, for power generation in off-grid situations, in gas turbines and in fuel cells. In view of the potential for a “substantial shift from an economy based on fossil carbon energy to one based on ammonia”, there is a discussion of the environmental impacts of the production and use of ammonia fuel, with the comment that it is “obviously important that humankind does not avoid one crisis revolving around CO2 emissions, by creating another crisis involving NH3 and NOx emissions.”
A report commissioned by Breakthrough Energy recommends a
wide range of projects “to accelerate Europe’s recovery and pave the way to
climate neutrality for Europe” (Capgemini, 2020). It recommends developing ammonia
fuel for maritime propulsion primarily in the context of long distance freight
transport, and cites a development project by the European ShipFC consortium.
This project has EU backing and will allow an offshore vessel, Viking Energy, retrofitted with a 2MW
ammonia fuel cell, to sail on the clean fuel for up to 3,000 hours annually,
thus demonstrating the feasibility of long-range zero-emission voyages with larger
ships. The ammonia fuel cell system is expected to be installed in Viking Energy in late 2023 (FCH, 2020).
Hansson et al., (2020), attempt to “assess the prospects
for ammonia as a future fuel for the shipping sector in relation to other
marine fuels.” They review the literature on ammonia including “production
pathways, costs, and environmental impact”, and note that while ammonia “has
been demonstrated as a fuel in compression ignition (CI) engines, spark
ignition (SI) engines, and fuel cells” there has as yet been no commercialized
marine operation of “ammonia driven propulsion technologies”. However one
previous study sees hydrogen, methanol, and ammonia as providing “pathways to
climate-neutral shipping by 2050” for the cargo sector. A detailed
consideration of a range of criteria including “availability, cost, energy density,
technical maturity, and environmental impact” of ammonia compared with other
fuels leads them to conclude that in the long term, “the use of hydrogen
represents a more cost-effective option for the shipping sector than ammonia”,
but that on shorter time scales “ammonia may be almost as interesting for
shipping related stakeholders as hydrogen and various biomass-based fuels”,
although many issues will need to be resolved before it can be used on a large
scale.
Bruce et al. (2020), consider the use in aviation of
hydrogen-based fuels, including ammonia. They note that an extensive
infrastructure for production and transport already exists, and that ammonia
need only be lightly pressurized to be liquid at ambient temperatures, and can
be transferred from the storage facility with relative ease. Pressurised
ammonia has a high specific energy density but this may require redesign of
on-board storage. Like other writers they note the toxicity of ammonia, its high
ignition temperature, and that combustion produces not only water, but also nitrous
oxides (NOx). They also state that while “fuels such as ammonia and methanol … are
likely to require fewer changes to airframe and engine design … when compared
to kerosene, these fuels have a poor energy density by volume” and this can limit their competitiveness in
long haul air travel.
Ammonia fuel may have a place in longer distance aviation
as an element in a hybrid fuel system. Since a great deal of the total energy
necessary for a flight is used in take-off and in reaching cruising height,
ammonia might be used directly as fuel in these flight stages, with another
power source, such as fuel-cell driven electric motors, for cruising and
landing. Much work on such systems has assumed that they will be hydrogen powered,
but since the findings may be relevant to ammonia-based hybrid power, brief mention
is included here. Airbus recently released three concepts for hydrogen-based zero-emission aircraft,
including a turboprop design; the largest seats up to 200 passengers, and the greatest
range is more than 2000 nautical miles (Airbus, 2020). A hybrid
hydrogen/electric system is outlined by Ceurstemont (2020), who cites research
including that of an EU consortium aimed at “supporting the demonstration of
radical aircraft configurations”. The HASTECS project aims to provide design
tools for the development of hybrid aircraft, concentrating on the problem of optimising
the overall hybrid power chain (HASTECS, 2019).
References
Airbus (2020), Airbus
reveals new zero-emission concept aircraft, press release, 21 September
2020
Bruce et al., 2020, Opportunities
for hydrogen in commercial aviation
Bruce S, Temminghoff M, Hayward J, Palfreyman D, Munnings
C, Burke N, Creasey S, CSIRO
Capgemini, 2020, “F I T
F O R N E T - Z E R O : 55 Tech
Quests to accelerate Europe’s recovery and pave the way to climate neutrality”,
Capgemini Invent
https://www.capgemini.com/resources/investments-in-next-generation-clean-technologies/
Ceurstemont, S., 2020, “How hybrid electric and fuel
aircraft could green air travel”, Horizon:
The EU Research & Innovation Magazine Nov. 10, 2020
https://techxplore.com/news/2020-11-hybrid-electric-fuel-aircraft-green.html
David, B., et al., 2016, Cracking ammonia, NH3 Fuel Conference 2016
https://www.ammoniaenergy.org/paper/cracking-ammonia/
FCH, 2020, “Major project to convert offshore vessel to
run on ammonia-powered fuel cell”, FCH (press release)
https://www.fch.europa.eu/sites/default/files/Press%20release%20ShipFC%20project%20%28004%29.pdf
Hansson et al., 2020, “The Potential Role of Ammonia as
Marine Fuel—Based on Energy Systems Modeling and Multi-Criteria Decision
Analysis”, Hansson J., S Brynolf, S., Fridell, E., Lehtveer, M., Sustainability, 2020
https://www.mdpi.com/2071-1050/12/8/3265
HASTECS, (2019), Hybrid
Aircraft; academic reSearch on Thermal and Electrical Components and Systems,
CORDIS EU research results
https://cordis.europa.eu/project/id/715483
Kobayashi et al., (2019), “Science and technology of
ammonia combustion”, Proceedings of the
Combustion Institute Volume 37, Issue 1, 2019, Pages 109-133
https://www.sciencedirect.com/science/article/pii/S1540748918306345#!
MacFarlane et al., 2020, “A Roadmap to the Ammonia
Economy”, Joule https://doi.org/10.1016/
j.joule.2020.04.004
Mitsubishi, 2019, “CO2-Free Energy (Ammonia)”, Mitsubishi Heavy Industries Technical Review
Vol. 56 No. 1
https://www.mhi.co.jp/technology/review/pdf/e561/e561080.pdf
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