In essence, they would create an environmentally friendly alternative to the current fossil fuels, especially for maritime transport and aviation. This solution could be even better than hydrogen alone, but there are still some question marks.
The newly announced ammonia engine from the British company Reaction Engines, which uses the intellectual property of several companies, is based on a system designed for the company’s Skylon spacecraft. The Synergetic Air-Breathing Rocket Engine (SABRE) engine was originally designed for hypersonic and space flight, but when combined with ammonia catalysts from the UK government-backed Science and Technology Facilities Council (STFC), the result is a solution that could work in otherwise difficult-to-off-grid sectors such as shipping, aviation and off-grid power generation.
As the above-mentioned press release states, aviation and shipping together currently account for 5 percent of global carbon emissions, and emissions from both sectors are only set to increase in the future. Reaction Engines’ lightweight and compact reactors would therefore make it possible to reduce emissions in these sectors. The basic idea is that the device would harness the heat generated by the engine to produce an ammonia-hydrogen mixture from pure ammonia, which could largely replace current aviation fuels. As the New Atlas article says, ammonia has several advantages over hydrogen: it is easier and cheaper to store and transport, but it has only 20 percent of the energy of hydrogen in terms of mass. But volume is another issue: in this respect, ammonia has 70 percent more energy than liquid hydrogen.
It is precisely because of the mass aspect that ammonia is not usually considered as a fuel for aviation. At the same time, because of the high volume requirements of hydrogen, it is not an ideal solution in this area either – more volume means bigger tanks, which means that if the current aircraft are fitted with this system, they will have less space for seats. The current solution is therefore an option that allows an airline operator to choose between weight and volume based on his own criteria.
The question is, of course, how “green” this particular option is. Reaction Engines stresses that emissions from these sectors can be reduced, which is true, but New Atlas adds that the combustion product of such a fuel will be nitrogen oxide compounds (NOx), which do not include CO2, but they are still a problem, and a big one. Nitrous oxides are “important in the formation of photochemical smog”, but they are also responsible for acid rain and are harmful to human health. It is not clear from the press release, so one can only hope that the company has come up with a solution to this problem.
New Atlas also mentions that ammonia production is not the greenest process and that green ammonia is currently in short supply and expensive. The latter requires green hydrogen, because if ammonia is produced in the way it is currently produced, i.e. by producing methane, then although the aircraft will not emit CO2, the methane produced in the production of the fuel used will be more of a greenhouse gas than carbon dioxide. The press release also mentions ‘green’ ammonia, but this would be expensive and not available in significant quantities.