Shipping is responsible for three percent of global CO2 emissions but has been slow to embrace technical change. Yet, says a new Ricardo report, there could be an answer in the shape of green ammonia.
With the world waking up to the global climate emergency, major industries are looking towards zero carbon emissions by 2050 or earlier. Shipping, responsible for between 2 and 3 percent of global greenhouse gas emissions, has been one of the slowest to be regulated and hence, to embrace technical change. Yet according to a new Ricardo report there could be an answer, in the surprising shape of ‘green’ ammonia.
The shipping industry, with its high investment costs and long replacement cycles for vessels and their engines, has traditionally been a slow mover when it comes to environmental initiatives. Recently, however, the International Maritime Organization has agreed to halve the sector’s greenhouse gas (GHG) emissions by 2050. The quantities are significant: if sea transport were a country, it would rank fifth in the world for GHG emissions, midway between Japan and Germany.
Decarbonised powertrains are at varying stages of development in almost every transport sector, with batteries, hydrogen and synthetic fuels all vying for advantage. But in shipping the route forward is less clear: battery power is currently inadequate for any but the shortest of ferry routes, and hydrogen has challenges due to infrastructure issues.
The liquefied natural gas fuel that has recently become more popular is helpful in avoiding the sulphur emissions of traditional heavy fuel oils and does slightly reduce CO2 impact thanks to the lower carbon content; however, truly climate-neutral synthetic fuels are still too expensive for this acutely cost-conscious sector and are not currently available.
One possibility gaining traction is carbon-free ‘green’ ammonia, and its potential is highlighted in a study published by Ricardo Energy & Environment in spring 2019 on behalf of the Environmental Defense Fund, a leading international non-profit organisation addressing environmental problems. The report, entitled Sailing on Solar: Could green ammonia decarbonize international shipping? builds a strong case for ammonia as a fuel which could not only decarbonise maritime transport but also help boost the economies of developing nations. The connection might appear obscure but, as we explain below, it could make perfect sense.
Ammonia as a green fuel
On the face of it, using green ammonia as a marine fuel ticks a great many boxes. The lifecycle of green ammonia is climate neutral if it is generated using carbon-free energy, and the fuel can be used in existing marine diesel engines with some modifications to the powerplant and fuel storage systems. There is a potential for the same fuel to be used later in fuel cell equipped vessels.
Ammonia is a known compound in industry. It is widely used in transport as a reductant, stored as AdBlue for use in the selective catalytic reduction (SCR) systems employed to reduce NOx from diesel exhausts. Despite its corrosiveness, ammonia is in general easier to store than hydrogen and does not require cryogenic freezing.
More compellingly still, is that ammonia is produced using renewable electricity, the whole fuel value chain becomes fully climate neutral. Abundant solar irradiation in many developing countries in the equatorial belt and southern hemisphere gives them the potential to set up green ammonia plants as part of extended solar farms. These could provide sustained demand for renewable energy and thus help the region or nation reach the critical mass required to increase the contribution from renewables in their energy mix. And that in its turn could help displace existing GHG-intense sources of generation, further reducing aggregate global GHG emissions.
In the Sailing on Solar report, experts from Ricardo Energy & Environment have detailed some case studies of possible installations, each of them close to busy shipping routes so as to allow easy refuelling of vessels and loading of bulk ammonia carriers. Among the locations examined are Morocco, Greece and Chile.
“in the green ammonia process, renewable electricity powers electrolysers that split water into its constituents of hydrogen and oxygen; the hydrogen is then stored ready for the next process”
How is green ammonia produced?
Ammonia (NH3) is a compound of hydrogen and nitrogen, and in conventional industrial methods the hydrogen is ‘reformed’ from carbon-based feedstocks like natural gas, oil or coal. In the green ammonia process, renewable electricity powers electrolysers that split water into its constituents of hydrogen and oxygen; the hydrogen is then stored ready for the next process, which sees it combined with nitrogen harvested from the air using an air separation unit, again a familiar technology.
The electrolyser plants for green ammonia production are made up of multiple modular units that can operate at low loads and can be stopped or started easily, says the report. These characteristics give them high operational flexibility, which is well suited to renewable electricity with fluctuating output.
The Haber-Bosch process for turning the hydrogen and nitrogen into ammonia is also an established one. The technique involves an exothermic reaction (one that creates heat) that works best when it continues uninterrupted, explains the report, but it is possible to design a Haber-Bosch plant with the ability to operate more flexibly and to reduce the load at times of lower electricity output from intermittent renewable sources. In terms of energy demand (excluding the desalination plant that provides the pure water), the electrolyser absorbs some 92 percent of the total, with the Haber-Bosch synthesis accounting for just 6 percent.
“the central advantage of using ammonia in any combustion application is that it contains no carbon atoms, so the fuel itself does not give rise to any carbon emissions”
Buffer storage tanks for both hydrogen and nitrogen allow the system to absorb surplus electricity supply, as well as enabling the process to continue even when renewable generation is low. In fact, says Nick Ash, one of the report’s authors, the electricity supply could drop by more than 92 percent and the Haber-Bosch process could continue operation near full load.
“ammonia can be kept under pressure at around 10 bar, or refrigerated to -33°C”
Green ammonia on board ship The central advantage of using ammonia in any combustion application is that it contains no carbon atoms, so the fuel itself does not give rise to any carbon emissions. The harmful sulphur and heavy metal emissions associated with heavy oil bunker fuels are avoided too.
With large diesel engines running on heavy fuel oil being so dominant in the shipping sector, these engines would be the natural entry point for the new marine fuel. Yet the report identifies three further potential shipboard applications: firstly, indirect use as a hydrogen carrier for a hydrogen fuel cell system; secondly, to react chemically in a solid oxide fuel cell system; and finally, and perhaps least promisingly, for direct combustion in gas turbines – as used in some military ships.
As always, the substitution of the new fuel may not be quite as simple as plug and play. Because ammonia can be difficult to combust, especially at low loads and/or speeds, a support fuel (probably a fossil fuel or hydrogen) will likely be needed to ensure smooth operation. And given the corrosive nature of ammonia, the main fuel storage and supply system would have to leak free and avoid certain materials such as copper, brass and zinc alloys, as well as rubber and some plastics.
The storage of ammonia is much less problematic than hydrogen, the only other current contender as a carbon-free fuel: ammonia can be kept under pressure at around 10 bar, or refrigerated to -33°C, though it is much less energy-dense than heavy fuel oil and occupies roughly four times the space of a fossil fuel tank giving the same range.
Combustion of ammonia either under stoichiometric or lean conditions can form NOx, notes Matthew Keenan, aftertreatment and chemistry technical specialist at Ricardo’s Shoreham Technical Centre. “NOx emissions are formed in the combustion process under high temperatures, above approximately 2000 K,” he explains. “This can be controlled, to an extent, by optimising the engine operating conditions, and additional NOx control will be achieved through the application of an SCR-based aftertreatment system.”
“combustion of ammonia either under stoichiometric or lean conditions can form NOx”
Spark-ignition stoichiometric NH3 combustion will have higher NOx emissions compared to lean compression-ignition and lean spark-ignition NH3 combustion owing to the higher peak combustion temperatures.
The aftertreatment is likely to be an SCR system, broadly similar to those already in use on many lean combustion engines. And no separate AdBlue supply will be needed as the reductant is contained in the principal fuel; even so, the aftertreatment and reductant supply may impact a vessel’s payload and packaging.
“the aftertreatment is likely to be an SCR system, broadly similar to those already in use on many lean combustion engines”
As for particulate matter (PM), so long the bane of the diesel engine, the Ricardo engine performance development department cautions that PM emissions can still be expected from NH3-fuelled engines. However, NH3 is expected to result in a much less visible emissions signature, with reduced black smoke. As with diesel engines, the lubricating oil will contribute to PM emissions, along with any hydrocarbon-based fuel used as a combustion enhancer.
Also, signal the specialists, the NH3 fuel itself may form solid nitrates which will contribute to PM emissions. Appropriate after treatment filtration systems may be required to minimise particulate matter.
The urgency of the global climate crisis and the slow pace of upgrades and replacements in the marine sector mean that low-carbon shipping needs to begin feeding into international fleets within the next five years, if not earlier. As ships have a typical lifespan of 20 to 30 years, it is important to act soon.
“the urgency of the global climate crisis and the slow pace of upgrades and replacements in the marine sector mean that low-carbon shipping needs to begin feeding into international fleets within the next five years”
MAN, a leading manufacturer of marine engines, is planning to develop one of its engine models to run on ammonia, targeting brake thermal efficiency in the region of 50 percent, notes the report. Furthermore, MAN also indicated that up to 3,000 existing engines could be retrofitted to run on ammonia. It is not clear from MAN’s plans whether their approach is compression- or spark-ignited. The firm anticipates that a relatively short timeframe of two to three years would be required to develop and test its engine for ammonia combustion, which indicates that it is technically achievable to have new and retrofitted existing vessels with ammonia-operated engines in the next decade.
“MAN, a leading manufacturer of marine engines, is planning to develop one of its engine models to run on ammonia, targeting brake thermal efficiency in the region of 50 percent”
Green ammonia, concludes the report, is a technically feasible solution for decarbonising international shipping, even though there are many development steps required prior to its introduction. It is a fuel that can be combusted in engines and potentially used for fuel cells in the future. The pathway to its deployment can begin using technologies familiar to the maritime sector: diesel or dual-fuel engines in new and existing vessels.
But to make a success of this pathway, what is needed is certainty – both for the marine industry in building and retrofitting such vessels, as well for a green ammonia supply industry to manufacture at scale.
This must be provided by strategic and policy measures adopted by the International Maritime Organization. This would encourage green ammonia and vessels that can accommodate it to be introduced within the timescales required to achieve the IMO’s decarbonization targets.
What is more, demand from shipping could unlock investment in the green ammonia supply chain, including low-carbon industry and renewable electricity. This represents a unique opportunity for sustainable economic development and distribution of bunkering infrastructure around the world – especially for developing economies rich in renewable energy potential.
Ammonia in other engines
Thanks to its hydrogen content, ammonia can operate in a variety of different combustion engines and energy conversion devices. As well as conventional reciprocating piston engines, ammonia can also be burned in gas turbines. However, its lower energy density than conventional jet fuel rules it out for long- and even medium-haul aviation applications. For the same reason, it is not a suitable road fuel but, says Ricardo, NH3 has the potential to be a viable fuel for stationary power. Fuel cell applications are still under study, and initial indications are that ammonia could operate equally well in both PEM fuel cells, with their higher precious metal content, and in solid oxide fuel cells which run at high temperatures. In both cases, ammonia could offer advantages over hydrogen, which has to be stored under much higher pressure.