Green ammonia is one of the shipping industry’s most promising low-carbon fuel alternatives, as it is a high-density means of “storing” green hydrogen on a ship. However, it must be used carefully to ensure that the benefit outweighs the harm, so a careful study prepared by the Maersk McKinney Moller Center for Zero Carbon Shipping. Several potential onboard emission sources need to be addressed to meet health, safety, air quality and greenhouse gas emissions requirements.
Ammonia poses health and safety concerns related to its potential toxicity in the event of a leak, and these hazards are at the forefront of any discussion of its use as a fuel. But there are many other possible challenges related to the ship’s exhaust. Ammonia produces NOx and N2O (laughing gas) when burned. NOx is a well-known and well-regulated air pollutant and has been associated with public health impacts. N2O is a powerful greenhouse gas, more than 250 times stronger than CO2. In addition, the combustion process will inevitably release some unburned ammonia into the exhaust stream (ammonia slip).
This means that the exhaust from a ship fueled with ammonia needs more or less after-treatment, adding to the cost and complexity — and increasing the need for stakeholders and engineers to work together to get it right, the centers said.
Ammonia slip is the most serious problem on the list as it is acutely toxic. Acceptable exposure limits vary between research studies, but the highest value the center could find was 110 parts per million. The leading class societies have different standards on acceptable ppm limits, ranging from 25 ppm for daily release/delivery to 350 ppm for automatic safety system activation. The center emphasized that setting the standard very low reduces the risk but also increases the size and cost of the required aftertreatment systems. In addition, duplication of different standards could impede progress.
“A common approach to setting standards is needed to avoid unnecessary uncertainty and the risk of different design and development approaches. The IMO must advance this discussion with the support of industry stakeholders,” the center warned.
The second challenge is laughing gas (N2O). Because it is such a potent greenhouse gas, even tiny amounts released in the exhaust could quickly outweigh the decarbonization benefits of ammonia propulsion. Hypothetically, an engine producing one gram of N2O per 180 grams of ammonia fuel (0.5 percent) would be as greenhouse gas intensive as a VLSFO-powered engine.
Minimizing the impact starts with engine design and engine tuning. According to the center, engine designers hope to achieve acceptable emissions of nitrogen oxides and ammonia in the exhaust without aftertreatment. Only the NOx remains, which would be removed from the exhaust gas flow with the well-known SCR technology.
If this approach is unsuccessful and the engine produces excessive N2O or ammonia slip, the emissions control system would require several steps. A two-stage catalytic treatment system is one possibility: an SCR system could handle the NOx, just like in existing modern diesel applications, and if properly designed could also scrub the N2O. A separate catalytic process would reduce the amount of ammonia slip in the exhaust stream.
Another, more novel possibility is a newly developed technology to treat the exhaust gas with plasma, which would control all three pollutants simultaneously. However, this approach is still in the development stage.
The ship’s fuel tanks, fuel systems and all ancillary equipment would also require measures to control ammonia emissions. A well-established way of doing this is by capturing ammonia vapor and directing it to a “water scavenger” – a fresh water tank that absorbs the ammonia gas and turns it into a mild ammonia solution. Any gas concentrated enough to be combustible could be sent to an auxiliary boiler.
“We believe now is the time for key stakeholders to come together and develop the necessary emissions management solutions that mitigate the identified emissions risks,” the center concluded. “With industry-wide collaboration in the development of engine and emissions management technologies and the design of ammonia-fueled vessels, the risks of ammonia emission should not be a showstopper for ammonia-based fuel pathways.”
This proactive approach to emissions is a lesson learned from the introduction of LNG as a marine fuel, as “methane slip was not fully understood and addressed in advance at the time,” the center noted.
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