SOMETHING OLD, SOMETHING NEW: STUDY EYES NH3 TOOLS

The Mærsk Mc-Kinney Møller Center for Zero Carbon Shipping (MMMCZCS) has published a paper Managing emissions from ammonia-fuelled vessels explaining the need for multiple different treatment technologies to manage emissions

Ammonia slip from fuel handling operations or engine exhaust gas is highly toxic and represents a safety risk for crew and passengers. Other exhaust emissions include NOx formed by incomplete ammonia combustion and N2O, a potent greenhouse gas (1 gram of N2O is equivalent to 265 grams of CO2). Additionally, CO2 emissions will occur if a fossil-based pilot fuel is used.

Different emission management technologies will be needed onboard to treat ammonia boil-off gas (BOG) from fuel tanks, ammonia mixtures from purging and venting operations, and combustion emissions from engines. Some of these technologies are already commercially available for maritime use, including reliquefication and selective catalytic reduction (SCR). Others will need to be adapted for ammonia as a fuel, including gas combustion units/boilers, catalysts, and water catchers.

Ammonia water catchers (chemical absorbers) can treat ammonia emissions from fuel systems resulting from purging operations or shutdowns. While such treatment is not required for LNG, it is usually released to the atmosphere, this is not suitable for ammonia due to safety concerns. Therefore, onboard treatment of ammonia or ammonia mixtures is needed.

A water catcher could consist of a system where the ammonia is vented to a knockout tank before the lines are purged with nitrogen. The liquid ammonia collected in the tank is pushed towards a recovery tank using nitrogen, while the vapours are mixed with fresh water within a water seal until there is sufficient volume for them to be returned to the engine for combustion.

Ammonia slip, N2O, and NOx emissions from engines could be reduced using SCR systems that use ammonia as a reducing agent rather than the more typical choice of urea. “For two-stroke engines, a high-pressure SCR integrated into the engine design can more efficiently reduce the potential increased NOx emission levels relative to conventional fossil fuels, due to higher temperature before the turbocharger,” states the paper.

A wide range of catalysts are used for reducing N2O in industrial processes, some without reducing agents, some using ammonia as a reducing agent. These technologies offer a potentially simple and compact solution where an ammonia dosing system ensures adequate ammonia for NOx and N2O reduction if ammonia slip levels within the exhaust are insufficient. The challenges for using catalysts to remove N2O include potentially low exhaust temperatures during certain engine operating conditions and the potential for sulphur from the pilot fuel to affect catalyst performance.

Plasma reduction systems are currently being developed for methane slip emission reduction and could also potentially be used to reduce ammonia slip. These systems consist of a catalyst and an absorbent-free after-treatment technology that produces a non-thermal plasma containing a high density of electrons with high energy. This plasma is obtained from a dielectric barrier discharge generated by applying a high voltage between an arrangement of electrodes separated by a dielectric material layer. Exhaust pollutants are converted into harmless molecules via a chain of complex chemical kinetic reactions. This technology is still in the early stages of development, so it is too early to estimate power consumption and reduction rates.

The paper describes three potential configurations for emissions management onboard vessels. A simple option consists of a fully pressurized fuel tank with no BOG, one emission management technology for the fuel handling system and one after-treatment technology for NOx emissions, assuming that ammonia slip and N2O emissions are managed in the engine design.

For a design based on a semi-refrigerated fuel tank (at 6-8 bar) or a fully refrigerated fuel tank (at ambient pressure), BOG treatment will need to be added to the design. The most complex configuration would also add an after-treatment technology to manage engine exhaust emissions where ammonia NH3 slip or N2O emission levels are above acceptable limits. N2O emissions could be treated as part of an SCR in combination with NOx emissions if properly dimensioned.

The paper states that without collaboration, specific parts of the vessel design for handling emissions will be developed in isolation, and interconnected systems and technologies could end up unnecessarily oversized, inefficient, or costly. Additionally, the operational ammonia limits defined in existing class guidelines vary. “Coordinated alignment on thresholds for adequate risk management is required to secure standardisation and industry guidance.”