Selective Catalytic Reduction for Marine Exhaust NOx Reduction

In the process of selective catalytic reduction a catalyst converts nitrogen oxides into diatomic nitrogen (N2) and water (H2O). Both of these are harmless when released, and are safe for the environment. The process for making this conversion is to combine the NOx with a reductant, typically ammonia (NH3), which then comes in contact with the NOx to produce the reaction that separates the NOx into the N2 and H20.  A chemical reaction takes place that separates the toxic NOx into the N2 and H2O. SCR systems can reduce the amount of NOx released by 70 to 95%, depending on the application used and the type of operation it is used on.

So far only SCR has demonstrated its capability to achieve the target NOx reduction in a reliable and repeatable way as a standalone technology.  Other technologies may have the theoretical potential to realize Tier III levels. However, it is highly probable that in practice, combinations of two or more of these technologies are necessary in order to achieve compliance at optimum general performance and for minimum total lifecycle cost.


SCR technology is based on the reduction of nitrogen oxides by means of a reductant (typically ammonia, generated from appropriate pre-cursor species such as urea) at the surface of a catalyst. For this purpose, the exhaust gas is led through a reactor, containing a sufficiently large number of catalyst blocks for providing the catalyst surface area required. The temperature of the exhaust gas (and hence also the catalyst) is thereby subject to constraints both on the upper side (in order to avoid oxidation of the reductant) and the lower side (for preventing the formation of undesired by-products such as ammonium sulphates, which may subsequently clog and deactivate the catalyst). The latter is particularly an issue with fuels containing higher fractions of sulphur, such as those present in typical heavy fuel oil (HFO) qualities available today, which calls for even higher minimum temperatures in the catalyst. On 2-stroke engines, due to their high efficiency, the temperatures after the engine are generally too low for an SCR unit to work properly, which is why the reactor needs to be put on the high-pressure side, before the turbine. Integrating the SCR reactor before the turbine involves both challenges and opportunities. The presence of an element with non-negligible heat capacity in this region has some impact on the dynamic characteristics of the turbocharging system, which needs to be controlled through appropriate measures. On the other hand, the reactor can be designed in a more compact way compared to a location downstream of the turbine, due to the higher density of the exhaust gas.

Using modern selective catalytic reduction methods it is now possible to remove 70 to 95% of the nitrogen oxides that are released into the environment. The amount that is removed really depends on the SCR method that is used and the type of operation it is used with.

Marine fleets are now very focused on NOx reduction in their operations. It has helped to improve environmental conditions around their operations, and has helped to improve the health of people living in affected areas.


These regulations set the stage for considerable further reductions of the permissible NOx emissions – with the first step (Tier II) already in force since January 2011. Tier III will become applicable after 2021, but only inside specifically designated emission control areas (ECAs), whereas, outside of these ECAs, the Tier II regulation will continue to apply.  This new regulation poses enormous challenges to the engine developers: They have to optimize their products for both requirements, and need to provide technologies allowing for switching between the resulting Tier II and Tier III operating modes of the engines in operation, thereby achieving NOx emissions reduction of more than 76% for 2-stroke engines operating at rated speeds of less than 200 rpm, when switching from Tier II to Tier III mode. Moreover, the Tier III requirements are not limited to compliance with respect to the cycle-weighted NOx emissions, but also include an additional not-to- exceed clause stipulating that the NOx values at the individual points of the test cycle must not be more than 50% higher than the weighted average.


Besides being a single toxin, NOx combines with other substances and creates smog when it is exposed to sunlight. This creates a major health hazard for people that live in affected areas. NOx penetrates into the lung tissue and can cause permanent damage to lung tissue. Children, the elderly, people with asthma and other breathing difficulties, and people that work outside are especially susceptible to the negative health effects of NOx. The way to reduce that health risk is for companies to employ NOx reduction measures.

Nitrogen oxides also combine with water which produces nitric acid. When released from the atmosphere it is called acid rain, and results in significant long term damage to any structure that is repeatedly exposed to it.

In 1997 the Kyoto Protocol classified NOx as a greenhouse gas, and also called for a world wide effort to reduce the amount that was being released into the atmosphere. In the United States this is regulated by the Environmental Protection Agency (EPA). They have set levels which can legally be released by companies. If the company does not comply with the regulations they can have sanctions and significant fines leveled against them.