Selective Catalytic Reduction
and Direct Ammonia Injection
The global objective to achieve a greener environment has also focused on the effects that fossil fuel burning installations, and therefore internal combustion engines, have on this environment.
Applying Selective Catalytic Reduction (SCR) technology in the exhaust systems associated with internal combustion engines is a positive step in reducing their carbon footprint. The SCR unit requires ammonia for the reduction process.
The more common SCR systems used onboard ships use liquid urea as the injected agent. The required ammonia is then extracted before reaching the SCR unit. This system carries a penalty in that;
• A large urea storage tank is required; and
• The ammonia extraction requires a length of exhaust line after injection and before the SCR unit to allow the chemical reaction to take place in the presence of exhaust gas temperature.
There are many benefits associated with injecting ammonia directly into the exhaust gas. However, the most important one is the fact that this installation does not have such a space and weight impact as the urea injection system.
A space and weight constraint is the single most important aspect that opens up the SCR application regime to include the many smaller vessels in the shipping and marine industry that significantly contributes to the fuel based sources of NOx-emissions.
On smaller vessels such as fast passenger ferries, the weight is critical and also the space is much more limited, making this the only feasible solution for SCR technology application.
The Mecmar direct ammonia injection system has essentially the same components as the typical urea injection system, but with the main difference of having no need for a urea storage tank and a simpler injection and control system that allows a more compact design.
Mecmar recognized the fact that ammonia is an unpleasant and aggressive gas when inhaled. Human safety therefore has to play a more important role when using pure ammonia in the system, specifically on passenger ferries where free ammonia can cause panic and further complicate an emergency situation.
The Mecmar design has therefore been focused on minimizing the probability of encountering free ammonia inside the vessel.
The ammonia bottles are stored astern the ves- sel, in a trunk into which a dedicated ammonia bottle module can be lowered. The stern trunk is well removed from the passenger area and all distribution lines between the external storage and the exhaust system is contained in a gas tight cofferdam arrangement that is monitored for ammonia leakage.
The design of the trunk isolates it from the deck area. At deck level the trunk is sealed off by means of a Butterworth hatch. At the base the trunk is vented to sea via a pressure relief valve. Any free ammonia, in the unlikely event of a leak, will therefore be vented below sea level where it can rapidly be diluted and dispersed.
The ammonia container uses a standardised design. The standard modular design will, however be vessel speci c. The bottle volume, and hence the module size, is based on the operational pro le that determines the required volume of ammonia needed for the route.
For easy handling and installation, the ammo- nia module is cylindrical and independent of angular orientation inside the trunk. It gets connected to the distribution piping system onboard by means of a leak-free instantaneous coupling at the base of the module. The receptacle part of the coupling station is situated at the base of the trunk. The coupling happens automatically under gravity when the module is lowered and locked into place. When the module is locked in position, the seal will be leak tested prior to opening the ammonia bottle to allow ow into the piping system.
The system can be scaled up and may also be installed on larger vessels where the space and weight is critical or sensitive. The main components are the same as for the systems already described, with the absence of the Urea storage tank, but including the following:
• Ammonia Bottle(s) Module(s)
• Ammonia Module Trunk
Mecmar fully subscribes to this application and believes that it will become the system of choice to supply the Ammonia required by the SCR process as applied to Marine Systems.