Selective Catalytic Reduction of NOx on ships 

The project verified the promising potential of maritime SCR installed at the high-pressure location (before turbocharger) at large 2 stroke ships through the following learnings:

• The kinetics of the SCR reaction was investigated at elevated pressure up to 5 bar in both a lab scale reactor using crushed catalyst and a real monolith catalyst element in a bench scale reactor. The results showed that the kinetics of the reaction over the crushed catalyst was insensitive to pressure. The rate expression determined at atmospheric pressure can thus be used also at elevated pressure. However, for the monolith the conversion of NOx decreased slightly with increasing pressure (at constant residence time in the catalyst) due to increased diffusion limitations in the gas film and in the catalyst pore system. Nevertheless, elevated pressure leads to a lower volumetric flow rate and so the overall effect of increased pressure is an increased conversion of NOx (for a constant mass flow rate of gas). The monolith reactor including diffusion limitations was modelled and good agreement between model and data was observed. A design tool for reactor design at elevated pressure has thus been developed.
• The kinetics of the undesired SO2 to SO3 oxidation reaction was investigated at elevated pressure up to 4.5 bar using a real monolith catalyst element in a bench scale reactor. Also for this reaction, the results showed that the kinetics of the reaction was insensitive to pressure and was close to first order in SO2 and zero order in SO3. The presence of NO2 showed a promoting effect at elevated pressure, but in practice this is not expected to lead to higher SO3 formation since any NO2 is quickly removed by the fast SCR reaction.
• The condensation potential/risk of ABS and ammonium sulphate (AS) was investigated with condensation probes at Alfa Laval’s testcenter in Aalborg at a full-scale setup, and at DTU using a bench scale reactor and a lab-scale soot blowing test setup. The soot blowing setup generated a very interesting finding with respect to significantly less sticky ammonium bisulphate fouling when mixed with engine soot, making removal with conventional soot blowing possible. These observations correlate with findings at Alfa Laval’s test center that deposits could be easily removed. Monolith tests showed catalyst deactivation by ABS pore condensation, with only partial regeneration possible at elevated temperatures. This shows the importance of operating the catalyst at a position where sufficiently high temperature is available, i.e. upstream the turbo charger, to avoid any ABS deposition in the catalyst.
• A Total Cost of Ownership (TCO) model comparing the capital and operating cost of low pressure SCR, high pressure SCR and the EGR cleaning technologies has been developed. It can be used to support the decision process regarding cleaning technology depending on the vessel size and sailing pattern/time in NECA zones. Initial simulations with the model indicate that high pressure SCR can be completive against EGR for sailing patterns with both shorter and longer time spent in NECA zones. Low pressure SCR suffered from higher operating cost, due to the need for exhaust reheating to prevent deactivation. The model also illustrates the importance of the precision of capital cost estimates for the competing technologies – CAPEX constitute approximately 10 times the yearly OPEX.