Selective Catalytic Reduction of NOx on Ships

Abstract

This thesis is dedicated to investigating the effects of installing a selective catalytic reduction (SCR) reactor upstream of a turbo charger on a two-stroke marine diesel engine, at which elevated pressure of up to 5 bar is present. The installation of an SCR reactor is of interest in order to comply with IMO Tier III regulation when sailing within NOx emission control areas.The first part of the thesis investigates the effect of increased pressure on the main SCR reaction, which was studied using a catalyst containing about 1 wt% V2O5/10wt%WO3/TiO2 supplied by Umicore Denmark ApS. The SCR reaction was studied across granulated catalysts (150-300 microns) in steady state experiments, for which the NOx conversion was found independent of the pressure when the residence time was kept constant. This shows that the kinetics of the SCR reaction were not affected by the increased pressure of up to 5 bar. NH3 temperature programmed desorption (TPD) experiments were also conducted for the granulated catalyst. The results showed that the adsorption of NH3 increased with pressure. The desorption proles were modeled using a Temkin isotherm (i.e.a coverage dependent desorption enthalpi). The steady state SCR experiments were also modeled, and the SCR reaction was found independent of the catalyst surface concentration of NH3, when the surface coverage was higher than 14%. This low surface coverage also explains why the SCR reaction was unaffected by the increased NH3 adsorption because the active part of the catalyst was already covered with NH3. At a marine engine, the catalyst will be used in the form of a monolith. Therefore, SCR experiments at elevated pressure using a monolith was also conducted. Using a constant residence time, the NOx conversion was found to be lowered by up to 5% points, at an increased pressure of 3 bar and above 250‰, due to increased mass transfer limitations. The change in mass transfer limitation is due to inverse proportionality between the binary diusion constant and the pressure.The second part of this thesis investigated how the oxidation of SO2 into SO3 was affected by increased pressure. SO2 oxidation is known to be slightly catalyzed by the vanadium-based SCR catalyst. When the residence time was kept constant, the conversion of SO2 was found independent of the pressure in the 1-4.5 bar range investigated. Hence the increased pressure does not aect the reaction kinetics. The reaction rate was found to be rst order in SO2 and zero order in SO3. The rate of SO2 oxidation was found to be promoted by NO2, probably due to a catalyzed reaction between NO2 and SO2.The last part of this thesis investigated the formation and condensation of ammonium sulfates. The sticky nature of ammonium bisulfate (ABS) was found to be prohibited by the presence of soot, probably because the soot absorbed ABS. The deactivation of SCR monoliths due to condensation of sulfates was also studied. The bulk condensation temperature was found to be best calculated by the expressionpresented by Muzio et al.[1]. Experiments where an SCR catalyst was exposedto SO3, H2O and NH3 showed the importance of taking pore condensation into account because the catalyst was deactivated above the bulk dew point temperature due to pore condensation. The deactivated catalyst was regenerated by elevating the temperature to 350-400‰, however, the regeneration was found insufficient at these temperatures.This thesis contributes with new insights into the possibility of using SCR reactors positioned upstream of a marine turbocharger, where increased pressure is also present. Because no direct pressure dependency was found for either the kinetics of the SCR reaction or the kinetics of the oxidation of SO2, the most dominating factor for deciding where to install the reactor is the correct temperature to ensure that ABS does not condense within the catalyst. Such temperatures will typically only be available upstream of the turbocharger at two-stroke marine diesel engines,and therefore also at an increased pressure of up to 5 bar.