Impact of SCR on particle emissions in HFO application

Introduction NOx and SOx emissions from ship exhausts are limited by IMO (International Maritime Organization) ship pollution rules. NOx emission limits are set for diesel engines depending on the engine maximum operating speed. Limits are set globally (Tier I and Tier II) and in addition for emission control areas (Tier III). Tier III standard is dated to 2016 and is expected to require the use of emission control technologies. SCR (selective catalytic reduction) is an available technology capable of meeting this requirement. This technology uses a catalyst and ammonia for the reduction of NOx to elemental nitrogen. On the other hand SOx limits are requiring the use of lower sulphur level fuels or after-treatment systems, like scrubbers, to decrease SOx emissions while allowing the use of inexpensive heavy fuel oil (HFO). The PM is expected to decrease indirectly through the SOx limitations (by reduction of sulphate particle emissions), but at the moment, no direct PM limitations exist. Experimental In this study, a partial flow test bench was used to study (smaller) SCR units with a proper exhaust gas from a medium speed diesel engine. A heavy fuel oil with a sulphur content of 2.5% was utilized as test fuel. Two different SCRs, having differences in structure ("SCR A "- an extruded honeycomb type and "SCR B" - a packed bed reactor), were tested similarly using engine load of 75%. In addition, different exhaust temperatures were utilized in testing. The effect of SCR on particle emissions was studied by collecting particles on filters both before and after the catalyst according to the ISO 8178 method. The particle filters were further analysed for sulphates and organic and elemental carbon. In addition ELPI was employed to study the particle number size distribution. For NOx a chemiluminescence analyser was in use and FTIR was used to measure the NH3 and SO2. Results and discussion The results show that similar NOx reduction efficiencies can be achieved with both SCR units at exhaust temperature of 340-450°C. However, at the same time, the PM results were found to vary significantly. Higher PM levels were measured downstream of SCR B. Also, a clearly higher nanoparticle mode was found downstream of the SCR B than what was measured without any catalyst. Recently, the size distribution result downstream of the SCR A showed a decrease in nanoparticle concentration compared to the one measured without any SCR (Lehtoranta et al. 2015). Both size distributions are presented in Figure 1. The sulphate formation was also found to enhance with the SCR B indicating sulphur has a significant role in nanoparticle formation. For the SCR operation the exhaust temperature was found to be important as well as the soot accumulation in SCR. Figure 1 Particle number size distributions measured without any SCR and downstream of "SCR A" (honeycomb styled) and "SCR B" (packed bed reactor).