Testing SCR in High Sulphur Application

NO(x) and SO(x) emissions from ship exhausts are limited by IMO (International Maritime-Organisation) ship pollution rules. NO(x) 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 NO(x) to elemental nitrogen. On the other hand SO(x) limits are requiring the use of lower sulphur level fuels or after-treatment systems, like scrubbers, to decrease SO(x) emissions. Scrubbers might become popular as they allow the use of inexpensive heavy fuel oil. The sulphur is usually considered as poison to catalysts. In SCR's a V2O5 catalyst has been widely employed due to its high activity and sulphur tolerance. Even so, sulphur related challenges do occur. At high temperatures the SO3 can result to an unwanted visible plume while at low temperatures the SO3 can react with the ammonia to form ammonium sulphates which deposit on and foul the catalyst. This brings certain requirements to the SCR optimization in high sulphur applications. Ships utilize large engines which require large catalyst volumes to deal with the emissions. Installations to large engine applications can be difficult and testing rather complex. Only minor (or none) tuning of the parameters is possible in real applications. In this study, a slipstream emission control test bench is utilized to test smaller SCR units with a proper exhaust gas from a medium speed diesel engine. The test bench has an advantage of easily tuned and controlled parameters (like temperature and exhaust flow). A heavy fuel oil with a sulphur content of 2.5% is utilized as test fuel. Two different SCR catalysts with a volume of 40 dm3 are tested using engine loads of 100%, 75% and 50%. In addition, different exhaust gas flow rates and temperatures, adjusted by the test bench, are utilized in testing. The test bench utilizes NO(x) sensors placed upstream and downstream of the test SCR reactor. In addition, the standard analyser to measure the NO(x) (chemiluminescence) was in use. FTIR was used to measure the NH3. Hydrocarbons, carbon monoxide and carbon dioxide were measure as well. The effect of SCR on particle emissions was studied by collecting particles on filters both before and after the catalyst. The particle filters were further analysed for sulphates and organic and elemental carbon. The results for both test catalysts show NO(x) conversions of near 80% at 100% load and even 95% at 75% load. The HC and PM emissions were also found to reduce over both catalysts. The organic carbon fraction of PM was reduced by the catalyst as well as the sulphates. While the organic carbon reduction can be explained by the oxidation over the catalyst the sulphates are believed to store in the catalyst. Overall the two catalysts showed nearly the same observations except in the case of a lower exhaust flow (i.e. lower space velocity) were the behaviours differed