| Selective Catalytic Reduction |
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| CATEGORIES ABOUT SELECTIVE CATALYTIC REDUCTION | |
| pollution control technologies | |
| chemical engineering | |
| air pollution control systems | |
| nox control | |
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Specialty Catalytic Converter s are required to make an SCR system work, the current options being a Vanadium based catalyst, or a catalyst with Zeolites in the washcoat. In some jurisdictions use of a Vanadium based catalyst are frowned on ( California ) and the Zeolite type preferred, however Vanadium offers better performance. SCR catalysts function well only within a narrow temperature window, and for OEM installations the Engine Control Unit is specially programmed to keep the exhaust gas temperature in that range. REDUCTANTS There is considerable discussion about which reductant is best. While Ammonia offers slightly better performance, it is poisonous, and a difficult substance to handle safely. Urea is safer to handle, however not quite as effective, however to date Urea has been a more popular choice for engine manufacturers. In both cases the reductant must be extremely pure - if it isn't the impurities can clog the catalyst. Typically SCR catalysts require frequent cleaning even with pure reductants, as the reductant can cake the inlet surface of the catalyst while the exhaust gas stream temperature it too low for the SCR reaction to take place. Research into reductant technology is continuing. A wide variety of suggestions have been made for alternative reductants, especially ones that have a wide distribution infrastructure in place. Due to the lack of a distribution infrastructure for both Ammonia and Urea the United States Environmental Protection Agency has been reluctant to certify any diesel engines fitted with SCR. In Europe SCR is a common choice for NOx control technology by the engine manufacturers, and a variety of Ammonia and Urea brands are available, the most popular of which is AdBlue . AMMONIA SLIP A common problem with all SCR systems is Ammonia slip. The term describes tailpipe emissions of Ammonia that occur when # exhaust gas temperatures are too cold for the SCR Reaction to occur. # Electronic Controller feeds too much reductant into the exhaust gas stream for the amount of NOx . A variety of strategies have been developed to deal with Ammonia slip, including the fitting of extra catalysts after the SCR catalyst. TECHNICAL PROBLEMS In order to ensure that the SCR ( Selective Catalytic Reduction ) remains free from contaminant, correct materials of construction must be used for both storage and dispensing. Manufacturers of the SCR have specified that without using compatible materials of construction ions can be passed from the dispensing materials and into the porous head on the SCR. This can render the SCR ineffective and reduce it’s life expectancy from +500000 km to less than 200000 km. Equipment which may prove suitable for Urea solution is often not compatible with AdBlue and this common assumption has led to a number of systems failing prematurely. To ensure that the AdBlue is not affected by incorrect material specification operators should refer to the DIN 70070 standard for production of AdBlue and CEFIC quality control document AUS32. The biggest issue with SCR is the necessity to tune the SCR system to the engine operating cycle. This requires running the engine through a simulation of the operating cycle of the machine it will be fitted to. The simulation can be run on an Engine Dyno , or on an actual piece of equipment during its normal work day ( Datalogging ). Even at best this tends to be inaccurate, as no two operators will use the equipment in the same way, and even when used for the same general purposes (i.e. a truck delivering goods to stores in a city) small differences in the route such as hills, one way streets, amount unloaded, etc., can make the engine loads different enough that effectiveness of the system will suffer. Research into how to defeat this issue is ongoing. SEE ALSO EXTERNAL LINKS
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