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Selective catalytic reduction (SCR)
SCR is a process where a gaseous or liquid reductant (most commonly ammonia or urea) is added to the exhaust gas stream and is absorbed onto a catalyst. The reductant reacts with NOx in the exhaust gas to form water vapor (H20) and nitrogen gas (N2).
DET’s catalysts are manufactured to make an SCR system work effective with the minimum requirement of a reductant. Our current options include a vanadium-based catalyst, as well as a catalyst with zeolites in the washcoat. The performance of Fe- and Cu- exchanged zeolite urea-SCR is approximately equal to that of vanadium urea-SCR if the fraction of the NO2 is 20% to 50% of the total NOx.
Zeolites offer improved durability at high temperatures, including an ability to withstand extended operation above 650 °C and brief exposure to temperatures of 750 to 850 °C. Thermal durability is particularly important for mobile SCR applications that incorporate the use of the DET diesel particulate filter with forced regeneration. Our Vanadium catalysts are quickly deactivated at temperatures above 600 °C. The reliance on an increased fraction of NO2 within the total NOx to increase low temperature (sub-350 °C) performance of zeolite catalysts typically requires the use of an upstream oxidation catalyst for NO to NO2, which in turn requires the use of fuels that are virtually sulfur-free to allow low-temperature oxidation of NO to NO2 and also to prevent the formation of sulfate particulate matter. Zeolite catalysts combined with an upstream oxidation catalyst also have a smaller overall volume than vanadium catalysts.
For specific lower temperature applications and applications that use higher sulfur fuels and where space is a less critical concern (large sea going vessels, power plant or Rail high level sulfur part of their propulsion systems), our vanadium catalysts are generally preferred. Vanadium catalysts are much more common than zeolites for controlling NOx emissions from stationary power plants. SCR catalysts function well only within a relatively narrow temperature window, and process control systems must be programmed to keep the exhaust gas temperature within that range, again when used in Conjunction with our Smarttrap™ and controller we are able to achieve this.
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