SCR for NOx control is currently used on stationary
>diesel engines, and has been proposed for mobile applications. SCR
>uses ammonia as a NOx reducing agent. The ammonia is typically
>supplied by introducing a urea/water mixture into the exhaust upstream
>of the catalyst. The urea/water mixture is stored in a separate tank
>that must be periodically replenished. These systems can be very
>effective, with NOx reductions of 70 to 90%, and appear to be tolerant
>of current U.S. on-highway diesel fuel sulfur levels.
>However, there is concern that applying current SCR technology to
>highway vehicles will require use of catalyst formulations that are
>sensitive to sulfur, such as those employing platinum, to deal with
>the broad range of operating temperatures typical of highway diesel
>engines in use. There is also potential for formation of ammonia
>sulfate, which is undesirable because it is a component of fine PM28 >
>In addition, SCR systems bring some unique concerns. First, precise
>control of the quantity of urea injection into the exhaust,
>particularly during transient operation, is very critical. Injection
>of too large of a quantity of urea leads to a condition of ``ammonia
>slip'', whereby excess ammonia formation can lead to both direct
>ammonia emissions (with accompanying health and odor concerns) and
>oxidation of ammonia to produce (rather than reduce) NOx.
>Second, there are potential hurdles to overcome with respect to the
>need for frequent replenishment of the urea supply. This raises issues
>related to supply infrastructure, tampering, and the possibility of
>operating with the urea tank dry.
>
>Third, there may be modes of engine operation with substantial NOX
>generation in which SCR does not function well. Finally, there is
>concern that SCR systems may produce N2O, a gas that has been
>associated with greenhouse-effect emissions.
You raise some good points. One ptentail problem I see with SCR's is that require high tempatures to work best (in the 300 to 600c range) which means a higher exhaust system temp must be maintained even longer which can lead to brush fires on SUV's in dry grass. It kinda remains to be seen what the light diesel future will be several years from now. The main reason that there is ULSD today is because it enable the usage of SCR's to control NOx emissions. Furthermore, the level of sulpher are to be continually reduced in coming years until
2013 when it will be mostly gone from fuel. Starting in 2010 Detriot is going to have to make diesel controls tamper proof which will require a bit of technology to enforce and it is supposed to disable engine if tampered with. This will not go over well with diesel owned that want to ship their trucks. I did notice one "nic" side effect of thisin that I heat with oil and in years past when wind was right you could smell furnace when it was running. The last oil I bought was low sulpher and the furnace has little order to it now.----------------- TheSnoMan.com