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2.4 SCR reductants
Two most c ommonly used SCR reductants are anhydrous ammonia and aqueous ammonia or urea. Pure a nhydrous ammonia i s extremely toxic and d ifficult t o s afely s tore, b ut n eeds n o
further c onversion t o operate w ithin an S CR. It is typically fa vored b y larg e in dustrial S CR operators. A queous a mmonia must b e hydrolyzed in o rder to b e u sed, b ut i t i s s ubstantially
safer to store and transport than anhydrous ammonia. Urea is the safest to store, but requires conversion t o a mmonia through t hermal d ecomposition i n order to b e u sed a s a n e ffective
reductant [DieselNet 2005].
The aqueous ammonia is also known as AdBlue, Urea Water Solution UWS and Diesel Exhaust Fluid DEF depending on manufacturers. Eberhard 1994 introduced the use of solid urea but it
has received v ery l ittle a cceptance. Hoffman 1996 suggested an alternative t o u rea u sing carbamate salt such as ammonium carbamate, NH
2
COONH
4
. Kelly et al., 2006 reported various
amines evaluated as SCR reductants which could potentially be generated from diesel fuel and nitrogen.
Alkemade et al., 2006
reviewed the best reductant to be used for SCR system. While ammonia offer slightly better performance, its toxicity and handling difficulty remain the biggest concern.
Urea is not as effective but safer to handle which has made it the popular choice for automotive manufacturers. Sullivan et al., 2005 suggested in both form of ammonia it has to be extremely
pure d ue t o the fact th at impurities c an c log t he c atalyst. An SCR c atalyst t ypically re quires frequent cleaning even with pure reductants as the reductants can cake the inlet surface of the
catalyst w hen the exhaust g as s tream te mperature i s to o l ow f or th e S CR r eaction to o ccur. Research in to reductant t echnology is c ontinuing an d a w ide variety o f alternative re ductant
have been explored especially the one with wide availability and a distribution infrastructure in place. [US EPA 2006]
2.4.1 Aqueous Ammonia
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