Aqueous Ammonia LITERATURE REVIEW

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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

Aqueous ammonia or water solutions urea remained the preferred choice for SCR application due to safe handling and commercial availability. AdBlue is a r egistered trademark for AUS32 18 aqueous U rea S olution 32.5 b y weight I t i s a s olution of h igh p urity urea 32.5in demineralised water 67.5 used as a supplementary operating fluid reducing agent in diesel powered vehicles using selective catalytic reduction SCR to improve exhaust emissions. AUS32 is primarily produced in Europe by BASF and AMI, however many other companies manufacture their own similar solution in varying quantities. [BASF 2003] AUS32 is carried onboard the vehicle in a tank separate to the fuel system and is sprayed into the engine exhaust gases in a special catalytic converter. A typical SCR system uses an amount of AUS32 equivalent to approximately 3 to 5 of the vehicle fuel consumption. In order to ensure effective working o f the SCR system, care must be taken to ensure purity of the catalyst and reducing agent. Any small contaminant can severely reduce the SCR system performance. The manufacturing quality control for AUS32 solutions is governed by DIN standard 70070 [Focus on Catalysts 8, 2, 2005] SCR systems u sing A dBlue a re currently fitted to many trucks and b uses m anufactured b y Mercedes Benz, Volvo Trucks, DAF Trucks and Iveco, however AdBlue usage as reducing agent is hindered b y its relative availability. S chemes a re u nderway i n E urope but to lesser extents i n Australasia and North America to improve the network distributors for AdBlue and other SCR additive. Internet based tool have been developed to map the locations of AUS32 filling stations reflecting plans for small scale use of SCR system in private vehicle as well as corporate fleets [Focus on Catalysts2, 3, 2006] . The t ypical aq ueous u rea s olutions fo r S CR system concentration at 3 2.5 fo rm an e utectic solution c haracterized b y t he lo west c rystallization p oint o f -11 O C. Th e eu tectic s olution i s advantages due to equal concentrations in liquid and solid phases during crystallization. With even p artial f reezing of the s olution in t he u rea t anks, c rystallization would not change t he concentration of the urea solution fed to the SCR system [BASF 2003]. 19 Figure 2.4.1a Urea solution freezing point [BASF 2003]. The 32.5 urea solution is a colourless liquid with a faint alkaline reaction. The freshly prepared solutions have a pH of 9 to 9.5. In solution the urea decomposes slowly in room temperature into am monia an d CO 2 . When th e s olution i s h eated, th e rate of d ecomposition in creases additionally producing biuret [BASF 2003]. Figure 2.4.1b Urea solution 32.5 decomposition [BASF 2003]. Fang and DaCosta 2003 highlighted possible side reactions from decomposition of urea in HDD application. Koebel et al., 2000 also presented problem related to urea during start up due to its freezing point at -11 O C which cause it to be heated if the surrounding temperature is lower. 20 Problem associated with urea spray have triggered for alternative solution to supply ammonia gas to the SCR system. Elmoe et al., 2006 suggested solid ammonia storage using Mg NH 3 6 Cl 2 which has high ammonia density very close to urea solution. Taturr et al., 2009 also provide alternative to urea with the use of ammonium carbamate [NH 2 -CO 2 -NH 4 ] in HD diesel which is capable to supply ammonia by heating at a capacity 3 to 4 times more than urea. Therefore, other alternatives than urea to supply ammonia to the SCR system are continuously explored.

2.4.2 Anhydrous ammonia.