Chapter 4 presents and discusses the results obtained from the ammonia gas and urea spray experiments
2.2 Diesel Oxidation Catalyst (DOC) and Diesel Particulate Filter (DPF)
Diesel Oxidation catalyst and particulate filter have been widely used for PM removal in diesel applications. DOC is one of the oldest technologies originated from the early two way catalyst for controlling CO, HC and PM. DOC works by oxidizing unburned species of fuel in the exhaust to h armless p roduct s uch a s C O2 and H2O. D OCs come in m etallic o r c eramic t hrough honeycomb substrates coated with an oxidizing catalyst such as platinum, palladium or both due to low temperature activity for HC conversions [MECA 2007]. Johnson T.V., (2010) highlighted the usage of DOC as being used in more vehicles than any other emission control device. Their critical p resent for t he p roper f unctioning of DPF a nd d eNOx s ystem was als o r eviewed an d continuously evolving.
Diesel Particulate Filters (DPF) are devices which remove diesel particulate matter (PM) or soot from the exhaust g as o f d iesel e ngines. It works by f orcing t he p articulate matter t o fl ow through a wall fl ow ceramic h oneycomb filter. The f ilters have alternate o pen a nd c losed channel as illustrated in figure 2.2. The exhaust gases contained PM or soot will enter the open channel, and gaseous CO2 and H2O will passes through the wall. Dry carbon soot particle size
9 larger than the monolith wall are trapped until the pressure drop across the DPF become too high.
Figure 2.2 Wall-Flow DPF (reproduced from Heck 2009)
However DPFs have limited capability and will eventually fully clog, therefore they need to be periodically regenerated by c ombustion o f t he t rapped P M. T he s oot r equires a m inimum temperature of 500OC for ignition in the absence of a catalyst which the engine exhaust does not frequently o r reliably re ach. A dditional s teps o r m echanism are n eeded t o c lean u p t he trapped PM, reduce the back pressure and restart the trapping cycle. (Heck 2009)
Konstandopoulos et al., (2000) suggested three method of facilitating the DPF regeneration in order t o maintain t he s atisfactory performance of DPF. They involved a ctive, e xternal an d passive regeneration. Th e active r egeneration i nvolved c hanging th e o peration o f th e d iesel engine w hile p assive approach involved m odification o f t he t rap c omposition. E xternal regeneration would be possible with the introduction of an external system to heat up the trap.
Magdi et al., (1999) evaluated the performance of DOCs and DPFs coupled with SCR system and reported exc ellent results for P M e mission. S CR w ith D OC c an ac hieved P M emission o f 0 .05 g/bhp-hr and combined PM, NOx and NHMC of less than 1.5 g/bhp-hr. DPF technology further reduced the PM emissions below 0.01 g/bhp-hr. Beeck et al., (2006) reviewed possible conflict from in tegration of S CR with DPF technologies b ased o n p ure t hermal an d c atalyzed DPF regeneration as s hown in fig ure 2 .2a. The b enefit o f F uel B orne Catalyst ( FBC) w as al so highlighted w hich p rovides fle xible t hermal management allo wing fas t an d c omplete DPF regeneration.
10 Figure 2.2a Possible architecture for NOx/PM control (Beeck et al. 2006)
Gurupatham et al., (2008) compared t he i ntegrated D OC-SCR-DPF, D OC-DPF-SCR an d c losed couple DOC-DPF-SCR as shown in figure 2.2b. The DPF forward system shows better PM active regeneration due to being closer to the engine and greater passive regeneration of DOC by NO2. However, DPF forward system disadvantage includes substantially delay of hot gas downstream reducing its SCR light off and the reduction of NO2 by SCR reactions because of soot oxidation by NO2 in the DPF. The close coupled DOC-DPF improved warm up time of DPF and SCR for cold start.
a.SCR Forward system
b.DPF forward system
c. DOC-DPF couple
Figure 2.2b Schematic of an advance diesel after treatment system architecture compared in Gurupatham et al., (2008)
Guo G. et al., (2010) introduced an SCR washcoat with wall flow on DPF called SCRF together with traditional SCR catalyst in light duty diesel application to perform NOx and PM reduction
11 simultaneously. However low washcoat loading on SCRF due to backpressure concern, cause the NOx reduction efficiency lower than SCRF placed upstream of SCR catalyst.
Figure 2.2c Advance diesel after treatment system with SCRF concepts (Guo et al., 2010)
Gieshoff et al., (2001) discovered that the SCR catalyst is affected by the unburned diesel fuel therefore s uggested a DOC b e placed upstream t o r emove u nburned h ydrocarbon. Koebel (2002) and Koebel (2001) also highlighted an increased NO2 level can be realized by placing an oxidation catalyst which promotes oxidation of NO. The oxidation catalyst placed upstream of the u rea i njection p oint decreased V 2O5 light o ff t emperature t o as lo w as 1 50OC. Th e disadvantages of this was an increased oxidation of sulphur dioxide and sulfate PM which result from using fuels of higher sulphur content and an increased of ammonium nitrate formation at temperature below 200OC.
Lambert et al., (2006) proposed to m ove the SCR upstream o f t he D PF to h andle c old s tart issues for p assenger c ar. Many a utomotive m anufacturers h ave a nnounced SCR sy stems for their latest SUVs and LDTs with undisclosed system configuration especially regarding the actual location of the SCR catalyst.
12 2.2.1 Effect of NO2/NO ratio on NOx conversion.
Chandler (2000) suggested that the composition of exh aust gases emission are mostly of NO (from 8 5-95%) an d s mall quantity o f N O2 (5-15%). It wa s r eported th at increasing t he N O2
fraction in the feed gas can im prove low temperature activity of the V2O5 as s hown in figure 2.2.1a
Figure 2.2.1a Effect of NO2/NO ratio on NOx conversion in V2O5/TiO2 catalyst (Chandler, 2000)
Gieshoff (2001) also reported similar performance with CU/ZSM-5 and other low temperature zeolite based catalysts. Narayanaswamy et al., (2008) simulated NO2/NO ratios up to three and implied good conversion over zeolite with excess NO2.
The significance of excess NO2 particularly over zeolite at lower temperature was discussed by Rahkamaa-Tolonen et al., (2005) who stated that excess NO2 will enhance the SCR reactions.
Takada et al., (2007) also s how go od N Ox c onversion w ith h igh N O2 level ( > 5 0%) in t heir modelling of reactions over zeolite at a temperature range from 500 to 550 K. Devadas et al., (2006) also supported excess NO2 particularly over zeolite an d reported b est performance a t NO2/NOx ratio of 75% which is much higher than the generally accepted optimum 50%.
13 However Cooper (2003) suggested that the amount on NO2 must be optimised by suitable sizing and formulation o f the o xidation c atalyst. I f t he NO2 level are too h igh, NOx c onversion efficiency decreases as shown by the red dash line and circles in figure 2.2.1b
Figure 2.2.1b Effect of NO2 from DOC on NOx conversion (Cooper 2003).
Cooper (2003) also suggested a large Pt loading Oxidation catalyst to increase the NO2/NO ratio to nearly 5 (over 80% NO2 in NOx) at around 280OC. As a result, the NOx conversion deteriorated significantly d ue t o d epletion o f am monia s ince t he required NO w as s ubstituted b y N O2 as shown in red line in figure 2.2.1b.