109
4.9.2 Transient analysis of 4 SCR brick with urea spray
The transient analysis was performed on the 4 SCR with urea spray in a similar way to the transient analysis for 4 SCR with 4 NH
3
gas. An example of a typical transient observation with urea spray and 4 SCR is shown in figure 4.9.2. In this case, the spray setting was adjusted and the incoming ammonia was
estimated at around 929, then 857 and then 785 ppm. The spray was potentially capable of supplying more ammonia than this but some remained as urea droplets and was not available for reaction. From
the figure shown, the incoming NOx was 539 ppm throughout and this was fully reacted as there was no NOx slip detected at the exit of the SCR. The trace up to 956 seconds can be divided into three different
regions.
Figure 4.9.2 Transient Analysis for urea spray with 4 SCR
Region A, represents the overall NH
3
reacted. The area under region A starts from the first urea spray injection and the spray rate changed twice until time reached 956 seconds. In this region, all of the
incoming NOx at 539 ppm was reacted. The ammonia slip was observed just after the 270 seconds. The total NH
3
reacted in this case can be found using;
mNOx 539
150 270
NH
3
246
100 200
300 400
500 600
700 800
900 1000
200 400
600 800
1000 1200
p p
m
timesec Transient Analysis 4 SCR with Spray
Refn 230708bL1
NH3 mNOx
Spray trigger
956 509
246
A - NH
3
Reacted
B - NH
3
Stored C - NH
3
Slipped
30 spray setting ms
34 32
929 857
785
390 318
110 Area for region A = Total NH
3
reacted = 539 ppm x 956 seconds
= 515 284 ppm.secs ≈ 8.62 grams
Area for region B + C = 390 x 150 + 318270-150 + 246956-270
= 265 416 ppm.secs ≈ 4.44 grams
Region B represents the amount of ammonia being stored by the 4 SCR bricks. The ammonia slip started rising around 270 seconds. It took another 686 seconds to reach the ammonia slip steady value of 246
ppm. Region C represents the ammonia slipped at the exit of the 4 SCR bricks. The steady value of 246 ppm is
reached at about 956 seconds. The amount of ammonia slippage can be found by integration of the area under ammonia slip curve between 270 to 956 seconds. This is obtained using numerical integration in
excel spreadsheet and converted to mass as shown in appendix 4.9.2a. The amount of ammonia slip was calculated and found to be 1.93 grams.
Similarly to the 4 ammonia gas study, at 270 seconds ammonia storage is approaching its maximum and ammonia desorption started. This is clearly shown by the exponential rise in the ammonia slip curve
in figure 4.9.2. Finally the ammonia stored under Region B = Area B+C – Area C
= 4.44 – 1.93 grams Ammonia Stored in the 4 SCR bricks
≈ 2.5 grams
The total ammonia reacted in the SCR system was found to be 8.62 grams, 2.5 grams was stored in the bricks while 1.93 grams slip at the back.
111
4.9.2.1 Time constants for urea spray.
The time constant for NOx reduction in this case is defined as the time where the concentration has fallen 0.3679 from its initial value.
[C] = 0.3679 [C]
o
[C] = 0.3679 [539 ppm] = 198.3 ppm Time 198.3 ppm = 7.5 seconds
Therefore time constant for NOx reduction is 7.5 seconds. However, this time constant is dominated by the time response from the MEXA analyzer since the NOx and NH
3
reaction in the SCR is occurring at a much faster rate.
The time constant for the ammonia rise is the time from where the ammonia slip just begins until 0.632 of its final steady value as described in the rising curve analysis. Therefore the time constant for
ammonia rise in this case is as follows:
NH
3
begin slip after 270 seconds 0.632 x 246 ppm = 155.5 ppm509 seconds
Time constant for ammonia rise = 509 – 270 = 239 seconds.
4.9.3 Comparison of the urea spray and ammonia gas transients.
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