112 Table 4.9.3 Comparison of the 4 gas with urea spray transient analysis.
Properties 4 gas with
4 SCR bricks Urea spray with
4 SCR bricks
1. NOx reduction time constant.
5 seconds 7.5 seconds
2. Ammonia storage time to onset of slip.
220 seconds 270 seconds
3. Time constant of rise in ammonia slip
159 seconds 239 seconds
4. Amount of ammonia reacted
7.02 grams 8.62 grams
5. Amount of ammonia stored
3.01 grams 2.50 grams
6. Amount of ammonia slipped
3.14 grams 1.93 grams
From the table 4.9.3, it was observed that the NOx reduction time constant for gas is slightly less than the urea spray case, but both times were attributable to the response time of MEXA analyser and should
be instantaneous. The ammonia storage, rise and slip times were different with 4 gas as compared with urea spray. In urea spray case droplet conversion is necessary while the 4 gas is readily available
for SCR reaction. The amount of stored and slipped are slightly higher with 4 gas case compared with the urea spray case.
4.10 Summary of the experimental and simulation results.
This investigation has compared the performance of SCR system with urea spray injection and ammonia gas. These studies involved the NO
2
NO ratio of approximately 6040 and shows all reactions with ammonia were complete after the two SCR bricks at a length of 182 mm.
To summarize the results the following concluding remarks could be made: • Some precaution and concern is needed when interpretations are made based on
measurements reading from a CLD based analyser involving NO and NO
2
.This is needed especially in the present of ammonia. The methodology suggested in this investigation however
enables amount consumed to be extracted. From known amounts of input from individual measurements upstream and downstream of the SCR, the data for NO, NO
2
and NH
3
can be extracted.
113 • In the urea spray studies, when the urea in the form of AdBlue solution was injected, about 200
ppm of NH
3
were released from the droplets of urea spray and reacting with NOx within the SCR bricks.
• From estimation, it was observed that in the range of 10 to 100 ppm of potential ammonia manage to pass through one SCR at a length of 91 mm in droplets form.
• From the CFD simulations using the porous medium approach and kinetics scheme published in the open literature, have shown some ability to predict the steady state tests investigated here.
• The model has been used to predict individual species along the SCR bricks length and some moderate agreement with the measurement has been achieved especially with the long bricks.
For short brick, space velocity was high and there were breakthrough of all species. • A transient analysis showed that the time constant for NOx reduction are quite close for gas and
spray but for the time constant for ammonia slip is higher in spray than gas. • NO
2
conversion efficiency was found higher than NO in all test cases which contradict with fast reaction kinetic.
114
CHAPTER 5: CONCLUSIONS AND FUTURE WORK
5.0 Conclusions and Future work: Introduction.
Despite the limitations of the MEXA gas analyser, and the need to derive a strategy for interpretation of the measurements made by it, a thorough investigation of SCR process has been
made in a specially designed exhaust system on an experimental test bed. The conclusion from the investigation include the development of the experimental techniques, the interference of NO
2
and NH
3
, the methodology, the transient response, the SCR and spray system performance and the significance of the main findings from the result chapter.
5.1 DPF-DOC Arrangement.
The DOC-DPF arrangement was tested for NO
2
to NO ratio to assist the SCR reactions. With this arrangement, the NO coming out from the engine was oxidized by the DOC but later reacted with
the trapped soot in the DPF, leaving less NO
2
out than before. With less NO
2
, the SCR reactions taking place were at the minimal level and leaving NOx out passing the system still at higher
readings. In the final arrangement used in this investigation, DPF-DOC was identified as the acceptable sequence upstream of the SCR. Utilizing this arrangement, higher NO
2
to NO ratio was achieved. In the literature, 50:50 NO
2
to NO ratio or higher was shown as the preferred condition to optimize the SCR reactions. Subsequently, in this investigation, a higher NO
2
to NO ratio was studied.
5.2 Experimental techniques.
The biggest obstacles in the beginning of this investigation were to establish suitable experimental techniques in order to complete the steady state study with the SCR system. Interferences within
the analysers were a particular problem because the continuous injection of the urea or ammonia gas was necessary in this investigation. The use of both urea spray and ammonia gas were
investigated. Interference and reaction between NO
2
and NH
3
on the NOx converter within the MEXA has resulted in significant loss of reliable directly measured test data.
This was overcome by a methodology that allowed all required parameters to be deduced. The spray used in this study was designed for heavy-duty application with the lowest possible setting utilized.
This caused intermittent problems especially with the low settings involved in light duty
