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4.1.1 Urea spray studies: Upstream Measurements 1 and 4 SCR bricks
At the location upstream of the SCR bricks with the MEXA in NH
3
NOx mode the following equation applies,
SUM upstream = [NH
3
+NO +NO
2
] Thus, for potential values,
Pot SUM upstream = [potential NH
3
+NO + NO
2
] Pot SUM is calculated from the potential ammonia and from NO and NO
2
measurement taken when the spray was off. The NOx upstream is measured without the spray injection and is assumed to
remain the same when the spray is injected due to the assumption that the gas phase reactions are negligible. The NO upstream can be measured, even with the presence of ammonia using the MEXA
analyser in NONO
2
mode. Similarly, the assumption is made that no gas phase reactions occur.
4.1.2 Urea spray studies: Downstream Measurements 1 and 4 SCR bricks
The measurement with the MEXA in NH
3
NOx mode downstream of the SCR bricks will give the SUM downstream, which effectively represent the NH
3
, NO and NO
2
coming out from the SCR bricks. Thus,
SUM downstream = [NH
3
+ NO + NO
2
] The NOx downstream can generally be measured only with the spray off, unless the ammonia slip is
very minimal. In the spray experiments, in all cases, the ammonia slip was significant. This is because the spray was designed for heavy duty vehicles and would not operate effectively at lower urea flow
setting. Therefore, all of the experiments with the spray were carried out under excess spray conditions. The NO downstream could be measured even in the presence of ammonia slip by using
MEXA in the NONO
2
mode. The NH
3
downstream reading is erroneous with the MEXA when the ammonia slip level is significantly above zero, which occurred in most of the experiments with spray.
However, [ SUM - NO ] = [ NH
3
+ NO
2
]. Thus, the two useful pieces of downstream information are the NO levels and [NH
3
+ NO
2
] levels and would be useful for CFD validation.
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4.1.3 Urea spray studies: Deduced value.
To deduce NH
3
, and NO
2
, it was necessary to use the difference between the potential SUM upstream and the SUM downstream in this case. Therefore, using the following equation
Pot SUM upstream – SUM downstream = [NH
3
+ NO + NO
2
] consumption in the catalyst. The implication of this is the assumption that SUM downstream is the true measurement of the
ammonia gas plus the NOx with no droplets or HNCO passing through the catalyst. This assumption may not be true for the 1 SCR, where ammonia in droplet form or possibly as HNCO at the catalyst
exit is unaccounted for. But, it should however be true for the 4 SCR bricks case. It is again reasonable to assume that these species can only be consumed if they react with one another, and
that they react on a mol NH
3
per mol NOx basis. It also neglects non mol to mol reactions and ammonia oxidation. There may be also additional reactions with urea by products that are
neglected. Therefore, ½ [NH
3
+ NO + NO
2
] consumed = NH
3
consumed = NOx consumed NO consumed can be found directly from
[NO upstream – NO downstream] Hence,
NO
2
consumed = [NOx consumed – NO consumed]
4.1.4 Urea sprays studies: Ammonia levels upstream of SCR bricks.
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