85
4.2.2 Ammonia gas studies: downstream measurements. 1 and 4 SCR bricks
The SUM measurements downstream of the SCR bricks were also valid using the MEXA in NOxNH
3
mode similar to the upstream measurements. The SUM measurement downstream is given as the equation below:
SUM downstream = [NH
3
+NO +NO
2
] downstream
NOx measurements downstream are only valid with no ammonia gas injection present or with very minimal ammonia slip. If the latter was true, for cases with more than 1 SCR bricks, then the
measured NOx level downstream was additional information available in these cases. The NO measurements downstream were always valid using the MEXA in the NONO
2
mode even with the presence of ammonia slip. The NO
2
values downstream with gas off can be deducted from NOx-NO and it is also available for cases where gas injection dosing was very low and where ammonia slip
was minimal. The NH
3
downstream measurements were erroneous with the MEXA at any ammonia levels significantly above zero. However, the following equation is true:
[SUM-NO] = [NH
3
+NO
2
].
Therefore the two useful pieces of downstream data are the NO levels and [NH
3
+NO
2
] levels and these could be used for CFD model validation. The NO
2
levels downstream are also available for the low dose cases where approximately zero ammonia slips occurred.
4.2.3 Ammonia gas studies: Deduced values.
The gaseous consumption in the catalyst could be easily obtained via deductions by the following equation:
SUM upstream - SUM downstream = [NH
3
+ NO + NO
2
] consumption in the catalyst.
86 Reasonably, it is safe to assume that these species can only be consumed in the SCR if NOx reacts
with NH
3
. Furthermore, it is reasonable to assume that a mol of NH
3
reacts with a mol of NOx thus neglecting non-mol to mol reactions and ammonia oxidation.
Consequently,
½ [NH
3
+NO +NO
2
] consumed ≈ NH
3
consumed ≈ NOx consumed
Therefore, the NO consumed can be found directly from,
[NO upstream-NO downstream]
and similarly the NO
2
consumed from,
NO
2
consumed = [NOx consumed-NO consumed]
4.2.4 Ammonia gas studies: Ammonia levels
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