73 Considering the MEXA analyser limitation in measuring the emission in this investigation, careful
interpretations are needed to analyze the results. Therefore a total of seven set of positive results have been identified and categorized according to the type of ammonia injected and the number of
SCR brick utilized. The remaining of the measurements was considered as loss and discarded from the analysis of the results. Two sets of result were obtained from urea spray test comprises of single
SCR brick and four SCR bricks. Four sets were from the 5 ammonia gas test which includes one through four bricks. Only one set of results were available from the 4 ammonia gas test.
3.11 Final Measurement Strategies.
As stated above due to the interference between NO
2
and NH
3
on the NOx converter erroneous measurements resulted when NH
3
was present in the gas stream. To circumvent this problem a measurement strategy was derived which enable measurements of all three gas, NO, NO
2
and NH
3
to be obtained upstream and downstream of the SCR. The EXSA was used to measure engine out emissions upstream of the DPFDOC. The MEXA was used upstream and downstream of the SCR.
The following measurement strategy was used to interpret the MEXA analyser readings. The NO and NO
2
measurements upstream of the SCR were made in the absence of ammonia and it was assumed that gas phase reactions prior to the SCRs were negligible. Therefore these readings were also valid
in the presence of ammonia. The SUM reading from the analyser in the NOxNH
3
mode in the presence of ammonia was valid, so the ammonia level could be found by manual subtraction.
In the presence of ammonia slip, downstream of the SCR brick only NO measurement is correct and reliable. However the readings of the SUM upstream minus the SUM downstream gives a measure
of NH
3
+ NOx consumed by the SCR bricks. Furthermore, an assumption can be made that NOx and ammonia are mainly consumed on a molmol basis during the SCR reactions.
Using this assumption neglects ammonia oxidation and the slow SCR reaction, but is valid as a first approximation for the temperature range of around 300
O
C in this investigation. Therefore, half of NH
3
+ NOx consumed is either ammonia or NOx consumed. NO consumed is available directly from
74 the difference between upstream and downstream measurements. Finally, NO
2
consumed is found from the difference between NOx consumed and NO consumed.
From the direct measurement of NO downstream, the slip of NH
3
+ NO
2
is found by subtraction of NO from the measurements of SUM NH
3
+ NO
2
+ NO. In the case of 4 and 5 ammonia gas in N
2
injection, the input level can be determined from a calibrated flow meter and the known exhaust mass flow rate. This information can be used to check upstream measurements. For urea spray
injection, the potential ammonia injected can be determined from the spray mass flow rate.
By comparison of this with the measured ammonia upstream of the SCR will indicate the mass of spray that has released its ammonia between the spray point and the emissions measurement
location. The magnitude of the potential SUM upstream potential NH
3
+ NO + NO
2
minus the measured SUM downstream should indicate the total consumption of all species NH
3
+ NO + NO
2
in the SCR bricks. This condition is valid with the assumption that no droplets pass through the SCRs.
The comparison between urea injection and NH
3
gas injection in the 1 SCR case would generally give some idea of what happened to the droplets within the SCR brick. Finally the tests were carried out
for 1, 2, 3 and 4 SCRs with ammonia gas injection but only 1 SCR and 4 SCR test cases were implemented using urea spray. All of the measurements were made as a function of ammonia level
input. The measurement capability of the MEXA analyzer in the investigation is summarized in table 3.11a.
75 Table 3.11a Measurement strategy when using Horiba MEXA 1170Nx Ammonia analyzer
NH
3
Gas case Sampling
Upstream SCR NH
3
Gas Case Sampling
Downstream SCR
Spray Case Sampling
Upstream SCR Spray Case
Sampling Downstream
SCR SUM =
NH
3
+NO+NO
2
OK OK
OK OK
NH
3
Subtraction SUM-NOx
OK If low NH
3
slip Subtraction
Potential SUM-NOx OK
If low NH
3
slip
NOx Measure with
gas off OK
If low NH
3
slip Measure upstream of spray
with spray off OK
If low NH
3
slip
NO Measure with
gas off OK
If low NH
3
slip Measure upstream of spray
with spray off OK
If low NH
3
slip
NO
2
Measure with gas off
OK If low NH
3
slip Measure upstream of spray
with spray off OK
If low NH
3
slip
Note: Downstream measurements with high NH
3
levels ideally need an ammonia scrubber which was not available for MEXA at the time of this study.
These restrictions, have resulted in different measurements mode either NH
3
NOx or NO
2
NO to be conducted in separate environments. After the final measurement strategies have been fully
develop the sampling locations of EXSA and MEXA analysers along the SCR exhaust system were finalized. The experiment was carried out according to the test matrix shown in table 3.11b.
Location
Measure
76 Table 3.11b Experimental Test Matrix with urea spray and NH
3
gas
Up DPF
1
st
module Spray
Gas 2
nd
module 3
rd
module SCR Bricks
length 4
th
module Test A
EXSA Capped
Spray capped
Lambda1 MEXA1
Single 1x 91 mm
Lambda2 MEXA2
Test B EXSA
capped Spray
capped Lambda1
MEXA1 Quad 4x
364 mm Lambda2
MEXA2 Test 1
EXSA NH
3
gas Capped
capped Lambda1
MEXA1 Single 1x
91 mm Lambda2
MEXA2 Test 2
EXSA NH
3
gas Capped
capped Lambda1
MEXA1 Double 2x
182 mm Lambda2
MEXA2 Test 3
EXSA NH
3
gas Capped
capped Lambda1
MEXA1 Triple 3x
273 mm Lambda2
MEXA2 Test 4
EXSA NH
3
gas Capped
capped Lambda1
MEXA1 Quad 4x
364 mm Lambda2
MEXA2
3.12 Summary of Final Experimental Procedures.
