five out of nine cases where it was possible to calculate spos. The average value of the oxygen and time corrected standard deviation (sgrid,t), across all of the cases, is 4.8% with a maximum of 11.3% and this is very similar to the spos results. This confirms that the distribution of fully corrected grid concentrations provides a reasonable measure of the positional uncertainty. If the maximum value of sgrid,t is removed as an outlier, the next highest value of 6.7% is much closer to the average value. The average and maximum SO2 concentrations for this case are 213 and 331 mg/m3, respectively.
These results for the best (Stack) sampling location suggest that the maximum standard uncertainty of ±5.8%, based on ISO 10396, is appropriate for abated acid gases, although the maximum uncertainties seen in practice can be higher. Even higher relative deviations, of up to 50%, were apparent when the outlet SO2 concentration was very low (< 21 mg/m3 or 7 ppm). However, these cases have been excluded from the current analysis, even though the homogeneity test was passed, since it is not possible to determine how much of the variation is related to the measurement uncertainty of the analyser at such low concentration levels. It follows that, as the monitoring technology improves, EN 15259 testing should be repeated.
Annual Guide 2023 I SOURCE TESTING ASSOCIATION Figure 3 Positional SO2 uncertainty assessment for coal fired plant with FGD
Table 2: Summary of SO2 duct survey results from coal fired power station SO2
Table 2. Summary of SO2
Plant type
Coal with FGD
Coal with FGD
Coal with FGD
Coal without FGD
SRM Stack
FGD Outlet
FGD Inlet
Stack Inlet
No. 9 7 15 5 sgrid sref spos sgrid,t
duct survey results from coal fi red power station RAW CONCENTRATION
Location units Mean Max Mean Max Mean Max # Mean Max Mean Max 11.1% 20.0% 11.1% 22.0% 5.9% 10.5% 5 4.8% 11.3% 213 17.2% 31.1% 17.5% 32.4% 8.1% 13.7% 3 7.4% 14.7% 298 7.9% 51.2% 8.7% 53.6% 1.7% 3.0% 9 3.2% 9.2% 1492 3.7% 6.0% 3.3% 5.4% 1.8% 2.6% 5 2.6% 3.6% 919 331 715 3638 1071
The results for the next case in Table 2 contain a further seven coal-fired units with sampling further upstream, between the FGD Outlet and the Stack location, and the positional uncertainties are unsurprisingly somewhat higher than the first case considered. The average value of spos increases from 5.9% to 8.1%, as shown in Table 2. Higher values can be obtained in these non-ideal sampling locations, as illustrated by the two highest values in this data set of 12.3% and 14.7%. A standard uncertainty of ± 8.7%, equivalent to a tolerance of ± 15% across the sample plane, is more appropriate in these situations.
The remaining two cases in Table 2 are related to higher absolute SO2 concentrations at the FGD Inlet (measured at 15 units) or the Stack Inlet at the five unabated units. The uncertainties are lower, as expected, being of the order of 2 to 3% of the higher mean concentration.
A selection of results from multiple coal fired sites with FGD is presented graphically in Figure 3. It can seen that spos and sgrid,t are in reasonable agreement when it is possible to evaluate spos and, in two cases where there is significant disagreement, sgrid,t is the lower and more representative value.
in Table 2. Higher values can be obtained in these non-ideal sampling locations, as illustrated by the two highest values in this data set of 12.3% and 14.7%. A standard uncertainty of ± 8.7%, equivalent to a tolerance of ± 15% across the sample plane, is more appropriate in these situations.
The results for the next case in Table 2 contain a further seven coal-fi red units with sampling further upstream, between the FGD Outlet and the Stack location, and the positional uncertainties are unsurprisingly somewhat higher than the fi rst case considered. The average value of spos
increases from 5.9% to 8.1%, as shown
The remaining two cases in Table 2 are related to higher absolute SO2
concentrations at the FGD Inlet (measured at 15 units) or the Stack Inlet at the fi ve unabated units. The uncertainties are lower, as expected, being of the order of 2 to 3% of the higher mean concentration.
in reasonable agreement when it is possible to evaluate spos in two cases where there is signifi cant disagreement, sgrid,t
lower and more representative value.
A selection of results from multiple coal fi red sites with FGD is presented graphically in Figure 3. It can seen that spos
and sgrid,t are and, is the
CORRECTED FOR O2 and t SO2 grid mg/m3
such as HCl and HF, which are also abated by absorption. This may also be the case for mercury since the oxidised portion of the mercury in the fl ue gas is absorbed in the FGD unit.
duct survey results from coal fi red power
Plant type
Coal with FGD
Coal with FGD
Coal with FGD
Coal without FGD
SRM Location Stack
FGD Outlet
FGD Inlet
Stack Inlet
No. 9 7 15 5 sgrid
RAW CONCENTRATION sref
spos stations
Table 3: Summary of O2 duct survey results from coal fired power stations O2
Although variations in oxygen are largely taken into account in the above analysis, the variations in the oxygen concentration are shown separately in Table 3. For the coal fired units, the relative positional uncertainty is about 2 to 4%, on average, with a maximum of about 6%. This is much lower than for Stack SO2 for coal fired units with FGD. Variation in O2 at the Stack location is governed by the residence time and fluid mixing between the Boiler and the Stack, in addition to air in-leakage. Variation in SO2 at the Stack location is governed by the performance of the FGD absorber along with the lower residence time and reduced fluid mixing between the FGD Outlet and the Stack. It follows that O2 is not a good surrogate for SO2 in these processes but SO2 is expected to be an acceptable surrogate for other acid gases, such as HCl and HF, which are also abated by absorption. This may also be the case for mercury since the oxidised portion of the mercury in the flue gas is absorbed in the FGD unit. Table 3: Summary of O2
CORRECTED FOR t sgrid,t
O2 grid %vol units Mean Max Mean Max Mean Max # Mean Max Mean Max 3.5% 8.8% 3.4% 8.4% 1.4% 2.4% 7 2.1% 3.6% 6.8% 13.5% 5.6% 10.6% 3.7% 7.7% 7 3.0% 5.5% 4.5% 13.2% 4.5% 12.3% 1.9% 5.5% 11 3.0% 5.1% 4.8% 10.6% 3.9% 8.8% 2.9% 4.8% 4 3.7% 5.2% 7.3 6.7 7.3 7.6 8.3 9.4 8.8 8.2
cases where this quantity could be derived. Also presented are the relative standard deviation (sgrid,t
coal fi red FGD units and 5 unabated coal fi red units are summarised in Table 4. As before, the relative standard deviations of both the grid and the reference points, based on raw concentration measurements, are presented, along with the positional uncertainty defi ned in EN 15259 (spos
Compliant duct survey test data for NOx ) for the
concentrations, for all of the cases, along with the mean NOx concentrations.
Table 4. Summary of NOx
Plant type
Coal with FGD
Coal with FGD
Coal with FGD
Coal without FGD
CCGT SRM Location Stack
FGD Outlet
FGD Inlet
Stack Inlet Stack No. 9 7 0 5 30
Table 4 Summary of NOx duct survey results from power stations NOx
sgrid sref spos
along with the positional uncertainty defined in EN 15259 (spos) for the cases where this quantity could be derived. Also presented are the relative standard deviation (sgrid,t) of the fully corrected grid concentrations, for all of the cases, along with the mean NOx concentrations.
) of the fully corrected grid
duct survey results from power stations RAW CONCENTRATION
sgrid,t
CORRECTED FOR O2 and t NOx grid mg/m3
units Mean Max Mean Max Mean Max # Mean Max Mean Max 3.3% 11.5% 3.2% 11.2% 0.8% 1.9% 7 2.2% 3.7% 5.3% 7.7% 6.6% 11.7% 2.6% 3.1% 2 3.9% 9.8% - - - - 6.0% 19.1% 5.8% 19.8% 2.0% 2.4% 3 2.5% 3.9% 3.7% 21.2% 3.5% 21.1% 2.1% 6.8% 17 2.1% 4.7% 477
405 Not
measured 530
34 537 491
Compliant duct survey test data for NOx from the same 16 coal fired FGD units and 5 unabated coal fired units are summarised in Table 4. As before, the relative standard deviations of both the grid and the reference points, based on raw concentration measurements, are presented,
from the same 16
676 69 None of the these units are fi tted with NOx
same basis, the maximum uncertainty value is 9.8% for a single coal fi red unit when sampling upstream of the stack but, once this is removed as an outlier, there are several sgrid,t
might be expected, the average relative standard uncertainty is similar in all cases and this is about 2.5% (based on sgrid,t
5% amongst the other coal fi red units.
None of the these units are fitted with NOx abatement so, as might be expected, the average relative standard uncertainty is similar in all cases and this is about 2.5% (based on sgrid,t). On the same basis, the maximum uncertainty value is 9.8% for a single coal fired unit when sampling upstream of the stack but, once this is removed as an outlier, there are several sgrid,t values at circa 5% amongst the other coal fired units.
abatement so, as ). On the
The final case in Table 4 summarises data from an additional 34 CCGT units. The maximum CCGT uncertainty of 4.7% corresponds with the only GT exhaust duct location within the data set and the next highest CCGT value of 4.3% (not tabulated) is from a supplementary fired Heat Recovery Steam Generator (HRSG) fitted with grid burners. In both of these situations, the homogeneity of NOx concentration is expected to be worse than for the standard CCGT case.
values at circa
Figure 3 Positional SO2 uncertainty assessment for coal fired plant with FGD with FGD
Figure 3. Positional SO2 uncertainty assessment for coal fi red plant
Although variations in oxygen are largely taken into account in the above analysis, the variations in the oxygen concentration are shown separately in Table 3. For the coal fired units, the relative positional uncertainty is about 2 to 4%, on average, with a maximum of about 6%. This is much lower than for Stack SO2 for coal fired units with FGD. Variation in O2 at the Stack location is governed by the residence time and fluid mixing between the Boiler and the Stack, in addition to air in-leakage. Variation in SO2 at the Stack location is governed by the performance of the FGD absorber along with the lower residence time and reduced fluid mixing between the FGD Outlet and the Stack. It follows that O2 is not a good surrogate for SO2 in these processes but SO2 is expected to be an acceptable surrogate for other acid gases, such as HCl and HF, which are also abated by absorption. This may also be the case for mercury since the oxidised portion of the mercury in the flue gas is absorbed in the FGD unit.
Although variations in oxygen are largely taken into account in the above analysis, the variations in the oxygen concentration are shown separately in Table 3. For the coal fi red units, the relative positional uncertainty is about 2 to 4%, on average, with
a maximum of about 6%. This is much lower than for Stack SO2 for coal fi red units with FGD. Variation in O2
Table 3: Summary of O2 duct survey results from coal fired power stations O2
Plant type
Coal with FGD
Coal with FGD
Coal with FGD
Coal without FGD
is expected to be an acceptable surrogate for other acid gases, Stack
is not a good surrogate for SO2 SRM No. 9
FGD Outlet
FGD Inlet
Stack Inlet
7 15 5 sgrid Location units Mean Max Mean Max Mean Max # Mean Max Mean Max in these processes but SO2 spos 3.5% 8.8% 3.4% 8.4% 1.4% 2.4% 7 2.1% 3.6% 6.8% 13.5% 5.6% 10.6% 3.7% 7.7% 7 3.0% 5.5% 4.5% 13.2% 4.5% 12.3% 1.9% 5.5% 11 3.0% 5.1% 4.8% 10.6% 3.9% 8.8% 2.9% 4.8% 4 3.7% 5.2% 7.3 6.7 7.3 7.6 8.3 9.4 8.8 8.2 Compliant duct survey test data for NOx from the same 16 coal fired FGD units and 5 unabated 5
at the Stack location is governed by the performance of the FGD absorber along with the lower residence time and reduced fl uid mixing between the FGD Outlet and the Stack. It follows that O2
is governed by the residence time and fl uid mixing between the Boiler and the Stack, in addition to air in-leakage. Variation in SO2
RAW CONCENTRATION sref
CORRECTED FOR t sgrid,t
O2 grid %vol at the Stack location for the standard CCGT case.
The fi nal case in Table 4 summarises data from an additional 30 CCGT units. The maximum CCGT uncertainty of 4.7% corresponds with the only GT exhaust duct location within the data set and the next highest CCGT value of 4.3% (not tabulated) is from a supplementary fi red Heat Recovery Steam Generator (HRSG) fi tted with grid burners. In both of these situations, the homogeneity of NOx
concentration is expected to be worse than
reasonable agreement when it is possible to evaluate spos results are bounded by (lower than) 5%.
and sgrid,t are in
A selection of results from multiple CCGT sites is presented graphically in Figure 4. It can seen that spos
A selection of results from multiple CCGT sites is presented graphically in Figure 4. It can seen that spos and sgrid,t are in reasonable agreement when it is possible to evaluate spos and the results are bounded by (lower than) 5%.
In most cases, it has not been possible to obtain meaningful results for CO since emission concentrations tend to be either close to zero or high and variable. During normal operation, the behaviour of unabated CO emissions is expected to be close to that of unabated NOx since both species are formed within the combustion system. However, greater variation in CO formation between individual burners is to be expected since, below a critical firing temperature, CO increases exponentially. For example, one of the CCGT units in this study was run at low load with an uneven outlet CO distribution, producing a fully corrected Stack CO concentration of 41 mg/m3 and a relative standard deviation (sgrid,t) of 4.7%. The equivalent NOx deviation was 2.5% thus illustrating the point that uneven CO production within the combustion system produces a higher variation at the Stack.
and the combustion system. However, greater variation in CO formation
In most cases, it has not been possible to obtain meaningful results for CO since emission concentrations tend to be either close to zero or high and variable. During normal operation, the behaviour of unabated CO emissions is expected to be close to that of unabated NOx
since both species are formed within the
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