SOURCE TESTING ASSOCIATION I Annual Guide 2023


The expanded positional uncertainty, Upos using a Student t-factor, tN-1;0,95,


expanded uncertainty is then Upos


, is then calculated


function of the number of grid points, N. For the most common power plant case of N = 20, the value of tN-1;0,95


essentially a coverage factor that accounts for the fi nite number of traverse points. If the positional uncertainty, Upos


= 2.093 * spos


. The t-factor is , is less than


50% of the maximum permissible uncertainty defi ned in EN 14181 [3], then the single measurement point closest to the average concentration can be selected, otherwise, grid sampling is required at that location. However, it needs to be recognised that the optimal sampling position may be different for different pollutants and a compromise sampling position may be required.


fi red power plant fi tted with FGD in the UK. Positional Uncertainty


When considering the standard uncertainty that is applicable for a single SRM measurement taken from within a ‘homogeneous’ concentration distribution, the various tolerances and approaches within the relevant standards, described in Section 2, can be considered. Table 1 gives an overview of the applicable tolerances within the available regulatory documents and standards.


When a tolerance is defi ned, specifying that the grid point concentrations must be within ± 10% of the mean concentration, for example, it is standard practice to assume a rectangular probability distribution, i.e., the true spatial variation has an equal probability of falling anywhere within the tolerance. The tolerance half-width (10% in this example) is then divided by √3 [=1.732] in order to obtain the standard uncertainty, giving ± 5.8% for the same example. The range of maximum relative standard uncertainty based upon tolerances is then ± 2.9% to ± 5.8%.


It should be noted that the criterion in the national document, MID EN 15259, is applicable only in situations where the homogeneity test would otherwise fail, as noted above.


can be obtained directly from duct survey test results. However, in many situations the spatial and temporal variations are of a similar magnitude (sgrid


The standard uncertainty defi ned in EN 15259 [spos Table 1: Summary of homogeneity criteria and tolerances Document ISO 10396


US EPA Method 7E


EN 15259 MID EN 15259


Pollutant and/or diluent


NOx, SO2, CO and O2 or CO2


Any one pollutant or diluent


Description Non-stratified Non-stratified


Minimally- stratified


Each relevant pollutant or a suitable surrogate Homogeneous


NOx, SO2, CO, TOC and O2


Homogeneous s2


only a positive square root can be evaluated. Table 1. Summary of homogeneity criteria and tolerances


≈ sref ) and spos Criterion ± 10% ± 5% ± 10% grid/s2 ref < 2.17 sgrid < 5%


Coverage factor


√3 √3 √3 - - Standard uncertainty ± 5.8% ± 2.9% ± 5.8% √(s2 grid - s2 ± 5.0%


In order to evaluate the spatial variation in all situations, both the reference and the grid concentrations are therefore first corrected to the standard oxygen reporting condition (O2rep), i.e., 6% O2 dry for solid fuel firing and 15% O2 dry for a Combined Cycle Gas Turbine (CCGT). The oxygen correction is given in (Annex C, EN 15259):


), i.e., 6% O2 dry


cgrid,i,rep = cgrid,i * (21 – O2rep) / (21 – O2grid,i) and cref,i,rep = cref,i * (21 – O2rep) / (21 – O2ref,i)


4


Since the oxygen is continuously measured at every point, this takes account of variations that are solely related to dilution since these are eliminated when later correcting emissions for compliance reporting.


The grid values are then finally corrected for temporal (t) variations using the results from the ref) = √(sgrid2 - sref2 )] cannot be determined since


is insuffi cient when considering homogeneity downstream of abatement systems, e.g., SO2


EN 15259 does not specify which pollutants are subject to a homogeneity survey and it is common practice to use a surrogate, such as O2 the O2


, to represent other pollutants. However,


homogeneity refl ects the overall combustion process and assessment is required for coal


again defi ned in the standard as a is 2.093. The


for solid fuel fi ring and 15% O2


dry for a Combined Cycle


Gas Turbine (CCGT). The oxygen correction is given in (Annex C, EN 15259):


cgrid,i,rep


and cref,i,rep


= cgrid,i * (21 – O2rep ) / (21 – O2grid,i ) = cref,i * (21 – O2rep ) / (21 – O2ref,i )


Since the oxygen is continuously measured at every point, this takes account of variations that are solely related to dilution since these are eliminated when later correcting emissions for compliance reporting.


The grid values are then fi nally corrected for temporal (t) variations using the results from the fi xed reference point, as follows, using the average of the oxygen corrected reference point measurements čref,rep


): cgrid,i,t = cgrid,i,rep * čref,rep / cref,i,rep


The standard deviation of the grid concentration distribution, having corrected for both temporal and oxygen variations, is then a direct measure of the standard uncertainty relating to the spatial variations alone (sgrid,t


). for the cases where this quantity could be derived.


and 22.0%, respectively. The positional uncertainty [spos √(sgrid2


corrected grid concentrations provides a reasonable measure


is 4.8% with a maximum of 11.3% and this is very similar to the spos


corrected standard deviation (sgrid,t results. This confirms that the distribution of fully


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.


In order to evaluate the spatial variation in all situations, both the reference and the grid concentrations are therefore fi rst corrected to the standard oxygen reporting condition (O2rep


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.


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 ,


this has an average value of 5.9% and a maximum value of 10.5% across the five out of nine cases where it was possible to calculate spos


and sref - sref2


. The average value of the oxygen and time ), across all of the cases,


units fitted with wet limestone-gypsum FGD absorbers, and five units without FGD, are summarised in Table 2. The relative standard deviations for both the grid and the reference points, based on raw concentration measurements, are presented, along with the positional uncertainty defined in EN 15259 (spos


Compliant duct survey test data for SO2 from 16 coal fired


Taking the case of coal fired FGD units with sampling at the Stack (the first row of data in Table 2), the average values of sgrid


are both 11.1% with maximum values of 20.0 =


)] takes temporal variations into account and


)


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