SOURCE TESTING ASSOCIATION I Annual Guide 2023
between individual burners is to be expected since, below a critical fi ring 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
thus illustrating the point that uneven CO production within the combustion system produces a higher variation at the Stack.
) of 4.7%. The equivalent NOx deviation was 2.5%
Higher uncertainties are applicable at sampling locations upstream of the Stack that are closer to the source of the emission variation, typically ±8.7% for abated processes which is equivalent to a tolerance of ±15% across the sampling plane. Lower uncertainties are applicable when sampling from the Stack from unabated processes, typically ±2.9%, equivalent to a tolerance of ±5% across the sampling plane.
coal fi red FGD units, ultra-low CO concentrations from CCGT units during normal operation or highly variable spatial and/or time-dependant variations in CO during abnormal plant operation. CO positional uncertainty is generally not assessed during normal plant operation but it is higher than for NOx
It is not generally possible to obtain a meaningful uncertainty assessment for ultra-low or highly time-dependent concentrations. For example, ultra-low SO2
due to less
homogeneity of CO within the combustion system (when CO is present at measurable concentrations). Since the Confi dence Interval for CO is only 10%, the positional uncertainty can represent a large proportion of the overall uncertainty budget and the required overall measurement uncertainty cannot then be achieved.
Figure 4 Positional NOx uncertainty assessment for CCGT plant CONCLUSIONS
Figure 4. Positional NOx
Positional uncertainty, related to a single SRM measurement taken at any random point within a sample plane that is deemed to be homogeneous according to EN 15259, can be characterised by the relative standard deviation of the grid concentrations obtained from a duct survey test, having fully corrected these for oxygen and temporal variations. This quantity is the most relevant for compliance purposes and is very similar in magnitude to the positional uncertainty obtained by direct calculation from the grid and reference point variations, as specified in EN 15259, which cannot be evaluated in all situations.
Conclusions
EN 15259 does not specify a maximum allowed deviation across the measurement plane but ISO 3096 specifies that all of the grid points must be within ±10% of the mean concentration. Assuming a coverage factor of Ö3 for this tolerance, based on an assumed rectangular probability distribution, this results in a standard uncertainty of ±5.8%.
Based on the duct survey data reviewed for multiple coal fired FGD units and multiple CCGT units, a standard uncertainty of ±5.8% is sufficient in most cases, including the measurement of abated species at coal fired plant, when sampling from the Stack location. Oxygen is not a suitable surrogate for abated acid gases such as SO2. However, SO2 is expected to be a suitable surrogate for other acid gases, such as HCl and HF, and possibly mercury. A standard uncertainty of ±5.8% is also sufficient to cover CO measurement at CCGT units, supplementary firing at CCGT units and also sampling from gas turbine exhaust duct locations during normal operation.
Positional uncertainty, related to a single SRM measurement taken at any random point within a sample plane that is deemed to be homogeneous according to EN 15259, can be characterised by the relative standard deviation of the grid concentrations obtained from a duct survey test, having fully corrected these for oxygen and temporal variations. This quantity is the most relevant for compliance purposes and is very similar in magnitude to the positional uncertainty obtained by direct calculation from the grid and reference point variations, as specifi ed in EN 15259, which cannot be evaluated in all situations. Revision of the standard is therefore recommended.
Higher uncertainties are applicable at sampling locations upstream of the Stack that are closer to the source of the emission variation, typically ±8.7% for abated processes which is equivalent
EN 15259 does not specify a maximum allowed deviation across the measurement plane but ISO 3096 specifi es that all of the grid points must be within ±10% of the mean concentration. Assuming a coverage factor of √3 for this tolerance, based on an assumed rectangular probability distribution, this results in a standard uncertainty of ±5.8%.
Based on the duct survey data reviewed for multiple coal fi red FGD units and multiple CCGT units, a standard uncertainty of ±5.8% is suffi cient in most cases, including the measurement of abated species at coal fi red plant, when sampling from the Stack location. Oxygen is not a suitable surrogate for abated acid gases such as SO2
. However, SO2 is expected to be a suitable surrogate for other
acid gases, such as HCl and HF, and possibly mercury. A standard uncertainty of ±5.8% is also suffi cient to cover CO measurement at CCGT units, supplementary fi ring at CCGT units and also sampling from gas turbine exhaust duct locations during normal operation.
uncertainty assessment for CCGT plant
The new plant sampling guidelines in EN 15259 do not guarantee complete homogeneity of concentration since this depends primarily on the proximity of the combustion or abatement system to the sampling location. It should be noted that it is usual practice to conduct only a single duct survey at base load operating conditions. Uncertainties related to changes in the concentration distribution as the plant load is varied, or year-on- year variation related to degradation of combustion or abatement systems, have therefore not been considered.
Finally, single point SRM sampling should always be from the same location when conducting a QAL2 calibration or an AST calibration check in order to minimise deviations caused by spatial concentration variation.
References
[1] Blank F T, Faniel N, Graham D P, VGB Statistical Guidelines for Emissions Compliance Evaluation, ENG/21/PSG/CT/2407/R Revision 1, December 2021.
[2] Graham D P, Jones H D, Spatial uncertainty contributions for emissions monitoring based on EN 15259, CEM 2022 Virtual Event, March 2022.
[3] EN 14181:2014, Stationary source emissions – Quality assurance of automated measuring systems, 2014.
[4] EN 15259:2007, Air quality - Measurement of stationary source emissions - Requirements for measurement sections and sites and for the measurement objective, plan and report, 2007.
[5] EN 14793:2017, Stationary source emissions – Demonstration of equivalence of an alternative method with a reference method, 2017.
[6] EN 13211:2001, Air quality - Stationary source emissions – Manual method of determination of the concentration of total mercury, 2011.
[7] ISO 10396:2007, Stationary source emissions – Sampling for the automated determination of gas emission concentrations for permanently installed monitoring systems.
[8] United States Environmental Protection Agency, Method 7E— Determination of nitrogen oxides emission from stationary sources (instrumental analyser procedure).
[9] Environment Agency, Method Implementation Document for EN 15259, Version 1.2 Jan 2012.
[10] Graham DP, Jones HD, EN15259 Duct Surveys: practical experience and limitations, CEM 2011: 10th International Conference and Exhibition on Emissions Monitoring, Prague, Czech Republic, 5 – 7 Oct 2011.
concentrations from
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