|Abstract Title:||The Importance of Measurement Uncertainty for Emissions Compliance|
|Presenter Name:||Mr Frans Blank|
|Co-authors:||Mr David Graham|
|Session Choice:||Standards and Quality|
Abstract Information :
The European Union's Industrial Emissions Directive (IED) specifies Emission Limit Values (ELVs) and emissions monitoring requirements for NOx, SO2, Dust and CO for Large Combustion Plants (LCPs). Continuous emission monitoring systems must be used to measure these pollutants for large plants and compliance is assessed against Hourly, Daily and Monthly emissions averages.
The Best Available Techniques (BAT) Reference document, the LCP BREF, defines more stringent emissions requirements with compliance assessed against Daily and Annual averages, applicable to existing plants from 2021. A wider range of species is regulated at substantially lower ELVs so the instrumental capability of the reference methods at these lower concentrations is investigated.
The IED specifies that the 95% Confidence Interval of a single measurement (Hourly average) shall not exceed a defined percentage of the Daily ELV. The Quality Assurance (QA) standards applicable to the Automated Measuring Systems (AMS) that are used for continuous emissions monitoring are also based on this Confidence Interval. AMS are subject to the QA standard EN 14181 which defines three Quality Assurance Levels - QAL1 (certification), QAL2 (calibration) and QAL3 (control) - along with an Annual Surveillance Test (AST calibration check).
The IED requires that the validated Hourly and Daily averages used for compliance assessment are determined by subtraction of a specified Confidence Interval from the reportable Hourly average concentration in order to allow for measurement uncertainty. Compliance is then assessed based on these validated averages for Hourly, Daily and Monthly ELVs, noting that there may only be a few hours of operation within a day or a few days of operation within a month. However, the Confidence Interval that is applicable for reporting Annual averages under the BREF needs to be determined.
There are two main reasons for applying Confidence Intervals. The first is the question of legal certainty and this is crucially important for compliance assessment - it needs to be clear to both the Competent Authority and the Operator when permit conditions are breached. The second is to account for the systematic uncertainty that is applicable to Annual average concentrations, noting that random uncertainties will tend to cancel out across a long averaging period.
This paper examines the most important systematic uncertainties that apply to emissions monitoring and reporting within the EN 14181 framework. For a top-level assessment, it can be assumed that the AMS random uncertainties under QAL1 can be neglected and any systematic uncertainties are accounted for by the QAL2 calibration by the Standard Reference Method (SRM). However, the SRM usually samples from a single point in a measurement plane even though there is some residual variation in concentration. This residual positional uncertainty, established under EN 15259, is systematic and therefore needs to be accounted for. Whilst the SRM is often assumed to be without bias, in reality, there is a residual systematic uncertainty that cannot be neglected as highlighted by inter-laboratory comparisons; although this is difficult to quantify The SRM uncertainty must also be considered. The QAL2 calibration is permitted to shift, year-on-year, within a defined AST tolerance, which gives rise to an additional systematic uncertainty. However, short-term QAL3 drift can be assumed to be random in nature and can be neglected. Using relatively conservative assumptions, it can be demonstrated that it is entirely appropriate to apply the IED Confidence Interval to the Annual emission concentration. This will provide the necessary legal certainty for compliance assessment in addition to simplifying emissions reporting and compliance management.