38 Air Monitoring
CONTINUOUS EMISSIONS MONITORING ON STACKS - DATA ACQUISITION AND HANDLING SYSTEMS AND THE NEW EUROPEAN STANDARD EN 17255
The Quality Assurance (QA) requirements for Continuous Emissions Monitoring Systems (CEMS), also known as Automated Measuring Systems (AMS), are specifi ed in the CEN standard EN 14181 which defi nes three Quality Assurance Levels - QAL1 (certifi cation), QAL2 (calibration) and QAL3 (control). QAL1 requires type approval of the AMS which must have a suitable certifi cation range when compared with the daily Emission Limit Value (ELV). QAL2 defi nes a straight-line calibration function, with a gradient and off set, that relates the AMS data to the test data obtained by an accredited test laboratory that uses a Standard Reference Method to independently measure each regulated pollutant, e.g., NOx
, SO2 , CO and dust.
The standard deviation of the diff erences between the individual test points and the calibration line is a measure of the data scatter and this is known as the variability.
A functional test of the AMS is required prior to the QAL2 and prior to the Annual Surveillance Test (AST) in which the test laboratory annually checks the ongoing validity of the QAL2 calibration. A Valid Calibration Range is defi ned by the maximum emission concentration measured during the QAL2, extendable using AST data, but a repeat QAL2 is required if the reportable emissions regularly exceed the Valid Calibration Range based on a weekly check.
The linear QAL2 calibration must be applied to the averaged raw AMS data followed by correction to standard reference conditions. Prior to reporting to the authorities, for compliance assessment against Emission Limit Values (ELV), the deduction of the allowed measurement uncertainty may be performed, according to national emissions reporting requirements.
QAL3 requires that the operator performs regular zero and span checks on the AMS, to ensure that the AMS drift remains within pre-defi ned tolerances, using any control chart approach to plot the zero and span drift over time; only the CUSUM approach, defi ned in EN 14181, generates a drift correction that can be applied to the AMS data.
Figure 1: DAHS functionality EN 17255 Part 1 – Requirements for the
handling and reporting of data Part 1 of the standard defi nes how raw emissions, peripheral and stack fl ow data are used to produce First Level Data (FLD) averages, typically a one-minute average calculated from data collected at no more than 10 second intervals. If applicable, a QAL3 drift correction (CUSUM) is applied prior to forming the FLD, and negative values are included in the FLD. However, the lower and upper limits of the measurement range are instead reported when the AMS is reading outside of its normal measuring range,
Introduction
All of the data transformation operations associated with EN 14181, and compliance reporting, are performed within the Data Acquisition and Handling System (DAHS) which is usually a commercial software package with the functionality shown in Figure 1. However, other options are allowed provided that they meet the requirements of the standard, particularly in relation to speed, accuracy, access, security and data validation.
Since the functionality of the DAHS is not covered by EN 14181, QA requirements for the DAHS are defi ned in EN 17255, as outlined in this article. EN 17255 is published in four parts, covering every aspect of DAHS operation as shown in Table 1, noting that Part 4 is awaiting fi nal publication at time of writing. The QA requirements for DAHS, specifi ed in EN 17255, follow the pattern established in EN 14181 for AMS. This article is based on presentations given by the authors at the Air Quality and Emissions (AQE) conference in 2022.
i.e., when the AMS signal is overly negative or is overly high (saturated or over-range) unless regulations specify otherwise; these FLD, and any subsequently derived averages, are then fl agged as being ‘outside of range’. Additional FLD are determined for the AMS status of each emission, peripheral and stack fl ow measurement, and also for the plant status to determine when the plant is in normal operation.
Part 1 then defi nes how the Short-term Average (STA) is calculated from the FLD averages, typically over a half-hour period for Waste Incineration plant or a one-hour period for Large Combustion Plant, and taking into account the AMS status. The two-thirds rule applies, i.e., there must be at least 40 minutes of valid FLD and normal plant operation within an hour, or 20 minutes within a half-hour period, to produce a valid STA. If the plant is not operating for at least two-thirds of the STA, then the STA is fl agged as ‘plant not in reportable state’.
The STA is then ‘calibrated’, by applying the QAL2 gradient and offset, and then corrected to standard reference conditions at a fi xed reference oxygen (O2
) concentration in the dry fl ue gas at
273.15 K and 101.3 kPa. Peripheral data are needed to perform this standardisation, including the oxygen content, water vapour content (if the AMS analyses a wet sample) and the temperature and pressure (for dust monitoring). If valid peripheral data are temporarily not available, substitute values can be used. Applying the QAL2 factors to the FLD produces the same result as for the STA. However, as shown in Figure 2, different results are obtained when standardising to reference conditions at the STA, rather than at the FLD which is common practice in some Member States. This is due to the non-linearity of the oxygen correction, noting that standardising at the STA, as now required by the standard, produces a lower result than standardising at the FLD, as also illustrated in Figure 2.
Validated short-term averages are calculated by subtracting the allowed measurement uncertainty from the STA prior to compliance assessment, e.g., by subtraction of the Confi dence Intervals specifi ed in the Industrial Emissions Directive. The way in which this is implemented varies between Member States.
Table 1: Structure of DAHS standard
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