Monitoring Requirements

Emissions monitoring underpins all of the compliance and reporting requirements of the Industrial Emissions Directive (IED) and is to be considered within BAT Conclusions. Monitoring provisions must be included in the permit conditions.

Compliance is determined on the basis of monitoring results which must be assessed at least annually and which must be publicly available.

For large combustion plant, detailed monitoring provisions for emission to air are specified in Annex V and the competent authority is required to determine the location of the sampling or measurement points to be used for emissions monitoring.

Practically, this is likely to be satisfied by the competent authority approving monitoring points that are proposed and justified by the Operator.

Monitoring and compliance requirements are included within the three year IED review process and the detailed provisions may be amended from time to time to take account of scientific and technical progress.

Monitoring provisions for large combustion plant

When and what to monitor Continuous monitoring is formally required by the IED, for plant with a rated thermal input higher than 100 MW, for SO2, NOx and Dust. CO is also required for gas fired plant. Oxygen, temperature, pressure and water vapour must also be measured in order to correct the emissions to the required reporting conditions, although water is not required if the pollutant is measured on a dry basis.

Continuous monitoring may not be required, by agreement with the competent authority, for SO2 and Dust when firing natural gas, for SO2 when firing oil with a known sulphur content on unabated plant, for SO2 when firing biomass with a demonstrably low sulphur content and for low load factor plant (with a remaining lifespan of fewer than 10,000 operating hours).

In these situations, six monthly periodic monitoring is required or an acceptable alternative procedure, e.g. calculation of SO2 from fuel sulphur content. Mercury, however, shall be measured at least once per year for coal and lignite fired plant – a new requirement.

How to monitor Sampling and analysis, and the quality assurance of CEMs (Continuous Emissions Monitoring systems), are subject to CEN standards. The major standards of interest are EN 14181 (the quality assurance of Automated Measuring Systems) and EN 15259 (defining measurement locations by the use of duct surveys). The IED also requires annual checking of CEMs using manual reference methods (the Annual Surveillance Test, or AST, defined in EN 14181).

The availability of the CEM systems is expected to be high. Any day with more than three hours of lost data, due to malfunction or maintenance of the CEMs is invalidated. If more than ten invalidated days in a calendar year, the competent authority shall require the operator to take adequate measures to improve the reliability of the CEMs.

Implications for the Quality Assurance of CEMs

The implications of the IED for the Quality Assurance of CEMs are considered in relation to the Quality Assurance Levels (QALs) specified in EN14181.

QAL1 – fitness for purpose The substantial reduction in ELVs for existing power plant may require replacement of CEMs if they are unsuitable for measuring at lower concentrations. The certified range of the instruments must be less than 2.5*Daily ELV for combustion plant. For the previous example of large gas turbines (page 8), the required certification ranges are shown in Table 1. The ELV for O2 is given as 10%, even though the process reference condition is 15%. The required span value is then close to 21% (air). The importance of the oxygen reading for gas turbines (operating at high excess air levels) needs to be recognised. The ELV for H2O is set at a typical process level of 6%, although emissions from gas turbines are often measured on a dry basis.

If the certified ranges are unacceptable, then new instruments may need to be purchased if the remainder of the QA checks cannot be satisfied. CO limits are specified for gas fired systems under the IED. A new CEM for CO measurement may therefore be required if one is not already fitted or if an existing instrument is unsuitable and has not previously been subject to QA requirements.

QAL2 and the AST – in-situ calibration A QAL2 is normally required every five years for combustion plant or following a significant change to the process or CEMs. Installation of abatement equipment is an example of a significant process change since the flue gas matrix is altered. Improved NOx control by combustion modification, however, can normally be regarded as a minor process change, although it may be prudent to perform an AST to verify the CEMs performance.

If the CEMs do not meet the revised QAL1 certification requirements, a QAL2 may be required if this is considered to be a major change to the CEMs (or at least an AST by agreement with the competent authority).

Various functional checks are required prior to conducting the QAL2 or AST, including a span check and a linearity test.

For combustion plant, the span value used for calibration of the CEMs is usually set at 2*ELV and the points used in the linearity test are set at 0, 20, 40, 60, 80 and 100% of span. The reduction in ELVs therefore requires a change in gas cylinder concentrations.

Since CO concentrations are very low during normal operation, it is often not possible to produce an adequate QAL2 calibration using reference methods and it is often necessary to instead use the linearity data to verify the instrument calibration.

In the analysis of QAL2 data, the test results are plotted against the CEM results and a straight line is fitted to the data. The QAL2 variability test compares the standard deviation of the differences, between the test points and the calibration line, with a theoretical standard deviation which is calculated from the ELV. A reduced ELV therefore makes it more difficult to pass the variability test. Similar tests are specified for the AST with similar implications.

QAL3 – zero and span drift check QAL3 is intended to provide an audited check of ongoing performance by conducting regular zero and span checks of the monitors and comparing the measured drift against predefined warning and action limits using a control chart approach, e.g. a Shewhart chart. One simple approach is to set control limits based on the ELV so these control limits may become tighter as the ELV is reduced.

Periodic review of these control limits is recommended in any case, in order to check that they are suitable for the actual instrument drift performance and in order to maintain the accuracy of the reported data. There is therefore a limited impact on QAL3 requirements.

Concluding remarks

The Industrial Emissions Directive specifies reduced emission limit values for large combustion plant and these represent a minimum standard for emission control. CEMs must be capable of accurately measuring these lower pollutant concentrations and they may have to be replaced if their certification is inadequate. The CEMs must, in any case, pass the other requirements of the Quality Assurance standard EN 14181, noting that the associated statistical tests are more stringent at reduced limit values.

Monitoring provisions are generally similar to the current requirements, although the European Commission has a mandate to update the monitoring requirements from time to time to reflect technology changes. The competent authority also has an increased responsibility with regards to the approval of sample port locations.

While the general compliance approach for NOx, SO2 and Dust has been harmonised for both new and existing large combustion plant, continuous CO monitoring is a new, formal requirement for gas fired plant. Mercury emissions must also be measured, at least annually, for coal and lignite fired plant.

Published: 03rd Jul 2013 in AWE International