To control emissions to the atmosphere, you need to know what those emissions actually are. The European standard EN14181 describes the quality assurance procedures needed to ensure that continuous emission monitoring systems give sufficiently accurate readings.
Plants that produce emissions to the atmosphere are regulated under strict European and national laws to protect the environment and human health. Such plants need a permit to operate, which usually requires monitoring of emissions.
Permanently installed analyser systems that continuously monitor and display the volumes of substances in the flue gas are known as CEM or CEMS (continuous emissions monitoring systems) or AMS (automatic measuring systems).
EN14181 is the European Committee for Standardisation (CEN) standard for quality assurance of CEMS, setting performance standards and procedures for testing and approval.
As well as providing the framework for legal compliance, conforming to EN14181 is required by plants participating in the EU emissions trading scheme, an aspect expected to grow in significance over the coming years.
Monitoring is usually carried out by extracting a small sample of flue gas, using a pump, into the CEMS system via a sample probe. The sample is transported through a line, or umbilical, to a manifold where individual analysers extract materials.
A data acquisition and handling system receives the signal output from each analyser, collecting and recording the emissions data.
Alternatively, using a more modern approach, Fourier transform infrared spectroscopy (FTIR) can be used to measure all pollutants at a single point. This method analyses the spectrum from the incoming infrared light, enabling identification of the molecules as well as concentrations and temperatures of substances.
The rate of flow also needs to be measured. Frequently, this is done using a pitot tube, the same technology as is used to determine the airspeed of aircraft. Other common technologies include ultrasonic, vortex shedding or optical scintillation measurement.
To comply with EN14181, readings must be accurate to within a certain percentage. This only applies to the CEMS itself; the system for data acquisition, storage, handling and calculation of measurement values is not considered part of the CEMS for the purposes of the standard, but will be the subject of a different CEN standard, yet to be published.
Three quality assurance levels
Process operators have overall responsibility for complying with EN14181 but they rely heavily on the advice from their CEMS manufacturer and from test laboratories.
The standard specifies three quality assurance levels (QALs) and an annual surveillance test (AST). These quality assurance levels follow a logical sequence to demonstrate the suitability of the CEMS, its proper installation, accurate calibration and correct operation.
QAL1 – outlines a procedure to demonstrate that the CEMS is suitable for the intended purpose before installation. This is normally done by the system manufacturer. MCERTS product certification at an appropriate range (1.5 or 2.5 times the daily average emission limit value) is taken as evidence of compliance with the QAL1 requirements.
When an operator installs a CEMS, it is essential to ensure that the sampling system is certified to national and European standards. The CEMS must be able to test for linearity, zero drift and span drift checks, as well as to check the entire system for leaks. The legislation specifies uncertainty allowances expressed as 95 percent Confidence Intervals (CIs).
Once a system has been installed, conditions on the site can increase the uncertainty in the measurements. Care must be taken to ensure that the CEMS selected will meet the QAL2 requirements in the specific application.
QAL2 – describes the procedure to prove that the CEMS has been installed correctly, verified and calibrated, and to check that the CEMS still meets the uncertainty requirements. Large parts of this procedure can be done by the operator, but checking calibration requires the involvement of a test laboratory.
This involves inspection and functional tests, which check performance characteristics such as response time and linearity.
The QAL2 test procedure is carried out when the CEMS is installed, then at specific intervals of three or five years depending on the application. It also needs to be carried out whenever there is a significant change in plant operation which alters the emissions, as well as after a repair or upgrade that requires re-calibration of the system.
Functional tests are a mandatory requirement of EN14181. The operator is responsible for ensuring that these take place not more than one month before QAL2 or AST. These can be carried out either by a test laboratory, or the process operator, with or without the help of the CEMS. These checks ensure that the system is operating correctly before comparison tests are started.
The system has to be calibrated against standard reference materials (gas or dust with traceable concentrations). This is to check whether the CEMS operates correctly so that any faults can be addressed and also to calibrate the CEMS if there is a bias in the readings.
The process operator has overall responsibility for ensuring that the functional checks are carried out, but they can be carried out by any party as long as the tests meet the requirements. Test laboratories are responsible for reporting the results of the functional tests, within QAL2 and AST reports.
QAL3 – consists of the ongoing control of the CEMS during the operation. This is normally done by the operator, either manually or automatically with the help of the CEMS. The plant operator is required to regularly measure the drift and precision of the CEMS and use the statistical data to determine when the CEMS needs maintenance.
The required frequency can be anything from weekly to several months apart. Many systems carry out QAL3 tests automatically. The data from these automatic zero and span tests is then used for the QAL3 procedure.
All CEMS must have the facility to allow operators and test laboratories to perform regular zero and span tests, or to extract the data from automatic zero and span tests.
AST
EN14181 describes the annual surveillance test (AST) of the CEMS. This is a procedure to show that the CEMS continues to function correctly and that the calibration is still valid. If the AST shows that the calibration function is no longer valid, a full QAL2 test is required.
With the exception of calibration tests, testing can be carried out by anyone with the required training.
The AST is a reduced version of a QAL2 test. Its purpose is to check whether the calibration function determined during the QAL2 tests is still valid.
Outsourcing testing
Using a third party to help ensure your CEMS gives sufficiently accurate reading is a cost-efficient approach. For instance ABB offers a functional test as an additional service to its standard six and 12 monthly service and calibration visits. Services like these can be carried out in combination with the standard service.
The tests include:
- Alignment and cleanliness
- Sampling systems inspection
- Leak testing
- Zero and span check
- Linearity
- Interferences
- Zero and span drift (QAL3 audit)
- Response time
All engineers carrying out functional checks are factory certified and have in-depth knowledge of the equipment, ensuring tests can be completed efficiently and accurately.
Equipment failures or a test failure can be investigated and resolved quickly, ensuring there is no delay prior to the QAL2 or AST.
The engineer responsible for the tests can back up all test data directly from the analyser, minimising any error in data collection.
The engineer will also review and save the QAL 3 data and write the report, ensuring that all data is accurately entered in the final report.
All stages are completed by the same engineer, eliminating any delay in handing over the final report.
