Continuous Emissions Monitoring Systems (CEMS) are used for stack monitoring and regulatory compliance purposes for a range of industrial processes in the United States (US) and, increasingly, within the European Union (EU), notably at coal fired power stations and cement works. Cross-comparison with stack-test results, obtained using a manual reference method, is central to the on-site proving of the CEMS on both continents. In the EU, the reference method is a wet chemistry approach (EN 13211), although a Sorbent Trap standard, with lower detection limits, is under development (TS 17286) based upon the preferred US reference method (US EPA Method 30B).
In the US, the reference method can alternatively be an Instrumental Analyzer (Method 30A) which is also possible in the EU if equivalence with EN 13211 is formally demonstrated to EN 14793, combined with regulatory approval from the Member State. However, this is difficult to establish since the performance of both Sorbent Trap and Instrumental Analyzers is generally better than the wet chemistry approach.
EU requirements are defined in the central Quality Assurance (QA) standard EN 14181:2014 which specifies three Quality Assurance Levels: QAL1 (Certification - CEMS type testing to EN 15267); QAL2 (Calibration - to EN 14181 using stack test results obtained by an accredited test laboratory) and QAL3 (Control - to EN 14181 by assessment of regular Zero and Span drift checks using reference materials). The QAL2 calibration is verified annually by means of an Annual Surveillance Test (AST) and the accredited test laboratory also conducts, or audits, various functional tests that are performed on the CEMS, including linearity and leak checks. EN 14181 is augmented, for mercury, by EN 14884.
US requirements are defined in regulations, e.g., Maximum Achievable Control Technology standards, and are based on prescriptive functional tests, typically including a daily calibration using reference materials, weekly integrity checks and quarterly linearity tests. On-board mercury generators are also subject to regular stability tests. Performance Specifications (12A and 12B) define the initial installation and CEMS proving requirements, including a Relative Accuracy Test Audit (RATA), as performed by a stack test laboratory, in order to determine the system bias. Clean Air regulations then specify the ongoing RATA frequency which is typically annual.
The traceability of reference materials underpins both the ongoing CEMS performance and their calibration using reference methods. The current situation in both the US and the EU will be reviewed briefly.