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Category: Emission Monitoring | Gas Analytics | Process Analytical Technology (PAT) | Environmental Measurement
Author: Dusko Kadijevic, Publisher Analytic Journal
Last updated: May 2026
Reading time: approx. 11 minutes
Target audience: Process engineers, environmental compliance officers, plant operators, instrumentation engineers at permitted industrial facilities
CEMS (Continuous Emission Monitoring System) refers to a system for the continuous, automated measurement of pollutant emissions from industrial facilities — with the objectives of demonstrating compliance with regulatory emission limits in real time, maintaining tamper-proof records, and where applicable feeding data back into process control. CEMS is not voluntary — it is a regulatory obligation whose scope and technical requirements are defined by the operating permit of the individual facility.
Before a CEMS system is designed, procured or commissioned, one document determines all subsequent decisions: the operating permit of the facility.
The regulatory authority specifies in this permit:
What is measured is therefore not a free decision of the plant operator — it results from a regulatory determination driven by facility type, fuel type, materials processed and local permit conditions.
Practical note: What is burned or processed in a facility determines directly which pollutants are relevant. Coal-fired power plants must monitor mercury because coal naturally contains it. Waste incineration facilities with mixed waste streams — including older materials, fluorescent lamps, amalgam from dental practices or quecksilver-containing instruments — may also receive mercury-containing inputs. Mercury thermometers from hospitals and medical practices are legally classified as hazardous waste and must be separately collected and disposed of by specialist contractors — they must not enter the general waste stream. In practice, however, it cannot be excluded that mercury-containing materials enter a facility through improper handling of older waste streams. Regulators respond to such operational experience with permanent monitoring requirements.
In Germany, only type-approved measurement systems (eignungsgeprüfte Messeinrichtungen) may be used for regulatory continuous emission monitoring. Each device must have passed a formal type-approval procedure before it can be used.
Type approval is conducted according to EN 15267 and national minimum requirements. Results are published by the Federal Environment Agency (UBA) in the Federal Gazette. All systems certified since 2009 are listed at www.qal1.de.
Equivalent frameworks in other regions:
| Region | Approval Framework | Authority |
|---|---|---|
| Germany | EN 15267 / UBA type approval | Umweltbundesamt (UBA) |
| EU (other member states) | EN 15267 — same norm, national implementation varies | National environmental authorities |
| UK | MCERTS (Monitoring Certification Scheme) | Environment Agency |
| USA | Performance Specifications (PS series), 40 CFR Part 60 Appendix B | US EPA |
Practical note — new technologies in the approval process: In one waste incineration facility, halogenated compounds (bromine, chlorine) had been determined by wet-chemical methods for many years. When a laser-based measurement system became available that could perform the same task with greater precision, lower maintenance demand and continuous output, it was still undergoing the type-approval process. The regulatory authority accepted installation of the new system on an agreed basis — provided the wet-chemical reference method continued in parallel until type approval was completed. The new system was technically superior and eventually received full approval. The lesson: regulatory authorities can be pragmatic when the technical superiority is clearly demonstrated and the transition arrangement is transparent.
| Component | Typical Measurement Principles | Typical Facilities |
|---|---|---|
| SO₂ | NDIR, UV photometry, FTIR | Power plants, cement works, refineries |
| NOₓ (NO, NO₂) | Chemiluminescence, NDIR, FTIR | Combustion plants, engines, gas turbines |
| CO | NDIR, TDLS | Combustion plants, coke ovens |
| CO₂ | NDIR | Power plants, greenhouse gas reporting |
| O₂ | Paramagnetic, ZrO₂, TDLS | Combustion control, all fired installations |
| HCl | FTIR, TDLS, wet-chemical | Waste incineration, chlorine chemistry |
| HF | FTIR, TDLS | Glass industry, aluminium smelters |
| Particulate matter | Scattered light, transmitted light, triboelectric | All fired installations, cement works |
| Mercury (Hg) | Gold trap technique, CVAAS, CVAF, sorbent methods | Coal and waste power plants, cement |
| VOC / TOC | FID | Coating plants, solvent processing |
| NH₃ | TDLS, FTIR | SCR denitrification systems |
| Flue gas flow | Differential pressure, ultrasonic, Pitot | All plants requiring mass flow balance |
Mercury (Hg) is one of the most demanding measurement targets in emission monitoring. It occurs in flue gas in several chemical forms — elemental Hg⁰, divalent Hg²⁺ (reactive) and particle-bound Hg — and requires different measurement principles depending on the regulatory requirement.
Gold trap technique: Mercury is adsorbed from the flue gas stream onto a gold surface (amalgamation). After a defined collection period, the gold trap is thermally regenerated and the released Hg is detected — typically by cold vapour atomic absorption spectrometry (CVAAS) or atomic fluorescence (CVAF). This method is highly sensitive and type-approved in Germany and recognised in US and EU frameworks.
Direct continuous measurement systems: Modern CEMS for mercury can measure continuously — either through in-situ laser spectroscopy or through extractive conditioning with subsequent CVAAS/CVAF detection.
When is mercury monitoring typically required or potentially necessary?
Practical note from plant operations: At one incineration facility, mercury appeared in the flue gas even though the operator had not expected any mercury-containing material streams. The regulatory authority subsequently ordered permanent Hg monitoring — as a direct response to this specific operational experience, not because of a general statutory requirement. This illustrates that even when an operator expects no obvious mercury input, a facility can be required to implement permanent Hg monitoring based on its operational history.
A frequently underestimated but regulatorily critical element of CEMS is the emission data acquisition and handling system (in Germany: Emissionsrechner or EDAS; in the US: DAHS — Data Acquisition and Handling System). It receives raw signals from all analysers, calculates emission mass flows and stores data in a tamper-proof manner.
Why the data system must be directly connected:
In Germany, the emission data system must be connected directly to the measurement instruments — connection via the process control system (DCS/PLC) is not permitted. The rationale: the DCS is an operational system that could theoretically be manipulated. The emission data system is a separate, tamper-proof unit with clearly regulated data paths and access controls. The same underlying principle — direct, tamper-proof data acquisition without routing through operational control systems — applies in the US (DAHS requirements under 40 CFR Part 75) and in the UK (EA Guidance M2), though the specific technical implementation requirements differ in detail.
Requirements for the emission data system:
Annual verification by independent third parties:
In addition to the operator's ongoing quality assurance (QAL3 daily checks), an annual verification by an accredited, independent inspection body is required (AST — Annual Surveillance Test under EN 14181). In Germany, accredited bodies such as TÜV and DEKRA frequently carry out this function. In the UK, MCERTS-accredited bodies perform the equivalent role; in the US, independent stack testing firms conduct RATAs (Relative Accuracy Test Audits). These annual checks confirm that the CEMS continues to operate within permitted tolerances under real operating conditions and that QAL3 documentation has been correctly maintained. Results must be submitted to the regulatory authority.
The quality assurance framework for CEMS is defined in EN 14181 across the EU and UK. The US equivalent uses Performance Specifications and Appendix F procedures under 40 CFR Part 60.
| Level | Designation | Content | Who carries it out | Frequency |
|---|---|---|---|---|
| QAL1 | Type approval | Demonstration of fundamental suitability of the measurement system | Accredited testing body | Once before commissioning |
| QAL2 | On-site calibration | Comparison of CEMS against reference measurement methods under real process conditions | Accredited measurement body | At commissioning, then every 3 years |
| QAL3 | Continuous quality control | Daily monitoring of measurement range stability using reference gases | Plant operator | Daily, automated |
| AST | Annual Surveillance Test | Annual review of QAL3 data by independent external body | Accredited inspection body | Annually |
US equivalent: RATA (Relative Accuracy Test Audit) under 40 CFR Part 60, Appendix F — functionally equivalent to QAL2, conducted annually.
CUSUM charts in QAL3:
For statistical monitoring of daily calibrations, CUSUM charts (Cumulative Sum Control Charts) are widely used. They detect systematic drift at an early stage — before intervention thresholds in conventional control charts would be triggered. CUSUM charts document not only whether an intervention was necessary but also whether it was statistically justified — important for demonstrating maintenance decisions to the regulatory authority.
Emission monitoring is frequently viewed solely as a compliance instrument. This is too narrow a perspective. CEMS data provide valuable information about the operational condition of the plant itself.
CO, CO₂, NOₓ and O₂ indicate whether combustion is clean and complete. Changes in these values can signal incorrect air-to-fuel ratio, worn burners, fouling or developing process faults — before limit values are breached or damage occurs. The process can be adjusted faster and energy is not wasted through inefficient combustion.
Important distinction: In this application, emission measurement is an indirect diagnostic lever — the process is not directly controlled via emission values, but the data serve as diagnostic information for assessing process condition. This is a significant difference from SIL safety applications: in functional safety (SIL), CEMS measurement systems are generally not suitable for direct safety control loops — dedicated, type-approved safety measurement circuits are required for that purpose.
Practical example with high ROI: At one industrial facility, CO₂ was calculated on the basis of individual annual measurements using conservative flat-rate assumptions that systematically overestimated actual output. After introducing continuous CO₂ monitoring, the actual emission quantity proved substantially lower. This enabled a cause-based calculation instead of a conservative flat-rate estimate — with direct economic consequences for cost allocation. The return on investment of the measurement installation was under 12 months.
CEMS systems are heavily regulated and subject to frequent normative updates. Operators must track the regulatory landscape continuously. For large facilities or multi-site industrial groups, it can make sense to have the planning, construction and commissioning of CEMS systems carried out by specialist service providers — retaining operational responsibility internally. Specialists in emission measurement technology know the current status of type approvals, accepted systems and the requirements of individual regulatory authorities. Internal competence should then focus on tendering, quality oversight and regulatory communication — not on technical detail planning.
The legal foundation in Germany is the Federal Immission Control Act (BImSchG) and its associated ordinances:
Type-approved measurement systems are published by the Umweltbundesamt in the Federal Gazette. Certificates are accessible at www.qal1.de.
The Industrial Emissions Directive (IED, 2010/75/EU) sets the European framework, requiring the use of Best Available Techniques (BAT) as defined in BAT Reference Documents (BREFs). Emission limit values follow BAT Conclusions.
EU-wide for CEMS: EN 14181 for quality assurance of automated measurement systems, EN 15267 for device certification.
Important note on variation within the EU: While the IED and EN 14181/15267 apply in all member states, the stringency of national implementation and enforcement varies considerably. Germany typically applies requirements stricter than the EU minimum. France, Italy, Spain and Poland implement the same framework but national technical detail requirements and enforcement intensity differ — particularly for older installations which may operate under transition arrangements. Operators at multi-country sites should not assume that German-level requirements apply across all EU locations.
In the United States, emission monitoring is governed by the Environmental Protection Agency (EPA) under the Clean Air Act:
Performance Specifications (PS series) define minimum CEMS performance requirements. RATA (Relative Accuracy Test Audit) is the US equivalent of QAL2 — annual comparison against certified reference measurement methods.
Flares — a US-specific regulatory topic:
In the United States, flaring of surplus process gases is widespread in petrochemicals, refineries and upstream oil and gas operations. In Germany and the EU, continuous flaring is strongly restricted — surplus gases must be recovered or treated.
For flares in the US, 40 CFR § 63.670 specifies requirements including:
CEMS systems at US flares must continuously capture gas flow, composition and heating value to demonstrate combustion efficiency — requirements substantially tightened since 2019.
Following Brexit, the UK applies independent requirements:
| Characteristic | Extractive CEMS | In-situ CEMS |
|---|---|---|
| Principle | Sample extracted, conditioned, analysed | Measurement directly in flue gas duct |
| Sample conditioning | Extensive — coolers, filters, pumps, lines | Not required |
| Response time | Slower (transport path) | Fast (seconds) |
| Maintenance | High (sampling system) | Medium (optics, window cleaning) |
| Condensable components | Manageable (heated lines) | Conditional |
| High particulate loading | Filtration possible | Critical — optical systems sensitive |
| Typical principles | NDIR, FTIR, chemiluminescence, gold trap | TDLS, UV-DOAS, transmitted light |
| Standard / Regulation | Issuer | Content |
|---|---|---|
| EN 14181 | CEN / national standards bodies | Quality assurance for automated measuring systems — QAL1, QAL2, QAL3, AST |
| EN 15267 | CEN | Certification of automated measuring systems for continuous emission monitoring |
| EN 17255 | CEN | Requirements for data acquisition and handling systems for CEMS |
| IED (2010/75/EU) | EU | Industrial Emissions Directive — framework for emission limit values and BAT |
| 40 CFR Part 60 | US EPA | New Source Performance Standards — CEMS requirements for new sources |
| 40 CFR Part 63 | US EPA | National Emission Standards for Hazardous Air Pollutants |
| 40 CFR § 63.670 | US EPA | Specific requirements for flare control devices |
| 40 CFR Part 75 | US EPA | Continuous emission monitoring in the Acid Rain Program |
| EA Guidance M2 | Environment Agency (UK) | Monitoring requirements for emissions from stacks |
| MCERTS | Environment Agency (UK) | Certification scheme for emission monitoring systems in the UK |
What is CEMS and why is it mandatory?
CEMS stands for Continuous Emission Monitoring System — a system for the continuous measurement of pollutant emissions from industrial facilities. It is not a voluntary measure but a regulatory obligation defined in the operating permit of the facility. Which substances must be monitored depends on facility type, fuel and processed materials.
Who determines what must be measured?
The regulatory authority — the competent environmental authority in the relevant jurisdiction. In Germany, this is the Länder authority under BImSchG; in the US, EPA and delegated state agencies under the Clean Air Act; in the UK, the Environment Agency under EPR. The operator has no discretion over the basic selection of measurement components.
What are type-approved measurement systems and where can I find the approved list?
Devices that have passed a formal type-approval procedure under EN 15267 and been published by the relevant authority. In Germany, the list is maintained by the Umweltbundesamt and accessible at www.qal1.de. In the UK, MCERTS-certified systems are listed by the Environment Agency. In the US, EPA Performance Specifications define minimum requirements rather than individual device approvals.
Why must the emission data system be directly connected to the analysers?
The process control system (DCS/PLC) is an operational system that could theoretically be manipulated. A direct connection to the emission data system — without routing through the DCS — ensures tamper-proof data integrity. This principle applies in Germany (EDAS), the US (DAHS under 40 CFR Part 75) and the UK (EA Guidance M2), though specific technical requirements vary.
What is the difference between QAL2 and RATA?
Both involve comparison of the CEMS against certified reference measurement methods under real operating conditions. QAL2 is the European designation under EN 14181; RATA (Relative Accuracy Test Audit) is the US designation under 40 CFR Part 60. They are functionally equivalent — the main difference is the regulatory framework and frequency requirements.
When is mercury monitoring typically required?
Mercury monitoring is typically required at coal and lignite power plants where mercury content in coal is well-established. It may be required at waste incineration facilities with mixed waste streams, cement works using substitute fuels, and facilities with operational history showing mercury in flue gas. Regulators may impose permanent mercury monitoring based on site-specific experience even when no obvious mercury sources are expected.
What are CUSUM charts and why are they used in CEMS?
CUSUM charts (Cumulative Sum Control Charts) are statistical tools for detecting systematic drift in measurement systems. In CEMS operation, they are used for daily QAL3 monitoring. They identify trends early and document statistically whether maintenance interventions were necessary and justified — important for demonstrating maintenance decisions to regulators.
Why are flares regulated differently in the US compared to Germany and the EU?
In Germany and the EU, continuous flaring of process gases is strongly restricted — surplus gases must be recovered or treated. In the US, flaring is widespread in petrochemicals, refineries and upstream oil and gas, and is regulated under 40 CFR § 63.670. US flares are subject to comprehensive CEMS requirements for combustion efficiency monitoring, requirements that have been substantially tightened since 2019.
Does EN 14181 apply equally across all EU member states?
The norm is the same across all EU member states. However, the stringency of national implementation and enforcement varies considerably. Germany typically applies requirements stricter than the EU minimum. Other member states implement the same framework but with different levels of technical detail and enforcement intensity — particularly for older installations under transition arrangements.
Can CEMS data be used for process control, not just compliance?
Yes — and this is an underutilised application. CO, CO₂, NOₓ and O₂ values from CEMS provide direct diagnostic information about combustion quality and process condition. Deviations can signal developing faults before limit values are breached. However, CEMS measurement systems are not suitable for direct safety control loops under SIL functional safety requirements — dedicated, type-approved safety measurement circuits are required for that purpose.
Sample conditioning systems for extractive CEMS
Filtration, cooling, heating, condensate removal and pump concepts for extractive CEMS systems are covered in a dedicated article on sample conditioning.
Further reading: "Sample Conditioning for Process Analysers — Filters, Coolers, Pumps" (forthcoming)
Dioxin and furan measurement
Dioxins and furans at waste incineration facilities are measured periodically by accredited testing bodies rather than continuously. This specialist measurement technology is a subject in its own right.
CEMS is a topic with many practical dimensions — from permit negotiations through system design to QAL3 documentation and the introduction of new measurement principles. If you have experience from plant operations, approval processes or the implementation of new technologies that you would like to share:
Feedback and practical reports: info@analyticjournal.de
Analytic Journal aims to be a platform for exchange among practitioners — not a one-way channel. Practical reports from plant operations, corrections and additions are explicitly welcome.
CEMS is not a question of technology selection — it is a regulatory obligation whose framework is defined by the operating permit. What is measured, which systems may be used and how data must be stored and transmitted is prescribed by the authority.
The technical challenge lies in operating robust, low-maintenance and long-term stable measurement systems within this framework — under the harsh conditions of real flue gas streams, with varying load profiles and under the constant scrutiny of quality assurance requirements.
New measurement principles — such as TDLS for HCl, NH₃ or mercury — are increasingly receiving type approvals and replacing older, more maintenance-intensive methods. The path to approval of new technologies requires persistence, but regulators are receptive when technical superiority is clearly demonstrated.
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Regulatory requirements are subject to change — current versions should be verified with the relevant authorities.
© 2026 Analytic Journal / Dusko Kadijevic. All rights reserved.
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Editorial Note
This article has been written to the best of our knowledge based on publicly available technical information, standards, and manufacturer data. Specifications may change — for binding information, consult current manufacturer datasheets and relevant standards bodies. Analytic Journal produces editorial content independently of manufacturers and advertising partners.
Author: Dusko Kadijevic — 20 years of experience as a PAT practitioner in the process industry
Dusko Kadijevic | Publisher Analytic Journal | analyticjournal.com
Written May 2026. All content formulated in our own words, created with AI assistance.
First published on 8 June 2026 on analyticjournal.com