CEMS

Continuous Emission Monitoring System (CEMS) is a system of devices and software designed to continuously sample, analyze, and report the pollutants emitted from a stationary source's stack or smokestack. CEMS are used to verify compliance with emission limits, support environmental permits, and provide data for process control and regulatory reporting.

A CEMS typically comprises a sample extraction or in-situ measurement system, pollutant analyzers, a dilution and conditioning subsystem, a stack flow or volume measurement, and a data acquisition and handling system that collects, stores, and transmits results. Commonly monitored pollutants include nitrogen oxides (NOx), sulfur dioxide (SO2), carbon monoxide (CO), carbon dioxide (CO2), and oxygen (O2), with additional measurements for hydrogen chloride (HCl), hydrogen fluoride (HF), particulate matter, and, where required, elemental mercury. The data are used to compute mass emissions (for example, kg/hr) and to support compliance reporting and performance monitoring.

Operation and QA: The system is calibrated and validated to maintain accuracy, with routine calibration using span and zero gases, drift checks, and performance tests. Data must be continuously collected and reported to regulatory authorities, with procedures for data availability, handling of missing data, and QA/QC documentation.

Regulatory framework: In the United States, CEMS are mandated under the Clean Air Act with EPA rules such as 40 CFR Part 60 and Part 75 that govern monitoring, calibration, data handling, and reporting for major and affected sources. Many other countries require CEMS under national or regional emission standards, and international directives exist for large combustion plants and industrial installations.

Impact: CEMS provide time-resolved emission data, supporting compliance demonstrations, emissions trading programs, and process optimization of combustion and control systems.