- Continuous emissions monitoring system
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CEM systems were historically used as a tool to monitor flue gas for oxygen, carbon monoxide, and carbon dioxide to provide information for combustion control in industrial settings.[1] They are currently used as a means to comply with air emission standards such as the United States Environmental Protection Agency's Acid Rain Program,[2] , other federal emission programs, or state permitted emission standards. Facilities employ the use of CEMS to continuously collect, record, and report the required emissions data.
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Continuous Emissions Monitoring Systems (CEMS)
The standard CEM system consists of a sample probe, filter, sample line (umbilical), gas conditioning system, calibration gas system, and a series of gas analyzers which reflect the parameters being monitored. Typical monitored emissions include: sulfur dioxide, nitrogen oxides, carbon monoxide, carbon dioxide, hydrogen chloride, particulate matter, mercury, volatile organic compounds, and oxygen. CEM systems can also measure air flow, flue gas opacity and moisture.
Operation
A small sample of flue gas is extracted, by means of a pump, into the CEM system via a sample probe. Facilities that combust fossil fuels often use a dilution-extractive probe to dilute the sample with clean, dry air to a ratio typically between 50:1 to 200:1, but usually 100:1. Dilution is used because pure flue gas can be hot, wet, and with some pollutants, sticky. Once diluted to the appropriate ratio, the sample is transported through a sample line (typically referred to as an umbilical) to a manifold from which individual analyzers may extract a sample. Gas analyzers employ various techniques to accurately measure concentrations. Some commonly used techniques include: infrared and ultraviolet adsorption, chemiluminescence, fluorescence, and beta ray absorption. After analysis, the gas exits the analyzer to a common manifold to all analyzers where it is vented out of doors. A Data Acquisition and Handling System (DAHS) receives the signal output from each analyzer in order to collect and record emissions data.
Another sample extraction method used in industrial sources and utility sources with low emission rates, is commonly referred to as the "hot wet" extractive method or "direct" CEMS. The sample is not diluted, but is carried along a heated sample line at high temperature into a sample conditioning unit. The sample is filtered to remove particulate matter and dried, usually with a chiller, to remove moisture. Once conditioned, the sample enters a sampling manifold and is measured using the same methods above. One advantage of this method is the ability to measure % oxygen in the sample, which is often required in the regulatory calculations for emission corrections. Since dilution mixes clean dry air with the sample, dilution systems can't measure % oxygen.
Quality Assurance
Accuracy of the system is demonstrated by several ways. An internal quality assurance check is achieved by daily introduction of a certified concentration of gas to the sample probe. The analyzer reading must be accurate to a certain percentage. The percent accuracy can vary, but most fall between 2.5% and 5%. In power stations affected by the Acid Rain Program, annual (or bi-annual) certification of the system must be performed by an independent firm. The firm would have an independent CEM system temporarily in place to collect emissions data in parallel with the plant CEMS. This testing is referred to as a Relative Accuracy Test Audit (RATA).
References
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