Continuous emissions monitoring system

Continuous emission monitoring systems (CEMS) 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 (EPA) 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.

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, airborne particulate matter, mercury, volatile organic compounds, and oxygen. CEM systems can also measure air flow, flue gas opacity and moisture.

In the U.S., the EPA requires a data acquisition and handling system to collect and report the data. SO
₂ emissions must be measured in pounds per hour using both an SO
₂ pollutant concentration monitor and a volumetric flow monitor. For NO_x, both a NO_x pollutant concentration monitor and a diluent gas monitor are required to determine the emissions rate in weight per volume or weight per heat value (for example lbs/million Btu, lbs/cuft, kg/kWh or kg/m³). Opacity must also be monitored. CO
2 measuring is not a current requirement, however if monitored, a CO
2 or oxygen monitor plus a flow monitor should be used. In monitoring these emissions, the system must be in continuous operation and must be able to sample, analyze, and record data at least every 15 minutes and then averaged hourly.[3]

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.[4]

Another sample extraction method used in industrial sources and utility sources with low emission rates, is commonly referred to as the "hot dry" 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 cannot measure % oxygen.[4]

Quality assurance

Accuracy of the system is demonstrated in several ways. An internal quality assurance check is achieved by daily introduction of a certified concentration of gas to the sample probe. The EPA also allows for the use of Continuous Emissions Monitoring Calibration Systems which dilute gases to generate calibration standards.[5] 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 will 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).

In the U.S., periodic evaluations of the equipment must be reported and recorded.[3] This includes daily calibration error tests, daily interference tests for flow monitors, and semi-annual (or annual) RATA and bias tests.[6] CEMS equipment is expensive and not always affordable for a facility. In such cases, a facility will install non-EPA compliant analysis equipment at the emissions point. Once yearly, for the equipment evaluation, a mobile CEMS company measures emissions with compliant equipment. The results are then compared to the non-compliant analyzer system.[7]

References

1. The Babcock & Wilcox Company. Steam: its generation and use. The Babcock & Wilcox Company. pp. 36–5. ISBN 0-9634570-1-2.
2. United States Code of Federal Regulations, Title 40, Part 72, Subpart A
3. "Continuous Emissions Monitoring Fact Sheet". US EPA. Archived from the original on February 11, 2009.
4. "Tier 2 Generation Capital Projects". www.pnm.com. Retrieved 23 February 2016.
5. "EPA Traceability Protocol for Assay and Certification of Gaseous Calibration Standards, "EPA-600/R93/224. Revised September 1993.
6. "Specifications and Test Procedures for Total Hydrocarbon Continuous Monitoring Systems in Stationary Sources" (PDF). www3.epa.gov. Retrieved 23 February 2016.
7. "Flue Gas & Emissions Analyzers | Nova Gas". Nova Gas. Retrieved 2016-02-23.