Menu
Home
People
Places
Arts
History
Plants & Animals
Science
Life & Culture
Technology
Reference.org
Continuous emissions monitoring system
open-in-new
<h2 id="operation">Operation</h2> <p>A small sample of flue gas is extracted, by means of a <a href="/facts/Pump/UtMn3Btt">pump</a>, into the CEM system via a sample probe. Facilities that <a href="/facts/Combustion/ohQoWj8z">combust</a> <a href="/facts/Fossil_fuels/BPgxAqB7">fossil fuels</a> often use a <a href="/facts/Concentration/wqBmz5od">dilution</a>-extractive probe to dilute the sample with clean, dry air to a <a href="/facts/Ratio/mFmhQt7T">ratio</a> 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 <a href="/facts/Manifold_(fluid_mechanics)/401KqWAN">manifold</a> from which individual analyzers may extract a sample. Gas analyzers employ various techniques to accurately measure concentrations. Some commonly used techniques include: <a href="/facts/Infrared/44XdJNcc">infrared</a> and <a href="/facts/Ultraviolet/kncBUqn5">ultraviolet</a> <a href="/facts/Adsorption/ARH9peYM">adsorption</a>, <a href="/facts/Chemiluminescence/9LLwID4d">chemiluminescence</a>, <a href="/facts/Fluorescence/qsJVT2qP">fluorescence</a> and <a href="/facts/Beta_particle/x8b3ttLn">beta ray</a> <a href="/facts/Absorption_(electromagnetic_radiation)/xBYKt7yS">absorption</a>. After <a href="/facts/Analysis/heFlRrYd">analysis</a>, 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 <a href="/facts/Data_logging/U3GglNdc">collect and record emissions data</a>.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> </p><p>Another sample extraction method used in industrial sources and utility sources with low emission rates, is commonly referred to as a "dry extractive", "hot dry" extractive, 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 <a href="/facts/Chiller/DpdaocSf">chiller</a>, to remove <a href="/facts/Moisture/vubw4SpX">moisture</a>. Once conditioned, the sample enters a sampling manifold and is measured by various gas analyzers, typically NO<i>x</i> and <a href="/facts/Oxygen/acyn6o8r">O2</a> (and sometimes CO) for combustion turbines and engines running natural gas or diesel. NO<i>x</i> analyzers typically work using <a href="/facts/Chemiluminescence/9LLwID4d">chemiluminescence</a>. <a href="/facts/Oxygen/acyn6o8r">O2</a> analyzers a magnetic field which attracts <a href="/facts/Oxygen/acyn6o8r">O2</a> to measure the concentration. The <a href="/facts/Oxygen/acyn6o8r">O2</a> causes movement of a suspended mirror within the analyzer which then changes the amount of light being reflected by that mirror onto a photocell. The amount of current required to move the mirror back to center is proportional to the <a href="/facts/Oxygen/acyn6o8r">O2</a> concentration. The ability to measure % <a href="/facts/Oxygen/acyn6o8r">oxygen</a> in the sample is required to perform the required calculations. </p> <h2 id="quality-assurance">Quality assurance</h2> <p>Accuracy of the system is demonstrated in several ways. An internal <a href="/facts/Quality_assurance/BhDeyhel">quality assurance</a> check is achieved by daily introduction of a <a href="/facts/Certified/sDRN8T1g">certified</a> <a href="/facts/Concentration/wqBmz5od">concentration</a> of gas to the sample probe. The CEMS measurement is then compared against the known concentration to arrive at a Calibration Error percentage. A zero gas reading is also taken and compared. If the calibration error % exceeds 2x the performance specification for 5 consecutive days or 4x the performance specification in 24 hours, the CEMS is considered out of control meaning the data can not be relied upon as accurate until it is brought back into control. Data substitution will be used for out of control periods. The data substitution method is generally not advantageous so it is critical to get the CEMS back into control as soon as possible. </p><p>The EPA also allows for the use of Continuous Emissions Monitoring Calibration Systems which dilute gases to generate calibration standards.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> The analyzer reading must be <a href="/facts/Accurate/JQ47xXDq">accurate</a> 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). </p><p>In the U.S., periodic evaluations of the equipment must be reported and recorded.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> This includes daily calibration error tests, daily interference tests for flow monitors, and semi-annual (or annual) RATA and bias tests.<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> 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.<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a><a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> </p> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>The Babcock & Wilcox Company (2005). Steam: its generation and use. The Babcock & Wilcox Company. pp. 36–5. ISBN 0-9634570-1-2. <a href="0-9634570-1-2" target="_blank">0-9634570-1-2</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>[Jahnke, James] Company (2001). Continuous Emissions Monitoring Systems:2nd Edition. Wiley. ISBN 978-0471292272. <a href="978-0471292272" target="_blank">978-0471292272</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>United States Code of Federal Regulations, Title 40, Part 60, 63, 72, and 75 <a href="/wiki/United_States_Code_of_Federal_Regulations" target="_blank">/wiki/United_States_Code_of_Federal_Regulations</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>"Federal Register :: Request Access". <a href="https://www.ecfr.gov/cgi-bin/retrieveECFR?gp=&SID=1134dfa8ef4327a6889f0bd9c267b205&mc=true&n=pt40.7.60&r=PART&ty=HTML" target="_blank">https://www.ecfr.gov/cgi-bin/retrieveECFR?gp=&SID=1134dfa8ef4327a6889f0bd9c267b205&mc=true&n=pt40.7.60&r=PART&ty=HTML</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>"Continuous Emissions Monitoring Fact Sheet". US EPA. Archived from the original on February 11, 2009. <a href="https://web.archive.org/web/20090211082920/http://epa.gov/airmarkets/emissions/continuous-factsheet.html" target="_blank">https://web.archive.org/web/20090211082920/http://epa.gov/airmarkets/emissions/continuous-factsheet.html</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>"Tier 2 Generation Capital Projects". www.pnm.com. Retrieved 23 February 2016. <a href="https://www.pnm.com/documents/396023/2708993/11B_Testimony+and+Exhibits+of+Chris+M.+Olson+pt2.pdf/dff3e6eb-25cc-4e03-bbc0-ae2acf7a3755" target="_blank">https://www.pnm.com/documents/396023/2708993/11B_Testimony+and+Exhibits+of+Chris+M.+Olson+pt2.pdf/dff3e6eb-25cc-4e03-bbc0-ae2acf7a3755</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>"EPA Traceability Protocol for Assay and Certification of Gaseous Calibration Standards, "EPA-600/R93/224. Revised September 1993. <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>"Continuous Emissions Monitoring Fact Sheet". US EPA. Archived from the original on February 11, 2009. <a href="https://web.archive.org/web/20090211082920/http://epa.gov/airmarkets/emissions/continuous-factsheet.html" target="_blank">https://web.archive.org/web/20090211082920/http://epa.gov/airmarkets/emissions/continuous-factsheet.html</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>"Specifications and Test Procedures for Total Hydrocarbon Continuous Monitoring Systems in Stationary Sources" (PDF). www3.epa.gov. Retrieved 23 February 2016. <a href="http://www3.epa.gov/ttnemc01/perfspec/ps-8a.pdf" target="_blank">http://www3.epa.gov/ttnemc01/perfspec/ps-8a.pdf</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>"CEMS | ESEGAS". ESEGAS. Retrieved 2025-05-07. <a href="https://esegas.com/product/cems/" target="_blank">https://esegas.com/product/cems/</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>"Flue Gas & Emissions Analyzers | Nova Gas". Nova Gas. Retrieved 2016-02-23. <a href="http://www.nova-gas.com/analyzers/flue-emissions" target="_blank">http://www.nova-gas.com/analyzers/flue-emissions</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> </ol>