CEM, continuous emissions monitors, monitor, stack, stack testing, tests

Continuous Sampling Techniques
*Continuous Instrumental Sampling Procedures.*
Continuous instrumental test methods are among the most useful and common stack testing techniques. Continuous instrumental test methods are usually integral to most large scale test programs. The majority of continuous methods are aimed at "criteria pollutants" including nitrogen oxides (NO + NO₂ = NO_X), sulfur dioxide (SO₂), carbon monoxide (CO) and volatile organic compounds (VOC); however, techniques exist for other compounds.
Continuous monitoring techniques also exist for the primary diluent gases such as oxygen (O₂) and carbon dioxide (CO₂). Using continuous methods allows us to constantly assess the state of compliance of the facility and to develop trends of emissions versus process parameters. USEPA Test Method procedures that utilize continuous instrumental analysis include: EPA Method 3A - Diluent Measurements of either oxygen and/or carbon monoxide. Requires a bias correction - alternative bias correction available for some O₂ analyzers. Oxygen can be measured using paramagnetic technology, fuel cells or zirconium oxide techniques. Carbon dioxide is generally measured using Non-Dispersive Infra-Red (NDIR) technology. EPA Method 6C - Determination of sulfur dioxide using continuous instrumental techniques. Even if you don't need to measure SO2, Method 6C spells out the QA/QC requirements for EPA Methods 3A and 7E as well. The principle of operation may be either through ultraviolet absorption (Bier's Law stuff) or through Pulsed Flourescence. The ultraviolet technique is less prone to interferences, but is not immune to them (We have found that HIGH concentrations of hydrocarbons influence these monitors). At normal source levels (ie a coal fired power plant or an oil fired process), they are your best bet - you should not experience any interferences. The pulsed fluorescence monitors are more sensitive and can measure lower concentrations, but are subject to "quenching" (interference) from carbon dioxide, oxygen and water vapor. This technology is improving in terms of elimination of interferences and this technique is often used in combination with dilution probes due to it's sensitivity. The quenching effect may be compensated for mathematically or by calibrating the instrument with a gas that has similar concentrations of the interfering gases as those experienced during sampling. EPA Method 7E - Determination of nitrogen oxides (NO_X = NO + NO₂) using continuous emissions monitoring techniques. Requires a bias correction. The only allowable principle of operation is the Chemilumenescent technique. Chemilumenescence measures only NO by reaction with excess ozone producing an NO₂ molecule in the cell for each NO, but also releasing a photon of a specific wavelength for each molecule which is measured by a photomultiplier tube. Total NO_X (NO + NO₂) is measured by catalytically converting any NO₂ in the sample to NO prior to analysis. Traditional NO_X analyzers use a stainless steel NO₂ to NO converter operating at approximately 650 degrees C prior to analysis. Other technologies include molybdenum converters operating at lower temperatures and newer dry chemical converters. Steel converters generally last longer, but may be subject to ammonia interferance; however, scrubbers may also be used to prevent this phenomenon. Method 10 - Continuous instrumental measurement of Carbon Monoxide. The principle of operation is non-dispersive infrared (NDIR) absorption. Traditional NDIR is subject to interference from carbon dioxide and water vapor. As a result, the test method was originally designed to incorporate an ascarite trap for absorption of CO₂ in the sample (which later had to be corrected for in the final concentration calculation). More recent technology has introduced gas-filter-correlation/NDIR (GFC/NDIR) technology which virtually eliminates these interferences. GFC/NDIR technology can be extended to a variety of other compounds including HCl. Method 20 - Continuous measurement of any of O₂, CO₂, NO_X and/or SO₂ specific to combustion turbines. Generally, this technique is used for New Source Performance Testing (NSPS) for oxygen and nitrogen oxides under 40CFR60, Subpart GG for combustion turbines (CT's). This technique requires a preliminary oxygen traverse for determination of the subsequent test points. Method 25A - Continuous determination of volatile organic compounds (VOC) as total hydrocarbons (THC). Analysis is on a "hot-wet" basis using a flame ionization detector (FID). New continuous emissions monitoring techniques are constantly being developed. Other interesting technologies include hydrochloric acid (HCl) continuous monitors and ammonia (NH3) monitoring systems, as well as systems capable of measuring multiple compounds such as fourier transform infrared (FTIR) spectroscopy and ultraviolet differential optical absorption spectroscopy (DOAS).