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EPA sets forth various methods for continuous monitoring of stack emissions, including:
Oxygen or carbon monoxide (or both) - Diluent measurements can be made using paramagnetic technology, fuel cells, or zirconium oxide techniques. Carbon dioxide is generally measured using Non-Dispersive Infra-Red technology.
Sulfur dioxide - Determined either through ultraviolet absorption or through Pulsed Flourescence.
Nitrogen oxides - Using Chemilumenescent technique that 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 NOX is measured by catalytically converting any NO2 in the sample to NO prior to analysis.
Carbon monoxide - Principle of operation is non-dispersive infrared (NDIR) absorption. Traditional NDIR is subject to interference from carbon dioxide and water vapor. More recent technology has introduced gas-filter-correlation/NDRI technology that virtually eliminates these interferences. The technology can be extended to a variety of other compounds, including HCl.
Oxygen, carbon dioxide, and nitrogen oxide or sulfur dioxide (or both) - Requires a preliminary oxygen traverse for determination of the subsequent test points.
Volatile organic compounds (VOC), as total hydrocarbons (THC) - Analysis if on a hot-wet basis uses a flame ionization detector.
A variety of techniques can be used to reduce VOC emissions. Using material containing an inherently low quantity of VOC compounds will reduce the release of VOCs. Also, the processes can be redesigned to reduce the quantities that are lost as fugitive emissions. When these techniques are inapplicable or insufficient, add-on control systems, or other techniques can be used:
Thermal oxidation
In a thermal oxidizer, the VOC-laden air stream is heated to gas temperatures several hundred degrees Fahrenheit above the autoignition temperatures of the organic compounds that need to be oxidized. Due to these very high temperatures, thermal oxidizers have refractory-lined combustion chambers (also called fume incinerators), which increase their weight and size considerably.
The VOC-laden gas stream is held at this temperature for residence times ranging from a fraction of a second to more than two seconds. Temperatures of the exhaust gas from the refractory-lined combustion chambers are often 1,000°F to 2,000°F. Thermal oxidizers usually provide VOC destruction efficiencies that exceed 95% and often even exceed 99%.
Thermal oxidizers have the broadest applicability of all the VOC control devices. They can be used for almost any VOC compound. Thermal oxidizers can also be used for gas streams having VOC concentrations at the very low concentration range of less than 10 ppm up to the very high concentrations approaching 10,000 ppm. Thermal oxidizers are rarely used on gas streams having VOC concentrations exceeding approximately 25% of the lower explosive limit (LEL).
