An in situ system measures a gas as it passes by the analyzer in a stack. Figure 5.12.2 shows an in situ probe-type analyzer. The measurement cavity is placed directly into the sample flow system to measure the gas received on a wet basis.
The most commonly accepted in situ analyzer is the zirconium oxide (ZrO2) oxygen analyzer. It is the most reliable method for measuring and controlling a combustion process. Since the introduction of ZrO2 analyzers, many other gases have been measured in situ with light absorption instrumentation, such as UV and IR spectrometers. The gases that can be measured with such spectrometers include CO, CO2, SO2 and NO.
Most of the first analyzers designed to measure these gases have disappeared. Most noticeably, across-the-stack technology units are no longer used in this country due primarily to the promulgation of Title 40, Part 60,
Appendix F of the Code ofFederal Regulations by the EPA in 1986. Appendix F requires that an analyzer be able to complete gas calibrations and that it be certified quarterly. An across-the-stack unit works like an opacity instrument in that it lacks the capability of using protocol gases for cylinder gas audits (CGA) as allowed by the EPA under Appendix F.
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