Continuous Emission Monitoring. James A. Jahnke

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Figure 3‐27 Example of a wet flue gas being diluted with dry air.

      Consider again from Figure 3‐27 that the diluted SO₂ concentration is measured to be 3 ppm. This means that for every 100 ml of gas analyzed, the volume of SO₂ equals 3 × 10⁻⁶ × 100 = 3 × 10⁻⁴ ml. However, because no SO₂ is in the dry air used for dilution, this must be the volume of SO₂ gas contained in the 1 ml of flue gas drawn into the dilution system. A volume of 3 × 10⁻⁴ ml in 1 ml is equivalent to a flue gas concentration of 3 × 10⁻⁴ × 10⁶ = 300 ppm. Because, in this example, the 1 ml of flue gas sample also contains 0.1 ml of water vapor, the 300 ppm value is actually a wet‐basis value.

To report a dry‐basis SO₂ emission concentration, the moisture content must be known. It can be calculated by using Equation 3‐11:

(3‐11)

      where

      A value for B_ws can be obtained by either installing some type of moisture monitor, using a value obtained by manual stack testing, or by estimating a value based on process parameters. Using a moisture analyzer is the most straightforward approach; however, this adds another analyzer to the CEM system. A common technique is to measure oxygen on both a wet and a dry basis and using the results to compute B_ws. Also, various calculation approaches have been proposed (Aldina 1985; McGowan 1976) that involve the manipulation of combustion source F factors. But frequently, a moisture value obtained from manual stack test measurements is set as a constant factor to make the appropriate corrections. This approach assumes that variation in the value will be small under normal source operating conditions.

It is not necessary to convert dilution measurements into a dry basis if emission measurements are to be reported on an actual or wet basis. If the emissions are to be reported in terms of a pollutant mass rate (pollutant mass/unit time) as in the acid rain program, the wet basis measurement can be used directly. Also, if emissions are reported in terms of ng/joule or lbs/million Btu, a moisture correction is not needed if CO₂ is monitored along with the pollutant. This is due to the form of the F factor equation used to calculate emission rate in ng/Joule or lbs/million Btu as discussed earlier (see Equation 3‐10). When CO₂ is monitored as a diluent and the F_c factor equation is used, moisture content cancels out in the ratio between the pollutant concentration and CO₂ concentration. This is also discussed in EPA Test Method 19 (U.S. EPA 2020b).

Lastly, one should be aware of inadequacies associated with the use of dry calibration gases to check the performance of wet systems. Cylinder gases used for calibration or audit purposes are dry gases, containing no water. The molecular weight of a dry gas used to establish the calibration of a dilution system will be different than the molecular weight of a flue gas containing percent levels of moisture. As discussed earlier, this will affect the dilution ratio and introduce bias into the measurement. Also, some analyzers are subject to interference by water vapor. Here, a dry calibration gas may not truly establish the calibration function for the analyzer and the wet flue gas measurements will be in error. Additionally, water can adsorb onto sample tubing and internal instrument surfaces and subsequently reduce the adsorption of other gases such as volatile organic compounds (Peeler et al. 1997). Daily calibration with dry gases can disrupt this effect and cause some disequilibrium in the measurements for a period of time after calibration.

SAMPLING INTERFACE/MONITOR CALIBRATION

      The entire extractive system sampling system and gas analysis system must be capable of being calibrated as a unit. In the design of the system, calibration gases should be able to be injected as close as possible at the probe – a recommendation made in fact by the EPA in its Appendix F quality assurance requirements and a requirement for cylinder gas audits and linearity checks. This is necessary to check for leaks or other operational problems in the system. In calibrating source‐level‐extractive systems, the analyzer should be calibrated at the same gas flow rate, pressure, temperature, and operating procedures that are used in monitoring the stack gas. In calibrating dilution systems, the calibration gas must enter the system before the critical orifice. Flooding the coarse filter or dilution probe with calibration gas СКАЧАТЬ