Continuous Emission Monitoring. James A. Jahnke
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gas concentration
Because the pollutant concentration is divided by percent CO2, any constant change in the dilution ratio cancels out in the concentration ratio. When reporting emission mass rates in pounds/hour, the pollutant mass rate equation is not expressed in a form where flue gas density effects on the dilution ratio cancel, and changes in the flue gas density must be taken into account.
Other Correction Methods.
In conjunction with Pennsylvania Power and Light (PPL), the Lehigh University Energy Research Center developed a dilution ratio calculation system called “DRCalc™” (Batug et al. 2004; Romero et al. 1999, 2002, 2005). After the dilution ratio is set initially, this system provides corrections for changes in the dilution air supply pressure, dilution air supply temperature, stack pressure and temperature, the sampled gas molecular weight, and the calibration gas molecular weight. Signals from temperature, pressure, and sensors, in addition to flue gas composition data, are sent to the DRCalc™ unit, where proprietary software is used to continually recalculate the dilution ratio. This system has been patented (Batug et al. 2004) and was successfully applied at PPL facilities (Sale 2000a, b). It also incorporates an algorithm for changes in the dilution probe flue gas temperature, f(T).
For the EPM dilution probe, an apparatus has been designed (Baugham 1996, 1997) that uses a regulator to adjust the flow of dilution air to maintain a consistent dilution ratio to compensate for gas density changes. The advantage of this apparatus is that it can easily retrofitted into existing systems without having to make computational changes in the data acquisition system. Atmospheric and stack pressure and sample temperature are monitored to provide inputs into the system. Equation 3‐6 is used to adjust for sample temperature changes. The system can also provide for changes in gas density as the flue gas moisture or oxygen concentrations vary.
The Electric Power Research Institute developed an algorithm for in‐stack and external dilution system corrections in their “CEMS Analyzer Bias and Linearity Effects (CABLE) study” (Berry 2000). The study was conducted to improve the accuracy of CEM systems used in the acid rain program and provided recommendations for the implementation of a correction algorithm based on Equation 3‐9. In this algorithm, the temperature dependence, f(T), was expressed empirically as f
Dilution Extractive Systems: Wet or Dry Measurement?
A question that frequently arises when considering the use of a dilution system is whether the emission values are given on a wet basis or a dry basis. Dilution systems that do not dry the diluted sample gas give concentration values (ppm) on a wet basis. If there is no moisture removal, there will still be moisture in the diluted sample. When the diluted concentration (measured with ambient air level analyzers) is scaled back up by multiplying it by the dilution ratio, the moisture content is scaled back up also. The concentration of the pollutant gas in the original wet sample is obtained.
Consider an example of 1 ml of flue gas that contains 0.1 ml of water vapor (10% H2O), which is diluted with 99 ml of dry air (Figure 3‐27). In the diluted sample, the percentage of water vapor is reduced to about 0.1% of the total volume. Essentially, the analyzer is measuring SO2 in a background gas containing 99.9% dry gas and 0.1% water vapor. Therefore, the flue gas SO2 concentration obtained is given on a wet basis. When the diluted concentration is scaled back up by multiplying it by the dilution ratio, the concentration of SO2 in the original wet sample is obtained.
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:
where
cd = dry‐basis concentration value (ppm or percent)
cw = wet‐basis concentration value (ppm or percent)
Bws = the moisture fraction of the flue gas
A value for Bws 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 Bws. 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 Fc 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