Continuous Emission Monitoring. James A. Jahnke

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СКАЧАТЬ mixtures. A molecular weight correction expression was developed by Appel (1994) for changes in flue gas composition valid under the condition that the dilution system is calibrated initially with single‐blend, nitrogen background cylinder gases:

(3‐6)

       Combined Corrections.

      If pressure, temperature, and molecular weight changes are affecting the dilution system response, the equations given above can be cascaded to obtain the following:

      (3‐7)

      Such an expression may not be necessary for all applications, but it does indicate that some insight must be put into the application and that more than one correction may be necessary.

       Theoretical Corrections.

      Although the theory of the critical orifice has been examined in detail, the construction and operation of dilution probe and its analogues may not satisfy all the underlying assumptions of the theory. Theoretical aspects of dilution systems were studied by Munukutla (1992) and Jahnke and Marshall (1994), Romero and Associates (1999), and Batug and Associates (2004). The extended formulation of Jahnke is provided here.

      Dilution system theory begins from the theoretical equation for critical flow (see ASME 1971; Green and Perry 2007; Sadegh and Worek 2017; Shapiro 1953). This equation is used to obtain a corrected dilution ratio, D, from a new set of gas conditions that differ from the conditions under which the system was originally calibrated for the initial dilution ratio, D_o (Jahnke and Marshall 1994; Munukutla 1992):

      (3‐8)

Instead of using Equation 3‐3 (c = c_measD_o) to calculate the source‐level concentration from the measured analyzer data, the following equation can be used to correct the data to the actual conditions of measurement:

(3‐9)

      This equation has been shown to adequately represent the effect of pressure and molecular weight changes on dilution system data (note: it is simplified by not incorporating changes in specific heat) (Jahnke and Marshall 1994; McGowan 1994; Miller 1994). Critical orifice theory predicts that the temperature variation . However, this expression is incomplete since it does not account for changes in the dilution air temperature. Baugham (1996) has done this and has developed an instrument control method that corrects for both probe and dilution air temperature variations. In later work, Romero et al. (1999) and Batug et al. (2004) applied computational fluid dynamic techniques to develop a model for evaluating temperature, pressure, and molecular weight effects in the EPM dilution probe.

Equation 3‐9 is especially useful for calculating molecular weight corrections that would apply to changing flue gas composition or cylinder gas audits using different gas blends. One can simply input the molecular weight of the initial calibration gas, M_o, and correct subsequent readings if the molecular weight of the test gas, M, is known.

      Note that these corrections may not be necessary if data are reported as an emission rate, E, in terms of lb/mmBtu (ng/J). If emissions are reported in units of lbs pollutant emitted/10⁶ Btu generated (or ng/J),

      the F factor expression can be used (see Appendix A).

(3‐10)

      where

       Fc = “F” factor in scf/106 Btu

       c = pollutant 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 СКАЧАТЬ