Continuous Emission Monitoring. James A. Jahnke
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Название: Continuous Emission Monitoring
Автор: James A. Jahnke
Издательство: John Wiley & Sons Limited
Жанр: Биология
isbn: 9781119434023
isbn:
Figure 3‐25 Principal factors causing changes in dilution systems.
Scenario 1 A dilution system is calibrated initially when the total stack pressure PT = Pbar + ps, the unit is on, the flue gas temperature = To, and the calibration gas has an average molecular weight = Mo. Typically, any variation from the nominal dilution ratio is adjusted out by setting the analyzers to the certified calibration gas concentration during calibration.
Scenario 2 A dilution system is calibrated at a stack absolute pressure of Po. A weather front moves in and the atmospheric pressure is reduced. A calibration check will indicate an upward drift in reading.
Scenario 3 A dilution system is installed on a cycling unit and is calibrated when the unit is off, at a temperature To. After the unit is turned on and the stack temperature increases, a calibration check will indicate a downward drift in reading.
Scenario 4 A dilution system is calibrated with an SO2 in N2 calibration gas. An auditor conducts a linearity test using a gas mixture containing SO2 and 20% CO2. The test gives a reading lower than expected.
These are all scenarios that may cause discrepancies in dilution system measurements. However, these discrepancies can be corrected (i) empirically or (ii) theoretically. Empirical corrections, based on experimental data from installed systems, have been used successfully in specific applications. Lacking experimental data, theoretical expressions can be used and have also been successful in correcting for the effect of gas density variation on the dilution ratio.
Empirical Corrections.
Empirical correction equations for pressure and temperature have been given by Jahnke and Marshall (1994) as follows:
Pressure Correction Equation.
Pressure changes may be caused by changes in either stack pressure or ambient pressure. It is not uncommon to observe an apparent reduction in pollutant gas concentrations when a weather front passes a CEM installation having a dilution system uncorrected for pressure. In one study, this pressure effect was noted as causing a 1% error for each 3.45 in. of water pressure change (Jahnke and Marshall 1994; see Figure 3‐26).
An empirical correction equation that has been used to account for pressure changes in the EPM dilution probe is given in Equation 3‐4:
Figure 3‐26 Pressure dependence of the critical orifice dilution system.
Source: Jahnke and Marshall (1994).
where
∆P is the difference between the stack absolute pressure when the measurement is made and the pressure when the system was calibrated.
This equation appears to be general for dilution systems that do not use mass flow controllers for the dilution air. It would be prudent, however, to check the validity of the empirical coefficients on the installed system and adjust them for any system‐specific characteristics. Procedures for doing this are given in Jahnke and Marshall (1994).
Temperature Correction Equation.
Temperature effects contribute approximately 1% error for each 50 °F change in temperature from the initial dilution probe calibration setting (Jahnke and Marshall 1994). The temperature effect, however, is nonlinear. Temperature fluctuations of this magnitude do not usually occur in most operating units. However, for cycling units and other units that operate infrequently, a common practice has been to perform calibration adjustments to the dilution system when the unit is cold (not operating) and assume that the calibration holds during start‐up and when the unit is hot. This is not a valid assumption and can lead to measurement error greater than 10% due to the nonlinearity of the temperature dependence.
Probe temperature dependence can be minimized by heating the probe, although this has not always been successful during start‐up due to cooling of the heater by initially cool flue gas. A better approach, particularly for cycling units, is to use an external dilution system outside of the stack, where the temperature can be better controlled.
Alternatively, but not as satisfactory as maintaining probe temperature, empirical temperature correction equations can be developed. One equation developed for the EPM dilution probe is given in Equation 3‐5.
This correction equation appears to be system specific (Jahnke and Marshall 1994) and should only be used as a first approximation for small changes in temperature (e.g. ± 50 °F). It is recommended that the user check the expression for his or her own installation and adjust it as appropriate. It does not appear to work well in cycling units where large swings in temperature are frequent.
Molecular Weight Correction Equation.
Since the flow through the critical orifice is dependent on gas density, molecular weight of the sample gas also affects the dilution ratio. Fluctuation of flue gas moisture and CO2 concentrations can cause errors in the dilution measurements since their concentrations are respectively low (18) and high (44) relative to the usual combustion gas molecular weight of ~30. Also, with regard to earlier Scenario 4, the use of multi‐blend calibration gases containing CO2 initially caused considerable confusion in conducting calibration error (drift) and linearity tests. The higher molecular weight of CO2 increases the density and will cause lower readings if the dilution system was previously calibrated with a cylinder gas containing nitrogen (MW 28) only, as a background gas. Errors due to fluctuations in molecular weight have been calculated to approach up to 7%, depending upon the initial calibration conditions and measurement conditions (McGowan 1994).
This issue was first resolved by Miller (1994) who calculated relative sonic velocities for СКАЧАТЬ