the Operation of Dilution Systems
In today's applications for CEM systems, new demands have been made for dilution systems and CEM systems in general. For example, in the acid rain program, relative accuracy specifications became more stringent at 10%, daily calibration error (drift) is limited to 2.5% of span, and a 5% linearity criterion has been established. Although relative accuracy specifications are not difficult to meet, it is found that a clear understanding of the operating characteristics of dilution systems is necessary to achieve the optimum performance necessary for meeting the drift and linearity test specifications.
Figure 3‐24 STI external dilution system design.
In dilution systems, the flue gas concentration is calculated from the dilution ratio as follows:
where
c = the calculated source‐level concentration of the gas (ppm)
cmeas = the analyzer response (ppm) to the diluted sample
Do = the dilution ratio at the time of calibration
This equation assumes that the dilution ratio remains the same while the reading cmeas is obtained as when the dilution system was calibrated initially. This is not always the case. If the absolute pressure changes from Po to P, the stack temperature changes from To to T, or the molecular weight of the sampled gas changes from Mo to M, the dilution ratio will change also. Pressure, temperature, and molecular weight all affect gas density, which affects the sonic flow of the gas through the orifice. A change in gas density will therefore affect the dilution ratio.
Problems arise when the original conditions at which the dilution system was first calibrated change for any subsequent measurements. Figure 3‐25 illustrates the principal factors that may cause a change in the dilution ratio.Consider the following scenarios illustrated in the figure:
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