Continuous Emission Monitoring. James A. Jahnke

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Continuous Emission Monitoring - James A. Jahnke


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      External Dilution Systems (Probes)

Schematic illustration of an in-situ dilution probe extractive system.

      Several external dilution system designs are available. Each design is associated with a particular manufacturer and different industrial sectors, such as coal‐fired power plants or pulp and paper plants, tend to favor one particular design over others. In CEM system upgrades, external dilution systems often replace in‐stack dilution probes, not so much for any inherent faults in the in‐stack probes, but more often for not having to remove the probe from the stack for maintenance or because the designs are newer and “trendy.”

       Cross‐Piece Fitting Design.

      In locating the dilution system outside of the stack at the end of the probe, the characteristic low flow rate of a dilution system would decrease the response time of the system to real‐time flue gas concentration fluctuations. This is in contrast to an in‐stack dilution probe, where the flue gas is diluted essentially at the probe tip and the diluted sample is transported relatively swiftly to the analyzers. A solution to this problem in external dilution probes is to add a venturi bypass eductor to transport the flue gas to the analyzer at a relatively high flow rate of 3–5 l/min. A slipstream from this flow is then extracted by the dilution eductor at a lower flow rate on the order of 30–50 ml/min. The flue gas sample is then diluted after the critical orifice (see the inset in Figure 3‐22) by mixing with the motive air flowing through the dilution eductor. It is necessary that the filter and other system components connected prior to sample dilution be heated to avoid condensation by flue gas moisture.

      Using a bypass eductor solves a response time problem; however, by bringing the flue gas sample in at a high flow rate, the low flow advantage of the in‐stack dilution probe is lost. Higher flow rates will transport higher levels of particulate matter through the probe to the probe filter than the lower flow of the dilution eductor in transport of the flue gas. This will require that the probe filter and a probe be periodically inspected and cleaned, and may require the installation of a blowback system.

       Modular Block Design.

Schematic illustration of external dilution system with cross-piece dilution unit. Schematic illustration of dilution system with modular block dilution unit.

      In the daily calibration verification of the CEM system, the calibration gas is sent through the calibration/purge gas line (shown in Figure 3‐23 at the bottom of the assembly) at a flow rate sufficient to flood the annulus outside of the filter, expelling the flue gas. The calibration gas is then extracted by the CEM system similarly to the sample gas, being conditioned in the same manner as the sample gas.

       The STI Probe.

      As mentioned previously, one advantage of an external dilution system is that an undiluted sample can be first sent to an O2 analyzer before it proceeds to the dilution system. This provides an alternative to using a CO2 analyzer for the diluent monitor. A separate sample line can be installed after the probe filter, but before the dilution orifice, to draw an undiluted sample. This sample can then be conditioned to remove water vapor and analyzed with an oxygen sensor.

      An alternative approach to external dilution systems is to transport the gas from the stack in the traditional manner of a source‐level extractive system and to dilute the gas in the CEM shelter. This does provide for rapid gas transport, but a heated sample line must be used. This approach loses one of the principal advantages of dilution systems – their ability to deliver the sample to the analyzers with unheated sample line.


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