External Dilution Systems (Probes)
Another approach to diluting the flue gas is not to dilute it in the stack, but outside of the stack in a dilution assembly contained in a protective housing close‐coupled to the stack. External dilution systems can be more easily serviced and temperature controlled than an in‐stack dilution probe; however, in principle, their response time is somewhat slower due to the length of time necessary to transport the sample from the probe tip to the dilution orifice. External dilution probes do offer several advantages over in‐stack dilution probes. They can be heated relatively easily to maintain a more constant temperature. In wet scrubber applications, they can resolve droplet and aerosol condensation problems that may be more difficult to address with an in‐stack critical orifice. For example, by sloping the stinger of an external system, droplets can run off into the stack before they reach the critical orifice. The external systems also offer a significant maintenance advantage by not requiring the probe to be extracted to examine or replace the filter or critical orifice.
Figure 3‐21 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 a system designed by M&C Tech Group, an assembly, close‐coupled to the stack, is unique by cleverly incorporating both the dilution eductor and a dilution capillary within a Swagelok cross fitting (Figure 3‐22). This makes for a compact design where the temperature can be easily maintained.
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.
Another M&C Tech Group external dilution probe design is shown in Figure 3‐23. This second‐generation design by M&C is more modular and robust than the design shown in Figure 3‐22. It is again compact and easily heated and has been successful especially in mercury monitoring applications. In this design, two modular heated blocks, located on the top of the assembly, enable the flue gas flow and sample dilution using two ejector pumps. In operation, the bypass eductor contained within the first block draws the sample through the probe filter. As in the external probe shown in Figure 3‐22, the dilution eductor in the second block draws a slipstream from the bypass eductor flow line at a lower flow rate. Again, clean, dry, conditioned air from the dilution air cleanup system provides the motive force for the pump vacuum and also serves to dilute the flue gas that it draws in from the slipstream. The flow rate of the sampled gas is controlled by a glass capillary serving as a critical orifice under the sonic flow conditions.
Figure 3‐22 External dilution system with cross‐piece dilution unit.
Figure 3‐23 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.
The first external dilution probe was developed to address plugging of dilution orifices by water droplets in the EPM probe by Sampling Technology, Inc. (STI) and is available from STI and Thermo Fisher Scientific (Figure 3‐24). This external dilution system, made of Torlon, is shown in Figure 3‐24 (Fischer 1993). It again uses an ejector pump to extract the sample from the stack and has been popular in pulp and paper industry applications, particularly after wet scrubbers. The flue gas sample extracted at low flow rates is first filtered by a tubular filter, where particulate matter and aerosols not dropping out in the probe are collected. A quartz capillary serves as the critical orifice, which controls the sample gas flow rate. The sample gas combines with the ejector pump air as in other dilution systems described.
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.