Is there an alternative to an expensive wet-extractive PM monitor on a stack with a wet FGD?

Modern coal-fired power plants use a range of methods to reduce the amount of pollution released to the atmosphere. Selective catalytic reduction is used to remove oxides of nitrogen (NOx), high-efficiency electrostatic precipitators remove particulate matter (PM) and wet flue-gas desulfurization (FGD) systems remove sulphur dioxide (SO2). An unfortunate side-effect of using a wet FGD is that the stack gases downstream are saturated with water drops. Consequently, typical instruments used for measuring PM emissions such as opacity monitors and in-situ light-scattering monitors cannot be used because they cannot distinguish between dust particles and water droplets.

Is there an alternative to an expensive wet-extractive PM monitor on a stack with a wet FGD?One approach is to use a heated extractive PM monitor such as the AMETEK Land 4650-PM EXN. This type of instrument removes a sample of gas from the stack, heats it to evaporate any condensed water and then measures the PM content using a light-scattering instrument. This approach is effective, but the equipment is expensive and, because it includes a sample system, it requires a lot more maintenance than a traditional instrument such as an opacity monitor. Another drawback is that a PM-CEMS requires a correlation test to calibrate the scattered light measurement against the amount of PM emitted from the stack. To perform the correlation test, a skilled team makes a series of reference-method measurements over several days and AMTEK Land 4500 MkIIIrelates them to the readings from the PM-CEMS. Because of the time taken and the level of skill needed, such tests are inevitably rather expensive.

An alternative approach is to mount an opacity monitor between the precipitator and the wet FGD. There is no condensed water at this point so it is possible to use a standard opacity monitor. This avoids the complication and cost of both the extractive analyzer and the correlation test. The drawback is that the wet FGD scrubs some of the residual PM from the flue gas, so the opacity monitor reads higher than it would if it could be mounted downstream of the FGD, so there is less operational flexibility for the plant to remain within its permitted emissions levels. Even so, the extreme efficiency of modern PM arrestment equipment means that this is a worthwhile trade-off for many site operators.

To learn more about AMETEK Land's range of opacity and dust monitors, click here
 

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