Dilution Extractive Systems

The newest, accepted measurement system is dilution-extractive CEM. By precisely diluting the sample system at stack temperature with clean, dry (lower than —40°F dew-point) instrument air, dilution-extractive systems eliminate the need for heat tracing and conditioning of extracted samples. Particulates are filtered out at the sample point. Thus, a dilution system measures all of the sample along with the water extracted with the sample on a wet basis. Figure 5.12.4 shows a schematic of a dilution-extractive system.

The dilution-extraction CEM system uses a stack dilution probe (see Figure 5.12.5) designed in Europe. A precisely metered quantity of flue gas is extracted through a critical orifice (or sonic orifice) mounted inside the probe. Dilution systems deliver the sample under pressure from the dilution air to the gas analyzers. Thus, the system pro tects the sample from any uncontrolled dilution from a leak in the sample line or system.

The environmental engineer selects the dilution ratios based on the expected water concentration in the flue gas and the lower limit of the ambient air temperature to avoid freezing. This kind of sample line is still considerably less expensive than the heated sample line for wet-extractive CEM.

When choosing the dilution ratio, the engineer must also consider the lowest pollutant concentration that can be detected by the monitoring device. These systems use a dilution ratio ranging from 12:1 to more than 700:1. Irrespective of the dilution ratio, the diluted sample must match the analyzer range.

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