Читать книгу Continuous Emission Monitoring - James A. Jahnke - Страница 95

The main problem associated with source‐level extractive systems is the need to filter and condition relatively large volumes of stack gas. This problem can be largely avoided by using dilution systems where gas is drawn into the probe at low flow rates, sometimes two orders of magnitude less than in a source‐level system (e.g. 0.05 vs 5 l/min.). This means that there will be less particulate matter to filter and less moisture to remove. Because the flow is relatively low, particles are more likely to follow the flue gas streamlines around the probe than to enter the probe.

Dilution systems are used in conjunction with ambient air level analyzers – a feature that can provide significant advantages to a source that has had previous experience with ambient air level analyzers or that is operating an ambient air network. In the case of analyzer problems, CEM system analyzers could be swapped for ambient air level analyzers or spares maintained for both purposes. Plant technicians already may be familiar with the operation and maintenance needs of the analyzers and may not require additional training.

There are two approaches to designing dilution systems. One approach is to dilute the stack gas in an in‐situ, “in‐stack” sample probe (Figure 3‐17). Here, the eductors and dilution orifice are contained in the probe itself. Another approach is to dilute the stack gas outside of the stack (Figure 3‐18) in a module containing the eductor and the dilution capillary or orifice. As with fully extractive‐system conditioning systems, using a dilution module outside of the stack, dilution can be performed at either the stack or the CEM shelter. If diluted at the stack, unheated sample lines can be used to transport the gas to the analyzers, but if diluted at the shelter, heat‐traced lines must be used.

Figure 3‐17 An in‐situ (in‐stack) dilution probe CEM system.

Figure 3‐18 An external dilution CEM system.

The dilution systems have seen widespread application in monitoring emissions from electrical utilities affected by EPA's acid rain program. In this program (U.S. EPA 2020a), emissions are reported in units of mass/time (i.e. lbs/hr or tons/year), calculated by multiplying a wet basis flow rate measurement and a wet basis pollutant gas measurement. Since dilution systems measure gases on a wet basis, the calculation is straightforward, not requiring a determination of flue gas moisture content. This feature, as well as the advantages inherent in sampling at low flow rates, has made it popular.