Four Types of Continuous Emission Monitoring Systems

Continuous emissions monitoring systems (CEMS) are required by some Environmental Protection Agency (EPA) regulations. A CEMS is defined as the equipment necessary to continuously monitor, analyze, and record the gas concentrations, particulate matter content, and/or flow rate of an emission. They include the samplers, analyzers, software, and recording devices necessary to show compliance with EPA regulations. There are four basic types of CEMS: source-level extractive, dilution extractive, point in-situ, and path in-situ.

Source-Level Extractive

• Source-level or direct-source extractive CEMS extract a sample from the flue gas which is transported by way of a heated sample line into a sample conditioner. The conditioner filters out particulate matter and dries the sample to remove moisture. Once conditioned, the sample is analyzed to determine concentrations using techniques that include infrared and ultraviolet absorption, beta ray absorption, fluorescence, or chemiluminescence (the analysis of light emitted by a chemical reaction.)

Dilution Extractive

• Dilution extractive CEMS operate similarly to the source-level CEMS, but before the sample is conditioned it is mixed with dry, clean air to a ratio between 50 to 1 and 200 to 1. The dilution extractive method is necessary when the flue gas is too wet, hot, or polluted for the analyzing equipment to handle. The principal drawback for this method versus source-level extraction is that it cannot be used to measure the oxygen concentration of the flue gas, since the sample is mixed with air.

Point In-Situ

• In-situ CEMS differ from extractive CEMS in that they use instruments to continuously monitor the flue gas directly and do not use an extracted, conditioned sample. Point in-situ CEMS continuously measure the concentrations directly from the stack gas at a single point or along a short path within the stack.

Path In-Situ

• Analysis and recording are an important part of a CEMS.

  Path or cross-stack in-situ CEMS, like the point in-situ CEMS, continuously measure the concentrations directly from the stack gas. However, instead of measuring the concentrations at a single point, they use a light beam projected across the entire path of the stream of gas to analyze the concentrations.

  Both in-situ techniques have the advantage of timeliness over the two extractive methods; there is no time-delay. They also avoid many of the sampling problems faced by the extractive methods such as absorption of sample components, moisture from condensation, and chemical reactions between the sample components.