Continuous Emission Monitoring, 3rd Edition

CHAPTER 6IN‐SITU GAS MONITORING SYSTEMS

Problems with extractive monitoring systems have led to the development of monitors that can directly measure source‐level gas concentrations in a stack. These so‐called “in‐situ” systems do not modify the flue gas composition and are designed to detect gas concentrations in the presence of particulate matter. In‐situ units are constructed to withstand environmental conditions at stack locations and have fewer subsystems than do extractive systems. In‐situ measurements can be made either over an integrated path across the stack or at a point within the stack, leading to two types of commercial designs: “integrated‐path” and “point” in‐situ systems – point monitors are also referred to as in‐stack or short‐path monitors and the integrated‐path monitors are also called cross‐stack monitors. As the terminology implies, point monitors measure essentially at point in the stack, whereas integrated‐path monitors measure the flue gas over a path extending from one side of the stack to the other.

In‐situ systems were developed in the late 1970s and were installed widely in electrical utilities and industrial sources. Although the systems performed relatively well at high concentrations, problems were found in measuring lower gas concentrations resulting from increasingly stringent emission control programs. Also, quality assurance requirements were established in the 1980s in the United States, specifying that quarterly assurance audits using ...