from a stack exit can make cross‐stack gas measurements. Four drones could measure on two diameters. UAV platforms can carry simple miniature sensors, readily available from hand‐held monitors, to measure flue gases in real time using a stand‐off probe or by flying through the stack. For compliance measurements, a probe can be inserted from the UAV “down” the stack, as it hovers at a fixed position next to the stack. The availability, at present, of miniature and micro analyzers provides many options for analysis using UAV platforms. Flares and smaller process stacks with limited access or without sampling platforms are seen as potential applications for this technology.
Performance specification and certification procedures have not been developed for remote sensing systems or UAVs; however, this technology is relatively new. Calibration procedures and precision and accuracy issues relative to in‐stack measurements must first be standardized for UAV data to be credible. Standards developed by independent standards bodies such as ASTM or ISO may provide a basis for future agency requirements. Because the regulatory applicability of remote and UAV pollutant gas measurements to stationary sources has not yet been established, they will not be discussed further, but do bear watching in the technical literature.
Parameter Monitoring Systems
Alternative approaches to emissions monitoring have been developed that do not require the use of analytical instrumentation, but rely instead on inputs from process sensors, such as thermocouples, pressure transducers, and fuel flow meters. Data from these sensors can be used in a variety of ways in environmental regulatory programs. The parameter information can be either used directly as a surrogate to substitute for concentration‐based emissions data or it can be incorporated into a model to predict emissions.
U.S. regulatory programs have long used parameter data such as pressure drop or temperature to monitor the performance of emission control equipment. The parameter data has been used either as a regulatory trigger to initiate enforcement action directly or as an indicator of noncompliance with permit conditions. Control equipment and unit operational parameters can also be used directly in continuous parameter monitoring systems (CPMS) as part of a continuous monitoring system (CMS). This regulatory approach does not require the use of continuous emission monitoring systems although a CEM system can be a part of a CMS. The U.S. air toxics standards make extensive use of this method.
A more recent approach has been used to develop emission models based on process parameter data. Models are developed by first correlating parameter data to emissions data. An initial study is performed by varying and monitoring process and control equipment parameters while monitoring flue gas emissions using reference methods or CEM systems. One can then correlate the data using engineering calculations, least squares methods, or neural net techniques to develop a model that “predicts” emissions from parameter data. Such predictive emission monitoring systems (PEMS) employ from 3 to 20 input parameters and have been applied to a variety of sources. They are most successful on sources with minimal variation in fuels and operating conditions.
Analytical Techniques Used in CEM System Instrumentation
The analytical techniques used in extractive and in‐situ CEM systems encompass a wide range of chemical and physical methods. These vary from chemical methods using simple electrochemical cells to advanced electro‐optical techniques such as wavelength modulation and Fourier‐transform infrared spectroscopy. Table 1‐1 summarizes the analytical techniques that are used in currently marketed CEM systems for gases. Table 1‐2 gives a summary of analytical techniques used in continuous emission monitoring systems for particulate matter (PM CEMS).
Techniques used for laboratory analysis, as well as techniques applied specifically for emissions monitoring, have been incorporated into commercially marketed systems. New analyzers have been developed using established electro‐optical methods, but are beginning to incorporate new light sources and detectors, such as tunable diode lasers, quantum cascade lasers, and diode arrays and new techniques such as cavity ringdown spectroscopy. The incorporation of microprocessors into today's analyzers has added useful features such as data storage, troubleshooting diagnostics, and external communication.
To lower the cost of CEM systems, CEM system manufacturers are employing multi‐gas techniques to avoid subsystem duplication that occurs when using single‐gas dedicated analyzers. One approach is to use multi‐gas methods such as dispersive, FTIR, or photoacoustic spectrometry. Another approach is to incorporate discrete, multiple, and interchangeable sensors into a single chassis.
TABLE 1‐1 Analytical Techniques Used in Continuous Emission Monitoring Systems for Gases and Volumetric Flow/Velocity
| Gases | Flow/Velocity | |
|---|---|---|
| Extractive | In‐situ | In‐situ |
| Absorption spectroscopy: | Path: | Path: |
| Differential absorption | Differential absorption – IR/UV | Acoustic velocimetry |
| Photoacoustic | Second‐derivative spectroscopy | Time‐of‐flight |
| Gas filter correlation | Wavelength modulation | |
| Fourier transform IR | Gas filter correlation | |
| Luminescence methods: | Point: | Point: |
| Fluorescence (SO2) | Differential absorption – IR/UV | Differential pressure |
| Chemiluminescence (NOx) | Gas filter correlation | Thermal sensing |
| Electroanalytical methods: | ||
| Polarography | ||
| Potentiometry | ||
| Calorimetry | ||
| Electrocatalysis (O2) | ||
| Paramagnetism (O2) | ||
| Methods for HAPS: | ||
| Differential absorption | ||
| Gas chromatography | ||
| Mass spectrometry |
|
