Читать книгу Continuous Emission Monitoring - James A. Jahnke - Страница 16

Remote sensing systems have no interface between the stack gases and the sensing instrument, other than the ambient atmosphere. They thus avoid problems associated with a stack or duct interface. These systems can detect emission concentrations merely by projecting light up to the stack (active systems) or by sensing the light radiating from the “hot” molecules emitted from the stack (passive systems). However, due to an inherent problem in defining the length of the measurement path in the plume, the accuracy of gas concentration data is poorer than that obtained by the extractive or in‐situ techniques. This problem is also an issue in making flue gas measurements using laser systems mounted on unmanned aerial vehicles (drones), a developing source monitoring technology.

The U.S. EPA has developed Method 9A for monitoring stack exit opacity, using a laser light detection and ranging technique (LIDAR). The method is particularly useful at night or under atmospheric conditions that are not favorable to a visible emissions observer (VEO) performing EPA test method 9. In a related development, a digital camera technique for measuring plume opacity has been standardized by ASTM in ASTM standard D7520‐16. Although not a continuous method, data using this method are being accepted by regulatory agencies for visible emissions compliance determinations.

Test methods and certification procedures have not been standardized for remote sensing systems used to monitor gaseous emissions. Since the regulatory applicability of remote pollutant gas measurements to stationary sources has not yet been established, these systems for source emissions monitoring are not extensively applied in commercial applications. They have, however, seen wider application in “fence‐line” monitoring, particularly at petroleum refineries and chemical plants. Attempts are frequently made to correlate emissions data with long‐path remote sensing data obtained along a plant perimeter. Such correlations typically devolve into research studies and have met with limited success as a regulatory tool.

Unmanned aerial vehicles (UAVs) (drones) offer significant potential in source monitoring applications (Villa et al. 2016). UAV systems can obtain grab sample for laboratory analysis, or, as part of an active remote system, incorporate a stationary mirror to reflect light projected from a ground‐based analyzer. Using lasers, two drones standing‐off 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.