The emerging PM_2.5 ambient particulate standards will likely lead to the need for characterization of the emissions of fine particulate from a variety of industrial sources, including coal-fired utility boilers. This will require a new class of highly sensitive measurement systems to monitor low levels of small particle mass loading in industrial exhaust stacks.

Physical Sciences Inc. (PSI) has demonstrated the feasibility of employing a forward scattering nephelometer, using a near-infrared wavelength to enable specific, direct monitoring of PM2.5 particles in coal combustion flue gases. The device utilizes the principles of optical extinction and forward scattering to determine the mass measurement, and is non-intrusive to the exhaust flow. In addition, we have incorporated advanced room-temperature infrared diode laser and optical fiber coupling technologies currently being implemented at PSI for industrial gas-phase species sensors.

In laboratory experiments, PSI showed that this device can be used to measure mass loadings of PM_2.5 particulate on a real-time basis. The first set of experiments measured light extinction and scattering in a flow of monodisperse latex spheres of known concentrations. In this way we could compare measurements of scattering and extinction in a relatively controlled environment to those predicted by Mie scattering theory. For the second set of experiments, the optical measurement device was attached to the base of a laboratory coal combustor. Two coals were burned and ash was drawn from the reactor at several different rates for both coals to measure extinction and scattering signals for varying ash flows. Particle size distribution measurements were made at the same flow conditions using a cascade impactor. These experiments showed that this technique was successful in enabling the determination of the mass of fine particulate material under 2.5 microns in diameter while being insensitive to the larger particulate in the gas flow.