Diode-laser-based sensors for flame diagnostics and combustion-emissions monitoring

Advanced diode-laser-based sensors for measuring the concentrations of hydroxyl (OH), atomic mercury (Hg), and nitric oxide (NO) have been developed to study flame chemistry and monitor combustion emissions. For each sensor, tunable laser radiation in the ultraviolet (UV) spectral region is generated by sum-frequency mixing the output of a high-power, fixed-frequency solid-state laser with the output of a tunable diode laser in a nonlinear optical crystal. Using the generated radiation to perform absorption spectroscopy, the sensors can be used to measure very low concentrations of each species with minimal interference from absorption by other molecules. Utilizing the most recent laser technology available, these sensors have capabilities never before achieved with diode-laser-based sensors for each species. Measurement rates are demonstrated that are 1-4 orders of magnitude greater than previous sensors. Wide spectral tuning ranges allow correction for broadband attenuation so that the sensors can be applied in particulate-laden environments such as coal-combustion exhaust. After extensive characterization in the laboratory, each sensor was applied in several practical combustion systems to demonstrate their performance in harsh environments. The first reported measurements with diode-laser sensors were performed in a model gas turbine combustor, a model scramjet combustor, a coal-fired boiler, and a bluff-body flame holder. Many aspects of these combustors were studied, including combustion instabilities behind a bluff-body flame holder, contaminant formation in the vitiated airflow of the scramjet combustor, and the formation of air pollutants in coal-combustion exhaust. The unique capabilities of these sensors provided valuable insight into these and other combustion phenomena in the combustors. This research demonstrates that these sensors are rugged enough for measurements in realistic applications to aid in furthering our understanding of the combustion process