Measurements of trace pollutants in combustion flows using room-temperature, mid-IR quantum cascade lasers (2002)

Measurements of CO and NO are reported using thermo-electrically cooled, mid-IR quantum cascade lasers operating near 4.6 and 5.26 μm, respectively. Measurements of NO in H2-air and CH4-air flames at temperatures of approximately 780 and 1600 K are presented along with room-temperature CO measurements. Sensitivities on the order of 1.5 ppm-m are achieved in the case of NO in a flame using a balanced ratiometric detection technique. Comparisons between measured and predicted water-vapor interference is discussed. Introduction and Motivation Advances in gas turbine combustor technology for aeroengine applications are driving combustor designs toward high pressure, ultra-lean operating points. Highpressure operation, up to 50 atm, is desired for improved thrust-to-weight ratio (i.e., a higher energy density heat release) and ultra-lean operation is desired to ameliorate the increase in NOx and particulate emissions associated with higher-pressure operation. The emissions reduction is also being driven by increasing regulatory pressure to limit NOx and particulate emissions during all flight phases, not just cruise conditions. Ultra-lean conditions can move the gas turbine toward stability margin boundaries, resulting in deleterious or potentially destructive combustion instabilities or flame blow-off. Combustor re-light at altitude is also more difficult at ultra-lean conditions, thereby substantially increasing the penalty associated __________ *Principal Scientist, Member AIAA Principal Research Scientist Principal Research Scientist, Senior Member AIAA Copyright © 2002 by Physical Sciences Inc. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. with blow-off. Also, operating at optimum ultra-lean conditions throughout the power envelope places severe demands on a fixed geometry combustor concept and suggests that adaptive operating modes involving fuel or air distribution may be required to ensure safe and lowemission performance. For these reasons, there are emerging requirements for sensors, actuators, and control technology suitable in advanced gas turbine combustors. Actuator technology should be capable of modulating fuel and/or air distributions to adapt to changing power demands or alter combustion dynamics to suppress instabilities. Sensors must be able to determine critical combustor and engine operating points such as turbine inlet temperature (T4), pattern factor, and emission levels. Finally, intelligent closed-loop control systems are required to interpret sensor input and tailor the actuator response to achieve the desired operating conditions. A basic sensor requirement that may become important in advanced actuator/control schemes is for continuous and real-time measurements of exhaust gas emission of CO and NO at bandwidths of a few Hz. The NO measurement provides for continuous optimization of the engine NOx emissions while the CO measurement provides for on-line determination of combustion efficiency and serves as a global health monitor diagnostics for fouled fuel and/or injectors. The sensor bandwidth requirement is set by the engine control system response time. Additionally, there are no commercial sensors capable of providing these measurements, and their need has been widely stated, most recently in Reference 1. Recently, tunable diode-laser absorption sensing has been employed for real-time measurement and closed-loop adaptive control of gas temperature and H2O concentration in the combustion region, CO and hydrocarbons in the exhaust region of laboratory and industrial-scale combustors, and in pulse detonation