Optical Oxygen Sensors using Tunable Diode Laser Spectroscopy: Application to Harsh Reactive Processes

We have developed specialized in situ O₂-sensors for applications in nearly all combustor types including waste incinerators, gas-fired power plants, and rotary kilns used for the combustion of special wastes. High pressure combustors up to 15 bar have also been addressed. To be able to transmit light through a multi-100-MW, up to 20m diameter combustor and then detect it, very robust, digital signal processing-based, real-time spectroscopic recovery algorithms had to be developed. These algorithms are able to extract the weak O₂ absorption signal and remove the strong, rapidly changing disturbances (up to 99.99% light loss and 1000% background radiation) caused by the combustion process. Powerful spectral simulation tools where developed to simplify an application specific selection of absorption lines to optimize the spectrometer performance. Furthermore, numerous technical tools like laser beam stabilization and fully automatic spectrometer alignment had to be developed to compensate for the strong thermal expansion movements of the combustor walls during load changes. Such tools also facilitate the initial spectrometer alignment without the need to shut down the plant. We have been able to measure simultaneously other species (CO, H₂O, CO₂, CH₄, etc), gas temperature, and gas residence time by temporal and spectral laser multiplexing techniques. Such data permits one to get a good picture of the local chemistry in the probe volume. Of significant importance for industrial applications is the fact that our data evaluation techniques are self calibrating; they required no sensor calibration with a reference gas. This is to be contrasted to 2f-modulation techniques which required periodic calibration.