Simultaneous laser-based in situ detection of oxygen and water in a waste incinerator for active combustion control purposes

A simultaneous in situ detection of oxygen and water vapor via electronic (A-band), respectively, rovibrational transitions (000 to 112 or 211) was performed in the furnace of a 20 MWth waste incinerator using AlGaAs-diode lasers at 760 and 812 nm. Temperature effects were minimized by choice of absorption lines with relatively high ground-state energies, which were for the case of water determined experimentally in a high-temperature absorption cell at up to 1000 °C. High-resolution absorption spectroscopy was employed by scanning the laser wavelengths synchronously via current modulation at 1 kHz. Fast transmission changes due to scattering losses caused by dust or soot could be corrected with an on-line transmission correction based on analog electronics. Compared with standard oxygen and water sensors, which were probing the flue gas duct, the laser absorption signals had a much higher time resolution and were found to respond much faster due to the avoided gas transport. A calibration procedure based on the reference signals was used to convert the in situ signals into absolute concentrations, check the linearity, and estimate the concentration resolution. Good linearity and a sensitivity in the order of 0.3 vol % oxygen or water was found.