Sensitive in situ detection of CO and O2 in a rotary kiln-based hazardous waste incinerator using 760 nm and new 2.3 μm diode lasers

Abstract CO and O 2 are key combustion parameters closely linked to the process stoichiometry and most frequently determined by gas sampling. This limits time resolution and hinders the extraction of representative concentration data from inhomogeneous flue gas streams. Especially, batch-fired processes like hazardous waste incineration in rotary kilns (RK) face fast and spatially confined CO fluctuations, making them difficult to optimize. To address this sensor deficiency, we used new 2.3 μm distributed-feedback diode lasers (DFB-DL) accessing the CO-2 ν -band to develop fast, sensitive, and spatially integrating in situ absorption spectrometers suitable for the harsh conditions in full-scale combustion processes. Spectrally multiplexing the 2.3 μm-DL with a 760 nm-DFB-DL for the O 2 -A-band we also developed a simultaneous in situ CO/O 2 spectrometer, which is most interesting for control strategies requiring the coverage of wide stoichiometry ranges. These new spectrometers were successfully tested for up to two weeks in a 3.5 MW th hazardous waste incinerator. The absorption path ( l  = 2.56 m, T  = 800–1000 °C) was located in the post-combustion chamber right at the RK exit. Direct absorption spectroscopy enabled calibration-free species detection. A new data acquisition system using a digital signal processor and voltage-controlled preamplifiers ensured 100% data throughput, enabled a real-time validity and transmission evaluation of individual laser scans, and thus permitted a real-time transmission compensation and hence an automatic dynamic range adaptation. This proved to be an effective method to avoid systematic errors found in PC-based systems under these rapidly fluctuating combustion conditions. With 1 s acquisition time we achieved for CO/O 2 an optical resolution (1 σ ) of 1.2 × 10 −4 /6 × 10 −5 OD corresponding to detection limits of 6.5 ppm CO and 250 ppm O 2 . Sensitivity and time resolution of the spectrometer was high enough to detect—even under fuel lean conditions—small periodic stoichiometry and CO changes caused by the periodic fuel feed of the RK.