This paper presents an optical TDLAS (Tunable Diode Laser Absorption Spectroscopy) method for the quantification of oxygen and exhaust gas concentrations in combustion systems. Such an in-direct diagnosis approach is expected to overcome many deficiencies in conventional direct measurements (e.g. simultaneity, fast response, compact size, etc.). In this study, we compares DAS (Direct Absorption Spectroscopy) with WMS (Wavelength Modulation Spectroscopy) techniques in TDLAS method by employing the diode lasers at 760 nm (visible range), 1599.1 nm, and 1789.4 nm (near-IR range) of DFB butterfly type.
DAS technique, which is popular for the diagnosis of large combustion systems in which the measuring distance is large (e.g. furnace in power plant), is validated by considering a lab-scale high temperature electrical furnace with different oxygen concentrations (5.06 % and 21 %). The measuring distance of the furnace is about 1.8 m. The absorption signals on oxygen are measured at high (~1000 K) and low (~296 K) temperatures, and the concentrations extracted from the signals are well matched with the real values at both temperatures.
Also, the WMS method is applied to quantify the concentrations of the diverse species in exhaust gas (e.g. O2, CO, NO) from small combustion systems (e.g. diesel engine). The gas concentrations are varied to show the generality of WMS method. The measurements confirm that the near-IR diode laser can successfully capture the signals from CO and NO even with narrow measuring distance. For example, the 10 cm distance is proven sufficient in obtaining the reliable signals.
Overall, the results in this paper show that the advantages of WMS method over conventional DAS technique in diagnosing the small combustion systems in which the measuring distance is short.