We report approaches towards quantitative absorption spectroscopy under very low transmission conditions using base band diode laser modulation spectroscopy. Water absorption lines in the 800 nm region were used to derive number-densities and gas temperatures.

Ein NIR-Diodenlaser-gestütztes Absorptionsspektrometer zum simultanen In-situ- Nachweis mehrerer Spezies und der Temperatur und die erstmalige simultane In-situ- Erfassung aller Majoritätsspezies und der Temperatur im Brennraum eines industriellen Verbrennungsprozesses wird beschrieben. Haupteinsatzzweck des Gerätes ist die aktive Kontrolle chemischer Prozesse oder z.B. die Überwachung der schnellen Lastwechsel in Verbrennungsanlagen. Im Spektrometer werden insgesamt drei Diodenlaser bei 760nm, 812nm und 1,65µm eingesetzt um O2, H2O, CH4 und CO2 innerhalb des gleichen Messvolumens zu erfassen. Die Temperatur wird simultan aus dem Mehrlinienspektrum von Wasser bei 812 nm bestimmt. Um die starken Deformationen der Kesselwände zu korrigieren, wurde zusätzlich ein in-situ-taugliches System zur automatischen Nachführung und Stabilisierung der Laserstrahllage entwickelt und erfolgreich eingesetzt. Mit diesem Spektrometer gelang es, einen kompletten An- und Abfahrvorgang eines 1000- MWth-Gaskraftwerkes mit einem Brennraumdurchmesser von 10m zu überwachen. Die Zeitauflösung betrug dabei 1,6s, die kleinste nachweisbare molekulare Absorption lag im Bereich von 10–4 bis 10–3 optische Dichte. Methan, das für die schnelle Erfassung von Zündverzögerungen entscheidend ist, konnte unter Verbrennungsbedingungen mit einer Nachweisgrenze unter 100 ppmV und einem dynamischen Bereich von mehr als zwei Größenordnungen nachgewiesen werden.

We demonstrate the first simultaneous in-situ detection of methane and water in a full-scale 1 GWth, gas-fired power plant with an absorption resolution in die order of 10−4 OD corresponding to less than 5 ppmV at 400K.

We have developed a diode-laser (DL)-based spectrometer and demonstrated, to our knowledge, the first simultaneous in situ detection of all major combustion species and the temperature in the same measurement volume for active combustion control purposes and to ensure a safe ignition procedure of large-scale multi-burner gas-fired combustion systems. Two distributed-feedback DLs at 760 nm and 1.65 mum were used to detect O-2, CH4, and CO2, while a Fabry-Perot DL at 812 nm served to extract absolute H2O concentrations and the temperature from multiline water spectra. Permanent alignment of the laser beams could be ensured, despite strong wall deformation. with a new active alignment control loop. We analyzed the instationary ignition procedure of a full-scale gas-fired power plant with a 10 m furnace diameter using the spectrometer. A time resolution of 1.6 s and a minimum detectable absorption better than 10(-3) OD could be achieved. CH4 could be detected with a dynamic range of more than two orders of magnitude and a detectivity in the 100 ppm range. A strong dependence of the CH4 signal on the burner height was found. This spectrometer is well suited to enable an on-line control of the furnace atmosphere and a rapid detection of ignition delays by unburned CH4.

A laser-based in-situ spectrometer using near infrared diode laser at 767 and 770 nm to detect potassium atoms in high-temperature environments is described. The device was used to investigate the thermal dissociation of KCl and to monitor K behind the flue gas filter (300mm diameter, 1000degreesC) of a 250kW-coal combustor, without the necessity of error prone gas sampling. With a response time of 2 s we achieved detection sensitivities in the order of 10(7) atoms/cm(3). The spectrometer should therefore be fast and sensitive enough to detect a malfunction of the flue gas filters via the release of potassium atoms.

In this paper we present two different schemes - resonant photoacoustic (PA) and direct absorption - for the sensitive detection of gaseous methane employing recently commercially available low-power distributed feedback (DFB) diode lasers at 1.65 mu m. The sensitivities achievable and the advantages and disadvantages of these systems are compared and discussed. The PA spectrum measured in the range of 1655.5 nm +/- 1.5 nm is compared to the data extracted from the HITRAN 92 database. A minimum detection limit of 60 ppm V or 6 ppm m at 1.6537 mu m and a practical sensitivity of 120 ppm V has been achieved. III addition to PA detection, absorption spectroscopy was employed to detect methane. The signal was quantitatively extracted by wavelength-modulation spectroscopy with second-harmonic detection. Combined temperature and current tuning of the diode laser was used to scan the laser over the methane absorption line. Under atmospheric pressure conditions an excellent linearity was achieved. From the scatter of a series of individual concentration measurements at different concentrations between 15 ppm V and 100 ppm V an average detection limit (SNR = 1) of 1.15 ppm m could be deduced. This corresponds to a practical sensitivity of about 7 ppm V (SNR = 3 and 47.5 cm absorption length).

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 ex?,experimentally in a high-temperature absorption cell at up to 1000 degrees 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.