Holger Teichert

Bio: Holger Teichert is an academic researcher from Heidelberg University. The author has contributed to research in topics: Absorption (electromagnetic radiation) & Spectrometer. The author has an hindex of 7, co-authored 12 publications receiving 716 citations.

Simultaneous in situ measurement of CO, H2O, and gas temperatures in a full-sized coal-fired power plant by near-infrared diode lasers.

Abstract: We present what is to our knowledge the first near-infrared diode-laser-based absorption spectrometer that is suitable for simultaneous in situ measurement of carbon monoxide, water vapor, and temperature in the combustion chamber (20-m diameter, 13-m path length) of a 600-MW lignite-fired power plant. A fiber-coupled distributed-feedback diode-laser module at 1.56 microm served for CO detection, and a Fabry-Perot diode laser at 813 nm was used to determine H2O concentrations and temperature from multiline water spectra. Despite severe light losses (transmission, <10(-8)) and strong background radiation we achieved a resolution of 1.9 x 10(-4) (1sigma) fractional absorption, equivalent to 200 parts in 10(6) by volume of CO (at 1450 K, 10(5) Pa) with 30-s averaging time.

Digital, phase-sensitive detection for in situ diode-laser spectroscopy under rapidly changing transmission conditions

Abstract: A new harmonic detection scheme for fully digital, fast-scanning wavelength-modulation spectroscopy (DFS-WMS) is presented. DFS-WMS is specially suited for in situ absorption measurements in combustion environments under fast fluctuating transmission conditions and is demonstrated for the first time by open-path monitoring of ambient oxygen using a distributed-feedback diode laser, which is doubly modulated with a fast linear 1 kHz-scan and a sinusoidal 300 kHz-modulation. After an analog high-pass filter, the detector signal is digitized with a 5 megasample/s 12-bit AD-converter card plugged into a PC and subsequently – unlike standard lock-ins – filtered further by co-adding 100 scans, to generate a narrowband comb filter. All further filtering and the demodulation are performed completely digitally on a PC with the help of discrete Fourier transforms (DFT). Both 1f- and 2f-signals, are simultaneously extracted from the detector signal using one ADC input channel. For the 2f-signal, a linearity of 2% and a minimum detectable absorption of 10-4 could be verified experimentally, with the sensitivity to date being limited only by insufficient gain on the 2f-frequency channel. Using the offset in the 1f signal as a transmission ‘probe’, we could show that the 2f-signal can be transmission-corrected by a simple division by the 1f-background, proving that DFS-WMS provides the possibility of compensating for transmission fluctuations. With the inherent suppression of additive noise, DFS-WMS seems well suited for quantitative in situ absorption spectroscopy in large combustion systems. This assumption is supported by the first measurements of oxygen in a high-pressure combustor at 12 bar.

Simultaneous diode-laser-based in situ detection of multiple species and temperature in a gas-fired power plant

Abstract: 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 μm were used to detect O2, 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.

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.

Combustion dynamics and control: Progress and challenges

Abstract: Combustion dynamics constitutes one of the most challenging areas in combustion research. Many facets of this subject have been investigated over the past few decades for their fundamental and practical implications. Substantial progress has been accomplished in understanding analysis, modeling, and simulation. Detailed laboratory experiments and numerical computations have provided a wealth of information on elementary dynamical processes such as the response of flames to variable strain, vortex rollup, coupling between flames and acoustic modulations, and perturbed flame collisions with boundaries. Much recent work has concerned the mechanisms driving instabilities in premixed combustion and the coupling between pressure waves and combustion with application to the problem of instability in modern low NO x heavyduty gas turbine combustors. Progress in numerical modeling has allowed simulations of dynamical flames interacting with pressure waves. On this basis, it has been possible to devise predictive methods for instabilities. Important efforts have also been directed at the development of the related subject of combustion control. Research has focused on methods, sensors, actuators, control algorithms, and systems integration. In recent years, scaling from laboratory experiments to practical devices has been achieved with some successebut limitations have also been revealed. Active control of combustion has also evolved in various directions. A number of experiments on laboratory-scale combustors have shown that the amplitude of combustion instabilities could be reduced by applying control principles. Full-scale terrestrial application to gas turbine systems have allowed an increase of the stability margin of these machines. Feedback principles are also being explored to control the point of operation of combustors and engines. Operating point control has special importance in the gas turbine field since it can be used to avoid operation in unstable regions near the lean blowoff limits. More generally, closed loop feedback concepts are useful if one wishes to improve the combustion process as demonstrated by applications to automotive engines. Many future developments of combustion will use such concepts for tuning, optimization, and emissions reduction. This article proposes a broad survey of these fast-moving areas of research.

Calibration-free wavelength-modulation spectroscopy for measurements of gas temperature and concentration in harsh environments

Abstract: We present a practical implementation of calibration-free wavelength-modulation spectroscopy with second harmonic detection (WMS-2f) for measurements of gas temperature and concentration in harsh environments. The method is applicable to measurements using lasers with synchronous wavelength and intensity modulation (such as injection current-tuned diode lasers). The key factors that enable measurements without the on-site calibration normally associated with WMS are (1) normalization of the WMS-2f signal by the first harmonic (1f) signal to account for laser intensity, and (2) the inclusion of laser-specific tuning characteristics in the spectral-absorption model that is used to compare with measured 1f-normalized, WMS-2f signals to infer gas properties. The uncertainties associated with the calibration-free WMS method are discussed, with particular emphasis on the influence of pressure and optical depth on the WMS signals. Many of these uncertainties are also applicable to calibrated WMS measurements. An example experimental setup that combines six tunable diode laser sources between 1.3 and 2.0 mum into one probe beam for measurements of temperature, H(2)O, and CO(2) is shown. A hybrid combination of wavelength and frequency demultiplexing is used to distinguish among the laser signals, and the optimal set of laser-modulation waveforms is presented. The system is demonstrated in the harsh environment of a ground-test scramjet combustor. A comparison of direct absorption and 1f-normalized, WMS-2f shows a factor of 4 increase in signal-to-noise ratio with the WMS technique for measurements of CO(2) in the supersonic flow. Multidimensional computational fluid-dynamics (CFD) calculations are compared with measurements of temperature and H(2)O using a simple method that accounts for the influence of line-of-sight (LOS) nonuniformity on the absorption measurements. The comparisons show the ability of the LOS calibration-free technique to gain useful information about multidimensional CFD models.

Experimental Diagnostics for the Study of Combustion Instabilities in Lean Premixed Combustors

Abstract: An improved understanding of the mechanisms of unstable combustion in lean premixed combustors is essential to the development of stable gas turbine combustion systems. To obtain such understanding, detailed experimental studies of the phenomenology of unstable combustion are required. A number of experimental diagnostic techniques for characterizing unstable combustion and the underlying instability mechanisms are discussed. This includes techniques based on pressure, chemiluminescence emission, infrared absorption, and laser-induced fluorescence measurements. The techniques themselves are discussed briefly; however, the primary objective is to present and discuss results illustrating how these techniques can be used to characterize the mechanisms of unstable combustion, to gain an improved understanding of unstable combustion, and to develop strategies for suppressing unstable combustion in lean premixed gas turbine combustors.