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In situ H2O and temperature detection close to burning biomass pellets using calibration-free wavelength modulation spectroscopy
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The design and application of an H2O/temperature sensor based on scanned calibration-free wavelength modulation spectroscopy (CF-WMS) and a single tunable diode laser at 1.4 µm is presented. The sensor probes two H2O absorption peaks in a single scan and simultaneously retrieves H2O concentration and temperature by least-squares fitting simulated 1f-normalized 2f-WMS spectra to measured 2f/1f-WMS signals, with temperature, concentration and nonlinear modulation amplitude as fitting parameters. Given a minimum detectable absorbance of 1.7 × 10−5 cm−1 Hz−1/2, the system is applicable down to an H2O concentration of 0.1 % at 1,000 K and 20 cm path length (200 ppm·m). The temperature in a water-seeded laboratory-scale reactor (670-1220 K at 4 % H2O) was determined within an accuracy of 1 % by comparison with the reactor thermocouple. The CF-WMS sensor was applied to real time in situ measurements of H2O concentration and temperature time histories (0.25-s time resolution) in the hot gases 2-11 mm above biomass pellets during atmospheric combustion in the reactor. Temperatures between 1,200 and 1,600 K and H2O concentrations up to 40 % were detected above the biofuels.
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... Finally, the exceptional performance of our proposed servo system under severe airflow shocks indicates its potential for various OF-CES setups in real-time in situ measurements, especially in scenarios where gas eruption occurs, such as in the detection of thermal runaway gases of lithium-ion batteries 24 or in monitoring the combustion and gasification of biomass. 25 ...
In various optical feedback cavity-enhanced spectroscopies (OF-CESs) based on absorption or scattering, conventional phase-locking methods are constrained by their ability to handle only minor phase deviations. This limitation is due to the source of an error signal for phase adjustment. This paper introduces a robust approach for phase-locking, which combines the shape and intensity of cavity transmission profiles to identify phase deviations. The advantage of this combination is that it can always generate a suitable error signal, irrespective of the phase's position in the entire 2π period. The outstanding performance of the corresponding servo loop under severe airflow shocks demonstrates that our approach significantly increases the feasibility of applying various OF-CES setups for real-time, in situ gas detection in harsh environments.
... However, its performance may be limited in applications involving optically opaque conditions and specifically effective in measuring atomic alkali metals. TDLAS has been employed to measure atomic potassium (K(g)) in various environments and expanded the technique's application to a pilot-scale biomass gasification reactor, thus investigating the reliability of TDLAS for K(g) measurement [4,[28][29][30][31][32][33]. Therefore, this work utilizes TDLAS to detect K(g) release during the combustion process. ...
... The wavelength scanning spectroscopy demonstrated in the DAS scheme exhibits high sensitivity performance, which is comparable to that of the wavelength modulation spectroscopy (WMS) based on continuous-wave (CW) lasers 22,23 . It is believed to benefit from the OFC related pulse modulation at RF level, and the sensitivity will be further enhanced by introducing kHz-level wavelength tuning as a second-stage modulation. ...
... NO sensors based on tunable laser absorption spectroscopy (TDLAS) have been proven to be a suitable candidate to monitor NO levels because of their capability to provide sensitive, accurate, in situ and absolute measurements for various conditions, even in harsh environments [12]. Accurate spectral line parameters of the targeted molecule (such as pressure broadening coefficient and line strength) are critical for TDLAS spectrometers in the context of a precise amount of fraction monitoring of trace species with a low signal-to-noise ratio [13][14][15]. ...
A tunable diode laser absorption spectroscopy (TDLAS)-based spectrometer employing a mid-infrared (Mid-IR) interband cascade laser (ICL) was developed and used to determine pressure broadening coefficients of two NO absorption transitions at 1914.98 cm−1 and 1915.76 cm−1 in the fundamental (1←0) band (R11.5 Ω1/2 and Ω3/2) for CO2, N2, Ar, O2, He, and H2. For the first time, a reliable and consistent set of six different pressure-broadening coefficients for the NO line has been measured by a consistent approach covering pressures from 100 to 970 mbar at a temperature of 294 K. Air pressure broadening has been calculated based on N2 and O2 coefficients. The stated pressure-broadening coefficients for N2, CO2, Ar, H2, O2, He, and Air have relative errors in the 0.5–1.5% range. For CO2 and H2, broadening results of NO (1←0) band (R11.5 Ω1/2 and Ω3/2) lines are reported for the first time. The results are also compared to previously available literature data. It was found that the broadening coefficients for O2 and Air are in agreement with literature values, whereas results for Ar and He show larger differences.
... According to calculation based on the high-resolution transmission (HITRAN) molecular absorption database, the LoD of 0.415% corresponds to a minimum detectable absorbance of ~ 0.0031. This sensitivity performance nearly approaches that of the wavelength modulation spectroscopy (WMS) 24,25 , which is a mature spectroscopic gas sensing technique using tunable narrow-linewidth CW lasers. But potential of much higher detection speed is exhibited benefiting from the fast pulse characteristic of OFCs. ...
... cm −1 (combined) and carried out temperature tests for flat flame burner and scramjet, respectively. Qu Zhechao et al. [37] proposed an in-reactor H 2 O temperature test based on calibration-free wavelength modulation spectroscopy (CF-WMS) using the 7153.74 cm −1 and (7154.35 ...
The rapidly changing and wide dynamic range of combustion temperature in scramjet engines presents a major challenge to existing test techniques. Tunable diode laser absorption spectroscopy (TDLAS) based temperature measurement has the advantages of high sensitivity, fast response, and compact structure. In this invited paper, a temperature measurement method based on the TDLAS technique with a single diode laser was demonstrated. A continuous-wave (CW), distributed feedback (DFB) diode laser with an emission wavelength near 1.4 μm was used for temperature measurement, which could cover two water vapor (H2O) absorption lines located at 7153.749 cm−1 and 7154.354 cm−1 simultaneously. The output wavelength of the diode laser was calibrated according to the two absorption peaks in the time domain. Using this strategy, the TDLAS system has the advantageous of immunization to laser wavelength shift, simple system structure, reduced cost, and increased system robustness. The line intensity of the two target absorption lines under room temperature was about one-thousandth of that under high temperature, which avoided the measuring error caused by H2O in the environment. The system was tested on a McKenna flat flame burner and a scramjet model engine, respectively. It was found that, compared to the results measured by CARS technique and theoretical calculation, this TDLAS system had less than 4% temperature error when the McKenna flat flame burner was used. When a scramjet model engine was adopted, the measured results showed that such TDLAS system had an excellent dynamic range and fast response. The TDLAS system reported here could be used in real engine in the future.
Forest fires spread via production and combustion of pyrolysis gases in the understory. The goal of the present paper is to understand the spatial location, distribution, and fraction (relative to the overstory) of understory plants, in this case, sparkleberry shrub, namely its degree of understory consumption upon burn, and to search for correlations between the degree of shrub consumption to the composition of emitted pyrolysis gases. Data were collected in situ at seven small experimental prescribed burns at Ft. Jackson, an army base in South Carolina, USA. Using airborne laser scanning (ALS) to map overstory tree crowns and terrestrial laser scanning (TLS) to characterize understory shrub fuel density, both pre- and postburn estimates of sparkleberry coverage were obtained. Sparkleberry clump polygons were manually digitized from a UAV-derived orthoimage of the understory and intersected with the TLS point cloud-derived rasters of pre- and postburn shrub fuel bulk density; these were compared in relation to overstory crown cover as well as to ground truth. Shrub fuel consumption was estimated from the digitized images; sparkleberry clump distributions were generally found to not correlate well to the overstory tree crowns, suggesting it is shade-tolerant. Moreover, no relationship was found between the magnitude of the fuel consumption and the chemical composition of pyrolysis gases, even though mixing ratios of 25 individual gases were measured.
The design and use of two-color tunable diode laser (TDL) absorption sensors for
measurements of temperature and H2O in a rotating detonation engine (RDE) are presented.
Both sensors used first-harmonic-normalized scanned-wavelength-modulation spectroscopy
with second-harmonic detection (scanned-WMS-2f/1f) to account for non-absorbing
transmission losses and emission encountered in the harsh combustion environment. One
sensor used two near-infrared (NIR) TDLs near 1391.7 nm and 1469.3 nm that were
modulated at 225 kHz and 285 kHz, respectively, and sinusoidally scanned across the peak
of their respective H2O absorption transitions to provide a measurement rate of 50 kHz and
a detection limit in the RDE of 0.2% H2O by mole. The other sensor used two mid-infrared
(MIR) TDLs near 2551 nm and 2482 nm that were modulated at 90 kHz and 112 kHz,
respectively, and sinusoidally scanned across the peak of their respective H2O transitions
to provide a measurement rate of 10 kHz and a detection limit in the RDE of 0.02% H2O
by mole. Four H2O absorption transitions with different lower-state energies were used to
assess the homogeneity of temperature in the measurement plane. Experimentally derived
spectroscopic parameters that enable temperature and H2O sensing to within 1.5–3.5% of
known values are reported. The sensor design enabling the high-bandwidth scanned-WMS-
2f/1f measurements is presented. The two sensors were deployed across two orthogonal and
coplanar lines-of-sight (LOS) located in the throat of a converging-diverging nozzle at the
RDE combustor exit. Measurements in the non-premixed H2-fueled RDE indicate that the
temperature and H2O oscillate at the detonation frequency (≈3.25 kHz) and that production of
H2O is a weak function of global equivalence ratio.
A novel strategy has been developed for analysis of wavelength-scanned,
wavelength modulation spectroscopy (WMS) with tunable diode lasers
(TDLs). The method simulates WMS signals to compare with measurements to
determine gas properties (e.g., temperature, pressure and concentration
of the absorbing species). Injection-current-tuned TDLs have
simultaneous wavelength and intensity variation, which severely
complicates the Fourier expansion of the simulated WMS signal into
harmonics of the modulation frequency (fm). The new method
differs from previous WMS analysis strategies in two significant ways:
(1) the measured laser intensity is used to simulate the transmitted
laser intensity and (2) digital lock-in and low-pass filter software is
used to expand both simulated and measured transmitted laser intensities
into harmonics of the modulation frequency, WMS-nfm (n = 1,
2, 3,…), avoiding the need for an analytic model of intensity
modulation or Fourier expansion of the simulated WMS harmonics. This
analysis scheme is valid at any optical depth, modulation index, and at
all values of scanned-laser wavelength. The method is demonstrated and
validated with WMS of H2O dilute in air (1 atm, 296 K, near
1392 nm). WMS-nfm harmonics for n = 1 to 6 are extracted and
the simulation and measurements are found in good agreement for the
entire WMS lineshape. The use of 1f-normalization strategies to realize
calibration-free wavelength-scanned WMS is also discussed.
The development and initial demonstration of a scanned-wavelength, first-harmonic-normalized, wavelength-modulation spectroscopy with n f detection (scanned-WMS- n f / 1 f ) strategy for calibration-free measurements of gas conditions are presented. In this technique, the nominal wavelength of a modulated tunable diode laser (TDL) is scanned over an absorption transition to measure the corresponding scanned-WMS- n f / 1 f spectrum. Gas conditions are then inferred from least-squares fitting the simulated scanned-WMS- n f / 1 f spectrum to the measured scanned-WMS- n f / 1 f spectrum, in a manner that is analogous to widely used scanned-wavelength direct-absorption techniques. This scanned-WMS- n f / 1 f technique does not require prior knowledge of the transition linewidth for determination of gas properties. Furthermore, this technique can be used with any higher harmonic (i.e., n > 1 ), modulation depth, and optical depth. Selection of the laser modulation index to maximize both signal strength and sensitivity to spectroscopic parameters (i.e., gas conditions), while mitigating distortion, is described. Last, this technique is demonstrated with two-color measurements in a well-characterized supersonic flow within the Stanford Expansion Tube. In this demonstration, two frequency-multiplexed telecommunication-grade TDLs near 1.4 μm were scanned at 12.5 kHz (i.e., measurement repetition rate of 25 kHz) and modulated at 637.5 and 825 kHz to determine the gas temperature, pressure, H 2 O mole fraction, velocity, and absorption transition lineshape. Measurements are shown to agree within uncertainty (1%–5%) of expected values.
The calibration-free wavelength modulation absorption spectrum is studied based on the second harmonic (2f) signal to remove the calibration procedure for tunable diode laser absorption gas sensing. The simulated 2f signal is obtain from the analysis of peak and trough from measured 2f signal line shape. The gas concentration is calculated by the linear fitting of measured and simulated 2f signals. The experiment on CO2 detection is carried out at transition of 6336.24cm-1 in a 10 cm absorption cell. The results prove that the calibration-free wavelength modulation absorption spectrum is suited for on-site gas sensing at various conditions. The average absolute error of gas concentration is 0.67% at modulation index ranging from 1.8 to 3.2. With gas concentration and pressure varied, the average absolute errors of gas concentration are measured to be about 0.98% and 0.74%.
We introduce the basics of an apodized 2f/1f wavelength modulation method for the spectroscopy of the R(9) transition line in the first overtone band of carbon monoxide ((CO)-C-12-O-16) in near-infrared (NIR) region around 2.33 mu m. Performance of the method is investigated for high gas concentrations beyond the optically thin limit to generalize common 2f/1f wavelength modulation spectroscopy (WMS) reported by Rieker et al. (Appl Opt 48:5546, [28]). Numerical simulations are performed based on real experimental parameters associated with a NIR spectrometer designed in our laboratory. The results primarily show a more linear response and less error than occurred in the common WMS-2f/1f method for an optically thick sample. It is also theoretically shown that the apodized method enables sharpening the spectrum without peak displacement compared to the common WMS-2f/1f method. The validity of the method is verified experimentally by the trace detection of an air-broadened R(9) CO absorption line centered at 4,294.637 cm(-1) at atmospheric pressure and room temperature. The effect of a so-called scaling k-factor on the sharpening of WMS-2f/1f signal is investigated through trace simulation and detection of CO and methane (CH4) lines in the scanning range of a distributed feedback laser. The obtained results show very good agreement between simulation and experiment.
A real-time, in situ water vapor (H2O) sensor using a tunable diode laser near 1,352 nm was developed to continuously monitor water vapor in the synthesis gas of an engineering-scale high-pressure coal gasifier. Wavelength-scanned wavelength-modulation spectroscopy with second harmonic detection (WMS-2f) was used to determine the absorption magnitude. The 1f-normalized, WMS-2f signal (WMS-2f/1f) was insensitive to non-absorption transmission losses including beam steering and light scattering by the particulate in the synthesis gas. A fitting strategy was used to simultaneously determine the water vapor mole fraction and the collisional-broadening width of the transition from the scanned 1f-normalized WMS-2f waveform at pressures up to 15 atm, which can be used for large absorbance values. This strategy is analogous to the fitting strategy for wavelength-scanned direct absorption measurements. In a test campaign at the US National Carbon Capture Center, the sensor demonstrated a water vapor detection limit of similar to 800 ppm (25 Hz bandwidth) at conditions with more than 99.99 % non-absorption transmission losses. Successful unattended monitoring was demonstrated over a 435 h period. Strong correlations between the sensor measurements and transient gasifier operation conditions were observed, demonstrating the capability of laser absorption to monitor the gasification process.
Spatially resolved laser absorption measurements of CO, CO2, and H2O within an ethylene-fueled direct-connect model scramjet combustor are presented. The sensors employ a variety of laser sources at midinfrared wavelengths to provide access to fundamental vibrational band absorption transitions for each species. Both scanned-wavelength modulation spectroscopy and scanned-wavelength direct-absorption are used, with particular attention paid to employing these methods in a manner that accounts for expected nonuniformities in temperature and composition throughout the combustor. Results for product temperatures and column densities offer insight on the ongoing combustion process downstream of fuel injection throughout the combustion-product plume, and on the significant temporal variations in the combustor. Additional tests measure the temperature and concentration of H2O in the cavity flameholder during a flame extinction event, which gives an upper bound of the cavity residence time. These measurements are the first use of these midinfrared wavelengths for measurements in a scramjet combustor, and this work introduces an important new tool for characterizing hydrocarbon combustion in scramjet engines.
A quantitative and simultaneous measurement of K, KCl, and KOH vapors from a burning fuel sample combusted in a single particle reactor was performed using collinear photofragmentation and atomic absorption spectroscopy (CPFAAS) with a time resolution of 0.2 s. The previously presented CPFAAS technique was extended in this work to cover two consecutive fragmentation pulses for the photofragmentation of KCl and KOH. The spectral overlapping of the fragmentation spectra of KCl and KOH is discussed, and a linear equation system for the correction of the spectral interference is introduced. The detection limits for KCl, KOH, and K with the presented measurement arrangement and with 1 cm sample length were 0.5, 0.1, and 0.001 parts per million, respectively. The experimental setup was applied to analyze K, KCl, and KOH release from 10 mg spruce bark samples combusted at the temperatures of 850, 950, and 1050 °C with 10% of O2. The combustion experiments provided data on the form of K vapors and their release during different combustion phases and at different temperatures. The measured release histories agreed with earlier studies of K release. The simultaneous direct measurement of atomic K, KCl, and KOH will help in the impact of both the form of K in the biomass and fuel variables, such as particle size, on the release of K from biomass fuels.
This paper presents how pelletizing die temperature and moisture content affect combustion behaviour of single wood pellet. Pine wood particles with two different moisture contents (i.e. 1 wt.% and 12 wt.%) were pelletized in a laboratory-scale single pelletizer (single die pellets) at die temperature of 20, 100, 150 and 200 °C. The pellets were combusted in a laboratory scale furnace at 800 °C. Time required for single pellet combustion generally increased with both increase of pelletizing temperature and moisture content of biomass. In addition, combustion behaviour of single die pellets was significantly different than those produced in a pilot scale pelletizing plant (semi-industrial scale pellet). That difference was due to variation in physical properties of pellets (e.g. density, and morphology).