Quantum cascade laser absorption sensor for in-situ, real-time and sensitive measurement of high-temperature SO2 and SO3.

We report a mid-infrared quantum cascade laser absorption sensor capable of measuring SO2 and SO3 simultaneously and sensitively at elevated temperatures. In the sensor development, the intense transitions of SO2 and SO3 in the mid-infrared region of 1129 cm-1 and 1398 cm-1 were exploited by two quantum cascade lasers. A high-temperature multipass cell was adopted to increase the absorption path length to 10 m. The quantitative concentrations of SOx were directly obtained from the calibration-free wavelength modulation spectroscopic method, which was validated at varied temperature and pressure conditions. From Allan deviation analysis, we achieved a minimum detection limit of 8 parts per billion (ppb) for SO2 and 3 ppb for SO3, with an average time of 100 s. Lastly, we successfully demonstrated the real-time and sensitive measurement of SO2 and SO3 during the oxidation reaction of SO2 by O3 at 460 K. Our laser sensor shows great potential for in-situ and real-time monitoring of SOx from combustion emissions.

Infrared spectral soot emission for robust and high-fidelity flame thermometry.

Spectral soot emission (SSE) in the visible spectrum is a popular technique for non-intrusive thermometry in sooting flames. However, its accuracy is restricted by uncertainties in the wavelength dependence of soot optical properties. We propose a novel infrared spectral soot emission method that successfully addresses this issue. Comprehensive light extinction experiments were firstly conducted to explore the spectral variation of soot optical property. The results indicated a wavelength independence of the soot absorption function provided the wavelength of the incident light is larger than 1000 nm, thereby indicating through the Kirchhoff law the potential of a robust thermometry using infrared (>1000 nm) spectral soot emissions. Proof-of-concept experiments were performed for sooting premixed flames of ethylene with different equivalence ratios. The results demonstrated that the new method provided more accurate temperature results compared with its visible-NIR counterpart, particularly at flame positions where nascent soot particles are present. The proposed method is, to our knowledge, the first infrared spectral soot emission-based thermometry, and is believed to offer a solution to improving the fidelity of SSE with a cost-effective optical setup.

Development and validation of a hybrid constraint spectral thermometry for laminar sooting flames.

A hybrid constraint spectral thermometry (HCST) method based on combined light extinction and spectral soot emission was developed for temperature measurements in sooting flames. Light extinction measurements were performed in the 635-1064 nm spectral range to constrain the fitting of the soot optical property E(m) as a function of the wavelength for separately measured spectral emission data. This hybrid constraint methodology helps to avoid complete reliance on either measurement, which significantly increases the immunity of the temperature determination to measurement noises. After numerically validating the performance of HCST, measurements were performed for a series of representative sooting premixed flames. Additional measurements were conducted with the proven tunable diode laser absorption technique to provide a reference temperature, and satisfactory agreements demonstrate the accuracy and robustness of the proposed HCST method.

A Laser-Based Multipass Absorption Sensor for Sub-ppm Detection of Methane, Acetylene and Ammonia.

A compact, sensitive laser-based absorption sensor for multispecies monitoring of methane (CH4), acetylene (C2H2) and ammonia (NH3) was developed using a compact multipass gas cell. The gas cell is 8.8 cm long and has an effective optical path length of 3.0 m with a sampling volume of 75 mL. The sensor is composed of three fiber-coupled distributed feedback lasers operating near 1512 nm, 1532 nm and 1654 nm, an InGaAs photodetector and a custom-designed software for data acquisition, signal processing and display. The lasers were scanned over the target absorption features at 1 Hz. First-harmonic-normalized wavelength modulation spectroscopy (f = 3 kHz) with the second harmonic detection (WMS-2f/1f) is employed to eliminate the unwanted power fluctuations of the transmitted laser caused by aerosol/particles scattering, absorption and beam-steering. The multispecies sensor has excellent linear responses (R2 > 0.997) within the gas concentration range of 1-1000 ppm and shows a detection limit of 0.32 ppm for CH4, 0.16 ppm for C2H2 and 0.23 ppm for NH3 at 1 s response time. The Allan-Werle deviation analysis verifies the long-term stability of the sensor, indicating a minimal detection limit of 20-34 ppb were achieved after 60-148 s integration time. Flow test of the portable multispecies sensor is also demonstrated in this work.

Spatially and temporally resolved temperature measurements in counterflow flames using a single interband cascade laser.

Spatially and temporally resolved temperatures are measured in counterflow diffusion flames with a tunable diode laser absorption spectroscopy (TDLAS) technique based on direct absorption of CO2 near 4.2 µm. An important aspect of the present work is the reduction of the beam diameter to around 150 µm, thus providing high spatial resolution that is necessary to resolve the high axial temperature gradient in counterflow flames. The temperature non-uniformity was taken into account through both hyperspectral tomography and the multiline technique with profile fitting, with the latter one being capable of providing temporally resolved data. The proposed methods were used to measure four counterflow flames with peak temperature ranging from 1654 to 2720 K, including both non-sooting and sooting ones.