Ruifeng Kan

An approach of open-path gas sensor based on tunable diode laser absorption spectroscopy

Tunable diode laser absorption spectroscopy (TDLAS) is a new method to detect trace-gas qualitatively or quantificationally based on the scan characteristic of the diode laser to obtain the absorption spectroscopy in the characteristic absorption region. A time-sharing scanning open-path TDLAS system using two near infrared distributed feedback (DFB) tunable diode lasers is designed to detect CH4 and H2S in leakage of natural gas. A low-cost Fresnel lens is used in this system as receiving optics which receives the laser beam reflected by a solid corner cube reflector with a distance of up to about 60 m. High sensitivity is achieved by means of wavelength-modulation spectroscopy with second-harmonic detection. The minimum detection limits of 1.1 ppm·m for CH4 and 15 ppm·m for H2S are demonstrated with a total optical path of 120 m. The simulation monitoring experiment of nature gas leakage was carried out with this system. According to the receiving light efficiency of optical system and detectable minimum light intensity of detection, the detectable optical path of the system can achieve 1-2 km. The sensor is suitable for natural gas leakage monitoring application.

Simultaneous multi-laser, multi-species trace-level sensing of gas mixtures by rapidly swept continuous-wave cavity-ringdown spectroscopy.

The greenhouse-gas molecules CO(2), CH(4), and H(2)O are detected in air within a few ms by a novel cavity-ringdown laser-absorption spectroscopy technique using a rapidly swept optical cavity and multi-wavelength coherent radiation from a set of pre-tuned near-infrared diode lasers. The performance of various types of tunable diode laser, on which this technique depends, is evaluated. Our instrument is both sensitive and compact, as needed for reliable environmental monitoring with high absolute accuracy to detect trace concentrations of greenhouse gases in outdoor air.

Remote open-path cavity-ringdown spectroscopic sensing of trace gases in air, based on distributed passive sensors linked by km-long optical fibers.

A continuous-wave, rapidly swept cavity-ringdown spectroscopic technique has been developed for localized atmospheric sensing of trace gases at remote sites. It uses one or more passive open-path optical sensor units, coupled by optical fiber over distances of >1 km to a single transmitter/receiver console incorporating a photodetector and a swept-frequency diode laser tuned to molecule-specific near-infrared wavelengths. Ways to avoid interference from stimulated Brillouin scattering in long optical fibers have been devised. This rugged open-path system, deployable in agricultural, industrial, and natural atmospheric environments, is used to monitor ammonia in air. A noise-limited minimum detectable mixing ratio of ~11 ppbv is attained for ammonia in nitrogen at atmospheric pressure.

H2S detection by tunable diode laser absorption spectroscopy

Hydrogen Sulfide is a highly toxic and flammable gas. It is generated as a common by-product of many industrial processes. Early detection of H2S at concentrations of 10 ppm in air is essential to prevent its toxic influence at higher concentrations. Tunable diode Laser based gas detectors are now being used in a wide variety of applications for safety and environmental interest. Their characteristics like non-intrusive, high selectivity, high sensitivity, and fast response time make them more appealing than conventional point sensors for a reliable H2S detection. A long open-path H2S sensor based on tunable diode laser absorption spectroscopy has been developed, using telecommunication near infrared distributed feed-back(DFB) tunable diode laser combining with wavelength modulation spectroscopy technology, a detection limit of 20 ppmldrm has been demonstrated, an on-board reference cell provides on-line sensor calibration. The sensor is very suitable for large area field H2S monitoring application.

Gas leakage monitoring with scanned-wavelength direct absorption spectroscopy

A natural gas leakage detector based on scanned-wavelength direct absorption spectroscopy is described. The sensor employs a multi-channel scanned-wavelength direct absorption strategy. It has the potential to simultaneously monitor methane and hydrogen sulfide in open path environment. Traditionally, scanned wavelength direct absorption spectroscopy is the technique choice for natural gas leakage applications because of its simplicity, accuracy, and stability. We perform the gas sensor using direct-absorption wavelength scans with isolated features at 1-kHz repetition rate and the center wavelength is stabilized at the center of the 2 \nu 3 band R(3) line of methane (1.65 \mu m) and the (\nu 1 +\nu 2 +\nu 3) combination band of hydrogen sulfide (1.57 \mu m), respectively. The influence of light intensity fluctuations can be eliminated by using scanned-wavelength direct absorption spectroscopy. Because of the fast wavelength scanning, the sensor has a response time of less than 0.1 s. The sensor can be configured to sense leakages in path-integrated concentrations of, for example, 100-ppm·m hydrogen sulfide and 10-ppm·m methane.

Analysis of algebraic reconstruction technique for accurate imaging of gas temperature and concentration based on tunable diode laser absorption spectroscopy

An improved algebraic reconstruction technique (ART) combined with tunable diode laser absorption spectroscopy(TDLAS) is presented in this paper for determining two-dimensional (2D) distribution of H2O concentration and temperature in a simulated combustion flame. This work aims to simulate the reconstruction of spectroscopic measurements by a multi-view parallel-beam scanning geometry and analyze the effects of projection rays on reconstruction accuracy. It finally proves that reconstruction quality dramatically increases with the number of projection rays increasing until more than 180 for 20 × 20 grid, and after that point, the number of projection rays has little influence on reconstruction accuracy. It is clear that the temperature reconstruction results are more accurate than the water vapor concentration obtained by the traditional concentration calculation method. In the present study an innovative way to reduce the error of concentration reconstruction and improve the reconstruction quality greatly is also proposed, and the capability of this new method is evaluated by using appropriate assessment parameters. By using this new approach, not only the concentration reconstruction accuracy is greatly improved, but also a suitable parallel-beam arrangement is put forward for high reconstruction accuracy and simplicity of experimental validation. Finally, a bimodal structure of the combustion region is assumed to demonstrate the robustness and universality of the proposed method. Numerical investigation indicates that the proposed TDLAS tomographic algorithm is capable of detecting accurate temperature and concentration profiles. This feasible formula for reconstruction research is expected to resolve several key issues in practical combustion devices.

Atomosphere boundary layer height determination and observation from ceilometer measurements over Hefei during the total solar on July 22, 2009 eclipse

Using an improved inexion point method (IIPM), we investigate atmosphere boundary layer (ABL) height evolution over Hefei during the total solar eclipse on July 22, 2009. A lidar ceilometer is used in ground-based observations. Estimations of ABL heights before, during, and after the solar eclipse are analyzed using the IIPM. Results indicate that the IIPM, which is less sensitive to background noise, is more suitable in detecting ABL height and temporal evolution. Data demonstrate that the total solar eclipse resultes in a decrease in ABL height, indicating a suppression of turbulence activity, similar to that observed during the sunset hours. Changes in ABL height are associated with a slow change in temperature, indicating a significant weakening of penetrative convection and a time lag between ABL response and the reduction in solar radiation.

Multi-wavelength sensing of greenhouse gases by rapidly swept continuous-wave cavity ringdown spectroscopy

The greenhouse gas molecules CH<inf>4</inf>, CO<inf>2</inf>, and H<inf>2</inf>O are detected by using a cavity ringdown laser spectrometer with rapidly swept optical cavity and multi-wavelength coherent radiation. This sensitive portable instrument is applicable to environmental monitoring.

The measurement of the H2S in the pre-desulfurization of natural gas in the Shengli oil field with the TDL

Hydrogen Sulfide, with the character of erosion and strong toxicity, is a kind of associated gas of nature gas. How to measure and monitor the hydrogen sulfide concentration becomes an important issue to be solved in nature gas transfer-process. Online measurement for the hydrogen sulfide concentration before the desulphurization remains very difficult in Bonan gas gathering station of SINOPEC Shengli Oil Field (SOF).TDL(Tunable Diode Laser) can relative easily select the absorption line of the detecting gas without the interference from other gas thus make the rapid and accurate hydrogen sulfide measurement a possible. In this paper, a hydrogen sulfide measurement system is designed and then be carried out in Bonan gas gathering station of (SOF) .The implemented experiments showed the system effectively solved some problems such as overfall, temperature and pressure. After comparing the hydrogen sulfide online detection sensor of the TDLAS industry with the long hydrogen sulfide detection tube, the linear fitted relationship with the correlation coefficient of 99.96% between them was attained. In order to meet the requests of industrial field anti-explosion, the open-path optical coupling technology was performed in china for the first time. All the results demonstrate that the system will be put into use and enjoy a high application in the near future.

Development of a Dew/Frost Point Temperature Sensor Based on Tunable Diode Laser Absorption Spectroscopy and Its Application in a Cryogenic Wind Tunnel

We have proposed a sensor for real-time and online measurement of dew/frost point temperature using tunable diode laser absorption spectroscopy (TDLAS) technique. Initial experiments have demonstrated its feasibility and technical advantages in comparison to a chilled mirror hygrometer (CMH). The TDLAS sensor we developed has a dew/frost point temperature range from −93 °C to + 14.5 °C, with a measurement uncertainly of less than 2%, and a response time of about 0.8 s, which is much faster than that of the chilled mirror hygrometer (ranging from several minutes to several hours). A TDLAS-based dew/frost point sensor has many advantages, such as rapid and continuous measurements, low frost point temperature sensing, high accuracy, and non-intrusiveness. Such a sensor would be useful for dew/frost point temperature determinations in various applications. In a cryogenic wind tunnel, real-time dew/frost point temperature measurements are helpful in preventing the formation of condensed liquid and ice, which can affect the model geometry and lead to unreliable test data.