Fiber Optic Gas Monitoring System for Coal Mine Safety

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Fiber Optic Gas Monitoring System for Coal Mine Safety Amiya Behera*, Bo Dong, Anbo Wang Center for Photonics Technology, Bradley Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA *Corresponding author: [email protected]

Abstract: A multiple gas monitoring system based on wavelength modulation spectroscopy has been designed and fabricated for atmospheric monitoring inside coal mines. The sensor system is evaluated for Methane detection from 0.01% to 50% concentration. OCIS codes: (060.2370) Fiber optics sensors; (300.6260) Diode lasers spectroscopy; (280.4788) Optical sensing and sensors

1. Introduction Monitoring and detecting harmful gases is of significant importance to coal mine industry. Over the last 40 years, tunable-diode-laser spectroscopy (TDLS) has become an established method for non-intrusive measurements of gas properties. Features like high precision, long-term stability, self-calibration, and maximum selectivity/very low cross sensitivity to other gases have made this technology suitable for wide range of applications. Wavelength modulation spectroscopy (WMS) is a derivative form of tunable-diode-laser spectroscopy that has been increasingly applied for measurements in harsh environments due to its improved sensitivity and noise-rejection capabilities over direct absorption [1], [2]. In this paper, a practical implementation of wavelength modulation spectroscopy is presented for simultaneous concentration measurement of multiple gases in harsh coal mine environment. 2. Theory Transmitted Intensity 𝐼𝑜𝑢𝑡 of light through an absorbing gas is given by Beer-Lambert law 𝐼𝑜𝑢𝑡 (𝜈) = 𝐼𝑖𝑛 𝑒 −[𝛼(𝜈)𝐶𝑙] ⁡

(1)

where 𝐼𝑖𝑛 is the incident light intensity, 𝛼(𝜈) is the absorption coefficient of the absorbing gas at frequency 𝜈, 𝑙 is the length through which light beam and gas interact, and 𝐶 is the gas concentration. In TDLS with WMS, the laser frequency 𝜈 is modulated by both a ramp and a high frequency sinusoidal driving current. As a result, 𝜈 and laser intensity 𝐼𝑖𝑛 vary with time. Using the analysis done by Duffin et al [1], for small absorbance (𝛼(𝜈)𝐶𝑙 ≪ 1), second harmonic or 2f component in 𝐼𝑜𝑢𝑡 at a particular frequency 𝜈1 is given by Eq. (2). 1 1 𝐼2𝑓 = − ∙ Δ𝐼(𝜈1 ) ∙ 𝛼 ′ (𝜈1 ) ∙ 𝐶𝑙 ∙ 𝛿𝜈 ∙ cos(2𝜔𝑡 − 𝜓) − ∙ 𝐼(𝜈1 ) 2 4 ∙ 𝛼 ″ (𝜈1 ) ∙ 𝐶𝑙 ∙ 𝛿𝜈 2 ∙ cos⁡(2𝜔𝑡 − 2𝜓)

(2)

where Δ𝐼 is the intensity modulation amplitude, 𝛿𝜈 is the frequency modulation amplitude, and 𝜓 is the phase shift between intensity and frequency modulation. Since⁡𝐼2𝑓 ∝ 𝐶𝑙, the concentration of a particular absorbing species can be directly estimated by measuring the 2f signal with a software or instrument lock-in amplifier. 3. System design The experimental system designed for simultaneous detection of multiple gases is illustrated in Fig. 1. In this design, three important gases to the coal mine industry are addressed, i.e. Methane (CH4), Carbon dioxide (CO2), and Carbon monoxide (CO). Three different DFB lasers are used with center wavelengths at 1651 nm, 1577 nm, and 1566 nm for CH4, CO2, and CO gas respectively. While the ramp signals fed to the laser driving currents have same frequencies for all the lasers, the sinusoidal signals have different frequency values, i.e. 𝑓1 , 𝑓2 , 𝑎𝑛𝑑⁡𝑓3 . These modulation frequencies are selected such that they are not multiples of each other. The optical signals from the lasers are combined by a couplers and transmitted to one or multiple sensor-heads by single mode fiber cables. The sensor head is simply an open gas chamber or cell which allows light to interact with surrounding air. Graded index fiber collimators are used on both ends of the chamber to keep optical transmission loss below 6 dB. Optical signal returned from the sensor-heads is then detected by optical receiver modules. A closed reference gas cell (RGC) containing known mixture of CH4, CO2, and CO is also used for improving gas sensor accuracy and locating correct absorption peaks in case of laser frequency drift. Three different types of signals are captured at the receiver module; (a) optical signal without any gas absorption (marked ‘Laser’), (b) optical signal from RGC (marked ‘RGC’), (c) optical signal from sensor (marked ‘Sensor’). All these signals are digitally sampled by an industrial panel PC for processing. By using

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Frontiers in Optics/Laser Science 2015 © OSA 2015

software lock-in amplifiers at different reference frequencies, i.e.⁡2𝑓1 , 2𝑓2 , 𝑎𝑛𝑑⁡2𝑓3, 2f signals from three different gas absorptions are separated and finally individual gas concentrations are calculated. All the electronic components are kept at surface level or in fresh air and the sensor-heads are placed at remote locations inside the mine.

Fig. 1. Design for multiple gas sensing system with sensor heads placed at two remote locations

4. Results and discussion The sensor system has been tested for CH4 monitoring by comparing its measured concentration value with a gas chromatograph (Shimadzu, Model GC-2014) and the result is depicted in Fig. 2. It is evident that the sensor system tracks the true concentration accurately and has a linear relation with it. It can measure CH4 concentration from 0.01% (i.e. 100 ppm) to 50% in real time and satisfies the lower explosive limit (LEL) criteria (5% concentration) on commercial methane sensors. Although the absorption signal from CO 2 and CO has also been measured, sensor performance against other commercial sensors is yet to be tested.

Fig. 2. Sensor system performance for Methane (CH4) concentration measurement

5. Conclusion Fabrication and testing of a multiple gas sensing system for coal mines has been carried out. Its performance for detecting methane (CH4) from 0.01% to 50% is also measured. The designed system has several features such as use of single sensor-head for multiple gases, remote sensing, real time monitoring, low cost, and fast response. 4. References [1]

K. Duffin, A. J. McGettrick, W. Johnstone, G. Stewart, and D. G. Moodie, “Tunable diode-laser spectroscopy with wavelength modulation: A calibration-free approach to the recovery of absolute gas absorption line shapes,” J. Light. Technol., vol. 25, no. 10, pp. 3114–3125, 2007.

[2]

G. Rieker, J. Jeffries, and R. K. Hanson, “Measurements of high-pressure CO2 absorption near 2.0 μm and implications on tunable diode laser sensor design,” Appl. Phys. B Lasers Opt., vol. 94, pp. 51–63, 2009.