3.3. DFB-based experimental platform

The system is designed mainly aimed at the gas gathering station, and the schematic diagram of the system is shown in Figure 2. Three butterfly-packaged distributed feedback (DFB) lasers are selected to detect CH4, C2H2, and C2H4 with the center output wavelengths of 1653, 1531, and 1621 nm, respectively. The light sources are controlled by the corresponding temperature,


Table 1. The parameters of absorption lines.

Figure 2. Schematic diagram of the experimental system.

current driver module, and signal generator module, respectively. Three modulation light beams are time-sharing output through a 3 1 optical switch which is controlled by a microprocessor and then the collimator and beam expander of the transmitter (THORLABS GBE10-C: ten times beam expander, 1050–1650 nm antireflective coating), passing through the measurement area to the corner cube mirror at the reflecting end. Then, returning to the receiving end along the parallel light path, the light beam containing the absorption signal is focused on the photosensitive surface of the photoelectric detector through an aspherical focusing lens and converted into electrical signals before entering the host control section. The amplified electrical signals are collected by the data acquisition card and transmitted to the microprocessor system after amplification by the preamplifier circuit. Finally, the online inversion of spectral data is carried out to obtain the gas concentration. Meanwhile, the early warning will be carried out according to the setting of the alarm limit. If the value exceeds the setting one; the system will send out light and sound alerting signal.

In order to decide the detection limit of the system, a calibration experiment was designed and shown in Figure 3. A calibrated absorption cell with a length of 1 m was placed on the laser path. In the calibration experiment, three gases CH4, C2H2, and C2H4 with the mixing ratios of 1%, 500, and 500 ppm are mixed in the absorption cell, and the corresponding absorption signals are displayed in Figure 4. The absorption lines of CH4 and C2H2 are independent, and there are no other spectral interferences, but there is a relatively weak absorption spectral line on the left of the absorption line of C2H4. Therefore, the absorption lines of CH4 and C2H2 are fitted using a single peak, and the absorption line of C2H4 is fitted by double peak in the fitting process. The absorbance A values of CH4, C2H2, and C2H4 absorption spectral lines are 0.076,

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respectively. According to the linear relationship between the direct absorbance and gas concentration, the obtained MDLs of CH4, C2H2, and C2H4 were 100, 40, and 50 ppm-m, respectively, which completely satisfied the gas gathering station leakage test requirements [47].

, respectively. The SNR of the absorption signals are 100, 12, and 10,

0.012, and 0.014 cm<sup>1</sup>

Figure 4. Direct absorption signal and fitting results.

Figure 3. Schematic diagram of calibration principle.

Figure 3. Schematic diagram of calibration principle.

Figure 4. Direct absorption signal and fitting results.

current driver module, and signal generator module, respectively. Three modulation light beams are time-sharing output through a 3 1 optical switch which is controlled by a microprocessor and then the collimator and beam expander of the transmitter (THORLABS GBE10-C: ten times beam expander, 1050–1650 nm antireflective coating), passing through the measurement area to the corner cube mirror at the reflecting end. Then, returning to the receiving end along the parallel light path, the light beam containing the absorption signal is focused on the photosensitive surface of the photoelectric detector through an aspherical focusing lens and converted into electrical signals before entering the host control section. The amplified electrical signals are collected by the data acquisition card and transmitted to the microprocessor system after amplification by the preamplifier circuit. Finally, the online inversion of spectral data is carried out to obtain the gas concentration. Meanwhile, the early warning will be carried out according to the setting of the alarm limit. If the value exceeds the

In order to decide the detection limit of the system, a calibration experiment was designed and shown in Figure 3. A calibrated absorption cell with a length of 1 m was placed on the laser path. In the calibration experiment, three gases CH4, C2H2, and C2H4 with the mixing ratios of 1%, 500, and 500 ppm are mixed in the absorption cell, and the corresponding absorption signals are displayed in Figure 4. The absorption lines of CH4 and C2H2 are independent, and there are no other spectral interferences, but there is a relatively weak absorption spectral line

setting one; the system will send out light and sound alerting signal.

Molecule Wavenumber (nm) Line strength at 300 K (cm<sup>2</sup> atm<sup>1</sup>

0.0206 0.0206 0.0368

C2H2 1531.5878 0.2916 0.23

CH4 1653.7282

212 Green Electronics

C2H4 1621.3600

1653.7256 1653.7225

Table 1. The parameters of absorption lines.

Figure 2. Schematic diagram of the experimental system.

) Δν (cm<sup>1</sup>

0.14

)

on the left of the absorption line of C2H4. Therefore, the absorption lines of CH4 and C2H2 are fitted using a single peak, and the absorption line of C2H4 is fitted by double peak in the fitting process. The absorbance A values of CH4, C2H2, and C2H4 absorption spectral lines are 0.076, 0.012, and 0.014 cm<sup>1</sup> , respectively. The SNR of the absorption signals are 100, 12, and 10, respectively. According to the linear relationship between the direct absorbance and gas concentration, the obtained MDLs of CH4, C2H2, and C2H4 were 100, 40, and 50 ppm-m, respectively, which completely satisfied the gas gathering station leakage test requirements [47].

#### 3.4. Results and discussion

The system had been field-tested at the testing ground of China Petroleum Pipeline Bureau. The environment temperature was 35�C, air relative humidity was 45%, and wind speed was 1 m/s during the experiment. We used the gas which was mixed with 90% methane, 5% ethylene, and 5% acetylene to simulate gas pipeline leakage in the experiment. The leakage position was about 2 m below the side of the laser beam. In order to measure the three gases simultaneously, lasers were switched every 10 seconds using an optical switch. Three kinds of gases were detected circularly in the order of CH4 ! C2H2 ! C2H4. The mixing ratios of the gases are exhausted 1 minute each time which was displayed in Figure 5. The reason for fluctuations is that the measured concentrations are the average of the paths along the line of sight. Due to the uncertainty of wind speed and gas diffusion in the measurement field, the concentration on the beam path fluctuates greatly. Meanwhile, this system is also equipped with an alarm limit for each gas, and the veracity of fire alarming system achieved 100%.

The system includes three DFB lasers which have an output power of about 20 mW higher than the other semiconductor lasers. Moreover, the optical fiber loss is less than 0.25 dB/km in this waveband. So the system can connect four pairs of transmitter and receiver units simultaneously. According to the requirements and distribution of gas pipeline, gas gathering device, housing, and other special places in the gas gathering station, the installation scheme including a host control machine and two pairs of transmitter and receiver units was designed and displayed in Figure 6. This system can be used to monitor the leakage of natural gas station in the range of 100 � 100 m.

The leakage detection system based on TDLAS can detect methane, ethylene, and acetylene rapidly and effectively in the open environment, and the response time of the three gases is less than 2 s. The accuracy of giving an alarm is 100%, which can be used in natural gas station and valve room gas leakage. Compared to other techniques, this technique has the advantages of safety in nature, no calibration, high accuracy, and little environmental effects. The MDLs for methane, acetylene, and ethylene gas are 100, 40, and 50 ppm-m, respectively, which meet the requirements for the detection of natural gas leakage in the petrochemical industry.

4. Carbon monoxide (CO) monitoring

Figure 6. Installation scheme of natural gas gathering and transferring station.

on the accuracy of gas monitoring [48].

4.2. Absorption line selection

CO is a kind of toxic, combustible, explosive gas and brings lots of hidden danger to the production and life of human beings. The research of coal spontaneous combustion suggests that a series of gases which could indicate the degree of oxidation and combustion of coal will be produced when coal seam is on fire. Using the relationship between the amount of indicator gases and the rate of change could predict coal seam fire at an early stage. Nowadays, CO is widely used as the main indicator gas for early warning of coal seam fire because the quantity of CO is closely related to the temperature of coal seam and the concentration change is obvious. In addition, the safety production under the mine has attracted much attention. In order to avoid accidents, gas monitoring has become a necessary means. The detection devices of the main gas constituents such as methane and carbon dioxide have been improved and widely used. With the improvement of security awareness, people have higher requirements

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The absorption intensity of CO in the mid-infrared region is two orders of magnitude higher than that of overtone band in the near infrared. With the development of mid-infrared lasers, high sensitivity detection of CO has been obtained by some researchers [49]. But for the long-

4.1. Introduction

Figure 5. Concentration curves of experiment results.

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Figure 6. Installation scheme of natural gas gathering and transferring station.
