Based on the Beer-Lambert law, the relationship between the incident intensity I<sup>0</sup> and the transmitted intensity I can be expressed as

$$I = I\_0 \exp\left(-kL\right) \tag{1}$$

where k is the absorption coefficient and L denotes the path length (in cm). In the near-infrared region, the gas absorption coefficient is usually very small, i.e., kL ≤ 0.05 [38]. Eq. (1) can thus be simplified as

$$I = I\_0(1 - kL) = I\_0[1 - \sigma(\nu)\mathbb{C}L] \tag{2}$$

where σ(ν) is the absorption cross section (in [cm<sup>2</sup> /molecule]) at frequency ν and C is the gas mixing ratio. The integrated absorbance AI (in [cm�<sup>1</sup> ]) can be written as

$$A\_I = \int A(\nu)d\nu = \int \ln(I\_0(\nu)/I(\nu))d\nu = \text{NL}\int \sigma(\nu)d\nu = \text{NLS} \tag{3}$$

N is the number of absorbing molecules (in [molecules/cm<sup>3</sup> ]); S is the molecule absorption line strength (in [cm<sup>2</sup> /(mol cm)]). Based on Eq. (3), the gas species mixing ratio can be retrieved from the integrated absorbance AI measured at temperature T and pressure P [39]:

$$\mathcal{L}\{\text{ppm}\} = \frac{N}{N\_T} \times 10^6 = \frac{A\_I P\_0 T}{N\_L \text{PT}\_0 \text{LS}} \times 10^6\tag{4}$$

where NL = 2.6868 � 1019 mol/cm<sup>3</sup> represents the Loschmidt number at T0 = 273.15 K and P0 = 760 Torr.

For gas mixing ratio detection, WMS is often adopted. The intensity of 2f signal can be expressed as [40]

$$I\_{2f} \propto I\_0 \sigma\_0 \mathbf{C} L \tag{5}$$

The extraction, transportation, and storage of natural gas have become an important part of social development. Equipment safety and high efficiency operation in gas transmission station are the keys to ensure the natural gas transportation. Once it is released, the serious safety accidents such as energy waste, environmental pollution, fire, and explosion will happen [42], which would directly threaten the safety of life and property of the countries and people [43]. The main component of natural gas is methane, accounting for 90%, and also contains a small amount of ethane, acetylene, butane, carbon dioxide, carbon monoxide, hydrogen sulfide, and so on. Traditional natural gas leakage detectors include flame ion detectors (FID), electronic detectors, electrochemical catalytic combustion detectors, and infrared absorption detectors [44]. However, these detectors are self-charging and have potential safety problems in the application of flammable, explosive, and other special environments. Moreover, these sensors are short in life, low in precision, poor in stability, and difficult in adjustment and often give the wrong results of measurements and misinformation. Recently, TDLAS technology has been widely used with the rapid development of narrow linewidth semiconductor laser technology [45]. The SRI International (Menlo Park, CA) company in America has developed a vehicular natural gas pipeline leakage detector, which improves the efficiency of pipeline leakage detection. However, they are all limited to the detection of methane and do not involve the detection

Environmental Application of High Sensitive Gas Sensors with Tunable Diode Laser Absorption Spectroscopy

http://dx.doi.org/10.5772/intechopen.72948

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In view of the area of natural gas, gathering station is large, and the pipeline system of natural gas is gathered; point and portable measurement is not suitable in this situation. We designed an open, continuous detection and alarm system which has the characteristics of fast response speed and high detection precision based on TDLAS technology. Moreover, this system also detects ethylene, acetylene, and other gases, which improves the measurement precision and

The near-infrared absorption band matches with the low loss window of optical fiber and is convenient for long-distance transmission and multipoint distributed detection by using fiber and fiber devices. Therefore, the absorption lines of selected CH4, C2H2, and C2H4 are 1653.72, 1531.59, and 1621.36 nm, respectively. There are three adjacent absorption lines at 1653.72 nm for CH4, which are close to each other and cannot be separated in the atmospheric pressure by consulting the HITRAN 2008 database. In the experiment, they are processed as one absorption line. The C2H4 absorption lines are not included in HITRAN database. A large amount of absorption lines of C2H4 from 1600 to 1650 nm can be found from the PNNL25C (Northwest Pacific National Laboratory) database which have been already experimentally verified in the

literature [46]. The parameters of three gases absorption lines are shown in Table 1.

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,

of other gases in natural gas.

reduces the probability of false alarm.

3.3. DFB-based experimental platform

3.2. Absorption line selection

When the reference signal and nonlinear least square multiplication method are introduced to fit the 2f signals of the target gas [41], Eq. (5) can be rewritten as

$$\mathbf{C}\_{\text{Mea}} = a \frac{I\_{\text{Mea}} \mathbf{C}\_{\text{Ref}} L\_{\text{Ref}}}{I\_{\text{Ref}} L\_{\text{Mea}}} \tag{6}$$

where a is fitting coefficient; CMea and CRef are the mixing ratios of the target gas to be measured and reference gas in the calibration cell, respectively; IRef and IMea denote the intensities of the two split laser beams; and LRef and LMea represent the calibration cell and the measurement optical path length, respectively. In general, the ratio of the 2f and 1f signals can be used to cancel any laser intensity differences. In this case, the mixing ratio from the following equation could be easily obtained:

$$\mathbf{C}\_{\text{Mea}} = \frac{\begin{pmatrix} \frac{I\_{2f}}{I\_{1f}} \end{pmatrix}\_{\text{Mea}} \mathbf{C}\_{\text{Ref}} L\_{\text{Ref}}}{\begin{pmatrix} \frac{I\_{2f}}{I\_{1f}} \mathbf{1}f \end{pmatrix}\_{\text{Ref}} L\_{\text{Mea}}} \tag{7}$$

where <sup>I</sup>2<sup>f</sup> I1f � � Ref and <sup>I</sup>2<sup>f</sup> I1f � � Mea represent the 2f/1f ratio value of the reference and target gas signals, respectively.
