Application of Portable CH₄ Detector Based on TDLAS Technology in Natural Gas Purification Plant

Abstract

Methane (CH₄) is the main pollutant in oil and gas production. The detection and accounting of CH₄ is an important issue in the process of greenhouse gas control and emission reduction in oil and gas industry. In this study, a portable CH₄ detector based on tunable diode laser absorption spectroscopy (TDLAS) technology was deployed. The three-dimensional distribution of CH₄ in a natural gas purification plant in Sichuan was obtained through vertical unmanned aerial vehicle (UAV) flight observations and ground mobile observations. According to the mass balance method, the emission of CH₄ on 30 m above ground level (AGL) and 60 m AGL in this site was about 0.012 kg/s (±42% at 1σ) and 0.034 kg/s (±47% at 1σ), respectively, in one day. The vertical distribution showed that the CH₄ concentration reached the maximum (2.75 ± 0.19 ppm) with height of 0 to 100 m AGL. The CH₄ concentration from 100 to 300 m AGL showed a downward trend with height. Atmospheric instability at high altitude and high wind speed promoted the diffusion of CH₄. The CH₄ concentrations of horizontal distribution on 30 m AGL and 60 m AGL were 2.48 ± 0.11 ppm and 2.76 ± 0.34 ppm. In the observation of mobile campaigns, the connecting equipment of natural gas treatment facilities was prone to leakage, such as in valves and flanges. CH₄ leakage was also detected at the torch mouth, especially when there was an open flame at the torch mouth. During the mobile movement investigation, the downwind measurement (OTM-33A) was applied to determine the overall CH₄ emission rate shortly after patrolling the site. This work plays a vital role in optimizing the operation and maintenance of natural gas production stations pipe network, ensuring human safety and minimizing greenhouse gas emissions.

1. Introduction

Methane (CH₄) is a potential greenhouse gas. Since the pre-industrial era, the emission of CH₄ from human activities has caused a radiation force of 0.97 W/m², compared with the emission of carbon dioxide (CO₂) of 1.7 W/m² [1]. The global emissions of CH₄ in the world is about 5.7 × 10⁸ t, of which 60% comes from anthropogenic emissions [2]. According to isotopic signature measurements of ice core and accumulated snow samples to assess pre-industrial CH₄ levels, the extent of the increase in anthropogenic fossil fuel CH₄ emissions may be underestimated by 25–40% [3,4]. Oil and natural gas systems (ONGs) are the leading CH₄ emission source in the field of energy supply, with emissions of 1677.3 MtCO₂eq in 2010 [5].Therefore, it is of great significance to improve the performance of CH₄ detection technology to avoid destructive losses and atmospheric pollutants caused by CH₄ leakage in the petroleum and petrochemical industries. From the producer’s point of view, natural gas leakage also means the loss of products and income.

The vehicle-mounted method has the advantages of having no need to enter the station, large detection coverage and accurate location of CH₄ leakage. MacKay et al. measured CH₄ emissions from 6650 locations in six major oil and gas producing areas in Canada by vehicle-mounted measurement [6]. Yacovitch et al. continuously measured and estimated the location of CH₄ emission at 224 locations in Banert through a mobile laboratory [7]. Thoma et al. used GMAP-REQ platform with a G1301-fc cavity ring-down spectrometer to measure the emissions from well sites in Texas, Colorado and Wyoming. The results showed that the median CH₄ emission rates were 0.43 g/s, 0.21 g/s and 0.79 g/s, respectively [8]. All the above studies are based on ground measurement to investigate CH₄ emissions. However, these mobile laboratories and ground investigation technologies are more time-consuming or may miss high leakage sources.

In recent years, airborne-based platforms, such as manned light aircraft, have effectively capture lofted plumes [9,10]. Karion et al. used airborne measurement technology to measure the regional CH₄ emissions from oil and gas operations in Banert shale, Texas, USA [11]. Allen et al. used the component-level and vehicle-mounted tracer method to detect the oil and gas production stations in the United States, and the average daily natural gas production of the covered production stations ranged from 5.6 × 10² to 1.33 × 10⁶ m³ [12]. Yuan et al. the airborne measurement method to measure CH₄ emission from two shale gas fields in the United States [13]. However, for the sake of public safety and property losses, it is not allowed to fly at ultra-low altitude or near the source. CH₄ emissions from shale gas producing areas in the United States are measured by the Picarro measurement system on a plane This expensive instrument is not easily accessible to most scientists, and it is too large to be carried for outdoor experiments [14]. Therefore, it is urgent to develop a CH₄ detector with high precision, low cost and real-time monitoring.

Laser-based techniques, also known as laser absorption spectroscopy (LAS), achieve selectivity for a target gas, typically CH₄ [15]. The most common LAS technology is called tunable diode laser absorption spectroscopy (TDLAS) [16]. In 2002, TDLAS techniques and dual-frequency modulated spectroscopy were used to analyze atmospheric gases [17]. Titchener et al. reported a new type of remote sensor using single photon lidar and TDLAS, which was used to continuously monitor the unorganized emission of industrial CH₄ [18]. Zhang et al. applied an NIR-TDLAS spectrometer to locate and quantify the unorganized emission of oil and gas fields [19]. In this study, we deployed a portable CH₄ detector based on TDLAS technology. On April 2023, we observed the three-dimensional CH₄ distribution of a natural gas purification plant in Sichuan Province, China. Combined with UAV flight and mobile monitoring activities, the vertical and spatial distributions of CH₄ in the natural gas purification plant was comprehensively observed. The mass balance method and Other Test Method 33A (OTM 33A) were used to calculate and evaluate CH₄ emission flux. The purpose of this study is to develop a new measuring system to investigate CH₄ emissions from natural gas purification plants and provide reliable basic data and a more scientific basis for policies to alleviate global warming.

2. Materials and Methods

2.1. Sampling Site

The sampling site was located in an oil and gas mine in Sichuan, China (Figure 1). The plant is mainly responsible for treating and purifying the primary natural gas extracted from oil and gas mines, with an average daily output of 4.45 × 10⁶ m³. The natural gas purification plant is divided into seven main links: pre-treatment, membrane separation, adsorption, desulfurization and decarbonization, dehydration and the production of liquefied natural gas.

2.2. Apparatus

According to Figure 2, we deployed a portable CH₄ detector (Tengyork, Shaoxing, China, http://www.tengyork.com/product/id/8vuz3.html, accessed on 20 November 2023) based on TDLAS technology, which is a method of scanning the laser wavelength around the specific absorption line. In the working state, the laser signal is absorbed by methane gas, and the concentration value of methane gas can be accurately deduced by the change of laser absorption spectrum. All detector signals were collected and converted by a data-acquisition device through a microcomputer. The detailed parameters of the portable CH₄ detector are shown in

Table 1. The performance parameters of CH₄ detector.

Category	Detailed Parameters
Size	13 (length) × 20 (width) × 29 cm (height)
Overall weight	2 kg
Operating voltage	12 V
Sampling flow	0.4~0.6 L/min
Range of detection	0~100 ppm
Measurement error	±1%
Precision	0.01 ppm
Time of response	≤1 s
Endurance	10 h
Pump	19 (width) × 53.2 mm (length), 23.1 g
Sheathing material	Carbon fiber
Cost of investment	$14,000

.

A six-rotor UAV was selected to carry the CH₄ detector in this field campaign (Figure 2), and the air inlet of the CH₄ detector was located 0.6 m above the UAV to alleviate the air turbulence effect caused by the downwash of the UAV rotor [20]. The detailed parameters of the UAV are shown in

Table 2. The performance parameters of UAV.

Category	Detailed Parameters
Size	60 (height) × 230 cm (width)
Material	Carbon fiber
Endurance	0.5 h
Maximum load weight	6 kg
Maximum flying altitude	500 m
Farthest plane flight distance	1000 m
Flight control system	DJI-A3
Image and control system	DJI-lightbridge2
Cost of investment	$20,000

. The meteorological parameters of the natural gas purification plant were observed by using a three-dimensional ultrasonic wind speed and direction meter (M307200) and a portable meteorological detector (Figure 1).

2.3. Experimental Process

2.3.1. Vertical Measurement of CH₄

To study the vertical distribution of CH₄ at different times, we carried out vertical flight experiments at 9:00, 11:00, 13:00, 15:00, 17:00 and 19:00 local time (LT), respectively, in April 2023. According to Figure 3, in each vertical flight, the UAV raised vertically to 300 m AGL and hovered for 1 min (min), then began to descend, hovering for 1 min at the heights of 200, 100 and 50 m AGL, respectively, and returning to the ground. According to Figure 1, the position of vertical flight is between area A and area C, which is indicated by a green five-pointed star.

2.3.2. Horizontal Measurement of CH₄

The experimenter controlled the UAV equipped with CH₄ detector to conduct detection around area A and area C at the speed of 1 m/s, respectively. The heights were determined to be 30 m and 60 m AGL, which can cover the potential leakage area to the greatest extent. When setting the detected height, the following factors should be considered: detected target, safe distance (stay away from natural gas equipment and pipelines) and environmental factors (such as meteorology, wind speed, airflow, etc.). The meteorological factors during the experiment are shown in Figure 4. ABLH is the abbreviation for atmospheric boundary layer height. According to the downward shortwave radiation flux (DSRF) and the wind speed, the PS classification was determined [21].

2.3.3. Measurement of CH₄ from the Torches and Chimneys

The pilot maneuvered the drone into the plume (about 50 m AGL) and measured CH₄ online by flying. The flight time lasted about 10 min. The UAV hovered over the vent, and the vertical distances between the UAV and the vent of the torch and chimney were controlled at 6 m and 2 m, respectively.

2.3.4. Mobile Observation of CH₄

The CH₄ mobile monitoring campaigns were periodically conducted on 13 April 2023, at six periods of one day (9:00, 11:00, 13:00, 15:00, 17:00 and 19:00 LT). The speed of mobile observation was controlled at 10 km/h to ensure that CH₄ in the atmosphere reacted with the device. The sample height was designed at 1 m AGL. The whole experimental route was approximately 2 km, including the daily inspection route in the factory, pipeline valves and flange joints of the natural gas purification devices.

2.3.5. Flux Quantification of CH₄

The most common flux estimation method for UAV is the mass balance method [19], which derives the net flux by integrating the measured concentration (above background level) across a vertical sampling plane downwind of the emitting source. The total flux (in moles/s) of CH₄ was calculated as:

$$\mathrm{Flux}={{\displaystyle \int }}_0^{\mathrm{Z}}{{\displaystyle \int }}_{\mathrm{A}}^{\mathrm{B}}\left({\mathrm{S}}_{\mathrm{ij}}-{\mathrm{S}}_0\right){\mathrm{n}}_{\mathrm{ij}}{\mathrm{U}}_{\perp \mathrm{ij}}\mathrm{dxdz}$$

(1)

where ${\mathrm{S}}_{\mathrm{ij}}$ is the mole fraction (moles/mole) of species S for each coordinate on the vertical plane AB (oriented perpendicular to the prevailing mean wind vector), and ${\mathrm{S}}_0$ is the measured (or assumed) background. The ${\mathrm{n}}_{\mathrm{ij}}$ term is the mole density of air (moles·m⁻³), which is determined using an ideal gas assumption. The ${\mathrm{U}}_{\perp \mathrm{ij}}$ term is the wind speed (m·s⁻¹) perpendicular to a downwind vertical horizontal plane AB. Fluxes are then integrated over the vertical and horizontal (AB) extent of the plane (or plume) to calculate flux through this plane.

The measurement of CH₄ emission from ground movement was usually carried out in the downwind of the station to obtain the concentration for the whole natural gas station or area. The Environmental Protection Agency (EPA) proposed another test method, 33A (OTM-33A) [22], a programmed version of the downwind measurement method, making it easy to apply for researchers or operational personnel [23]. The emission rate was calculated as:

$$\mathrm{Q}=2\mathsf{\pi }\times \overline{\mathrm{U}}\times {\mathsf{\sigma }}_{\mathrm{Y}}\times {\mathsf{\sigma }}_{\mathrm{Z}}\times {\mathrm{C}}_{\mathrm{peak}}$$

(2)

where $\mathrm{Q}$ is the CH₄ emission rate (kg/h), $\overline{\mathrm{U}}$ is the average wind speed (m/s), and ${\mathsf{\sigma }}_{\mathrm{Y} }$ and ${\mathsf{\sigma }}_{\mathrm{Z}}$ are the horizontal and vertical dispersion coefficients, respectively, determined based on the average local atmospheric stability class and the measured downwind distance. The CH₄ background concentration was determined as the lowest 5% of all concentration data and subtracted. A Gaussian fit of the concentration enhancement versus the wind direction was performed with the vertex of the fit as ${\mathrm{C}}_{\mathrm{peak}}$. The above procedures were performed using the MATLAB program code provided by the OTM-33A manual [24].

2.4. Data Quality Assurance

Before the measurement, the CH₄ detector was calibrated in the laboratory to improve its accuracy. The known concentrations of the CH₄ standard gases were passed into the CH₄ detector until stable gas concentration data were obtained, and the stabilization time was 3 min. To verify the reliability of the CH₄ detector, the standard gases produced by the reference device at six concentrations (0, 2, 4, 6, 8 and 10 ppm) were measured by a sensor. Then, standard curves were made to determine the calibration effect. After calibration, the CH₄ detector shows good agreement with a linear regression correlation coefficient (R²) up to 0.99.

3. Results and Discussion

3.1. High-Altitude Emission of CH₄

The vertical distribution of CH₄ concentrations in the natural gas purification plant is shown in Figure 5. The CH₄ concentration gradually increased to the maximum (2.75 ± 0.19 ppm) from 0 to 100 m AGL and then began to decrease. The concentration of CH₄ at 60 m AGL (2.76 ± 0.34 ppm) was higher than 30 m AGL (2.48 ± 0.11 ppm). The CH₄ concentration at 300 m AGL was 2.52 ± 0.11 ppm, which was lower than the ground. The density of CH₄ is lower than that of air, and it generally rises near the ground. Leakage of equipment near the ground may also be one of the reasons. Flange connection is mostly used between natural gas pipelines or between pipelines and equipment, which is also the most vulnerable part for natural gas leakage. During the mobile observation at the site, CH₄ leakage was found in many flanges, joints and grease holes (Figure 6).

The daily variation of CH₄ at different heights is shown in Figure 5B. At 11:00 LT, the CH₄ concentration began to decrease, and the relative increase of atmospheric boundary layer height and wind speed after sunrise acted as the diffusion agent for atmospheric CH₄ molar fraction [25]. The CH₄ concentration decreased to the minimum at 13:00 LT, which was similar to the daily cycle observed in Shanghai, China [26]. As shown in Figure 4, the DSRF reached the maximum value of 604 W/m⁻² at 13:00 LT. Active photochemical reactions can reduce the concentration of CH₄. The dominant loss of CH₄ was through oxidation in the atmosphere via the hydroxyl radical (OH). Active photochemical reactions can consume a certain concentration of oxygen. CH₄ is a reductive gas, and it is easily oxidized in the atmosphere [27]. The CH₄ concentration began to rise at 17:00 LT, but its growth rate was lower than 19:00 LT. At 19:00 LT, the atmosphere was relatively stable (PS = D), which hindered the vertical diffusion of CH₄ and increased it [28]. In addition, the low wind speed and ABLH were also reasons for the high CH₄ concentration at night [29,30].

The CH₄ concentration of chimneys and flares in the natural gas purification plant is shown in Figure 5. The CH₄ concentrations at the discharge ports of the chimney (A) and torches (B and C) were 2.24 ± 0.03, 2.60 ± 0.35 and 2.59 ± 0.03 ppm respectively. The CH₄ concentration of the two torches was higher than that of the chimney. In the third observation of torch B, it was found that the CH₄ concentration increased obviously, reaching a maximum of 5.97 ppm, indicating that there was a leak. Compared with torch B, the change of CH₄ concentration in torch C was relatively stable.

3.2. Spatial Distribution of CH₄

The measurement of CH₄ from ground movement was usually carried out in the downwind of the station to obtain the CH₄ concentration for the whole natural gas station or area.

The observation results of CH₄ movement in the natural gas purification plant are shown in Figure 7. The average CH₄ concentrations at 9:00 LT and 11:00 LT were 2.39 ± 0.38 ppm and 2.17 ± 0.05 ppm, respectively, both of which were lower than those at 19:00 LT (2.64 ± 0.27 ppm). At 13:00 LT, the CH₄ concentration exceeded 3.0 ppm in 33 places outside area A and inside area C. Interestingly, the CH₄ concentration at 13:00 LT was the lowest (2.47 ± 0.06 ppm) in a single day according to the Figure 5, which showed that the mobile observation supplemented the high-altitude monitoring data to some extent. According to Figure 8, the CH₄ concentration in the station increased obviously at 15:00 LT and 17:00 LT, with 47.2% and 43.5% of CH₄ exceeding 3 ppm, respectively. The CH₄ concentration of gas pipeline valves were relatively high in the natural gas purification unit area (Area A). The valve is affected by the temperature, pressure, erosion and vibration corrosion of natural gas, and the defects in the production and manufacture of the valve will inevitably lead to CH₄ leakage in the process of use [31]. In the natural gas purification unit area (area C), the maximum concentration of CH₄ was 394.48 ppm, and the leakage point was located at the product gas delivery flange. Flange connection is the main form of natural gas pipeline and equipment connection, and its leakage is also one of the most important forms of natural gas station leakage. The sealing of the flange mainly depends on the pre-tightening force generated by the bolt connected with it, and the gasket can achieve enough working sealing specific pressure to prevent natural gas leakage [32]. For the natural gas pipeline, the transmission medium has the characteristics of corrosion, high pressure and vibration during transportation, which will lead to the failure of the flange seal o for the natural gas pipeline and leakage [32].

The mobile movement observation of CH₄ can achieve a large coverage in the detection range. However, due to the randomness of CH₄ emission, short-term “snapshot” on-site mobile detection may not capture this CH₄ emission signal. It is necessary to take longer-term (2 weeks or more) CH₄ emission detection measures in the future [33].

3.3. Flux Quantification of CH₄

The spatially-interpolated CH₄ enhancement projected onto the flux planes is shown in Figure 9. Having obtained the spatially interpolated flux planes downwind (with the mean background subtracted to represent net downwind enhancement) in Figure 9, Equation (1) was used to calculate CH₄ flux by integrating across the horizontal and vertical extents of the 2D sampling plane perpendicular to the prevailing wind. In the observation of mobile measurement, the background concentration of CH₄ was determined to be 2.03 ppm (the lowest 5% of all concentration data). The concentration of CH₄ at 60 m AGL (2.76 ± 0.34 ppm) in the natural gas purification plant was higher than 30 m AGL (2.48 ± 0.11 ppm). In the Lagrange mass balance method, the amount of air entering and leaving an air volume is measured with the difference of measured values, thus giving the net surface flux in the volume. According to

Table 3. Mean CH₄ flux and one standard deviation uncertainties.

Units in kg/s	Height of Flight
	30 m AGL	60 m AGL
Mean flux	0.012	0.034
total uncertainty	0.00504	0.01598
Wind uncertainty	0.0009576	0.0003995
Background uncertainty	0.0031752	0.0131036
Measurement uncertainty	0.0001008	0.0003196
Downwind uncertainty	0.0008064	0.00204544

, for the flight on 30 m AGL, the flux was derived to be 0.012 kg/s (±42% at 1σ) with an uncertainty dominating in both the background variability (accounting for 63% of the total flux uncertainty) and the wind measurement variability (accounting for 19% of the total flux uncertainty). For the flight on 60 m AGL, the flux was derived to be 0.034 kg/s (±47% at 1σ) with an uncertainty dominating in the background variability (accounting for 82% of the total flux uncertainty).

3.4. Treatment and Preventive Measures for Equipment Leakage

In view of the natural gas leakage in the flange, the bolts can be tightened by de-pressurizing and venting. If it is necessary to stop gas transmission, the valves on both sides of the leaking part must be closed, the old gasket should be replaced after venting, and then the bolts should be tightened again. In the case that gas transmission can not be stopped, flange plugging treatment technology is needed to deal with it. According to the actual leakage situation, in order to avoid more leakage, it is best to use a wound metal flange gasket or metal ring gasket. In view of the valve leakage, it can be dealt with by fastening bolts. There is too much purge gas in the emergency torch, so it is necessary to install a more reliable automatic ignition system and adjust the purge gas to the minimum required level. Preventive measures can be taken against the leakage of natural gas pipelines. For example, the cleaning of natural gas pipeline in the future can avoid the existence of iron sulfide powder in the pipeline. When the peak gas consumption comes in winter, it is necessary to prepare for standby gas to protect the gas pipeline from flying dust due to the excessive gas flow rate, resulting in excessive wear and tear of the pipeline.

4. Conclusions

In this study, a lightweight (<3 kg), low-cost ($14,000), and portable CH₄ detector based on TDLAS technology was developed. We made many aerial and ground investigations on a natural gas station in Sichuan, China by using a self-developed CH₄ detector. The results show that, in vertical detection, the CH₄ concentration increased with height at 0 to 100 m AGL and decreased at 100 to 300 m AGL. In the horizontal measurement, the CH₄ emission flux at 30 m AGL and 60 m AGL were approximately 0.012 kg/s (±42% at 1σ) and 0.034 kg/s (±47% at 1σ), respectively, in a day. However, this calculation method is not necessarily accurate. Uncertainty contributing to the flux was dominated by ambient variability in the background (inflow) concentration (>60%) and wind speed (>20%), with instrumental error contributing about 1–2%. CH₄ leakage was detected at the torch mouth, especially when there was an open flame at the torch mouth. During the mobile observation, we found leakage at the flange and valve some of the time.

The UAV equipped with CH₄ detector can go deep into some areas that are difficult for personnel to reach in order to carry out monitoring activities, thus improving the working efficiency. With the aid of mobile observation, CH₄ emission from natural gas stations can be better detected. Another advantage is that the time resolution of the instrument is 1 s, which captures the fine and dynamic spatial changes of CH₄. This study is helpful for oil field production enterprises in carrying out effective and reliable CH₄ emission reduction.

In addition, this study was conducted at a certain time in the day, which may affect the applicability of this measurement method in other places. Ground-based meteorological sensors may not provide the necessary spatial and vertical resolution, and winds should ideally be measured onboard the UAV in the future. Besides, it is necessary to expand the temporal and spatial coverage of this study to answer the complete distribution of CH₄ and the emission factors of components in normal and abnormal activities.