Evaluation of Methane Emissions in Daily Operations and Accidents: A Case Study of a Local Distribution Company in China

Abstract

As the second-largest contributor to historical global warming, methane emissions must be controlled to slow down temperature increase and achieve climate benefits. Due to a lack of knowledge about the specific sources and processes, a quantitative approach will lead to inaccurate estimation. In this paper, a typical local distribution company with more than 100 years of operation history was chosen. Detailed procedures of pipeline constructions and accidents were investigated, and critical steps leading to methane emissions were clarified. Then emission quantification methodologies for all processes were proposed, including a new pipeline connection, new regulator connection, emergency repair and third-party damages. As a basis for emission estimation, the distribution of parameters, such as diameter, length and pressure, was counted. Then the emission rates of all projects were calculated, and emission factors were established. The average emission rates of the new pipeline connection, new regulator connection, emergency repair, third-party damage (medium pressure) and third-party damage (low pressure) were 234 kg, 147 kg, 217 kg, 17,282 kg and 62 kg, respectively. In addition, the total methane emission in China from these sources was estimated to be about 5 × 10⁴ t, which is large enough to attract attention. The work in this paper aims to establish a reasonable framework to evaluate venting methane emissions from the distribution process of natural gas.

1. Introduction

Methane, with its GWP20 = 84 (global warming potential over 20 years) and its GWP100 = 28, is the second-largest contributor to historical global warming [1,2,3], accounting for 19% of global greenhouse gas emissions [4]. It was emphasized in an IPCC report [5] that methane emissions have to dramatically reduce to limit the temperature increase to 1.5 °C above pre-industrial levels. In terms of anthropogenic methane emissions by source, emissions from natural gas systems are the highest [6]. An EDF (Environmental Defense Fund) study [7] revealed that gas emission must be controlled below 2.7%, 1.5% and 0.8%, respectively, in order to achieve climate benefit when gas is used to replace coal, petroleum and diesel. Elucidating methane emissions from natural gas systems will make great contribution to emission reduction.

Methane emissions in the natural gas industry chain, including production, processing and supplying, arise from venting, fugitive emissions and incomplete combustion [8,9]. The sites of production and processing are relatively concentrated, and the processes are well-managed, making it easier to carry out research on them. A majority of research to date has concentrated on the estimation of methane emission from production and processing [10,11,12,13,14]. However, due to the long distance and complex processes of the gas supply chain including transmission, storage, distribution and utilization (shown in Figure 1), emissions associated with the gas supply chain are hard to characterize [15]. Due to a lack of knowledge about the specific sources and processes, the quantitative approach will lead to inaccurate estimation, which is shown in the discrepancies between top-down and bottom-up inventories [16,17,18], and this may cause the underestimation of methane emissions [16]. Consequently, wide data dissemination and sharing are crucial for methane emission evaluation and constraint [19,20].

From Figure 1, it can be seen that methane emission sources of venting are much more than those of fugitive emissions and incomplete combustion. The clarification and separation of emission sources contribute to the difficulty in emission estimation. Moreover, the considerable discrepancies of each process such as pipeline repairs and third-party damages lead to the skew in emission data. As a result, characterization and estimation of methane emission in the gas supply chain have focused on fugitive emissions from pipelines and infrastructures [21,22,23,24]. Very little research about venting emission, especially from distribution pipelines, was reported, and it is the objective of the study, as shown in the red dashed-line oval in Figure 1. Given the strain that, as a considerable consumer of natural gas, China emitted 1620 Mt CO₂ eq (GWP100) in 2019 [4], accounting for 16.5% of global methane emissions, taking China as an example to carry out this research is appropriate.

In this paper, detailed procedures of pipeline constructions and rehabilitation were investigated for a local distribution company, and critical steps leading to methane emissions were clarified at first. This work helps classify the multifarious projects, which may lead to methane emission and is the basis to establish emission estimation methods. Then emission quantification methodologies for blowdown, purging and third-party damages are proposed. Similar to other segments in the natural gas industry chain, the range in emission rates across sources is highly skewed. To minimize bias caused by data selection, all cases in the whole year 2019 were investigated. As a basis for emission estimation, the distribution of parameters, such as diameter, length and pressure, was counted. Then the emission rates of all projects were calculated, and emission factors based on them were established. Moreover, the total methane emission in China from these sources is estimated, which is large enough to attract attention from the governors. The work in this paper aims to establish a reasonable framework to evaluate venting methane emissions from the distribution process of natural gas.

2. Methods

2.1. Scope of Study

2.1.1. Description of the Distribution System

Within the distribution system of the company discussed, pipelines in operation accounted up to 8800 km in 2019. The length of high-pressure (HP, 0.8 MPa), medium-pressure A (MP-A, 0.4 MPa), medium-pressure B (MP-B, 50 kPa) and low-pressure (LP, 2 kPa) pipelines is 230 km, 1370 km, 420 km and 6780 km, respectively. In terms of material, steel, cast-iron, polyethylene (PE) and galvanized steel pipe accounted for 1880 km, 3270 km, 3160 km and 490 km, respectively. The gas constituent (in volume) is 93.77% CH₄, 3.56% C₂H₆, 0.67% C₃H₈, 0.13% n-C₄H₁₀, 0.11% i-C₄H₁₀, 1.23% N₂ and 0.53% CO₂. The total distribution amount in 2019 was 1 BCM.

2.1.2. Emission Source Identification

①

New pipeline connection

“New pipeline connection ” is a kind of construction intended to connect newly finished pipes with existing pipes or to replace aged pipes. The construction plans are always well prepared. Detailed steps are as follows:

(1)

Isolation: after the construction of new pipes (red dashed line in Figure 2) and strength and tightness tests are finished, to shut off all valves (V1, V2, V3, V4, V5 in Figure 2) on adjacent existing pipes leading to new pipes so as to establish an “isolated” network (dash–dot oval) to minimize the impact on customers outside the “isolated” section.

(2)

Blowdown: to de-pressurize the “isolated” network by releasing gas into the atmosphere through venting holes on regulators or valves until the pressure is slightly higher than atmospheric in order to reduce risk in the following operation processes.

(3)

Accurate isolation: to drill holes on existing pipes and insert stopples (shown as BB1, BB2 in Figure 2) to achieve a smaller cut-off section.

(4)

Construction: to perform constructions such as welding, etc., and then new pipes are physically connected with existing pipes.

(5)

Purging: to take out stopples and then purge the air remaining within new pipes into the atmosphere through the drilled holes or venting holes on neighboring regulators or valves. During this step, methane concentration at the vent is monitored continually with detecting equipment. When it rises to 85%, workers close upstream valve and wait for 1 min and then open the valve again while checking the methane concentration three times (once a minute) to ensure no air remains.

During the new pipeline connection procedure, methane emission happens in the blowdown and purging steps.

②

New regulator connection

“New regulator connections” are usually carried out after new regulators, which are connected to new blocks or buildings, are installed and adjusted and prior to delivering gas. The procedures of new regulator connection and new pipeline connection are almost the same, as illustrated in Figure 3, but the shape parameters of pipes, such as diameter and length, are different in these two categories, so they are analyzed separately to achieve a more accurate result.

During the new regulator connection procedure, methane emission happens in the blowdown and purging steps.

③

Emergency repair

After receiving alarm calls, emergency staff will reach the spot within half an hour and then start repairing. The detailed steps are almost the same as those for new pipeline connection. For emergency repair, the construction steps are temporarily determined.

During the emergency repair procedure, methane emission happens in the blowdown and purging steps.

④

Third-party damage and repair

The response to third-party damage of MP pipes is similar to that for emergency repair. The difference is that free leakage before rehabilitation cannot be ignored, and this part of gas emission has to be taken into account. The repair of LP pipes is relatively simple, without the blowdown and purging steps, so gas emission during repair can be ignored.

During the third-party damage and rehabilitation procedure of MP pipes, methane emission happens in free leakage and the blowdown and purging steps. During the third-party damage and rehabilitation procedure of LP pipes, only methane emission in free leakage step is taken into account.

2.2. Emission Quantification Methodology

2.2.1. Data Source

Pipes related to a construction or an accident are of different pressure, diameter and length. Therefore, information for gas emission quantification of a new pipeline connection, new regulator connection, emergency repair, third-party damage to MP pipelines and third-party damage to LP pipelines in the year 2019 is summarized from recorded filings of a local distribution company in Shanghai, including gauge pressure, diameter, length and the number of these pipes. In total, over 600 pipeline construction projects’ third-party damages are studied. In addition, information on damaged pipes and the time of alarm calls for third-party damages was collected. Pressures of pipes were determined by upstream regulators settings and the distance between the pipe and regulators, so pipes which are connected may be under different pressures.

2.2.2. Gas Emission from Blowdown

Gas emission from blowdown is the difference between the amount of gas stored in the “isolated” network under operation pressure and atmospheric pressure. Therefore, the emission can be calculated according to Equation (1):

$$\Delta M={\displaystyle \sum _{\mathrm{i}}\frac{V_{\mathrm{i}}}{R{T}_{\mathrm{i}}}\Delta P_{\mathrm{i}}}$$

(1)

where $\Delta M$ is the gas emission in terms of mass (kg); $\Delta P_{\mathrm{i}}$ is the gauge pressure of the pipe section i (Pa); $T_{\mathrm{i}}$ is the gas temperature of the pipe section i; $V_{\mathrm{i}}$ is the volume of the pipe section i (m³); and $R$ is the gas constant.

2.2.3. Gas Emission from Purging

①

Model for simulation

Theoretically, the purging step is always described as one-dimensional non-steady compressible flow accompanied by component variations. However, the venting pipe is usually vertical to the main pipe, and its length cannot be ignored. Therefore, two-dimensional CFD (computational fluid dynamics) models were established to evaluate the total gas emission and to quantify the influence of various parameters such as distance between upstream valve and downstream venting pipe, operation pressure, pipe diameter, etc.

In the Fluent software, the transient simulation method is adopted. The pipe inlet is the pressure inlet, and the species are 100% methane. The pipe wall is at a constant temperature wall of 288 K. The atmospheric environment boundary is the pressure outlet with a pressure of 1 bar, and the backflow is 21%O₂ + 79%N₂ at 288 K. Initial gas components in the pipe are 21%O₂ + 79%N₂. To describe the energy transmission, the energy model is on. The standard k-ω model is chosen for viscous calculation. Species transport model is adopted to describe the species change at selected points during the simulation process.

As discussed before, the purging period is composed of the minimum venting time until the methane concentration at the vent rises to 85% and additional 3 min for concentration checking. The diameter of the venting pipe is 50 mm, which is the standard for most purging steps. CFD simulations were performed for several diameters (DN100, DN200, DN300, DN500, DN700) and lengths (L = 50 m, 100 m, 200 m, 500 m). The mass of vented gas until the minimum venting time and until the additional 3 min was calculated.

If pipes related to the purging step are under different operation pressure, the pressure corresponding to the longest length of pipes is selected to calculate gas emission during the purging step.

②

Simulation results

Figure 4 shows the gas emission during the purging step for p = 50 kPa and p = 0.2 MPa as an example. The red dashed line in Figure 4 representing 64 kg and 128 kg can be considered the approximate emission for most cases. The approximate values for other gauge pressures were calculated in the same way, as 200 kg for p = 0.65 MPa, 149 kg for p = 0.25 MPa, 85 kg for p = 0.1 MPa and 77 kg for p = 0.08 MPa.

③

Simulation verification

A purging process is measured and simulated to verify the accuracy of the model. The pipe works under the pressure of 0.07 MPa, with a length of 390 m and a diameter of 200 mm. The venting pipe is connected to the end of the pipe, with a length of 0.5 m and a diameter of 50 mm. Installed on the venting pipe, the equipment measures the dynamic pressure of the venting flow.

The dynamic pressure of the venting flow during the purging is both measured and simulated, as shown in Figure 5. Apart from the fluctuation caused by the high-speed flow, the measured value and the simulation result show strong coincidence, which verifies the rationality and accuracy of the simulation model.

2.2.4. Gas Emission from Third-Party Damage

According to the damaged condition, third-party damages can be classified into two kinds: broken pipe and a hole of various shape on the pipe. The calculation methods for this two cases are somewhat different.

①

Free leakage of completely broken pipes

When a pipe is completely broken by third-party, huge amount of gas will escape from the leaking point, and the suddenly increasing flow rate tends to give rise to friction between flow and pipe walls. The model shown in Figure 6 was incorporated to analyze gas flow within broken pipes. The cross-sections I, II and III stand for a location upstream of the leaking point where pressure remains unchanged during leakage, the leaking point and the atmosphere, respectively. The distance L between cross-sections I and II has a strong influence upon resistance despite it being quite difficult to determine for a specific third-party damage accident.

Equations to describe adiabatic flow between cross-sections I and II are as follows:

Momentum equation:

$$\frac{\mathrm{d}P}{\mathrm{d}x}+\frac{\mathrm{d}}{\mathrm{d}x}\left(\rho u^2\right)=-\lambda \frac{1}{d}\cdot \frac{1}{2}\rho u^2$$

(2)

Continuity equation:

$$\rho u={\mathrm{Const}=\mathrm{C}}_1$$

(3)

State equation:

$$\frac{P}{{\rho }^{\mathsf{\kappa }}}={\mathrm{Const}=\mathrm{C}}_2$$

(4)

where $P$ is absolute pressure (Pa); $\rho$ is density (kg/m³); $u$ is velocity (m/s); $d$ is the inner diameter of the pipe (m); $\lambda$ is the friction coefficient; and $\kappa$ is the specific heat ratio of gas.

If gas flow from the leak is sub-sonic, the flow rate $Q_{\mathrm{m}}$ can be described as Equation (5):

$$Q_{\mathrm{m}}=3600\frac{C_{\mathrm{D}}}{{\rho }_0}\frac{\pi }{4}{d}_{\mathrm{h}}^2{\left(\frac{P_0}{P_2}\right)}^{\frac{1}{\mathsf{\kappa }}}\sqrt{2\cdot \frac{\kappa }{\kappa -1}{P}_2\cdot {\rho }_{\mathrm{int}}\left[1-{\left(\frac{P_0}{P_2}\right)}^{\frac{\mathsf{\kappa }-1}{\mathsf{\kappa }}}\right]}$$

(5)

where $Q_{\mathrm{m}}$ is flow rate (kg/s); $P_0$ is atmospheric pressure (Pa); $P_2$ is absolute pressure at cross-section II (Pa); $C_{\mathrm{D}}$ is the discharge coefficient and taken as 1 when the pipe is broken completely; ${\rho }_0$ is the density of the gas under the reference state (kg/Nm³); ${\rho }_{\mathrm{int}}$ is the density of the gas within the pipeline (kg/m³); and $d_{\mathrm{h}}^{}$ is the equivalent diameter of the damaged hole and taken as the diameter of the pipe when completely broken.

If gas flow from the leak is supersonic, flow rate $Q_{\mathrm{m}}$ can be described as Equation (6):

$$Q_{\mathrm{m}}=3600\frac{C_{\mathrm{D}}}{{\rho }_0}\frac{\pi }{4}{d}_{\mathrm{h}}^2{\left(\frac{2}{\kappa +1}\right)}^{\frac{1}{\mathsf{\kappa }-1}}\sqrt{2\cdot \frac{\kappa }{\kappa +1}{P}_2\cdot {\rho }_0}$$

(6)

Then the leaking flow rate can be solved by Equations (2)~(4) and Equations (5) or (6).

②

Free leakage of partially broken pipes

When the third-party damage does not break a pipe completely, a hole of various shape will be formed, and gas will escape from the hole at a very high velocity. For such kinds of third-party damage, the gas flow rate can be directly calculated according to Equations (5) or (6) since the discharge is driven by the difference between operation pressure and atmospheric pressure.

According to the management of the company, if the accident involving MP pipes happens during business hours (8:00~17:00), the time between the alarm call and valves being shut is about 60 min, and if the accident happens outside of business hours, it will be 120 min. The response duration to accidents regarding LP pipes is 30 min. The distance L of incidents for MP pipes is determined according to the layout of related pipes, mostly 2 km and 4 km, and that for LP pipes is taken as 500 m, considering the layout of communities within which the pipeline runs.

3. Results and Discussion

3.1. Filing Data Analyses

The number of new pipeline connections, new regulator connections, emergency repairs, third-party damages (MP) and third-party damages (LP) was 134, 138, 71, 8 and 298, respectively, in the year 2019. The distribution of pressure and length of these emission sources are shown in Figure 7. From Figure 8, it can be seen that 0.05 MPa and 0.2 MPa are the most common pressures in all these emission sources, and 0.2 MPa accounts for more than 50% in new pipeline connections, new regulator connections and third-party damages (MP). The lengths of related pipes in different cases differ from each other dramatically, and the dominating length ranges of new pipeline connections, new regulator connections, emergency repairs and third-party damages (MP) are 1000–5000, 0–50, 1000–5000 and 500–1000.

The distribution of diameter of emission sources is shown in Figure 8. Two calculation method are adopted herein, by length and by times. The results show the difference in these two methods, which is because pipes of some diameters are usually longer than others, such as DN300-DN500. Third-party damage (LP) happens to low-pressure pipes, so the diameters of these pipes are mostly below DN50. Apart from this, most pipes related are bigger than DN50 and smaller than DN500.

3.2. Methane Emission Rate

Figure 9 gives the distribution of the methane emission rate of these three kinds of construction. The emission rates vary within a very broad range because of the differences of network structure. Taking new pipeline construction as an example, the minimum emission is 56 kg, while the maximum is 1355 kg. Fortunately, due to pipeline construction specifications such as valve spacing requirements, most emission rates are concentrated between 0 and 300 kg, which will be of great accuracy and convenience to emission estimations. The average emission rates of new pipeline connections and emergency repairs are very close, respectively 234 kg and 217 kg, and the average emission of a new regulator connection is 147 kg. The reason is that pipes for new regulator connections are always shorter than pipes in existing networks. From these data, it can be known that methane emission from construction is closely related to the shape parameters of pipelines, and the average value can accurately estimate the emissions. The ratio of emission from purging and blowdown shows an upward trend with emission rate because the emission rate of purging is relatively stable, while that of blowdown is approximately proportional to the length of the pipelines.

As a result of stringent administrations in recent years, third-party damages to MP pipelines decreased dramatically, with only eight third-party damages in the whole year. LP pipes, most of which are buried adjacent to buildings to deliver gas to customers, lack information regarding their location and construction filings. Lack of effective coordination between different utilities is also a reason why third-party damages to LP pipelines happen so frequently, at 298 times a year. Figure 10 gives the distribution of the methane emission rate of these two kinds of third-party damages. The emission rates also vary within a very broad range because of the difference of network structure and response time. Most emission rates of third-party damages to LP pipelines are concentrated at between 5 and 10 kg. The number of third-party damages to MP pipelines is too small to show the regular distribution of emissions. The average emission rates of third-party damages to LP pipelines and MP pipelines are 62 kg and 17,282 kg, respectively. Though third-party damage to MP pipelines happen quite rarely, the amount of methane emission from it is enormous compared to other emission sources.

3.3. Total Methane Emission and Emission Factor

Listed in

Table 1. Methane emission statistics.

Emission Source	Numbers	Blowdown (kg)	Purging (kg)	Leakage (kg)	Total (kg)
New pipeline connection	134	18,114	13,189	0	31,303
New regulator connection	138	5074	15,150	0	20,224
Emergency repair	71	10,004	5378	0	15,382
Third-party damage (MP)	8	0	731	137,526	138,257
Third-party damage (LP)	298	0	NA	18,607	18,607
Total	649	33,192	34,448	156,133	223,773

are methane emission statistics of different steps from four kinds of emission sources during the whole year. Although the frequency of third-party damage of MP pipelines is the least, it accounts for the most emission. Except for this emission source, methane emissions are all around 20,000 kg. Total methane emission for all these emission sources is 223.8 t. Estimating methane emission of these sources in China according to the number of customers and the amount of gas consumption, it is 44,536 t and 59,083 t, respectively, which accounts for 0.08% and 0.1% of methane emission in China, and that amount is big enough to attract the public’s attention. The blowdown and purging steps contribute almost the same to methane emission and should draw attention of an equal degree.

Lamb et al. [25] reported that the methane emission rate derived from dig-ins and blowdowns was 0.149 Gg/year in Indianapolis for 6521 km length of main pipelines. A report [6] showed that the methane emission factor of pipeline blowdowns and dig-ins was 1.2 kg/km and 19.0 kg/km, respectively. Methane emission factors of pipeline blowdown and dig-ins were 20.01 kg/km and 18.89 kg/km, respectively, in [26]. Almost all blowdowns happen to MP pipelines. If methane emission is estimated by the total length of MP pipelines, it is 18.5 kg/km according to analyses in this paper, similar to that from [26]. However, if this figure is calculated using the total length of the distribution system, it is 3.3 kg/km, much closer to the figure from [6]. Furthermore, the emission of dig-ins is 76.8 kg/km for MP pipelines, 2.7 kg/km for LP pipelines and 17.7 kg/km for the whole distribution system. These three figures differ from each other greatly, and only that for the whole distribution system is consistent with former research results.

The ratios of MP and LP pipelines in cities are in a certain range but differ from each other undoubtedly. Moreover, the frequency of routine maintenance and accidents depends on the ages of pipelines and the management. Therefore, factors related to the length of pipelines, especially for the whole distribution system, may lead to great error. As shown in Figure 9 and Figure 10, emissions from different cases are strongly skewed, which is the result of the strong relationship between emission rate and network structure (lengths, diameters, operation pressures of pipes influenced). Undoubtedly, calculation according to case-specific filings is the first choice, but when detailed information is not available, emission factors calculated by times of events rather than length of the system are of great importance to evaluate methane emission conveniently and reflect the technical status. Emission factors are established and listed in

Table 2. Methane emission factor.

Segment/Source	Factor (kg/Event)	Segment/Source	Factor (kg/Event)
New pipeline connection	233.6 (195.8–271.4)	Blowdown	96.8 (78.3–115.3)
New regulator connection	146.5 (130.6–162.5)	Purging	98.1 (95.2–101.0)
Emergency repair	216.6 (173.4–259.8)	Leakage (MP)	17,190.7 (1602.0–2779.5)
Third-party damage (MP)	17,282.1 (1694.0–32,870.2)	Leakage (LP)	62.4 (43.3–81.5)
Third-party damage (LP)	62.4 (43.3–81.5)
Uncertainty limits: estimated as the 95% confidence limit

.

4. Conclusions

①

Detailed procedures of pipeline constructions and rehabilitation were investigated for a local distribution company, and emission sources were identified including new pipeline connections, new regulator connections, emergency repairs and third-party damages and repairs.

②

By analyzing filing data, it was found that 0.05 MPa and 0.2 MPa are the most common pressures. The lengths of pipes in different cases differs from each other dramatically, and the most common length ranges of new pipeline connection, new regulator connection, emergency repair and third-party damage (MP) are 1000–5000, 0–50, 1000–5000 and 500–1000. The diameters of pipes involved in third-party damage (LP) are mostly under DN50, and apart from this, most pipes related are larger than DN50 and smaller than DN500.

③

The emission rates vary within a very broad range because of the difference of network structure. The average emission rates of new pipeline connections, new regulator connections, emergency repairs, third-party damages (MP) and third-party damages (LP) are 234 kg, 147 kg, 217 kg, 17,282 kg and 62 kg, respectively.

④

Total methane emission for all these emission sources is 223.8 t. Estimating methane emission of these sources in China according to the number of customers and the amount of gas consumption, it is 44536 t and 59083 t, respectively, which accounts for 0.08% and 0.1% of methane emission in China.