Study on the permeability behaviour of hydrogen doped natural gas in polyethylene pipeline

The use of non-metallic PE pipes to transport hydrogen energy can avoid the problem of hydrogen embrittlement in metal pipes, but due to material characteristics, PE pipes exhibit gas leakage behavior. Therefore, this article conducted a study on the permeability of polyethylene (PE100) as a gas pipeline material for hydrogen-doped gas, mainly analyzing the effects of hydrogen content and gas pressure on the permeability of PE100 gas. We ensure that the raw materials are uniform and free from pore defects through microstructure observation and industrial CT characterization. The experimental results show that with the increase of hydrogen content in the hydrogen-doped gas, the gas permeability coefficient of the sample significantly increases. When the hydrogen content is 60%, the gas permeability coefficient increases by 83% compared to when no hydrogen is added. Further increase in hydrogen content results in no significant change in the gas permeability coefficient; As the gas pressure increases, the gas permeability coefficient of the s ample gradually decreases. This study will provide data support for the safe transportation of hydrogen energy and promote the development of the hydrogen energy industry.


Introduction
The depletion of fossil fuels and the gradual deterioration of the environment have forced people to accelerate the development of new energy.Hydrogen energy, as a clean and renewable energy source, has attracted much attention [1,2].At the same time, hydrogen energy technology has gradually developed, bringing in technologies, such as 'coal to hydrogen' and 'methanol to hydrogen'.Hydrogen has gradually become the main alternative energy source, entering industrial production and daily life.
At present, hydrogen production plants are scattered and far away from user terminals.Therefore, how to transport hydrogen energy to terminals safely and economically has become one of the key issues restricting the development of hydrogen energy.Transporting pure hydrogen or hydrogen-doped natural gas through existing pipelines is an economical and safe way: clean wind and water energy are used to produce hydrogen by water electrolysis, and then hydrogen is mixed with gas, transporting through existing gas pipeline networks [3,4].Through this approach, the problem of clean energy waste can be solved, and it is expected to activate the whole hydrogen energy industry chain.The steel used in the current gas pipeline network is usually low-strength steel, such as API 5L A, API 5L B, X42, and X46; The non-metallic polymer materials used include polyethylene (PE 63, PE 80, PE 100), polyvinyl chloride (PVC), and other polymer materials [1].Although the use of non-metallic pipes can avoid the problem of hydrogen embrittlement in metal pipes, non-metallic pipes still have serious problems.Gas permeation and leakage cause loss and waste of gas.Hydrogen-doped natural gas, as a flammable and explosive gas, can cause explosion accidents after leakage, endangering personal safety and causing property damage.Therefore, studying the permeation behavior of hydrogen-doped natural gas in nonmetallic pipelines is of great significance for ensuring the safety of hydrogen transportation.
At present, scholars have conducted relevant research on gas dissolution-diffusion theory [5,6], method for material gas permeability testing [7], pipeline permeability testing equipment [8], and numerical simulation for non-metallic pipe gas permeability [9], and have carried out projects such as the NaturalHy project (EU) and VG2 project (Netherlands).However, due to differences in actual usage conditions and pipe manufacturing levels of different countries, it is necessary to research the safety of hydrogen-doped natural gas pipeline transportation based on actual situations.Unlike hydrogen leakage caused by metal degradation, PE materials exhibit certain gas permeation phenomena due to their structural characteristics.Gases can pass through PE materials through processes such as "surface adsorption-dissolution-diffusion-desorption processes" [10].The permeability of PE materials to gases depends on factors such as gas composition, pressure, and the structural properties of PE materials.The NaturalHy project [11] of the European Union studied the gas permeability characteristics of highdensity polyethylene pipes in a hydrogen-doped natural gas environment.The results show that regardless of the hydrogen content in hydrogen-doped natural gas, the permeability coefficient of hydrogen is always about 5 times higher than that of methane.Based on experimental results, it can be calculated that during pipeline transportation, the leakage rate of hydrogen-doped natural gas with a hydrogen content of 20% will be about twice that of pure natural gas transportation; Klopffer et al. [12] pointed out that due to the smaller molecular volume of hydrogen compared to methane molecules, hydrogen has a greater diffusion coefficient in materials; The research results of Flaconneche et al. [13] indicate that gas particle size is directly related to its diffusion coefficient in the material; Fujiwara's research found that an increase in air pressure can promote the contraction of the free volume of the material, leading to a decrease in solubility and diffusion coefficients, thereby reducing the permeability coefficient of the gas [8].
At present, there is a lack of research results on the hydrogen permeation of PE materials in China, which has become a key constraint on the development of hydrogen-doped gas.To promote the use of hydrogen-doped gas, this article conducts a study on the gas permeability characteristics of typical nonmetallic gas pipeline PE 100 under the composition and pressure of hydrogen-doped gas through a pressure difference gas permeability testing system.During the application of hydrogen-doped natural gas, there may be issues of gas permeation and leakage, so research mainly focuses on the permeability coefficient of the gas.

Experimental Materials and Methods
This section will introduce the raw materials and methods used in the experiment.

Raw materials
The polyethylene pipe sample used in this study was taken from a PE 100 gas pipe, and the hydrogen permeation samples were taken from the pipe wall with a size of 50×50×5 mm, as shown in Figure 1a).In addition, after sampling and spraying gold, SEM observation was conducted, as shown in Figure 1b), and it was found that the PE100 material sample was dense and without pores.

Testing methods
The hydrogen permeation characteristics of polyethylene pipe fittings were tested using the pressure difference method.Three samples were tested in each experiment, and the average of the test results was taken.The basic process of testing is as follows: first, we extract air from the low-pressure chamber to reduce its pressure to below 10 Pa, and then inject experimental gas on one side of the high-pressure chamber for testing and recording, to detect the changes in air pressure between the two chambers.At the same time, during the experiment, the temperature of the experimental chamber is ensured to be constant through circulating water, and the hydrogen in the experimental environment is promptly removed to reduce the interference of environmental hydrogen concentration in the experiment.
The testing equipment used in the experiment is the G-TRANS gas transmittance tester.The equipment is equipped with a gas detection sensor.When gas diffuses and permeates from the highpressure to the low-pressure side, the gas that permeates to the low-pressure side is detected by the sensor placed in the lower chamber.The amount of penetration is proportional to the signal size of the sensor.By analyzing and calculating the electrical signal of the sensor, the gas permeability and other indicators of the sample are obtained [14].
The phase changes of raw materials and materials before and after the experiment are characterized by X-ray diffraction (XRD), and the microstructure of the materials is mainly observed through scanning electron microscope (SEM)and Computed tomography (CT).

Characterization of raw polyethylene samples
XRD was used to analyze the phase of polyethylene pipe samples, with a testing angle range of 2θ= 10~70°, with a scanning step of 0.04°/s.The experimental results are shown in Figure 2. The phase of polyethylene can be determined by the characteristic diffraction peaks, and the sharp diffraction peaks indicate that the polyethylene sample has a high crystallinity.
Using industrial CT for three-dimensional characterization of pipe fittings samples, it was found that the extruded polyethylene pipe fittings were uniform and dense, without holes, as shown in Figure 3.This characterization can effectively avoid the impact of bubble introduction on gas permeability testing during the production process.Figure 3. CT result of PE100 sample.

Effect of gas composition on hydrogen permeation characteristics
We mix different proportions of hydrogen-doped natural gas and use it as a gas source to test the gas permeability characteristics of the sample.The test temperature is selected as room temperature of 23℃, and the test pressure is 0.1 MPa, that is, the pressure difference between the high-pressure chamber and the low-pressure chamber is 0.1 MPa.
The gas permeability Q can be measured by Equation ( 1), and then the gas permeability coefficient P can be obtained by Equation (2).
Among them, ∆p/∆t is the arithmetic mean value of the pressure change of the low-pressure side gas per unit time when stable transmission occurs, in Pa/h; V is the volume of the low-pressure chamber, in cm 3 ; S is the test area of the sample, in square meters; T is the test temperature, in K; p 1 -p 2 is the pressure difference on both sides of the sample, in Pa; T 0 and p 0 are the temperature (273.15K) and pressure (1.0133× 10 5 Pa) under standard conditions, respectively.
Among them, d is the thickness of the sample, in cm.The gas permeameter can calculate the diffusion coefficient D using the "high vacuum method", in cm 2 /s.The calculation formula is as shown in Equation ( 3): Among them, L is the thickness of the sample, in cm; θ 0 is the lag time, in seconds.In testing software programs, the relationship between pressure and time can be calculated [15].The gas permeability coefficient P and diffusion coefficient D have the following relationship with the solubility coefficient S as shown in Equation (4): Therefore, after obtaining the data of P and D, the solubility coefficient of the gas in plastic materials can be calculated in cm 3 /(cm 2 •s•cmHg).
The test results are shown in Table 1 The gas permeability coefficient is related to material type, environmental conditions, gas size, polarity, and compressibility [16].
From the data in Table 1, it can be seen that as the proportion of hydrogen in the mixed gas increases, the permeability P of the mixed gas gradually increases, and the diffusion coefficient D shows an increasing trend with the increase of hydrogen proportion, while the solubility coefficient S does not change significantly.We compare the permeability coefficients of each group of mixed gases with the permeability of methane gas, as shown in Figure 4. From Figure 4, it can be seen that when the hydrogen content in hydrogen-doped gas is less than 60%, the gas permeability coefficient significantly increases with the increase of the hydrogen ratio; When the hydrogen ratio is 60%, the permeability of the mixed gas increases by about 83%; As the hydrogen doping ratio further increases, the change in permeability coefficient fluctuates, but the change is not significant compared to the hydrogen doping ratio of 60%.Table 1.Gas permeation test results of PE.The ratio of gas permeability with different hydrogen doping ratios to pure methane gas permeability was calculated.Under the same environmental conditions, the permeability coefficient of hydrogen is greater than that of methane, so the permeation or leakage rate of hydrogen in pipelines will be higher than that of natural gas.In hydrogen-doped gas, methane and hydrogen coexist, and the interactions between different components of gas and polymers are different, which may result in varying degrees of gas permeation due to different dissolution and diffusion modes.Due to the smaller molecular volume of hydrogen compared to methane molecules, hydrogen has a greater diffusion coefficient in materials.Gas particle size is directly related to its diffusion coefficient in the material: for small molecule gases, their permeability coefficient is mainly determined by the diffusion coefficient; When the particle size and diffusion coefficient of gas molecules are similar, their permeability coefficient is determined by the dissolution coefficient of gas molecules in the material.

Characterization of raw polyethylene samples
Using a mixture of 20% hydrogen gas as the gas source, the gas permeation characteristics of PE100 materials under different pressures were studied.Due to the general air pressure of the distribution gas pipeline being around 0.5 MPa, gas permeability evaluations were conducted at 0.1, 0.2, 0. Figure 5 shows the gas permeability test results of polyethylene under different gas pressures.From the experimental results, it can be seen that as the pressure of the mixed gas increases, the gas permeability coefficient of the material gradually decreases.At the same time, it can be observed that under the experimental pressure, the gas diffusion coefficient of the material slightly decreases, but the change is not significant; The solubility coefficient of the material also shows a decreasing trend with increasing air pressure.By comparison, the influence of air pressure on solubility is greater than that of the gas diffusion coefficient.Pressure can affect the gas permeability characteristics of polymers, and external pressure may compress the free volume in the material, thereby reducing gas diffusion inside the material.It also affects the solubility of gas molecules in the material [17,18].Performing XRD analysis on the sample after the experiment, and the results are shown in Figure 6.
From the results, it can be seen that there is no significant difference in the phase and microstructure of the samples tested under different pressure conditions.

Discussion on Gas Permeation of PE samples
Polyethylene pipes are prepared by extrusion, during which the material undergoes two significant changes: 1) the extrusion flow reduces the free volume inside the polyethylene material; 2) The extrusion flow during the extrusion process is accompanied by shear flow induced alignment of polyethylene molecular chains.According to literature reports [19], the gas leakage process occurs almost entirely through the transition of free volume in the amorphous region, therefore, a decrease in free volume will hinder gas permeation, as shown in Figure 7.In addition, during the extrusion process, the orientation arrangement of molecular chains will also induce the orientation arrangement of the crystallization zone, which will form an obstruction effect on the gas, similar to the "isolation wall", increasing the path of gas diffusion.At the same time, the orientation of molecular chains inevitably leads to a tighter chain arrangement, thereby affecting the free volume of the composite material.
Since hydrogen molecules are smaller than methane molecules, the diffusion coefficient of hydrogen in the material is relatively high, as the main influencing factor of the permeability coefficient, the permeability coefficient of hydrogen is greater than that of methane.When the hydrogen content in the gas increases, the permeability coefficient of the mixed gas significantly increases.However, the free volume inside the material is limited.When hydrogen gas increases to a certain extent (in this study, it is 60%), the permeability coefficient no longer increases with the increase of hydrogen gas ratio; When the pressure increases, the aggregation and collision of gas molecules increase, which will affect the diffusion rate of gas.Therefore, different results were obtained in the study.After calculation, the permeability coefficient did not increase with the increase of pressure but showed the opposite pattern.

Conclusions
By conducting gas permeation research on the gas pipeline material PE100 using hydrogen-doped gas, the following conclusions have been drawn: As the hydrogen content in the mixed gas increases, the gas permeability coefficient of polyethylene material significantly increases.When the hydrogen content reaches 60%, the gas permeability coefficient increases by 83%; Further increase in hydrogen content results in less significant changes in the gas permeability coefficient.As the gas pressure increases, the solubility coefficient and diffusion coefficient of polyethylene materials decrease, but the overall change is not significant.Both lead to a decrease in the gas permeability coefficient of the material.The mixed gas and lower gas pressure have no significant effect on the phase and microstructure of the material.Based on this study, pipeline wall thickness can be designed based on the composition of the transported gas.It can economically reduce gas leakage and ensure safe use.

Figure 1 .
Figure 1.Appearance and SEM morphology of PE sample: (a) Appearance and (b) SEM morphology.

Figure 4 .
Figure 4. Permeability coefficient at different hydrogen contents.The ratio of gas permeability with different hydrogen doping ratios to pure methane gas permeability was calculated.Under the same environmental conditions, the permeability coefficient of hydrogen is greater than that of methane, so the permeation or leakage rate of hydrogen in pipelines will be higher than that of natural gas.In hydrogen-doped gas, methane and hydrogen coexist, and the interactions between different components of gas and polymers are different, which may result in varying degrees of gas permeation due to different dissolution and diffusion modes.Due to the smaller molecular volume of hydrogen compared to methane molecules, hydrogen has a greater diffusion coefficient in materials.Gas particle size is directly related to its diffusion coefficient in the material: for small molecule gases, their permeability coefficient is mainly determined by the diffusion coefficient; When the particle size and diffusion coefficient of gas molecules are similar, their permeability coefficient is determined by the dissolution coefficient of gas molecules in the material.

Figure 5 .
Figure 5. Gas permeation test results of PE100 under different pressures.

Figure 6 .
Figure 6.XRD results of PE samples under different pressures.

Table 2 .
MPa in the experiment.The gas permeation results of PE100 material under different air pressures are shown in Table2.Gas permeation test results of PE100 under different pressures.