D1C.3 The influence of the existing natural gas distribution networks on the purity of hydrogen
Description
As part of the national HyDelta research programme, a study was conducted into the influence that the use of the existing natural gas distribution grid could have on hydrogen quality. For this purpose, the contaminants that occur when using natural gas distribution grid have been mapped out and measured. The focus of this study is on the release of the natural gas components from the pipe materials and the contaminants from the environment that reduce the purity of hydrogen gas.
The hydrogen gas picks up absorbed natural gas components, such as THT, aromatics and alkanes, from the pipe material.
Desorption of THT and other natural gas components decreases as the amount of hydrogen flowing through the gas grid increases. The quantities of these substances that can be absorbed by the hydrogen per time unit depend on the equilibrium concentration and the time that the gas is stagnant and therefore also on the extent of flushing. The desorption rate also depends on the amount of THT, alkanes or aromatics in the pipe material and attached to the wall, which clearly decreases over time. Based on the results obtained, desorption rates were determined for the various pipe materials. These desorption ratesare used to calculate desorption coefficients for each of the pipe materials investigated, which could then be used to calculate the concentrations of THT, aromatics and alkanes that could be expected after a certain period of time.
After approximately 50 days and approximately 50 flushings with the inner pipe volume, the THT content was below the minimum THT content for odourisation (10 mg/m3) in most cases.
The permissible concentrations of natural gas components in the hydrogen gas depend on the application.
Contamination from the environment reduces the purity of hydrogen gas in plastic pipes. This is caused by an unavoidable natural process called permeation. Permeation does (practically) not occur in steel pipes. Permeation occurs regardless of the type of gas transported, meaning that even the natural gas of today is contaminated with components from the environment such as oxygen, nitrogen and water. The results of this study can be used to estimate the speed at which the hydrogen gas is contaminated. The basis for the prognosis for the pipe material PVC (rigid as well as impact-modified PVC) and PE (1st, 2nd and 3rd generation PE) has been established using the results shown in the table below. The estimate also depends on the pipe dimensions, the temperature, and the length of time the hydrogen gas remains in the pipe. For the study, a temperature of ~23°C was taken into account as the worst-case scenario. Ultimately, whether the degree of contamination is acceptable for the intended application will have to be assessed in consultation with the end user.
As an indication, an estimate has been provided for two practical circumstances with deliberately selected conditions that will result in a high relative oxygen increase as a result of permeation. This concerns two thin-walled pipes with a small diameter and a low hydrogen content (low gas pressure). It is assumed that the pipe will be completely disconnected from the distribution grid so that no diffusion of oxygen through the pipe grid can occur. For a DN 16 PE pipe with 20 mbar pure hydrogen gas, the oxygen content in the hydrogen will increase from 0% to 0.296% in the first 24 hours. For a DN 16 PVC pipeline with 20 mbar pure hydrogen gas, the oxygen content in the hydrogen will increase by 0.077% in the first 24 hours. For pipes with a larger diameter, larger wall thickness or higher hydrogen pressure, the relative oxygen increase will be (much) lower.
Table 1. Conservatively chosen permeation coefficient for the various materials.
Component |
PVC |
PE |
Unit |
Oxygen |
19.1 |
77.7 |
[(ml∙mm)/(m2∙day∙bar)] |
Nitrogen |
19.1 |
23.3 |
[(ml∙mm)/(m2∙day∙bar)] |
Water |
3.06 |
0.17 |
[(ml∙mm)/(m2∙day)] |
In this report, THT is referred to as a natural gas component because, although it is not naturally present in natural gas, it is inherently associated with natural gas in the distribution grid and is included as such in ISO 6976.
Notes
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D1C_3_HyDelta_Eerste_Tranche_effect_bestaand_aardgas_distributienet_op_kwaliteit_H2_EN.pdf
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