Concentration distribution pattern of hydrogen-blended natural gas under static operating conditions

The use of in-service natural gas pipelines for hydrogen blending transportation has become a research hotspot and focus in the field of hydrogen energy and natural gas transportation. So far, however, there has always been controversy regarding whether hydrogen-blended natural gas (HBNG) can spontaneously stratify during pipeline transportation and shutdown static processes. In order to clarify this issue, this paper takes the extreme operating condition of HBNG being stationary in pipelines as an example to derive a mathematical model of HBNG concentration distribution under the action of gravity field based on the principle of thermodynamic minimum energy. Then, the mathematical model is solved using the Peng-Robinson real gas equation of state. Finally, the distribution patterns of hydrogen concentration in methane-hydrogen binary gas mixtures at four typical scenario heights and five hydrogen blending ratios are studied. And the following research results are obtained. First, under the influence of gravity, hydrogen concentration difference occurs between the top and bottom of the pipeline, but it is significantly affected by height difference. For horizontal pipelines at the meter level, natural gas risers in general urban buildings at dozens of meters level, and natural gas risers in high-rise buildings at the hundred-meter level, the variation of hydrogen concentration with height is slight and the hydrogen stratification can be completely ignored. Second, for HBNG pipelines with kilometer level drop, hydrogen stratification can still be ignored when the hydrogen blending ratio does not exceed 20%. However, when the hydrogen blending ratio is very high (such as 50%), the hydrogen concentration difference between the top and bottom of the pipeline exceeds 1%. In this case, it is necessary to evaluate whether hydrogen stratification can be ignored based on the actual engineering situations. Third, the concentration distribution pattern of each gas component with the drop height in a certain pipeline of the West-to-East Gas Transmission Project considering true gas composition, hydrogen blending ratio of 20% and pipeline drop of 200 m reveals that the hydrogen stratification in HBNG with such drop height can be ignored in practical engineering. In conclusion, for evenly mixed HBNG, it is most likely to stratify under extreme static conditions, so if there is no extreme kilometer drop and extreme high hydrogen blending ratio, stratification of HBNG pipelines can be completely ignored. This research result theoretically clarifies the current controversy regarding the stratification phenomenon in HBNG, which is of important guiding significance for the safe transportation of HBNG pipelines. © 2024 Natural Gas Industry Journal Agency. All rights reserved.