**6. Conclusions**

The main objective of this paper was to analyze the possibilities of pipeline transmission of hydrogen and methane/hydrogen mixtures. This analysis has been performed to determine the impact of hydrogen content on the conditions of natural gas transmission, the main component of which is methane. It should be emphasized that hydrogen will play an increasingly important role as an energy carrier in the global economy, particularly for energy storage. The economic environment for the use of hydrogen should be favorable in the coming years. A steady increase of renewable energy content in the total energy balance in all regions of the world, and the growing irregularities in power generation and usage, will have an important role in the field of hydrogen utilization. Hydrogen pipeline transmission is the most effective method for transporting significant amounts of hydrogen over long distances, in particular for its storage, when the appropriate storage site for hydrogen is located at a considerable distance from the source of its generation due to geological conditions (suitable locations for salt cavern). In addition, technological and technical issues related to pipeline transmission of natural gas with an increased hydrogen content should be considered, where a significant change in thermodynamic parameters may also affect the operational conditions of installations associated with the transmission system. An additional problem is the impact of the increased molar fraction of hydrogen on the pipe material.

**Author Contributions:** Conceptualization, T.W. and M.Ł.; formal analysis, T.W.; funding acquisition, M.Ł. and A.S.; investigation, T.W. and S.K.; methodology, T.W.; supervision, M.Ł. and A.S.; validation, T.W. and A.O.; visualization, T.W.; writing—original draft, T.W.; writing—review and editing, S.K. and T.W.

**Funding:** This work received funding from the Statutory Research of Natural Gas Department at Drilling Oil & Gas Faculty, no. 11.11.190.555.

**Conflicts of Interest:** The authors declare no conflict of interest.
