The transition of industrial economies from reliance on fossil fuels to primary usage of renewable sources for energy has compelled the development of new methods for production, transport, and utilization of hydrogen gas. Potential uses include electricity storage and generation, transportation, and residential end use. However, natural gas still accounts for a large proportion of primary energy consumption for heating and electricity in many parts of the world. Interest has grown recently in blending hydrogen produced using renewable energy into existing natural gas pipelines and distribution grids, so that the significant capital investments made in these systems can continue to generate revenue while supporting the energy transition. In the case that technological and materials issues related to compressors and pipeline integrity are addressed, the physical and chemical differences between hydrogen and natural gas will still significantly affect pipeline flow transients, energy capacity, and economics. With hydrogen blending, the much lighter yet more energy-dense gas may be injected in time-varying quantities at multiple locations throughout a pipeline system that delivers flows to consumers who also have highly time-varying consumption. Gas composition must be predicted and monitored in order to accurately model flow physics, to account for the form in which energy is actually delivered to consumers, and to quantify the effects on the efficiency with which a pipeline delivers energy. In this paper, we review the results of recent studies to extend steady-state and transient pipeline flow modeling, simulation, optimization, and control methods developed for homogeneous processed natural gas to the setting of heterogeneous gases.

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