  We develop an explicit second order staggered finite difference
discretization scheme for simulating the transport of highly heterogeneous gas
mixtures through pipeline networks. This study is motivated by the proposed
blending of hydrogen into natural gas pipelines to reduce end use carbon
emissions while using existing pipeline systems throughout their planned
lifetimes. Our computational method accommodates an arbitrary number of
constituent gases with very different physical properties that may be injected
into a network with significant spatiotemporal variation. In this setting, the
gas flow physics are highly location- and time- dependent, so that local
composition and nodal mixing must be accounted for. The resulting conservation
laws are formulated in terms of pressure, partial densities and flows, and
volumetric and mass fractions of the constituents. We include non-ideal
equations of state that employ linear approximations of gas compressibility
factors, so that the pressure dynamics propagate locally according to a
variable wave speed that depends on mixture composition and density. We derive
compatibility relationships for network edge domain boundary values that are
significantly more complex than in the case of a homogeneous gas. The
simulation method is evaluated on initial boundary value problems for a single
pipe and a small network, is cross-validated with a lumped element simulation,
and used to demonstrate a local monitoring and control policy for maintaining
allowable concentration levels.
