Assessing the option for blending hydrogen directly into a transmission pipeline: blending hydrogen into an existing gas network allows relatively direct access to a market for hydrogen and targets CO2 emissions via existing connections. Blending has been field tested with medium-pressure small-diameter public networks at relatively low volume flow rates; however, scaling up from medium-pressure distribution diameters to transmission diameters and high pressures would require significant investment with sizeable equipment and disruptive intervention.

Assessing the implications of various flow conditions and mixing solutions: selecting the best by recognising strengths and weaknesses from a practical implementation, manufacturing, and operational perspective. Solutions include using the pipeline as a mixer or mixing in pipework bypassing an inline-block valve, with and without compression. The results are backed up by extensive analytical modelling using Computation Fluid Dynamics (CFD) using high-performance computers and open-source codes. The assessment criteria include % hydrogen contacting the inside surface of the transmission pipeline and a coefficient of variance for mixing n diameters downstream of the mixing point.

Considerable economies can be obtained by injecting the hydrogen directly into the pipeline and using the pipeline itself as a mixer component. Transmission pipelines are predominantly made of steel and can operate at high stresses. Materials subject to these operating conditions can be sensitive to hydrogen. To mitigate against this, there is a requirement to limit steel exposure to hydrogen/natural gas mixtures above predefined concentrations.

The challenges with using the pipeline as a mixer component are achieving adequate mixing within a reasonable number of diameters downstream from the injection point, not exposing the inside surface of the pipeline to unacceptable percentages of hydrogen, and designing a nozzle, or nozzles that are practical from a manufacturing perspective, allow inline inspection and don’t introduce a structural weakness, or have onerous construction or operational implications.