Modeling hydrogen-natural gas blends in transmission pipelines: insights from the Portuguese network

• H2 blending assessed in high and medium-pressure natural gas transmission pipelines • Low H2 concentrations lead to persistent stratification in high-pressure pipelines • Medium-pressure natural gas pipelines enable faster, more uniform H2 mixing • Velocity ratio VH₂/VNG ≥ 12 prevents hydrogen stratification • 90° T-junction injection with d/D = 1/7–1/8 delivers optimal hydrogen mixing As global efforts to mitigate climate change accelerate, hydrogen has gained attention as a low-carbon energy carrier. Blending hydrogen with natural gas in existing pipelines has been proposed as a transitional strategy to reduce emissions while avoiding the economic and technical challenges of large-scale infrastructure replacement. Hydrogen blending is limited by the risk of hydrogen embrittlement, which can be intensified by density-driven stratification, leading to locally elevated hydrogen concentrations and an increased likelihood of pipeline failure. These effects are strongly influenced by hydrogen concentration, flow conditions, and injection configuration, governing jet penetration and mixing behavior. Although widely studied, most investigations are conducted at specific gas network pressures, limiting the transferability of their findings. Consequently, the influence of pipeline pressure on hydrogen mixing performance using practical injection configurations remains insufficiently quantified. To address this, computational fluid dynamics (CFD) simulations were used to evaluate hydrogen–natural gas blending in medium- and high-pressure pipelines via T-junction injection without static mixers, using Portuguese gas infrastructure as a case study. A parametric analysis assessed the effects of hydrogen concentration, natural gas velocity, injection angle, and injector diameter on blending efficiency, quantified using the coefficient of variation (CoV). Results indicate that medium-pressure injection achieves faster and more uniform mixing than high-pressure injection, with CoV values below 5% over shorter downstream distances. Blending improves when the hydrogen-to-natural-gas velocity ratio exceeds 12 and when larger injector diameters are used. Perpendicular injection (90°) consistently provides the most uniform mixtures, indicating favorable conditions for safer hydrogen blending in existing networks.