From Underground Leakage to Pre-Ignition Flammable Cloud Formation in Buried Hydrogen-Blended Natural Gas Pipelines: A Review and Perspective on Urban Safety

Hydrogen-blended natural gas (HBNG) is widely regarded as a transitional pathway for decarbonizing urban gas systems. However, the coupled evolution from buried pipeline leakage to pre-ignition flammable cloud formation has not yet been systematically integrated across research stages. This review synthesizes experimental, numerical, and data-driven studies on leak source-term dynamics, subsurface migration through porous media, surface breakthrough and escape, accumulation in semi-enclosed spaces, and pre-ignition flammable cloud development. Hydrogen blending modifies the density, diffusivity, flammability limits, and ignition sensitivity of the gas mixture, thereby influencing breakthrough time, stratification behavior, and the available early-warning window before ignition. The hazard evolution is jointly governed by pipeline pressure, leak orifice size, burial depth, soil heterogeneity, soil moisture content, spatial confinement, and ventilation conditions. Six major research gaps are identified, including fragmented stage-specific investigations, limited full-scale multiphysics experimental data, insufficient characterization of heterogeneous soils, inadequate high-resolution gas-cloud measurements, weak integration with quantitative risk assessment, and delayed full-lifecycle integrity management. To address these gaps, this review proposes a coherent, mechanism-informed analytical framework for urban HBNG pipeline safety and further provides a numerical parameter-transfer example showing how surface breakthrough outputs can be converted into aboveground velocity, mass flux, and species-concentration boundary conditions. This framework integrates dynamic mechanistic parameters into high-consequence area zoning, sensor placement, ventilation interlocking, and full-lifecycle integrity management, thereby supporting safer engineering deployment of HBNG systems.