Study on multicomponent leakage and diffusion characteristics of hydrogen-blended natural gas in utility tunnels

With the rapid development of utility tunnels and hydrogen energy, hydrogen-blended natural gas (HBNG) enters gas compartments just around the corner. However, all safety requirements for gas compartments in current standards are designed for natural gas, and the applicability to HBNG is unclear. In addition, most of the existing diffusion models assume that the CH4/H2/Air mixture is single-component homogeneous gas, which cannot accurately describe the diffusion behavior differences between CH4 and H2. In this paper, an improved diffusion coefficient model for CH4/H2/Air non-ideal multi-component mixtures is proposed, which considers the interaction between CH4 and H2 components. After that, a 200 m long and narrow three-dimensional utility tunnel model is established to study the HBNG concentration distribution, alarm response time, and explosion hazardous area under different ventilation conditions. The effects of hydrogen blending ratio (HBR), leakage diameter, pipeline operating pressure, leakage location, and ventilation frequency on leakage and diffusion characteristics of utility tunnels are analyzed. The results show that under natural ventilation, the HBNG diffuses symmetrically to both sides with the leak hole as the center and covers the whole compartment after 1500 s. Normal mechanical ventilation reduces the distribution range and HBNG concentration, and the leaked gas covers the compartment downstream within 250 s. With the increase of HBR, the CH4/H2 mixture concentration increase at each monitoring point downstream of the leakage location, which causes the advance of the alarm response time. When the leakage reaches a steady state, the CH4/H2 mixture concentration for HBR of 5%, 10%, 15%, and 20% is 2.15%, 4.14%, 7.76%, and 10.97% higher than that of natural gas. The leakage location affects the hazardous area range, but little affects the CH4/H2 mixture concentration at the final steady state. The larger leakage diameter and the higher pipeline operating pressure can accelerate the HBNG diffusion rate in utility tunnels. When the pipeline operating pressure increases to 1.6 MPa, the current minimum accident ventilation frequency of 12 times/h is insufficient to reduce the CH4/H2 mixture concentration below the lower explosion limit (LEL). Increasing the ventilation frequency can effectively reduce the leaked gas concentration in utility tunnels, so it is recommended to increase the minimum accident ventilation frequency from 12 times/h to 15 times/h. The research results can guide the design and safety management of HBNG pipelines in utility tunnels.