Depressurization Simulation of Methane-hydrogen Mixtures in Pipelines

The reduction of carbon dioxide generated using natural gas for electric energy production or domestic usage is the main factor suggesting the addition of hydrogen. An increase in the use of hydrogen mixed with methane is one possible technology for mitigating greenhouse effects. Besides mechanical problems related to hydrogen embrittlement generated by hydrogen absorption, the presence of hydrogen changes the thermodynamic and transport properties of the natural gas. Different equations of state for an optimum simulation are tested for both steady-state and dynamic calculations. Depressurization of a pipeline can be performed as a scheduled or emergency operation. Hydrogen mixtures with natural gas are considered at ambient temperature and high pressures ranging from 150 bar to 250 bar. The reliable simulation of relief conditions (flow rate, pressure, and temperature) is a preliminary fundamental step for evaluating gas dispersion effects. This study analyzes the depressurization of a large-diameter pipeline, designed for natural gas transportation, now transporting methane-hydrogen mixtures across a distance of 20 km. Mixtures containing up to 10 % of hydrogen in moles are considered. The event of a leak at 15 km from the inlet point is considered. Differences between pure methane and its mixtures with hydrogen are evaluated and discussed. The methods presented allow the calculation of the discharged gas flow rate and its duration as fundamental data required for the evaluation of dispersion in the ambient and HSE evaluation.