[Objective] As a green energy resource, hydrogen-doped compressed natural gas (HCNG) is progressively gaining attention. A potential large-scale method of hydrogen transportation is mixing hydrogen with natural gas using the natural gas supply pipeline. However, owing to the difference in the properties of natural gas and hydrogen mixture, leakage and explosion accidents possibly occur during the transportation and utilization of HCNG. To ensure the safe promotion and utilization of HCNG, it is necessary to mix inert gas suppressants into HCNG. Therefore, this study aims to investigate the explosion characteristics of HCNG in a 0.5 m×0.5 m×0.5 m square explosion vessel. The suppressing explosion experiments are conducted using the explosion suppressants carbon dioxide and heptafluoropropane.[Methods] First, a study on the explosion characteristics of HCNG with a hydrogen volume fraction in the range of 0 % to 50 % is conducted. Subsequently, the HCNG with the 50 % volume fraction hydrogen is selected to investigate the explosion suppression based on the suppressants, carbon dioxide and heptafluoropropane. The profiles of critical explosion parameters such as maximum explosion overpressure, maximum overpressure delay time, and flame propagation velocity are analyzed under different volume fractions of suppressants.[Results] Results show that the maximum explosion overpressure and flame propagation velocity increase with increasing hydrogen doping ratio. With increasing volume fraction of suppression gas, the maximum explosion overpressure rapidly decreases, and the delay time to reach the maximum overpressure is prolonged. The high-speed image acquisition results indicate significant stretching of the flame surface. The spherical flame propagation velocity is considerably decreased by the suppressant. Subsequently, the typical volume fraction interval of the two suppressants is selected for comparing their explosion suppression effects. The explosion suppression effects of carbon dioxide and heptafluoropropane on the HCNG are quantified.[Conclusions] The suppression effect of heptafluoropropane is found to be generally comparable with that of carbon dioxide at twice the volume fraction. Complete explosion suppression is attained using 10 % volume fraction heptafluoropropane or 20 % volume fraction carbon dioxide, and the maximum explosion overpressure is decreased by 98.0 % and 94.4 %, respectively. The reasons for the suppression effect of heptafluoropropane being considerably superior to that of carbon dioxide are discussed based on the physical and chemical differences between the two suppressants. In particular, heptafluoropropane demonstrates better abilities of oxygen isolation, combination of explosive reactive radicals, and absorption of reaction heat than those of carbon dioxide. The current results can serve as a guide for improving the transportation safety of HCNG.
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