Flame spreading mechanisms over the methane hydrate surface

This study investigates the flame spreading mechanisms on the surface of methane hydrate using numerical simulations with the Fire Dynamics Simulator and Smokeview (FDS-SMV). Methane hydrate, a promising alternative for natural gas transportation, can pose a fire hazard during marine transport. Experimental studies have revealed two distinct flame spreading behaviors depending on surface temperature - low-speed and high-speed spreading - but the mechanisms underlying these differences remain unclear. To address this, simulations were conducted over a temperature range from 193 K to 213 K, encompassing the hydrate dissociation and self-preservation thresholds. The simulation incorporates two key reactions: methane hydrate dissociation and methane-air combustion. Results show that when the surface temperature exceeds the dissociation threshold, a thin uniform methane-air premixed layer forms prior to ignition, enabling rapid flame propagation at approximately 500 mm/s. Conversely, at lower temperatures, the non-premixed type is formed, and the flame spreading is governed by localized dissociation driven by heat conduction from the leading flame edge, resulting in a significantly reduced flame speed and extended spreading time. Temperature distributions within the hydrate were analyzed and compared with experimental data, revealing consistent trends and confirming the influence of surface temperature on flame behavior. The study concludes that the transition between premixed type and non-premixed type flame spreading modes is governed by whether pre-dissociation occurs, offering valuable insights into fire safety assessments for methane hydrate transport and storage.