Experimental study on combustion of a methane hydrate sphere

Abstract

The combustion behavior of a methane hydrate sphere under normal gravity is experimentally investigated. The initial diameter of the sphere is 20 mm. Variation in temperature at the center of the sphere (T _c) is measured with a K-type thermocouple at ignition temperatures (T _c,i) from 193 to 253 K at 20 K intervals. Variation in the near-surface temperature of the sphere (T _s) is measured at ignition temperatures (T _s,i) from 233 to 263 K at 10 K intervals. Two combustion phases are observed. When the hydrate is ignited, a stable flame envelope is formed around the sphere (phase 1). In phase 1, the surface of the sphere is dry. After a few seconds, water formed by dissociation of the methane hydrate appears on the surface and methane bubbles are formed by methane ejected from inside the sphere (phase 2), thus destabilizing the flame and causing local extinction. Methane bubbles move down along the surface and merge into a large methane bubble at the bottom of the sphere. This bubble bursts, releasing methane to form a temporary flame, and the water drops from the hydrate sphere. Water on the surface is cooled by the hydrate inside, and an ice shell confines the methane gas that dissociated inside the sphere. Because the dissociation occurs continuously inside the hydrate, the inner pressure gradually increases and at some instant, the ice cracks and methane gas is ejected from the cracks, which results in a micro-explosion with a flame. In phase 1, the surface temperature is below the freezing point of water, and so the surface remains dry and a stable flame envelope is formed; in phase 2, the surface temperature is above the freezing point, and so water appears on the surface. When the temperature at the center of the sphere is lower (193, 213, or 233 K), some methane hydrate remains even after flame extinction because heat transfer from the flame decreases in phase 2 as a result of local extinction. The diameter of the sphere decreases during combustion in accordance with the d-square law, which indicates that the heat of dissociation is supplied by the flame and methane is supplied by the dissociation of methane hydrate in the sphere, as in single-droplet combustion.