Non-stationary combustion of natural and artificial methane hydrate at heterogeneous dissociation

Abstract

Dissociation of natural and artificial methane hydrate at combustion was studied experimentally. Thermal imaging and Particle Tracking Velocimetry (PTV) methods were used to analyze the temperature field and gas velocity. The laminar air flow rate varied from 0 to 1.8 m/s. Previously, when modeling the combustion of gas hydrate, a simplified model for a standard laminar velocity profile had been considered. Expectedly, in the combustion region (in the vicinity of the wall) the gas flow velocity was much lower than that at the outer surface of the boundary layer, and calculations showed the presence of the stoichiometric ratio line. The presence of this line in the area of combustion was thought to result in high flame spread speed. The use of the Particle Tracking Velocimetry method and the experimental values of the methane injection rate have led to an unexpected result. High air velocities (maximum of 3–5 m/s) due to free convection have been found in the combustion region. At that in the combustion region, there is a significant excess of oxidant (oxygen) and the mole fraction of CH4 is 10–30 times lower than necessary for the maximum reaction rate. It is shown that high combustion rates (high flame spread speed) may be associated with extremely inhomogeneous dissociation inside the powder and inhomogeneous rate of methane injection over the layer surface. At combustion, “mushroom-like" pulsating flame and methane “bubbles” are formed. The frequency of pulsations increases with the growth of external flow rate. The influence of the initial concentration of methane in natural hydrate on its combustion has been considered. The studies prove that the description of the combustion process must take into account the free convection and heterogeneity of dissociation of methane hydrate in the powder volume.