Effect analysis on flame characteristics in the combustion of methane hydrate spheres under natural convective flow conditions

Abstract

Natural gas (methane) hydrates have great potential as fuels. It is necessary to study the flame characteristics to evaluate its energy efficiency when directly burned as a fuel and the risk of storing and transporting natural gas as the form of hydrate. Here, the flame characteristics in the combustion of methane hydrate spheres under natural convective flow conditions were investigated. Physical parameters related to the flame, such as height, brightness, oscillation frequency, and temperature were used to characterize the combustion process. The results showed that the combustion process was divided into four stages according to the changes in flame characteristics. The flame height was mainly affected by the mass change rate and the diameter of methane hydrate sphere. Further, the combustion of methane hydrate spheres was unstable, as reflected in the oscillation of the flame height with a period of ~0.1 s. A ‘dark region’ was found near the methane hydrate sphere surface, characterized by a dark flame and low temperatures. Increasing the initial center temperature had little effect on the flame characteristics, causing only a slight increase in flame height. The diameter of the hydrate spheres, on the other hand, had a significant effect on the combustion behavior. As the sphere diameter increased, the flame height increased dramatically, the oscillation frequency decreased, and the maximum flame temperature remained unchanged.