Combustion Of Methane Hydrate Research Articles

Co-combustion of methane hydrate granules and liquid biofuel

The use of fossil hydrocarbons is accompanied by such problems as the depletion of energy resources and high levels of anthropogenic emissions. One of the options for solving these problems can be the involvement in the fuel sector of composite mixed fuels using gas hydrates and vegetable liquid bio-fuels. In this research we test a hypothesis that gas hydrate with added rapeseed oil will make an energy-efficient and environmentally friendly composite fuel. According to the experimental findings, the co-combustion of gas hydrates and liquid biofuels shows high potential. It shortens the ignition delay by 1.5 times at 700 °C in the combustion chamber and by half at 800 °C compared with the combustion of methane hydrate alone. It also produces 18–32% less carbon monoxide, 12–22% less nitrogen oxides, and 14–33% less sulfur oxides than the direct combustion of rapeseed oil. On the basis of the results obtained, we have developed a predictive mathematical model simulating the heat transfer in a layer of composite fuel. A methane hydrate and rapeseed oil mixing scheme is proposed for combustion chambers. We also provide recommendations on how to use the research findings in a number of energy-related applications.

Dissociation and Combustion of a Layer of Methane Hydrate Powder: Ways to Increase the Efficiency of Combustion and Degassing

The interest in natural gas hydrates is due both to huge natural reserves and to the strengthened role of environmentally friendly energy sources conditioned by the deterioration of the global environmental situation. The combustion efficiency increase is associated with the development of understanding of both the processes of dissociation and combustion of gas hydrates. To date, the problems of dissociation and combustion have, as a rule, been considered separately, despite their close interrelation. Usually, during combustion, there is a predetermined methane flow from the powder surface. In the present paper, the combustion of methane hydrate is simulated taking into account the non-stationary dissociation process in the powder layer. Experimental studies on the methane hydrate dissociation at negative temperatures have been carried out. It is shown that due to the increase in the layer temperature and changes in the porosity of the layer over time, i.e., coalescence of particles, the thermal conductivity of the layer can change significantly, which affects the heat flux and the dissociation rate. The flame front velocity was measured at different external air velocities. The air velocity and the vapor concentration in the combustion zone are shown to strongly affect the combustion temperature, flame stability and the flame front velocity. The obtained results may be applied to increase the efficiency of burning of a layer of methane hydrate powder, as well as for technologies of degassing the combustible gases and their application in the energy sector.

An Experimental Study of Combustion of a Methane Hydrate Layer Using Thermal Imaging and Particle Tracking Velocimetry Methods

In this paper, the combustion of methane hydrate over a powder layer is experimentally studied using thermal imaging and Particle Tracking Velocimetry (PTV) methods. The experiments are carried out at different velocities of the external laminar air-flow from zero to 0.6 m/s. Usually, simulation of methane hydrate combustion is carried out without taking into account free convection. A standard laminar boundary layer is often considered for simplification, and the temperature measurements are carried out only on the axis of the powder tank. Measurements of the powder temperature field have shown that there is a highly uneven temperature field on the layer surface, and inside the layer the transverse temperature profiles are nonlinear. The maximum temperature always corresponds to the powder near the side-walls, which is more than 10 °C higher than the average volumetric temperature in the layer. Thermal imager measurements have shown the inhomogeneous nature of combustion over the powder surface and the highly variable velocity of methane above the surface layer. The novelty of the research follows from the measurement of the velocity field using the PTV method and the measurement of methane velocity, which show that the nature of velocity at combustion is determined by the gas buoyancy rather than by the forced convection. The maximum gas velocity in the combustion region exceeds 3 m/s, and the excess of the oxidizer over the fuel leads to more than tenfold violation of the stoichiometric ratio. Despite that, the velocity profile in the combustion region is formed mainly due to free convection, it is also necessary to take into account the external flow of the forced gas U0. Even at low velocities U0, the velocity direction lines significantly deviate under the forced air-flow.

Methane hydrate combustion by using different granules composition

Dissociations of methane gas hydrate during gas combustion were investigated. The anomalous behavior of a sample was revealed after termination of combustion and long stay of powder at the ice melting temperature (“secondary self-preservation of methane”). Increasing storage temperature and transport of natural gas hydrate are the important scientific and technical problems. The effect of combustion on decay of gas methane clathrate is considered using the important key parameters: powder layer height, external heat flux, granules composition. Existing methods of combustion modeling use a simplified one-dimensional case of kinetic combustion without taking into account the granule size and thickness of the hydrate layer. In the present research, the boundary dissociation conditions at non-stationary combustion were determined from the experiment data. The heat flux before combustion was constant and it was controlled by the ambient air temperature. During combustion, the heat flux increased by an order, and its value was determined experimentally. Currently, when simulating non-stationary combustion of СH4 gas hydrate, constant flows of combustible gasses were used. In reality, magnitudes of mass gas flows vary by several orders in time. As a result, CH4 combustion occurs under the non-stationary and non-isothermal regime in violation of stoichiometric ratio. The proposed model allowed us to connect the non-stationary gas flows with decomposition kinetics of gas clathrate. Thus, chemical reaction kinetics is controlled by means of the internal transfer processes. The existing predictions for describing the decomposition at negative temperatures did not take into account the structural parameters of particle surface as well. As a result, these simplified models incorrectly described the decomposition process. It is shown that non-uniform distribution of particle sizes and thick powder layer will result in unsteady dissociation and hydrate burning and this will lead to a decrease in combustion efficiency.

523 メタンハイドレートの燃焼に与えるメタン濃度の影響(GS5-1燃焼)

523 メタンハイドレートの燃焼に与えるメタン濃度の影響(GS5-1燃焼)

Transient combustion of a methane-hydrate sphere

A solution is presented for the spherically symmetric, transient vaporization and combustion of methane-hydrate in a three-phase configuration with rate control by liquid-phase diffusion. The simulation has two moving boundaries due to a solid methane-hydrate core that melts, a transient water shell with small methane gas bubbles, and a quasi-steady gas phase with Stefan convection and advection, diffusion, and chemical reaction. First, a model for melting and vaporization without oxidation is considered. Then, the combustion process is considered at an infinite chemical reaction rate (i.e., a “thin flame” case).The characteristics of the methane-hydrate combustion are examined at different ambient temperatures, pressures and compositions and values of methane-to-water mass ratio. Different values of products in the environment are examined, considering individual particles burning in an environment which has been heated by the presence of other burning droplets, as in-situ production methods are currently developed. An appropriate characteristic time scaling is identified for particles of initial radius of 100 µm or less, considering these will be relevant for combustion of grounded hydrates.

The features of self-preservation for hydrate systems with methane

Dissociation of hydrate systems is studied experimentally: gas hydrate of methane produced artificially in the reactor–crystallizer; natural gas hydrates of methane; water–methane–isopropanol systems in the air atmosphere. Artificial gas hydrate of methane and natural hydrates demonstrate the ranges of abnormally low dissociation rate. At that the boundaries of the temperature windows of self-preservation differ significantly for natural and artificial hydrate systems. Despite the similar structures of elementary hydrate cells of natural and artificial gas hydrates, the dissociation rate of natural samples was significantly lower than the dissociation rate of artificial powders. Moreover, natural hydrates had the expanded time period of the stable thermodynamic state. The mechanism of gas hydrate dissociations depends not only on the driving forces and structural characteristics, but also on the average initial diameter of the powder particles. The creep properties of gas hydrates and limits of their strength are associated with microstructural characteristics, and the dissociation rate depends on the size of the grains. The temperature fields of separate granule surface were obtained via many-times magnification of thermal images. Temperature distribution over the surface is significantly non-uniform, and this characterizes non-uniform hydrate dissociation within the granule volume. Dissociation kinetics for the natural and artificial samples was studied at different heat fluxes. The maximal heat flux and maximal dissociation rate were achieved at combustion of methane hydrate. Both instantaneous and average dissociation rates were measured.