The Study Of Methane Hydrate Combustion Characteristics Under Different Airflow Conditions

With the growth of energy demand and the increasing requirements of environmental protection,the global demand of natural gas is increasing.As the most abundant natural gas resource,methane hydrate is receiving more and more attention due to its lower carbon emissions and wide range of applications.However,before the large-scale development and utilization of methane hydrate,there are still many scientific problems that need to be solved urgently.Studying the combustion characteristics of methane hydrate is of great significance to engineering practice such as methane hydrate in-situ mining,methane hydrate storage and transportation process safety,and methane hydrate terminal application.While the combustion of methane hydrate under natural convection has many problems,such as unstable flame and low heating value.Forced air-flow can improve the combustion efficiency of methane hydrate,but the current research on this problem is not yet in-depth.In this paper,experimental studies combined with numerical simulation methods were used to study the influence of forced airflow on the combustion characteristics of methane hydrate.First,the methane hydrate combustion experiment under the natural convection environment was carried out,methane hydrate particles were taken as the research object,and obtain some combustion characteristic parameters,such as the hydrate weight and flame characteristics（height,temperature,structural characteristics,pulsation characteristics）during the combustion process with the combustion time were also obtained.By comparing with the combustion characteristics of methane hydrate under natural convection environment,the effects of two airflow forms（external horizontal airflow and internal vertical airflow）and the airflow velocity on the combustion characteristics of methane hydrate were explored.Then,by using computational fluid dynamics software,a numerical model of methane hydrate combustion was constructed to obtain combustion characteristics such as combustion temperature field,flow field,and component concentration field and to study the mechanism of vertical airflow velocity and horizontal airflow on the combustion of methane hydrateThe results show that the process of methane hydrate combustion can be divided into six stages according to significant difference of combustion characteristics.They are combustion development stages,intensive combustion stage,steady combustion stage,decay stage,eruption phenomenon and re-decay stage.The combustion of methane hydrate under natural convection condition is seriously unstable,the flame can be observed obviously only in the first20 seconds,the heat cannot be used.The combustion of methane hydrate under the internal vertical airflow condition is much more stable,the flame height is more uniform,and the stable combustion stage is about 48 s,accounting for 80%of the total combustion time.With the increase of the vertical airflow velocity,the maximum flame height first increases and then decreases.The combustion of methane hydrate under the external horizontal airflow condition is also unstable and there is also eruption phenomenon.With the increase of the airflow velocity,the flame gradually approaches the hydrate upper wall,and the heat transfer to the hydrate increases.Therefore,the dimensionless gas mass and the combustion rate both show an upward trend.The flame temperature,flame structure and flame height obtained by numerical simulation are extremely close to the experimental results,which proves the accuracy of the model.The maximum flame temperature appears near the fuel-oxygen ratioα=0.5,and where the burning rate is also high.The former is conducive to full combustion,and the latter can provide more combustion heat.Exerting small velocity of horizontal airflow（u₁≤0.3 m/s）and vertical airflow(φ_air<0.3)contributes to the decrease of excessive fuel near the fuel inlet,thereby improving the combustion and reducing flame liftoff height.However,as the velocity of vertical airflow continues to increase,fuel dilution effect takes the leading role,the methane concentration in the fuel gas gradually decreases,and the flame temperature gradually decreases.As the velocity of horizontal airflow continues to increase,due to the dilution effect of airflow,the fuel-oxygen ratio at a high position can no longer reach 0.5,and the fuel gas is seriously insufficient.The flame gradually changes from vertical development to horizontal development,and the flame inclines significantly.The research results can not only enrich the basic theory of heterogeneous combustion,but also promote the large-scale development and application of the methane hydrate industry.