| Under the energy era background of strict control of combustible greenhouse gas emissions and vigorous development of clean and low-carbon combustion,the effective utilization of a large number of associated gases formed by coal mining-ultra low concentration methane(ULCM)in coal mine with a volume concentration of no more than 6 vol% has very important practical and long-term significance for increasing China’s energy supply and reducing pollutant emissions.This paper aims at realizing stable combustion of ultra-low concentration methane.Based on the lack of research on the flow rate range,lean combustion limit and other stable combustion characteristics in the study of porous media premixed combustion,the lack of research on the combustion temperature reduction and enhanced stability of porous media burners and the lack of research on the stable combustion characteristics of the unique gas source characteristics of ultra-low concentration methane,through the experimental research of self built experimental system,theoretical analysis and FLUENT combustion numerical simulation,the stable combustion characteristics of ultra-low concentration methane in molten salt heat storage porous media burner are studied comprehensively.The main research results and conclusions are as follows:The stable combustion characteristics of ultra-low concentration methane with cylindrical and conical V-shaped structures are expounded.Aiming at the research object of stable combustion of ultra-low concentration methane,experiments on combustion temperature distribution,lean combustion limit,stable combustion speed range,periodic fluctuation combustion temperature distribution and pollutant emission combustion characteristics are carried out,research shows: Compared with the equidistant cylindrical structure,it is found that the lean combustion limit of the conical V-shaped salt free optimized structure type is at least 1 vol%,and the stable combustion characteristics of the conical V-shaped structure with low combustion temperature,high flow rate,very low lean combustion limit,wide stable combustion flow rate range and low NO content emission are obtained.It is found that with the increase of the porosity in the combustion reaction zone of the conical V-shaped structure,the uniformity,concentration and linearity of the combustion temperature distribution in the corresponding zone are improved,which is due to the weakening of the gas-solid heat transfer capacity and the enhancement of the sphere radiation capacity.However,it is found that the flame surface is easy to move up and the combustion temperature changes greatly in the V-shaped structure combustion process.The above research results can provide a basis for the experimental analysis in the following experimental chapters.The stable combustion characteristics of ultra-low concentration methane in the composite structure of heat storage molten salt are expounded,and the stationary combustion heat balance model of the composite structure of heat storage molten salt porous media is established.The experimental method of adding heat storage molten salt into porous media burner and the flame stationary preheating method with small flow,short heating time and high heating efficiency are proposed.Through experiments,it is found that the lean combustion limit of the heat storage molten salt composite structure is 1.25 vol%,showing a change trend that the combustion temperature distribution of this structure is significantly reduced,the brightness and width of the flame surface are reduced,and the flame surface moves up and stagnates.The mechanism of reducing the heat return flow through the heat storage of molten salt to achieve low combustion temperature,low NO emission and improve the lean combustion limit is revealed.It is found that the combustion stability can be effectively enhanced by reasonably changing the heat capacity and heat wave rate of salt containing structure.Based on the thermodynamic theory,the stationary combustion heat balance model of the composite structure of regenerative molten salt is constructed,and the internal cycle relationship between energy input,heat transfer and loss in the stationary combustion process is shown.At the same time,the experimental analysis shows that the combustion reaction temperature decreases,the preheating zone distance increases and the combustion zone distance decreases with the increase of the length of the salt burner at a fixed power.It is found that the temperature of molten salt fluctuates below the phase transition line with low power fluctuation,while the temperature of molten salt fluctuates near the phase transition line with high power fluctuation.Using the FLUENT combustion reaction simulation program based on the added two temperature model,the influence laws of four factors,including operation mode,physical parameters,structure shape and industrial size,on the stable combustion of ultra-low concentration methane with heat storage molten salt composite structure are revealed.The research on this law deepens the scientific understanding of the stable combustion characteristics of ultra-low concentration methane containing molten salt.The influence mechanism of high temperature flame zone under the change of operating parameters is analyzed.It is found that with the increase of premixed initial temperature,the enthalpy flame becomes closer to the inlet and the influence range of temperature distribution becomes larger,and the conical V-shaped structure is more advantageous in the overall emission.It is found that the heat capacity of salt and the extinction coefficient of sphere are the main controlling physical factors,which play a leading role in the high-temperature flame zone,while the thermal conductivity of salt and the latent heat of phase transition have relatively small effects.Through fluid simulation,the influence of flow velocity and resistance change inside the salty conical V-shaped structure and salty cylindrical structure is analyzed.It is found that the conical V-shaped shape has lower gas velocity and flow resistance than the salty cylindrical shape,which is helpful to reduce the energy consumption of burner operation.The effects of different scales of the same salt containing conical V-shaped structure on the stable combustion characteristics(temperature distribution and resistance distribution)are simulated and analyzed.It is found that the array split small-size conical structure burner has significant advantages in low combustion temperature,resistance loss and NO emission compared with the large-size conical structure.Therefore,the split combustion with small-size array conical V-shaped structure can provide a new idea for the development of new industrial burners.The variation characteristics of main components of ultra-low concentration methane in coal mines are expounded.On this basis,the effects of typical components(water vapor,hydrogen and carbon dioxide)on ultra-low concentration stable combustion are revealed.The results show that: It is found that the peak combustion temperature and no content decrease with the increase of water vapor mole fraction in ultra-low concentration methane.The results show that the influence coefficient of combustion efficiency decreases gradually with the increase of methane concentration,and both obey the power function distribution,indicating that water has a certain influence on combustion.The results show that with the increase of hydrogen mole fraction,the surface radiation efficiency and combustion efficiency decrease,and the influence degree of combustion efficiency is also related to the proportion of oxygen.It is found that with the increase of carbon dioxide content,the stable combustion power range decreases gradually,the blowing out area increases,and the tempering area decreases gradually,which indicates that the carbon dioxide composition has an adverse effect on combustion. |
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