MILD Combustion Characteristics Analysises And Numerical Simulations

The burning of fossil fuels is the main way for mankind to obtain energy.With the growth of the world’s population and economic development,people’s demand for energy is increasing,but the pollutants produced by the burning of fossil fuels have put tremendous pressure on the global climate and environment.The contradiction between mankind’s need for development and the ecological pressure is becoming more and more intense.Therefore,it is necessary to develop new combustion technology with high combustion efficiency and low environmental pollution.In recent years,MILD（moderate or intense low-oxygen dilution）combustion technology has attracted attention due to its extremely low pollutant emissions and high combustion efficiency.Moreover,MILD combustion is compatible with oxy-fuel combustion,and the combined MILD-oxy combustion technology can combine the advantages of the two combustion technologies.However,whether it is MILD combustion or MILD-oxy combustion,the understanding of its basic characteristics is not comprehensive enough.Therefore,in this paper,the methods of experiment and numerical simulation are used to study the MILD combustion technology.Thermogravimetric analyzer is used to experimentally study the combustion characteristics of bituminous coal and anthracite under different conditions.Since the combustion process of gas fuel is relatively simple,numerical simulation method is used to study the methane MILD combustion characteristic.Further,the pulverized coal MILD combustion characteristics are studied.These sutdies provide some scientific guidelines for MILD combustion technology.Firstly,in this paper,the thermogravimetric analysis on the pulverized coal combustion characteristics under different oxygen concentration is carried out.The influence of atmosphere,heating rate and oxygen concentration on the combustion characteristics of bituminous coal and anthracite is studied.The experimental results show that high oxygen concentration or high heating rate can improve the combustion and burnout characteristics of pulverized coal.In O₂/CO₂ atmosphere,5%oxygen concentration makes the pulverized coal in an environment with high concentration CO₂and low concentration O₂.The burn-out temperature of pulverized coal increases,and the char gasification reaction gradually appears.Increasing the heating rate increases the char gasification reaction.Then,the combustion characteristics of methane MILD and conventional combustion are studied and the numerical effects of the Eddy Dissipation Concept（EDC）parameters in the two combustion modes are studied.The results show that the original EDC constants are suitable for conventional combustion prediction.While,the predicted temperature,and therefore the NO_x production in MILD combustion,are overestimated with the original EDC constants.The numerical result with modified EDC parameters can better predict experimental data of mehtane MILD combustion.Because the structure of the burner can change the organization of fuel and oxidant,it will affect flue gas flow in the furnace.Then,by changing the structure of the burner,the influence of the organization of oxidant and fuel on the methane combustion MILD is studied.The results show that methane MILD burners should try to use single fuel outlet,because more fuel outlets disperse the initial kinetic energy of the fuel,which is not conducive to the diffusion of fuel to a large space.The arrangement of high momentum nozzle deviating from the outlet is easier to entrain the flue gas in the furnace and enhance the flue gas recirculation.So,it is beneficial to enhance the MILD combustion performance.Based on the experiment of International Flame Research Foundation（IFRF）,the pulverized coal MILD combustion is numerically simulated.The difference between MILD combustion and MILD-oxy combustion is studied and the MILD-oxy combustion characteristics under different oxygen concentrations are discussed.The results show that,under the same oxygen concentration,CO₂ as a diluent has better MILD combustion performance than N₂.In MILD-oxy combustion,the increase in oxygen concentration increases the chemical reaction and weakens the dilution effect of the circulating flue gas,so the MILD combustion performance deteriorates.In addition,when the excess oxygen coefficient is constant,increasing inlet CO₂ concentration is beneficial to improve MILD combustion performance.However,too high inlet CO₂ concentration will lead to insufficient combustion and excessive CO emissions.When the excess oxygen coefficient is constant,since changing the oxygen concentration in MILD-oxy combustion changes the inlet velocity of fuel and oxidant at the same time,the inlet velocity is very important for MILD combustion.Therefore,based on the IFRF pulverized coal MILD combustion experiment,the influence of inlet velocity is also studied on MILD-oxy combustion.The results show that whether it is MILD combustion or MILD-oxy combustion with different oxygen concentrations,increasing the inlet velocity can improve the MILD combustion performance.Increasing the inlet velocity can suppress the peak temperature in the furnace,make the species distribution,temperature distribution in the furnace more uniform and increase the share of convective heat transfer in the total heat transfer.The increase of the inlet velocity changes the flow in the furnace,which enhances the flue gas recirculation,the chemical reaction rate slows down.For pulverized coal,increasing inlet velocity enhances the MILD combustion performance,and the inhibition of the char reaction is stronger.In addition,as the oxygen concentration increases,the temperature in the furnace rises,and the char gasification reaction is enhanced.