Experimental And Numerical Study Of MILD Combustion With Air Preheating

Confronted with the shortage of environmental resource around the world, MILDcombustion which has high efficiency and low emission has attracted the internationalcombustion field’s attention. In particular, MILD combustion means flameless with thediffused or partially premixed combustion system. This paper has studied the influence ofpreheated air temperature on the characteristics of MILD combustion on a pilot scalefurnace of20kw through experiments and numerical simulation.The experiments have been carried out by taking liquid gas and methane as fuelrespectively to study the influence of the preheating air temperature. It is found thatair-preheating results in the rise of mean value of temperature inside the furnace and thedecrease of temperature gradient, which makes temperature field uniform. Besides,air-preheating results in the air imported initial momentum increasing and the ability ofentraining fuel gas and high-temperature ash for air has been strengthened. With airpreheated, the temperature of combustible mixture increases, the concentration of oxygen inthe mixture reduces at the same time, the concentration of oxygen inside the furnace islower and the distribution of oxygen is more uniform in MILD combustion.The concentration of CO is much higher in the preheating MILD combustion mode. Ithas little effect on the concentration of CO in the tail flue gas, Meanwhile, the samecondition occurs in the concentration of NO. Preheating air can enhance mixture. It haslittle effect on the flue gas in the furnace tail.Realizable k-ε turbulence mode and EDC combustion model which has a detailchemical mechanism of methane has been adopted. Different preheating temperature(303Kand673K) has been adopted in CH4MILD combustion simulation. The results have shownthat the temperature in furnace is higher in preheating condition than that in normalcondition. The contrary condition appears in the concentration of oxygen. It remains a goodagreement between experiment data and simulation apart from the near burner inlet region.The peak value of CO shifted toward to the center of furnace when compared to theexperiment. The simulation is much more than the experiment data of NO in preheatingcondition. The contrary condition appears in the normal temperature. The concentration of NO is low in the whole range.It appears low temperature enriched oxygen near burner region. It is indicated that thecharacteristic of MILD combustion is the low chemical reaction rate. It exists intensity fluegas entrainment in the inlet and exit of furnace. The effect of turbulence is much more thanthe one of chemical reaction in that area, resulting in the discrepancy between experimentand simulation.