Review on MILD Combustion of Gaseous Fuel: Its Definition, Ignition, Evolution, and Emissions

Abstract

Combustion of fossil fuels has been continuously generating over 70% of the total energy for our human society, concurrently emitting vastly greenhouse gases and ecologically harmful pollutants. To mitigate the adverse issue, combustion technologies of a new type were successfully developed and debuted around 1990, associated with various terms of "flameless oxidation (FLOX)", "flameless combustion (FLC)", "high-temperature air combustion (HiTAC)", and "moderate and intense low-oxygen dilution (MILD) combustion". These combustion processes may be regarded as similar or the same class of combustion, because they all involve high preheat and deep dilution of individual reactants (nonpremixed) or the reactant mixture (premixed), prior to main combustion reactions, with high-temperature flue gases (burnt products) internally and/or externally. Very significantly, all of these combustion technologies can simultaneously achieve high efficiency and ultralow emissions of nitric oxides (NOx), light, and noise. This class of combustion has been frequently unified as "MILD combustion". Highly worthy to note, as a particular type of combustion, the MILD combustion has thus far been most extensively explored, resulting in many hundreds to 1000 academic articles published by various energy-related journals and conferences. The present review summarizes the research progress in the gaseous-fuel MILD combustion on its definition, ignition, evolution, and NOx emissions, specifically, through its aspects associated with open jet flames in hot coflow and industrial flames in combustion chamber or furnace. Also, discussions are provided on the relevant terminologies, existing problems, and further fundamental issues.

Experimental and Numerical Study on Moderate or Intense Low-Oxygen Dilution Oxy-Combustion of Methane in a Laboratory-Scale Furnace under N2, CO2, and H2O Dilutions

“…The MILD combustion arises from the reactant auto-ignition, and so, its establishment in furnace requires a high environmental temperature or wall temperature ( T w ) that is greater than the reactant auto-ignition temperature ( T ai ), i.e., T w > T ai . ,, Therefore, for the in-furnace MILD combustion, a traditional bluff-body flame is needed to heat up the furnace. When T w > T ai , the oxidant stream is switched from the outer annulus to the inner annulus for realizing the MILD combustion.…”

“…The place of local temperature T * and the local oxygen fraction f O2 * at the beginning of the main reaction zone is shown in Figure . The classification map of Mi et al is obtained for the JHC flame of burning CH 4 /H 2 at the beginning of the reaction zone (indicated by red stars in Figure ) under various simulation conditions. Clearly, although all the cases are in flameless combustion (FLC) regime, f O2 * for f O2,O = 30% under the N 2 dilution is significantly higher than (almost twice) those under the CO 2 and H 2 O dilutions.…”

“…To address these problems, a combination of the oxy-combustion with MILD combustion, i.e., the MILD oxy-combustion, has been undertaken. Note that the MILD combustion has received much attention in the past three decades or so . It applies high-speed reactant jets to strongly entrain the surrounding flue gas and hence enhance the mixing and dilution of the reactants.…”

“…It applies high-speed reactant jets to strongly entrain the surrounding flue gas and hence enhance the mixing and dilution of the reactants. As a result, the local reactant concentration is reduced while the temperature is heightened, thus realizing the uniform and stable MILD combustion. − The MILD combustion can be characterized mainly with the following: , (1) uniform temperature distribution and no visible flame front; (2) high combustion stability, low temperature fluctuation, and extremely low noise; and (3) perhaps most importantly, ultralow NO x emissions. …”

“…Note that the MILD combustion has received much attention in the past three decades or so. 6 It applies highspeed reactant jets to strongly entrain the surrounding flue gas and hence enhance the mixing and dilution of the reactants. As a result, the local reactant concentration is reduced while the temperature is heightened, thus realizing the uniform and stable MILD combustion.…”