MILD Combustion of Solid Fuels: Its Definition, Establishment, Characteristics, and Emissions

Abstract

Moderate or intense low-oxygen dilution (MILD) combustion has the advantages of high thermal efficiency and ultralow NO x emissions. This technology has attracted extensive attention from the combustion community since 1990 or so. The early development of MILD combustion technologies required highly preheating the combustion air externally, thus limiting the range of its industrial applications, especially for burning solid fuels. Later, this technical limitation was gradually overcome, particularly becoming possible to burn solid fuels without preheating under MILD conditions. Significant progress has since been made in solid-fuel MILD combustion. Some examples include the development of solid-fuel MILD burners utilizing low-volatile residual char and high-volatile biomass, low emissions of pollutants such as NO x and fine particles, MILD oxy-fuel combustion, and pilot-scale demonstrations and industrial applications. Solid fuels adopted for MILD combustion comprise coal, residual char, biomass, and sludge. This paper reviews the research progress achieved so far for solid-fuel MILD combustion. The definition of solid-fuel MILD combustion is first introduced. Then, the establishment approach is discussed. The combustion features and NO x emission mechanisms are investigated in detail. The advantages of MILD oxy-fuel combustion are also presented. The future development of solid-fuel MILD combustion is proposed finally. This review covers the fundamentals and applications of MILD combustion of solid fuels.

Effects of H2O Dilution on NOx Emissions of Methane-Added Ammonia Oxidation

“…Ariemma et al indicated that the increased OH production due to CH 4 oxidation consumes NH 2 radicals, which inhibits the DeNO x process. To mitigate high NO x emissions from NH 3 /CH 4 combustion, various combustion technologies have been adopted, including gliding arc-assisted combustion, , rich-lean combustion, , pressurized combustion, and flameless combustion. , In many studies, a small percentage of NH 3 is adopted as the fuel-N source. , As the NH 3 proportion increases, the concentrations of OH, N, and HNO radicals are altered, thereby influencing NO x formation characteristics. Zhang et al and Filipe Ramos et al both noted that the NO x emissions reach the peak value when the volume fraction of NH 3 in the fuel ( V NH 3 ) equals 0.5.…”

Numerical Analysis on the Quantitative Separation of Different NOx Formation and Reduction Routes during MILD Oxy-Coal Combustion with the Improved Nitrogen Chemistry Model

“…Oxy-coal combustion (OCC) technology has garnered wide attention with the push for carbon neutrality. However, OCC has several drawbacks, including ignition delay, incomplete combustion loss, and low combustion stability. , Moderate and intense low-oxygen dilution (MILD) combustion, which extensively dilutes the oxidizer and preheats the fuel through intensively internal flue gas recycle (FGR), leads to a lower peak temperature, more uniform temperature distribution, extended reaction zone, and reduced reaction rate. , This technology not only improves thermal efficiency but also inhibits NO x emissions. By integrating the MILD combustion with the OCC technology, the MILD-OCC technology effectively mitigates the deficiencies of conventional OCC technology. , …”