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

Moderate or intense low-oxygen dilution (MILD) combustion has the advantages of high thermal efficiency and ultralow NOx 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 NOx 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 NOx 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.