In the past 3 decades or so, the innovative combustion technologies associated with so-called “moderate and intense low-oxygen dilution (MILD) combustion” have been researched and developed to help mitigate greenhouse gases and harmful pollutants from burning fossil fuels. The MILD combustion is characterized by ultra-low emissions of nitric oxides (NO x ) and carbon monoxides, uniform temperature and large reaction zone, invisibility and silence, and broadly usable fuels. Currently, it has been successfully used in steel and metallurgy industries worldwide, offering a high thermal efficiency and ultra-low NO x emissions. It also has a high potential to apply in the power generation industries, such as gas turbines, most of which fire liquid fuels. However, this type of combustion for burning liquid fuels has not been well-understood, thus requiring further research and development (R&D). To facilitate the R&D, the present review is undertaken on the past progress made in research into the liquid fuel MILD combustion. Specifically, it summarizes the findings from the previous studies of both open fuel jet flames in hot coflow and enclosed furnace combustion of liquid fuels. Likewise, it analyzes the combustion characteristics of burning liquid fuels and evaluates the challenges in operating with different liquid fuel types. The review is concluded by suggesting some future research directions for the combustion itself, the computational fluid dynamics modeling, and the comprehensive design methods of its applications.

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.