Research progress of moderate or intense low-oxygen dilution (MILD) combustion of pulverized coal in high-speed jet

Developing a cleaner and more efficient combustion technology of pulverized coal is an important way to deal with the increasingly serious environmental pollution. Moderate or intense low-oxygen dilution (MILD) combustion of pulverized coal is regarded as a new combustion technology with great potential due to its advantages of reducing NO_x emission and improving the uniformity of heat flux in the furnace. More importantly, the pulverized coal MILD technology combined with the oxy-fuel combustion technology will be an important path to reduce carbon emission in the process of coal utilization in the future. The history, characteristics and current research status of the MILD combustion of pulverized coal worldwide are summarized, and the key issues and research challenges of the MILD combustion technology of pulverized coal with a low preheating temperature are put forward. The MILD combustion technology of pulverized coal originates from that of gaseous fuels, which strengthen the entrainment of high-temperature flue gas and then dilute the reactants to make the flame become blurred or even invisible. Under the MILD conditions, the peak temperature in the combustion region is reduced and the uniformity of temperature distribution is improved, therefore significantly inhibiting the generation of thermal NO_x and fuel NO_x. The MILD combustion of pulverized coal can be realized by preheating air to high temperatures or increasing the jet velocity greatly, and the second way is more economical and feasible. However, the mechanisms and characteristics of the MILD combustion of pulverized coal under the condition of strong shear and turbulent mixing, when the jet velocity is as high as around 100 m/s, still need to be further investigated. Under the strong shear and entrainment of high-speed jets, the dispersion of coal particles, which is directly related to the achievement of the MILD combustion mode, is quite different from that in the conventional coal combustion. In addition, a strong turbulent mixing will affect the heating, devolatilization and char combustion of pulverized coal, therefore changing the ignition delay, the flame structure and the generation of pollutants. Based on the in-depth understanding of the above key issues and combined with the research on the MILD combustion of gaseous fuels, a systematic and accurate criterion for the MILD combustion of pulverized coal can be further developed, so as to better guide the application of the MILD combustion technology of pulverized coal.