Experimental study of coal dust ignition characteristics at oil-free start-up of coal-fired boilers

Highlights: • Oil-free start-up of coal-fired boilers can be implemented in practice. • The ignition characteristics of bituminous and brown coal dust were determined. • Ambient temperature 500 °C is minimal for coal dust ignition (sizes about 100 µm) • Low-temperature ignition of high reactivity (V^daf ≥ 50%) coals is guaranteed during 1–4 s. The conditions and characteristics of ignition of bituminous and brown coal widely used in thermal power engineering as the main fuel have been studied experimentally. The studies were carried out under the heating of fine coal particles in the muffle furnace. Such conditions are similar to those of coal dust ignition in muffle burners used in the implementation of advanced technology of oil-free start-up of coal-fired boilers. This technology is characterized by positive economic and environmental indicators in comparison with the widespread technology of coal-fired boilers start-up by liquid fuel combustion. Processes of ignition were studied in a wide range of temperature variation (400–1000 °C) for several grades of coal with various combustion heats and volatile matter content and different particle sizes: 40, 140, 250 µm. Minimum air temperatures necessary for ignition of fine particles of coal, as well as dependences of the main characteristic of the process, i.e. ignition delay time, on ambient temperature have been determined. The ambient temperature required to ignite coal particles with the sizes of about 100 µm is 500 °C for coals with relatively high volatile content (more than 30%) and 600 °C for coals with low volatile content (less than 30%). Approximation expressions are given for the obtained dependences. The results of the experiments serve as a basis for predicting the conditions and characteristics of coal dust ignition instead of liquid fuel and for justifying the size of the muffle burners for the development works and for optimization of their operation modes in the conditions of coal-fired boiler start-up.