Thermal Stresses Based Fragmentation Model for Pulverized Coal Particles Gasification by Low Temperature Air Thermal Plasma

Novel system for boiler start-up and flame support based on coal particles ignition and gasification by low temperature air thermal plasma is planned to supplement existing heavy oil system in Serbian thermal power plants. It is expected that the new system will reduce air pollution and that will be more cost effective compared with the existing one. Coal gasification plasma system consists of a pulverized coal burner equipped with a plasma generator which is able to produce air thermal plasma with temperatures in range 5000 – 7000 °C. This, locally introduced high temperature plasma stream, enhances coal particle ignition, devolatilisation, and partial char oxidation producing high thermal energy gaseous mixture for further combustion inside the boiler furnace.

Numerical simulation using comprehensive CFD code ANSYS FLUENT was performed in order to optimize operating parameters of novel thermal plasma coal treatment technology before its implementation in real scale boilers. Available combustion sub-models are able to predict particle ignition, devolatilisation, and char oxidation with satisfying accuracy, while, pulverized coal particle fragmentation is usually neglected due to its limited influence on combustion process under common conditions in the furnace.

However, thermal plasma produces high thermal stresses inside entrained coal particles. This thermal stresses lead to phenomenon known as “thermal shock” that intensifies particle fragmentation process, during which initial particles break into 8 – 10 smaller fragments, which makes also important enhancement on devolatilisation process and consequently significantly accelerating subsequent char oxidation.