Development of a 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.
The main aim of this work is development of fragmentation model based on calculations of the thermal stresses inside pulverized coal particles and its implementation in ANSYS FLUENT combustion model using User Defined Functions (UDFs). Fragmentation model calculates induced thermal stresses for different particle residence times taking into account particles of different radius. Calculated unsteady stress field is used to obtain principal stresses values. Model further assumes that particle fragmentation occurs when major principal stress exceeds particle uniaxial tensile strength.
Numerical model accuracy was checked comparing numerically calculated temperatures with temperature values measured inside novel semi industrial scale experimental plasma burner test facility.
Key words: Serbian lignite, low temperature thermal plasma, coal particle fragmentation numerical modeling, experimental validation.
Rastko Jovanović*, Dejan Cvetinović, Predrag Stefanović, Predrag Škobalj, Zoran Marković
University of Belgrade, VINCA Institute of Nuclear Sciences, Laboratory for Thermal Engineering and Energy, P.O.Box 522, 11001 Belgrade, Serbia
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