Development of Particle Size and Particle Velocity in a Pressurized Pulverized Coal Flame

摘要

Combined cycles with pressurized combustion of pulverized coal in combination with a high temperature gas turbine have a high efficiency potential. However, the separation of the molten particles from the flue gas before entering the gas turbine is still an unsolved problem. Especially the very fine particles can not be removed by separation units under consideration. The origin of the small particles is not well understood. They may result from coal ash directly or from newly build-up material from the combustion process. The aim of the present investigation is a better understanding of the particle history within the pressurized swirl flame. Measurements were carried out in the Pressurized Pulverized Coal Combustor (PPCC) of the Aachen University with its thermal power of up to 400 kW and a pressure in the range from atmospheric to 10 bar. The radial profiles of the axial and tangential velocities and of the particle sizes are measured by Laser Doppler Anemometry (LDA) and Phase Doppler Anemometry (PDA), respectively. Since the swirl burner of the combustor can be moved axially during operation measurements of axial development of these quantities can be evaluated. Because the PDA measurement technique can only be applied for spherical particles, samples were taken from the combustor by a suction probe and analyzed by a scanning electron microscope. The photographs show clearly that most of the particles are of spherical shape. However, a rather large amount are well below 1 m, which can not be detected by the PDA technique. The experimental results of velocity and particle size distribution from pressurized combustion of Rhenish lignite will be presented and discussed comparing the measurements with numerical results using a CFD approach.