ICHMT DIGITAL LIBRARY ONLINE

ISSN 961-91393-0-5

Year 2000

Volume 1 - Thermal Sciences 2000 Proceedings of the International Thermal Science Seminar Bled, Slovenia, June 11 – 14, 2000

604 pages

Volume price - $120.00

Mirko S. Komatina
Mechanical Engineering Faculty, Belgrade University, 27. marta 80, 11000 Belgrade, Serbia and Montenegro

Mladen S. Ilic
Laboratory for Thermal Engineering and Energy Institute for Nuclear Sciences — Vinca, P.O. Box 522 Belgrade, Serbia, 11001, Yugoslavia

Simeon N. Oka
Laboratory for Thermal Engineering and Institute of Nuclear Sciences — VINCA, Belgrade 11001, Serbia and Montenegro

During combustion in a fluidized bed, processes of heating, drying, devolatilization, combustion of volatiles, char combustion, fragmentation and attrition are occurred. Determination of temperature of burning coal particles is necessary for creating reliable mathematical model, and for well predicting of No_x and SO₂ emissions and probability of ash melting. In order to analyze and model processes of heating, drying, devolatilization, combustion of volatile and char combustion, reliable data of heat transfer coefficient are needed.
The coal particle temperature, during different phases of combustion process in fluidized bed is very difficult to determine and this problem is not solved completely. There are significantly differences between different models from literature, and between models from literature and experimental data also. These differences occur on account of not enough correct or precise determination of coal particle temperature.
The measurements of temperature of large particles, which results are shown in this paper were done with an aim of obtaining new, reliable data on heat transfer coefficient and coal particle temperature in hot fluidized bed.
The temperature was measured by 0.5 or 1mm Cr-Ni thermocouple imbedded in the particle center. The particles were previously shaped into spherical form, to eliminate uncertainties associated with differences in shape.
There were done two groups of experimental investigations. The first group was performed with aim to determine heat transfer coefficient between the large particle and the hot fluidized bed. The data obtained from this group of experiments would be used for modeling of processes in fluidized bed. The second group of measurements was done with original coal particles. The size of particles was chosen in the range of coal particle sizes usually fed in real fluidized bed combustors.
The first group of measurements was performed with spheres of different materials (copper, aluminium, brass and graphite) with diameters d_c=5, 10, 15 and 20mm. Metal and graphite spheres were used since their thermophysical properties are well known, they could be shaped relatively easily in sphere and they are homogeneous materials with the exception of graphite spheres. Air was used as fluidization gas in the experiments with metal spheres. Also, graphite spheres (99% carbon and small porosity) were used since they are similar to coal particles. In this case fluidization was achieved with nitrogen in order to eliminate chemical reactions. Investigations were performed with bed temperatures of T_b=300, 500, 600 and 750°C.
The second group experiments were done with two characteristic types of Yugoslav coals: lignite Kosovo and brown coal Bogovina, diameter of d_c=5, 7 and 10mm. Investigations were performed in inert atmosphere. with bed temperature in the range T_b=590÷710°C.
As inert material for both group of experiments was used silica sand with mean diameter of d_p=0.25, 0.50 and 0.90mm (in experiments with metal sphere and graphite) and d_p=0.25mm in experiments with coal. Fluidization velocity was in the range v_f=0.063÷0.855m/s.
On the base of the numerous measured temperatures of metal and graphite spheres, a correlation of heat transfer coefficient between the large particles and hot fluidized bed was obtained, which encompass experimental results with accuracy of ±15%.
On basis experimental measurements temperature of coal particle was analyzed changed temperature of coal particle during processes of heating, drying and devolatilization. These experimental results with coal particles would be used for testing our developed mathematical model of coal combustion.

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