Some Aspects Of Fluidized Bed Coal Combustion And Their Effect On FBC Modeling.

Fluidized bed combustion offers a clean and relatively inexpensive method for coal combustion. There are various physical and chemical processes occurring in the fluidized bed combustor, some of which are investigated in this work. Devolatilization and combustion of a single coal char particle, minimum fluidization velocity at high temperatures, fluidization regimes, and SO(,2) release rates in FBC are the areas examined in this work. The mode of release of volatiles can be instantaneous or uniform depending on the relative magnitudes of mixing time and devolatilization time. The burnout time of large coke particles (\u3e 2 mm) can be as high as 600 - 800 seconds in FBC and must be considered in relation to the residence times of these particles for estimating combustion efficiency. In this work, the minimum fluidization velocity is shown to decrease with an increase in temperature for small particles (less than about 2 mm for sand with hot air as a fluidizing medium) but increase with an increase in temperature for large particles (\u3e 2 mm for sand). For large particle fluidized beds as in FBC, the operation is found to be partly in the slow bubble regime and partly in the fast bubble regime, unlike fluidized beds using FCC catalyst where the fast bubble regime exists. Sulphur in coal exists in both organic and inorganic forms. At low temperatures organic sulphur is released first in pyrolysis, followed by the release of inorganic sulphur. Organic sulphur release rate is comparable to the devolatilization rate since organic sulphur is released in the form of volatiles and immediately oxidized in the gas phase. Inorganic sulphur release rate is proportional to the char combustion rate. An overall FBC model is developed here taking into account devolatilization and combustion of coal, sulphur dioxide capture by limestone, attrition and elutriation of char and limestone, bubble hydrodynamics, etc. The model is able to simulate most of the important performance characteristics. Model predictions are compared with the experimental data of the Babcock & Wilcox 6\u27 x 6\u27 fluidized bed combustor and the agreement is good