Combustion of Shelled Corn in a Small-Scale Fluidized Bed

Abstract

Fluidized bed combustion is an attractive method of extracting energy from a wide variety of fuels. This study analyzed operating parameters for shelled corn combustion in a 15.2 cm diameter, 44-kW atmospheric fluidized bed burner. Variables studied include sand particle size distribution, fuel particle size, moisture, air/fuel ratio, and bed depth. Increasing moisture from 16% to 22% reduced bottom bed temperature slightly, without significantly changing the rest of the bed profile temperatures. Increasing bed depth from 0.58 m to 0.71 m reduced the freeboard temperature while increasing heat energy in the sand bed. A system based reaction rate constant was developed and mass loss profiles and residence times of the various particle sizes were determined. Solving an energy balance for the system, k values (reaction rates) gave 0.3 min-1 for whole shelled corn and 0.4 min-1 or greater for particles sizes smaller than 5.7 mm. Combustion characteristics, chemical and physical properties, and fluidization behavior of whole and cracked shelled corn are presented. In particular, the activation energy for ground corn was found to be 68,630 J/mol using ASTM E 1641-04. Combustion efficiencies ranged from 93-99% for particles smaller than 5.7 mm and approximately 85% for the whole shelled corn. It should be noted that feed rates were the fixed variable, so excess air was lower for the whole shelled corn. More studies to control the air-fuel ratio are needed. Design strategies for the fluidized bed combustor including bed material and size, available air flow, and air/fuel ratios are discussed.