Thermogravimetry as a Tool for Determining Combustion Efficiency and Calcium Utilization of a Fluidized Bed Combustion Furnace

Coal, the most abundant natural fossil fuel, is projected to be a major source of energy well into the next century. Because of this, much experimentation an development has been directed toward using this fossil fuel effectively. Coal's disadvantage is that much of it is high in sulfur. Consequently, its use in fossil fuel power plants poses a major emissions problem.

This work was performed to learn how thermogravimetry can be used to determine the combustion and sulfur scrubbing efficiency of a fluidized bed combustion furnace used in these power plants.

In this furnace design, a fuel mixture of coal and limestone is used. The limestone is added to remove the sulfur through an in situ scrubbing technique. The fluidized bed design employs high pressure air to both feed the fuel and provide oxygen for combustion. This method causes the fuel to levitate, essentially simulating a fluid mass. This levitation of the fuel mixture provides for more efficient combustion and also enhances the reaction of the calcium oxide with the sulfur oxides that evolve from the coal. Analysis of both the fly ash and bed ash resulting from the combustion should provide a measure of the effectiveness of sulfur oxide removal and combustion efficiency.

The Perkin-Elmer TGA 7 [1] was employed to analyze these ashes for unburned fuel and unreacted calcium oxide. These analyses were performed using three different atmospheres—nitrogen, oxygen and carbon dioxide. Nitrogen was used to remove water and any calcium carbonate. Oxygen was used to oxidize and thereby determine the amount of any unburned fuel. Carbon dioxide was used to recombine calcium carbonate from any unreacted calcium oxide and thereby determine the calcium oxide content.