The following diagram illustrates the process that produces synthetic gypsum from byproducts of energy generation at the Tennessee Valley Authority’s Cumberland fossil plant.
1. Coal is pulverized into a fine powder.
2. To provide the energy that drives the generation of electricity, coal powder is blown into the furnace and burned. During the burning process, the coal is mixed with air and the combustion process releases all of the chemicals as well as the unburned dirt and clay locked up in the coal. Ash is formed from the unburnable portion of the coal.
3. Some of the chemicals, such as sulfur, are released and the unburned dirt and clay are carried through in the flue gas.
4. Fly ash is collected in the precipitators by electric currents that attract the ash to wires.
5. Fly ash falls into hoppers and is removed and stored in silos for use by the cement industry.
6. Limestone rock (or calcium carbonate) is finely ground with water to form a slurry.
7. Limestone slurry is stored so that it can be pumped to the scrubber as needed. Limestone slurry is converted to gypsum slurry in the scrubber.
8. Limestone slurry is delivered as a fine spray near the top of the scrubber where it comes in contact with the flue gases from the furnace. Due to the sulfur content of the coal burned, these flue gases contain sulfur dioxide as a gas. The limestone slurry reacts with the sulfur dioxide in the flue gases to form calcium sulfite initially. Air forced through the system forces the chemical reaction from calcium sulfite to calcium sulfate dihydrate, which is chemically the same as natural rock gypsum. As crystals of calcium sulfate or gypsum form in the scrubber, these heavier crystals sink toward the bottom of the tank where they are continuously pumped out into the effluent slurry tank.
9. The gypsum slurry is pumped through a series of lines to a valve station.
10. At the valve station the gypsum slurry can be routed to storage or to de-watering.
11. Synthetic materials are delivered to a processing plant where the slurry is de-watered on a series of vacuum belt filters from a solution that is about 85% water to a manageable material that is about 5-7% free moisture.
12. This material is then sent to a storage area.