For the majority of TVA's history, coal played a major role in the generation of electricity. As recently as 2007, almost 60% of all power generated by TVA came from coal-fired plants. Today, coal represents about 20% of our generation mix. This percentage will drop further with the retirement of Paradise Fossil Plant in 2020 and Bull Run Fossil Plant in 2023.
The process of burning coal generates coal combustion residuals (CCR), commonly known as coal ash. Safely managing this legacy of decades of coal generation is a responsibility TVA takes seriously.
Coal combustion residuals is a term the utility industry and government uses to describe all the components of coal ash. It is regulated by the Environmental Protection Agency, which has determined that CCR byproducts are non-hazardous. TVA also conducts environmental studies of CCR disposal sites under the oversight of the Tennessee Department of Environment and Conservation (TDEC) Order.
This graphic illustrates how each CCR component is created in the coal combustion process. The components that make up CCR are described below.
Bottom ash, about the size of beach sand, is too heavy to be carried by the flue gasses. It is collected in a hopper at the bottom of the boiler furnace. Bottom ash is commonly used as construction filler for embankments, road bases and pavement. It is also used in cement and many cities put it out on winter roads for traction as a more eco-friendly alternative to salt.
Note: Cyclone furnaces, such as Paradise Unit 3, are quenched with water, which generates boiler slag – smooth pellets that have a glassy appearance. Boiler slag is commonly used in asphalt roof shingles.
Fly ash, which looks and feels like talcum powder, travels with the flue gasses until it is captured by electrostatic precipitators.
Fly ash is mixed with concrete to build homes, stores, bridges, skyscrapers, roads, and dams. Anywhere concrete is used, you’ll find fly ash. It improves concrete’s superior strength and longevity. Builders use 40-70 percent fly ash mixes depending on the strength needed. The higher percentage of ash, the stronger the concrete.
Prior to the exhaust going up the stacks, a limestone slurry is sprayed into the flue gases, “scrubbing” out the sulfur and oxides. This process removes 95 percent of the sulfur dioxide (SO2) from the exhaust. Synthetic gypsum is a byproduct of this chemical reaction. The gypsum is very high-quality and is used in more than 40 percent of wallboard manufactured in the U.S. It is used in self-leveling floor compounds, glass manufacturing and farmers even use it to improve soil quality.