Today about half of the energy produced in the U.S. is generated from burning coal. As a result, several byproducts have received increased attention from the public. These include SO2, SO3, mercury, CO2, and flyash.
SO2 is removed from the gas put off by burning coal, called flue gas, by a process called flue gas desulfurization. This process effectively removes about 95-99.9% of SO2 from the flue gas.
SO3 is a nuisance because it turns the smoke (mostly water) from the stack a blue color. This is usually a concern for the public because a dark or blue-tinted smoke emission is equated with increased pollution. SO3 is removed by injecting powdered limestone into the flue gas stream. Again, this is very, very effective.
Mercury naturally occurs in coal, which is released as a gas when burned. This is very difficult to remove from the flue gas, and no suitable, effective method has been developed yet.
The same is true for CO2. CO2 is tough to remove from the flue gas, and no method that has been developed is both practical and not cost-prohibitive.
Flyash is a byproduct of burning coal that can be captured very effectively by Electrostatic Precipitators (ESP). The problem is that you still have the flyash to get rid of.
Flyash is made of many chemicals, mainly silicon dioxide, which is present in 2 forms – Amorphous and Crystalline. Certain environmental toxins are also present in fly ash, such as arsenic, barium, beryllium boron, chromium, lead, manganese, and other heavy metals 1. Nevertheless, the construction industry considers flyash one of the most important and advantageous waste materials from burning coal. Nearly 30% of the coal is converted into ash, of which 75% is in fine fly ash form and the remaining 25% is in the form of coarse bottom ash1. But there are many uses for flyash – mainly as an additive to concrete, for which it was widely used prior to 2008.
What happened in 2008 you ask? Well, on Dec. 22, 2008, a containment dike ruptured at Kingston Fossil Plant near Kingston, Tenn., and sent 1.1 billion gallons (4.2 billion L) of coal flyash slurry over 300 acres (122 hectares) of surrounding land, damaging homes, and flowing into nearby rivers. This spill was the most significant flyash release in U.S. history. Cleanup costs were estimated to run between $525 million and $825 million, not including potential long-term cleanup. Before the Tennessee spill, the EPA categorized flyash as “special waste,” meaning it was exempt from federal hazardous waste regulations under Subtitle C of the Resource Conservation and Recovery Act2.
Because of the spill and increasing public pressure, the EPA considered relabeling flyash as a Hazardous Waste. This would have cost the industry billions of dollars because, as we know, Hazardous Waste cannot be treated the same as regular waste. Before 2008, flyash was put into settling ponds using water as the medium to transport the flyash through pipes (sluicing) and then recovering the water for reuse. After 2008, flyash settling ponds pretty much went away. The EPA came out with a new ruling in April 2015 that limited the use of flyash settling ponds, and basically, most plants are closing them down or have already done so3. Luckily, the EPA did not label flyash as hazardous waste; therefore, it is still used for making cement and can also be landfilled.
The biggest issue facing the coal power industry is the perception that coal is bad for the environment and that the byproducts of burning coal increase greenhouse gas emissions. To some extent, the industry is fighting back with new technologies that clean up the flue gas emissions and help explain to the public the vital role coal power plants play in maintaining grid resiliency. The industry, if it is to survive, must continue this fight. Unfortunately, Flyash is one of the things that gets a bad rap. It is not a harmful byproduct of burning coal. On the contrary, it is a byproduct that can be used in other ways that benefit both the environment and construction costs.