Scrubbers and Environmental Controls
Appalachian Power parent company American Electric Power (AEP) has invested more than $5 billion since 2004 to retrofit a number of its coal-fired power plants with environmental controls. About $2 billion of that is within Appalachian Power. The effort is aimed at reducing nitrogen oxide (NOx) and sulfur dioxide (SO2) emissions, created when coal is burned in the process of generating electricity.
For Appalachian Power, the most significant environmental control construction efforts were:
- A flue gas desulfurization (FGD) system, commonly called a scrubber, at the 1,300-megawatt Mountaineer Plant in Mason County, W.Va.; and
- FGD systems on all three generating units at the 2,900 megawatt John E. Amos Plant in Putnam County, W.Va.
AEP and Appalachian Power also have invested in the mid-range NOx-reduction technology known as a selective non-catalytic reduction system (SNCR), as well as selective catalytic reduction (SCR) systems to reduce NOx emissions.
Why Environmental Controls Are Needed
Approximately 98 percent of electricity produced in our region is generated at plants fueled by coal. Appalachian Power uses coal to generate about 70 percent of the power it produces. Coal is abundantly available from local sources, and its low cost relative to other fuel options enables companies to provide electricity at reasonable prices. When coal burns, particles of ash and various gases are emitted. These flue gas emissions include NOx and SO2, formed when nitrogen and sulfur in the coal combine with oxygen during combustion.
Types of Environmental Control Equipment
Watch a video and read below to learn about the different types of power plant environmental control equipment found at many AEP and Appalachian Power plants.
- Low-NOx burners control the way coal burns to limit formation of NOx, effectively reducing emissions by up to 60 percent.
- Selective catalytic reduction systems (SCRs) remove most NOx emissions from treated flue gas. These systems use ammonia, manufactured on-site as needed, in a chemical reaction to covert the NOx to harmless nitrogen and water. View a fact sheet on SCRs (PDF).
- Selective non-catalytic reduction systems (SNCRs) provide mid-range removal of NOx from emissions. An SNCR system injects urea solution to create a chemical reaction that converts NOx into nitrogen and water. View a fact sheet on SNCRs (PDF).
- Electrostatic precipitators (ESPs) remove 99 percent of the fly ash from power plant emissions. In the ESP, fly ash receives an electric charge. The charge attracts the particles to a collector plate, effectively preventing the ash particles from being discharged into the air.
- Flue gas desulfurization systems (FGD systems or scrubbers) remove most of the SO2 emissions from treated flue gas. Lime, crushed limestone or other chemical reagents are mixed with water and sprayed into the boiler flue gas containing SO2. A chemical reaction then forms calcium sulfate or gypsum crystals. View a scrubber fact sheet (PDF), or view a fact sheet on gypsum, a scrubber by-product (PDF).
Scrubbers in Operation: White Cloud Cleaner Air
Visually, a coal-fired generating plant without a scrubber may appear cleaner than one with a scrubber. Emissions from a non-scrubbed unit rise almost invisibly from the stack, along with hot flue gas, high into the atmosphere. In contrast, a dense, white plume billows from the stack of a unit fitted with a scrubber.
While more visible, the plume from a scrubbed unit is made up mostly of water vapor, and contains lower concentrations of pollutants than the nearly invisible flue gas stream of non-scrubbed units.
The scrubber captures SO2 by spraying a water and limestone mix into the flue gas stream. In the process, the hot flue gas turns some of the water into vapor, and the vapor rises from the stack in a billowy, white cloud. Meanwhile, the limestone neutralizes the SO2, forming gypsum. This harmless substance then is sent to a landfill.
The scrubber captures almost all of the SO2 produced from burning coal. That makes our air cleaner. It also gives plants the flexibility to use locally available high-sulfur coal, which helps keep fuel costs low.