In 2015, 33.2 percent of the electricity generated in the United States came from coal. This is not surprising, as the United States has more recoverable coal than any other nation.
Coal is made up of compressed pieces of dead plant material that have been subjected to heat and pressure over millions of years. The heat and pressure environment determines what type of coal is formed:
- Lignite (brown coal)
- Bituminous coal (soft coal)
- Anthracite (hard coal)
A typical coal-based power plant produces energy by first burning coal to heat water in boiler tubes. The water becomes steam and is cycled through a steam turbine. The turbine drives a generator shaft to create electricity. Modern coal plants include advanced pollution control technologies to manage the release of trace chemical elements that are released during the combustion process. Coal-based power plants can range in size from less than 50 megawatts (MW) to more than 3,000 MW.
The industry is utilizing emerging technologies to transform how we generate electricity from from traditional fuel sources, such as coal and natural gas. Innovative technologies for generating energy from coal address the twin goals of generating energy efficiently while minimizing emissions. All but the smallest power plants use emission controls to limit nitrogen oxides (NOx). To control sulfur dioxide (SO2), power plants use low-sulfur coal or apply a process called flue gas desulfurization (FGD).
All power plants control particulate matter with fabric filters (baghouses) or electrostatic precipitators. Numerous plants are testing mercury control equipment, as well.
- Carbon Capture and Storage (CCS)
CCS is a process by which carbon dioxide (CO2) is separated from emission sources, transported, and injected into suitable underground geologic locations. The CO2 is captured, or “separated,” from flue gas by means of a chemical or physical process. The captured CO2 is then compressed (i.e., pressurized) in order to change the gas into a liquid. The liquid CO2, also referred to as a supercritical fluid, is denser than in its original gaseous state and is easier to transport by pipeline. CO2 in small volumes also can be transported as a liquid in tanks by ship, road, and rail. The CO2 can then be injected into depleted oil and gas reservoirs or into deep underground saline formations for storage; or, it can be injected into depleting oil reservoirs to extract more oil and then to store the unused CO2.
It is estimated that the United States has an abundance of underground storage capacity, but these potential storage areas are not evenly distributed around the country. Even with a very aggressive research, development, and deployment effort, CCS technologies are not expected to be commercially available until around 2025, with widespread commercial deployment following by five to 10 years. Many carbon capture compression technologies are ready for pilot and small-scale demonstration projects now, and a few large-scale storage demonstrations are underway.
- Integrated Gasification Combined Cycle (IGCC)
Coal gasified by steam and air under high pressure and temperatures produces a synthetic gas (syngas), which is used as a fuel in an IGCC power plant. This technology uses a combined cycle process with a gas turbine driven by the combusted syngas. The exhaust gases are heat-exchanged with water steam to generate superheated steam to drive a steam turbine.
Since most of the electricity is generated from the gas turbine, IGCC shows promise in providing high system efficiencies with ultra-low pollution levels. There are currently two IGCC plants in operation for electricity generation in the United States: the Polk County Gasification Project in Florida, and the Wabash River Coal Gasification plant in Indiana. Additional IGCC plants are under serious consideration.
- Fluidized Bed Combustion (FBC)
FBC is a method of burning coal (and/or other solid fuels), which is fluidized by an upward flow of air, on a bed of either inert material (usually sand) or sorbent material (usually limestone). The velocity of the air is such that the gases and solid produce a "bubbling" or "circulating" mixture. In a bubbling bed design, the velocity is relatively low in order to minimize the solids carryover from the combustor.
Circulating FBCs, however, employ high velocities to promote circulation of the solids. The circulating FBC maintains a continuous, high-volume recycle rate which increases the residence time, compared to the bubbling bed design. Because of this feature, circulating FBCs often achieve higher combustion efficiencies and better sorbent utilization than bubbling bed units.
This technology eliminates the need for external SO2 emission controls, such as FGD, since the limestone serves as an SO2 sorbent during the coal combustion process.
- Advanced Pulverized Coal Combustion
This type of coal combustion is achieved through the use of specially developed high strength alloys, which enable the use of the supercritical and ultra-supercritical steam (high pressures and temperatures) necessary to achieve higher combustion efficiencies—close to 45 percent in some cases.
In the future, the application of new advanced materials to pulverized-coal-based power plants should enable efficiencies of 55 percent to be achieved. Higher efficiencies in combustion reduce CO2 emissions because less coal is used per kilowatt-hour to generate power.
- Low-NOx Burners (LNB)
These burners are designed to reduce the level of NOx released during the combustion of coal, or any fossil fuel. NOx forms from oxidation of nitrogen bound in the coal, and from thermal fixation of atmospheric nitrogen in the combustion flame. The burner technology works by limiting the amount of oxygen in the initial stages of combustion when fuel-bound nitrogen is released, and by reducing the flame temperature to reduce thermal NOx formation. By controlling how the coal is burned ("staged combustion"), NOx emissions are reduced by 37 to 68 percent.
- Selective Catalytic Reduction (SCR)
SCR is another form of NOx control technology. SCR uses ammonia or urea, along with catalysts, in a post-combustion vessel to transform NOx into nitrogen and water. While this technology is capable of reducing NOx to the levels required by government regulations, it is costly and presents its own potential environmental issues (e.g., release of ammonia, a severe respiratory irritant).
- Selective Non-Catalytic Reduction (SNCR)
SNCR is a relatively new form of NOx control technology in coal-based power plants. SNCR is a post-combustion process that converts NOx emissions in the flue gas into elemental nitrogen and water by injecting a nitrogen-based chemical reagent, most commonly urea or ammonia. The most common injection location, due to temperature requirements, is at the top and backpass of the boiler. This technology can typically reduce NOx emissions by 35 to 50 percent without significant impacts on performance. SNCR is currently used in more than 15 utility-scale boilers in the United States and Europe.
- Flue Gas Desulfurization (FGD)
FGD is a chemical process to remove SO2 from flue gas. The goal of this process is to chemically combine the sulfur gases released in coal combustion by reacting them with a sorbent, such as limestone, lime, or ammonia.
As the flue gas comes in contact with the slurry of calcium salts,SO2 reacts with the calcium to form hydrous calcium sulfate (or gypsum). The gypsum by-product can then be used by wallboard manufacturers, making the FGD process economical while also reducing environmental impacts from future gypsum mining.