In the United States, 52% of the electricity comes from coal generation.
A typical coal fired power plant, like the one in this picture, heats water to 540 deg C to produce high pressure steam.
To produce 109 kWh/year of power, the plant burns 14,000 tons of coal every day.
Preparation of the Fuel
The coal that goes to a typical coal-fired power plant has minimal processing. It is crushed to approximately 5 cm in size before it is loaded into the inlet hopper.
Coal contains many impurities. Coal in the midwest typically contains a lot of sulfur. The pyritic sulfur, from the mineral FeS, is relatively easy to remove. The coal can be ground and washed with water. The heavier FeS particles fall to the bottom and can be separated from the floating coal particals.
The coal is combined in air and ignited in the boiler. The carbon is oxidized to CO and CO2. Nitrogen and sulfur contaminants are oxidized to their highest oxides. Mercury is volatilized. Other minerals are left in the ash.
Power plants use water for cooling. Heat from the combustion process is absorbed by water. The high pressure steam that results drives a boiler. Because of evaporation, the water returned to the environment has a higher concentration of impurities.
A coal-fired power plant emits roughly twice the amount of carbon dioxide for the energy produced as a natural gas-fired power plant. Burning coal releases 2,249 lbs of carbon dioxide, 13 lbs of SO2 and 6 lbs of NOx per MWh.
Coal fired power plants are responsible for 18 % of the total nitrogen oxides released by people in the US.
Important nitrogen oxides include:
(a) Nitric oxide (NO) results from the combination of O2 and N2 in lightning strikes and it is produced by the combustion of nitrogen compounds in coal. It is a radical and so is very reactive in the atmosphere. Its unpaired electron can be removed through oxidation to NO+ or the molecule can be reduced to NO-.
(b) Nitrogen dioxide (NO2) is a brown gas, responsible for the color of photochemical smog. It is a radical and so is reactive in the atmosphere. Nitrogen dioxide is in equilibrium with its colorless dimer, N2O4.
(c) Dinitrogen tetraoxide (N2O4) has a very weak N-N bond and is in equilibrium with NO2. It is a diamagnetic substance. It reacts with water to form nitrous and nitric acids and it oxidizes many metals.
(d) Other neutral nitrogen oxides include N2O5 and N2O3.
(e) Nitrogen trioxide (NO3) is a radical with an unpaired electron on one of the oxygen atoms. Its reactivity is similar to that of the hydroxy radical (HO).
The neutral nitrogen oxides are commonly grouped as NOx. These are toxic to humans and other organisms and are important in the production of ground-level ozone and photochemical smog. The scheme below summarizes the reactions of nitrogen oxides and the formation of nitric acid in the atmosphere.
Burning coal produces 59 % of the sulfur oxides (and sulfuric acid) released in the US. The initial product is SO2. This is further oxidized and hydrolyzed in the atmosphere.
Sulfuric acid from coal power plants is the primary source of acid rain.
Metals in the Exhaust
Coal-fired power plants are the largest source of human-caused mercury emissions in the U.S. (43 tons of mercury per year). Mercury is toxic to human, particularly to young children.
Power Plants in Illinois
Coal is important to Illinois. Coal underlies 37,000 square miles of Illinois or about 65 percent of the state's surface. Our coal resources hold more BTUs (British thermal units) than all of Saudi Arabia's and Kuwait's oil reserves combined. Most of the coal (90 %) is used to generate electricity. Almost half of the electricity used in this state comes from coal. Coal makes up nearly 85% of the U.S. fuel resources and, at present rates of consumption, should last for more than 250 years.