What is Coal?
Coal is a combustible rock--a rock that burns. It is composed mostly of carbon and hydrocarbons. (A hydrocarbon is a molecule consisting of some combination of carbon and hydrogen, such as methane, CH4).
Coal is a fossil fuel, which means it was created over millions of years from dead plants trapped under layers of earth. The heat and pressure turned the plant remains into what we call coal today. Petroleum and natural gas are also fossil fuels, formed in similar ways.
Fossil Fuels and the Carbon Cycle
A fundamental point to realize about all fossil fuels is that the energy we release by burning them came originally from the sun. How's that?
Plants grow as a result of photosynthesis, a process where carbon dioxide (CO2), water (H2O), and energy from the sun combine to create simple sugars, such as glucose (C6H12O6), and oxygen (O2). Photosynthesis is an endothermic chemical reaction (meaning that it requires the net input of energy to occur). The sun provides this necessary energy, which is used to create chemical bonds. The simple sugars created in photosynthesis may later be converted into other types of molecules that make up all the "matter/stuff" of a plant, including specialized carbohydrates, such as cellulose. (Source: Virtual Chembook, Elmhurst College, 2003, retrieved August, 2011).
Over millions (and often hundreds of millions) of years, heat and pressure causes the chemistry of the dead plants to change somewhat, and some carbon dioxide and oxygen are released, but the energy from the sunlight is generally retained. So, we can think of coal as a bundle of carbon and hydrocarbon molecules held together by bonds that were formed from the sun's energy millions of years ago. It is this very energy that makes coal so useful to us now.
To release this energy, we burn the coal. This is an exothermic chemical process called combustion. It releases energy stored in the chemical bonds that hold the molecules together. Remember Smokey the Bear? (He's still around right? Did I just date myself? Moving on...) The fire triangle has three necessary components for combustion (fire) to begin: fuel, oxygen, and heat. Once the fire gets started, a chain reaction takes over between the hydrocarbons in the fuel and available oxygen. Some energy is used to break the bonds in the fuel, but even more energy is released when the new bonds form with the oxygen. Overall, the reaction is exothermic--energy is released. In complete combustion of a pure hydrocarbon, the hydrocarbon is converted to carbon dioxide (CO2), water vapor (H2O), and heat (and light). Note that hydrocarbons usually have impurities (e.g. nitrogen, sulfur, mercury), and thus other byproducts usually result from the combustion reaction.
See the reaction below for complete combustion of a hydrocarbon, and the reaction for complete combustion of methane. (Methane is a hydrocarbon composed of one carbon and four hydrogen atoms). Note that the "extra" heat energy released as a byproduct provides heat for the continued combustion process. Combustion will continue to occur until either the heat, fuel source, or oxygen is insufficient to continue the reaction.
Please note that the reactions above describe complete combustion, which means that all of the fuel is completely converted to the given byproducts. In reality, this process is rarely so simple. When incomplete combustion occurs, other byproducts such as carbon monoxide (a silent, tasteless, and odorless deadly gas) and carbon (e.g. soot) result. In addition, there are often impurities in the hydrocarbon that result in additional byproducts. For example, a lot of coal contains traces of sulfur, which forms sulfur dioxide (SO2) when combusted. Sulfur dioxide emissions from power plants are proven to cause so-called "acid rain," which became a major nonmarket issue in the 1980's in the U.S. Coal also often contains traces of mercury, which is released when combustion occurs. Coal combustion is the second leading source of mercury pollution worldwide (just a little bit behind artisanal and small scale gold mining), and mercury is a major human health hazard. Interestingly, the chemical content of the air used in the combustion reaction can be a problem as well. Our atmosphere is mostly nitrogen, and a byproduct of combustion with air will be nitrogen dioxide (NO2). In short, the products of combustion depend on the specifics of all the compounds involved in the reactions, and the combustion of coal nearly always results in unwanted byproducts.
While we're on this topic, another interesting consideration is the amount of greenhouse gases formed during the combustion process. When we burn a fuel, a reaction takes place between a hydrocarbon and oxygen that yields carbon dioxide and water. When we burn one pound of coal, we produce about two and half pounds of CO2. How can that be?
The atomic weight of carbon is 12 and oxygen is 16 (grams per mole), giving carbon dioxide a total molecular weight of 44. So, each atom of carbon results in 3.7 times its weight in CO2. (44/12 = 3.7)
The typical carbon content for coal ranges from more than 60 percent for lignite to more than 80 percent for anthracite, according to the EIA. Let's consider coal that is around 70% carbon. One pound of this coal results in about (0.70 lb carbon/lb coal) x (3.7 lb CO2/lb carbon) = 2.6 pounds of CO2.