Scientists estimate that approximately 20 feet of decompressed plant matter may have been needed to form one foot of bituminous coal. Bituminous coals or soft coals are the largest group of coals. It is the most widely used type of coal in the world. It has been named so due to the presence of a gluey and tar-like substance known as bitumen in it.
Bituminous coals are created when a sub-bituminous coal is exposed to high pressure and heating. Based on their volatility, bituminous coals are divided into low-volatile bituminous, medium-volatile bituminous and high-volatile A, B and C bituminous.
High grade bituminous coals are found in Appalachian regions of Ohio, Pennsylvania, Kentucky, and West Virginia. The highest production of the coal is in West Virginia, followed by Kentucky and Pennsylvania and Ohio.
Substantial deposits are also found in Colorado, and the midwestern states of Indiana, Illinois, and Missouri. Low-grade bituminous coals are found in Alaska, Kansas, Mexico, Utah, and Wyoming. It has been found that bituminous coal cover about 52 percent of the United States' coal reserves. It is around 100 to 300 million years old.
They have moisture content of about 2 to 17%, carbon content of about 45 to 85%, ash content of about 2 to 14% and contains 0.5 to 2 percent nitrogen. Their sulfur is of about 0.5 to 5 % and chlorine of about 340 ± ppm. The sulfur that is present in the coal brings acid rain.
Some metals like lead and mercury, hydrocarbons like methane, alkanes, alkenes, benzenes and others are also released. Some unburned substances like carbon monoxide and various organic compounds are released even under proper boiler operating conditions.
Coals have heating values of 10,000 to about 15,000 British thermal units per pound (Btu/lb), which is much higher than that of lignite or sub-bituminous coal. Bituminous coals have lower fixed carbon and higher volatile content than that of anthracite.
Bituminous coals have two subtypes; thermal or steam coal and metallurgical coal. Metallurgical or coking coal is baked in ovens in the absence of oxygen at temperatures as high as 1,100 °C (2010°F). This process is called pyrolysis.
By this process, the coal is melted and the volatile constituents and impurities are driven off. Finally, the end product is pure liquified carbon which solidifies into coke.
Mostly, bituminous coal contains a glassy material called vitrinite. It is composed of macerals, which comes from woody plant tissue.
Bituminous coals are used as fuels to produce steam that runs locomotive trains. They were also previously used as a fuel for domestic water heating.
Due to the serious pollution issues from bituminous coals, the electric power plants in the US and some European countries have set up cleaning devices to minimize air pollution emissions or are using sub-bituminous coal that has less amount of sulfur in it. France, on the other hand, uses nuclear power for the generation of electricity.