Under huge pressures and amidst the Earth’s core heat, decayed and decaying organic materials formed complex compounds primarily composed of hydrogen and carbon atoms. These complex, carbon-based compounds are more commonly referred to as fossil fuels. The three most commonly available fossil fuels are coal, petroleum, and natural gas.

Fossil Fuels

Petroleum Products

Most petroleum products, which include petrol, propane, and heating oil, are derived from crude oil drilled from beneath the ground. The energy that derives from petroleum products comes from burning the fuels, or oxidizing the fuels. Carbon dioxide, the molecule responsible for global warming, results from the burning of petroleum sources.

In order to be converted into petrol and other useful fuels, crude petroleum must be cracked, which is a process similar to a distillery where different boiling points of a complex liquid are reached and the constituent materials boil off separately. Petroleum refineries are huge chemical processing plants where crude oil is cracked. Following the cracking process, various fuels and other petroleum products are further separated.

Crude oil products are classified according to their carbon chain lengths. The products with longer chain lengths generally have lower boiling points, which makes them easier to separate during the refinement phase. As the carbon chains increase in complexity, the materials get thicker, or more viscous. The combustion properties also change, in particular the temperature and pressure under which the product burns most efficiently.

Natural Gas

Natural gas is drilled from beneath the surface of the earth, just like petroleum, but it comes in gaseous form instead of liquid or solid. Natural gas is generally favored because it burns cleaner than petroleum products and coal.

Natural gas is a gaseous mixture of hydrocarbon compounds, mostly methane, which is usually found near oil deposits, but sometimes completely independent of crude oil deposits.

In combustion, methane (CH4) combines with oxygen to form water and heat and carbon dioxide.

Coal

Coal is mined from the ground and comes in a solid, blackish, oily form that’s easy to burn but very difficult to burn cleanly. Coal comes in a wide range of qualities and specific heat values (the amount of heat that a given weight of coal can output, during combustion).

Classifications of coal are (from soft to hard) lignite, sub-bituminous, bituminous, and anthracite. The harder coals are more costly than the softer because harder coals have more energy per weight, and the market bears a higher price. As far as which industries use which types of coals, the combustion machines are set up to optimize a certain type of coal, so it’s the machines, not the end uses, that determine which type a given industry uses.

Combustion Processes

Every fossil fuel must be burned in order to extract energy. The only kind of energy that is available through combustion is heat energy. In most combustion processes, however, the end goal isn’t heat, but electrical energy or energy that can be used for transportation. To produce electrical energy, the heat from combustion is used to spin a turbine that creates electricity via generators.

To produce energy for transport, the heat from combustion is translated into mechanical energy, or torque and power on a shaft which is connected to a vehicle’s wheels through a transmission (gears).

A turbine is simply a fan; when air pressure or steam pressure is set against the turbine blades it causes the shaft to spin. A generator is like an electric motor, except operating in reverse. When a generator's shaft is forced to turn, two output wires provide electrical power.

Hydrogen and carbon content in fossil fuels

All fossil fuels are composed of hydrogen and carbon, hence the term hydrocarbons to refer to these energy sources. The vast proportion of the usable energy comes from hydrogen, whereas the carbon generates the vast majority of the waste.

When carbon burns completely in an oxygen atmosphere, the product is a lot of carbon dioxide (CO2), the gas for global warming. Other byproducts of fossil fuel combustion are carbon monoxide (CO), nitrogen oxides (NOx), sulfur oxides (SOx), and various particulates like mercury that cause undue environmental harm. In general, the less carbon a molecule contains (in relation to the amount of hydrogen), the cleaner the burning process.

Efficiency limitations of fossil fuels

The driving force of any heat engine (machine used to derive usable power from the heat of combustion) is a temperature differential. According to Carnot’s law, the efficiency of any combustion process is proportional to the difference in temperature between the heat source (the combustion chamber) and the heat sink (the exhaust environment).

In other words, you want to burn the fuel at as high a temperature as possible in order to achieve the most efficiency. You can also increase efficiency by lowering the temperature of the exhaust environment as much as possible as well, but in most cases this is not a practical variable.

Liquid petroleum gas (LPG)

LPG is similar to natural gas, but it is liquefied, making distribution and use much easier and less expensive. LPG consists of a group of hydrogen rich gases - propane, butane, ethane, ethylene, propylene, butylene, isobutylene, and isobutane. It’s derived from a refining process from either crude oil or natural gas. The most commonly available form of LPG is propane.

Utility Scale Power Plants

At the heart of every thermal power plant is the combustion of fossil fuel energy sources. Coal is by far the most prevalent type of power plant in India. In all cases (including nuclear), heat is generated and that is used to boil water (or other liquid), which in turn is used to rotate a turbine or generator.

Coal is mined, then cleaned and degassed, then transported to the power plants. Coal is loaded into the hopper which feeds the raw fuel through a pulverizer that turns the hard chunks into a fine dust. A blower forces this dust into the combustion chamber (along with the prescribed amount of oxygen) where it burns at a carefully controlled temperature and pressure. The resulting heat converts liquid water into a very high energy steam, which in turn spins a large turbine connected to an electrical generator. The steam is condensed back into liquid water form, which re-circulates through the system via a feed pump.