Fossil fuels: oil and coal
Oil, or petrol, together with coal and natural gas take place in the deposit of biogenic origin existing in the crust of the earth.
Petrol is a fossil fuel, of organic nature, known and used since ancient times. The products obtained from refining petrol have become indispensable to the modern society. It is considered to be as valuable as gold, petrol being gold the “black gold”.
Oil is found in tightly sealed deposits deep underground. Because of the gases that gather in the deposits the pressure rises, thus being possible to extract petrol either through natural eruption due to the very big pressure inside the deposit or by mechanic pumping out of the deposit.
Oil is a complex mixture that is different from one deposit to another. The main classes of substances found in oil are: hydrocarbons, organic compounds made of oxygen, nitrogen or sulphite and some metals.
Petrol is a thick liquid whose colour varies from yellow to black. It has a certain smell and is immiscible in water.
Raw petrol is extracted from deposits and is transported using trains or ships (special freighters called petrol tanks) to the refineries. There, in special installations it is separated into its components, meaning it is refined. The process that is the base of refining is distillation.
The products obtained from refining petrol are transported, the easiest and fastest way being through high capacity tubes towards centres where they are either sold as fuels or refined again.
Almost all oils burn in air generating heat, which can be directly used, or converted into other forms of energy. Oils can be used as fuels for heating, lighting, powering combustion energies etc.
Coal is a fossil fuel formed in swamp ecosystems, where plant remains were saved by water and mud from oxidization and biodegradation. It is a sedimentary rock, formed by carbonization (in lack of oxygen) of plant remnants from geological epochs. The internal condensation of prehistoric plants took place millions of years ago, through two important processes:
Biochemical phase produced by bacteria and funguses which converts the cellulose and the lignin from plants
Geochemical phase produced at high temperatures and pressures, resulting in a long time the bituminous coal and the anthracite
Coal as fuel
Coal is primarily used as a solid fuel to produce electricity and heat through combustion. World coal consumption is about 6.2 billion tons annually, of which about 75% is used for the production of electricity.
When coal is used for electricity generation, it is usually pulverized and then burned in a furnace with a boiler. The furnace heat converts boiler water to steam, which is then used to spin turbines which turn generators and create electricity.
Approximately 40% of the world electricity production uses coal. The total known deposits recoverable by current technologies, including highly polluting, low energy content types of coal (i.e., lignite, bituminous), might be sufficient for 300 years' use at current consumption levels, although maximal production could be reached within decades.
A more energy-efficient way of using coal for electricity production would be via solid-oxide fuel cells or molten-carbonate fuel cells (or any oxygen ion transport based fuel cells that do not discriminate between fuels, as long as they consume oxygen), which would be able to get 60%–85% combined efficiency (direct electricity + waste heat steam turbine). Currently these fuel cell technologies can only process gaseous fuels, and they are also sensitive to sulfur poisoning. As far as gaseous fuels go, one idea is pulverized coal in a gas carrier, such as nitrogen. Another option is coal gasification with water, which may lower fuel cell voltage by introducing oxygen to the fuel side of the electrolyte, but may also greatly simplify carbon sequestration.
Coal dust is a fine powdered form of coal. Because of the brittle nature of coal, coal dust can be created during mining, transportation, or by mechanically handling coal.
The risks are that coal dust suspended in air is explosive. Coal workers' Pneumoconiosis, or black lung disease, is caused by inhaling coal dust, typically dust produced in coal mining.
High prices of oil and natural gas are leading to increased interest in "BTU Conversion" technologies such as gasification, methanation and liquefaction. Coal gasification breaks down the coal into its components, usually by subjecting it to high temperature and pressure, using steam and measured amounts of oxygen. This leads to the production of syngas, a mixture mainly consisting of carbon monoxide (CO) and hydrogen (H2).
Gasification is also a possibility for future energy use, as the produced syngas can be cleaned-up relatively easily leading to cleaner burning than burning coal directly (the conventional way). The cleanliness of the cleaned-up syngas is comparable to natural gas enabling to burn it in a more efficient gas turbine rather than in a boiler used to drive a steam turbine. Syngas produced by gasification can be CO-shifted meaning that the combustible CO in the syngas is transferred into carbon dioxide (CO2) using water as a reactant. The CO-shift reaction also produces an amount of combustible hydrogen (H2) equal to the amount of CO converted into CO2. The CO2 concentrations (or rather CO2 partial pressures) obtained by using coal gasification followed by a CO-shift reaction are much higher than in case of direct combustion of coal in air (which is mostly nitrogen). These higher concentrations of carbon dioxide make carbon capture and storage much more economical than it otherwise would be.
Liquefaction - Coal-To-Liquids (CTL)
Coals can also be converted into liquid fuels like gasoline or diesel by several different processes. Coal would be gasified to make syngas (a balanced purified mixture of CO and H2 gas) and the syngas condensed using Fischer-Tropsch catalysts to make light hydrocarbons which are further processed into gasoline and diesel. Syngas can also be converted to methanol, which can be used as a fuel, fuel additive, or further processed into gasoline via the Mobil M-gas process.
All of the liquid fuel production methods release carbon dioxide (CO2) in the conversion process, far more than is released in the extraction and refinement of liquid fuel production from petroleum. If these methods were adopted to replace declining petroleum supplies, carbon dioxide emissions would be greatly increased on a global scale.
Environmental effects of coal
release of carbon dioxide and methane, both of which are greenhouse gas
waste products including Uranium, Thorium, and other heavy metals
interference with groundwater and water table levels
impact of water use on flows of rivers and consequential impact on other land-uses
subsidence above tunnels, sometimes damaging infrastructure
rendering land unfit for the other uses.
In 2006, China was the top producer of coal with 38% share followed by the USA and India.
At the end of 2006 the recoverable coal reserves amounted around 800 or 900 gigatonnes. The United States Energy Information Administration gives world reserves as 998 billion short tons (equal to 905 gigatonnes), approximately half of it being hard coal. At the current production rate, this would last 164 years. At the current global total energy consumption of 15 terawatt, there is enough coal to provide the entire planet with all of its energy for 57 years.