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PowerPedia:Hydrocarbon

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1 Three types of hydrocarbons
2 The number of hydrogen atoms
3 Petroleum
4 Burning hydrocarbons
5 External articles and references

Hydrocarbons are any chemical compound that consists only of the elements carbon (C) and hydrogen (H). They all contain a carbon backbone, called a carbon skeleton, and have hydrogen atoms attached to that backbone. (Often the term is used as a shortened form of the term aliphatic hydrocarbon.) Most hydrocarbons are combustible. The simplest hydrocarbon is methane (swamp/marsh gas), a hydrocarbon with one carbon atom and four hydrogen atoms: CH4. Ethane is a hydrocarbon (more specifically, an alkane) consisting of two carbon atoms held together with a single bond, each with three hydrogen atoms bonded: C2H6. Propane has three carbon atoms (C3H8) and butane has four carbons (C4H10). Usually carbon backbone is represented as molecular graph in which only carbon atoms are represented as vertices and bonds as edges. Molecular graphs contain the structure of the hydrocarbon in which missing hydrogen atoms can be added in a unique way. Hydrocarbons are extensively studied in mathematical chemistry.

Three types of hydrocarbons

There are essentially three types of hydrocarbons. The first are aromatic hydrocarbons, which have at least one aromatic ring. The second are saturated hydrocarbons, also known as alkanes, which don't have any double, triple or aromatic bonds. The third are unsaturated hydrocarbons, which have one or more double or triple bonds between carbon atoms. The last can be divided into alkenes and alkynes.

The number of hydrogen atoms

The number of hydrogen atoms in hydrocarbons can be determined, if the number of carbon atoms is known, by using these following equations:

Alkanes: C_nH_2n+2
Alkenes: C_nH_2n (assuming only one double bond)
Alkynes: C_nH_2n-2 (assuming only one triple bond)
Cyclic hydrocarbons: C_nH_2n

Each of these hydrocarbons must follow the 4-hydrogen rule which states that all carbon atoms must have the maximum number of hydrogen atoms that it can hold (the limit is four). A carbon atom has a bonding capacity of 4 and therefore must make 4 bonds, whether it be with hydrogen or an adjoining carbon atom. Note, an extra bond removes 2 hydrogen atoms and only saturated hydrocarbons can attain the full four. This is because of the unique positions of the carbon's four electrons.

Petroleum

Liquid geologically-extracted hydrocarbons are referred to as petroleum or mineral oil, while gaseous geologic hydrocarbons are referred to as natural gas. Petroleum (from Greek petra – rock and elaion – oil or Latin oleum – oil ) or crude oil is a black, dark brown or greenish liquid found in porous rock formations in the earth. The American Petroleum Institute, in its Manual of Petroleum Measurement Standards (MPMS), defines it as "a substance, generally liquid, occurring naturally in the earth and composed mainly of mixtures of chemical compounds of carbon and hydrogen with or without other nonmetallic elements such as sulfur, oxygen, and nitrogen." All are significant sources of fuel and raw materials as a feedstock for the production of organic chemicals and are commonly found in the Earth's subsurface using the tools of petroleum geology. The extraction of liquid hydrocarbon fuel from a number of sedimentary basins has been integral to modern energy development.

Petroleum is found in porous rock formations in the upper strata of some areas of the Earth's crust. It consists of a complex mixture of various hydrocarbons, largely of the alkane series, but may vary much in appearance and composition. Petroleum is used mostly, by volume, for producing fuel oil and petrol (gasoline), both important "primary energy" sources (IEA Key World Energy Statistics). Petroleum is also the raw material for many chemical products, including solvents, fertilizers, pesticides, and plastics. 88% of all petroleum extracted is processed as fuel; the other 12% is converted into other materials such as plastic. Since petroleum is a non-renewable resource, many people are worried about peak oil and eventual depletion in the near future. Due to its continual demand and consequent value, oil has been dubbed black gold. The combining form of the word petroleum is petro-, as in petrodiesel (petroleum diesel).

Hydrocarbons are mined from tar sands, oil shale and potentially extracted from sedimentary methane hydrates. These reserves require distillation and upgrading to produce synthetic crude and petroleum. Oil reserves in sedimentary rocks are the principal source of hydrocarbons for the energy, transport and petrochemical industries. Hydrocarbons are of prime economic importance because they encompass the constituents of the major fossil fuels (coal, petroleum, natural gas, etc.) and plastics, paraffin, waxes, solvents and oils. In urban pollution, these components--along with NOx and sunlight--all contribute to the formation of tropospheric ozone. The concentration of hydrocarbon vapours can be harmful if inhaled.

Burning hydrocarbons

Hydrocarbons are one of Earth's most important natural resources. Hydrocarbons are currently the main source of the world’s electric energy and heat sources (such as home heating) because of the energy produced when burnt. Hydrocarbons are all substances with low entropy (meaning they hold a lot of energy potential), which can be released and harnessed by burning them. Often this energy is used directly as heat such as in home heaters, which use either oil or natural gas. The hydrocarbon is burnt and the heat is used to heat water, which is then circulated in pipes around the building heating every room. A similar principle is used to create electric energy in power plants. Hydrocarbons (usually coal) are burnt and the energy released in this way is used to turn water in to steam, which is used to turn a turbine that generates energy. In an ideal reaction the waste would be only water and carbon dioxide but because the coal is not pure or clean there are often many toxic byproducts such as mercury and arsenic. Also, incomplete combustion causes the production of carbon monoxide which is toxic because it will bind with hemoglobin more readily than oxygen, so if it is breathed in, oxygen can not be absorbed, causing suffocation. Incomplete combustion also has a byproduct of carbon in the form of soot.

Flue gas emissions from fossil fuel combustion refers to the combustion product gas resulting from the burning of fossil fuels. Most fossil fuels are combusted with ambient air (as differentiated from combustion with pure oxygen). Since ambient air contains about 79 volume percent gaseous nitrogen (N2), which is essentially non-combustible, the largest part of the flue gas from most fossil fuel combustion is uncombusted nitrogen. The next largest part of the flue gas is carbon dioxide (CO2) which can be as much as 10 to 15 volume percent or more of the flue gas.

A typical flue gas from the combustion of fossil fuels will also contain some very small amounts of nitrogen oxides (NOx), sulfur dioxide (SO2) and particulate matter. The nitrogen oxides are derived from the nitrogen in the ambient air as well as from any nitrogen-containing compounds in the fossil fuel. The sulfur dioxide is derived from any sulfur-containing compounds in the fuels. The particulate is composed of very small particles of solid materials and very small liquid droplets which give flue gases their black, smoky appearance. The steam generators in large power plants and the process furnaces in large refineries and petrochemical plants burn very considerable amounts of fossil fuels and therefore emit large amounts of flue gas to the ambient atmosphere. The table below presents the total amounts of flue gas typically generated by the burning of fossil fuels such as natural gas, fuel oil and coal. The data in the table were obtained by stoichiometric calculations. It is of interest to note that the total amount of flue gas generated by coal combustion is only 10 percent higher than the flue gas generated by natural gas combustion.

EXHAUST FLUE GAS GENERATED BY COMBUSTION OF FOSSIL FUELS (In SI metric units and in USA customary units)

Combustion Data

Fuel Gas

Fuel Oil

Coal

Fuel properties:

Gross caloric value, MJ / Nm³
Gross heating value, Btu / scf
Gross caloric value, MJ / kg
Gross heating value, Btu / gallon
Gross caloric value, MJ / kg
Gross heating value, Btu / pound

Molecular weight
Specific gravity
Gravity, °API
Carbon / hydrogen ratio by weight

weight % carbon
weight % hydrogen
weight % oxygen
weight % sulfur
weight % nitrogen
weight % ash
weight % moisture

Combustion air:
Excess combustion air, %

Wet exhaust flue gas:
Amount of wet exhaust gas, Nm³/ GJ of fuel
Amount of wet exhaust gas, scf / 10⁶ Btu of fuel
CO₂ in wet exhaust gas, volume %
O₂ in wet exhaust gas, volume %
Molecular weight of wet exhaust gas

Dry exhaust flue gas:
Amount of dry exhaust gas, Nm³/GJ of fuel
Amount of dry exhaust gas, scf / 10⁶ Btu of fuel
CO₂ in dry exhaust gas, volume %
O₂ in dry exhaust gas, volume %
Molecular weight of dry exhaust gas

43.01
1,093

294.8
11,600
8.8
2.0
27.7

241.6
9,510
10.8
2.5
29.9

43.50
150,000

0.9626
15.5
8.1

303.1
11,930
12.4
2.6
29.0

269.3
10,600
14.0
2.9
30.4

25.92
11,150

61.2
4.3
7.4
3.9
1.2
12.0
10.0

323.1
12,714
13.7
3.4
29.5

293.6
11,554
15.0
3.7
30.7

Note: Nm³ at 0 °C and 101.325 kPa, and scf at 60 °F and 14.696 psia.

Coal reserves will last for decades and possibly beyond 2100. Mostly in response to climate concerns, clean coal technology is currently under development. For example, the UK and China have signed an agreement to develop such technology with carbon dioxide emissions capture and storage in both China and the EU by 2020. Similar research is being conducted in the U.S. and other countries. Mounting evidence links the use of Hydrocarbons in the form of fossil fuels to environmental pollution and Global Warming.