ABSTRACT
There are three major groups of fossil fuels: coal, oil, and natural gas, which were formed over millions of years� The age at which they were formed is described as the carboniferous period, which is named after carbon, the main constituent of fossil fuels� It is accepted that as living matter died, it sank to the bottom of swamps, lakes, and seas, which then formed spongy material, after being covered by sand and other mineral matter to turn in time into a rock type described as sedimentary� With the accumulation of more rocky material and the combined action of temperature, pressure, catalysts, and bacteria over a long time, coal, oil, and natural gas were formed�
The major fraction of commonly used organic fuels is hydrocarbons� These are by definition compounds that are made up of hydrogen and carbon only� The carbon atom is considered to be relatively flexible in its association with other atoms, especially those of hydrogen� The carbon atom has four electrons in its outer shell that share four more electrons to make up to eight electrons, to form compounds, that is, with a valence of four� The simplest of these are the open chain normal alkane paraffinic compounds having the general formula, CnH2n+2�
Some hydrocarbon compounds are of the type in which some of the carbon atoms are unsaturated, sharing electrons with other similar carbon atoms� Both saturated and unsaturated hydrocarbon fuels can also be found in a ring type format� Another form contributing to the proliferation of these member fuels results from the different arrangements of the carbon and hydrogen atoms within basically the same molecule to form a very large number of isomers� These are of the same compound and have significantly different chemical properties� However, the corresponding differences in their physical properties often tend to be much less�
3.2.1 Paraffinic Series, CnH2n+2 (Saturated, All with Single “C” Bonds)
All these compounds have names ending with “-ane,” added to a part that identifies the number of carbon atoms� The most common of these, as shown in Table 3�1, include the simplest member methane, the main component of natural gas and most biogases, as well as propane and butane, the main components of liquefied petroleum gases (LPG)� These are usually gaseous at ambient conditions� Higher members such as pentane, hexane, heptane, and octane are liquid, whereas those of much higher carbon members tend to become solid, such as in paraffin waxes (Table 3�2)�
n-Hexane: The three-dimensional arrangement of the atoms within the molecules increases in complexity as the number of carbon atoms is increased� However, for simplicity, these are represented in a planar form� Also, when written, the hydrogen atoms may not be written in full, as shown for normal hexane earlier�
Hydrocarbon molecules may lose a hydrogen atom to form an atomic reactive complex that is unstable on its own and is known as “radical,” which is usually given the general symbol “R�” These radicals will combine with different atoms or other radicals to form completely new compounds with different properties� Radicals are named with a prefix indicating the parent compound and ending with “-yl�” For example, methyl is the radical derived from methane, isopentyl from isopentane, and hexyl from n-hexane�
3.2.2 Olefin Series, CnH2n (Unsaturated with One Double Bond between Two C Atoms)
All olefins have names ending with “-ene” but may also end with “-ylene,” such as propene or propylene, C3H6� The olefins, mainly as a result of their
double bond, tend to be less stable than normal paraffins, chemically reactive compounds, and have good combustion characteristics� They will unite somewhat readily with hydrogen to form the corresponding normal paraffin compound� Olefins with two double bonds are described as diolefins� These unsaturated open-chain compounds have the general formula CnH2n−2 and tend to have less desirable properties as fuels�
3.2.3 Acetylene Series, CnH2n−2 (Unsaturated with One Triple Bond between Two C Atoms)
These are highly reactive compounds and mostly synthetic fuels� An important member of this group is acetylene, C2H2, which is widely burned with oxygen to provide very high temperature flames that are employed widely in metal welding and cutting� They are also important raw materials in the manufacture of a wide range of chemicals�
3.2.4 Naphthenes or Cycloparaffins, CnH2n (Closed Chain with Single and Saturated Bonds)
The molecular structure of naphthenes or cycloparaffins is in the form of a closed chain and all carbon bonds are single and saturated, so that, although they have the same general formula as the olefins, they are rather more similar to the corresponding paraffins� Hence, they are also known as cyclic paraffins� Their physical properties tend to be close to those of the normal paraffins, but their chemical properties and hence their combustion properties are closer to those of isoparaffins�
H ─ C ≡ C ─ H
3.2.5 Aromatics, CnH2n−6 (Unsaturated Ring Compounds)
Aromatics are a group of unsaturated ring hydrocarbon compounds having the symbol CnH2n−6� Benzene is a common aromatic member, C6H6� Although unsaturated, the double bonds alternate in position between the carbon atoms, making them more stable than other unsaturated compounds� Thus, they strongly resist autoignition�
A wide range of compounds are formed by the attachment of groups to the benzene ring or by having the attachment of multiple rings to each other, such as in naphthalene� Aromatics are considered to be toxic and potentially cancer causing� They are being increasingly restricted for their use as fuels or their presence even in low concentrations in the products of combustion� The deprivation from a benzene ring of one of its hydrogen atoms results in the formation of a phenyl radical� For example, toluene is formed by having a benzene molecule with one of its hydrogen atoms replaced by a methyl radical�
Isomer compounds: Isomers are compounds that have the same number of C and H atoms but with different molecular structural arrangements� Usually, the physical properties of isomers of the same parent compound tend to be somewhat similar, but their chemical behavior and associated combustion properties can be substantially very different� In organic compounds, the molecular structure is much more important in determining the characteristics of a substance than the actual number of its H and C atoms�
In the paraffin series when the structure is a simple straight chain, it is described as being normal and given a prefix n-such as n-pentane� However, for the more complex branched structure of an isomer, it is prefixed by iso�
Because often numerous isomers can be formed out of one parent compound, a specific isomer needs to be unambiguously identified� This is done
by identifying the number, location, and name of the attached groups in the branched chain� For example, the iso-octane shown below is identified as 2,2,4-trimethyl pentane indicating that there are three branches of methyl group radicals attached to a pentane structure� Two of the radicals are at the location of the second carbon atom and the third is at the fourth carbon location� Numerous other isomers can be formed as it often takes place during the course of chemical reactions and in refining� The larger the size of a fuel molecule the greater is the number of its possible isomers�
Of course, there are enormously large numbers of a variety of organic compounds that may be considered as fuels besides hydrocarbons� An important set of these are those having oxygen in combination with carbon and hydrogen� Other elements such as nitrogen, sulfur, chlorine, phosphorous, silicon, and various metals form their own corresponding fuel families� They represent truly diversified and incredibly large number of compounds� The main groups of oxygenated hydrocarbon compounds associated with fuels are the following:
1� Alcohols, R – O – H, for example, methyl alcohol, CH3OH
2� Aldehydes, R – C – H|| O
,
for example, formaldehyde, HCHO 3� Peroxides, R – O – O – H,
for example, methyl peroxide, CH3OOH
4� Ethers, R – O – R′, for example, methyl-ethyl ether, CH3OC2H5
5� Acids, R – C – O – H|| O
,
for example, formic acid, HCOOH
6� Ketones, R – C – R′|| O
,
for example, methyl-ethyl ketone, CH3COC2H5
The chemical structure of organic fuels influences greatly both their chemical and physical properties� Hence, these control their suitability as fuels for certain applications and combustion devices� Examples of such properties are as follows:
• Volatility characteristics and associated boiling points (these determine whether the fuel is to be gas, liquid, or solid at ambient conditions)
• Heating value (the specific combustion energy release reckoned on either mass or volume bases)
• Ignition temperatures (the minimum temperatures needed for selfignition in the presence of an adequate supply of external energy source for ignition)
• Flame speed (the propagation rate of the reaction front within the fuel-oxidant mixture and its variation with concentration, temperature, and pressure)
• Chemical stability, explosiveness, decomposition, and reactiveness • Formation of carbon deposits and gumming • Compatibility with other materials
There are some serious limiting factors that have restricting influence on the availability, suitability, and wide-scale exploitation of various fuel resources� These factors may include the following:
• The extent of lack of abundance of fuel supplies and associated costs • Ecological and resource depletion impacts • The distribution and requirements of infrastructure • Safety and associated health concerns • The need for exploration and reliable forecasting of the fuel resource • Numerous social and political potentially influencing factors
However, through much research and development efforts, the impact of these limitations on the fuel and energy fields is being increasingly managed and focused�
Practical fuels are usually made up of complex mixtures of common components that vary widely in complexity and concentration� Such variations influence the values of the physical and chemical properties of the fuel and its suitability for any specific application� The properties change with the size of the molecule and the carbon to hydrogen ratio� Table 3�3 shows the overall C/H mass ratio for a number of common fuels�
1� What do you consider to be the main desirable features of common fossil fuels that helped to make them attractive and used extensively over recent decades?
