Power Stations
BY
DR. MOSTAFA A. H. ABDELMOHIMEN
Mechanical Engineering Department
College of Engineering
King Khalid University, Saudi Arabia
Chapter 4: Fuel and Combustion
Classification Of Fuels
Fuels can be classified according to whether :
1. They occur in nature called primary fuels or are prepared called secondary fuels ;
2. They are in solid, liquid or gaseous state. The detailed classification of fuels can be given in a summary form as follows :
Solid Fuels
➢ Coal. Its main constituents are carbon, hydrogen, oxygen, nitrogen, sulphur, moisture and ash. Coal passes through different
stages during its formation from vegetation. These stages are enumerated and discussed below :
Plant debris—Peat—Lignite—Brown coal—sub-bituminous coal—Bituminous coal—Semibituminous coal—Semi-anthracite
coal—Anthracite coal—Graphite.
➢ Peat. It is the first stage in the formation of coal from wood. It contains huge amount of moisture and therefore it is dried for
about 1 to 2 months before it is put to use.
➢ Lignites and brown coals. These are intermediate stages between peat and coal. They have a woody or often a clay like
appearance associated with high moisture, high ash and low heat contents.
➢ Wood charcoal. It is obtained by destructive distillation of wood. During the process the volatile matter and water are expelled.
➢ Coke. It consists of carbon, mineral matter with about 2% sulphur and small quantities of hydrogen, nitrogen and phosphorus. It
is solid residue left after the destructive distillation of certain kinds of coals. It is smokeless and clear fuel and can be produced
by several processes. It is mainly used in blast furnace to produce heat and at the same time to reduce the iron ore.
➢ Briquettes. These are prepared from fine coal or coke by compressing the material under high pressure.
Liquid Fuels
The chief source of liquid fuels is petroleum which is obtained from wells under the earth’s crust. These
fuels have proved more advantageous in comparison to sold fuels in the following respects.
Advantages :
1. Require less space for storage.
2. Higher calorific value.
3. Easy control of consumption.
4. Staff economy.
6. Easy handling and transportation.
7. Cleanliness.
8. No ash problem.
Gaseous Fuels
➢ Natural gas. The main constituents of natural gas are methane (CH4) and ethane (C2H6). It has calorific value nearly 21000
kJ/m3. Natural gas is used alternately or simultaneously with oil for internal combustion engines.
➢ Coal gas. Mainly consists of hydrogen, carbon monoxide and hydrocarbons. It is prepared by carbonisation of coal. It finds
its use in boilers and sometimes used for commercial purposes.
➢ Coke-oven gas. It is obtained during the production of coke by heating the bituminous coal. The volatile content of coal is
driven off by heating and major portion of this gas is utilized in heating the ovens. This gas must be thoroughly filtered
before using in gas engines.
➢ Blast furnaces gas. It is obtained from smelting operation in which air is forced through layers of coke and iron ore. After
filtering it may be blended with richer gas or used in gas engines directly. The heating value of this gas is very low.
➢ Producer gas. It results from the partial oxidation of coal, coke or peat when they are burnt with an insufficient quantity
of air. It has low heating value and in general is suitable for large installations. It is also used in steel industry for firing
open hearth furnaces.
➢ Sewer gas. It is obtained from sewage disposal vats in which fermentation and decay occur. It consists of mainly marsh
gas (CH4) and is collected at large disposal plants.
Basic Chemistry
➢ Atoms. It is not possible to divide the chemical elements indefinitely, and the smallest particle which can take part in a
chemical change is called an ‘atom’. If an atom is split as in nuclear reaction, the divided atom does not retain the
original chemical properties.
➢ Molecules. It is rare to find elements to exist naturally as single atom. Some elements have atoms which exist in pairs,
each pair forming a molecule (e.g. oxygen), and the atoms of each molecule are held together by stronger inter-atomic
forces.
Combustion Equations
➢ In a combustion chamber proportionate masses of air and fuel enter where the chemical reaction takes place, and then the
combustion products pass to the exhaust.
➢ By the conservation of mass the mass flow remains constant (i.e., total mass of products = total mass of reactants), but the
reactants are chemically different from the products, and the products leave at a higher temperature.
➢ The total number of atoms of each element concerned in the combustion remains constant, but the atoms are rearranged into
groups having different chemical properties.
➢ This information is expressed in the chemical equation which shows
(i) the reactants and the products of combustion,
(ii) the relative quantities of the reactants and products.
➢ The oxygen supplied for combustion is usually provided by atmospheric air, and it is necessary to use accurate and consistent
analysis of air by mass and by volume.
➢ It is usual in combustion calculations to take air as 23.3% O2, 76.7% N2 by mass, and 21% O2, 79% N2 by volume.
so that for each mole of O2 in a reaction we will have:
Combustion Equations
Thus, on the (simplistic) assumption that N2 will not undergo any chemical reaction, the chemical reaction of the
combustion of propane is replaced by:
Combustion Equations
➢ The minimum amount of air that supplies sufficient O2, for the complete combustion of the fuel is called theoretical air
or stoichiometric air.
➢ When complete combustion is achieved with theoretical air, the products contain no O2, as in theoretical reaction.
➢ In practice, it is found that if complete combustion is to occur, air must be supplied in an amount greater than
theoretical air. This is due to the chemical kinetics and molecular activity of the reactants and products.
➢ Slightly insufficient air results in CO being formed; some hydrocarbons may result from larger deficiencies.
➢ The parameter that relates the amount of air used in a combustion process is the air-fuel ratio (AF),which is the ratio of
the mass of air to the mass of fuel. The reciprocal is the fuel-air ratio (FA). Thus:
Combustion Equations
➢ Again, considering propane combustion with theoretical air, the air- fuel ratio is:
5 + 5 ∗ 3.76 ∗ 29 5 ∗ 4.76 ∗ 29
𝐴Τ𝐹 = = = 15.69 𝑘𝑔𝑎𝑖𝑟 Τ𝑘𝑔𝑓𝑢𝑒𝑙
(3 ∗ 12 + 8 ∗ 1) 44
Where we have used the molecular weight of air as 29 kg/kmol and that of propane as 44 kg/kmol. If, for the combustion
of propane, A/F > 15.69, a lean mixture occurs; if A/F < 15.69, a rich mixture results.
Combustion Equations
Combustion Equations
Combustion Equations
Combustion Equations
