In combustion reactions, rapid
oxidation of combustible elements of the fuel results in energy release as
combustion products are formed. The three major combustible chemical elements
in most common fuels are carbon, hydrogen, and sulfur. Although sulfur is
usually a relatively unimportant contributor to the energy released, it can be
a significant cause of pollution and corrosion.
The emphasis in this section is on
hydrocarbon fuels, which contain hydrogen, carbon, sulfur, and possibly other
chemical substances. Hydrocarbon fuels may be liquids, gases, or solids such as
coal.
Liquid hydrocarbon fuels are
commonly derived from crude oil through distillation and cracking processes.
Examples are gasoline, diesel
fuel, kerosene, and other types of fuel oils. The compositions of liquid fuels
are commonly given in terms of mass fractions. For simplicity in combustion
calculations, gasoline is often considered to be octane, C8H18, and diesel fuel
is considered to be dodecane, C12H26.
Gaseous hydrocarbon fuels are obtained
from natural gas wells or are produced in certain chemical processes. Natural
gas normally consists of several different hydrocarbons, with the major
constituent being methane, CH4. The compositions of gaseous fuels are
commonly given in terms of mole fractions.
Both gaseous and liquid
hydrocarbon fuels can be synthesized from coal, oil shale, and tar sands. The composition
of coal varies considerably with the location from which it is mined. For
combustion calculations, the makeup of coal is usually expressed as an ultimate
analysis giving the composition on a mass basis in terms of the relative
amounts of chemical elements (carbon, sulfur, hydrogen, nitrogen, oxygen) and
ash. Coal combustion is considered further in Chapter 8, Energy Conversion.
A fuel is said to have burned completely
if all of the carbon present in the fuel is burned to carbon dioxide, all
of the hydrogen is burned to water, and all of the sulfur is burned to sulfur
dioxide. In practice, these conditions are usually not fulfilled and combustion
is incomplete. The presence of carbon monoxide (CO) in the products
indicates incomplete combustion. The products of combustion of actual combustion
reactions and the relative amounts of the products can be determined with
certainty only by experimental means. Among several devices for the
experimental determination of the composition of products of combustion are the
Orsat analyzer, gas chromatograph, infrared analyzer, and flame
ionization detector. Data from these devices can be used to
determine the makeup of the gaseous products of combustion. Analyses are
frequently reported on a “dry” basis: mole fractions are determined for all gaseous
products as if no water vapor were present. Some experimental procedures give
an analysis including the water vapor, however.
Since water is formed when
hydrocarbon fuels are burned, the mole fraction of water vapor in the gaseous
products of combustion can be significant. If the gaseous products of
combustion are cooled at constant mixture pressure, the dew point
temperature (Section 2.3, Ideal Gas Model) is reached when water vapor
begins to condense. Corrosion of duct work, mufflers, and other metal parts can
occur when water vapor in the combustion products condenses.
Oxygen is required in every
combustion reaction. Pure oxygen is used only in special applications such as
cutting and welding. In most combustion applications, air provides the needed
oxygen. Idealizations are often used in combustion calculations involving air:
(1) all components of air other than oxygen (O2) are lumped with nitrogen
(N2). On a molar
basis air is then considered to be 21% oxygen and 79% nitrogen. With this
idealization the molar ratio of the nitrogen to the oxygen in combustion air is
3.76; (2) the water vapor present in air may be considered in writing the
combustion equation or ignored. In the latter case the combustion air is
regarded as dry; (3) additional simplicity results by regarding the
nitrogen present in the combustion air as inert. However, if high-enough
temperatures are attained, nitrogen can form compounds, often termed NOX, such as nitric
oxide and nitrogen dioxide.
Even trace amounts of oxides of
nitrogen appearing in the exhaust of internal combustion engines can be a
source of air pollution.
The minimum amount of air that
supplies sufficient oxygen for the complete combustion of all the combustible
chemical elements is the theoretical, or stoichiometic, amount of
air. In practice, the amount of air actually supplied may be greater than or
less than the theoretical amount, depending on the application. The amount of
air is commonly expressed as the percent of theoretical air or the percent
excess (or percent deficiency) of air. The air-fuel ratio and
its reciprocal the fuel-air ratio, each of which can be expressed on a
mass or molar basis, are other ways that fuel-air mixtures are described.
Another is the equivalence ratio: the ratio of the actual fuel-air ratio
to the fuel-air ratio for complete combustion with the theoretical amount of
air. The reactants form a lean mixture when the equivalence ratio is
less than unity and a rich mixture when the ratio is greater than unity.
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