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10.5 Combustion Processes and Fuels
The primary source of energy in a heating boiler is the combustion of
a fossil fuel—coal, oil, or gas—or waste materials. The use of peat,
garbage, sawdust, petroleum coke, and other waste products is in-
creasing, but it is still a small fraction of the total fuel burned in this
country.
Combustion is a process of burning—combining the fuel with oxygen
and igniting the mixture. The result is heat release, absorbed through
radiation, convection, and, to some degree, conduction.
10.5.1 The combustion process
The combustion process follows basic principles called the three T’s of
combustion. The first one is time—the time required for the air to
properly mix with the fuel and for the combustion process to be com-
pleted. It is critical when waste materials are being combusted in con-
junction with standard fuels. The second is temperature—the temper-
ature at which the fuel will ignite, oxidation is accelerated, and the
process of combustion begins. Ignition temperatures are well estab-
lished for standard fossil fuels but must be carefully considered when
waste or other organic-type materials are being burned. The third is
turbulence—the process of thoroughly mixing the air and fuel so that
each particle of fuel is in contact with the right amount of oxygen and
combustion can continue to completion. The turbulence must be vio-
lent enough to ensure good contact between the fuel and the oxygen.
Assuming there is enough combustion air to work with, inadequate
turbulence is the most common cause of incomplete combustion. In-
adequate turbulence can result in the generation of excessive amountsof carbon monoxide, and combustion may continue well beyond the
furnace portion of the boiler.
10.5.2 The chemical reaction
In its simplest form, the combustion of natural gas (methane, CH4)
with air as a source of oxygen, the chemical reaction can be written
CH 2(4N O) ⇒ CO 2H O 8N heat 422222
This describes a perfect and complete or stoichiometric combustion
process. In practice, the process is never perfect or complete. Some
carbon monoxide is formed, and some contaminants, such as sulfur,
are present and enter into the process. Sulfuric and nitric acids and
nitrous oxides are often formed, along with other undesirable com-
pounds.
10.5.3 Excess air
Because the combustion process is never perfect and perfect mixing of
air and fuel is never achieved, every combustion process requires ex-
cess air. Excess air is the additional air that must be added to the
theoretically perfect mixture to ensure as complete a combustion pro-
cess as is practically possible. The larger the amount of excess air, the
lower the combustion efficiency. Often overlooked is the possibility of
condensation in the boiler or flue that has too much excess air. It can
be reasoned that turbulence is a most important factor in the com-
bustion process. Almost all of the newest boiler developments have
been in burner design, in an attempt to improve the mixing of air and
fuel to minimize excess air, to maximize combustion efficiency, and to
minimize the generation of nitrous oxides.
10.5.4 Combustion efficiency
The combustion efficiency is the ratio of fuel heat input minus the
stack loss (through the chimney or vent), pided by the fuel heat
input. Typical efficiencies for mechanically fired boilers range from 75
to 83 percent for new installations at full-load conditions. Firing at
reduced capacity may reduce the combustion efficiency. Therefore it is
desirable to match the boiler to the load as closely as possible or to
use multiple boilers.
The overall thermal efficiency is the gross output in Btu/h pided