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Combustion Analysis 63
SULFUR IN FUEL FORMS ACID COMBUSTIBLES
Sulfur in some fuels can end up as sulfuric Because combustibles in the flue gas are un-
acid when the flue gas temperatures drop too burned fuels, this represents fuel flowing out of
low. Boiler damage and corrosion from sulfu- the stack.
ric acid has been a problem and a challenge for Scientists have observed on occasion that
many years, causing large (energy wasting) safety combustibles are composed of equal parts carbon
margins in stack temperature to be used to avoid monoxide and hydrogen. Hydrogen has a heating
damage. value of 61,100 Btu/lb. Carbon monoxide has a
In the past, temperatures were maintained heating value of 4,347 Btu/lb.
above the approximate levels listed below to pre-
vent formation of SO and SO which combines
3
2
with moisture to form acids. COMBUSTION EFFICIENCY
In practice combustion efficiency is thought
• Natural Gas 250 F of as the total energy contained per pound of fuel
• No. 2 Heating Oil 275 F minus the energy carried away by the hot flue
gases exiting through the stack, expressed as a
• No. 6 Fuel Oil 300 F
percentage.
• Coal 325 F Combustion efficiency is only part of the
• Wood 400 F total efficiency. Radiation loss from hot exposed
boiler surfaces, blowdown losses and electrical
losses in pumps and fans are examples of other
As the chief concern with acid formation is
kinds of losses that must be considered in de-
cold surfaces, minimum metal temperatures are
termining total efficiency. However in most fuel
used as a more precise means to control corrosion
burning equipment, the most effective way to
Figure 7.4 shows minimum metal temperature
reduce wasted fuel is to improve combustion effi-
guidelines for air heaters.
ciency. To do so, it is necessary to understand the
fundamentals of combustion.
STOICHIOMETRIC COMBUSTION
The three essential components of combus-
tion are fuel, air and heat. In fossil fuels, there
are really only three elements of interest: carbon,
hydrogen and sulfur. During combustion, each
reacts with oxygen to release heat:
C + O → CO + 14,093 Btu/lb.
2
2
H + 1/2 O → H O + 61,095 Btu/lb.
2
2
S + O → SO + 3,983 Btu/lb.
2
2
Pure carbon, hydrogen and, sulfur are rarely
used as fuels. Instead, common fuels are made
up of chemical compounds containing these el-
Figure 7.4
ements. Methane, for example, is a hydrocarbon
Minimum metal temperatures for air heaters.
