Page 74 - Boiler plant and distribution system optimization manual

Chapter 7
Combustion Analysis
COMBUSTION ANALYSIS
Understanding the combustion process is
very important for safe and efficient operation of
a plant. Perfect combustion is the proper mixture
of fuel and air under exacting conditions where
both the oxygen and the fuel are completely con-
sumed in the combustion process. Having just
the right amount of oxygen (no more, no less) is
called the stoichiometric point, simply the ideal
air to fuel ratio for combustion.
Anything above the ideal amount of air sup-
plied to the combustion process is called Excess
Air, and is wasteful.
A CO analysis alone does not provide a safe
2
indication of the combustion air/fuel setting.
Additional requirements of either smoke or CO
is recommended as the same CO measurements
2
can occur on either side of stoichiometric.
Excess air is the preferred term to describe
the combustion setting on the safe side of stoi-
chiometric. Using oxygen measurements is the
best way find excess air.
By understanding a few simple instruments
most of the potentially hazardous conditions
can be reduced. The key is to properly measure
smoke, oxygen, carbon monoxide, draft and gas
pressure.
CAUTION
A most dangerous approach, when dealing
with combustion systems is thinking the systems
will always be correct and not considering that it
can be affected by small and seemingly unrelated
external forces.
The following is a list of key parameters con- •
sidered as important safety measurement areas:
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• Input gas pressure
• Draft (over fire and boiler exhaust)
• Carbon Monoxide
• Stack temperature
• Smoke
• Combustibles
Too much smoke is one of the most com-
mon indicators of excessive fuel wastage and can
cause major problems.
STACK FIRES
Buildup of combustibles in the exhaust sys-
tem and chimneys which can cause fires and
explosions. To a lesser extent a build up of soot
in the exhaust system can block the normal pas-
sage of flue gases further restricting the amount
of oxygen supplied for combustion progressively
compounding the problem.
SOURCES OF PROBLEMS
Four basic combustion zone conditions that
prevent clean, efficient combustion are:
• Insufficient combustion air applied to the
flame to permit clean combustion at an ac-
ceptable combustion efficiency
• Non-uniform delivery of fuel/or combus-
tion air to the combustion zone.
Insufficient temperature of the combustion
zone to permit proper burning of the fuel.