Page 79 - Boiler plant and distribution system optimization manual

64 Boiler Plant and Distribution System Optimization Manual

gas that burns as follows:

CH +2O →
4
2
CO + 2H O + 1,013 Btu/Ft 3
2
2
Pure oxygen is also rarely used for combus-
tion. Air contains about 21 percent oxygen and
79 percent nitrogen by volume and is much more
readily available than pure oxygen:

CH + 2O + 7.53N →
2
2
4
CO + 2H O + 7.53N + 1,013 Btu/Ft 3
2
2
2
Figure 7.5—Stoichiometric point and air rich
In this example, one cubic foot of methane and fuel-rich combustion shown as a function of
(at standard temperature and pressure) will burn the air fuel ratio.
completely with 9.53 cubic feet of air contain-
ing 21 percent oxygen and 79 percent nitrogen. bustibles appear in the flue gas, it is common
This complete burning of fuel, with nothing but practice to supply some amount of excess air. In
carbon dioxide, water, and nitrogen as the end the era of cheap energy it was not uncommon to
product is known as stoichiometric combustion run a burner with a large amount of excess air in
(Figure 7.5). The ratio of 9.53 cubic feet of air to order to avoid smoking. Today this is becoming
one cubic foot of methane is known as the stoi- known for the highly wasteful practice it really is.
chiometric air/fuel ratio. The heat released when
the fuel burns completely is known as the heat of
combustion. HOW DO YOU ACHIEVE OPTIMUM
COMBUSTION EFFICIENCY?

ExCESS AIR Too little excess air is inefficient because it
permits unburned fuel, in the form of combusti-
The Importance of Excess Air bles, to escape up the stack. But too much excess
As most combustion equipment operators air is also inefficient because it enters the burner
know, it is extremely undesirable to operate a at ambient temperature and leaves the stack hot,
burner with less-than-stoichiometric combustion thus stealing useful heat from the process. This
air. Not only is this likely to result in smoking but leads to the fundamental rule:
it will significantly reduce the energy released by
the fuel. “Maximum combustion efficiency achieved when the
If a burner is operated with a deficiency of correct amount of excess air is supplied so the sum of
air, carbon monoxide and hydrogen will appear both unburned fuel loss and flue gas heat loss is min-
in the products of combustion. CO and H are the imized .”
2
result of incomplete combustion and are known
as combustibles. Anything more than a few hun- Measuring Combustibles
dred parts per million of combustibles in flue gas Combustible analyzers are available to ac-
indicates inefficient burner operation. curately measure CO and H concentrations in
2
In actual applications, it is impossible to flue gas to (+/–) 10 ppm or less. Carbon monox-
achieve stoichiometric combustion because burn- ide analyzers are often used in control systems
ers can not mix fuel and air perfectly. To insure because of their greater accuracy and calibration
that all of the fuel is burned and little or no com- stability.