Civil Engineering Reference
In-Depth Information
6.8
How to Improve Boiler Efficiency
Boiler efficiency is influenced by many factors. As shown in Fig.
6.11
, efficiency
varies according to fuel, load, and the existence of stack gas recovery systems such
as economizers, air preheaters, and condensing systems.
Average efficiency values range between 75 % and 90 %, and they can be
obtained by a proper control of the operating parameters (the higher values within
this range if recovery systems are installed and if the Net Heating Value is used as
input in the fuel).
Of course, efficiency is improved when losses are kept as low as possible. This
happens mainly when excess air and flue gas stack temperature are reduced.
In what follows, suggestions are made about how to minimize industrial boiler
losses.
Excess air, ranging between 10 % and 60 %, but necessary to insure complete
combustion and safe operation of the boiler, and high temperature of stack flue gas
(473-573 K; 200-300
C; 392-572
F) are responsible for the most part of the stack
losses.
Losses due to the latent heat of stack water vapor mixture depend on fuel and
operating conditions. These losses cannot be reduced because no means are gener-
ally available to permit the water vapor mixture to condense before passing from
the boiler into the stack (see condensing boilers).
Excess air can be reduced by improving the combustion control system which
regulates the supply of air and fuel to meet load demand variations. Notice that a
CO
2
content in stack flue gas higher or lower than standard values signals anoma-
lous boiler operation due to lower or higher excess air values.
Fig. 6.11
Ranges of boiler operating efficiencies
