POWER December 2015 - 18

Optimize Boiler Combustion
Using Improved Flue
Gas Measurement
The presence of unburned fuel in a
power plant's ash indicates an inefficient
combustion process, as some of
the energy in the fuel is being wasted.
Unburned fuel also poses a risk to
safety; if ignited, an explosion could
occur, with potentially disastrous consequences.
Therefore, it is important
for plant operators to optimize the furnace
combustion process to the greatest
extent possible.
One common problem found during optimization
is insufficient combustion air
in the furnace. Maintaining the correct
level of excess air for combustion can be
complicated by variations in the fuel and
the air-such as atmospheric pressure, humidity,
and changes in the heating value
of the fuel-and the performance of the
boiler itself.
Optimization Challenges
Most large utility boilers use oxygen analyzers
installed at the boiler economizer
exit gas ducts to monitor and make adjustments
to excess air. Over time, many
boilers develop significant air in-leakage
between the furnace exit and the oxygen
analyzers. Any air that seeps into the furnace
post-combustion does not take part
in combustion, yet it registers on the oxygen
analyzers as excess oxygen.
As a result, the operator could modify
the air-fuel ratio incorrectly based on the
assumption that the total excess oxygen
measured was involved in the combustion
process. This improper adjustment would
lead to increased fuel usage and higher
concentrations of unburned fuel outside of
the desired boiler combustion zone, which
could pose a safety hazard.
CO events (areas of poor combustion)
are most often localized and occur in
pockets throughout the boiler. The best
way to identify the cause of these pockets
is by developing a spatial grid across the
unit (Figure 1).
The furnace produces discrete process
zones of flue gas created by the stream
pattern. A combustion stream remains intact
and separate from other combustion
streams as a result of being compressed
by the walls of the furnace and by adjacent
combustion streams. Some diffusion
(mixing) between streams may occur in
18
1. Streaming through the process. Discrete zones can be created due to stream
patterns inside a boiler. Courtesy: ABB
slow-moving regions, but overall, the relatively
high-velocity combustion streams
remain intact. This results in a nonuniform
combustion pattern throughout the
boiler. Evaluating the combustion gas
composition from each burner offers the
only way to optimize overall combustion
conditions.
For optimum combustion to be
achieved, there must be sufficient excess
air present to burn all of the fuel, but not
too much to reduce efficiency or increase
NOx
production. For this reason, measuring
oxygen alone will not provide an accurate
picture of combustion efficiency.
Monitoring More Than Just Oxygen
ABB's new AZ40 analyzer (Figure 2) continuously
monitors waste gases to measure
both excess air and unburned fuel.
With that information, operators can adjust
the amount of excess air to obtain
optimal combustion.
The AZ40 analyzer monitors the oxygen
and unburned combustibles in terms of
carbon monoxide equivalent (COe) using
two sensors. The oxygen reading indicates
the percent by volume measurement of the
net oxygen. Typically, this reading is the
primary indicator for combustion control.
The COe measurement is often used to optimize
combustion control by further trimming
the oxygen adjustment.
The localized concentration of COe
present in the boiler directly relates to
heat rate/efficiency, total fuel consumption,
loss on ignition, localized slagging,
www.powermag.com
and fouling. By effectively measuring COe
and O2
, plant engineers and operators can
ensure optimal combustion that results in
reduced fuel costs, reduced maintenance,
and reduction of NOx
.
Different strategies can be implemented
to model boiler inefficiency. In
general, a strategy that correctly targets
partial grids to identify pockets
of high CO and excess air can be very
effective. By pinpointing the location
of poor combustion, an operator can
home in on the problematic area that is
leading to inefficiency and can better
utilize the O2
measurement in order to
fine-tune the burners to further optimize
combustion.
Adequately monitoring areas of poor
combustion requires a matrix of probes
horizontally (sometimes vertically)
protruding at different lengths into
the furnace (usually from both sides).
The object is to extract flue gas samples
from multiple points with multiple
AZ40 analyzers.
The analyzer consists of a sensor assembly,
probe with filter assembly,
transmitter, and interconnecting cable
assembly. The sensor assembly flange
mounts to a duct or process wall, with
the probe extending into the flue gas
stream. Available in lengths up to 2,460
mm (96.8 inches) and suitable for applications
up to 1,650C (3,000F), the probe
continuously draws in the sample, which
is then analyzed by the sensor. Electrical
outputs from the sensors are fed through
POWER | December 2015
http://www.powermag.com
POWER December 2015

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