POWER August 2010 - 54

PLANT O&M
1. Excess oxygen in flue gas. This is an example of how secondary combustion can
mask air in-leakage. The graph shows secondary combustion with postcombustion leakage resulting
in zero measured leakage via in-furnace (point A) and economizer (point B) while actual
leakage (and minimum oxygen level) is 2.5% (point C). Source: Storm Technologies Inc.
100
7
6
5
4
3
2
1
Area for secondary
combustion
Postcombustion
leakage zone
2. Typical zones for leakage. This
figure highlights zones where air in-leakage
typically occurs as well as where the potential
for secondary combustion is located. Source:
Storm Technologies Inc.
Oxygen probes
A
B
C
Ideal combustion with no leakage
Secondary combustion with leakage
Precombustion
leakage zone
Measured
airflow
tive amount of margin air. There are two approaches
for determining the amount of air
in-leakage:
The degree to which air and fuel are mixed
cannot be measured directly. Therefore, instead
of measuring airflow into the furnace,
the resulting level of oxygen is measured as
an indicator of the mixing process and how
much air is left over. Though this solution is
practical for new and good-condition boilers,
the drawbacks become apparent when leakage
upstream of the probes increases.
Precombustion and
Postcombustion Leakage
Because on balanced draft units the furnace
is under a negative pressure (typically -0.5inch
water column at the furnace exit), the
furnace is susceptible to ambient air entering
the envelope of the furnace. For the
purposes of this article, we will divide leakage
into two categories: precombustion and
postcombustion.
Precombustion leakage includes any ambient
air that enters the furnace and theoretically
participates in the combustion process.
Sources of this type of leakage would include
bottom ash hoppers, wall blower
penetrations, gaps in tube membranes, and
access doors. Though not directly harmful
to the combustion process, this air takes the
place of overfire or secondary air because
primary air is usually controlled on a mass
flow basis.
Tuning for good combustion, low NOx
fireside corrosion, and slagging is heavily de,
54
pendent
on where and how air is introduced
to the fuel. Leakage air is introduced to the
fuel in a haphazard manner. It usually leaks
in and then follows up the walls due to the
lack of penetration velocity. Therefore, typically
leakage air will prove a hindrance for
tuning and good combustion.
The other type of leakage is postcombustion
leakage. As it sounds, this is any
air that enters the furnace envelope and
does not participate in the combustion process.
Sources of this leakage include sootblower
penetrations, access doors, tube
penetrations, gaps in the tube membranes,
penthouse cracks, and economizer hopper
leakage. This leakage is particularly dangerous
because it artificially raises the excess
oxygen indication at the economizer
outlet. Because most units are run on an
automatic oxygen trim controller, the control
system will see an increase in oxygen
and reduce the air input into the furnace
to maintain the oxygen setpoint. Unless a
large access door or other source of major
air leaks is opened in the right place, this
process will not be noticeable to even the
most attentive operator. Air in-leakage is a
problem that usually increases slowly over
time (Figure 2).
Two Leakage Monitoring Methods
As stated earlier, the oxygen measurement
is just an indirect measurement of the relawww.powermag.com
■
Mass flow only indication. This measurement
approach does precisely meter for
the proper air/fuel ratios but does not measure
leakage air and does not indicate the
amount of air/fuel mixing.
■ Oxygen only indication. This measurement
approach includes leakage air and provides
a relative indication of air/fuel mixing,
assuming good furnace conditions,
but cannot distinguish between leakage
air and actual margin and is susceptible to
stratifications in the measurement grid due
to combustion-related issues or leakage.
Note the complimentary relationship between
the two measurement approaches:
Each system's pros can compensate for the
cons of the other method. We'll now use that
observation to develop a new approach for
monitoring boiler air leakage.
Prerequisites for Using an Online
Leakage Monitoring System
The online leakage monitoring system provides
real-time leakage indication based on
the mass flow of air and fuel and the indicated
excess oxygen. A robust system of
measurement can be implemented based on
the following criteria.
Total Combustion Airflow Is Accurately
Measured on a Mass Flow Basis.
Secondary air and primary air can be measured
separately or together. However, the
system will not be accurate if some of the
POWER | August 2010
Excess oxygen in flue gas (%)
Normal
combustion
Furnace exit
Secondary
combustion
Leakage
Economizer outlet

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POWER August 2010

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