POWER June 2015 - 59

FUNDAMENTALS
Table 1. Ash fusion analysis example. Source: Storm Technologies Inc.
Initial deformation (F)
Environment
Oxidizing
Reducing
1,975
1,975
Softening (F) Hemispherical (F)
2,050
2,030
burner can, in fact, slag up the furnace exit.
Some fuels are more unforgiving than others.
The coal ash iron content is a huge factor.
Ash that has as much as 15% to 20% iron
content will have an ash fluid temperature
in a reducing atmosphere as much as 500F
lower than the same ash in an oxidizing atmosphere.
Current plant operations with a
strong regulatory limit on NOx
tend to drive
operators to operate at low excess oxygen
levels. This practice, combined with fuel and
air imbalances, can result in conditions in
which flue gas lanes can have zero free oxygen
and, therefore, are technically operating
in a reducing atmosphere.
So, how can a reducing atmosphere or secondary
combustion be created at the furnace
exit? Here are six of the most common causes of
boiler slagging and fouling in our experience:
■ Low furnace excess oxygen
■ Extreme stratifications of the FEGT flue
gas lanes
■ High primary airflows
■ Burner damage and deficient mechanical
condition/tolerances
■ Poor coal pulverizer performance
■ Inconsistent fuel properties and chemistry
Low Furnace Excess Oxygen
The No. 1 cause of furnace slagging is low
furnace oxygen content. Most boilers are
designed for 115% to 120% theoretical combustion
air. This is generally expressed as
15% to 20% excess air. For coal furnaces, the
oxygen levels would be 3% to 3.8%. Note
the location of the oxygen analyzers at the
2. Location, location, location. Furnace
oxygen content instrument readings can
be inaccurate due to the location of the analyzers.
Source: Storm Technologies Inc.
Air in-leakage locations
Furnace
Penthouse and
convection pass
Ductwork before APH
Air preheater (APH)
Ductwork after APH
Burner belt
Oxygen
analyzers
location
2,135
2,100
Fluid (F)
2,750
2,410
economizer exit in Figure 2. This location is
often subject to reading higher oxygen levels
than the actual furnace oxygen content, due
to air in-leakage between the furnace and the
air heater flue gas inlet.
It is extremely important to apply the attention
necessary to optimize the furnace
burner belt " inputs, " because combustion
must be completed within the furnace cavity.
Of absolute importance is providing sufficient
combustion airflow to the fuel before
the products of combustion exit the furnace.
One of the most common causes of slagging
and fouling is secondary combustion at the
upper furnace. The most common cause of
secondary combustion is insufficient excess
oxygen within the burner belt.
Why is this so common? There are two
reasons. First, most U.S. boilers have some
age on them, and the boiler settings have
allowed air in-leakage to increase over the
years. Because the oxygen analyzers are
usually located at the economizer outlet, the
excess oxygen that is measured at the economizer
exit includes any ambient air that has
leaked into the boiler setting after combustion
should have been completed. This lack of
excess free oxygen at the furnace causes the
active combustion to stretch out and actively
continue into the superheater section. The
flue gas temperature, due to such secondary
combustion, can and has been measured to be
well over 1,000F above optimum.
The second factor is that when the coal
ash has an iron content of over about 10%,
the melting temperature of the ash is lower
in a reducing atmosphere. In other words, not
only does the secondary combustion elevate
the FEGT, but also, if the coal ash contains
significant amounts of iron, the fusion temperature
can be drastically lower as a result
of the ash chemistry. That is, the ash will melt
at a much lower temperature in a reducing atmosphere
from what the fusion temperature
would be in an oxidizing atmosphere. As
noted previously, the ash fusion temperature
can be reduced by as much as 500F.
These two factors combined are particularly
serious for eastern U.S. plants that burn
bituminous fuels. The iron content in the ash
has not been a large factor with Powder River
Basin fuels, but secondary combustion affects
all boilers and all fuels.
Extreme Stratifications of the
FEGT Flue Gas Lanes
The limited residence time of large utility
boilers demands that the furnace inputs of
fuel and air be optimized (Figure 3). If not
properly controlled, fuel/air inconsistencies
can contribute to slagging and fouling issues
due to secondary combustion and elevated
FEGTs. Optimizing the fuel and air inputs to
the furnace and making certain that the fur3.
Time is not on your side. With residence times as short as a couple of seconds,
there is not a lot of time for heat to be absorbed in the furnace. When secondary combustion is
present, the furnace exit gas temperature can exceed the ash fusion temperature, which is the
primary reason that furnace slagging occurs. Source: Storm Technologies Inc.
Unacceptable
combustion
Flame carryover into the
superheater can elevate the FEGT
1,000F above optimum
1-2 seconds
residence time
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