POWER June 2015 - 60

FUNDAMENTALS
4. A bridge to nowhere. This superheater
was slagged over because the furnace
exit gas temperature was about 1,000F above
optimum. Courtesy: Storm Technologies Inc.
5. Don't out-kick your coverage. When primary airflow is too high, fuel gets driven
deep into the furnace. Source: Storm Technologies Inc.
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High primary
air velocity
nace exit is an oxidizing atmosphere are the
first steps in reducing furnace slagging.
Fuel input optimization includes ensuring
that:
■ Coal fineness meets the following guidelines:
At least 75% passes a 200-mesh
screen and less than 0.2% remains on a 50mesh
screen with representative and isokinetically
removed coal fineness samples.
■ Coal distribution to each burner must be
balanced plus or minus 10%.
Combustion air optimization includes ensuring
that:
■ Primary airflow quantities are optimized
and air/fuel ratios are repeatable.
■ Measured and controlled secondary airflow
is uniformly distributed to the individual
burners.
■ Measured and controlled overfire airflow
is optimized.
FEGT and excess oxygen can be measured
with a water-cooled high-velocity thermocouple
(HVT) probe. The measurements by HVT
probe should be a minimum of 3% excess oxygen
with maximum temperatures about 100F
to 150F below the ash fusion temperature. It
is when the FEGT approaches the ash fusion
temperature that slagging occurs.
Often, the most useful data obtained by using
a water-cooled HVT probe is the furnace
exit, excess oxygen levels, and profiles. All
points at the upper furnace should be oxidizing
and preferably above 3% excess oxygen.
The word " slagging " is usually used to describe
slagging in the furnace, whereas fouling
is generally used to describe cinders or ash that
have carried over into the convection pass and
created flow obstructions due to the deposition.
As discussed previously, fouling of the convection
pass, SCR, and air heater are the result of
ash accumulations on the leading edges of superheater
and reheater tubing that is removed
by long retractable sootblower operation.
Minimizing slagging and fouling begins
by optimizing the burner belt combustion
performance. This is necessary because there
60
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POWER | June 2015
Optimized
primary air
velocity
is only about 1 or 2 seconds of residence time
between the top of the burner belt and the superheater
flue gas inlet. At the furnace exit,
the superheater and reheater tube spacing
becomes closer and closer, resulting in narrowing
gas flow lanes.
The typical FEGT is about 2,150F to
2,250F, assuming that the prerequisites for optimum
burner belt combustion are present for
the inputs. In the superheater shown in Figure
4, the peak furnace flue gas temperatures were
well above the melting temperature of Alloy
310 stainless steel (about 2,780F). The active
secondary combustion peak temperatures
were truly about 1,000F above the FEGT with
optimized burner belt inputs. When optimized,
the FEGT was a uniform 1,950F to 2,100F
across the boiler width. Prior to optimization,
temperatures of 2,850F to 3,100F were present.
These extremes have been documented in
numerous cases.
The cause of the high temperature in this
case was threefold. First, primary air velocities
were high, which drove the fuel deep into
the furnace, away from the secondary air provided
at the burners. Second, the burner belt
suffered from combustion air starvation due
to the overfire airflow being too high and exceeding
20% of total airflow, with only about
115% of total theoretical airflow to the boiler.
In other words, the burner belt was deeply
staged at sub-stoichiometric excess oxygen
levels. Third, fuel fineness and distribution
was not optimized. The flue gas temperatures
were over 3,000F at the superheater gas side
inlet. At this temperature, the ash condition
was fluid, and it only took a couple of shifts
to completely slag the furnace exit.
It is common to find between 0.5% and 1%
oxygen rise from the furnace to the air heater
inlet flue gas. Why? Well, for one reason, the
average 500-MW pulverized coal-fueled boiler
is more than 30 years old. Therefore, the
potential for air infiltration is increased due to
age alone, even when diligent and thorough
maintenance repairs are practiced. The only
excess oxygen that matters from a slagging
and fouling viewpoint is the excess oxygen
present at the furnace exit. Keep in mind that
the available residence time from the top to the
burner belt may be less than 1.5 seconds.
High Primary Airflows
High primary airflow, especially on wall6.
Out-of-spec. Damaged burners must be repaired in order to optimize combustion. Courtesy:
Storm Technologies Inc.
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POWER June 2015

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