POWER June 2015 - 58

FUNDAMENTALS
Typical Causes of Slagging and
Fouling Problems in Boilers
Boiler slagging and fouling, and the resulting frequent operation of sootblowers,
are some of the major operations and maintenance factors that can
negatively affect power plant reliability and efficiency.
Richard F. (Dick) Storm, PE
B
oiler slagging and fouling are among
the most common causes of maintenance
headaches at coal-fired power
plants. Though you cannot totally eliminate
the problem, following proper operation and
maintenance practices can drastically reduce
the time and trouble of addressing it.
Demystifying Slagging and
Fouling
Let's review what slagging is before getting
into the causes and corrections of slagging
and fouling.
Slag is molten ash and incombustible byproducts
that remain following coal combustion.
When the material cools to a certain
temperature, it can stick to furnace components,
such as waterwalls, which is called slagging.
A pulverized coal-fueled boiler is designed
with a large furnace cavity that can
tolerate liquid phase slag on the waterwalls.
The furnace exit, however, should be at a
low enough temperature that the slag will be
cooled below its softening temperature.
A typical bituminous fuel ash fusion temperature
is determined using the American Society
for Testing and Materials (ASTM) standard
D1857. To conduct the test, a cone of ash is
placed into a laboratory furnace and the furnace
is slowly heated. The furnace temperature
is noted at four points as the ash cone deforms.
The temperature of the first point-when
the ash cone point becomes blunted-is called
the " initial deformation temperature. " As the
furnace is heated more, the temperature when
the ash becomes soft and the height (H) of
the cone equals the width (W), is recorded.
This value is referred to as the " softening
temperature. " Heating continues, resulting in
the ash cone sagging further until H = 1/2 W.
This temperature is called the " hemispherical
temperature. " Finally, when the ash cone becomes
a liquid, the temperature is noted and
referred to as the ash " fluid temperature. "
Modern laboratories use more advanced
furnaces than when the method was first developed,
but the reporting of the ash fusion
temperatures is still completed using the
58
same four levels of ash fusion: initial deformation,
softening, hemispherical, and fluid.
The purpose of the laboratory test is to determine
the approximate condition of the ash
when it is in various parts of a boiler furnace.
For slagging and fouling, the most important
issue is to have the furnace gases or " products
of combustion " leaving the furnace at a
temperature so that the ash is not too sticky.
A good approximation is to have the furnace
exit gases about 100F to 150F cooler than the
ash softening temperature.
I have seen furnaces where the furnace
exit gases are above the fluid temperature,
and it is possible to operate a boiler with
liquid-phase ash flowing through the superheater
and reheater, but it is not advisable for
reasons of coal ash corrosion and the need for
near-continuous long retractable sootblowing
to mitigate the ash deposits.
" Fouling " typically refers to deposits that
occur in the convection pass after the gases
exit the furnace. Fouling is generally attributed
to ash cinders and accumulations that
form on the leading edges of the superheater
and reheater tubes (Figure 1), especially the
outlet legs, which are above 1,000F metal
surface temperature. The deposits are dislodged
by sootblowing.
When the long retractable sootblowers are
used to blow the ash deposits free, the ash
particles entrain into the flue gas stream and
create cinders, which can block selective catalytic
reduction (SCR) catalyst flow paths,
plug air heater baskets, and bridge across
boiler tubing in the convection pass. Usually,
the areas of the boiler that are designated as
being susceptible to slagging are from the
burner belt to the furnace exit.
Heat flows from hottest to coolest and,
therefore, to produce 1,000F to 1,100F superheated
steam and reheated steam outlets, the
furnace exit gas temperature (FEGT) must be
above about 1,500F at the reheater gas inlet to
drive the heat flow into the reheater and superheater
to create the desired steam temperatures.
Consequently, the sweet spot for the FEGT of
a pulverized coal-fueled boiler should be about
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2,150F to 2,250F to achieve desired steam temperatures
without slagging. Lower than 2,150F
it becomes difficult to achieve design steam
temperatures. Above 2,250F on a bulk gas basis,
it becomes close to the melting temperature
of the ash in some fuels.
Fuels with extremely high ash fusion
temperatures are thought of as being " boiler
friendly " and forgiving. Fuels with lower ash
fusion temperatures require more precise
combustion tuning and increased sootblowing
to mitigate slag deposits.
Let's take a look at the coal ash fusion
D1857 analysis shown in Table 1 as an example.
Given the ASTM D1857 temperatures
of the ash fusion analysis, the ash condition
can be estimated in the furnace and at the furnace
exit, if the temperatures are known.
In this example, the coal ash analysis shows
a fluid temperature in a " reducing atmosphere "
of 2,410F. So, if there are lanes of products of
combustion that are fuel rich and still actively
burning, then this is in fact a " reducing atmosphere "
for the particular lane of fuel-rich
products of combustion. In practice, this could
result from one burner that is fuel heavy and air
lean. The products of combustion from that one
1. The line of demarcation. The tube
spacing of all boilers becomes progressively
more restrictive as the heat transfer process
changes from radiant in the furnace to convective
in the back pass. Source: Storm Technologies
Inc.
Rear
Front pendant
RH 9' CTRS
SH division
panels 8' CTRS
Platen SH
18' CTRS
Preferred HVT
test plane
Furnace
Windbox
Corner
firing
system
Stagging
Fouling
POWER | June 2015
pendant RH
4' CTRS
Front pendant
SH 4' CTRS
Rear pendant
SH 4' CTRS
Economizer
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POWER June 2015

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