POWER August 2010 - 22

8. An unprotected stack. The protective coating on the stack liner must be selected
based on the actual flue gas temperature entering the stack. A well-applied coating system
can prevent catastrophic corrosion that is expensive to repair and may require a long outage.
Courtesy: Bril Inc.
layers. Proper surface preparation includes
sand blasting the entire interior stack surface
to a near-white condition per code
SSPC-SP-10 to ensure that the mastic material
will properly adhere.
351F to 400F. At these mid-range temperatures,
the stack interior does not require
an internal coating of mastic protection,
but it does require external insulation and
lagging to prevent moisture condensation
on the outside stack surface.
401F to 850F. For gas temperatures in
this range, no internal or external protection
is required for the prevention of corrosion.
851F and above. At these elevated
temperatures, the internal lining of the
stack must be protected with refractory.
The refractory material should match the
chemistry of the acids within the flue gas.
The refractory recipe is usually three parts
acid-resistant aggregate and one part
lumnite cement. The refractory is typically
pneumatically or gun applied and is usually
2 inches thick over a reinforcing material
such as road-mesh or chicken-wire
mesh. The reinforcing mesh is held to the
stack interior using stand-offs such as slab
spacers, t-slot studs, or studs and nuts.
sphere. Their height and diameter are selected
to match the specific requirements
of each plant, and they are usually a lowmaintenance
item.
Stacks may be low maintenance, but
they are not no maintenance. The cost
of preventing corrosion may be as little
as $10,000, but the cost of repair or replacement
could be many times that or
even put your plant out of commission
until the stack problem is corrected. It
pays to invest in stack liner protection
(Figure 8).
At a typical 800-MW power plant, the
exit gas temperature leaving the boiler
is approximately 700F. The flue gas then
passes through the economizer and the
selective catalytic reduction system to remove
the nitric oxides in the gas. The gas
then passes through a heat exchanger or
air preheater that transfers the thermal energy
from the exhaust gases to the incoming
boiler combustion air to improve the
overall unit combustion efficiency. The flue
gas temperature leaving the air preheater
will be reduced to around 350F before next
passing through a series of air quality equipment
steps to remove SO2
, particulates, and
perhaps mercury until the gas reaches the
induced draft fan on a balanced draft unit.
This fan pulls the flue gases from the boiler
through this equipment and sends it to the
stack. On the upstream side of the boiler,
a forced draft fan provides combustion air
22
to the boiler combustors, pulverizers, and
other equipment.
A typical small industrial facility like
a hospital or school has boilers that are
much smaller (75 MW or less capacity).
Flue gas leaving these boilers is much
lower in temperature (typically between
351F and 500F), and the unit may not
be outfitted with an air preheater. These
small facilities may have a heat exchanger
such as an economizer but not an air
heater. The economizer extracts heat from
the flue gas to heat the water needed in
the boiler. The gas leaving the economizer
is usually above 350F, but, as is often the
case with the smaller boiler systems, the
flue gas goes directly to the stack.
Corrosion Protection for Stacks
The rules for determining the required
level of stack protection against corrosion
are based on the temperature of the flue
gas entering the stack at full load.
130F to 350F. At these low temperatures
it is recommended that the stack
interior be painted or sprayed with a highsolid
type nonasphaltic mastic type coating
three-sixteenth to a quarter-inch wet
thickness. This type of coating will prolong
the life of the stack interior from hot sulfuric,
hydrochloric, and hydrofluoric acid
solutions as well as from vapors present in
the flue gas. The application of this mastic
is usually made by painters, not by brickwww.powermag.com
The
Cost of Repairs Can Be Steep
Stacks rarely need repair and are lowmaintenance
items that only require
attention if they have corrosion issues.
However, a small investment made early
in their lives can save on expensive repair
costs in the future.
Here's an example of the potential
savings. Installing a mastic coating, as
described above, on a 7-foot-diameter,
96-foot-high stack operating at 300F will
require scaffolding, sand blasting, priming,
and coating with the mastic and will
take approximately six days to complete.
Compare that maintenance cost to the
$250,000 required to replace the stack and
the lost revenue during the 30 days the
plant will be out of service for repairs.
Also, pay attention to your exit flue
gas temperature, especially if you change
the size of the economizer or air preheater,
because the exit gas temperatures
may change. This temperature change
could affect the level of stack protection
required to prevent corrosion. ■
-Contributed by Gary Bases, president
of BRIL Inc., an independent consulting
firm specializing in brick, refractory,
insulation, and lagging. Bases is also
the author of The Bril Book(a guide to
brick, refractory, insulation, and lagging
systems) and The Bril Book II (a technical
manual with application drawings for
the power generating industry).
POWER | August 2010

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POWER August 2010

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