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Acid Dew-Point Corrosion:
Causes,
Corrective Action, and Inspection
The combustion of
most fossil fuels, natural gas being one exception, produces
flue gases that contain sulfur dioxide, sulfurtrioxide and
water vapor. At some temperature, these gases condense to
form sulfurous and sulfuric acids. The exact dew-point
depends on the concentration of these gaseous species, but
it is around 300F. Thus surfaces cooler than this
temperature are likely locations for dew-point corrosion.
Any point along the flue-gas path, from combustion in the
furnace to the top of the chimney, is a possible site.
The obvious locations
are openings to the furnace, support penetrations through
the roof, leaks around superheater, reheater and economizer
penetrations, and of course, the air preheater. In boiler
terminology, "acid dew-point" refers to the sulfuric-acid
dew-point, as this is the highest dew-point temperature.
Both sulfurous acid and hydrochloric acid condense at lower
temperatures. For hydrochloric acid, the dew-point may be as
low as 130F. While the precise dew-point for sulfuric acid
depends on the sulfur-trioxide concentration, at 10 parts
per million sulfur trioxide in the flue gas, the dew-point
is 280F.
Dew-point corrosion is
exacerbated in coal-fired boilers by the presence of fly
ash. Fly ash accumulates throughout the flue-gas path, and
the resultant deposit acts like a sponge to collect both
moisture and acid, especially during shutdown cycles.
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