can occur on surfaces that are
lower in temperature than the dew point of the flue gas to which they
are exposed. Air heaters and economizers are particularly susceptible
to corrosive attack. Other cold-end
components, such as the induced draft fan, breeching, and stack, are less
frequently problem areas. The accumulation of corrosion products often
results in a loss of boiler efficiency and, occasionally, reduced capacity
due to flow restriction caused by excessive deposits on heat transfer
particle emission, commonly termed "acid smut" or "acid
fallout," is another cold-end problem. It is caused by the production
of large particulates (generally greater than 100 mesh) that issue from
the stack and, due to their relatively large size, settle close to the
stack. Usually, these particulates have a high concentration of condensed
acid; therefore, they cause corrosion if they settle on metal surfaces.
most common cause of cold-end problems is the condensation of sulfuric
acid. This chapter addresses problems incurred in the firing of sulfur-containing
fuels. Sulfur in the fuel is oxidized to sulfur dioxide:
A fraction of the sulfur dioxide, sometimes as high as 10%, is
oxidized to sulfur trioxide. Sulfur trioxide combines with water to form
sulfuric acid at temperatures at or below the dew point of the flue gas. In a
boiler, most of the sulfur trioxide reaching the cold end is formed according to
the following equation:
The amount of sulfur trioxide produced in any given situation
is influenced by many variables, including excess air level, concentration of
sulfur dioxide, temperature, gas residence time, and the presence of catalysts.
Vanadium pentoxide (V2O5) and ferric oxide (Fe2O3),
which are commonly found on the surfaces of oil-fired boilers, are ef-fective
catalysts for the heterogeneous oxidation of sulfur dioxide. Catalytic effects
are influenced by the amount of surface area of catalyst exposed to the flue
gas. Therefore, boiler cleanliness, a reflection of the amount of catalyst
present, affects the amount of sulfur trioxide formed.
The quantity of sulfur trioxide in combustion gas can be
determined fairly easily. The most commonly used measuring techniques involve
either condensation of sulfur trioxide or adsorption in isopropyl alcohol.
Figure 22-1 is a curve showing the relationship of sulfur trioxide concentration
to dew point at a flue gas moisture content of 10%. Higher flue gas moisture
increases the dew point temperature for a given sulfur trioxide-sulfuric acid
concentration. Cold-end metal temperatures and flue gas sulfur trioxide content
can be used to predict the potential for corrosion problems.
At the same sulfur content, gaseous fuels such as sour natural
gas, sour refinery gas, and coke oven gas produce more severe problems
than fuel oil. These gases contain more hydrogen than fuel oil, and
their combustion results in higher flue gas moisture. Consequently,
dew points are raised. With any type of fuel, corrosion and fouling
potentials rapidly increase below gas temperatures of 140 degrees F
(60 degrees C), which is the typical water dew point for flue gases.
Cold-end corrosion and deposition are usually much less severe
in coal-fired boilers than in oil-fired units. Usually, coal ash is
alkaline, so it increases the pH of the deposits formed in cold-end
sections. Thus, the extent of the corrosive attack by sulfuric acid
is diminished. Also, the high level of ash present when coal is fired
results in a lower concentration of acid in the ash particle. At the
same sulfur content, coal firing dew points are generally 20-40 degrees
F lower than oil firing dew points.
The most common cause of deposition within air preheaters is
the accumulation of corrosion products. Most air preheater deposits contain at
least 60% iron sulfates formed by the corrosion of air heater tube metal.
Therefore, a reduced corrosion rate frequently reduces the fouling of air
A regenerative air preheater can reduce cold-end problems when
installed instead of a recuperative air preheater on a new or existing boiler.
In the regenerative air preheater design, heat transfer surfaces are below the
acid dew point for much shorter periods of time.
Most modern regenerative air preheaters are equipped with
steam or compressed air sootblowers and fixed or oscillating water washing
nozzles. In boilers equipped with multiple units, individual air preheaters can
be isolated and washed on-line. Suitable drain connections must be provided as
well as a system for treating the wash water prior to disposal. Washing is
generally continued until the pH of the wash water is above 4.5. The wash water
effluent is a relatively low pH stream with a high soluble iron content. Most
air preheaters are washed with untreated water. Some operators add caustic soda
or soda ash to neutralize the deposits and lower the loss of air heater metal
The average cold-end temperature of an operating air preheater
is the sum of combustion air inlet temperature and flue gas outlet temperature,
divided by two. The average cold-end temperature is generally used in the
assessment of potential problems and the selection of air preheater size and
materials of construction. The average cold-end temperature of an operating air
preheater must be maintained in accordance with the manufacturer's
specifications. Corrosion-resistant materials are used in some regenerative air
preheater cold sections to obtain the lowest possible stack gas temperature and
consequently the highest boiler efficiency.
Steam Coil Air Preheaters
In some installations, heating coils are placed between the
forced draft fan outlet and the air preheater inlet to accommodate seasonal
fluctuations in incoming combustion air temperature. These heat exchangers are
commonly termed "steam coil air preheaters." They maintain the average
cold-end temperature of the air preheater above the acid dew point. Where steam
coils are used, the temperature of the combustion air entering the air heater is
independent of the am-bient temperature.
Steam coil air preheaters are also installed when boilers are
changed from coal or gas firing to oil firing. Steam coils are installed because
oil firing requires maintenance of an air preheater average cold-end temperature
that is higher than that normally specified for the firing of natural gas or
coal. The operation of steam coil air preheaters results in an increase in the
heat rate of the steam plant. Combustion air bypasses around the air heater and
hot air recirculation have also been used to control average cold-end
temperatures. Both of these methods reduce boiler efficiency.
Minimizing Air Infiltration
The operation of a boiler at or below 5% excess air can result
in a marked reduction in flue gas sulfur trioxide content and dew point. An
experimentally determined relationship for one boiler is shown in Figure 22-2.
The infiltration of air into the flame zone or into an area where the catalytic
oxidation of sulfur dioxide is occurring increases the potential for cold-end
problems. Therefore, maintenance and inspection procedures should be directed
toward minimizing air infiltration.
Minimizing Flue Gas Moisture Content
As previously stated, the dew point is not only affected by
the partial pressure of sulfuric acid in the flue gas but also by the partial
pressure of water in the flue gas. The minimum obtainable flue gas moisture
content is determined by the moisture content of the fuel and combustion air and
by the hydrogen content of the fuel.
The moisture content of coal is somewhat controllable through proper
handling and storage procedures. Handling and storage specifications
can be written limiting the moisture content of fuel oil. Factors that
increase flue gas moisture content include:
|| boiler tube leaks
|| steam coil air preheater leaks
||excessive boiler or air heater soot blowing
||leaking water wash nozzles
|| instrumentation leaks
When two fuels (such as coal and oil,
oil and natural gas, or blast furnace gas and coke oven gas) must be
fired simultaneously, certain ratios produce the highest dew points.
The worst ratio on a Btu-fired basis is 1:1.
When a fuel is fired that has a higher hydrogen content than
the base fuel normally used, the flue gas produced has a higher moisture
content, resulting in an increased dew point. When possible, fuels of different
hydrogen content should be fired separately.
Figure 22-3 graphically depicts the
influence on sulfuric acid dew point that results when natural gas and a
sulfur-containing fuel are fired simultaneously in a single boiler.
Many chemical solutions have been devised to control cold-end
deposition and corrosion. These solutions can be divided into two broad
classifications: fuel additives and cold-end additives. Fuel additives are
compounds that are added directly to the fuel or combustion process. Cold-end
additives are fed into the back of the boiler after steam-generating surfaces so
that they spe-cifically treat only the lower-temperature areas.
Magnesium and magnesium/aluminum based fuel additives are used
to reduce sulfur trioxide in the flue gas. These compounds function primarily by
altering the effectiveness of the iron and vanadium catalysts. They are fed to
liquid fuels, most commonly residual fuel oil. Alkaline fuel oil additives, such
as magnesium, also increase the pH of deposits formed on cold-end surfaces,
thereby reducing corrosion.
Cold-end corrosion and deposition can be controlled more
economically and effectively through the use of cold-end additives. Cold-end
additives include sulfuric acid neutralizing agents and corrosion inhibitors.
Magnesium Compounds. Alkaline magnesium compounds, such as magnesium oxide and
magnesium carbonate, are fed to reduce the sulfur trioxide content of flue
gases. These compounds are fed in high-temperature areas, such as primary
superheater sections. The reaction product formed, magnesium sulfate, often
increases deposition within air preheaters.
The main benefit of magnesium compound injection is a
reduction in air preheater corrosion. Often, the level of fouling is not
appreciably altered, because the corrosion product fouling is replaced by
magnesium sulfate fouling. Therefore, where magnesium compounds are used,
suitable water wash nozzles must be present to permit periodic removal of
Additives that remove sulfur trioxide from flue gas must be
fed in stoichiometric quantities with respect to the amount of sulfur trioxide
to be removed. Therefore, higher levels of sulfur in the fuel require higher
feed rates for protection. Coal-fired boilers require less treatment than
oil-fired boilers for a given sulfur level in the fuel.
Corrosion Inhibitors. Corrosion inhibitors can be added to the cold end of
the boiler to alleviate problems associated with the condensation of sulfuric
acid. These materials do not neutralize the sulfuric acid in the flue gas; they
prevent surface corrosion where the sulfuric acid condenses. Fouling of the air
preheater is reduced because the quantity of corrosion products is reduced.
Although the dosage of inhibitor required to achieve the desired effect
increases with increasing acid content in the flue gas, the relationship is not
The compositions of inhibitor-type cold-end additives are
usually proprietary. Products are available in powder and liquid form. Liquid
solutions are injected upstream of the problem area with atomizing spray
Justification for cold-end additives is generally based on the
benefits obtained by higher unit heat rates and lower maintenance costs for the
equipment in the cold-end section. The feed of cold-end additives enables the
unit to operate with a lower rate of steam flow to the steam coil air preheaters,
resulting in an increase in unit heat rate. If average cold-end temperature is
controlled with bypasses, the bypassed air flow can be reduced so that an
improvement in boiler efficiency is obtained. A smaller improvement in heat rate
is gained through reduction of fan horsepower, which reduces the average
pressure drop across the air preheater.
Acid Smut Control. Cold-end additives can be used to reduce acid smut problems.
In some in-stances, it is believed that smut is created when fly ash particles
agglomerate to form larger particles. These particles adsorb sulfuric acid mist
and become highly acidic. Fly ash deposits often accumulate in low-temperature
areas of breeching. During soot blowing or load changes, some of the deposited
fly ash can be entrained in the flue gas stream and carried out the stack. The
large particles then settle in the vicinity of the stack. Magnesium-based fuel
additives have been beneficial in reducing acid smut problems by increasing the
pH of the deposits.
Evaluation and Monitoring Techniques
Corrosion Rate Measurement. Various devices are available to assess the impact of
additive application on corrosion rates. Table 22-1 shows a selection of
monitoring methods. In some cases, problems in the breeching, induced draft
fans, and stack can be measured by corrosion coupons placed within the flue gas
For air heater corrosion and fouling problems, some provision
must be made to maintain a corrosion specimen temperature that is within the
ranges typically found in the operating air preheater. Corrosion probe methods
(see Table 22-1) are used to simulate corrosion rates in an operating air
Figure 22-4 shows a multipoint corrosion probe. The temperature to be
maintained on the specimen is determined by calculation of average cold-end
temperature and measurement of dew point.
22-1. Comparison of corrosion rate assessment methods
|Corrosion coupon installed on surfaces
||loss of weight of coupon
||simple; measures corrosion directly
||not clear if location of coupon represents
|Air-cooled corrosion probe for flue gas test
||loss of weight of coupon
||simple; can determine corrosion as a function
||temperature cannot be closely controlled
without relatively high expense
|Multipoint corrosion probe
||short-range loss of iron
||low to moderate
||can determine corrosion over a wide temperature
range; rapid indication
||difficulty extrapolating short-range to
|Sampling of flue gas
||flue gas sulfur trioxide
||direct measurement of SO3; then dew point
temperature is accurate
||more technical knowledge required to operate
equipment; not a direct indication of corrosion
|Electrical conductivity, dew point and rate of
acid buildup meter
||temperature at which acid condenses on probe;
rate of acid deposition below the dew point
||quick; gives some idea of corrosion problem
||low sulfur trioxide gives inaccurate and
nonreproducible results; high dust loadings interfere with rate of acid
buildup measurements; does not measure surface coating additive effect
The electrical conductivity dew point meter is useful in
problem assessment work and some results monitoring. In addition to corrosion
monitoring, this meter provides an indication of the deposition rate by
measuring the increase in the conductance of an acid-containing film with
respect to time. The electrical conductivity meter is acceptable for results
monitoring only where a sulfur trioxide removal additive is used.
The dew point
meter is shown in Figure 22-5.
Normally, direct measurement of sulfur trioxide is used only
for Environmental Protection Agency tests because of its cost and complexity.
All of these evaluation techniques and tools are used by suppliers of
proprietary cold-end treatments. They enable the engineer to define the problem
and measure results properly.