Impurities in fuels can cause deposit formation and fireside metal surface
corrosion. Compounds of aluminum, barium, copper, iron, magnesium, manganese,
and silica have all been used to control combustion fouling and corrosion.
The most severe problems are generally found in combustion equipment firing
fuels that significantly deviate in composition from the fuel on which
the equipment design was based.
NATURE OF FIRESIDE DEPOSITION
Fireside fouling of combustion equipment is caused by the
deposition of fuel ash components. Table 21-1 shows analyses of typical high,
medium, and low ash liquid fuels. Oils containing more than 0.05% ash are
considered high ash oils; those containing less than 0.02% ash are considered
low ash oils.
21-1. Typical residual oil analyses.
|Specific Gravity, at 60 °F
|Viscosity SSF at 122 °F, sec
|Calorific Value, Btu/gal
|Bottom Sediment & Water, %
Sulfur emission regulations have severely restricted the use
of high sulfur oils. Generally, high sulfur oils (greater than 1.0% sulfur) have
high ash contents. These oils are usually imported from the Caribbean area.
Prior to 1972, most East Coast boilers were burning high sulfur, high ash oils.
The combustion of high ash oils produced
troublesome deposits on boiler convection surfaces such as steam generating,
superheat, reheat, and economizer sections. The firing of high ash oils
(even in those units which were originally designed to burn coal) produced
convection surface deposition that was difficult to remove by soot blowing.
The most troublesome oil ashes are those which contain a
vanadium/sodium ratio of less than 10:1. In Table 21-1, the fuel with the medium
ash content has a relatively low vanadium/sodium ratio and produced an
tenacious deposit on superheater surfaces (as shown in Figure 21-1).
Vanadium reacts with oxygen to form various oxides in the
furnace. When formed, vanadium pentoxide condenses within the furnace when gas
temperatures approach its solidification point.
Sodium also reacts with oxygen and sulfur trioxide to form
fouling compounds. Thermo-dynamics favor the formation of sodium sulfate rather
than the relatively unstable sodium oxide.
Nickel can also contribute to deposition by forming oxides.
Aluminum can be present in the oil in the form of cracking catalyst fines
(aluminum that is introduced during the refining of oil). Although it is
generally not a troublesome component, iron is occasionally present in fuels at
relatively low levels. When waste fuels are burned, some contaminants (such as
lead) can cause extremely dense and tenacious deposits.
Black liquor has been used as a boiler fuel for many years in
the Kraft paper industry. It contains a significant amount of combustible
material, along with sodium salts. Black liquor burning can produce relatively
adherent, low bulk density sodium sulfate deposits on recovery boiler convective
surfaces. In some instances, it is advisable and economically beneficial to
control or limit deposition with magnesium-based additives. These materials are
blended compounds which contain magnesium oxide and/or aluminum oxide.
The combustion of coal tar in boilers often produces
objectionable amounts of sodium salts and/or iron compounds on boiler convective
surfaces. Often, ash deposits are similar in composition to coal ash.
Combustion of solid fuels such as coal and bark (also referred
to as "hog fuel") can also lead to fireside slagging. Sodium, calcium,
silica, iron, and sulfur content are of primary concern in the burning of solid
fuels. Other metal oxides, such as alumina, titania, and potassium oxide, can
also aggravate slagging.
Figure 21-2 shows a heavy accumulation of slag in a
In addition to proper boiler furnace design, operational
considerations should be explored to prevent fireside slagging.
testing has been used by boiler manufacturers to assess slagging potential (see
Figure 21-3). Where design and operational improvements are impractical or
insufficient, chemical treatments (such as combustion catalysts and antifoulants)
should be considered.
TREATMENT FOR FOULING
Additives are used to control fouling by elevating the melting
point of the deposits, by physically diluting deposits, or by providing
a shear plane to assist in removal by soot blowing. Additives used to
control fouling contain magnesium, silica, manganese, and/or aluminum.
Figure 21-4 illustrates the
effect of treatment.
The melting point of untreated ash constituents can be as low
as 1000°F. The introduction of appropriate metal oxides elevates the melting
point of ash components by several hundred degrees. The additive components most
commonly used to raise the deposit melting point are magnesium and/or aluminum.
Dosages depend on ash levels in the fuel and the ratio of various ash
Fuel additives are intended to control fouling by forming a
friable, nonadherent deposition that can be removed by soot blowing.
When melting temperatures are raised, the physical
characteristics of the deposits are altered. Often, the heaviest deposition
occurs in areas where the gas temperature is lower than the melting temperature
of a deposit. Therefore, a treatment program designed solely to elevate the
melting point of the deposit will not solve the problem, and it is necessary to
introduce additive components that change the physical characteristics of the
deposit, making it more friable. Additives used for this purpose contain
compounds of magnesium or aluminum. Aluminum is usually the most effective
material for increasing friability of deposits.
Newer technology has been developed that offers certain
advantages over magnesium or magnesium/aluminum combination treatments.
Silicon-based materials have been used to control fouling and form friable
nonadherent oxides. The silicon component acts like a sponge to adsorb low
melting point oxides, preventing their agglomeration and subsequent deposition.
The silicon-based materials contribute far fewer solids than traditional
treatment. This reduces fouling and minimizes loading on the solids collection
Additive dosage should be fixed at the minimum level necessary
for proper conditioning of depo-sits. Overfeeding of additives, particularly
magnesium oxide, can cause troublesome deposition. Magnesium oxide can react
with sulfur trioxide in the flue gas to form magnesium sulfate deposits. These
deposits generally form in convection areas, such as primary superheaters, steam
generating sections, and economizers.
Boiler design has proven to be a very important consideration
in the determination of magnesium additive levels. The addition of aluminum to
magnesium-based additives reduces magnesium sulfate deposition.
The greatest need for fuel additives to control deposition is
in boilers previously used to fire other fuels, such as natural gas or coal.
Generally, units used to fire coal have sufficient soot blower placement for
adequate removal of properly conditioned deposits.
Natural gas-burning units generally do not have the desired
soot blowing equipment. It is advisable to contact the manufacturer so that the
proper number of soot blowers and their location can be specified. It is
virtually impossible to burn a fuel containing greater than 0.2% ash without the
use of soot blowers, even with the aid of a conditioning additive.
Boilers designed for oil firing incorporate design parameters
that allow for relatively trouble-free operation from a fireside standpoint.
Combustion gas velocity, tube spacing, furnace gas exit temperature, and
economizer configuration are adjusted to account for oil ash characteristics.
The most common problems associated with oil-design boilers are air preheater
fouling and corrosion. Deposition and corrosion can occur when the oil used is
of lesser quality than specified for the boiler, or if multiple oils are used.
Fuel ash corrosion in high-temperature areas can cause extensive
boiler damage. The corrosion is caused principally by complex oxide-slags
with low melting points. Corrosion by slag components, such as sodium
vanadyl vanadate, progresses rapidly between 1100 and 1650°F.
Sodium sulfate is also a primary corroding medium that can be
present with sodium vanadyl vanadate. It also is the primary corroding material
of gas turbine blades. High-temperature corrosion can proceed only if the
corroding deposit is in the liquid phase and the liquid is in direct contact
with the metal.
The liquid-phase deposits corrode by fluxing of the protective
oxide layer on the metal surface as shown in Figure 21-1. Deposits also promote
the transport of oxygen to the metal surface. Corrosion is caused by the
combination of oxide layer fluxing and continuous oxidation by transported
Tube spacers, used in some boilers, rapidly deteriorate when
oil of greater than 0.02% ash content is fired. Because these spacers are not
cooled and are near flue gas temperatures, they are in a liquid ash environment.
Fuel oil additives can greatly prolong the life of these components.
High-temperature corrosion caused by sulfidation is a common
problem in gas turbines. The corroding medium is sodium sulfate. Sodium enters
the gas turbine either with the fuel or combustion air. The sodium content of
the fuel can be lowered by means of water washing. It is relatively difficult to
remove all sodium-containing mists or particulates at the intake of the gas
turbine. Due to salt water mist ingestion, blade sulfidation problems are common
in gas turbines used for marine propulsion.
Reheating furnaces in the iron and steel industry have
suffered wastage of metallic recuperators. The problem is more prominent where
steel mills have converted reheat furnaces from natural gas to residual oil
firing. The gas inlet temperature at the recuperator is high enough to provide
an environment for high-temperature corrosion.
Firing of residual oil in refinery process heaters has caused
numerous corrosion failures of convection tube support members. The tube
supports are generally not cooled by water or air and, therefore, are at a
temperature close to that of the gas. In some cases, tube supports fabricated
from nickel and chromium alloys have been installed to alleviate this problem.
Additives containing magnesium and aluminum oxides have been
successful in controlling these and other high-temperature corrosion problems.
The additives function by elevating the melting point of the deposits. Enough
magnesium oxide must be added to enable the metallic constituents to remain in
the solid phase as they contact the metal surface.
In conjunction with an additive program to control
high-temperature corrosion, it is advisable to operate the unit at minimum
excess air levels. The corrosion rate is influenced by the oxygen content of the
combustion gas. The second phase of the corrosion mechanism is oxidation;
therefore, the partial pressure of oxygen in the combustion gas can be lowered
to reduce corrosion rates.
In residual oil-fired equipment, it is often necessary to feed
a 1:1 ratio of magnesium and vanadium to achieve low corrosion rates. This level
of feed is generally higher than that which is necessary to control fouling.
Combustion catalysts have been used for all types of fuels.
The combustion catalyst functions by increasing the rate of oxidation
of the fuel. Some fuels are difficult to burn within a given fixed furnace
volume. Combustion catalysts are applied to these fuels to comply with
particulate and opacity regulations. Combustion catalysts are also used
to improve boiler efficiency by reducing carbon loss in the flue gas.
Most soluble fuel additives contain metallo-organic complexes
such as sulfonates, carbonyls, and naphthenates. These additives are
in a very convenient form for feeding. Most dry fuel additive preparations
are used to treat fuels with high ash content, such as coal, bark, or
black liquor. Metal oxides are used for this purpose.