A boiler operator has many problems to overcome. The only ones with which
LMGI will be concerned are those related to the chemistry of the hot gas path.
Because of the nature of boiler operations, boilers will normally burn the
poorest quality fuels. These fuels will consequently have many contaminants
and higher sulfur levels than fuels seen in gas turbine or diesel engine use.
These fuels contain primarily the contaminants vanadium, sodium, and sulfur.
The two metals when combusted and at high temperatures cause both corrosion
and hard slag deposits. Sulfur combustion can cause acid corrosion problems.
Fireside problems due to these contaminants
can exist in three separate areas of a boiler. In the firebox area and
convection section (superheater and reheater), where temperatures may reach
or exceed 600° C, deposits due to vanadium, sodium, lead and potassium can
accumulate and reduce heat transfer. Corrosion of metal tubes and refractory
can occur if these deposits remain in a molten or semi-molten state. In the
colder areas of the boiler (economizer, air preheater, and stack) where
temperatures may fall below the acid dew point (below 150° C), sulfuric acid
formed from combustion of sulfur in the fuel can condense on metal surfaces
and destroy them through acidic attack. Another major problem that may arise
in a heavy fueled boiler is smoking and soot emissions due to incomplete
combustion. These problems can be substantially reduced or eliminated by
chemical modification of the combustion products of the fuels by using
fireside additives, primarily magnesium based. There is no substitute for
appropriate equipment design and correct engineering procedures (and in fact
mechanical problems should be investigated and eliminated before beginning an
additive treatment program) in boiler operations. However, good fireside
additives correctly applied have earned their place in the boiler field. Even
with improved designs, it may still be necessary to treat a particularly
contaminated fuel with magnesium additives to lessen the effects of corrosion
and deposits. The use of magnesium additives has been an accepted practice
for many years.
Hard Slags and Hot Corrosion
When vanadium and sodium are combusted
they form vanadium pentoxide and sodium oxide/sulfate (just which depends on
other boiler conditions). Normally vanadium pentoxide has a melting point of
675° C. However, when even traces of sodium salts are mixed with the vanadium
pentoxide, even lower melting points result. It is quite possible for the
melting point of these mixtures to be at or below the operating temperature
of a modern boiler. When at or below the melting point, these compounds will
be molten. When in a molten state, the compounds will be sticky. Think of the
situation with ice and water (below the melting point, above the melting
point respectively). If ice is thrown against a wall it doesn't stick. If
water is thrown against the wall it does stick (although obviously most of
the water will run off). It is the same situation with the vanadium
pentoxide/sodium sulfate mixtures in boilers. When they are molten, they
stick to the tubes and other surfaces in the boiler. The amount of material
that "sticks" to the surfaces will be dependent upon the quantity
of material in the fuel to form the sticky deposits and the temperature of
the boiler. The hotter the boiler (or the section of the boiler) the more
likely the materials will be molten. This is part of the reason the
superheater sections often have the heaviest deposits.
As these deposits accumulate on the
tubes in particular, the cooling effect of the water inside the tubes lowers
the effective temperature seen by the deposit. When the melting point (freeze
point in this direction) is reached the molten material solidifies into a
hard slag. But the slag itself acts as an insulator keeping the outside of
the deposit hot. This allows more deposit to form on the outside as it
remains above the melting point of the deposit. But as the deposit grows, it
insulates the inner portion from the heat so it cools and solidifies. This
cycle can continue until a deposit can grow to be quite thick. Gravity can
affect the way the deposit grows. As it gets thicker, gravity may cause it to
drip or fall off the tubes. This can lead to some interesting deposit growths
in severely slagged boilers. As the slag deposits grow they can
"bridge" the tubes, completely closing off the air passages.
The most obvious effect of the deposits
is to make the boiler very difficult to clean. When the boiler is brought
down, the slags cool into hard deposits. The immediate problem for the
operator is the reduction in heat transfer into the water tubes. Slag is an
insulator so heat will not be transferred easily into the tubes. As the heat
transfer is reduced, the operator will typically increase the amount of fuel
burned to try to generate more heat to restore the proper steam rates. This
fact provides a method to determine if a boiler is slagging up during
operation. The amount of fuel consumed by the boiler per hour (or day) will
increase as time passes. Also, since the heat generated by combustion is not
being absorbed by the water to form steam, the heat will pass deeper into the
boiler. Temperatures in the superheater, reheater, economizer, air preheater,
and/or stack will all increase. The rise in any of these temperatures is most
likely due to the buildup of slag deposits on the tubes. It has been calculated
that for each 55° C increase in stack temperature, a 10 to 15° C loss in
steam temperature may result.
While the slag deposits are molten and
in contact with the tube metal, the protective oxide coating of the tube
metals may be dissolved and the tube corroded. When this occurs, tube
thinning can result. If the thinning becomes severe enough, a tube blowout or
leak may result. Corrosion of tubes only occurs while the deposits are
molten. And as suggested earlier, as the deposits grow in thickness and the
inner portion of the deposit solidifies, corrosion will actually lessen since
the deposit is no longer molten.
Cold - End Corrosion
When sulfur contained in the fuel is
combusted it forms primarily sulfur dioxide. About 2 - 5 % of the sulfur
dioxide formed is further oxidized to sulfur trioxide in the presence of
appropriate catalysts and additional oxygen. This formation occurs when the
dioxide is in contact with an iron or vanadium oxide (slag) surface at
temperatures of 500 - 600° C , and if there is extra oxygen in the flue gases
to react with it. The trioxide that forms in the flame quickly decomposes
back to the dioxide due to thermodynamic considerations. Most of the sulfur
trioxide forms after the flame. When sulfur trioxide is present it will
condense with water vapor to form sulfuric acid when the acid dew point is
reached. This acid collects on iron surfaces causing corrosion. Because the
acid dew point will only be reached in colder parts of the boiler this is
called cold-end corrosion.
Although all sulfur in the fuel will
form the dioxide as it is combusted along with oxygen in the air, a catalyst
is required to effectively form the trioxide form of sulfur. Unfortunately
for boiler operators, hot iron surfaces and the vanadium pentoxide slags are
two of these effective catalysts. Various salts of nickel can also act as
catalysts.
Virtually no sulfur trioxide is formed
in a boiler except by the above reaction utilizing a catalyst. Unfortunately
for the boiler operator the catalyst can be any iron or slag-covered surface
at the appropriate temperature. The temperature range at which this occurs is
in the range of many boiler superheater and reheater sections. Therefore,
almost any boiler that burns a sulfur-containing fuel will generate varying
quantities of sulfur trioxide. If the temperature in any part of the boiler
drops below the acid dew point (which can be any temperature below about 150°
C, depending on the concentration of sulfur trioxide in the flue gas), the
sulfur trioxide can condense with water vapor to form highly corrosive
sulfuric acid.
When this corrosive acid condenses on
iron, such as those in an air preheater or a cold stack, it will attack the
iron or refractory surfaces and eventually can result in failure of the
corroded components. This is known as acid corrosion or cold-end corrosion.
It is apparent, therefore, that reduction of sulfur trioxide is a favorable
goal for a boiler operator to pursue. This is difficult to achieve since
either sulfur and/or air would need to be reduced.
Smoke and Plumes
Smoke and plumes in boiler operation is
a widespread problem. There are many causes of smoke and plumes in boilers.
One cause of this problem is the drive by the operators to reduce the amount
of excess air as much as possible. The reason for this is economic. Since air
is only 21% OXYGEN, there is a lot of "other" gas pulled into the
boiler along with the oxygen to support combustion. The heat from burning
fuel needs to warm all this "air" in the combustion process. This
takes away from usable heat that would go into boiling water. To keep this
loss of heat to a minimum, boiler operators try to minimize the excess air to
that level that just supports combustion with just a small amount extra. This
excess air is that amount of air above the stoichiometric amount of oxygen
needed to exactly produce carbon dioxide and water vapor from the carbon and
hydrogen in the fuel. Remember that air is only about 21% OXYGEN, the rest of
the air does not participate in the combustion process. In fact the nitrogen
present can lead to air pollution by forming NOx. Unfortunately as the amount
of excess air is reduced, the efficiency of combustion is adversely affected.
When a critical point is reached, a smoky exhaust plume may be seen. This is
caused by unburned hydrocarbons that carry out the stack.
Other causes of plumes and smoking in
boilers may be mechanical in origin. For example worn or improperly adjusted
oil guns in burners may cause poor combustion. And if black smoke results,
this is lost BTU's going up the stack as the full heat has not been liberated
from the fuel. This impacts boiler efficiency.
A blue-white, persistent plume signals
the presence of sulfur trioxide. This is actually fine droplets of condensed
sulfuric acid. The plume itself may be thick or thin, it may be voluminous or
not. The characteristic that always accompanies this sulfuric acid plume is
persistence. The plume may carry off great distances.
White plumes, are most likely water
vapor. A reddish colored plume, is most likely nitrogen oxides. Both of these
arise from the combustion process. Water vapor is of course not a problem.
Nitrogen oxide plumes may be a sign the burners need upgrading to newer types
of burners that don't produce as much of these pollutants.
LMGI Magnesium Solutions
For every problem there is a solution.
Most of the problems that were listed above
can be solved with the appropriate application
of a magnesium product from LMGI.
The mechanism of the solutions
are described in this section.
Hard Slags and Hot Corrosion
Other than living with these problems,
an operator has three choices to reduce or eliminate these: A: he can change fuel to one that is cleaner
(normally very costly if possible at all; B: he
can reduce excess oxygen to a level where lower valence (a chemical term
reflecting the oxidation state of various elements, in this case vanadium),
higher melting point compounds will be formed (in actual practice this is
nearly impossible to achieve as severe smoking will occur first); or C: use an additive. The additive is the best
choice for two reasons: additive metals are able to increase the melting
point of the ash components to a level above the system temperatures and the
additive metal modifies the ash that does form to a soft, powdery and
extremely friable form that will not form hard deposits. These soft powdery
ashes are very easy to remove from boiler surfaces, often with normal soot
blowing. Upon shutdown, these deposits can often be removed with water
washing.
Magnesium additives are chosen most
often for this purpose. When magnesium is added to the fuel in a highly
available and reactive form, it will react within the flame and combine with
vanadium pentoxide. The combination results in a higher melting form of
magnesium orthovanadate. This compound melts at temperatures in excess of
those found anywhere in the boiler. When the deposits are no longer molten,
they are no longer sticky. Hence slag will not form. When the deposits are no
longer molten, they are no longer corrosive. The surface oxide dissolving
mechanism no longer occurs.
Experiments performed in the industry,
found that an additive should treat not only the vanadium, but it was also
useful to add magnesium for any sodium, potassium and/or lead in the fuel. By
treating for the sodium too, beneficial magnesium will help to dilute and
separate the deposit components.
The amount of magnesium required for a
product that forms extremely small magnesium oxide particles when passing
through the flame is between 0.1 and 0.5 parts for each part of contaminant
metal in the fuel. The magnesium products from LMGI do exactly that when
passing through the flame. The exact amount of magnesium required will depend
on the actual boiler parameters: temperature, contaminants, duty cycle,
sodium to vanadium ratio, boiler size, excess air, and other factors.
Many slurry suppliers (the most
widespread method of treating boilers) must recommend 1 part of magnesium for
each part of contaminant metal (vanadium, sodium, potassium). The increased
reactivity, smaller resulting particle size, and substantially larger number
of particles formed from oil-soluble materials allow this class of materials
to be used at lower treating levels. Oil soluble additives are just more
efficient in finding the particles of vanadium in the flame and gas path.
This makes appropriate oil soluble products up to 10 times more effective
than less reactive slurries.
Cold - End Corrosion
Just as with hot corrosion problems,
there are multiple methods of solving the cold end corrosion problem. The
operator has four solutions in this case. These are:
In order to prevent the acid from
forming before it reaches the colder sections of the boiler requires an
interruption of the reaction that forms sulfur trioxide from sulfur dioxide
(sulfur dioxide will always form). To achieve this an oil soluble additive is
added to the fuel. The very small particles of magnesium oxide that are
formed from an oil soluble additive coat the internal metal surfaces (iron,
vanadium, and nickel) of the boiler removing them as potential catalysts for
the sulfur dioxide to sulfur trioxide reaction. If no catalytic surface is
available, little sulfur trioxide will be formed. This has the benefit of
reducing the trioxide formed in this reaction. And highly reactive, small
particles of magnesium oxide have an additional benefit, they will travel
throughout the boiler to neutralize any sulfur trioxide or sulfuric acid that
is formed.
LMGI recommends treating cold-end
corrosion problems with magnesium. The exact amount will need to be
determined experimentally. The actual amount of sulfur trioxide formed will
depend on other factors (the amount of vanadium in the fuel, the temperature
of the boiler, excess oxygen levels, size of the boiler, etc.
Smoke and Plumes
If the smoking problem is caused by a
mechanical problem, the operator should check and adjust his boiler
components. There may be a slight increase in fuel needed to maintain
temperatures as the amount of excess air is increased. If after adjustment
the boiler still smokes and there are no additional mechanical problems, the
use of an effective combustion improver may help. The catalytic action of
iron, manganese, and several other metal elements have been useful in solving
this problem. The carbonaceous components of the fuel are more completely
burned and soot and smoke levels can be reduced. Improved combustion helps
the operator obtain more BTU's per pound of fuel.
Differences Between
LMGI Oil - Soluble Additives
and Slurries / Powders
LMGI oil-soluble additives are totally
dissolved in the fuel and are inherently stable in storage. This fact
distinguishes them from other types of additives, specifically slurries,
powders, emulsions, and aqueous solutions, all of which have problems of
stability in storage and in fuel. Such additives also have limited chemical
reactivity, which reduces their efficiency. Most additives used in treating
boiler problems are based upon magnesium oxide, hydroxide or other magnesium
salts. Although these products work for their intended purposes, they often
create more problems for the boiler operator than they solve. Several
problems are inherent in slurry type additives that are not problems with
oil-solubles. These include:
Slurries are suspensions of
relatively large particles in either water or oil. As such, they
experience settling problems when stored over even relatively short
periods. If kept for long periods, they must be continuously or
intermittently stirred. LMGI Oil-soluble products are inherently stable
in storage and require no mixing or stirring to maintain them in a
usable state.
Slurries, because of their
inherent instability, have to be injected into the fuel just prior to
combustion and must not enter fuel storage or they will settle out. LMGI
Oil-soluble additives are stable in fuel in storage. As such, oil
solubles allow treatment of the fuel at any point in the fuel handling
system. This provides extra flexibility in fuel and additive handling.
CAUTION: Excessive storage periods should still be avoided if possible.
Although product stability for several weeks is expected, unknown fuel
properties or detrimental fuel storage practices may make storage for
longer periods unwise. Testing at use temperature in actual fuel is
recommended before storing additized materials for longer periods.
Slurries contain granular
particles that may damage fuel nozzles and pumps in boiler applications.
Their basic incompatibility in the fuel may cause uneven treatment if
large slugs of additives are not well mixed into the fuel. LMGI
oil-soluble additives contain no abrasive particles to erode nozzles and
pumps. They are completely soluble in the hydrocarbon fuel and assure
more intimate contact and mixing with potential problem causing
contaminant species when combusted.
Because of the slurries'
stability limitations, extra handling (manpower) and expensive mixing
systems may be required to properly use the additive. With LMGI
oil-soluble products, only a simple chemical injection pump that can
handle fuel oil pressures is needed to assure proper injection of the
additive into the fuel (three to five pipe bends are needed for adequate
mixing).
Slurry type additives are
not as effective as LMGI oil-soluble additives. Third party literature
stating that oil-soluble products are from 3 to 5 times more effective
than slurries exist. The problem with the effectiveness of slurries is
inherent in their manufacture. The major component of most slurry
products is magnesium oxide (MgO). In the manufacture of this oxide, it
is often heated to high temperatures, or calcined, to drive off excess
water and prepare it for use. This heating, sometimes to temperatures as
high as 1850° C (3360° F), has in effect, "pre-reacted" the
particle. Also as a final step to preparing the particle of magnesium oxide
for use, the materials are ground to a relatively large particle size.
This results in a relatively large, unreactive particle. Reactivity is,
to a great extent, related to the surface area of the particles, MgO
slurry particles tend to not be reactive. Effective surface area, which
affects reactivity, varies with additive particle size. Liquid Minerals
oil-soluble additives, are in an unreacted nanoparticle magnesium form
until they are burned with the fuel. Combustion results in a "flash
calcination" of the additives. At this time they form very high
surface area, very small size particles. These particles are of very
high reactivity due to this fine size and high surface area. Therefore
less additive metal, in oil-soluble form, is required to obtain equivalent
results. This allows lower ash loading of the system compared to the
large amounts of ash contributed by (higher dosing) slurry additives.
The goal of treating is to have additive particles at their highest
reactivity in the combustion flame and immediately thereafter. It is
believed slurries decrease in reactivity upon burning, while LMGI
oil-solubles increase in reactivity upon burning.
Slurries form larger
particles than oil-solubles. The tendency of a particle to be blown
through a boiler is related to particle size. The smaller the particle
size, the more likely it will reach the section of the boiler where
problems occur. Large slurry particles may collect in the firebox area
of the boiler, but small particles from LMGI oil-soluble additives are blown
throughout the boiler, coating boiler surfaces more efficiently to
reduce SO3 formation and preventing slag from collecting on heat
transfer surfaces. Large particles do have one benefit: they coat
firebox surfaces faster, if that is desired. This can be used to
increase the temperature in the superheater by forcing the heat past the
firebox due to the additive coating. Oil-soluble additives have been
found to have fewer tendencies to do this in actual field tests.
Powders have even more problems with
particle size (they often have particles larger than 20 microns) and require
complicated handling equipment. Their use in boilers, particularly in the
cold end where they can plug flue gas passages should be avoided when
possible.
Emulsions and aqueous solutions also
suffer from the problem of fuel instability since they are not miscible in
hydrocarbon fuel. In addition, there is a limited amount of useful metallic
element (magnesium) that can be put into an emulsion or an aqueous solution due
to water solubility limitations. This seriously limits their value as
additives. Although in some contexts the introduction of large amounts of
water can be beneficial, most disciplined studies have found that these
benefits are off set by the fuel required to vaporize the water.
When Liquid Minerals first evaluated the
use of additives for this application we desired to avoid as many of these
problems as possible. The use of oil-soluble additives, is more expensive on
a cost per gallon basis, but NOT on a cost per treatment basis. LMGI
oil-soluble additives are more convenient and more effective than other types
of additives. LMGI produces the most technically advanced oil-soluble
additives available, and combined with our technical expertise, can solve
virtually any problem present in the combustion of heavy and/or contaminated
fuels.