Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR BURNING SULFUR CONTAINING FUELS
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to the field of burning sulfur-containing fuels and to
reducing the
production of SOX and NO,, therein.
Related Art
Over the past several years, power generation processes and other combustion
processes for
burning sulfur-containing fuels have been subject to increasingly strict
emissions restrictions,
particularly for NO,, and SOX. Sulfur-containing fuels that are popular for
power generation
but especially problematic for emissions include coal, petcoke and heavy fuel-
oil boilers.
Current methods of removing SOX from, e.g., coal fired boilers are very
expensive. It is
anticipated that within a few years, most of those boilers in the U.S. will
require de-
sulfurization equipment. NOX removal techniques are similarly expensive,
complex and
difficult to operate. The preferred embodiments of the present invention
disclose cost-
effective methods to remove SOX and NOX.
Thus, a problem associated with processes for burning sulfur-containing fuels
that precede the
present invention is that they produce a level of SOX emission that is
unacceptable in view of
existing environmental regulations.
Yet another problem associated with processes for burning sulfur-containing
fuels that precede
the present invention is that they produce a level of NOX emission that is
unacceptable in view of
existing environmental regulations.
Still another problem associated with processes for burning sulfur-containing
fuels that precede
the present invention is that they have not been successively modified to
provide adequate
combustion characteristics resulting in adequate reduction of SOX formation
sufficient to meet
environmental guidelines without expensive and complex SOX treatment
apparatus, such as
scrubbers, etc.
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Another problem associated with processes for burning sulfur-containing fuels
that precede the
present invention is that they have not been successively modified to provide
adequate
combustion characteristics resulting in adequate reduction of NOX formation
sufficient to meet
environmental guidelines without expensive and complex NOX treatment
apparatus.
An even further problem associated with processes for burning sulfur-
containing fuels that
precede the present invention is that they have not been provided with a means
for chemically
preventing the formation of SOX and concurrently provided with a mechanism to
avoid problems
associated with slagging or other fouling of the combustion equipment.
Another problem associated with processes for burning sulfur-containing fuels
that precede the
present invention is that they have not been provided with a means for
chemically preventing the
formation of NOX and concurrently provided with a mechanism to avoid problems
associated
with slagging or other fouling of the combustion equipment.
For the foregoing reasons, there has been defined a long felt and unsolved
need for a process for
burning sulfur-containing fuels that facilitates an inexpensive, non-intrusive
method for reducing
the formation of SOX while at the same time maintaining the operability and
safety of the
combustion process.
SUMMARY OF THE I1NVENTION
A process for burning a sulfur-containing fuel to produce a flue gas is
disclosed. The process
comprises introducing a sulfur-containing fuel into a combustion chamber,
introducing at least
one oxygen enriched oxidant stream into the combustion chamber, and
introducing potassium
carbonate into the combustion chamber. The sulfur-containing fuel is burned to
produce the flue
gas and potassium sulfate.
An aspect of the present invention is to provide a process for burning sulfur-
containing fuels that
produces a level of SOX emission that is within acceptable levels in view of
existing
environmental regulations.
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Yet another aspect of the present invention is to provide a process for
burning sulfur-containing
fuels that produces a level of NOX emission that is within acceptable levels
in view of existing
environmental regulations.
Still another aspect of the present invention is to provide a process for
burning sulfur-containing
fuels that can be successively modified to provide adequate combustion
characteristics resulting
in adequate reduction of SOX formation sufficient to meet environmental
guidelines without
expensive and complex SO, treatment apparatus, such as scrubbers, etc.
Another aspect of the present invention is to provide a process for burning
sulfur-containing
fuels that can be successively modified to provide adequate combustion
characteristics resulting
in adequate reduction of NOX formation sufficient to meet environmental
guidelines without
expensive and complex NOX treatment apparatus.
An even further aspect of the present invention is to provide a process for
burning sulfur-
containing fuels that provides a means for chemically preventing the formation
of SO, and
concurrently provides a mechanism to avoid problems associated with slagging
or other fouling
of the combustion equipment.
Another aspect of the present invention is to provide a process for burning
sulfur-containing
fuels that provides a means for chemically preventing the formation of NOX,
and concurrently
provides a mechanism to avoid problems associated with slagging or other
fouling of the
combustion equipment.
Another aspect of the present invention is to provide a process for burning a
sulfur-containing fuel
to produce a flue gas, the process comprising: introducing a sulfur-containing
fuel into a combustion
chamber at a fuel inlet; oxygen-enriching at least one oxidant stream;
introducing a primary oxidant
stream into the combustion chamber at a primary oxidant inlet positioned
proximate to or coincident
the fuel inlet and mixing it with the sulfur-containing fuel to define a first
combustion zone;
introducing a secondary oxidant stream into the combustion chamber at a
secondary oxidant inlet
positioned so that the secondary oxidant enters the combustion chamber in the
primary combustion
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zone; introducing a tertiary oxidant stream into the combustion chamber at a
tertiary oxidant inlet
positioned away from the primary oxidant inlet and away from the secondary
oxidant inlet, the
tertiary oxidant entering the combustion chamber to define a secondary
combustion zonethe
tertiary oxidant stream completing combustion of the sulfur-containing fuel;
introducing potassium
carbonate into the combustion chamber via the tertiary oxidant stream; and
burning the sulfur-
containing fuel to produce the flue gas and potassium sulfate.
Another aspect of the present invention is to provide a process of the present
invention wherein the
total oxygen content of the oxidant entering the combustion chamber exceeds 21
%.
Another aspect of the present invention is to provide a process of the present
invention wherein the
total oxygen content of the primary oxidant exceeds 21 %.
Another aspect of the present invention is to provide a process of the present
invention wherein the
total oxygen content of the secondary oxidant exceeds 21 %.
Another aspect of the present invention is to provide a process of the present
invention wherein the
total oxygen content of the tertiary oxidant exceeds 21 %.
Another aspect of the present invention is to provide a process of the present
invention wherein the
potassium carbonate is introduced into the combustion chamber in an amount
sufficient to exceed
the stoichiometric requirement needed to react with the sulfur in the fuel by
between 0% and 50%.
Another aspect of the present invention is to provide a process of the present
invention wherein at
least half' of the sulfur in the sulfur-containing fuel is converted to
potassium sulfate.
Another aspect of the present invention is to provide a process of the present
invention wherein the
potassium carbonate is introduced into the combustion chamber through the fuel
inlet.
Another aspect of the present invention is to provide a process of the present
invention wherein the
total oxygen content of the primary oxidant exceeds 21 %.
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Another aspect of the present invention is to provide a process of the present
invention wherein the
total oxygen content of the secondary oxidant exceeds 21 %.
Another aspect of the present invention is to provide a process of the present
invention wherein the
total oxygen content of the tertiary oxidant exceeds 21 %.
Another aspect of the present invention is to provide a process for burning a
sulfur-containing fuel
to produce a flue gas, the process comprising: introducing a sulfur-containing
fuel into a combustion
chamber at a fuel inlet; introducing a primary oxidant stream containing more
than 21 % oxygen into
the combustion chamber at a primary oxidant inlet positioned proximate to or
coincident the fuel
inlet and mixing it with the sulfur-containing fuel to define a first
combustion zone; introducing a
secondary oxidant stream containing more than 21 % oxygen into the combustion
chamber at a
secondary oxidant inlet positioned so that the secondary oxidant enters the
combustion chamber in
the primary combustion zone; introducing a tertiary oxidant stream containing
more than 21 %
oxygen into the combustion chamber at a tertiary oxidant inlet positioned away
from the primary
oxidant inlet and away from the secondary oxidant inlet, the tertiary oxidant
entering the
combustion chamber to define a secondary combustion zone; the total oxygen
content of the oxidant
entering the combustion chamber exceeding 21 %; introducing potassium
carbonate into the
combustion chamber through the tertiary air inlet in an amount sufficient to
exceed the
stoichiometric requirement needed to react with the sulfur in the fuel by
between 0% and 50%; and
burning the sulfur-containing fuel to produce the flue gas and potassium
sulfate; wherein at least
half of the sulfur in the sulfur-containing fuel is converted to potassium
sulfate.
These and other aspects, advantages and features of the present invention will
be apparent from
the detailed description that follows.
BRIEF DESCRIPTION OF THE DRAWINGS
In the detailed description that follows, reference will be made to the
following figures:
Fig. 1 is a schematic illustration of a first preferred embodiment of a
process for burning a sulfur-
containing fuel;
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Fig. 2 is a schematic illustration of a second preferred embodiment of a
process for burning a
sulfur-containing fuel;
Fig. 3 is a schematic illustration of a third preferred embodiment of a
process for burning a
sulfur-containing fuel; and
Fig. 4 is a graph illustrating data of theoretical data expected from burning
a sulfur containing
fuel according to a preferred embodiment of a process for burning sulfur-
containing fuel.
DESCRIPTION OF PREFERRED EMBODIMENTS
In its simplest application, a process for burning a sulfur-containing fuel to
produce a flue gas is
disclosed. The process comprises introducing a sulfur-containing fuel into a
combustion
chamber, introducing an oxidant stream into the combustion chamber and mixing
it with the
sulfur-containing fuel to define a combustion zone, and introducing potassium
carbonate into the
combustion chamber. The sulfur-containing fuel is burned to produce the flue
gas and
potassium sulfate.
In the preferred embodiments, a combustion subassembly uses at least two, and
sometimes
three, oxidant streams. In the example in which coal is the sulfur-containing
fuel, oxygen
enrichment is employed to reduce NOR, as is more fully described in United
States Patent
Publication Number US 2004-0185404 Al.
In the preferred embodiments, a process designed to reduce SOX emissions in
boilers,
particularly in coal-fired boilers, is disclosed. The process includes
introducing potassium
carbonate in the combustion process, at the burner level or above the burners.
When used in
conjunction with oxygen enrichment, NO,, reduction can be achieved, to an even
greater
degree than is expected by using oxygen enrichment alone. By this process, SOX
levels can
be reduced to a few ppm, even for high-sulfur fuels such as Midwestern coals
and pet coke.
At the same time, the NO,, reducing effect of the oxygen enrichment is
enhanced by the
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potassium carbonate, resulting in a low NO. process. To avoid slagging effect
of the high
temperature on the potassium carbonate, a staged combustion process is most
preferred.
Laboratory test data illustrates just a portion of the expected benefits in
removing sulfur from
pet-coke combustion using potassium carbonate. Test results show that the flue
gas of both
air-combustion (traditional) and oxy-combustion (oxygen enriched) contained
approximately
6 ppm SO,,. This is extremely low in view of the fact that the pet-coke used
contained
approximately 3-6% sulfur. The reaction taking place in the combustion process
is:
K2C03 + SO, -> K2S04 + C02. (1)
Recent calculations made of the adsorption of SO2 by K2C03 in a pulverized
coal boiler fired
with Illinois No. 6 coal are illustrated as follows. It was assumed that one
million pounds per
hour of coal, corresponding to approximately 1000 MW power production, was to
be burned
with 10% excess air. An elemental composition of the parent coal reveals:
Element Wt% daf
C 77.32
H 5,33
N 1.49
0 8.88
S 6.98
The coal was assumed to have 10% ash, and moisture was neglected. Note that
the sulfur
composition for this coal is high (approximately 7 wt.% daf). The adsorption
rate was
assumed to be limited by the diffusion of SO2 to the surface of the particle.
The mass transfer
rate is:
N502 = h, (C5o2,g - Cs02,5) (2)
where N 02 is the molar flux of SO2 to the particle surface per external
surface area of
particle, hm is the convective mass transfer coefficient, and CS02 is the
concentration of gas in
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either the bulk gas phase or at the surface. Under diffusion-limited
conditions, Cso2s is
essentially zero, and equation (2) becomes very simple. The mass transfer
coefficient is
calculated from the Sherwood number, which is 2.0 for small particles:
Sh = hn,dn (3)
DSO2-air
where dp is the particle diameter, assumed to be 50 microns in this
calculation. The
diffusivity of SO2 was calculated from the Chapman-Enskog theory for kinetic
gases. The
parameters for air were used, since they are similar to post-combustion gases.
The diffusivity
changes as a function of temperature.
For this calculation, a temperature profile was assumed. The particle and gas
temperature
started at 2000 K and then decreased in a linear fashion to 1000 K after one
second. This is
thought to approximate the conditions in most pulverized coal boilers.
The initial concentration of SO2 was calculated from the flow rates of coal
and air, assuming
that all of the sulfur in the coal ended up as SO2. This yielded a calculation
of about 4510
ppm. The differential equation for the change in SO2 concentration in this
case is:
dCs02 = n Ap NsOZ (3)
dt
where np is the particle number density (number of particles per cubic meter),
Ap is the
external area per particle (4 tr p2), and Ns02 is from equation (2).
The resulting SO2 profile is shown in Fig. 4. As shown, the calculations
showed only 70%
conversion of SO2 to K2SO4. Actual laboratory data from petroleum coke yield
much better
results, however. The data indicates a conversion of greater than 95%.
Although the reason
for the difference is not fully understood, it is believed that perhaps some
of the potassium
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species may have vaporized, which would increase conversion of SO2 (because
diffusion to a
particle surface would not be necessary).
It is further believed that using K2C03 particles will facilitate adsorbing
the SO2 from hot
post-flame gases. Although, in a preferred embodiment described herein, K2C03
is injected
with the coal, it is possible that this arrangement will cause the K2C03 to
become too hot.
Excessive temperatures are expected to the K2C03 to melt and perhaps become
sticky,
therefore causing a deposition problem in the combustion chamber. However,
because the
data seem to indicate that there may have been some vaporization and
consequent
enhancement in the sulfur conversion to sulfur carbonate, it is possible that
the vaporization
may be beneficial.
In a more preferred embodiment, the K2CO3 is injected above the flame zone
(primary
combustion zone) in order to reduce fouling effects downstream. Thus, in the
preferred
embodiment illustrated in Fig. 3, potassium carbonate is introduced with the
tertiary air, in a
second combustion zone. Not only does this arrangement overcome the slagging
of
potassium carbonate that may occur when it is introduced directly into the
flame, it provides
an enhanced NOx reduction. The data suggests that NOX formation is decreased
by the
addition of the potassium carbonate, in a reaction of the type:
K2C03 + NOX -> K2N03 + CO2. (4)
The mechanism by which this NOX reaction occurs is not fully understood.
However, in
combination with the oxygen enrichment, this preferred embodiment seems to
yield
synergistic results.
Referring now to Figs. 1 through 3, three preferred embodiments of a burner
are shown in
schematic fashion. As shown schematically in Fig. 1, a combustion chamber 20
is shown
having a first or primary combustion zone 22 and a second or secondary
combustion zone 24.
The first of the three inlet streams, the primary stream 26, combines the
primary oxidant air with
the solid, pulverized fuel, and thereby conveys the pulverized solid fuel into
the combustion
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chamber 20 in the primary combustion zone 22. In an application where the fuel
is not solid, the
primary inlet stream can be eliminated. The secondary stream 28 introduces the
secondary
oxidant into the burner, around or near the primary stream 26, and into the
primary combustion
zone 22. The tertiary stream 32, is injected, if necessary, in the secondary
combustion zone 24,
to complete combustion. It is understood that in these apparatus, multiple air
streams of each
type thus described (primary, secondary and tertiary) can be utilized - indeed
multiple burners
can be used; the following description will refer to each in the singular for
simplicity).
As shown in Fig. 1, oxygen enrichment is employed iii the primary and
secondary oxidant
streams, and the potassium carbonate is introduced with the fuel. As shown in
Fig. 2, oxygen
enrichment is employed in all three oxidant streams, and the potassium
carbonate is introduced
with the fuel. As shown in Fig. 3, oxygen enrichment is employed in all three
oxidant streams,
and the potassium carbonate is introduced with the tertiary oxidant into the
secondary
combustion zone.
Flue gas 34 is formed and exhausted from the combustion chamber 20. Thus, the
first
combustion zone is the zone where the fuel reacts around the burner level.
Secondary zones are
sometimes desirable if 02 is provided downstream from the burner before the
furnace exit to
provide more complete combustion downstream. The oxygen equivalent amount of
oxidant is
adjusted in the oxidant streams (primary, secondary and, if applicable,
tertiary oxidant) to
maintain a predetermined amount of excess oxygen in view of the stoichiometric
balance needed
to complete combustion. This amount of excess oxygen is preferably maintained
so that the 02
content of the flue gas is maintained between 1.5 percent and 4.5 percent, and
more preferably
between 2.5 percent and 3.5 percent, and most preferably about 3.0 percent.
For purposes of this
application, all 02 contents are stated by volume of dry gas (excluding H20).
Thus, the preferred embodiments disclose processes designed to reduce NOX and
SOX
emissions in boilers, particularly in coal-fired boilers. These embodiments
comprise
introducing potassium carbonate in the combustion process, at the burner level
or above the
burners, in conjunction with oxygen enrichment. By using this process, the SOX
levels can be
reduced to a few ppm, even for high-sulfur fuels such as Midwestern coals and
pet coke. At
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the same time, the NO. reducing effect of the oxygen enrichment will be
significantly
enhanced by the potassium carbonate, resulting in a low NOX process. Due to
the slagging
effect of the high temperature on the potassium carbonate, a staged combustion
process may
be preferred. Potassium sulfate can be scrubbed from the flue gas and can be
sold as a
fertilizer.
Fig. 1 illustrates a first preferred embodiment. The boiler using a solid
fuel, such as pet-coke
or coal, and utilizes three oxidant streams - primary for fuel transport,
secondary for
combustion, and tertiary for staged combustion. Note that, as adapted to a
liquid fuel-burning
apparatus, the primary oxidant stream may be unnecessary.
As shown, the process works to reduce NOX emissions by controlling temperature
at the
burner level, and further due to the introduction of the potassium carbonate
in the boiler at the
same level with the fuel. By controlling the temperature and limiting it from
becoming too
high, to avoid NOX production, potassium carbonate slagging will be reduced or
perhaps
completely avoided. Oxygen is injected at the primary/secondary oxidant level,
in order to
initiate the combustion process faster and more efficient than with air alone
(particularly
under fuel-rich conditions).
It is noted that, as less air will be used at the primary/secondary oxidant
level, the combustion
will be less efficient. Under these circumstances, oxygen offers a clear way
to balance this
effect, due to the enhanced reactivity when compared to air combustion.
Additionally, the
presence of the oxygen in the primary combustion zone is even more desirable
when low-
volatile fuels are implemented, such as anthracite or pet-coke. Finally, the
use of an oxygen-
enriched oxidant in the primary combustion zone will heat the fuel quicker,
and will allow the
nitrogen to be released in pure form, rather than being transformed in nitric
oxide.
The preferred embodiment illustrated in Fig. 2 shows an alternative process
for improving
combustion efficiency by improving the oxygen-fuel mixing at the burner level
between the
fuel and oxidant. In the embodiment of Fig. 2, oxygen enrichment is introduced
at the
tertiary oxidant level as well, to enhance combustion at the secondary
combustion zone.
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Referring now to Fig. 3, potassium carbonate is injected into the boiler at
the tertiary oxidant
level. By injecting the potassium carbonate in the secondary combustion zone,
the higher-
temperature environment at the burner level is avoided. The potassium
carbonate can be
injected through the air stream, or even better, through the oxygen stream
(where an oxygen
lance is used), due to the higher flow velocities, yielding better mixing with
the flue gas
stream. Alternately, oxygen can be introduced only at the primary/secondary
oxidant level,
for NO,, control.
It is preferred that the quantities of potassium carbonate used be selected to
comport with the
stoichiometry defined by the sulfur content in the fuel. In a preferred
embodiment, the
potassium carbonate is introduced into the combustion chamber in an amount
sufficient to
exceed the stoichiometric requirement needed to react with the sulfur in the
fuel by between
about 0% and about 50%. In a more preferred embodiment, the excess is between
about 10%
and about 50%. In a most preferred embodiment, the excess is between about 20%
and about
35%. As shown by the data, the process results in at least half of the sulfur
in the sulfur-
containing fuel being converted to potassium sulfate. Oxygen is used such as
to replace less
than 10-20% of the overall oxidant, in a relationship between the
primary/secondary oxidant
stream and tertiary stream such as to minimize the NO% formation and unburnt
fuel in the ash.
Thus, in a preferred embodiment, a process for burning a sulfur-containing
fuel to produce a flue
gas is disclosed. A sulfur-containing fuel is introduced into a combustion
chamber at a fuel
inlet. A primary oxidant stream containing more than 21% oxygen is introduced
into the
combustion chamber at a primary oxidant inlet positioned proximate to or
coincident the fuel
inlet and mixing it with the sulfur-containing fuel to define a first
combustion zone. A
secondary oxidant stream containing more than 21% oxygen is introduced into
the combustion
chamber at a secondary oxidant inlet positioned so that the secondary oxidant
enters the
combustion chamber in the primary combustion zone. A tertiary oxidant stream
containing
more than 21 % oxygen is introduced into the combustion chamber at a tertiary
oxidant inlet
positioned away from the primary oxidant inlet and away from the secondary
oxidant inlet. The
tertiary oxidant enters the combustion chamber to define a secondary
combustion zone.
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The total oxygen content of the oxidant entering the combustion chamber
exceeds 21%.
Potassium carbonate is introduced into the combustion chamber through the
tertiary air inlet in
an amount sufficient to exceed the stoichiometric requirement needed to react
with the sulfur in
the fuel by between 0% and 50%. The sulfur-containing fuel is burned to
produce the flue gas
and potassium sulfate. At least half of the sulfur in the sulfur-containing
fuel is converted to
potassium sulfate.
While in the foregoing specification this invention has been described in
relation to certain
preferred embodiments thereof, and many details have been set forth for
purpose of illustration,
it will be apparent to those skilled in the art that the invention is
susceptible to additional
embodiments and that certain of the details described herein can be varied
considerably without
departing from the basic principles of the invention.
