FIELD OF THE INVENTION
The present invention relates to an alternate sorbent feeding system for
Circulating fluidized Bed Combustion (CFBC) boilers for effective capture of
pollutant sulphur dioxide.
BACKGROUND OF THE INVENTION
Circulating fluidized bed combustion (CFBC) is an efficient method of burning
various grades of fuels and in particular, the fuels containing high sulphur for
example coal, lignite, anthracite coal, which can be burnt along with sorbent for
example, limestone or dolomite for absorbing sulphur dioxide in-situ in the
Circulating fluidized bed combustor. The sorbent consumption rate is determined
by the calcium to sulphur molar ratio which is normally maintained between 1.5
and 3.5. Though the pollutant sulphur dioxide can be absorbed by the sorbent
for example, limestone or dolomite in CFBC boilers, its consumption rate can be
reduced only if it is effectively utilized. In the CFBC boilers, the sulphur dioxide
absorption by the sorbent is high when the boiler operates between 800 and 850
DegC. The other factors for selection of the sorbent are average size of the
sorbent, surface area, pore surface area, the reaction rate of the sorbent with
sulphur dioxide, etc. Hence, it is necessary that the consumption of the sorbent
be reduced for effective removal of sulphur dioxide in a CFBC plant.
The CFBC boilers are generally operated at temperature between 700-900 Deg.C
and at fluidizing velocity between 2.50 m/sec and 6.0 m/sec. Air from a primary

air fan is admitted from the bottom of CFB combustor to fluidize the bed
material from the bottom. Secondary air delivered from a fan is admitted above
the air distributor plate for secondary combustion of the fuel in the combustor.
Normally, the entrained particles consists of the fuel, fluidized bed material and
sorbent from the combustor are collected in at least one cyclone which is either
cooled or un-cooled, and recycled to the combustor through a seal pot to sustain
the process continuity. Accordingly, the combustion efficiency and heat transfer
to water wall of the enclosure are improved. The enclosure of the cooled cyclone
are made of tubes through which either water or steam passes. The un-cooled
cyclone is lined with a refractory and insulating material to withstand the high
temperature (usually in the range of 750°C to 1000°C). In case of a solid fuel,
less than 6 mm size is stored in a bunker and fed at a desired rate to the
combustor through a drag link feeder at the outlet of a seal pot. The sorbent is
crushed to size less than 1.0 mm in a ball mill or an impact crusher and stored in
a silo/bunker and fed at a regulated rate by gravity with air assistance to the
outlet of the seal pot so that the fuel and sorbent are fed at a lower elevation of
the combustor.
The sorbent is crushed to less than 1.0 mm size since the absorption of sulphur
dioxide shows a better result if the surface area of the sorbent particles is more.
The fine sorbent particles once admitted into the combustor dissociate carbon
dioxide and form oxides of either calcium or magnesium. The oxides
subsequently react with the sulphur dioxide and form their respective sulphates.
The particles are carried away along with the flue gas and the coarse particles

are collected in the separator/cyclone and recycled to the combustor through the
seal pot. The fine size particles including un-reacted sorbent leave out of the
cyclone along with the fuel ash and the fluidizing material.
According to the method disclosed in US patent No. 6,912,962, the un-reacted
sorbent present in the fly ash and bottom ash of the CFBC boiler is regenerated
by mixing water or aqueous sodium at about 100 Deg C and the un-reacted
sorbent is converted into a hydration product in a mixing chamber.
US patent No. 7374590 discloses a process in which the sulphur is removed by a
unique pre-combustion treatment. The coal is placed in a container that can be
sealed and depressurized. The depressurization will be sufficient to remove all
the fluids, be that gaseous or liquid, entrapped in the coal. This is believed to
result in fracturing the coal, i.e. creating fractures in the form of small cracks,
faults, or channels in the coal. The number of fractures in the coal should be
adequate and with suitable cross section sizes in order to allow a sufficient
amount of the aqueous composition supersaturated with calcium carbonate to
penetrate the fractures. Once the coal has been depressurized, it is then
contacted with the aqueous silica colloid composition supersaturated with
calcium carbonate for a time sufficient to infuse the fractures with the dissolved
calcium carbonate. Such a physical state of the solid fuel enables the calcium
carbonate to intimately associate with the coal, and allows further fracturing of
the coal through crystallization of the calcium carbonate with the fractures. The
coal is stirred or agitated to intimately mix with the aqueous composition.

The sorbent feed rate is determined based on various factors for example,
sulphur content of the fuel, the sulphur dioxide removal level, the
composition/purity of the sorbent, size of the sorbent particles, quality of the
sorbent to absorb sulphur dioxide, temperature of combustion and distribution of
the sorbent in the combustor, etc.
The distribution of the sorbent in the combustor is one of the major criteria for
effective absorption of the sulphur dioxide. The sorbent particles should be
sufficiently mixed throughout the entire cross section and height of the
combustor so as to absorb sulphur dioxide released from the fuel. Since the
average size of the sorbent is very less compared to the fuel, the sorbent
elutriate at much higher rate than the fuel at high fluidizing velocity which
shortens the residence time of the sorbent in the combustor. The sorbent
particles admitted into the furnace along with fuel in seal pot elutriate with the
flue gas and travel along the wall of the combustor and leave out to the cyclone.
Hence, the sorbent may not be distributed to the entire cross section of the
combustor. It is also observed in CFBC boilers that although the concentration of
the fuel particle at the core region of the boiler is less, the flue gas concentration
with sulphur dioxide is substantially uniform across the entire cross section of the
boiler. Accordingly, the fuel gas moving upwards in the core region of the boiler
normally fail to contact with the sorbent particles, and hence removal of the
sulphur dioxide present in the flue gas is not corresponding to the extent of gas
moving close to the water wall.

OBJECT OF THE INVENTION
It is therefore an object of the invention to propose a sorbent feeding system in
circulating fluidized bed combustion (CFBC) boilers for distributing a sorbent
across the entire cross section of the combustor for effectively capturing of
sulphur dioxide to reduce the sorbent consumption in CFBC boilers.
Another object of the invention is to propose a sorbent feeding system in
circulating fluidized bed combustion (CFBC) boilers for distributing a sorbent
across the entire cross section of the combustor for effectively capturing of
sulphur dioxide to reduce the sorbent consumption in CFBC boilers, which
ensures a free-flow of fine sorbent particles to the combustor.
A still another object of the invention is to propose a sorbent feeding system in
circulating fluidized bed combustion (CFBC) boilers for distributing a sorbent
across the entire cross section of the combustor for effectively capturing of
sulphur dioxide to reduce the sorbent consumption in CFBC boilers, which
eliminates the need for segregation between coarse and fine particles when
injected in to the combustor.
Yet another object of the invention is to propose a sorbent feeding system in
circulating fluidized bed combustion (CFBC) boilers for distributing a sorbent
across the entire cross section of the combustor for effectively capturing of
sulphur dioxide to reduce the sorbent consumption in CFBC boilers, which allows
independent sorbents feeding in case the fuel feeding system temporarily
experiences operational set back.

A further object of the invention is to propose a sorbent feeding system in
circulating fluidized bed combustion (CFBC) boilers for distributing a sorbent
across the entire cross section of the combustor for effectively capturing of
sulphur dioxide to reduce the sorbent consumption in CFBC boilers, which
eliminates provision of an additional opening in the water wall of the combustor
for uniform feeding of the sorbents.
A still further object of the invention is to propose a sorbent feeding system in
circulating fluidized bed combustion (CFBC) boilers for distributing a sorbent
across the entire cross section of the combustor for effectively capturing of
sulphur dioxide to reduce the sorbent consumption in CFBC boilers, which
ensures an equal distribution of the feed rate of sorbent in each transportation
line.
SUMMARY OF THE INVENTION
Accordingly, there is provided an alternate sorbent feeding system for Circulating
fluidized Bed Combustion (CFBC) boilers for effective capture of pollutant sulphur
dioxide. The system comprises a sorbent distribution device which distributes the
sorbent equally to a plurality of pneumatic transportation lines for example,
between 4 to 20, depending on the size of the combustor; a plurality of air and
sorbent mixing nozzles disposed in each pneumatic transportation line, a source
for delivering high pressure air between 0.2 kg/cm2 (g) and 0.5 kg/cm2 (g) to
transport the sorbent pneumatically from the outlet of a sorbent bunker to the
combustor for uniform feeding of the sorbent across the entire cross section of
the combustor. The system is configured to be flexible with the provision of an

expansion attachment corresponding to the expected expansion of the
combustor; means for erosion prevention/minimization arranged in each of the
pneumatic transportation lines; a plurality of support devices disposed along the
routing of the transportation lines; means for allowing a mixing of the sorbent
pneumatic transportation line; a source of delivering the secondary air close to
the combustor; and at least one corrosion-resistant sorbent entry nozzle in the
combustor for wide distribution of the sorbent in the combustor.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Schematic diagram of a Circulating Fluidized Bed Combustion
(CFBC) Boiler according to the prior art.
Figure 2 - Schematic plan view of a secondary air injection port arrangement
in a Circulating Fluidized Bed combustor of prior art.
Figure 3- Schematic diagram of a sorbent feeding system along with a
secondary air injection device in a combustor according to the
resent invention.
Figure 4 - Schematic plan view of the sorbent feeding system along with a
secondary air injection device in a combustor according to the
present invention.

DETAIL DESCRIPTION OF THE INVENTION
Circulating fluidized bed combustions (CFBC) boiler (Fig:l) consists of a
combustor (1) connected to a primary air fan (2) through an air preheater (3)
and an air distributor plate (4). Air is delivered from a secondary air fan (5),
admitted to the combustor (1) through a plurality of air injection ports (6) to
fluidize the bed materials and to supply air for a secondary combustion of the
fuel. The elutriated solid particles are collected in at least one cyclone (7) and
recycled to the combustor (1) through a seal pot (8). A first blower (9)
connected to the seal pot (8) assists to transfer the particles to the combustor
(1). Coal stored in a first bunker (10) admitted at the outlet of the seal pot (8) to
feed to the combustor (1) through a drag link chain feeder (11) and an air lock
feeder (12). Combustion takes place along the entire height of the combustor (1)
and the sulphur present in the fuel is converted into sulphur dioxide. A sorbent
for example, limestone/dolomite, is added from a second bunker (13) to capture
the sulphur dioxide emitted during the combustion. The sorbent is fed to the
outlet of the seal pot (8) through a rotary feeder (14) for admitting to the
combustor (1). The coarse particles settled in the combustor (1) are drained
through an ash cooler (16) to recover the sensible heat of the hot ash. The flue
gas leaving from the cyclone (7) is allowed to let out through a vortex finder (17)
including a flue gas duct (18) to a back pass (19). The heat available in the flue
gas is recovered in a super heater (20), a reheater (21), an econimser (22) and
an air preheater (3). The flue gas is then released to the atmosphere after
separating the solid particles in an electrostatic in an electrostatic precipitator
(23).

Fig: 2 shows an arrangement for secondary air admission to the combustor (1)
through the ports (6) disposed above the air distributor (4).
The present invention can be better described with reference to the
accompanying Figs: 3 and 4. The sorbent feeding system of the Circulating
Fiuidized Bed Combustion Boiler, comprises a rotary airlock feeder or a drag link
feeder (14) connected to each hopper of the second bunker/silo (13). The
feeders (14) have multiple outlets (25) allowing pneumatic feeding of the
sorbent to the combustor (1) through a first plurality of nozzles (24). The
pneumatic transportation air is supplied from a third blower (15). The feed rate
of the sorbents to each of the transportation lines is distributed equally in the
multiple outlets (25) available at the outlet of the feeder (14). The secondary air
delivered from a secondary air fan (5) and, admitted along with the sorbent
through a second plurality of nozzles (26) to the combustor (1). The
configuration of the second plurality of nozzles (26) enables entry for the sorbent
including the secondary air, as shown in Fig: 4, which allows the sorbent to be
dispersed across the combustor (1). An erosion resistant means (27) is adopted
to minimize the erosion while transporting the solid fuel pneumatically to the
combustor (1). The delivery pressure of the third blower (15) is selected so as to
overcome the pressure of the valves and ducts, the first plurality of nozzle (24),
the pneumatic transportation of the solids to the combustor (1), the pressure
loss at the entry port in the combustor (1), and the pressure prevailing at the
elevation of the combustor (1) wherein the sorbent is fed.

WE CLAIM
1. An alternate sorbent feeding system to effectively capture pollutant
sulphur dioxide in a circulating fluidized bed combustor (CFBC) boiler,
comprising a combustor (1) connected to a primary air fan (2) through an
air preheater (3) and an air distributor plate (4); a secondary air fan (5)
for supplying air through a plurality of ports (6) into the combustor (1) for
fluidizing the bed material including permitting a secondary combustion of
the solid fuel, the solid fuel stored in a bunker (10) fed to the combustor
(1) through a drag-link chain feeder (11) and an air-lock feeder (12), the
entrained solid particles being collected in at least one cyclone (7) and
recycled to the combustor (1) through a seal pot (8) which is connected
to a first blower (9), the combustion resulting in production of the
pollutant sulphur dioxide out of the sulphur present in the fuel, an ash
cooler (16) recovering the sensible heat from the coarser particles settled
in the combustor (1), before the flue gas being allowed to emit to the
atmosphere through a vortex finder (17), a flue gas duct (18), and a back
pass (19), wherein the heat available in the flue gas being recovered in a
superheater assembly (20, 21, 23, 3); the CFBC boiler being injected with
sorbent in the form of limestone or dolomite during the combustion in the
combustor (1) by adapting the alternate sorbent feeding system, the
system comprising:
- a sorbent bunker (13) connected to the drag link feeder (11) via at least
one rotary feeder (14) for the sorbent;

- a second blower (15) connected to a first plurality of air-mixing nozzles
(24) to restrict the pressure drop in the sorbent feed line below a
minimum, the second plurality of nozzles (26) allowing injection of sorbent
material into the combustor (1), the second plurality of nozzles (26)
disposed above the air distributor plate (4) and injecting the sorbent
including the secondary air into the combustor (1) enabling an uniform
dispersion of the sorbent across the combustor (1); and
- the sorbent feed line and the secondary air line connected to the second
plurality of nozzles (26) at an angle between 30° and 45° with an
elevation between 2 and 5 meters.

2. The system as claimed in claim 1, wherein the second plurality of nozzles
(26) are connected to the combustor (1) at an angle between 45 and 60
Degree.
3. The system as claimed in claim 1, wherein the second plurality of nozzles
(26) are configured to mix the sorbent and the secondary air at the outlet
so that the sorbent particles are mixed and carried till the centre of the
combustor (1).
4. The system as claimed in any of the preceding claims, wherein the second
plurality of nozzles (26) are provided with an erosion-resistance means
(27) in the form of a erosion-resistance coating at select area to minimize
erosion due to sorbent particles.

5. The system as claimed in claim 1, wherein the ratio of the solid to air
mixture is maintained up to 3.
6. An alternate sorbent feeding system to effectively capture pollutant
sulphur dioxide in a circulating fluidized bed combustor (CFBC) boiler, as
substantially herein described with reference to the accompanying
drawings.

The invention relates to an alternate sorbent feeding system to effectively
capture pollutant sulphur dioxide in a circulating fluidized bed combustor (CFBC)
boiler, comprising a combustor (1) connected to a primary air fan (2) through air
preheater (3) and an air distributor plate (4); a secondary air fan (5) for
supplying air through a plurality of ports (6) into the combustor (1) for fluidizing
the bed material including permitting a secondary combustion of the solid fuel,
the solid fuel stored in a bunker (10) fed to the combustor (1) through a draglink
chain feeder (11) and an air-lock feeder (12), the entrained solid particles
being collected in at least one cyclone (7) and recycled to the combustor (1)
through a seal pot (8) which is connected to a first blower (9), the combustion
resulting in production of the pollutant sulphur dioxide out of the sulphur present
in the fuel, an ash cooler (16) recovering the sensible heat from the coarser
particles settled in the combustor (1), before the flue gas being allowed to emit
to the atmosphere through a vortex finder (17), a flue gas duct (18), and a back
pass (19),wherein the heat available in the flue gas being recovered in a
superheater assembly (20, 21, 23, 3). The CFBC boiler being injected with
sorbent in the form of limestone or dolomite during the combustion in the
combustor (1) by adapting the alternate sorbent feeding system, the system
comprising a sorbent bunker (13) connected to the drag link feeder (11) via at
least one rotary feeder (14) for the sorbent; a second blower (15) connected to

a first plurality of air-mixing nozzles (24) to restrict the pressure drop in the
sorbent feed line below a minimum, a second plurality of nozzles (26) allowing
injection of sorbent material into the combustor (1), the second plurality of
nozzles (26) disposed above the air distributor plate (4) and injecting the sorbent
including the secondary air into the combustor a(1) enabling an uniform
dispersion of the sorbent across the combustor (1); and the sorbent feed line
and the secondary air line connected to the second plurality of nozzles (26) at an
angle between 30° and 45° with an elevation between 2 and 5 meters.