U.S. patent application number 11/685447 was filed with the patent office on 2008-09-18 for direct sorbent preparation/feed apparatus and method for circulating fluidized bed boiler systems.
This patent application is currently assigned to Alstom Technology Ltd. Invention is credited to James F. Durant, Robert Van Etten Jacobs.
Application Number | 20080223267 11/685447 |
Document ID | / |
Family ID | 39598393 |
Filed Date | 2008-09-18 |
United States Patent
Application |
20080223267 |
Kind Code |
A1 |
Jacobs; Robert Van Etten ;
et al. |
September 18, 2008 |
DIRECT SORBENT PREPARATION/FEED APPARATUS AND METHOD FOR
CIRCULATING FLUIDIZED BED BOILER SYSTEMS
Abstract
A sorbent conditioning and feed apparatus for a circulating
fluidized bed combustion system includes a dryer apparatus
configured to dry unprocessed sorbent material transported thereto
from a raw storage container; a sorbent crushing device configured
to reduce the particle size of dried sorbent material discharged
from the dryer apparatus; and a circulating fluidized bed (CFB)
boiler configured to receive processed sorbent material conveyed
from the sorbent crushing device; wherein the dryer apparatus
utilizes an untempered hot air source diverted directly from a
primary air stream input to the circulating fluidized bed
boiler.
Inventors: |
Jacobs; Robert Van Etten;
(Windsor, CT) ; Durant; James F.; (Windsor,
CT) |
Correspondence
Address: |
CANTOR COLBURN, LLP
20 Church Street, 22nd Floor
Hartford
CT
06103
US
|
Assignee: |
Alstom Technology Ltd
Baden
CH
|
Family ID: |
39598393 |
Appl. No.: |
11/685447 |
Filed: |
March 13, 2007 |
Current U.S.
Class: |
110/245 |
Current CPC
Class: |
F23J 7/00 20130101 |
Class at
Publication: |
110/245 |
International
Class: |
F23G 7/00 20060101
F23G007/00 |
Claims
1. A sorbent conditioning and feed apparatus for a circulating
fluidized bed (CFB) combustion system, comprising: a dryer
apparatus configured to dry unprocessed sorbent material
transported thereto from a raw storage container; a sorbent
crushing device configured to reduce the particle size of dried
sorbent material discharged from the dryer apparatus; and a
circulating fluidized bed boiler configured to receive processed
sorbent material conveyed from the sorbent crushing device; wherein
the dryer apparatus utilizes an untempered hot air source diverted
directly from a primary air stream input to the circulating
fluidized bed boiler.
2. The apparatus of claim 1, wherein the temperature of the
untempered hot air source is at least about 400.degree. F.
3. The apparatus of claim 1, wherein the dryer apparatus is further
configured to remove sorbent fines from the unprocessed sorbent
material prior to input to the sorbent crushing device.
4. The apparatus of claim 3, wherein the sorbent fines removed by
the dryer apparatus are diverted directly to the CFB boiler.
5. The apparatus of claim 3, wherein the sorbent fines removed by
the dryer apparatus are diverted to the CFB boiler using a portion
of boiler system secondary air discharged from the dryer
apparatus.
6. The apparatus of claim 1, wherein the sorbent crushing device
comprises a roll crusher.
7. A direct limestone conditioning and feed apparatus for a
circulating fluidized bed (CFB) combustion system, comprising: a
dryer apparatus configured to dry unprocessed limestone material
transported thereto from a raw storage container; a roll crushing
device configured to reduce the particle size of dried limestone
material discharged from the dryer apparatus; and a circulating
fluidized bed boiler configured to receive processed limestone
material conveyed from the roll crushing device; wherein the dryer
apparatus utilizes an untempered hot air source diverted directly
from a primary air stream input to the circulating fluidized bed
boiler.
8. The apparatus of claim 7, wherein the temperature of the
untempered hot air source is at least about 400.degree. F.
9. The apparatus of claim 7, wherein the dryer apparatus is further
configured to remove limestone fines from the unprocessed limestone
material prior to input to the roll crushing device.
10. The apparatus of claim 9, wherein the limestone fines removed
by the dryer apparatus are diverted directly to the CFB boiler.
11. The apparatus of claim 9, wherein the limestone fines removed
by the dryer apparatus are diverted to the CFB boiler using a
portion of boiler system secondary air discharged from the dryer
apparatus.
12. The apparatus of claim 7, wherein the dryer apparatus comprises
a fluidized bed dryer.
13. A method of conditioning and feeding sorbent material for a
circulating fluidized bed combustion system, the method comprising:
transporting raw, unprocessed sorbent material from a raw storage
container to a dryer apparatus; diverting untempered hot air
directly from a primary air stream input to the circulating
fluidized bed boiler, the untempered hot air used to dry the raw
sorbent material; directing dried raw sorbent material from the
dryer apparatus to a crushing device configured to reduce the
particle size of the dried raw sorbent material discharged from the
dryer apparatus; and conveying processed sorbent material from the
crushing device to the CFB boiler.
14. The method of claim 13, wherein the temperature of the
untempered hot air source is at least about 400.degree. F.
15. The method of claim 13, further comprising using the dryer
apparatus to remove sorbent fines from the raw sorbent material
prior to directing the dried raw sorbent material from the dryer
apparatus to a crushing device.
16. The method of claim 15, wherein the sorbent fines removed by
the dryer apparatus are diverted directly to the CFB boiler.
17. The method of claim 15, wherein the sorbent fines removed by
the dryer apparatus are diverted to the CFB boiler using a portion
of boiler system secondary air discharged from the dryer
apparatus.
18. The method of claim 13, wherein the dryer apparatus comprises a
fluidized bed dryer.
19. The method of claim 13, wherein the sorbent crushing device
comprises a roll crusher.
20. The method of claim 13, wherein the sorbent comprises
limestone.
Description
TECHNICAL FIELD
[0001] The present disclosure relates generally to circulating
fluidized bed (CFB) combustion systems and, more particularly, to
an improved direct sorbent preparation/feed apparatus and method
for a CFB boiler system.
BACKGROUND
[0002] Fluidized bed combustion (FBC) is a combustion technology
used in power plants primarily to burn solid fuels. FBC plants are
more flexible than conventional plants in that they can be fired on
coal, coal waste or biomass, among other fuels. The term FBC covers
a range of fluidized bed processes which include Circulating
Fluidized Bed (CFB) boilers, Bubbling Fluidized Bed (BFB) boilers
and other variants. Fluidized beds suspend solid fuels on
upward-blowing jets of air during the combustion process, resulting
in a turbulent mixing of gas and solids. The tumbling action, much
like a bubbling fluid, provides a means for more effective chemical
reactions and heat transfer.
[0003] During the combustion of fuels that have a sulfur containing
constitutent, coal for example, sulfur is oxidized to form
primarily gasous SO.sub.2. In particular, FBC reduces the amount of
sulfur emitted in the form of SO.sub.2 by a desulfurization
process. A suitable sorbent, such as limestone containing
CaCO.sub.3, for example, is used to absorb SO.sub.2 from the flue
gas during combustion. In order to promote both combustion of the
fuel and the capture of sulfur, FBC combustion operates at
temperatures lower than conventional combustion systems. FBC
systems operate in a range typically between about 780.degree. C.
and about 1000.degree. C. Since this allows coal to combust at
cooler temperatures, NO.sub.x production during combustion is lower
than other coal combustion processes. Fluidized-bed boilers evolved
from efforts to find a combustion process able to control pollutant
emissions without external emission controls (such as
scrubbers).
[0004] CFB boiler systems are generally associated with limestone
feed systems for sulfur capture. Processed limestone fed to a
boiler is typically conditioned by means of size reduction machines
to specific size ranges to allow for the desulfurization process to
proceed efficiently. If the particles are too large, the
desulfurization process will not be efficient because there is
insufficient limestone particle surface area to react with the flue
gas. On the other hand, if the particles are too small, the
limestone will be carried out of the desulfurization zone with the
flue gas before it can react to remove the sulfur. Typically,
limestone is fed to the boiler with a median particle diameter in
the range of (as an example, but not limited to) about 100 to about
400 microns. In order to achieve this particle size range,
unprocessed, raw limestone is reduced in both size and moisture
content by size reducing machines. Presently, there are various
machines available for crushing limestone, including for example,
hammer mills, roll crushers and roller mills. Regardless of the
type of equipment used for limestone crushing, the particles are
dried either before or during crushing in order to produce a freely
flowing material.
[0005] Traditionally, limestone is prepared separately from the
boiler system, either on-site or by the limestone supplier.
Prepared limestone is conveyed to a storage system in the boiler
house from which it is thereafter metered and injected into the
boiler. Experience has shown that the cost of prepared limestone
using separate on-site systems or supplied from off-site vendors is
expensive. In the case of separate, on-site systems a separate
building and auxiliary fuel is used to dry the limestone. On the
other hand, a limestone preparation and feed system may also be
integrated with the boiler system itself, resulting in a
significant reduction in capital and operating costs. Specifically,
CFB boilers may be equipped with an integrated limestone
preparation and feed system that resides in the boiler building.
Such a system that dries and prepares limestone as needed is also
referred to a Just-In-Time (JIT) limestone system.
[0006] Systems using roller mills have also been designed and
installed on CFB boilers. Roller mill systems utilize hot air (mill
air) for drying of limestone and transportation to the CFB. Mill
air is typically obtained from the primary air stream as the mills
utilize air at elevated pressures. For most CFB boilers, hot
primary air in the range of about 400.degree. F. to about
600.degree. F. is typically available. The hot air is used to dry
the entering limestone, sweep the limestone out of the mill, and
convey it to the boiler. However, in order to protect the
mechanical components of roller mills, the operating temperatures
thereof are kept relatively low, for example in the range of about
170.degree. F. to about 200.degree. F.
[0007] Furthermore, limestone generally contains low quantities of
moisture (e.g., less than 5%). For proper system operation, the air
temperature entering the roller mills is a function of the
acceptable mill mechanical operating temperature, the limestone
moisture content, and the ratio of air to limestone. For a JIT
system as described above, air temperatures entering the mill will
generally be less than about 250.degree. F. Therefore, given the
available primary air temperature in the range of about 500.degree.
F., the mill air obtained from the primary air stream is therefore
tempered in order to produce a mill exit air temperature, typically
in the range of about 180.degree. F. to 225.degree. F.
[0008] Experience has shown that the use of tempering air
effectively reduces heat recovery from the flue gas. In turn, the
reduced heat recovery lowers boiler efficiency, as an example, in
the range of about 0.5% to about 1.5%, thereby increasing equipment
size and operating costs. Although the installation of a JIT
limestone system using roller mills on a CFB boiler provides an
initial benefit with respect to reduced capital cost for the CFB
boiler, there is a tradeoff with respect to an increase in long
term operating costs (e.g., as high as 15% to 30% compared to the
cost of the CFB boiler). Accordingly, it would be desirable to be
able to improve heat recovery and thus increase boiler efficiency
for a CFB boiler equipped with a JIT limestone system.
SUMMARY
[0009] According to aspects illustrated herein, a sorbent
conditioning and feed apparatus for a circulating fluidized bed
combustion system includes a dryer apparatus configured to dry
unprocessed sorbent material transported thereto from a raw storage
container; a sorbent crushing device configured to reduce the
particle size of dried sorbent material discharged from the dryer
apparatus; and a circulating fluidized bed (CFB) boiler configured
to receive processed sorbent material conveyed from the sorbent
crushing device; wherein the dryer apparatus utilizes an untempered
hot air source diverted directly from a primary air stream input to
the circulating fluidized bed boiler.
[0010] According to other aspects illustrated herein, a direct
limestone conditioning and feed apparatus for a circulating
fluidized bed combustion system includes a dryer apparatus
configured to dry unprocessed limestone material transported
thereto from a raw storage container; a roll crushing device
configured to reduce the particle size of dried limestone material
discharged from the dryer apparatus; and a circulating fluidized
bed (CFB) boiler configured to receive processed limestone material
conveyed from the roll crushing device; wherein the dryer apparatus
utilizes an untempered hot air source diverted directly from a
primary air stream input to the circulating fluidized bed
boiler.
[0011] According to other aspects illustrated herein, a method of
conditioning and feeding sorbent material for a circulating
fluidized bed combustion system includes transporting raw,
unprocessed sorbent material from a raw storage container to a
dryer apparatus; diverting untempered hot air directly from a
primary air stream input to the circulating fluidized bed boiler,
the untempered hot air used to dry the raw sorbent material;
directing dried raw sorbent material from the dryer apparatus to a
crushing device configured to reduce the particle size of the dried
raw sorbent material discharged from the dryer apparatus; and
conveying processed sorbent material from the crushing device to
the CFB boiler.
[0012] The above described and other features are exemplified by
the following figures and detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Referring now to the figures, which are exemplary
embodiments, and wherein the like elements are numbered alike:
[0014] FIG. 1 is a schematic block diagram of an existing sorbent
preparation/feed apparatus, including a roller mill which utilizes
tempered air to dry and separate sorbent material;
[0015] FIG. 2 is a schematic block diagram of an improved direct
sorbent preparation/feed apparatus and method for a CFB boiler
system, utilizing untempered air for sorbent drying, in accordance
with an embodiment of the invention; and
[0016] FIG. 3 is a schematic diagram of a CFB boiler system
incorporating an improved direct sorbent preparation/feed
apparatus, utilizing untempered air for sorbent drying, in
accordance with a further embodiment of the invention.
DETAILED DESCRIPTION
[0017] Disclosed herein is an improved direct sorbent
preparation/feed apparatus and method for a CFB system. Briefly
stated, a sorbent preparation/feed apparatus utilizes an alternate
crushing device (i.e., one that does not need the use of tempered
air) with a separate, high temperature dryer apparatus in order to
supply dry, sized limestone on a just-in-time basis to a CFB boiler
without reducing boiler efficiency.
[0018] As indicated above, CFB boilers equipped with roller mill
JIT limestone systems employ tempering air in order to control the
entering air temperature, which results in reduced heat recovery
from the flue gas. The reduced heat recovery in turn lowers boiler
efficiency (e.g., on the order of about 1%) and thus increases
equipment size and operating costs. As will be illustrated from the
embodiments discussed herein, a JIT system incorporating one or
more roll crushers and utilizing a separate limestone drying
apparatus eliminates the use of tempering air, thus allowing for
maximum heat recovery from a boiler air heater.
[0019] As used herein, "Primary Air" (PA), in the context of FBC
boilers, refers to combustion air delivered to the bed fluidization
grate at the bottom of the furnace. Primary air is generated at
relatively high pressures. In addition, some primary air may be
diverted to other equipment, as well as enter the furnace at
locations other than the grate. Furthermore, in the context of FBC
boilers, "Secondary Air" (SA) refers to combustion air delivered
through openings in the furnace walls. Secondary air is generated
at pressures lower than primary air, and is used to fluidize the
bed and stage combustion for emission control. "Tempered Air"
refers to hot air that is cooled (i.e., tempered) with relatively
cold air, such as ambient air for example. "Untempered hot air"
refers to hot air leaving an air heater of the FBC system. Both
primary air and secondary air leaving the air heater are considered
untempered hot air streams. "Unprocessed Sorbent" refers to raw
sorbent delivered to the FBC boiler conditioning (size reduction
& drying) system. Unprocessed sorbent has not been conditioned
to the proper size and moisture content required for feeding to the
FBC boiler.
[0020] Referring initially to FIG. 1, there is shown a schematic
block diagram of an existing sorbent preparation/feed apparatus
100. As is shown, the apparatus 100 utilizes a roller mill 102 to
both dry and separate the raw sorbent material. In order to control
the temperature of the air utilized by the roller mill 102, hot
primary air (PA) exiting the boiler system air heater 104 is mixed
with tempering air (e.g., from cold primary air entering the air
heater 104) so as to produce tempered primary air for input to the
roller mill 102. As further illustrated in FIG. 1, dampers 106 may
be utilized in the hot primary air path and the tempering air path
in order to produce the tempered air of a desired temperature and
pressure. For purposes of illustration, FIG. 1 further illustrates
additional inputs and outputs of the air heater 104, including cold
input secondary air (SA), hot secondary air output to a CFB boiler
(not shown), hot gas input to the heater 104 from a boiler backpass
heat exchanger 108, and exiting warm flue gas.
[0021] One reason for requiring tempered air in a conventional
roller mill preparation/feed apparatus 100 relates to the
temperature limits of the crushing device (roller mill) itself. In
addition, the velocity of the air conveying the conditioned
limestone from the roller mill to the CFB boiler is controlled. If,
for example, the velocity of the conveying air is too low, sorbent
particles may settle out in the pipe, leading to blockage.
[0022] Accordingly, FIG. 2 is a schematic block diagram of an
improved direct sorbent preparation/feed apparatus 200 for a CFB
boiler system, utilizing untempered air for sorbent drying, in
accordance with an embodiment of the invention. In lieu of a roller
mill, the apparatus 200 utilizes a sorbent dryer 202 and separate
roll crusher device 204. The wet, raw sorbent is received into the
sorbent dryer 202, wherein hot, untempered primary air (e.g., on
the order of 400.degree. F. or higher is directly input to the
dryer 202 to dry the wet, raw sorbent.
[0023] In an exemplary embodiment, the sorbent dryer 202 is
configured to discharge warm air, which carries sorbent fines (fine
particles) directly to the boiler, thus bypassing the roll crusher
device 204. In this sense, the sorbent dryer 202 has a particle
separation capability. The remaining dry, raw sorbent that is not
already of a fine particulate size is input from the sorbent dryer
202 to the roll crusher 204. As such a device does not have the
same temperature restrictions as a roller mill, the need to temper
the air used to dry the particles is eliminated. Once the dry, raw
sorbent particles are conditioned (i.e., reduced to the desired
size), they are conveyed (e.g., pneumatically or mechanically) to
the boiler (not shown in FIG. 2) in accordance with a direct (JIT)
feed system.
[0024] FIG. 3 is a schematic diagram of a CFB boiler system 300
incorporating an improved direct sorbent preparation/feed apparatus
(such as apparatus 200 of FIG. 2, for example), utilizing
untempered air for sorbent drying, in accordance with a further
embodiment of the invention. As is shown, the system 300 further
includes a CFB boiler 302, a raw sorbent (e.g., limestone) storage
facility/container 304 and cyclone 306. Raw, wet limestone is
conveyed from the storage container 304, is metered and conveyed to
the dryer 202. In an exemplary embodiment, limestone metering may
be controlled by sulfur reduction signals received from the
associated boiler control system. Again, hot, untempered air from
the primary air system is directly used as the source of heat to
dry the limestone, wherein only enough hot air is diverted from the
primary air stream to the dryer 202 as may be needed to dry the
limestone. The dryer 202, having the capability of operating at
high temperatures, is therefore is not subject to tempering.
[0025] As indicted above, the dryer 202 may include a particle
separation capability so as to remove sorbent fines with exiting
warm air to the boiler, thereby preventing an unnecessary feeding
of sorbent fines into the roll crusher 204. One exemplary type of
dryer in this regard may be a fluidized bed dryer. However, other
types of high-temperature dryers may also be used. In any event,
warm air containing evaporated water and limestone dust (sorbent
fines) is conveyed directly to the boiler 302 as part of the
combustion air. On the other hand, limestone discharged from the
dryer 202 is conveyed to one or more roll type crushers 204
arranged in series where it is crushed to a desired size and
discharged. Downstream of the roll crushers 204, the dried, sized
limestone is conveyed (e.g., pneumatically or mechanically) to the
CFB boiler 302. Although not specifically depicted in FIG. 3, the
system 300 may also include surge hoppers and metering systems
downstream of the roll crusher 304 to enhance system operation and
flexibility.
[0026] Through the use of the above described CFB boiler system and
improved direct sorbent preparation/feed apparatus embodiments, the
total amount of pressurized primary air in the system may be
reduced. Because the elimination of tempering air reduces the
amount of primary air needed to dry the sorbent, the total primary
air is reduced and replaced by lower pressure secondary air.
[0027] Moreover, the above exemplary embodiments further improve
particle size distribution of the conditioned limestone to the CFB
boiler. As discussed, limestone (sorbent) to the boiler requires a
specific range of sizes for optimum sulfur capture. If the size
distribution becomes coarser, the quantity of unused sorbent rises.
Larger particles are not sufficiently broken down in the furnace
before removal, thus resulting in a greater amount of unused
sorbent. On the other hand, if the size distribution becomes too
fine, the quantity of unused sorbent rises due to reduced residence
time for the sulfur capture reactions.
[0028] Accordingly, improved particle size distribution is achieved
when small particles are removed (classified) prior to entering the
roll crusher. If allowed to enter the crusher, many initially small
particles are made even smaller, thus increasing the amount of
unused sorbent. By using a separate sorbent dryer with direct
contact of hot air (e.g., a counter flow flash dryer), entrainment
of the smaller particles can be achieved thus avoiding additional
size reduction in the crusher.
[0029] Further, it will be appreciated that the disclosed
embodiments reduce the amount of high pressure air (primary and
tempering) in the CFB system. This reduction is obtained primarily
by using higher temperature air to dry the sorbent. The reduction
lowers total power consumption of the air fans, which in turn
improves boiler economics. For example, the use of primary air at a
maximum air heater outlet temperature maximizes energy recovery
from the flue gas, thus increasing boiler efficiency with respect
to a JIT system using tempering air. As the boiler efficiency is
significantly increased, a long-term savings in plant operating
costs is realized. By way of example, the boiler efficiency may be
increased in the range of about 0.5% to about 1.5%.
[0030] While the invention has been described with reference to
various exemplary embodiments, it will be understood by those
skilled in the art that various changes may be made and equivalents
may be substituted for elements thereof without departing from the
scope of the invention. In addition, many modifications may be made
to adapt a particular situation or material to the teachings of the
invention without departing from the essential scope thereof.
Therefore, it is intended that the invention not be limited to the
particular embodiment disclosed as the best mode contemplated for
carrying out this invention, but that the invention will include
all embodiments falling within the scope of the appended
claims.
* * * * *