U.S. patent application number 12/994927 was filed with the patent office on 2011-08-11 for method for producing pulverized coal.
This patent application is currently assigned to PAUL WURTH S.A.. Invention is credited to Beno t Junk, Claude Junk, Guy Junk, Louis Schmit, Georges Stamatakis.
Application Number | 20110192079 12/994927 |
Document ID | / |
Family ID | 40228023 |
Filed Date | 2011-08-11 |
United States Patent
Application |
20110192079 |
Kind Code |
A1 |
Schmit; Louis ; et
al. |
August 11, 2011 |
METHOD FOR PRODUCING PULVERIZED COAL
Abstract
Method for producing pulverized coal, the method comprising the
steps of heating a drying gas, preferably an inert gas, in a hot
gas generator (26) to a predefined temperature; feeding the heated
drying gas into a pulverizer (20); introducing raw coal into the
pulverizer (20), the pulverizer (20) grinding the raw coal into
pulverized coal; collecting a mixture of drying gas and pulverized
coal from the pulverizer (20) and feeding the mixture to a filter
(34), the filter (34) separating the dried pulverized coal from the
drying gas; collecting the dried pulverized coal for further use
and feeding the drying gas from the filter (34) to a recirculation
line (38) for returning at least part of the drying gas to the hot
gas generator (26); and determining an oxygen level in the drying
gas, preferably in the recirculation line (38), and comparing the
determined oxygen level to a predetermined oxygen level threshold.
According to a preferred embodiment of the invention, if the
determined oxygen level is higher than a predetermined oxygen
threshold, water is injected into the heated drying gas before it
is fed into the pulverizer (20), the volume of water injected being
calculated so as to reduce the oxygen level below the predetermined
oxygen level threshold.
Inventors: |
Schmit; Louis; (Luxembourg,
LU) ; Stamatakis; Georges; (Canach, LU) ;
Junk; Guy; (Ettlebruck, LU) ; Junk; Claude;
(Vianden, LU) ; Junk; Beno t; (Ettelbruck,
LU) |
Assignee: |
PAUL WURTH S.A.
Luxembourg
LU
|
Family ID: |
40228023 |
Appl. No.: |
12/994927 |
Filed: |
June 2, 2009 |
PCT Filed: |
June 2, 2009 |
PCT NO: |
PCT/EP2009/056763 |
371 Date: |
April 26, 2011 |
Current U.S.
Class: |
44/621 |
Current CPC
Class: |
B02C 23/04 20130101;
F26B 21/06 20130101; F26B 17/103 20130101; C10B 57/10 20130101;
C21B 5/003 20130101; B02C 23/34 20130101 |
Class at
Publication: |
44/621 |
International
Class: |
C10L 9/00 20060101
C10L009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 2, 2008 |
LU |
91 451 |
Claims
1. Method for producing pulverized coal, the method comprising the
steps of: heating a drying gas in a hot gas generator to a
predefined temperature; feeding the heated drying gas into a
pulverizer; introducing raw coal into the pulverizer, the
pulverizer turning the raw coal into pulverized coal; collecting a
mixture of drying gas and pulverized coal from the pulverizer and
feeding the mixture to a filter, the filter separating the dried
pulverized coal from the drying gas; collecting the dried
pulverized coal for further use and feeding the drying gas from the
filter to a recirculation line for returning at least part of the
drying gas to the hot gas generator determining an oxygen level in
the drying gas and comparing the determined oxygen level to a
predetermined oxygen level threshold wherein the oxygen level in
the drying gas is determined during a grinding cycle, in which
heated drying gas is fed through the pulverizer and raw coal is
introduced into the pulverizer, and if, during the grinding cycle,
the determined oxygen level is higher than the predetermined oxygen
level threshold, a volume of water is injected into the heated
drying gas before it is fed into the pulverizer, the volume of
water injected being calculated so as to reduce the oxygen level
below the predetermined oxygen level threshold.
2. Method according to claim 1, wherein, in the recirculation line,
a volume of fresh air is injected into the drying gas, and wherein,
if the determined oxygen level is higher than the predetermined
oxygen level threshold, the volume of fresh air injected into the
drying gas is reduced.
3. Method according to claim 2, wherein, if the volume of fresh air
injected reaches zero and the oxygen level is still higher than the
predetermined oxygen threshold, a volume of water is injected into
the heated drying gas before it is fed into the pulverizer, the
volume of water injected being calculated so as to reduce the
oxygen level below the predetermined oxygen level threshold.
4. Method according to claim 1, wherein the predetermined oxygen
threshold is chosen to be between 0 and 14 volume %.
5. Method according to claim 4, wherein the predetermined oxygen
threshold is chosen to be between 5 and 12 volume %.
6. Method according to claim 1, comprising: determining an exit
temperature of the mixture of drying gas and pulverized coal
exiting the pulverizer; and controlling the exit temperature by
controlling a volume of water injected into the heated drying gas
before feeding it into the pulverizer, the volume of water injected
being calculated so as to bring the exit temperature to a preferred
working temperature.
7. Method according to claim 6, wherein the method comprises: a
startup cycle wherein heated drying gas is fed through the
pulverizer without introducing raw coal, the exit temperature being
kept below a first temperature threshold, and a grinding cycle
wherein heated drying gas is fed through the pulverizer and raw
coal is introduced into the pulverizer, the exit temperature being
kept at a preferred working temperature, wherein during the startup
cycle, said drying gas is heated to a temperature above the first
temperature threshold and a volume of water is injected into the
heated drying gas, the volume of water being calculated so as to
reduce the temperature of the heated drying gas to obtain an exit
temperature below the first temperature threshold; and at the
beginning of the grinding cycle, the volume of water injected into
the heated drying gas is reduced so as to compensate for a drop in
exit temperature.
8. Method according to claim 6, wherein the volume of water
injected into the heated drying gas is reduced at a rate determined
by the exit temperature.
9. Method according to claim 1, wherein the volume of water
injected into the heated drying gas is reduced at a rate determined
by a pressure drop measured across the pulverizer.
10. Method according to claim 7, wherein, during the grinding cycle
and after compensation for the drop in exit temperature, the method
comprises the steps of: reducing the heating of the drying gas; and
reducing the volume of water injected into the heated drying gas to
maintain the preferred exit temperature.
11. Method according to claim 1, wherein, in the recirculation
line, at least part of the drying gas is extracted as exhaust
gas.
12. Method according to claim 1, wherein, in the recirculation
line, fresh air and/or hot gas is injected into the drying gas.
13. Method according to claim 1, comprising: continuous monitoring
of the exit temperature and comparing the measured exit temperature
to a maximum temperature; and if the measured exit temperature
exceeds the maximum temperature, increasing the volume of water
injected into the heated drying gas.
14. Method according to claim 1, wherein the drying gas is heated
in a hot gas generator powered by a lance burner.
15. Method according to claim 1, wherein water is injected into the
heated drying gas by means of a water injection device arranged
between the hot gas generator and the pulverizer.
16. Method according to claim 7, wherein the volume of water
injected into the heated drying gas is reduced at a rate determined
by the exit temperature.
Description
TECHNICAL FIELD
[0001] The present invention generally relates to a method for the
production of pulverized coal, in particular for use in the
metallurgical industry.
BACKGROUND
[0002] In the metallurgical industry, pulverized coal is generally
injected as combustible into blast furnaces. It is important, in
order to ensure good functioning of the blast furnace, that the
pulverized coal is of good quality, i.e. that the pulverized coal
has the right consistence, size and humidity level. The pulverized
coal is generally produced in a grinding and drying installation,
wherein raw coal is ground in a pulverizer and dried to the right
humidity level before the resulting pulverized coal is fed to a
hopper for storage or direct use in a blast furnace. It is known to
subject the freshly ground coal to a stream of hot gas so as to dry
the pulverized coal. The pulverized coal can e.g. be entrained by
the hot gas from the pulverizer to a filter, where the pulverized
coal is then separated from the gas and fed to the hopper. Part of
the gas is recirculated and heated before it is reintroduced into
the pulverizer.
[0003] For the correct functioning of the grinding and drying
installation, it is important to monitor the oxygen level in the
drying gas, generally downstream of the filter. If the oxygen level
becomes too high, the combination of drying gas and pulverized coal
may become an explosive mixture with potentially dangerous
consequences. Generally, in the recirculation line, i.e. in the
line returning the drying gas back to the pulverizer, exhaust
gasses are extracted from the drying gas and fresh air is
injected.
[0004] In known grinding and drying installations, the oxygen level
in the drying gas is monitored and, if the measured oxygen level is
found to be too high, the amount of fresh air introduced into the
drying gas in the recirculation line is reduced. This allows
lowering the oxygen level in the drying gas.
[0005] However, in some circumstances, e.g. if the raw coal is very
dry and/or if the installation is run under reduced load, the
reduction of the amount of fresh air introduced into the drying gas
may not be enough to sufficiently reduce the oxygen level. Indeed,
once the amount of fresh air introduced into the drying gas is
reduced to zero, i.e. no more fresh air is introduced, the oxygen
level may in such circumstances still be too high. In order to
avoid any damage to the installation it may then be necessary to
shut down the grinding and drying installation. Such a shut down
not only leads to a loss of production, but also to extra costs
relating to the replacement or conditioning of the drying gas.
BRIEF SUMMARY
The Invention Provides an Improved Method for Producing Pulverized
Coal, which does not Present the Drawbacks of The Prior Art
Methods
[0006] More specifically, the present invention proposes a method
for producing pulverized coal, the method comprising the steps of:
[0007] heating a drying gas, preferably an inert gas, in a hot gas
generator to a predefined temperature; [0008] feeding the heated
drying gas into a pulverizer; [0009] introducing raw coal into the
pulverizer, the pulverizer grinding the raw coal into pulverized
coal; [0010] collecting a mixture of drying gas and pulverized coal
from the pulverizer and feeding the mixture to a filter, the filter
separating the dried pulverized coal from the drying gas; [0011]
collecting the dried pulverized coal for further use and feeding
the drying gas from the filter to a recirculation line for
returning at least part of the drying gas to the hot gas generator;
[0012] determining an oxygen level in the drying gas, preferably in
the recirculation line, and comparing the determined oxygen level
to a predetermined oxygen level threshold.
[0013] According to a preferred embodiment of the invention, the
oxygen level in the drying gas is determined during a grinding
cycle wherein heated drying gas is fed through the pulverizer and
raw coal is introduced into the pulverizer and if, during the
grinding cycle, the determined oxygen level is higher than a
predetermined oxygen threshold, water is injected into the heated
drying gas before it is fed into the pulverizer, the volume of
water injected being calculated so as to reduce the oxygen level
below the predetermined oxygen level threshold. The injection of
water into the drying gas during the grinding cycle allows
increasing the overall volume of the drying gas, thereby reducing
the relative oxygen volume. The water injection therefore allows
reducing the oxygen level to an acceptable level and thereby avoids
any damage to the installation or the need to shut down the
grinding and drying installation.
[0014] According to a preferred embodiment, the method further
comprises injecting, in the recirculation line, fresh air into the
drying gas wherein, if the determined oxygen level is higher than
the predetermined oxygen level threshold, the volume of fresh air
injected into the drying gas is reduced.
[0015] Advantageously, the method comprises first reducing the
volume of fresh air injected into the drying gas, and then, if the
volume of fresh air injected reaches zero and the oxygen level is
still higher than the predetermined oxygen threshold, injecting
water into the heated drying gas before it is fed into the
pulverizer, the volume of water injected being calculated so as to
reduce the oxygen level below the predetermined oxygen level
threshold.
[0016] Preferably, the predetermined oxygen threshold is chosen to
be between 0 and 14 volume %, preferably between 5 and 12 volume
%.
[0017] According to a further aspect of the present invention, the
method comprises the further steps of determining an exit
temperature of the mixture of drying gas and pulverized coal
exiting the pulverizer; and controlling the exit temperature by
controlling a volume of water injected into the heated drying gas
before feeding it into the pulverizer. By controlling the amount of
water injected into the drying gas upstream of the pulverizer, the
temperature of the drying gas entering the pulverizer can be
adjusted rapidly so as to take into account temperature differences
occurring due to raw coal with different levels of humidity being
introduced into the pulverizer. It is thereby possible to maintain
the temperature of the drying gas exiting the pulverizer, hereafter
referred to as exit temperature, as constant as possible.
[0018] The present aspect is of particular advantage during a
startup phase of the installation, wherein the method comprises a
startup cycle wherein heated drying gas is fed through the
pulverizer without introducing raw coal, the exit temperature being
kept below a first temperature threshold, and a grinding cycle
wherein heated drying gas is fed through the pulverizer and raw
coal is introduced into the pulverizer, the exit temperature being
kept at a preferred working temperature. According to an important
aspect of the invention, the method comprises: [0019] during the
startup cycle, heating said drying gas to a temperature above the
first temperature threshold and injecting a volume of water into
the heated drying gas, the volume of water being calculated so as
to reduce the temperature of the heated drying gas to obtain an
exit temperature below the first temperature threshold; and [0020]
at the beginning of the grinding cycle, reducing the volume of
water injected into the heated drying gas so as to compensate for
the drop in exit temperature.
[0021] During a startup phase of the installation, drying gas is
generally fed through the installation before raw coal is
introduced into the pulverizer. This allows the individual
components to be heated to the desired working temperature. By
controlling the amount of water injected into the drying gas
upstream of the pulverizer during this startup phase, the drying
gas, which may be heated to a temperature above the maximum
tolerated exit temperature, can be cooled down again so that the
temperature downstream of the pulverizer does not exceed the first
temperature threshold.
[0022] When the raw coal introduction is then started, a sudden
drop in exit temperature occurs due to the addition of cold and wet
material. By overheating the drying gas in the hot gas generator
and subsequently cooling it through water injection, the
temperature of the drying gas entering the pulverizer can be
quickly adapted to the new operating conditions. A reduction of the
quantity of injected water allows a rapid temperature increase of
the drying gas entering the pulverizer so as to compensate for the
temperature drop due to the introduction of the raw coal. As a
consequence, the transition time, wherein pulverized coal is
produced at lower temperature is considerably reduced or even
avoided. The amount of unusable coal slurry is also considerably
reduced, thereby increasing the efficiency of the installation.
[0023] The volume of water injected into the heated drying gas can
be determined based on the exit temperature. Alternatively, the
volume of water injected into the heated drying gas can be
determined based on a pressure drop measured across the pulverizer.
It is not excluded to use other measurements, alone or in
combination, to determine the volume of water to be injected into
the heated drying gas.
[0024] Preferably, during the grinding cycle and after compensation
for the drop in exit temperature, the method comprises the further
steps of reducing the heating of the drying gas; and reducing the
volume of water injected into the heated drying gas to maintain the
desired exit temperature. This allows reducing consumption of
energy once the installation is running. Indeed, the importance of
the overheating and subsequent cooling of the drying gas is
particularly important during the startup phase of the
installation, wherein it allows providing a buffer to compensate
for the drop in temperature occurring when the introduction of raw
coal is started. Once the installation is running, only smaller
temperature drops might occur and the buffer can be reduced. During
normal operation of the grinding and drying installation, there is
hence no need to over heat the drying gas in the hot gas generator
and subsequently cooling it to the working temperature.
[0025] In the recirculation line, part of the drying gas can be
removed as exhaust gas. Apart from fresh air, hot gas can also be
injected into the drying gas in the recirculation line.
[0026] The method may also comprise continuous monitoring of the
exit temperature and comparing the measured exit temperature to a
maximum temperature, wherein, if the measured exit temperature
exceeds the maximum temperature, the volume of water injected into
the heated drying gas is increased. This allows using the water
injection means used for general process control, to be used for
emergency cooling also.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The present invention will be more apparent from the
following description of one not limiting embodiment with reference
to the attached drawing, wherein
[0028] FIG. 1 shows a schematic representation of a grinding and
drying installation used for carrying out the method according to
the present invention.
DETAILED DESCRIPTION
[0029] FIG. 1 shows a grinding and drying installation for
producing pulverized coal using the method according to the present
invention.
[0030] Such a grinding and drying installation 10 comprises a
pulverizer 20 into which raw coal is fed via a conveyor 22. In the
pulverizer 20, the raw coal is crushed between internal mobile
pieces (not shown) or any other conventional grinding means into a
fine powder. At the same time, a hot drying gas is fed through the
pulverizer 20 to dry the pulverized coal. The drying gas enters the
pulverizer 20 through a gas inlet 24. Upstream of the pulverizer
20, the grinding and drying installation 10 comprises a hot gas
generator 26 in which a drying gas can be heated to a predefined
temperature. Such a hot gas generator 26 is powered by a burner 27,
such as e.g. a multiple lance burner. The heated drying gas is
carried from the hot gas generator 26 to the pulverizer 20 via a
conduit 28. As the heated drying gas passes through the pulverizer
20, from the gas inlet 24 to an outlet 30, pulverized coal is
entrained. A mixture of pulverized coal and drying gas is carried
from the pulverizer 20, via a conduit 32, to a filter 34, where the
pulverized coal is again removed from the drying gas and fed to a
pulverized coal collector 36, ready further use. The drying gas
exiting the filter 34 is fed to a recirculation line 38 for feeding
it back to the hot gas generator 26. The recirculation line 38
comprises fan means 40 for circulating the drying gas through the
installation. The fan means 40 may be located upstream or
downstream of a line 42, e.g. a stack, which is used to extract
part of the drying gas from the recirculation line 38.
[0031] The recirculation line 38 further comprises gas injection
means 44 for injecting fresh air and/or hot gas into the
recirculation line 38. The injected fresh air and/or hot gas is
mixed with the recycled drying gas. The injected fresh air allows
reducing the due point of the drying gas and the injected hot gas
is used to improve the thermal balance of the grinding and drying
circuit.
[0032] According to an important aspect of the present invention,
the installation 10 comprises water injection means 46 arranged
downstream of the hot gas generator 26 and upstream of the
pulverizer 20. The importance of the water injection means 46 will
become clear in the description herebelow.
[0033] The water injection means 46 helps to regulate the dew point
of the drying gas by regulating the oxygen level therein. In the
recirculation line 38, part of the drying gas is extracted via the
line 42 and fresh air may be injected via the gas injection means
44. In conventional installations, the oxygen level is monitored
for safety reasons by means of an oxygen sensor 45 and, if the
oxygen level is found to be too high, the gas injection means 44 is
instructed to reduce the amount of fresh air introduced into the
dying gas. A problem however occurs when the gas injection means 44
reaches its shut-off point, i.e. when the gas injection means 44 is
completely turned off and no fresh air is injected into the dying
gas. If the oxygen level is then still found to be too high, the
volume of fresh air injected into the dying gas cannot be further
reduced and a shutdown of the installation becomes necessary.
[0034] According to the present invention, the oxygen level in the
drying gas can be reduced by injecting water into the drying gas by
means of the water injection means 46. When the oxygen level
measured by the oxygen sensor 45 is too high, the water injection
means 46 can be instructed to increase the volume of water injected
into the drying gas, thereby reducing the oxygen level downstream
of the filter 34.
[0035] Preferably, the oxygen level is first reduced by the
conventional method of reducing the volume of fresh air injected
into the dying gas by the gas injection means 44 and if this is not
sufficient, the oxygen level is then further reduced by increasing
the volume of water injected into the drying gas by the water
injection means 46.
[0036] Another function of the water injection means 46 may be to
help regulate the temperature of the drying gas at the exit of the
pulverizer 20. In operation, the drying gas is heated to a
predefined temperature in the hot gas generator 26 and fed through
the pulverizer 20. The temperature of the drying gas is reduced in
the pulverizer 20 as the heat from the drying gas is used to dry
the pulverized coal. The level of humidity of the raw coal
determines the temperature loss of the drying gas. In order to
prevent damage to the filter 34, the temperature of the mixture of
pulverized coal and drying gas exiting the pulverizer 20, hereafter
referred to as the exit temperature, is monitored, e.g. by means of
a temperature sensor 48.
[0037] In order to maintain a correct exit temperature, the
temperature of the drying gas entering the pulverizer needs to be
controlled, which is generally achieved by controlling the output
power of the burner 27 of the hot gas generator 26. Unfortunately
this process has a relatively slow response time, meaning that once
the installation has determined that the exit temperature is too
high or too low and the burner 27 has been made to react in
consequence, some time passes before the exit temperature reaches
the correct exit temperature again.
[0038] The response time is particularly important during a startup
phase of the installation. Indeed, initially, heated drying gas is
fed through the installation before the raw coal is introduced.
This allows the installation to heat up and reach the ideal working
conditions. When, after a certain time, raw coal is then introduced
into the pulverizer 20, the exit temperature suddenly drops well
below the desired exit temperature. Conventionally, the burner 27
then reacts by further heating the drying gas so as to reach the
desired exit temperature. The desired exit temperature is then
however only obtained after a long delay and any pulverized coal
obtained in the meantime may have to be discarded because it has
not been sufficiently dried. Indeed, during a transition period
wherein the exit temperature is too low, unusable coal slurry is
generally obtained instead of dried pulverized coal.
[0039] According to the present invention, during the startup
phase, the burner 27 is set to heat the drying gas well above the
desired exit temperature. The heated drying gas is then subjected
to controlled cooling by injecting water into the heated drying gas
through the water injection means 46, whereby the drying gas is
cooled so that the desired exit temperature can be achieved. After
a certain heat-up time of the grinding and drying installation,
when the raw coal is introduced into the pulverizer 20, the exit
temperature suddenly drops well below the desired exit temperature.
Instead of compensating for this sudden drop by adapting the
heating temperature of the burner 27, the amount of water injected
into the drying gas by the water injection means 46 is reduced. The
heated drying gas is hence cooled less and the desired exit
temperature can be kept stable. The reaction time of this procedure
is considerably lower than the conventional one, thereby
considerably reducing or avoiding a transition period wherein the
exit temperature is too low and the production of unusable coal
slurry.
[0040] It should be noted that this method shows its most dramatic
advantages during the startup phase, i.e. during a transition
period shortly after raw coal is initially introduced into the
pulverizer. The present method is however also advantageous during
normal operation of the installation. When a reduction of the
humidity in the raw coal occurs, the exit temperature can be
quickly brought back to the desired exit temperature should a
sudden drop in temperature occur.
[0041] In order to optimize energy consumption, it is advantageous
to gradually reduce both the heating and the subsequent cooling of
the drying gas once the exit temperature has stabilized. If no such
subsequent cooling is required, the water injection system can be
switched off.
[0042] Advantageously, the water injection means 46 is also used
for an emergency cooling. The method may comprise continuous
monitoring of the exit temperature and comparing the measured exit
temperature to a maximum temperature. When the measured exit
temperature exceeds the maximum temperature, the water injection
means 46 is instructed to increasing the volume of water injected
into the heated drying gas, thereby reducing the temperature of the
drying gas entering the pulverizer 20 and consequently also the
temperature of the drying gas exiting the pulverizer 20.
* * * * *