Note: Descriptions are shown in the official language in which they were submitted.
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Method for producing pulverized coal
TECHNICAL FIELD
The present invention generally relates to a method for the production of
pulverized coal, in particular for use in the metallurgical industry.
BACKGROUND ART
In the metallurgical industry, pulverized coal is generally injected as com-
bustible 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 pulver-
ized 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.
For the correct functioning of the grinding and drying installation, it is im-
portant 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 danger-
ous 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.
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
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reduced. This allows lowering the oxygen level in the drying gas.
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.
OBJECT OF THE INVENTION
The object of the present invention is to provide an improved method for
producing pulverized coal, which does not present the drawbacks of the prior
art methods. This object is achieved by a method as claimed in claim 1.
GENERAL DESCRIPTION OF THE INVENTION
To achieve this object, the present invention proposes a method for pro-
ducing pulverized coal, the method comprising the steps of:
- heating a drying gas, preferably an inert 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 grinding 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
dry-
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ing gas to the hot gas generator;
-
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.
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.
According to a preferred embodiment, the method further comprises in-
jecting, in the recirculation line, fresh air into the drying gas wherein, if
the
determined oxygen level is higher than the predetermined oxygen level thresh-
old, the volume of fresh air injected into the drying gas is reduced.
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.
Preferably, the predetermined oxygen threshold is chosen to be between
0 and 14 volume (Yo, preferably between Sand 12 volume (Yo.
According to a further aspect of the present invention, the method com-
prises the further steps of determining an exit temperature of the mixture of
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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.
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:
-
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 tem-
perature of the heated drying gas to obtain an exit temperature below the
first temperature threshold; and
- 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 tempera-
ture.
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 tempera-
ture. 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
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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.
When the raw coal introduction is then started, a sudden drop in exit tem-
5 perature 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 intro-
duction of the raw coal. As a consequence, the transition time, wherein pulver-
ized 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.
The volume of water injected into the heated drying gas can be deter-
mined 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 measure-
ments, alone or in combination, to determine the volume of water to be
injected
into the heated drying gas.
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 reduc-
ing consumption of energy once the installation is running. Indeed, the impor-
tance of the overheating and subsequent cooling of the drying gas is particu-
larly 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
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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.
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.
The method may also comprise continuous monitoring of the exit tempera-
ture 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
The present invention will be more apparent from the following description
of one not limiting embodiment with reference to the attached drawing, wherein
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 OF PREFERRED EMBODIMENTS
Figure 1 shows a grinding and drying installation for producing pulverized
coal using the method according to the present invention.
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
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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.
The recirculation line 38 further comprises gas injection means 44 for in-
jecting 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 dew point of the drying gas and the injected hot gas is
used to improve the thermal balance of the grinding and drying circuit.
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.
The water injection means 46 helps to regulate the dew point of the dry-
ing 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 drying 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
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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.
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.
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.
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 tempera-
ture 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.
In order to maintain a correct exit temperature, the temperature of the dry-
ing 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,
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some time passes before the exit temperature reaches the correct exit tempera-
ture again.
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.
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.
It should be noted that this method shows its most dramatic advantages
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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
5 back to the desired exit temperature should a sudden drop in temperature
occur.
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
10 water injection system can be switched off.
Advantageously, the water injection means 46 is also used for an emer-
gency 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.
REFERENCE SIGNS
10 grinding and drying installation 34 filter
pulverizer 36 pulverized coal collector
22 conveyor 38 recirculation line
24 gas inlet 40 fan means
26 hot gas generator 42 line
27 burner 44 gas injection means
28 conduit 45 oxygen sensor
outlet 46 water injection means
32 conduit 48 temperature sensor
