AU2014333245B2

AU2014333245B2 - Drying Method and Drying Facility for Low-Grade Coal

Info

Publication number: AU2014333245B2
Application number: AU2014333245A
Authority: AU
Inventors: Masaki Kataoka; Yuji Nakamura; Takayuki Noguchi
Original assignee: Tsukishima Kikai Co Ltd
Current assignee: Tsukishima Kikai Co Ltd
Priority date: 2013-10-11
Filing date: 2014-09-02
Publication date: 2016-09-22
Anticipated expiration: 2034-09-02
Also published as: AU2014333245A1; JP2015074765A; JP6052130B2; WO2015053011A1

Abstract

[Problem] To dry low-grade charcoal, particularly low-grade charcoal having high water content, and to suppress the spontaneous ignition property of the low-grade charcoal. [Solution] In the drying of low-grade charcoal by an indirect heating dryer (3), the heating conditions for the indirect heating dryer are adjusted in such a manner that the temperature of the surface of the low-grade charcoal at the outlet of the dryer becomes higher by 0 to 5ºC than the dew point of a carrier gas discharged from the dryer and is 70 to 95ºC and the oxygen concentration in the carrier gas discharged from the dryer is more than 10% and 15% or less on the wet gas basis.

Description

DESCRIPTION

DRYING METHOD AND DRYING FACILITY FOR LOW-GRADE COAL

Technical Field [0001]

The present invention relates to a technique of drying low-grade coal, particularly low-grade coal with a high water content, and suppressing spontaneous combustion of the dried coal.

Background Art [0002]

Nowadays, because of an energy situation of developing countries, a demand for coal, which is mainly bituminous coal, is on the rise.

[0003]

Accordingly, instead of bituminous coal, low-grade coal whose amount of usage is small, such as lignite, sub-bituminous coal, or brown coal, whose reserves are large and which is relatively inexpensive, is drawing attention. However, due to reasons such that a transportation cost per energy of the low-grade coal is high since the low-grade coal has a high water content, and further, when the low-grade coal is dried, spontaneous combustion easily occurs, it is difficult to say that the low-grade coal is sufficiently used, and thus the low-grade coal has been used only in the vicinity of a coal mine.

[0004]

As a main cause of spontaneous combustion of coal, there can be primarily cited existence of active groups which easily react with oxygen.

[0005]

As a main cause of facilitating the reaction between oxygen and the active groups, there can be cited a specific surface area, a water content, existence of FeS, an oxygen partial pressure, a temperature, or the like.

[0006]

Accordingly, in order to suppress spontaneous combustion of low-grade coal, the low-grade coal has been turned into briquette after being dried (Patent Document 1).

[0007]

However, in order to turn the low-grade coal into briquette, pulverizing processing and high-pressure processing are required, which increases a cost of facility, and in addition to that, a power cost, and a maintenance cost such as parts-replacement cost also become expensive, resulting in that a merit in using the low-grade coal is reduced, when compared to a price of widely-used bituminous coal. Further, since only the specific surface area as the main cause of facilitating the reaction between oxygen and the active groups is reduced, an effect of suppressing the spontaneous combustion is small.

[0008]

Although there has been proposed, as another method, a method in which active groups which easily react with oxygen are extracted by chemicals, a facility becomes complicated, and thus the method is not economical.

[0009]

Although there has been proposed, as still another method, a method in which a surface of coal is coated with heavy oil or the like to cut off air (oxygen), if the coating is peeled off due to impact or the like in the middle of transportation, an effect of the coating is lost. Further, to uniformly coat the surface of coal with an expensive coating agent, is not economical in terms of not only facility but also maintenance cost.

[0010]

Meanwhile, it is known that, when a temperature of coal is increased under an oxygen atmosphere as a method of suppressing the spontaneous combustion of coal, a chemical structure, which easily bonds with oxygen, in the vicinity of a surface of coal, bonds with oxygen, resulting in that spontaneous combustion to be occurred thereafter can be suppressed (Patent Document 2). Specifically, by making a large part of active groups which easily react with oxygen react with oxygen, the spontaneous combustion is suppressed.

[0011]

Actually, the method in Patent Document 2 requires two-stage processing including heating processing at 100 to 350°C in an inert gas, and oxidation processing under an oxygen concentration of 1 to 10 volume%, which increases a processing time, and thus the method is impractical.

[0012]

Incidentally, in a conventional drying method for low-grade coal, drying through direct heating with hot air by using a flash dryer, hot air rotary drying or the like, has been mainly employed. When the direct heating with hot air is employed, an oxygen concentration can be increased, but, a product temperature of coal is not increased, so that a chemical structure, which easily bonds with oxygen, in the vicinity of a surface of coal, is difficult to bond with oxygen, and thus there is no effect of suppressing the spontaneous combustion. Further, in the method of performing direct drying by using heating water vapor and the like, it is possible to increase a product temperature of coal, but, since no oxygen exists, there is no effect of suppressing the spontaneous combustion. Therefore, in the conventional drying methods, after performing drying, dried coal is cooled by a cooling machine to delay a rise in temperature caused by spontaneous combustion, to thereby suppress the spontaneous combustion.

[0013]

Further, in the conventional coal drying, the drying has been performed until when a water content reaches a critical water content, from a viewpoint of preventing a coal dust explosion and combustion, and operation has been performed so that a product temperature of coal is not increased too much.

[Prior Art Document] [Patent Document] [0014]

Patent Document 1: Japanese Patent Application Laid-open No. 201137938

Patent Document 2: Japanese Examined Patent Application Publication No. Sho 63-67518

Disclosure of the Invention Problems to Be Solved by the Invention [0015]

The present invention is made for solving the above-described problems, and an object thereof is to suppress spontaneous combustibility by drying low-grade coal, particularly low-grade coal with a high water content.

Means for Solving the Problems [0016]

The present invention solving the above-described problems is as follows.

Note that low-grade coal in the present invention has a meaning including brown coal such as Victoria coal, North Dakota coal, or Beluga coal, sub-bituminous coal such as West Banko coal, or Binungan coal, lignite, and the like.

[0017] <Invention described in claim 1> A drying method for low-grade coal, includes drying low-grade coal using an indirectly heating dryer, in which the indirectly heating dryer has drying conditions as follows: a surface temperature of the low-grade coal at an outlet of the dryer is higher, by 0°C to 5°C, than a dew point of a carrier gas discharged from the dryer and is 70°C to 95°C, and an oxygen concentration in the carrier gas discharged from the dryer ranges from greater than 10% to 15% on a wet gas basis.

Here, regarding meaning of the description “the indirectly heating dryer has drying conditions”, as long as the first-half condition stipulated in the present invention is satisfied, adjustment of a carrier gas amount, adjustment of a steam amount in the indirectly heating dryer, adjustment of a number of rotations of the indirectly heating dryer, and the like are unnecessary, and it is also possible to previously fix an operation amount to a set value, and it is of course possible that, in order to satisfy the first-half condition stipulated in the present invention, a variation value in the condition is acquired, and the operation amount is controlled to achieve a target value of the condition through the adjustment of the carrier gas amount, the adjustment of the steam amount in the indirectly heating dryer, the adjustment of the number of rotations of the indirectly heating dryer, and the like. Practically, it is desirable to employ the latter control method. This also similarly applies to an invention of claim 2.

[0018] (Operation and effect)

In the present invention, the low-grade coal is dried by the indirectly heating dryer in a constant-rate drying zone in which a water content of the coal reaches a critical water content. Since the drying is performed by the indirectly heating dryer, a product temperature of the low-grade coal can be increased by adjusting the dew point of the carrier gas (by adjusting the carrier gas amount), and, for example, the product temperature at the outlet of the dryer can be increased to 70°C to 95°C. In addition to that, since the drying processing is performed under the drying atmosphere with high oxygen concentration ranging from greater than 10% to 15% on the wet gas basis, it is possible to make active groups in the low-grade coal favorably react with oxygen, which enables to reform the low-grade coal into low-grade coal exhibiting a high effect of suppressing the spontaneous combustibility.

Here, a reason why the upper limit of the drying temperature is set to 95 °C, is because if the temperature exceeds 95 °C, drying in a falling-rate drying zone is performed. In the falling-rate drying zone, there is a possibility that the temperature of the coal is increased to equal to or greater than the dew point, resulting in that combustion may be occurred by oxygen-enriched carrier gas. On the other hand, a reason why the lower limit of the drying temperature is set to 70°C or more, is because if the temperature is less than 70°C, an oxidation reaction becomes slow.

[0019] <Invention described in claim 2> A drying method for low-grade coal, includes drying low-grade coal using an indirectly heating dryer, in which the indirectly heating dryer has drying conditions as follows: a surface temperature of the low-grade coal at an outlet of the dryer is higher, by 0°C to 5°C, than a dew point of a carrier gas discharged from the dryer and is 80°C to 95°C, oxygen is enriched in the carrier gas, and an oxygen concentration in the carrier gas discharged from the dryer ranges from greater than 10% to 15% on a wet gas basis.

[0020] (Operation and effect)

When a steam tube dryer is used as the indirectly heating dryer, a steam tube dryer with a structure capable of making a carrier gas flow, is used widely. By designing such that the oxygen-enriched air is used as the carrier gas which conveys water vapor dried and vaporized from the low-grade coal, and the carrier gas is moved from an inlet side to the outlet side of the indirectly heating dryer, a drying rate becomes high, when compared to a case where air is used as the carrier gas. As a result of this, even under the same drying time, the oxidation reaction on the surface of the coal can be efficiently facilitated, and the effect of suppressing combustion is improved.

[0021] <Invention described in claim 3>

In the drying method for the low-grade coal described in claim 1 or 2, an air-slide conveyor is used when conveying dried coal of the low-grade coal from the outlet of the dryer, and in a process of conveying the dried coal on a deflecting perforated plate of the air-slide conveyor, a direction of blowing air through openings of the perforated plate is set to be directed to a dried-coal conveying direction and directed upward by 10 degrees to 20 degrees with respect to a horizontal direction, to make the surface of the low-grade coal and oxygen bring into contact with each other.

[0022] (Operation and effect)

By classifying, through wind power, a fine powder of the dried coal, it is possible to convey the dried coal in which the specific surface area being one of the main causes of the spontaneous combustion and the dust explosion is reduced, toward a target place. In this case, it is desirable to use the air-slide conveyor having the deflecting perforated plate on which the dried coal is conveyed, as will be described later.

If the angle of direction of blowing air through the openings of the perforated plate is 10 degrees to 20 degrees, particularly 14 degrees to 16 degrees, driving power of a conveying blower is small, and an amount of conveyance per unit width becomes large. It is optimum to employ the air-slide conveyor, since the air-slide conveyor exhibits advantages such that it realizes good solid-gas contact, and it requires relatively small driving power.

[0023] <Invention described in claim 4>

In the drying method for the low-grade coal described in claim 3, a carrier gas processed by a dust collector which extracts, through a dry system, dust in the carrier gas for the indirectly heating dryer, is set to a conveying medium for the air-slide conveyor.

[0024] (Operation and effect)

In the first place, the carrier gas discharged from the indirectly heating dryer has a large content of water vaporized from the coal, so that when compared to air, the carrier gas has a low O2 concentration (on the wet gas basis), and has a temperature which is about the same as a surface temperature of the dried coal.

Therefore, by designing such that the carrier gas as described above is supplied as the conveying medium for the air-slide conveyor, it is possible to facilitate the oxidation reaction on the surface of the dried coal (the reaction proceeds because of the high temperature of the supplied gas), while suppressing the spontaneous combustion (the combustion is difficult to occur because of the low oxygen concentration).

[0025] <Invention described in claim 5>

In the drying method for the low-grade coal described in claim 3, when a gas blowout rate from the openings of the deflecting perforated plate is set to u m/s, an opening ratio is set to m, and a gas density is set to pkg/m3, a value of 1/2 pmu3 is 30 to 200.

[0026] (Operation and effect)

It is possible to obtain a practical amount of conveyance per unit width with respect to a certain gas blowout amount from the openings of the perforated plate. A preferable range of the value of 1/2 pmu3 is 40 to 100.

[0027] <Invention described in claim 6> A drying facility for low-grade coal, includes: an indirectly heating dryer drying low-grade coal; a measuring device measuring a temperature of dried coal at an outlet of the dryer; a dew point measuring device measuring a dew point of a carrier gas discharged from the dryer; an oxygen concentration measuring device measuring an oxygen concentration of the carrier gas; a unit of adjusting a surface temperature of the dried coal at the outlet of the dryer by associating the surface temperature with the dew point measured by the dew point measuring device, and a unit of adjusting the oxygen concentration in the carrier gas discharged from the dryer on a wet gas basis.

[0028] (Operation and effect)

Basically, operation and effect similar to those of claim 1 are exhibited. <Invention described in claim 7>

The drying facility for the low-grade coal described in claim 6, further includes an air-slide conveyor conveying the low-grade coal discharged from the indirectly heating dryer by setting a carrier gas processed by a dust collector which extracts, through a dry system, dust in the carrier gas discharged from the indirectly heating dryer, to a conveying medium.

[0029] (Operation and effect)

Operation and effect similar to those of claim 4 are exhibited. <Invention described in claim 8>

The drying facility for the low-grade coal described in claim 6 or 7, further includes: a dust collector extracting, through a dry system, dust in the carrier gas discharged from the indirectly heating dryer; a nitrogen-containing gas producing apparatus producing a nitrogen-containing gas supplied to the dust collector from air; and a unit of supplying an oxygen-containing exhaust gas discharged from the nitrogen-containing gas producing apparatus, as the carrier gas for the indirectly heating dryer.

[0030] (Operation and effect)

From a viewpoint of prevention of environmental pollution, installation of the dust collector which extracts, through the dry system, the dust in the carrier gas discharged from the indirectly heating dryer, becomes necessary.

In this case, it is desirable to use, as the dust collector, a dust extractor which uses pulse air. The pulse air has to be N2 gas with low oxygen concentration, in order to prevent dust explosion at a time of cleaning. The required N2 gas is desirably obtained by the nitrogen-containing gas producing apparatus.

As an example of the nitrogen-containing gas producing apparatus, there can be cited a PSA (Pressure Swing Adsorption) apparatus, in which the N2 gas is produced through a method of either oxygen adsorption in the air (in this case, exhaust gas is nitrogen-enriched) or nitrogen adsorption in the air (in this case, desorbed gas is nitrogen-enriched). The adsorbed nitrogen-enriched gas is used as the pulse air, and the oxygen-enriched gas which becomes unnecessary at this time is set to the carrier gas for the steam tube dryer (STD), resulting in that the carrier gas required for the suppression of combustibility can be obtained without separately providing an additional equipment.

Effect of the Invention [0031]

As described above, according to the present invention, it is possible to dry low-grade coal, particularly low-grade coal with a high water content, and to suppress spontaneous combustibility. Therefore, it is possible to use inexpensive low-grade coal, and to realize improvement of energy situation.

Brief Description of Drawings [0032]

Fig. 1 is a flow chart illustrating one example of drying processing of the present invention;

Fig. 2 is a schematic perspective view of an example of an indirectly heating dryer (steam tube dryer (STD));

Fig. 3 is a partial perspective view of a perforated plate of an air-slide (mesh) conveyor;

Fig. 4 is a sectional view of the perforated plate of the air-slide (mesh) conveyor;

Fig. 5 is a relation diagram between a dew point and an oxygen concentration;

Fig. 6 is an explanatory diagram regarding spontaneous combustibility;

Fig. 7 is an explanatory diagram illustrating influence of a product temperature of coal;

Fig. 8 is an explanatory diagram illustrating influence of an atmospheric oxygen concentration;

Fig. 9 is an explanatory diagram illustrating an amount of conveyance per unit width of air-slide in which an angle of blowout nozzle is changed;

Fig. 10 is an explanatory diagram illustrating a relation between a flow rate of air supplied to the air-slide (l/2pmu3) and conveying performance;

Fig. 11 is a flow chart of an example in which a PSA apparatus is incorporated; and

Fig. 12 is a flow chart of another example in which a PSA apparatus is incorporated.

Best Mode for Carrying out the Invention [0033]

Hereinafter, the present invention will be further described.

[0034]

The entire drying processing for low-grade coal of the present invention can be carried out by an embodiment in Fig. 1, for example.

Further, as a drying processor, it is possible to preferably use a steam tube dryer (STD) 3.

[0035]

In advance of the explanation of the entire processing, an example of the steam tube dryer (STD) 3 being an indirectly heating dryer applied to the embodiment of the present invention will be described based on Fig. 2, in order to deepen understanding.

[0036]

In a rotating shell 30 which can freely rotate around an axial center in the steam tube dryer 3 illustrated in Fig. 2, a plurality of heating tubes 31 are arranged in parallel with the axial center between both end plates, heating steam as a heat medium is supplied to these heating tubes 31 through a heat medium inlet pipe 51 attached to a rotary joint 50 to be flowed through the respective heating tubes 31, and then drain of the heating steam K is discharged via a heat medium outlet pipe 52.

[0037]

Further, a charging apparatus 33 having a screw and the like for charging a material to be processed in the rotating shell 30, is provided to the steam tube dryer 3. Low-grade coal (LRC) put into the rotating shell 30 from one end side of the rotating shell 30, through a charging hole 53 of the charging apparatus 33, is brought into contact with the heating tubes 31 heated by the heating steam K to be dried. Along with this, since the rotating shell 30 is disposed with a falling gradient, the coal is sequentially moved smoothly in a discharge port 54 direction, and the dried coal (DC) is continuously discharged from the other end side of the rotating shell 30.

[0038]

As illustrated in Fig. 2, the rotating shell 30 is disposed on bases 36, and is supported, via tires 34, by two sets of support rollers 35 disposed, with an interval provided therebetween, to be parallel with the axial center of the rotating shell 30. In accordance with the falling gradient and a diameter of the rotating shell 30, a width and a longitudinal-direction inclination angle between the two sets of support rollers 35 are selected.

[0039]

Meanwhile, in order to rotate the rotating shell 30, a driven gear 40 is provided in a periphery of the rotating shell 30, and when a drive gear 43 engages with the driven gear 40, rotational force of a motor 41 is transmitted via a speed reducer 42, resulting in that the driven gear 40 rotates around the axial center of the rotating shell 30. Further, in an inside of the rotating shell 30, a carrier gas is introduced from a carrier gas inlet 61, and the carrier gas is discharged from a carrier gas discharge port 62, by being accompanied by steam formed when water contained in the low-grade coal (LRC) is vaporized.

[0040]

Note that the entire configuration of the above-described steam tube dryer 3 is only one example, and the present invention is not limited by the above-described configuration.

[0041]

In the present invention, the drying processing can be executed in the following manner, for example, as in Fig. 1 in which the entire processing flow is illustrated.

[0042]

The low-grade coal (LRC) previously pulverized to about 10 mm or less, for example, by a pulverizer 33A is put into the charging apparatus 33, and then supplied to the indirectly heating dryer 3. The supplied low-grade coal is indirectly heated to be dried by the steam supplied to the indirectly heating dryer 3, and the coal is turned into the dried coal (DC) and discharged from the outlet of dryer 54.

[0043]

At the outlet of dryer 54 of the indirectly heating dryer 3, a temperature measuring device 21 measuring a temperature of the dried coal is disposed. Although there is no limitation in the temperature measuring device as long as it is a thermometer capable of measuring a product temperature of the dried coal, a surface temperature measuring device 21 of the dried coal, such as a non-contact thermometer is disposed. Note that the product temperature of the dried coal to be measured may be either an internal temperature or a surface temperature. In the present example, a temperature measuring device which measures a surface temperature, is employed. A measured value is sent to a drying condition adjusting unit (not-illustrated) by the temperature measuring device 21.

[0044]

The carrier gas is pressurized by a forced draft fan 22, and while monitoring an oxygen concentration to achieve a desired oxygen concentration by using an oxygen concentration meter (O2 meter) 23A, air and oxygen are supplied from the carrier gas inlet 61 of the charging apparatus 33. Here, as oxygen to be enriched in air, it is possible to use oxygen adsorbed in a separately-provided not-illustrated oxygen PSA (Pressure Swing Adsorption) apparatus, or exhaust gas obtained after adsorbing nitrogen in a nitrogen PSA apparatus. In particular, when the facility according to the present invention is disposed in a thermal power station, it is preferable to utilize an oxygen-containing gas discharged from a nitrogen PSA apparatus which generates nitrogen for nitrogen purge used in the thermal power station.

[0045]

The carrier gas is discharged by being accompanied by water vapor vaporized from the low-grade coal (LRC) in the indirectly heating dryer 3 and a slight amount of coal dust, and subjected to dust extraction in a dry dust collector 24.

In the dry dust collector 24, a pulse gas supply device (not-illustrated) which blows down the coal dust is provided, and the pulse gas is supplied at a fixed interval, or in accordance with a pressure difference between in front of and behind a filter medium.

[0046]

The carrier gas discharged from the dry dust collector 24 is discharged from an exhaust fan 25 while measuring an oxygen concentration of the carrier gas by using an oxygen concentration meter (O2 meter) 23B. The oxygen concentration is calculated on the wet gas basis, and concretely, it is possible to use a zirconia-type oxygen concentration meter, for example. Note that a position of disposing the oxygen concentration meter 23B is not limited to the outlet side of the dry dust collector 24, and it is also possible to provide the oxygen concentration meter 23B in a carrier gas flow path between the indirectly heating dryer 3 and the dust collector 24, for example. A measured value of the oxygen concentration meter 23B is sent to the drying condition adjusting unit (not-illustrated).

[0047]

The drying condition adjusting unit (not-illustrated) compares the measured value as a result of measurement in the temperature measuring device 21 and a previously set setting range (or set value) of the temperature of the dried coal, and adjusts a flow rate, a temperature, and a pressure of the steam supplied to the indirectly heating dryer 3, a number of rotations of the indirectly heating dryer 3, and the like, so that the measured value falls within the setting range (the measured value becomes the set value). The number of these adjustment items is not limited to one, and the items can be appropriately combined to perform adjustment.

Further, the measured value sent from the oxygen concentration meter 23B is compared with a previously set setting range (or set value) of the oxygen concentration of the carrier gas, and an amount of enrichment of oxygen with respect to the carrier gas is adjusted, so that the measured value falls within the setting range (the measured value becomes the set value).

Note that the drying condition adjusting unit can also be divided into two units, including one for the temperature adjustment of the dried coal, and one for the adjustment of oxygen concentration of the carrier gas.

[0048] A drying rate of the dried coal (DC) is adjusted by the pressure of steam supplied to the indirectly heating dryer 3, or an amount of the low-grade coal (LRC) supplied to the indirectly heating dryer 3.

[0049]

When the spontaneous combustibility is further suppressed, it is desirable that the dried coal (DC) is supplied to the air-slide (mesh) conveyor 26 provided with the perforated plate 26A having the openings 26a and vertically dividing an inside of a main body with a hollow structure, and while conveying the dried coal by a drying exhaust gas 27 supplied to the air-slide conveyor 26 by a blower 27a, oxidation of active groups in the low-grade coal is proceeded by maintenance of temperature and by oxygen in the gas, to thereby suppress the spontaneous combustibility. Although the drying exhaust gas supplied to the air-slide conveyor 26 can be used as it is, it is also possible to use the gas preheated with steam drain after being used for the drying and the like.

[0050]

The fine powder discharged from the air-slide conveyor is returned to the position in front of the dust collector 24 via a flow path 28, and the dried dried coal (DC) is stored in a not-illustrated bunker to be supplied, as fuel, to a fluidized bed boiler, for example. Further, as another example, the dried coal taken out from the bunker is pulverized by a pulverizer, and then supplied to a combustion burner.

[0051]

Incidentally, there is known the method, as one of the methods of suppressing the spontaneous combustion of coal, in which the active groups in the coal which easily react with oxygen are previously reacted with oxygen, as described above.

In order to make the active groups in the coal which easily react with oxygen react with oxygen, the higher the temperature or higher the atmospheric oxygen concentration, the faster the reaction.

[0052]

However, when the temperature is too high, or the oxygen concentration is high, the oxidation reaction proceeds rapidly, which may cause combustion of the coal in the middle of reaction.

[0053]

According to a lot of pieces of test data obtained by the present inventors, there is a possibility of occurrence of a coal dust explosion of the low-grade coal when the oxygen concentration is 16% or more. Meanwhile, the spontaneous combustion of the low-grade coal is controlled by the coal temperature, the atmospheric oxygen concentration, and an elapsed time.

[0054]

Incidentally, when coal is subjected to drying processing by using the indirectly heating dryer, a product temperature of the coal and an atmospheric dew point substantially match, up to a point of time at which a water content reaches a critical water content, being a boundary between a constant-rate drying zone and a falling-rate drying zone of coal. Therefore, when a surface temperature of the coal at an outlet of the dryer falls within a range of 0 to +5°C with respect to a dew point of the carrier gas discharged from the dryer, it can be judged that the coal is subjected to the constant-rate drying.

[0055]

Therefore, the present invention tries to suppress the spontaneous combustion, when the coal is subjected to the drying processing up to the constant-rate drying zone by using the indirectly heating dryer, by previously and gradually oxidizing the active groups which easily react with oxygen within the ranges of the coal temperature and the atmospheric oxygen concentration in which there is no chance that the rapid oxidation reaction occurs to reach a critical region.

[0056]

Here, a relation between the oxygen concentration (on the wet gas basis, which similarly applies to the description hereinafter) when the drying is performed up to the falling-rate drying zone by using only the air as the carrier gas supplied to the indirectly heating dryer, and the dew point, is illustrated in Fig. 5. When only the air is used, the oxygen concentration and the dew point in the carrier gas are inversely proportional to each other, and accordingly, as the dew point is increased, specifically, as the product temperature of the coal is increased, the oxygen concentration in the gas is reduced.

[0057]

Specifically, the atmospheric oxygen concentration (on the wet gas basis, which similarly applies to the description hereinafter) when the product temperature of coal is desired to be set to about 90°C, for example, becomes about 7%. Under such circumstances, it is not possible to facilitate the oxidation reaction.

[0058]

In the present invention, when the low-grade coal with a high water content is dried to improve a heating value of the coal, the dew point in the drying atmosphere (which is substantially the same as the temperature of the dried coal) is controlled through adjustment with the oxygen-enriched air as the carrier gas.

[0059]

It is of course possible that when the drying temperature is raised, the oxidation reaction can be facilitated, which contributes to the suppression of combustibility. However, in the constant-rate drying zone in which the dew point of carrier gas becomes equal to the coal temperature (drying temperature), the oxygen concentration on the wet gas basis is reduced as the drying temperature is raised. Therefore, even if the drying temperature is raised, the oxygen concentration is low, and thus it is difficult to efficiently perform the suppression of combustibility.

[0060]

Therefore, the atmospheric oxygen concentration is set to 12% or less. Meanwhile, during a residence time in the dryer, there is a possibility that a sufficient effect of suppressing the spontaneous combustion is not exhibited. In such a case, the coal is dried, and then conveyed in an atmosphere with high oxygen concentration while maintaining the temperature of the dried coal in the process of conveying the dried coal, for example, conveyed by using the air-slide (mesh) conveyor as described above, to thereby make the active groups in the coal which easily react with oxygen react with oxygen.

[0061] A type of the conveying equipment is not particularly limited to the air-slide (mesh) conveyor, and it may be any one of a screw conveyor, a belt conveyor, a vibrating conveyor and the like, or a combination thereof.

[0062]

When the above description is summarized, an operable scope of the present invention is represented as in Fig. 6. It is additionally described that the present inventors determine an operable scope Z1 (scope of claim 1) and an operable scope Z2 (scope of claim 2) of the present invention based on a lot of experiments, other than experiments to be described hereinafter.

[0063]

The oxygen concentration in the drying atmosphere is 16% or less, and practically, the concentration as high as possible of 15% or less is preferable, by taking a safety factor into consideration.

[0064]

There is no particular limitation in the method of supplying gas with 15% or less of oxygen concentration to the conveying equipment, and it is also possible to supply the gas simply through the nozzle, or it is also possible to supply the gas via the perforated plate of the air-slide conveyor and the like, in order to further facilitate the contact between the coal and the gas.

[0065]

When the air-slide is used, each of openings 26a of the perforated plate 26 is directed to the dried-coal conveying direction and directed upward with a blowout angle 0 of 10 degrees to 20 degrees with respect the horizontal direction, and the blowout angle of 14 degrees to 16 degrees is particularly preferable since conveying power can be further reduced.

[0066]

As already described above, from a viewpoint of prevention of environmental pollution, installation of the dust collector which extracts, through the dry system, the dust in the carrier gas discharged from the indirectly heating dryer 3, becomes necessary. In this case, it is desirable to use, as the dust collector, a dust extractor which uses pulse gas. The pulse gas has to be N2 gas with low oxygen concentration, in order to prevent dust explosion at a time of cleaning. The required N2 gas is desirably obtained by the nitrogen-containing gas producing apparatus.

As an example of the nitrogen-containing gas producing apparatus, there can be cited a PSA (Pressure Swing Adsorption) apparatus, in which the N2 gas is produced through a method of either oxygen adsorption in the air (in this case, exhaust gas is nitrogen-enriched) or nitrogen adsorption in the air (in this case, desorbed gas is nitrogen-enriched).

[0067]

When a concrete example of a case of nitrogen adsorption is cited, it is illustrated in Fig. 11 in which a nitrogen adsorption PSA70 having an adsorbing filler 70a is provided, air is taken into the nitrogen adsorption PSA70 to conduct N2 adsorption, and the adsorbed N2 is stored in a temporary storage tank 73 as N2-enriched gas by a vacuum pump 72, and supplied to a bag filter dust collector (dust extractor) 24 provided with a pulse gas supply device. As described above, the adsorbed nitrogen-enriched gas can be used as pulse air for backwash. The pulse gas supply device 74 is schematically illustrated.

[0068]

The oxygen-enriched gas which becomes unnecessary at this time is stored in a temporary storage tank 71 from an upper part of the PSA apparatus 70, and is set to the carrier gas for the steam tube dryer (STD) 3, resulting in that the carrier gas with the oxygen concentration required for the suppression of combustibility can be obtained without separately providing an additional equipment.

[0069]

On the other hand, when a concrete example of a case of oxygen adsorption is cited, it is illustrated in Fig. 12 in which adsorbed O2 is stored in the temporary storage tank 71 as O gas by the vacuum pump 72, and is set to the carrier gas for the steam tube dryer (STD) 3.

[0070]

The nitrogen-enriched gas which becomes unnecessary at this time is stored in the temporary storage tank 73 from the upper part of the PSA apparatus 70, and when backwash of the bag filter dust collector (dust extractor) 24 provided with a filter cloth 24a by using pulse gas is conducted, the nitrogen-enriched gas can be used as the pulse gas for the backwash.

[0071]

The present invention will be described based on examples.

Examples [0072]

Low-grade coal (LRC) subjected to experiments is brown coal produced in Indonesia in which a total water content is 59.2%, and a content of water dried by air is 13.2%. As a comparative example regarding spontaneous combustibility, bituminous coal produced in Australia (referred to as BC, hereinafter) was used.

[0073]

In measurement of index for the spontaneous combustibility, a coal sample was heated to 110°C under a nitrogen atmosphere, the atmosphere was then switched to an oxygen atmosphere, and a raise in temperature was observed. Specifically, if the temperature rises in a short period of time, this means that the spontaneous combustibility is high.

[0074] (Example 1: Influence of product temperature of coal on spontaneous combustibility)

The spontaneous combustibility was examined regarding an undried product, and cases where surface temperatures of dried coal are 70°C and 92°C, respectively, in which an oxygen concentration of drying exhaust gas is about 15%, and a water content of the dried coal is about 22%.

[0075]

Numerals and symbols in the drawing denote the following. S00: Undried product

S70: Surface temperature of dried coal: 70°C

S92: Surface temperature of dried coal: 92°C BC: Bituminous coal produced in Australia [0076]

When the oxygen concentration in the drying atmosphere is 15%, the spontaneous combustibility was improved when the product temperature of the dried coal is 70°C, and further, when the product temperature of the dried coal is 92°C, the spontaneous combustibility was further suppressed, when compared to that of the bituminous coal produced in Australia.

[0077] (Example 2: Influence of atmospheric oxygen concentration)

The spontaneous combustibility was examined regarding cases where the oxygen concentration of the carrier gas discharged from the dryer (referred to as discharged carrier gas, hereinafter) was adjusted to about 5%, 10%, and 15%, respectively, under heating conditions in which the water content of the dried coal becomes about 22%, and the surface temperature of the dried coal becomes 92°C.

[0078]

Numerals and symbols in the drawing denote the following. S00: Undried product S5%: Oxygen concentration of discharged carrier gas: 5% S10%: Oxygen concentration of discharged carrier gas: 10% S15%: Oxygen concentration of discharged carrier gas: 15% BC: Bituminous coal produced in Australia [0079]

When the oxygen concentration of the discharged carrier gas is 5% in the case where the product temperature of dried coal is 92°C, no improvement of the spontaneous combustibility was confirmed. When the oxygen concentration of the discharged carrier gas is 10%, improvement of the spontaneous combustibility was confirmed, and when the oxygen concentration in the drying atmosphere is 15%, the spontaneous combustibility was further suppressed, when compared to that of the bituminous coal produced in Australia.

[0080] (Example 3: Blowout angle of nozzle of deflecting perforated plate and conveying performance) A blowout angle of a nozzle of a deflecting perforated plate with respect to a horizontal direction and conveying performance of dried coal were examined.

[0081]

The used dried coal is obtained by pulverizing brown coal produced in Indonesia to 10 mm or less (coal with a maximum particle diameter of about 30 mm is also mixed), and drying the pulverized coal until when a water content of the coal reaches 25%. To a horizontally-disposed air-slide, air preheated to 80°C was supplied to make the value of l/2pmu3 satisfy 100, and an amount of conveyance per unit width of the air-slide was measured while changing an angle 0 of a blowout nozzle. A result thereof is represented in Fig. 9. The blowout angle of nozzle is preferably 14 degrees to 16 degrees.

[0082] (Example 4: Rate of air supplied to air-slide and conveying performance)

The blowout angle 0 of air-slide of 16 degrees was employed, and an amount of conveyance per unit width of the air-slide was measured while changing an amount of supply of air per unit area to the air-slide (while changing the value of l/2pmu3).

[0083] A result of the measurement is represented in Fig. 10. The used coal is similar to that of the example 4. The conveying performance increases substantially in proportion to the value of l/2pmu3. However, when the value of l/2pmu3 is 30 or less, coal of large particle was remained on the air-slide. Therefore, the value of l/2pmu3 is preferably 30 or more.

The value of l/2pmu3 is preferably as small as possible since it is in proportion to the power of the blower for air-slide, and the value also exerts influence on exhaust gas processing equipment of the air-slide.

[0084]

However, since the maximum particle diameter after pulverization changes depending on a type of the pulverizer, the value of l/2pmu3 is appropriately selected, and it is sufficient that the value is about 200 at maximum.

Explanation of Numerals and Symbols [0085] 3 ··· indirectly heating dryer (steam tube dryer (STD)), 21 ··· temperature (product temperature) measuring device of dried coal, 23A-23B ··· oxygen concentration meter, 24 ··· dust collector, 26 ··· air-slide (mesh) conveyor, 27 ··· drying exhaust gas for conveyor, 30 ··· rotating shell, 31 ··· heating tube, 33 ··· charging apparatus, 54 ··· outlet of dryer, 61 ··· carrier gas inlet, 70 ··· PSA apparatus, LRC ··· low-grade coal, DC ··· dried coal

Claims

CLAIMS:

1. A drying method for low-grade coal, comprising drying low-grade coal using an indirectly heating dryer, wherein the indirectly heating dryer has drying conditions as follows: a surface temperature of the low-grade coal at an outlet of the dryer is higher, by 0°C to 5°C, than a dew point of a carrier gas discharged from the dryer and is 70°C to 95°C, and an oxygen concentration in the carrier gas discharged from the indirectly heating dryer ranges from greater than 10% to 15% on a wet gas basis.
2. A drying method for low-grade coal, comprising drying low-grade coal using an indirectly heating dryer, wherein the indirectly heating dryer has drying conditions as follows: a surface temperature of the low-grade coal at an outlet of the indirectly heating dryer is higher, by 0°C to 5°C, than a dew point of a carrier gas discharged from the dryer and is 80°C to 95°C, oxygen is enriched in the carrier gas, and an oxygen concentration in the carrier gas discharged from the indirectly heating dryer ranges from greater than 10% to 15% on a wet gas basis.
3. The drying method for the low-grade coal according to claim 1 or 2, wherein an air-slide conveyor is used when conveying dried coal of the low-grade coal from the outlet of the dryer, and in a process of conveying the dried coal on a deflecting perforated plate of the air-slide conveyor, a direction of blowing air through openings of the perforated plate is set to be directed to a dried-coal conveying direction and directed upward by 10 degrees to 20 degrees with respect to a horizontal direction, to make the surface of the low-grade coal and oxygen bring into contact with each other.
4. The drying method for the low-grade coal according to claim 3, wherein a carrier gas processed by a dust collector which extracts, through a dry system, dust in the carrier gas for the indirectly heating dryer, is set to a conveying medium for the air-slide conveyor.
5. The drying method for the low-grade coal according to claim 3, wherein when a gas blowout rate from the openings of the deflecting perforated plate is set to u m/s, an opening ratio is set to m, and a gas density is set to pkg/m3, a value of 1/2 pmu3 is 30 to 200.
6. A drying facility for low-grade coal, comprising: an indirectly heating dryer drying low-grade coal; a measuring device measuring a temperature of dried coal at an outlet of the dryer; a dew point measuring device measuring a dew point of a carrier gas discharged from the dryer; an oxygen concentration measuring device measuring an oxygen concentration of the carrier gas; a unit of adjusting a surface temperature of the dried coal at the outlet of the dryer by associating the surface temperature with the dew point measured by the dew point measuring device; and a unit of adjusting the oxygen concentration in the carrier gas discharged from the dryer on a wet gas basis.
7. The drying facility for the low-grade coal according to claim 6, further comprising an air-slide conveyor conveying the low-grade coal discharged from the indirectly heating dryer by setting a carrier gas processed by a dust collector which extracts, through a dry system, dust in the carrier gas discharged from the indirectly heating dryer, to a conveying medium.
8. The drying facility for the low-grade coal according to claim 6 or 7, further comprising: a dust collector extracting, through a dry system, dust in the carrier gas discharged from the indirectly heating dryer; a nitrogen-containing gas producing apparatus producing a nitrogen-containing gas supplied to the dust collector from air; and a unit of supplying an oxygen-containing exhaust gas discharged from the nitrogen-containing gas producing apparatus, as the carrier gas for the indirectly heating dryer.