It is known that reducing the moisture of a coking coal before introducing it into a coke oven can serve to improve the quality of the product coke and the productivity, the life and the heat efficiency of the coke oven.
It is also known that drying a thermal coal before inserting it into a boiler improves the heat efficiency and enables the size of the exhaust gas treating system to be reduced.
This invention provides an efficient and economical coal-moisture control method by applying a tube dryer to high rank coal.
Tube dryers are constituted in such a manner that a plurality of tubes are axially disposed in an inclined rotary cylinder. In one known type of tube dryer, known as a "coal-in-tube" type tube dryer, coal to be dried is passed through the tube, while a heating medium such as steam is passed along the outer surface of the tube so that the coal to be dried is indirectly heated. In another known type of tube dryer, known as a "steam-in-tube" type tube dryer a heating medium such as steam is passed through a tube, while coal to be dried is passed along the outer surface of the tube so that it is indirectly heated.
Hitherto, the coal-in-tube type of tube dryer has been used only for drying high water content brown coal (water content: 60 to 70 wt%) to a level of 20 to 30 wt%.
One such conventional tube dryer is known, in which a stirring means formed by a flat bar (width 20 to 30 mm) and designed in a spiral form is inserted into each tube. The stirring means increase the probability that the coal is brought into contact with the inner surface (heat transfer surface) of the tube.
DE-C-611988 discloses a Schultz dryer, which is a coal-in-tube type tube dryer, for use in the drying of material, for example, of coal for briquetting.
As another coal-in-tube type, a structure disclosed in U.S. Patent No. 3,765,102 is known. This structure is, as shown in Figs. 9 and 10, constituted in such a manner that a stirring means formed by flat bars 2 in the form of longitudinal fins attached to the inside of a tube 1 is provided for the purpose of enlarging the contact area with the heat transfer surface by picking or carrying up the granule layer. Reference numeral 3 represents a tube sheet, reference numeral 4 represents a support ring, and reference numeral 5 represents a dam element.
Since such long and complicated stirring means needs to be disposed along the tube, the weight is enlarged, and the changing, cleaning and maintenance work becomes complicated.
The system, which is disclosed in the above described U.S. Patent, wherein a dam element is disposed at the outlet portion of the tube for the purpose of controlling the coal retention time, is disadvantageous in that the particles become non-uniform, a part of the coal is excessively dried, and complete discharge is difficult to achieve. As a result of this, fine particles tend to be stacked, causing corrosion to occur.
Coal-in-tube type of tube dryers have been developed for drying brown coal and have not been used for high rank coal such as coking coal.
High rank coal such as coking coal contains only 10 to 20 wt% water even if the same is stocked in outdoor pile, and is thus lower in moisture than low rank coal such as brown coal. Since this high rank coal, when dried, becomes dustier as the moisture level is reduced, the water content of the coal is preferably dried to 4 to 7 wt% at the outlet portion of the dryer.
Further, coking coal generally has lower flowability than brown coal.
Therefore, in comparison to the brown coal case, the quantity of water to be evaporated can be limited to only 1/2 to 1/3 of that in the brown coal case, if the same amount of coal is used. Therefore, a necessity arises that the quantity to be treated should be increased by up to two to three times. However, the conventional type of tube dryer can not treat a sufficient quantity of coal due to the flow resistance caused by the too small diameter of the tube thereof and the too complicated shape of the stirring means mounted in the space within the tubes.
As an example of the steam-in-tube type dryer, a coal dryer disclosed in Japanese Patent Unexamined Publication No. 63-3089 is known, wherein austenitic acid-resisting stainless steel is used in the inlet portion thereof, an austenitic-ferric stainless steel is used in the intermediate portion thereof, and carbon steel is used in the outlet portion thereof.
Since, in a steam-in-tube type of tube dryer, the coking coal charge ratio is usually 10 to 20% and the heat transferring area cannot be efficiently used, the capacity of such apparatus needs to be enlarged. Furthermore, the route through which the coal passes is too complicated, as a result of which, adhesion and accumulation of fine particulate material generated will cause corrosion.
According to the above-mentioned Japanese Patent Unexamined Publication No. 63-3089, it is known that, when the temperature of the wall of the tube which is brought into contact with the coal layer (the inner temperature) is lowered to a low temperature (120°C or lower, sometimes it is 130°C or lower), components (SO₄²⁻ or Cl⁻) derived from coal cause corrosion. Furthermore, according to the above-mentioned Japanese Patent, a high grade material such as austenitic-ferric stainless steel is used so as to prevent such corrosion. Although such material described above can endure such corrosion and its life can be prolonged, it is not economical since the cost of materials becomes too high (such materials cost substantially 20 times as much as carbon steel).
Generally in the steam-in-tube type dryer, steam condensate which has been heat-exchanged with the coal is present in the tube thereof. Therefore, the temperature of the wall of the tube portion which is brought into contact with condensed water having a relatively high resistance to heat transfer is lowered, causing corrosion.
Since the structure of the steam-in-tube type tube tends to retain condensed water on the heat transferring surface, the tube needs to be made of a high grade material or the steam pressure needs to be raised, so as to keep the temperature of the shell high, both of these being uneconomical.
Many coal-in-tube type dryers have been used as dryers for high water-content brown coal, but there have not been reports of such corrosion at a low temperature region because brown coal has a different composition relative to that of the coal (mainly coking coal) to be treated according to the present invention.
The invention provides a coal-moisture control process or a method for effectively controlling the moisture in coal of the type having a relatively high coal rank, such as coking coal, in which method a tube dryer is used, such tube dryer being so constituted that the coal or the like to be dried is passed through the tube inside, while any heating medium is passed outside this tube.
A phenomenon, according to which coal is raised in the direction of rotation, the thus-raised angle exceeds the angle of repose and the granule layer of coal falls, is called a kiln action.
The inventors have found through experiments that the kiln action can be stably generated by arranging the average charge ratio in the tube to be 15% or more, preferably, to be 30 to 50%. As a result, a sufficient retention time in the tube can be secured and the heat transfer quantity can be improved.
The inventors have also achieved the present invention based on the finding that corrosion at the time of treatment of coking coal can be prevented and sufficient corrosion resistance can be obtained in the case of a low grade material such as carbon steel SGP by arranging the steam temperature in the tube dryer to be 130°C or higher, and the tube wall temperature (inner surface temperature) to be 120°C or higher, preferably 130°C or higher.
Furthermore, it has been found that, if the rotational speed of the dryer is raised, the coal flow velocity in the tube can be increased due to the characteristics of the tube dryer, and even if the coal flow velocity is increased, a desired drying capability can be maintained by making the retention time longer by way of lengthening the tube length or by raising the steam pressure (or the temperature).
The present invention has been achieved in view of the foregoing points. An object of the present invention is to provide a coal-moisture control process whereby a treatment quantity can be increased, with a necessary drying capability being maintained, by using a tube dryer of a coal-in-tube type, which type has been used previously only for drying coal having a high water content, such as brown coal, the tubes preferably being constituted in such a manner that they have a diameter twice to three times of that of the conventional tubes, and, preferably, the stirring means in this tube being omitted or the configuration thereof being simplified.
Another object of the present invention is to provide a coal-moisture control process in which the generation of corrosion can be prevented even in the case of general carbon steel by means of controlling (increasing) the number of revolutions of a tube dryer, maintaining the steam temperature at a high temperature exceeding 130°C with the coal drying capability maintained, and securing the tube wall temperature above 120°C, preferably 130°C or higher, that is, above the corrosion temperature region.
The present invention provides a coal-moisture control process comprising introducing a mono species or a mixture of several species of coal containing 45 wt% or less on a dry ash free basis of volatile matter, having 75 wt% or more of particles of a size of 3 mm or less, and having a water content of 20 wt% or less on a wet ash free basis into a plurality of tubes disposed in and along an axial direction of an inclined rotational cylinder of a tube dryer, the tubes each having an inner diameter of 150 mm or more, so controlling the ratio of a coal layer with respect to a cross-sectional area of said tubes that it is from 15 to 40% on average, and passing a heating medium, for example, steam along the outer surfaces of said tubes for the purpose of indirectly heating said coal so as to dry it so that the water content on the wet ash free basis is made 4 to 7 wt%.
The invention further provides a coal-moisture control process comprising introducing a mono species or a mixture of several species of coal containing 45 wt% or less on a dry ash free basis of volatile matter, having 75 wt% or more of particles of a size of 3 mm or less, and having a water content of 20 wt% or less on a wet ash free basis into a plurality of tubes disposed in and along an axial direction of an inclined rotational cylinder of a tube dryer, the tubes each having an inner diameter of 150 mm or more, passing a heating medium, for example, steam along the outer surfaces of said tubes for the purpose of indirectly heating said coal so as to dry it, the conditions being such that the temperature of a tube shell is 120°C or higher, and so controlling the rotational speed of said tube dryer that the coal is dried to a water content of 4 to 7 wt%.
Moreover, the invention provides a coal-moisture control process comprising introducing a mono species or a mixture of several species of coal containing 45 wt% or less on a dry ash free basis of volatile matter, having 75 wt% or more of particles of a size of 3 mm or less, and having a water content of 20 wt% or less on a wet ash free basis into a plurality of tubes disposed in and along an axial direction of an inclined rotational cylinder of a tube dryer, the tubes each having an inner diameter of 150 mm or more, so controlling the ratio of a coal layer with respect to a cross-sectional area of said tubes that said ratio is 50% or more at an inlet portion of said tubes, and passing a heating medium, for example, steam along the outer surfaces of said tubes for the purpose of indirectly heating said coal so as to dry it so that the water content on the wet ash free basis is made 4 to 7 wt%.
If the ratio of the coal layer with respect to the cross-sectional area of the tube is below 15%, the kiln action cannot be generated. Therefore, the area contacting with the inner surface (heat transfer surface) of the tube is too small and the coal layer cannot be stirred sufficiently, causing the heat transfer efficiency to be limited to low, while the flow velocity in the tube becomes too large with respect to that in the case where the kiln action is generated. That is disadvantageous because the length of tube needed for the coal to be dried sufficiently may become excessive.
In general, water content in coal is high at the inlet portion of the tube, causing the flow velocity to remain small and the charge ratio to be high, while at the outlet portion, the water content in the coal becomes lowered, causing the flow velocity to become too large, and the charge ratio to be low. In order to assuredly generate the kiln action at the outlet portion, the charge ratio or proportion of the coal layer at the inlet portion of the tube should be 50% or more in the case of coals having the water contents specified above, although depending upon the water content in the coal.
If the charge ratio exceeds 50%, in average the flow velocity of coal is reduced, causing the flow to become unstable. As a result of this, the tube may be clogged or a non-uniform distribution of the charge ratio occurs. Therefore, the charge ratio is preferably limited so that it does not exceed 50% even locally, except at the inlet portion, and the average charge ratio is preferably limited to 40% or less.
Moreover, it is preferred that the ratio of the quantity of the coal with respect to the cross-sectional area of the tube is controlled, by reducing the rotational speed of the tube dryer near its lowermost level, by monitoring the level of fine particles of said coal in an inlet hood of the tube dryer so as to prevent a rise in the fine particles level as well as by monitoring the quantity of said coal discharged from the tube dryer so as to prevent reduction in said quantity of the coal discharged from said tube dryer, and that the quantity of supply of the coal to be dried to the tube dryer is controlled at a substantially constant level.
In one type of coal-moisture control process according to the present invention a mono species or a mixture of several species of coal, to be dried, containing 45 wt% or less on a dry ash free basis of volatile matter, 75% or more of particles of a size of 3 mm or less, and having a water content of 20 wt% or less on a wet ash free basis is introduced into a plurality of tubes disposed along the axial direction of an inclined rotational cylinder of a tube dryer having stirring means inserted into the tubes, a heating medium such as steam is passed along the outer surface of the tubes for the purpose of indirectly heating the coal so as to dry it so that the water content on the wet ash free basis is made 4 to 7 wt%.
The stirring means may preferably be formed by a spiral wire material (shown in Figs. 1 to 3), by a structure (shown in Figs. 4 and 5) in which a plurality of annular dams in the transverse direction of the tube are disposed at regular intervals, or by a structure (shown in Figs. 6 and 7) in which a plurality of flat-bars are disposed in axial direction of the tube in such a manner that they can be in contact with the inner surface of the tube.
In the coal-moisture control process according to the present invention the rotation speed of the tube dryer is preferably controlled so as to make the temperature of the heating medium such as steam in the tube dryer exceed 130°C.
Furthermore, in the coal-moisture control process according to the present invention the tube shell temperature may be made 120°C or higher instead of making the temperature of the heating medium such as steam in the tube dryer 130°C or higher.
In order to prevent or suppress corrosion of the inner surface of the tube, the temperature of the tube shell (the temperature of the inner surface) may be raised to 120°C or higher. However, there is no industrially convenient method to measure the temperature of the tube wall in the rotating dryer, and if any, it has a poor reliability. Since the pressure of steam is uniform through the inside of the dryer of a coal-in-tube type tube dryer and the temperature drop inside the tube shell is limited to a very small level, the temperature of steam at the inlet portion of the dryer at which measurement can be performed rather easily may be measured for the control as an alternative to measuring the tube shell temperature. Since the generated condensed water is immediately separated from the tube shell and is discharged from the dryer, the temperature of the same is substantially the same as that of the steam in the dryer. Therefore the temperature of the condensate at the outlet portion of the dryer may be measured so as to use it to perform control.
When a dryer which is designed with a rated drying capability is used to treat coal of a type having a relatively lower water content than a water content at the inlet portion for which the dryer was designed, corrosion of the tubes can occur if the temperature of the steam is lowered for the purpose of preventing excessive drying.
Therefore, with the temperature of the steam kept at 130°C or higher, at which temperatures corrosion cannot occur, the revolution speed of the dryer may be controlled so as to bring the water content in the coal at the outlet portion to a predetermined level.
If the coal water content is kept low, flowability is improved and the coal charge ratio in the tube is lowered. However, a sufficient drying capability can be secured by providing stirring means which can further stably generate the kiln action.
The content of the volatile matter of the coking coal on a dry ash free basis may be 45 wt% or less but is preferably to be 30 wt% or less. The water content of the same on a wet ash free basis may be 20 wt% or less but is preferred to be 8 to 12 wt% in average.
Furthermore, the inclination of the inclined rotational cylinder is preferred to be 5 to 15°. If it is below 5°, the coal flow velocity in the tube is too small, causing the quantity treated to become also too small, while the retention time becomes longer. As a result of this, the coal at the outlet portion becomes over-dry. If an inclined rotational cylinder of an inclination of 5° or less is used for processing the coal of the above described water content range, the dryer needs to be a cylinder having a large diameter but having a short overall length. Such dryer causes manufacturing and transportation problems. Raising the rotational speed for the purpose of increasing the quantity of the coal to be treated will cause a problem to be described later. On the other hand, if it exceeds 15°, the size of the supporting device such as a stopper for supporting a thruster or a shaft bearing portion in the axial direction of the dryer becomes too large, causing manufacturing and economical problems.
It is preferable for the rotational speed of the inclined rotational cylinder to be 5 to 25 rpm, the diameter to be 600 to 6000 mm, and the number of the tubes to be six or more. If the rotational speed is less than 5 rpm, similarly to the case where the inclination is 5° or less, the quantity treated by a tube becomes too small, causing the problem that the shape of the dryer becomes a cylinder of large diameter and short length. On the other hand, if it exceeds 25 rpm, the life of the rotational portions such as the bearing or the like can be shortened and the power needed becomes too large. In addition, the tube disposed away from the rotation shaft is much affected by centrifugal force, which has the result that generation of the kiln action is prevented.
If the inner diameter of the tube is less than 150 mm, the quantity treated by one tube may be excessively low, and the retention time becomes long, causing the drying capability to become excessively high. Therefore, such dryer causes much more serious manufacturing and transportation problems relative to the case where the inclination is 5° or less.
If the diameter of the cylinder is less than 600 mm, it is difficult to dispose in it a plurality of tubes having an inner diameter of 150 mm. If it exceeds 6000 mm manufacturing and transportation problems occur, and the excessive number of the tubes to be disposed therein makes it difficult for coal to be uniformly supplied into the tubes.
If the number of the tubes is less than five, a proportion or ratio of the total cross-sectional areas of the tubes with respect to the cross-sectional area of the dryer, that is, the ratio of the coal passage with respect to the diameter of the dryer, is lowered, causing an economical problem.
Preferred embodiments of the invention will be described in detail below with reference to the drawings, of which:
Unless otherwise specified, the material, shape and the relative positions of the component devices described with reference to the drawings are not intended to limit the scope of the present invention, but are employed as an example.
Fig. 1 shows an example of a dryer in which the method according to an embodiment of the present invention is carried out. Reference numeral 10 represents an inclined rotational cylinder which is designed to be able to be rotated around the axis thereof by a driving means (omitted from the illustration). This inclined rotational cylinder 10 is disposed in such a manner that it is downward inclined by approximately 10° when viewed from the coal supplying side.
The material to be dried such as coal is supplied to an inlet hood 11, and is dried by a heating medium flowing outside a plurality of tubes 1, at which the same is heat-exchanged through the walls of the tubes 1 while the coal passes through the tubes 1. Lastly, dried coal is discharged from the outlet end of the tubes 1. The tubes 1 are disposed between tube plates 3 at both ends of the inclined rotational cylinder 10. The heating medium, such as steam under pressure of several atmospheres, is supplied into the cylinder 10 via a rotational coupling 12, and flows to the bottom and downstream portion of the cylinder 10 after it has heated the outside of the tube 1 and has been condensed. Then, it passes through a drain pipe 13 in accordance with rotation of the cylinder 10, and is discharged through a rotational coupling 14.
In the method according to the present invention, since coal having a water content lower than that of brown coal is dried, the drying efficiency can be improved by employing the following means. First, the diameter of the tube 1 is preferably designed to be 150 to 300 mm for the purpose of increasing the quantity treated although the diameter is substantially 100 mm in the case of the tube for brown coal. In accordance with the quantity treated, the rotational speed of the tube dryer is adjusted and the coal charge ratio in the tube may be arranged to be 15 to 40% for the purpose of stabilizing generation of kiln action. If the quantity treated is relatively small, the heat transfer area becomes relatively large. Therefore, the water content in the coal at the outlet portion is controlled by lowering the quantity (pressure) of the heating medium to be supplied.
As shown in Figs. 2 and 3, by inserting a stirring means 16 constituted by spiral steel bars 15 having circular cross section into the tube, the kiln action can be generated at the layer of the coal to be dried, causing the heat conducting efficiency to be improved. Furthermore, the flow velocity of coal along the axial direction of the tube 1 is restricted so as to secure the retention time for the purpose of further improving treatment capability. This stirring means 16 is so constituted that it is prevented from separation from the tube by means of a stopper 17 disposed at the outlet end portion of the tube 1, but it can be freely rotated within tube 1. Reference numeral 18 represents a passage through which the heating medium passes, and reference numeral 20 represents a hopper for separating the dried coal from the exhaust gas (composed or consisting of evaporated water and air) and for taking out the same.
As the coal to be dried moves forward through the tube 1, water content thereof is reduced, causing generally the flow velocity to be raised. Therefore, the charge ratio of the coal to be dried in the tube becomes lower and this leads to the fact that the kiln action becomes less likely to be generated. By using the stirring means 16 according to the present invention, however, the kiln action can be maintained even in the case where the coal has low water content and the charge ratio is low. Therefore, a sufficient coal-moisture controlling capability and drying capacity can be maintained. It is sufficient that the length of the stirring means 16 along the tube 1 is, depending upon the physical properties of the coal to be dried, inserted in the downstream half portion of the tube in the case of a coking coal. The diameter of the spiral needs to be half or more of the inner diameter of the tube 1. The pitch for the spiral is preferred to be 1/4 times to twice of the diameter of the tube 1. Furthermore, the stirring means 16 may not be rotated within the tube, and the same may be fixed to the stopper 17 or the like.
As an alternative to the stirring means 16 shown in Figs. 1 to 3, several annular or ring-like plates 21 spaced from each other at a several hundred milli-meters pitch by spacers 22 made of steel bars may be inserted in the tube 1. The size of the annular plate 21 is preferably smaller that the diameter of the tube 1 and it is able to rotate for the purpose of preventing adhesion and accumulation of the coal to be dried. It is insufficient to have only one sheet of annular plate 21 at the downstream end for the stable generation of the kiln action. Therefore, two or more plates 21 are desired. The remaining structure and the operation are the same as the case shown in Figs. 1 to 3.
In another example, as an alternative to the stirring means 16 shown in Figs. 1 to 3, several flat bars 23 are laid in the axial direction, with the same positioned in close contact with the inner surface of the tube 1, and are secured by a steel bar 24 having a circular cross section to form a stirring means, as shown in Figs. 6 and 7, the thus-secured stirring means being inserted into the tube 1. The stirring means in Figs. 6 and 7 is so constituted that, in contrast to the conventional example shown in Figs. 9 and 10 in which the material to be dried is picked up by itself, the base portion of the fine particle layer is supported for the purpose of assisting generation of the kiln action. Therefore, the width of the flat bar 23 can be limited small and the number of the same may be, for example, two or three. Furthermore, only a few of the steel bars 24 may be provided. Therefore, the flow rate is scarcely reduced. The remaining structure and the operation are the same as those shown in Figs. 1 to 3.
In a method according to a preferred embodiment of the present invention, corrosion of a tube material can be prevented by the following operation: first, as shown in Fig. 8, the pressure of the steam to be supplied is, in proportion to the water content in the coal at the outlet portion, controlled by a control valve 25 or the like. However, when the temperature of the steam becomes a predetermined temperature (for example 130°C) above a temperature below which corrosion can occur, the control valve 25 is controlled to keep the steam at the predetermined temperature, and simultaneously the rotational speed of the dryer is controlled by using an inverter 27 for a driving motor 26 for the purpose of bringing the water content in the coal at the outlet portion to a predetermined level. Reference numeral 28 represents a water content detector, reference numeral 29 represents a temperature detector, reference numeral 30 represents a rotational-speed adjuster, reference numeral 31 represents a rotation drive shaft, and reference numeral 32 represents a bearing.
As described above, according to the present invention, a coal-in-tube type of tube dryer, which has been hitherto used only for brown coal, is used for the purpose of performing coal-moisture control of coal having a relatively high coal rank such as coking coal. The coal-moisture control can be more efficiently and economically performed than in the case of the other type of tube dryers and other types of drying method.
In the method according to the present invention, tube corrosion can be prevented and tubes can be made from low-cost materials, so that the coal-moisture control process can be performed efficiently and economically, by way of controlling the temperature of the heating medium such as steam in the tube dryer above or no less than 130°C, or making the temperature at the inner surface of the tube above or no less than 120°C.