DESC:FIELD OF THE DISCLOSURE
The present disclosure relates to a coal drying system.

BACKGROUND
Coal obtained after mining and processing operations has high moisture content, which affects its quality and calorific value. With the coal prices rising, there is an immediate need to maximize the coal efficiency for use in generating electricity or in other process operations as fuel. In order to maximize the efficiency, coal is dried prior to burning. This increases the calorific value of the coal, thereby resulting in increased efficiency.

Various systems and methods are known for drying coal before use. The conventional coal drying systems used at a power plant facilitate drying of coal by using hot air, flue gases from the power plant, steam from the power plant or energy from microwaves. These coal drying systems are however plagued with several drawbacks. The system using hot air requires ambient air to be heated by means of flue gases or excess heat generated in the power plant, thereby requiring a heat exchanger as well as large volume of ambient air supply, for removing the moisture from coal. Furthermore, the hot air used for removing the moisture carries fines of coal and therefore requires treatment before being released into the atmosphere. The efficiency of this process depends on air humidity. The hot air must essentially be dry to minimize the air volume required. Humid areas require more volume of air to remove the moisture as compared to dry areas and thus the system becomes site specific. This can increase the operational cost of the power plants. US Patent No. 4493157 discloses one such method for drying coal by passing ambient air, in which the air is passed countercurrent to the coal to effect an exothermic oxidation that produces heated air and dried coal.
Further, the coal drying systems using flue gases as the drying medium, require nearly 90% of flue gases generated at the power plant. Thus, the operational cost for diverting the flue gases through the coal drying system prior to disposal through the stack is high. Also, flue gases from power plants are generally at a temperature in the range of 140 to 150 ºC. The flue gases lose heat while drying the coal. The temperature of the flue gases may fall below the acid dew point temperature of sulfur i.e., 120 ºC, in which scenario all the ducts, pipes and equipments subsequent to the coal drying system must be protected by an acid-proof lining to avoid corrosion. This increases the capital cost of the power plant. This is also the reason why supplying nearly all of the flue gases from the power plant to the coal drying system becomes unviable.

Another conventional method for drying coal is disclosed in US Patent No. 7666235 in which coal is conveyed through a microwave-energized heating chamber to achieve a controlled aggregate moisture content target range. The microwave heating chamber boils the water without heating the coal itself above about 90° C. The method using microwave energy for drying coal involves high electric power consumption, making it costly and impractical for large-scale applications.

Hence, there is felt a need to alleviate the afore-mentioned drawbacks associated with the conventional coal drying systems and provide a coal drying system which is efficient, cost-effective and reduces energy consumption.

OBJECTS
Some of the objects of the coal drying system of the present disclosure are aimed to ameliorate one or more problems of the prior art or to at least provide a useful alternative and are listed herein below.
An object of the present disclosure is to provide a coal drying system which is efficient, safe-to-use and cost-effective.

Another object of the present disclosure is to provide a coal drying system which reduces the volume of drying medium required and effectively reduce wastage of heat energy by recovering latent heat from the evaporated moisture, thereby reducing the operating costs of the system.

An additional object of the present disclosure is to provide a coal drying system which minimizes power penalty on thermal power plants.

Other objects and advantages of the present disclosure will be more apparent from the following description when read in conjunction with the accompanying figures, which are not intended to limit the scope of the present disclosure.

SUMMARY
In accordance with the present disclosure, there is provided a system for drying coal, said system comprising:
a jacketed conveyor including conveying means, said conveying means being at least partially encased by jacketing means,
wherein, said conveying means are adapted for receiving a coal feedstock and a flow of moisture sweeping medium such that said coal feedstock and said moisture sweeping medium traverse in a controlled countercurrent flow pattern, and said jacketing means are adapted for passing there through primary drying medium, such that said primary drying medium evaporates moisture from said coal feedstock. The moisture is carried away by said moisture sweeping medium to provide dried coal.

The moisture sweeping medium can be hot flue gases and said primary drying medium can be low pressure steam.

The conveying means can be a screw conveyor.

In accordance with the present disclosure, said jacketed conveyor comprises a hopper for supplying said coal feedstock.

A heat exchanger is provided in operative communication with said conveying means for receiving moisture-laden partially cooled flue gases, said heat exchanger being adapted to condense said moisture-laden partially cooled flue gases to remove the moisture and obtain cooled dry flue gases.

The temperature of said moisture sweeping medium is in the range of 130 – 170 °C and the temperature of said primary drying medium is in the range of 120 – 160 °C.

In accordance with the present disclosure, there is provided a power plant comprising a system for drying coal, said system comprising:
a jacketed conveyor including conveying means, said conveying means being at least partially encased by jacketing means,
wherein, said conveying means are adapted for receiving a coal feedstock and a flow of moisture sweeping medium such that said coal feedstock and said moisture sweeping medium traverse in a controlled countercurrent flow pattern, and said jacketing means are adapted for passing there through a primary drying medium, such that said moisture sweeping medium sweeps away evaporated moisture from said coal feedstock due to heat from primary drying medium. The moisture is carried away by said moisture sweeping medium to provide dried coal.

In accordance with the present disclosure, there is provided a method for drying coal, said method comprising the following steps:
passing coal feedstock through a conveying means;
receiving a flow of a moisture sweeping medium in said conveying means such that said coal feedstock and said moisture sweeping medium move countercurrent to each other;
passing a primary drying medium through a jacketing means, said jacketing means being adapted to at least partially encase said conveying means;
evaporating moisture in said coal feedstock by means of said primary drying medium; and
sweeping said moisture in said moisture sweeping medium while flowing countercurrent to said coal feedstock to provide dried coal.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
The coal drying system of the present disclosure will now be described with the help of the accompanying drawing, in which:

FIGURE 1 illustrates a schematic of a preferred embodiment of the coal drying system in accordance with the present disclosure.

DETAILED DESCRIPTION
The system of the present disclosure will now be described with reference to the embodiment shown in the accompanying drawing. The embodiment does not limit the scope and ambit of the disclosure. The description relates purely to the example and preferred embodiment of the disclosed method and its suggested application.

The coal drying system and the various features and advantageous details thereof are explained with reference to the non-limiting embodiment in the following description. Descriptions of well-known parameters and processing techniques are omitted so as to not unnecessarily obscure the embodiment herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiment herein may be practiced and to further enable those of skill in the art to practice the embodiment herein. Accordingly, the examples should not be construed as limiting the scope of the embodiment herein.

The present disclosure relates to a system for drying coal and method thereof. The system of the present disclosure comprises a jacketed conveyor including a conveying means. The conveying means are adapted to carry a coal feedstock having high moisture content. A flow of a moisture sweeping medium is passed over the coal feedstock in the conveying means, countercurrent to the coal feedstock. The conveying means are at least partly encased by a jacketing means. The jacketing means comprises a passage adapted to receive a flow of a primary drying medium. The primary drying medium indirectly heats the coal feedstock to evaporate the moisture thereof. The flow rate of the moisture sweeping medium in the conveying means is modulated such that it carries away the moisture to provide dried coal.

Referring to FIGURE 1 of the accompanying drawing, therein is illustrated a preferred embodiment of the system of the present disclosure; the system being generally referenced by the numeral 100. The system 100 can be used with a power plant using coal to generate electric energy. The system 100 comprises a jacketed conveyor 103. The jacketed conveyor 103 includes a conveying means 104. In one embodiment, the conveying means 104 is a screw conveyor. The coal feedstock having high moisture content is received via a hopper 102 and is conveyed over the screw conveyor 104. A flow of hot flue gases is passed over the coal feedstock in countercurrent direction to the movement of the coal feedstock. The hot flue gases are received in the screw conveyor 104 at an inlet port 106 via a blower 114. The blower 114 receives flue gases from the power plant’s post ESP (electrostatic precipitator) and introduces the flue gases to the screw conveyor 104 through the inlet port 106. The flue gases are at a temperature in the range of 130 – 170 °C.

The screw conveyor 104 is at least partially enclosed by jacketing means 128. The jacketing means 128 cover the outer cylinder of the screw conveyor 104 and is adapted to allow heat transfer with the coal feedstock conveyed on the screw conveyor 104. The jacketing means 128 comprise a fluid inlet 126 for receiving a flow of low pressure steam from the LP turbine of the power plant. The low pressure steam has a temperature in the range of 120 – 160 °C. The low pressure steam provides enough heat to dewater the coal. This steam induces minimum penalty to the power plant due to its usage in the drying plant rather than in the power plant.

Due to injection of low pressure steam in the jacketing means 128, the coal feedstock is indirectly heated by the steam. Due to heat transfer, the moisture in the coal feedstock vaporizes into steam, thereby dewatering the coal. The moisture thus vaporized, is swept from the screw conveyor 104 by the incoming hot flue gases injected by the blower 114 via the inlet port 106. The flue gases absorb the moisture to become moisture-laden partially cooled flue gases. The low pressure steam from the jacketing means 128 loses the heat to become heat-deficient low pressure steam.

The screw conveyor 104 comprises a coal outlet 108 for removing the dried coal and an outlet port 112 for discharging the moisture-laden partially cooled flue gases. A heat exchanger 116 is provided in operative communication with the screw conveyor 104 for receiving the moisture-laden partially cooled flue gases. The latent heat from the evaporated moisture can be recovered and transferred to a heat transfer medium in the heat exchanger 116. The coal particulates carried by the moisture-laden partially cooled flue gases may be removed by passing the stream through a cyclone separator (not shown in fig.) and/or a filter (not shown in fig.) prior to receiving at the heat exchanger 116.

The filtered moisture-laden partially cooled flue gases can be used for pre-heating boiler feed water received from the power plant cycle 124; the moisture in the gases condensing in the process to produce cooled dry flue gases which are discharged at a flue gas outlet going back to the stack in the power plant 122. The condensed moisture is discharged at 120 and can be used in the power plant. The pre-heated water is received at an appropriate point in the power cycle 118 of the power plant. The cooled dry flue gases can be released through a stack of the power plant (not shown in fig.). In the said process, the flue gas requirement is minimal and thus the operational costs are very low.

The heat-deficient low pressure steam is discharged through a fluid outlet 110 in the form of condensate. A condenser 130 can be provided in operative communication with the jacketing means 128 to receive the heat-deficient low pressure steam from the fluid outlet 110. In the condenser 130 the heat-deficient low pressure steam is further condensed to produce a condensate stream. This condensate stream may be recycled in the power plant cycle as boiler feed water. The condensed moisture from the heat exchanger 116 may be used in power plant operations.

Preferably, according to the present disclosure, the flue gases and the low pressure steam available at the power plant are at a temperature of about 130 to 170 and 120 to 160o C, respectively. At this high temperature, the moisture carrying capacity of the flue gases is high, thereby requiring reduced volume of flue gases for the drying operation. Due to reduced requirement of the flue gases, the blower costs and piping costs are reduced, therefore minimizing the operational costs of the power plant.

EXAMPLES
Trials were conducted using the system 100 of the present disclosure. Table 1 lists the steam consumption.
Table 1: Steam (primary drying medium) Consumption
Parameter Trial # 1 Trial # 2 Trial # 3
Steam Consumption
(low pressure steam conditions) 1.0 to 1.4 kg steam / kg moisture evaporated 1.0 to 1.4 kg steam / kg moisture evaporated 1.0 to 1.4 kg steam / kg moisture evaporated

The process parameters, the energy consumption and the moisture content of the dried coal were monitored during the trials. The results are listed in the Table 2.

Table 2: Trial results
Parameter Trial # 1 Trial # 2 Trial # 3
Flue Gases Consumption 1.5 to 4 % of the total volume of flue gas in the power plant 1.5 to 4 % of the total volume of flue gas in the power plant 1.5 to 4 % of the total volume of flue gas in the power plant
Moisture content of coal feedstock 20 to 35% 20 to 35% 20 to 35%
Moisture content of dried coal 10 to 20% 10 to 20% 10 to 20%
Penalty due to usage of steam 1 to 5 % 1 to 5 % 1 to 5 %
Auxiliary electricity consumption 1 to 5 % 1 to 5 % 1 to 5 %

The results showed that the coal moisture content can be reduced to about 10% with minimal flue gases, steam and electricity consumption from the power plant.

TECHNICAL ADVANCEMENT
The coal drying system, as described in the present disclosure, has several technical advantages including, but not limited to, the realization of:
- the system is efficient, safe-to-use and cost-effective;
- the system reduces the volume of drying medium required and effectively reduces wastage of heat energy by recovering latent heat from the evaporated moisture, thereby reducing the operating costs of the system; and
- the system minimizes power penalty on a thermal power plant.

Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the invention to achieve one or more of the desired objects or results.

Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the invention as it existed anywhere before the priority date of this application.

The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the invention, unless there is a statement in the specification specific to the contrary.

The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein. ,CLAIMS:1. A system (100) for drying coal, said system comprising:
a jacketed conveyor (103) including conveying means (104), said conveying means (104) being at least partially encased by jacketing means (128),
wherein, said conveying means (104) are adapted for receiving a coal feedstock and a flow of moisture sweeping medium such that said coal feedstock and said moisture sweeping medium traverse in a controlled countercurrent flow pattern, and said jacketing means (128) are adapted for passing there through a primary drying medium, such that said primary drying medium evaporates moisture from said coal feedstock, wherein said moisture is carried away by said moisture sweeping medium to provide dried coal.

2. The system as claimed in claim 1, wherein said moisture sweeping medium is hot flue gases.

3. The system as claimed in claim 1, wherein said primary drying medium is low pressure steam.

4. The system as claimed in claim 1, wherein said conveying means (104) is a screw conveyor.

5. The system as claimed in claim 1, wherein said jacketed conveyor (103) comprises a hopper (102) for supplying said coal feedstock.

6. The system as claimed in claim 1, wherein a heat exchanger (116) is provided in operative communication with said conveying means (104) for receiving moisture-laden partially cooled flue gases, said heat exchanger being adapted to condense said moisture-laden partially cooled flue gases to remove the moisture and obtain cooled dry flue gases.

7. The system as claimed in claim 1, wherein the temperature of said moisture sweeping medium is in the range of 130 – 170 °C.

8. The system as claimed in claim 1, wherein the temperature of said primary drying medium is in the range of 120 – 160 °C.

9. A power plant comprising a system for drying coal, said system comprising:
a jacketed conveyor (103) including conveying means (104), said conveying means (104) being at least partially encased by jacketing means (128),
wherein, said conveying means (104) are adapted for receiving a coal feedstock and a flow of moisture sweeping medium such that said coal feedstock and said moisture sweeping medium traverse in a controlled countercurrent flow pattern, and said jacketing means (128) are adapted for passing there through a primary drying medium, such that said primary drying medium evaporates moisture from said coal feedstock, wherein said moisture is carried away by said moisture sweeping medium to provide dried coal.

10. A method for drying coal, said method comprising the following steps:
passing coal feedstock through a conveying means;
receiving a flow of a moisture sweeping medium in said conveying means such that said coal feedstock and said moisture sweeping medium move countercurrent to each other;
passing a primary drying medium through a jacketing means, said jacketing means being adapted to at least partially encase said conveying means;
evaporating moisture in said coal feedstock by means of said primary drying medium; and
sweeping said moisture in said moisture sweeping medium while flowing countercurrent to said coal feedstock to provide dried coal.