Cross-Reference To Related Applications

[0001] This application claims the benefit of Australian Provisional Patent Application No. 2012904352 filed 5 October 2012, which is incorporated herein by reference.

Technical Field

[0001] The present invention relates to drying particulate matter such as lignite particles, and in particular relates to a method and device for passing moist particulate matter such as raw coal through a fluidised bed for drying.

Background of the Invention

[0002] Coal is an important energy resource and has been used for a long period of time. However, continuing exploration and exploitation has lead to high quality coal resources becoming more scarce. One important index of the quality of coal is the moisture content; the higher the moisture content, the lower the quality. Typically, parties such as power generators wishing to purchase coal require coal to have a low moisture content, such as a moisture content around 10%. In order to improve the quality of moist coal, it may thus be necessary to perform a drying process in order to reduce the moisture content.

[0003] Lignites (brown coals) have a moderate to high moisture content. In one of the world's largest deposits in Victoria, Australia, lignite moisture content ranges from around 55% to 65% by weight. The geological age of lignite is short, with lignite typically arising in shallow buried layers. Lignite typically has high moisture, and produces minimal heat upon combustion. The high moisture content and relatively low energy content generally makes long distance transport of lignite uneconomic. A further complication is that lignite is flammable and can spontaneously ignite, so that handling and transporting lignite can require separating the lignite from air.

[0004] High moisture content in bituminous coal, sub-bituminous coal, lignite or brown coal carries a range of disadvantages. When used for power generation, a higher moisture content of the fuel reduces thermal efficiency of the power plant, as shown in Figure 1. The upper curve in Figure 1 illustrates thermal efficiency against moisture content for a 500 MW plant with parameters 168 b/540 C/540 C, Tstack 185C, with the lower curve illustrating same for a 1000 MW plant with parameters 275 b/580 C/600 C, Tstack lOOC. Lower efficiency undesirably leads to higher fuel usage to maintain output, increasing C02 emissions. Dryer fuel reduces fuel requirements, reduces C02 emissions, and reduces the capital cost of the power plant. Thus, energy efficient coal drying is critical to the reduction of greenhouse emissions for existing and future uses of lignite.

[0005] When available coal has a high moisture content, pre-drying of the fuel can thus be desirable. Dried coal or lignite also weighs less and is thus easier to transport. Removing water from coal or lignite can also tend to remove salts, which is usually beneficial when using the dried coal in boilers. To allow lignite to be processed into gaseous and liquid products as well as high-grade solid fuels, its moisture content usually must be reduced to 10 to 20 % wt, depending on the production goal.

[0006] A range of techniques have been proposed to dry coal or lignite. These include steam fluidised bed drying, integrated drying gasification, densified brown coal, mechanical thermal expansion, and hydrothermal dewatering.

[0007] Steam fluidised bed drying involves evaporative drying of the coal in a superheated steam fluidised bed. Figure 2a illustrates one proposed steam fluidised bed drier, with figures 2b and 2c illustrating the process principles of using such a drier, respectively using vapour recompression and vapour condensation. In the system of Figure 2b steam condensate output from the heat exchanger is passed to a coal preheater. In the system of Figure 2c steam condensate output from the heat exchanger is passed out of the plant as a by-product. In existing lignite steam fluidized bed drying the boiler or other heat source produces steam in the fluidized bed dryer heating tube, which passes through the lignite. A resulting condensing steam mixture is discharged into a drain tank or de-aerator, such as a soda recovery unit, from where it is returned to the boiler to form a soft cycle. [0008] Densified brown coal is obtained by attritioning of as-mined coal to produce moist, 'plastic', fine-grained, coal 'clay' that subsequently is air dried to a low moisture content coal and represents a "black coal equivalent" product. This technique offers the potential for recovery of water content. Run-of mine brown coal is milled to

approximately 50 mm lumps and then fed to a low intensity shearing attritioning mill from which the coal emerges with a mean particle size of approximately 10 μηι. This reduction in particle size effectively releases the water that has been trapped within the porous structure of the raw coal. The plastic attritioned coal is then extruded into pellets or blocks of the required dimensions. The moist product is then conveyed for drying, which takes place by evaporative loss of water to the atmosphere.

[0009] Another approach to brown coal densification involves a pyrolysis process, combining mechanical, thermal and chemical processes that accelerate the natural transformation from brown coal to black coal equivalent. Outputs of this process are hydrocarbon gases, water and a metallurgical coke equivalent.

[0010] Mechanical thermal expression involves compressing coarse crushed coal, heated in the range of 150 to 200 °C by condensing steam, at mechanical pressures typically of about 60 bar to squeeze the water from the brown coal, and produce a dried brown coal product.

[0011] In hydrothermal dewatering, wet coal is heated under pressure to -250-300 °C and the coal structure breaks down and shrinks, releasing the water as a liquid. A combination of high temperature and pressure (300 C and 100 bar) decarboxylates the coal. When CO ² is ejected the coal physically shrinks, expelling liquid water from its interstices in the process. The decarboxylation reaction is exothermic and contributes energy to the process. This process uses about 2% of the energy content of the wet brown coal for the overall process.

[0012] Another approach to brown coal drying is microwave drying. Coal is largely transparent to microwave energy, whereas water is highly absorbent of microwave energy and so microwave energy can be efficiently delivered to both free and inherent water in the coal.

[0013] Microbial hydrolysis has also been proposed to extract the water from brown coal without using heat. The microbial gasification process is a digestion system, the first stage of which is hydrolysis and then use of a series of bacteria.

[0014] Brown coal presents a range of possible uses, particularly when dry. Many countries have large supplies, making brown coal a low cost primary energy resource. Brown coal's high reactivity is suitable for gasification and reductant applications.

Brown coal is used, or could be used, in applications such as: Power generation, Coal-to- liquids, Gas-to-liquids, Minerals processing, and export-quality brown coal.

[0015] Key parameters that affect drying rate and final moisture content of a dried brown coal include temperature, pressure, drying medium flow rate, relative humidity, residence time in dryer, and particle size of coal to be dried. This has led to a plethora of techniques that have been developed to dry brown coal. One important distinguishing feature of drying techniques is the form in which the water comes out of the process and/or can be recovered, in particular as a gas that can be condensed back to a liquid (evaporative techniques) or as a liquid (dewatering techniques).

[0016] Nevertheless, safety and transport challenges arise with dried lignite or coal. Dried coal has a tendency for spontaneous combustion and conditions permitting spontaneous combustion must be avoided. Coal handling also has a tendency for generation of undesirable coal dust.

[0017] Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present 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 present invention as it existed before the priority date of each claim of this application. [0018] 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.

Summary of the Invention

[0019] According to a first aspect the present invention provides an apparatus for drying particulate matter, the apparatus comprising:

a steam supply device for producing steam; and

a fluidised bed dryer comprising:

a solids inlet for receiving particulate matter,

at least one steam inlet for injecting steam into the dryer so as to agitate and fluidise a bed of particulate matter proximal to the or each steam inlet, heat transfer tubing positioned within the fluidised bed dryer and configured to pass steam from the saturated steam supply device through the tubing to transfer heat to the particulate matter, and further configured to allow condensate within the tubing to flow under gravity back to the steam supply device so as to effect a gravity assisted loop; and

a solids outlet for permitting particulate matter to exit the fluidised bed.

[0020] According to a second aspect the present invention provides a method for drying particulate matter, the method comprising:

delivering particulate matter into a fluidised bed dryer;

heating a steam supply device so as to produce steam for circulating within a gravity-assisted closed loop which comprises heat exchange tubing within the fluidised bed dryer configured to heat particulate matter within the dryer and further configured to permit condensate to flow under gravity back to the saturated steam supply device;

injecting steam into the fluidised bed dryer so as to fluidise a bed of particulate matter within the dryer; and

dried particulate matter exiting from the dryer. [0021] In some embodiments of the invention, an enclosed circulating circuit is formed between the heat exchange tubing and the saturated steam supply device, and condensate forming in the heat exchanger circularly flows into the saturated steam supply device while the saturated steam supply device continuously provides the built-in heat exchanger with saturated steam.

[0022] In some embodiments of the invention, the steam supply device is a saturated steam supply device and comprises a steam drum and a heating device for providing saturated steam for the steam drum, and the enclosed circulating circuit includes a heat- releasing circuit and a heat-absorbing circuit, wherein the steam drum is situated in both the heat-releasing circuit and the heat-absorbing circuit, and serves as an interconnection between the heat-releasing circuit and the heat-absorbing circuit. In such embodiments, in the heat-releasing circuit, the saturated steam in the steam drum may be delivered to the heat exchanger within the fluidised bed and be transformed into saturated liquid in the heat exchanger, with the saturated liquid flowing around the circuit to the steam drum. Further, in such embodiments in the heat-absorbing circuit the saturated liquid in the steam drum may be delivered to the heating device to be transformed into saturated steam via phase change, with the saturated steam flowing back around the circuit to the steam drum. A steam tank may be provided to gather saturated steam from the steam drum and may comprise or be associated with a fluidized fan to drive saturated steam to the heat transfer tubing positioned within the fluidised bed dryer.

[0023] In some embodiments of the invention, a liquid level inside the steam drum is at a height which is lower than a height of an outlet port of the heat exchanger in the fluidised bed, to effect gravity-assisted, and optionally unpowered, circulation of liquid from the fluidised bed to the steam supply device.

[0024] In some embodiments of the invention, an atmosphere within the dryer comprises saturated steam, to ensure an inert environment lacking free oxygen, to avoid oxidation of the products and minimise fire or explosion risk. The steam injected into the dryer so as to agitate and fluidise the bed of particulate matter is preferably superheated steam. To provide superheated steam used to fluidise the fluidised bed, a superheating steam supply device is preferably provided in order to take steam and superheat the steam to a superheated temperature, for example the superheated steam may be at atmospheric pressure (approx. 1 bar) and in the range 110-130 degrees Celsius, preferably 120 degrees Celsius.

[0025] In some embodiments of the invention, the particulate matter to be dried may comprise any suitable material, whether inorganic, organic chemicals, starch, PVC, and could for example comprise coal or lignite.

[0026] In some embodiments of the invention, the particulate matter may be preheated before entering the fluidised bed dryer. An excess steam discharge from the steam tank is preferably connected to a particle pre-heater, to provide heat to pre-heat the particulate matter.

[0027] The fluidised bed dryer is preferably sufficiently elevated relative to the boiler to ensure sufficient gravity-assistance of the closed loop heat transfer steam. The fluidised bed dryer is preferably also sufficiently elevated to allow dried particulate matter to fall from the dryer under gravity to a briquetting or molding machine to avoid having to lift the dried particulate matter prior to briquetting. For example the briquetting or molding machine may comprise a dried coal silo located below the solids outlet of the fluidised bed dryer, a dried coal feeder located below the dried coal silo, and a packing or briquetting machine located below the dried coal feeder.

[0028] In some embodiments of the invention, steam exiting the fluidised bed dryer is recycled, via one or more of: a cyclone, a filter bag and/or a steam box. Preferably, steam exiting the fluidised bed dryer is first recycled by a cyclone which removes dust and produces a cyclone-recycled steam, with the cyclone-recycled steam in turn being passed to a filter bag for further dust removal to produce filtered steam. The cyclone- recycled steam may be reheated before being passed to the filter bag, for example by passing a discharge of the steam drum boiler to a cyclone-recycled steam reheater. The filtered steam produced by the filter bag is preferably returned to the steam tank. In such embodiments, coal dust separated from the steam by the cyclone and/or filter bag is preferably discharged to a discharge material packing-briquetting device.

[0029] The steam supply device may produce saturated steam at a raised pressure, for example in the range 3-5 bar, and at a temperature in the range of 130-180 degrees Celsius. For example the saturated steam may be at greater than 140 degrees Celsius, such as in the range 160 - 180 degrees Celsius, and may be at about 170 degrees Celsius.

[0030] Air passing to an air intake of the boiler is preferably preheated by a boiler air preheater, which is preferably configured to obtain heat from a discharge of the steam drum boiler.

[0031] A gas discharge channel of the steam drum boiler is preferably provided with a boiler precipitator and a boiler induced draft fan.

[0032] By providing a gravity-assisted closed loop between the saturated steam supply device and the heat transfer tubing within the fluidised bed dryer, condensed liquid within the heat transfer tubing suffers minimal cooling loss, and heat released by the gas-to- liquid phase change is more efficiently used to achieve efficient drying. Moreover, the temperature of the saturated steam within the steam drum can be simply controlled by controlling pressure within the steam drum.

Brief Description of the Drawings

[0033] An example of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 illustrates thermal efficiency against moisture content for two brown coal power plants;

Figures 2a-2c illustrate a previous proposed steam fluidised bed drier and the process principles thereof;

Figure 3 is a schematic of a system in accordance with the present invention for drying lignite; Figure 4 illustrates a fluidized bed dryer (FBD) in accordance with one embodiment of the present invention; and

Figure 5 illustrates the functionality of the closed loop gravity assisted circulation.

[0034] List of elements in Figures

1 Boiler

2 Steam Drum

3 Fluidized Bed Dryer

4 Raw Coal Feeder

5 Raw Coal Pre-Heater

6 Dried Coal Silo

7 Fluidized Fan

8 Cyclone

9 Circulated Steam Re-Heater

10 Air Pre-heater

11 Forced Draft Fan

12 Dust remove filter

13 Steam Induced draft Fan

14 Steam Tank

15 Boiler Induced Draft Fan

16 Chimney

18 Control Valve

19 Briquetting

20 Dried Coal Feeder

21 Drain

22 Flue Gas Precipitator

23 Control Valve Description of the Preferred Embodiments

[0035] Figure 3 is a schematic of a system for drying lignite and lignite coal. This system exploits a natural circulation of steam arising from phase change heat flow. A coal conveyor belt delivers coal or lignite to the coal hopper (17). This coal may for example comprise 40-70% water. This coal is then passed through a coal preheater (5) in order to preheat the coal prior to the coal being delivered to the fluidised bed dryer (3) to a temperature between 30 degrees Celsius and 70 degrees Celsius, for example to 50 degrees Celsius. Such a preheater (5) may in other embodiments be omitted, or in still further embodiments the preheater (5) may also effect partial drying of the coal to aid operation of the fluidised bed dryer (3).

[0036] Coal preheated by preheater (5) is then passed through the coal feeder (4) into the steam fluidized bed dryer (3). Dried coal from the fluidised bed dryer (3) is controllably released via the discharge control valve (18) to fall into the dry coal hopper (6), and then through the dry coal feeder (20) into the molding machine (19), resulting in briquetted product coal of reduced moisture content.

[0037] A fluidised bed of coal is produced in dryer (3) by injecting high pressure steam, driven by fan (7). The steam may be superheated, or may be non-superheated steam recycled from other processes of the plant. Injecting the high pressure steam into the base of the dryer (3), beneath incoming coal, turbulently agitates coal within the dryer 3 to produce the fluidised bed of steam and coal particles within the dryer (3). Such a fluidised bed provides for efficient heat transfer into each particle, so that moisture within each particle is converted into steam and due to the high factor of expansion when water turns to steam the moisture largely evacuates from the particle. The stream of steam driven by fan (7) is obtained from steam box (14). Further, the steam is saturated steam and the dryer (3) is held above ambient pressure to ensure that no oxygen is present within the fluidised bed dryer (3), to avoid auto-combustion of coal or lignite within the dryer.

[0038] To improve heat transfer into the particles passing through the fluidised bed dryer (3), an indirect heat exchanger steam boiler (1) is provided. Together, the steam injected by fan (7) and the heat from heat exchanger boiler (1) effect rapid heat transfer into the coal particles, enabling continuous evaporation of water and improved coal throughput.

[0039] Steam rising out of dryer (3), carrying fine coal powder, is passed to a cyclone (8). This steam is typically at a temperature of about 110 degrees Celsius. Cyclone (8) is configured to separate the coal powder fines from the steam. Coal thus extracted by cyclone (8) is passed to coal hopper (6).

[0040] Steam rising out of cyclone (8) will, despite the separation effected by the cyclone (8), continue to carry very fine coal particles and may be too dirty to release to the atmosphere. Accordingly, in this embodiment the steam from cyclone (8), which typically will have cooled to about 105 degrees Celsius, is passed to a heater (9) for indirect heat transfer from boiler (1) to be superheated to about 115 degrees Celsius, and then passed to a bag filter (12). Superheating the steam/powder mixture delivered to bag filter (12) helps avoid condensation occurring within the bag filter (12), as any such condensation would likely obstruct the filter membrane. In providing the bag filter (12), the present embodiment effects higher quality environmental performance of the drying system. Filter (12) is further beneficial in minimising the amount of steam-borne dust passed to coal preheater (5).

[0041] Pump (13) creates a negative differential pressure across the bag filter (12) thereby drawing steam through the filter membrane and out of the filter (12), and delivering this filtered steam into a steam box (14). Steam from the steam box (14) is then re-used as an input to pre-heater (5), and is also used by fan (7) to steam fluidise the fluidised bed of coal within dryer (3), so that steam may repeatedly cycle around the loop of elements (7), (3), (8), (9), (12), (13) and (14). Steam which condenses in steam box (14) falls into trap (21), and the water from the trap (21) may be removed by suitable pipes or recycled as desired. Steam box (14) is particularly beneficial in avoiding excess emissions of steam from the process, and further provides for load balancing of the steam cycles. Moreover, by reusing steam the latent heat present in the recycled steam is retained within the cycle instead of being discharged, improving energy efficiency and reducing energy consumption of the overall process.

[0042] In alternative embodiments, the steam from fan (13) may be released to the atmosphere, for example via stack (16), rather than being recycled.

[0043] A phase-change boiler (1) heats water and, through the drum (2), produces saturated steam at a temperature of about 170 degrees Celsius, which is delivered into heat exchange coils within the fluidized bed dryer (3) to indirectly heat raw coal present in the proximity of the heat exchange coils. Depending on operating conditions the saturated steam could instead be heated to an alternative temperature as required, for example in the range of 130 degrees Celsius to 180 degrees Celsius. As heat passes from the steam to the coal the steam condenses, with the condensate formed then falling under gravity back to the boiler drum (2) and boiler (1), thereby forming a gravity-assisted closed cycle of phase change heating. Such a cycle is particularly beneficial as hydrostatic pressure, condensation, and gravity ensure circulation without the need for water pumps or the like, without loss or chemical treatment of condensed water.

Moreover, providing for such a circulating heat transfer arrangement gives a high degree of freedom to adapt to a variety of variables, including flow of the fan, power bed uniformity, dense tubing bundles, and variable heat transfer area. Suitable control over coal flow rates controls the amount of time coal spends in the drier (3), and permits selection of how much moisture content reduction will occur. For example, incoming coal of 60% moisture content may spend about 30 minutes within the fluidised bed to be reduced to 5% moisture content.

[0044] Boiler (1) additionally generates flue gas, which is used to provide heat for the steam reheater (9) and the air preheater (10). The flue gas is then passed to the boiler precipitator (22), through the boiler fan (15) and ejected via chimney (16).

[0045] The present embodiment of the invention thus beneficially does not require a boiler feed pump, drain tank, or other soft recovery. The system is simple thus having reduced chance of faults or leaks, requires less equipment and involves almost no soda loss. The elevated position of the fluidized bed dryer (3) not only provides for the gravity assisted phase change heat transfer cycle, but also advantageously leaves reserve space beneath the drier (3). Consequently, dried coal falling from drier (3) via discharge control valve (18) can simply fall under gravity into hopper (6), coal feeder (20), and briquetting device (molding machine) (19). This is in contrast to other driers which require that the coal be lifted to be fed into the drier, and then be lifted a second time after drying to be fed into a briquetting device. The second lifting step not only requires additional power but also delays the time at which briquetting occurs. The present embodiment recognises that it is beneficial to undertake briquetting as soon as possible while the dried coal powder is still at elevated temperatures to effect improved molding, with the present embodiment permitting rapid briquetting immediately after drying, unlike alternative systems involving a time-consuming second lifting step before briquetting can occur. A single vertical path of the coal also reduces a footprint of the drying plant.

[0046] The system of Figure 3 thus provides for continuous evaporative drying of coal passing through fluidised bed dryer (3). This configuration is particularly beneficial and in this embodiment a transverse cross sectional area of 70 square metres permits coal throughput of around 3 million tonnes per year, in contrast to previous systems in which a ten times larger bed area of 700 square metres permits throughput of only about 1 million tonnes per year.

[0047] Heating the lignite in the fluidised bed with saturated steam ensures an anaerobic environment with little or no chance of auto-combustion, while use of a fluidised bed ensures heterogeneity of the dried coal product. Use of saturated steam heated from the boiler drum enables the drum pressure to be controlled in a simple manner, by controlling the temperature of saturated steam, thus avoiding the need for more complicated temperature and pressure reduction systems.

[0048] Figure 4 illustrates a fluidized bed dryer (FBD) 403 in accordance with one embodiment of the present invention, and which may be utilised in the system of Figure 3, or in an alternative system. Steam (460) is injected to agitate coal within the dryer to form a fluidised bed, and then exits via steam outlet (440). Heating steam from a boiler drum is delivered into a vertical header (412) within the bed, and is typically within the range of 130 to 180 degrees Celsius, such as being at 160 degrees Celsius. This inbuilt header minimizes the tubes that go through the wall of the drier (403), having only one inlet (410) and one outlet (450), giving less chance of leakage. This is a particularly beneficial feature of this embodiment. The heat exchange tubes, indicated generally at (420), are fed from the single inlet (410) via header (412) and arranged in parallel across the dryer (403) and slant downwards to take advantage of gravity, such that when the steam cools down and condenses water moves from left to right in the figure down the sloped pipes (420) to collect in the right side vertical header (452) which passes the condensate to the single outlet (450). During this phase change, large amounts of heat are released and efficiently transferred to the agitated raw coal in the fluidised bed by the superheated steam outside the heat exchange tubes and within the fluidised bed, providing a significant heat source for drying the raw coal. The extent of drying can be selected by controlling the retention time of coal within the dryer (403). Dried coal then falls under gravity from the dryer (403) out a dried coal outlet.

[0049] In figure 4 it is further noted that the raw coal inlet (430) is lower than and distal from the steam outlet (440), to minimise the amount of solid coal dust particles carried out of outlet (440). Steam and fine coal dust exits port (440) and can be recycled via cyclone and/or filter bag as previously discussed herein.

[0050] Figure 5 further illustrates the function of the dryer (403) in a circulation system. A saturated steam supply device comprises a steam drum (541) and a steam drum boiler (545), which maintain the contained saturated steam and saturated liquid within a range of 130-180 degrees Celsius, for example 160 degrees Celsius. Saturated steam generated inside the steam drum (541) flows to the inlet port (410) of dryer (403). Condensed liquid flows from outlet (450) of dryer (403) via a control valve (544) and returns to steam drum (541) to complete an enclosed circulating circuit. Saturated liquid and steam co-exist in steam drum (541). Consequently, the heat carried in the condensate discharged from outlet (450) of dryer (403) is efficiently retained in the system with little heat loss.

[0051] The liquid in steam drum (541) is provided with heat by the steam drum boiler (545), maintaining a dynamic balance in the steam drum (541). Thus, as shown in Figure 5, this system includes two closed loop circuits with the steam drum (541)

interconnecting the two loops. Steam drum (541) and dryer (403) form a heat-releasing circuit. The heat-releasing circuit circulates under the assistance of gravity and differential pressure, as the fluid level h2 exiting dryer (403) is higher than the fluid level hi in the steam bed (541). The height differential between hi and h2 can be selected and/or configured according to factors such as the desired speed of flow. Therefore, in this embodiment, no pump or other powered device is provided in order to assist circulation within the heat-releasing circuit.

[0052] It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described.

[0053] For example, the saturated liquid and/or gas in the heat-releasing and heat- absorbing circuits and passed through the heat exchange pipes (420) may be a substance other than water, for example oil.

[0054] The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.