METHOD TO IMPROVE THE EFFICIENCY OF REMOVAL OF LIQUID WATER FROM SOLID BULK FUEL MATERIALS

Abstract

The invention provides methods to efficiently reduce the water concentration of raw solid fuels, including low rank coals such as brown coal, lignite, subbituminous coal, and other carbonaceous solids. Efficiently drying these materials at low temperatures significantly reduces greenhouse gas emissions and allows the production of low-rank coals for gasification and liquifaction.

Claims

1. A method of treating a solid carbonaceous material comprising: compacting a solid carbonaceous material under a force of at least about 5000 lb/in.sup.2 to form a compacted carbonaceous material; and, contacting the compacted carbonaceous material with a working fluid, wherein the working fluid causes evaporative drying of the compacted carbonaceous material to form a dried carbonaceous material.

2. The method of claim 1, wherein the solid carbonaceous material is selected from the group consisting of brown coal, lignite, subbituminous coal and mixtures of these materials.

3. The method of claim 1, wherein the solid carbonaceous material has a top size between about 0.1 mm and about 6 mm.

4. The method of claim 1, wherein the solid carbonaceous material has a moisture content between about 15 weight percent and about 65 weight percent.

5. The method of claim 1, wherein the solid carbonaceous material has a temperature between about 17 C. and about 66 C.

6. The method of claim 1, wherein the force is between about 5000 lb/in.sup.2 and about 50000 lb/in.sup.2.

7. The method of claim 1, wherein the working fluid is selected from the group consisting of unsaturated air, nitrogen, inert gas, flue gas, superheated steam and mixtures of these fluids.

8. The method of claim 1, wherein the contacting comprises applying the working fluid to the compacted carbonaceous material in at least one of a stockpile of the compacted carbonaceous material and a rotary dryer containing the carbonaceous material.

9. The method of claim 1, wherein the contacting comprises contacting the compacted carbonaceous material on a conveyor with a working fluid flowing past the conveyed material.

10. The method of claim 1, further comprising collecting dust created in the contacting step and returning the collected dust to a solid carbonaceous material for the compacting step.

11. The method of claim 1, further comprising compacting the dried carbonaceous material to form a shaped compacted material.

12. The method of claim 11, wherein the shaped compacted material is a briquette of ovoid shape with a minor dimension between about 6 mm and about 100 mm.

13. The method of claim 11, wherein the shaped compacted material has a moisture content between about 7 weight percent and about 17 weight percent.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG. 1 shows a schematic drawing of a low-temperature drying process integrated into a typical fossil-fired power plant operation in which a source of waste heat is available to heat the working fluid.

[0037] FIG. 2 shows a schematic drawing of low-temperature drying process that can be independently sited where no waste heat is available. Ambient air provides the working medium.

[0038] FIG. 3 shows a schematic drawing of low-temperature drying process that can be independently sited and uses an external heat source to provide warm air for drying.

[0039] FIG. 4 shows a schematic drawing of an ambient-temperature drying process that can be independently sited where material is stored in a covered stockpile. The stockpile is managed to accept compacted material on a continuous basis and be reclaimed as required.

[0040] FIG. 5 is a graph showing the results of a study comparing the relative drying rates of raw lignite and compacted lignite.

DETAILED DESCRIPTION OF THE INVENTION

[0041] The present invention provides a novel method to treat solid carbonaceous materials such as lignite and subbituminous coal used to fire boilers, combustors, stokers, and to feed coal gasifiers. This method takes advantage of the fact that a significant proportion of the water contained in pores of low-rank coal (often as much as 74% of the total water) can be efficiently evaporated without the difficulties of the conventional thermal drying systems. Conventional drying operations must heat the solids to evaporation temperatures to remove water held in the interior of the material. Because the rate of drying is greater with the present invention, lower temperatures can be efficiently used, significantly reducing material oxidation.

[0042] The method continuously compacts and collapses the porous material to express water held in pores, and transfers the expressed water to the surface of the processed material. Water residing on the surface is efficiently evaporated in the presence of the working fluid. By avoiding the difficulties associated with heat and mass transfer into and out of a particle, the present invention provides the superior heat and mass transfer only available when water is placed in direct contact with a working fluid. The efficiency gains can make both ambient-temperature and elevated-temperature drying systems practical in many applications.

[0043] The present invention includes the following subsystems: [0044] 1. Raw solid fuel preparation. [0045] 2. Material compaction. [0046] 3. Working fluid management. [0047] 4. Drying. [0048] 5. Dust collection. [0049] 6. Optional secondary compaction to form the dried product into desired shapes, as may be required for certain commercial or industrial applications.

[0050] Each of these subsystems is described in detail below.

Raw Solid Fuel Preparation

[0051] The raw solid fuel preparation subsystem receives crushed material of traditional trade top size, typically about 50 mm. In one preferred embodiment, the minus-50 mm raw solid fuel is comminuted by a hammer mill, roll crusher, or other appropriate device to produce a product of approximately 5 mm top size. The optimum particle size required to provide the desired compaction properties is experimentally determined for a particular application and feed source. However, feed to the compactor may have a top size that typically varies between about 0.1 mm and about 19 mm. Preferably, the top size is about 0.5 mm. The crushed material may include carbonaceous materials such as brown coal, lignite, subbituminous coal, waste coals and mixtures of these materials. Preferably, this raw feedstock contains between about 15 weight percent moisture and about 65 weight percent moisture, and more preferably about 35 weight percent moisture. Preferably, the temperature of this raw feedstock is between about 17 C. and about 66 C., and more preferably the feedstock is at ambient temperature.

Primary Material Compaction

[0052] The prepared bulk raw material is compacted with sufficient force to mobilize and transfer waters held in fractures, voids, and pores from the interior of the solid particle to the surface of the solid particle. Preferably, the compaction of the pre pared bulk raw material is conducted in a continuous manner. The compaction force produces a pressure of between about 5000 lb/in.sup.2 and about 50000 lb/in.sup.2, and more preferably the compaction force produced is about 30000 lb/in.sup.2.

[0053] In the preferred embodiment, a roller press is used to compact the feed material using a specific roll force between about 5 kN/cm and about 150 kN/cm of roll width. Water driven from the interior to the surface of particles by these compaction forces therefore becomes readily available for contact with a working fluid.

Working Fluid Management

[0054] The working fluid can be unsaturated air, nitrogen, inert gas, flue gas, superheated steam, or other substances that are compatible with the dried material. The working fluid management system generates a substance containing less than 100% relative humidity. In a preferred embodiment, the substance is air containing less 100% relative humidity that is collected and contacted with the wet material using natural convection, fans, or blowers. In these cases, the entire drying system is independent of external heat sources. The material can contact the working fluid in stockpiles and drying vessels such as a rotary dryer. In another embodiment, the working fluid can be heated by an external source. Supplied heat may be transferred to the working fluid by a heat exchanger. The heat exchanger is configured to suit the application. Sources of external heat may include, for example, condenser cooling water, flue gas desulfurization sludge, gasifier cooling water, syngas cooling water, heat recovery steam generator, or other forms of heat that would otherwise be rejected to the environment. In yet another embodiment, the working fluid generated by an external heat source can be hot flue gas that is tempered with air, or other material that is at a lower temperature than the combustion gas. In yet another related embodiment, a purpose-built boiler or combustor can be used to heat the working fluid.

Drying

[0055] The compacted product, usually in flake or pellet form, is transferred to a vessel where feed particles can be efficiently contacted with the unsaturated working fluid. In a preferred embodiment that works with coal or other heat-sensitive applications, the drying vessel is an indirect rotary dryer. Indirect rotary dryers transfer heat into the wet compressed material in two ways. First, heat is transferred by convection. This is accomplished by passing hot working fluid over the wet material. Second, heat is transferred by conduction by contacting the wet compressed material with a hot surface (shell of the rotary dryer). Both sources of heat evaporate water. In other applications that work with materials that are not heat sensitive, a direct rotary dryer may be used. Direct dryers use heat supplied by the hot working fluid alone, and do not heat the material by conduction. The working fluid used in a direct dryer is typically hotter than the working fluid used in an indirect rotary dryer.

[0056] In another embodiment, unsaturated air can be directed across a stockpile of compacted material. Fans or natural convection can used to accelerate the air to increase the rate of drying.

[0057] In another embodiment, material can be conveyed on a vibrating pan conveyor fitted with a perforated screen deck. Working fluid enters upward though the perforations and flows past the conveyed material. The conveyor device is sufficiently long to provide the required residence time to dry the material. Saturated vapor is removed from the top of the conveyer. Additional methods including fluid bed dryers and other vessels of commercial configuration are available. The present invention is not limited to the type of style of drying vessel as long as it is compatible with the process material.

Dust Collection

[0058] Vapors emanated from the dryer often contain dust that must be removed before venting to the atmosphere. Standard dust separation and collection devices such as electrostatic precipitators, bag houses or wet scrubbers may be used to separate fine particles from water vapors as dictated by the application. Collected fine particles may be recycled to the compaction subsystem as desired, so that all, or nearly all, feed material is processed without waste.

Secondary Material Compaction

[0059] Applications that require the finished product to be of a specified shape, such as a briquette, can be accommodated by compacting the dried product as described above. Formed products are typically used where the dried material is transported, or used in stoker furnaces where a coarse particle size distribution is required.

[0060] In instances in which a shaped final product is desired, the product is preferably a briquette of ovoid shape with a minor dimension of at least about 6 mm, but less than about 100 mm, and more preferably having a minor dimension of about 50 mm. These shaped products are preferably formed bulk materials having a void space of between about 12 volume percent and about 60 volume percent, and more preferably having a void space of about 30 volume percent void space. Preferably, the shaped products are formed such that upon being subjected to coking conditions, they form coke that has a compressive strength between about 100 lb/in.sup.2 and about 2,000 lb/in.sup.2, and more preferably a compressive strength of about 800 lb/in.sup.2. Preferably, the shaped products have a total moisture content between about 7 weight percent and about 17 weight percent, and more preferably a total moisture content of about 12 wt %.

[0061] FIG. 1 shows a schematic of the overall system of a preferred embodiment of the invention. A source of raw solid fuel (1) supplies material (2) to the raw solid fuel comminution circuit (3) where the feed is crushed and sized. The prepared raw feed (4) and collected dust (12) are feed to the compaction circuit (5) where they are compressed under high pressure to force water from its internal pores to produce a flake product with water adhering to the surface of the compacted material (6). The compacted material is fed to the dryer (7) where it is mixed with heated air (19), evaporating the water residing on the surface of the compacted material. The resulting vapors and dust (8) are passed to a dust collection circuit (9) where the dust and water vapor are separated. Dust-free vapor (10) is vented to the atmosphere (11). Dust (12) is conveyed to the compaction circuit. Dryer product (13) containing substantially less moisture than the feed, but within the application product specifications, is conveyed to a dried product storage point (14) where it is available for use or additional processing. A source of waste heat (15) capable of supplying sufficient power to satisfy the evaporative load provides a hot flow input (16) and accepts a hot flow return (17). A sufficient temperature drop exists between the input and return flows to impart the required energy to the working fluid (21). An ambient air source (20) provides the cool working fluid (21) to the heat exchanger (18) where it is heated to a specified temperature by the circulating hot in and hot out flows. The heated working fluid (19) passes to the dryer where it contacts the wet feed material.

[0062] FIG. 2 shows a schematic of the overall system of a preferred embodiment of the present invention. A source of raw solid fuel (21) supplies material (22) to the raw solid fuel comminution circuit (23) where the feed is crushed and sized. The prepared raw feed (24) and collected dust (212) is feed to the compaction circuit (25) where it is compressed under high pressure to force water from the feed's internal pores to produce a flake product with water adhering to the surface of the compacted material (26). The compacted material is fed to the dryer (27) where it is mixed with unsaturated ambient-temperature air (215) thus evaporating the water residing on the surface of the compacted material. The resulting vapors and dust (28) are passed to a dust collection circuit (29) where dust and water vapor is separated. Dust-free vapor (210) is vented to the atmosphere (211). Dust (212) is conveyed to the compaction circuit. Dryer product (213) containing substantially less moisture than the feed, but within the application product specifications, is conveyed to a dried product storage point (214) where it is available for use or additional processing. A source of ambient-temperature air (215) is fed into the dryer.

[0063] FIG. 3 shows a schematic of the overall system of a preferred embodiment of the invention. A source of raw solid fuel (31) supplies material (32) to the raw solid fuel comminution circuit (33) where the feed is crushed and sized. The prepared raw feed (34) and collected dust (312) is feed to the compaction circuit (35) where it is compressed under high pressure to force water from the feed's internal pores to produce a flake product with water adhering to the surface of the compacted material (36). The compacted material is fed to the dryer (37) where it is mixed with heated air and flue gas (322) thus evaporating the water residing on the surface of the compacted material. The resulting vapors and dust (38) are passed to a dust collection circuit (39) where dust and water vapor are separated. Dust-free vapor (310) is vented to the atmosphere (311). Dust (312) is conveyed to the compaction circuit. Dryer product (313) containing substantially less moisture than the feed, but within the application product specifications, is conveyed to a dried product storage point (314) where it is available for use or additional processing. A source of ambient air (319) provides combustion air (320) and tempering air (321) to the process. A source of fuel (315) is supplied (16) to a furnace (317) where it is combusted to provide hot flue gas (318). The flue gas is mixed with tempering air (321) to provide a warm gas (322) of the specified temperature for drying purposes.

[0064] FIG. 4 shows a schematic of a preferred embodiment of the invention. A source of raw solid fuel (41) supplies material (42) to the raw solid fuel comminution circuit (43) where the feed is crushed and sized. The prepared raw feed (44) is feed to the compaction circuit (45) where it is compressed under high pressure to force water from the feed's internal pores to produce a flake product with water adhering to the surface of the compacted material (46). The compacted material is stacked out in a covered stockpile (47). A source of ambient, unsaturated air (48) is available to sweep (49) over the stockpiled material thus evaporating the water residing on the surface of the compacted material. The resulting vapors (410) are released as a gas to the atmosphere (411). Dried product (412) containing substantially less moisture than the feed, but within the application product specifications, is reclaimed to a dried product storage point (413) where it is available for use or additional processing.

[0065] Additional objects, advantages, and novel features of this invention will become apparent to those skilled in the art upon examination of the following examples thereof, which are not intended to be limiting.

EXAMPLES

Example 1

[0066] A detailed study of lignite (high-moisture lignite from North Dakota) was undertaken to assess the relative drying rates of raw material and compacted product. Experiments were conducted on materials spread out on a flat tarpaulin at ambient conditions. Samples of raw lignite and compacted lignite were taken periodically from the spread out material and assayed for total moisture. The measured moisture values were normalized as percent of the total water evaporated to compare the results on an equal basis. Drying conditions were 31 C., and 23% relative humidity. Results are plotted in FIG. 5.

[0067] These results demonstrate the increased drying rates possible by compacting the raw lignite. Table 1 summarizes the ratio of drying rates between raw and compacted lignite processed at ambient conditions of 31 C., 23% relative humidity.

TABLE-US-00001 TABLE 1 Ratio of Relative Drying Rates for North Dakota Lignite Drying Time, % Moisture Removed hr Raw Compacted Ratio 0.5 4 16 4.0 1.0 9 22 2.4 1.5 13 26 2.0 2.0 16 28 1.8

Example 2

[0068] A detailed study of one LRC (high-moisture lignite from South East Asia) was undertaken to assess the relative drying rates of raw material and compacted product processed by an indirect rotary dryer (180 mm diameter3000 mm long). The heated portion of the drying tube was 2000 mm long, the cooling zone was 600 mm long, and the feed zone was 400 mm long. Test conditions were identical for the compacted and raw materials. The results are summarized in Table 2.

TABLE-US-00002 TABLE 2 Rotary Indirect Dryer Test Results Compacted Raw Test Parameter Material Material Feed moisture, wt % 45 46 Product moisture, wt % 16 38 Residence time, min 20 20 Sweep gas temperature, at 130 130 feed point, C. Shell temperature, C. Less than 110 Less than 110 Maximum material Less than 100 Less than 100 temperature, C. Sweep gas flow rate, L/min 700 700 Material feed rate, kg/min 10 10

[0069] These data show the compacted material dries to a lower moisture content that is about half that of the raw material under identical drying conditions. The increased drying rate afforded by compaction effectively doubles the capacity of the drying equipment. If cost savings are more important than capacity, the cost of the drying equipment will be half.

Example 3

[0070] An experiment was conducted to measure the relative drying rates of South East Asia lignite held under warm, moist conditions that would be expected in a covered stockpile located in non-condensing, warm, humid tropical climates. Table 3 lists results.

TABLE-US-00003 TABLE 3 Moisture Content of Lignite Stored in Warm, Humid Atmosphere 30 C., >60% Relative Humidity Compacted Raw Lignite Lignite Moisture, Time, hrs Moisture, wt % wt % 0 40 43 48 36 37

[0071] These test data show that the compacted material lost 50% more moisture than the raw material in 48 hours.

Example 4

[0072] Samples of lignite produced from North Dakota were processed by the GTLE process to form briquettes of low moisture content. These briquettes were then processed at high temperature and gas conditions that are typical of those found in the reaction zone of a solid feed gasifier.

[0073] The compressive strength of the briquettes and associated coke produced from the gasification conditions were measured. Tests were also conducted on briquettes formed from North Dakota lignite by standard industrial processes typically used to form home heating fuels and other industrial products. Test results are listed in Table 4.

TABLE-US-00004 TABLE 4 Compressive Strength of Briquettes and Coke Produced from LRCs Compressive Strength, Sample Briquette lb/in.sup.2 Process Description Moisture, wt % Briquette Coke Method North Dakota 16.3 1,405 711 Present Lignite #5 A Invention North Dakota 10.9 944 676 Present Lignite #5 B Invention North Dakota 12.0 1,211 898 Present Lignite #6 A Invention North Dakota 9.98 767 755 Present Lignite #6 B Invention North Dakota 8.24 735 506 Present Lignite #6 C Invention German Lignite 10.81 n/a 247 Industrial Process

[0074] These results demonstrate the increase of compressive strength from 247 lb/in.sup.2 for coke produced from briquettes formed from industrial processes to over 500 lb/in.sup.2 for coke produced from briquettes formed by the present invention.

[0075] The foregoing description of the present invention has been presented for purposes of illustration and description. Furthermore, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, and the skill or knowledge of the relevant art, are within the scope of the present invention. The embodiment described hereinabove is further intended to explain the best mode known for practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with various modifications required by the particular applications or uses of the present invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.