Inclined Fluidized Bed System for Drying Fine Coal

FIG. 1 shows a typical coal drying process employing inclined fluidized beds.

FIG. 2 shows in two views 2A and 2B a typical inclined fluidized bed bench scale equipment.

FIG. 3 shows the particle size distribution of tested crushed feed coals.

FIG. 4 shows experimental TGA weight loss curves for heating Usibelli coal.

FIG. 5 shows experimental TGA weight loss curves for heated Eagle Butte coal.

FIG. 6 shows inclined fluidized bed cold flow experimental results using Eagle Butte coal.

FIG. 7 shows inclined fluidized bed cold flow experimental results using Usibelli coal.

FIG. 8 shows moisture and temperature conditions during a typical larger test run.

The present invention represents a process to thermally dry fine coal to produce a low-moisture product that is stabilized against moisture reabsorption, dust formation, and spontaneous combustion. Thus, the shipping weight is reduced and further surface treatment is unnecessary. The unique control capabilities of the inclined fluidized bed allow efficient operation of such process.

According to the preferred embodiment of present invention, recycled carbon dioxide, produced from partial decarboxylation of coal and representing an inert gas, dries fine coal to a low moisture content. An inclined fluidized bed operating at plug flow conditions provides excellent gas-solid contact while minimizing elutriation from the dryer. The plug flow nature of the inclined fluidized bed allows drying, tar mobilization, quenching, and cooling to occur in separate zones by control of the appropriate reactor temperature profile and solids residence time; thus producing a zonal inclined fluidized bed. The tar mobilization and subsequent quenching with carbon dioxide seals off the micropores so that moisture reabsorption is prevented. The final cooling with carbon dioxide avoids autogenous heating and leaves the product dried coal in a stabilized form so that further transfer can be simply done, such as pressing into briquettes for easy handling and shipping.

FIG. 1 shows a typical block flow sheet for the process showing the preferred embodiment. The process begins with feed coal, 1, which usually is predried if the initial moisture content is over 30%. Predrying avoids mechanically feeding difficulties entering the first inclined fluidized bed (IFB), 2. This coal passes through the first IFB, 2, and is fluidized by hot carbon dioxide, 3, entering its bottom plenum, 4. The exit gases, 5, from the first IFB, 2, are treated to remove fines, 6, and then cooled to remove water, 7, before the gas is compressed by blower action, 8. This gas stream, 9, now essentially carbon dioxide, is recycled, 10, back to the plenum of the second IFB, 11. The second IFB, 11, is fed by dried coal, 12, exiting from the first IFB, 2. As the dried coal exits the second IFB, 12, as product, it is briquetted, 13, before storage. Part of the gas stream, 14, exiting the second IFB, 12, flows directly to the first IFB fluidizing gas plenum, 3. The remaining off-gas from the second IFB flows through a heat exchanger, 15, in the coal combustor, 16, for heating before re-entering the inlet plenum stream, 3. The coal combustor is fed coal fines, 17, that maybe recycled from the fines removal equipment, 6, and combustion air, 21. The resulting combustor stack gases, 18, and ash, 19, are produced for disposal and in particular this flue gas is environmentally acceptable as is. Some excess carbon dioxide may be vented, 20, if leaks in the system do not compensate for the needed carbon dioxide produced in the first IFB, 2.

In an alternate formulation, the combustor gas, 18, may be employed as part of the dryer gas, 3, going to the first IFB, 2. Further, it may be used as the gas for a predryer, if employed.

In a further alternate formulation, the carbon dioxide, 10, needed as the input fluidizing gas for the second IFB can be obtained from bottled sources heated to acceptable inlet conditions; thus, recycle is not employed, and all the gas is vented, 20. In this situation, which is common for small bench-scale operation, water removal, 7, is not employed and compression of the gases, 8, is not needed since the bottle gas is at sufficient pressures to operate the system.

In a further alternate formulation, the product, 13, is not briquetted, but the dry fine coal is stored for further use, shipped via transportation equipment, or utilized directly, such as for a coal-fired power plant.

The equipment is standard except for the inclined fluidized beds, 2, and 11. FIG. 2B shows a typical drawing of an inclined fluidized bed scaled to bench operation. Main characteristics are the lower gas plenum, 25, although shown using the same inlet gas, 26, can use different gas streams along the bed length. A further optional feature could be independently controlled heaters in each inlet gas zone for necessary temperature control. Similarly, the exit gas stream, 27, is collected into one stream, but can be kept separate if desired. The design of the exist gas plenum chamber, 28, FIG. 2B is purposely to keep the pressure drop constant so that horizontal mixing of the gas fluidizing stream is minimized; thus, separate exit gas streams of different compositions are possible to collect. Further this upper plenum area, 28, is by design widened with multiple exit apertures, 32, to reduce the gas velocity and allow a disengaging space for larger entrained particles to remain in the bed region. The inlet coal, 29, enters the bed and moves approximately horizontal in plug flow as a shallow bed to the discharge position, 30, efficiently contacting the gas fluidizing stream. The inclination angle of the bed is measured from the horizontal inlet toward the outlet and is normally expressed as a positive angle in degrees. The shallow bed height can be generally controlled by the discharge baffle height, 31. This shallow bed keeps the concentration of the contacting gas essentially constant and maximizes the temperature and humidity gradients for efficient dryer operation. The plug flow prevents undesirable back-mixing. The velocity of the fluidizing gas is desirably kept at or slightly below that needed for minimum fluidization to reduce solids entrainment and to produce the desirable plug flow operation. The residence time of the material depends upon the slope of the installed inclined fluidized bed, the feed rate, and the velocity of the fluidizing gas. In the drying of coal, these appropriate parameters can be experimentally determined such that the coal product has the desired characteristics. Scaling the size of inclined fluidized beds is straight-forward because of its simple design.

The two inclined fluidized beds are used for convenience, and the residence time of the coal for the system is determined by which bed is most critical. In most designs, the first inclined fluidized bed determines the system residence time for these beds since its operating parameters are more critical. It is possible to use only one inclined fluidized bed if the inlet gas plenum is divided so that cool carbon dioxide can be employed in the final zone which then serves as cool-down region for the processed coal. This is referred to as a zonal inclined fluidizing bed.

The inclined fluidized bed serves as a dryer, reactor, and cooler for the processed coal. The fluidization of the coal particles allows efficient heat and mass transfer between the solid surface and the bulk gas phase. The equipment is operated in a plug-flow regime in order to effectively serve as a dryer. The shallow fluidized bed along with gas cross flow provides maximum humidity gradient for high mass transfer rates and allows minimum fluidization gas velocity to reduce carry-over fines to a minimum.

The reactor zone of the inclined fluidized bed performs the decarboxylation and partial coal pyrolysis reactions where carbon dioxide for recycling is produced while mobilizing coal tars. The residence time is short along with a high heating rate to maximize tar production among the many possible pyrolysis reactions. Next, a rapid cooling of the coal occurs with exposure to lower temperature carbon dioxide, and serves to quench the tar in the coal micropores to prevent future moisture reabsorption and spontaneous combustion.

The inert gas medium during this process is carbon dioxide in order to prevent explosions of ultra-fine coal and spontaneous combustion of dried coal. Further, with this final treatment the coal is left with carbon dioxide in its internal pore space. This helps to prevent moisture from reentering the pores and to exclude oxygen. Because the moisture reabsorption is exothermic, any oxygen present tends to enhance the potential for spontaneous combustion; thus, maintaining a carbon dioxide internal pore gas requirement prevents the conditions needed for spontaneous combustion.

Another advantage to this system is that the stabilized dried product coal is in excellent condition to briquette for easier handling. The operation for forming briquettes, which is simply performed with the warm product from the second inclined fluidized bed, handles the coal fines as well as the normal fine dried coal.

Further, excess fines, removed from the exit gas stream of the first inclined fluidized bed which are not burned in the combustor, can be combined in this step and also formed into briquettes.

In order to dry coal, it is necessary first to investigate its characteristics in order to determine the necessary temperature settings for the inclined fluidized bed operations. In this test of the process two crushed coals were employed: Eagle Butte from Campbell County, Wyoming, and Usibelli from near Healey, Alaska. The feed coals were crushed to minus 590 microns (minus 28 mesh) to produce an average particle diameter of 70 microns for the Eagle Butte coal and 80 microns for the Usibelli coal by wet screen analysis. Since wet coal fines tend to aggregate during dry screening, wet screen analysis was employed to better characterize the fines distribution. FIG. 3 shows the particle size distributions obtained for these coals. Both coals are high-moisture subbituminous coals with "as received" moisture contents of 29% and 22% for the Eagle Butte and Usibelli coals, respectively. Coincidentally, both coals have a heating value of 8470 Btu/lb. Table 1 gives proximate, ultimate, and heating value analyses of the two coals.

Controlled tests of the rate of volatile loss from the coals as they were heated at different heating rates are summarized in FIGS. 4 and 5. The heating rate parameters on these graphs do not significantly affect the results. In all cases the moisture is effectively removed by 200.degree. C.

At higher temperatures gases other than water are emitted as pyrolysis becomes important. Further gas analysis by component indicated that hydrogen gas has maximum rates of evolution just above 400.degree. C. Methane has a broader evolution peak with a maximum near 500.degree. C. Ethene has a maximum rate of evolution near 400.degree. C. but also evolves at a lower rate to 800.degree. C. Carbon dioxide has a broad evolution profile starting near 100.degree. C. and extending to 1000.degree. C. with a maximum near 400.degree. C. Hydrogen is not formed in significant amounts below 500.degree. C. These results are valid for both coals. These conversion studies indicate that for both coals significant pyrolysis conversion starts at near 250.degree. C. with predominately carbon dioxide formed as the gaseous product below 400.degree. C.; however, as the carbon dioxide forms, these pyrolysis reactions will also produce considerable liquid tar.

From the above information the preferred embodiment optimum operating conditions are to keep the bed temperature below 200.degree. C. (392.degree. F.) for only drying. This will evolve moisture without allowing any significant pyrolysis to occur. Then rapid heating to near 350.degree. C. (662.degree. F.) will evolve carbon dioxide and mobilize tar. Quenching to below 250.degree. C. (482.degree. F.) will stop the pyrolysis, and slow the flow of the tar.

A series of cold flow experiments were run to determine the solids residence time relationship to the gas-flow conditions with the slope of the inclined fluidized bed as a parameter. If too low a gas velocity is employed, the material will plug the inclined fluidized bed. The correlation was made using a solid Reynolds number thus:

where N.sub.RE is the solids Reynolds number, D.sub.S is the average diameter of the solid particles, V.sub.G is the fluidizing gas velocity, P.sub.S is the solid particles density, and u.sub.G is the gas viscosity. Units are appropriately picked to make this solids Reynolds number dimensionless. FIGS. 6 and 7 show the results of these cold-flow test correlations. These allow operating conditions to be rapidly obtained for a wide range of process conditions.

With the previous information obtained in Example 1, bench drying runs were made at various slopes of the inclined fluidized bed. The feed rate was approximately ten pounds per hour, controlled by a mechanical feeder, for these small scale tests, and carbon dioxide from the process was not recycled, but instead a separate pressured supply of carbon dioxide was used. Tables 2 and 3 give the results for a series of four hour runs with an occasional twelve hour run utilized. The experimental yield values are presented as percentages of the total feed coal as summarized in Table 1.

The product coal can be safely handled in a number of ways including briquetting, direct bagging, transfer by mechanical or other means to a storage area, or even as feed stock for additional coal processing.

It is evident that the product coal has been dried to a very low moisture content for in all instances the moisture content was below 1.5%. The heating values of the Eagle Butte dried product coals tested in the range of 11,800 to 12,600 Btu/lb. Compared to the feed value of 8,470 Btu/lb, this is a significant enhancement in product value. Similar improvement would be expected for the Usibelli dried coal product from the information shown in Table 3. Additionally, the process stability allowed operation over an extended time period.

To further test the characteristics of the product coal, moisture reabsorption, dust content, and spontaneous heating tests were performed.

The moisture reabsorption test exposed samples of product coal to 95% relative humidity at 30.degree. C. for five days. Typical results were that the new level of equilibrium moisture after reabsorption was approximately half that of the feed coal. The higher the average drying temperature, the lower the new equilibrium moisture value became. In actual instances 95% relative humidity may not always be encountered and lower values better represent more realistic conditions. At 50% relative humidity at 30.degree. C. for five days, the new equilibrium moisture level was only about one-third that of the feed coal, and indicated the success of the pyrolysis tar mobilization and quenching to prevent moisture reabsorption.

Dust tests were performed using opacity meter measurements on product samples of both coals. These test results confirmed that the dried coal products contained very low levels of dust compared to the feed samples.

Spontaneous heating test were run under the standard conditions: 70.degree. C. starting temperature with heating exposed to 160 cc/min oxygen saturated with moisture. Ignition time or a 300.degree. C. coal temperature ended each test. Table 4 gives the results which show that the product coal self-heats quicker by a factor of two to three when compared to the feed. This produces a better product combustion for future use but also makes the final carbon dioxide pore treatment important for storage safety.

A further verification of the process is that the bed temperature shown in Tables 2 and 3, which is an average of several test positions, falls generally in the range of the previously determined expected value of approximately 350.degree. C. (662.degree. F.).

It is noted that although some bed inclination angles would be preferred because of lower fines carry-over, the drying operation can be successfully operated over a wide range of such angles.

A series of larger test were performed on a pilot plant process system that was designed for approximately 100 pounds per hour feed rate of coal. This feed coal was Eagle Butte with the properties given in Example 1. The system was designed for mild coal gasification, and the drying aspects were only the first part of the process; however recycle carbon dioxide was employed. Therefore, two inclined fluidized beds were employed; the first was principally a coal dryer, the second the mild coal gasification unit. The results shown in FIG. 8 represents approximately a 24 hour pilot plant run for the first inclined fluidized bed and gives comparable results to the previous smaller scale experiments. In this instance the inflection point on the bed temperature curve occurred at approximately the midpoint of the bed; thus, indicating the start of significant pyrolysis forming carbon dioxide.

Since the product coal was normally not separately removed but continued directly on to mild coal gasification, the drying bed temperature was not raised to the pyrolysis tar mobilization temperature. Nearly complete moisture removal, however, was easily obtained as shown in FIG. 8. This drying curve well illustrates the characteristic sections associated with free, physically bound, and chemically bound moisture.

The test parameters for the illustrated number 117 run were: coal feed rate, 119 lb/hr; coal residence time, 3 min; recycle gas flow, 92 scfm; fluidizing gas temperature, 540.degree. F.; dryer zone temperatures, .degree.F.: No. 1, 128; No. 2, 151; No. 3, 284.

The recycle carbon dioxide generally tested out at better than 95%, after moisture and fines removal from the dryer exit gas, even after many hours operation of the pilot plant. For this run the dryer produced 5.5% fines, 29.8% moisture, and 0.9% gas, with a basis of 100% for the feed and all percentages are by weight. It is to be noted that the percentage of fines as presented represents the fines produced only in the dryer; for these pilot plant operations the feed coal had had its fines significantly removed before processing.

The product coal can be safely handled in an appropriate manner as indicated in Example 2.

It is noted that this feed coal in Table 1 analyzed at 29.2% moisture; therefore, essentially complete removal was obtained.

The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that other 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 are modifications 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 herein is for the purpose of description and not of limitation.