Patent Grant
5087269
This invention represents a continuation-in-part of Ser. No. 07/332,138, filed Apr. 3, 1989 and now abandoned, entitled Drying Fine Coal in an Inclined Fluidized Bed, the disclosure of which is herein incorporated by reference. 1. Field of Invention The present invention relates to a process using an inclined fluidized bed for drying and stabilizing coal fines in an environmentally acceptable and safe manner to improve heating value and handling characteristics. 2. Background Coal is dried for a variety of reasons, such as to save on transportation costs, to increase the heating value, to increase the net dollar value, to prevent handling problems caused by freezing weather, to improve coal quality particularly when used for coking, briquetting, and producing chemicals, to improve operating efficiency and reduce maintenance of boilers, and to increase coke oven capacity. However, drying of coal causes increased dust formation as the dry coal is more friable. Further, reabsorption of moisture must be considered a potential problem. Dry coal is generally preferred in many coal operations. In World War II the Germans determined that dry coal improved pyrolysis in Lurgi-Spulgas ovens, while the French found that the capacity of coking ovens could be increased by using said coal. Thus, increased tonnages of dry coal were being sold in the United States up to the 1970's when stringent emission standards elevated its cost to an uneconomic level. Another trend in the coal mining industry was its increased mechanization resulting in an increased percentage of coal fines. Because coal fines have a greater relative surface area, they are very susceptible to water absorption. In order to market such fines, drying was necessary. Difficulties in coal drying abound. Besides the stringent emissions standards adding an economic burden, numerous explosions and fires have occurred when low-cost air is employed as the drying medium. Coal dust fines are more susceptible to dust explosions than are larger particles (Hertzberg et al., "Domains of Flammability and Thermal Ignitability for Pulverized Coals and Other Dusts: Particle Size Dependences and Microscopic Residue Analysis," 18th International Symposium on Combustion Proceedings, Pittsburgh, Penn, 1982). Often dry coal is treated with heavy oil before shipping to prevent dust formation and the reabsorption of moisture. Many proposed processes for upgrading coal involve fine grinding and separations in liquids media. The resulting cleaned coal is difficult to handle using conventional techniques because of fine particles and high moisture contents. Additional drying is sometimes employed; however, moisture reabsorption, dust formation with its fire and explosion hazards, and spontaneous heating often result in unstable products. Typical processes include that of Greene, U.S. Pat. No. 4,725,337, which discloses a process for drying and removing impurities from low rank coal and peat by subjecting the coal to a recycled superheated gaseous medium to desorb the moisture from the coal and produce superheated gases. Another is McMahon, U.S. Pat. No. 4,304,571, which discloses a method for increasing the Btu-value of a solid fuel, for instance, coal, by subjecting it to hydrothermal treatment in the presence of an added decarboxylation catalyst, such as soluble salts of vanadium, copper, nickel or other similar metal. Ruyter et al., U.S. Pat. No. 4,285,140, uses a process for dewatering and upgrading low rank coal by heating a pressurized mixture of coal and water at 150.degree.-300.degree. C. After the water is separated, the coal is further heated to 300.degree.-400.degree. C. under pressure to vaporize additional moisture. Ottoson, U.S. Pat. No. 4,495,710, discloses a process for the rapid fluidized bed heating of coal to mobilize tar with subsequent cooling using a recycle stream. Comolli, U.S. Pat. No. 4,249,909, discloses a hot gas, fluidized bed wicking up process where coal hydrocarbons prevent moisture reabsorption. The general problem of coal drying represents removing three types of moisture: free, physically bound, and chemically bound. Free moisture is found in the very large pores and interstitial spaces of coal and maybe removed by mechanical means as it exhibits the normal vapor pressure expected of water at that temperature. Physically bound moisture is more difficult to remove as it is held tightly in small coal capillaries and pores. Because of this, its vapor pressure and specific heat are reduced over that expected of free moisture. Chemically bound moisture is characterized by a bonding between surfaces and water. Monolayer and multilayer bonding are commonly identified. Sometimes a fourth type of moisture is identified which comes from the decomposition of organic compounds. It is really not moisture held in coal but is produced during coal decomposition. Coal drying can be characterized by typical drying curves that exhibit distinct rate regions. Firstly, a transient region occurs as equilibrium conditions are sought while the material heats. This is followed by a largely constant rate portion of drying where the material temperature is relatively constant during the unbound moisture removal, and the drying rate is generally determined from only the particle size and moisture content, be it coal or some other material. The final region is a period of decreasing rate as the material temperature increases and the physically and chemically bound moisture is removed. For this drying regime the particle size, temperature, and residence time are important parameters. Often the drying rate becomes diffusion controlled, and since diffusivity increases with temperature, higher temperatures are employed to continue drying the materials. Because coal needs to be ideally dried to a very low moisture content, appropriate design for operating in this diffusion controlled region is important. During the constant rate period, the heat and mass transfer rates are directly proportional to the driving forces of temperature gradient and humidity gradient respectively; the appropriate proportionality constants, however, are usually experimentally determined. Maintaining near maximum values of said gradients become important when effective drying equipment is designed. Adding oil to dry coal is a common method to prevent moisture reabsorption and autogenous heating. Thus, using 1.5 to 2.0 gallons of No. 6 oil per ton of coal has been shown to be effective for this purpose (Bauer, "Thermal Drying of Western Coal--A Review Paper," Western Regional Conference on Gold, Silver, Uranium, and Coal Proceedings, Rapid City, SD, September 1980). Processes such as oil addition, however, increase operating costs. Willson et al., "Low-Rank Coal Slurries for Gasification," Fuel Processing Technology, 1987, 15: 157-172, describe a variety of drying techniques to upgrade low rank coals. Included were hot water and steam drying under pressure and hot-gas drying using a rotary kiln, Roto-Louvre dryer or a Perry turbulent entrainment dryer. In this study two bituminous coals, Illinois No. 6 and Pittsburgh No. 8, and Wyoming subbituminous coal were employed. When dried directly in hot gases, the dried coal reabsorbs moisture and returns to nearly the original equilibrium moisture level. In contrast, both steam and hot-water drying produced dried coal in which moisture reabsorption was significantly reduced. At these drying temperatures, 270.degree.-330.degree. C., and under pressure, it was concluded that residual tar in the dried coal significantly helped in reducing the moisture reabsorption. However, the high energy requirements will likely rule out this process for drying ultra-fine, modern-mined coal. Ultra-fine coal adds two additional problems to any effective thermal drying processes--fines carryover and explosions. Since indirect heating is inefficient as it requires large heat transfer surfaces with a separate heating medium that escalate capital costs, and leads to high maintenance requirements and low throughput, an inert atmosphere is needed with a low gas velocity. Smith, U.S. Pat. No. 4,170,456, discloses a method for inhibiting the spontaneous combustion of coal char by treating with carbon dioxide to deactivate the char surface to oxygen. The temperature ranged used was 10.degree.-149.degree. C. Since coal char and dried coal are similar, this carbon dioxide treatment would likely reduce the pyrophoric nature of dried coal. After World War II fluidized bed dryers were adapted to coal drying; however, critical control of both coal and gas flow was required in order to avoid fires and explosions. McNally Flowdryer, Dorr-Oliver Fluo-Solids Dryer, Link-Belt Fluid Flow Dryer, and Heyl and Patterson fluidized bed dryers are all well known. Typically fluidized bed dryers have a coal-fired zone, using stokers or pulverized coal pneumatically injected, where fluidizing air is heated and its oxygen content reduced. Another zone acts as the dryer where the pressure drop across the gas distributor is large relative to the pressure drop across the bed in order to assure good dryer gas distribution. In some installations, gas from the coal is recycled to further reduce the oxygen concentration. Coal distribution is controlled by a feeder-spreader device, such as a roll feeder, multiple screw feeders, or grate. These fluidized bed dryers are potentially hazardous when air or mixtures of air and recycled gas are employed. The oxygen concentration is critical to avoid explosive conditions, and special safety equipment, such as sprinkler systems, blowout doors, and automatic fail-safe shutdown devices, is common. Additionally, the moisture content of the dry coal is often held to 5-10%, or 0.5-1.0% surface water, to make the drying operation less hazardous and to avoid excessive formation of dust. After removal of the surface water, the rising bed temperature becomes the control parameter to keep it safely below auto-ignition conditions. Equipment to control particulate emissions from fluidized beds include combinations of cyclones, electrostatic precipitators, bag filters, and wet scrubbers. Cyclones are ineffective with particle sizes below five microns, so their operation is usually restricted to extraction of large particle dust loading prior to removal of fine dust particles by subsequent equipment. However, cyclones employed at the gas stream dew point or with water-spraying, can be nearly as effective as wet scrubbers. Electrostatic precipitators when successfully used must be kept free of condensation, and in addition, are subject to malfunctions and frequent maintenance. Flash dryers use entrained fluidized beds to dry particles under residence times of one second or less. This short residence time gives a high capacity with a low inventory of coal, and makes them less hazardous than conventional fluidized bed dryers. However, particle fines entrainment due to the required high gas velocity is a problem, and requires additional separation equipment. Conventional dryers, such as Multi-Louvre and Cascade, use many flights and vibrating shelves to control coal flow in the dryer. With these, maintenance is a major cost when compared to fluidized bed dryers. Roto-Louvre is a variation on a rotary drum dryer. Modern development is exploring a number of technologies to improve coal drying processes. Hot water dewatering and decarboxylation both employ a high pressure treating reactor for altering coal micropore structures to prevent moisture reabsorption, but then additional drying becomes necessary. Vapor recompression principles can reduce energy requirements by compressing water vapor to a higher pressure so that recycle heating can be employed. In essence much of the heat of vaporization of the water removed from the coal can be recovered. Pilot plant testing has been employed but high capital and maintenance costs are a definite drawback. The multistage fluidized bed process achieves good thermal efficiency by recompressing water vapor from the first stage and using it to heat and fluidize the second stage. A portion of the first-stage water vapor is recycled to fluidize the bed while steam tubes provide heating. Solar drying processes use a slurry of coal that is pumped to shallow ponds. The coal then is stockpiled for further air drying. The slurry requires large amounts of water and ponds require large amounts of land. The process is effective only in dry climates. The Fleissner process, developed in 1927, dries coal by heating with high pressure steam. High steam temperatures change the coal structure and release water and carbon dioxide leaving a hydrophobic coal remaining for final drying. However, high steam pressures require elevated capital costs. The Koppelman process heats coal some 400.degree. C. above evaporative drying conditions so that partial pyrolysis occurs releasing oil; this process requires, however, extensive water cleanup because of the pyrolysis. The product coal can be almost completely dried, but hot water is typically used to cool the coal so approximately 5% water is present in the final product. This process produces enhanced heating value coal, so potentially longer transportation costs can be economically tolerated. Unfortunately, extruders are required because of the high pressure and this is a severe economic disadvantage. Existing coal dryers can be grouped into three basic types: fluidized bed, entrained bed or flash, and shallow moving bed. The later can be further subdivided into Multi-Louvre, vertical tray and Cascade, continuous carriers, and drum type. McNally Flowdryer, Link-Belt Fluid-Flo dryer, Heyl, Patterson fluid bed dryer, and Dorr-Oliver Fluo-Solids dryer all use fluidized beds with hot air or hot gases. Flash dryers, for instance Combustion Engineering's type, use entrained bed drying with hot gas. Dryers using a shallow bed are Link-Belt Multi-Louvre, McNally fine coal Cascade, McNally Vissac, and Link-Belt Roto-Louvre. The present invention has several objectives; they include overcoming the deficiencies of the aforementioned prior art, providing an improved process for drying coal including coal fines, providing an improved process for upgrading coal, providing coal which is not subject to spontaneous combustion, and providing dried coal which does not readily reabsorb moisture. Coal is processed in an inclined fluidized bed dryer with staged or zonal temperature control. The inert fluidizing gas is largely carbon dioxide in later treatment stages, but may be contain other combustion products is earlier stages. The carbon dioxide, which is ideally recycled, is produced by partial decarboxylation of the coal. The coal is heated sufficiently to mobilize coal tar by pyrolysis, which seals micropores upon quenching with carbon dioxide to enhance stabilization.
This invention was made with Government support under DE-AC21-87MC24268 awarded by the Department of Energy. The Government has certain rights in this invention.
| Number | Name | Date | Kind |
|---|---|---|---|
| 708604 | Welch | Sep 1902 | |
| 3755912 | Hamada et al. | Sep 1973 | |
| 4031354 | D'Souza | Jun 1977 | |
| 4249909 | Comolli | Feb 1981 | |
| 4495710 | Ottoson | Jan 1985 | |
| 4725337 | Greene | Feb 1988 |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 332138 | Apr 1989 |
