Method of stabilising coal

Abstract: A process for reducing the spontaneous combustion tendencies of thermally beneficiated low rank coals employing heat, air or an oxygen containing gas followed by an optional moisture addition. Specific reaction conditions are supplied along with knowledge of equipment types --a fluidized bed reactor (20) or a vertical tower reactor (30)-- that may be employed on a commercial scale to complete the process.

Description: The Government of the United States of America has certain rights in this invention pursuant to Contract No. DE-FCC22-90PC89664 awarded by the United States Department of Energy. FIELD OF THE INVENTION The present invention is directed to the processing of coal; and more specifically preventing the spontaneous combustion of thermally beneficiated low rank coal. BACKGROUND OF THE INVENTION There are continuing efforts in the coal industry to develop technologies resulting in fuels derived from coal which, as compared to raw coal, burn cleaner, have higher heat (BTU) content, and are more cost-efficient to transport. In coal industry parlance, such technologies are referred to as "clean coal" technologies. Due to the plentiful reserves of low sulfur low rank coals, one area of development related to clean coal technologies is "thermally beneficiated low rank coal". This term means coal which has been processed at elevated temperatures to generate a product with a reduced moisture content and a higher heat value per unit of weight. Such thermally beneficiated low rank coals have shown a tendency to spontaneously combust. Although raw coal also has a tendency to spontaneously combust, this tendency in raw coal is much less pronounced than that exhibited by thermally beneficiated low rank coals. This problem impedes the commercialization of thermally beneficiated low rank coals, because it does not allow them to be stored, shipped and handled using the same techniques used with raw coal. The present invention addresses this problem and provides a method to stabilize commercial scale quantities of thermally beneficiated low rank coals against spontaneous combustion to a degree whereby they can be handled in a manner similar to raw coal. The term stability used herein is defined as the resistance to spontaneous combustion and the term stabilization is defined as processes which produce the resistance to spontaneous combustion. It is to be understood that the term "coal", as used herein, shall include but not be limited to, peat, lignite, sub-bituminous and bituminous ranked coals. However, the beneficiated coal primarily contemplated by this invention is thermally beneficiated sub-bituminous and lignite coal. Coal has a tendency to spontaneously heat and combust after it is mined. This tendency is exhibited when the coal is stored in large piles; in rail cars, storage silos, storage bunkers or in like storage facilities. Spontaneous heating and combustion of coal is the result of a combination of heat released during surface oxidation and heat released by hydration, i.e. the absorption of moisture. Both the oxygen and moisture are supplied by atmospheric air. If the coal is stored in a manner in which heat from oxidation and hydration is generated faster than it can be dissipated, the temperature of the stored coal increases until the combustion temperature of the coal is reached and combustion occurs. The natural insulating qualities of the stored coal facilitates the retention of heat and its attendant spontaneous combustion. The coal industry has adapted itself to handle and use raw coal within the general constraints of the coal's natural tendency to spontaneously heat and combust. One of the methods for preventing spontaneous combustion is to move or use the coal before it is allowed to sit in large storage for more than a week. For raw coals, this short storage time does not allow the temperature to the point where spontaneous combustion occurs. The spontaneous combustion problem is exacerbated in the case of thermally beneficiated low rank coals. Some of the thermally beneficiated low rank coals have had a substantial portion of their internal water content removed; without the heat dissipation capacity supplied by the water in the parent coal, these coals have a tendency to spontaneously combust that is greater than that of raw coal. Many of the thermally beneficiated low rank coals can spontaneously combust within one or two days of being placed in a large storage pile. To remove this barrier to the commercialization of the new thermally beneficiated low rank coals, they must be stabilized to inhibit spontaneous combustion. Ideally, they should be stabilized to the point where they have the same stability as raw coal. This will permit the new thermally beneficiated low rank coals to be used with the same handling systems and with the same handling procedures as raw coal, and will thereby greatly increase the practical value of these thermally beneficiated fuels. The inventors recognized and faced the issue of spontaneous combustion in connection with operating a demonstration facility built to produce a thermally beneficiated low rank coal, SynCoal®. U.S. Patent No. 4,810,258, issued March 7, 1989, to Greene, describes the SynCoal® product. U.S. Patent No. 4,725,337, issued February 16, 1988, also to Greene, describes the process for making SynCoal®. This technology is referred to as the Advanced Coal Conversion Process (ACCP). The ACCP technology was first used to produce SynCoal® in bench tests, and in a pilot plant operated in 1986, prior to the issuance of U.S. Patent Nos. 4,725,337 and 4,810,258, described above. To further develop the ACCP technology, a 300,000 ton per year demonstration facility was constructed in 1990-92 at Western Energy Company's Rosebud Coal Mine near Colstrip, Montana. The United States Department of Energy supported the ACCP Project through its Clean Coal Technology Program. One of the ultimate objectives of the Clean Coal Program is to foster the commercialization of projects that provide fuels with characteristics that allow them to replace imported, higher cost fuels, thereby reducing dependence on imported fuels. The problem of the spontaneous combustion tendency of SynCoal®, was recognized during initial operations at the demonstration facility. Spontaneous combustion occurred within days of placing SynCoal® in air permeable storage silos or in open piles. By repeating ACCP pilot tests in 1992, it was shown that the 1986 pilot plant produced SynCoal® which was equal in reactivity to that of the demonstration facility. The spontaneous heating characteristic was not identified at the pilot plant stage because the pilot plant generated SynCoal® in smaller quantities and at a lower rate than the demonstration facility. This low rate of production allowed enough time for the beneficiated coal to stabilize passively prior to it's being covered by subsequent layers of SynCoal®. As an initial remedy to this problem of spontaneous combustion, a technique of "pile management", i.e. periodic handling and moving of the SynCoal® stored in piles or bins was developed. Based on actual observations, SynCoal® spread at depths of less than 45.7 cm (18 inches) reached a peak temperature within approximately 2 days. High heat production was sustained for approximately 10 days, followed by a period of steady decline in pile temperatures. After being piled and held for over 3 months, spontaneous combustion did not occur, and apparently, a stable coal product was achieved. These results indicated that stability can be achieved through pile management, allowing oxidation and rehydration to occur along with sufficient heat dissipation. By expanding on the concept of pile management, the inventors proceeded to develop a stabilization process from a bench scale to pilot scale. The inventors piloted a 453 kg (1,000 pounds) per hour process that produced air stabilized SynCoal® with about seven day stability. It remained a thermally beneficiated coal and retained its higher heat value per unit of weight. The present invention stabilizes coal by using hot air or air with a reduced oxygen concentration to oxidize reactive sites on the surface of the coal. The oxidation step is followed by the addition of moisture to the coal product to bring the coal to a stable moisture level. Once the reactive sites of the coal have been oxidized and the coal adequately hydrated, the coal is stabilized and spontaneous combustion retarded. The adjustment of final product moisture content may be omitted if a lower moisture coal is desired and a less stable coal is acceptable. The subject invention does not claim the novelty of oxidizing thermally beneficiated coals followed by rehydration. This invention teaches industrial scale methods of completing the stabilization including knowledge of maximum processing temperatures that may be utilized that minimizes the risk of process fires and the duration of processing necessary to obtain a stability level that allows handling and transporting the product using conventional means. Fortunately, 100% stability is not required, only stability that will allow handling in a manner similar to raw coals, which allows for up to 7 days before use or rehandling. In general this 7 days before use is the time-frame meant to be comparable to raw coal used in commercial application. Economical commercial application of oxidative stabilization requires the smallest possible reaction chamber in order to minimize construction and operating costs. If the process can be completed in less time, the processing equipment can be scaled down resulting in reduced equipment costs and reduced operating costs. PRIOR ART The prior art teaches ways to thermally beneficiate and stabilize coal, but the prior art fails to teach or suggest enough information to apply the stabilization techniques on a commercial scale. Most notable is a lack of knowledge of the necessary treatment times (residence times) that will result in an adequate stability and a lack of knowledge of the optimum reactor styles for completing the oxidation step. In addition, much of the prior art was developed on a small laboratory scale; and due to complications that are not present on a small scale, actually teach processing conditions that are unsafe on a large scale. Numerous prior patents claim treatment temperatures over 149°C (300°F), which, if applied in the presence of high (greater than 18%) concentrations of oxygen, will inevitably result in process fires. The prior art discusses a process for thermally beneficiating coal which process is improved upon by the present invention. U.S. Patent Nos. 4,725,337 and 4,810,258, noted above, describe the SynCoal® process and the SynCoal® product. The SynCoal® process removes a substantial portion of naturally contained water and impurities from low rank coal, while keeping much of its volatile combustible content. The resulting improved product, SynCoal®, not found in nature, has a higher BTU content per unit of weight than raw coal feedstock. Prior art related to processes or treatments inhibiting spontaneous combustion potential of coals or char includes U.S. Patent No. 3,723,079, issued March 27, 1973 to Seitzer . The patent describes a process for stabilizing dried coal by treating it with oxygen, and then rehydrating it. The Seitzer patent: (1) teaches processing temperatures well above those in the subject patent; (2) does not supply necessary residence times; (3) does not teach knowledge of reactor type; (4) teaches different rehydration ranges; and (5) does not teach the option of omitting rehydration. U.S. Patent No. 4,213,752, issued July 22, 1980 also to Seltzer , describes a method of inhibiting spontaneous combustion in conjunction with a drying step that supplies its own heat source by partial combustion of the coal being processed using a drying gas stream containing 5-20% oxygen. This Seitzer patent: (1) teaches processing temperatures in a range well above those in the subject patent; (2) does not teach necessary processing times; (3) does not teach rehydration ranges; and (4) utilizes a significantly different technology than the subject patent and other prior art. U.S. Patent No. 3,896,557, issued July 29, 1975 also to Seitzer , describes a method of inhibiting spontaneous combustion in conjunction with a drying step using a drying gas stream with 7-9% oxygen. This Seitzer patent: (1) does not teach processing temperatures or times; (2) uses a much lower oxygen concentration; (3) leaves a significant amount of moisture in the coal; and (4) does not teach rehydration ranges. U.S. Patent No. 4,192,650, issued March 11, 1980 also to Seitzer , describes a method of inhibiting spontaneous combustion utilizing steam. This Seitzer patent does not teach oxidation treatment and only rehydrates using steam. U.S. Patent No. 4,170,456, issued October 9, 1979 to Smith , describes a method of inhibiting the spontaneous combustion of coal char by air treatment followed by carbon dioxide treatment. The Smith patent: (1) teaches processing temperatures in a range well above those in the subject patent; (2) does not supply necessary residence times; (3) does not teach knowledge of reactor type; (4) does not teach rehydration ranges; and (5) does not teach a treatment for stabilization without carbon dioxide. U.S. Patent No. 4,396,394, issued August 2, 1983, to Li et al , describes the method of inhibiting spontaneous ignition of dried coal by cooling it, or by partially oxidizing it prior to cooling followed by the application of a deactivating fluid. The Li et al patent: (1) does not teach any knowledge of processing temperatures or times; (2) does not teach knowledge of reactor type; (3) does not teach rehydrating ranges; and (4) teaches the application of a deactivating fluid. U.S. Patent No. 4,645,513, issued February 24, 1987, to Kubota et al , also teaches a stabilization method. The Kubota et al patent: (1) teaches processing temperatures in a range well above those in the subject patent; (2) does not supply necessary residence times; (3) does not teach knowledge of reactor types; and (4) does not teach rehydration ranges. U.S. Patent No. 4,402,706, issued September 6, 1983 to Wunderlich , describes a method of inhibiting the spontaneous combustion of coal with oxygen treatment in a reactor. The Wunderlich patent: (1) uses a partially dried coal and completes the drying during stabilization; (2) teaches processing temperatures in a range above those in the subject patent; (3) does not supply necessary residence times; (4) teaches a reactor type that not be effective on a full range of particle sizes and will experience process fires if operated in the claimed temperature range; and (5) does not teach rehydration ranges. U.S. Patent No. 3,918,929, issued November 11, 1975 to Schmalfeld et al , describes a method of inhibiting the spontaneous combustion of briquetted coal by oxygen treatment in a reactor. The Schmalfeld et al patent: (1) teaches processing temperatures in a range much higher than the subject patent; (2) does not supply necessary residence times; (3) does teach knowledge of reactor type but the subject patent teaches that the Schmalfeld et al style of reactor will experience process fires if operated in the claimed temperature range; and (4) does not teach rehydration ranges. There also exists a wealth of prior art dating back about 60 years that teaches the application of deactivating fluids. The subject patent does not claim the need for a deactivating fluid. SUMMARY OF THE INVENTION The primary objective of the present invention is to provide a method for reducing the spontaneous combustion tendency of thermally beneficiated low rank coals to levels comparable to natural raw coal. It is a further objective of the present invention to provide thermally beneficiated coals with a reduced tendency for spontaneous combustion. It is a further objective of this invention to provide optimum processing conditions that will allow economically feasible application of a stabilization process on a commercial scale. It is a further objective of this invention to identify processing equipment and process conditions that may be economically applied to commercial quantities of coal. At least 90718 kg (100 tons) per day of coal is a commercial quantity; and more likely commercial quantities are 907185 kg-9071850 kg (1,000 to 10,000 tons) per day. One of the keys to applying oxidative stabilization is to recognize that the stabilization cannot be completed in short periods of time. The rate of oxidation can be increased by increasing the processing temperature, but care must be taken when increasing the processing temperature to avoid the condition where the coal simply ignites causing process fires. The maximum possible processing temperature is dependent on the quality of the heat rejecting inherent in the equipment used to conduct the reaction and the oxygen content in the gas used to supply oxygen to the product. Operation with a reduced oxygen gas stream allows higher processing temperatures, but the lower oxygen content increases the required residence time. Processing with a gas oxygen content approaching that of ambient air will be the most economical option. Once the maximum processing temperatures are established, the corresponding minimum residence time for a desired product stability is naturally fixed along with the necessary reactor size for any given volume of coal flow. According to the teaching of the present invention, a method for stabilization and prevention of the spontaneous combustion of commercial quantities of coal having flow rates of at least 100 tons a day, is characterized in separating the coal into a coarse coal stream and a five coal stream and in subjecting the coal to a direct contact with an oxygen containing gaseous medium in a temperature controlled reactor with the coal temperature in a range between 140°F and 250°F (60°C and 121°C) for a specific controlled reaction time in a range between 30 minutes and 2 hours. The resulting coal will have oxidized active sites and, therefore, will be stable for a time period comparable to that of a raw coal used in commercial applications. In one aspect of the present invention designed for the coal having a particle size under 0.75 inches (1.9 cm) in diameter, the reactor is a fluidized bed reactor and the oxygen containing gaseous medium is an air heated to a temperature in a range between 200°F and 350°F (93°C and 177°C). The coal temperature is maintained in a range between 200°F and 250°F (93°C and 121°C) for at least 30 minutes. Preferably, coal temperature is 230°F (110°C) and the reaction time is approximately 45 minutes. In another aspect of the present invention designed for the coal having a particle size greater than 0.065 inches (0.165 cm) (10 mesh) in diameter, the reactor is a vertical tower reactor and the oxygen containing gaseous medium is an air heated to a temperature in a range between 140°F and 250°F (60°C and 121°C). Preferably, the air enters the reactor at its bottom, while the coal enters the reactor at its top and moves down the reactor, while the coal enters the reactor at its top and moves down the reactor to exit at its bottom. The temperature of the coal in the reactor is maintained in a range between 140°F and 190°F (60°C and 88°C) (with an average of about 170°F (77°C)), with the reaction time of at least 60 minutes. Preferably, for the coal particles size greater than 0.065 inches (0.165cm), the oxidation temperature is about 150°F (66°C), the peak coal temperature is about 180°F (82°C), and the reaction time is approximately 90 minutes. The coal to be stabilized has pores and reactive sites. When it is successively oxygenated at elevated temperatures and is cooled thereafter repeatedly, the alternate heating and cooling facilitates in the diffusing of oxygen into the pores of the coal, and allows for more complete oxygenation of the coal. After cooling, the oxidized coal can be rehydrated in order to bring the coal to its natural moisture level (between 5 and 15%) It is important that the rehydration step may be omitted when a very dry coal is desired with a stability lower than the optimum one. For better results, the unstabilized coal is sized, in order to separate the coal into a coarse coal stream and a fine coal stream. The split is made somewhere between 0.065 inches (0.165 cm) and 0.75 inches (1.9cm)by crushing larger particles or by agglomerating finer particles. The sized coal streams are then directed to respective reactors specifically designed for the sized coal. The fine coal (less than 0.75 inches (1.9cm) in diameter) is stabilized in a fluidized bed, while the coarse coal (larger than 0.065 inches (0.165cm) in diameter) is stabilized in a vertical tower reactor. About 1.0-1.5% oxygen by weight will be absorbed into the coal, and for each pound of oxygen absorbed, between 2500 and 5600 BTUs (630 - 1411 kilogram-calorie) will be released. In a fluidized bed reactor, the quantity of heat released is relatively manageable because of good mixing and contact; and the quantity of gas required to fluidize the bed provides a good heat dissipation. In a moving packed bed or tower style reactor with the product slowly flowing down a vertical shaft and the gas stream flowing up, the heat generated is not efficiently rejected and can act as a preheater for incoming coal. Because of the preheating effect, the maximum operating temperature in a tower type reactor is significantly lower than the maximum operating temperature for a fluidized bed reactor. These and other objects of the present invention will become apparent from a reading of the following specification taken in conjunction with the enclosed drawings. It is to be understood that the term "air", as used herein, shall include gas streams with slightly reduced oxygen concentrations. Some applications of the invention may use a fraction of the oxygen in an air stream to burn a fuel in order to heat the gas stream or may utilize a recycle stream for efficient use of heat. Either option will result in a slightly reduced oxygen concentration in the inlet gas stream. In no case would an oxygen concentration less than 17% be desirable because the resulting reduced reaction rates would increase the necessary reactor size. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a flow chart showing the air oxidation stabilization process as incorporated into the Rosebud SynCoal® process. Figure 2 shows a schematic view of the horizontal fluidized bed used in the invention to oxidize the thermally beneficiated coal. Figure 3 is a schematic view of the vertical tower used in the invention to oxidize the thermally beneficiated low rank coal. PREFERRED EMBODIMENTS The following embodiments would be typical of a stabilization process step retrofitted into the ACCP demonstration facility described in the Background of the Invention set forth above. Figure 1 provides a flow chart describing the addition of the stabilization process into the SynCoal® ACCP demonstration facility. In the original configuration, SynCoal® drying/conversion 10 and cooling 11 equipment dries, converts, and cools the coal, and the product is then moved via path (A) to the cleaning equipment 12, prior to storage and loadout. In the improved process with the invention's stabilization process step, the product goes from the drying 10 and cooling 11 equipment to the stabilization equipment 13 via path (B). The stabilized product may be moved from the stabilization equipment 13 to the rehydration equipment 14 through alternate path (D). The alternate path would provide stabilized and rehydrated product to the SynCoal® cleaning equipment by alternate path (E) prior to loadout and storage. In the stabilization process, the coal is sized using either a screening step or a crusher. The sized coal is fed to one of two styles of reactors described below. The oxidized coal is fed to a rehydrator via path D and finally to the cleaning system via path E. Optionally, the rehydration step may be bypassed via path C if a drier but less stable product is desired. Within the oxidation step, the coal is screened and then directed to one of two reactor designs. The fine coal is best handled in a fluid bed reactor while the coarse coal fraction is best handled in a moving packed bed or tower reactor. The fluidized bed reactor 20 (Figure 2) works best with coal sized under 0.75 inches (1.9cm) in diameter due to the ease of fluidizing the smaller particles. The smaller the particles, the lower the fluidization velocity and hence the lower the horsepower requirement to move the hot gas. The tower reactor 30 (Figure 3) works most efficiently with coal sized larger than 0.065 inches (0.165cm) (10 mesh) in diameter. Hot gas contact with the coal is inhibited unless the finest particles are excluded, because the material has a tendency to pack and prevent even gas distribution. The size at which the separation is made can be selected based on construction cost and operating efficiency. The fluidized bed reactor 20 (Figure 2) uses air heated at a temperature of about 200° - 300°F (93°C - 149°C), and oxidizes the coal at a temperature of 200 - 250°F (93°C - 120°C) for 30 minutes to one hour. The hot air enters the intake 21 and passes through a plurality of ports 22 to the fluidized bed 23. The heated air rises up through the bed 23 and exits through the gas discharge duct 24. The unstabilized coal enters through the inlet chute 26 and falls into the bed 23. The oxidized product exits the bed through the valve/chute combination 27/28, when the valve 28 is opened. It is to be understood that the term "air", as used herein, includes gas streams with slightly reduced oxygen concentrations. Some applications of the invention may use a fraction of the oxygen in an air stream to burn a fuel in order to heat the gas stream or may utilize a recycle stream for efficient use of heat. Either option will result in a slightly reduced oxygen concentration in the inlet gas stream. In no case would an oxygen concentration less than 17% be desirable because the resulting reduced reaction rates would increase the necessary reactor size. The size of the processing equipment is always dependent on the flow rate of product and the required residence time. In the case of the ACCP demonstration facility, the fluidized bed used in this invention is sized to process about 38 tons (43,466 kg)/hour of fine fraction from the screening process. The fluid bed is about 47 feet (14.3m) long, 7 feet (2.1m) wide and holds a bed of coal about 4 feet (1.2m) deep. In the fluidized bed reactor, the oxidation can take place in a period of 30 minutes, at the maximum possible processing temperature of 250°F (121°C). To allow a margin of error in operations so that process fires are minimized, a processing temperature of 230°F (110°C) can be applied for approximately 45 minutes. The tower reactor (Fig. 3) uses air heated at a temperature of 35 about 140° - 250°F (60°C - 121°C), and oxidizes the coal at a temperature of 140 - 190°F (60°C - 88°C) with and average of 170°F (77°C) for one to two hours. The hot air enters the intakes 36 and passes through a plurality of ports 37 into the tower 33. The heated air rises up through the tower and disengages the coal in the freeboard section 38 then exits through the gas discharge duct 39. The coarse unstabilized coal 31 enters through the inlet chute 32 and falls into the tower 33. The oxidized product exits the tower through the valve/chute combination 34/35, when the valve 35 is opened. The size of the processing equipment is always dependent on the flow rate of product and the required residence time. In the case of the ACCP demonstration facility, the tower used in this invention is sized to process about 38 tons/hour of coarse fraction from the screening process. The tower is about 9 feet in diameter and 60 feet high. About 10 feet of the tower height is freeboard. In the tower reactor the oxidation can take place in a period of one hour, with a peak processing temperature of 190°F (88°C). To allow a margin of error in operations so that process fires are minimized, an average processing temperature of 150°F (66°C) with a peak coal temperature of 180°F (82°C) can be applied for approximately 90 minutes. The final stages of the oxidation reaction is diffusion limited. It is believed that within the product's pores, a high nitrogen concentration occurs due to oxygen depletion. The overall oxidation reaction then depends on oxygen in the air, around the product particle, diffusing into the pores. A method of combating the diffusion limited process is to alternately heat, then cool, and then reheat the product. During the alternate heating and cooling cycles, a further completion of the oxidation reaction is accomplished. The cooling stage forces fresh air to be drawn into the product pores as the interstitial gases contract. As an example, hot gas is provided for 20 minutes, followed by cold gas for 5 minutes, followed by hot gas for 17 minutes, followed by a final cool down gas for 3 minutes. A total of 45 minutes. To obtain the most stable product, the moisture level of the treated coal must be adjusted after the oxidation reaction is completed. The improved treatment method entailing alternate heating, cooling and reheating of the coal to aid in the defusing of oxygen into the pores of the coal is applied by means of alternating zones in a long fluidized bed and by recycling a fraction of the tower discharge coal. SynCoal® from the demonstration facility has a natural rehydrated moisture level of about 7%. The rehydration step is completed on a slow moving conveyor belt. Any thermally beneficiated coal will reabsorb some moisture upon exposure to air. If the heat of oxidation and heat of rehydration are rejected, the product moisture level will increase to some equilibrium state. The extent of rehydration and the length of time required to complete the rehydration is dependent on the nature of the raw coal, the type and severity of the thermal beneficiation process, the ambient temperature, and the ambient air humidity. This level of rehydration can be determined for any thermally beneficiated coal by placing a small representative portion of the product in contact with normal ambient air for a period of at least one month. The sample should be small enough that any heat of oxidation and rehydration will be rejected to the air; a sample size of about 100 lbs. (45.4kg) would suffice. The product should be shaded from the sun to avoid radiative drying. The sample will air oxidize and rehydrate. Once an equilibrium level is reached, the coal's moisture will vary with the ambient air humidity. Preferably, a sample for the rehydrated moisture level measurement should be taken from the test coal during a period of high humidity. The resultant moisture level would be the target moisture level in the process equipment; it will likely fall between 5 and 15%. The moisture addition may be conducted in commercially available mixers or on a slow moving conveyor belt. A minimum exposure time of 5 minutes is required to allow the moisture to be absorbed by the coal. Longer exposure times and multiple water addition points increases the ability to precisely adjust the moisture level especially when excess moisture is added to allow evaporative cooling. When moisture is added to the coal, heat will be released and the bulk coal temperature will increase. This heat must be cooled to the minimum possible temperature because the residual oxidation rate is dependent on the final product temperature. The most effective method of cooling is to pass ambient air through the product in a fluidized or semi-fluidized state. The product's temperature will, within minutes, drop to within 15°F (8°C)of the air temperature. At the demonstration facility, excess moisture beyond the target rehydrated moisture level is added. The product is then sent to a pneumatic cleaning system where the air stream will remove mineral impurities. The excess moisture is evaporated and the cooling effect of the evaporation acts to remove the heat of hydration and any residual heat from the oxidation reaction. The adjustment of final product moisture content may be omitted if a lower moisture coal is desired and a less stable coal is acceptable. EMPIRICAL RESULTS FROM AIR STABILIZATION TEST TRIALS Pilot tests using two types of stabilization equipment were conducted at the SynCoal® demonstration facility. In a horizontal fluidized bed, manufactured by Heyl & Patterson Inc., air at about 350 degrees F. (176°C) was used to oxidize SynCoal® at about 230 degrees F (110°C). The volumetric percent oxygen concentration was 20%. The pilot fluidized bed processed between 400 and 1,000 pounds (181 and 454 kg) per hour. This was about a 1/100 scale test compared with the commercial scale. The hot air came into contact with the SynCoal® for about 45 minutes in the fluidized bed prior to cleaning. In a vertical tower, designed and manufactured by the inventors at the ACCP demonstration facility, 140 to 250 degrees F. (60°C to 121°C) air was used to oxidize the SynCoal® at an average temperature of about 150 degrees F (66°C). The coal entered the tower at about 120 degrees F (49°F), the temperature then increased to about 180 degrees F (82°C) in the middle of the tower, and then exited the tower at about 140 degrees F (60°C). The pilot tower reactor processed between 400 and 1,000 pounds (181 and 454 kg) per hour which was also about 1/100 scale compared to a commercial scale. Charts 1 and 2 show the results of test batches made with pilot scale stabilization reactors. These test results show that SynCoal® produced with the present invention has a stability of about seven days compared to a normal stability of about 1 day. The improved stability is competitive with naturally occurring low rank coal, and is adequate for the commercialization of stabilized SynCoal®. FURTHER EMBODIMENTS The embodiments illustrated and discussed in this specification are intended only to teach those skilled in the art the best way known to the inventor to make and use the invention. Nothing in the specification should be considered as limiting the scope of the present invention. Changes can be made by those skilled in the art to produce equivalent systems without departing from the invention. The present invention should only be limited by the following claims and their legal equivalents. For example, the method of the invention can be used on thermally beneficiated low rank coals other than SynCoal®. Beneficiated coals and processed solid carbon fuels, and beneficiated coal in the briquetted or pelletized form other than SynCoal®, can be stabilized using the present invention process. Also, waste coals, such a culm and gob, can be beneficiated by the SynCoal® process, and stabilized by the present invention process. Note that the present invention's process steps can be executed as part of a larger beneficiation process, or in a different sequence within the process than as indicated in Figure 1 herein. The steps of the present invention can also be combined with other process steps, instead of being executed as separate process steps. For example, the air stabilization step may be combined with the drying step, by using some natural air in the drying step, rather than using only a completely inert atmosphere in the drying step. Alternatively, the present invention may partially rehydrate the product before oxidation, and then rehydrate the product further after oxidation. Tower Style Reactor Pilot Test Results Test Pile Number Reactor Residence Time Average Processing Temperature Duration Before Spontaneous Combustion Stability Improvement Over Baseline Comment 9342 Control 0 na <1 day - Untreated 9344a 360 min 144 F (62°C) 7 Days 560% 9344h 120 min 147 F (64°C) Did not SC At least 600% Note 9344i 120 min 151 F (66°C) 7 days 470% 9344k 120 min 157 F (69°C) Did not SC At least 600% Note 9344q 90 min 147 F (64°C) Did not SC At least 600% Note Note: Well stabilized-small test piles sometimes did not spontaneously combust; instead they would become permanently stabilized. This was not an indication of completely stabilized coal; rather it indicated very good stability in combination with a small test pile. Larger test piles would have combusted. Fluid Bed Style Reactor Pilot Test Results Test Pile Number Reactor Residence Time Average Processing Temperature Duration Before Spontaneous Combustion Stability Improvement Over Baseline Comment 9342s 30 min 240 F(116°C) Did not SC At least 600% Note 9342aa 45 min 220 F(104°C) Did not SC At least 600% Note 9342ad 45 min 220 F(104°C) Did not SC At least 600% Note 9344ae 70 min 240 F(116°C) Did not SC At least 600% Note Note: Well stabilized-small test piles sometimes did not spontaneously combust; instead they would become permanently stabilized. This was not an indication of completely stabilized coal; rather it indicated very good stability in combination with a small test pile. Larger test piles would have combusted.

Claims: A method for stabilising and preventing the spontaneous combustion of commercial quantities of particulate coal subject to spontaneous combustion, at a flow rate of at least 100 tons per day , comprising subjecting the coal to a direct contact with a gaseous medium having an oxygen concentration not less than 17% in either a temperature controlled fluidized bed reactor (20) or a vertical tower reactor (30), with the coal temperature in a range between 60° and 121 °C for a specific controlled reaction time in a range between 30 minutes and 2 hours, yielding coal with oxidised active sites and being stable for a time period comparable to that of a raw coal used in commercial applications, characterized in that said method comprises separating said coal into a coarse coal stream and a fine coal stream, wherein the separating takes place between 1.9 cm (0.75 inches) and 0,165 cm (0.065 inches), and stabilising said fine coal stream in said fluidized bed reactor (20) and said coarse coal stream in said vertical tower reactor (30). The method of claim 1 wherein the fluidized bed reactor (20) is used with air heated to a temperature in a range between 93°C to 177°C, and the coal is being oxidized at a temperature range between 93°C and 121°C. The method of claim 2 wherein the coal temperature is 110°C and the reaction time is approximately 45 minutes. The method of claim 1 wherein the vertical tower reactor (30) is used with an air heated to a temperature in the range between 60° C and 121°C entering the reactor (30) at its bottom (36), and the coal having a particle size greater than 0.165 cm in diameter, enters the reactor (30) at its top (32) and moves down the reactor (30) to exit at its bottom, the temperature of the coal in the reactor (30) being maintained in a range between 60°C and 88°C with an average of about 77°C, with the reaction time of at least 60 minutes. The method of claim 4 wherein the oxidation temperature is of about 66°C, the coal temperature of about 82°C and the reaction time is of approximately 90 minutes. The method of claim 1 characterized in that the coal, having pores and reactive sites, is successively oxygenated at elevated temperatures and is cooled repeatedly, such that the alternate heating and cooling facilitates in the diffusing of oxygen in the pores of the coal, thereby allowing for more complete oxygenation of the coal. The method of claim 1 characterised in the cooling of the oxidised coal. The method of claim 7 characterised in the rehydration of the oxidized and cooled coal to its natural moisture level.

Inventors: RICHARDS JEFF M, VIALL ARTHUR J, RICHARDS, JEFF M., VIALL, ARTHUR J.
Applicants: WESTERN SYNCOAL CO, WESTERN SYNCOAL COMPANY
Application year: 1996
Main classification: C10L9
IPC codes: C10L9