Thermal coal upgrading process
A process for thermally upgrading coal in a vertical processor wherein the coal is gravity fed and a heated inert gas is introduced into the processor. The temperature of the gas, the size of the coal and the rate of movement of the coal are controlled to efficiently remove moisture from the coal and to not remove volatile organic compound.
The present application is related to provisional patent application Ser. No. 60/759,513 filed Jan. 17, 2006.
BACKGROUND OF THE INVENTION1. Field of the Invention
The subject invention is an improved thermal coal upgrading process which efficiently treats higher moisture content coals (and other similar materials but herein referred to as coal) with relatively low grade (“waste”) heat sources from an interconnected host facility. The subject invention involves a solids size separation means, a solids to gas contacting means, a means of controlling the solids discharge from the solid to gas contacting means, a fine solid feeding means and a hot solids and fines solids mixing means.
2. Background of the Invention
Due to the inventors' extensive history with and knowledge of the SynCoal® process this invention is compared to the original SynCoal Process for explanatory purposes. ( SynCoal® is a registered trademark for both the process and the product that results from the process although the ® is not generally included herein for brevity.) The original SynCoal process was patented by Monroe Greene, U.S. Pat. No. 4,725,337 issued Feb. 16, 1988 and U.S. Pat. No. 4,810,258 issued May 7, 1989. The demonstration SynCoal facility operated throughout the 1990s and received coal sized to accommodate the material handling components of this demonstration facility. Coal was screened to eliminate material larger than 1½ inch and reduce the amount of material smaller than ½ inch. The oversized and undersized material was returned to the mine stockpile, a situation that cannot be sustained on a large commercial basis either at a mine site, or at an end consumer's site without significant efforts to mix untreated coal with the upgraded coal.
The gas to solids contacting components selected for the original SynCoal facility for treating the coal were vibrating, fluidized bed (VFB) processors (aka conveyor dryers) requiring relatively high differential pressure process gas fans. While the VFBs provide an effective mechanism for contacting the coal with the process gas, they proved to be difficult to maintain due to excessive thermal and mechanical stresses and resulted in removal of significant quantities of fine material from the coal and consumed large amounts of electric power for the high pressure process gas fans. Additionally, subsequent efforts to develop larger facilities found it was nearly impossible to expand the productive capacity of the individual VFB processor units.
The method for cooling the upgraded coal in the original application was ineffective and expensive. The first stage of cooling used a water spray intended to flash cool the coal. The second stage used a VFB to contact the upgraded coal with a cool, saturated gas. The entire system delivered an upgraded product at about 180° F., well above the safe temperature limit for storage and required continuous inerting to prevent product spontaneous combustion. These limitations led to extensive efforts to identify better equipment and process options. The cooled, dried coal was screened with each sized fraction delivered to individual vibratory gravity separation systems known as air tables or jigs. The particular gravity separation components were prone to mechanical failures and did not allow for unit capacity increases for larger volume applications.
The inventors are aware of the following prior art:
Bowling '539 teaches drying coal rapidly in a fluid bed process using combustion gases from a steam generation system where the finer sized dried coal is combusted in the steam generation system and the coarser sized dried coal is meant to be transported (presumably in conventional bulk transport and handling systems) to other users. Actual commercial field experience has taught the present inventors that this process will result in a dusty, low density product that is susceptible to spontaneous combustion, making conventional handling of the dried coarse coal product problematic. Bowling does not teach process residence times, heating rates, contact velocities or operating pressures. Bowling's focus upon rapid drying, making a transportable product, lack of heating rate or contact velocities are distinct departures from the subject invention.
Norman '763 teaches mixing warm dried fine coal (0.5 to 10.0% moisture @ 175°-480° F. (80° to 250° C.), presumably produced with a rapid heating rate process with coarse run of mine coal (25 to 50% moisture @ ambient) to rehydrate and cool the dried coal, producing a stable (passivated) mixture. The ratio of dried coal to raw coal can vary from 1:1 to 1:10, depending on the moisture content and temperature of the dried coal. The objective of this patent is to stabilize dried coal for storage and handling. Actual commercial field experience has taught the present inventors that mixing dried and undried coal produces an extremely dusty product, making conventional handling of the mixed coal problematic. The present invention slowly heats coarse coal particles and then mixes them with fine coal reducing the moisture content and producing an upgraded coal product that can be physically cleaned with no intention of making a product that will be handled with conventional bulk handling or storage techniques resulting in distinct differences between the processes.
Siddoway '913 teaches a rapid drying and cooling process using fluidized bed technology. A portion of the process feed coal is rapidly dried in a fluidized bed dryer and then mixed with the balance of the feed to the process. The mixture is then fed to a fluidized bed cooler. Preferably, the finer material is fed to the fluidized bed dryer and the coarser material mixed with the dried coal prior to delivery to the fluidized bed cooler. Fines collection and distribution is discussed together with process gas treatment. Of particular interest is the use of raw coal to assist in the cooling of the dried coal. Convective and evaporative cooling is discussed, but in the context of the application to fluidized bed technology. The desired goal is to produce a product more resistant to spontaneous combustion by reducing the temperature of the dried product. Mixing raw coal with dried coal is one of the process steps in the process of the present invention. The difference is that the present invention process maximizes gas contact time, minimizes contact gas velocity which minimizes compressive energy input. Additionally, the present invention teaches the use of fines as the cooling medium as opposed to the use of coarse material and cool gas as taught by Siddoway. The present invention produces a product temperature of 140° F., while the reference predicts a product temperature of less than 100° F. Rapid fluid bed drying only the fine coal fraction and the higher contact gas velocities are distinct departures from the process of the present invention.
BRIEF SUMMARY OF THE INVENTIONIt is an object of the present invention to provide an improved thermal coal upgrading process to efficiently remove moisture from coal without removing the volatile organic compound (VOC).
It is a further object of the present invention to provide an improved thermal coal upgrading process which is economical.
In accordance with the teachings of the present invention, there is disclosed a process for thermally upgrading the coal The coal is separated into coarse and fine fractions. A vertical processor is provided having a plurality of baffles therein. The coarse fraction of coal is introduced into the top of the processor wherein the coarse coal moves to the bottom of the processor by gravity. A moderately high temperature process gas is provided. The process gas is introduced into the processor under rows of baffles where the gas is uniformly distributed at a low velocity in a cross flow manner throughout the coal in the processor. The coal flows by gravity around the baffles gently agitating the coal and the process gas heats the coal to remove moisture from the coal which alters the combustion and physical characteristics of the coal. The heated, altered coal is removed from the processor. The heated altered coal is mixed with a controlled amount of the fine fraction coal wherein the coarse fraction of the coal is cooled and the fine fraction of the coal is heated.
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.
Generally, all the relevant prior art is focused upon creating a dried coal that has bulk material characteristics that allow conventional bulk transportation in open top rail cars and trucks and storage in open stockpiles. Additionally, the relevant prior art processed a selected coarse fraction of the feedstock requiring alternative use of the non-selected finer sized material. The subject invention does not attempt to achieve these characteristics but focuses upon processing the entire range of feed coal size at a slow heating rate with low contact gas velocity resulting in minimal fines elutriation and volatile organic compound evolution (loss) while maintaining the granular product characteristic that is amenable to physical cleaning. Based upon their extensive commercial field experience, the subject inventors do not expect to be able to handle the upgraded coal product in conventional railcars, open storage piles or high capacity bulk handling systems.
In the present invention, coal has been previously crushed to eliminate oversized material larger than approximately three inches. It is preferred that the coal not exceed two inches. The preferred processor design maximum feedstock size specifications is two inch by zero. A screen is used to divert a portion of the finer coal for use in the cooling process. Reducing the fines in the thermal process will reduce particulates in the exit gas stream and reduce the power required to force the process gas through the upgrading processor system, and optimized the cooling of the dried and altered coal.
An improved tower processor is used to gently mix the coal particles while exposing them to low velocity, moderately high temperature process gas in a cross flow manner that results in small pressure differentials between the process gas inlets and outlets thereby minimizing the compressive power requirements. As the coal dries and descends through the processor, the coal particles will break up by thermal forces and attrition, shifting the size distribution of the coal to a smaller average particle size. The tower processor is crossed with alternating levels of inverted V inlet and outlet baffles extending the full width of the tower.
The coal is treated using a moderately high temperature, inert process gas in intimate contact with the coal. The coal to gas contact time can be varied widely, generally from 20 minutes to several hours but is idealized to be about 60 minutes versus the 10 to 15 minute contact time in the original application. Although a relatively wide range of temperatures (450° F. to 900° F.) and “inert” gas compositions (combustion flue gas to steam) can be applied, ideally inlet gas temperatures would be attemperated to approximately 700° F. and oxygen content maintained as low as possible within reasonable margins of safety, to minimize or prevent devolatilizing the coal while providing the process heat required. Preferably, the oxygen content is less than 5% by volume. The higher the difference between the inlet and outlet process gas temperature, the smaller the volume that is needed to heat the coal and the lower the gas velocities resulting in less fine material carryover and lower process gas compressive power required. This relationship can be described as the gas to coal ratio. In the preferred embodiment, as the coal is heated and the moisture vaporized into the process gas stream (make gas), the combined process and make gas temperature is about 225° F. at the processor exit. Using the current invention, coal in the processor is heated at a rate of not to exceed 10° F. and preferably less than 3° F. per minute resulting in more thorough upgrading without creating significant quantities of volatile organic compounds (VOCs). Empirical testing (shown in Table 1) by the inventors has shown that VOCs evolved by this process are negligible as long as the moisture content of the coal is not reduced to less than 1.5%. Reduction to lower moisture contents results in the production of very small quantities of VOCs.
Within these temperature ranges, a variety of fuel combustion processes, external sources such as steam boilers, cement/lime kilns, hydrocarbon refining processes, and coal liquefaction and gasification processes can provide “waste” heat and/or combustion gas to the improved process which can in turn provide an improved solid fuel feedstock to these operations. Additionally any steam condensation or water cooling requirements of the host facilities can be integrated into the top of the thermal processor by placing condensing tubes in the baffles of the processor to improve the overall thermal efficiency of the process. The process gas may be reheated by indirect exchange with a waste heat source and recycled to the processor.
The heat energy that is required to be transferred from the process gas to the coal is dictated by the amount of moisture to be removed. The gas flow rate (velocity) is determined by coal's residence time in the processor and the volume of process gas required per unit of coal to be processed. The long residence and slow rate of heating results in more uniform and complete upgrading without producing significant quantities of VOCs.
The process gas velocities are extremely low when compared to other processor operations. This minimizes the size of the particle that will be removed from the coal being processed and the process gas fan power required (see Table 2). The power required to drive the gas through the subject process is nearly an order of magnitude less than that required for the original VFB operation. The VFB's in the original facility had bed velocities of approximately 10-12 fps and combined with the vibratory bed action carried particles as large as 0.0234 inches (˜28 mesh) into the exit duct work where the higher velocities entrained the particles requiring the need for extensive dust collection equipment. This system resulted in a 45 inch water column (iwc) total system differential pressure. The current invention system design has particle elutriation velocities less than 5 fps, preferably, less than 2 where the process gas exits the coal bed and a total system pressure drop of less than 5 iwc. At this low elutriation velocity, coal particles larger than 0.0035 inch (170 mesh) should not be entrained and carried out of the coal bed.
Stokes Law defines an expression where particles falling in a viscous fluid by their own weight teach a point where the frictional force combined with the buoyant force exactly balance the gravitational force termed the terminal or settling velocity. This velocity translates directly into the elutriation velocity.
Centrifugal fan horsepower is a function of the pressure drop and the mass flow through the gas flow path. The reduction in system pressure requirement reduces process gas fan power requirements by 9 times over the previous experience. Additionally, for any fixed flow path, pressure drop is a function of the square of the gas velocity and mass flow is a function of the gas velocity and the area of the flow path. Therefore, for a fixed flow path and gas density, fan horsepower requirements are a direct function of the cube of the velocity.
The fine coal removed prior to the processor feed is mixed with the thermally treated product, cooling the hot coal while removing some of the moisture from the fine coal, resulting in a lower temperature product than was attained using the combination of quench and VFB mechanisms. The fine coal can be fed to the mixing system through a stand pipe that allows excess fines to overflow into the processor feed bin so that the fine coal used for cooling is always in controlled proportion to the processor product. The mixing process can be accomplished using a screw auger or pug mill where hot coal product from the processor is mixed with the coal fines. The combination of sensible and latent heat can reduce the product to a more acceptable temperature typically less than 180° F. and preferably approximately 140° F. The final product temperature can be varied with different ratios of fine coal to processor product.
Heating the coarser coal the processor, and then mixing the dried, altered coal with the raw coal fines heat the fine particles while cooling the dried, altered coal. This cooled product has virtually the same physical and chemical characteristics as the original SynCoal prior to physical coal cleaning. As a result, similar to the original SynCoal, the product has enhanced cleaning characteristics and can be fed to a commercial dry gravity separation process to reduce mineral components of the feed coal further improving the product's combustion and environmental characteristics.
The improved process of the present invention produces a nearly identical final product (but at a safer lower temperature) as compared to the original process. The final product of the present invention provides improved combustion efficiency, boiler capacity and reduced environmental emissions as shown previously. Many heavy metal contaminants (such as mercury) that form air toxic emissions have been found to be associated with pyrite minerals (
From a side by side test with two nearly identical commercial power plants—burning a raw Montana Powder River Basin (NPRB) coal and the other blending at average of 12.3% SynCoal with the same raw NPRB coal over a three month period, the following results were observed.
Laboratory mineral separation testing on a typical Montana NPRB sub-bituminous coal demonstrated that mercury content is associated with pyrite content.
The ash, sulfur, mercury and air toxic removal shown by the laboratory testing is as follows:
The process of the present invention is simplified, minimizing maintenance, energy use and operator interface dramatically reducing operating staffing and costs compared to the original application. In the preferred embodiment, the integrated application of the improved process at a thermal host provides for the use of “waste” heat and minimizes capital and operating energy costs providing significant economic advantage to this application of the process over the current competitors.
Obviously, many modifications may be made without departing from the basic spirit of the present invention. Accordingly, it will be appreciated by those skilled in the art that within the scope of the appended claims, the invention may be practiced other than has been specifically described herein.
Claims
1. A process for thermally upgrading coal comprising the steps of:
- separating the coal into coarse and fine fractions,
- providing a vertical processor having a plurality of baffles therein,
- introducing the coarse fraction of coal into the top of the processor wherein the coarse coal moves to the bottom of the processor by gravity,
- providing moderately high temperature process gas,
- introducing the process gas into the bottom of the processor under rows of baffles wherein the gas is uniformly distributed at a low velocity in a cross flow manner throughout the coal in the processor, the coal flows by gravity around the baffles gently agitating the coal while the direct contact with the moderately high temperature process gas heats the coal to remove moisture from the coal altering the combustion and physical characteristics of the coal,
- removing the heated, altered coal from the processor, and
- mixing the heated altered coal with a controlled amount of the fine fraction coal wherein the coarse fraction of the coal is cooled and the fine fraction of the coal is heated releasing some of the moisture contained in the fine fraction.
2. The process of claim 1, wherein the coarse fraction of the coal does not exceed approximately three inches.
3. The process of claim 1, wherein the baffles are alternating levels of inverted V inlet baffles and outlet baffles extending transversely across the processor wherein the coal is gently mixed as the coal descends by gravity through the processor reducing compressive power requirements.
4. The process of claim 1, wherein the process gas is heated by waste heat from an external source.
5. The process of claim 4, wherein the process gas is exhaust gas from a combustion process.
6. The process of claim 4, wherein the process gas is reheated by indirect exchange with a waste heat source and recycled to the processor.
7. The process of claim 1, wherein the temperature of the process gas ranges from 450° F. to 900° F.
8. The process of claim 7, wherein the temperature of the process gas is approximately 700° F.
9. The process of claim 1, wherein the oxygen content of the process gas is less than 5% by volume.
10. The process of claim 1, wherein the coal in the processor is heated at a rate of less than 10° F. per minute.
11. The process of claim 10, wherein the coal in the processor is heated at a rate of less than 3° F. per minute.
12. The process of claim 1, wherein a minimum amount of volatile organic compounds (VOC) are evolved from the coal.
13. The process of claim 1, wherein the process gas exits the coal at a velocity of less than 5 fps.
14. The process of claim 13, wherein the process gas exits the coal at a velocity of less than 2 fps.
15. The process of claim 1, wherein the product has a temperature of less than 180° F.
16. The process of claim 15, wherein the product has a temperature of less than 140° F.
17. The process of claim 1, wherein the residence time of the coal in the processor is approximately 60 minutes.
18. A continuous process for thermally upgrading low rank coal comprising the steps of:
- providing a tower means for passing the coal via gravity from the top of the tower, the tower having baffle means with the tower for channeling the coal downwardly within the tower,
- providing a source of heated gas, and
- introducing the heated gas in outlet means within the tower wherein the heated gas mixes within the tower heating the downwardly descending coal to remove moisture from the coal, the heated gas passing through outlets in the tower.
19. The process of claim 18, wherein the temperature of the heated gas ranges from 450° F. to 900° F.
20. The process of claim 18, wherein the coal in the processor is heated at a rate of less than 3° F. per minute.
21. The process of claim 18, wherein the residence time of the coal in the processor is approximately 60 minutes.
Type: Application
Filed: Jan 11, 2007
Publication Date: Feb 19, 2009
Inventors: Harry E. Bonner (Sheridan, WY), Roger B. Malmquist (Butte, MT), Ray W. Sheldon (Huntley, MT)
Application Number: 11/652,194
International Classification: C10G 1/08 (20060101);
