A large portion of the world's electric power is generated from burning fossil fuels such as coal. The four primary types of coal (ranked from high to low) are anthracite, bituminous, sub-bituminous and lignite. Higher-rank coals typically contain less moisture and fewer pollutants than lower-rank coals. Coal is typically dried to enhance its rank and heating value (kJ, BTU per pound). In addition to enhancing rank and heating values, drying coal provides additional benefits. For example, once moisture has been removed after drying, coal is lighter and can be transported more easily and with less expense. Thus, coal drying is an important step in electric power generation.
Various coal drying methods and systems have been used in the past several decades including rotary kilns, cascading whirling bed dryers, elongated slot dryers, hopper dryers, traveling bed dryers and vibrating fluidized bed dryers. Many of these methods and systems require high temperatures and pressures. Because large amounts of energy are needed to reach these high temperatures and pressures, drying lower-rank coals with these methods can be economically impractical. Thus, efforts have been made to develop coal drying methods using lower temperatures and pressures. Many low temperature methods utilize fluidized bed technology, but are able to dry coal only to a limited extent. Subsequent high temperature steps are sometimes used to further dry coal processed at low temperatures. One issue encountered with fluidized bed drying of coal is the production of fines that become entrained in the fluidizing medium. In an environment where oxygen, and in some cases the ignition energy, is readily available, these fines can spontaneously combust. Thus, these drying methods typically use inert fluidizing gases such as nitrogen, carbon dioxide and steam to provide an environment with limited oxygen in order to prevent combustion.
Efforts have also been made to increase the efficiency of coal drying systems by using waste heat streams as heat sources. Waste heat streams include coke cooling gas, flue gas, stack gas, and steam condensate from power generation turbines. One or more waste heat streams can be used alone to provide heat to coal drying systems or in conjunction with primary heat sources, typically provided by the combustion of fossil fuels.
While past innovation has provided advancement of coal drying techniques, further improvements in coal drying efficiency and cost are desired. Even small improvements in coal drying efficiency can have huge, beneficial effects. A five percent increase in efficiency can mean tens of millions of dollars in savings per year for an average size power plant.
A method for drying particulate matter includes delivering particulate matter to a dryer, circulating a fluidizing gas through the dryer, heating the particulate matter in the dryer to remove water from the particulate matter, and removing dried particulate matter from the dryer. The method also includes removing water vapor and fluidizing gas from the dryer, removing fine particulates and water vapor from the fluidizing gas, and redirecting the fluidizing gas to the dryer after removing water vapor from the fluidizing gas.
A coal drying system includes a fluidized bed dryer and a fluidizing gas loop in fluid communication with the fluidized bed dryer. The fluidized bed dryer has a coal inlet for delivering coal to the dryer, a gas inlet for receiving a fluidizing gas, a heat exchanger for heating coal and fluidizing gas, a gas outlet for removing water vapor and fluidizing gas, and a coal outlet for removing dried coal from the dryer. The coal inlet and coal outlet receive an inert gas during coal delivery and removal, respectively, to prevent ingress of oxygen into the dryer. The fluidizing gas loop includes a heat exchanger for heating the fluidizing gas, a bypass for directing fluidizing gas to an upper portion of the dryer, a dust collector for removing fine particulates from the fluidizing gas, a condenser for removing moisture from the fluidizing gas, a fan for circulating the fluidizing gas through the fluidizing gas loop, a vent outlet for removing gas from the loop and a makeup gas inlet for adding fluidizing gas to the loop. The fan has a seal and an inert gas is directed at the seal to prevent ingress of oxygen into the fluidizing gas loop.
A modular fluidized bed dryer includes first and second dryer modules. Each dryer module has a plenum section with a gas inlet, a gas distribution plate section, a middle housing section with a heat exchanger, and an upper housing section with a particulate matter inlet and a gas outlet. The first dryer module and the second dryer module are welded together so that the plenum sections of the first and second dryer modules, the middle housing sections of the first and second dryer modules, the upper housing sections of the first and second dryer modules and the gas distribution plate sections of the first and second dryer modules are connected to form the modular fluidized bed dryer.
The present invention provides an improved method and system for drying particulate matter, including coal. While various types of particulate matter can be dried using the present invention, the embodiments described herein refer specifically to the drying of coal. Drying coal presents certain challenges (i.e. spontaneous combustion). However, the methods and systems described for drying coal can also be used for drying other types of particulate matter. Though the following embodiments explicitly refer to coal drying, it should be understood that the method and system of the present invention is not limited solely to coal drying, but includes other types of particulate matter (e.g., biomass, peat, solid waste, etc.) as well.
In one embodiment, a method for drying coal utilizes a closed loop system employing waste heat sources and an inert fluidizing gas. By drying particulate matter using a fluidized bed with an inert fluidizing gas, the level of oxygen present in the drying system can be tightly controlled to prevent combustion within the system. In a closed loop arrangement, only the inert fluidizing gas is delivered to the system to dry the particulate matter. Oxygen is generally kept out of the system. Small amounts of oxygen can sometimes enter when coal is added to or removed from the system and during the drying process when the coal physically breaks down and releases oxygen trapped within it. Additional control over the oxygen level within the system is maintained by a series of mechanisms that prevent ingress of oxygen into the system. Small amounts of inert gas are applied to the devices and sealing surfaces of the system where oxygen has the potential to enter (e.g., fan shaft seals, rotary airlocks, etc.). By utilizing these mechanisms, the oxygen level within the system can be controlled more tightly than in previous systems. In some cases, incorporating the small amounts of inert gas at various sites in the system can provide and maintain the proper level of inert fluidizing gas within the system to allow for steady state operation. In other cases, only small amounts of “makeup” gas need to be added to the system.
In one embodiment, the inert fluidizing gas is recycled and used again for fluidizing the particulate matter. In order to recycle the inert fluidizing gas, the moisture released from the coal and taken up by the fluidizing gas must be removed from the fluidizing gas before it is reintroduced to the coal. One way of removing moisture from the fluidizing gas is to condense the water vapor carried by the fluidizing gas so that the water vapor and the gas can be separated. This condensing step allows the system to recycle the fluidizing gas for additional use and operate with increased efficiency and lower costs. In systems where the fluidizing gas is not recycled, large amounts of the fluidizing gas need to be purchased or generated. Purchasing or generating large amounts of inert gas is costly. Recycling the fluidizing gas allows the system to operate at lower cost levels. Additionally, the recycled fluidizing gas still has an elevated temperature just before it returns to the fluidized drying bed. As its temperature is above ambient, less energy is needed to reheat the fluidizing gas to the necessary drying temperature. Thus, recycling the fluidizing gas reduces costs related to both the purchase or generation of fluidizing gas and the energy needed to heat the fluidizing gas.
In one embodiment, the drying method and system utilize relatively low drying temperatures within the fluidized bed dryer. By using relatively low drying temperatures, a wider range of heat sources can be used to dry coal according to the present invention, not just those heat sources providing high levels of thermal energy. When combined with the method and system of the present invention, the low bed temperature provides for reduced potential of in-bed combustion of coal during drying as well as lower levels of gasification. A low temperature drying bed also provides a more efficient drying process.
In addition to the lower thermal energy needed to dry coal, the drying method and system of the present invention allow the use of smaller and more efficient equipment for subsequent processing steps. For example, in one embodiment the drying method and system significantly reduces the particle size of friable coal such as lignite. This particle size reduction can translate into power and cost savings during subsequent processing steps. Because the particle size of the coal has been reduced, smaller secondary grinding and milling equipment can be used. Smaller secondary equipment can cost less to manufacture and requires less power to operate and grind or mill the dried coal. The amount of power can be reduced by sixty to ninety percent when friable coal is dried according to the present invention before grinding or milling.
Fluidized bed dryer 12 can be generally divided into three separate sections. Plenum section 16 is generally located at the bottom of fluidized bed dryer 12. Fluidizing gas enters fluidized bed dryer 12 at plenum section 16. Plenum section 16 typically does not contain coal during the drying process. Distribution plate 18 separates plenum section 16 from middle housing section 20. Once established, the fluidized bed occupies a substantial portion of middle housing section 20. Middle housing section 20 can also contain heat exchangers or heating coils that transfer heat to the fluidized coal during the drying process. Upper housing section 22 is generally located at the top of fluidized bed dryer 12. The fluidized bed also occupies a substantial portion of upper housing section 22. Fluidizing gas typically exits fluidized bed dryer 12 from upper housing section 22.
Various types of coal can be dried using the method and system of the present invention. Low-rank coal, such as lignite, and higher-rank coals, such as bituminous and sub-bituminous coal, and other moisture-laden coals can be effectively dried. The surface moisture of “wet” coal introduced into fluidized bed dryer 12 can vary depending on the type of coal. Wet coal that can be dried using the method and system of the present invention typically has an incoming surface moisture between about 0.5% and about 10%. Wet coal with a surface moisture greater than 10% can still be dried according to the present invention. Wet coal can also contain internal moisture in addition to surface moisture. Besides variances in surface moisture, the particle size of the wet coal can vary greatly. Depending on the particle size of the incoming wet coal, the temperature within fluidized bed dryer 12 and the flow of fluidizing gas through fluidized bed dryer 12 are adjusted to create and maintain a fluidized bed of coal. Coal with particle sizes (diameters) ranging from 5 microns to greater than 1 inch can be dried using the method and system of the present invention. The maximum particle size of coal that can be dried according to the present invention is determined by the overall system's ability to transport large coal particles within fluidized bed dryer 12.
Wet coal is introduced into fluidized bed dryer 12 at coal inlet 24. A fluidized bed of coal is created within fluidized bed dryer 12 as described below. The fluidized coal releases moisture. Dried coal exits fluidized bed dryer 12 at coal outlet 26. Outlet 26 can be an overflow weir, an underflow device such as a rotary airlock or horizontal screw conveyor located at the end of the bed, or a combination of these devices. Dried coal removed from fluidized bed dryer 12 at coal outlet 26 can go through additional processes, such as milling or grinding steps or mineral oil coating, before the coal is burned to produce energy.
Fluidizing gas enters fluidized bed dryer 12 at gas inlet 28. Gas inlet 28 is generally located at or near the bottom of fluidized bed dryer 12 so that fluidizing gas can flow through dryer 12 and create a fluidized bed of coal during the drying process. Various fluidizing gases can be used according to the present invention. Typically, an inert gas is chosen. Suitable inert fluidizing gases include nitrogen, carbon dioxide and low-oxygen flue gas. In coal drying system 10 illustrated in
Gas exits fluidized bed dryer 12 at gas outlet 30. Gas outlet 30 is generally located in upper housing section 22, above the fluidized bed. Fluidizing gas generally flows from gas inlet 28 through plenum section 16, middle housing section 20 and upper housing section 22 to gas outlet 30. As the fluidizing gas flows through middle housing section 20 and upper housing section 22, the gas mixes with coal in these sections to create a fluidized coal bed. Moisture from the outer surface and internal core of the coal evaporates in the fluidized bed. As the fluidizing gas passes through the fluidized bed, the gas picks up and absorbs the moisture released from the coal. The gas can also carry very fine coal particles (fines) either present with the wet coal stream as it enters the dryer or released from the coal during drying. When the gas exits fluidized bed dryer 12 at gas outlet 30, the gas contains more moisture and fines than the gas contained when it entered fluidized bed dryer 12 at gas inlet 28. Gas exiting fluidized bed dryer 12 at gas outlet 30 flows into fluidizing gas loop 14.
One or more bed heat exchangers 32 can be located in middle housing section 20 of fluidized bed dryer 12. Bed heat exchanger 32 can have a tubular configuration with tubes either in horizontal or vertical orientation (relative to the bed of fluidizing coal particles), or consist of plate coils. In both cases, the tubes or coils are normally connected to common inlet and outlet supply headers. Other suitable heat exchanger configurations are also possible. Bed heat exchanger 32 provides heat to the fluidized coal in middle housing section 20 via conductive heat transfer with coal particles in direct contact with the heated surface, or via convective means with heat transfer to the fluidizing gas. Heating the fluidized coal increases the rate at which moisture contained within the coal vaporizes. Typical fluidized bed temperatures are generally between about 15° C. (60° F.) and about 120° C. (250° F.). However, bed temperatures as high as about 200° C. (400° F.) can be used according to the present invention. Bed heat exchanger 32 is optional. In some embodiments, the fluidizing gas contains enough thermal energy to heat the fluidized coal and bed heat exchanger 32 can be omitted.
Thermal energy is provided to bed heat exchanger 32 by one or more heat sources 34. Heat source 34 can be any primary or secondary heat source. Heat source 34 generally provides heat between about 38° C. (100° F.) and about 315° C. (600° F.). Heat provided by primary heat sources includes heat generated by burning fossil fuels such as oil, natural gas or coal. Secondary heat sources include waste heat streams from other locations in a power plant. Waste heat streams include heated cooling water, condensate, saturated and/or superheated steam and heat transfer fluids heated by other power plant activities (e.g., cooling coke, etc.). Thermal energy is provided by heat source 34 to bed heat exchanger 32, which heats the fluidized coal. The cooled residual heat stream leaving bed heat exchanger 32 is removed from the fluidized bed dryer 12 and disposed of or reused for other purposes within the power plant.
Fluidizing gas loop 14 includes dust collector 36, condenser 38, gas vent valve 40, gas inlet valve 42 and one or more fans 44. Fluidizing gas loop 14 can also optionally include gas loop heat exchanger 46, which can be heated from the same heat sources as bed heat exchanger 32 or another heat source.
Gas exits fluidized bed dryer 12 at gas outlet 30 and enters fluidizing gas loop 14. The gas exiting fluidized bed dryer 12 contains fines and moisture. Coal drying system 10 illustrated in
Dust collector 36 removes fines from the gas after the gas has exited fluidized bed dryer 12. The fines removed from the gas can be routed to and combined with the dried coal exiting fluidized bed dryer 12 through coal outlet 26, returned to dryer 12 for reprocessing or kept as a separate stream for other uses or disposition. Because the amount of fines is relatively small compared to the amount of dried coal removed via coal outlet 26, any moisture carried by the fines is relatively insignificant when the fines are combined with the dried coal. The partially reconditioned gas (without the fines) continues through fluidizing gas loop 14.
Dust collector 36 can take various forms. Suitable dust collectors 36 include, but are not limited to, cyclones, multiclones, baghouses, electrostatic precipitators and wet scrubbing units. Baghouses include mechanical-shaker baghouses, reverse-air baghouses and reverse-jet baghouses. Wet scrubbing units include venturi scrubbers, countercurrent spray towers, co-current packed towers and countercurrent packed towers. Dust collector 36 can be a single unit or a combination of units functioning cooperatively to remove fines from the gas in order to recondition it.
Condenser 38 removes moisture from the gas after the fines have been removed. Condenser 38 is typically a surface condenser, although other condensers and shell and tube heat exchangers that convert water vapor into water can also be used. Condenser 38 removes at least a substantial Portion of water vapor from the gas. Under normal conditions, the amount of moisture condensed is equivalent to the amount of moisture evaporated from the coal in dryer 12. The dried gas exits condenser 38 and continues through fluidizing gas loop 14. The condensed water vapor exits condenser 38 as liquid water separately from the gas. In some cases the liquid water can be reused for additional purposes such as water cooling or provision of makeup or removed from the power plant. An alternate configuration of condenser 38 allows for isolation of the cooling media from the gas in loop 14 and employs the use of a cross-cooling heat exchanger between the water used within the condenser itself and the cooling source. In such a case, the cooling source can include chilled water, refrigerant and other media, as well as cooling water. This latter configuration prevents or eliminates the potential for contamination of the cooling media itself with dust or other undesirable constituents which could be captured in the condensing step.
After leaving condenser 38, the gas continues through fluidizing gas loop 14. Fluidizing gas loop 14 includes gas vent valve 40 and gas inlet valve 42 to control the pressure of coal drying system 10. Gas vent valve 40 allows gas to leave coal drying system 10. Coal drying system 10 generally operates with a pressure in upper housing section 22 of dryer 12 at or near atmospheric pressures (760 mm Hg), usually between about 755 mm Hg and about 775 mm Hg. Gas vent valve 40 allows gas to exit fluidizing gas loop 14 and coal drying system 10 in order to maintain necessary or preferred operating pressures. When pressures in fluidized bed dryer 12 or other areas of coal drying system 10 become too high, gas is bled out of the system through gas vent valve 40. Gas inlet valve 42 allows fluidizing gas to enter coal drying system 10. When pressures in fluidized bed dryer 12 or other areas of coal drying system 10 become too low, “makeup” gas is added to the system through gas inlet valve 42. Gas vent valve 40 and gas inlet valve 42 can operate independently of one another, but normally operate in a coordinated fashion and in conjunction with the objective of maintaining reduced oxygen levels in coal drying system 10.
Fluidizing gas loop 14 includes one or more fans 44 to circulate gas through fluidizing gas loop 14. Fans 44 are typically located in areas of fluidizing gas loop 14 where additional gas velocity and pressure is needed to maintain overall system flow (e.g., before heat exchangers). As shown in
Gas loop heat exchanger 46 is used to heat or pre-heat the new or recycled fluidizing gas before the gas enters fluidized bed dryer 12. Gas loop heat exchanger 46 is heated by one or more primary or secondary heat sources. Heat source 34 can provide thermal energy to gas loop heat exchanger 46 just as it provides thermal energy to bed heat exchanger 32. Alternatively, gas loop heat exchanger 46 can receive thermal energy from a different heat source. Other heat sources for heat exchanger 46 can include the previously mentioned primary or secondary heat sources and the returning media from heat source 34 after its use in bed heat exchanger 32. Gas loop heat exchanger 46 is optional depending on the type of fluidizing gas selected and the operating temperatures of fluidized bed dryer 12. For example, when the fluidizing gas is flue gas, the flue gas may enter the system at a high enough temperature that does not require further elevation before the gas fluidizes the coal in fluidized bed dryer 12. Additionally, where temperatures within fluidized bed dryer 12 are low, bed heat exchanger 32 can sometimes provide enough thermal energy so that the fluidizing gas does not need to be preheated before it reaches fluidized bed dryer 12. Operation of coal drying system 10 can include the addition of heat to the system by bed heat exchanger 32, gas loop heat exchanger 46 or both bed heat exchanger 32 and gas loop heat exchanger 46.
As shown in
In addition to dampers 48, distribution plate 18 can also be used to modify the flow of fluidizing gas in fluidized bed dryer 12. Distribution plate 18 can utilize directional flow to facilitate the removal of oversized or large particles so that they do not affect the fluidizing or drying processes. A variety of plate designs are possible which direct gases into the lower boundary of the fluidizing layer of particles. Plates with nozzles, angular perforations, or slots and assembled upper pieces can effectively create a directional flow component with the introduction of fluidizing gas. The directional gas flow component can be arranged to direct larger sized coal particles towards a discharge area within or toward coal outlet 26 (the discharge end of dryer 12). The directional flow configuration can also reduce the potential for backsieving of fluidized coal particles into the compartments of plenum section 16 of fluidized bed dryer 12. This directional plate design can also serve to separate oversized material if the flow pattern is arranged in such a fashion to direct flow to a separate oversized material discharge mechanism (e.g., an internal screw or rotary airlock discharge device).
Fluidized bed dryer 12 optionally contains baffles 50 to enhance the drying process. Baffles 50 are used to reduce backmixing effects and narrow residence time distribution for particles within fluidized bed dryer 12. Baffles 50 ensure uniform treatment of coal particles before they are discharged. Baffles 50 serve to minimize the cross-flow of particles back and forth between respective zones in dryer 12, and on balance allow more of the particles to migrate as intended in the dryer, from the point of feed (coal inlet 24) to the discharge area (coal outlet 26). In one embodiment, baffles 50 are arranged with minimal open areas near the bottoms of baffles 50 to allow the intended directional migration of oversized coal particles without obstruction. Baffles can be designed to extend above the fluidized layer in such a fashion that particle eruptions (as would occur with the emergence of a large gas bubble from the top of the fluidizing layer of particles) are contained within the same zone or area of the bed from which they originate. The extension of the baffle can even be arranged to meet the top of upper housing section 22 of dryer 12, allowing for the separate collection and processing of the gas exiting fluidized bed dryer 12 from gas outlets 30, which can be beneficial in some cases.
Fluidized bed dryer 12 can also be arranged in subdivisions or stages. Staged treatments allow different areas of the fluidized bed to focus on particular treatments. For instance, one stage can accelerate classification of the coal, while a second stage accelerates particle size reduction of the coal, and a third stage cools the coal before it is removed from fluidized bed dryer 12. Stages and subdivisions offer the opportunity to provide improved process control.
As a result of the fluidizing gas flow direction and the moisture released from the coal during the drying process, upper housing section 22 of fluidized bed dryer 12 can contain high levels of water vapor during operation. If left unchecked, this water vapor can condense on relatively cooler surfaces within upper housing section 22 and cause undesired accumulations of fines on the upper surfaces of fluidized bed dryer 12, or even in undesirable locations within fluidizing gas loop 14 or dust collector 36 (e.g., the surfaces of bags, thus causing a fouling or caking effect in a baghouse, if used). To prevent this from occurring, an additional supply of heated inert gas is delivered to upper housing section 22. The heated inert gas can be the same gas as the fluidizing gas or any other heated inert gas. This gas is used to suppress the absolute and relative humidity of gas exiting fluidized bed dryer 12 through gas outlets 30, and thus prevent or at least minimize the condensation effects.
Bypass gas loop 52 is an additional element of fluidizing gas loop 14. While some of the fluidizing gas enters fluidized bed dryer 12 through gas inlets 28, some of the fluidizing gas bypasses gas inlets 28 and continues to bypass gas loop 52. Typically, between about zero percent and about twenty percent (v/v) of the fluidizing gas bypasses gas inlets 28 and proceeds to bypass gas loop 52. Optionally, bypass gas loop 52 can include bypass heat exchanger 54, which heats the fluidizing gas to an even higher temperature than that provided by gas loop heat exchanger 46. The addition of exchanger 54 can be beneficial as the volume of bypass gas can be reduced, creating savings in terms of reduced gas handling equipment sizing and overall operating cost. The bypass fluidizing gas enters fluidized bed dryer 12 in upper housing section 22. Because this gas is generally warmer than the fluidizing gas already present in fluidized bed dryer 12, the relative humidity in upper housing section 22 is reduced. This decrease in relative humidity prevents condensation of water vapor on surfaces within upper housing section 22 and in downstream equipment such as dust collector 36, and allows more water vapor to exit fluidized bed dryer 12 at gas outlets 30. By eliminating or reducing condensation within fluidized bed dryer 12 and downstream areas such as dust collector 36, consequences such as fouling and scaling caused by condensed water exposure can be reduced, if not entirely eliminated.
Coal drying system 10 illustrated in
Oxygen control feature 56a is associated with coal inlet 24. One example of coal inlet 24 with oxygen control feature 56a is a rotary airlock as shown in
Oxygen control feature 56b is associated with coal outlet 26. Examples of coal outlets 26 include, but are not limited to, rotary airlocks, screw conveyors and overflow weirs.
Multiple discharge points can be arranged for discharging the dried coal. In most cases, depending on the intended purpose, it can be beneficial to separate the dried coal from the fluidized coal prior to reaching the 56b rotary airlock device. Usually, a combination of underflow devices (e.g., actuated underflow gates or flaps, rotary screw conveyors, underflow rotary airlocks) and an overflow mechanism are employed. The overflow can consist of a simple weir, over which the fluidizing solids at the discharge area of the dryer are intended to flow over. The weir can be arranged in an adjustable fashion (operating in a fashion like an elongated horizontal ball valve), a bolted plate with pre-drilled bolting holes for relocating the plate to a higher or lower position, or similar. The underflow arrangement can be operated in an intermittent fashion simply to clear oversized particles or on a more continuous basis to take more of the normal dryer throughput. In the latter case, the device can be operated with speed control to maintain a constant fluidized bed level based on the measured differential pressure of the fluidized layer (an indication of the theoretical height of the layer). In this case, the overflow arrangement serves more to prevent overfilling of the dryer. The discharging solids from the overflow weir can be handled separately from the underflow arrangement (e.g., in the case where it is desirable to handle oversized material in a different fashion downstream such as reprocessing, recrushing, etc.), or combined into one stream and discharged from a common device such as rotary airlock coal outlet 26.
Oxygen control features 56c and 56d are associated with dust collector 36. Where dust collector 36 is a baghouse, oxygen control feature 56c can be a baghouse pulse jet system. A baghouse pulse jet system delivers pulsed jets of inert gas through the baghouse filter in the opposite direction of fluidizing gas flow. The pulsed jets prevent the baghouse filter from becoming clogged with fines. Inert gas is used instead of environmental air so that oxygen is not blown back into the system by the fluidizing gas. Reverse flow baghouses can simply use the inert gas already present in the gas loop (after it has been discharged from the baghouse) for cake control on the bags. Oxygen control feature 56d can be associated with the outlet of dust collector 36 in a fashion similar to that of oxygen control feature 56b and coal outlet 26. Fines from dust collector 36 exit through a rotary airlock. Pocket purges prevent environmental air from entering dust collector 36 and entering fluidizing gas loop 14.
Oxygen control features 56e and 56f are generally associated with mechanical seals. Fan shaft seals for fans 44a and 44b can allow minute amounts of environmental air to enter coal drying system 10. To prevent these seals from allowing environmental air to slip through, tiny pulsed jets or a light stream of inert gas are applied to the seal area. Pulsed jets of inert gas can be suitable for components that do not operate continuously (e.g., turn on and off during the drying process). Persistent light streams of inert gas can be suitable for components that run continuously. Like the purge (sweep) gas described above, the inert gas for oxygen control features 56e and 56f can also be of the same type as the fluidizing gas. The pulsed jets and streams of inert gas sweep environmental air away from areas in which the air might enter coal drying system 10.
The various oxygen control features 56 prevent oxygen from entering coal drying system 10 and/or introduce additional inert gas into the system. An additional benefit of oxygen control features 56 is that the additional inert gas can replace gas lost from system 10 during processing. Some of the inert fluidizing gas is lost to the environment at coal outlet 26. The inert gas leaves fluidized bed dryer 12 along with the coal and is not easily recoverable. In other systems, this lost gas would typically be replaced by “makeup” gas delivered to the system through gas inlet valve 42. However, because inert gas is already being added to coal drying system 10 as part of the oxygen control element, the amount of makeup gas entering through gas inlet valve 42 can be reduced or even eliminated. In essence, coal drying system 10 utilizes some of the makeup gas to also prevent ingress of oxygen into the system. In a demonstration facility processing up to 7300 kg of wet feed per hour, makeup gas quantities were between about 45 kg per hour and about 200 kg per hour (depending on the targeted oxygen level in fluidizing gas loop 14, among other conditions).
As shown in
When combined with the closed loop design, oxygen control features 56 allow tight control of the oxygen content within coal drying system 10. While the system needs to have less than about nine or ten percent oxygen (v/v) in order to operate safely, coal drying system 10 can control the level of oxygen present in the system to virtually any desired value. Levels of six percent oxygen (v/v), three percent oxygen (v/v) and lower are possible for coal drying system 10 illustrated in
Additional features in coal drying system 10 include pressure sensor 60, moisture sensor 62 and sight glasses 64. Pressure sensor 60 measures the pressure within fluidized bed dryer 12. Pressure sensor 60 communicates with a controller (not shown) that operates gas vent valve 40 and gas inlet valve 42. Gas vent valve 40 bleeds gas out of coal drying system 10 when the pressure is too high and gas inlet valve 42 allows new fluidizing gas to enter coal drying system 10 when the pressure is too low. Moisture sensor 62 measures the water vapor content of the gas exiting fluidized bed dryer 12. Moisture sensor 62 communicates with a controller (not shown) that operates valves that control the amount of fluidizing gas entering or bypassing gas inlets 28. When the water vapor content of the gas leaving fluidized bed dryer 12 is too high, additional fluidizing gas is delivered to bypass gas loop 52 to enter fluidized bed dryer 12 at upper housing section 22 to reduce the relative humidity within the dryer. When the water vapor content of the gas leaving fluidized bed dryer 12 is low, a smaller amount of fluidizing gas is delivered to bypass gas loop 52 and more gas is used to fluidize the coal in the dryer. This allows coal drying system 10 to maintain the desired level of absolute or relative humidity of the gas exiting the dryer and delivered to dust collector 36.
In some embodiments, the walls of fluidized bed dryer 12 contain one or more sight glasses 64. Sight glasses 64 facilitate monitoring of the fluidization quality in different sections of fluidized bed dryer 12. An operator can observe various locations or stages within fluidized bed dryer 12 to determine whether any temperature or gas velocity or distribution adjustments need to be made. Due to the coal fluidization within fluidized bed dryer 12, inside surfaces of sight glasses 64 may become coated with coal particles, especially in high-moisture release or coal loading areas, obscuring an operator's view of the fluidized bed. The inner surfaces of sight glasses 64 can be equipped with wipers or inert gas nozzles to physically remove attached coal particles which make viewing difficult.
Coal drying system 10 can also be configured to allow for clean-in-place (CIP) operation. CIP allows for quick cleaning of coal drying system 10 without disassembly or other invasive cleaning procedures. Middle housing section 20 and upper housing section 22 of fluidized bed dryer 12 can be emptied of coal using pulses of dry gas, such as the fluidizing gas, which direct the dryer contents towards coal outlet 26. Plenum section 16 can also be cleaned using gas pulses, directing any fine particles that manage to pass through distribution plate 18 to an outlet within plenum section 16. Dust collector 36 can also be emptied using pulses of dried gas. Cleaning of fluidized bed dryer 12 and dust collector 36 can be facilitated by recirculating a cleaning gas through each. Suitable cleaning gases include nitrogen, carbon dioxide and, as mentioned, the inert fluidizing gas itself (if taken from a suitable high pressure location within fluidizing gas loop 14 or compressed beyond normal operating pressures).
Coal drying system 10 illustrated in
As described above, coal is deposited into fluidized bed dryer 12 via coal inlet 24. Fluidizing gas enters fluidized bed dryer 12 through gas inlet 28. The fluidizing gas is delivered to fluidize the coal inside fluidized bed dryer 12. The coal is heated in fluidized bed dryer 12 by the fluidizing gas (preheated by gas loop heat exchanger 46), bed heat exchanger 32 or both. As a result of the heat applied to the fluidized coal, moisture present in the coal vaporizes. The fluidizing gas carries the water vapor out of the fluidized bed dryer 12 at gas outlet 30. Particulate material (fines) is removed from the fluidizing gas by dust collector 36. Water vapor is removed from the fluidizing gas by condenser 38. Once particulate material and water vapor have been removed from the fluidizing gas, the fluidizing gas is redirected to the dryer to fluidize additional coal. Dried coal is removed from fluidized bed dryer 12 via coal outlet 26.
Utilizing coal drying system 10 in conjunction with method 70 dries the coal added to the system. In addition to drying the coal, coal drying system 10 and method 70 reduces the particle size of the coal added to fluidized bed dryer 12. Many coals, in particular low-rank coals like lignite, fracture during the drying process. By drying the coal according to method 70, the average particle size of the coal can be reduced by up to sixty percent. This reduction in particle size provides additional benefits. First, reducing the particle size of the coal can reduce the dead space between adjacent coal particles thereby reducing the volume needed for storage. Second, dried coal is sometimes milled or ground following method 70 and before combustion. Reducing the particle size of the coal in turn reduces the amount of energy needed for secondary milling and grinding steps. Reducing the particle size of the coal also reduces the size requirements of the milling and grinding equipment. Reductions in energy consumption upwards of seventy-five percent or greater can be observed for subsequent milling or grinding.
According to the system and method of the present invention, coal can be dried with a thermal energy input of between about 2740 kilojoules (kJ) and about 3260 kJ per kilogram of water evaporated (˜1180-1400 BTU per pound of water evaporated). The amount of thermal energy expended to dry the coal depends upon a variety of factors including the initial moisture content of the wet coal, the temperature of the wet coal fed into fluidized bed dryer 12, ambient conditions (atmospheric temperature and humidity), available utility conditions (heat sources and electrical power available for operating the system) and the desired moisture of the dried coal. Higher thermal energy inputs are observed for outlet moistures below about fifteen percent (w/w) (including internal moisture).
A significant amount of the energy consumed by coal drying system 10 is used to operate condensing step 82. Removing water vapor from the fluidizing gas can require between about 80% and about 110% of the combined amount of thermal energy used by fluidized bed dryer 12 and/or gas loop heat exchanger 46. The amount of energy required for condensing step 82 depends upon a variety of factors including the temperature of the wet coal fed to the dryer, the moisture levels of coal entering and exiting the dryer, available utility conditions, the amount of heat introduced to the system from system components (fans, etc.), heat losses and the amount and condition of fluidizing gas exiting the system.
While condensing step 82 consumes a relatively significant amount of energy, recycling the fluidizing gas in a closed loop offers huge cost savings in other areas. The fluidizing gas used in coal drying system 10, can flow through the system once, be partially recycled or nearly completely recycled (assuming losses only for gas that leaves the system with the dried coal). Generating or purchasing fluidizing gas for coal drying system 10 can be expensive. Recycling the fluidizing gas by removing water vapor (condensing step 82) after it exits fluidized bed dryer 12 reduces the need for generating or purchasing additional gas as the reconditioned and recycled fluidizing gas can be used to dry additional coal. Overall, utilizing a closed loop system with recycled fluidizing gas can provide an efficiency increase over existing coal drying systems and methods on the order of five to ten percent. This increase in efficiency can translate into tens of millions of dollars in savings per year for an average size power plant.
As described above, coal is deposited into fluidized bed dryer 12 via coal inlet 24 (rotary airlock). Oxygen control feature 56a purges coal inlet 24 with an inert gas to prevent oxygen from entering fluidized bed dryer 12. Fluidizing gas enters fluidized bed dryer 12 through gas inlet 28 and circulates to remove moisture from the coal inside fluidized bed dryer 12. As a result of the heat applied to the fluidized coal, moisture present in the coal vaporizes. The fluidizing gas carries the water vapor out of the fluidized bed dryer 12 at gas outlet 30. Particulate material (fines) is removed from the fluidizing gas by dust collector 36. An inert gas is applied to dust collector 36 to remove fines from dust collector filters (oxygen control feature 56c) and/or to prevent oxygen from entering dust collector 36 during removal of the particulate material (oxygen control feature 56d). Water vapor is removed from the fluidizing gas by condenser 38. Once particulate material and water vapor have been removed from the fluidizing gas, the fluidizing gas is redirected to the dryer to fluidize additional coal. Fans 44 redirect the reconditioned fluidizing gas back to fluidized bed dryer 12. Fans 44 contain a seal and oxygen control feature 56. Oxygen control feature 56e or 56f directs inert gas towards the fan shaft seals to prevent ingress of oxygen into coal drying system 10. Dried coal is removed from fluidized bed dryer 12 via coal outlet 26 (rotary airlock). Oxygen control feature 56b purges coal outlet 26 with an inert gas to prevent oxygen from entering fluidized bed dryer 12. The closed loop design and oxygen control features 56 allow the tight control of the oxygen content within coal drying system 10.
In many embodiments, fluidized bed dryer 12 has significant size with large dimensions and a large footprint. In one contemplated installation, a footprint of approximately 8.2 meters by 17.7 meters was determined to be appropriate for processing about 100 metric tons of wet coal per hour. Due to the large size of fluidized bed dryers 12, they are typically either constructed or assembled at the power plant or other manufacturing site in which they will operate. Often, one or more large crews of skilled construction engineers are required to assemble dryer 12 once it has been designed. In addition to the engineers, large quantities of all of the various construction materials, tools and other equipment must to be sent to the power plant site, taking up space. An additional feature of coal drying system 10 is the modular capability of fluidized bed dryer 12. Fluidized bed dryer 12 can be manufactured as separate modules at a manufacturing worksite, delivered to the installation site and then more easily assembled into a modular fluidized bed dryer 12 at the installation site. Dryer modules can be erected by skilled craftsmen at a permanent manufacturing site with dedicated tools and equipment, better ensuring a high-quality and consistent product. Dryer modules can be shipped individually assembled or in a relatively small number of “pieces” by regular transportation means to the installation site for final assembly. This modular aspect provides for reduced assembly time at the installation site and allows fabrication of identical or nearly identical modules that can be welded together to form fluidized bed dryer 12.
Upper housing section 22 can include gap 108 between sections 22a and 22b. Due to the dimensions of fluidized bed dryer 12 and dryer modules 106 and the weight of construction materials used in their construction, additional support structures may be required. In these instances, gap 108 separates upper housing sections 22a and 22b so that support bar 110 (shown in
Middle housing section 20 includes apertures 122, which allow for easy installation and removal of bed heat exchangers 32. Easy installation and removal of bed heat exchangers 32 is useful as fluidized bed dryer 12 can operate with or without bed heat exchangers 32 in middle housing section 20. Bed heat exchangers 32 are not shown in dryer module 106 in
Plenum section 16 and middle housing section 20 include left edge 126 and right edge 128. Middle housing section 20 also includes top edge 130. As is the case with upper housing section 22, welds are made along edges 126, 128 and 130 during assembly of fluidized bed dryer 12 at the installation site. For example, left edge 126 of plenum section 16 is welded to an end cap module while right edge 128 is welded to the left edge of an adjacent module's plenum section 16. Left edge 126 of middle housing section 20 is welded to an end cap module while right edge 128 is welded to the left edge of an adjacent module's middle housing section 20. Top edges 116 of middle housing section 20 are welded to bottom edges 116 of upper housing sections 22a and 22b.
The present invention provides a particulate matter drying system and a method for drying particulate matter. The drying system and method take advantage of a closed loop drying design to dry particulate matter, such as coal, safely and efficiently. Wet particulate matter is fluidized in a dryer with a fluidizing gas to transfer moisture from the particulate matter to the fluidizing gas. Fine particles and water vapor are removed from the fluidizing gas so it can be recycled and reused to fluidize and dry additional particulate matter. Oxygen control features prevent oxygen from entering the drying system to reduce the potential for spontaneous combustion when particulate matter like coal is dried. According to the present invention, particulate matter can be dried efficiently using a closed loop system while maintaining strict control over the amount of oxygen present in the system. The present invention also provides a modular drying system. Dryer modules can be constructed at a site different from the installation site, shipped to the installation site and assembled to complete the drying system. The system modularity allows skilled manufacturers to produce the modules at a manufacturing site with its own equipment without having to travel to the installation site. This allows for a higher quality product and consistent system builds. Earlier drying systems do not possess all of these capabilities.
While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.
This application is a U.S. National Phase filing of International Application No. PCT/US2009/004639, entitled “CLOSED LOOP DRYING SYSTEM AND METHOD,” filed on Aug. 12, 2009, which claims priority from U.S. Provisional Application Ser. No. 61/188,736, entitled “CLOSED LOOP COAL DRYING APPARATUS AND METHOD,” filed on Aug. 12, 2008.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US2009/004639 | 8/12/2009 | WO | 00 | 2/23/2011 |
Number | Date | Country | |
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61188736 | Aug 2008 | US |