Patent Application
20130036955
Low grade fuels, such as coal, burn in three stages including volatile release, char combustion, and gas and residual particulate combustion. Most conventional systems attempt to perform all three stages in a single module, such as the cyclone combustor depicted in
Coal-fired power plants generate nearly half of the electricity in the United States. The other main energy sources are the fossil fuels natural gas and oil; nuclear power; hydroelectric power; and the renewable sources including solar, wind, geothermal, tidal, and biomass sources.
There are major problems with each of these sources. Fossil fuel supplies are being depleted, while concerns about carbon dioxide are rising. Nuclear technology is expensive, and there are concerns about the safety of both the plants and the spent fuel rods. Hydroelectric power is limited to areas with adequate water supplies and acceptable environmental impacts. Green technologies are promising, but these sources are not yet capable of supplying a major portion of the nation's power needs. Beyond these problems, there is increasing world-wide competition for power, particularly in China and India. The net result is that all power sources must be considered to meet growing demand.
As this demand grows, the relative importance of these various sources is changing. In particular, there is strong pressure to decrease the use of coal in the Unites States. Even under the most severe forecasts, however, coal will remain a major contributor to the nation's energy supply at least for the next several decades.
The major objection to coal use is environmental impact: the burning of coal releases large amounts of emissions. To maintain or even increase the use of coal, highly effective emissions controls are therefore necessary. Unfortunately, coal emissions are extremely complex, and are therefore expensive and difficult to control.
The underlying problem is the structure of the coal itself. Coal consists mostly of carbon, which is why coal is black. In addition to carbon, coal also contains a mixture of highly reactive carbon compounds called volatiles. Finally, coal also contains inert mineral matter that was laid down when the seam was formed. Coal therefore contains some of every chemical element on earth.
This complex structure in turn yields a complicated combustion process. The first step is the emission and burning of the volatiles. The loss of the volatiles leaves char, which burns more slowly but quite intensely. The loss of the char then leaves the mineral matter, in the form of ash.
In practical systems, however, the combustion is not complete, leaving unburned fuel, volatile organic compounds, and soot. There are also inorganic gas emissions, notably oxides of nitrogen and sulfur. Finally, the mineral matter can form solids that are suspended in the exhaust gas, as well as vaporized, highly toxic metals, notably mercury.
An effective emissions control strategy must therefore address all of these complex, diverse problems. Without such a process, it will not be possible to use the nation's plentiful coal reserves.
One approach is to remove the coal contaminants before combustion. Without contaminants such as mineral matter, the “clean coal” product can thus burn with minimum emissions. Unfortunately, advanced coal cleaning processes are expensive and difficult, thereby requiring subsidies or other incentives. Pre-combustion clean coal technologies are therefore not currently in general use.
Because it is thus not practical to clean the coal before combustion, the only alternative is to burn the coal, and clean the emissions during and/or after the combustion process. This approach includes advanced combustor designs; electrostatic precipitators; scrubbers for sulfur oxides; fabric filters; catalysts; and various additives for selected products, notably mercury and nitrogen oxides.
The overall result is that “clean coal” today essentially amounts to a more or less a conventional combustor 2, seen in
As regulations tighten, the existing technologies must become more and more effective, and thus more and more expensive. Furthermore, as EPA imposes new regulations, more components become necessary: mercury controls are currently under way, and selenium control is not far behind. This increasingly complex and expensive piecewise approach to emissions control is simply not sustainable. Another approach is therefore necessary.
It is an object of the invention to provide a combustor for low quality fuels having reduced emissions.
It is another object of the invention to provide a three stage combustor for separating the fuel into components.
It is still another object of the invention to provide a first stage combustor separating coal into char and small particles.
It is yet another object of the invention to provide a three stage combustor having a devolatilization chamber, a cyclone combustor and a fluidized bed.
These and other objects of the invention will become apparent to those of ordinary skill in the art after reading the disclosure of the invention.
The system controls oxygen and fuel mixtures by having three distinct components, one for each of the three stages. Under this approach, the combustion process can be matched to the specific characteristics of the fuel component being used.
The first two components preferably use “entrained flow” conditions, a particularly effective technique in which the coal is ground so finely that it flows in the combustion air stream. An enhancement of this basic entrained flow system is a sudden expansion, which slows and mixes the air and fuel, thus improving combustion. A conical expansion zone is called a “quarl.” If the quarl stops before reaching the outside wall, a “recirculation” zone forms between the quarl end and the outer wall. The spinning hot gases in this zone help to stabilize the flame.
A further enhancement is to rotate the incoming air to produce a “swirl.” At sufficiently high swirl numbers, the combusting fuel and air mixture produces a strong central recirculation zone that extends into the combustor body. This recirculation zone provides rapid fuel heating, thereby greatly aiding combustion stability and efficiency.
The third component preferably uses “fluidized bed” conditions, a particularly effective technique in which the fuel is burned in an upflowing stream of gas, typically including a non-combusting fill material. In this application, the fill material is preferably limestone for sulfur capture.
1. Devolatilization Module
The first component is a devolatilization module 10 coupled to a second char extraction/volatile collection module. The devolatilization module produces two separate streams. A first stream is a partially burned, highly fuel rich volatile stream for later combustion, as will be explained later. This stream contains most of the gas yield. A second stream contains concentrated, partially burned char particles. This stream contains minimum gas phase products that are burned in the char combustor module, also to be explained later.
As seen in
a. Volatile Generation/Combustion Section
The Volatile Generation/Combustion Section 14 produces maximum volatile flame stability to prevent a flame-out and maximum volatile yield (which also yields minimum NOx) by constricting the combustor at the end of the volatile flame front (essentially a mirror image of the quarl zone), thus yielding a compression of the central recirculation zone and a mounting point for matched antennae for electromagnetic (microwave) enhancement of the flame. Microwaves have been used in other types of systems to extend the lean limits of combustion, in an attempt to limit NOx. The system uses microwaves to extend the limits to the opposite end, to make the most fuel rich system possible for maximum volatile yield. A sonic amplifier section couples directly to sonic drivers. Like microwaves, sound is also known to influence flames. In particular, low hundreds of Hz sound is quite effective at stabilizing poorly mixed flames. Orthogonal sources of low kHz sound and upper tens of kHz sound are also quite useful in the presence of particles. One set of sonic drivers is diametrically opposed to another and emits a first frequency while a second set of sonic drivers is diametrically opposed to another and emits a second frequency. The four sonic drivers can be evenly distributed about the char chamber 14. Combinations of microwaves and sound, or no microwaves and no sound can be used.
This section produces maximum particle separation to the periphery, thereby yielding maximum particle loading to the subsequent char chamber 20 through a first outlet, or side port, 16, and minimum particles and gases to the subsequent gas phase combustor through a second outlet, or end port, 18 and ideal mixing of combustion and separation air for the second section of the devolatilization chamber.
b. Char Extraction/Volatile Collection Section.
Air is added to the output of the Volatile Generation/Combustion Section 14 to keep the char burning, but only slightly, thereby keeping any ash particles in a dry, powder (not melted) state. The char extraction chamber expands rapidly, with the first outlet 16 in a side wall removing the particles with minimum amounts of carrier gas. This feeds the char combustor 20, described later. The far end of the chamber 15 is confined to produce a cyclone action resulting in maximum particle collection and maximum cleaning of the volatile stream. The volatile stream is extracted along an second outlet 18 along the central axis. Sound may be applied along the central axis to enhance flame stability, keep particles out of the exhaust stream, and disrupt soot formation.
2. Char Combustion Module
The char that emerges from the devolatilization phase is essentially solid carbon. Solid carbon burns by first forming carbon monoxide at the surface, which then burns quickly. Char combustion is thus inherently a slower process than volatile combustion.
The immediate problem is to match the speeds of the devolatilization and char combustion steps. Conventional systems try to perform both functions in a single unit, with the result that neither function is performed well. Instead, the system uses separate chambers to perform these functions.
A particularly effective means of burning char is a cyclone combustor 20, as depicted in
First, cyclones often have unsteady flames, leading to difficult control, poor efficiency, and excessive emissions. The underlying problem is that conventional systems introduce the raw fuel into a highly turbulent mixture of extremely hot gases. The fuel therefore burns rapidly but erratically.
The char combustor 20 uses several unique flame stabilization techniques to improve the combustion process. First, the system feeds the cyclone combustor with char, not raw coal. Under this approach, there are no volatiles to compete with the char for the available oxygen. Therefore, the characteristic flickering and popping of conventional systems transitions to a steady, uniform glow, indicating the desired stable combustion. The cyclone chamber may have a bulbous bottom 24, seen in
For maximum benefit, the char fuel is introduced in a unique geometry, seen in
The overall geometry is two or more parallel tubes, as shown in
In addition to orientation, the relative sizes of the devolatilization and char combustion chambers are also important. Again, the underlying problem is that devolatilization and volatile combustion is fast, but char combustion is slow. Devolatilization chambers 10 can thus be small, but char combustors 20 must be relatively large. In conventional utility systems, this difference causes scaling problems in the design phase. Furthermore, this difference also causes turn-down problems while following varying loads in the operational phase.
To avoid these problems, the system may use multiple devolatilization chambers 10 to feed a single char combustion chamber 20. This arrangement is easily scaled to any desired size, with no loss of effectiveness or efficiency. Furthermore, during operation, some of the devolatilization chambers can be shut down at part load, thus thereby providing unmatched turn-down capability.
The second major problem with conventional cyclones is that they produce excessive levels of nitrogen oxides, or NOx. This problem is so severe that cyclone combustors are not in common use in the United States. The char combustion chamber has several means of reducing NOx in the cyclone. These techniques follow from the well-known NOx formation mechanisms. First, because the volatiles contain most of the fuel bound nitrogen, and these exit through the end port 18, not the side port 16, the char combustor produces essentially no NOx from fuel bound nitrogen. The fuel bound nitrogen NOx from the volatile stage is addressed in the following fluidized bed module. Likewise, this absence of volatiles virtually eliminates prompt NOx.
The majority of NOx is therefore thermally generated. To reduce this component, the system uses two features. First, the reactivity of char is much less than the reactivity of volatiles. Therefore, the combustion of char is relatively mild, thus generating relatively little thermal NOx. Second, the char chamber is operated at fuel rich, or low oxygen, conditions. This arrangement yields low temperatures that reduce thermal NOx. Furthermore, reducing conditions convert much of any generated NOx back to nitrogen and oxygen. This “reburn” technique is continued in the third stage, as discussed later.
Finally, the system also uses staging to introduce the oxygen in multiple steps, thereby avoiding local zones of excessive heat. While some conventional systems use staging, only the unit has an optional segregated stage for complete combustion, followed by a return path through a reducing atmosphere.
In addition to NOx control, the char combustion chamber provides the option of high temperature combustion, specifically temperatures sufficient to melt the waste ash into slag. Currently, coal ash is typically produced in a dry powder form. Although some ash is used for cement manufacture or other purposes, most ash is simply dumped in landfills or collected in sludge ponds. In these locations, the ash can either escape in bulk, or leach out into the water supply. In either case, the net result is severe environmental pollution, primarily due to contamination by mercury, selenium, and other heavy metals in the ash. Melting the ash, which also contains silica, instead produces a vitrified form that traps these toxins.
While other systems can produce slag, only this system has the provisions necessary to keep the NOx levels under control at the required elevated temperatures. Furthermore, only the system includes a second stage with enhanced swirl just for slag production. This configuration can also provide the option of converting existing ash to slag, thereby cleaning up the toxic ash pits that are common in coal burning regions. The second stage slag producer is mentioned above, with reference to
Finally, while other systems produce slag in more or less spherical shapes, the system has two unique slag formation features. First, the slag is extruded through a rectangular port that forms slag blocks upon solidification. These blocks are thus quite stable at the disposal site, unlike conventional slag that can shift and thus pose a threat to workers or any subsequent users of the disposal site.
Of course, the slag is molten when it passes through the extrusion mold. To maintain the rectangular shape, the slag must therefore be quenched. In the system, this quenching is done in a conventional water bath. The net effect is thus similar to lava from a volcano entering the sea. Like lava, the slag produces immense clouds of positively charged steam. In the system, this positively charged steam is then used to trap small ash particles in a vapor trap. A suitable vapor trap is disclosed in copending U.S. application Ser. No. 13/471,918, herein incorporated by reference.
The system may be used with oxygen rich combustion. High oxygen levels are useful for two reasons. First, coal and other poor quality fuels can undergo “gasification” to produce “synthetic gas” or “syngas,” which is then burned or used as a feedstock. Although gasification has been used for years, only the present system provides separate combustion stages. This allows the control of the competing volatile and char reactions, as described above. In particular, the system can provide either oxygen or normal air to either stage as desired. For example, relatively cheap air can be used to drive out the volatiles, leaving the char for gasification. An immediate extension of this concept is the control of the air and/or oxygen levels to produce coke in the second stage. Alternatively, oxygen can be added in the volatile stage, depending on the chemistry of the specific coal type (the volatile composition and percent varies with the coal type). In either case, the uniformity of the second stage char provides an easily controlled system.
An emerging use of this feature is the ability to burn coal under conditions of minimal nitrogen, as needed for various proposed “carbon capture and sequestration” (CCS) technologies. Although these technologies are still under development, the ability of the system to operate smoothly and reliably under initial high oxygen concentrations is a great advantage over conventional systems.
Another option is the use of sound. As discussed in the first stage above, and the third stage below, sound greatly aids combustion efficiency and stability. All of these advantages likewise apply here. The char combustor, however, also uses sound for slag control. The underlying problem is that slag varies greatly from coal seam to coal seam, because the mineral matter varies from seam to seam. This variation is so great that some seams are described as “slagging” while others are described as “non-slagging.” The physical difference is that some slags flow freely, while others are so viscous that they accumulate in the combustor. The application of sound, however, causes even the most viscous slag to flow to the exit ports, thus allowing the system to function even with “non-slagging” coals.
3. Residual Gas and Particle Combustion Chamber
The products of the two previous modules 10, 20 are mainly gases, with small particles entrained in the flow. This third module burns these gases and entrained particles, and thus completes the combustion process. The output of this module is thus essentially complete combustion of all carbon compounds, notably soot, volatile organic compounds (VOC) and highly carcinogenic species, such as dioxins and furans. The exhaust gas from the third stage, after cooling during the steam generation process, is thus immediately ready for the subsequent vapor trap module. In particular, the output does not contain the large amounts of particulates that are produced in conventional systems, thereby greatly reducing or even eliminating the need for downstream particulate control technology.
One way of producing highly effective combustion at low temperatures (for low NOx generation) is the use of a fluidized bed. The operating principle of a fluidized bed is that an upflowing stream of combustion air supports the burning solid fuel. The result is thorough combustion due to high turbulence and complete mixing. A common enhancement is to add sand, limestone, or other non-combusting solids to the bed to reduce peak temperatures and capture pollutants, notably sulfur oxides.
There are several types of fluidized beds. Stationary or bubbling beds use low air velocity to suspend larger particles in an essentially fixed bed. Smaller particles are entrained into the exhaust. Recirculating beds use high air velocity to suspend and eventually entrain particles of all sizes. A cyclone separator then grades the entrained particles by size. Unburned particles are then returned to the bed to complete combustion, while spent particles are removed from the system. Stationary beds have the advantages of lower power to drive the air, reduced erosion on heat exchanger pipes, and entrainment of only smaller particles, thereby reducing the need for downstream cyclones, filters, or other particle collection devices.
As noted earlier, coal has multiple combustion steps. In a conventional fluidized bed, the coal shrinks as it burns, and thus progresses up through the bed while ash is released. The bed must therefore be deep. There are also problems with ignition, flame stability (sometimes requiring a week or more to reach steady operation), and ash removal.
Conversely, in the residual gas and particle combustion chamber of the system, the fuel is mostly gaseous, the volatiles having been generated in the devolatilization chamber, and the off-gas (mostly CO) generated in the char combustor. The only solids are microscopic, and they flow along with these gases. This configuration allows for thin beds that are easy to build and require only low flow velocity to obtain fluidization. The immediate operational benefits are easier starting, rapid attainment of operating temperature, stable operation, and no ash removal problems.
The gases coming from the volatile generator and the char combustor are very hot. Heat exchanger tubes can be placed in these streams before the gases enter the fluidized bed combustor. The immediate design benefits of placing the tubes outside the bed are decreased erosion, decreased thermal NOx, enhanced space utilization, and lower net heat stress. The operational benefits are the options of using the tubes as (1) preheaters for the subsequent fluidized bed, or (2) post-heaters to provide the high temperatures required for ultra-supercritical steam systems, as required for the newest, highest efficiency turbine configurations.
A common procedure in coal fired fluidized beds is to use limestone as a non-combusting additive to reduce NOx (standard low temperature combustion technique) and trap sulfur oxides. In particular, the reaction of limestone and sulfur oxides yields synthetic gypsum, a commercially valuable product.
This approach is used with the limestone being ground before entering the bed. Grinding the limestone produces many particles that are too small to be confined in the fluidized bed. If these particles are added to the bed, they will increase the particle loading on the downstream particle traps (ESP, filters, or the vapor trap described below). This additional loading must be avoided because it: (1) decreases total particle collection, (2) increases processing costs, and (3) adds to the volume, and thus the cost, of final disposal. It is therefore preferable to blow air through the ground limestone before it enters the bed. The separated large particles then proceed to the bed, while the fine dust is collected and sold for agricultural purposes.
Conventional fluidized beds are typically cylindrical in cross section to provide uniform flow and combustion. The gas fuel of the invention, however, enables the use of other geometries. Specifically, a rectangular geometry is useful, with the incoming limestone fed in along one side. The limestone then progresses uniformly across the bed, where it exits on the opposite side from the entrance. This geometry provides uniform conversion of the incoming pure limestone to completely processed gypsum at the exit. Additional enhancements include the addition of flow straightening screens or grids to keep the flow uniform (plug versus laminar), as well as gradual deepening of the bed towards the exit to provide complete sulfur oxide capture even with partially processed limestone.
Conventional synthetic gypsum has several problems. First, when formed in a limestone scrubber, conventional gypsum is wet, and therefore requires expensive drying before it can be used. Conversely, the gypsum in the present process is dry, and thus ready for use. Another problem is that conventional synthetic gypsum is formed at low temperatures, and at low flow rates, and therefore contains mercury and selenium, both of which are toxic. In the system, these toxic elements are removed downstream, and thus do not contaminate the gypsum. Finally, conventional synthetic gypsum can contain large amounts of ash, depending on the specific plant technology. Conversely, gypsum from the system is essentially ash free. The net result is that gypsum from the system has major market advantages over gypsum from conventional systems.
As noted, sound greatly influences combusting systems. Three approaches are utilized with the invention. Low frequency (low hundreds of Hz) oscillations move the inert bed particles, any incoming (small) particles, and the burning gases. These sound waves are applied from the sides of the bed, and propagate parallel to the bed. Because the particles are suspended by the upflowing fluidizing air, these sound waves are easily capable of displacing the particles in the horizontal direction. The net result is great improvements in mixing and turbulence, and thus great improvements in combustion. Upward waves are not helpful here because they would lift the entire bed.
Mid frequency (low kHz) move particles relative to gas. These sound waves are applied vertically downward to compress the bed, and to confine large particles to the bed. In any fluidized bed, the particles near the top are loosely distributed in a “freeboard.” The bed in this region thus performs more like an entrained flow system, which is not desired. By known acoustic radiation pressure principles, the downward force is greater than the returning wave pulse, so that the net result is the desired confinement. Once confined, any large fuel particles are burned out completely. Large inert particles are confined to the bed, and therefore proceed immediately to the gypsum trap. Only fine particles can pass through the sound, thereby reducing the load on the downstream particle traps to a minimum.
For comparison, centrifugal force systems have been used to obtain these benefits. The advantages of the sonic system over centrifugal systems are (1) greatly reduced cost, (2) simpler operation, and (3) selective suppression with frequency control.
High frequency sound (Upper tens of kHz) moves the gas past the particles, and thus aids combustion.
All three frequencies can be used together, just one at a time or any combinations of any two.
To protect the sound sources from excessive heat, the sound is generated away from the bed and piped in using a “wave guide” technique as marketed in a Bose stereo system. This approach also provides a means of treating large beds, without excessive sound attenuation.
A particular advantage of mixed frequencies is enhanced heat transfer in the tubes. The limiting problem here is that a boundary layer of gas forms around the tubes, thus reducing heat transfer. The application of sound disrupts this boundary layer, while also increasing local mixing. The net result is improved heat transfer to the tubes, thus improving overall system efficiency without increasing erosion or causing other types of tube damage. The required sound can be generated by speakers or by horns. Additional enhancements include focused sound, as well as selected dampening to control excess noise.
An example of speakers 34 used with a fluidized bed 30 is depicted in
Speakers are useful along the entire height of the bed, beginning at the base of the fluidized bed. The sonic waves agitate the bed to improve performance and do not suffer from the disadvantages of mechanical vibrators. The speaker system is inexpensive and the vibrations can traverse the entire bed, thereby providing maximum, uniform treatment of the entire reactor vessel. The active component of this sound is horizontal. The frequency is in the low hundreds of Hz. Natural frequency resonance is acceptable, and unavoidable, for standing waves. Traveling wave components, with appropriate phase linking, eliminate undesirable “dead zones” near the nodal points.
4. Combined System
The char combustor, seen in
The products from the devolatilization chamber and the char combustor are then collected and burned in the residual gas and particulate combustion chamber 30. This chamber may be a fluidized bed, preferably using limestone to convert sulfur oxides to synthetic gypsum. One option is the placement of heat transfer tubes before the residual gas and particulate combustion chamber 30 to control peak temperatures, decrease erosion, and improve overall system efficiency. Another option is sonic enhancement for improved bed performance and ideal combustion. The products of this chamber are: (1) high pressure steam to drive turbines or other equipment, and (2) a low pollutant exhaust stream that can be released immediately or subjected to additional cleaning.
While the invention has been described with reference to preferred embodiments, variations and modifications would be apparent to tone of ordinary skill in the art. The invention encompasses such variations and modifications.
This application claims benefit of provisional application No. 61/522,976, filed on Aug. 12, 2011, the entire contents of which are herein incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61522976 | Aug 2011 | US |
