Patent application title: ROTARY REGENERATIVE CHEMICAL LOOPING COMBUSTION
J. Edward Cichanowicz (Saratoga, CA, US)
Lawrence J. Muzio (Laguna Niguel, CA, US)
IPC8 Class: AC10L906FI
Class name: Fuel and related compositions coal treating process or product thereof removal of undesirable
Publication date: 2009-10-29
Patent application number: 20090265978
Chemical looping combustion of coal utilizing a rotating regenerative
assembly to expose coal particles to alternating conditions of oxidation
1. A method of chemical looping combustion, comprising: passing a stream
of solid coal particles by a heated metal or metal oxide substrate or
substrates of a rotating regenerative assembly and exposing the coal
particles materials and the substrate or substrates to alternating
conditions of oxidation and regeneration, such that at least a portion of
the coal particles pyrolyze, gasify, and combust within the rotating
2. The method of claim 1, wherein another portion of the coal particles do not combust within the rotating regenerative assembly and exit the rotary regenerative environment as solid partially reacted char that continues to burn in an oxidizing environment downstream of the rotating regenerative assembly.
3. The method of claim 2, wherein the partially reacted char, after experiencing residence time for burnout, is directed to an environmental control system of either a new or existing power station for removal of combustion products including particulate matter, sulfur oxides, and nitrogen oxides.
4. The method of claim 1, wherein the method operates in a continuous manner.
5. The method of claim 2, wherein the method operates in a continuous manner.
6. The method of claim 3, wherein the method operates in a continuous manner.
7. The method of claim 1, wherein the rotary regenerative assembly is configured to provide a residence time for the coal particles of at least 0.15 seconds.
8. The method of claim 2, wherein the rotary regenerative assembly is configured to provide a residence time for the coal particles of at least 0.15 seconds.
9. The method of claim 4, wherein the rotary regenerative assembly is configured to provide a residence time for the coal particles of at least 0.15 seconds.
10. A rotary regenerative chemical looping combustion apparatus configured to oxidize coal particles at atmospheric pressure.
11. The rotary regenerative apparatus of claim 10, further comprising materials for absorbing and regenerating oxygen selected from Fe, Cu, Mn, Co, and/or Ni.
12. The rotary regenerative apparatus of claim 10, further comprising a carrier material selected from Al and/or Ti.
13. The rotary regenerative apparatus of claim 10, further comprising a carrier material selected from grid, honeycomb, plate, and/or corrugated geometries.
14. The rotary regenerative apparatus of claim 10, wherein the apparatus is configured for continuous operation.
15. The rotary regenerative apparatus of claim 14, wherein the apparatus is configured to provide a residence time for coal particles of at least 0.15 seconds.
16. The rotary regenerative apparatus of claim 10, wherein the apparatus is configured to provide a residence time for coal particles of at least 0.15 seconds.
17. A method of retrofitting an operating coal-fired power plant, comprising: retrofitting a coal-fired power station with regenerative chemical looping such that the coal-fired power plant passes a stream of solid coal particles by a heated metal or metal oxide substrate or substrates of a rotating regenerative assembly and exposes the coal particles materials and the substrate or substrates to alternating conditions of oxidation and regeneration, such that at least a portion of the coal particles pyrolyze, gasify, and combust within the rotating regenerative assembly; and such that another portion of the coal particles do not combust within the rotating regenerative assembly and exit the rotary regenerative assembly as solid particle char that continues to burn in an oxidizing environment downstream of the rotating regenerative assembly.
18. The method of claim 17, wherein the method operates in a continuous manner.
19. The rotary regenerative apparatus of claim 18, wherein the rotary regenerative assembly is configured to provide a residence time for the coal particles of at least 0.15 seconds.
20. The rotary regenerative apparatus of claim 17, wherein the rotary regenerative assembly is configured to provide a residence time for the coal particles of at least 0.15 seconds.
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority to U.S. Provisional Patent Application Ser. No. 61/043,390, filed Apr. 8, 2008. The disclosure of U.S. Provisional Patent Application Ser. No. 61/043,390 is hereby incorporated herein by reference in its entirety.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
THE NAMES OF THE PARTY TO A JOINT RESEARCH AGREEMENT
INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This disclosure is directed to chemical looping combustion.
(2) Description of Related Art Including Information Submitted under 37 CFR 1.97 and 1.98
Chemical looping combustion can employ a metal oxide circulated between two oxidizing and reducing reactors.
BRIEF SUMMARY OF THE INVENTION
At least some aspects and embodiments of this disclosure are directed to a method of chemical looping combustion, including: passing a stream of solid coal particles by a heated metal or metal oxide substrate or substrates of a rotating regenerative assembly and exposing the coal particles materials and the substrate or substrates to alternating conditions of oxidation and regeneration, such that at least a portion of the coal particles pyrolyze, gasify, and combust within the rotating regenerative assembly. In at least some embodiments, another portion of the coal particles do not combust within the rotating regenerative assembly and exit the rotary regenerative environment as solid partially reacted char that continues to burn in an oxidizing environment downstream of the rotating regenerative assembly. In at least some embodiments, the partially reacted char, after experiencing residence time for burnout, is directed to an environmental control system of either a new or existing power station for removal of combustion products including particulate matter, sulfur oxides, and nitrogen oxides. In at least some embodiments, the method operates in a continuous manner. In at least some embodiments, the rotary regenerative assembly is configured to provide a residence time for the coal particles of at least 0.15 seconds, and, in at least some embodiments at least 0.5 seconds. At least some aspects and embodiments of this disclosure are directed to a rotary regenerative chemical looping combustion apparatus configured to oxidize coal particles at atmospheric pressure. In at least some embodiments, the rotary regenerative apparatus also includes materials for absorbing and regenerating oxygen selected from Fe, Cu, Mn, Co, and/or Ni. In at least some embodiments, the rotary regenerative apparatus also includes a carrier material selected from Al and/or Ti. In at least some embodiments, the rotary regenerative apparatus also includes carrier material selected from grid, honeycomb, plate, and/or corrugated geometries.
At least some aspects and embodiments of this disclosure are directed to a method of retrofitting an operating coal-fired power plant, comprising: retrofitting a coal-fired power station with regenerative chemical looping such that the coal-fired power plant passes a stream of solid coal particles by a heated metal or metal oxide substrate or substrates of a rotating regenerative assembly and exposes the coal particles materials and the substrate or substrates to alternating conditions of oxidation and regeneration, such that at least a portion of the coal particles pyrolyze, gasify, and combust within the rotating regenerative assembly; and such that another portion of the coal particles does not combust within the rotating regenerative assembly and exit the rotary regenerative assembly as solid particle char that continues to burn in an oxidizing environment downstream of the rotating regenerative assembly.
Other exemplary embodiments and advantages of this disclosure can be ascertained by reviewing the present disclosure and the accompanying drawings.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
This disclosure is further described in the detailed description that follows, with reference to the drawings, in which:
FIG. 1 is a cross-sectional representation of an apparatus/system for chemical looping of solid carbon particles in accordance with at least some aspects of this disclosure; and
FIG. 2 is a cross-sectional representation of particles entering and passing through a looping target in accordance with at least some aspects of this disclosure.
DETAILED DESCRIPTION OF THE INVENTION
Exemplary embodiments of this disclosure are described herein by way of example.
At least one aspect of this disclosure is directed to a method, apparatus, and system for chemical looping combustion of solid carbon particles.
Chemical looping combustion can employ a metal (Me) or a reduced metal oxide (MexOy-1) that can be associated with a fully oxidized form of the same metal (MexOy), that can be circulated between two reactors. One reactor can serve as an oxidizing system and/or provide for an oxidizing step while the second can serve as a reducing system and/or provide for a reducing step. A benefit of chemical looping can be the utilization of fossil fuel and release of heat in a manner to deliver a high CO2 content product gas, while requiring a fraction of the parasitic energy load that characterizes presently available CO2 control options.
Chemical looping work has historically focused on utilizing fluidized bed reactors that can provide the alternating oxidizing and reducing environment, with moving media to transfer the carrier between reactors.
An alternative approach, as described by Noorman et al. (Noorman, S. et. al., `Packed Bed Reactor Technology for Chemical Looping Combustion", Ind. Eng. Chem. Res., 46, 4212-4220, 2007) described fixed beds that are utilized in an alternating oxidizing and regenerating mode.
This disclosure describes innovative methods, systems and apparatus, utilizing coal-based fuels, to alternatively expose oxidizing air to surfaces that can retain oxygen, and subsequently to a reducing environment, that can be conducted without "batch" or start/stop operation, or recirculating a large mass of solid reacting particles.
These methods, systems and apparatus can be based on the innovative application of two devices or methods with significant large-scale utility experience--one an apparatus well known to the power industry for over 50 years, and the second a special purpose material that has experienced widespread application in the U.S. since the year 2000. This apparatus and special material are the regenerative air heater, and the high surface area catalyst substrate, similar to that utilized in processing combustion products with selective catalyst reduction (SCR) NOx reduction.
The method, system, and apparatus, described in this disclosure as rotary regenerative chemical looping combustion (RRCLC) can provide an innovative option to deploy chemical looping to oxidize coal particles.
The utilization of a rotary regenerative reactor has been proposed before, but for application with gaseous fuels at elevated pressure (see Gauthier, T., "ENCAP SP4 Chemical Looping Workshop", Castoer-Encap-Cachet-Dynamis Workshop", 2006).
However, applying this concept to coal particles is not obvious, as prior discussion of chemical looping and coal has utilized fluidized beds, which feature extremely long residence times. Significantly, and unlike Gauthier, 2006, the methods, systems and apparatus disclosed in this disclosure can operate at near atmospheric pressure, enabling retrofit to the bulk of the existing coal-fired power plant inventory.
The material that can alternately absorb and regenerate oxygen can be incorporated on a high surface area substrate, the latter material similar to that utilized in commercial practice for SCR catalysts. This high surface area material can then be configured into an assembly resembling the "baskets" of a regenerative heat exchanger to alternately expose the reactive surfaces to air and fuel.
This disclosure describes an exemplary basis of how to configure this device as a retrofit option to existing coal fired power stations, while utilizing the current environmental control system and steam generator with modest modification.
This innovative concept is not limited by the type and selection of materials used to absorb and regenerate oxygen, or the material used as a carrier or substrate. Any of the materials described by Noorman (2007) or Cao (Cao, Y. et. al., "Investigation of Chemical Looping Combustion by Solid Fuels. 1. Process Analysis", Energy & Fuels 2006, 20, 1836-1844.) are candidates for this purpose, as well as other materials studies by numerous other investigators. These include, for example, in reduced metal form, Fe, Cu, Mn, Co, Ni, and others.
The carrier material can be any carrier material conventionally utilized in SCR catalyst or other substrates: Al, Ti, or others. In particular, the carrier material can, for example, be formed in a manner in the geometry and composition of the substrates utilized for catalyst for selective catalyst reduction (SCR)NOx control. These can be, for example, grid-type or honeycomb geometry shapes, or plate-type, any series of corrugated shapes, or any type of geometry that presents a high specific surface area. Such materials are characterized by extremely high surface area, as determined by numerous geometrical factors. For the grid-type geometry, these are the pitch or cell opening, and wall thickness. For the plate-type geometry, relevant factors include the plate spacing or hydraulic diameter, and plate thickness.
The substrate materials can be specially processed to offer specific pore size distribution, and physical features such as resistance to erosion. The experience in the manufacture and design of these shapes derived with SCR NOx reduction can be beneficial in avoiding erosion, or the sticking of ash particles to surfaces, this latter issue can be of great interest for materials where the particle temperature is near the temperature of ash fluidization or slagging.
At least one innovative embodiment of chemical looping combustion can enable retrofit of conventional coal-fired power stations with regenerative chemical looping, delivering low cost control of CO2 and other emissions species.
The innovation of rotary regenerable chemical looping combustion (RRCLC) can be utilized to enable continuous operation. Existing experience and expertise in the following areas can provide the basis of the concept: the extensive theoretical work addressing absorbing materials conducted to date by various investigators; the manufacture and testing of high temperature substrates by manufacturers of industrial catalysts; the engineering experience of rotating heat transfer equipment; and the evaluation of heat transfer surfaces to generate steam with minimal contribution of radiation heat transfer. However, this background work has not yet prompted the presently disclosed embodiments.
The prior work did not acknowledge or exploit the significance of studies conducted with drop-tube test apparatus, that are intended to define how individual coal particles pyrolize, gasify, and combust.
This innovative concept of RRCLC can be accommodated into the configuration of an existing or new coal-fired power station, for example, through the following equipment or design modifications.
Rotary Regenerative Equipment
The rotating regenerative assembly commonly used for Ljungstrom or Rothemuhle type air preheaters will be used to expose the active materials and substrate to alternating conditions of oxidation and regeneration.
The premise of the rotary regenerative chemical looping combustion for coal--heretofore not recognized as a viable concept--is that a stream of coal particles flowing through heated walls will pyrolyze, gasify, and "combust". The inspiration for this mechanism is the research exploring pyrolysis of individual coal particles, conducted in "drop tube" furnaces. These furnaces have been used to establish the rate of coal particle pyrolysis and burnout. Much of this work is conducted not in air, but in artificial atmospheres of oxygen in background gases such as argon. The purpose of these artificial atmospheres is to separate the contribution to NOx of coal-bound nitrogen compared to atmospheric nitrogen. In essence, the drop-tube furnace work simulates the coal heating and oxidation that will occur in RRCLC better than it simulates conventional coal firing in a boiler.
Kobayashi (1976) (Kobayashi, H., Howard, J. B., and Sarofim, A. F., "Coal Devolatilization at High Temperatures", Proceedings of the Sixteenth Symposium (International) on Combustion, 1976) depicts the type of drop-tube furnace typically utilized by researchers. The details of this furnace are described in the referenced papers--but for the present disclosure, it is noted that coal particle heating by a hot gas and thermal radiation from closely spaced walls is similar between the drop-tube furnace and the environment proposed for coal particle chemical looping. Coal particle pyrolysis and burnout within the substrate can be expected to be faster than in the drop tube furnace, as turbulent flow can characterize at least the inlet portion of the flow field, and increase rates of mass transfer.
FIG. 1 projects a exemplary system for chemical looping with coal, based on the physics and chemistry within a drop-tube furnace, and thus circumventing (or alternatively supplementing) the need for separate gasification as a pre-processing step. The premise of the mechanism is that coal particles 20 enter the channel formed between walls 30, 32 of the substrate, are preheated by the recycled CO2, and exposed to further heating from the walls 30, 32. The temperature of these walls 30, 32 is unknown, but these walls will have exited the absorption side of the reactor, and may be near peak temperatures of 800° C. to 1200° C. Similar to the events in a drop-tube furnace, three zones are envisioned, defined primarily by the fluid dynamics in the channel, described as follows:
Turbulent Flow 40. The first zone 40 can be characterized by turbulent flow, with coal particle temperature driven by the gas temperature of the conveying media. Coal particles 20 will experience incipient thermal radiation, and be subject to initial heating. Oxygen may diffuse from the walls, but only in small quantities as the concentration of reducing reactants is small.
Transition to Laminar Flow 42. The flow field reverts to laminar, dictated by the small physical opening or characteristic size of the substrate. Thermal radiation continues to heat particles so that CO and H2 evolve in larger concentrations. These species can diffuse to the wall, accelerating the reduction reactions, and the liberation of oxygen.
Laminar Flow 44. The flow field becomes fully laminar, and significant CO and H2 evolve, promoting and accelerating coal particle reaction. Heat is now liberated in significant quantities from the coal particles. With the elevated temperature the remaining carbon can gasify via the following reactions:
a. C+CO2=2CO (Boudard)b. C+H2O═CO+H2 (Water-Gas)c. CO+H2O CO2+H2 (Water-Gas Shift)d. CH4+H2O═CO+3H2O (Steam Reforming)
The final temperature of the product gas and wall can depend on many factors, including coal particle density, radiation from the coal particle, gas flow, content of active material, etc.
The concept of installing catalyst within a rotary reactor was proposed by Gauthier (2006). However, Gauthier did not describe the extension of this concept to the direct utilization of coal. In addition, the work described by Gauthier (2006) and further elaborated upon by Lebas ("CLC combined cycles Performances and reactors; ENCAP Seminar on CLC; 6 Dec. 2006, Gothenburg; E. Lebas et al.) requires a continuous separate process stream of steam to participate in the absorption and desorption steps. Indeed, the background work with chemical looping and coal was mostly carried out in fluidized bed reactors that offer a significant residence time for coal particle pyrolysis and burnout. For example, up to 40 seconds of residence time has been estimated as typifying coal particles in a chemical looping reactor. This prior experience with fluidized beds would not suggest that a rotary reactor designed for gas-based fuels would offer a practical means to deploy chemical looping for coal. Finally, both Gauthier (2006) and Lebas (2006) suggest that the work be conducted at elevated pressure, for the purpose of supporting combustion turbine utilization of the effluent. These preconditions can complicate the design and operation of a rotary regenerative device.
However, the results of drop tube furnace studies--which may more closely simulate the mechanism of coal particle pyrolysis and burnout than simulate conventional coal combustion--provide a basis for the process environment of a substrate in a rotary reactor that will prompt pyrolysis and burnout. For example, Kobayashi reports that about 50% of the coal particle weight is lost to either pyrolysis or burnout, within 50 msec, when the drop tube furnace wall is approximately 1200° C. The rotary regenerative reactor can be configured utilizing-basic assumptions about inlet gas velocity and practical height for a commercial system, for example, a residence time of at least 0.15 seconds, or for example at least 0.5 seconds and up to several seconds could be experienced. In addition, coal is known to contain moisture both associated with the surfaces of the particles and inherent to the composition, which when undergoing heating generates steam required for the absorption and desorption.
Further, it is not necessary that all of the carbon burnout be completed within the confines of the regenerative surface--once the nearly pure O2 stream is generated, final carbon burnout can be accomplished downstream of this device. Consequently, designing the rotary reactor to reflect the process conditions of the drop tube furnace will provide the basis for a practical design. Accordingly, several aspects of the concept described by Gauthier (2006) and Lebas (2006)--the need for a separate moisture process flow stream, the need to operate at an elevated pressure above atmospheric, the need to complete all combustion reactions within the confines of the rotary regenerative device, not recognizing the relationship between the fundamental drop-tube furnace work and the thermal environment of the regenerative material, and not recognizing the ability to use recycled partial combustion products or a separate feed of gaseous fuel to seed to regeneration--demonstrate that the prior art did not recognize the potential to utilize pulverized coal in this manner.
There are numerous configurations in which the process can be operated. Specifically, because the processes can vary along the flowpath through the RRCLC, the geometry and chemical composition of the material (substrate and active oxygen carrier) can be varied along the flowpath in order to try and optimize the chemical looping process.
A commercial rotating reactor can be configured to enable practical operation. Such configuration can enable maintaining the ability to rotate a large mass of equipment at high temperature. The utilization of special ceramic materials, and applying cooling to bearings or other rotating components can be applied. Further, the in-leakage of air into the process, or cross-leakage of gases between the absorption and regeneration side, can be controlled with active seals, and/or creating a sheath or envelope of gaseous fuels to minimize the in-leakage of oxygen. These fuels can be either generated on-site by the gasification reactions, or be comprised of a small stream of methane or natural gas.
Boiler Modifications to Accommodate Heat Transfer
FIG. 2 presents a schematic of a power station that has retrofit one embodiment of rotary chemical looping combustion. The modifications to the existing power station start with removing the conventional air preheater, and splitting or bifurcating the gas flowpath at the junction of the boiler economizer exit, and the inlet of the particulate matter (PM) collector. FIG. 1 describes two flow paths: Path 1 for the oxygen-depleted airflow that raises steam, and Path 2 for the products of coal-based regeneration. The key elements of these two flow paths are described by numbers for the coal-based regeneration effluent, and by letters for the heated nitrogen effluent.
Combustion Air/Nitrogen Effluent
Path 1 within FIG. 2 depicts the flow path for the oxygen-depleted effluent, exiting the regenerative reactor.
Air enters the rotary regenerative chemical looping reactor (RRCLR) at point (A), and progresses through the reactor contacting the oxygen-absorbing material. As a consequence, the heated effluent, which is predominantly nitrogen, exits the RRCLR at point (B) at temperatures approaching 1200° C. This effluent, although predominantly nitrogen, can contain trace amounts of the inorganic material in coal that migrate through the clearances in the rotary reactor element seals. From point (B), the heated effluent can pass through an optional heat exchanger (C), depending on the effluent temperature and the water and steam circuit design of the boiler.
The temperature exiting the fuel side and entering the boiler can be at least the minimum temperature that will generate steam conditions. Also, in at least some embodiments, gases such as CO and/or H2 can be recycled to the inlet of the rotary device to assist with ignition and prompting the desorption reactions. In fact, the recycling of these products or partial combustion--or alternatively the introduction of methane, natural gas, or another separate fuel stream--can be beneficial to effect desorption within the time frame for practical system design. This auxiliary fuel source can, for example, be 1% or less of the entire heat throughput of the process, and can be utilized without materially affecting either the process economics or applicability.
Subsequent to this optional heat exchanger, the effluent can enter the former radiant zone of the boiler. In the cavity formed by the radiant zone, additional heat exchangers (D) can be installed, optionally as an extension of the existing pendant superheater sections. The gas leaving these heat exchangers (D) can enter the inlet of the former convective section (E), where the superheater and reheater are located. The heat transfer surfaces in these superheater and reheater sections can be modified, depending on the volume of gas flow--specifically, how much reduction in volume is incurred due to separation of oxygen. Subsequent to these heat exchangers at (E), the gas can enter the final series of heat exchangers at (F). Point F represents the exit of the economizer, as equipped in the original boiler, and additional heat exchangers (G) could be utilized to improve and/or maximize boiler thermal efficiency.
The predominantly-nitrogen effluent can thereafter be directed to the stack, utilizing a new flue duct (H) that bypasses the former ductwork and environmental control equipment.
Coal Regeneration Products, Gasification along with Devolatilization and Pyrolysis
The products of coal regeneration are described by Path 2, within FIG. 2. Coal can be introduced, in pulverized form, at (1), as conveyed in a media of CO2 that is recycled from the regeneration stream. The coal particles can enter the RRCLR, and as described by the mechanism in FIG. 1, undergo devolatilization, pyrolysis, generating primarily CO and H2. The CO and H2 can migrate to the surface, regenerate the absorbant material, liberating oxygen for reaction and production of an almost exclusive CO2 stream. These products of reaction, possibly at a temperature of up to 600° C. depending on the oxidizing material in the reactor, can exit the RCLR at point (2). This location will generally coincide with the extraction point where a fraction of this predominantly-CO2 effluent is recycled to point (3), providing the media to transport, preheat, and inject coal particles. The CO2 product stream, laden with inorganic or ash material, SO2, NOx, and other trace species, is transferred through additional flue gas ductwork retrofit for this embodiment. Depending on the temperature of this product gas, an optional heat exchanger (5) can be employed for feedwater heating, or some other means to contribute to boiler thermal efficiency.
The product gas can enter the environmental control system of the host unit, which typically include either an electrostatic precipitator (ESP) of fabric filter for particulate matter (PM) removal (6), flue gas desulfurization (FGD) for SO2 control (7), and optionally a selective catalytic reduction (SCR)NOx control system (not shown). This product gas stream can feature a significantly lower flow rate than the original "flue gas" flow for which these environmental controls were configured-perhaps 30-60% of the design flow value. As a consequence, the lower flow rate can improve the effectiveness of the PM device, as either a fabric filter or ESP can experience a more favorable air/cloth ratio or specific collecting area (SCA), respectively. Regarding the impact of this lower flow rate on FGD performance--the residence time in the absorber vessel is greater, but the significantly elevated concentration of SO2 may inhibit removal effectiveness. In summary, the significantly lower flow rate of product gas that is produced by the regeneration reactor can improve performance of the environmental control system in many aspects.
An additional aspect of this process is removal of moisture (8) and treatment or reuse of the water (9), and compression (10) to generate CO2 as a supercritical fluid for transport to the sequestration site.
The bulk of the applications proposed have relied upon fluid bed technology and fluidization processes to generate a carrier of oxygen and carbon from fuel. The most prominent method chosen is referred to as an "interconnected fluidized bed system", including three separate fluid bed reactors, for commercial application (see Lyngfelt, A., Leckner, B.; Mattison, T., A Fluidized Bed Combustion Process with Inherent CO2 Separation: Application of Chemical Looping Combustion, Fuel, 2004, 83, 1749.).
Some advantages of fluid bed processes including the utilization of such reactors and the associated solids handling are well established, particularly in the chemical industry. In addition, the last 25 years has witnessed the evolution of fluid bed combustion technology for power generation.
However the approach of utilizing fluid beds--although addressing the needs of the process--has several drawbacks. First, as with any solid bed reacting process, a significant amount of parasitic power is necessary to transport the solid particle oxygen carriers between the three reactors. Second, the need to maintain temperature and process conditions within three separate reactors requires an effective process monitoring and control strategy. The third and perhaps most significant disadvantage of utilizing fluid bed technology for chemical looping is that particles within the bed in one reactor must be separated from the gas stream and retained--the term elutriation is used to describe the carryover of such particles in the gas stream to a following reactor. This elutriation is problematic in two ways. First, it increases particulate matter content in the product gas, which must be removed in a particulate collector. Perhaps more important, it represents a loss of active materials that must be replenished. The economic penalty of this loss--even if only several percentage points--can add up to a considerable operating cost penalty.
A second approach employing packed bed reactors has been proposed by Noorman (2007). The use of conventional packed beds is also commonplace in chemical processing, but less so in power generation applications. Noorman (2007) has mathematically shown that in concept such a reactor can be built to satisfy the needs of oxygen adsorption, regeneration, and CO2 removal efficiency. However, the use of such reactors in an alternatively oxidizing and reducing capacity requires intermittent operation. The use of intermittent operation has yet to be deployed in the power industry for continuous operation, as practical operation is challenged by managing extremely large volumes of gas, in which the flow rate must be continually initiated and terminated. Although Noorman (2007) mathematically demonstrates that such on/off operation is feasible, the flow and product gas composition will be highly irregular, and the challenges of handling particulate laden material in this manner significant.
Although researchers have identified the concept of utilizing a rotary regenerative air heater, modified to include monolith materials coated with metal carriers, as a means to deploy chemical looping, such applications have been described for gaseous fuels, and not for coal. One barrier is the burnout time for coal particles can generally exceed the residence time that the gases being processed will incur in the path through the monolith. A second barrier would be the ability of the coal-fueled process to initiate desorption steps, which generally requires some content of CO, H2, methane, or natural gas. However, it can be recognized based on fundamental drop tube experiments with coal particles that the particle burnout, although not complete, is adequate to be sufficiently carried out in the residence time offered by the monolith in a rotary regenerative environment. As long as 60-70% of the heat is evolved, as demonstrated by drop-tube studies, then the predominant form of unreacted chemical energy leaving the rotary regenerative environment is the carbon in the solid particle char. This material can continue to burn in an oxidizing environment downstream of the monolith; this option would not be practical with completely gaseous fuel due to safety and the inability to control heat transfer, or control the products of combustion as proposed for the gas-fired applications.
It is also noted that a gas-fired application would not be expected to handle partially reacted fuel--in such a case, it would be vented to the stack.
However, FIG. 2 of the present disclosure shows that the gas with the partially reacted char, after experiencing residence time for burnout, can be directed to the environmental control system of either a new or existing power station, for removal of combustion products such as particulate matter, sulfur oxides, and nitrogen oxides.
Thus, the methods, systems, and apparatus as disclosed can employ continuous operation, and can include a rotary regenerative reactor utilized at atmospheric conditions.
The deployment of regenerative chemical looping combustion is applicable to a broad set of coal-fired power stations in the U.S., and abroad. An aspect to the broad applicability is selecting and configuring the process equipment to operate at near atmospheric gas pressure, to enable broad applicability to conventional boilers and gas handling equipment.
In at least some embodiments, the coal particle size can be such that at least seventy percent of the coal passes through a two-hundred mesh screen.
The foregoing exemplary embodiments have been provided for the purpose of explanation and are in no way to be construed as limiting this disclosure. This disclosure is not limited to the particulars disclosed herein, but extends to all embodiments within the scope of the appended claims, and any equivalents thereof.
Patent applications by J. Edward Cichanowicz, Saratoga, CA US
Patent applications by Lawrence J. Muzio, Laguna Niguel, CA US
Patent applications in class Removal of undesirable
Patent applications in all subclasses Removal of undesirable