Patent Application
20070251432
Not Applicable
Not Applicable
1. Field of Invention
The present invention relates to the use of nuclear response measurements of fuel flow and fuel quality combined with a process control system for optimization of air and coal stoichiometry at each burner in a coal fired boiler.
2. Description of the Related Art
In the field of coal fired boilers used especially by utility companies and industrial boiler operators, it is well known that increasingly stringent emissions limits continue to apply pressure to reduce NOx emissions from coal fired boilers. Years of investigation by utilities, boiler suppliers, and controls suppliers have determined that stoichiometries local to the burners must be maintained to achieve very low NOx emissions without negatively effecting combustion efficiency or boiler performance. Common barriers to lower NOx emissions include poor coal and air distribution which may also lead to high unburned carbon, high COx, boiler slagging, and oxygen and/or steam temperature imbalances.
To date, all low NOx firing systems are based on a pre-defined balance of air and coal at the burners. Deviations from the design air/fuel balance at individual burners results in burners operating at a fuel lean or fuel rich condition. A fuel lean burner produces high NOx levels at elevated O2 resulting in a flue gas with high COx, high NOx, and increased LOI due to burners operating with poor stoichiometries. A fuel rich burner produces large amounts of COX, high LOI, and longer flames while lowering the oxygen level in the flue gas. Many coal fired boilers with poor air/fuel distribution experience problems such as: emission problems; increased unburned carbon in fly ash; distorted oxygen profile at the boiler outlet; uneven steam temperature profiles; flame impingement; increased slagging; and water wall heat waste.
The inventor of the present invention is a co-inventor of the subject matter disclosed in U.S. Pat. No. 7,006,919, titled “Real time continuous elemental measurement of bulk material,” issued on Feb. 28, 2006. In that patent, various methods and an apparatus for continuous real-time measurement of bulk material using gamma irradiation and neutron irradiation is disclosed. The '919 device includes a dual-energy gamma attenuation (DGA) device for monitoring bulk material flow and for producing a spectrum that is compared to a baseline spectrum to produce a relative weight/impurity ratio. A prompt gamma neutron activation analysis (PGNAA) device monitors the same bulk material flow and produces a spectrum that is compared to a library of spectrums to produce a relative component ratio. The relative component ratio is processed with the relative weight/impurity ratio to produce an absolute weight and impurity value, which is then processed with the relative component ratio to produce absolute component, or analyte, values.
The DGA analysis technique involves bombarding a bulk material with gamma rays from two gamma ray emitters of sufficiently different energies. The gamma rays interact with the bulk material resulting in the attenuation of the number of gamma rays transmitted through the bulk material. The gamma rays are typically detected by a scintillation crystal (typically NaI). The sum of these released gamma rays at these specific energies is referred to as an energy spectrum. The technology relies on the fact that elements with different atomic numbers attenuate gamma rays at specific energies in different ways. Thus, for low-energy gamma rays (i.e., those generated by a low energy gamma emitter such as Am-241), the attenuation of gamma rays is largely dependent on the atomic number of the atoms/elements present in the bulk material. For high-energy gamma rays (i.e., those generated by a high-energy gamma emitter such as Cs-137), attenuation is independent of the atoms/elements in the bulk material. Analysis of the energy spectrum leads to a determination of the bulk elemental composition of the bulk material.
DGA based sensors are known in the art. DGA devices are based on the premise that analyzed material will attenuate different energy gamma rays in fixed repeatable ways. A DGA device consists of a gamma energy source arrangement consisting of dual energy gamma emitters. The gamma emitters are chosen in such a way that the material to be analyzed will attenuate the different energy gamma rays in ways that are conducive to measuring one or more specific properties of the material being measured. One such application of DGA technology uses gamma ray sources to interrogate coal, with the assumption that the material of which the coal is composed will attenuate the differing energy gamma rays to produce a measurement that is conducive to determining coal ash content and density.
The PGNAA technique involves bombarding a bulk material sample with neutrons from a neutron emitter (typically Cf-252). The neutrons collide with atoms/elements in the sample, emitter housing, and/or an external moderator and are captured by the nuclei of atoms/elements present in the sample. The capture process often involves the release of gamma rays at energies specific to the captured atom/element. These gamma rays are detected typically by a scintillation crystal (typically NaI). The sum of the detected gamma energy at these specific energies is an energy spectrum. Analysis of the energy spectrum provides analytical information on the proportion of the various elements present in the bulk material.
As discussed in the '919 patent, various PGNAA based sensor systems are known. One such analyzer is that described in U.S. Pat. No. 4,582,992, titled “Self-Contained, On-Line, Real-time Bulk Material Analyzer,” issued to Atwell, et al., on Apr. 15, 1986, which uses PGNAA technology in an attempt to determine the elemental content of the bulk material. The described analyzer uses an arrangement of neutron sources and gamma ray detectors in an enclosed assembly to perform its analysis. A similar device, described in U.S. Pat. No. 6,362,477, titled “Bulk Material Analyser for OnConveyor Belt Analysis,” issued to Sowerby, et al., on Mar. 26, 2002, uses PGNAA technology in a bulk material on-conveyor belt arrangement to analyze bulk material. Again, this analyzer uses a neutron source and gamma ray detectors in an enclosed assembly to perform its analysis.
The present invention is a method and an apparatus for continuous real time heating value/coal flow balancing of coal from a coal feeder to a burner. The apparatus includes a dual-energy Gamma Attenuation (DGA)/multi-energy Gamma Attenuation (MGA) device for measuring coal quality at a specific location (i.e., on the coal feeder tube between the coal silo and the mill) in a coal fired plant in order to control the parameters of the plant according to the measured coal quality. By strategically placing the DGA/MGA device, continuous accurate real-time coal quality information is accomplished for making individual adjustments in order to improve stoichiometry to optimize performance of the system.
A system incorporating the apparatus of the present invention is includes a coal silo for receiving and burning coal. A DGA/MGA device includes at least one DGA/MGA source and at least one DGA/MGA detector in a fixed position relative to the coal silo. The analyzed coal flows onto a coal feeder conveyor or is otherwise moved to a mill. The coal is pulverized before being combined with air by way of at least one actuated damper. The dampers are electrically or electromechanically operated to open and close to adjust the volume of air introduced into the pulverized coal. The resulting product is then delivered to at least one burner.
Quality information pertaining to the coal is determined by the DGA/MGA device and then sent to a processor. The processor is provided for varying the control of various system components based upon the coal quality information received from the DGA/MGA device.
A time delay is established between the time the coal is passed through the coal silo feeder tube, at which point the coal is analyzed by the DGA/MGA device, and the time that the coal is processed through the dampers. The processor uses the coal quality information pertaining to the coal and the known time delay to adjust the actuated dampers, thereby controlling the amount of air mixed with the coal for optimal burner stoichiometry. Thus when a selected volume of coal is analyzed and particular settings at the dampers are required for that volume, the processor waits a predetermined time equal to the time delay before adjusting the dampers.
The above-mentioned features of the invention will become more clearly understood from the following detailed description of the invention read together with the drawings in which:
The present invention provides a method and an apparatus for continuous real time heating value/coal flow balancing of coal from a coal feeder to a burner. The apparatus includes a dual-energy Gamma Attenuation (DGA)/multi-energy Gamma Attenuation (MGA) device for measuring coal quality at a specific location in a coal fired plant in order to control the parameters of the plant according to the measured coal quality. By strategically placing the DGA/MGA device, continuous accurate real-time coal quality information is accomplished for making individual adjustments in order to improve stoichiometry to optimize performance of the system.
The DGA/MGA device of the present invention is illustrated generally at 10 in the figures. As described below, the DGA/MGA device 10 is mounted relative to a coal silo feeder tube 28 to determine the quality of the material flowing to the coal feeder 30. This coal quality information is fed to a processing component 18 where control logic is used to adjust air/coal dampers 34 feeding the burners 36 into the boiler (not shown), resulting in improved burner stoichiometry.
Coal feeders of the prior art meter the flow of coal to the mills based on volume or weight, as opposed to quality or heating value. Variability in fuel quality across the feeders and subsequent burners coupled with air (O2) introduction based upon design conditions rather than actual on-line parameters produces the imbalances discussed above, such as poor air/fuel distribution. In the present invention, density (flow rate) and coal quality (heating value) are measured continuously and real time adjustment is provided for air introduced in the system just prior to combustion. The discrete control of this fuel/air balance allows for improved stoichiometries local to all burners 36.
Accordingly, a method and apparatus for continuous real time heating value/coal flow balancing are illustrated generally in the figures. A system incorporating the apparatus of the present invention is illustrated in
The analyzed coal flows onto a coal feeder conveyor 30 or is otherwise moved to a mill 32. The coal 24 is pulverized before being combined with air by way of at least one actuated damper 34. The dampers 34 are electrically or electromechanically operated to open and close to adjust the volume of air introduced into the pulverized coal 24. The resulting product is then delivered to at least one burner, shown schematically at 36.
Quality information pertaining to the coal 24 is determined by the DGA/MGA device 10 and then sent to a processor 18. The processor 18 is provided for varying the control of various system components based upon the coal quality information received from the DGA/MGA device 10. To this extent, the processor 18 includes at least one input/output (I/O) control 20 in communication with existing boiler controls 22 and/or other peripheral devices to control at least the dampers 34 in response to the coal quality information obtained via the DGA/MGA device 10. The I/O control 10 is a conventional input and/or output device including but not limited to a plant network card or an Ethernet card.
A time delay is established between the time the coal 24 is passed through the coal silo feeder tube 28, at which point the coal 24 is analyzed by the DGA/MGA device 10, and the time that the coal 24 is processed through the dampers 34. The processor 18 uses the coal quality information pertaining to the coal 24 and the known time delay to adjust the actuated dampers 34, thereby controlling the amount of air mixed with the coal for optimal burner stoichiometry. Thus when a selected volume of coal 24 is analyzed and particular settings at the dampers 34 are required for that volume, the processor 18 waits a predetermined time equal to the time delay before adjusting the dampers 34. Therefore, as any given volume of coal 24 is processed through the dampers 34, the dampers 34 are adjusted for that volume of coal 24, as determined by the DGA/MGA device 10 and the processor 18. Additional boiler distribution controls and information is obtained as necessary from a boiler distribution control system. Manual override functionality is accomplished by way of existing boiler controls 22.
The present invention may further include a Full Stream Elemental Analyzer (FSEA) 38 such as that described in the aforementioned '919 patent. When incorporated in such a system, the FSEA system 38 is in communication with the processor 18 and allows for improvements in the coal quality information as well as provides more specific elemental and slagging indices concerning the coal stored in the silo and that has been previously measured. These indices are stored in a database 40 associated with the FSEA system 38 and communicated to the processor 18.
From the foregoing description, it will be recognized by those skilled in the art that a method and an apparatus for continuous real time heating value/coal flow balancing of coal from a coal feeder to a burner has been provided. The apparatus provides for a DGA/MGA device disposed for measuring coal quality at a specific location—between a coal silo and a mill—in a coal fired plant in order to control the parameters of the plant according to the measured coal quality. By strategically placing the DGA/MGA device, continuous accurate real-time coal quality information is accomplished for making individual adjustments in order to improve individual burner stoichiometries to optimize performance of the system.
While the present invention has been illustrated by description of several embodiments and while the illustrative embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant's general inventive concept.
