Patent Application
20070190473
This invention generally relates to a premixed duct burner and more particularly, to a premixed duct burner for use with gas turbines or reciprocating engines, which has high thermal efficiency and low NOx emissions.
Power generating devices, such as gas turbines and reciprocating engines, can be implemented to meet the power requirements of industrial and commercial users. In these types of applications, heat recovery is a desirable goal. Heat recovery is commonly achieved through steam generation and may accomplished by passing hot exhaust gasses generated by a device, such as turbine exhaust gas (TEG), through a waste heat boiler or heat recovery steam generator (HRSG). In the simplest systems, there is no supplemental fuel added to the exhaust. These combined power generation and heat recovery systems are known as “combined heat and power” (CHP) systems.
Improved operating flexibility, higher thermal efficiency, and an increased heat-to-power ratio can all be obtained by adding supplemental firing to the CHP system. This may be achieved by installing supplemental firing mechanisms, typically in the form of duct burners, between the turbine and the waste heat boiler or HRSG. These burners elevate the temperature of the TEG to increase downstream heat recovery, and can provide more control over the amount of energy recovered relative to the power produced by the power generating device. The marginal efficiency (conversion of fuel energy to steam, hot water, or other thermal output) of duct burners is very high.
Presently, ultra low NOx burners (UNLBs) used in boiler applications have lower NOx and CO emissions than conventional duct burners. This may be due, at least in part, to the structural and operating characteristics of conventional duct burners. For example, conventional duct burner technology is installed as an array (or grid) of burners that do not block the complete cross section of the TEG duct. While this approach may keep pressure losses low in some applications, it limits the amount of turbine exhaust that can interact with supplemental fuel, since all of the added fuel can not immediately mix and burn with all of the TEG. Therefore, for a given heat input, the flame temperature in the immediate vicinity is higher than the temperature that would be reached in a premixed or rapidly mixed flame. This higher temperature leads to higher NOx emissions.
One prior duct burner described in U.S. Pat. No. 6,508,056 of Brandon (“Brandon”) uses a metal fiber surface burner. Brandon does not teach fully premixing the full exhaust stream with fuel, and therefore makes no attempt to minimize stack oxygen in order to maximize thermal efficiency. Also, the Brandon burner does not substantially block all of the exhaust gas flow, which limits the amount of exhaust gas that passes through the burner. This limits the amount of supplemental heat that can be added by the duct burner.
It would therefore be desirable to provide a fully premixed duct burner with ultra-low NOx (ULN) emissions.
The present invention provides a premixed duct burner for use with power generators. In one embodiment of the present invention, a fully premixed duct burner is provided that receives exhaust gas from a power generating device, such as a gas turbine or reciprocating engine, and can be used as part of a CHP system. As used herein the term “fully premixed” should be understood to mean that substantially all of the fuel and exhaust gas that will pass through the burner flame zone are mixed prior to combustion. The burner may include a burner surface adapted to combust the mixture on or above the surface in blue-flame mode. Substantially all exhaust gas passes through the burner with the exception of some small fraction of exhaust gas that may pass through cooling holes or slots. The burner assembly can also include an array of burner modules that receive substantially all of the exhaust gas and fully premix the exhaust gas with burner fuel prior to combusting the mixture on or above an array of burner surfaces.
In another embodiment, a duct burner assembly is provided for use with a power generating device that generates exhaust gas. The duct burner includes a premix section that is fluidly coupled to an exhaust duct of the heat generating device for receiving substantially all of the exhaust gas; one or more injectors that are operatively disposed within the premix section for injecting burner fuel and fully mixing the exhaust gas received by the premix section with the burner fuel; and a burner surface for receiving the fully mixed exhaust gas and fuel combination, and combusting the combination on or above the burner surface.
In another embodiment of the invention, a duct burner assembly is provided for use with a power generating device that generates exhaust gas. The duct burner assembly includes an array of burner modules, the array including one or more premix sections that are fluidly coupled to an exhaust duct of the heat generating device for receiving substantially all of the exhaust gas; one or more injectors that are operatively disposed within the one or more premix sections for injecting burner fuel and fully mixing the exhaust gas with the burner fuel; and an array of burner surfaces for receiving the fully mixed exhaust gas and fuel combination, and combusting the fuel on or above the burner surface.
These and other aspects, features, and advantages of the present invention will become apparent from a consideration of the following specification and the attached drawings.
Embodiments of the present invention will now be described in detail with reference to the drawings, which are provided as illustrative examples of the invention so as to enable those skilled in the art to practice the invention. Where certain elements of the present invention can be partially or fully implemented using known components, only those portions of such known components that are necessary for an understanding of the present invention will be described, and detailed descriptions of other portions of such known components will be omitted so as not to obscure the invention. Embodiments of the present invention are illustrated in the Figures, like numerals being used to refer to like and corresponding parts of various drawings.
Burner assembly 10 further includes a burner surface 18, which is located downstream of the gas injectors 16. In one embodiment, burner surface 18 is located at a sufficient distance from gas injectors 16 to ensure adequate mixing of exhaust gas and burner fuel before combustion. A sufficient distance will depend on various factors, such as the desired application, number and type of injectors, the pressure of exhaust gas, the geometry of the assembly, and the like. One of ordinary skill in the art could certainly and accurately determine a sufficient distance on a case-by-case basis using conventional knowledge and/or testing techniques without undue experimentation.
In one embodiment, burner surface 18 is a surface stabilized burner capable of operating in blue flame mode. Burner surface 18 may be a porous burner surface, such as a sintered or unsintered metal fiber or ceramic fiber mat. For example, burner surface 18 may be formed from a sintered fiber mat, such as that disclosed in U.S. Pat. No. 5,439,372 of Duret et al., which is assigned to the present assignee and is incorporated herein by reference. As shown in
In operation, exhaust gas from a power generating device, such as a gas turbine or reciprocation engine, passes from exhaust duct 26 into enclosing duct 22, where it is combined with fuel from the injectors 16. The combined exhaust gas and fuel mixture passes downstream to burner surface 18. The exhaust gas and fuel is fully mixed when it passes through the burner surface 18. The mixture combusts on or above the burner surface 18, preferably burning in blue-flame mode. The burner surface 18 can be modulated in a known manner to control thermal turndown by varying the amount of fuel that is added to a fixed amount of turbine exhaust. In one embodiment, for a fixed amount of turbine exhaust, the burner can be operated with nominally 2:1 turndown, with the operating limits being defined by the lean extinction limit of the flame at the low-fire limit and by operation at near the stoichiometric fuel-air ratio at the high-fire limit. In applications where ultra-low NOx emissions are desired, the thermal turndown in the constant flow case would be less.
In an alternate embodiment, the system allows for varying the amount of exhaust gas that enters the enclosing duct 22 by use of louvers or other known flow control devices. By varying the amount of exhaust gas flow, the thermal turndown of the burner could be increased considerably beyond what is possible in the constant flow case. However, this type of air flow control is typically not utilized to vary heat input from a duct burner. It is more common to achieve a wide operating range with respect to thermal turndown by individually controlling multiple duct burner modules, with each module operating in on-off mode. It is also possible to operate with no modulation.
The surface stabilized burner provides good flame stability at the ultra-lean conditions that are typically implemented to achieve low NOx emissions. Those skilled in the art will also appreciate that full premixing of exhaust gas and fuel minimizes the residual oxygen content of the exhaust stream downstream of the duct burner surface 18 for a given peak flame temperature, with the ultimate result being that thermal efficiency of this approach would be higher than that of conventional burners.
Outer duct 220 is illustrated with a portion cut away in order to show the operative arrangement of the array of burner surfaces 18. Outer duct 220 is fluidly coupled to an exhaust duct 260 carrying exhaust gases from a gas turbine or reciprocating engine, such that a substantial majority of the exhaust gas is passed through the burner modules 10. In one embodiment, the outer duct 220 and premix section 120 receive 80% or more of the exhaust gas. Exhaust gas that does not pass through the premix section 120, may be diverted around the various burner modules 10 in a conventional manner to provide cooling by use of conventional cooling holes or slots. In one embodiment, approximately 10-20% of the exhaust gas is diverted around the burner surfaces 18 as bypass cooling air. Fuel injectors 16 are operatively disposed throughout in the premix section 120 for dispersing gaseous burner fuel. The injectors 16 disperse burner fuel into the exhaust gas to ensure that substantially all of the exhaust gas passing through the burner surfaces 16 is fully premixed with burner fuel prior to combustion. In one embodiment, three separate injectors 16 are used for each row of burner modules 10 in the array. The injectors 16 laterally extend through the enclosing duct 220 and provide for a substantially uniform distribution of burner fuel in the premix section 120. The invention, however, is not limited to any particular number, arrangement or type of injectors, and in other embodiments, any number, arrangement or type of injectors may be used. In one embodiment, the injectors 16 disperse burner fuel perpendicular to the flow of exhaust gas in the premix section 120 (e.g., perpendicular to the direction shown by arrow 240).
The burner surfaces 18 in array 180 are located downstream of the gas injectors 16. In the preferred embodiment, burner surfaces 18 are located at a sufficient distance from gas injectors 16 to ensure a full mix of exhaust gas and burner fuel before combustion. In
In operation, exhaust gas from a power generating device, such as a gas turbine or reciprocating engine, passes from exhaust duct 260 into enclosing duct 220, where it is combined with fuel from the injectors 16. The fuel and exhaust gas are thoroughly combined in each of the one or more flow paths formed within duct 220. The combined exhaust gas and fuel mixture passes downstream to burner surfaces 18. The exhaust gas and fuel are premixed prior to passing through the burner surfaces 18. The mixture combusts on or above the burner surfaces 18, preferably burning in blue-flame mode. The burner assembly 100 can be modulated by selectively turning on and off one or more of the burner modules 10 to control thermal turndown. Also, individual burners can be individually modulated in a known manner to control thermal turndown. It is also possible to operate with no modulation.
The surface stabilized burners provide good flame stability at the ultra-lean conditions that are typically implemented to achieve low NOx emissions. The array 180 provides a nearly complete blockage of the duct 260, requiring that nearly all of the exhaust gas to be mixed with fuel and pass through the flame zone of the burner modules. Those skilled in the art will also appreciate that full premixing of exhaust gas and fuel minimizes the residual oxygen content of the exhaust stream downstream of the duct burner surface 18 for a given peak flame temperature, with the ultimate result being that thermal efficiency of this approach can be higher than that of conventional burners. Another result is that very low NOx emissions, as well as minimal emissions of CO and unburned hydrocarbons, can be achieved by operating at conditions near the lean flammability limit. These reduced emissions are achieved simultaneously with a very high level of exhaust gas residual oxygen burnout and supplemental fuel heat input.
It is understood that the invention is not limited by the exact constructions or methods illustrated and described above but that various changes and/or modifications may be made without departing from the spirit and/or the scope of Applicant's inventions.
