Abstract:
In the operation of gas turbine engines, it is an ever increasing goal to reduce the amount of harmful elements contained within the emissions of the engine. It is also desirable to provide a method and system that is capable of being utilized to retrofit existing gas turbine engines. In particular, it is of primary importance to reduce the amounts of nitrogen oxides contained within the emissions. Many times, reduced emissions comes at the cost of decreased flame operability. The present invention provides airflow to the pilot fuel line of a combustor in order to reduce the total harmful emissions, yet at the same time allow improved flame stability.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/251,902 filed Dec. 6, 2000 and titled “EMISSIONS IMPROVEMENT IN GAS TURBINE COMBUSTORS.” 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention generally relates to generally to industrial and power generation systems, and more particularly to a method and system for modifying industrial and power generation systems with reduced emissions and improved flame operability.  
           [0003]    The conventional gas turbine combustor, as used in a gas turbine power generating system, requires a mixture of fuel and air which is ignited and combusted uniformly. Generally, the fuel injected from a fuel nozzle into the inner tube of the combustor is mixed with air for combustion, fed under pressure from the air duct, ignited by a spark plug and combusted. The gas that results is lowered to a predetermined turbine inlet temperature by the addition of cooling air and dilutent air, then injected through a turbine nozzle into a gas turbine.  
           [0004]    It is well known within the art that exhaust gases produced by combusting hydrocarbon fuels can contribute to atmospheric pollution. This occurrence is attributed to the development of localized high temperature zone, which can exceed 2,000° C. Exhaust gases typically contain many undesirable pollutants such as nitric oxide (NO) and nitrogen dioxide (NO 2 ), which are frequently grouped together as Nitrogen Oxides (NO x ), unburned hydrocarbons (UHC), carbon monoxide (CO), and particulates, primarily carbon soot.  
           [0005]    It is also known that the amount of undesirable pollutants can be reduced and controlled by design modifications, clean-up of exhaust gases and/or regulating the quality of fuel. The formation of oxides of nitrogen involves the direct oxidation of nitrogen and oxygen, and the rate of the chemical reaction producing this by-product is an exponential function of temperature which is particularly dependant on the temperature in the main combustion zone. Therefore, a small reduction in temperature within the main combustion zone can result in a large reduction in the quantity of oxides of nitrogen.  
           [0006]    Gas turbine engines emit higher levels of Oxides of Nitrogen (NO x ) at high power operation. This is caused by high peak flame temperatures existing in the combustion chamber. In most gas turbine dry low NO x  combustors, emissions are reduced by premixing fuel and air. The fuel and air must be premixed to a very lean mixture to reduce the peak flame temperature. Typically, the amount of airflow required to reduce the NO x  levels below 25 parts per million (ppm) is greater than 30 times the amount of fuel flow. Combustors operating with such lean mixtures operate very close to lean extinction limits and tend to have poor operability. Frequently, a richer, piloted region in the combustor is used to improve the operability of dry low emission combustors. In these systems, most of the NOx emissions are produced in the pilot region.  
           [0007]    U.S. Pat. No. 5,303,542 issued to Hoffa discloses a method of reducing emissions that maintains flame temperature within predetermined limits by increasing the combustor airflow during periods of increased fuel flow and by increasing the burner area when the airflow has reached an upper limit. Increases in burner area are countered by decreasing airflow until the airflow reaches a lower limit, at which time the procedure repeats itself. When the fuel flow is decreased, the airflow is reduced until it reaches its lower limit, at which time the burner area is decreased, allowing the airflow to rise to its upper limit, at which time, the procedure repeats itself.  
           [0008]    Accordingly, what is needed is a method and system for combusting hydrocarbon fuels that allows for improved operability and stability of the flame, while still providing reduced emissions.  
         SUMMARY OF THE INVENTION  
         [0009]    In one aspect of the present invention, a method of combusting hydrocarbon fuel is disclosed comprising injecting an air stream into an air assist valve resulting in an air assist valve air stream; allowing the air assist valve air stream and pilot fuel to flow through a pilot fuel tube resulting in a pilot fuel-air mixture stream; allowing fuel to flow from a premix fuel line to a premixer resulting in a premix fuel stream; combining the pilot fuel stream, premix fuel stream and an air stream resulting in a mixture stream; and igniting the mixture stream. The temperature and composition of the mixture stream are selected to control simultaneously the amounts of NO x  formed in the main combustor and the stability of the flame in the main combustor, thereby controlling the total amount of NO x  emitted.  
           [0010]    According to another aspect of the invention, a system for combusting a hydrocarbon fuel is disclosed comprising a combustor having an upstream end and a downstream end; a premix area connected to the upstream end of the combustor for receiving and substantially premixing fuel and air prior to delivery to the upstream end of the combustor. The premix area is comprised of a centerbody extending longitudinally through the premix area, at least one fueling pathway positioned radially within the centerbody, which receives air assisted pilot fuel and allows the air assisted pilot fuel to exit through a pilot fuel outlet pathway, at least one premix fueling pathway positioned radially within the centerbody which receives premix fuel, and at least one pilot air pathway positioned radially within the centerbody and allows air to exit through a pilot air outlet pathway. This system may also include an air assist line in communication with the pilot fuel. The air assist line may also have a check valve to prevent fuel entering the source of the air assist line. The air assist line may be in communication with a bleed air valve from the core compressor.  
           [0011]    According to an embodiment, a system for combusting a hydrocarbon fuel is disclosed comprising a combustor having an upstream end and a downstream end; a pilot fuel line in communication with an air assist line, wherein fuel from the pilot fuel line is injected with air from the air assist line and allowed to flow through a pilot flow tube to a premix area. A check valve is controllably in communication with the air assist air stream. A premix fuel line is connected to the premix area and radially located around a centerbody. The premix area is connected to the upstream end of a combustor and receives and premixes fuel from the pilot fuel tube and premix fuel tube. The premix area is comprised of a centerbody extending longitudinally through the premix area; a fueling pathway is positioned radially within the centerbody and receives air assisted pilot fuel. A pilot air pathway is positioned radially within the centerbody. The system also comprises an igniter located centrally within the centerbody.  
           [0012]    The fuel supplied to the combustor may be varied between the premix fuel line and the pilot fuel tube. According to an embodiment, about 0-50 percent of the fuel is supplied through the pilot fuel line and about 50-100 percent of the fuel is supplied through the premix fuel line. Preferably, less than twenty percent (20%) of the fuel may be supplied through the pilot fuel line and greater than eighty percent (80%) supplied through the premix fuel line.  
           [0013]    The system may also have a combustor liner in communication with a heat shield. The heat shield may also be in communication with a combustor cap, wherein the heat shield is interposed between the combustor liner and combustor cap.  
           [0014]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a cross-section of a natural gas combustor that uses one embodiment of the present invention;  
         [0016]    [0016]FIG. 2 is a cut-away view of the natural gas combustor of FIG. 1;  
         [0017]    [0017]FIG. 3 is a perspective view of the natural gas combustor of FIG. 1;  
         [0018]    [0018]FIG. 4 is a schematic diagram showing the aerodynamic effect of air assist; and  
         [0019]    [0019]FIG. 5 is a plot of test data showing improved emissions achieved with the use of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims.  
         [0021]    The present invention generally provides an easily adaptable method and system that is capable of retrofitting industrial and power generation systems to reduce emissions while providing improved operability. The present invention allows for minimized air flow to achieve reduced No x  emissions without increasing CO emissions.  
         [0022]    While the prior art typically utilizes a mixture of fuel and air to a lean mixture to reduce the peak flame temperature, this results in lean premixed combustors operating very close to the lean extinction limits and tend to have poor operability. Frequently, a richer, piloted region in the combustor is used to improve operability, but this often comes at the cost of increased emissions. The present invention allow for reduced emissions and improved operability.  
         [0023]    For the purposes of promoting an understanding of the principles of the invention, reference is made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.  
         [0024]    The system may be an add-on feature to current generation combustors allowing for generation combustors to be retrofitted in a manner that will reduce emissions. The system may utilize an air assist injected into the pilot fuel line. An embodiment according to the present invention is depicted in FIG. 1. Air is injected through an air assist  10 . The air assist valve air stream is less than 10 times the pilot fuel stream which can flow through a pilot fuel line  14 . The air assist valve air stream may be injected by several methods. These may include, without limitation, the use of bleed air from the gas turbine engine core compressor and/or the use of an air assist pump. An air assist pump may be required, such as when bleed air from the core compressor is used. The need for an air assist pump depends upon the required fuel pressure, and the pressure loss in the recuperator (which is bypassed by the bleed air). As would be understood by those of skill in the art, an external air pump could be used to supply all the air-assist  10  valve air stream without using any core compressor bleed air. The air injection can be turned on when emission reduction is needed and turned off at other operating conditions.  
         [0025]    A pilot fuel tube  12  can require a tee to the existing pilot fuel line  14 . The air assist line  10  can be attached to the tee. According to an alternate embodiment, a small mixing chamber with connections from pilot fuel and air assist source is used. The air assist line may have a check valve to prevent fuel entering the source of air assist  10 . Air assisted fuel can be led through the pilot fuel tube  12  to an upstream end of a combustor  27 . Another fuel line, a premix fuel line  20 , may allow fuel to flow to an area within a premix area  22 , but outside of a centerbody  23 . The premix area  22  can be contained within the upstream end of the combustor  27 . The centerbody  23  may extend longitudinally through or within the premix area  22 .  
         [0026]    A combustor cap may contain premixer area  22  and centerbody  23  at the upstream end of the combustor  27 . A pilot air outlet pathway  21  may be positioned radially within the centerbody  23  and receive pilot air, and may allow pilot air to exit through the pilot air outlet pathway  21 .  
         [0027]    A fueling pathway  24  can receive fuel from the pilot fuel tube  12  and allow air to exit through the pilot fuel outlet  25 . An igniter  18  can create a flame and burn the product from the pilot air outlet pathway  21  and the pilot fuel outlet pathway  25 . The resulting mixture stream can enter the combustor  27  which may be encased by a combustor liner  28 , and surrounded by a heat shield  30 .  
         [0028]    [0028]FIG. 2 depicts a cut-away view of the natural gas combustor of FIG. 1. As shown, the combustor liner  28  allows for the receipt of the mixture stream and radially surrounds the heat shield  30 . The premix area  22  can be contained in the upstream end of the combustor and is upstream from the combustor liner  28 . The centerbody  23  may also be located in the upstream end of the combustor and can contain the pilot air pathway  16  which can surround the fueling pathway  24 . Fuel from the pilot fuel tube  12  may be introduced into the premix area  22  through the fueling pathway  24  and allowed to exit through the pilot fuel outlet pathway  21 . Fuel from the premix fuel line  20  may be introduced into the premix area  22 , but not within the centerbody  23 . The igniter, not shown, may placably fit within the igniter pathway  19  contained within the centerbody  23 .  
         [0029]    [0029]FIG. 3 depicts a perspective view of the natural gas combustor of FIG. 1. As shown, the combustor line  28  may be radially surrounded by the heat shield  30 . The heat shield can be in communication with the premix area  22 , which is surrounded by the combustor cap  26 . The premix fuel line  20  can be in communication with the premix area  22 . The pilot fuel line  14  can be in communication with the premix area  22 .  
         [0030]    [0030]FIG. 4 depicts the effect of the present invention on flame location and shape. As shown, the mixture stream may be ignited by the igniter  18 , and the NOx production zone that results as in the present invention creates a smaller and leaner zone, Air assist NOx production Zone  32 . Previously, a larger and richer NOx production zone resulted, as shown by the non-air assist NOx Production Zone  34 . The Air assist NOx production zone is also located farther away from the pilot fuel injectors, which can be used to reduce acoustic emissions.  
       EXAMPLES  
       [0031]    [0031]FIG. 5 depicts the effect of air assist as measured in a Parallon 75 microturbine power generation system using the configuration depicted in FIG. 1. Gaseous emissions were measured at full load conditions. The combustor was operated with twenty percent (20%) of the fuel supplied through the pilot and the remaining eighty percent (80%) through the premixer. Metered air assist flow was varied from about zero to 40 lb/hr flow rate. As depicted, the NO x  emissions at 0 pph air assist flow was measured to be about 25 ppm. As the air assist flow was increased to about 40 pph, the NO x  emissions were reduced to about 6 ppm, without increasing Carbon Monoxide (CO) emissions. Further reduction in NOx emissions have been observed by reducing the pilot fuel flow split and/or by increasing the air assist flow.  
         [0032]    It should be understood, of course, that the foregoing relates to preferred embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.