Patent Application
20100011771
The invention relates generally to combustors, and more particularly to a coanda injection system for axially staged low emission combustion devices.
A gas turbine employed in a gas turbine plant or a combined cycle plant is operated to achieve higher operational efficiency under higher temperature and higher pressure conditions, and this tends to increase emissions (for example, NOx) in an exhaust gas stream. Although various factors for generation of NOx are known, the dominant one is flame temperature in a combustor. NOx emissions are directly proportional to the flame temperature in a combustor.
There are some conventional techniques for reducing NOx in an exhaust gas stream from a combustor. One conventionally adopted method involves injection of steam or water into the high-temperature combustion area in a combustor for reducing the flame temperature during the combustion. Although this method is easy to perform, it suffers from problems in that a large amount of steam or water is required, resulting in reduced plant efficiency. Additionally, injection of a large amount of steam or water into a combustor increases combustion vibrations, partial combustion products, and reduces life.
Taking the above defects into consideration, a dry type premixed lean combustion method has been developed, in which fuel and combustion air are injected in a premixed mode and burned under lean fuel conditions in a single stage combustor. Even though reduction of NOx emissions is achieved, the operability range of the combustor is reduced due to the premixed injection mode. The usage of a single stage combustion in a combustor may not guarantee lower NOx emissions.
Multi-stage combustion may be used to achieve reduced NOx emissions and better operability range of a combustor. In such conventional systems, the additional premixers are provided in an environment of the later stages of the combustor having reacting gas flows from one or more primary nozzles. The presence of premixers disturbs the flow pattern of hot gases in the later stages of the combustor resulting in higher pressure drops across the combustor. Cooling of such premixers is also difficult due to elevated temperatures and the introduction of flammable mixtures in later stages of combustors.
Accordingly there is a need for a system that is employed in gas turbines that achieves reduced NOx emissions from the axially staged combustor without compromising the dynamics and operability of the combustor.
In accordance with one exemplary embodiment of the present invention, a low emission combustor is disclosed. The combustor includes a combustor housing defining a combustion chamber having a plurality of combustion zones. A liner sleeve is disposed in the combustion housing with a gap formed between the liner sleeve and the combustor housing. A secondary nozzle is disposed along a centerline of the combustion chamber and configured to inject a first fluid comprising air, at least one diluent, fuel, or combinations thereof to a downstream side of a first combustion zone among the plurality of combustion zones. A plurality of primary fuel nozzles are disposed proximate to an upstream side of the combustion chamber and located around the secondary nozzle and configured to inject a second fluid comprising air and fuel to an upstream side of the first combustion zone. The combustor also includes a plurality of tertiary coanda nozzles. Each tertiary coanda nozzle is coupled to a respective dilution hole. The tertiary coanda nozzles are configured to inject a third fluid comprising air, at least one other diluent, fuel, or combinations thereof to one or more remaining combustion zones among the plurality of combustion zones. The one or more remaining combustion zones are located to a downstream side of the first combustion zone.
In accordance with another exemplary embodiment, a gas turbine having a low emission combustor is disclosed.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
As discussed in detail below, certain embodiments of the present invention disclose a low emission combustor having a combustor housing defining a combustion chamber including a plurality of combustion zones. A liner sleeve is disposed in the combustion housing with a gap formed between the liner sleeve and the combustor housing. A liner having a plurality of dilution holes is disposed within the liner sleeve. A secondary nozzle is disposed along a center line of the combustion chamber and configured to inject a first fluid including air, at least one diluent, fuel, or combinations thereof (also referred to as “pilot injection”) to a downstream side of a first combustion zone among the plurality of combustion zones. A plurality of primary nozzles are disposed proximate to an upstream side of the combustion chamber and located around the secondary nozzle and configured to inject a second fluid including air and fuel (also referred to as “main injection”) to an upstream side of the first combustion zone. The amount of the first fluid is typically less than the second fluid.
The combustor also includes a plurality of coanda tertiary nozzles, each coanda tertiary nozzle coupled to a corresponding dilution hole. The coanda tertiary nozzle is configured to inject a third fluid including air, at least one other diluent, fuel, or combinations thereof to one or more remaining combustion zones (later stages) among the plurality of combustion zones located downstream of the first combustion zone. The coanda tertiary nozzles operate in variable premix mode based on the fuel supply to the coanda tertiary nozzles. The coanda tertiary nozzle includes a coanda device configured to mix the air, fuel and diluents. The coanda tertiary nozzles facilitate to provide heat in the later stages of the combustor resulting in improvement of operability, and emissions abatement. The provision of the coanda tertiary nozzles on the liner facilitates to minimize the pressure drop in the later stages of the combustor and thus maximize efficiency across the combustor. It should be noted herein that in the embodiments discussed below, even though it may not be explicitly stated, “air” may also be considered to mean a combination of air and diluents. Similarly “fuel” may also be considered to mean a combination of fuel and diluents.
As discussed in detail below, embodiments of the present invention function to reduce emissions in combustion processes in various applications such as in ground power gas turbine combustors, gas ranges and internal combustion engines. In particular, the present invention discloses a low emission combustor having a plurality of axial combustion zones/stages provided with a plurality of coanda nozzles configured to allow mixing of the air, diluents, and fuel based on a “coanda effect”. Turning now to drawings and referring first to
Referring to
A secondary nozzle 28 (also referred to as “pilot nozzle”) is disposed aligned with a centerline 30 of the combustion chamber 17. The secondary nozzle 28 is configured to mix air and the fuel and inject a first fluid (also referred to as “pilot fluid”) to a downstream side 32 of a first combustion zone 34 of the combustion chamber 17. The first combustion zone 34 is designed to operate in lean conditions for minimization of emissions such as NOx. In certain embodiments, the fuel may include hydrocarbons, natural gas, or high hydrogen gas, or hydrogen, or biogas, or carbon monoxide, or syngas, or inert gas, or water vapor, or oxidizers along with predetermined amount of diluents. Diluents may include nitrogen, carbon dioxide, water, steam, or the like. In one embodiment, the secondary nozzle 28 is a coanda type nozzle. A plurality of primary nozzles 36 is disposed on an upstream side of the combustion chamber 17 and located around the secondary nozzle 28 and configured to inject a second fluid (also referred to as “main fluid”) including air, fuel, and/or diluents to an upstream side 38 of the first combustion zone 34 of the combustion chamber 17. In one embodiment, the primary nozzle 34 may be a coanda nozzle. It should be noted herein that the amount of first mixture of air and fuel is less than the amount of second mixture of air and fuel. It should be noted herein that in some embodiments, the combustor 12 does not include a secondary nozzle.
In the illustrated embodiment, the combustor 12 is operated in a premixed mode. Fuel feed is split between the primary nozzles 36 and the secondary nozzles 28. Flame resides completely within the downstream combustion zone 32 of the combustion chamber 16. The venturi assembly 26 enhances fuel-air mixing during the premixed mode for the fluids entering the downstream combustion zone 32.
In the exemplary embodiment, a plurality of coanda tertiary nozzles 40 is also provided to the combustor 12. Each coanda tertiary nozzle 40 is coupled to a respective dilution hole 23 provided in the liner 22. The tertiary nozzles 40 are configured to inject a third fluid including air, fuel, one or more diluents, or combination thereof to a second combustion zone/stage 42 disposed to a downstream side of the first combustion zone 34. The number of zones/stages in the combustor may vary depending upon the application. The coanda tertiary nozzles 40 are configured to allow mixing of the fuel and air based on a “coanda effect”. As used herein, the term “coanda effect” refers to the tendency of a stream of fluid to attach itself to a nearby surface and to remain attached even when the surface curves away from the original direction of fluid motion. A gap 44 formed between the liner 22 and combustor housing 20 allows passage of air to the tertiary nozzles 40 provided to the dilution holes 23 of the liner 22. In particular, the nozzle 40 employs the coanda effect to enhance the mixing efficiency of the device that will be described below with reference to subsequent figures. It should be noted herein that in some embodiments, the liner 22 may not be provided with dilution holes. In such embodiments, other suitable provisions may be provided in the liner 22 to accommodate the coanda tertiary nozzles 40. The provision of the coanda tertiary nozzles 40 to the liner 22 does not disturb the flow pattern of hot gases in the later stages of the combustor resulting in lower pressure drops across the combustor. It should be noted herein that the coanda type tertiary nozzles 40 may be used for the later stages of the combustor 12 regardless of the type of the primary and secondary nozzles 36, and 28 or whether there is even a secondary nozzle used in the combustor.
Referring to
It is known conventionally to use multi-stage combustion to achieve better operability range. However, it is difficult to provide additional premixers in later stages of combustors due to higher pressure drops and the need for placing premixers in an environment including reacting gas flows from the primary nozzles. Cooling of such premixers is also difficult due to elevated temperatures and introduction of flammable mixtures in later stages of combustors. The provision of the exemplary coanda nozzles will minimize the pressure drop and thus maximize efficiency across the combustor. The coanda nozzles act as dilution devices when fuel is not delivered to the nozzles. Therefore these nozzles do not need special cooling. The coanda nozzles do not hold flame, and will not disturb the combustion flow. The coanda nozzles are also virtually flash back resistant. The coanda nozzles provides enhanced premixing of air and fuel and can be easily retrofitted to existing dilution holes in the liner of combustor. The shearing action of the flowing fuel in the air stream forces (pulls along) more air through the coanda nozzle. Thus more air flows through the coanda nozzle resulting in lower local flame temperature and better mixing of air and fuel. When no fuel is supplied to the coanda tertiary nozzles 40, more air is supplied through the primary fuel nozzles, thereby reducing the local fuel air ratio in the combustor. The local flame temperature is reduced resulting in reduction of the local thermal NOx production. When axial staging is used in combustors, more air is forced through the Coanda tertiary nozzles, and thereby reducing the thermal NOx production.
More details pertaining to coanda devices are explained in greater detail with reference to U.S. application Ser. No. 11/273,212 incorporated herein by reference. The various aspects of the tertiary nozzle 40 described hereinabove have utility in different applications such as combustors employed in gas turbines and heating devices such as furnaces. In addition, the nozzles 40 may be employed in gas range appliances. In certain embodiments, the nozzles 40 may be employed in aircraft engine hydrogen combustors and other gas turbine combustors for aero-derivatives and heavy-duty machines.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
