Patent Application
20130186094
The invention relates to fuel nozzles and, more particularly, to an axial flow fuel nozzle for a gas turbine including a plurality of annular passages to facilitate mixing.
Gas turbine engines generally include a compressor for compressing an incoming airflow. The airflow is mixed with fuel and ignited in a combustor for generating hot combustion gases. The combustion gases in turn flow to a turbine. The turbine extracts energy from the gases for driving a shaft. The shaft powers the compressor and generally another element such as an electrical generator. The exhaust emissions from the combustion gases generally are a concern and may be subject to mandated limits. Certain types of gas turbine engines are designed for low exhaust emissions operation, and in particular, for low NOx (nitrogen oxides) operation with minimal combustion dynamics, ample auto-ignition, and flame holding margins.
In existing low NOx combustor nozzles, a liquid fuel circuit directly injects fuel and water in a recirculation zone (combustion zone). Rich burning of fuel produces high temperatures, which cause the formation of higher emissions. Existing designs also use atomizing air and water together for NOx reduction. It would be desirable to provide a simple design with better liquid fuel atomization in a premixing passage to reduce emissions while also making better use of curtain air.
In an exemplary embodiment, an axial flow fuel nozzle for a gas turbine includes a plurality of annular passages for delivering materials for combustion. An annular air passage receives compressor discharge air, and a plurality of swirler vane slots are positioned adjacent an axial end of the annular air passage. A first annular passage is disposed radially inward of the annular air passage and includes first openings positioned adjacent an axial end of the first annular passage and downstream of the swirler vane slots. A second annular passage is disposed radially inward of the first annular passage and includes second openings positioned adjacent an axial end of the second annular passage and downstream of the first openings.
In another exemplary embodiment, an annular air passage receives compressor discharge air, and a plurality of swirler vane slots are positioned adjacent an axial end of the annular air passage. The annular air passage delivers curtain/atomizing air to a premix area downstream of the swirler vane slots via the swirler vane slots. An annular liquid fuel passage is disposed radially inward of the annular air passage and delivers liquid fuel to the premix area. An annular water passage is disposed radially inward of the annular liquid fuel passage and delivers water to the premix area, where the water serves to cool the fuel nozzle and facilitates mixing of the liquid fuel and compressor discharge air.
In yet another exemplary embodiment, a method of premixing fuel and air for combustion in a gas turbine includes the steps of flowing compressor discharge air through an annular air passage and through a plurality of swirler vane slots positioned adjacent an axial end of the annular air passage to a premix area downstream of the swirler vane slots; delivering one of (1) fuel, (2) water, and (3) a mix of fuel and water via a first annular passage disposed radially inward of the annular air passage to the premix area; and delivering one of (1) water and (2) air via a second annular passage disposed radially inward of the first annular passage to the premix area.
In one embodiment, the first annular passage 66 is coupled with a source of liquid fuel. In this context, the first openings 68 are positioned relative to the annular air passage 62 such that air passing through the swirler vane slots 64 at least partially atomizes the liquid fuel flowing through the first openings 68. In this arrangement, the second annular passage 70 may be coupled with a source of water. In this context, the second openings 72 are positioned relative to the first openings 68 such that water passing through the second openings 72 impacts the liquid fuel flowing through the first openings 68. The area upstream of the swirler vane slots 64 adjacent the first and second openings 68, 72 serves as a premix area.
In an alternative operation, the second annular passage 70 may be coupled with a source of air. In this context, the second openings 72 are positioned relative to the first openings 68 such that air passing through the second openings 72 impacts the liquid fuel flowing through the first openings 68. The second openings 72 may be oriented such that air passing through the second openings 72 creates an annular air layer along a distal end of the nozzle center body. The annular air layer or air curtain serves to cool the center body and also atomizes the liquid fuel jet.
The first annular passage 66 may still alternatively be coupled with a source of mixed liquid fuel and water. The use of water serves to make the system cooler, thereby reducing carbon deposits. Additionally, water serves to cool flame temperatures and reduce NOx emissions. Air in the second annular passage 68 serves to clean the surface downstream of fuel input, which can reduce concerns with regard to flame holding.
During a gas operation, all three passages may be coupled with sources of air only.
The vane slots 64 produce shear and increase gas mixing. A greater angle (e.g., greater than 45°) strengthens the center recirculation by increasing swirl, which is desirable for flame stability. The fuel holes 68 are preferably placed such that high velocity air in the air passage 62 serves to break the fuel jet. The momentum ratio can be easily controlled by controlling the number of holes 68 and slots 64. The addition of water also serves to break the fuel jet and reduces NOx while also cooling the liquid fuel and preventing clogging (anti-cocking).
With reference to
With continued reference to
The air passage 62 is traditionally used for cooling the nozzle center body 82. As shown in dashed line, the nozzle center body may also be tapered, wherein a larger center body diameter can be better for flame stabilization. The passage 62 drives compressor discharge air through the swirler vane slots 64. With the structure of the described embodiments, this air is diverted such that it is used to first atomize the liquid fuel jet and then cool the center body and center body tip by forming a layer of only air at the center body and tip. During gas operation, this air can be used for further mixing as it creates a shear layer above the hub with the main swirler air. It is possible to have a fuel hole pattern that generates a slightly hub-midspan rich gas fuel air mixing profile. That is, with curtain air mixing with the main air, it is possible to adjust the fuel-air mixing profile.
The next radially inward passage 66 may be for liquid fuel, or, as noted, during the gas operation it may be purged with air. The circuit may contain only liquid fuel or emulsion fuel (liquid fuel mixed with water).
The next radially inward passage 70 is preferably for water, which water cools the liquid fuel from beneath to avoid carbon formation/cocking problems. As shown, the holes 72 are placed such that water flowing through the holes hits the fuel jet and removes any low velocity region (to avoid flame holding just behind the jet) with water behind the fuel jet. The water helps to break the fuel jet. At a downstream location, water mixing with fuel and while burning serves to reduce local temperatures and reduce NOx formation.
Liquid fuel orifices 68 and water orifices 72 may be placed near each other such that water may have better chance to impact/mix with the liquid fuel. As noted, in an alternative embodiment, atomizing air may be included with low-pressure ratio instead of water. Cold atomizing air may cool the liquid fuel passage from beneath and will help atomization of the liquid fuel jet.
Generally, the design provides an inexpensive way to incorporate liquid fuel with better atomizing and premixing (resulting in lower emissions). The design also enhances gas fuel operations and cooling of the center body tip. The improved atomization and premixing serves to decrease concentrated burning and resulting high temperatures, thereby reducing NOx emissions. By providing the curtain air for gas side premixing, with a shear layer, it is possible to have rapid mixing near the center body tip. The design may also reduce the requirement of water and may eliminate use of atomizing air thereby improving the heat rate on liquid fuel operation.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
