Patent Application
20090170043
The present invention relates to an arrangement for preparation of a fuel for combustion including a burner, a combustion chamber associated with the burner and in which combustion of a fuel is to take place in use of the arrangement as well as means for supplying liquid fuel to the arrangement through an internal passage in the burner for said combustion, solid portions of the burner body being heated by said combustion in use of the arrangement.
This arrangement may be designed for preparation of a fuel for combustion in a burner in various heating applications, for example for producing steam and hot water, such as in different types of boilers used in process industries or district heating plants. Such an arrangement is also used in gas turbine engines, and this particular application of the invention will primarily be discussed hereinafter for illuminating the invention and the problems to be solved thereby but not in any way restricting the scope of the invention.
In common to all such burners is the demand by environmental authorities for increasingly low emissions of primarily NOx. The key issue for obtaining low emissions of NOx and other pollutants is to obtain a sufficient distribution and evaporation of said liquid fuel in the combustion process avoiding hot spots resulting in higher emissions of NOx as well as spots with combustion at too low temperatures resulting in high emissions of CO. The main task of an arrangement for preparation of a fuel as defined in the introduction is therefore to provide a sufficiently uniform distribution of the fuel by evaporation thereof before the fuel is mixed with air/oxidant. However, in some cases a “defined” or “controlled” non-uniform distribution may also be accepted where a NOx/turndown trade-off would be a possible solution.
Different ways have so far been chosen for obtaining this. Water or steam has been injected in combination with the liquid fuel in particular to limit the flame temperature and as a secondary effect to produce a more refined spray. Another way to proceed has been to utilize a higher feed pressure of the fuel obtaining a well atomized spray and increasing the time for mixing/evaporation from the injection point to combustion. A third route has been to convert the liquid fuel to a gas in a separate reactor using steam or combustion in oxygen poor environments.
The alternative last mentioned appears to be the most attractive one from the point of view of obtaining low emissions of NOx, since it is easier to obtain a homogenous mixture of air and fuel when starting the mixing process by mixing air and a gas as fuel than air and atomized liquid fuel to be evaporated. The hot gas fuel can be better distributed throughout the entire amount of air, so that lower temperature combustion results and by that lower emissions of NOx. Moreover, possible combustion of not evaporated and/or unmixed liquid fuel will also result in higher emissions of NOx.
Russian patent document 2106574 discloses a said arrangement in which a pipe containing liquid fuel is exposed to hot combustion gases in a space in which the flame of the burner is located evaporating the fuel inside the pipe. The hot gas fuel is then mixed with air and conveyed into said space for combustion. A disadvantage of this arrangement is that as a consequence of the thin pipe walls, very hot zones of the pipe will result, which involves a risk of coking of the fuel producing deposits on the internal walls of the pipe. Furthermore, the pipe is unprotected and exposed to corrosion and wear, so that the lifetime thereof will be limited. There is also risk of occurrence of vibrations of the pipe extending freely in said space. Furthermore, it is difficult to control the heating and evaporation of the liquid fuel, since the thin walls of the pipe are very sensitive to changes of the operation conditions of the burner.
The object of the present invention is to provide an arrangement of the type defined in the introduction that is improved in at least some respect compared to such arrangements already known.
This object is according to the invention achieved by providing such an arrangement in which said internal passage is located inside said solid portions of the burner body for receiving heat energy evaporating said fuel from these body portions, and which comprises means for conveying the vaporized fuel to the combustion chamber to take part in the combustion.
It has been realized that heat absorbed by such solid portions of the burner body during combustion results in the temperature of these portions being well suited for evaporation of liquid fuel. This means that liquid fuel requiring less pumping capacity for raising it to injection pressure than gas fuel may be fed to the burner and in spite of that low emissions of NOx are possible thanks to the advantageous evaporation of the liquid fuel passing through said internal passage. The conversion of liquid fuel to gas fuel takes place efficiently close to the combustion chamber. Such solid portions of the burner body will not be heated to such high temperatures as those prevailing close to the flame in the combustion chamber reducing the risk of coking of the fuel inside said internal passage, and the solid portions of the burner body will not be that sensitive to changes in the operating conditions of the burner, so that temperature changes thereof will be slow. This means that control of the evaporation process will not be overly sensitive and may be accomplished reliably.
Another advantage of evaporating the liquid fuel in an air free integrated part of the burner is the elimination of any risk of fire in the fuel conduits.
According to a preferred embodiment of the invention said internal passage extends according to a prolonged path inside said solid burner body portions thereby to increase the surface area of the walls of the passage to enhance heat exchange with the fuel. Such a path improves the likelihood of obtaining evaporation of all the liquid fuel flowing through the passage. The prolongation may for instance be obtained by arranging for at least a portion of the internal passage to follow a path that spirals inwardly.
According to another embodiment of the invention said burner body is designed to be allowed to be split into at least two parts at a location enabling inspection and/or cleaning of surfaces of said internal passage. According to another embodiment at least one of said at least two parts is removable from the rest of the burner body for exposing said internal passage for enabling inspection and/or cleaning thereof, and said supply means is connected to a part of the burner body other than said at least one removable part. This means that no disconnection of said means for supplying fuel to the burner has to be performed for enabling inspection and/or cleaning of said internal passage, simplifying the maintenance and making it less costly and possible to be carried out more frequently.
According to another embodiment of the invention the arrangement comprises an insert member which participates in forming said internal passage and is removably inserted in said solid portions of the burner body. Accordingly, inspection and/or cleaning of said internal passage may be facilitated by removing the entire insert from the rest of the burner. It is also possible to replace the insert by another during said maintenance thereby reducing the time for which the burner is out of service. In this arrangement, it is preferred that said supply means is connected to said solid portions of the burner body and through these to said internal passage thereby to allow removal of said insert member from the burner body without disturbing the connection of the supply means to the burner body. This further reduces the time for which the burner is out of service. Furthermore, different evaporation times and fuel temperatures can be achieved by exchange of the insert member and the length of the passage machined in the insert member.
According to another embodiment of the invention said internal passage is within a member of said burner body that forms by an end face thereof a base of a space in which the flame of the burner is located, the flame extending from the base towards said combustion chamber. Such a part of the burner body is well suited to evaporating liquid fuel, since its temperature, from heat energy received from the flame, is within a range well suited to the evaporation. Furthermore, the temperature of said member will change slowly, so that other devices for controlling different members (such as valves and the like in the power plant or the like in which the burner is arranged) may easily adapt such control to the temperature change of said member. The temperature of said burner body member decreases the greater the distance from said space. The path chosen for said internal passage may take this into account. The path chosen may also take into account the evaporation temperature of the particular liquid fuel to be used in the burner. It is pointed out that other solid portions of the burner may also be used to accommodate said internal passage. For example, there could be used guide vanes or other members that surround the space containing the burner flame. It is also possible for the arrangement to have more than one said internal passage, such as one extending through said burner body member and one through the guide vanes or other members surrounding said space.
According to another embodiment of the invention in which said internal passage is within said burner body member, said conveying means comprises an opening of said internal passage in said end face for injecting by an injector means a part of the evaporated fuel into said space. The burner body member may then belong to a pilot burner part of said burner, which will then inject a gas fuel instead of liquid fuel into said space resulting in substantially lower emissions of NOx emanating from the pilot fuel, which emissions constitute a considerable part of the emissions at start up of the burner and at low load operation.
According to another embodiment of the invention said conveying means is adapted to convey at least a part of said fuel evaporated in said internal passage to said space thorough openings into said space at locations circumferentially distributed around said space. Such location of said openings is advantageous for obtaining a homogenous mixture of fuel and air in said combustion space. The circumferentially distributed openings may be arranged in said end face and/or in lateral burner walls surrounding said space having said end face as base.
According to another embodiment of the invention the arrangement comprises pilot gas injecting members arranged circumferentially distributed around said burner space, and said conveying means is adapted to convey at least a part of said fuel evaporated in said internal passage to said space through said pilot gas injecting members. It may be that the fuel evaporated in said internal passage is the only gas fuel entering said space through said pilot gas injecting members. Thus, no separate supply is necessary for said pilot gas injecting members, and these are fully supplied with gas fuel by means of the liquid fuel supplied to said internal passage.
According to another embodiment of the invention the arrangement comprises main gas injecting members arranged adjacent to said burner space, and said conveying means is adapted to convey at least a part of said fuel evaporated in said internal passage to said space through said main gas injecting members. This means that liquid fuel which is easier to pump may be used to feed the main gas injecting members with gas fuel.
According to another embodiment of the invention said main gas injecting members are arranged circumferentially distributed around said burner space and/or axially distributed along said burner space. Such distribution is advantageous for obtaining a homogenous mixture of the gas fuel with air when using known so-called swirlers for burners, especially in gas turbine engines. According to another embodiment of the invention said conveying means is adapted to convey at least a part of said fuel evaporated in said internal passage as the only gas fuel to enter said space through said main gas injecting members, thereby simplifying the construction of the burner.
According to another embodiment of the invention said conveying means is adapted to convey at least a part of said fuel evaporated in said internal passage as the main fuel supply for said combustion to said burner space using said distributed openings. It is advantageous when said conveying means is adapted to convey at least a part of said fuel evaporated in said internal passage as the only fuel for said main fuel supply, so that the burner only has to be fed with said liquid fuel and yet obtains a sufficiently homogenous mixture of gas fuel and air to achieve low emissions of NOx.
According to another embodiment of the invention the arrangement comprises a temperature sensor adapted to measure the temperature of said solid burner body portions at least one location, a valve means communicating with said internal passage, and a control unit adapted to, upon start up of the arrangement, keep said valve means closed for injecting substantially all said fuel passing through said internal passage into said space through said injector means, and, when receiving information from said sensor that the temperature has exceeded a predetermined level, open said valve means to convey at least a part of said fuel evaporated in said internal passage through said distributed openings into said burner space. This means that upon start up the fuel may be partially injected through said injector means as a liquid into said burner space using a conventional liquid atomising nozzle, and when the temperature in said solid burner portions has increased sufficiently- to produce full evaporation of the liquid fuel in the internal passage only gas fuel will be injected into the space through the injector means, and by opening the valve means this vaporized fuel may then also be used to supply the burner space using said distributed openings.
According to another embodiment of the invention said control unit is adapted to open said valve means when said temperature sensor senses a temperature above 100° C., preferably 150-400° C. The value of the predetermined temperature is dependant upon the evaporation temperature of the particular liquid fuel used and should be such that all the liquid fuel will be evaporated in said internal passage when said predetermined temperature is reached.
According to another embodiment of the invention said valve means is adapted to open and close the fuel flow path to said pilot gas injecting members, the arrangement comprises a further valve means adapted to communicate with said internal passage, and said control unit is adapted to control said further valve means to start to open at a certain load on the arrangement and to assume an opening degree varying with said load so as to convey at least a part of said fuel evaporated in said internal passage to said main gas injecting members.
According to another embodiment of the invention said internal passage extends through guide vanes or other members surrounding a space for containing the burner flame for injecting said fuel as a gas into said space. In this embodiment, the conveying means may be arranged to convey liquid fuel vaporised in said internal passage to said space as a main fuel supply in the form of gas. It is also possible to have the internal passage extend first through the guide vanes or other members for preheating said liquid fuel and then through said burner body member for complete evaporation of the fuel, which is then conveyed to said space as the main gas supply.
The arrangement may also comprise means for mixing fuel and an oxidant in the burner or combustion chamber in use of the arrangement.
According to another embodiment of the invention said supply means is adapted to supply a liquid fuel wherein all major components of the fuel have an evaporation temperature below 250° C. This means that it may be ensured that the liquid fuel will be evaporated in said solid burner body portions, since this temperature may easily be obtained in such solid burner body portions surrounding the space in which the flame of the burner is located. It is then preferred that the supplying means is adapted to supply a liquid fuel having a substantially uniform composition in the sense that the majority of the fuel's components have evaporating temperatures close to one other. Suitable fuels with a high degree of purity and low boiling point are environmentally friendly fuels, so-called bio-fuels, such as alcohols (methanol, ethanol) and dimethyl ether (CH3—O—CH3).
According to another embodiment of the invention said internal passage has heat transfer enhancing means, such as turbulators, for speeding up said evaporation of said liquid fuel, and at least a part of said internal passage may also be provided with a surface designed to prevent adherence of fuel components thereto, such as by being smooth and/or having a repellent coating, which may prevent deposition on the walls of the internal passage should any components of the fuel start to coke. The repellent coating may also provide corrosion protection in case the fuel and the walls of the internal passage should give rise to reduction/oxidation over time.
According to another embodiment of the invention said arrangement is designed to be part of a gas turbine engine, which is a suitable application for an arrangement of this type for obtaining low emissions of NOx during operation of the engine.
According to another embodiment of the invention the arrangement is designed to be part of a boiler for heating applications.
Further advantages as well as advantageous features of the invention appear from the following description.
With reference to the appended drawings, below follows a specific description of embodiments of the invention cited as examples.
In the drawings:
a shows an arrangement according to the invention which is a modification of the embodiment shown in
As already mentioned, the present invention is directed to an arrangement for preparation of a fuel for combustion for different applications where a liquid fuel is supplied to the burner of the arrangement, and one such application is in a gas turbine engine as used in for instance a power plant as schematically illustrated in
The general function of an arrangement for preparation of a fuel for combustion for instance in a gas turbine engine of this type will now be explained with reference to
Main liquid fuel nozzles 16 are circumferentially distributed around said burner space 9 for supplying to the space atomized liquid fuel for evaporation and later combustion. Pilot gas injector nozzles 17 are also distributed around the burner space 9 on face 10. Finally, a main gas fuel supply for combustion takes place at locations circumferentially distributed around the burner space 9 in the space 18 between sector-shaped guide vanes 19 which guide air from the compressor into said burner space while mixing it with said main gas fuel. This mixing will result in a sufficiently homogenous mixture of gas fuel and air to obtain a steady combustion process thanks to the so-called radial swirler arrangement used for this.
The flame 20 established in the burner space 9 in operation of the arrangement will heat the solid burner body member 11, so that a temperature gradient is developed therein along the length of this member, which means that the temperature will increase in the direction of the arrow 21. The present invention is based on the use of the heating by the flame of solid burner body portions for evaporating liquid fuel supplied to the arrangement for obtaining lower emissions of NOx from said combustion. This is contrary to earlier attempts to avoid evaporation in the liquid fuel channel inside the member 11 for preventing deposits occurring on the channel walls.
Depending on the boiling point or evaporation temperature of the liquid fuel used and the super-heating required, different sections, as illustrated by 22-24, of the solid burner body member 11 may be used to achieve the evaporation. Liquid fuels wherein all major components of the fuel have an evaporation temperature below 250° C. are suitable fuels to use, since the temperature range in member 11 is typically 100-400° C. Suitable fuels are high-quality liquid hydrocarbon fuels, alcohols, such as methanol or ethanol, and dimethyl ether. It is pointed out that although dimethyl ether is a gas at atmospheric pressure, it is a liquid at pressures above approx. 5 bar, which is the pressure required for injection in a gas turbine. The liquid fuel should be of a quality that prevents deposits on the walls of internal passage 13 when evaporated. The internal passage 13 for the liquid fuel inside the member 11 preferably extends according to a prolonged path for increasing the surface area of walls of the passage for enhancing heat exchange with the fuel, see arrow 26 in region 25 in
The internal passage 13 (including the region 25) is preferably also provided with other heat transfer enhancing means, such as turbulators. It may also be appropriate to design the inner wall surfaces thereof to prevent adhesion of fuel components thereto, such as by being smooth and/or having a repellent coating. It is preferred to use turbulators when using “pure” fuels, and the said coating when using “dirty” fuels. Within a range between these two extremes of fuel these two features may be combined.
It is illustrated in
In
The first valve means 33 could be located in the burner body and thermally-actuated. This would avoid the need for external control of the valve.
The arrangement comprises a further, second valve means 34 which communicates with said internal passage, and which comes into operation as the gas turbine engine accelerates and more fuel is required. The control unit 31 is adapted to begin to open said second valve means once a certain load on the arrangement is reached, and to assume a degree of opening corresponding to said load so as to convey at least a part of the fuel evaporated in said internal passage to said main gas injecting members at 18. It would also be possible to include a separate evaporator loop in the burner body member to feed the main gas injection.
The burner may also be provided with an auxiliary electrical evaporator element arranged to heat said internal passage 13 for producing the necessary evaporation energy until such time as the heat conducted from the flame 20 into the burner body member 11 is adequate for this purpose.
In the embodiments disclosed above a swirling effect is used to mix fuel and air and the burner is then in some sense round, implying a circumferential distribution of fuel to be preferable. However, if the fuel/air mixing is arranged for example through vortices the injection point of the fuel may be arranged radially or axially rather than circumferentially, as the shape of the burner in this case may be rectangular, such as squared.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0519520.1 | Sep 2005 | GB | national |
This application is the US National Stage of International Application No. PCT/EP2006/065353, filed Aug. 16, 2006 and claims the benefit thereof. The International Application claims the benefits of British application No. 0519520.1 filed Sep. 24, 2005, both of the applications are incorporated by reference herein in their entirety.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2006/065353 | 8/16/2006 | WO | 00 | 3/20/2008 |
