METHOD FOR COMBUSTING A FUEL, AND COMBUSTION APPLIANCE

Abstract

A method for combusting a fuel includes, providing an oxidizer mass flow, providing a first oxidizer partial mass flow, combusting a fuel in the first oxidizer partial mass flow thus providing a mass flow of combustion products, providing the mass flow of combustion products to a duct, and ducting the mass flow of combustion products through the duct in a flow direction. A flow of a first supplementary fluid which is essentially aligned with the flow of combustion products is selectively provided as one of an oxidizer and a premixed fuel/oxidizer mixture. A further mass flow is discharged laterally offset with respect to a discharged mass flow of first supplementary fluid and across the flow cross section of the duct, to provide mass flows of supplementary fluids as laterally stratified layers of the respective supplementary fluids.

Claims

1. A method for combusting a fuel, the method comprising: providing an oxidizer mass flow; providing a first oxidizer partial mass flow; combusting a fuel in the first oxidizer partial mass flow thus providing a mass flow of combustion products; providing the mass flow of combustion products to a duct, and ducting the mass flow of combustion products through said duct in a flow direction, discharging a mass flow of a first supplementary fluid into said duct; and discharging the mass flow of the first supplementary fluid in a discharge direction essentially aligned with the flow direction such as to provide a flow of the first supplementary fluid essentially aligned with the flow of combustion products, wherein the first supplementary fluid is selectively provided as one of an oxidizer and a premixed fuel/oxidizer mixture.

2. The method according to claim 1, comprising: discharging at least one further mass flow of a further supplementary fluid into the duct, wherein the further supplementary fluid is selectively provided as one of an oxidizer and a premixed fuel/oxidizer mixtureg; discharging the at least one further mass flow essentially aligned with the flow direction; and wherein the at least one further mass flow is discharged laterally offset with respect to the discharged mass flow of first supplementary fluid and across a flow cross section of the duct, such as to provide the mass flows of supplementary fluids as laterally stratified layers of the respective supplementary fluids.

3. The method according to the claim 2, comprising: discharging at least two further mass flows of further supplementary fluids, wherein each further mass flow is discharged with a different lateral offset with respect to the flow of the first supplementary fluid and across the flow cross section of the duct, such as to provide the mass flows of supplementary fluids as laterally stratified layers of the respective supplementary fluid.

4. The method according to claim 3, comprising: providing each layer of further supplementary fluid laterally adjacent at least one other layer of supplementary fluid.

5. The method according claim 1, wherein the discharge of at least one further mass flow of a further supplementary fluid comprises: discharging each mass flow of supplementary fluid at a different location along the flow direction.

6. The method according to claim 1, performed in a combustion appliance and comprising: operating the combustion appliance at different thermal loads; providing each mass flow of a supplementary fluid as a mass flow of oxidizer when the combustion appliance is operated at or below a first threshold value of a thermal load parameter; and providing the mass flow of at least one supplementary fluid as a premixed fuel/oxidizer mass flow when the combustion appliance is operated above the first threshold value of the thermal load parameter.

7. The method according to claim 1, comprising: providing a supplementary fluid as oxidant when the combustion appliance is operated at or below a respective threshold value of a load parameter; and providing the supplementary fluid as a premixed fuel/oxidizer mixture when the combustion appliance is operated above a respective threshold value of the load parameter, such that a number of supplementary fluids which are provided as a premixed fuel/oxidizer mixture increases stepwise with increasing thermal load at which the combustion appliance is operated and decreases with a decreasing thermal load at which the appliance is operated.

8. The method according to claim 1, comprising: choosing a respective threshold value of the load parameter higher the further downstream of the flow of combustion products the respective supplementary fluid is discharged.

9. The method according to claim 1, performed as a combustion of fuel in a gas turbine engine, the method being comprising: operating the gas turbine engine at different loads of the gas turbine engine; providing each of the mass flows of a supplementary fluid as a mass flow of oxidizer when the gas turbine engine is operated at or below a first threshold value of a load parameter of the gas turbine engine; and providing the mass flow of at least one supplementary fluid as a premixed fuel/oxidizer mass flow when the gas turbine engine is operated above the first threshold value of the load parameter of the gas turbine engine.

10. The method according to claim 9, comprising: providing a supplementary fluid as oxidant when the gas turbine engine is operated at or below a respective threshold value of a load parameter of the gas turbine engine; and providing the supplementary fluid as a premixed fuel/oxidizer mixture when the gas turbine engine is operated above the respective threshold value of the load parameter of the gas turbine engine, such that a number of supplementary fluids which are provided as a premixed fuel/oxidizer mixture increases stepwise with increasing load of the gas turbine engine and decreases with a decreasing load of the gas turbine engine.

11. The method according to claim 1, comprising: choosing a respective threshold value of the load parameter of the gas turbine engine higher the further downstream of the flow of combustion products the respective supplementary fluid is discharged.

12. A combustion appliance, comprising: a first combustion stage; a duct provided to receive combustion products from the first combustion stage; and a fuel/oxidizer premix device, the fuel/oxidizer premix device, at a discharge end thereof, being in fluid communication with a discharge appliance provided in the duct, wherein the discharge appliance is arranged and configured to discharge a fluid received from the fuel/oxidizer premix device into a flow of combustion products in the duct and essentially aligned with a flow direction of the flow of combustion products.

13. The combustion appliance according to claim 12, comprising: at least one further fuel/oxidizer premix device, wherein the further fuel/oxidizer premix device, at a discharge end thereof, is in fluid communication with a further discharge appliance provided in the duct, wherein the further discharge appliance is arranged and configured to discharge a fluid received from the further fuel/oxidizer premix device into a flow of combustion products in the duct and essentially aligned with a flow direction of the flow of combustion products, wherein each discharge appliance is disposed and configured to discharge the respective fluid laterally offset with respect to the fluid discharged from each other discharge appliance in a throughflow cross section of the duct, such that discharge flows from the discharge appliances will be provided without an overlap in a throughflow cross section of the duct.

14. The combustion appliance according to claim 13, wherein the discharge appliances which are in fluid communication with different fuel/oxidizer premix devices are provided in the duct with a mutual offset in a throughflow direction of the duct.

15. A gas turbine engine comprising: at least one combustion appliance as claimed in claim 12, wherein the at least one fluid discharge device is provided adjacent and upstream an expansion turbine inlet.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The subject matter of the present disclosure is now to be explained in more detail by means of selected exemplary embodiments shown in the accompanying drawings. The figures show

[0038] FIG. 1 a depiction of a combustion appliance illustrating a method as disclosed above as carried out at a low load;

[0039] FIG. 2 a depiction of the combustion appliance illustrating a method as disclosed above as carried out at a low intermediate load;

[0040] FIG. 3 a depiction of the combustion appliance illustrating a method as disclosed above as carried out at a high intermediate load;

[0041] FIG. 4 a depiction of the combustion appliance illustrating a method as disclosed above as carried out at a high load; and

[0042] FIG. 5 a diagram showing qualitatively an exemplary course of the fuel/oxidizer ratio of the different fluid participating in the combustion.

[0043] It is understood that the drawings are highly schematic, and details not required for instruction purposes may have been omitted for the ease of understanding and depiction. It is further understood that the drawings show only selected, illustrative embodiments, and embodiments not shown may still be well within the scope of the herein disclosed and/or claimed subject matter.

EXEMPLARY MODES OF CARRYING OUT THE TEACHING OF THE PRESENT DISCLOSURE

[0044] The method for combusting a fuel as disclosed above, and an exemplary combustion appliance suitable for carrying out the method, are lined out in detail by way of an example shown in FIGS. 1 through 4. A combustion appliance 1 generally comprises a first combustion stage 11. A generic burner 12 is provided in fluid communication with and upstream of first combustion stage 11. Burner 12 may be of any known type known to the skilled person, such as, but not limited to, any type of diffusion burner or premix burner as for instance, but not limited to, disclosed in EP 321 809, EP 780 629, WO93/17279 or EP 945 677. The depiction of burner 12 is thus to be understood symbolic, and generally speaking any device suitable for receiving an oxidizer and a fuel and preparing a combustible fuel/oxidizer flow thereof may be applied. An oxidizer mass flow is provided. A first partial mass flow 101 of said oxidizer, for instance compressed air form a compressor of a gas turbine engine, is provided to burner 12. Further, a mass flow of fuel 102 is provided to burner 12. The combustible fuel/oxidizer flow is combusted as illustrated by flame 13. A mass flow 103 of combustion products is generated and flows in a flow direction generally denoted by the arrow at 103 downstream inside the combustion appliance 1 and is received by a duct 14. Within duct 14 mass flows 104, 105 and 106 of supplementary fluids are discharged into duct 14. The discharge of the mass flows of supplementary fluids is effected on the one hand axially staged in the flow direction of the mass flow of combustion products 103. Supplementary fluid mass flow 104 is discharged upstream of supplementary fluid mass flow 105, which in turn is discharged upstream of supplementary fluid mass flow 106. Further, the discharge of the mass flows of supplementary fluids is effected in a laterally staged manner, i.e., staged across the direction of the flow of combustion products 103, or across a flow cross section of duct 14. That is, with respect to the flow of combustion products each mass flow of a supplementary fluid is provided laterally offset from the other mass flows of a supplementary fluid, and adjacent to another mass flow of supplementary fluid. In the shown exemplary embodiment, a mass flow of a supplementary fluid 104 is discharged most upstream the flow of combustion products and the deepest in duct 14, or the closest to a center of duct 14, while a mass flow of a supplementary fluid 106 is discharged the furthest downstream the flow of combustion products and closest to the walls of duct 14. A mass flow of a supplementary fluid 105 is discharged in and across the flow direction between them. In order to discharge the mass flows 104 and 105 of supplementary fluids in an inner region of duct 14, for instance a suitable piping system may be provided as a discharge appliance. Mass flow 106 of supplementary fluid may for instance be discharged through a piping system, or through orifices, for instance circumferentially arranged slits, provided in the walls of duct 14. Other appliances for discharging the mass flows of supplementary fluids at suitable locations in duct 14 are conceivable. Generally, any appliance which is able to discharge the mass flows of supplementary fluids within the duct with the discharge characteristics set forth below may be found suitable. However, the discharge appliances need to be able to withstand the temperature of the flow of combustion products 103. Further, the mass flows of supplementary fluids are discharged in a discharge direction which is at least essentially aligned with the flow of combustion products and oriented downstream the flow of combustion products. The mass flows of discharged supplementary fluids are furthermore provided in directions at least essentially aligned with and parallel to each other. Thus, rapid intermixing between the supplementary fluid mass flows, or between the mass flows of supplementary fluids, is at least largely voided and will only occur comparatively far downstream the respective discharge appliance. Thus, stratified layers 114, 115 and 116 of supplementary fluids are formed. Downstream duct 14 flue gas mass flow 107 is discharged from duct 14. It goes without saying that mass flow 107 is the sum of all mass flows provided to duct 14, i.e. the mass flow 103 of combustion products plus the sum of all mass flows 104, 105 and 106 of supplementary fluids. Further, flue gas mass flow 107 may also comprise coolant. Flue gas mass flow 107 may for instance be guided to an expansion turbine of a gas turbine engine, or another device where a flow of heated fluid is required.

[0045] As noted above, combustion appliance 1 comprises a first combustion stage 11, with at least one burner 12, and a duct 14 for receiving a flow of combustion products form the first combustion stage 11. It is noted that a multitude, that is at least two, burners and/or first combustion stages may be provided to provide a common mass flow of combustion products 103 to the duct. Likewise, a mass flow of combustion products provided by a single burner or jointly provided by a multitude of, i.e. at least two, burners in a first combustion stage or first combustion stages may be distributed to a multitude of, i.e. at least two, ducts 14, where supplemental fluid is discharged into the flow of combustion products inside a duct. For instance, the first combustion zone may be provided by a conventional combustion chamber of a gas turbine engine. An annular duct or annularly distributed ducts, equipped with suitable means for discharging supplementary fluid into a flow of combustion products, may be provided upstream an expansion turbine of the gas turbine engine. Discharge appliances for discharging the mass flows 104, 105 and 106 of supplementary fluids are provided in the duct, wherein “in the duct” in this respect may also mean provided at a wall of the duct such as to discharge a fluid into the duct. As noted, the specific structure of the discharge appliances is of minor interest within the frame of the present disclosure, as long as they are suitable to discharge the mass flows of supplementary fluids as required by the herein disclosed method. Although the discharge appliances are not denoted by reference numerals, for the sake of ease and clarity of the depiction, their location is perfectly clear by the depiction of the flows 104, 105 and 106 discharged therefrom. The discharge appliances are provided with a mutual offset in the intended direction of the flow within duct 14, as well as they are arranged with a mutual lateral offset, that is, in a throughflow cross section of the duct, or, in another aspect, are provided to discharge the mass flows of supplemental fluids with a mutual lateral offset. It will become apparent in the light of the description below that each mass flow of a supplemental fluid is provided to a discharge appliance by a fuel/oxidizer premix device, such that each supplemental fluid may be selectively provided as oxidizer or as a premixed fuel/oxidizer mixture. Each fuel/oxidizer premix device is, at a downstream end thereof, in fluid communication with at least one discharge appliance. In particular each discharge appliance is in fluid communication with one fuel/oxidizer premix device only.

[0046] When operating combustion appliance 1 in accordance with the herein disclosed method, as mentioned above, a mass flow of fuel 102 is supplied to burner 12 and is com busted with a first oxidizer partial mass flow 101 in first combustion zone 11.

[0047] In particular at low thermal loads of the combustion appliance, that is, at a comparatively low overall fuel mass flow provided to combustion appliance 1, all mass flows 104, 105 and 106 of supplementary fluids are provided as mere oxidizer, with nor fuel added. Accordingly, all the fuel provided to combustion appliance 1 is com busted in first combustion zone 11, and in first oxidizer partial mass flow 101. First oxidizer partial mass flow 101 may represent 50%, or about 50%, of the overall oxidizer mass flow provided to combustion appliance 1. As only first oxidizer partial mass flow 101, in this case, participates in the actual combustion process, the equivalence ratio of the actual combustion is comparatively high already at a comparatively low overall fuel mass flow. Thus, already at low part load conditions a stable combustion is achieved. The combustion temperature reaches already at low part load conditions a level at which a good and complete burn-out of the fuel, resulting in low emissions of products of incomplete combustion, such as for instance carbon monoxide and unburnt hydrocarbons, is achieved.

[0048] It will be appreciated that load may in this respect to be understood as a mass flow specific thermal load, which may, on the one hand be considered as a mass flow specific heat release rate, correlated with the overall fuel/oxidizer ratio or equivalence ratio of the combustion appliance. On the other hand, if thermal load is understood as a parameter describing the conditions of combustion, the temperature of the oxidizer provided needs to be taken into account in determining the thermal load. Control of the combustion appliance may in this respect be performed based upon an appropriate load parameter, considering the most relevant influencing variables. For instance when the combustion appliance is operated in a gas turbine engine, similar overall equivalence ratios may be found at different compressor pressure ratios, and thus the operation modes lined out below may also be controlled dependent on a load parameter of the gas turbine engine. For the ease of description below a “load” in a generic sense will be referred to. The skilled person will readily be able to determine an appropriate load parameter or a set of appropriate load parameters for controlling the operation of the combustion appliance.

[0049] As load rises while fuel is only supplied as fuel mass flow 102 to burner 12, while mass flows 104, 105 and 106 of supplementary fluids are provided as pure oxidizer, with no fuel therein, and thus fuel is only com busted in first combustion zone 11, the temperature of flame 13 as well of the mass flow of combustion products 103 rise. This may yield in a dramatic increase in the thermally induced formation of nitric oxides and the pollutant emissions of the combustion appliance. Thus, above a certain threshold load fuel is added to mass flow 104 of supplementary fluid. Mass flow 104 of supplementary fluid is thus provided as a premixed fuel/oxidizer mixture. Reference is in this respect made to FIG. 2. The oxidizer mass flow contained in mass flow 104 may account for about 25%, or at least approximately 25%, of the total oxidizer mass flow provided to the combustion appliance. Mass flow 104 of supplementary fluid, provided as a premixed fuel oxidizer mass flow, will auto-ignite upon contact with the hot combustion gases from the first combustion stage and is com busted in flame 15, and/or downstream thereof. In particular when a lean premixed fuel/oxidizer mixture is provided, the thermally induced formation of nitric oxides is rather low. Due to the already hot combustion products 103 from first combustion stage 11 the premixed fuel/oxidizer mixture will be reliably com busted over a large range on premix equivalence ratios. Mass flow 104 of supplementary fluid is discharged the most upstream duct 14, and thus the residence time of the fuel, or fuel residues, respectively, in duct 14 is sufficiently high to allow for a complete burn-out of the fuel provided therein. Fuel mass flow 102 may be controlled to maintain the equivalence ratio in first combustion stage 11 at least essentially constant. In another embodiment, fuel mass flow 102 may be controlled to maintain the temperature of the first stage combustion products at least essentially constant, for instance in a 1800K range, or at least approximately at 1800K. This temperature level ensures reliable auto-ignition of any fuel/oxidizer premix flow discharges into the flow of first stage combustion products 103, while the thermally induced nitric oxides formation in the first combustion stage is still maintained at an acceptable level.

[0050] The richer the fuel/oxidizer mixture provided as the mass flow 104 of supplementary fluid is, the higher will the nitric oxide generation become. Thus, for load parameters exceeding a further threshold level also mass flow 105 of supplementary fluid will be provided as a lean premixed fuel/oxidizer mixture. The oxidizer mass flow contained in supplementary fluid mass flow 105 may account for about 15%, or at least approximately 15%, of the total oxidizer mass flow provided to the combustion appliance. Mass flow 105 of supplementary fluid is discharged into duct 14 such as to form a fluid layer 115 adjacent fluid layer 114 formed by mass flow 104. As illustrated in FIG. 3, the fuel/oxidizer mixture provided as supplementary fluid mass flow 105 auto-ignites and fuel therein contained is combusted in flame 16, and downstream thereof. If a suitable load parameter exceeds still a further threshold value, also mass flow 106 of a supplementary fluid is provided as a premixed fuel oxidizer mixture. The oxidizer mass flow contained in supplementary fluid mass flow 106 may account for about 10%, or at least approximately 10%, of the total oxidizer mass flow provided to the combustion appliance. As illustrated in FIG. 4, the fuel/oxidizer mixture provided as supplementary fluid mass flow 106 auto-ignites and fuel therein contained is combusted in flame 17, and downstream thereof.

[0051] In considering a total oxidizer mass flow provided to the combustion appliance this, as a matter of course, also takes into account the oxidizer partial mass flow which participated in generating the mass flow of combustion products provided to the combustion appliance.

[0052] Each of the mass flows 104, 105 and 106 of a supplementary fluid may be selectively provided as pure oxidizer or as a premixed fuel/oxidizer mixture in selectively providing fuel to the respective premix device, or not. It is conceivable that the mutual ratio of premix fuel mass flows provided in each of the mass flows of a supplementary fluid, if said supplementary fluid is provided as a fuel/oxidizer mixture, is constant. This enables to provide the combustion appliance with only one premix fuel control valve, and for the individual premixed devices only shut-off valves need to be provided.

[0053] As becomes apparent from the explanations above with respect to FIGS. 1 through 4, additional fuel is provided to duct 14 the further downstream the higher the load threshold value for activating the fuel supply is. The temperature in duct 14 increases in a downstream flow direction. In turn, the residence time of the premix fuel, or the combustion products generated therefrom, respectively, is the shorter the higher the threshold load value for activating the supply of fuel is. In other words, the higher the combustion temperature is, the shorter is the residence time. This enables on the one hand a complete burn-out of the fuel before the resulting flue gas mass flow 107 is discharged at a downstream end of duct 14, while on the other hand the thermally induced formation of nitric oxides is limited.

[0054] The disclosed method of a stratified axially staged combustion thus provides a combustion process with low pollutant emissions throughout a large load range, and furthermore exhibits an excellent part load operation behavior and accordingly provides a good turndown ratio. While duct 14 is shown with a widening cross section in a downstream direction, such as to adapt for the increasing mass flow and temperature, embodiments are conceivable in which the duct is provided with a constant flow cross-section, resulting in performing the method in an accelerating flow.

[0055] FIG. 5 shows an exemplary qualitative course of the fuel/oxidizer, or, in the specific example, fuel/air ratio FAR of the different fuel/oxidizer flows over the relative load P of a gas turbine engine. The ordinate, vertical axis, is shown without scaling, as only the relative change of the fuel/air ration of the different combustion stages is to be illustrated. As becomes apparent, at low loads fuel is only supplied to the first combustion stage 11. At a certain threshold load, fuel is provided to the mass flow 104 of supplementary fluid, while the fuel/air ratio FAR.sub.11 of the first combustion stage is gradually reduced and further is maintained constant over the load. In this respect, the fuel com busted in the first oxidizer partial mass flow in first combustion stage 11 provides a basic load, while load control at a load above approximately 10% is achieved in controlling the mass flows of fuel provided to supplementary fluid flow 104, 105 and 106. The fuel/air ratio FAR.sub.104, FAR.sub.105, and FAR.sub.106 in a supplemental fluid mass flows 104, 105 and 106 is increased up to a value where nitric oxides generation starts to become unacceptable. At higher load, consequently fuel is provided to a consecutive mass flow of supplemental fluid. At approximately 10% relative load, fuel air ratio FAR.sub.104 of the mass flow of supplementary fluid 104 is increased up to approximately 20% relative load, when an increasing fuel mass flow is added to the mass flow of supplementary fluid 105. Fuel/air ratio FAR.sub.104 is gradually decreased and/or maintained constant at higher loads. Fuel/air ratio FAR.sub.105 of the mass flow of supplementary fluid 105 is increased up to a certain level. To further increase load, fuel is provided to the mass flow of supplementary fluid 106, and the respective fuel/air ratio FAR.sub.106 increases up to the gas turbine engine rated power. Between approximately 80% relative load and 100% relative load, fuel/air ratio FAR.sub.106 is approximately constant. This is due to the fact of an opening variable inlet guide vane of the gas turbine engine, which yields in an increasing air mass flow. Thus, although the fuel mass flow is increase, the fuel/air ratios remain approximately constant in this load range, or actually experience a slight decrease, due to the increasing compressor pressure ratio and the accordingly increasing temperature of the combustion air, which allow less fuel to be combusted to reach a certain combustion temperature. Qualitatively, similar observation will be made applying a different appropriate load parameter. It is readily appreciated that these observations can easily be generalized to a fuel/oxidizer ratio instead of the more specific fuel/air ratio.

[0056] While the subject matter of the disclosure has been explained by means of exemplary embodiments, it is understood that these are in no way intended to limit the scope of the claimed invention. It will be appreciated that the claims cover embodiments not explicitly shown or disclosed herein, and embodiments deviating from those disclosed in the exemplary modes of carrying out the teaching of the present disclosure will still be covered by the claims.

LIST OF REFERENCE NUMERALS

[0057] 1 combustion appliance

[0058] 11 first combustion stage

[0059] 12 burner

[0060] 13 flame

[0061] 14 duct

[0062] 15 flame

[0063] 16 flame

[0064] 17 flame

[0065] 101 first oxidizer partial mass flow

[0066] 102 mass flow of fuel

[0067] 103 combustion products, mass flow of combustion products

[0068] 104 supplementary fluid, mass flow of supplementary fluid

[0069] 105 supplementary fluid, mass flow of supplementary fluid

[0070] 106 supplementary fluid, mass flow of supplementary fluid

[0071] 107 flue gas mass flow

[0072] 114 supplementary fluid layer

[0073] 115 supplementary fluid layer

[0074] 116 supplementary fluid layer

[0075] FAR fuel/air ratio, fuel/oxidizer ratio

[0076] FAR.sub.11 fuel/air ratio, fuel/oxidizer ratio of first combustion stage

[0077] FAR.sub.104 fuel/air ratio, fuel/oxidizer ratio if supplementary fluid 104

[0078] FAR.sub.105 fuel/air ratio, fuel/oxidizer ratio if supplementary fluid 105

[0079] FAR.sub.106 fuel/air ratio, fuel/oxidizer ratio if supplementary fluid 106