Patent Application
20100300109
The present invention concerns the field of combustion technology. A method is proposed, whereby MBtu fuels with highly reactive components can be safely and cleanly burned in a sequential reheat burner, as found e.g. in a gas turbine.
In standard gas turbines, the higher turbine inlet temperature required for increased efficiency results in higher emission levels and increased material and life cycle costs. This problem is overcome with the sequential combustion cycle. The compressor delivers nearly double the pressure ratio of a conventional compressor. The compressed air is heated in a first combustion chamber (e.g. via an EV combustor). After the addition of a first part, e.g. about 60% of the fuel, the combustion gas partially expands through the first turbine stage. The remaining fuel is added in a second combustion chamber (e.g. via an SEV combustor), where the gas is again heated to the maximum turbine inlet temperature. Final expansion follows in the subsequent turbine stages.
In so-called SEV-burners, e.g. sequential environmentally friendly v-shaped burners, generally of the type as for instance described in U.S. Pat. No. 5,626,017, regions are found, where self-ignition of the fuel occurs and no external ignition source for flame propagation is required. Spontaneous ignition delay is defined as the time interval between the creation of a combustible mixture, achieved by injecting fuel into air at high temperatures, and the onset of a flame via auto-ignition. A reheat combustion system, such as the SEV-combustion chamber, also called SEV-combustor, can be designed to use the self-ignition effect. Combustor inlet temperatures of around 1000 degrees Celsius and higher are commonly selected.
For the injection of gaseous and liquid fuels into the mixing section of such a premixing burner, typically fuel lances are used, which extend into the mixing section of the burner and inject the fuel(s) into the oxidizing stream (22) of combustion air flowing around and past the fuel lance. One of the challenges here is the correct distribution of the fuel and obtaining the correct ratio of fuel and oxidizing medium.
SEV-burners are currently designed for operation on natural gas and oil. The fuel is injected radially from a fuel lance into the oxidizing stream and interacts with the vortex pairs created by vortex generators, as for instance described in U.S. Pat. No. 5,626,017, thereby resulting in adequate mixing prior to combustion in the combustion chamber downstream of the mixing section.
The present disclosure deals with a method for fuel injection in a sequential combustion system having: a first combustion chamber and, downstream thereof, a second combustion chamber. In between the first and second combustion chambers is a premixing chamber having a longitudinal axis that includes at least one vortex generator. Located downstream of the vortex generator is a mixing section and a fuel lance having a vertical portion and a horizontal portion parallel to the longitudinal axis provided within said mixing section. The fuel has a calorific value of 5-20 MJ/kg. In the mixing section, the fuel and the oxidizing stream coming from the first combustion chamber are premixed to a combustible mixture. The method includes injecting the fuel in such a way that the residence time of the fuel in the mixing section is reduced in comparison with a radial injection of the fuel from the horizontal portion of the fuel lance.
In the accompanying drawings preferred embodiments of the invention are shown in which:
Currently, burners for the second stage of sequential combustion are designed for operation on natural gas and oil. In light of the above mentioned problems, the fuel injection configuration should be altered for the use of MBtu-fuels in order to take into account their different fuel properties, such as smaller ignition delay time, higher adiabatic flame temperatures, lower density, etc.
The objective goals underlying the present invention is therefore to provide an improved stable and safe method for the injection of MBtu fuel for the combustion in such second stage burners or premixing chambers as known for example from U.S. Pat. No. 5,626,017.
In other words, the present invention pursues the purpose by providing a method for fuel injection in a sequential combustion system comprising a first combustion chamber and downstream thereof a second combustion chamber, in between which at least one vortex generator (e.g. swirl generator as disclosed in U.S. Pat. No. 5,626,017) is located, as well as downstream of the vortex generator a premixing chamber having a longitudinal axis, with a mixing section and a fuel lance having a vertical portion and a horizontal portion, extending into said mixing section. Said fuel lance can for instance be of the type disclosed in EP 0 638 769 A2, or any other fuel lance type known in the state of the art. The fuel to be injected, preferably a MBtu-fuel, has a calorific value of 5,000-20,000 kJ/kg, preferably 7,000-17,000 kJ/kg, more preferably 10,000-15,000 kJ/kg. In said premixing chamber, or in its mixing section, respectively, the fuel and the oxidizing stream (combustion air) coming from the first combustion chamber are premixed to a combustible mixture. The fuel is injected in such a way that the residence time of the fuel in the premixing chamber is reduced in comparison with a radial injection of the fuel from the horizontal portion of the fuel lance. Thereby, the creation of the combustible mixture and its spontaneous ignition is postponed.
Experience from lean-premixed burner development indicates that the SEV burner has to be redesigned in order to cope with the radically different combustion properties of MBtu (MBtu fuel input=Million Btu; 1 Btu=amount of energy required to raise one pound of water 1° F.) such as H-richness, lower ignition delay time, higher adiabatic flame temperature, higher flame speed, etc. It is also necessary to cope with the much higher volumetric fuel flow rates caused by densities up to 10 times smaller than for natural gas. Application of existing burner designs to such fuels results in high emissions and safety problems. The MBtu fuels, which are gaseous, cannot be injected radially into the oncoming oxidizing stream because the blockage effect of the fuel jets (i.e. stagnation zone upstream of jet, where oncoming air stagnates) increases local residence times of the fuel and promotes self ignition. Furthermore, the shear stresses are highest for a jet perpendicular to the main flow. The resulting turbulence may be high enough to permit upstream propagation of the flame. It is important to avoid recirculation zones around the fuel lance, which might be filled with fuel-containing gas and could lead to flashback or thermo-acoustic oscillations. When injecting the fuel, it should be ensured, that the combustible mixture is not combusted prematurely.
In a first preferred embodiment of the present invention, the fuel contains H2 or any other equivalently highly reactive gas. A gas with a substantial hydrogen content has an associated low ignition temperature and high flame velocity, and therefore is highly reactive. Preferably the fuel is synthesis gas (or Syngas), which per se is known as having a high hydrogen content, or any other synthetic flammable gas, as e.g. generated by the oxidation of coal, biomass or other fuels. Syngas is a gas mixture containing varying amounts of carbon monoxide, carbon dioxide, CH4 (main components are CO and H2 with some inert like CO2 H2O or N2 and some methane, propane etc.) etc. and hydrogen generated by the gasification of a carbon containing fuel to a gaseous product with a heating value. Examples include steam reforming of natural gas or liquid hydrocarbons to produce hydrogen, the gasification of coal and in some types of waste-to-energy gasification facilities. The name comes from their use as intermediates in creating synthetic natural gas (SNG). This kind of fuel has rather different characteristics from natural gas concerning the calorific value, the density and the combustion properties as e.g. volumetric flow, flame velocity and ignition delay time. Syngas typically has less than half the energy density of natural gas. In a gas turbine with sequential combustion, significant adjustments are thus necessary in order to cope with these differences.
According to a further embodiment of the present invention, at least a portion of the fuel is injected from the fuel lance with an axial component greater than zero in flow direction with reference to the longitudinal axis of the premixing chamber. Preferably, the radial component of the fuel jet is also greater than zero. The injection holes can be inclined such that the angle of injection a of fuel from the horizontal portion of the fuel lance between the fuel jet and the longitudinal axis is between 10 and 85 degrees, preferably between 20 and 80 degrees, more preferably between 30 and 50 degrees, most preferably between 40 and 60 degrees with respect to the longitudinal axis of the premixing chamber. Preferably, the fuel jet has an axial as well as a radial component. Fully radial injection results in excessive fuel jet/air interactions in the mixing section and thereby results in a high risk for premature self-ignition, whereas a fully axial injection leads to bad mixing of fuel and air.
Another measure for improving burner safety is to re-shape the downstream side of the fuel lance. Reducing the bluffness of the downstream side of the lance diminishes, or even eliminates, the recirculation zone that currently exists behind this device (fuel trapped in such a recirculation zone has a very high residence time, greater than the ignition delay time).
An alternative approach achieving the same or similar or equivalent effect, e.g. the reduction of residence time of fuel in the premixing chamber or the mixing section, respectively, would be, to inject at least a portion (or all) of the MBtu fuel into the mixing section further downstream of the fuel lance, nearer to the burner exit, via a series of injection holes in one or more additional injection devices (using considerations stated above, preferably with a fuel jet inclination comprising both axial and radial components) distributed along the circumference of the mixing section tube on its periphery. For instance, the MBtu fuel can be supplied via a device or plenum located downstream of the fuel lance near the entrance to the second combustion chamber and thereby closer to the second combustion chamber than to the at least one vortex generator, which is located upstream of the fuel lance. Preferably, the combustible mixture of air and fuel is created close to the entrance to the combustion chamber to minimise residence time. As well as minimizing alterations of the standard fuel lance, this method also reduces the residence time of the MBtu fuel in the mixing section, thereby diminishing the risk of flashback. Preferably, also the additional injection devices have injection holes inclined in a way to enable fuel jets with axial components.
Preferably but not imperatively, the fuel lance contains more than 4 injection holes. More preferably, it injects at least 8, preferably at least 16 fuel jets into the premixing chamber The diameter of each injection hole is preferably reduced (while e.g. the total content of fuel to be injected remains constant). This results in a greater number of fuel jets with smaller diameters dispersed over the area of the mixing section, which again results in an adapted mixing of fuel with oxidizing medium.
Furthermore, it can be of advantage, if the fuel is injected not only with a radial and an axial component with respect to the longitudinal axis of the fuel lance, but also with a tangential component with respect to the periphery of the cylindrical fuel lance tube. Depending on whether the tangential injection of the fuel is in the direction of swirl created in the oxidizing stream by the vortex generator(s) or against said swirl direction, different mixing properties can be achieved.
According to another preferred embodiment, whether or not the fuel jet has an axial component or the number of injection holes is increased or whether or not one or more additional injection devices are provided upstream of the fuel lance, air and/or N2 and/or steam, preferably a non-oxidizing medium or inert constituent such as N2 or steam in order to prevent back firing, can be provided as a buffer between the injected fuel and the oxidizing stream. Such a “dilution” or shielding of the gaseous fuel improves the stability of combustion and contributes to the reduction of flashback typical for high-H2-concentrations. Preferably the buffer is or builds a circumferential shield around the fuel jet. The carrier-/shielding properties of N2 or steam permit greater radial fuel penetration depths, which results in improved fuel distribution. The carrier provides an inert buffer between fuel jet and incoming combustion air, such that there is initially no direct contact between fuel and air (oxygen) in the stagnation region on the upstream side of the jet. Steam is even more kinetically-neutralising than N2. Furthermore, its greater density promotes even greater fuel jet penetration. This technique can also be employed with more axially-inclined jets, so as to firstly prevent contact between oxidant and fuel prior to a certain level of fuel spreading, and secondly to utilize the momentum of the carrier to increase the fuel penetration and thus improve fuel distribution throughout the burner.
For this purpose, N2 and/or steam can also be premixed with the fuel before injection, or can be injected separately concomitantly with the fuel or in an alternating sequence. The air and/or N2 and/or steam, preferably a non-oxidizing medium such as N2 or steam, can be injected from the fuel lance itself, together or separate from the fuel, or from one or more injection devices downstream of the fuel lance.
As already mentioned above, it can be of advantage to inject at least some of the fuel (with or without carrier air, N2 or steam) from the downstream side of the fuel lance. The fuel momentum could serve to prevent the formation of any recirculation regions. If desirable, the same effect could be achieved by injection of only air or N2 or steam.
According to another preferred embodiment, two different fuel types are injected, preferably from different injecting devices or different injection locations, into the premixing chamber. A second fuel type (e.g. natural gas or oil) can serve as a backup or startup. Of course, at least one of the two fuel types is an MBtu-fuel. If the two fuel types are injected from at least two different injection devices or locations, at least one fuel type advantageously is injected with an axial component with respect to the longitudinal axis of the premixing chamber.
In the sequential combustion system, it is advantageous, if the gas is at least partially expanded in a first expansion stage between the first combustion chamber and the second combustion chamber. In a gas turbine, said expansion preferably is achieved by a series of guide-blades and moving-blades. Preferably, a first expansion stage is provided downstream of the first combustion chamber and a second expansion stage downstream of the second combustion chamber.
Alternatively, it may be of advantage if a portion of Mbtu fuel is injected axially via the trailing edge of the vortex generators, and the remainder of the fuel via the fuel lance (using any of above concepts) and/or one or more further downstream injection devices. Apart from improving overall mixing and burner safety, this method frees up valuable space in the main fuel lance, thereby permitting a second fuel (e.g. natural gas or oil) to be used as backup (or startup). In an extreme case of this alternative, all MBtu fuel is injected via the vortex generators such that the lance remains in its original guise and therefore does not affect standard natural gas and oil operation (i.e. tri-fuel burner).
Further embodiments of the present invention are outlined in the dependent claims.
Referring to the drawings, which are for the purpose of illustrating the present preferred embodiments of the invention and not for the purpose of limiting the same,
The downstream side 8 of the horizontal portion 7 makes up the free end of the fuel lance 5 facing the second combustion chamber 2. Said free end of the horizontal portion 7 of the fuel lance 5 can have a frusto-conical shape. This reduction of the bluffness of the downstream side of the fuel lance 5 contributes to a reduction or elimination of the recirculation zone existing behind the lance. Fuel trapped in such a recirculation zone has a very high residence time, potentially greater than the ignition delay time.
Said two vortex generators 3 (swirl generators) are illustrated as two wedges in the figure. The hot gases entering the premixing chamber 4 are swirled by the vortex generators 3 such that mixing is possible and recirculation areas are diminished or eliminated in the following mixing section 17. The resulting swirl flow promotes homogenization of the mixture of combustion air and fuel. The mixing section 17, being generally formed as a cylindrical or rectangular duct or tube, has a length L1 of 100 mm to 350 mm, preferably 150 mm to 250 mm and a diameter of 100 mm to 200 mm. The fuel injected by the fuel lance 5 into the hot gases that enter the premixing chamber 4 as an oxidizing stream 22 initiates mixing and subsequent self-ignition. Said self-ignition is triggered at specific mixing ratios and gas temperatures depending on the type of fuel used. For instance, when MBtu-fuels are used, self-ignition is triggered at temperatures around 800-850 degrees Celsius, whereas flashback temperature depends on H2 content. For the above mentioned combustion chamber the main parameter which controls flashback is ignition delay time, which goes down with increasing temperature.
A mixing zone is established in the mixing section 17 around the horizontal portion 7 of the fuel lance 5 and downstream of the fuel lance 5 before the entrance 13 into the second combustion chamber 2, if further injection devices 10, as depicted in
The injection holes 9 are located on a circle line around the circumference of the generally hollow cylindrical horizontal portion 7 of the fuel lance 5. In the state of the art, the injection holes 9 are arranged in a way that the fuel is injected fully radially with respect to the axis of the cylindrical horizontal portion 7 of the fuel lance 5 and/or the longitudinal axis A of the generally cylindrically shaped mixing section 17 or the premixing chamber 4. However, according to a preferred embodiment of the invention, the fuel is injected into the oxidizing stream 22 with a significant axial component in flow direction F with respect to the longitudinal axis A of the premixing chamber 4.
Said injection holes 9 can have a diameter of about 1 mm to about 10 mm. In the state of the art, the fuel lance 5 has at most 4 injection holes 9. However, the fuel lance can be equipped with any number of holes between 2 and 32, possibly even more. In order to improve the mixing properties, more than 4, for instance 8, or even more, e.g. up to 16 or even up to 32 injection holes 9 can be provided on the fuel lance 5. By increasing the number of injection holes 9, with a constant amount of fuel to be injected, the diameter of each injection hole 9 can be reduced, which leads to a more directed fuel jet 11 coming from each injection hole 9 and thereby to a greater injection pressure. By achieving a more directed fuel jet 11, the fuel is distributed further downstream of the fuel lance 5, thereby shifting the ignition zone to a position further downstream and closer to the entrance 13 of the second combustion chamber 2. This is desired as the residence time of the fuel in the premixing chamber 4 is thereby reduced. By increasing the number of injection holes 9 it must be noted that this measure can cause a smaller fuel penetration and consequently as a result, worse mixing.
As depicted in
Injection angle α′ is defined as the angle between the fuel jet injection direction and the direction of the inner surface of the tube or cylindrical wall 23, respectively, of the mixing section 17 in an axial plane thereof. Said angle α′ can have any value of zero or greater and at the most 180, preferably 90 degrees. The injection angle α, α′, whether from the fuel lance 5 or an additional injection device downstream of the fuel lance, depends on different factors, such as the type of fuel used, whether or not a buffer such as N2 or steam is employed, on the gas temperature etc. It is possible to provide injection holes 9 directed at different injection angles α′ in a single injection device 10, such that the fuel is injected into different directions simultaneously. The fuel jets 11 from the additional device(s) 10 can also have tangential components as discussed in
In
In
| Number | Date | Country | Kind |
|---|---|---|---|
| 07150153.0 | Dec 2007 | EP | regional |
This application is a continuation of International Application No. PCT/EP2008/067581 filed Dec. 16, 2008, which claims priority to European Patent Application No. 07150153.0, filed Dec. 19, 2007, the entire contents of all of which are incorporated by reference as if fully set forth.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2008/067581 | Dec 2008 | US |
| Child | 12816112 | US |
