Patent Application
20120317986
The present application hereby claims priority under 35 U.S.C. Section 119 to Swiss Patent application number 01039/11, filed Jun. 20, 2011, the entire contents of which are hereby incorporated by reference.
The present invention relates to the field of combustion technology, particularly in relation to gas turbines. It refers to a method for operating a combustion device. It also refers to a combustion device for implementing the method.
Burner systems, which are operated in accordance with the concept of lean premix combustion, have low pollutant emissions but also a distinctly limited stability range. In addition to flashback into the mixing zone and to lift-off or quenching of the premix flame, thermoacoustic vibrations lead to distinct limitations in the operating behavior.
The operating behavior of premix burners can be improved by means of a staged fuel supply (see, for example, printed publications EP0797051 A2 or EP1205653 B1 or EP1292795 B1 or EP1344002 B1 or W02007/082608, which are incorporated by reference).
The fuel staging via a central device installed in the burner is shown in
By adjustment of the fuel proportion via the central fuel injection 11, 13 (fuel stage no.1) in relation to the fuel injection in the burner air slot 12, 14 (fuel stage no.2), the operating range of the burner 10 can be broadened with regard to quenching or lift-off of the premix flame and also with regard to flashback. The fuel staging also offers the possibility of optimizing the operating range of the burner 10 with regard to thermoacoustic vibrations and pollutant emissions.
Shown in
The parameters for the fuel staging—if the burners are part of a gas turbine—are determined by tuning runs during commissioning of the gas turbine. The parameters of the fuel staging can be predetermined in this case as a function of the output of the gas turbine, the ambient conditions, such as ambient temperature and air humidity, and the fuel composition. The parameters must also be predetermined with an adequate reserve so that even with transient operating states, such as loading or unloading of the gas turbine or changes to the gas composition, a reliable operation is ensured. Since these transient operating states in gas turbines very seldom occur, the premix burner cannot be operated to the greatest possible extent with minimum pollutant emissions.
A method for operating a premix burner with staged fuel injection is now known from printed publication EP1205653 B1, which has at least one first stage and at least one second stage, arranged downstream of the first stage, for introducing fuel into a combustion air flow, wherein pulsations of a combustion initiated by the burner and/or emission values of the combustion are detected, and the fuel feed to the first stage and second stage is controlled as a function of the detected pulsations and/or emission values. During steady-state operation, the fuel feed in this case is controlled in such a way that the operating point lies (permanently) below the maximum of the pulsations. During transient operation, on the other hand, the fuel feed is controlled in such a way that the operating point can lie (permanently) above the maximum of the pulsations. As a result, the reliability of the gas turbine may be limited during transient operation.
A method for controlling the ratio of the fuel flows in a first fuel supply line and in a second fuel supply line to a combustion device is known from printed publication W02007/082608, in which it is first of all determined whether the value of a first parameter—which shifts the operating point of the device towards a first undesirable operating range—has exceeded a predetermined limit value. If this limit value has been exceeded, the ratio of the fuel flows is changed so that the value of the first parameter falls short of the limit value. If this limit value has not been exceeded, it is determined whether the value of a second parameter—which shifts the operating point of the device towards a second undesirable operating range—has exceeded a predetermined limit value. If this limit value has been exceeded, the ratio of the fuel flows is changed so that the value of the second parameter falls short of the limit value. If this limit value has not been exceeded, the aforesaid steps are repeated in order to hold the values of the first and second parameters below the respective limit value. This type of controlling is comparatively costly and complicated.
The present description is directed to a method for operating a combustion device, having at least one premix burner equipped with fuel stagings. The at least one premix burner is connected to a combustion chamber. The method includes changing, during transient operating states, the fuel stagings of the at least one premix burner in accordance with combustion-induced pulsations which occur in the combustion chamber. The method also includes continuously determining a pulsation level of the pulsations which occur in the combustion chamber and changing the fuel stagings of the at least one premix burner, lowering the pulsation level if the pulsation level exceeds a predetermined maximum value. The method also includes resetting, after a predetermined time interval, the fuel stagings to an undisturbed value which determines the respective operating point if within said time interval the predetermined maximum value is not exceeded.
The disclosure is also directed to a combustion device including a combustion chamber and at least one premix burner connected to the combustion chamber and equipped with fuel stagings. The fuel staging can be changed by a control unit in accordance with combustion-induced pulsations which are picked up by measuring devices arranged in the combustion chamber, the control unit is connected to a timer by which a time interval for controlling the stagings is predefined.
The invention shall subsequently be explained in more detail based on exemplary embodiments in conjunction with the drawing. In the drawings:
It is therefore an object of the invention to disclose a method for controlling a premix burner equipped with fuel stagings, which with unchanged effectiveness is made appreciably simpler with regard to the avoidance of pulsations.
It is also an object of the invention to disclose a combustion device for implementing the method. The object is achieved by the appended independent claims.
The method according to the invention is based on a combustion device which has at least one premix burner which is equipped with fuel staging and is connected to a combustion chamber, wherein during transient operating states the fuel staging of the at least one premix burner is changed in accordance with the combustion-induced pulsations which occur in the combustion chamber. According to the method, a pulsation level of the pulsations which occur in the combustion chamber is continuously determined, the fuel staging of the at least one premix burner is changed for lowering the pulsation level if the pulsation level exceeds a predetermined maximum value, and after a predetermined time interval the fuel staging is reset to the undisturbed value which determines the respective operating point if within this time interval the predetermined maximum value is not exceeded.
In one embodiment of the method according to the invention, the fuel staging, in the event of the maximum value of the pulsation level being exceeded, is changed by a predefined value. This change is carried out in a particularly rapid manner, as a result of which a particularly simple process execution is achieved.
According to another embodiment the fuel staging, in the event of the maximum value of the pulsation level being exceeded, is changed step-by-step by a plurality of predefined values until either the predetermined maximum value of the pulsation level is fallen short of or an extreme value for the fuel staging is achieved. As a result of this, the fuel staging can be adjusted to the conditions in a more flexible and finer manner.
In a further embodiment of the method according to the invention, the fuel staging, with a constant rate of change, is reset to the undisturbed value which defines the respective operating point. During transient states, this undisturbed value usually deviates from the initial value of the fuel staging since the device has moved towards another operating point within the lapsed time interval.
In yet another embodiment, the fuel staging of the at least one premix burner is changed at least one additional time for lowering the pulsation level if the pulsation level exceeds the predetermined maximum value again within the predetermined time interval. This may be particularly necessary when the operating point is changed particularly quickly during transient operation.
In particular, in the method according to the invention, the overall fuel flow is kept constant while the fuel staging is being changed.
The combustion device according to the invention for implementing the method according to the invention has a combustion chamber and also at least one premix burner which is connected to the combustion chamber and equipped with fuel staging, wherein the fuel staging can be changed by a control unit in accordance with the combustion-induced pulsations which are picked up by measuring means arranged in the combustion chamber. The device is characterized in that the control unit is connected to a timer by means of which a time interval for the controlling is predefined. The timer can be an external timer or part of the control unit.
In one embodiment of the device according to the invention, the at least one premix burner is supplied with fuel via at least two distribution lines in the course of the fuel staging, the at least two distribution lines are connected to means of adjustment which can be operated by the control unit in such a way that a constant overall fuel flow can be apportioned to the at least two distribution lines in different ways.
The measuring means preferably comprise at least one sensor, which is arranged in the combustion chamber, for picking up pressure fluctuations.
Using the method which is proposed here, by means of dynamic controlling, the operating range of a burner system with fuel staging is optimized. The controlling relates basically to transient operating states of a combustion device. The method can be used for controlling all burners which have internal fuel staging. The combustion device is preferably implemented in a gas turbine which customarily includes a measuring sensor, or a group of measuring sensors, in the combustion chamber for detecting flame-generated pulsations.
Reproduced in
The control valves 27 and 28 are operated by a control unit 29 which receives input signals from a plurality of sensors S1, . . . ,S4 which are arranged in (or on) the combustion chamber 23 and pick up combustion-induced (pressure) pulsations which occur in the combustion chamber 23. Also associated with the control unit 29 is a timer 30 which gives the control unit 29 a fixed (possibly also adjustable) time interval At after which the control unit 29 automatically undertakes a change of the fuel staging via the control valves 27 and 28. In the example of
It is the aim of the controlling to adjust the fuel staging(s) inside the burners for a limited time period Δt so that a reliable operation of the combustion device or of the gas turbine is made possible during transient operation. After this time period Δt, the controller is deactivated and the combustion device or gas turbine can be operated again with optimum fuel staging.
A schematic representation of the parameters which are relevant to the controlling is shown in
The process can also be repeated a number of times depending upon the extent of the pulsation level. After a predetermined time period Δt with a sufficiently low pulsation level (e.g. at time point t3), the fuel ratio is reset to the value for the fuel stagings with a gradient ds/dt (rate of change), which is optimum for the new operating point. After this (after time point t4), the control system is deactivated. If within the time period Δt the limit value pmax of the pulsation level p of the flame-generated pulsations is exceeded again (time point t2 in
Depending upon the boundary condition or manner of the transient process, different fuel stagings can be created for the operational reliability of the gas turbine. In such a case, the adjustment of the fuel staging can be repeated at a plurality of intervals Δt until the flame-generated pulsations fall below the limit value or a maximum value for the fuel staging smax is achieved. If the pulsations increase further even at the maximum value of the fuel staging, the gas turbine can be shut down as a result of a protective load shedding, for example.
Overall, by using the invention, particularly a controlling of a gas turbine with at least one premix burner with two or more fuel stagings, which is optimized in steady-state operation for the reduction of pollutant emissions, is made possible by at least one of the fuel stagings being adjusted as a function of combustion-induced pulsations during transient operating states.
The controlling can apply in this case to gaseous and/or liquid fuels.
The controlling is especially applicable to transient processes, such as frequency back-up control, change of the gas composition or of the gas initial pressure and also load changes with low pollutant emissions.
The adjustment of the fuel staging can be carried out in this case in a plurality of equal or different steps.
It is also conceivable, however, to use the controlling for adjustment of the steady-state operating curve of the fuel staging if continuous operation at the optimized fuel staging is not possible on account of increased combustion-induced pulsations.
| Number | Date | Country | Kind |
|---|---|---|---|
| 01039/11 | Jun 2011 | CH | national |
