Patent Publication Number: US-6336806-B1

Title: Method for combustion of a liquid fuel in a combustion system, and a combustion system for carrying out the method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of combustion technology. It relates to a method for burning a liquid fuel in a combustion system. 
     2. Description of the Related Art 
     Thermoacoustic oscillations represent a danger to all types of combustion applications. They lead to high-amplitude pressure fluctuations, to constriction of the operational range, and can increase undesirable emissions. This affects, in particular, combustion systems with little acoustic damping, such as those used in gas turbines. Active control of the combustion oscillations may be required to guarantee high performance with regard to pulsations and emissions over a wide operating range. 
     Various techniques for controlling and suppressing combustion instabilities by means of an active control system have already been proposed, in which, using either an open or a closed control loop, the supply of fuel and/or combustion air to the burner or to the burners is controlled or modulated in a defined manner. A prior, not previously published application from the applicant relates, for example, to active control of the instabilities in a premixing burner and is illustrated, for example, in FIG. 1 of EP-B1-0 321 809. In such a premixing burner, the fuel flows in the two outer fuel lines ( 8 ,  9  in FIG. 1 of EP-B1-0 321 809) are modulated asymmetrically in an open loop at frequencies between 0.3 Hz and 5 kHz, preferably between 5 Hz and 200 Hz. The modulation process is carried out with the aid of two fuel valves which are inserted in the fuel lines. 
     A disadvantage with the use of mechanically moving, electrically driven fuel valves is that they have mechanically moving parts which not only limit the range of the modulation frequency but, at the modulation frequencies which are used, are also subject to increased wear and whose functional reliability is subject to limitations. Another disadvantage is the relatively high power required by the valves themselves, which makes considerable additional circuit measures necessary. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a combustion method and a combustion system which effectively suppress thermoacoustic combustion instabilities in a simple and effective manner and without any mechanically moving parts by modulation over a wide frequency range. 
     The essence of the invention is for the liquid fuel to be injected with electrostatic assistance. The application of an electrical voltage to the fuel allows the spray angle and the range of the injected fuel spray, the atomization and the resultant droplet size distribution, with the vaporization and distribution of the fuel to be influenced. If this influencing process is carried out in accordance with a predetermined, selected time function, the combustion flame and thus the instantaneous rate at which heat is released can be modulated appropriately. Since the instantaneous rate at which heat is produced represents the most important single factor in the production and suppression of thermoacoustic combustion instabilities, the instabilities can effectively be suppressed by suitable selection of the time function and other modulation variables. 
     According to one preferred embodiment of the method according to the invention, a high voltage of several kV is used for modulation and is, in particular, a pulsating DC voltage, with a periodic function being used as the time function. 
     In this case, the high voltage can on the one hand be modulated in an open control loop. This is justified in particular if the instabilities in the combustion chamber have a response which is constant over time and can be suppressed or attenuated by selection of a suitable fixed time function. 
     If, on the other hand, the instabilities vary with time or as a function of various changing operating parameters, it is expedient and advantageous to monitor the hydrodynamic instabilities continuously by measuring suitable variables, in particular the instantaneous rate at which heat is released or the pressure pulsations, and to carry out the modulation process in a closed control loop, including the measured variables. 
     The combustion system according to the invention, which comprises a combustion chamber and at least one injection nozzle through which liquid fuel is injected into the combustion chamber, is distinguished in that first means are provided in order to apply an electrical voltage to the liquid fuel during the injection process. 
     One preferred embodiment of the combustion system according to the invention is distinguished in that the injection nozzle comprises a nozzle bore and a fuel channel which leads to the nozzle bore, in that the fuel in the fuel channel has the modulating electrical voltage applied to it, in that the injection nozzle comprises a nozzle body in which the nozzle bore is arranged, in that the fuel channel is formed between the nozzle body and an insert which is inserted with a gap into a recess in the nozzle body and is electrically insulated from the nozzle body, and in that the modulating electrical voltage is applied between the nozzle body and the insert. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     The invention will be explained in more detail in the following text with reference to exemplary embodiments and in conjunction with the drawing, in which: 
     FIG. 1 shows a schematic arrangement of a combustion system according to a preferred exemplary embodiment of the invention having a closed control loop; and 
     FIG. 2 shows a longitudinal section through an injection nozzle, suitable for electrostatic modulation, as shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a schematic illustration of the arrangement of a combustion system according to a preferred exemplary embodiment of the invention and having a closed control loop. The combustion system  10  comprises a combustion chamber  11  in whose interior  12  an injection nozzle  13  for injection of a fuel spray  16  of liquid fuel is arranged. The liquid fuel (for example oil) is supplied to the injection nozzle  13  from the exterior via a fuel line  14 . The necessary combustion air enters the combustion chamber through an air inlet  15 . The fuel nozzle  13  is connected via supply lines to a controlled voltage source  17 , which emits a pulsed or clocked DC voltage in the high-voltage range of several kV, for example 10 to 20 kV. The clock frequency is governed predominantly by the thermoacoustic combustion instabilities which can occur in the combustion chamber  11 , and is selected such that these instabilities are suppressed or are at least highly attenuated. The frequencies may in this case be in the range from several Hz to several kHz. The controlled voltage source  17  may on the one hand be permanently set to the optimum clock frequency or time function. The control loop for suppressing the instabilities is then an open control loop. The voltage source  17  may, however, also—as is shown in FIG. 1 —be controlled by a sensor element, for example a pressure transducer  19 , which records the instabilities (pressure fluctuations) in the combustion chamber  11  and passes on appropriate signals via a signal line  18  for control purposes to the voltage source  17 . In this case, the control process is carried out in a closed control loop. However, the instantaneous rate at which heat is released may also be measured, for example by measuring the OH radiation intensity, and used for control purposes, instead of the pressure pulsations when using the pressure transducer  19 . 
     The pulsed high voltage from the voltage source  17  modulates the fuel spray  16  in the injection nozzle  13  (indicated in FIG. 1 by the periodic vertical lines in the fuel spray). The application of electrostatic voltages to the liquid droplets allows various parameters relating to the fuel spray  16  to be influenced, such as 
     the spray angle and the range of the spray; 
     the atomization and the droplet size distribution resulting from it; and 
     the vaporization of the fuel droplets and the fuel distribution. 
     For their part, these parameters influence—by varying the ignition delay times, the local equivalence relationships, etc.—the point of fuel consumption and thus the flame position. If the high voltage on the injection nozzle is varied (by the pulsation), this allows the instantaneous rate at which heat is produced in the combustion chamber  11  to be modulated, this being the most important single factor for the production, and thus for the active suppression as well, of the thermoacoustic combustion instabilities. 
     The high voltage can be applied to the injected fuel in an injection nozzle  13 , a longitudinal section of which is shown in FIG.  2 . The injection nozzle  13  comprises a nozzle bore  23  and a fuel channel  22  which leads to the nozzle bore  23 . The modulating electrical voltage is applied to the fuel in the fuel channel  22 . To this end, the injection nozzle  13  comprises a nozzle body  20  in which the nozzle bore  23  is arranged. The fuel channel  22  is formed between the nozzle body  20  and an insert  21  which is inserted with a gap into a recess in the nozzle body  20  and is electrically insulated from the nozzle body  20 . The modulating electrical voltage U, which is connected via two terminals  24 , is applied between the nozzle body  20  and the insert  21 . The fuel flowing through the fuel channel  22  thus passes through a DC voltage field oriented at right angles to the flow direction and is modulated in the manner described above in accordance with the time function of the applied voltage. 
     The advantages of such an injection nozzle which can be modulated electrostatically over mechanical solutions such as fast-switching valves or rotating devices are obvious: 
     there are no moving parts; high modulation frequencies up to several khz can thus be achieved without any problems; 
     the simple design without any moving parts results in high reliability and low construction and servicing costs; 
     the fuel mass flow rate remains constant, that is to say no mechanical work is carried out, and the power requirement is thus low; 
     the acoustic impedance of the fuel line  14  cannot influence the response of the modulation device since the nozzle bore  23  terminates the fuel line  24 ; and 
     the control process can be carried out without any problems both in an open control loop and in a closed control loop.