In the light of international efforts for reduce pollutant emissions from heating systems, particularly gas turbines, burners and methods of operation for burners have been developed in recent years which have particularly low nitrous oxide (NOx) emissions. In such cases emphasis has frequently been placed on the fact that such burners are able to be operated not just with one fuel but where possible with a wide variety of fuels, for example oil, natural gas and/or synthetic gas (e.g. coal gas) as required or even in combination in order to increase security of supply and flexibility during operation. Such burners are described for example in EP 0 276 696 B1.
An associated problem is that of a stable combustion in the burner, which is based on a stable location of the combustion zone in the burner. This alters drastically if changes occur in the composition of the fuel, meaning for example a combustion gas having a high proportion of high-grade saturated hydrocarbons, such as C2+ alkane, ethane or propane. Under such circumstances there is a danger of flame blowbacks in the burner. Patent WO 03/062618 A1 thus in particular monitors the C2+ alkane of the inflow combustion gas through infrared absorption. To avoid a blowback, with an increased proportion of C2+ alkane, the combustion gas characteristic is regulated by intervening for example in the combustion gas supply, but also by injecting water or steam.
The problem in designing burners for all possible different operating conditions and fuels, especially when there is also a variation in the fuel composition or if there are fluctuations in fuel quality, lies in the fact that the volumes needed during operation in each case (fuel mass flow) of the individual fuels are completely different, making it difficult to use the same supply system and the same spray openings for all fuels. The use of different supply systems for liquid and gaseous materials is thus known in the prior art. A further problem then presents itself however if alternate gaseous fuels with entirely different specific calorific values, for example natural gas and coal gas, are to be used. The completely different volume ratios when using these two fuels and the different chemical processes during their combustion demand a modification and expansion of the known systems.
Modern low-NOx combustion systems are usually based on the so-called “jet in crossflow” mixing-in concept. Low-pollutant combustion, especially with low NOx emissions, can in this case be undertaken by corresponding designs of the mixing-in of the fuel into the cross-flow combustion air. An important design variable in such cases is the penetration depth of the fuel jets into the cross-flow air. This mechanical design adapted in the best possible manner is then correspondingly only used for a specific fuel composition. The invention now uses as its starting point the problem that, with a temporary alteration to the fuel composition or if the fuel is changed, the result can be an alteration of the mixing field, which, with an unfavorable mixture, usually leads to increased NOx emissions.
Based on this observation, an object of the invention is to specify a method for operation of a burner with which a low nitrous oxide combustion is possible even if there is a change to the fuel composition. A further object of the invention is to specify a suitable device for executing the method.
The first object is achieved in accordance with the invention by a method for operation of a burner, in which a fuel is supplied to the burner, sprayed into the combustion air and mixed with the combustion air into a fuel/air mixture and burned in a combustion chamber, in which case, to reduce the nitrous oxide emissions, a fuel characteristic is set explicitly to a predetermined nitrous oxide emission, with a change in a parameter characterizing the fuel being determined and with a penetration depth of the fuel jets into the combustion air adapted to the change being effected.
In this case the invention starts from the knowledge that the influence of fuel composition fluctuations on the NOx emissions should if possible not be compensated for by expensive burner-side measures or by expensive adaptations to the mechanical design in the combustion chamber, in order to achieve sufficient flexibility and timely adaptation to the predetermined nitrous oxide emissions. Constructive measures could only reduce the sensitivity to fluctuations in fuel quality—and thereby mixing field variations—to a limited extent, but could not eliminate them completely. This is attributable to the fact that, for a quite specific fuel composition in each case (fuel characteristic) the injection and mixing-in facilities of the burner are “optimized”. Designs with a plurality of injection spray points for the fuel—well distributed over the cross section through which the flow is conducted—or the use of static mixers for setting a desired mixture field, in the absence of the required flexibility—especially with shorter-term variations in fuel composition—are not suitable for guaranteeing compliance with permitted emission limits for NOx emission during operation of the burner or of the combustion system. On the other hand the invention starts from the knowledge that, by spraying the fuel into the combustion air with a most favorable possible penetration level of the fuel jets, the mixing field will be adjusted in respect of a low-emission combustion. This mixing field can even be maintained during operation, taking into account the parameter characterizing the fuel.
Thus the invention proposes for the first time achieving an especially low nitrous oxide combustion by monitoring the fuel characteristic of the fuel composition, in order, if required, to use suitable measures to once again set an optimum low-pollutant operating mode as regards nitrous oxide if a parameter characterizing the fuel changes. This creates the opportunity of keeping the nitrous oxide emissions below a predetermined limit using a parameter which characterizes the fuel, with a penetration depth of the fuel jets adapted to the change being effected in the combustion air.
Preferably in this case the fuel is sprayed into the combustion air and mixed with the combustion air. Fuel and combustion air are mixed in the burner, with the best possible penetration depth of the fuel rays into the combustion air having to be ensured for the injection of the fuel into the combustion air. This means that the mixing field can be adjusted in respect of a low-pollutant combustion and can be maintained even during operation taking into account the parameter characterizing the fuel.
In an especially preferred embodiment a change in the parameter characterizing the fuel is registered and transferred to a control system. In this case the parameter characterizing the fuel is preferably continuously detected and evaluated in the control system. The parameter characterizing the fuel can in this case be determined by a suitable measurement of the fuel flow during operation and in this way the timing of the parameter characterizing the fuel can be stored and evaluated.
Preferably the fuel characteristic is set explicitly, with the value being set to a reference or required value of the parameter characterizing the fuel at which the predetermined pollutant emission occurs. In this case characteristic performance data determined in advance can already be stored in the control system, with said data representing the relationship between the fuel composition and the nitrous oxide emissions. Alternatively however an in-situ measurement of both the current nitrous oxide emission values and also the fuel composition is simultaneously measured and transferred to the control system.
In an especially preferred embodiment the Wobbe index is determined from a parameter characterizing the fuel. What is referred to as the Wobbe index is a normal standard used to characterize the fuel composition and temperature. The Wobbe index allows a comparison of the heat content of different fuels relative to their volume, especially combustion gases, to be made at different temperatures. Since a combustion system, such as a gas turbine for example, is operated such that in the final analysis heat energy is released in a combustion chamber and that the fuel flow is set by controlling the volume flow, fuels with different fuel composition but still with relatively similar Wobbe indexes are generally supplied by the same fuel supply system to the burner. Variations in the fuel composition lead to variations in the nitrous oxide emissions, with a setting of the Wobbe index enabling a permitted highest nitrous oxide emissions in the operation of the gas turbine.
In the inventive method the Wobbe index of the fuel is preferably determined by evaluating
the relationship
            W      ⁢                          ⁢      I        =                  L        ⁢                                  ⁢        H        ⁢                                  ⁢        V                              S          ⁢                                          ⁢                      G            ·                          T              /                              T                Ref                                                          ,with LHV being the lower heat value of the fuel, T its absolute temperature and SG the specific gravity of the fuel relative to the air under standard conditions, and TRef being a reference temperature.
In this case, for setting the desired Wobbe index, the temperature of the fuel is preferably set explicitly with respect to a predetermined nitrous oxide emission. According to the above formula the Wobbe index is related in a relatively simple manner to the current fuel temperature, namely inversely proportional to the square root of the fuel temperature. This means that, if the Wobbe index changes, i.e. the Wobbe index deviates from a predetermined required value with low nitrous oxide emissions, the desired Wobbe index and thereby the desired nitrous oxide emission can be set by a corresponding regulation of the fuel temperature. Depending on the situation the fuel can be warmed up or cooled down to the required value to enable a temperature setting of the required value of the Wobbe index to be used to achieve the desired NOx emission.
It is however also possible for a medium to be mixed in with the fuel for setting the fuel characteristic. A modification of the Wobbe index is also especially to be achieved in this way in order to ensure a low-pollution operation of the burner. The preferred medium considered for injection into the fuel is water, steam or nitrogen, but also hydrocarbons with a high heat value for example.
As an alternative to the Wobbe index, the so-called impulse flow density ratio can also be determined and evaluated as a parameter characterizing the operating state. Impulse flow density ratio: The mixture quality with “jet in crossflow” depends, with a given geometry, on the impulse flow density ratio, i.e. on the quotient of the impulse flow density ratio of the jet and on the impulse flow density ratio of the crossflow.Impulse I=M·c=p·c·A·c Impulse density I=I/A (surface)=pc2 
Impulse flow density of the air is essentially given by ambient conditions and gas turbine performance. Impulse flow density of the fuel, apart from depending on the gas turbine performance also only depends on the fuel composition. The heating value gives the mass flow and thereby with the density for fixed geometry the impulse flow density. The impulse flow density is thus not a fuel characteristic but a variable which depends on the fuel composition. This variable can also be regulated to a desired required value if it changes by monitoring the fuel composition during the operation of the burner.
Preferably the method is applied when a burner of a gas turbine is operated. The demands for low-pollutant combustion during operation of gas turbines, especially with stationary gas turbines for energy production, have increased continuously in recent years. The method In accordance with the invention makes low-emission operation possible, with the regulation measures described above already being able to be undertaken on the fuel side during the operation of the gas turbine system if there are variations in the composition of the fuel. Expensive constructive measures on the burner side can be dispensed with.
A liquid or gaseous fuel is preferably used for operation. The method can be used example with oil, natural gas or with a synthetic gas, e.g. coal gas.
An object of the invention directed to a device is achieved by a device for carrying out the method with an analysis device for analysis of the current fuel composition during burner operation and with a checking and control system for determining a deviation and for setting the parameter characterizing a fuel to a required value, at which the predetermined pollutant emission is present.
The advantages of the inventive device are produced in the same way as the above-described advantages relating to the method.
The checking and control system is preferably designed in this case for setting the fuel temperature of the fuel, i.e. a heating up or cooling down the fuel as required.
The checking and control system also preferably includes means for controlled injection of an inert medium, especially steam, water or nitrogen or of a hydrocarbon, into the fuel.
The same reference symbols have the same meaning in the figures.