Method of operating a burner

The invention is directed to a method of operating a burner. The method includes feeding an auxiliary gas into a liquid fuel upstream of an injection orifice during the ignition and during part load of the burner, and interrupting the feeding of the auxiliary gases within high load ranges of the burner.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a method of operating a burner. 
2. Discussion of Background 
So-called solid-jet atomizers, inter alia, are used in order to atomize 
liquid fuels. In such a nozzle, the liquid fuel is ejected under high 
pressure from a prechamber through a circular injection orifice of a 
certain guide length. The resulting fuel jet disintegrates in a more or 
less static environment to form a fine spray. In order to produce a 
sufficiently fine spray for the combustion, however, a relatively high 
fuel pressure is required, as is applied only during full load of a 
gas-turbine plant. On the other hand, on account of the low fuel flow 
rate, only a low fuel pressure is required, for example, during the 
ignition of a combustion chamber or when running the plant up to speed 
after the ignition. However, since the atomization of the liquid fuel by 
means of a solid-jet atomizer naturally leads to relatively large droplets 
during part load of the gas-turbine plant, the conventional solid-jet 
atomizers are not suitable for the part-load operation of a gas-turbine 
plant. 
In order to nonetheless obtain fine droplets, the nozzle may additionally 
be provided with a so-called ignition stage. This ignition stage is a 
second atomizer which is designed for correspondingly low flow rates and 
therefore ensures sufficiently fine atomization of the liquid fuel during 
part load. Such a solution is disclosed by the textbook "Atomization and 
sprays", by A. Lefebvre, West Lafayette, Ind. 1989, page 120, FIG. 4.21. 
However, this nozzle, with its two fuel feed lines and with two fuel ducts 
which lie radially one above the other and to which fuel is admitted 
according to the fuel mass flow required, requires a relatively large 
construction space. In addition, the components used are naturally of 
intricate design, as a result of which the nozzle is more susceptible to 
trouble. The use of more than two atomizers in one nozzle adds to said 
disadvantages. In addition, a corresponding number of fuel feed lines with 
various control valves are necessary, as a result of which not only the 
design input but also the costs increase. During changeover to the 
atomizer required in each case, discontinuity occurs in the fuel flow and 
this may lead to extinction of the burner. 
Also disclosed by the textbook "Atomization and sprays", by A. Lefebvre, 
West Lafayette, Indiana 1989, on pages 142-144 and FIG. 4.50, is an 
atomizer. for liquid fuels, in which atomizer an auxiliary gas is 
introduced into the liquid flow upstream of the injection orifice. To this 
end, a gas tube is arranged in the interior of the liquid-fuel tube, which 
gas tube ends upstream of the injection orifice and is provided with a 
plurality of discharge orifices for the auxiliary gas. The auxiliary gas 
is injected into the liquid flow at a low velocity and at a pressure only 
marginally higher than that of the liquid flow. The auxiliary gas issuing 
into the liquid forms gas bubbles, the effect of which is to produce 
relatively thin shreds and ribbons of liquid in the liquid flow. Since 
such liquid flows of smaller diameter are easier to break up into a fine 
spray, the atomization of the liquid fuel is improved in this way. The 
total volumetric flow to be atomized is increased by the injection of the 
auxiliary gas into the liquid-fuel tube, so that sufficient atomization of 
the fuel can be achieved by means of a solid-jet atomizer even during part 
load. 
The disadvantage of such an atomizer, however, is that it cannot be used 
during full load of the gas-turbine plant, i.e. at high fuel pressure. In 
order to ensure the injection of the auxiliary gas into the liquid fuel 
and thus the operability of the atomizer during this operating state, the 
auxiliary gas must be highly compressed. However, this is very expensive 
and is not possible without an external supply of energy. Therefore such 
atomizers have not been widely adopted hitherto. 
SUMMARY OF THE INVENTION 
Accordingly, one object of the invention, in attempting to avoid all these 
disadvantages, is to provide a simple method of operating a burner, which 
method is suitable for all operating states. 
According to the invention, this is achieved in a method in that which the 
auxiliary gas is fed only during the ignition and during part load of the 
burner and the feeding is interrupted within high load ranges. 
The atomizer nozzle, which is known per se, can now be adapted in an 
optimum manner not only to ignition and low load conditions but also under 
high load conditions and of course during full load. In this way, the 
respective range of use of such an atomizing nozzle or a burner equipped 
with it is considerably widened. Only in this way is its use in a 
combustion chamber operated with varying combustion-air pressures, such as 
in the case of a gas turbine for example, made possible. 
In general, lower emission combustion can be achieved by the premixing, 
taking place in the burner, of the liquid fuel with the auxiliary gas. An 
additional advantage is obtained when using the method in axially mounted 
combustion chambers in which the burners are arranged at different 
geodetic heights. By the feeding of the auxiliary gas, the uneven 
distribution of the liquid fuel, which otherwise occurs in particular 
during the ignition, can be markedly reduced and thus the operability of 
the combustion chamber can be increased. 
It is especially expedient if the auxiliary gas continues to be fed even 
when the supply of liquid fuel to the burner is interrupted. In this way, 
the atomizing nozzle can additionally be purged and thus its carbonization 
prevented. 
Furthermore, it is advantageous if the auxiliary gas is delivered to the 
burner from a pressure vessel or an auxiliary compressor. In accordance 
with the actual conditions of use of the burner, there is therefore a 
suitable source for the auxiliary gas in each case. 
Compressed air is fed as auxiliary gas in an especially advantageous 
manner. When required, either ambient air is compressed or compressed air 
from the pressure vessel, which is already filled before the ignition of 
the burner, is used for this purpose. The use of ambient air as auxiliary 
gas is especially favorable because it is always available. 
Depending on availability, the invention may also be realized with inert 
gases, such as nitrogen for example, with ignition gases (e.g. propane) or 
with fuel gases (e.g. natural gas).

Only the elements essential for understanding the invention are shown. 
Elements of the plant which are not shown are, for example, the compressor 
and the gas turbine. The direction of flow of the working media is 
designated by arrows. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawing, a plurality of burners 1 are arranged in the 
gas-turbine plant (not shown) and are operated with a liquid fuel 2, more 
precisely fuel oil. Other suitable fuels may of course also be used. 
Each burner 1 consists of an outer air tube 3 and an atomizing nozzle 4 
arranged coaxially in the interior of the air tube 3, both the air tube 3 
and the atomizing nozzle 4 leading into a combustion chamber 5 of the 
gas-turbine plant. The atomizing nozzle 4 has a liquid-fuel tube 6 with an 
interior space 7, a fuel feed line 8, and a circular injection orifice 9. 
Arranged in the interior space 7 of the atomizing nozzle 4 is a gas tube 
11, which is connected to a feed line 10 and has a plurality of discharge 
orifices 12 leading into the interior space 7. The interior space 7 is 
narrowed in the direction of the injection orifice 9, i.e. it is formed 
with a guide piece 13 for the fuel oil 2. The feed line 10 has a control 
valve 14 with which the gas tube 11 can be opened or blocked. 
During operation of the gas-turbine plant, each burner 1 is supplied with 
fuel oil 2 via the corresponding fuel feed line 8. In the process, the 
fuel oil 2 first of all passes into the interior space 7 of the 
liquid-fuel tube 6, where it is delivered further by the fuel pressure in 
the direction of the injection orifice 9. During both the ignition action 
and the part-load operation of the burners 1 or the gas-turbine plant, 
compressed air, serving as auxiliary gas 15, is directed into the fuel oil 
2 in the interior space 7 via the feed line 10 and the discharge orifices 
12 arranged in the gas tube 11. This injection is effected at a low 
velocity and at a pressure of about 0.1 to 3.0 bar, which is only 
marginally. The volumetric flow and thus the fuel pressure are increased 
by the additional air 15, so that improved atomization of the fuel oil 2 
can be achieved even during both the ignition action and the part-load 
operation of the burners 1. In addition, the auxiliary gas 15 entering the 
liquid fuel oil 2 forms air bubbles, the effect of which is to squeeze the 
fuel oil 2 into the form of thin shreds and ribbons of liquid fuel. Since 
the individual portions of the fuel oil 2 therefore have a relatively 
small initial diameter, especially fine atomization can be achieved when 
injecting the fuel oil 2 through the injection orifice 9. 
The air serving as auxiliary gas 15 is extracted in a precompressed state 
from the compressor section of the gas-turbine plant and, if required, is 
brought to the requisite pressure via an auxiliary compressor 16. The air 
15 may of course also be fed from a pressure vessel 17. 
With increasing load of the gas-turbine plant, the fuel flow rate through 
the burner 1 steadily increases. In a similar manner to the fuel flow 
rate, the fuel pressure in the burner 1 and in the atomizing nozzle 4 
increases. When the fuel pressure required for sufficient atomization is 
reached, the air feed is interrupted by closing the control valve 14. As 
can be appreciated by one of ordinary skill in the art, the control valve 
14 is re-opened when the fuel pressure is lowered. When the control valve 
14 is closed, i.e. during high fuel pressure, the fuel oil 2 is divided by 
means of the circular injection orifice 9 into a fine spray suitable for 
the combustion. 
Air 15 continues to be fed to the burner 1 even during an interruption in 
the supply of liquid fuel 2 to the burner 2, e.g. during temporary use of 
the burner 1 as ignition burner or pilot burner or as stage burner of a 
sequential combustion chamber, or when the gas-turbine plant is being shut 
down. In. this way, purging of the atomizing nozzle 4 is ensured and its 
carbonization is thus prevented. 
Other auxiliary gases, such as, inert gases (nitrogen) or ignition gases 
(propane) or fuel gases (natural gas), may of course also be used as an 
alternative to the air 15 used. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.