Gas turbine combustor and operation method thereof for a diffussion burner and surrounding premixing burners separated by a partition

A gas turbine combustor having a fuel injection nozzle for diffusion combustion disposed in a central portion of the combustor and an annular premixed nozzle disposed in an outer peripheral portion of the fuel injection nozzle for injecting a mixed gas of fuel and air. The annular premixed nozzle includes an annular premixing chamber and a plurality of premixed fuel nozzles for injecting fuel into the annular premixing chamber. The annular premix nozzle is divided by a partition (108) to form a plurality of premixing chambers each adjacent to the fuel injection nozzle for diffusion combustion. Each of the premixing chambers includes at least one of the premixed fuel nozzles. A mechanism is provided for supplying fuel to a predetermined number of the premixing chambers in a low load operation of the gas turbine and for supplying fuel to all of the premixing chambers in a high load operation.

BACKGROUND OF THE INVENTION 
The present invention relates to a gas turbine combustor and, more 
particularly, to a gas turbine combustor which is suitable for reduction 
in emission of nitrogen oxides (hereunder referred to as NOx). 
NOx which occurs at time of combustion of natural gas, kerosene, gas oil 
(light oil), etc. is thermal NOx and occurs through oxidization of 
nitrogen in the air. The occurrence of the thermal NOx depends highly on 
temperature. In general, in a gas turbine in which low nitrogen content 
fuel is used, reduction of flame temperature is a principal concept of low 
NOx combustion. A combustor of a gas turbine is different from a burner 
used in boilers, etc.. That is, the fuel flow rate changes according to 
the gas turbine load. On the other hand, air flow rate is substantially 
fixed, and the fuel air ration, which is a mass flow ration between fuel 
and air, changes greatly between a partial load and a 100% rated load. 
Further, at the rated load, in which the fuel flow rate is a maximum, a 
lot of air, up to twice as much as the theoretical air flow rate necessary 
to effect complete combustion of fuel, is supplied. Therefore, a lean burn 
which forms a flame with much excess air can be employed, and the lean 
burn is a leading technology in a method of reduction of NOx adapted in 
the current combustor. 
Combustion methods of gas fuel are classified into a diffusion combustion 
method of burning fuel while mixing the fuel with air and a premixed 
combustion method of premixing fuel and air and then jetting the mixture 
from a nozzle to burn it. The diffusion combustion method is excellent in 
stability of the flame and is able to form a flame in a wide range of fuel 
air ratios. However, since the fuel is burned while being mixed with air, 
the fuel air ration changes greatly, spatially, within the flame, so that 
even if lean burn is tried, a part of the fuel is burned under the 
condition of fuel rich combustion. Therefore, the flame temperature is 
raised partially and a lot of NOx is apt to occur. 
In premixed combustion, fuel and air are mixed before they are introduced 
into the combustion chamber. Therefore, with the premixed combustion 
method it is easier to provide a uniform fuel air ration within the flame 
than it is with diffusion combustion, and the formation of partial 
temperature elevation due to lock of uniformity of mixing can be avoided, 
so that an effect of reduction of NOx is large. However, the fuel air 
ratio range in which the flame is stably formed and the conditions of 
jetting velocity are narrower than with diffusion combustion. In 
particular, in an operation of a turbine from starting to 100% load or 
operation of load interruption at emergency, since the fuel flow rate 
changes widely, it is difficult to operate the gas turbine only by 
premixed combustion, so that a two step combustion method is employed in 
which diffusion combustion is effected from the staring to a partial load 
and then premixed combustion is started when the turbine goes beyond the 
partial load. For combustors employing the two step combustion method, 
there are two kinds, one of which is a combustor in which combustion 
chambers are provided independently for diffusion combustion and for 
premixed combustion, respectively, which is disclosed in JP A 61-22106 and 
JP A 61-22127, for example. An example of the other kind is a combustion 
in which the diffusion combustion and the premixed combustion are effected 
in the same combustion chamber, which is disclosed in JP A 63-161318. 
Further, since the fuel air ratio increases and the flame temperature 
elevates according to an increase in the gas turbine load or an increase 
in the fuel flow rate, NOx increases according to the increase in load. 
When the two step combustion method is employed, the NOx emission amount 
can be reduced after staring of the premixed combustion, but the NOx 
occurrence amount increases during the diffusion combustion before 
starting of the premixed combustion. In order to suppress NOx occurrence 
over a wide load range, the premixed combustion is started at a load as 
low as possible, but it is necessary to suppress NOx occurrence in the 
diffusion combustion. 
In order to form a premixed flame, it is necessary to set the injection 
velocity of the mixture and the fuel air ratio within a range. When the 
injection velocity becomes larger and the fuel air ratio becomes smaller, 
the flame is blown out, and when the injection velocity becomes lower and 
the fuel air ratio becomes larger, the flame comes into the mixture 
injection nozzle, and so-called back fire takes place. The amounts of fuel 
and air necessary to form the flame are determined by the outlet diameter 
of the nozzle. If the outlet diameter of the nozzle is made larger, the 
amount of fuel and air necessary to inject at a velocity necessary for 
stable combustion increases, and a gas turbine load at which a premixed 
combustion can be effected becomes higher. If the outlet diameter of the 
nozzle is made smaller, a premixed flame can be formed stably by a small 
amounted of fuel and air, but an amount of fuel in which the premixed 
combustion can be effected becomes small and a ratio of the amount of fuel 
to be burned by the diffusion combustion increases, so that the amount of 
NOx occurrence increases. Therefore, when it is intended to reduce NOx 
during a high load operation, the premixed nozzle is made larger, so that 
a gas turbine load at which the gas turbine is operated by premixed 
combustion becomes high. 
As an example of a combustor for addressing the above-mentioned subjects, a 
combustor provided with a mechanism for adjusting an air flow rate for 
diffusion combustion and premixed combustion, and reducing NOx, CO, etc. 
over a wide load range by optimizing the air flow rate distribution is 
already proposed in JP A 60-91141, JP A 60-218535, JP A 61-153316 and JP A 
61-52523, for example. A combustor in which a plurality of premixed 
nozzles are provided and the number of the premixed nozzles in use is 
changed according to load also is proposed in JP U 2-100060 (Laid-Open 
Utility-Model Application), for example. 
The gas turbine can be operated with less NOx emission. However, in order 
to improve further the performance, there is the following subject to be 
addressed. When the combustion chamber is divided, each of diffusion 
combustion and premixed combustion is independently effected so that 
flames can be prevented from interfering with each other and stable flames 
can be formed. However, combustion air also is divided into air for 
diffusion combustion flame and air for premixed combustion flame, and a 
ratio of the premixed combustion can not be increased. In case diffusion 
flame and premixed flame are formed in the same combustion chamber and the 
combustion air is used commonly for both diffusion combustion and for 
premixed combustion, for example, when the gas turbine load is low, there 
is a problem to be solved as the fuel air ratio in diffusion flame becomes 
small by air for premixed flame and unburned components become easy to 
exhausted. 
Further, in order to reduce NOx further in a wide range of loads, it is 
necessary that the premixed nozzle be made large to increase the ratio of 
the premixed combustion. The premixed combustion is started from a low 
load, and NOx occurred during the diffusion combustion is made as small as 
possible. 
Of the above-mentioned prior art, in the combustor provided with an air 
flow adjustment mechanism, when the air amount for premixed combustion is 
increased in order to effect premixed combustion at a low load, injection 
velocity of premixed fuel air also becomes low and there is a possibility 
of back fire occurring. Therefore, in particular, when the premixed nozzle 
outlet diameter is made large, it is insufficient to reduce the load at 
which the premixed combustion starts. 
On the other hand, in the combustor which is provided with a plurality of 
premixed nozzles and the number of the nozzles in use is changed according 
to load, since the construction is such that the plurality of annular 
premixed nozzles are provided concentrically with an axis, when the number 
of the premix nozzles in use increases, the next nozzle has to be ignited 
with a relatively low temperature premixed flame, so that there is a 
problem that ignitability of the premix nozzle of the second stage or 
later stages worsen. Further, there are problems such that an area in 
which flames of adjacent premixed nozzles contact each other is large, the 
premixed flames interfere with each other, pressure change (combustion 
vibration) becomes large and the life of the combustor shortens. 
SUMMARY OF THE INVENTION 
An objective of the present invention is to provide a gas turbine combustor 
which can effect stable combustion over a wide load range and reduce NOx. 
Another objective of the present invention is to provide a combustor in 
which diffusion combustion and premixed combustion are effected in the 
same combustion chamber and combustion stability and operation are 
improved. 
A gas turbine combustor according to the present invention has a fuel 
injection nozzle for diffusion combustion, disposed in a central portion 
of the combustor and an annular premixed nozzle for injecting a gas 
mixture of fuel and air, arranged in an outer periphery of the fuel 
injection nozzle. The annular premixed nozzle is divided circumferentially 
to form a plurality of premixing chambers and the combustor is constructed 
so that fuel is supplied into a predetermined number of premixing chambers 
and the plurality of premixing chambers at the time of low load operation 
of the gas turbine, and fuel is supplied into all the plurality of 
premixing chambers at the time of high load operation. 
Further, a stabilizer for generating eddies in the mixture gas can be 
provided in the vicinity of the outlet of the gas mixture in the premixed 
nozzles. 
Further, air flow adjusting means for adjusting air flow rate for each 
premixing chamber, supplied into the plurality of the premixing chambers 
can be provided. 
Further, a gas turbine combustor according to the present invention has a 
fuel injection nozzle for diffusion combustion, disposed in a central 
portion of the combustor and an annular premixed nozzle for injecting a 
gas mixture of fuel and air, arranged in a periphery of the fuel injection 
nozzle. A stabilizer which is a resistor of circular flow is provided in 
the vicinity of the gas mixture outlet of the premixed nozzle. The 
diffusion combustion fuel nozzle is disposed at an upstream side of the 
mixture, the diffusion combustion nozzle is radially spaced from the 
premixed nozzle so that circulation flows, sufficient to form stably 
diffusion flames, are formed therebetween, and a conical partition wall is 
provided between the diffusion combustion fuel nozzle and the premixed 
nozzle. 
Further, the diffusion combustion fuel nozzle is characterized in that the 
air is sufficient to generate swirling flows in the fuel for diffusion 
combustion is swirled by the air and injected. 
Further, the above-mentioned partition wall is characterized in that the 
partition wall is cooled by combustion air for diffusion combustion and 
the cooling air is introduced in the vicinity of the premixed nozzle and 
then injected into the combustion chamber. 
It is preferable that the radial distance between the diffusion combustion 
fuel nozzle and the premixed nozzle is 0.2 to 0.4 in ratio to an inner 
ring of the premixed nozzle and a spread angle of the conical partition 
wall is 30.degree. and 60.degree. from a face of the outlet of the 
premixed nozzle. 
Further, another feature of the present invention, utilizing a method of a 
gas turbine combustor having a plurality of premixed nozzles and a 
plurality of flow control systems for supplying fuel into the premixed 
nozzles and changing the number of the premixed nozzles to be operated 
according to load, resides in that when the number of the premixed nozzles 
in operation is increased, a fuel amount for the premixed nozzles operated 
before and after the increase is decreased and fuel is supplied into the 
newly operated premixed nozzles, and when the number of the premixed 
nozzles in operation is decreased, a fuel amount for the premixed nozzles 
to be stopped to operate is decreased, and a fuel amount for the remaining 
premixed nozzles which are operating is increased. 
According to the gas turbine combustor of the present invention, the 
annular premixed nozzle disposed in the periphery of the fuel injection 
nozzle for diffusion combustion arranged around the central portion of the 
combustor is circumferentially divided to form a plurality of premixing 
chambers and when the gas turbine operates at a low load, that is, when 
fuel flow rate is small, fuel is supplied to only a predetermined number 
of the premixing chambers of the plurality of the premixing chambers, so 
that a fuel air ratio in the premixing chambers in use can be raised a 
magnification of the division number. Therefore, by increasing the 
division number of the premixed nozzle, premixed combustion can start at a 
lower load and a NOx value at a partial load can be reduced. 
Further, all the premixed gas injected from each of divided premixed 
nozzles can contact with flames of the diffusion combustion nozzle 
provided at the central portion. Therefore, each of the premixed nozzles 
can be ignited by the diffusion flame which is higher in temperature than 
the premixed flame, and the number of the premixed nozzles in use can be 
increased more stably during increase in the load. 
Further, when a plurality of premixed nozzles are used, an area in which 
adjacent premixed flames contact each other is small, interference of 
premixed flames with each other becomes less and combustion vibration is 
reduced. 
Further, by providing a flame stabilizer at mixture outlets of the premixed 
nozzles, stabilization of flame and ignitability can be improved further. 
Further, by providing the air flow adjusting means for adjusting air flow 
for each of the premixing chambers supplied into the plurality of 
premixing chambers, the air flow rate can be increased and a desired fuel 
air ratio can be maintained even under such conditions of operation that 
the fuel air ratio reduces at low load. In this case, compensation for the 
air flow rate can be small as compared with an air flow rate compensation 
in the premixed nozzle which is not divided, and the air flow rate becomes 
easy to be controlled according to a change in load, so that stability of 
the flame can be improved and NOx can be reduced. 
Further, according to the present invention, by providing a conical 
partition wall between the diffusion combustion nozzle and the premixed 
nozzle and constructing the combustor so that the diffusion combustion 
nozzle is disposed upstream of the premixed nozzle, mixing of diffusion 
fuel and a lot of air injected from the premixed nozzle can be delayed and 
diffusion flame of a relatively high fuel air ratio and high temperature 
can be formed in the central portion of the combustor. Further, by 
radially separating the diffusion combustion fuel nozzle and the premixed 
nozzle so that circulation flow sufficient to stably form diffusion flame 
is formed therebetween, stability of the diffusion combustion flame can be 
further improved. By setting the radial distance between the diffusion 
combustion fuel injection nozzle and the premixed nozzle to 0.2-0.4 in 
ratio to the inner ring of the premixed nozzle, the above-mentioned 
circulation flow can be suitably formed. 
Another advantage of forming the partition wall in a conical form is in 
that the air flow rate for cooling a combustion chamber wall can be 
reduced because the surface area of the combustion chamber wall upon 
forming the combustion chamber for diffusion combustion can be made small, 
compared with a cylindrical combustion chamber. In the low NOx combustor, 
it is necessary to use the air as much as possible for premixed 
combustion, and when cooling air is reduced and premixed air is increased, 
NOx can be reduced according to a decrement of the cooling air and an 
increment of the premixed air. 
In order to reduce further cooling air for the partition wall, air is 
impinged on the surface of the partition wall on an air supply side, that 
is, on the side opposite to the combustion chamber, whereby a boundary 
layer of air on the surface is disturbed and the heat transfer coefficient 
is raised to effectively cool the surface. The cooling air is injected 
into the combustion chamber from a portion close to the premixed nozzle so 
that the air as much as possible can be used for combustion of premixed 
flame. Where a stabilizer which is a bluff body, in which a resistor for 
air is provided at the central portion of the premixed gas flow and the 
flame is stabilized (held) with circulating flows of high temperature 
combustion gas formed at the downstream side of the resistor is used, 
flames are formed along the resistor and propagate from the edge portion 
toward the outer periphery. In the premixed gas flow also, combustion is 
delayed in the outer peripheral portion as compared with the central 
portion, and also air from an adjacent portion of the premixed nozzle in 
addition to air from the interior of the premixed nozzle has the effect of 
lower the temperature of the premixed flame. 
Further, according to a method of the gas turbine combustor relating to 
another feature of the present invention, when the number of the premixed 
nozzles in use is increased, the fuel flow rate of the premixed nozzles 
operated before and after the increase of the nozzle number is decreased 
and fuel is supplied into the newly operated premixed nozzles, and when 
the number of the premixed nozzles is decreased, the fuel flow rate of the 
premixed nozzles, the operation of which is stopped, is decreased and fuel 
for the remaining premixed nozzles which are operating is increased, so 
that an abrupt change due to a change in fuel supply amount according to a 
change in the number of premixed nozzles in operation can be suppressed 
and a rapid change of load can be prevented.

DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 shows an embodiment of the present invention. 
A gas turbine comprises an air compressor, a combustor and a turbine. Air 
from the air compressor is introduced into the combustor, and used for 
combustion of fuel to turn into high temperature gas which is introduced 
into the turbine. The combustor comprises a combustion chamber 1, a fuel 
nozzle 2A for diffusion combustion, a premixed nozzle 3, dividing plates 
30 dividing the premixed nozzle 3 into a plurality, and premixing chambers 
formed by the dividing plates 30. Air 4 for combustion is introduced from 
a downstream side of the combustor to an upstream side through a cooling 
liner surface forming the combustion chamber 1. Air supplied into the 
combustion chamber 1 includes diffusion fuel dispersing air 4A, partition 
wall cooling air 4B and premixed combustion air 4C. Fuel is injected into 
the combustion chamber from the diffusion combustion fuel nozzle 2A and 
premixed combustion fuel nozzles 2B. 
The diffusion combustion fuel nozzle 2A is arranged in the central portion 
of the combustor, and the premixed nozzle 3 which is annular is arranged 
on the outer periphery thereof by a partition wall 5. The diffusion 
combustion fuel nozzle 2A is important to be spaced radially from the 
premixed nozzle 3 so that combustion gas of diffusion flame 9B circulates 
sufficiently from a downstream side of diffusion flame 9B to a fuel outlet 
side of the diffusion combustion fuel nozzle 2A. By occurrence of such 
circulation flow, stability of the diffusion flame 9B can be improved. 
The diffusion combustion fuel nozzle 2A injects fuel from its fuel 
injection port, and the fuel is injected into the combustion chamber 1 
together with diffusion fuel dispersing air from flow passages 6 provided 
on the outer peripheral portion of the fuel injection port. The diffusion 
fuel dispersing air 4A is swirled by swirling vanes provided at outlets of 
the flow passages 6 to be swirling flow which is introduced into the 
combustion chamber 1. When fuel is directly injected as swirling flow 8 
without using the diffusion fuel dispersing air 4A, momentum of a swirling 
component changes depending on a gas turbine load or a fuel flow rate. In 
order to avoid this phenomena and keep swirling momentum of some extent 
irrespective of the fuel flow rate, the diffusion fuel dispersing air 4A 
is introduced to the swirling flow 8. The diffusion fuel dispersing air is 
sufficient to be an amount of air necessary to impart swirling momentum to 
injected fuel flow, the amount is 10% or less of an amount of air 
introduced into the combustor, usually, 2 to 3%. Further, in this 
embodiment, the diffusion fuel dispersing air 4A is injected toward the 
center of the combustor. 
The annular premixed nozzle 3 comprises a plurality of premixed combustion 
fuel nozzles 2B, a mixing portion 3A of air introduced at an inlet of the 
premixed nozzle, and an annular flame stabilizing ring 3B provided at an 
outlet thereof. Fuel 9A for premixed flame and air is mixed in the mixing 
portion 3A and injected into the combustion chamber 1. Circulation flows 
10 are formed downstream of the flame stabilizing ring 3B at the outlet of 
the nozzle, the circulation flows 10 become combustion gas of high 
temperature during premixed combustion, and premixed flame 9A is 
stabilized by this combustion gas. 
In diffusion combustion, air from the premixed nozzle 3 is used for 
diffusion combustion is employed in low load and at turbine starting. As 
the load becomes high, both the premixed combustion and the diffusion 
combustion are effected. Therefore, for the diffusion flame 9B used in a 
low load operation or in an operation at t small fuel flow rate, air from 
the premixed nozzle 3 is excessive. When all the air is introduced into 
the reaction area of the diffusion combustion, misfire takes place or an 
exhaust amount of unburned substances increases. In the combustor of this 
embodiment, air from the premixed nozzle 3 is divided by the stabilizing 
ring 3B into two, one of which is inner periphery air (4D) and the outer 
is outer periphery air (4E). In this manner, when the air flow is divided, 
the outer periphery air (4E) is delayed in mixing with fuel for diffusion 
combustion, air used for diffusion flame 9B formed in the central portion 
of the combustor is decreased, and a stable flame is easy to be formed. 
A conical partition wall 5 is arranged between the diffusion combustion 
fuel nozzle 2A and the premixed nozzle 3. By this partition wall 5, 
diffusion combustion fuel is delayed in mixing with combustion air 4, that 
is air from the premixed nozzle 3 and the stability of the diffusion flame 
9B formed in the central portion of the combustor is secured. 
Detailed construction of the partition wall 5 is shown in FIGS. 2 and 3. 
FIG. 2 is a view in which the partition wall 5 is viewed from the upstream 
side of the combustor. FIG. 3 is an enlarged view of the partition wall 5. 
The partition wall 5 is provided with a multi-hole plate 11 substantially 
in parallel with the partition wall 5 on the upstream side thereof. Air 
jet flow from the multi-hole plate 11 impinges on the partition wall 5. A 
distance sufficient to cool the partition wall through the impingement of 
jet flow from the multi-hole plate 11 is provided between the partition 
wall 5 and the multi-hole plate 11, and cooling air 4B passes through a 
flow path 12 and injected into the combustion chamber 1 from a portion in 
the vicinity of premixed nozzle 3. 
In this embodiment, a spreading angle .alpha. of the conical partition wall 
5 is 34.degree., the height h is 0.32.times.H wherein H is the diameter of 
the inner ring of the annular premixed nozzle, that is, h/H=0.32. The 
angle .alpha. and the ratio h/H are preferable to be in a range of 
0.2-0.4, respectively. By setting h/H in such a range, circulating flow of 
combustion gas of the diffusion flame 9B as mentioned above is formed 
sufficiently. 
FIG. 4 shows a comparison of characteristics of CO exhaust between a 
combustor in which the fuel injection outlet port for the diffusion 
combustion fuel is disposed on the same face as the premixed nozzle outlet 
port, that is, the angle .alpha.=0.degree. in the combustor of the 
embodiment as shown in FIG. 1. In the combustor of .alpha.=0.degree., the 
CO exhaust amount becomes large at a certain range of fuel air ratio. That 
is, it is important that the diffusion combustion fuel nozzle is 
positioned to be at an upstream side and fuel therefrom is dispersed well 
before the fuel is mixed with air from the premixed nozzle. 
A second embodiment of the invention is now explained referring to the 
drawings. 
FIG. 5 shows a low NOx combustor for a gas turbine of the second 
embodiment. The combustor is provided with a fuel nozzle 105 for diffusion 
combustion in the central portion and an annular premixed nozzle 103 at an 
outer peripheral side. The premixed nozzle 103 is divided in a direction 
towards the circumference thereof into two by a dividing plate 108, 
whereby a premixing chamber 104A of upper half and a premixing chamber 
104B of lower half are formed. 
High pressure air 109 from a compressor (not shown) passes through between 
an outer cylinder 101 and an inner cylinder 102 forming therein a 
combustion chamber and is branched into air for the premixed nozzle 103 
and air for the diffusion fuel nozzle 105. The air for the premixed nozzle 
103 passes through swirlers 107 provided at the inlet of the premixed 
nozzle 103, and is premixed with fuel from the premixed nozzles 106A, 106B 
in the premixing chambers 104A and 104B, and the premixed gas is burned at 
the outlet of the premixed nozzle 103 to form premixed flames 116A, 116B. 
Further, the premixed flames are stabilized by swirling of air having 
passed through the swirlers 107. 
On the other hand, air for the diffusion combustion fuel nozzle 105 passes 
through between the premixed nozzle 103 and the diffusion combustion fuel 
nozzle 105, and the air is swirled by a swirler 119 and injected into the 
combustion chamber together with fuel. The diffusion combustion fuel is 
burned while being mixed with air injected from the premixed nozzle 103 to 
form diffusion flames 115. Therefore, air for the diffusion combustion 
fuel nozzle 105 is sufficient to be an amount of air necessary to spread 
fuel jet flow so that the diffusion combustion fuel can be mixed with air 
jetted from the premixed nozzle 103 and the air can be an amount less than 
an amount of air necessary for diffusion combustion. 
Fuel 110 is divided into fuel supplied for each nozzle by a fuel flow 
controller 112 on the bases of gas turbine load 114. Namely, fuel 110C for 
diffusion combustion is supplied into the diffusion combustion fuel nozzle 
105, with opening of fuel control valve 111C. That is, the fuel rate is 
adjusted by control signal 113C from the fuel flow controller 112. In the 
same manner, premixed combustion fuel 110A (or 110B) is supplied into the 
premixed combustion fuel nozzle 106A (160B). That is, the fuel flow rate 
is adjusted by control signal 113A (113B) from the fuel flow controller 
112, and mixed with air in the premixing chamber 104 (104B). The fuel 
nozzles 106A, 106B for supplying fuel into the premixing chambers 104A, 
104B are five in number, respectively, however, any number of nozzles can 
be applied so long as the mixing degree of fuel and air does not worsen. 
Next, combustion control operation of the above-mentioned combustor is 
explained. As shown in FIG. 6, in an operation under low load, fuel is 
supplied to only the diffusion combustion fuel nozzle 105 to effect only 
diffusion combustion operation. When the turbine reaches a load at which 
premixing combustion is started, the diffusion combustion fuel is 
decreased and fuel is supplied into the premixed combustion fuel nozzles 
106A of the upper half by the decrement of the diffusion combustion fuel, 
and the combustor is operated by the diffusion combustion fuel nozzle and 
the premixed combustion fuel nozzles 106A of the upper half. In a further 
high load operation, fuel for the premixed combustion fuel nozzle 106A of 
the upper half is reduced to a half and the same amount of fuel is 
supplied into the premixed combustion fuel nozzles 106B of the of the 
lower half, whereby all the fuel nozzles 106A and 106B are operated, and 
the load is raised to a full load while being controlling so that the fuel 
air ratios in the two premixed nozzles are equal to each other. The fuel 
air ratio of the premixed nozzles and NOx concentration at the outlet of 
the combustor at this time are shown by solid lines in FIGS. 7 and 8. 
In FIG. 7, an upper limit and a lower limit of fuel air ratio in which 
stable premixed combustion can be effected are shown. It is necessary to 
operate the premixed nozzle in this range defined by the upper and lower 
limit values, so that if there is a difference in the fuel air ratio 
between the plurality of premixed nozzles 6A, 6B when the nozzles are 
operated, an operable range of load becomes narrow. Therefore, it is 
necessary to control the fuel air ratio so that all the fuel air ratios of 
the premixed nozzles 6A, 6B in operation are equal to each other. Further, 
when fuel is supplied to only the premixed nozzles 6A of upper half, the 
fuel air ratio of each of the premixed nozzles 6A, 6B becomes twice the 
fuel air ratio of each of all the premixed nozzles 6A, 6B, so that by 
dividing the premixed nozzle 103 into two, premixed combustion can be 
started at a load corresponding to a fuel flow rate about 1/2 the flow 
rate in the premixed nozzle which is not divided. As a result, an 
operation load under which only diffusion combustion is effected is 
lowered, as shown in FIG. 8 NOx value can be decreased on in the diffusion 
combustion operation, and low NOx can be realized in a wide range of the 
load. Further, in this embodiment, the premixed nozzle 103 is divided into 
2, and by increasing the number of divisions the load at which premixing 
starts can be further lowered and the NOx value only in diffusion 
combustion operation can be further decreased. For reference, 
characteristics of NOx in case of the premixed nozzle which is equally 
divided into 3 are shown by dotted line in FIGS. 7 and 8. 
Next, another embodiment of the present invention is explained referring to 
FIG. 9. 
In FIG. 9, a combustor of this embodiment, as compared with the combustor 
of the second embodiment, is not provided with the swirler 107 at the 
inlet of the premixed nozzle 103, and is provided with stabilizer 117 at 
the outlet of the premixed nozzle 103 instead thereof. The stabilizer 117 
is formed in an annular configuration corresponding to the annular 
premixed nozzle. The cross section thereof is an isosceles triangle, the 
apex of which is oriented to the upstream side. The stabilizer 117 holds 
the premixed flames 116A, 116B by circulation flows generated in the 
downstream side of stabilizer 117. 
Holding or stabilizing of the premixed flame by the stabilizer 117 can 
decrease NOx more than holding stabilizing of swirling flows by the 
swirler 107 shown by the second embodiment because a fuel air ratio range 
in which stable combustion is possible is large and the fuel air ratio of 
premixed gas can be made small. A flow direction of premixed gas at the 
outlet of the premixed nozzle 103 is deflected in the direction of the 
diameter by the stabilizer 117. Namely, gas flow in the premixed nozzle 
103 around the axis is deflected further toward the axis by the stabilizer 
117. Conversely, the gas flow in the premixed nozzle 103 at the outer 
diameter side is deflected further toward the outer diameter side. 
Therefore, in the case where the stabilizer is applied to a combustor 
which is provided with a plurality of premixed nozzles arranged 
concentrically with an axis as shown in the prior art, since the premixed 
nozzles are placed in the diameter direction, flames at the outlets of 
adjacent premixed nozzles violently interfere with each other. As a 
result, there is a problem that combustion vibration becomes violent. 
Therefore, such a nozzle is necessary to be divided circumferentially and 
prevent interference between the premixed flames, as in the present 
invention. 
Next, a combustor of another embodiment of the present invention is 
explained referring to FIG. 10. The combustor of this embodiment, as 
compared to the combustor of the second embodiment, has a premixed nozzle 
103 which is bent in a L-letter shape in an axially vertical section and 
has an inlet directed to the outer diameter side, and an air flow 
adjustment mechanism 118 disposed at the inlet. Further, a drive mechanism 
(not shown) also is provided for axially moving the air flow adjustment 
mechanism 118. 
The air flow adjustment mechanism 118 is shifted axially, whereby the inlet 
opening area of the premixed nozzle 103 changes, and a flow rate of air 
entering the premixed nozzle 103 can be changed. Therefore, when a fuel 
air ratio of the premixed nozzle such as the premixing starting load 
becomes low, the fuel air ratio of the premixed nozzle can be increased by 
decreasing the premixed combustion air flow rate by utilizing the air flow 
adjustment mechanism 118. Further, as a load or a fuel flow rate 
increases, air for premixed combustion can be increased, so that change in 
the fuel air ratio can be small and the NOx is decreased further to 
stabilize the combustion. 
Further, in case of bending the premixed nozzle 103 in a L-letter shape in 
the section in order to mount the air flow adjustment mechanism 118, as in 
this embodiment, since the premixed nozzles are arranged in the diameter 
direction in the prior art, the position of each nozzle inlet directed to 
the outer diameter side is necessary to be shifted to the axis direction. 
As a result, the inner diameter side nozzle and the outer diameter side 
nozzle are different from each other in the length of the premixing 
chamber, respective nozzles are different in flow rate characteristics, 
mixing characteristics, etc., and there is a problem that uniform 
combustion is difficult. However, the problem is solved by dividing the 
premixed nozzle in the circumferential direction, as in the present 
invention. 
According to the present invention, mixing of fuel and a lot of air is 
delayed, so that a diffusion flame of high temperature can be formed in 
the central portion of the combustor, and divided air for diffusion 
combustion is swirled, so that high temperature circulating flows can be 
formed in the downstream side of the diffusion combustion fuel nozzle, 
whereby stability of the diffusion flame can be improved further and 
unburned components, such as CO, can be reduced further. 
Further, by making the partition wall in a conical shape, the surface area 
of the combustion chamber for diffusion combustion can be made small, an 
amount of cooling air can be reduced by the decrement, whereby NOx can be 
reduced by increasing air for premixed combustion. 
Further, according to the present invention, the premixed nozzle can be 
used partially, so that the premixing starting load can be lowered, and 
low NOx operation is possible in a wide load range.