BACKGROUUND OF THE INVENTION
Field of the Invention
[0001] The present invention relates to a combustor for a gas turbine engine. More specifically,
the present invention relates to a combustor for a gas turbine engine, capable of
making the combustor of the gas turbine engine achieve desired stable, low-pollution
combustion under partial load.
Description of the Related Art
[0002] Private power generation equipment has progressively come into wide use recently,
and gas turbine engines are used widely as power units for private power generation
equipment. Different time zones have different power demands, respectively, on private
power generation equipment. Therefore, private power generation equipment does not
operate always in a maximum-load operating mode and are often required to operate
in a partial-load operating mode.
[0003] Consequently, the gas turbine engine for private power generation equipment is required
to maintain satisfactorily stable combustion and to reduce the rate of emission of
harmful gases including NO
x (nitrogen oxides) and CO (carbon monoxide) for environmental conservation.
[0004] In order to meet the requirements, some techniques have been proposed previously,
such as a first method that injects water or steam into a combustion chamber and a
second method that reduces air for lean premixed combustion through engine extraction.
[0005] The first method needs an additional mechanism for injecting water or steam into
the combustion chamber. The additional mechanism enlarges the gas turbine engine and
the gas turbine engine cannot reduce the rate of NO
x emission greatly so as to compensate for the disadvantage of the additional mechanism.
[0006] Although the second method can reduce the rate of NO
x emission in a wide range of load on the gas turbine engine even in lean premixed
combustion having a narrow stable combustion range, work necessary for extraction
reduces the thermal efficiency of the gas turbine engine.
SUMMARY OF THE INVENTION
[0008] The present invention has been made in view of those problems in the related art
and it is therefore an object of the present invention to provide a combustor for
a gas turbine engine, capable of achieving stable combustion and of reducing the rate
of emission of harmful substances, such as NO
x and CO, in a wide range of load without enlarging the gas turbine engine, without
complicating the constitution of the gas turbine engine and without reducing the thermal
efficiency of the gas turbine engine.
[0009] A combustor for a gas turbine engine of a lean premixed combustion type according
to the present invention includes: a fuel jetting device configured to jet a premixed
fuel into a combustion chamber so that the premixed fuel burns in a premixed combustion
zone which is formed in the combustion chamber; and an additive fuel supply unit disposed
downstream of the premixed combustion zone so as to additionally supply fuel into
the combustion chamber, the additive fuel supply unit including first and second additive
fuel supply devices disposed in a two-stage arrangement such that the second additive
fuel supply device is positioned downstream of the first additive fuel supply device.
[0010] In the combustor according to the present invention for a gas turbine engine, it
is preferable that a main air passage formed in a liner head to supply air into the
premixed combustion zone has a sectional area not lager than a sum of sectional areas
of air passages in the first and second additive fuel supply devices of the additive
fuel supply unit.
[0011] In the combustor according to the present invention for a gas turbine engine, it
is preferable that a sectional area of an air passage in the first additive fuel supply
device is not larger than a sectional area of an air passage in the second additive
fuel supply device.
[0012] In the combustor according to the present invention for a gas turbine engine, it
is preferable that the first and second additive fuel supply devices of the additive
fuel supply unit are respectively provided with diffusion nozzles to supply fuel.
[0013] The combustor according to the present invention for a gas turbine engine achieves
desired stable, low-pollution combustion under load varying in a wide load range and
discharges an exhaust gas including reduced amount of harmful substances, such as
NO
x and CO.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The above and other objects, features and advantages of the present invention will
become more apparent from the following description taken in connection with the accompanying
drawings, in which:
Fig. 1 is a schematic sectional view of a combustor U in a preferred embodiment according
to the present invention for a gas turbine engine; and
Fig. 2 is diagram comparatively showing the respective load characteristics of harmful
gas emissions of a combustor in a present example of the present invention and of
a combustor in a comparative example.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] A combustor in a preferred embodiment according to the present invention will be
described by way of example with reference to Figs. 1 and 2.
[0016] Referring to Fig. 1 showing a combustor U in a preferred embodiment according to
the present invention included in a gas turbine engine, the combustor U has a combustor
casing including a flow sleeve 1, namely, an outer sleeve, and an inner tube 2, namely,
an inner sleeve, extended inside the flow sleeve 1.
[0017] One end of the flow sleeve 1 and one end of the inner tube 2 are closed by an end
wall 3. The other end of the flow sleeve 1 is an air inlet 1a and the other end of
the inner tube 2 is an exhaust outlet 2a.
[0018] The interior of the inner tube 2 is divided into a liner head 4 on the side of the
end wall 3 and a combustion chamber 5 on the side of the exhaust outlet 2a by a partition
wall 2b having a shape resembling an annular plate disposed at a predetermined longitudinal
position.
[0019] The liner head 4 is provided with a predetermined number of openings 4a through which
combustion air taken through the air inlet 1a into the flow sleeve 1 flows into the
inner tube 2. A fuel jetting device 6 is disposed inside the liner head 4. The fuel
jetting device 6 prepares a premixed fuel by mixing fuel supplied by a fuel supply
system, not shown, and combustion air and jets the premixed fuel into the combustion
chamber 5. The premixed fuel jetted by the fuel jetting device 6 burns in a predetermined
premixed combustion zone V in the combustion chamber 5.
[0020] The combustor U is provided with a two-stage additive fuel supply unit 7 in addition
to the fuel jetting device 6. The two-stage additive fuel supply unit 7 supplies fuel
additionally into the combustion chamber 5 while the gas turbine engine is in a partial-load
operation. In Fig. 1, a character F indicates diffusion flames, namely, premixed flames.
[0021] The fuel jetting device 6 includes a pilot combustion unit 10 in a central part of
an inner opening of the partition wall 2b disposed on the upstream side of the combustion
chamber 5, and a main combustion unit 20 surrounding the pilot combustion unit 10.
[0022] The pilot combustion unit 10 has a central pilot fuel nozzle 11, and a combustion
air passage 12 surrounding the pilot fuel nozzle 11. A premixed fuel produced by mixing
fuel jetted through the pilot fuel nozzle 11 and combustion air is jetted into the
combustion chamber 5.
[0023] The pilot fuel nozzle 11 has a closed end and a side wall provided with radial jetting
holes 11a. Fuel is jetted radially through the radial jetting holes 11a. The combustion
air passage 12 is defined by the outside surface of the pilot fuel nozzle 11 and the
inside surface of an inner hat-shaped tube 13 surrounding the pilot fuel nozzle 11.
A swirler 14 is disposed at a predetermined position in the combustion air passage
12. The fuel jetted into the combustion air passage 12 through the pilot fuel nozzle
11 is mixed into swirling combustion air currents produced by the swirler 14. The
premixed fuel flows into the combustion chamber 5.
[0024] The main combustion unit 20 includes a main fuel nozzle 21 and a main premixed fuel
passage 22. A premixed fuel produced by mixing the fuel jetted through the main fuel
nozzle 21 and combustion air flows through the main premixed fuel passage 22 into
the combustion chamber 5.
[0025] The main fuel nozzle 21 surrounds the inner hat-shaped tube 13. The fuel is jetted
through the main fuel nozzle 21 toward the inlet 22a of the premixed fuel passage
22.
[0026] The main premixed fuel passage 22 is defined by the outside surface of the inner
hat-shaped tube 13 and an outer hat-shaped tube 23 of a diameter greater than that
of the inner hat-shaped tube 13. A space between the flanges of the hat-shaped tubes
13 and 23 forms the inlet 22a of the premixed fuel passage 22.
[0027] A swirler 24 is disposed in the inlet 22a of the premixed fuel passage 22.
[0028] The open end of the outer hat-shaped tube 23 is joined to the inside edge of the
partition wall 2b disposed inside the inner tube 2 to form a premixed fuel outlet
22b, namely, the outlet of the main premixed fuel passage.
[0029] A premixed fuel produced by mixing the fuel jetted through the main fuel nozzle 21
and combustion air is swirled. The swirling premixed fuel is jetted through the premixed
fuel outlet 22b into the premixed combustion zone V.
[0030] Combustion air taken through the openings 4a of the liner head 4 into the inner tube
2 flows through the main premixed fuel passage 22 and the swirler 14 into the premixed
combustion zone V. The present specification defines the sum of the respective sectional
areas of the main premixed fuel passage 22 and the swirler 14 as a main air passage
area S.
[0031] Generally, there are limited lean air-fuel ratios that enable the gas turbine engine
to achieve stable combustion at low rate of emission of NO
x and CO under low loads. According to the present embodiment, the main air passage
area S is determined such that the air-fuel ratio of the premixed fuel in the premixed
combustion zone V while the gas turbine engine is in a partial-load operation under
a predetermined load, such as a load equal to 50% of a maximum load, is nearly equal
to the limited air-fuel ratio for an operation under a predetermined load nearly equal
to a maximum load, such as a load equal to 80% of the maximum load. The main air passage
area S is smaller than an ordinary main air passage area.
[0032] The two-stage additive fuel supply unit 7 supplies fuel additionally for after burning
into a predetermined zone downstream of the premixed combustion zone V, in which stable
low-NO
x, low-CO combustion is carried out, in the combustion chamber 5 during a partial-load
operation to suppress the generation of NO
x. The two-stage additive fuel supply unit 7 has a predetermined number of additive
fuel supply tubes 31 penetrate parts corresponding to the predetermined zone of the
inner tube 2 defining the combustion chamber 5, and additive fuel supply nozzles 32
having end parts of a predetermined length inserted into the additive fuel supply
tubes 31.
[0033] The additive fuel supply nozzles 32 may be either of diffusion nozzles as shown in
Fig. 1 and premixed combustion nozzles, not shown.
[0034] The two-stage additive fuel supply unit 7 includes a first additive fuel supply device
7A and a second additive fuel supply device 7B. The first additive fuel supply device
7a and the second additive fuel supply device 7B are disposed at a first longitudinal
distance L1 and a second longitudinal distance L2, respectively, from the premixed
fuel outlet 22b. The distance L2 is greater than the distance L1.
[0035] The first distance L1, the second distance L2, the numbers of the tubes 31 and the
additive fuel supply nozzles 32 of the additive fuel supply devices 7a and 7B, the
pattern of arrangement, such as a straight pattern or a zigzag pattern, of the additive
fuel supply nozzles 32, the ratio between the main air passage area S and the sum
of the sectional areas of the air passages of the two-stage additive fuel supply unit
7, and the ratio between the respective sectional areas of the first additive fuel
supply device 7A and the second additive fuel supply device 7B are selectively determined
so that the rate of emission of NO
x and CO may be small and stable combustion can be achieved.
[0036] The ratio D1 of the main air passage area S to the total sectional area of the two-stage
additive fuel supply unit 7 is, for example, 5/5, and the ratio D2 of the total sectional
area of the first additive fuel supply device 7A to the total sectional area of the
second additive fuel supply device 7B is, for example, 4/6.
[0037] The ratio D1 is reduced to 4/6 or such to widen the range of low-load operation.
[0038] The ratio D2 is reduced to 3/7 or such to widen the range of low-load operation.
[0039] The main air passage area S of the combustor U in this embodiment is determined so
that the combustor U can achieve stable combustion at a low rate of emission of NO
x and CO while the gas turbine engine is in a partial-load operation, and the fuel
additionally supplied by the two-stage additive fuel supply unit 7 burns in the combustion
chamber 5. Therefore, a proper air-fuel ratio in accordance with a given load can
be determined so that the generation of NO
x and CO can be suppressed.
[0040] Thus the combustor U can maintain stable combustion at a low rate of emission of
NO
x and CO for the operation of the gas turbine in a wide load range.
[0041] Performance of combustors in a present example according to the present invention
and a comparative example will be described in connection with Fig. 2. Shown in Fig.
2 are load characteristics of NO
x and CO concentrations of the exhaust gas discharged from a combustor in the comparative
example provided with only the first additive fuel supply device 7A and load characteristics
of NO
x and CO concentrations of the exhaust gas discharged from a combustor in the present
example provided with the first additive fuel supply device 7A and the second additive
fuel supply device 7B. In Fig. 2, H1 shows the allowable upper limit of NO
x concentration, and H2 shows the allowable upper limit of CO concentration.
[0042] As obvious from Fig. 2, the NO
x concentration of the exhaust gas discharged from the combustor in the present example
is lower than that of the exhaust gas discharged from the combustor in the comparative
example in the entire load range.
[0043] It is known from Fig. 2 that the additive fuel supply devices 7A and 7B are effective
in reducing the NO
x and the CO concentration of the exhaust gas. The CO concentration of the exhaust
gas discharged from the combustor in the present example is below the allowable upper
limit H2 of CO concentration in the entire load range, which proves that the additive
fuel supply devices 7A and 7B are effective in reducing the CO concentration of the
exhaust gas. The CO concentration of the exhaust gas generated by the combustor in
the comparative example is higher than the allowable upper limit H2 of CO concentration
at loads below a predetermined load E1. Therefore, the combustor in the comparative
example should not be operated at loads below the predetermined load E1 from the viewpoint
of environmental conservation. Thus the two-stage additive fuel supply unit 7 including
the additive fuel supply devices 7A and 7B expands the load range in which the combustor
can operate without causing problems in environmental conservation.
[0044] Although the invention has been described in its preferred embodiment with a certain
degree of particularity, obviously many changes and variations are possible therein.
It is therefore to be understood that the present invention may be practiced otherwise
than as specifically described herein without departing from the scope thereof.