Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1 49,686
COMBUSTION TURBINE COMBUSTOR HAVING AN
IMPROVED HEAVY-OIL FUEL PREPARATION ZONE
CROSS-REFERENCES TO RELATED APPLICATIONS
Canadian Patent Application No. 405,430,
entitled "Combustion Turbine Combustor Having An
Improved Fuel-Rich Fuel Preparation Zone" and filed
June 17, 1982 by D. E. Carl, et al.
Canadian Patent Application No. 409,000,
entitled "Combustion Turbine Comb-ustor Having A Heavy
Oil Fuel Preparation Zone With Boundary Protection"
and filed August 9, 1982 by D. E. Carl, et al.
BACKGROUND OF THE INVENTION
The present invention relates to combustion
turbines and combustors employed therein and more
particularly to an improved fuel preparation zone
structure for a pre-mixed, pre-vaporized combustor.
In general terms, the typical prior art combus-
tion turbine comprises three sections: a compressor sec-
tion, a combustor section, and a turbine section. Air
drawn into the compressor section is compressed, increasing
its temperature and density. The compressed air from the
compressor section flows through the combustor section
where the temperature of the air mass is further increased.
From the combustor section the hot pressurized gases flow
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into the turblne section where the energy of the expanding
gases is transformed into rotational motion of the turbine
rolor .
A typical com'oustor section compr'ses a plural-
ity of combustors arranged in an annular array about thecircumference of the combustion turbine. In conventional
combustor technology pressurized gases flowing from the
compressor section are heated by a diffusion flame in the
combustor before passing to the turbine section. In the
diffusion flame technique, fuel is sprayed into the up-
stream end of the combustor by a nozzle. The flame is
maintained immediately downstream of the nozzle 'oy strong
aerodynamic recirculation. The lack of thorough mixing of
the fuel results in pockets of high fuel concentration and
correspondingly high combustion reaction temperatures
(approximately 4500R) in Ihose pockets. Because the
reaction temperature is hlgh, hot gases flowing from the
combustion reaction must be diluted downstream by cool
(approximately 700R) air so as to prevent damage to tur-
bine components positioned downstream. In addition, theflame diffusion technique produces emissions with signifi-
cant levels of undesirable chemical compounds including
NOX and CO.
Increasing environmental awareness has resulted
in more stringent emission standards for ~x and CO. The
more stringent standards are leading to developmen~ of
improved combustor technologies. One such improvement is
a premixing, prevaporizir.g combustor. In this type of
combustor, fuel is sprayed into a fuel preparation zore
where it is thoroughly mixed to achieve a homogeneous con~
centration which is everywhere within definite limits of
the mean concen~ration. Additionally, a certain amount of
the fuel is vaporized in ~he fuel preparation zone. Fuel
combustion occurs at a point downstream 'rom the fuel
3~ preparation zone. T!le substantially uniform fuel concen-
~ration achieved :l 'he fuel preparation zone results in a
uniform reaction tempe~ature which may be limited to
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appro,~l!ra e'y 'OGOF to 3000F. Due to -the uniformity and
-thoroughness of combustion, the premixing, prevaporizing
combustor produces Lower levels of NOy and CO than does a
conventional combustor using the same amount of fuel.
To date, premixing, prevaporizing combustors
have generally been fueled by clean fuel oils, such as No.
2 fuel oil, as opposed to heavy oil fuels. A clean fuel
oil is characterized by low ash content, low nitrogen
content and a low boiling point, so that the fuel will
readily vaporize in a fuel preparation zone operating at
normal temperatures. Heavy oil is generally avoided as a
fuel due ~o the problems which typically accompany lts
use.
Heavy oils are characterized by the short auto-
ignition times of the longer hydrocarbon chains, making
flashba-k a greater problem in heavy oils than in clean
oils. Elashback is the propagation of flame from the
point of combustion back into the fuel preparation zone.
If permitted to continue uncorrected, the presence of
flame in the fuel preparation zone will damage the com-
bustor to the extenl that the turbine must be shut down
and the combustor repaired or replaced. Because of the
higher boiling points, it is often impractical to com-
pletely vaporize a heavy oil. Furthermore, fully mixing a
heavy oil p-ior to combustion typically causes substantial
deposition of coke on the walls surrounding the fuel pre-
para~ion zone. Deposits of coke on the walls of a combus-
tor create flow obstructions and create irregular gas flow
patterns within the combustor, inhibiting performance of
the combustor. On the other hand, failure to mix a heavy
oil prior to combustion produces a non-uniform fuel con-
centration which can result in extremely high reaction
temperatures and damage to combustor components, espe-
cially to a catalytic element in a catalytic combustor.
~ence, the known prior art combustors do not ap-
pear to meet the need for a premixing, prevaporizing
combustor capable of successfully utilizing heavy oil
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fuels. ;~Ieatry oil fuels, not widely used in the past be-
cause o the problems des~ribed above, are attractive as
an alternative source of energy.
SUMMARY_ F THE_INVFNTIOM
Accordingly, a combustion turbine combustor
comprises an enclosure or basket having apertures îor
permitting the flow of compresso1- discharye gases into the
enclosure, a zone disposed within the enclosure downstream
of at least one of the ape~tures for mixing and vaporizing
the fuel prior to initiation of combustion, fuel injection
means di;,posed within the enclosure in the downstream end
of the mixin~ zone, and a combus-tion zone having means for
supporting combustion of the fuel mixture. The downstream
position of the fuel injection means reduces co~e deposi-
tion on the combustor walls and shortens the dwell time of
the fuel mi~ture in the combustor prior to initiation of
combustion. Use of a mul_iple-point fuel injector maxi-
mizes îuel mi~ing in the short space provided therefor.
Combustion may be by flame or by catalyst. When combus-
tion is by catalyst, a catalytic element is preferablystructured in t~o elements to reduce the potential îor
damage due to an imbalance in fuel concentration.
BRIEF DESCRlPTIOM OF T~E DR~.WTNGS
Figure l schematically shows a catalytic comblls-
tor arranged to operate a gas turbine in accordance withthe principles of the invention;
Figure 2 shows an elevational view of a catalytic
combustor disposed as shown schematically in Figure 1;
Figure 3 shows a sectior of the combustor of
3C Figure 2 arranged in accorc;ance w~th the principles of the
invention;
Figure a shows a cross-section of the combustor
of Figure 3;
Figure 5 shows an alternate embodiment of the
combustion means of Figure 3.
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D~-SC~ rN-C~ 0~ THE PREEER~ED E BODIr~ENT
More oa-ticularly, there is shown in Figure 1 a
generalized schematlc representation of a combustion tur-
bine combustor and combustor control system. A turbine or
general~y cylindrical catalytic combustor 10 is combined
with a plurality of like combustors (not shownj to supply
hot motive gas to the inlet of a turbine (not shown) as
indicated ~y reference character 12. The combustor ~i~
includes a catalytic unit 1' which supports catalytic
combustion (oxidation) of fuel-air mixture flowing through
the com~ustor 10.
The combustor 10 includes a zone 11 irto which
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fuel, such as oil, is injected by~nozzle /means 16 from a
fuel valve 17, where fuel-air mixin~ occurs in preparation
lS for entry into the catalytic unit la. Typically, the
îuel-air mix temperature (for example 800F) required for
cataly~ic reaction is higher than the temperature (for
example 700~) of the compressor discharge air supplied to
the combustors from the enclosed space outside the combus-
tor sheils. The deficiency in air supply temperature intypical cases is highest during startup and lcwer load
operation.
A primary combustion zone 18 is accordingly
provided upstream rom the fuel pre~aration ~one 11 within
the combustor 10. Nozzle means 20 are provided for in-
jecting fuel from a primary fuel valve 22 into the primary
combustion zone 18 where conventional flame combustion is
supported by primary alr enterirg the zone 18 from the
space within the turbine casing through o enings in che
combustor wall.
As a result, a hot gas flow is supplied to the
fuel preparation zone '1 where it can be mixed with the
fuel and air mixture to provide a heated îuel mi~ture at a
sufficiently high temperature to enable proper catalytic
unit operation. In this ar;^angement, the fuel injected by
the nozzle means 16 for com'oustion in the catalytic unit
is a secondary uel îlow. The secondarv fuel flow is
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mixed with secondary air and primary combustion products,
which su?ply the preheating needed to raise the tempera-
ture of the mixture to the level needed for entry to the
catalytic unit.
It should be noted that a combustor structured
according to the principles of the invention is not limit-
ed to the catalytic structure described herein. Other
combustors structured according to the principles of the
invention include catalytic combustors having no primary
combustion zone for preheating the gas flow and non-
catalytic combustors. A non-catalytic combustor structured
consistent with the principles of the invention comp.ises
nozzle means injecting fuel into a fuel preparation zone
for fuel-air mixing. Combustion of the fuel-air mixture
occurs at a flameholder or in an open section in a co~us-
tion zone downstream of the fuel preparation zone, produc-
ing a hot gas flow which is supplied to the turbine inlet.
The description hereinafter is directed expressly to a
catalytic combustor but applies equally well to a non-
catalytic type combustor.
In Figure 2 there is shown 3 structurally de-
tailed catalytic combustion system 30 embodying the prin-
ciples described for the combustor 10 of Figure 1. ~hus,
the combustion system 30 generates hot combustion products
which pass through stator vanes 31 to drive turbine blad~s
(not shown). A plurality of combustion systems 30 are
disposed about the rotor axis within a turbine casing 32
to supply the total hot gas flow needed to drive the tur-
bine.
In accordance with the prirciples of the inven-
tion, combustor 30 in_ludes a combustor basket 40, a cata-
lytic unit 36 and a t~ansition duct 38 which directs the
hot gas to the annular --pace through which it passes to be
directed against the turbine blades. ~he combustor 30
further comprises a fuel preDaration ~one internal to the
combustor baske; 40 at reference charact-r 34.
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A fuel preparation zone of a combustor is shown
in section in Figure 3. The fuel preparation zone 42
comprises a fuel injector means 44 positioned in the
downstrea.m end oE the fuel preparation zone 42. Placing
the fuel injector means 44 at the downstream end oi the
zone ~12, rather than at the upstream end as typically
found in prior art combustors, improves the reliability
and performance of the combustor when the combustor is
fueled with heavy oil.
The downstream position of the fuel injector
means 44 eliminates a prelimina.ry mixing section of the
fuel preparation zone of prior ar-t combustors. The pre-
liminary mixing section is an open space between the fuel
injector and a static mixing structure, which structure
could be either a flameholder, a catalytic element or a
separate mixing structure. The introduction of heavy oil
fuel into the preliminary mixing section can result in
substantial coke deposition on the walls of the fuel
preparation zone, obstructing gas flow and inhibiting
performance of the combustor. The downstream position of
the fuel injector means 44 reduces the dwell. time of the
fuel mixture in the fuel preparation zone 42. The reduced
dwell time reduces coke deposition by the heavy oil fuel
and also reduces the potential .for autoignition, or flash-
back, due to the shorter period of time between fuelinjection and fuel ignition. Due to the downstream position of the fuel in-
jector means 44, the low degree of mixedness of the fuel
mixture prior to combustion can result in the problems
associated with an imbalance in fuel concentration. One
approach to minimizing this problem is to use a multiple-
point fuel injector means 44 to yield substantial fuel
mixing immediately upon injection. The structure for one
form of multiple-point fuel injector means is depicted in
Figure 3 and Figure 4.
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As showr. ln Figure 3 and Figure 4, the uel
injector 'L4 comprises a plurality of truncated cone-like
s-tructures ~5, dispose~ with axes parallel to the combus-
tor axis of flow. Fuel is injected .rom a feeder tube ~8
protruding through an opening 50 in the truncated end of
each cone a6, whereupon the fuel is atomized by air flow,
shown by the arrow at 52, passing tnrough the opening 50.
The air flow 52 is accelerated through ~he opening 50,
thereby substantially vaporizing and mixing the fuel
iO injected by the tubes a~3.
The fuel feeder tubes 48 communicate fuel from a
fuel manifold, such as an outer fuel manifold 5a and an
inner fuel manifold 56, the two manifolds being concen-
trically disposed. A single fuel supply line 5~ supplies
fuel from a fuel valve (reference character 17 of Fig. l)
to the dual fuel manifolds 54, 56. Hence, by appropriate
structure of the fuel injector means 44, potential negative
effects of the downstream position of the fuel injector
means are diminished, permitting use of heavy oil fuels in
a premixing, prevaporizing combustor.
Incomplete mixing of the fuel-air mixture can
also cause problems within the catalytic unit (36 of Fig.
2) positioned downstream of the fuel injector means 44.
Pock-ts of high fuel concentration resulting from incom-
plete fuel mixing tend to react to a higher temperature
corresponding to that consentration, which temperature may
exceed the maximum temperature which the single cat~lytlc
element of typical prior art combustors is capable of
withs-tanding without damage.
To minimize tr.e risk of damage to the catalytic
unit, the catalytic element of the present combustor is
structured in two segments. In this configuration, the
fuel-air mixture undergoes only ?artial reaction within
the first catalytic segment 60, ensuring that the reaction
temperature remains within he li.nitations o~ the catalytic
element. ~ space 62 between the î~rst segment 60 and the
second catalytic segment 6a permits co;npiete Cuel mixing
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before combustion is completed within -the second catalytic
segment 6~. The dual catalytic element struc-ture thereby
reduces the sens~tivity o the catalytic unit to incom-
plete fuel mixing, fu~ther improviny performance of a
premixing, prevaporizing combustor fueled with heavy oil.
The precise structure and nature of the catalytic element
is well known in the ?rior art and not critical to the
operation of the combustor described herein.
As an alternative to the dual catalytic elemen~
structure, the outer annulus of the catalytic unit may be
structured so as to induce turbulence in the fuel-air
mixture flowing therethrough. This may be accomplished by
varyiny the diameter of the catalytic unit along its
length so as to ensure turbulence and resultant fuel
mixing.
The catalytic unit 36 may be replaced by a
flameholder unit 70, depicted in elevation in Figure 5, in
a non-cata].ytic combustor. The flameholder unit 70 c:om-
prises a flameholder 72 of any appropriace shape such as a
sphere, an ignition means 74 for initiating the combustion
reaction in response to command from a turbine control
(not shown) and an ignition tube 76 for communicating
between the ignition means 74 and the flamehol.der 72 and
for structural support of the flameholder 72. The struc-
ture of a flameholder unit, as presented generally above,is well known in tne prior art. It is noteworthy that the
flameholder unit 70 adapts well to the characteristics of
the invention described heretoîore.
An incompletely mixed -^uel-air mixture entering
the flamenolder un t 70 does not pose a threat of damage
to the flameholaer unit 70 as in the case of the catalytic
unit 3~ properly structured flameholder is by nature
a flow obstruction whose aercdynamic wake creates a well
stirred reaction ~one. ~ence, pockets of hiyh fuel con-
centrat on are thoroughly m_xed by Ihe flameholder 72. Inadditlon, the reducec. level Gf mixedr.ess upstream of the
flameholder may act to stabi'ize the combustion reaction
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by preventincJ flashback. Thus, the principles of the in-
vention apply to a non-catal~ytic combustor as well as to a
catalytic co~bustor, en2bling both types of combustor., to
operate by use of heavy oll fuel.
