Note: Descriptions are shown in the official language in which they were submitted.
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COMBUSTION TURBINE COMBUSTOR HAVING AN
IMPROVED FUEL-RICH FUEL PREPARATION ZONE
BACKGROUND OF THE INVENTION
... .. .
The present invention relates to combustion tur-
bines and combustors employed therein and more particular-
ly to an improved fuel preparation zone structure for a
pre-mixing, pre-vaporizing combus~or.
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 th~ combus~or section the hot pressurized gases flow
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into ~he turbine section where the energy of the expanding
gases is transformed into rotational motion of the turbine
rotor.
A typical combustor section comprise~ a plural-
5 ity of combu~tors arranged in an annular array about the
circumference of the combustion turbine. In conventional
combustor technology pressurized gases flowing from the
compressor section are heated by a difusion flame in the
combustor before passing to the turbine sec ion. In the
diffusion flame technique, fuel is sprayed into the up
stream end of the combustor by means of a nozzle. The
~lame i~ maintained immediately downstream of the no7zle
by strong aerodynamic recirculation. The lack of thorough
mixing of the fuel results in pockets of high fuel concen-
tration and correspondingly high combustion reactiontemperatures, on the order of approximately 450~R.
3ecause the reaction temperature is high, hot gases flow-
ing from the combustion reaction must be diluted down-
stream by cool (approximately 700~R) air so as to prevent
damage to turbine components positioned downstre m. In
addition, the flame diffusion technique produces emissions
with significant levels of undesirable chemical compounds
including NOx and CO.
Increasing environmental awareness has resulted
in more stringent emission s-tandards for NO~ and CO. The
more stringent standards are leading to ~evelopment of
improved combustor technologies. One such impro~ement is
a premixing, prevaporizlng combustor. In this t~pe of
combustor, fuel is sprayed into a fuel preparation zone
where it is thoroughly mixed to achieve a homogeneous
conentration which is everywhere within definite limits
of the mean concentration. Additionally, a certain amount
of the fuel is vap~rized in the fuel preparation zone.
Fuel combustion occurs at a point downstream from the fueL
preparatlon zonc. The substantially unlform fuel concen-
tration achieved in the fuel preparation zone results in a
uniform reaction temperature ~hich may be li.mited to
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approximately 2000F ~o 30C0F. Due to the uniormity and
thoroughness of combustion, the pre mixing, prevaporizing
combustor produces lower levels of NO~ and CO than does a
conventional combustor uslng the same amount o fuel.
One problem with premixing, prevaporizlng com~
bustors is the destructive potential for flashback, or
sudden propagation of flame from the polnt of combustion
back into the fuel preparation zone. If permitted to
continue uncorrected, the presence of flame in the fuel
10 preparation zone will damage the con~ustor to the e~tent
that the turbine must be shut down and the combustor re-
paired or replaced. This phenomenon has been classically
define~ to involve the competing relationship between the
flow velocity and the flame velocity of the combustible
15 ga~es. Because the flame velocity is ordinarily charac-
teristic o the fuel used, the flow velocity is more
readily within the control of the designer. For further
elaboration of the classical approach to flashback see 3.
Lewis, G. von Elbe, Combustlo_, Flames ~nd Explosions of
20 Gases, Ac~demic Press, New York (1961).
Elame stability in a combustor was later 52mi-
quantified by S. L. Plse and A. M. Mellor. See "Review Of
5Flashback Reported In Prevaporizing/Premixing Combustors,"
32 Combustion and Flame 193-203 (1978). The formuia
25 sug~ested by Plee and Mellor for stability i5 as follows:
1/9 = m/(P AD~ exp (T/C))
where:
.
m - mass flow rate of combustible gases in the fuel
preparation zone
P = absolute pressure of the fuel pxepaxation zone
A = cross-sectionaL area of the fuel preparation zone
D - cross-sectional diameter of the fuel preparation
7one
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T = absolute temperature of the fuel preparation zone
a, b, ~ c = constants, greater than zero
It was reported that the value of 1/~ for a fuel
preparation zone is representative of the stability of the
combustion flame. With larger values of 1/3 the fuel
preparation zone is less prone to flashback. From the
equation it can be determined that flame stability is
promoted by a reduction in the cross-sectional dimensions
of the fuel preparation zone.
Reduction of the cr.oss-sectional ar~a, however,
places severe restrictions on the designer by limiting
fuel nozzle selection to smaller nozzles. In general,
smaller noz~les suffer from more limited availability,
greater expense, an~ a greater pressure drop across the
nozzle as compared to larger nozzles. Pressure loss
within the combustor is to ba minimized so that the work-
ing efficiency of the combustion turbine is maintained.
The danger of flashback becomes especially acute
when fuel concentration in the fuel preparaticn zone is
high, as in the case of a fueL-rich fuel preparation.
Operating a combu~tor in a fuel-rich configuration can
reduce NOX ~missions by effectively depleting the oxygen
which would ordinarily be avallable to combine with nitro-
gen to form NOX. Without specific safeguards directed at
preventing fiashbac~, use of a fuel-rich fuel preparation
zone would not be possible in a premixing, prevaporizing
combustor.
Thus, the known prior art does not appear to
meet the need for preventing flashbac~ in a premi.~ing,
prevaporizing combustor without reducing the cross-
sectional area of the fuel preparation zone.
SUMMA~Y OF THE INVENTION
Accordingly, a combus~ion turbine combustor com-
prises an enclo~ure or basket having apertures for permit-
~ing the fl.ow of compressor discharge gases lnto the en~closure, fuel injection means wittl1n the enclosure down-
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stream of one or more of the apertures, a fuel preparationzone downstream of the fuel injection means, a structure
within the fueL preparation zone for mlxing and vaporizing
the fuel to obtain a homogeneous fuel mixture, means
within the fuel preparation zone for accelerating the flow
of the mixture through a co~ustion zorle, and a cor~ustion
zone for supporting co~bust:ion of the fuel mixture The
mi~ing structure i5 arranged to provide acceleration of
the fuel mixture through the combustion zone to permit use
of a high fuel concentration within the fuel preparation
zone and yet diminish the danger of flashback. The means
of combustion may be by flam~ or by catalyst; in both
ca es, the increased flow veloclty through the combustion
zone by virtue of the arrangement of the mixing structure
serves to prevent 1ashback.
BRIEF_DESCRIPTION OF THE DRAWINGS
Figura 1 schematically shows a catalytic combus-
tor arranged to operate a gas turbine in accordance with
the principles of the invention;
Eigure 2 shows an elevational view of a cataly-
tic combustor;
Figure 3 shows a fuel preparation zone for a
combustor arranged in accordance with the principles of
the invention;
Fi~ure 4 shows a static mixer of Figure 3 in
section;
Figure 5 shows a side view of an alternative
internal structure for the static mixer shown in Figure 3.
DESCRIPTION OF THE PREFERRED EMBODI~ENT
Mors par~icularly, there is shown in Figure 1 a
generalized schematic representation of a combustion tur-
bine combustor and combu~tor control system. A tu~bine or
generaLly cylindrical catalytic combustor 10 i5 combined
with a pluralit-~ o~ like combustors (not shown) to su~ply
hot ~otlve gas to the inlet of a turbine (~ot shown) as
indicated by reference charact-r 12. The combustor 12
includes a catalytic uni~ 1~ which suppo~ts catal~tic com-
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bustion (oxidation) of fuelwair mixture 10wing through
the combustor 10.
The combustor 10 includes a zone 11 into which
fueL, such as oil, is injected by nozzle means 16 from a
S uel valve 17, where fuel-air mlxing occurs in preparation
for entry into the cataly-tic unit 14. Typically, the
fuel-air mix temperature (for example 800F) required for
catalytic reaction is higher than the temperature (for
sxample 700Fj of the compressor discharge air supplied to
the combu~tors from the enclosed space outside the combus-
tor shells. Th~ deficiency in air supply temperature in
typical cases is highest during startup and lower load
operation.
A primary combustion zone 18 is accordingly
provid~d up-~tream from the fuel preparation ~one 11 within
the combustor 10. No7zle means 20 are provided for in-
jecting fuel from a primary fuel valve ~2 into the primary
combustion zone 18 where conventional flame combustion is
supported by primary air entering the ~-one 18 from the
space within the turbine casing throu~h openings in the
combustor wall.
As a result, a hot gas flow is supplied to the
îuel preparation zone 11 where it can be mixed with the
fuel and air mixture to provide a heated fuel mixture at a
sufficiently high temperature to enable proper catalytic
unit operation. In this arrangement, the fuel inject2d by
the nozzle means 16 for combustion in the catalytic unit
is a secondary fuel flow. The secondary fuel flow i5
mixed with secondary air and primary combustion products,
~hich supply the preheating needed to raise the tempera-
ture of the mixture to the level needed for entry into the
catalytic unit.
It should be notsd that a combustor structured
according to the principles of the invention ls not l.imit-
ed to the catalytic struc.ure described above. Othercombustors structured according to the principles of the
invention include catalytic combustors having no primary
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combustion zone or preheating the gas flow and non-
catalytic combustors. A non-cataLytic combustor (not
shown) structured consistent with the principles of the
invention compris2s nozzle means injecting fuel into a
fuel prepa~ation zone for fuel-air mixin~. Combustion of
the ~uel-air mixture occurs at: a flameholder or in an open
section in a co~bustion zone downstream OI the fuel prep
aration zone/ producing a hot gas flow which is supplied
to the turbine inlet. The description hereinafter is
directed expressly to a catalytic combustor but applies
egually well to a non-catalytic combustor.
In Figure 2 there is shown a structurally de-
tailed catalytic combustion systam 30 embodying the prin-
ciples described for the combustor 10 of Figure 1. Thus,
the combustion system 30 generates hot combustion products
~hich pass through stator vanes 31 to drive turbine blades
(not shown). A plurality of combustion systems 30 are
disposed about the rotor axis within a turbine casing 32
t~ supply the total hot gas flow needed to drive the
turbine.
In accordance wi~h the principles of the inven-
tlon, the combus~or 30 includes a combustor enclosure or
basket 40, a catalytic unit 36 and a transition duct 38
which directs tha hot gas to the annular space through
which it passes to be directed against the turbine blades.
Tho combustor 30 further comprises a fuel preparation zone
internal to the combustor basket AO at reference char~c^
ter 34.
A fuel preparation zone of the combustor 30 of
Eigure 2 is shown in section in Figure 3. The fuel pre-
paration zone comprises one or more nozzle means 42 for
injecting fuel into the fuel preparatlon zone, a prelimi-
nary mixing area 44, and a static mixer 46. Initial
fuel-air mixing occurs in the area 44 when fuel is sprayed
into a flow of compressor discharge air. Complete mixing
of the fuel~-air mlx~ure to obtain a uni orm ~oncentration
of the fuel throughout the mixture occurs as he mix~ure
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flows thro~gh the static mixing s~ructure ~6. The static
mixing structure 46 is arrang~d to provide efficient fuel
mixing while minimizing lcss o pressure across the mixing
str-ucture 46.
The structure of the static mixlng structure 46
is utili7ed to provide to the designer a means for con-
trolling the flo~ velocity of the gaseous mixture, supple-
me~tlng the control inherent in the choice of the cross-
sectional dimension of the combustor. The significance of
this control element is de~monstrated by the following
equation for flow velocity:
.
V ~ m/dKA
where:
V = flow velocity of the combustible gases in the
fuel preparation zone
m = mass flow rate of combusti~le gases
d = density of the combustible gases in the fuel
preparation zone
K = void fraction of the fuel preparation zone
20~ = cross~sectional area of the fuel preparation
~one
The void fracti~n (K) equals the ratio of un-
obstructed cross-sectional area to total cross-sectional
are of the fuel preparation zone. Thus, the void frac~
25 tion for an unobstructed fuel preparatlon zone equals 1.O.
.~s can be seen from the a~ove equation, the flow velocity
oî the gaseous mixture may be increased by decreasing the
void fraction of the fuel preparation zone. The static
mixing structure 46 provides a convenient means for de-
creasing the void fraction of the fuel preparation zone
and thereby increasing the flow velocity of the gaseous
mixture, without decreasing the cross-sectional dimensions
of the fuel preparation zone. Use o,^ the static mixing
structure a6 in this way permits a high fuel concentration
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(fueL-rich) fuel preparation zone without constraining the
col~bustor dimensions available to the designer.
Flgure 4 shows a cross-section of the static
mixing structure 46 disclosed in Figure 3. The lnternal
5 arrangement of the static mixing struc~ure '16 comprises a
plurality of layers of corrugated material, such as metal
alloy, arranged to define a plurality of passageways 50.
The layers of corrugated material may be arranged in sev-
exal continuous sections (not shown), so that when the
sections are disposed end-to-end to form a plurality of
continuous passageways through the several sections, tho
passageways of any two adjacent sections form angles of
90 or more with respect to one another. The thickness of
the corrugated material may be chosen according to the
above e~quation to provide the desired fiow velocity.
Figure 5 shows an elevation of an alternative
arrangement for the static mixing structure g6 of Figure
3. The structure disclosed is essentialLy a flat bar
twisted 360~ to create a spiral defining dual passageways.
This structure alone may be utilized or, alternatively, it
may be coupled with a second 360 spiral in the reverse
direction to increase the degree o mixedness within the
static mixing structure 46. In both cases, the thic~ness
of the metal bar is chosen to provide an appropriate void
fraction as set forth in the equation above.
Hence, a premixing, prevapori7ing combustor m~y
be structured according to the principles of the invention
to minimize the chance of flashbac~ and thereby enable the
combustor to operate in a fuel-rich configu~ation. By
appropriate design of the static mixing s~ruc_ure, îlo~-
velocity OI the fuel-air mixtura through the combustion
zone is increased, decreasing the risk of flashback ~ith-
out altering the cross-sectional dimensions of the fuel
preparation zone.
