Externally vaporizing system for turbine combustor

The externally vaporizing system of the present invention incorporates an auxiliary burner which is supplied with compressor discharge air at a rate which is regulated to the main fuel flow rate. The auxiliary burner is operated at approximately stoichiometric fuel-air ratio, in order to provide very-hot, nearly-inert gases for vaporizing the main fuel supply. The main fuel is sprayed in the auxiliary burner exit gas stream where rapid mixing and evaporation occur. The resulting vaporized fuel/inert gas mixture (at about 800.degree. F., preferably) is then ducted and distributed to the individual main combustor fuel injectors where it is injected into the premixing ducts.

BACKGROUND OF THE INVENTION 
The present invention relates to turbine engines, and, in particular, 
relates to a combustor therein. 
Many of the most commonly used fuels, such as heavy hydrocarbons, display 
physical and chemical properties which require great care in the 
implementation of a prevaporizing and premixing process. For example, 
complete vaporizing of commercial number 2 heating oil requires that the 
fuel temperature be raised to approximately 650.degree. F. Further 
increases in temperature simply assure the gassified state of the fuel. 
However, at temperatures as low as 900.degree. F., the vaporized fuel 
begins to break down, chemically forming molecules of both higher and 
lower molecular weight than that of the original fuel. The new heavy 
molecules are highly undesirable components as they cause clogging of the 
vaporizer fuel passages and injectors and produce high particulate 
emission levels when they burn. Thus, the range of temperature available 
for the vaporization process is quite narrow and requires very careful 
control. 
The chemical breakdown (pyrolysis) limit on fuel vaporization temperature 
leads to a second important problem. Although the fuel may be 
prevaporized, its heat content is insufficient to prevent some degree of 
recondensation if it is mixed with a stream of colder air. Although this 
partial recondensation results in extremely fine fog-like liquid droplets, 
the combustion properties of the system are degraded in comparison with 
the all gas-phase process. In addition, agglomeration can take place 
within the fog to produce larger droplets still and fuel can condense 
along the walls of the apparatus further degrading the combustion 
characteristics of the system. 
The premixed combustion of prevaporized liquid fuels has been the object of 
previous inventions. In U.S. Pat. No. 4,008,041, careful control of 
temperature during the vaporization process was accomplished by 
utilization of an intermediate heat transfer fluid. In U.S. Pat. No. 
4,089,638, the need for an intermediate fluid was eliminated by locating a 
fuel vaporization coil downstream of the point of flame initiation but 
sufficiently early in the reaction to avoid subjecting the coil to 
excessive temperatures. 
Numerous studies have shown that low levels of nitrogen oxides (NO.sub.x) 
and smoke emissions, as well as reduced flame radiation levels, can be 
achieved with lean premixed-prevaporized (LPP) combustion systems. One 
prior combustor is a single stage lean premixed-prevaporized combustor 
with conventional flameholder/reactor. Another is a parallel stage 
combustor with lean premixed-prevaporized catalytic reactor main stage. 
Several problems have been identified with this arrangement: 
(1) A relatively large number of well metered fuel injection points are 
needed in order to obtain an initially uniform spatial fuel distribution. 
The fuel injectors, therefore, tend to be mechanically complex, and prone 
to fouling, which can cause the spatial uniformity to deteriorate with 
operation; 
(2) At modern high-pressure ratio gas turbine operating conditions, 
ignition delay times are very short--on the order of 1-2 milliseconds, so 
potentially destructive combustion in the premixing duct is a concern. The 
duct must be very short, aerodynamically clean, and flow surges must be 
avoided; and 
(3) A high degree of fuel vaporization must be achieved in order to realize 
the LPP combustion benefits. Complete vaporization is difficult to obtain 
without encountering autoignition when current heavier fuels are used. 
Further, because of dwindling petroleum resources and increased demands, 
future fuels are expected to have increased boiling ranges, which will 
make it more difficult to achieve a high degree of vaporization. 
The use of a normally gaseous fuel (such as natural gas or propane) largely 
overcomes these problems, since (1) the gaseous fuel metering orifices are 
much larger and therefore less prone to fouling, and (2) the premixing 
duct then only needs to be long enough to achieve the desired degree of 
fuel-air mixing uniformity. 
The present invention is directed toward providing a combustor in which the 
undesirable characteristics of prior liquid fuel systems are overcome in 
light of the advantages of a purely gaseous system. 
SUMMARY OF THE INVENTION 
The present invention sets forth an externally vaporizing system for a 
liquid fueled lean premixed-prevaporized combustion system wherein heat is 
supplied to the fuel by direct mixing with hot inert gases thereby 
achieving a more compact and simple system. 
The externally vaporizing system incorporates an auxiliary burner which is 
supplied with compressor discharge air at a rate which is regulated to the 
main fuel flow rate. The auxiliary burner is operated at approximately 
stoichiometric fuel air-ratios, in order to provide very-hot, nearly-inert 
gases for vaporizing the main fuel supply. The main fuel is injected into 
the auxiliary burner exit gas stream where rapid mixing and evaporation 
occur. The resulting vaporized fuel/inert gas mixture (at about 
800.degree. F., preferably) is then ducted and distributed to the 
individual main combustor fuel injectors where it is injected into the 
premixing ducts feeding the main combustor. 
It is therefore one object of the present invention to provide an 
externally vaporizing system for a combustor; 
It is therefore another object of the present invention to provide an 
externally vaporizing system that is very compact; 
It is therefore another object of the present invention to provide an 
externally vaporizing system such that completely vaporized fuel can be 
generated at all engine operating conditions; and 
It is therefore another object of the present invention to provide an 
externally vaporizing system such that no valves, or small passages are 
required in the main vaporized fuel injection. 
These and many objects and advantages of the present invention will be 
readily apparent to one skilled in the pertinent art from the following 
detailed description of a preferred embodiment of the invention and the 
related drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a prior art parallel staged combustor 10 is shown 
which has a lean premixed-prevaporized catalytic reactor main stage 12. A 
liquid fuel injector 14 outputs through a first set 16 of nozzles and a 
second set 18 of nozzles. Following first set 16 is a premixing duct 20 
after which is a catalytic reactor 22 which vaporizes the premixed fuel. 
Combustion of the premixed and prevaporized fuel occurs in combustion 
chamber 24. A pilot burner 26 having second set 18 of injectors is 
provided in combustor 10. 
FIG. 2 illustrates a single stage combustor 28 having a fuel injector 30 
inputting fuel into a premixing duct 32 having an igniter 34. Complete 
burning occurs in a combustion chamber 36 when additional air is added. 
The present invention is illustrated in FIG. 3 as a combustor 38. Combustor 
38 is situated downstream of a diffuser section 40 and upstream of a 
turbine section 42, not shown, being conventional. 
The combustor 38 includes an inner annular casing 44 and an outer annular 
casing 46 forming an annular space 48 therebetween. Disposed within the 
annular space 48 is an annular combustion chamber 50. The combustion 
chamber 50 is constructed with an inner annular wall 52 and an outer 
annular wall 54 defining an annulus 56 therebetween being a conventional 
combustion chamber 50. The walls 52 and 54 also form an annular outlet 58 
at the downstream end of the combustion chamber 50 for directing 
combustion products into the turbine section 42. The upstream portion 60 
of the annulus 56 is the primary combustion zone. Mounted on the outer 
annular casing 46 and extending into the primary combustion zone 60 is 
ignition means 62 which is shown herein to be a conventional spark 
igniter. The ignition means 62 is mounted in the outer annular casing 46. 
Additionally, a plurality of premixing ducts/swirlers 64 are uniformly 
spaced about combustion chamber 50. An output end 66 of premixing 
ducts/swirlers 64 is connected to primary combustion zone 60. An input end 
68 has fuel injectors 70 therein. 
A plurality of externally vaporizing systems 72 are positioned about 
casings 46. Although, one externally vaporizing system 72 may be 
sufficient. Externally vaporizing system 72 has a flow control section 74, 
an auxiliary burner section 76, a main stage fuel injector section 78 that 
feeds into a vaporized fuel manifold 80. The vaporized fuel is then input 
into a plurality of fuel injectors 70. A single vaporized fuel manifold 80 
may feed several fuel injectors 70. 
Externally vaporizing system 72 draws a small fraction of input air 82, 
under pressure, into flow control section 74 wherein a throttle plate 84 
is connected to a control means, not further shown, that moves in response 
to operating conditions. The input air 82 enters auxiliary burner 76 
having a pressure atomizing fuel nozzle 86 and an igniter 88 wherein an 
approximate stoichiometric fuel-air ratio is established in order to 
provide very hot, nearly inert gases for vaporizing and mixing main fuel 
input by a plurality of injectors 90 in the main stage fuel injector 
section 78. The resulting vaporized fuel/inert gas mixture at about 
800.degree. F., preferably, is then input to fuel manifold 80 where it is 
distributed to individual main combustor fuel injectors 70 where it is 
injected into the premixing ducts/swirlers 64. 
The externally vaporizing system 72 may be very compact since about only 
one percent of the main combustor air flow is needed in the auxiliary 
burner to completely vaporize and heat the main fuel to about 800.degree. 
F.; completely vaporized fuel can be generated at all engine operating 
conditions because externally vaporizing system 72 is essentially 
independent of externally vaporizing system 72 is essentially independent 
of the main combustor 38; because the fuel input to premixing 
ducts/swirlers 64 is vaporized to a maximum level, no valves or small 
passages are required in the main vaporized fuel injectors 70 which would 
normally be prone to fouling and plugging. 
Clearly, many modifications and variations of the present invention are 
possible in light of the above teachings and it is therefore understood, 
that within the inventive scope of the inventive concept, the invention 
may be practiced otherwise than specifically claimed.