Method for reducing nitrous oxide emissions from a gas turbine engine

A simplified method is provided for turbulently mixing separate flows of fuel and water prior to delivery to the combustor of a gas turbine engine to reduce the temperature to which pressurized air is heated by the combustion of fuel, thereby reducing nitrous oxide emissions in the products of combustion. The method employs no complex homogenization equipment and no emulsifying agents are intentionally added to either the fuel or the water.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to gas turbine engines and, more particularly, to a 
simplified method for reducing nitrous oxide emissions through the 
technique of water injection. 
2. Description of the Prior Art 
In this era of environmental awareness it is anticipated that regulations 
covering air pollution will become increasingly restrictive and that 
compliance with industrial emission standards will become more difficult 
to attain. These environmental considerations will have an impact upon the 
development of industrial gas turbine engine power plants and may require 
the reduction of exhaust emission levels consistent with available 
technology at realistic costs. However, the trend in gas turbine engine 
development is toward higher temperature engines which, though they are 
inherently more efficient, also tend to produce higher emission levels of 
nitrous oxide (NO.sub.x). 
It is generally accepted that NO.sub.x formation increases exponentially 
with flame temperature. It has also been generally acknowledged that 
NO.sub.x formation can be reduced by introducing water in the form of 
liquid or steam into the combustion process to reduce the temperature to 
which the air is heated by the combustion of fuel in the primary zone of 
combustion. Because of the exponential increase of NO.sub.x formation with 
flame temperature, relatively large reductions in NO.sub.x can be achieved 
with relatively low water flow rates. Furthermore, the specific method of 
water injection in gas turbine engines does not appear to be particularly 
important. Water has been injected separately into the combustor through 
distinct water nozzles as a liquid and as steam. It has also been injected 
into the combustor in the upstream, downstream and "side stream" 
directions through separate water passages in the fuel injector. It has 
even been introduced through dual-flow nozzles wherein the water and fuel 
were injected coaxially into the combustor. However, although these 
methods of injecting water have been successful in controlling NO.sub.x, 
they have, on occasion, produced some problems with hardware life due to 
local temperature gradients in the region where the water is being 
injected. In fact, instrumented sector tests have demonstrated that in 
using the upstream method of injecting water through the nozzle, combustor 
metal temperature variations increase from a normal 260.degree. C. 
temperature variation with no water injection to a 427.degree. C. 
variation with the amounts of water injection necessary to achieve 
significant NO.sub.x reductions. While these temperature variations are 
the measured results of one particular series of engine sector tests, they 
are representative of the trend in temperature variations to be found in 
other gas turbine engine combustors. 
More recently, a concept for emulsifying the fuel and water together and 
injecting the mixture through the normal (or enlarged) fuel nozzles has 
been used successfully. This has considerable advantages over the systems 
relying on separate injection of fuel and water since complexity is 
minimized, separate nozzles may be eliminated, and costs reduced 
accordingly. 
There is an old axiom that fuel and water won't mix. However, they 
will--but only temporarily. They then separate at a rate that appears to 
be a function of the specific gravity of the fuel. As the specific gravity 
approaches unity (where fuel has the same density as water), the 
separation rate becomes much slower. To achieve satisfactory fuel-water 
emulsion, current practice has been to process the two separate liquids 
through a homogenizer where each is pressurized to a very high level and 
then sprayed through extremely small orifices into impingement against a 
hard impact block in a common mixing chamber. The impact breaks each fluid 
into extremely fine particles which become intimately mixed, or 
emulsified, into one homogeneous fluid. The subsequent separation rate is 
apparently slowed by the intimacy or fineness of the emulsion. This 
homogenizing equipment is, of course, very bulky and costly. 
Since water suppression of NO.sub.x is simply a function of water 
concentration, the emulsion concept is only one means employed to assure 
that each fuel nozzle is supplied with the same quantity of fuel and water 
as are all the others. Since all nozzles are supplied by a common pressure 
source (usually a fuel manifold), then all will flow the same rate of 
fluid, be it fuel, water or a fuel-water emulsion. If separation occurs 
prior to combustion, then some nozzles will flow more fuel (or water) than 
others and unacceptable temperature distributions will result inside the 
engine. In fact, it has been found that fuel variations between nozzles in 
excess of 10 percent are generally undesirable. In short, the fuel and 
water need be mixed or emulsified only to the extent required to assure 
uniform distributions throughout the manifolded fuel nozzles. Since 
state-of-the-art fuel-water emulsifiers or homogenizers are inherently 
complex, heavy and costly, it would be advantageous to develop a simple 
emulsifier which merely meets requirement of uniform fuel distribution 
among the manifolded nozzles. 
SUMMARY OF THE INVENTION 
Accordingly, it is the primary object of the present invention to provide a 
method for reducing NO.sub.x emissions from gas turbine engine combustors 
through a simplified concept of water injection. 
It is yet another object of the present invention to provide a method of 
operating a gas turbine engine with water injection which minimizes the 
variation in fuel percentage between a plurality of nozzles by prolonging 
the fuel-water separation time. 
These and other objects and advantages will be more clearly understood from 
the following detailed descriptions, the drawings and specific examples, 
all of which are intended to be typical of, rather than in any way 
limiting on, the scope of the present invention. 
Briefly stated, these objects as well as additional objects and advantages 
which will become apparent from the following specification and the 
appended drawings and claims are accomplished by the present invention 
which provides a method for reducing nitrous oxide emissions from the 
combustion products of a gas turbine engine. The method comprises 
turbulently mixing separate flows of fuel and water in a water:fuel ratio 
in the range of 0.6 to 1.4 by weight. The mixing is conducted under 
conditions which are sufficient to produce turbulence with a Reynolds 
number of at least l500 in the resulting mixture. The resulting mixture is 
then delivered, within thirty seconds from the time the fuel and water are 
mixed, to a plurality of combustor nozzles for combustion.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawings wherein like numerals correspond to like elements 
throughout, attention is first directed to FIG. 1 which is a graphical 
representation of the exponential relationship of NO.sub.x generation 
(represented by a nondimensional NO.sub.x index) with flame temperature in 
a gas turbine engine. Line 10 represents the locus of test data relating 
to this phenomenon. It is generally recognized that NO.sub.x control can 
be achieved by injecting liquid water or steam into the combustion process 
to lower peak flame temperatures and, since the relationship between 
NO.sub.x generation and flame temperature is exponential, it can be 
appreciated from FIG. 1 that relatively small amounts of water injection 
can produce large reductions in NO.sub.x. The invention soon to be 
described relates to the reduction of NO.sub.x in gas turbine engines and 
embodies the NO.sub.x reduction-through-water-injection concept depicted 
in FIG. 1. 
While it is recognized that gas turbine engines are, by now, well 
understood in the art, a brief description of a representative engine will 
enhance appreciation of the interrelationship of various components in 
light of the invention soon to be described. To that end, attention is now 
directed to FIG. 2 wherein a gas turbine engine of the marine or 
industrial variety, depicted generally at 12 and embodying the method of 
the present invention, is diagrammatically shown. This engine may be 
considered as comprising a core engine 14 and an independent power turbine 
16. The core engine includes an axial flow compressor 18 having a bladed 
rotor 20. Air enters inlet 22, is compressed by compressor 18 and is then 
discharged to a combustor 24 where it is normally mixed with fuel and 
combusted to provide high energy combustion gases to drive the core engine 
turbine 26. Turbine 26, in turn, drives rotor 20 through shaft 28. The hot 
gases of combustion then pass through and drive the power turbine 16 
which, in turn, drives an energy-absorbing device (not shown) through 
power shaft 30. Power is thus obtained by the action of the hot gases of 
combustion driving power turbine 16. The products of combustion are then 
collected and exhausted through discharge nozzle 32 which, in some 
applications, may be a propulsive nozzle. The above description is typical 
of many present-day gas turbine engines of the industrial power generation 
or marine propulsion variety and is not meant to be limiting on the 
present invention since it will soon become readily apparent from the 
following description that the method of the present invention is capable 
of application to any gas turbomachine, whether of the turbojet, turboprop 
or turboshaft variety. The foregoing description of the operation of the 
engine of FIG. 2 is, therefore, merely meant to be illustrative of one 
type of application. 
The present invention provides a simplified method for emulsifying fuel and 
water to reduce gas turbine engine NO.sub.x emissions and to achieve a 
more uniform fuel-water distribution within the combustor of the gas 
turbine engine. Directing attention now to FIG. 3, there is depicted 
schematically an apparatus which embodies the method of the present 
invention. Surprisingly, it was discovered that when water was added to a 
gas turbine engine fuel delivery system through a simple plumbing "T" 34, 
sufficient homogenization occurred. In particular, T-section 34 is 
provided with a first inlet 36 for receiving a flow of fuel under pressure 
and a second inlet 38 for receiving a flow of water under pressure. Pipes 
or tubes 40, 42 are connected to inlets 36, 38, respectively, and comprise 
portions of the respective fuel and water delivery systems. The water and 
fuel flows are pipe flows, not sprays or mists, and are generally normal 
to each other at their point of confluence within the "T" to produce 
turbulent mixing therein. A single outlet 44 discharges the mixture into a 
pipe 46 by which the mixture is carried to a flow splitter 48. Each 
splitter leg 50 communicates with a generally semicircular fuel manifold 
52 delivering the resulting mixture to a plurality of combustor nozzles 54 
disposed within the upstream end of combustor 24 (see FIG. 2) in the usual 
manner of a gas turbine engine. 
When the fuel and water are introduced into the "T" at conditions 
sufficient to produce turbulent mixing with a Reynolds number of at least 
1500 sufficient homogenization occurs without the intentional addition of 
any emulsifying agents to either the fuel or the water such that the 
proportions of fuel and water in the resulting mixture which is delivered 
to each nozzle 54 are sufficiently uniform. In tests performed using a 
mixing "T" having a 0.375" diameter water inlet, a 0.625" diameter fuel 
inlet and Diesel No. 2 as the fuel, a 1% variation in fuel content was 
measured at the discharge of splitter legs 50. In one manifold section 54 
having fifteen nozzles, a 3.5% fuel variation (maximum minus minimum, 
divided by average) was recorded at the nozzle discharge. With JP4 fuel, 
the fuel variation at the nozzles was 10%. For yet unexplained reasons, 
when the water inlet diameter was reduced to 0.31", the percent fuel 
variation (for Diesel No. 2)between splitter legs 50 increased from 1% to 
as high as 7.8%, yet still within acceptable limits. Experimental accuracy 
may account for some of this difference. 
The flow velocity rates in a gas turbine engine are such that in this 
particular embodiment, the elapsed time that it takes to travel from the 
"T" element (which, practically speaking, is located just upstream of the 
fuel manifolds) to the most distant nozzle 54 is only a few seconds, well 
within the "stay" time of the mixture. However, other embodiments of the 
present invention may employ a "T" element or other mixing chamber at a 
location which is somewhat removed from the combustor. Experimentation has 
shown that although the "stay" time of the mixture could be longer than 
one minute, the best results insofar as nozzle fuel variation percentages 
are achieved if the time between the mixing of the fuel and water and the 
combustion of the resulting mixture does not exceed thirty seconds. 
Tests conducted utilizing the present invention showed that NO.sub.x 
reductions attained with the use of the fuel-water mixtures were somewhat 
greater than those obtained with separately injected water. FIG. 4 shows a 
comparison of the NO.sub.x emissions measured during these tests. For 
example, a NO.sub.x reduction of 50% would require a water-to-fuel ratio 
of 0.6 when the water is injected separately. The same NO.sub.x reduction 
could be obtained with a ratio of 0.4 when the mixture is emulsified in 
accordance with the method of the present invention. 
The method of the present invention was incorporated into a gas turbine 
engine, such as the representative industrial-type gas turbine engine of 
FIG. 2, having a fuel flow delivery system 56 comprising, in part, a tank 
58 from which fuel is pumped and routed through a fuel control 60 of a 
known variety which is responsive to operator throttle input and which 
senses and compensates for measured engine parameters. Fuel from such a 
control is routed to the inlet side 36 of "T" 34. Water is pumped to 
T-inlet 38 from a tank 62 and through a valve 64 which is interlocked with 
the output of the fuel control to maintain the desired water-fuel mixture 
ratio. Such a control apparatus is within the skill of those familiar with 
such control art and is beyond the scope of the present invention. 
It was also discovered that when the water-fuel ratio for Diesel No. 2 
exceeds 0.7 by weight, the consistency of the resulting mixture begins to 
change from suspended water drops in fuel to a homogeneous mixture having 
the characteristics of a pseudoplastic fluid. The change from suspended 
water drops in fuel to the homogeneous mixture is completed as the ratio 
approaches 1.0 by weight. Whereas the normal mixture of suspended water 
drops tends to begin separation almost immediately, the homogeneous 
mixture tends to remain mixed for somewhat longer periods. Experimentation 
has shown that the homogeneous mixture may require as long as two to eight 
hours to completely separate. Clearly, this reduced tendency to separate 
means that a more uniform fuel distribution may be obtained between the 
plurality of nozzles and that the mixing "T" can be located further from 
the nozzles. Tests performed with JP4 fuel showed that the homogeneous 
mixture begins to form at a water-to-fuel ratio of 0.6 by weight and is 
fully formed at a water-fuel ratio of 1.4 by weight. 
The phenomenon causing the homogeneous mixture to form can probably be 
explained by the large difference in surface tension of the two fluids. 
Application of high shear stress to the two-phase mixture results in more 
subdivision of the fuel phase than the water phase. Initially, the fuel is 
a continuous phase while the water is in relatively large drops that 
settle rapidly. As water addition continues, a point is reached where a 
phase reversal occurs and the water becomes the continuous phase having 
the fuel suspended therein as relatively small drops with little 
opportunity to coalesce and separate. The mixture thus formed is probably 
a pseudoplastic fluid rather than a Newtonian fluid (such as the fuel or 
water alone). The process can be reversed by adding large quantities of 
fuel to the mixture while the shearing action continues. 
It should become obvious to one skilled in the art that certain changes can 
be made to the above-described invention without departing from the broad 
inventive concepts thereof. For example, the present invention is meant to 
embrace any arrangement whereby one flow of fuel and one flow of water, 
are combined turbulently without necessity of complex homogenizers. While 
a simple plumbing "T" section has been demonstrated to be effective, this 
is merely one example of a means for directly combining the two fluids 
through turbulent mixing to produce a homogeneous mixture. It is intended 
that the appended claims cover this and any other variations in the 
present invention's broader inventive concepts.