Catalytic combustion chamber with pilot stage and a method of operation thereof

A catalytic combustion chamber is provided with at least two catalytic combustion zones arranged in flow series. In a first mode of operation fuel is supplied from first fuel injectors, positioned upstream of the first catalytic combustion zone, into the catalytic combustion chamber and is burnt in the first catalytic combustion zone in order to preheat the subsequent catalytic combustion zones. In the second mode of operation the supply of fuel to the first fuel injectors is reduced and fuel is supplied from second fuel injectors positioned between the first catalytic combustion zone and the second catalytic combustion zone into the space between the first catalytic combustion zone and the second catalytic combustion zone. This prevents the first catalytic zone becoming overheated, and reduces the possibility of the second and third catalytic combustion zones becoming overheated and allows the optimum catalyst to be selected for the first catalytic combustion zone.

THE FIELD OF THE INVENTION 
The present invention relates to combustion chambers, in particular to 
catalytic combustion chambers for gas turbine engines. 
BACKGROUND OF THE INVENTION 
The use of catalytic combustion chambers in gas turbine engines is a 
desirable aim, because of the benefits in the reductions of combustion 
chamber emissions, particularly nitrogen oxides (NOx). The reduction in 
NOx is due to the lower operating temperatures and the use of much weaker 
fuel and air ratios than conventional combustion chambers. 
In catalytic combustion chambers it is known to use ceramic, or metallic, 
honeycomb monoliths which are coated with a suitable catalyst. It is also 
known to use honeycomb monoliths which contain a suitable catalyst or are 
formed from a suitable catalyst. 
It is also known to arrange several of the honeycomb monoliths in flow 
series such that there is a progressive reduction in the cross-sectional 
area of the cells of the honeycomb from one honeycomb monolith to an 
adjacent honeycomb monolith, in the direction of flow. The honeycomb cell 
size may vary and the cross-sectional area for flow may vary. The smaller 
honeycomb cell size has the effect of providing a high geometric surface 
area per unit volume, which may increase the available catalyst area per 
unit volume, which in turn may increase the catalytic reaction rate per 
unit volume and hence reduce emissions of unburned hydrocarbons. 
In catalytic combustion chambers there is an optimum temperature range at 
which catalytic reaction on the catalyst will occur. At temperatures below 
the optimum temperature range the rate of catalytic reaction will be very 
low, whilst at temperatures above the optimum temperature range the 
catalytic reaction diminishes due to damage to the catalyst, for example 
because of sintering, or phase transition e.g. palladium oxide changes to 
palladium, and lose its activity. However the catalytic activity of the 
catalyst is never likely to be zero. Different catalysts have different 
optimum temperature ranges. Thus some catalysts have good lower 
temperature capabilities, i.e. will operate at relatively low temperatures 
around 350.degree. C. to 400.degree. C., but have poor higher temperature 
capabilities. Other catalysts have good higher temperature capabilities, 
but poor lower temperature capabilities. Also a gas turbine engine 
operates over a wide operating range. Currently there is no known catalyst 
which has an acceptable level of activity across the entire operating 
temperature range of a gas turbine engine combustion chamber. This makes 
it necessary to have a series of catalyst coated honeycomb monoliths 
arranged in series in a combustion chamber, with catalysts having good 
lower temperature capabilities on the first honeycomb monolith and 
catalysts having progressively increasing higher temperature capabilities 
such that the catalyst on the last honeycomb monolith has the best higher 
temperature capability. Thus there may be two or more catalyst coated 
honeycomb monoliths arranged in flow series in a catalytic combustion 
chamber. Usually it is arranged that the temperature downstream of the 
last catalyst coated honeycomb monolith is sufficient to support 
homogeneous gas phase reactions. 
In catalytic combustion chambers hydrocarbon fuel and air are mixed and 
supplied to the catalyst coated honeycomb monoliths, or honeycomb 
monoliths formed from, or containing catalyst. The hydrocarbon fuel and 
air mixture diffuses to the catalyst coated surfaces of the honeycomb 
monoliths and reacts on the active sites, at and within the surface. 
In one known catalytic combustion chamber a pilot combustor, or pre-burner, 
is provided to burn some of the fuel to preheat the first catalytic 
combustion zone to the optimum temperature range. A main fuel injector 
positioned upstream of the first catalytic combustion zone, is provided to 
supply fuel to the first catalytic combustion zone. The second and 
subsequent catalytic combustion zones receive unburned fuel from the first 
catalytic combustion zone. 
It has been proposed to provide a catalytic combustion chamber with a pilot 
combustor, or pre-burner, to burn some of the fuel to preheat the first 
catalytic combustion zone to the optimum temperature range. A main fuel 
injector, positioned upstream of the first catalytic combustion zone, is 
provided to supply fuel to the first catalytic combustion zone. An 
additional fuel injector, positioned between the first and second 
catalytic combustion zones, is provided to supply additional fuel to the 
second catalytic combustion zone. 
A problem associated with catalytic combustion chambers is that there is a 
possibility that one or more of the catalytic combustion zones, may become 
overheated leading to deactivation of the catalyst. It is also necessary 
to ensure that the temperature downstream of the last catalytic combustion 
zone is sufficiently high to maintain homogeneous gas phase reactions. 
SUMMARY OF THE INVENTION 
The present invention seeks to provide a method of operating a catalytic 
combustion chamber which overcomes the above mentioned problem. 
Accordingly the present invention provides a method of operating a 
catalytic combustion chamber, the catalytic combustion chamber comprising 
a first catalytic combustion zone and at least a second catalytic 
combustion zone spaced from and positioned downstream of the first 
catalytic combustion zone, means to supply air to the first catalytic 
combustion zone, means to supply fuel to the first catalytic combustion 
zone and means to supply fuel to the space between the first and second 
catalytic combustion zones, the method comprising: 
(a) supplying fuel to the first catalytic combustion zone in a first mode 
of operation, 
(b) reducing the supply of fuel to the first catalytic combustion zone and 
supplying fuel to the space between the first and second catalytic 
combustion zones in a second mode of operation. 
The catalytic combustion chamber may comprise a third catalytic combustion 
zone spaced from and positioned downstream of the second combustion zone. 
There may be means to supply fuel to the space between the second and third 
catalytic combustion zones. 
The supply of fuel to the space between the first and second catalytic 
combustion zones may be reduced and fuel is supplied to the space between 
the second and third catalytic combustion zones in a third mode of 
operation. 
The supply of fuel to the first catalytic zone may be reduced to 10% or 
less of the total fuel supplied to the combustion chamber and 90% or more 
of the total fuel supplied to the combustion chamber is supplied to the 
second catalytic combustion zone. 
The supply of fuel to the first catalytic zone may be terminated and all 
the fuel is supplied to the second catalytic combustion zone. 
The advantage of the present invention is that it prevents overheating of 
the catalyst at least in the first catalytic combustion zone. Also it 
allows catalysts with very low lower temperature capabilities to be used 
to enhance the light off characteristics of the combustion chamber. 
The present invention also provides a catalytic combustion chamber 
comprising a first catalytic combustion zone and at least a second 
catalytic combustion zone spaced from and positioned downstream of the 
first catalytic combustion zone, means to supply air to the first 
catalytic combustion zone, first fuel injector means to supply fuel to the 
first catalytic combustion zone, second fuel injector means to supply fuel 
to the space between the first and second catalytic combustion zones, 
valve means to control the supply of fuel to the first fuel injector means 
and to control the supply of fuel to the second fuel injector means such 
that the valve means switches between a first position which allows the 
supply of fuel to the first catalytic combustion zone and a second 
position which reduces the supply of fuel to the first catalytic 
combustion zone and supplies fuel to the space between the first and 
second catalytic combustion zones. 
The catalytic combustion chamber may comprise a third catalytic combustion 
zone spaced from and positioned downstream of the second combustion zone. 
There may be third fuel injector means to supply fuel to the space between 
the second and third catalytic combustion zones. 
The valve means may comprise a first valve to control the supply of fuel to 
the first fuel injector means and a second valve to control the supply of 
fuel to the second fuel injector means. 
Preferably the first catalytic combustion zone comprises a catalyst 
suitable for catalysing combustion reactions at a first temperature range, 
the second catalytic combustion zone comprises a catalyst suitable for 
catalysing combustion reactions at a second temperature range and the 
first temperature range is at a lower temperature than the second 
temperature range. Alternatively the first and second catalytic combustion 
zones may comprise catalysts for catalysing combustion reactions at 
substantially the same temperature range. 
The third catalytic combustion zone may comprise a catalyst suitable for 
catalysing combustion reactions at a third temperature range, and the 
third temperature range is at a higher temperature than the second 
temperature range. 
Preferably in the second position the valve means terminates the supply of 
fuel to the first catalytic zone and all the fuel is supplied to the 
second catalytic combustion zone. 
Each catalytic combustion zone comprises a catalyst coated ceramic 
honeycomb monolith, a catalyst coated metallic honeycomb matrix, a 
honeycomb monolith formed from catalyst material or a honeycomb monolith 
containing catalyst material. 
The catalytic combustion chamber may be tubular or annular. 
A pilot combustor may be arranged to preheat the first catalytic combustion 
zone.

DETAILED DESCRIPTION OF THE INVENTION 
A gas turbine engine 10, which is shown in FIG. 1, comprises in flow series 
an intake 12, a compressor section 14, a combustion section 16, a turbine 
section 18 and an exhaust 20. The gas turbine engine 10 operates 
conventionally in that air is compressed as it flows through the 
compressor section 14, and fuel is injected into the combustor section 16 
and is burnt in the compressed air to provide hot gases which flow through 
and drive the turbines in the turbine section 18. The turbines in the 
turbine section 18 are arranged to drive the compressors in the compressor 
section 14 via shafts (not shown). 
The combustion section 16 comprises one or more catalytic combustion 
chambers 22 as shown more clearly in FIG. 2. The catalytic combustion 
chamber 22 shown in FIG. 2 is a tubular combustion chamber, and there are 
a plurality of the tubular combustion chambers arranged coaxially around 
the axis of the gas turbine engine 10, but it may be possible to use a 
single annular combustion chamber or other arrangements. The tubular 
catalytic combustion chamber 22 comprises an annular wall 24 which has an 
inlet 26 at its upstream end for the supply of compressed air, from the 
compressor section 14, into the tubular catalytic combustion chamber 22, 
and an outlet 28 at its downstream end for the delivery of hot gases 
produced in the combustion process from the tubular catalytic combustion 
chamber to the turbine section 18. The inlet 26 may be provided with swirl 
vanes, or other suitable mixing devices, to enable the fuel and air to be 
mixed thoroughly. 
A first catalyst coated honeycomb monolith 30 is positioned at the upstream 
end of the tubular catalytic combustion chamber 22 and forms a first 
catalytic combustion zone. A second catalyst coated honeycomb monolith 32 
is spaced from and positioned downstream of the first catalyst coated 
honeycomb monolith 30 and forms a second catalytic combustion zone. A 
third catalyst coated honeycomb monolith 34 is spaced from and positioned 
downstream of the second catalyst coated honeycomb monolith 32 and forms a 
third catalytic combustion zone. 
The first catalytic coated honeycomb monolith 30, the first catalytic 
combustion zone, is coated with a catalyst which has a good lower 
temperature capability, that is it requires a relatively low lower 
temperature to enable the catalytic combustion reaction to occur at lower 
temperatures to enable heat to be generated to heat up the second catalyst 
coated honeycomb monolith 32. The second catalyst coated honeycomb 
monolith 32, the second catalytic combustion zone, is coated with a 
catalyst which has low temperature capability or intermediate temperature 
capability. The third catalyst coated honeycomb monolith 34, the third 
catalytic combustion zone, is coated with a catalyst which has good higher 
temperature capabilities, that is it has a relatively high higher 
temperature to enable the catalytic combustion reaction to occur at higher 
temperatures and is capable of withstanding much higher temperatures 
before it becomes deactivated. 
A fuel supply 36 is provided to supply fuel to the tubular catalytic 
combustion chambers 22. The fuel supply 36 is arranged to supply fuel to a 
plurality of first fuel injectors 38, each one of which is positioned at 
the upstream end of one of the tubular catalytic combustion chambers 22. 
There may be more than one first fuel injector 38 for each tubular 
combustion chamber 22. The first fuel injectors 38 are arranged to inject 
fuel into the tubular catalytic combustion chambers 22 upstream of the 
first catalytic combustion zone, the first catalyst coated honeycomb 
monolith 30. The fuel supply is arranged to supply the fuel to the first 
fuel injectors 38 via a fuel pump 40, a fuel pipe 42 and a valve or valves 
44. It may be necessary to provide mixing devices to ensure that there is 
intimate mixing of the fuel and air before before the fuel reaches the 
first catalytic combustion zone 30. 
The fuel supply 36 is also arranged to supply fuel to a plurality of second 
fuel injectors 46. There may be more than one second fuel injector 46 for 
each tubular combustion chamber 22. The second fuel injectors 46 are 
arranged to inject fuel into the tubular catalytic combustion chambers 22 
to the space between the first catalytic combustion zone, the first 
catalyst coated honeycomb monolith 30 and the second catalytic combustion 
zone, the second catalyst coated honeycomb monolith 32. The fuel supply is 
arranged to supply the fuel to the second fuel injectors 46 Via the fuel 
pump 40, the fuel pipe 42 and a valve or valves 48. It may be necessary to 
provide mixing devices to ensure that there is intimate mixing of the fuel 
and air before before the fuel reaches the second catalytic combustion 
zone 32. 
The fuel supply 36 may also be arranged to supply fuel to a plurality of 
third fuel injectors 50. There may be more than one third fuel injector 50 
for each tubular combustion chamber 22. The third fuel injectors 50 are 
arranged to inject fuel into the tubular catalytic combustion chambers 22 
to the space between the second catalytic combustion zone, the second 
catalyst coated honeycomb monolith 32 and the third catalytic combustion 
zone, the third catalyst coated honeycomb monolith 34. The fuel supply is 
arranged to supply the fuel to the third fuel injectors 50 via the fuel 
pump 40, the fuel pipe 42 and a valve or valves 52. It may be necessary to 
provide mixing devices to ensure that there is intimate mixing of the fuel 
and air before before the fuel reaches the third catalytic combustion zone 
34. 
In operation in a first mode of operation, at start up and at powers up to 
a predetermined power, the valve, or valves, 44 are opened and fuel is 
supplied from the fuel supply 36 to the first fuel injectors 38 such that 
substantially all the fuel is supplied from the first fuel injectors 38 
into the catalytic combustion chambers 22 upstream of the first catalytic 
combustion zone 30. The fuel is burnt in the first catalytic combustion 
zone 30 to produce heat to heat the second and third catalytic combustion 
zones 32 and 34 up to the required temperature range for the selected 
catalysts. Any unburned fuel leaving the first catalytic combustion zone 
30 is burnt in the second catalytic combustion zone 32 or in the second 
catalytic combustion zone 32 and the third catalytic combustion zone 34. 
Whatever fuel remains on leaving the third, or last, catalytic combustion 
zone 34 is then burnt in a homogeneous combustion zone 54 which produces 
minimal levels of NOx. For example as the fuel supply is increased from 
say idle power to 4% power substantially all the fuel is supplied to the 
first fuel injectors 38 and no fuel is supplied to the second fuel 
injectors 46, or the third fuel injectors 50. 
In the second mode of operation, at powers above the predetermined power, 
the valve, or valves, 44 are completely closed to terminate the supply of 
fuel to the first fuel injectors 38 and the valve, or valves, 48 are 
opened and fuel is supplied from the fuel supply 36 to the second fuel 
injectors 46 such that all the fuel is supplied from the second fuel 
injectors 46 into the catalytic combustion chambers 22 between the first 
catalytic combustion zone 30 and the second catalytic combustion zone 32. 
Thus in the second mode of operation no fuel is supplied to the first 
catalytic combustion zone 30, and thus the first catalytic combustion zone 
30 does not become overheated at high power operation, and also the second 
and third catalytic combustion zones 32 and 34 respectively may not become 
overheated. Furthermore this enables the catalyst in the first catalytic 
combustion zone 30 to be optimised for lower temperature capabilities 
without fear of being overheated. 
Alternatively in the second mode of operation, at powers above the 
predetermined power, the valve, or valves, 44 are partially closed to 
reduce the supply of fuel to the first fuel injectors 38 and the valve, or 
valves, 48 are opened and fuel is supplied from the fuel supply 36 to the 
second fuel injectors 46 such that most of the fuel is supplied from the 
second fuel injectors 46 into the catalytic combustion chambers 22 between 
the first catalytic combustion zone 30 and the second catalytic combustion 
zone 32. Thus in the second mode of operation only a small amount of fuel, 
for example up to 10%, is supplied to the first catalytic combustion zone 
30, and thus the first catalytic combustion zone 30 and does not become 
overheated at high power operation, and the second and third catalytic 
combustion zones 32 and 34 may not become overheated. Furthermore this 
enables the catalyst in the first catalytic combustion zone 30 to be 
optimised for lower temperature capabilities without fear of being 
overheated. 
For example at powers above 40% power the valve 48 is opened to gradually 
increase the supply rate of fuel to the second fuel injectors 46 and the 
supply rate of fuel to the first fuel injectors 38 increases transiently 
while combustion in the catalytic combustion chamber 22 stabilises. 
Thereafter the valve 44 is either partially or fully closed to reduce the 
supply rate, or terminate the supply, of fuel to the first fuel injectors 
38. 
It is also possible in a third mode of operation at very high powers to 
open the valve, or valves, 52 such that some additional fuel is supplied 
to the third fuel injectors 50. It may be possible at very high powers to 
close or partially close the valve, or valves 48 to terminate or reduce 
the supply rate of fuel to the second fuel injectors 46 and the valve, or 
valves, 52 are opened and fuel is supplied from the fuel supply 36 to the 
third fuel injectors 50 such that some of the fuel is supplied from the 
third fuel injectors 50 into the catalytic combustion chambers 22 between 
the second catalytic combustion zone 32 and the third catalytic combustion 
zone 34. By partially opening the valves 52 it provides a method of 
controlling the catalytic combustion process such that the temperatures of 
each of the catalysts does not exceed the value which may cause damage to 
the catalysts and intermediate power levels may be achieved. 
The aim of the catalytic combustion chamber is to achieve a sufficiently 
high temperature downstream of the last catalytic combustion zone such 
that homogeneous gas phase reactions are maintained in the homogeneous gas 
phase combustion zone 54. 
The present invention has been described with reference to catalytic 
combustion zones comprising catalyst coated honeycomb monoliths. It is 
possible to use catalytic combustion zones comprising catalyst coated 
metallic honeycomb matrix, for example a metallic matrix comprising one or 
more corrugated metal strips interleaved with one or more smooth metal 
strips which are wound into a spiral or are arranged concentrically. A 
suitable metal for forming the metallic matrix is an 
iron-chromium-aluminium alloy which may contain yttrium for example 
FeCrAlloy (Registered Trade Mark). It is also possible to use catalytic 
combustion zones comprising honeycomb monoliths formed from catalyst 
material or honeycomb monoliths containing catalyst material. It is also 
possible to use catalytic combustion zones comprising catalyst coated 
ceramic honeycomb monoliths. 
It may also be possible to provide a pilot combustor 56 upstream of the 
first catalytic combustion zone 30 to preheat the first catalytic 
combustion zone 30 up to its operating temperature range, as is shown in 
FIG. 2. If a pilot combustor is provided, then in the first mode of 
operation, a small portion of the total fuel supplied to the combustion 
chamber is supplied to the pilot combustor. Alternatively other heating 
devices may be provided to preheat the first catalytic combustion zone up 
to the required operating temperature range, for example a heat exchanger 
may be used to heat the air supplied to the first catalytic combustion 
zone. 
The invention is applicable to tubular, annular or other types of 
combustion chamber. 
It may be possible to use a single valve to control the flow of fuel to the 
first and second fuel injectors, rather than two valves as described. 
It may be possible to only have the first and second catalytic combustion 
zones, or only to supply fuel to the first and second fuel injectors and 
possibly the pilot combustor. Although fuel pumps have been used in the 
description, it may not be necessary to provide fuel pumps to supply the 
fuel from the fuel supply to the fuel injectors. 
It may be possible to arrange that the catalysts on the first and second 
catalytic combustion zones have substantially the same operating 
temperature range.