Electrically-heated combustion catalyst structure and method for start-up of a gas turbine using same

This invention relates to an electrically-heated catalyst (EHC) and a start-up method of a gas turbine engine for combusting a hydrocarbonaceous fuel/oxygen-containing gas mixture using this electrically-heated catalyst. The catalytic structure is electrically heated to a predetermined temperature prior to start up of the turbine so as to reduce emissions during the start-up of the system. The EHC unit is a stacked or spirally wound layering of flat and corrugated thin metal foils which forms a plurality of axially-extending, longitudinal channels. The channels are preferably coated on one surface with a catalytic material, leaving the other surface free from the reaction to act as a heat sink, making the design an IHE (integral heat exchange) catalytic unit. The preferred embodiment of the EHC has electrodes outside of the fuel/oxygen-containing mixture stream, and uses electrical power having a predetermined voltage in the range of 100 to 200 volts to heat the unit. A method for using the EHC in the start-up of a gas turbine is also disclosed wherein an electrical power is applied to heat the EHC a predetermined temperature prior to the fuel/oxygen-containing mixture being introduced and may be left on for a certain period of time after the introduction of the fuel/oxygen-containing mixture. The EHC may be maintained at the desired predetermined temperature by modulating the applied voltage. The electrical power is terminated when any one of several conditions are met including when the heat of the catalytic reaction is sufficient to maintain the catalyst at its steady-state condition or when a certain period of time has elapsed.

BACKGROUND OF THE INVENTION 
This invention relates in general to electrically-heated catalyst 
structures and methods for starting up a gas turbine engine. Catalytic 
combustion systems have been proposed for turbines or engines which burn 
hydrocarbonaceous fuels. The catalytic combustion systems are used to 
reduce the amount of uncombusted hydrocarbons and other undesirable 
combustion by-products such as carbon monoxide and nitrous oxides (NOx). 
Typically, catalytic combustion systems are quite effective in reducing 
the amount of these undesirable emissions once steady-state operating 
conditions are achieved. However, during the initial start-up of the 
turbine or engine, the amount of emissions may be above the desired 
limits. 
The amounts of undesirable emissions are usually higher during start-up 
conditions because the catalyst is not at a temperature at which it is 
most effective. One method which has been used to achieve lower emissions 
is to more quickly bring the catalyst up to its desired operating 
temperature by preheating the air that is supplied with the 
hydrocarbonaceous fuel to the catalytic combustion system. Another way to 
achieve lower emissions more quickly is to preheat the catalyst. 
Typically, a catalytic structure is a metallic or ceramic substrate, coated 
with a catalytically reactive substance and placed into a fuel/air stream. 
The catalysts are typically Group VIII noble metals or the platinum group 
metals and react with the fuel/air mixture passed over the catalyst 
structure. The reaction rate of the fuel/air mixture over the catalyst is 
temperature dependent, typically being low or non-existent at low 
temperatures, most efficient and controllable in a particular higher 
temperature range, while above certain known high temperatures the 
catalyst suffers deactivation and/or the reaction becomes uncontrollable. 
Therefore, it is important to design the catalytic structure so that it 
will withstand the operating temperatures of the system it is installed 
in, can be quickly brought up to its most efficient operating temperature 
range and yet, be maintained within this desired operating temperature 
range so as to prevent any catalytic deactivation and/or runaway reaction. 
Until the catalyst reaches an ideal operating temperature range, the 
combustion of the fuel in the fuel air mixture is likely to be incomplete. 
It has been shown that by heating the catalyst structure prior to or 
concurrently with the introduction of the fuel/air mixture, the catalyst 
can be brought up to a satisfactory operating temperature range, with a 
concurrent increase in the amount of fuel combusted. This, in turn, leads 
to a reduction in the amount of emissions during the start-up period. It 
is known in the art to electrically heat a catalytic structure to reduce 
emissions during start up of a gas turbine. For example, U.S. Pat. No. 
5,440,872 to Pfefferle describes a catalyst designed to lower the start up 
emissions of a gas turbine using a catalytic combustion system. Pfefferle 
'872 uses a "microlithic catalyst" with a very low mass so that heat-up 
occurs quickly. Pfefferle also proposes the use of electrical heating to 
raise the catalyst temperature prior to introducing fuel. Because the 
catalyst operates at the adiabatic combustion temperature of the fuel/air 
mixture, the system is limited to temperatures in the range of 600.degree. 
K. to 1250.degree. K. (327.degree. C. to 977.degree. C.). This is a low 
temperature limiting this technology to gas turbines with low turbine 
inlet temperatures. The technology of the present invention is not limited 
to low combustor outlet temperatures and has been demonstrated at 
combustion outlet temperatures as high as 1500.degree. C. 
A further example of an electrically-heated catalyst is disclosed in U.S. 
Pat. No. 5,070,694 to Whittenberger. Whittenberger '694 describes an 
electrically-heated catalytic structure comprised of alternating strips of 
brazing material and thin metal strips, all of which are fused to a 
central electrode. The foil unit is made catalytically reactive by dipping 
it in a bath which contains slurries of the catalytic coating, and then it 
is spirally wound and encased in an electrically conductive outer shell. 
Current is then passed from the outer shell to the center electrode to 
heat the structure. This device is designed for use in an automotive 
application and uses a voltage source in the range of 12 to 24 volts and a 
start-up temperature of approximately 650.degree. C. The structure of the 
catalyst in whittenberger '694, in which all of the surfaces are 
catalytically reactive and are not insulated from one another, proscribes 
its use in an application using high voltages since short circuiting and 
uneven heating may occur in this structure. In contrast, the structure of 
the present invention provides insulative barriers between the current 
carrying members to ensure there is no short circuiting. In addition, the 
structure of the present invention can be operated at voltages of about 
100 volts or higher with the structure being evenly heated with no damage 
from arcing or overheating. 
The above examples of electrically-heated catalyst units have used a 
centrally located electrode to complete the electrical flow path. It is 
known that in applications using catalytic structures in gas turbines, any 
irregularity in the structure of the catalyst bed such as an electrode, 
can cause flow disruptions or irregular flow patterns leading to hot spots 
or even premature ignition of the fuel/air mixture and thereby destroying 
the structure. An electrically-heated catalytic structure that does not 
use a central electrode is described in U.S. Pat. No. 5,232,671 to Brunson 
et. al. Brunson '671 describes a spirally wound structure having two 
groups of catalytically reactive foils separated by an insulating barrier. 
Each of the foils in the separate groups is connected to one pole of the 
voltage source, and all the foils are connected to each other in the 
center of the structure. This method of connection puts each foil in 
parallel to the other foils, minimizing the available resistance of the 
foils. The foils in Brunson '671 are also connected in the center of the 
structure by a pin or other type of crimping device. In contrast, the 
technology of the present invention has the foils arranged in series with 
each other to maximize the available resistance. An embodiment of the 
present invention also provides a structure without a central supporting 
member such as an electrode or a pin. 
U.S. Pat. No. 5,250,489 to Dalla Betta et. al. discloses a catalytic 
structure that is formed into a spiral shape and which has integral heat 
exchange. In contrast to the present invention, this disclosure is not an 
electrically-heated catalyst (EHC), and the disclosure does not teach or 
suggest any requirements regarding the electrical considerations and 
insulative properties required in an EHC. 
SUMMARY OF THE INVENTION 
This invention relates to an electrically-heated catalyst (EHC) and a 
start-up method of a gas turbine engine for combusting a hydrocarbonaceous 
fuel/oxygen-containing gas mixture using this electrically-heated 
catalyst. More specifically, this invention is a catalytic structure which 
is electrically heated to a predetermined temperature prior to start up of 
the turbine, or any other system it is used with, so as to reduce 
emissions during the start-up of the system. In one preferred embodiment 
of the invention, the electrical power may be maintained for a certain 
period of time after the fuel/oxygen containing mixture is introduced. 
In general, the electrically-heated catalyst structure of the present 
invention is comprised of at least two heat resistant, thin metal foil 
strips that are corrugated or corrugated and flat. These foils are 
attached to each other at an attachment point and the two foils are 
stacked one on top of the other. In a preferred embodiment, the two foils 
are wound together starting at the attachment point so as to form a spiral 
structure. In an alternative embodiment, at least one set of the stacked 
foils, or more preferably, a plurality of the foil sets, comprise the 
catalyst structure. In both embodiments, the foils are stacked together 
such that a plurality of adjacent longitudinal channels are formed for 
receiving and passing the fuel/oxygen-containing gas mixture through the 
catalyst structure. At least some portion of the foils have an 
electrically insulative coating on at least some portion of the foil 
surfaces. In addition, a combustion catalyst is coated on at least one 
side of a portion of the foils such that at least a portion of the 
longitudinal channels have at least a portion of their surface coated with 
the catalyst. Coating only a portion of the channel surfaces with catalyst 
provides for integral heat exchange in which the heat of the catalytic 
reaction is transferred through both conduction and convection to both the 
catalytically reacting gas streams and to the non-reacting gas streams. 
The catalyst structure is electrically heated by attaching two electrodes 
to the catalyst structure and by connecting these electrodes to a source 
of electrical power. In one preferred embodiment of the invention, the 
ends of the two foils making up the structure which are not attached 
together at the attachment point are located at the periphery of the 
spiral structure and electrodes attached to the foils and connected to the 
electrical power. Upon application of the electrical power, a current will 
flow through some portion of foils from the first electrode to the second 
electrode, thereby heating the foils. In an alternative embodiment, a 
center electrode is attached at the attachment point of the two foils and 
one or more electrodes are attached to the foils at the periphery of the 
catalyst structure. Either a series or parallel circuit design can be 
employed in either embodiment, however, the series circuit design is 
preferred. 
Another aspect of the invention relates to a method of starting up a gas 
turbine engine, employing an electrically-heated catalyst structure such 
as that described above to affect combustion of the hydrocarbonaceous 
fuel/oxygen-containing gas mixture used to power the turbine, whereby the 
amount of unburned hydrocarbons and carbon monoxide emissions are 
minimized and runaway catalytic reactions and catalyst decomposition is 
avoided. In this aspect of the invention electrical power is first applied 
to the electrically-heated catalyst structure to preheat the catalyst to 
within about 100.degree. C. of the steady-state operating temperature of 
the catalyst structure in combusting the fuel/oxygen-containing gas 
mixture, at which point the preheated catalyst structure is contacted with 
the fuel/oxygen-containing gas mixture to initiate the combustion reaction 
and the electrical power heating the catalyst structure is then turned off 
when any of the following conditions are met: 
(i) the heat of reaction released by the reaction of the fuel and oxygen on 
said catalyst structure is sufficient to maintain said catalyst structure 
at its steady-state operating temperature, or 
(ii) the fuel/oxygen-containing mixture at the combustor outlet reaches a 
predetermined temperature limit, or 
(iii) a predetermined period of time has elapsed since said contacting step 
was initiated, or 
(iv) the partially combusted fuel and oxygen-containing mixture at the 
catalyst exit reaches a predetermined temperature. 
Various objects and advantages of this invention will become apparent to 
those skilled in the art from the following detailed description of the 
preferred embodiment, when read in light of the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, there is illustrated in FIG. 1a an 
electrically-heated catalyst (EHC) combustion structure or unit 10 in 
accordance with the present invention. As seen from FIG. 1a, the EHC unit 
10 of the present invention is a generally cylindrical, monolithic 
structure comprised of an electrically-heated and electrically-conducting 
catalyst support and a combustion catalyst. In one embodiment, the 
structure is wound or otherwise assembled into a spiral such that the 
combusting gas mixture flows through a plurality of longitudinally 
extending channels. Electrodes are connected to the catalyst support and 
an electrical voltage is applied causing resistive (joule) heating of the 
catalyst structure. A catalytic material is placed on only a portion of 
the longitudinally-extending channels which allows the heat being produced 
on the catalyst-coated surfaces to be transferred via integral heat 
exchange through the catalyst support to the gas mixture flowing through 
adjacent channels. 
In general, the design of an EHC combustion structure is concerned with 
four major criteria: 1) the physical geometry and dimensions of the 
structure; 2) the electrically-heated portion of the structure; 3) the 
catalytic portion of the structure; and 4) the heat transfer portion of 
the structure. Prior to discussing the details of these four criteria, it 
is important to understand the systems in which such an 
electrically-heated catalytic combustion structure is used. 
One type of system that an EHC can be used in is the gas turbine engine. 
Referring to FIG. 2, there is shown a combustion gas turbine comprised 
generally of a compressor 32, a catalytic combustion chamber 34, and a 
turbine 36. Outside air 31 is supplied to the compressor 32 which produces 
compressed air having a predetermined higher pressure and higher 
temperature. The compressed air is passed through a bypass valve 41 which 
is actuated by any suitable actuator 42 such as a solenoid or a pneumatic 
actuator. Normally, the bypass valve 41 is set to control the amount of 
air directed to the catalytic combustion chamber 34 to control the 
operating conditions in this combustion chamber to the within the desired 
operating regions. The air preheating section 43 is used during start up 
of the gas turbine to obtain the required inlet temperature to vaporize 
the liquid fuel and to be within the combustor operating windows, 
typically 200 to 400.degree. C. Heat may be provided in this preheating 
section 43 by electrical resistive (joule) heating. Normally, this 
preheating section 43 will be placed in service when the gas turbine is 
initially started. During other operating conditions, the preheating 
section 43 may not be in use. Next, the compressed air is mixed with a 
suitable hydrocarbonaceous fuel in a fuel mixing section 44. A fuel spray 
nozzle 45 is used for supplying the hydrocarbonaceous fuel to the inlet of 
the fuel mixing section 44. After uniform mixing of the hydrocarbonaceous 
fuel and air, it is passed to the catalyst 10 which is electrically heated 
in accordance with the present invention. After the compressed air and 
hydrocarbonaceous fuel have reacted together in the presence of the 
catalyst, the resulting higher temperature and pressure 
combustion-products gas mixture is passed to the turbine 36 where the 
energy of this gas is converted into rotational energy of the turbine 
shaft 37. The rotational energy of the turbine shaft 37 is used to drive 
the compressor 32 as well as any other output device such as a generator 
49. A starter motor 48 can also be connected to shaft 37 to start the gas 
turbine. 
Typically, a gas turbine engine having an EHC unit 10 is used in industrial 
applications for reducing the emissions resulting from the process of 
converting hydrocarbonaceous fuels into energy. However, an EHC unit 10 
could also be designed for use in non-industrial applications, such as for 
use in automotive vehicles. Emissions from automotive vehicles are now 
also receiving heightened scrutiny as a source of pollution. Therefore, 
the use of an EHC unit 10 according to the present invention in an 
automotive application would be advantageous and effective. Regardless of 
the type of application an EHC is used, the following four design criteria 
must be considered. 
The Physical Geometry and Dimensions of the EHC Structure 
Referring to FIG. 1a, the EHC of the present invention is shown generally 
at 10. In this Figure, the EHC unit is shown as composed of a flat foil 14 
and a corrugated foil 12. 
To form the EHC unit 10, the flat foil 14 and the corrugated foil 12 are 
stacked one on top of the other, joined together along one edge at the 
attachment point 20, as shown if FIG. 1b and wound into a spiral shape. In 
general, the EHC can be formed from any set non-nesting foils, either flat 
and corrugated or both corrugated such that when stacked together the 
corrugations do not nest but form well defined longitudinal channels. 
The foils can be wound in a variety of ways, depending upon the desired 
shape and application in which the EHC unit 10 is to be used. Generally, 
the attachment point 20 will be at the center of the catalytic structure 
10. FIG. 1b shows a detail of the attachment point 20 when at the center 
of the catalytic structure. Conventional methods including, but not 
limited to, welding, brazing and crimping can be used for joining the 
corrugated foil 12 and the flat foil 14 together. Alternatively, the two 
foils can be joined together by inserting adjacent ends of the corrugated 
foil 12 and the flat foil 14 in a longitudinally-extending slot formed in 
a cylindrical rod (not shown). The joined foils are then wound around the 
rod to form a cylindrical, monolithic structure. In general, the key 
requirement of any attachment or joining method is to connect the two 
foils in a manner which allows current to flow in series or in parallel 
between the two foils. In addition, the attachment procedure should 
include good manufacturing practices such as surface preparation of the 
foils, e.g., by sandblasting to ensure a solid connection. 
When the flat foil 14 and the corrugated foil 12 are placed together, and 
wound into a spiral shape as shown in FIG. 1a, a plurality of channels are 
thereby formed which extend longitudinally (or axially) with respect to 
the axis of the EHC unit 10, and along the whole length of the EHC unit 
10. The selection of the corrugation pattern determines the shape and size 
of the opening of each air channel 16. The shape of the corrugation also 
determines the air flow pattern in each channel 16. Two typical shapes of 
corrugations include a herringbone pattern and a straight corrugation, 
which are shown from the top view in FIGS. 1c and 1d, respectively. These 
corrugation types can be combined in a variety of combinations to form the 
EHC unit by stacking the foil together in a straight layered structure or 
by stacking two or more foils and then winding into a spiral to form the 
EHC unit. Since a great amount of heat is generated in a catalytic 
reaction, it is important the have turbulent air flow in the channels, and 
the corrugations should be designed to account for this need. The surface 
area of the corrugated foil 12 may also determine the amount of 
catalytically reactive substance present in the EHC unit 10 if the 
catalyst is deposited on the corrugated foil 12. Therefore, the 
corrugations should also be designed with this in mind. 
In general, the corrugations should occur at least twice per 
circumferential pass on a spiral wound catalytic structure, with the size 
of the channel openings 16 and the overall length determined by structural 
stability considerations and the type of application in which the EHC is 
used. The size of the channel openings 16 is a function of the corrugation 
height and peak-to-peak distance of the corrugated foil 12. For example, 
for an EHC used in a gas turbine having a power output of about 30 kW the 
EHC unit would have a diameter of 50 mm to 150 mm and length of 50 to 200 
mm and the corrugation height should range from about 0.8 mm to 1.6 mm 
while the peak-to-peak distance should range from about 1 mm to about 3 
mm. If a herringbone pattern is used, the channel length of each straight 
section of the corrugated should range from about 10 to 30 mm and these 
patterns should have a channel angle of about 3.degree. to 20.degree.. As 
an example only, a corrugated foil 12 having a herringbone pattern with 
channel lengths of 20 mm and a channel angle of 6.degree. could be used. 
This herringbone pattern would also have a corrugation height of 1.20 mm 
and a peak-to-peak period of 2 mm. In a straight corrugation pattern, the 
corrugation height would also be 1.2 mm with a peak-to-peak period of 2 
mm. 
As an alternative embodiment to the spiral wound EHC unit 10, one or more 
sets of foils could be stacked together to form the EHC unit 10. In 
addition, although only the herringbone and straight types of corrugation 
shapes having specific sizes have been discussed, it is envisioned that 
other shapes of corrugations of differing sizes can be used in making the 
EHC unit 10 of the present invention. In particular, corrugated foils can 
be stacked together to form an asymmetric structure such as those 
described by Dalla Betta et. al. in U.S. Pat. No. 5,512,250 in which the 
tortuosity of the catalytically coated channels is different then the 
tortuosity of the non-catalyst-coated channels. The structures of '250 
patent are incorporated herein by reference. 
Electrical Design Considerations 
To provide the electrically heated function of the EHC unit 10, the flat 
foil 14 and the corrugated foil 12 must be made from a material which is 
electrically conductive, usually a ceramic or metallic material, and which 
can also withstand the operating temperatures of the catalytic unit 10. A 
metallic material is the preferred material. Specific types of metallic 
alloys include, but are not limited to, iron/chromium/aluminum (Fe/Cr/Al), 
nickel/chromium/aluminum (Ni/Cr/Al), or any alloy containing aluminum. In 
the preferred embodiment, an aluminum-containing alloy is used and more 
preferably, a (Fe/Cr/Al) alloy containing 10 to 20% Cr (by weight) and 5 
to 10% Al (by weight or a (Ni/Cr/Al) alloy containing 10 to 20% Cr (by 
eight) and 5 to 10% Al is used. 
As noted previously, the corrugated foil 12 and flat foil 14 are joined 
together along one edge in a manner that will allow electrical current to 
pass unimpeded from one foil to the other, such as by brazing, welding, 
crimping, etc. The location of the attachment point 20 can best be seen by 
referring to FIG. 1b. The joined edges are placed at the center of the 
spiral, leaving two free edges at the periphery of the structure when 
corrugated foil 12 and flat foil 14 are wound together. Although in the 
preferred embodiment, corrugated foil 12 and flat foil 14 are of the same 
dimensions and alloy, it is envisioned that many different shapes and 
alloys could be used. 
Winding flat foil 14 and corrugated foil 12 together with attachment point 
20 in the center of the structure results in the unattached end of 
corrugated foil 12 and flat foil 14 at the outer periphery of the EHC unit 
10. The end of the foils may be located anywhere on the periphery. FIG. 1a 
illustrates a preferred location with the ends diametrically opposed. 
Separation of the ends of the foils is generally desired to minimize 
shorting across the electrodes. 
FIG. 1a illustrates a preferred embodiment of the EHC unit 10 with two 
electrodes at the outer periphery. In general, the EHC unit 10 may be 
formed from any number of foils using peripheral electrodes and center 
electrodes. FIG. 3a illustrates an embodiment using three foils with two 
peripheral electrodes. FIGS. 3b through 3e illustrate a number of 
embodiments of three foil units with a center electrode and at least one 
peripheral electrode. 
After the EHC unit 10 has been formed, corrugated foil 12 is attached to 
electrode 22 at its free end, and flat foil 14 is attached to electrode 24 
at its free end. In general, electrodes 22 and 24 should be constructed 
from materials that can be attached to corrugated foil 12 and flat foil 
14, provide low electrical resistance, and withstand the operating 
temperatures of the EHC unit 10. 
One arrangement of a electrode 22 or 24 which could be used with the EHC 
unit 10 and which forms another embodiment of the invention is shown in 
FIG. 5a. In this design, the main electrode body, indicated generally at 
50, consists of two flat metal weld strips 51 and 56 and a generally 
semicylindrical shaped connector part or socket 52 secured to one end of 
one of the weld strips 51 or 56. The connector part 52 further includes a 
center aperture or orifice for the insertion of an electrical lead 54. The 
electrical lead 54 is attached to the electrode 22 by any suitable low 
resistance method, such as by soldering with a high melting point braze, 
such as braze BAg-8. The corrugated foil 12 is attached to electrode 22 by 
sandwiching corrugated foil 22 between the weld strips 51 and 56, and then 
welding, crimping, soldering or otherwise providing a physical and 
electrical connection along edge 57. Generally, the material used for the 
weld strips 51 and 56 should be made from an oxidation resistant metal 
such as a Fe/Cr/Al alloy or Ni/Cr/Al alloys such as Alfa-IV produced by 
Allegheny Ludlum of Pennsylvania, Riverlite 20-5SR produced by Kawasaki 
Steel of Japan, or Haynes 214 produced by Haynes International of Indiana. 
Suitable materials for the leads 54 include, but are not limited to 
aluminum, silver, nickel, copper, stainless steel or and aluminum 
containing alloy. Similar materials and methods of attachment can be used 
for the center electrode when one is used. 
In general for a current flow in the range up to 200 amps, the electrode 
connector 52 should have a wall thickness in the range of about 0.1 mm to 
about 3 mm. The flat electrode weld strips 51 and 56 should have a 
thickness in the range of about 0.5 mm to about 3 mm. In a preferred 
embodiment, the electrode connector 52 has a wall thickness of about 1 mm 
and the weld strips have a thickness of about 2 mm. 
As an example, three sets of like electrodes 22 and 24 were made with the 
connector part 52 having a wall thickness of 0.6 mm, 0.9 mm, and 1.4 mm. 
In all three sets, the weld strips 51 and 56 had a thickness of 2 mm after 
welding to the foil. A silver lead 54 was inserted into the cylindrical 
hole 53 in the connector part 52, and brazed in place with BAg-8 alloy. 
These electrodes were then connected to a power supply to pass current 
through the silver lead and into the foil. Measured temperatures at 
locations A and B as shown on FIG. 5a are given in the following Table 1: 
TABLE 1 
______________________________________ 
Electrode Dimensions 
Flow Location Temperature Rise with Current 
connector 
weld strip 
Point A 50 Amps 100 Amps 
200 Amps 
part(mm) 
(mm) or B C C C 
______________________________________ 
0.6 2 A 200 &gt;600 Not tested 
0.6 2 B 50 360 Not tested 
0.9 2 A 10 70 400 
0.9 2 B 20 70 460 
1.4 4 A 5 30 170 
1.4 4 B 5 70 330 
______________________________________ 
These data show that high electric current results in resistive heating of 
the connector and electrode components. For high temperature oxidation 
resistant alloys, relatively thick lead connector components and 
electrodes components are required to maintain a low operating temperature 
at air flow and air temperature conditions typical of those present in a 
gas turbine combustor. For electrical currents of 100 amps, the connector 
should be in the range of 0.9 mm thick and the total electrode thickness 
in the range of 2 mm while for currents as high at 200 amps, the connector 
should be in the range of 1.4 mm thick and the electrode 4 mm thick. 
Similarly, the electrical leads bringing the electrical power to the EHC 
unit must also not be prone to resistive heating. The most common 
conductor material used for such leads would be copper due to its high 
conductivity. However, as shown in FIG. 5b, copper oxidizes very rapidly. 
For this test, a copper rod 1.5 mm in diameter and approximately 30 mm in 
length was placed in an air furnace and heated at 550.degree. C. 
Periodically, the copper rod was removed from the furnace and weighed. The 
decrease in weight was due to formation and loss of a copper oxide. A 
similar test with a silver rod is also shown. These results are 
significant because a recuperated gas turbine would have a combustor inlet 
temperature of 500 to 600.degree. C. and leads at the inlet of an EHC unit 
and copper leads would oxidize rapidly. 
An alternative lead material is an oxidation resistant stainless steal such 
as Type 309 stainless steel or FeCrAl alloys or NiCrAl alloys. These 
alloys typically have a higher resistance and would be restively heated 
when used as leads for the EHC unit. Comparison data is shown in FIG. 5c 
where the calculated steady-state temperature is shown for each of the 
lead materials and diameters in a cross flow of air at 350.degree. C. 
typical of the conditions or during start up. FIG. 5c shows that a 3 mm 
diameter 309 stainless steel lead would rapidly overheat even at currents 
of 50 amps. To minimize resistive heading of the 309 ss lead, the lead 
diameter would have to be 10 mm. Other iron or nickel based alloys would 
behave similarly. 
Silver is not generally considered a good lead material due to its expense 
and its low melting point. However, its properties make it uniquely useful 
as a lead material for EHC applications. As shown in FIG. 5b, it is very 
stable to oxidation at 550.degree. C. and at substantially higher 
temperatures. FIG. 5d shows that the very high conductivity of silver 
results in very low resistive heating so that a 1 mm diameter lead could 
be used at 50 amps current, 2 mm diameter leads at 100 to 150 amps and 3 
mm diameter leads for currents above 250 amps. Surprisingly, the 
960.degree. C. melting point of silver does not appear to be a problem in 
this application since no portion of the leads or connectors approach this 
temperature. In addition, very good low electrical resistance joints can 
be made between the silver lead and an iron based brazing alloys such as 
BAg-8 with 72% Ag-28% Cu and a melting point of 880.degree. C. Other braze 
alloys can be used depending on the operating temperature of the lead and 
connector components at the braze joint. 
While silver works quite well as a lead material, alloys of silver and 
other oxidation resistant elements can be used. Suitable alloying elements 
include platinum, palladium, rhodium, gold, copper and nickel. Also, the 
silver electrical current carrying lead can be a wire, a strip, or a 
specially designed connector to connect to the EHC unit and to penetrate 
the combustor wall and connect to the electrical power supply. 
In one preferred embodiment of this invention, two foils are used to form 
the EHC unit 10. Two foils allows the free ends of the foils to be located 
at the outer periphery of the EHC unit 10 so as to facilitate attachment 
of the electrodes and power leads. It is desirable to have both electrodes 
at the periphery of the catalyst structure, so as to not interfere with 
the fuel/air mixture entering the EHC unit 10. While not as important for 
EHC units 10 used in automotive exhaust emissions control applications, it 
is important in a gas turbine application. An electrode or any 
irregularity in the gas flow can act as a flameholder that can stabilize a 
flame upstream of the catalyst, thereby overheating the catalyst and 
destroying it. However, more than two foils can be used and still have the 
electrodes 22 and 24 at the outer periphery of the EHC unit 10. FIG. 3a 
illustrates an arrangement using three foils that, when wound together to 
form a spiral, will have the electrodes at the outer periphery. In the 
embodiment shown in FIG. 3a, only two of the three foils are current 
carrying members. 
Alternatively, the EHC unit 10 of the present invention can be constructed 
having a center electrode and one or more peripheral electrodes. FIGS. 3b 
through 3e schematically illustrates several alternate embodiments of the 
EHC unit 10 which use a center electrode. The center electrode is in the 
same location as the attachment point 20 of the foils. The configurations 
illustrated in FIGS. 3a through 3e use three different foils laid adjacent 
to each other. The foils will then be rolled about the center electrode to 
form the spiral structure. Similarly to the peripheral electrodes 22 and 
24, the center electrode can be attached to the foils as shown in FIG. 5a. 
The three foil designs shown can be arranged in parallel or series 
electrical circuits using any number of the foils as current carrying 
members. For example, FIG. 3b illustrates a three foil EHC unit having 
each of the foils used as current carrying members in a parallel circuit, 
with three peripheral electrode connections and a center electrode. FIG. 
3c illustrates a three foil EHC unit having one foil used as a current 
carrying member, with one peripheral electrode and a center electrode. The 
foils that are not used as current carrying members are electrically 
connected to the center electrode, thereby referencing the voltage seen 
between the foils to the center electrode. FIG. 3d illustrates a three 
foil EHC unit having one foil used as a current carrying member, with one 
peripheral electrode and a center electrode. The foils not used as current 
carrying members are not electrically connected to the circuit, thereby 
referencing the maximum voltage to the power supply voltage. Lastly, FIG. 
3e illustrates a three foil EHC unit having all three foils connected in 
series to each other. The maximum voltage is seen across all three foils. 
This is the arrangement with the highest resistance, and the lowest 
current draw for a given voltage. 
In general, the number of foils used as current carrying devices should be 
selected depending upon the available power supply and desired heat 
dissipation in the EHC unit 10. At least one side of the foils is normally 
coated with a insulative dielectric layer, and the configuration can be 
chosen to limit the maximum voltage seen between two layers. The following 
Table 2 gives values of current and maximum voltage (Vmax) for the 
different three foil configurations for a total power to the EHC unit of 
10 kilowatts. 
TABLE 2 
______________________________________ 
DESIGN RESISTANCE(ohms) 
VOLTS AMPS Vmax 
______________________________________ 
3a 1.10 105 95 30 
3b 0.183 43 234 .about.12 
3c 0.550 74 135 74 
3d 0.550 74 135 .about.26 
3e 1.650 128 78 42 
______________________________________ 
Each of the foils used in the EHC designs of FIGS. 3a-3e can be the same or 
different. For example, the circuit of FIG. 3b can utilize three different 
foil corrugations and when wound into s spiral, would appear essentially 
as shown in FIG. 6. In this Figure, an electrode is attached to each foil 
and connected to the same connector of the power supply. As noted above, 
this parallel electrical circuit would have a low resistance and would 
draw a high current. 
To obtain a high resistance and low current, the series circuit of FIG. 3e 
could be used. This design places an electrode at the center which may lie 
undesirable. A series circuit EHC with current flowing through all foils 
is shown in FIG. 4 where six foils are stacked together and connected as 
shown. When wound into a spiral, the EHC will have electrode at the 
periphery and will have a high resistance due to series circuit design. 
This design and any alternative design which employs a series circuit 
current flow coupled with electrodes located on the periphery of the 
catalyst structure comprises the most preferred embodiment of the 
invention. 
When an electrical current is passed through the foils, the electrical 
resistance of the foils causes the EHC unit 10 to heat up. The heat is 
conducted to the catalyst layer and the catalyst is brought up to a 
temperature where it becomes reactive with the fuel/air mixture passing 
over the surfaces of the foils. The resistance of the EHC unit 10 has to 
be matched to the available power source. In automotive exhaust emission 
control applications, the EHC unit 10 should be designed to heat up very 
rapidly and be able to operate with an electrical input of 12 to 24 volts. 
The operator of an automotive vehicle should be able to enter the vehicle, 
insert the key and start the engine with little or no delay. Therefore, 
automotive EHC units should be designed to heat up very rapidly, and 
control emissions from the cold engine. Since the available voltage source 
is usually a battery with a low voltage, i.e., 12 to 24 volts, and a rapid 
heat up is desired, the EHC unit 10 should be made with a material of low 
resistance. As an example, for a 12 volt system and a power requirement of 
2.5 kW, to quickly heat the catalyst, a total resistance of approximately 
0.06 ohms and a current of 208 amps will be required. Alternatively, for 
hybrid vehicles having higher voltage sources such as 150 volts or higher, 
the resistance required for a 2.5 kW circuit would be 9 ohms and the 
current would be 17 amps. The EHC unit 10 for use in a gas turbine will 
not be limited to a battery voltage, and can therefore be constructed so 
as to have a high resistance with a consequent lower current for the same 
power dissipation. 
The EHC unit 10 is heated by passing an electrical current through the 
foils. This necessitates the foils being insulated from one another, and 
the foils are generally coated with a dielectric (insulative) layer on at 
least one side to prevent the foils from shorting out against each other. 
The dielectric coating used on the surfaces of the foils in the EHC unit 
10 must be chosen to withstand the voltages in use and the high operating 
temperatures of the catalytic reaction. Typically, the EHC unit 10 may 
operate in a voltage range of about 100 volts or higher. Using these 
higher voltages necessitates a dielectric coating with a high breakdown 
voltage for use between the foil layers. Suitable dielectric coatings are 
any ceramic material including, but are not limited to, alumina, zirconia 
or other electrically insulating ceramic materials. 
Catalyst Design Considerations 
Prior to assembling the corrugated 12 and flat or corrugated 14 catalyst 
support foils into the final spiral shape, the catalytic material and 
other coatings are applied in a series of several steps. A detailed 
portion of an EHC unit 10 showing the various coatings on both a flat foil 
14 and a corrugated foil 12 is shown in FIG. 7. In general, these steps 
include first oxidizing both the corrugated 12 and flat 14 foils and 
coating the foils with a dielectric or washcoat (72 or 74). In addition, 
at least a portion of one side of either foil is coated with a catalyst 
which optionally, and preferably, may be incorporated into the washcoat 
(see above) such that the washcoat layer 74 includes the catalyst. In 
addition to the washcoat and catalyst layers, both foils may be coated 
with a dielectric coating (not shown). 
As noted previously, the catalyst support foils are preferably 
aluminum-containing alloys. Both the flat 14 and corrugated 12 foils are 
first oxidized at temperatures in the range of 80 to 110.degree. C. in air 
so that the aluminum forms alumina whiskers, crystals or a layer on the 
surface which provides a rough and chemically reactive surface for better 
adherence of the washcoat layer or dielectric coating layer. In general, 
the surfaces of foils that will have different applied electrical 
potential will be coated with a dielectric coating to electrically 
insulate these two foils from each other. The other side of these foils 
can be coated with a washcoat layer and catalyst, depending on the 
requirements of the catalyst design. In some case, it may be desirable to 
coat the second side with a dielectric coating or even to apply a 
dielectric coating layer underneath the washcoat and catalyst layer to 
provide improved electrical insulation between foils. For example, as 
shown in FIG. 7, the flat foil 14 is coated on one side with a dielectric 
coating 72 while the opposite side is coated with a washcoat and catalyst 
layer 74. In contrast, the corrugated foil 12 is coated on both sides with 
a dielectric coating 72. 
In general, the choice of the type of washcoat used on the different types 
of support foils depends on the design of the EHC unit 10, (two foils, 
three foils, etc.), the choice of catalyst used, and where the catalyst is 
applied, as will be discussed in greater detail below. Therefore, these 
decisions are left to one skilled in the art to decide for each particular 
application of the invention. 
In general, both the dielectric layer and the washcoat plus catalyst layer 
72 or 74 may be applied by spraying, direct application, dipping the 
support foils into the washcoat, or by any other suitable manner. 
Typically, layers 72 or 74 will have a thickness in the range of about 3 
to 50 micrometers. Materials suitable for the dielectric layer include, 
but are not limited to, silicon oxide, aluminum oxide, zirconium oxide, 
titanium oxide or mixtures of these oxides. These oxides may contain 
additions to stabilize the structure at high temperature such as yttrium, 
calcium and magnesium additions to zirconium oxide. 
The washcoat plus catalyst layer 74 is coated on at least a portion of one 
side of one of the support foils. Materials suitable for use as the 
washcoat include, but are not limited to, aluminum oxide, aluminum oxide 
containing additives such as barium, lanthanum, silicon or other 
components to prevent or reduce thermal sintering and loss of surface 
area, zirconium oxide with or without additives such as silicon. The 
washcoat would preferably possess a moderate to high surface area, from 2 
to 200 m.sup.2 /g. 
The catalyst, active for the reaction of fuel with oxygen, is deposited 
within and on the surface of the porous washcoat layer. Suitable catalytic 
materials include, but are not limited to, Group VIII noble metals or the 
platinum group metals consisting of palladium, ruthenium, rhodium, 
platinum, osmium, and iridium. For methane or methane containing fuels, 
the preferred catalyst is palladium or platinum or a mixture of palladium 
and platinum. For other fuel such as gasoline, diesel fuel, alcohol fuels 
or a variety of other hydrocarbon fuels, palladium and platinum are the 
preferred catalysts. However, for the other fuels less active catalysts 
can be used including base metal oxide catalysts such as copper, cobalt, 
manganese, chromium, nickel or other active base metal oxide catalyst 
either as the pure oxide, in admixture with other elements or dispersed on 
a second oxide. 
The catalyst may be incorporated onto the washcoat layer in a variety of 
different methods using metal complexes, compounds, or dispersions of the 
metal. The compounds or complexes may be water or hydrocarbon soluble. The 
liquid carrier generally needs only to be removable from the catalyst 
carrier by volatilization or decomposition while leaving the catalyst 
metal in a dispersed form in the washcoat layer. For the preferred mixed 
palladium/platinum catalyst, palladium ammonium nitrite can be mixed with 
platinum ammonium nitrite in water and an excess of nitric acid to form a 
solution which is sprayed onto a SiO.sub.2 /ZrO.sub.2 washcoat-treated 
support foil followed by drying and calcination in air at high 
temperature. 
An alternative preparation procedure for the washcoat plus catalyst layer 
is to prepare a mixture of the washcoat and catalyst components and apply 
this mixture to the foil surface in a single operation. Alternatively, the 
catalyst can be deposited onto the washcoat solid by impregnation or other 
suitable procedure and the heat treated to fix the catalyst on the 
washcoat surface. 
Heat Transfer Design Considerations 
Coating the catalyst support foil with catalyst on only one side provides a 
catalytic surface with a very efficient heat exchange mechanism to the 
non-catalyzed opposite surface. As exemplified by the simplified catalyst 
structure, indicated generally at 80 in FIG. 8, if only one side of a flat 
support foil 14 is coated with the catalyst 84, then both a reacting gas 
flow 86 and a non-reacting gas flow 88 will result when the fuel/air 
mixture is passed over the catalyst structure 80. The heat generated from 
the catalytic reaction of the fuel and air on the catalyst surface is 
transferred to both the reacting gas flow 86 as well as through the flat 
support foil 14 to the non-reacting gas flow 88 on the opposite side of 
the support foil. Thus, in this simplified arrangement, the catalyst 
structure has "integral heat exchange" since the heat generated by the 
catalytic reaction is transferred by both conduction and convection to 
both the reacting gas flow 86 and the catalyst support flat foil 14 which 
functions as a heat sink. The heat in the support flat foil 14 is then 
removed by the non-reacting gas stream 88 located on opposite side of the 
catalyst coating 84. 
This concept of integral heat exchange can be applied to the EHC unit 10 of 
the present invention. In these cases, if either the flat foil 14 or the 
corrugated foil 12 is coated with a catalyst on only one side and then 
rolled into a spiral structure, the resulting structure is one having a 
plurality of channels with only a portion of each channel being coated 
with the catalyst. The fuel/air mixture flowing over the surfaces of the 
portion of the channels having the catalyst coating results in the 
reaction of the fuel and air to generate heat. The heat is transferred to 
the fuel/air mixture flowing in that channel as well as being transferred 
through the channel walls (comprised of the washcoat plus catalyst layer 
and support foil) to the fuel/air mixture stream flowing in the adjacent 
channel. This can be seen by referring to FIG. 7, where channel 97 has a 
catalyst containing washcoat coating 74 in a portion of that channel. The 
heat generated in the catalyst layer 74 will be transferred to the 
fuel/air stream flowing in channel 97 as well as through the channel walls 
comprised of dielectric layer and corrugated foil 12 to the non-reacting 
fuel/air stream flowing through channels 78 (the fuel/air stream flowing 
in channels 78 are not undergoing catalytic reaction since no catalyst 74 
is present in channels 78.) 
An example of a catalytic structure utilizing integral heat exchange is 
illustrated in U.S. Pat. No. 5,250,489, issued to Dalla Betta et. al., the 
disclosure of which is hereby incorporated in its entirety. It is 
important to limit the temperature of the walls of the catalyst structure 
to avoid overheating and melting the substrate, and to prevent a runaway 
catalytic reaction. As combustion occurs at the catalyst surface, the 
temperature of the catalyst and the metal substrate will rise and the heat 
will be conducted and dissipated in the gas flow on both the catalytic 
side on the non-catalytic side of the channels in an IHE catalytic 
structure, as described above. This will help to limit the temperature of 
the catalyst substrate and will aid in maintaining the catalyst-coated 
wall temperature in the range of about 700.degree. C. to 1000.degree. C. 
In general, the heat transfer design of the EHC unit 10, including an IHE 
design must limit the temperature of the catalyst substrate to a maximum 
temperature above which catalyst and support foil damage will occur. While 
this maximum temperature depends on the type of material used for the 
support foil and the type of catalyst used, this maximum temperature is 
typically about 1100.degree. C. 
A number of heat transfer configurations can be designed for a catalyst 
structure. For example, a catalyst structure can be designed such that 
some of the channels are completely coated with the catalyst with the 
adjacent channels having no catalyst coating. Alternatively, a catalyst 
structure can be designed such that only some of the channels have a 
portion of their walls coated with the catalyst layer. Finally, all of the 
channels can be partially coated with the catalyst. This latter type of 
configuration where all channels are at least partially coated with the 
catalyst is a preferred embodiment because most or all of the fuel should 
be catalytically combusted and yet the heat generated by the catalytic 
reaction can be effectively removed. 
Having now described the structure of the EHC unit 10 in accordance with 
the present invention, a method for starting up a gas turbine using the 
inventive EHC unit 10 will be explained below. 
Start-up Method for a Gas Turbine 
In general, the method for starting up a gas turbine includes the steps of 
applying electrical power to the catalyst structure to heat the catalyst 
to a predetermined temperature limit and then introducing a 
fuel/oxygen-containing gas (such as air) mixture into the catalyst 
structure. The electrical power to the catalyst structure may be 
terminated when one of several conditions are met including, but not 
limited to, when the heat of the catalytic reaction is sufficient to 
maintain the catalyst at steady-state condition or when a certain period 
of time has elapsed. 
The general method described above can be best described using FIG. 9a and 
9b. The gas turbine system consists of a compressor 32 which compresses 
air 31 to produce a high pressure flow that proceeds through diverter 
valve 41, electric preheater 43, combustor 34 and then on to power turbine 
36 which produces the mechanical power to drive the load 49, typically an 
electrical generator. The combusted fuel and air exits the gas turbine 
through the exhaust 98. The energy to drive the gas turbine is generated 
in combustor 34 where fuel is injected through injector 45 and mixed with 
air to form a fuel/air mixture which flows over catalyst 10 and is 
combusted in the catalyst and in the downstream post catalyst combustion 
zone 35. 
Fuel injector 45 injects the fuel into the flowing air stream and should 
produce a relatively uniform fuel/air mixture at the catalyst inlet 33. 
For gaseous fuel such a methane or natural gas, the fuel injector will 
inject the fuel and cause it to be mixed with the air and can be any of 
the designs familiar to those skilled in the art. For a liquid fuel such 
as gasoline, diesel fuel or alcohol fuels, the fuel injector would 
preferably inject the fuel as a spray of small droplets and cause the fuel 
to vaporize to form a uniform gaseous mixture at the catalyst inlet. 
Alternatively, fuel injector 45 could consist of a prevaporizer that forms 
a fuel vapor that is then injected into the air stream as a gas. The fuel 
injector can take the form of an inline device, placed essentially in the 
air flow as shown schematically in FIG. 9b, it can incorporate air mixing 
devices such as swirlers, partial blocking mixing devices, reversal of the 
air flow or other designs familiar to those skilled in the art. The 
general method of control can utilize a variety of sensors that will be 
described. Thermocouples or resistance temperature measurement devices 90 
can be located in the catalyst unit 10. These thermocouples can be 
attached to the metal foil surfaces in the catalyst either the 
catalyst-coated foil or the non-catalyst-coated foil. Alternatively, a 
temperature sensor can be located just downstream of the catalyst 92 to 
measure the gas temperature exiting the catalyst. These temperature 
sensors would be connected to the controller 100. The controller would 
also be electrically connected to power controller 50 as shown in FIG. 9a 
which controls power to the preheated that heat the air going into the 
combustor 34. In addition, the controller is connected to power controller 
52 which is connected to the EHC catalyst and controls the electrical 
power sent to the EHC catalyst. The controller 100 also controls the 
diverter valve 41 and the fuel control valve 51. The electronic controller 
100 may be used to automatically control the start-up method by processing 
the signals described above and other signals relating to the operating 
conditions of the gas turbine system and then providing output signals to 
several of the components used in the system. For example, the electronic 
controller 100 could be used to terminate the electrical power supplied to 
the catalyst structure whenever a certain condition is sensed. 
Alternatively, the electronic controller 100 could be used to operate the 
bypass valve 41 or the fuel supply nozzle 45. 
While it is envisioned that this start-up method may be automatically 
controlled based on sensed operating parameters, the type of electronic 
controller 100 or other device used to achieve the automatic control is 
not critical to the inventive method. However, in general, the controller 
100 may be embodied as a conventional microprocessor or similar computing 
apparatus which can be programmed to generate one or more electrical 
output signals in response to a plurality of electrical input signals. The 
controller 100 may include a central processing unit (CPU) having analog 
and/or digital electronic calculation and logic circuitry for effecting 
arithmetic operations and processing data under the control of one or more 
algorithms. Also the CPU would receive input signals from a number of 
sensors which measure the various operating parameters of the EHC unit 10 
and the system it is used in, such as a gas turbine system. The controller 
100 may also include one or more memory modules, such as read-only memory 
(ROM) for storing predetermined data, random-access or data memory (RAM) 
for storing calculated or input data, and control memory for storing the 
control algorithms. 
The first step in the inventive start-up method of the gas turbine is to 
use a starter 48 to rotate the compressor/turbine system causing air to be 
driven through the compressor 32, combustor 34 and turbine 36. At the 
desired rotational speed, power is applied to the electrical heater 43 by 
activating power controller 50. Also, at some predetermined conditions, 
power controller 52 is activated to apply electrical power to the EHC 
catalyst unit 10. At some predetermined point, selected either by time or 
by reaching some set temperature in temperature sensors 90 or 92, fuel 
flow is initiated by activating fuel control valve 51 to introduce fuel 
into the preheated air flow and to initiate combustion in the catalyst 10 
and combustor 34 supplying power to turbine 36. 
In a preferred embodiment of the invention, the catalyst substrate is 
electrically heated until the catalyst temperature is within about 
100.degree. C. of the normal steady-state operating temperature of the 
catalyst before the fuel/air mixture is introduced. As may be expected, 
the steady-state operating temperature will be different for each type of 
catalyst. However, as an example, for the preferred mixed 
palladium/platinum catalyst, the steady-state operating temperature of 
this catalyst is between about 700.degree. C. to about 900.degree. C. More 
preferably, the fuel/air mixture is not introduced until the catalyst 
substrate temperature is within about 50.degree. C. of the normal 
steady-state operating temperature of the catalyst. 
The amount of electrical power supplied to the catalyst structure can be 
constant, modulated, or a combination of both. Alternatively, the power 
may be modulated in response to variances in the measured catalyst 
substrate temperature. For example, the electrical power to the EHC and 
the fuel flow can be controlled on a preset time sequence with the EHC 
power turned on for a preset period of time and then the fuel flow turned 
on and the EHC power turned off at a preselected time. Alternatively, the 
EHC power can be controlled based on measure temperatures such as from 
temperature sensor 90 or 92. The EHC power can be turned on at a high 
level until the catalyst substrate is near its steady-state operating 
temperature, then the EHC power modulated to hold the catalyst at its 
steady-state temperature and the fuel turned on. As the fuel reacts on the 
catalyst, the catalyst temperature would rise rapidly and the controller 
would turn off the EHC power. A variety of different control schemes can 
be devised by those skilled in the art and these schemes can be 
incorporated into the control algorithms of controller 100. 
The effectiveness of these control schemes would be determined by measuring 
the total emissions from the gas turbine system by monitoring the CO, UHC 
(unburned hydrocarbons) and NOx levels in the exhaust 98 of FIG. 9a. 
Once the desired catalyst substrate temperature is achieved, the fuel/air 
mixture is introduced into the catalytic combustion chamber 34. The 
amount, rate, temperature, and pressure at which both the fuel and air is 
introduced into the combustion chamber 34 is dependent on a number of 
factors, particularly on the design and operating characteristics of the 
gas turbine engine. The size of the EHC catalyst is determined by the 
size, air flow characteristics and fuel air ratio of the gas turbine. 
Independent of the size of the gas turbine and EHC catalyst, the normal 
steady-state, operating temperature of the catalyst is typically in the 
range of about 700.degree. C. to about 1000.degree. C. In addition, the 
theoretical adiabatic combustion temperature for the fuel/air mixture may 
be in the range of about 1100.degree. C. to about 1600.degree. C. 
The electrical power to the EHC unit 10 is terminated when one of several 
conditions occurs. In one embodiment, this condition may be when the heat 
of the catalytic reaction between the fuel and air is sufficient to 
maintain the catalyst at its steady-state operating temperature. This 
condition occurs when the measured catalyst substrate temperature begins 
to rise due to reaction of the fuel. The controller 100 would sense this 
rise in temperature and decrease or shut off EHC power. 
In another embodiment, additional thermocouples may be used to measure the 
substrate temperature of the catalyst layer, including thermocouples 
located at the inlet, middle and outlet portions of the EHC unit 10. In 
this embodiment, these three catalyst substrate temperatures are 
monitored, and the power is turned off when one or any combination of them 
reach a predetermined limit. In any event, the substrate temperature 
should not be allowed to exceed the maximum catalyst substrate 
temperature, approximately 1050.degree. C. In yet another embodiment, the 
thermocouple measuring the combusted fuel/air mixture at the outlet of the 
catalyst structure 10 or the thermocouple used downstream 93 can be used 
to terminate the power. For example, power may be shut off if either of 
these gas outlet temperatures exceeds the steady-state outlet gas 
temperature. In the preferred embodiment described herein, this 
temperature would be 750 to 800.degree. C. 
EXAMPLES 
The following examples describe both the structure of the inventive EHC 
unit 10 and their performance under start-up conditions. 
Example 1 
This example describes the preparation of an EHC unit 10 and its use under 
start-up conditions without electrically preheating the catalyst surface 
prior to the introduction of the fuel/air mixture. 
EHC Unit Preparation: 
A first, SiO.sub.2 -ZrO.sub.2 washcoat was prepared by first mixing 20.8 g 
of tetraethylorthosilicate with 4.57 cc of 2 mM nitric acid and 12.7 g of 
ethanol. The mixture was added to 100 g of zirconia powder having a 
specific surface area of 100 m.sup.2 /gm. The resulting solid was aged in 
a sealed glass container for approximately 24 hours and then dried. A 
major portion of this dried solid was calcined in air at 1000.degree. C. 
while the remaining portion was calcined in air at 500.degree. C. A sol 
was prepared by mixing 152 g of the SiO.sub.2 /ZrO.sub.2 powder calcined 
at 1000.degree. C. and 15.2 g of the SiO.sub.2 /ZrO.sub.2 powder calcined 
at 500.degree. C. with 3.93 g of 98% H2SO4 and 310 cc of distilled water. 
Finally, the mixture was milled using ZrO.sub.2 grinding media for eight 
hours to produce the SiO.sub.2 /ZrO.sub.2 sol. 
The SiO.sub.2 sol consisted of a commercially available colloidal silicon 
oxide sol produced by PQ Corporation, Ashland, Mass. and designated Nyacol 
Colloidal Sol 2034. 
A flat foil catalyst support was made from a Fe/Cr/Al alloy with a 
composition of 20% Cr, 5% Al and the balance Fe. A metal foil strip of 50 
micrometers thickness and 75 mm width was first treated at 900.degree. C. 
in air to form a surface oxide coating. One side of the flat foil was 
sprayed with the colloidal sol SiO.sub.2 to a thickness of about 5 
micrometers. The coated flat foil was then calcined in air at 1050.degree. 
C. Next, the opposite side of the flat foil was sprayed with the SiO.sub.2 
/ZrO.sub.2 washcoat to a thickness of about 40 micrometers. The coated 
foil was again calcined in air at 1050.degree. C. A mixed 
palladium/platinum catalyst was next applied to the side of the flat foil 
coated with the SiO.sub.2 /ZrO.sub.2 washcoat by first preparing a 
palladium/platinum solution. The palladium/platinum solution was made by 
dissolving Pd(NH3)2(NO2)2 and Pt(NH3)2(NO2)2 in water and an excess of 
nitric acid to form a solution of about 0.1 g Pd/ml and a Pd/Pt ratio of 
about 6. The palladium/platinum solution was sprayed onto the SiO.sub.2 
/ZrO.sub.2 coated side of the flat foil to a loading of about 0.25 g Pd/g 
of SiO.sub.2 /ZrO.sub.2. 
A corrugated support foil was also made from the same 50 micrometer thick 
Fe Cr Al alloy foil. A strip measuring 76 mm in width and having straight 
corrugations with a height of about 1.2 mm was first treated at 
900.degree. C. in air to form a surface oxide coating. Both sides of the 
corrugated foil were sprayed with the colloidal SiO.sub.2 sol to a 
thickness of about 5 micrometers. The coated corrugated foil was then 
calcined in air at 1050.degree. C. 
The catalyst structure was then assembled by first sandblasting the ends of 
both foils to remove the oxide coating. One end of both the corrugated 
foil and the flat foil were inserted into a longitudinally-extending slot 
in the center of a cylindrical rod. The assembly was then welded along the 
length of the rod. The two foils were then wound around the center rod to 
form a cylindrical catalyst structure. The remaining ends of the foils 
were also cleaned by sandblasting and welded to a flat electrode structure 
similar to that shown in FIG. 5a. A silver lead was attached to the outer 
electrode and the center cylindrical rod (which was also functioning as an 
electrode.) The resulting spiral wound catalyst structure measured 50 mm 
in diameter and 75 mm in length and had a center electrode and a 
peripheral electrode. In addition, the resulting catalyst structure had a 
catalyst-coated wall in half of the channels. 
The EHC unit 10 was installed in a combustion reactor test system having 
provisions for passing air at a controlled flow rate through the catalyst 
structure and for preheating the air before it entered the combustion 
reactor. In addition, the combustion reactor system included provisions 
for injecting liquid fuel into the flowing air stream and for uniformly 
mixing the fuel and air before it enters the combustion reactor. 
Electrical leads were inserted through the walls of the combustion reactor 
and connected to the center electrode and the peripheral electrode. 
Outside of the reactor, the electrical leads were connected to a DC power 
supply. Additional equipment was installed to measure the performance of 
the EHC unit including a thermocouple for measuring the substrate 
temperature of the catalyst-coated flat foil, a thermocouple for measuring 
the temperature of the partially combusted gas mixture immediately after 
it leaves the catalyst structure, and a third thermocouple for measuring 
the temperature of the combusted gas mixture at the outlet of the reactor, 
which was approximately 80 cm downstream of the catalyst unit. In 
addition, a water cooled gas sampling probe was installed in the reactor, 
approximately 82 cm downstream of the catalyst structure, to measure the 
composition of the gas at the outlet of the combustor test rig. 
Start-up Test Apparatus: 
To evaluate the performance of the EHC unit and the start-up procedure, a 
test rig was used to simulate the combustion system shown in FIGS. 9a and 
9b. This test system consisted of the following items: 
An air compressor and a flow control system to provide the desired air 
flow; 
An electric heater similar to electric heater 43 to preheat the air stream 
entering the combustor 34; 
A combustion reactor essentially similar to that shown in FIG. 9b 
containing a fuel injector, the catalyst and a post-catalyst reaction 
zone. 
Electrical leads penetrated the combustor wall to electrically connect to 
the electrodes of the EHC and to connect to a DC power supply that could 
control the electrical power to the EHC unit; 
Thermocouples installed within the EHC to measure substrate temperature, a 
thermocouple just downstream of the catalyst to measure the temperature of 
the gas exiting the EHC unit, and a thermocouple at the end of the 
combustion section 35; and 
A gas sampling probe was located at the end of combustion section 35 to 
measure the composition of the combustor exhaust, particularly CO and UHC 
(unburned hydrocarbons). 
Start-up Test Conditions: 
The following steps were used to test the EHC unit described above under 
start-up conditions: 
1. The air flow through the combustion reactor was set to 240 standard 
liters per minute (SLPM). This value is consistent with the air flow 
present in a gas turbine at ignition speed when adjusted for the required 
size of catalyst necessary for typical catalystic combustion performance. 
2. The air was preheated to 450.degree. C. 
3. Electrical power was NOT applied to the catalyst. 
4. Isooctane fuel was supplied at a flow rate of 7.9 g/min. The fuel 
injection was started at the time 75 seconds at shown in FIG. 10a. 
5. The temperatures of the catalyst substrate and combusted gas mixture and 
the emissions were monitored throughout this test. 
The results of this test are shown in FIGS. 10a through 10C. FIG. 10a shows 
the temperature profile in the catalyst substrate as the isooctane reacts 
on the catalyst and increases the catalyst temperature. In addition, FIG. 
10a shows that the temperature of the combusted gas mixture at both the 
outlet of the EHC unit 10 and the outlet of the reactor also increase due 
to the catalytic reaction occurring within the reactor. The calculated 
combustion reactor outlet temperature is that temperature that would occur 
if complete combustion of the fuel takes place in the reactor. The 
combustor outlet temperature is below the calculated combustor outlet 
temperature due to heat loss through the walls of the combustor test rig. 
FIGS. 10b and 10c show the total amount of uncombusted hydrocarbons (UHC) 
and carbon monoxide (CO) emissions which resulted under these test 
conditions. Unburned fuel will escape through the catalyst structure and 
exit the combustion reactor at very high levels until the temperature 
rises sufficiently high to fully combust the fuel and CO. Integration of a 
concentration versus time curve for each type of emission provides the 
total amount for that emission. FIG. 10b and 10c shows that the integrated 
emissions for the UHCs were 1353 mg while the integrated emissions for the 
CO was 82 mg. These start-up levels are very high and would result in an 
unacceptably high total emissions. 
Example 2 
This example shows the use of an EHC unit 10 with electrical preheating of 
the catalyst surface prior to the introduction of the fuel/air mixture. 
EHC Unit Preparation: 
The catalyst and test system used in this test are the same as in Example 
1. In this case, electrical power is applied to the catalyst prior to 
introducing the fuel. 
Start-up, Test Conditions: 
1. The air flow through the combustion reactor was set to 240 standard 
liters per minute (SLPM). This value is consistent with the air flow 
present in a gas turbine at ignition speed. 
2. The air was preheated to 450.degree. C. 
3. Electrical power is applied to the catalyst unit to achieve a catalyst 
substrate temperature of 750.degree. C. 
4. The flow of isooctane fuel was started at a flow rate of 7.9 g/min. The 
fuel injection was started at approximately 18 seconds as shown on FIG. 
11a. When the catalyst temperature reaches 800.degree. C. from reaction 
with the fuel, the electrical power was shut off. 
5. The temperatures of the catalyst substrate and combusted gas mixture and 
the emissions were monitored throughout this test. 
The results of this test are shown in FIGS. 11a through 11c. FIG. 11a shows 
that the catalyst substrate temperature increased to a maximum of about 
900.degree. C. when the fuel was initially injected and then returned to 
its steady-state operating temperature of about 800.degree. C. FIG. 11a 
also shows higher temperature for the combusted gas mixture at both the 
outlet of the catalyst structure and at the outlet of the combustion 
reactor. 
FIGS. 11b and 11c show the integrated total emissions for the UHCs and CO 
respectively. Under these operating condition, the total amount of UHCs 
was 4.2 mg and the total amount of CO was approximately 13 mg. These 
results show that by electrically preheating the catalyst, the emissions 
during start-up conditions can be substantially reduced. More 
specifically, the start-up conditions used in Example 2 resulted in a 
99.7% reduction in the UHC emissions and a 84% reduction of the CO 
emissions. Based on these results, it is clear that the use of an EHC unit 
in the start-up of a gas turbine can substantially reduce the total 
integrated emissions during start-up. 
Example 3 
This example illustrates that the exact sequence and timing of fuel on 
versus electrical power off is important to the total reduction of the 
emissions. If power is turned off too soon, before the fuel reaction rate 
is fast enough, then the catalyst temperature may decrease and the 
initiation of homogeneous combustion after the catalyst will be delayed. 
In addition, significant fuel will escape the combustion chamber and 
appear as undesired emissions. If the electrical power is turned off too 
late, then the catalyst may overheat and be damaged. 
EHC Unit Preparation: 
The catalyst and test systems used in Example 3 are similar to those 
described in Examples 1 and 2. In this test, the fuel used was natural gas 
consisting mainly of methane with minor constituents of higher 
hydrocarbons. The EHC unit was a two foil, spirally wound EHC unit using 
one flat foil and one corrugated foil. The corrugated foil in this test 
was formed into a herringbone pattern with a corrugation height of 1.20 mm 
and a peak to peak period of 2 mm. The herringbone pattern had channel 
lengths of 20 mm and a channel angle of 6. The corrugated foil in this 
test had one side coated with the catalyst, unlike the previous test, 
where the flat foil was coated with the catalyst. The washcoat preparation 
and application was the same as that described in Example 1. 
Start-up Test Conditions: 
The following conditions were used to test the EHC unit in this example: 
1. The air flow through the combustion reactor was set to 240 SLPM. 
2. The air was preheated to 450.degree. C. 
3. Electrical power was applied to the EHC unit to achieve a catalyst 
substrate temperature of 950.degree. C. 
4. Natural gas fuel was supplied at a flow rate of 7.9 g/min. The fuel was 
injected at the time 18 seconds. 
5. Fuel was turned on and the EHC power turned off after a selected time 
interval after the fuel was turned on. 
6. The temperatures of the catalyst substrate and combusted gas mixture and 
the emission were monitored throughout this test. Table 3 and FIG. 12 
contain the data from this test. 
TABLE 3 
______________________________________ 
Integrated 
Delay from Fuel 
Emissions T Outlet T Outlet 
on to EHC UHC CO Wall at Fuel 
Wall Min 
Run Power Off (sec) 
(mg) (mg) on to CAT 
During Start 
______________________________________ 
1 No EHC power 
4200 49 550 550 
2 15 seconds 3500 40 930 810 
3 18 seconds 110 22 930 900 
4 21 seconds 20 23 930 930 
______________________________________ 
These data demonstrate that there is a threshold period of time that the 
power to an EHC unit 10 should be left on that results in a significant 
reduction in undesired emissions. Emissions of UHC, during the start 
transient, can be reduced from 4200 mg to 20 mg and CO from 49 to 23 by 
increasing the time that EHC power is kept on after fuel is turned on. 
Because of the IHE structure of the catalyst, only a portion of the fuel 
is combusted in the catalyst. If the temperature exiting the catalyst is 
too low to initiate homogeneous combustion, then this unburned fuel will 
exit the combustor and be emitted to the atmosphere. This is clearly the 
case in run 1 where the catalyst substrate temperature was only 
550.degree. C. and required a very long time to be heated by the reacting 
fuel to its steady-state temperature as shown in FIG. 12. Applying EHC 
power raises the catalyst substrate temperature to its steady-state value, 
in this example 930.degree. C. However, if the EHC power is removed too 
soon as in runs 2 and 3, then the catalyst substrate temperature will 
decrease, not providing the required catalyst outlet temperature. If the 
EHC power is kept on for a long time after the fuel is turned on and 
begins to react on the catalyst, then the catalyst temperature may rise 
far above the steady-state temperature. These data clearly show that there 
is an optimum time for which EHC power should be maintained after fuel 
flow is started for achieving minimum emissions. In addition, these 
results show that using a control system to modulate the EHC power to 
maintain a relatively constant catalyst substrate temperature is a 
preferred operating mode. 
In accordance with the provisions of the patent statutes, the principle and 
mode of operation of this invention have been explained and illustrated in 
its preferred embodiment. However, it must be understood that this 
invention may be practiced otherwise than as specifically explained and 
illustrated without departing from its spirit or scope.