GB1578665A - Minimizing no production in operation of gas turbine combustors - Google Patents

Description

(54) MINIMIZING NOX PRODUCTION IN OPERATION OF GAS TURBINE COMBUSTORS (71) We, EXXON RESEARCH AND ENGINEERING COMPANY, a Corporation duly organised and existing under the laws of the State of Delaware, United States of America, of Linden, New Jersey, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The need for gas turbine combustion operations which meet air pollution requirements and maximize fuel utilization is of sufficient importance to have prompted a great deal of experimentation in the area.It is known that controlled mixing of excess air in the second stage of a two stage combustion system is the key to limiting NOX formation.

In a gas turbine engine, inlet air is continuously compressed, mixed with fuel and then burned in a combustor. Quantities of air greatly in excess of stoichiometric amounts are compressed and used to keep the combustor liner cool and to dilute the combustor exhaust gases so as to avoid damage to the turbine blades and nozzle. Generally, primary sections of the combustor are operated near stoichiometric conditions which produce combustor gas temperatures up to approximately 4,0000 F. Further down the combustor, secondary air is added which raises the air-fuel ratio and lowers gas temperatures so that the gases exiting the combustor are in the range of 2,000"F. The fuel injection pressure varies and it is typically 600 PSI for full power and as low as 60-100 PSI for idle conditions.

It is known that NOX formation is thermodynamically favored by high temperatures. Kinetic studies indicate that the rate of NO formation has a high activation energy (approx. 115 k cal/mole) so that the major formation of NO must take place in the high temperature primary combustion zone of conventional turbines. Since NO formation reaction is so very highly temperature dependent, decreasing peak combustion temperatures provide an effective means of reducing NOX emissions from combustion equipment. Operating the combustion in a very lean condition (i.e., high excess air) is one of the simplest ways of achieving low temperatures and consequently, low NOX emissions.The problems of very lean ignition and combustion are ones that have been encountered and solved for many automotive emission control systems and for industrial fume-solvent incineration systems. In both of these cases, catalysts are used to promote and complete the combustion process. In a similar way, catalysts can be used with gas turbines to provide efficient combustion in lean systems. This invention, therefore, relates to methods of operating gas turbine combustors while minimizing the formation and discharge of pollutants such as NOX.

The present invention provides a method for combusting fuel in a gas turbine comprising a fuel combustion zone, the method comprising: (a) partially combusting fuel with air within an open cannular combustor located within a primary non-catalytic section of the combustion zone to form a hot partially burned effluent which emanates from an opening in said cannular combustor, the amount of air present within said cannular combustor being in the range of from 50 to 70% of the stoichiometric requirement for complete combustion of the said fuel; (b) quenching said hot partially burned effluent by mixing with additional air within said primary non-catalytic section of said combustion zone without continued high temperature combustion, the amount of said additional air being sufficient to support subsequent combustion of the partially burned fuel contained in said effluent; and (c) passing said mixture of quenched effluent and additional air over an oxidation catalyst in said combustion zone at a temperature above the catalyst light-off temperature to complete the combustion of said fuel.

Preferably, the said open cannular combustor is a perforated cannular combustor.

The invention also provides a gas turbine comprising a fuel combustion zone in which fuel is combusted by a method as described above.

The effect of step (b) is both to quench the hot partially burned primary zone effluent and to provide a sufficient mix of the partially burned fuel with secondary air so that combustion is completed on contact with the oxidation catalyst under conditions which do not favor the formation of NOX. The operation of gas turbines according to the method of the invention provides, in addition to NOX reduction, the further benefits of improved fuel efficiency and minimization of CO and unburned hydrocarbon emissions.

The catalyst employed can have any of a variety of forms and compositions and can be any catalyst which promotes the oxidation of fuels in the presence of molecular oxygen.

The oxidation catalyst may comprise a Group VIII noble metal component (e.g. platinum) or a mixture of Group VIII noble metal components which will ignite the mixture of partially burned fuel and additional air, e.g. at a temperature in the range of from 350 to 12000F. The noble metal component(s) may be in the form of screens or supported on ceramic substrates.The catalyst may also comprise at least one-noble transition metal component (e.g. copper-nickel) disposed downstream (with respect to the direction of gas flow relative to the catalyst) of the Group VIII noble metal component for raising the combustion temperature, e.g. to about 1600"F. The non-noble catalyst component may comprise transition metals, or mixtures of alloys thereof in the form of screens or supported on ceramic substrates, or rare earth metal oxides on ceramics.

The catalyst may additionally comprise at least one non-noble transition metal component (e.g. a nickel-chromium alloy such as that commercially available under the registered Trade Mark "Nichrome") disposed downstream of the previous catalyst component(s) for raising the downstream temperature to from 1600 to 24000 F. The latter non-noble catalyst component may comprise transition metals or alloys thereof as screens or supported on ceramic substrates, or rare earth metal oxides on ceramics. The latter exhibit resistance to degradation of their physical properties at the high temperatures generated during the completion of combustion in contact with the catalyst.

If suitably selected catalyst components are employed, it will be seen that the complete combustion of the partially burned fuel is effected at a temperature in the range of from 350 to 24000 F.

In place of the catalysts mentioned above, the mixture of partially burned fuel and additional air may be passed directly into contact with a non-noble oxidation catalyst such as a nickel-copper catalyst, nickel oxide on ceramic, rare earth metal oxides or Nichrome screens to complete the fuel oxidation. If the amount of additional air mixed with the hot partially burned effluent from the cannular combustor provides an equivalence ratio of about 0.3 (where the equivalence ratio is 100 divided by the percentage of the stoichiometric air provided), the fuel oxidation would be completed at a temperature of about 2400"F.

The specific temperature range employed for completing the combustion depends upon the durability and activity properties of the catalyst component(s) and also on the mode of operation of the gas turbine. For example, the operational modes of an aircraft gas turbine include the range of from idle (low temperatures) to full power (high temperatures) to take-off (peak temperatures).

When the catalyst comprises a Group VIII noble metal component and a non-noble metal component, the former component is preferably disposed upstream of the latter, preferably in a thin zone which is adequate to initiate, or light-off, the combustion of the mixture of partially burned fuel and added air at a relatively low combustion temperature, and the combustion is then completed in contact with the non-noble metal component at a relatively higher combustion temperature.A dual catalyst system of this type takes advantage of the higher rates of reaction promoted by Group VIII noble metals (e.g. platinum) at relatively low temperatures, while high temperature deactivation problems normally associated therewith can be avoidede.g. by providing that the Group VIII noble metal catalyst has a volume which is low enough to avoid its exposure to temperatures in excess of 1500"F. The use of three catalyst components in series may be preferred in order not to exceed the physical property limitations of the catalyst. The catalyst component metals are selected from all the Groups of the Periodic Table.

The catalyst promotes the completion of combustion of the last 50% or less of the total fuel oxidation in the combustion zone.

The space velocity of gas passing in contact with the catalyst may be in the range of from 50,000 to 50,000,000 V/V/h (volumes of gas per unit volume of catalyst per hour), preferably from 500,000 to 5,000,000 V/V/h.

The materials from which the open cannular combustor may be constructed may be selected from suitable ceramics or high temperature-resistant alloys such as those known by their registered Trade Marks "Inconel" and "Hastelloy".

The operation of a gas turbine comprising a fuel combustion zone in which fuel is combusted by the method of the invention results in the combustion of fuel to drive the turbine to produce combustion gas containing amounts of NOX below 10 ppm, preferably below 5 ppm and most preferably below 1 ppm.

The invention is now further described by way of non-limitative examples and with reference to the accompanying drawings, in which: Figure 1 depicts a schematic representation of a fuel combustion zone for combusting fuel in a gas turbine in accordance with the method of the invention; Figure 2 is a graph illustrating air to fuel ratio on the abscissa and adiabatic flame temperatures for the various zones designated in Figure 1 by reference numbers (1), (2), (3), (4) and (5) plotted against equilibrium and kinetically-limited NOX emission indexes defined as pounds NOX (as NO2) per 1000 pounds of fuel; and Figure 3 is a schematic drawing illustrating comparatively how the open cannular combustor employed in the method of the invention reduces NOX emissions.

Referring first to Figure 1, it will be seen that the fuel combustion zone comprises an open cannular combustor enclosing a zone 1 surrounded by a casing, spaced apart from the cannular combustor, to define an annular space having an upstream zone 2 for the passage of dilution and quenching air. Fuel is injected into the interior of the open cannular combustor from a fuel injector and the fuel is partially burned within the combustor with from 50 to 70% of the stoichiometric air requirement to produce a hot partially burned effluent. The latter passes out of the open cannular combustor into a downstream zone 3 of the annular space wherein it mixes with and is quenched by the dilution and quenching air without continued high temperature combustion.The dilution and quenching air is preheated and enters zone 2 at e.g. 4000 F, and is provided in such an amount as to be sufficient to support combustion of the partially burned fuel contained in the effluent from zone 1. The mixture of air and quenched effluent on passing through zone 4 has a temperature of about 1300"F, and passes therefrom through a zone 5 containing an oxidation catalyst at a temperature above the catalyst light-off temperature to complete the combustion of the fuel, e.g. at a maximum combustion temperature of about 1700"F. After zone 5, the effluent stream would be at an equivalence ratio of about 0.3 and at about 1700"F.

In zone 3, the hot partially burned effluent is diluted with the dilution and quenching air in such a manner as to avoid going through the stoichiometric combustion mixture regime (equivalence ratio, 8=1.0). The mechanical design of the cannular combustor is important to the proper operation of the hybrid combustion zone. The cannular combustor can be made out of high-temperatureresistant alloys such as Hastelloy X or ceramic. Preferably, it should have small ports or chimneys to inject the effluent formed in zone 1 rapidly into the air stream in zone 3 and also to create local turbulence to mix the two streams rapidly. It has been shown that flame propagation does not occur if the quenching diameter (hole size) is below 0.12 inches or if the velocity of the effluent leaving the zone 1 is of the order of 100 feet/second.Alternatively, the open cannular combustor can be made out of a porous material such as that commercially available under the trade name "Rigimesh" which, during operation, would be effectively cooled by the passage thereover of the dilution/quenching air and at the same time would allow the hot partially combusted effluent to flow to the outside of the combustor.

Reference is now made to Figure 2 wherein the graphs show the variation of a number of parameters using the aviation kerosine well-known by the name "Jet A". If the partial combustion in zone 1 occurs at 70% stoichiometric air, then the equilibrium NOX emission index (EINox) is about 2.2 lb. per 1000 lb fuel and the adiabatic flame temperature is about 3400"F for combustion air preheated to 4000F and supplied at a pressure of 4 atmospheres. The process for NOX production is kinetically limited and therefore the actual EINoI would be much lower than the maximum of 150 lb per 1000 Ib fuel predicted at equilibrium, as indicated by the broken line kinetic curve corresponding to a 2 milliseconds reaction time.The "prompt" or instantaneous NOX level would be higher in the case of the rich primary combustion conditions in zone 1 and can approach 10% of the equilibrium value of 2.2 lb/1000 lb fuel, i.e. 0.22 lb per 1000 lb fuel. This prompt NOX value presents an expected value for the type of combustion proposed here (and is equivalent to about 3 ppm). The equilibrium conditions for zones 2, 3, 4 and 5 are also shown in Figure 2. As will be understood, the kinetic limitations will keep the actual EINox values well below the equilibrium values for the zones 2, 3, 4 and 5 shown in Figure 2.

Reference is now made to Figure 3 wherein the benefits of the use of the open cannular combustor in the primary combustion zone (as employed in the practice of the method of the invention) are demonstrated by comparison with the operation, under substantially identical overall conditions, with two other primary zone combustors which would not be employed in the practice of the method of the invention.

The fuel employed was propane, and the combustion was effected at atmospheric pressure with no preheat of the air supplied for combustion.

The variables in this comparison include the percent stoichiometric air in the primary combustion zone, and different types of physical barriers between the primary combustion zone and the additional dilution air which was mixed with the hot partially burned effluent produced in the primary combustion zone. The overall percent stoichiometric air was kept constant at 400% (0.25).

In Figure 3(a), there is shown a first arrangement (not used in the practice of the present invention) wherein fuel is burned with no physical barrier between the primary combustion zone and the dilution air. When the equivalence ratio, , was 1.0 (i.e. 100% stoichiometric air) in the primary combustion zone, it was found that 1.3 Ibs. NOX per 1000 lbs. fuel were produced in the exhaust gas resulting from combustion of the primary zone effluent with the dilution air. When the stoichiometric air in the primary zone was reduced to 67% (1.49), 2.0 Ibs NOX per 1000 Ibs fuel were produced (i.e. an increase relative to primary zone operation with stoichiometric air).

The foregoing two sets of operating conditions were repeated with an open ended tube to separate and define the primary combustion zone from the dilution air, as shown diagrammatically in Figure 3(b). Such an arrangement would not be employed in the practice of the method of the present invention. With 100% stoichiometric air in the primary zone, 2.4 Ibs NOX per 1000 lbs fuel were produced. When the primary air supply was reduced to 67% stoichiometric (=1.49), the NOX level was reduced to 1.6 Ibs per 1000 Ibs fuel, a reduction of about 33% compared to the stoichiometric combustion result.

In Figure 3(c), the foregoing two sets of operating conditions were repeated with an open cannular combustor which would be used in the practice of the method of the invention and which was similar to that shown as defining zone 1 in Figure 1. The open cannular combustor was perforated with small holes and made from Hastelloy X. When the air supplied to the primary combustion zone was 100% stoichiometric (8=1.0), the NOX production was 1.7 Ibs per 1000 lbs fuel. At 67% stoichiometric air, the NOX production was reduced to 1.0 Ibs per 1000 lbs fuel, a reduction of about 40% compared to the stoichiometric combustion result.

The method of the invention was demonstrated on a larger scale in a 3.08 cm (2 in.) diameter cannular combustor using an open-ended perforated Hastelloy X can. Combustion air was split into primary air for fuel-rich combustion within the can, and secondary air for cooling the can as well as changing the stoichiometry of the fuel-rich gaseous mixture to the lean side prior to impinging on the catalyst.

The fuel employed was JP-4 kerosine. The results of one of these experiments is given in the following Table. It should be noted that better than 99% combustion efficiency was achieved. The centerline temperature going into the catalyst bed was 1153"K (1615"F.), well above the catalyst light-off temperature. The residual trace quantities of CO and unburned light hydrocarbons were easily oxidized over the catalyst to achieve on the order of 99.9% combustion efficiency. The quantity of NOX was equivalent to 2.2 g/kg of fuel or 0.11 lb./l06Btu which is below current U.S. environmental standards.

TABLE Combustor Pressure (ATM) 3.3 Primary and Secondary Air Preheat ("K) 400 Primary Equivalence Ratio 1.5 Overall Equivalence Ratio 0.3 Reference Velocity (m/s)(1) 24.4 JP-4 Flow Rate (g/sec) 2.718 Primary Air Flow Rate (g/s) 26.66 Secondary Air Flow Rate (g/s) 106.61 Primary Injector Velocity (m/s) 65.6 Sec. Air Vel. Around Combustor (m/s) 70 Sec. Air Inj. Vel. at Combustor (m/s)(2)Discharge 19.5 Temp.Profile* at Catalyst Bed Inlet ("K) Thermocouple (C) 924 Thermocouple (D) 1083 Thermocouple (E) 1153 Thermocouple (F) 1143 Concentration Profile* at Catalyst Bed Inlet CO COZ O2 NOx HC PPM % % PPM PPM Probe (C) 395 4.5 14.4 32 50 Probe (D) 345 4.5 14.4 33 70 Probe (E) 350 4.6 14.8 26 120 Probe (F) 365 4.2 14.6 29 48 '''Calculated for air preheat of 400 K (260"F.) in 5.08 cm (2.0 in) diameter catalyst chamber.

2)No heat addition except for 400"K preheat.

*The positions (C), (D), (E) and (F) of the thermocouples and probes are described and illustrated in U.S. Air Force Technical Report AFAPL-TR-76-8, entitled, "Development of a Catalytic Combustor for Aircraft Gas Turbine Engines" by Siminski and Shaw, published in U.S.A. on 22 September 1976.

WHAT WE CLAIM IS: 1. A method for combusting fuel in a gas turbine comprising a fuel combustion zone, the method comprising: (a) partially combusting fuel with air within an open cannular combustor located within a primary non-catalytic section of the combustion zone to form a hot partially burned effluent which emanates from an opening in said cannular combustor, the amount of air present within said cannular combustor being in the range of from 50 to 70% of the stoichiometric requirement for complete combustion of the said fuel; (b) quenching said hot partially burned effluent by mixing with additional air within said primary non-catalytic section of said combustion zone without continued high temperature combustion, the amount of said additional air being sufficient to support subsequent combustion of the partially burned fuel contained in said effluent; and (c) passing said mixture of quenched effluent and additional air over an oxidation catalyst in said combustion zone at a temperature above the catalyst light-off temperature to complete the combustion of said fuel.

2. A method according to claim 1 in which the said open cannular combustor is a perforated cannular combustor.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. given in the following Table. It should be noted that better than 99% combustion efficiency was achieved. The centerline temperature going into the catalyst bed was 1153"K (1615"F.), well above the catalyst light-off temperature. The residual trace quantities of CO and unburned light hydrocarbons were easily oxidized over the catalyst to achieve on the order of 99.9% combustion efficiency. The quantity of NOX was equivalent to 2.2 g/kg of fuel or 0.11 lb./l06Btu which is below current U.S. environmental standards. TABLE Combustor Pressure (ATM) 3.3 Primary and Secondary Air Preheat ("K) 400 Primary Equivalence Ratio 1.5 Overall Equivalence Ratio 0.3 Reference Velocity (m/s)(1) 24.4 JP-4 Flow Rate (g/sec) 2.718 Primary Air Flow Rate (g/s) 26.66 Secondary Air Flow Rate (g/s) 106.61 Primary Injector Velocity (m/s) 65.6 Sec. Air Vel. Around Combustor (m/s) 70 Sec. Air Inj. Vel. at Combustor (m/s)(2)Discharge 19.5 Temp.Profile* at Catalyst Bed Inlet ("K) Thermocouple (C) 924 Thermocouple (D) 1083 Thermocouple (E) 1153 Thermocouple (F) 1143 Concentration Profile* at Catalyst Bed Inlet CO COZ O2 NOx HC PPM % % PPM PPM Probe (C) 395 4.5 14.4 32 50 Probe (D) 345 4.5 14.4 33 70 Probe (E) 350 4.6 14.8 26 120 Probe (F) 365 4.2 14.6 29 48 '''Calculated for air preheat of 400 K (260"F.) in 5.08 cm (2.0 in) diameter catalyst chamber. 2)No heat addition except for 400"K preheat. *The positions (C), (D), (E) and (F) of the thermocouples and probes are described and illustrated in U.S. Air Force Technical Report AFAPL-TR-76-8, entitled, "Development of a Catalytic Combustor for Aircraft Gas Turbine Engines" by Siminski and Shaw, published in U.S.A. on 22 September 1976. WHAT WE CLAIM IS:

1. A method for combusting fuel in a gas turbine comprising a fuel combustion zone, the method comprising: (a) partially combusting fuel with air within an open cannular combustor located within a primary non-catalytic section of the combustion zone to form a hot partially burned effluent which emanates from an opening in said cannular combustor, the amount of air present within said cannular combustor being in the range of from 50 to 70% of the stoichiometric requirement for complete combustion of the said fuel; (b) quenching said hot partially burned effluent by mixing with additional air within said primary non-catalytic section of said combustion zone without continued high temperature combustion, the amount of said additional air being sufficient to support subsequent combustion of the partially burned fuel contained in said effluent; and (c) passing said mixture of quenched effluent and additional air over an oxidation catalyst in said combustion zone at a temperature above the catalyst light-off temperature to complete the combustion of said fuel.

2. A method according to claim 1 in which the said open cannular combustor is a perforated cannular combustor.

3. A method according to claim 1 or claim 2 substantially as hereinbefore

described.

4. A gas turbine comprising a fuel combustion zone in which fuel is combusted by a method according to any one of claims I to 3.