WO1999009310A1

WO1999009310A1 - LOW NOx GAS TURBINE WITH HEAT EXCHANGER

Info

Publication number: WO1999009310A1
Application number: PCT/US1998/016831
Authority: WO
Inventors: Reagan Houston
Original assignee: Reagan Houston
Priority date: 1997-08-15
Filing date: 1998-08-13
Publication date: 1999-02-25

Abstract

The NOx production of a gas turbine unit is kept below 1 part per million by thoroughly premixing the fuel and air upstream of the compressor (13) and heating the fuel air mixture to near or above its auto ignition temperature by one or more heat exchangers (24, 122) transferring heat from combustion gases (31, 119) and/or the turbine expander exhaust gas (54, 84). The heat exchangers may be of the tube type (24) and/or a rotating ceramic wheel (122). An automatic control adjusts the speed of a rotating ceramic wheel heat exchanger and adjust fuel supply in response to sensing various operating parameters to avoid production of excessive amounts of NOx, flame out and flash back.

Description

LOW NOx GAS TURBINE WITH HEAT EXCHANGER

TECHNICAL FIELD

This invention relates to a burner for a gas turbine and more particularly to a burner construction producing even burning of the fuel-air mixture while minimizing NOx production to below 1 part per million (ppm).

RELATED DISCLOSURE DOCUMENT

Some embodiments and features of this invention are revealed in Disclosure Document Number 420,295 filed in the United States Patent and Trademark Office on June 2, 1997.

BACKGROUND OF THE INVENTION

Current design gas turbines use burners which burn essentially all of the fuel with part of the required air. This generates temperatures over 2300 degrees F. in the flame and considerable NOx, commonly 50 to 200 parts per million (ppm). NOx in exhaust gases is highly undesirable from a pollution standpoint. The rich fuel air mixture and attendant high flame temperature in current design gas turbine burners is required for flame stability. The part of the air not burnt with the fuel is blended in with the burnt gases to bring the gas temperature down to about 1600 degrees F., the temperature at which combustion gas is typically fed to the turbine-expander.

Flame temperature may be lowered somewhat by injecting steam or air directly into the flame. This lowers the flame temperature so that only 25 to 50 ppm of NOx is produced.

Flash back is an important design consideration in gas turbine design when the fuel and air are mixed ahead of the burner. Flash back is controlled by keeping the flow velocity of the fuel-air mixture faster than the flame velocity of the fuel-air mixture. The flame velocity of typical hydrocarbons is about 1000 feet per minute for stoichiometric mixtures at room temperature. In operation of a gas turbine the temperature of the air entering the combustion chamber will be elevated by compression to about 300 to 400 degrees F., which will increase flame velocity. A fundamental problem in gas turbine burner design is that the fuel-air mixture must be heated to a temperature such that the flame speed is as great or greater than the velocity of the burning fuel-air mixture passing through the burner combustion chamber. Accordingly, a reliable and preferably inexpensive mechanism is needed to prevent and control flash back in a gas turbine unit.

Another important design consideration for gas turbine units is flame blow out due to lean fuel-air mixtures. The prevention of flame blow out is commonly referred to as flame stabilization. In industrial burners, flame stabilization has been achieved by mixing the hotter products of combustion with the cooler fuel-air mixture entering the combustion chamber. Where flame temperature must be kept low to keep the formation of NOx below 1 part per million, gas recirculation is believed to be insufficient to stabilize the flame. Another known method of flame stabilization is to flow a rich fuel- air mixture through a porous refractory plate or a metal screen. For achieving combustion with very low NOx formation, neither of these methods of flame stabilization is considered satisfactory.

OBJECTS AND SUMMARY OF THE INVENTION

It is a primary object of this invention to provide a gas turbine burner which produces exhaust gases having less than 1 ppm of NOx. It is a further object of this invention to provide a gas turbine burner which burns a preheated fuel-air mixture at an even temperature without the production of hot spots in which NOx is generated.

I have found that the fuel-air mixture can be preheated in a heat exchanger to above the auto ignition temperature (about 1025 degrees F. for methane and less for most other fuels). In my burner design, the fuel-air mixture burns partly within the heat exchanger tubes without causing thermal expansion problems. The fuel-air mixture is heated to a temperature which exceeds the auto ignition temperature of the air-fuel mixture by at least 100 degrees F. and preferably 200 degrees F.

Another objective of this invention is to prevent flashback from the burner by maintaining the fuel-air flow velocity into the combustion chamber at well above the flame velocity.

A further objective of this invention is to provide combustion at a temperature higher than the turbine inlet blades can withstand but below the temperature where NOx formation becomes rapid, and to do this without local hot spots

Another objective of this invention is to use a highly porous refractory to stabilize the combustion flame, provide thermal mass and, by radiation, preheat the incoming fuel- air mixture.

BRIEF DESCRIPTION OF THE DRAWINGS

Several embodiments of the invention are illustrated in the drawings, in which

Figure 1 is a schematic of a gas turbine in which a fuel-air mixture is fed to the intake of a compressor and the fuel-air mixture passes through tubes in a heat exchanger which are heated by combustion gases passing to a turbine,

Figure 2 is a schematic showing a gas turbine in which the fuel-air mixture heated and blended by the compressor passes through tubes of a heat exchanger heated by the turbine exhaust gases and also heated by hot combustion gases passing from the burner to the turbine;

Figure 3 is a schematic of a gas turbine in which a torus shaped heat exchanger is concentric with the compressor and turbine,

Figure 4 is a schematic of a gas turbine in which heat of the turbine exhaust gases is transferred to the fuel air mixture leaving the compressor by a rotating wheel of porous ceramic material; Figure 5 is a longitudinal section of a mixing chamber and part of a combustion chamber of a burner for a gas turbine unit;

Figure 6 is schematic of a gas turbine having a rotating heat exchanger wheel for transferring heat from combustion gases flowing from the combustion chamber to the turbine to the fuel-air mixture entering the combustion chamber;

Figure 7 is a schematic of another embodiment of the invention;

Figure 8 is an enlarged section of the heat exchanger illustrated in Figure 2;

Figure 9 is a section of a flow control mechanism for a combustion chamber of a gas turbine unit taken along line IX-D in Figure 10,

Figure 10 is a section taken along the line X-X in Figure 9,

Figure 11 is a longitudinal section through of a combustion chamber having a pair of transverse porous ceramic walls and an alternate design flow control valve and

Figure 12 is a section taken on the line XTI-XII in Figure 11.

DETAILED DESCRIPTION OF THE INVENTION

Referring to Figure 1 , the illustrated gas turbine includes an fuel-air inlet duct 12 connected to the intake of a compressor 13 which is connected in driven relation to a turbine 14 by a shaft 16. The shaft is also connected to a load or work unit such as an electric generator. I prefer to deliver at least 85% of the required fuel by way of the fuel- air inlet duct 12, which may include baffles so that it serves as a mixing chamber. The compressed fuel-air mixture passes from the compressor 13 to a burner or combustion chamber 17 by way of a conduit 18 interconnecting a discharge of the compressor 13 to an opening or inlet 19 at the bottom of a heat exchanger section of the combustion chamber 17. The combustion chamber 17 includes a start-up burner 21 to which a fuel line 22 is connected. Alternatively, an electric igniter may be used in place of the start up burner 17. Also, alternatively, I may use 2 or more combustion chambers. The fuel- air mixture from the compressor flows from the combustion chamber inlet to a combustion cavity 23 at the top of the combustion chamber 17 by way of a plurality of tubes 24 supported only at their lower ends by a tube sheet 26. The tubes 24 are supported in cantilever fashion by the tube sheet 26. Combustion gases from the burning cavity 23 flow around the radially outer surfaces of the tubes 24 en route to a discharge opening 31 to which a conduit 32 is connected. In the alternative, the conduit 18 could be connected to the opening 31 and the conduit 32 could be connected to the opening 19, in which case the incoming fuel air mixture would flow on the outside of the tubes 24 en route to the combustion cavity and the combustion gases would pass through the inside of the tubes 24 en route to the conduit 32. The conduit 32 delivers the combustion gases to an inlet of the turbinel4. Exhaust gases from the turbine 14 are discharged through an exhaust conduit 33. During operation of the gas turbine of Figure 1, the fuel and air entering the compressor 13 are mixed, compressed to about 120 psi and heated to about 500 degrees F. by the compressor. The heated fuel air mixture is further heated to near or above an ignition temperature as it flows through the tubes 24 of the heat exchanger section of the combustion chamber 17 to the combustion cavity 23. The heating of the fuel-air mixture and thorough mixing prior to combustion, permits a lean fuel-air mixture to be used without danger of flame burn out. Flash back is avoided because the flow velocity of the fuel-air mixture in the tubes is maintained greater than the flame velocity of the fuel air mixture. Thus combustion does not occur in the lower end of the heat exchanger section of the combustion chamber. A relatively lean fuel-air mixture is used and the temperature of the combustion gases in the combustion cavity will be in the temperature range of 1600 to 2200 degrees F., which is below the 2300 degrees F. temperature level at which formation of NOx in the combustion gases starts to be rapid. The gases leave the combustion cavity 23 and flow on the outside of the tubes 24 in the heat exchanger section of the combustion chamber 17 and exit the heat exchanger section at the discharge opening 31. The transfer of heat from the combustion gases to the incoming fuel air mixture in the tubes 24 reduces the temperature of the combustion gases as much as 300 degrees as they pass to the turbine 14 by way of conduit 32. This is a desirable result because the temperature of the combustion gases entering the turbine 14 should not be greater than about 1600 degrees F. with present gas turbine materials.

With some fuels, the exit temperature of the fuel-air mixture leaving the compressor may be above the ignition temperature of the fuel-air mixture due to its composition and the heat of compression. I have found that harmful combustion does not usually occur because actual ignition requires a small fraction of a second after the ignition temperature is reached. During that small increment of time, the fuel-air mixture exits the compressor.

The gas turbine unit shown in Figures 2 and 8 is believed to be more efficient than that shown in Figure 1 because it incorporates a combined combustion chamber and dual heat exchanger structure 36 which includes a housing 37 for two heat exchanger sections 38, 39 and a combustion cavity 41. The heat exchanger sections 38, 39 share tubes 42 which pass through both sections. The tubes 42 are supported by a bottom tube sheet 43 and a central tube sheet 44. The central tube sheet 44 divides the housing 37 and forms the junction between the two heat exchanger sections 38, 39. Alternatively, the two heat exchangers may be formed separately and they may be made of metal or ceramic material. In operation of the gas turbine unit shown in Figures 2 and 8, fuel and air are delivered to an intake manifold 46 on a compressor 47. The manifold may include baffles to induce mixing of the fuel and air. A duct 48 interconnects the outlet of the compressor and an inlet opening 49 in the housing 37 for delivery of the compressed and heated fuel-air mixture to the heat exchanger section 38 where it is heated further by the exhaust gases discharged by a turbine 51. The turbine 51 , which is connected to the compressor by a shaft 52, includes an exhaust opening 53 to which a conduit 54 is connected. The conduit 54 delivers exhaust gases to one end of the heat exchanger section 38 by its connection to an inlet opening 56 in the housing 37. As the fuel-air mixture flows through the tubes 42 toward the combustion chamber 41 , the exhaust gases from the turbine, flowing around the outside of the tubes 42 in the opposite direction and exiting through an outlet opening 57 and an exhaust conduit 58, heat the fuel-air mixture to a higher temperature. As the fuel-air mixture flows through the upper heat exchanger section, it is heated to an ignition temperature by the combustion gases from the combustion cavity 41 flowing downwardly along the outside of the tubes 42 and exiting through an outlet opening 59 to which one end of a conduit 61 is connected. The other end of the conduit 61 is connected to an inlet of the turbine 51. As shown in Figure 8, annular rings 62 are installed in the tubes 42 near the tube sheet 44. The rings 62 may be positioned above the plane defined by the top of the tube sheet 44. The annular rings 62 serve as velocity increasing orifices, to prevent flash back into the lower heat exchanger section 38. The higher velocity of the fuel-air mixture established by the orifices substantially exceeds any flame speed anticipated in operation of the gas turbine unit. As illustrated, the annular rings have a tapered inside diameter portion on their lower intake ends and a cylindrical inside diameter portion on their upper discharge ends which terminates in a 90 degree shoulder. The orifices provided in the dual heat exchanger of Figure 8 provide a trouble free mechanical mechanism for maintaining the flow velocity of the fuel-air mixture at the orifices well above the range of flame velocities of the mixture expected during operation to the gas turbine unit. By using the heat exchanger section 39, it is possible to achieve a combustion temperature in the combustion chamber 41 that is only slightly higher than the permissible temperature of the combustion gases delivered to the turbine-expander 51

Referring to Figure 3, a gas turbine unit is illustrated which has a torus shaped combined heat exchanger and burner 66 surrounding a compressor 67 and a turbine 68, which are interconnected by a shaft 69. This embodiment of the invention operates in substantially the same manner as the gas turbine unit shown in Figure 1.

In the embodiment of the invention illustrated in Figure 4, the fuel-air mixture heated and compressed by a compressor 76 is delivered to a housing 77 of a heat exchanger 78 by an interconnecting conduit 79. A heat exchange element in the form of a porous ceramic wheel 81 is rotatably mounted in the housing by a shaft 82 driven by a suitable power unit such as an electric motor, not shown. An exhaust conduit 84 connects an exhaust of a turbine-expander 86 to the housing 77 of the heat exchanger 78. As illustrated, the openings in the bottom of the housing 77 to which the conduits are connected are on diametrically opposite sides of the axis of the heat exchanging wheel 81. The fuel-air mixture from the compressor 76 is further heated as it passes through the left side of the ceramic wheel 81, as viewed in Figure 4, and then passes into the lower end of a tube type heat exchanger section 88 of a combined heat exchanger and combustion chamber 89. The fuel-air mixture flows upwardly through tubes 90 of the heat exchanger 88 and into a combustion cavity 91. The fuel air mixture is heated to an ignition temperature as it flows through the tubes 90 by the heat transferred to the outer surfaces of the tubes 90 by the combustion gases flowing around and along the outer surfaces of the tubes 90 en route to an outlet opening to which an upper end of a conduit 92 is connected. The lower end of the conduit 92 is connected to the inlet of the turbine- expander 86. The exhaust gasses from the turbine heat the porous ceramic wheel 81 as it passes through it and exits through an exhaust manifold 93. The ceramic disk or torus of the ceramic wheel 81 is made of a heat absorbing material or composition which remains structurally stable at the temperature range of the gases exhausted from the turbine- expander 86, The ceramic torus of the ceramic wheel 81 may be made in wedge shaped sections and the wheel structure may include a cage like frame secured to the wheel shaft which holds the wedge shaped sections in assembly. The small holes in the ceramic wheel allow the fuel-air mixture to flow through and the wheel has enough thermal mass to carry heat from the products of combustion gases to the incoming fuel-air mixture. The pressure drop across the ceramic wheel is relatively small and thus leakage around the wheel is small, with negligible effect on heat transfer or exit gas velocities. Gas velocities with the present invention will be less than with typical prior art gas turbine units. Lower velocities will produce 1) a lower pressure drop across the ceramic heat transfer wheel, 2) a longer time for CO and hydrocarbons to burn at the lower temperatures and 3) little or no erosion or wear on the exposed ceramic. Heating the fuel air mixture by use of the ceramic wheel aids measurably in flame stabilization. In order to avoid formation of NOx in the combustion process, it is important that the burning temperature be below the 2300 degree F. level. The thorough mixing in the compressor 76 results in an even burning temperature and avoidance of the formation of hot spots (in which NOx forms). The thorough mixing and preheating of the fuel-air mixture permits a leaner mixture to be used without danger of flame blow out. The leaner mixture burns at a desired lower temperature, thus avoiding formation of NOx above a 1 ppm level.

Preferably, the combustion chamber is lined with ceramic material because essentially all the air will be mixed with the fuel and will not be available for transpiration cooling. Thermal expansion is mitigated by using ceramic fiber between the ceramic pieces and between the ceramic pieces and metal parts. Gas flow rates in the combustion chamber are expected to be below 1000 feet per minute and may be as low as 300 feet per minute.

Referring to Figure 5, a novel mixing chamber 101 is shown connected in upstream relation to a combustion chamber 102. The mixing chamber 101 is a cylindrically shaped conduit having mixing baffles 103, 104 and 106 fixedly secured thereto in oblique relation to the flow of the fuel-air mixture through the mixing chamber 101. Tests of typical flow conditions, such as Reynolds number of 20,000 show that a length to diameter ratio (L/D) of 3 gives good mixing and an L/D ratio of 5 gives excellent mixing. The Reynolds number is defined as D times G/u, where D is the diameter in centimeters, G is the weight flow in grams per second and u is viscosity in poises. The length and diameter of the mixing chamber shown in Figure 5 are indicated by the letters L and D. The length of the mixing chamber is determined by the distance L that the fuel line 107 is spaced from the combustion chamber 102. Although excellent mixing is obtained by using a mixing chamber having a L/D ratio of 5 or more, such mixing chambers are frequently undesirably long for a particular gas turbine unit By using the mixing baffles 103, 104 and 106, the mixing chamber can be shortened and still produce excellent mixing Danger of flash back is greatly decreased by provision of a constπction 111 at the downstream end of the mixing chamber, at the entrance or inlet 112 to the combustion chamber 102 The combustion chamber is lined with a ceramic lining 108 and a spherically shaped plate 109 of porous ceramic material is installed in the combustion chamber 101 with its convex side facing upstream Ceramic material is stronger m compression and thus the spheπcally shaped plate 109 will be stronger than a flat ceramic plate in the same position in the combustion chamber 102 All of the fuel- air mixture flowing through the combustion chamber passes through the ceramic plate 109

The single shaft gas turbine unit illustrated in Figure 6 includes compressor 116 connected to a turbine-expander 117 by a shaft 118 A combined heat exchanger and combustion chamber 119 includes a housing 121 having an upper cylindrically shaped part in which a porous ceramic wheel 122 is rotatably mounted and a pair of taped plenums 123, 124 connected to the bottom of the upper part of the housing 121 below the ceramic wheel 122 The plenum 123 has its bottom end connected to an outlet of the compressor 116 by a manifold or conduit 126 and its upper end, which defines one half of a circle, is connected to the bottom of the cyhndπcally shaped upper part of the housing 121 The lower end of the plenum 124 is connected to the intake of the turbine- expander 117 by a manifold or conduit 127 and the upper end of the plenum 124, which is semi-circular in shape, is connected to the lower end of the upper part of the housing 121 Thus the upper ends of the plenums 123, 124 completely enclose the lower end of the upper cyhndπcally shaped part of the housing 121 The lower face of the cyhndncally shaped ceramic wheel 122 covers the open upper ends of the plenums 123, 124 and thus the fuel air mixture flows through the left side of the rotating porous ceramic wheel 122 to the combustion cavity 128 between the upper face of the wheel 122 and the top of the housing 121 where combustion occurs. The combustion gases pass downwardly through the right hand side of the wheel 122 and into the plenum 124 from whence it flows to the turbine-expander 117 and out its exhaust conduit 129. The porous ceramic performs duel functions, both of which are highly desirable in a gas turbine unit. The ceramic wheel 122 heats the fuel air mixture by as much as 300 degrees F. (from 500 degrees F. to 800 degrees F. for instance) before it enters the combustion chamber. This permits the fuel air mixture to be relatively lean in fuel without risk of flame out. Heat from the combustion gases leaving the combustion cavity is transferred to the ceramic wheel 122 thus reducing the temperature of the combustion gases passing to the turbine- expander 117 by as much as 600 degrees F.. This is highly desirable because the combustion gasses in the combustion cavity are expected to be as high as 2200 degrees F. and the maximum temperature for gases entering the turbine expander is approximately 1600 degrees F.. The ceramic wheel 122 is driven by a variable speed electric motor 131 connected to the shaft 132 of the wheel 122. The speed of the ceramic wheel 122 can be adjusted up or down to increase or decrease the rate of heat transfer from the combustion gases to the incoming fuel-air mixture. This speed adjustment, which is independent of the speed of the compressor and turbine-expander, can be made automatically by an automatic control system, such as hereinafter described in relation to Figure 7, in response to operating parameters of the gas turbine unit, such as temperature of the fuel- air mixture leaving the compressor or temperature of the combustion gases passing to the turbine expander. The combustion chamber 119 is equipped with the usual ignition device 133 for start up purposes. The ignition device 133 may also be used to restart the gas turbine unit in case of a flame out. Fuel and air may be premixed and fed to the intake of the compressor 116 by way of a mixing conduit 134, or fuel may be delivered to an intermediate stage of the compressor 116 by way of a fuel inlet conduit 136. Delivery of liquid fuel to an intermediate stage of the compressor 116 has a cooling effect on the air being compressed as the liquid fuel vaporizes. Fuel may alternatively be delivered to the compressed air leaving the compressor 116 by a fuel line 137 connected to conduit 126. The compressor used in the Figure 6 embodiment of the invention has a relatively high compression ratio, such as about 15, and a relatively high discharge temperature, such as about 100 degrees F. below the auto ignition temperature for the fuel air mixture being compressed. The ceramic rotor 122 may be driven by the motor 131 at speeds ranging from 0 to 100 revolutions per minute depending on the amount of heat it is desired to transfer from the combustion gasses to the incoming compresses fuel-air mixture. The ceramic wheel may be made of foam ceramic and the free void fraction in the wheel is 40 to 95 %. When using a heavy wheel having a void fraction of 40 to 70%, the desired gas flow rate is 300 to 3000 feet per minute. When using a more open ceramic wheel, such as one having a void fraction of 70 to 95%, the desired gas flow rate is 1000 to 10,000 feet per minute.

With low load or idle conditions, the temperature of the fuel-air mixture can be expected to drop, in which case it is desirable to increase the preheat of the fuel-air mixture entering the combustion chamber to prevent a flame out. In using this invention, the amount of preheat can be controlled by changing the rotational speed of the ceramic wheel. At full load, there may be sufficient preheat from the compression of the compressor and it may be desirable to stop rotation of the ceramic wheel.

The gas turbine unit illustrated in Figures 7, 9 and 10 includes a compressor 141 connected in driven relation to a turbine-expander 142 by a drive shaft 143. Combustion air is delivered to a mixing chamber 144 connected to the intake of the compressor 141 and fuel is delivered to the mixing chamber 144 by a fuel delivery mechanism 145 connected to the mixing chamber 144 by a fuel line 146. A thoroughly mixed and compressed fuel-air mixture from the compressor 141 is delivered to an inlet opening 147 in the lower wall of a cylindrical shaped housing 148 of a ceramic wheel heat exchanger 149 by a manifold or conduit 150 interconnecting an outlet of the compressor 141 and the inlet opening 147. The ceramic wheel heat exchanger 149 includes a wheel

151 having a torus 152 of porous ceramic material having appropriately sized openings to permit the flow of the fuel-air mixture and combustion gases therethrough. The torus

152 may be made up of wedge shaped sections held in assembly by a cage secured to an air cooled shaft 153 of the wheel 151. The shaft 153 is supported in the upper and lower walls of the housing 148 by bearings, not shown, and is driven by an electric motor 154 drivingly connected thereto. The speed of the motor 154 is regulated by an automatic control 156 connected to the motor 154 by a lead 157. The fuel air mixture from the compressor 141 passes through the left side of the porous torus 152, as viewed in Figure 7, and passes to an inlet opening in the left end of a metering valve 161 by way of a conduit 162 connected to an outlet opening 158 in the upper wall of the wheel housing 148. The outlet end of the metering valve 161 is secured to a combustion chamber 163 which has an outlet connected to an inlet opening 164 in the upper wall of the wheel housing 148 by a conduit 166. Combustion gases from the combustion chamber 163 pass through the right side of the porous torus 152, as viewed in Figure 7, through an outlet opening 165 in the lower wall of the wheel housing 148 and to an inlet to the turbine- expander 142 by way of a conduit 167. Combustion gases are discharged by the turbine expander 142 by an exhaust pipe 159.

Referring to Figures 9 and 10, the metering valve 161 includes a cylindrically shaped valve housing 168 in which a half cylinder shaped valve element 171 is rotatably mounted by a shaft 172 rotatably mounted in an end wall 173 of the valve housing 168 and in coaxial relation with the valve housing 168. The shaft 172 is rotated by an electric actuator 174 in response to signals from the automatic control 156, shown in Figure 7, to which it is connected by a lead 176. The valve elementl71 includes a flat longitudinally extending wall 177, a pair of half-moon shaped end walls 178,179 secured to longitudinally opposite ends of the flat wall 177 and a semi-cylindrical wall 181 secured to the flat wall 177 and the end walls 178, 179. The inner end of the shaft 172 is secured as by welding to the end wall 178. A pocket for the valve element 171 is formed inside the cylindrically shaped valve housing 168 by longitudinally spaced parallel walls 182, 183 which are half moon shaped with aligned straight edges lying substantially in a plane through the axis of the shaft 172. The walls 182,183 are perpendicular to the axis of the housing 168 and are welded to the interior surface of the housing 168 at their arcuate edges. A longitudinally extending flat plat or wall 184 has its longitudinally opposite ends welded to the moon shaped wall 182, 183 adjacent their aligned straight edges. The upper edge of the wall 184, as viewed in Figure 9, is welded to the interior surface of the housing 168 and extends radially inward from the interior surface of the housing 168 to approximately the axis of the housing 168. As shown in Figure 10, the valve element 171 may be rotated from its full line position, in which one half of the maximum flow through the valve is permitted, to a position shown in broken lines 186, in which only one fourth of the maximum permitted flow is allowed. When the valve element 171 is rotated to the position shown in broken lines 187, the valve element permits three-fourths of the maximum available flow and when the valve element 171 is rotated to the position shown by broken line 188, one hundred percent of the possible flow through the valve is permitted. In this last mentioned full flow position of the valve element 171, one half of the cross section of the valve housing 168 is blocked and the other half is available for flow of the fuel-air mixture to the inlet 191 to the combustion chamber 163, which is lined with a ceramic lining 192 which helps to maintain an even combustion temperature.

Referring again to Figure 7, temperature sensors 194, 196, 197 and 198 are mounted on the conduits 150, 162, 166 and 167, respectively, and connected to the automatic control 156 by leads 199, 201, 202 and 203. The fuel supply mechanism 145 is connected to the automatic control by lead 204 and electric power is supplied to the control from a source, not shown, by a lead 206. A photo-electric cell 208 is mounted on the inlet end of the metering valve housing 168 for detecting flash back. The photoelectric cell is connected in signal delivery relation to the automatic control 156 by a lead 209. Upon detection of a flash back, the control 156 adjusts the fuel delivery quantity to eliminate the flash back. The control 156 senses operating parameters of the gas turbine unit and automatically adjusts the speed of the ceramic wheel 151, the fuel supplied by the fuel delivery mechanism 145 and the flow rate through the metering valve 161 to achieve the desired level of power operation of the gas turbine unit without risk of flame blow out or flash back and without production of NOx above 1 ppm.

Figures 11 and 12 illustrate a modified form of a gas turbine combustion chamber 250 and an alternate flow control valve 254. A conduit 251 from the discharge of a compressor, not shown, is connected to a valve housing 252 of the valve 254, which in turn is secured to the combustion chamber 250. A photo cell 253 is mounted on the valve housing 252 for detecting flash back. The valve 254 includes a pivotable flow control element in the form of a flat plate 302 for controlling flow of the fuel air mixture through inlet 260 to the combustion chamber 250. The flat plate 302 is selectively rotated to various flow control positions by a shaft 256 interconnecting an electric actuator 255 and the plate 302 in response to signals from an automatic control system, not shown, to which the actuator 252 is connected by a lead, not shown. As shown in Figure 12, the plate 302 may be pivoted from the one half open position, in which it is shown in solid lines, to a full open position, shown by broken lines 304, or to a three- fourth open position, shown by broken lines 306. The automatic control and actuator 255 may be designed and constructed to pivot the plate 302 to various position between closed and full open. Refeπing again to Figure 11, the combustion chamber 250 has a lining 272 of refractory material which extends from its inlet 260 to its discharge conduit 253. A pair of porous ceramic walls or plates 270, 271, which are spherically shaped, are placed in series in the combustion chamber 250. The longitudinally space plates 270, 271 provide additional stability to an even temperature combustion of lean fuel-air mixtures. This invention has application in dual shaft turbines as well as single shaft turbines. The use of two heat exchangers to heat the fuel-air mixture en route from the compressor to the combustion chamber insures even temperature burning without production of hot spots, thus avoiding formation of objectionable quantities of NOx. Using the combustion gases to heat the incoming fuel-air mixture in the upstream heat exchanger advantageously reduces the temperature of the exhaust gases passing to the turbine-expander.

Claims

What is claimed is:

1. A gas turbine unit comprising: a compressor having an inlet for receiving combustion air and an outlet for discharge of compressed combustion air, a turbine-expander connected in driving relation to said compressor and having an inlet for receiving combustion gases and an exhaust, a combustion chamber having an inlet and an outlet, a first conduit means interconnecting said outlet of said compressor and said inlet of said combustion chamber, a second conduit means interconnecting said outlet of said combustion chamber and said inlet of said turbine-expander whereby combustion gases are delivered to said turbine-expander, fuel delivery means connected at a point of delivery to said gas turbine unit operable to deliver at least 85% of the operating fuel to said combustion air into or upstream of said compressor, a first heat exchanger operatively associated with said first conduit means downstream of said point of delivery and with said second conduit means whereby sufficient heat is transfe╧Çed from combustion gases passing from said combustion chamber to said turbine-expander to the mixture of fuel and air passing to said combustion chamber to raise the temperature of said fuel-air mixture to not less than 100 degrees F. below the auto ignition temperature of said fuel-air mixture is transfe╧Çed to said fuel-air mixture.

2. The gas turbine unit of claim 1 wherein said fuel delivery means delivers all the required operating fuel to the combustion air entering said inlet of said compressor.

3. The gas turbine unit of claim 1 wherein said first heat exchanger heats said fuel-air mixture to a temperature level more than 100 degrees above the auto ignition temperature of said fuel-air mixture.

4. The gas turbine unit of claim 1 wherein said first heat exchanger is a tube type heat exchanger having a housing plurality of tubes through which said fuel-air mixture pass en route to said combustion chamber and wherein combustion gases flow along the outside of said tubes en route to said turbine-expander

5. The gas turbine unit of claim 4 wherein said first heat exchanger includes a tube sheet supporting said tubes at their ends receiving said fuel-air mixture, said tubes being without support at their other ends

6. The gas turbine unit of claim 1 and further comp╧Çsing a second heat exchanger operatively associated with said first conduit means upstream of said first heat exchanger and downstream of said point of delivery of said fuel, said second heat exchanger being connected to said exhaust of said turbine-expander whereby heat from combustion gases exhausted by said turbine-expander is transfe╧Çed to said fuel-air mixture.

7 The gas turbine unit of claim 6 wherein said heat exchangers are tube type heat exchangers.

8. The gas turbine unit of claim 7 wherein said heat exchangers are in end to end se╧Çes relation and share a housing and a plurality of parallel tubes which are supported in said housing by a pair of tube sheets spaced from one another in the longitudinal direction of said tubes

9. The gas turbine unit of claim 1 wherein said first heat exchanger is a rotary heat exchanger including a cylindrically shaped housing having first and second axially opposite ends, a wheel including a disc of porous heat resistant material and a drive shaft rotatably supported in said housing m coaxial relation to said housing and said wheel, a first inlet opening in said first end of said housing, said first conduit means being connected to said first inlet opening, a first outlet opening in said second end of said housing, first flow directing means connecting said first outlet opening to said combustion chamber, a second inlet opemng in said second end of said housing, second flow directing means connecting said combustion chamber to said second inlet opening, a second outlet opening in said first end of said housing, said second conduit being connected to said second outlet opening, said first inlet opening being substantially aligned with said first outlet opening and said second inlet opening being substantially aligned with said second outlet opening, said first inlet opening and said first outlet opening being positioned on said housing in diametrically opposite relation to the position of said second inlet opening and said second outlet opening and variable speed power means connected to said shaft for rotating said wheel, said wheel when rotated during operation of said gas turbine unit being operative to transfer heat from combustion gases passing to said turbine-expander to said fuel-air mixture passing to said combustion chamber.

10. The gas turbine unit of claim 9 and further comprising an automatic control connected in controlling relation to said power means and including a temperature sensor operatively associated with said first conduit means, said automatic control being operable to automatically adjust the rotational speed of said wheel to heat said fuel-air mixture to at least within 200 degrees F, of its auto ignition temperature.

11. The gas turbine unit of claim 9 wherein said automatic control includes a photo electric cell operatively associated with the first conduit means for detecting flash back and wherein said automatic control automatically adjusts fuel delivery upon detection of a flash back.

12 The gas turbine unit of claim 9 wherein said fuel delivery device delivers fuel to the combustion air entering the inlet of said compressor.

13. The gas turbine unit of claim 9 wherein said fuel delivery device delivers fuel to an intermediate stage of said compressor.

14. The gas turbine unit of claim 1 and further comprising a metering valve including a valve housing having an inlet connected to said first conduit means downstream of said heat exchanger and having an outlet connected to said inlet of said combustion chamber, a valve element shiftably mounted in said valve housing for incremental adjustment from less than one-fourth open to full open positions.

15. The gas turbine unit of claim 14 and further comprising a power operated actuator for shifting said valve element and an automatic control connected in controlling relation to said power operated actuator and having sensors for monitoring operating parameters of said gas turbine unit, said control shifting said valve element in response to detecting predetermined changes in said operating parameters.

16. A metering valve operable to meter flow of a fuel air mixture to a combustion chamber of a gas turbine unit comprising; a cylindrically shaped valve housing having an inlet opening at one end and an outlet opening at its other end, a first half moon shaped plate secured to said outlet end of said valve housing at right angles to the axis of said valve housing, said first half moon shaped plate occupying substantially one half of the cross section area of said cylindrically shaped valve housing and presenting an inward facing edge at the diameter of said valve housing, a second half moon shaped plate secured to the interior of said valve housing in axially spaced and parallel relation to said first half moon shaped plate, said second half moon shaped plate being spaced from said inlet end of said valve housing and presenting an edge in coplanar relation to said inward facing edge of said first plate, a radially and longitudinally extending wall having its radially outer edge in abutting relation to the interior surface of said valve housing and its longitudinally opposite ends secured, respectively, to said edges of said plates, said wall terminating at its radially inward end near said axis, a valve element mounted in said valve housing for rotary movement about said axis including a half cylinder structure defined by a semi-cylinder shaped wall, a pair of half moon shaped end walls secured, respectively, to longitudinally opposite ends of said semi-cylinder shaped wall and a longitudinally extending flat wall secured to said semi-cylinder shaped wall and to said half moon shaped walls and a shaft rotatably mounted in said inlet end of said valve housing for rotation about said axis and having an inner end rigidly secured to said half cylinder structure, said valve element being rotatable between open and closed positions.