EP0317110B1

EP0317110B1 - Low nox cogeneration process

Info

Publication number: EP0317110B1
Application number: EP19880310217
Authority: EP
Inventors: Ronald D. Bell
Original assignee: Radian Corp
Current assignee: Radian Corp
Priority date: 1987-11-18
Filing date: 1988-10-31
Publication date: 1992-03-04
Also published as: ES2030871T3; DE3868865D1; JPH01193513A; EP0317110A3; EP0317110A2

Description

DESCRIPTION

This invention relates to cogeneration and is more particularly concerned with a cogeneration process which ensures low NOx content of the evolved gases.
Some combination processes generate effluent gases having an unacceptable NOx content. Thus, oxides of nitrogen are one of the principal contaminants emitted by combustion processes. In every combustion process, the high temperatures at the burner result in the fixation of some oxides of nitrogen. These compounds are found in stack gases mainly as nitric oxide (NO) with lesser amounts of nitrogen dioxide (NO₂) and only traces of other oxides. Since nitric oxide (NO) continues to oxidise to nitrogen dioxide (NO₂) in the air at ordinary temperatures, there is no way to predict with accuracy the amounts of each separately in vented gases at a given time. Thus, the total amount of nitric oxide (NO) plus nitrogen dioxide (NO₂) in a sample is determined and referred to as "oxides of nitrogen (NOx)".
Oxides of nitrogen emissions from stack gases, through atmospheric reactions, produce "smog" that stings eyes and causes acid rains. For these reasons, the content of oxides of nitrogen present in gases vented to the atmosphere is severely limited by various state and federal agencies.
Cogeneration is a process which emits stack gases of undesirable NOX content.
Cogeneration is the simultaneous production of both useful thermal energy (usually steam) and electrical energy from one source of fuel. One or more gas turbines followed by a waste heat boiler using natural gas as fuel for both the turbines and to heat the exhaust gases from the turbines represent a typical system.
In recent years, the cogeneration market has expanded rapidly due in part to the Public Utility Regulatory Policy Act of 1978. PURPA gave financial incentive to cogenerators that sell excess electrical power and requires that utilities purchase power from cogenerators. It also allows utilities to own up to 50% of a cogeneration facility and receive the benefits of this status.
One problem with this system is the level of NOX emissions generated with the combined firing cycle. Cogeneration plants using conventional gas turbines and auxiliary fuel fired heat recovery boilers to produce electricity and steam are being subjected to stringent NO_X emission standards requiring levels below the 150 ppm range. New Source Performance Standards (NSPS) strictly limit NOX emission. To meet the regulations for NOX emissions, several methods of NOX control have been employed. These can be classified as either equipment modifications or injection methods. Injection methods include injection of either water or steam into the combustion zone to lower the flame temperature and retard the formation of NOX, since the amount of NOX formed generally increases with increasing temperatures, or injection of ammonia to selectively reduce NOX. Water or steam injection, however, adversely affects the overall fuel efficiency of the process as energy is absorbed to vaporize the water or heat the steam that otherwise would go toward heating the turbine gas and be ultimately converted into usable electricity or steam. A process involving the injection of ammonia into the products of combustion is shown, for example, in Welty, U.S 4,164,546. Examples of processes utilizing ammonia injection and a reducing catalyst are disclosed in Sakari et al, U.S. 4,106,286; and Haeflich, U.S. 4,572,110. Selective reduction methods using ammonia injection are expensive and somewhat difficult to control. Thus, these methods have the inherent problem of requiring that the ammonia injection be carefully controlled so as not to inject too much and create a possible emission problem by emitting excess levels of ammonia. In addition the temperature necessary for the reduction of the oxides of nitrogen must be carefully controlled to get the required reaction rates.
Equipment modifications include modifications to the burner or firebox to reduce the formation of NOX. Although these methods do reduce the level of NOX, each has its own drawbacks. Combustion equipment modification affects the performance of the turbines and limits the range of operation. Moreover, cogeneration plants of this type employed for generating electric power and steam are being subjected to increasingly stringent NOX emission standards, and a statisfactory emission control system is required to minimize the undesirable emissions exhausted to the atmosphere. A selective catalytic reduction system is presently considered by some authorities to be the best available control technology for the reduction of NOX from the exhaust gas of a cogeneration plant, and as a consequence is required equipment. Currently available selective catalytic reduction systems used for the reduction of NOX employ ammonia injection into the exhaust gas stream for reaction with the NOX in the presence of a catalyst to produce nitrogen and water vapor. Such systems typically have an efficiency of 80-90 percent when the exhaust gas stream is at temperature within a temperature range of approximately 316°-371°C (600°-700°F). The NOX reduction efficiency of the system will be significantly less if the temperature is outside the stated temperature range and the catalyst may be damaged at higher temperatures.
The turbine exhaust temperature of most gas turbine cogeneration plants, at full or rated load of the gas turbine engine, is conventionally between 413°C (775°F) and 566°C (1050°F). Since the exhaust gas temperature is above the optimum temperature range of the usual selective catalytic reduction system, it is necessary to reduce the temperature of the exhaust gas stream before it passes through the system. Current practice is to provide steam superheater and/or steam generating tubes upstream to cool the gas to a preselected desired nominal temperature before it passes through the system. This imposes various operating limitations on the cogeneration plant which either seriously limit the operating range of the gas turbine engine or require an undesirable exhaust gas bypass or other mechanism for diverting a portion of the exhaust gas stream. Where supplementary firing is provided to increase steam production, the supplementary firing is conventionally carried out with an excess of air.
In EP-A-0 047 346, a unitary compact structure is disclosed in which fuel is burned for the reduction of gases and complete high temperature and low temperature heat recovery is accomplished in the same structure. The device is especially characterised in that total longitudinal flow of gases is broken up into shorter sections so that the successive sections can be changed in direction, whereby common walls can be provided between two sections and a smaller overall volumne required for the total structure.
US-A-4 706 612 discloses a process which comprises combusting fuel to produce a gaseous stream of combustion products, passing said gaseous stream to a turbine to generate electricity, and to produce a gaseous exhaust stream, adding fuel to said gaseous exhaust stream from the turbine and passing said gaseous exhaust stream to a NOx emission reduction unit, removing heat from said treated stream, and venting the resultant cooled stream to the atmosphere.
In a first aspect, the invention provides for a process for reducing the NOx level in the exhaust gases from a cogeneration plant, and the process of the present invention is characterised in that, said additional fuel is added to said exhaust stream at exit from the turbine to provide a fuel-rich combustible gas stream; and said treatment in the NOx emission reduction unit comprises the steps of: combusting or catalytically treating said fuel-rich combustible gas stream in a reducing artmosphere to produce a heated, oxygen depleted gaseous stream, using at least a portion of the heat in said oxygen-depleted gaseous stream to convert water into steam, adding additional air to said oxygen-depleted gaseous stream to produce a stoichiometric excess of oxygen in the resultant stream relative to fuel present therein, and passing said resultant stream over a catalytic reactor containing an oxidising catalyst to produce an oxidised gaseous stream.
In a second aspect, the present invention provides for a cogeneration system having reduced levels of NOx in its exhaust gases, comprising fuel supply means arranged to supply fuel at the exit from the turbine; and said NOx emission reduction unit comprises: combustion means or a catalytic bed for treating said fuel-rich combustible gas stream in a reducing atmosphere to produce a heated, oxygen-depleted gaseous stream, a boiler, connected to receive said heated, oxygen-depleted gaseous stream and having means to output a cooled, oxygen-depleted gaseous stream, for converting water into steam, air supply means for adding additional air to said cooled, oxygen-depleted gaseous stream to produce a stoichiometric excess of oxygen in the resultant stream relative to fuel present therein, and a catalytic reactor, containing an oxidising catalyst, connected to receive said resultant stream and to the heat exchanger downstream of the NOx emission reducing unit, for producing an oxidised gaseous stream from said resultant stream.
An advantage offered by the present invention is the ability to reduce the level of NOx in the exhaust from a cogeneration system to low levels without the use of ammonia. Furthermore, the NOx emissions are controlled without adversely affecting the operation of the turbine, including its fuel efficiency. Consequently, a cogeneration system according to the present invention is more economical and more readily controlled than prior art systems.
In order that the present invention may be fully understood, embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which:-

Figure 1 is a diagramatic flow sheet of a cogeneration system according to the present invention;
Figure 2 is a diagramatic flow sheet of a cogeneration system embodying a gas turbine showing a novel method of handling the exhaust from the turbine; and
Figure 3 is a similar flow sheet showing recirculation of the gas.

Referring now to Figure 1, (the drawings), the reference numeral 10 designates a combustor or burner which receives fuel such as gas or naptha and compressed air and bums the air-fuel mixture to produce a gaseous effluent which passes into a duct 12 which directs it to a gas turbine 14 which is coupled to a generator (not shown), to produce electrical power. The turbine exhaust gas leaves through a duct 16 into which are introduced further amounts of fuel, the amount depending upon the fuel-air ratio in the exhaust gas from the turbine. Since there will be ample air, only fuel is injected at this point. The amount of fuel added is selected so that there will be 10 to 25% stoichiometric excess fuel relative to the available oxygen in the exhaust gas from the gas turbine. The fuel added is ordinarily gas, such as natural gas. Thus, the exhaust gaseous stream from the turbine is treated, i.e., has fuel added to it, to produce a fuel-rich, fuel-air mixture containing 10% to 25% excess of fuel over the oxygen stoichiometrically present. The thus-treated exhaust gas from the turbine is then passed to an afterburner 18 wherein it is burned at a temperature of about 1093 to 1649°C (2000 to 3000°F). A residence time of 0.5 seconds is required to ensure that the desired reduction of the oxides of nitrogen will occur. A greater residence time can be employed, e.g., 1 minute or more, but serves no useful purpose. Alternatively, as seen in Figure 2, the afterburner of unit 18 can be replaced by a reducing catalytic treatment. Thus, the fuel-enriched exhaust gas from the turbine at about 427°C (800°F) to about 566°C (1050°F) is passed to a catalytic treatment zone 18, wherein the fuel-rich stream is passed over a reducing catalyst, e.g., platinum-rhodium in the zero-valent state supported on a carrier such as alumina, silica or a metal alloy. The making of such catalysts is well known to persons skilled in the art. Catalyst volumes will vary depending on the particular catalyst used. Ordinarily, the quantity of catalyst and the flow rate are such that the space velocity is typically in the range of 30,000 to 50,000 hr.⁻¹ preferably 30,000 to 40,000 hr.⁻¹.
As mentioned, Applicatn has disclosed in Mc Gill et al 4,405,587, oxides of nitrogen can be reduced by reaction in a reducing atmosphere at temperatures in excess of 1093°C (2000°F), for example 1093° to 1649°C (2000° to 3000°F). Combining this observation with a gas turbine operation to generate electricity, maximum utilization of the turbine exhaust as useful heat to generate steam can be achieved. Products from the afterburner or the catalytic treatment zone 18 pass to and through a waste-heat boiler wherein they are cooled to a temperature of 260-316°C (500-600°F). Thus, the heated gaseous stream passes into a duct 20 and is led to a waste-heat boiler 22 which effectively utilizes the heated gaseous stream to produce steam and simultaneously to cool the stream. The gaseous effluent from the catalytic-treatment step, when it is used, which is ordinarily at a temperature of 538-760°C (1000-1400°F), passes, as mentioned, to and through a waste-heat boiler wherein the effluent is cooled to a temperature of about 260-316°C (500-600°F). The afterburner 18 when used and the waste-heat boiler 22 can, of course, be combined in the form of a fuel-burning boiler wherein the added fuel and the exhaust gas from the turbine are combusted to produce steam directly.
In any case, the boiler 22 discharges a waste effluent gas into a duct 24. Because of the addition of fuel to the duct 16 and the burning or catalytic treatment of the turbine exhaust gas in the presence of this fuel with significantly less than the stoichiometric requirement of oxygen, i.e., under reducing conditions, the exhaust gas in duct 24 from the boiler 22 contains not only combustion gases, but some unburned fuel. It is, however, low in NOX and the treatment of the gases flowing through the system has brought about a reduction of any NOX formed, or a suppression of the formation of the NOX, without the use of ammonia or like treating system widely used in the prior art. In order, however, to utilize to a maximum the heat potential of the gas and any fuel which it may contain, air is added to the stream in conduit 24 and the resulting gaseous stream is passed to a gas treatment unit 26 wherein the gas stream is passed over an oxidizing catalyst. The amount of air is added in an amount relative to the stream in conduit 24 such that the resulting stream will contain oxygen soichiometrically in excess of the amount needed to burn any fuel which may be present in the stream, e.g., 10% to 50% excess. Thus products at the boiler discharge temperature, e.g., 260-316°C (500°-600°F) are mixed with air and passed over an oxidizing catalyst. Either noble metal catalysts such as platinum or palladium or base metal oxides, such as copper oxide, chrome oxide, or manganese oxide, or the like, may be used for this purpose. The noble metal catalysts, e.g., platinum or palladium catalysts, are most suitably the noble metals deposited in the zero valent state upon a support, such as alumina, silica, kiesel-guhr, or a metal alloy, and the like. The metal oxide catalysts are also most suitably the metal oxides supported on supports of this character. The making of such catalysts is well known to persons skilled in the art. Catalyst volumes will vary depending on the particular catalyst used. Ordinarily, the quantity of catalyst and the flow rate are such that the space velocity is typically in the range of 30,000 to 50,000 hr.⁻¹. Data indicate that NOX levels in the parts per billion range can be realized by the combined reduction-oxidation operations of this invention. The oxidized gaseous effluent from the unit 26 passes into a conduit 27 which leads an economizer or a low-pressure, waste-heat boiler, or the like, indicated at 28, and the heat content of the oxidized gaseous effluent is extracted to the maximum amount economically feasible. The cooled gas at a temperature of about 149 to 204°C (300 to 400°F) is then discharged through an outlet conduit 30 into a stack 32 and vented to the atmosphere with the assurance that the vented effluent will comply with NOX emission standards. It will have a NOX content of less than 50 ppm.
In a preferred form of the present invention, as seen in Figure 3, treated gaseous effluent from conduit 30, i.e., the low NOX effluent following passage of the gas and the subsequent heat removal in the economizer, is cycled to combustor 10 wherein the gas stream to be fed to turbine 14 is prepared. Thus, as seen in Figure 3 of the drawing, when effluent, e.g. flue gas, is to be re-cycled, an appropriate valve (not shown) controls the recycle rate. At least a portion of the effluent is diverted into line 36 which conducts the diverted effluent to combustor 10. In turbine operation, air is added to the combustor for combustion of the fuel and a large excess of air is also added to keep the flame temperature in the combustor from becoming so high that the generated gas will damage the blades of the turbine. It has been observed that the recycled effluent gas acts as a heat sink in the combustor and makes it possible readily to prevent the turbine from over-heating. Thus, the large excess of air ordinarily added to control the flame temperature can be eliminated so that there is less oxygen in the effluent gas and there is an important saving of fuel needed to produce reducing conditions.
As mentioned, in view of the presence of cycled effluent in combustor 10, the oxygen content of the turbine exhaust will be significantly lower, which will correspondingly lower the fuel requirement for the subsequent reducing step. The quantity of effluent diverted can vary but, for best results the quantity of recycled effluent, e.g. flue gas, added to the combustor will be such as is required to produce a turbine exhaust at a temperature of 427-538°C (800-1000°F) with 1-2% O₂.
One aspect of the invention is that the gas turbine 14 furnishes the total of the combustion-supporting air for the afterburner 18, if one is used, and that care is taken to maintain reducing conditions during this combustion, or during the catalytic treatment at 18, by appropriate control of the supply of fuel. Another aspect of the invention is that heat recovery in a turbine cogeneration system is maximized in a higly economical manner and that NOX content is kept at a minimum without resort to elaborate equipment reconstruction, without heat loss by injecting water into the exhaust gases from the turbine, and without ammonia injection or catalytic reduction in the presence of ammonia.
It will, of course, be understood in the foregoing description of the drawing, reference to a combustor or heater, to turbine, afterburner, boiler, waste-heat boiler, economizer, gas treatment unit, and the like, can utilize standard equipment well known to persons skilled in the art. The gas treatment units, for example, can be any containers adapted for gas passage and containing an appropriate catalyst. The turbine 14, for example, can be of the type which produces substantially the same quantity of exhaust gas throughout the range of its operation, as, for example, a single cycle, single shaft gas turbine.
Minimizing the formation of oxides of nitrogen in cogeneration, in accordance with the invention, offers several advantages over the current state of the art. This process does not require that a potentially obnoxious gas, such as ammonia, be injected into the system; the reaction conditions do not require that a narrowly-controlled temperature be maintained for the reduction of oxides of nitrogen to occur; the operating conditions are compatible with conventional cogeneration conditions; and greater NOX reduction efficiencies can be achieved.
The following examples will serve more fully to illustrate the features of the invention.

EXAMPLE I

In a typical operation, utilizing an afterburner, a combustor is fed with natural gas to produce a combustible mixture which is combusted at a temperature of 2000°F, to produce a stream of combustion products which are fed to a turbine to generate electricity. The exhaust stream from the turbine, at a temperature of 427°C (800°F). contains about 14% oxygen. Natural gas at ambient temperature is injected into this exhaust stream to give the resultant stream a fuel content such that the fuel is 10% in stoichiometric excess relative to the oxygen present. The resultant stream is then combusted at a temperature of 1816°C (3300°F). and since the fuel is in excess, the combustion takes place in a reducing atmosphere. Heat present in the combustion products is at least partially converted into steam by heat exchange with water, e.g., in boiler tubes, and the resulting gaseous stream, which is of course, oxygen depledted, has a temperature of 260°C (500°F). To this oxygen-depleted stream is then added air at ambient temperature in an amount such that the resultant stream has an oxygen content which is 50% stoichiometrically in excess relative to any fuel present in the oxygen-depleted stream to which the air is added. The resultant oxygen-rich stream is then fed through a bed of platinum black supported on alumina with a space velocity of 50,000 hr.⁻¹. At this point the gaseous stream being processed has a temperature of 260°C (500°F). This temperature increases across the catalyst bed to about 399°C (750°F). Heat is then extracted by appropriate heat exchange to leave a final stream to be vented having a temperature of about 177°C (350°F). and a NOX content of less than 50ppm.

EXAMPLE 2

In another typical operation, a combustor is fed with natural gas to produce a combustible mixture which is combusted at a temperature of 427-538°C (800-1000°F). to produce a stream of combustion products which are fed to a turbine to generate electricity. The exhaust stream from the turbine, at a temperature of 800°F. contains about 14% oxygen. Natural gas at ambient temperature is injected into this exhaust stream to give the resultant stream a fuel content such that the fuel is 10% in stoichiometric excess relative to the oxygen present. The resultant stream is then passed over a platinum-rhodium catalyst (<1% supported on alumina) at a space velocity of 40,000 hr.⁻¹ and, since the fuel is in excess, the treatment takes place in a reducing atmosphere. This catalytic treatment causes the temperature of the stream to rise to 1400°F. Heat present in the combustion products is at least partially converted into steam by heat exchange with water, e.g., in boiler tubes, and the resulting gaseous stream, which is of course, oxygen depleted, has a temperature of 260°C (500°F). To this oxygen-depleted stream is then added air at ambient temperature in an amount such that the resultant stream has an oxygen content which is 25-50% stoichiometrically in excess relative to any fuel present in the oxygen depleted stream to which the air is added. The resultant oxygen-rich stream is then fed through a bed of platinum (1<% supported on alumina) with a space velocity of 50,000 hr.⁻¹. At this point the gaseous stream being processed has a temperature of 260°C (500°F). This temperature increases across the catalyst bed to about 399°C (750°F). Heat is then extracted by appropriate heat exchange to leave a final stream to be vented having a temperature of about 177°C (350°F). and a NOX content of less than 50 ppm.

EXAMPLE 3

In another operation, a combustor is fed with natural gas an combustion air to produce a combustible mixture which is combusted at a temperature of 927°C (1700°F). to produce a stream of combustion products which are fed to a turbine to generate electricity. The exhaust stream from the turbine, at a temperature of 427-538°C (800-1000°F). contains about 14% oxygen. Natural gas at ambient temperature is injected into this exhaust stream to give the resultant stream a fuel content such that the fuel is 10% in stoichiometric excess relative to the oxygen present. The resultant stream is then combusted at a temperature of 1816°C (3300°F). and, since the fuel is in excess, the combustion takes place in a reducing atmosphere. Heat present in the combustion products is at least partially converted into steam by heat exchange with water, e.g., in boiler tubes, and the resulting gaseous stream, which is of course, oxygen depledted, has a temperature of 260°C (500°F). To this oxygen-deplented stream is then added air at ambient temperature in an amount such that the resultant stream has an oxygen content which is 25-50% stoichiometrically in excess relative to any fuel present in the oxygen-depleted stream to which the air is added. The resultant oxygen-rich stream is then fed through a bed of platinum black (<1% supported on alumina) with a space velocity of 50,000 hr.⁻¹. At this point the gaseous stream being processed has a temperature of 260°C (500°F). This temperature increases across the catalyst bed to about 399°C (750°F). Heat is then extracted by appropriate heat exchange to leave a final stream to be vented having a temperature of about 177°C (350°F). and a NOX content of less than 50 ppm. In order to regulate combustion in the combustor preceding the turbine so that the gaseous effluent fed to the turbine is at a temperature of 927°C (1700°F)., 60-65% of the final effluent stream is cycled to provide a ratio of 1.75:1.0 of recycled flue gas to combustion gases.

EXAMPLE 4

In still another operation, a combustor is fed with natural gas to produce a combustible mixture which is combusted at a temperature of 927°C (1700°F). to produce a stream of combustion products which are fed to a turbine to generate electricity. The exhaust stream from the turbine, at a temperature of 427°C (800°F). contains about 14% oxygen. Natural gas at ambient temperature is injected into this exhaust stream to give the resultant stream a fuel content such that the fuel is 10% in stoichiometric excess relative to the oxygen present. The resultant stream is then passed over platinum-rhodium (<1% supported on alumina) at a space velocity of 30,000 hr.⁻¹ and, since the fuel is in excess, the treatment takes place in a reducing atmosphere. This catalytic treatment causes the temperature of the stream to rise to 760°C (1400°F). Heat present in the combustion products in at least partially converted into steam by heat exchange with water, e.g., in boiler tubes, and the resulting gaseous stream, which is of course, oxygen depleted, has a temperature of 260°C (500°F). To this oxygen-depleted stream is then added air at ambient temperature in an amount such that the resultant stream has an oxygen content which is 50% stoichiometrically in excess relative to any fuel present in the oxygen-depleted stream to which the air is added. The resultant oxygen-rich stream is then fed through a bed of platinum black (<1% supported on alumina) with a space velocity of 50,000 hr.⁻¹. At this point the gaseous stream being processed has a temperature of 260°C (500°F). This temperature increases across the catalyst bed to about 399°C (750°F). Heat is then extracted by appropriate heat exchange to leave a final stream to be vented having a temperature of about 177°C (350°F). and a NOX content of less than 50 ppm. In order to regulate combustion in the combustor preceding the turbine so that the gaseous effluent fed to the turbine is at a temperature of 927°C (1700°F)., 65% of the final effluent stream is cycled to provide a ratio of 1.75:1.0 of cycled effluent to combustion gases.

Claims

1. A process which comprises combusting fuel to produce a gaseous stream of combustion products, passing said gaseous stream to a turbine (14) to generate electricity, and to produce a gaseous exhaust stream (16), adding fuel to said gaseous exhaust stream (16) from the turbine (14) and passing said gaseous exhaust stream (16) to a NOx emission reduction unit (18, 22, 26) removing heat from said treated stream, and venting the resultant cooled stream to the atmosphere;
characterised in that
said additional fuel is added to said exhaust stream (16) at exit from the turbine (14) to provide a fuel-rich combustible gas stream;
said treatment in the NOx emission reduction unit (18, 22, 26) comprises the step of:
combusting or catalytically treating said fuel-rich combustible gas stream in a reducing atmosphere to produce a heated oxygen-depleted gaseous stream (20), using at least a portion of the heat in said oxygen-depleted stream to convert water into steam, adding additional air to said oxygen-depleted stream (24) to produce a stoichiometric excess of oxygen in the resultant stream relative to fuel present in said resultant stream, passing said resultant stream over a catalytic reactor (26) containing an oxidising catalyst to produce an oxidised gaseous stream.

2. A process as defined in claim 1 or claim 2, wherein the gaseous exhaust stream (16) at the exit from the turbine (14) is at a temperature 413 to 566°C (775 to 1050°F).

3. A process as defined in claim 1 or 2, wherein said fuel is added to said gaseous exhaust steam (16) in an amount 10% to 25% stoichiometrically in excess of the oxygen present in the resultant combustible gas stream.

4. A process as defined in claim 1, 2 or 3, wherein said combustible gas stream is combusted at a temperature of 1093 to 1649°C (2000 to 3000°F).

5. A process as defined in claim 4 wherein said combustible gas stream has a residence time 0.5 second during its combustion.

6. A process as defined in any proceeding claim, wherein said fuel-rich stream is combusted.

7. A process as defined in any preceding claim, wherein said fuel-rich stream is catalytically treated.

8. A process as defined in claim 7, wherein said fuel-rich gas stream is catalytically reated at a temperature of 427 to 566°C (800 to 1050°F).

9. A process as defined in claim 8 wherein the space velocity of aid fuel-rich gas stream while being catalytically treated is about 30,000 to 50,000 hr.⁻¹.

10. A process as defined in any preceding claim, wherein said oxygen-depleted stream is cooled to a temperature of about 316 to 371°C (500 to 600°F) during said conversion of the water to steam.

11. A process as defined in any preceding claim, wherein the space velocity of said resultant stream passing over said oxidising catalyst is about 30,000 to 50,000 hr.⁻¹.

12. A process as defined in any preceding claim, wherein said air is added to said oxygen-depleted stream (20) is an amount to provide a stoichiometric excess of oxygen present in the resultant stream of 10 to 25%.

13. A process as defined in any preceding claim, wherein the cooled gas (30) vented to the atmosphere is at a temperature of about 177 to 260°C (350 to 500°F).

14. A process as defined in any preceding claim, wherein the cooled gas (30) vented to the atmosphere has a NOx content less than 50 ppm.

15. A process as defined in any preceding claim further including the step of cycling at least a portion of said oxidised stream to said step of combusting fuel.

16. A process as defined in claim 15, wherein said cycled gas stream is 60 to 65% of said oxidised stream.

17. A cogeneration system, comprising combustion means (10) for combusting fuel and producing a gaseous stream (12) of combustion products; a turbine (14) connected to receive the gaseous stream (12) from said combustion means (10), generate electricity, and produce a gaseous exhaust stream (16); fuel supply means for adding fuel to said gaseous exhaust stream (16) from the turbine (14) to provide a fuel-rich, combustible gas stream; a NOx emission reduction unit (18, 22, 26) connected downstream of said turbine (14) to receive said gaseous exhaust stream (16) and added fuel; a heat exchanger (28) connected downstream of said NOx emission reduction unit for removing heat from said treated stream (27); and a vent (32) connected downstream of said heat exchanger (28) for venting the resultant tooled stream (30) to the atmosphere;
characterised in that,
said fuel supply means is arranged to supply fuel at the exit from the turbine (14); and
said NOx emission reduction unit (18, 22, 26) comprises:

- further combustion means (18) or a catalytic bed (18) for treating said fuel-rich combustible gas stream (16) in a reducing atmosphere to produce a heated, oxygen-depleted gaseous stream (20),

- a boiler (22), connected to receive said heated, oxygen-depleted gaseous stream (20) and having means to output a cooled, oxygen-depleted gaseous stream (24), for converting water into steam,

- air supply means for adding additional air to said cooled, oxygen-depleted gaseous stream (24) to produce a stoichiometric excess of oxygen in the resultant stream (24) relative to fuel present therein, and

- a catalytic reactor (26), containing an oxidising catalyst, connected to receive said resultant stream and to the heat exchanger (28) downstream of the NOx emission reducing unit (18, 22, 26), for producing an oxidised gaseous stream (27) from said resultant stream (24).

18. A cogeneration system as defined in claim 17, wherein said vent (30) is a stack (30).

19. A cogeneration system as defined in claim 17 or 18, further including gas cycling means (36) for cycling at least a portion of said oxidised gaseous stream (27) to said combustion means (10).