Patent Application
20060096297
The invention relates to a method and an apparatus for operating a burner of a heat engine, in particular of a gas turbine installation, having a burner inlet, to which a mixture of a fuel and an oxygen-enriched carrier gas is fed for combustion within a combustion chamber which adjoins the burner inlet in the direction of flow.
As part of the desire to reduce the emission of greenhouse gases into the atmosphere, considerable efforts are being undertaken in particular in the field of power generation to reduce in particular the emission of CO2 as a result of the combustion of fossil fuels. In this context, development work is being carried out on a gas turbine concept for generating electrical power which is known by the acronym “AZEP” (advanced zero emissions power) to achieve combustion of fossil fuels virtually without any nitrogen oxides and, moreover, offers the possibility of completely avoiding the free discharge of CO2 into the atmosphere.
The abovementioned gas turbine concept is based on the combustion of fossil fuels, in particular gaseous fuels, such as for example methane, in the presence of a mixture of recirculated exhaust gas, in which CO2, H2O and oxygen are used as oxidizing agents and not, as in conventional combustion processes, to form a fuel/air mixture which contains considerable quantities of nitrogen.
With the proposed combustion concept based on the AZEP principle, it is possible, for example during combustion of methane (CH4) as the gaseous fossil fuel used in the presence of oxygen as oxidizing agent, to obtain substantially only CO2 and H2O as combustion products. The water content contained in the exhaust-gas stream can if necessary be completely separated from the exhaust-gas stream with the aid of condensers which are simple and inexpensive to implement, so that ultimately it is possible to obtain an exhaust-gas stream which is made up of virtually pure CO2, assuming that the combustion is complete and any CO is oxidized to CO2 in the presence of oxygen. The CO2 exhaust-gas stream is in part fed back to the gas turbine process as part of part-stream recirculation as CO2 mass flow for the combustion, while the remaining residual exhaust-gas stream is taken for suitable utilization or is converted into compressed form for final land filling in suitable geological strata, which means that it is completely withdrawn from acting as a harmful greenhouse gas in the open atmosphere.
Successful and inexpensive implementation of the AZEP gas turbine concept requires a reliable and inexpensive method of producing oxygen. What are known as oxygen separation devices, which are equipped with a membrane which conducts oxygen ions and electrons, known as an MCM (mixed conducting membrane), are suitable for this purpose. A preferred apparatus for producing and preparing an oxygen-enriched carrier gas stream which, having been mixed with fuel, is ignited and burnt within a combustion chamber, is described in EP 1 197 257 A1, in which a partial exhaust-gas stream which emerges from the combustion chamber is introduced into an oxygen separation device and emerges in the form of an oxygen-enriched carrier gas stream. Since the oxygen-enriched carrier gas stream consists exclusively of CO2, H2O and pure oxygen as oxidizer, this mass flow is also referred to below as the oxidizer mixture.
The oxidizer mixture obtained with the aid of the oxygen separation device is then mixed with gaseous fuel, preferably CH4, and, as a fuel/oxidizer mixture, ignited for combustion within the combustion chamber.
To increase the efficiency of the oxygen separation device, it is important to produce the maximum possible driving gradient between the permeate side and retentate side of the MCM membrane. Therefore, the recirculated exhaust-gas stream which is to be enriched with oxygen, on the permeate side, is passed through the oxygen separation device at a high flow rate along the MCM membrane, with the result that the oxygen concentration on the permeate side is reduced and the driving gradient between the retentate side, through which a preheated, oxygen-containing gas is flowing, and the permeate side is increased. Despite the oxygen enrichment, the oxygen content within the oxygen-enriched carrier gas stream which emerges from the oxygen separation device is too low to achieve stable and effective combustion of the fuel/oxidizer mixture which forms with the aid of conventional combustion techniques.
Even using diffusion burners, which are known for a high combustion stability even at an oxygen ratio of λ=1 (λ indicates the ratio of actual oxygen supply to the minimum oxygen demand for complete combustion), the highly dilute oxygen content within the oxidizer mixture means that the combustion results which can be achieved are unsatisfactory. In particular, it has been found that the ignitability of a fuel/oxidizer mixture of this type, even at mixture temperatures of between 400° C. and 750° C., using premix burner techniques which are known per se, is much lower than in cases using conventional fuel/air mixtures. Moreover, it has been found that the ignition delay times of a fuel/oxidizer mixture using the AZEP technique are at least an order of magnitude greater than those which are measured when using conventional combustion techniques. Furthermore, the time-delayed ignition behavior also has a detrimental effect on the residence times required for complete combustion of the ignited fuel/oxidizer mixture within the combustion chamber, and moreover the CO and unburnt hydrocarbon contents which are formed during the combustion are well above the levels obtained with conventional combustion technology. Finally, the reduced ignitability or reactivity of the fuel/oxidizer mixture also has a reducing action on the flame velocity, with the result that what are known as the lean extinction limits are reduced.
The generally critical effects on the combustion behavior when using a fuel/oxidizer mixture which have been listed above are even more serious if account is taken of the fact that the maximum temperature of the exhaust-gas products formed as a result of the combustion should be relatively low, for example at most 1250° C., in order to avoid damage to the components provided along the exhaust-gas recirculation path, such as for example MCM membrane within the oxygen separation device and the high-temperature heat exchanger for preheating the oxygen-containing gas to be fed to the oxygen separation device on the retentate side.
It is also important to keep the thermal stresses, caused by the combustion process, on the individual components which come into contact with the exhaust gases at a substantially constant, relatively low temperature level (less than 1250° C.), in order to be able to substantially preclude thermally induced material degradation within the individual components. For this reason, conventional premix combustion methods, which are based exclusively on aerodynamic stabilizing effects and require the use of very hot flame temperatures, are unsuitable.
Although the use of catalytic combustion techniques is able to help with maintaining the required temperature conditions, the steam which is formed within the exhaust gases, under lean fuel mixture conditions, contributes to increasing the ignition temperature for the catalytic oxidation of the fuel. In order nevertheless to achieve increased ignition temperatures within the fuel/oxidizer mixture, it is necessary to ensure that the temperature of the oxidizer mixture which is mixed with the fuel upstream of the burner inlet is increased, with the result that the temperature level of the recirculating exhaust-gas stream automatically has to be increased. In this context, it ultimately remains a problem that the catalyst material which is required for catalytic oxidation of the fuel, typically platinum or palladium, is applied to support materials, such as for example aluminum oxide, silicon oxide or zirconium oxide, which are or start to become unstable at such high temperatures with a steam content of greater than 50%.
The invention is based on the object of developing a method and an apparatus for operating a burner of a heat engine, in particular of a gas turbine installation, having a burner inlet, to which a mixture of a fuel and an oxygen-enriched carrier gas is fed for combustion within a combustion chamber which adjoins the burner inlet in the direction of flow, in such a manner that the drawbacks and technical difficulties which have been mentioned above in connection with the prior art can be avoided. In particular, it is to be possible, using conventional combustion techniques, to provide a working gas for operation of a burner based on the AZEP principle which allows stable and complete combustion within the combustion chamber while complying with all the temperature limits imposed by the components required to provide the working gas.
The solution to the object on which the invention is based is described in claim 1 and claim 10. Features which refine the concept of the invention in an advantageous way form the subject matter of the subclaims and are also to be found in the description with reference to the exemplary embodiments.
As was mentioned in the introduction, the reduced ignitability or reactivity of the fuel/oxidizer mixture on account of the highly dilute oxygen content within the oxidizer mixture contributes to unsatisfactory results being achieved during the combustion operation with the aid of burner techniques which are known per se. To avoid all the negative effects on the combustion process of the fuel/oxidizer mixture provided which have been described above, the method in accordance with the preamble of patent claim 1 according to the invention provides the following method steps:
a first oxidizer mixture, i.e. an oxygen-enriched carrier gas stream, which is obtained in a manner known per se, as mentioned above, is provided upstream of the burner inlet. Fuel, preferably gaseous fossil fuel, such as for example methane, is admixed with this first oxidizer mixture to form a first fuel/oxidizer mixture. A second oxidizer mixture, the composition of which is identical to that of the first oxidizer mixture, is likewise provided.
In a second method step, the first fuel/oxidizer mixture is fed to a catalytic converter unit, within which the fuel content contained in the first fuel/oxidizer mixture is at least partially catalytically oxidized. If the gaseous fossil fuel used is methane (CH4), the catalyst products formed are hydrogen H2 and substantially steam and carbon dioxide (CO2). In particular the formation of hydrogen subsequently contributes to drastically increasing the reactivity of the fuel/oxidizer mixture, which has an advantageous effect in terms of shortening the ignition time of the fuel/oxidizer mixture. However, before the catalyzed, first fuel/oxidizer mixture is ignited and burnt, it is mixed with the second oxidizer mixture which was initially provided, to form a second fuel/oxidizer mixture. Only after the second fuel/oxidizer mixture has formed is the latter ignited and burnt within the combustion chamber.
The method according to the invention therefore provides a combination of catalysis of a fuel-rich or rich fuel/oxidizer mixture to form reactive hydrogen and subsequent combustion of a lean fuel/oxidizer mixture, for example within a conventional premix burner. The lean depletion of the catalyzed, first fuel/oxidizer mixture is effected by admixing the second oxidizer mixture which has been provided, with the result that the fuel content by volume is reduced.
The measures of the invention lead to a number of advantages: the ignition temperature of the catalytically oxidized fuel, which is fed to a catalytic converter unit in the form of a rich fuel/oxidizer mixture, is significantly reduced. The reduction in the ignition temperature, as well as the stabilization of the flame front which is formed within a combustion chamber downstream of a premix burner, is attributable to the formation of hydrogen which is formed during the catalytic oxidation of the fuel CH4.
Furthermore, an apparatus for operating a burner of a heat engine, in particular of a gas turbine installation, having the features of the preamble of claim 10, is used to carry out the method according to the invention, which apparatus according to the invention is distinguished by the fact that at least one means for feeding in the oxidizer mixture and a fuel feed unit are provided in the region of the burner inlet. The means for feeding in the oxidizer mixture and the fuel feed unit are arranged in the region of the burner inlet, in such a manner that substantially complete mixing between the gaseous fuel and the oxidizer mixture supplied is ensured. Furthermore, a catalytic converter unit, through which the fuel/oxidizer mixture which forms flows, is provided in the region of the burner inlet downstream of the means for feeding in oxidizer mixture and of the fuel feed unit. Finally, there is at least one bypass line which bypasses or passes through the catalytic converter unit, connecting the region of the burner inlet upstream of the catalytic converter unit to the region of the burner inlet downstream of the catalytic converter unit. Exclusively pure oxidizer mixture is passed through the at least one bypass line, and this pure oxidizer mixture, downstream of the catalytic converter unit, is admixed to the catalyzed fuel/oxidizer mixture for the purpose of targeted lean depletion of the fuel content.
It is advantageous for the oxidizer mixture to be produced using means which are known per se by some of the exhaust gases which emerge from the combustion chamber being introduced as carrier gas into an oxygen separation device by means of a recurculation line, with the oxygen-enriched carrier gas emerging from the oxygen separation device and being made available in the usual way via a heat exchanger in order to be introduced into the burner inlet, as described in EP 1 197 257 A1.
The oxidizer mixture is provided uniformly both for feeding into the bypass line in order to bypass the catalytic converter unit and for introduction into the means for feeding in oxidizer mixture, by which some of the oxidizer mixture is mixed with fuel and fed to the catalytic converter unit as fuel/oxidizer mixture.
Further details, relating to both the method principle according to the invention and the apparatus designed according to the invention for carrying out the method, for operating a burner of a heat engine, in particular of a gas turbine arrangement, are to be found below with reference to the exemplary embodiments.
The invention is described by way of example below, without restricting the general concept of the invention, on the basis of exemplary embodiments and with reference to the drawing in which:
a-d show alternative embodiments of the catalytic converter unit in the form of cross-sectional illustrations.
As has already been mentioned above, an oxygen separation device with an MCM membrane, into which some of the exhaust gas which emerges from the combustion chamber is recirculated and, as oxygen-enriched carrier stream, fed back to the burner inlet as oxidizer mixture via a preheating unit, is used to produce the oxygen-enriched carrier gas, i.e. the oxidizer mixture. An apparatus of this type is described, for example, in EP 1 197 257 A1.
Based on this initial statement, reference will be made below to a burner inlet 1, which is diagrammatically depicted in
Downstream of the inlet region of the burner inlet 1, there is a support structure 3 which, as seen in the direction of flow, has a multiplicity of through-passages 4, 5, of which one group of flow passages 4 is lined in thin-walled form with a catalyst material, for example Pt or Pd, while the other group of through-passages 5 consists of the material of the support structure itself, preferably an inert material, for example AlO3, SiO2 or ZrO. Immediately upstream of the catalytic converter unit 3, in each case in the flow region ahead of the through-passages 4 lined with catalyst material, there is a fuel feed unit 6, through which gaseous fossil fuel, preferably methane (CH4), is fed into the burner inlet to form a first fuel/oxidizer mixture 7. In this case, a first part-stream passes from the oxidizer mixture 2 provided by the oxygen separation device into the inflow region of the gaseous fuel provided by the fuel feed unit, to form the first fuel/oxidizer mixture 7. The remaining part of the oxidizer mixture 2 which is provided flows through the through-passages 5 designed as bypass lines.
The first fuel/oxidizer mixture 7 which is formed includes a gas mixture consisting of oxygen, carbon dioxide, water and methane as fuel, the so-called oxygen ratio λ1 of which, i.e. the ratio of the actual oxygen supply to the minimum oxygen demand for complete combustion, is less than 1, preferably 0.25. Within the catalytic converter unit, comprising the through-passages 4 lined with catalyst material, this relatively fuel-rich or rich gas mixture comes into surface contact with the catalyst material, for example rhodium (Rh), platinum (Pt), palladium (Pd) or nickel (Ni), with the result that the fossil fuel gas is at least partially catalytically oxidized and chemically converted. Hydrogen and carbon monoxide or carbon dioxide are formed as chemical by-products of the exothermic chemical reactions, with the result that the process temperature within the catalytic converter unit rises to temperatures between 550 and 1000° C., and the entire stream of substance passing through the catalytic converter unit and therefore also the support structure 3, consisting of the first fuel/oxidizer mixture and the second oxidizer mixture, which passes through the through-passages 5, is correspondingly heated. The heat quantity which is released by the exothermic reaction, and the resulting process temperature, depend on the oxygen content within the fuel/oxidizer mixture and can be influenced by controlling the oxygen content.
As will become clear below, in particular with reference to
Upper operating temperature limits, which are in the range between 700 and 900° C., preferably 750° C., are imposed by the choice of materials with regard to support structure and catalyst material. If these temperature limits are exceeded for process reasons, material degradation or detachment of the catalyst material from the corresponding support structure is likely, which limits the operating life of the catalytic converter unit. However, to maintain an optimum operating temperature, the ratios of the individual constituents of the fuel/oxidizer mixture which reacts chemically within the catalytic converter unit, i.e. fuel, oxygen, CO2 and water, have to be set in a targeted way to achieve a desired cooling action.
Passive cooling by selecting the proportion of the volume formed by the bypass lines passing through the support structure, within which no exothermic chemical reactions take place, is also possible, with the result that the oxidizer mixture which flows through the bypass lines or the through-passages 5 can be considered as a cooling flow.
Immediately after the support structure 3 within the burner inlet 1, as seen in the direction of flow, the catalyzed first fuel/oxidizer mixture is mixed with the pure oxidizer mixture which has been supplied through the through-passages 5, with the result that the volumetric content of the fuel within the second fuel/oxidizer mixture which forms in the region 8 of the burner inlet 1 decreases, so that the second fuel/oxidizer mixture is much leaner than the first fuel/oxidizer mixture fed to the catalytic converter unit. The mixing of the two streams is so efficient and fast that no ignition phenomena occur before complete mixing has taken place.
The requirement that the mixing of the catalyzed fuel/oxidizer mixture which emerges from the catalytic converter unit and the pure oxidizer mixture supplied via the bypass lines or through-passages 5 be as effective and fast as possible is derived from the very high reactivity of the hydrogen contents formed by the catalytic oxidation which have to be mixed with the oxidizer mixture supplied.
Should ignition phenomena nevertheless occur before complete mixing of the two streams has taken place, they would lead to what are known as hot spots, which produce a heterogeneous temperature profile within the region 8 of the burner inlet 1 and would ultimately lead to local overheating of material at those components which come into contact with the exhaust-gas streams formed.
The aim of rapid mixing downstream of the support structure 2 is to produce a complete, uniformly mixed, lean-depleted, catalyzed fuel/oxidizer mixture with an oxygen ratio λ2>1, which is ultimately burnt within a combustion chamber 9 that adjoins the burner inlet 2.
The combustion of the lean catalyzed fuel/oxidizer mixture is typically carried out within an inherently standard premix burner or as part of catalytic combustion. To explain the latter combustion variant, a further catalytic converter unit 10, which initiates the catalytic combustion, is provided upstream of the entry to the combustion chamber 9.
During catalytic oxidation of gaseous fossil fuel using a rich fuel/oxidizer mixture, the method according to the invention provides for controlled generation of highly reactive hydrogen, which considerably increases the reactivity of the catalyzed fuel/oxidizer mixture which forms. The actual combustion of the catalyzed fuel/oxidizer mixture which has been lean-depleted by the additional admixing of an oxidizer mixture is effected by the presence of hydrogen with very short ignition delay times and at temperatures of less than 1250° C.
Therefore, the ignition temperature and the lean extinction temperature within the combustion chamber can be significantly reduced by the targeted conversion of fossil fuel, preferably CH4, into hydrogen and carbon monoxide or carbon dioxide/steam. With the aid of the method according to the invention for operating a burner, it is possible to keep the exhaust-gas temperatures below those which would lead to destruction of all the components within the oxygen-enriching device.
A flow-guiding means 11, which is connected upstream of the support structure 3 as seen in the direction of flow and is illustrated separately from the support structure 3 purely to provide a better view, is used to spatially separately supply the respective streams into the through-passages 4 and 5, which have been divided into groups. Normally, the flow-guiding means 11 is fixedly joined in a gastight manner directly to the left-hand front of the support structure 3 facing it as seen in the direction of flow. The flow means 11 has an arrangement of compartments which is matched to the arrangement of the through-passages 4 and 5 in rows within the support structure 3, with this arrangement of compartments providing inlet side flanks 12, 13 which are open on alternate sides relative to the direction of flow through the through-passages 4 and 5 in the support structure 3. For example, in the exemplary embodiment shown in
The flow diversion in each case takes place in rows within the flow-guiding means 11, corresponding to the arrangement of the through-passages 4 in rows. This ensures that only the fuel/oxidizer mixture 7 flows through the through-passages 4 lined with catalyst material. The pure oxidizer mixture is supplied in the same way via the inlet side flanks 13 of the flow-guiding means 11. The flow of the respective streams which enter via the inlet side flanks 12, 13 is in each case diverted by a closed side wall, which lies opposite the open inlet side flanks in each row plane in the direction of incoming flow and diverts the respective streams into the direction of passage through the through-passages 4, 5.
To achieve intimate mixing of the catalyzed fuel/oxidizer mixture with the pure oxidizer mixture which is as efficient as possible and commences immediately after the respective streams have emerged through the support structure 3, before any of the ignition phenomena can precede complete mixing, the through-passages 4, 5 are to be arranged as closely adjacent to one another as possible and are to be provided with the minimum possible dimensions in cross section. Therefore, during mixing, it is appropriate to select the mixing length to be as short as possible, i.e. complete mixing of the streams which emerge from the through-passages 4, 5 should take place as close as possible to the emergence of the flow from the support structure 3.
a-d illustrate details from cross sections through the support structure 3 which show regions of through-passages 4 lined with catalyst material on the inner wall and through-passages 5 whose passage walls consist solely of the material of the support structure 3 itself, and in this respect have no catalytic effect on the stream passing through the through-passages 5. The level of the catalyzing reactions and the associated release of exothermic energy within the support structure 3 can be influenced by suitable selection of the ratio between the catalyzing surface area and the inert surface areas along the through-passages.
For example, the exemplary embodiment shown in
Finally, if the arrangement of through-passages shown in the exemplary embodiments illustrated in
To summarize, the following statements can be made in connection with the method according to the invention and the apparatus required to implement it:
1. The method according to the invention allows stable combustion of a gaseous fossil fuel using the AZEP principle, forming exclusively CO2 and water.
2. The targeted production of hydrogen within the catalyzed fuel/oxidizer mixture considerably reduces the ignition temperature of the mixture, with the result that further combustion of the fuel/oxidizer mixture under lean conditions is completely possible even using conventional premix burners with or without catalyzed combustion.
3. With the aid of the support structure provided within the burner inlet and the flow-guiding means provided upstream of the support structure, as seen in the direction of flow, it is possible to completely and effectively mix the catalyzed fuel/oxidizer mixture with the pure oxidizer mixture before ignition phenomena occur.
| Number | Date | Country | Kind |
|---|---|---|---|
| 103 18 669.7 | Apr 2003 | DE | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/CH03/00478 | Jul 2003 | US |
| Child | 11256113 | Oct 2005 | US |
