COMBUSTION INSTALLATION

Technical field

The present invention relates to a combustion installation comprising a combustion device and a membrane device arranged to separate oxygen from a gas mixture.

Description of the prior art

It is desirable to decrease unwanted exhausts from combustion installations and it is for instance desirable to reduce nitrogen oxides as far as possible.

It is also desirable to decrease exhausts of carbon dioxide that is produced during combustion. It is possible to separate carbon dioxide from a combustion gas but as the concentration of carbon dioxide usually is low and as the combustion gas comprises other components, such as oxygen and nitrogen, it is relatively complicated to separate the carbon dioxide from the combustion gas.

One possibility to facilitate the separation of carbon dioxide is to perform the combustion in another medium than air, from which medium carbon dioxide more easily can be separated. If air is not used as combustion medium it is necessary to provide oxygen to the medium. It is, however, relatively expensive to provide pure oxygen in the necessary volumes. One possibility for providing oxygen is to use a suitable membrane which is arranged to separate oxygen from a gas mixture, which gas mixture usually is air.

Such membranes are described in US-A-5,1 18,395.

One type of such membranes is called "mixed conducting membrane

(MCM)". This type of membrane device comprises an MCM material and works without a voltage being applied. Such a membrane device works through the partial pressure of oxygen being lower on the side

of the filter to which oxygen is transported. Oxygen ions are here directed in the one direction through the membrane and electrons are directed back through the membrane in the opposite direction.

EP-A-658,367 describes as well different types of such membranes. This document describes different combustion installations with a membrane device from which oxygen is extracted. The oxygen- depleted gas which is produced in the membrane device is led to one or more combustion devices and combustion gases from the com- bustion devices are used to drive a turbine.

The Norwegian published patent application NO-A-972,631 describes the use of an MCM in combustion processes. According to the described processes compressed air is directed to an MCM reactor. The MCM reactor comprises a membrane device, which separates oxygen from the air. The air from which oxygen has been separated is led away via a heat exchanger. The separated oxygen is used during combustion and combustion. The combustion gases comprise mainly steam and carbon dioxide. The steam may be condensed which makes it possible to separate the carbon dioxide. As nitrogen essentially not takes part in the combustion process the exhaust of unwanted nitrogen oxides is avoided. The diluted air, heated by heat exchange expands in the turbine, which generates power output.

In the PCT application WO 01/92703 a combustion installation is described in which fuel is combusted without nitrogen. A compressor is arranged to compress air to create a first flow of compressed air. The first flow of compressed air from the compressor passes an MCM reactor in which the compressed air is heated by a second flow of combustion gases, and oxygen from the air is transported over to the other flow of combustion gases. The combustion gases which have been cooled off and to which oxygen has been added are led back to a combustion chamber. In the combustion chamber fuel is added which is combusted with the oxygen, which was added from the air in the MCM reactor. The flow of air in the MCM reactor is counter- current to the flow of combustion gases.

The PCT-application WO 2004/094909 describes a method and a device for operating a burner of a heat engine, especially a gas turbine. A first oxygen-enriched carrier gas flow called a first oxidizer mixture is provided, to which the fuel is admixed so as to form a first fuel/oxidizer mixture, and a second oxygen enriched carrier gas flow called a second oxidizer mixture is provided. The first fuel-oxidizer mixture is catalyzed so as to form a catalyzed first fuel/oxidizer mixture in which the fuel is at least partly oxidized. The catalyzed first fuel/oxidizer mixture is mixed with the second oxidizer mixture in order to form a second fuel/oxidizer mixture, which is ignited and burned.

To maintain circulation of this gas mixture, some kind of pumping de- vice is needed. It is a technical problem to find a suitable equipment for this, mainly because of the high temperature of the gas. Above documents do not take into account this problem.

The ejector in a system works as a compressor. An ejector has no moving parts, there is no greasing needed, which means that an installation in a high temperature environment is favourable.

The ejector consists of a suction manifold, a pipe ending with a nozzle, a mixing chamber and a diffuser.

The gas, which is to be pumped, flows through the suction manifold. A narrow pipe, in which the fuel or fuel mixture flows, is preferably placed concentrically in the suction manifold. Both these parts of the ejector discharge preferably concentrically into the mixing chamber, whereas the narrow pipe ends with a nozzle.

Attached to and downstream the mixing chamber follows the diffuser as the finishing component of the ejector.

The nozzle could be of subsonic, sonic or supersonic (Laval) type. The ejector is the part of a system, where the colder high pressure fuel, called primary flow, actuating fluid or suction flow, mixes with

the hot low pressure recycle gas, which is called secondary flow, induced fluid or injection flow.

The main tasks of the ejector are to maintain the pressure needed by the system to re-circulate a specific secondary mass flow for a re- forming process.

The pressure of the recycle gas is affected by pressure losses in the system. These losses are compensated by the pressure increase in the ejector. The primary high pressure fluid expands supersonically out through the nozzle (for the nozzle designed as a Laval nozzle) to the mixing chamber.

This causes suction in the suction manifold, which entrains the secondary low pressure flow into the mixing chamber. Now the primary flow transfers part of its momentum (rn-v) to the secondary flow and the two flows mix until uniformity in reference to velocity, pressure, temperature and chemical composition is reached. This mixed high speed flow enters the diffuser, where part of the kinetic energy is converted into pressure with the aim to reach a higher pressure value than at the recycle inlet. The mixing chamber could be designed on the basis of two principles: constant area mixing or constant pressure mixing. In the first case the cross sectional area of the mixing chamber is hold constant from inlet of the mixing tube to the diffuser inlet. In the second case they hold the pressure constant along the mixing cham- ber length which causes a convergent chamber profile.

Description of the invention

The invention is to arrange at least one ejector in order to pump the flow of an oxygen containing gas through the described combustion installation.

According to a first aspect of the present invention a combustion installation is provided, comprising a combustion device with a space for combustion of a fuel and an oxygen-containing gas as combustion gas, a nozzle means for injection of the fuel into the inner space of

the combustion device, and a membrane device for supplying oxygen to the combustion gases for production of the oxygen-containing gas. The combustion device further comprises at least a first inlet for the oxygen-containing gas, an outlet for the combustion gases and a catalyst arranged between the nozzle means and the outlet of the combustion device. The membrane device is arranged between the outlet of the combustion device and the first inlet of the combustion device so that the combustion gases are arranged to be directed from the outlet of the combustion device into the membrane device, be supplied with oxygen and be directed back to the first inlet of the combustion device as the oxygen-containing gas. The combustion installation is characterised in that it comprises at least a first ejector arranged to pump the flow of combustion gases

With a combustion installation according to the present invention flow of the oxygen containing gas is provided through the injection of fuel or a mixture of fuel and a carrying gas. Thus, no additional device is required in order to provide the flow of the oxygen containing gas through the combustion device.

The use of an ejector to inject the fuel into the inner space of the combustion device also provides for a fast and efficient mixing of the fuel with the oxygen containing gas.

The combustion device may be arranged to inject a carrier gas together with the fuel into the inner space of the combustion device. By arranging the nozzle means in this way it is possible to alter the speed of the flow without altering the amount of fuel being injected through the nozzle means. The carrier gas is preferably a gas which does not take part in the combustion and may e.g. be steam.

In case the nozzle means comprises a plurality of nozzles, each nozzle may form part of an ejector. The flows from the different ejectors are then combined into a common flow through the combustion de- vice.

The inner space between the catalyst and the outlet of the combustion device may form a combustion compartment for combustion of the fuel and the oxygen-containing gas.

The combustion device may comprise a second inlet being connected to the outlet of the membrane device, wherein the second inlet is arranged next to the combustion device. With a second inlet more efficient combustion of the fuel and the oxygen-containing gas may be achieved. It is then possible to allow only a part of the oxygen- containing gas to be mixed with the fuel and to pass the catalyst.

The combustion installation may comprise at least a second ejector including the second inlet. In this way the flow of the oxygen containing gas from the second inlet may be driven by the flow of gas from the first ejector.

The combustion device may comprise a length axis which extends from the second inlet of the combustion device to the outlet of the combustion device, wherein the second inlet is ring-formed and en- circles the length axis.

The first inlet and the second inlet may be arranged so that at least half of the gas from the membrane device is directed into the second inlet. This has proved to be advantageous for the catalytic reaction in the catalyst.

The first inlet and the second inlet are preferably arranged so that 50 to 80 percent and most preferred 55 to 65 percent of the gas from the membrane device is directed into the second inlet.

The membrane device may comprise a first compartment with an inlet and an outlet and a second compartment with an inlet and an outlet, which membrane device is arranged to allow oxygen to pass between the first compartment and the second compartment. There are also other ways of arranging membrane devices but the described way of arranging the membrane device is the conventional

way of arranging membrane devices in. The purpose of the membrane device is, however, for the first compartment to be separated from an oxygen reservoir using an oxygen-transparable membrane. Arranging the second compartment with an inlet and an outlet makes it possible to arrange for the oxygen-containing gas to flow through the second compartment and thereby allowing oxygen to be transported over to the compartment.

The first inlet of the combustion device and the outlet of the combus- tion device may be connected to the outlet of the first compartment of the membrane device and the inlet of the first compartment of the membrane device, respectively, wherein the membrane device is arranged in such a way that an oxygen-containing gas, which passes the second compartment of the second membrane device is heated by the combustion gas passing the first compartment of the membrane device and which has been heated during combustion of the fuel in the said combustion device and in such a way that oxygen passes from the second compartment of the membrane device to the first compartment of the membrane device.

The combustion device may comprise a first heat exchanger comprising a first and a second compartment, wherein the outlet of the combustion device is connected to the inlet of the first compartment of the membrane device via the first compartment of the first heat exchanger. By arranging such a heat exchanger effective heat exchanging is possible between the first compartment and the second compartment at the same time as a part of the heat exchanging may take place outside the membrane device. As the membranes, that are used in membrane devices, usually are very expensive, it is de- sirable to limit the size of the membrane. As the effectiveness of the membrane usually is dependent on the temperature and as too high temperatures may damage the membrane it is desirable to avoid excessively high or excessively low temperatures in the membrane device. By arranging the first heat exchanger connected to the outlet of the combustion device the combustion gases will be cooled before

they reach the membrane device. This decreases the risk of too high temperatures in the membrane device.

The combustion installation may comprise a second heat exchanger comprising a first and a second compartment, wherein the first inlet of the combustion device is connected to the outlet of the first compartment of the membrane device via the first compartment of the second heat exchanger. In this way a higher temperature is achieved in the membrane device thereby an increased efficiency of the oxy- gen transport in the membrane device is achieved.

It is possible to have only the second heat exchanger and to dispose with the first heat exchanger.

The second compartment of the first heat exchanger may be connected to the outlet of the second compartment of the membrane device. In this way combustion gases will flow in the counter-current direction to the air which is heated by the combustion gases and which passes the second compartment of the heat exchanger

The following preferred embodiments of the invention will be described with reference to the appended drawings.

Short description of the drawings

Fig. 1 shows schematically a combustion installation with a compressor, turbine and a generator according to an embodiment of the present invention. Fig. 2 shows in great detail a combustion device Fig. 3 shows schematically a combustion installation with a compressor, turbine and a generator according to an alternative embodiment of the present invention.

Fig.4 shows a schematic picture of an ejector and its main components.

Description of preferred embodiment

In the following description of preferred embodiments of the invention similar parts in the different figures will be denoted with the same reference numeral.

In Fig.1 a combustion installation 1 is shown partly in cross-section. The combustion installation 1 comprises a combustion device 2 with an inner space 3 for combustion of the fuel and an oxygen-containing gas to a combustion gas in order to produce heat. A fuel supply pipe 4 is connected to the combustion device. The combustion device 2 comprises a first inlet 5 for the oxygen-containing gas and an outlet 6 for combustion gases. In Fig. 1 an optional second inlet 30 to the combustion device 2 is also shown. The combustion installation 1 also comprises a membrane device 7, which is shown in cross- section in Fig. 1. The membrane device 7 comprises a first compartment 8 with an inlet 9 and an outlet 10 and a second compartment 11 with an inlet 12 and an outlet 13. An oxygen-permeable membrane 29 divides the first compartment 8 of the membrane device 7 from the second compartment 1 1 of the membrane device 7. The outlet 10 of the first compartment 8 of the membrane device 7 is connected to the first inlet 5 of the combustion device 2 via a combustion gas pipe 15. The outlet 6 of the combustion device 2 is correspondingly connected to the inlet 9 of the first compartment 8 of the membrane de- vice 7 via an oxygen gas pipe 14. The combustion installation 1 comprises a compressor 16 being connected to the inlet 12 of the second compartment 1 1 of the membrane device 7 via an air supply pipe 17. The combustion installation further comprises a turbine 18, which is connected to the outlet 13 of the second compartment 1 1 of the membrane device 7 via a pipe 19. The compressor 16 comprises a compressor inlet 20. The turbine 18 comprises a shaft 21 , which is arranged to drive the compressor 16 and a generator 22. A bleed off pipe 23 for draining a part of the combustion gases is connected to the combustion pipe 14 and to a cooler 24. The cooler 24 comprises a cooling-water inlet 25 and a cooling-water outlet 26. The cooler 24 also comprises a condensed-water outlet 28 for water in the com

bustion gases which has condensed in the cooler 24 and a carbon dioxide outlet 27 for carbon dioxide which has been separated from the combustion gases. The function of the cooler 24 will not be described in detail here as coolers 24 in which carbon dioxide and wa- ter are separated are well-known from the art.

During operation the turbine 18 drives the compressor 16 and the generator 22. The compressor 16 takes in air through the compressor inlet 20. The air is compressed in the compressor 16 and is di- rected by the air supply pipe 17 to the inlet 12 of the second compartment 11 of the membrane device 7. When the compressed air passes the second compartment 11 of the membrane device 7 the air will be heated by the combustion gases passing the first compartment 8 of the membrane device 7. When the compressed air passes the second compartment 11 of the membrane device 7 oxygen in the air will pass the membrane 29 into the first compartment 8 of the membrane device 7. Combustion gases which are transported into the first compartment 8 of the membrane device 7 via the inlet 9 will be supplied with oxygen and thereby be transformed into an oxygen- containing gas while simultaneously being cooled off by the air passing though the second compartment 11 of the membrane device 7.

The oxygen-containing gas is transported from the outlet 10 of the first compartment 8 of the membrane device 7 via pipe 15 to the combustion device 2. Fuel is transported into the combustion device 2 through the fuel supply pipe 4. The fuel, which is injected into the combustion device 2 through the fuel supply pipe 4, is injected in a first ejector, which pumps the oxygen-containing gas into the com- bustion device 2. After the oxygen-containing gas has been mixed with fuel which has been transported through the fuel-supply pipe 4 combustion of the fuel with oxygen takes place so that combustion gases are formed. The main part of the combustion gases are directed via the combustion gas pipe 14 back into the membrane de- vice 7 through the inlet 9 of the first compartment 8 of the membrane device 7. However, material is added to the combustion gases

through the injection of fuel in the combustion device 2. In order to keep the pressure in the combustion gas pipe 14 constant over time part of the combustion gases must be directed away through the bleed off pipe 23. The combustion gases in the bleed off pipe 23 are directed to the cooler 24, where the combustion gases are cooled off using cooling water, which passes the cooler 24 from the cooling water to the cooling water outlet 26.

The air passing the second compartment 1 1 of the membrane device 7 will be heated by the combustion gases passing the first compartment 8 of the membrane device 7. The flow direction of air in the membrane device 7 is preferably counter-directed to the flow direction of the combustion gases. Thus, the highest temperature in the first compartment 8 and the highest temperature in second compart- ment 1 1 are found in the same end of the membrane device 7. The flows in the different compartments of the membrane device 7 are preferably chosen so, that they are essentially equally big. This means that the combustion gases are cooled down to the order of same temperature as the temperature of the compressed air at the inlet 12 of the second compartment 11 of the membrane device 7. The heated air leaving the outlet 13 of the second compartment 11 of the membrane device 7 is directed via the turbine pipe 19 to the turbine 18, where the heated air drives the turbine 18, which in turn drives the compressor 16 and the generator 22.

The oxygen-containing gas leaving the outlet 10 of the first compartment 8 of the membrane device 7 is directed to the first inlet 5 of the combustion device 7 and the second inlet 30 of the combustion device in case it exists. The combustion device 2 will now be described in further detail with reference to Fig. 2.

In Fig. 2 the combustion device 2 is shown in cross-section. A nozzle means 31 in the form of a nozzle is arranged at the first inlet 5. The nozzle means 31 is connected to the fuel supply pipe 4, which in turn is connected to the fuel tank (not shown). The first inlet 5 and the second inlet 30 are connected to the oxygen-containing gas pipe 15.

A catalyst 32 is arranged after diffuser 33. The pipe 37 is the mixing chamber for mixing of the fuel and the oxygen-containing gas. Before outlet (6) is a combustion compartment 34 for combustion of the fuel and the oxygen-containing gas. The combustion arrangement de- scribed forms an ejector 46 as a pump device.

It is possible to arrange the nozzle means to inject a carrier gas together with the fuel. The carrier gas is preferably a gas which does not take part in the reaction. The carrier gas may be e.g. steam. By altering the flow of the carrier gas the pump effect of the ejector 46 may be altered without altering the amount of fuel being injected into the combustion device 2. Thus, the flow speed in the combustion device 2 may be altered independently from the amount of fuel being injected into the combustion device 2.

The second inlet 30 is arranged downstream the catalyst 32 between the catalyst 32 and the outlet 6 of the combustion device 2. Between the catalyst 32 and the combustion compartment 34 there is an outlet compartment 50 having a outlet 36.

The combustion installation forms a second ejector 47 including the outlet 36 and the second inlet 30.

The first inlet 5 and the second inlet 30 are arranged in such a way that 50 to 80 percent and preferably 35 to 65 percent of the oxygen- containing gas is directed into the second inlet. Accordingly 20 to 50 percent and preferably 35 to 45 percent of the oxygen-containing gas is directed into the first inlet 5

Fig. 3 shows an alternative configuration of combustion installation 1 , where heat exchange between the compressed air and the circulating gas from the combustion device 2 is showed as separate heat exchangers. Only differences between the combustion installation 1 in Fig. 1 and the combustion installation 1 in Fig. 3 will be described.

The combustion installation 1 comprises a first heat exchanger 40 with a first compartment 41 and a second compartment 42. The outlet 6 of the combustion device 2 is connected to the inlet 9 of the first compartment 8 of the membrane device 7 via the first compartment 41 of the first heat exchanger 40. Correspondingly the outlet 13 of the second compartment 1 1 of the membrane device 7 is connected to turbine pipe 19 via the second compartment 42 of the first heat exchanger 40.

The combustion installation 1 comprises a second heat exchanger 43 with a first compartment 44 and a second compartment 45. The first inlet 5 of the combustion device 2 is connected to the outlet 9 of the first compartment 8 of the membrane device 7 via the first compartment 44 of the second heat exchanger 43. Correspondingly the inlet 12 of the second compartment 1 1 of the membrane device 7 is connected to the air supply pipe 17 via the second compartment 45 of the second heat exchanger 43.

When the combustion gases pass the first compartment 41 of the first heat exchanger 40 the combustion gases will be cooled down as heat is transferred to the air passing the second compartment 42 of the first heat exchanger 40. The combustion gases that come in contact with the membrane 29 are thereby cooled down. Correspondingly heat will be transferred from the combustion gases pass- ing the first compartment 44 of the second heat exchanger by air passing the second compartment 45 of the second heat exchanger 43.

The above-described embodiments may be modified in many ways without departing from the spirit and the scope of the present invention. For example it is not necessary to have both a first inlet and a second inlet into the combustion device.

Fig. 4 shows the main components of an ejector. Fuel inlet pipe 51 ends in direction of flow with a nozzle 52. The primary flow of an ejector goes through this pipe and nozzle. The oxygen-containing

gas in above described combustion installation is the secondary flow of an ejector and goes through suction manifold 53. The mixing of primary flow and secondary flow takes place in the mixing-chamber 54. The pressure recover of the gas mixture from mixing-chamber 54 takes place in the diffuser 55.