WO2002059524A1

WO2002059524A1 - A method and an apparatus for cycle fluid treatment in an oxygen fired semi-closed power cycle

Info

Publication number: WO2002059524A1
Application number: PCT/NO2002/000030
Authority: WO
Inventors: Tord Peter Ursin; Anton Malcolm Hove; Oscar Fredrik Graff
Original assignee: Aker Technology As
Priority date: 2001-01-23
Filing date: 2002-01-22
Publication date: 2002-08-01
Also published as: NO314925B1; NO20010403L; NO20010403D0; WO2002059524A8

Abstract

The present invention relates a novel method for heat utilization and oxygen supply in a semi-closed Oxyfuel process suitable for power production with very low emissions of carbon dioxide and oxides of nitrogen to atmosphere. Oxygen is transported to a nearly nitrogen free cycle fluid through oxygen selective membranes and heat from the cycle fluid is utilized to generate steam which is fed back to the cycle in a manner which facilitates usage of the oxygen enriched cycle for oxidation of a carbon containing fuel.

Description

A METHOD AND AN APPARATUS FOR CYCLE FLUID TREATMENT IN AN OXYGEN FIRED SEMI-CLOSED POWER CYCLE

The present invention relates to a method for cooling and for oxygen enrichment of a cycle fluid in a power producing process allowing very low emissions of carbon dioxide to the atmosphere .

An increasing demand for electric power combined with in- creasing environmental awareness has initiated extensive research for developing cost effective and environmentally friendly power generation. Several renewable power sources are available, but at present only nuclear and hydrocarbon fuelled power plants can supply the bulk of the power being demanded. Nuclear power plants suffer from safety risks and problematic radioactive waste disposal . Future development of nuclear power plants seems very limited, mostly due to lack of political acceptance. Thus, power plants based on fossil fuels are called upon to fill most of the energy gap. However, a continuous development of scientific data on the Greenhouse effect and political agreements such as the Kyoto protocol from 1997, is generating an increasing push towards limiting and reducing greenhouse gas emissions. As a result of this trend, several countries seek to limit their C0₂ emissions and establish annual maximum emission levels. In this endeavour, carbon dioxide (C0₂) emissions from thermal power plants is a main concern. In several regions, fossil fuelled power plants are also experiencing strict limits on other atmospheric emissions such as oxides of nitrogen.

Several processes for power production from fossil fuels with greatly reduced C0₂ emissions are known in the art. These processes produce concentrated and pressurised C0₂ suitable for sequestration or industrial usage. Sequestration of the C0₂, produced from a large-scale power plant, will most likely be achieved by injection as gas, liquid or hydrates into subterranean formations or into deep sea- water. A commercial value for the produced C0₂ may be obtained when used for enhanced oil recovery in producing oil fields. A particularly promising type of processes known in prior art is based on combustion of a carbon containing fuel with oxygen in nearly nitrogen free gas mixtures mostly composed of water vapour and C0₂. The combustion products will comprise mainly water vapour and C0₂. The temperature in the combustion process may be controlled by recirculation of combustion products, thus forming a cycle where the cycle fluid mainly consists of combustion products. The energy in the combustion products may be converted to electric power via gas turbines, steam turbines or reciprocating engines. The energy may also be trans- ferred to a heat carrying medium, e.g. hot water for district heating. The inventory in the power cycle is kept constant by constantly removing a suitable amount of cycle fluid. Water may easily be separated from the cycle fluid, and the rest, predominantly C0₂, may be disposed of as de- scribed above. Other pollutants normally present in the combustion products, such as oxides of nitrogen, may be similarly disposed of . Power cycles of this type may be called "Oxyfuel cycles" , and they are particularly well suited for power production with zero atmospheric emis- sions.

Both investment cost and energy consumption are very high for generation of oxygen at the purity and quantity required in Oxyfuel cycles . Novel technology for oxygen gen- eration is being developed. This is based on non-porous materials through which oxygen ion migration is possible. These materials typically require temperatures above 500°C and operate by a mechanism in which an oxygen partial pressure differential or a voltage differential across the ma- terial is the main driving force behind the migration of oxygen ions. Known materials allow oxygen selective processes of this type, such that significant amounts of other common gases, such as nitrogen, will not pass through the material. Thorough discussions and references on this technology are given in US pat. no. 5 753 007 and US pat. no. 5 976 223. This group of materials has here been called oxygen selective ion transport membranes. Other materials, such as some carbon and glass compounds allow selective passage of oxygen molecules. The selectivity for these materials are presently typically too low to prevent undesirable dilution of the cycle fluid. Such materials has here been called oxygen molecule selective membranes.

Most of the prior art has required the use of a source of highly concentrated oxygen, ref . US pat. no. 5 724 805, US pat. no. 5 956 937, US pat. no. 5 247 791 and SE pat. no. 9601898. In order to reduce the cost of oxygen, it is a goal to include the use of oxygen selective ion transport membranes in Oxyfuel cycles. This implies that a way to achieve a positive oxygen partial pressure differential and the required temperature, must be found. An Oxyfuel process utilizing oxygen selective ion transport membranes in an Oxyfuel cycle is described in PCT/NO97/00172. A conventional heat recovery system is proposed to utilize the heat emitted by the cycle. These are costly and more economical ways for the utilization of this heat energy are demanded.

No steps to facilitate the use of this oxygen enriched cycle fluid as oxidant in a combustion process has been described. It is an objective to use as little excess oxygen as possible compared to the stoichiometric amount, but still achieve a complete combustion of the fuel. This is particularly difficult if the oxygen is supplied to the combustion zone mixed with cycle fluid, i.e. in a non- concentrated form.

The subject invention relates to a method for oxygen sup- ply, steam generation and steam usage in a power cycle with greatly reduced atmospheric emissions of C0₂. Brief Description of the Invention

The subject invention presents a method for solving the problems described above. Oxygen selective ion transport membranes are included to transfer oxygen to the cycle fluid of an Oxyfuel power cycle. The higher cycle fluid temperatures in an Oxyfuel power cycle, occurring because of different thermodynamic behaviour compared to air based cycles, is taken advantage of in the integration of the oxygen selective ion transport membranes. The concentration of oxygen in the cycle fluid is increased by condensing water vapour from the fluid.

The oxygen enriched cycle fluid is passed to a combustion zone where it is used to combust a carbon containing fuel . This combustion process is facilitated by injection of steam into the combustion gas flows. The method proposes to generate steam for the usage described above by using heat from the cycle fluid. Thereby, the subject invention pres- ents a method that significantly reduces the investment cost for generating oxygen and utilizing heat in Oxyfuel cycles .

Brief Figure description

Fig. 1 is a schematic of the main principles of the present invention.

Fig. 2 is a schematic flow diagram of a specific embodiment of the present invention.

Fig. 3 is a schematic flow diagram of a specific embodiment of the present invention shown in an Oxyfuel power cycle using a gas turbine.

The invention also allows production of heat and/or steam usable for distribution to district heating or nearby steam consumers . Detailed Description

Fig. 1 shows the main principles of the invention. A line containing hot cycle fluid 11, is shown going to the Apparatus 1 where it is fed to the cycle fluid side. The cycle fluid is the cycle fluid of a semi-closed power cycle. The term semi-closed cycle is used to denote a cycle where a part of the cycle fluid is circulated in one or more loops and where material is added and removed at rates which keeps the inventory of cycle fluid largely constant. The cycle fluid is mainly composed of combustion products such as carbon dioxide and water. In addition, it may contain smaller parts of not combusted or partially combusted fuel, oxygen, fuel impurities and gas that has leaked into the cycle. The cycle fluid is cooled and oxygen enriched when it passes through the apparatus. After it exits the apparatus in cycle fluid outlet line 12 the oxygen enriched cycle fluid is passed to a zone where a fuel containing the element carbon is combusted using the oxygen in the cycle fluid as the main oxidant . The cycle fluid in line 12 may be passed directly to a combustion zone or may be further cooled, condensate removed and compressed before being passed to a combustion zone. The hot cycle fluid coming into the apparatus in cycle fluid supply line 11 is the products of an Oxyfuel type of combustion process, and may be the same combustion zone to which line 12 leads. It is also possible to include the apparatus in power production processes with multiple combustion zones either in series or in interconnected cycles. In that case, the combustion processes will be of an Oxyfuel type where the majority of the oxidant is supplied by the apparatus or apparatuses. A net flow of energy is removed from the cycle fluid exiting the combustion zone, before being passed to the apparatus. This energy may be used to produce electricity, mechanical work or heat. In order to reduce the size of the apparatus, it may be desirable to operate the cycle fluid side at a pressure significantly above ambient.

The line 21 passes to the oxygen supply side of the appara- tus . This line contains an oxygen carrying gas, preferably ambient air. A means for inducing the desired flowrate of gas from the inlet to the outlet on the oxygen supply side of the apparatus may be installed upstream, downstream or may be included in the apparatus. Alternatively, a compres- sor increasing the total pressure in supply line 21, and an expander recovering energy from the fluid in discharge line 22 may be installed. The oxygen carrying gas is heated before being passed to a section of the apparatus where, at least in part, an oxygen selective membrane or membranes 8, divide the cycle fluid and oxygen supply side of the apparatus. The temperature and partial pressure of oxygen in the oxygen carrying gas is such that oxygen is passed from the oxygen supply side to the cycle fluid side. If such a temperature is not possible to achieve by heat exchange with the cycle fluid, a means for heating the cycle fluid or oxygen carrying gas may be installed in or upstream the apparatus. The oxygen depleted gas exiting the oxygen supply side of the ion transport membrane or membranes, is cooled before being passed to the outlet of the oxygen sup- ply side, 22. It will not be necessary to include heat transfer surfaces for heating and cooling the oxygen carrying gas, if a membrane material that operates at ambient temperatures can be found. Further, other gases than air may be used as oxygen carrying gas and, if available, other oxygen selective membranes than oxygen selective ion transport membranes may be used to transport oxygen to the cycle fluid.

Line 31 is passed to the steam side of the apparatus. This line contains a pressurised liquid mostly composed of water. The liquid is heated by heat exchange with one or both of the other sides in the apparatus and evaporated. As the steam exits the apparatus in line 32, it may be super- heated, saturated or it may contain a minor part liquid water. At least part of the fluid in line 32 is then mixed with the cycle fluid. It is preferable to mix the fluid such that at least part of the water in the fluid is disso- ciated due to contact with reacting gases in the combustion process. This implies that water vapour should be mixed into reacting gas flows where the temperature is sufficient for dissociation to occur. It has been discovered that the dissociation products will positively interact with the combustion of carbon monoxide and thus results in a more complete combustion of the fuel . This can be very important in Oxyfuel cycles, which have significantly less oxygen than conventional power cycles. It is even more important when oxygen is supplied to the combustion zone mixed with cycle fluid, because the oxygen concentration to be supplied to the combustion equipment may not be adjusted freely. Part of the steam exiting the apparatus in line 32 may also be used in a conventional steam power cycle or as a source of heat. In addition, the steam may, in conven- tional manner, be expanded through a standard steam turbine before it is mixed with the cycle fluid. Surfaces for heat transfer to a conventional steam power cycle may also be included in the apparatus .

The apparatus is constructed such that it allows heat exchange between the three sides included in the apparatus to achieve the heating and cooling processes described above. The apparatus may perform this task by use of two or more heat exchangers with at least two sides or it may perform it by use a single heat exchanger with three sides.

Fig. 2 shows a schematic flow diagram of an embodiment of the present invention. The Apparatus 1 is shown with a hot end on the top and a cold end on the bottom. The cycle fluid supply 11 is shown coming from a power producing machine 2. The cycle fluid is divided into two parts before being supplied to the apparatus, both are cooled to a temperature where most of the water vapour has condensed, be- fore exiting on the bottom of the apparatus. One part cycle fluid is cooled without being oxygen enriched and exits in discharge line 13. Condensed fluid in this line may be mixed with the liquid in condensate line 33. Discharge line 13 contains mainly C0₂ and water, and is disposed of in a manner preventing most of the C0₂ entering the atmosphere. This may include a compressor train consisting of a series of coolers, scrubbers and compressors in order to achieve the injection pressure required for injection into subter- ranean formations. The pressure may also be generated by a C0₂ liquefaction system and C0₂ pumps. Oxygen is transferred by the oxygen selective ion transport membranes 8 from the oxygen supply side to one part of the cycle fluid and the oxygen concentration is further increased by cool- ing to a temperature where part of the cycle fluid condenses. The cycle fluid condensate is led from the apparatus in line 33. The oxygen enriched cycle fluid exits the apparatus in line 12 and is fed to power producing machine 2. A supply of fuel containing the element carbon is sup- plied to power producing machine 2 in line 51 and is oxidised by oxygen supplied through line 12. Steam supplied to power producing machine 2 in steam discharge line 32 takes part in the combustion process, lowering the outlet temperature and the concentration of carbon monoxide in the cycle fluid in line 11. A line supplying a smaller amount of oxygen to the power producing machine 2 may be added, if required, to achieve a more stable and complete combustion of the fuel .

A part of the condensed cycle fluid, mostly water, exiting the apparatus in line 33 is discarded through line 34 while another part 31 is pumped to a higher pressure by pump 3. Water supply line 31, exiting the pump, is connected to the cold end of the apparatus wherein the water is evaporated before exiting the apparatus in steam discharge line 32. The steam side of the apparatus may partly consist of a multitude of tubes providing a flow path from the cold end of the apparatus to the hot end of the apparatus . These tubes are arranged to provide a heat transfer surface in a conterflow configuration with the cycle fluid and/or the oxygen supply side. Injection of chemicals or removal of contamination on the steam side, may be performed in order to purify or condition the water-based fluid.

Air is supplied to the cold end of the apparatus in line 21. A means for inducing airflow through the apparatus is installed upstream line 21. The air is heated by heat ex- change with hot, oxygen depleted air exiting the oxygen selective ion transport membranes 24. Thereafter the air is heated by heat exchange with the cycle fluid to a temperature which makes the ion transport membranes operational. The hot air 23 then flows past the ion transport membranes 8 and oxygen is transferred from the air to a part of the cycle fluid. The oxygen depleted air 24 exiting the ion transport membranes is then cooled by heat exchange with air as described above. Cooled, oxygen depleted air is discarded to the atmosphere in line 22.

In order to cool the cycle fluid to a temperature where most of the water is condensed, a fourth side is added to the apparatus. This side is supplied with a cooling medium at a suitably low temperature, in line 41, which absorbs heat and exits in line 42.

Fig. 3 shows a schematic flow diagram of a specific embodiment of the present invention. The apparatus is shown as main component 4, heat exchanger 5 and gas-liquid separator 6. The apparatus is shown used in a power cycle with a gas turbine as the power producing device 2. The cycle fluid supply comes from the gas turbine in cycle fluid supply line 11 which contains 60 mol% water vapour and 39 mol% carbon dioxide at 0,1 barg and 700°C. The cycle fluid is divided in two streams and both are cooled by heat transfer to the air and steam sides before exiting the bottom of the apparatus 4. One stream of cycle fluid, which is cooled without being oxygen enriched, exits in cycle fluid dis- charge line 13, while one stream is oxygen enriched and contain 11 mol% oxygen when it exits in cycle fluid transfer line 14. The fluid in discharge line 13 is disposed of in a manner preventing most of the C0₂ entering the atmos- phere .

Transfer line 14 passes to gas-liquid separator 6 where the cycle fluid is cooled to 20°C. In order to perform this cooling of the cycle fluid, gas-liquid separator 6 is sup- plied with a cooling medium at 10°C in line 41. The cooling medium absorbs heat and exits in line 42. By condensing fluid from the cycle fluid, the oxygen concentration in the gaseous cycle fluid exiting gas-liquid separator 6 in line 12 is raised to 26 mol% oxygen. The gaseous cycle fluid ex- iting gas-liquid separator 6 further contains 2,2 mol% water vapour and 71 mol% carbon dioxide and is at 0 barg and 20°C. Part of the liquid condensed in gas-liquid separator 6 in condensate line 33 is discharged through line 34, while the rest is passed to pump 3. If desired, an external source of liquid may be connected to the pump suction. The pump increases the pressure to 100 barg and the liquid is supplied to apparatus 4 in water supply line 31. Line 31 enters on the cold end of the apparatus and the liquid is heated and evaporated by heat exchange with the cycle fluid before it exits in steam discharge line 32 at 400°C.

Oxygen enriched cycle fluid in cycle fluid outlet line 12 is fed to the compressor part of a gas turbine. This gas turbine is of conventional configuration but is specially designed to operate with the given cycle fluid. A combustion chamber is supplied with oxygen enriched cycle fluid at 35 barg and natural gas through line 51. Oxygen is consumed by combusting the natural gas, and steam is supplied to the combustion chamber in steam discharge line 32. At least part of the steam supplied to the combustion chamber dissociate, which results in a lower concentration of carbon monoxide in the cycle fluid exiting the combustion chamber. A further effect is to lower the outlet tempera- ture to the desired 1350°C. The cycle fluid is then passed to an expander where energy is extracted. After exiting the expander, the cycle fluid is supplied to the apparatus 4 through cycle fluid supply line 11.

Air is supplied to the cold end of heat exchanger 5 in air supply line 21. A means for inducing airflow through the apparatus by increasing the total pressure to 0,1 barg is installed upstream line 21. The air is heated by counter- current heat exchange with hot, oxygen depleted air exiting the oxygen selective ion transport membranes in line 24. The air exits heat exchanger 5 at 535°C and is passed through line 23 to apparatus 4 where it is heated to 675°C by heat exchange with the cycle fluid. The hot air then flows past the oxygen selective ion transport membranes and oxygen is transferred from the air to a part of the cycle fluid. It is here preferable for the cycle fluid and the air on the two sides of the ion transport membranes to flow in a configuration as close to counter-current as possible in order to maximise the partial pressure differential. The oxygen depleted air exiting the ion transport membranes 8 is then passed to heat exchanger 5 in line 24 and is there cooled by heat exchange with air as described above. Cooled, oxygen depleted air is discarded to the atmosphere in discharge line 22 at 50°C and with 2,6 mol% oxygen.

The above described power cycle has a predicted net electrical efficiency of 47 %.

The essential characteristics of the present invention are described completely in the present document . The given examples illustrate possible usage of the subject invention, but does not seek to include all possible setups for usage of the invention. It should be emphasized that not all as- pects of the method need to be implemented for the method to be advantageous. The given examples state specific values at various locations in the cycle. It is obvious to the competent reader that large deviations in these values may be made without departing from the scope of the invention. Further it will be obvious that many features normally present in power producing apparatus will be required, but are not shown on the schematic diagrams.

Claims

C l a i m s

1. Method for cooling and oxygen enrichment of a cycle fluid in power producing process allowing very low emissions of carbon dioxide to the atmosphere characterised in

transporting a flow of cycle fluid mainly composed of water and carbon dioxide (11) on a cycle fluid side, transporting a flow of gas carrying oxygen (21) on an oxygen supply side and transporting a flow of a fluid mostly composed of water (31) on a steam side,

transferring heat from said cycle fluid to said fluid mostly composed of water, transferring oxygen from said gas carrying oxygen to said cycle fluid to produce an oxygen-depleted gas (22) on the oxygen supply side and an oxygen enriched cycle fluid stream (12) on the cycle fluid side,

at least part of said cycle fluid (11) being passed, directly or indirectly, from a power producing device, and at least part of said cycle fluid (12) being passed, directly or indirectly, to a power producing device,

said fluid mostly composed of water changing from a mostly liquid phase (31) to a mostly gaseous phase (32) and being passed to the cycle fluid in said power producing process.

2. Method according to claim 1, characterised in that said fluid mostly composed of water in gaseous phase (32) is led to a reaction zone where at least part of the water in said fluid dissociate and reduce the concentration of carbon monoxide in the fluid exiting said combustion zone (11) .

3. Method according to claim 1, characterised in that the oxygen concentration in the cycle fluid is increased by removing condensed liquid (33, 34) from said cycle fluid after said cycle fluid has been cooled to a temperature where most of the water vapour in said cycle fluid has condensed.

4. Method according to claim 3, characterised in that at least part of said condensed liquid from the cycle fluid (33) is in fluid communication with said steam side inlet (31) .

5. Method according to claim 1, characterised in that said cycle fluid (21) is generated in a combustion process using said oxygen transported to said cycle fluid as the main oxidant source in combustion of a car- bon containing fuel (51) .

6. Method according to claim 1, characterised in that the cycle fluid flow is split into at least two streams, where at least one is not oxygen enriched and is passed from said power producing process (13) .

7. Method according to claim 1, characterised in that said gas carrying oxygen (21) is air.

8. Method according to claim 1, characterised in that oxygen selective ion transport membranes (8) oper- ating at an elevated temperature at least in part divide said oxygen supply side from said cycle fluid side.

9. Method according to claim 8, characterised in that a net transport of oxygen from said oxygen supply side to said cycle fluid side is achieved by a mechanism in which a differential in oxygen partial pressure over said ion transport membranes (8) is the main driving force .

10. Method according to claim 8, characterised in that a means for heating the cycle fluid (11) is installed upstream said oxygen selective ion transport membranes .

11. Method according to claim 8, characterised in that a plurality of surfaces is arranged to achieve a net heat transfer from said cycle fluid side, a net heat transfer to said steam side, a net heat transfer to said oxygen supply side upstream said oxygen selective ion transport membranes (8) and a net heat transfer from said oxygen supply side downstream said oxygen selective ion transport membranes (8) .

12. Method according to claim 1, characterised in that oxygen molecule selective membranes at least in part divide said oxygen supply side from said cycle fluid side and a net transport of oxygen from said oxy- gen supply side to said cycle fluid side is achieved by a mechanism in which a differential in oxygen partial pressure over said oxygen molecule selective membranes (8) is the main driving force.

13. Method according to claim 1, characterised in that said power producing device (2) comprises at least one gas turbine .

14. Method according to claim 1, characterised in that said power producing device (2) comprises at least one reciprocating engine.

15. Method according to claim 1, characterised in that said power producing device (2) comprises at least one steam turbine fed with steam from a steam boiler extracting heat from said cycle fluid (211) .

16. Method according to claim 1, characterised in said cycle fluid side is passed through ducting at close to atmospheric pressure, said oxygen supply side is partially or wholly passed on the inside of tubes, said tubes at least in part passing through said ducting.

17. Method according to claim 1, characterised in said cycle fluid side is passed through ducting at close to atmospheric pressure, said steam side is partially or wholly passed on the inside of tubes, said tubes at least in part passing through said ducting.