Method for storing electric energy by production, storage, and dissociation of methane having closed carbon circuit

Abstract

The invention relates to a method for storing electric energy, which comprises the steps a) production of methane from water and soot using electric energy, b) storage of the methane, c) dissociation of the methane into hydrogen and soot, with the hydrogen being used for energy generation, or energy generation by conversion of the methane into soot and water in a cyclic bromination-oxidation process,
wherein the soot formed in the dissociation of methane or in the cyclic bromination-oxidation process in step c) is collected and, in a renewed pass through the method, is used for methane production in step a), so that a closed carbon circuit is formed, and also a system comprising a power-methane conversion plant in which electric power is converted together with soot and water into methane and also a methane-power conversion plant in which methane is converted into hydrogen with elimination of soot.

Claims

1. A method for the production of methane using electric energy and subsequent energy generation, which comprises the steps a) production of methane from water and soot using electric energy, b) storage of the methane, c) dissociation of the methane into hydrogen and soot, with the hydrogen being used for energy generation or, energy generation by conversion of the methane into soot and water in a cyclic bromination-oxidation process, characterized in that the soot formed in the dissociation of methane or in the cyclic bromination-oxidation process in step c) is collected and, in a renewed pass through the method, is used for methane production in step a), so that a closed carbon circuit is formed.

2. The method as claimed claim 1, characterized in that the storage of the methane is effected by feeding into a gas grid.

3. The method as claimed in claim 1, characterized in that the dissociation of the methane and the energy generation according to step c) are carried out at a different place than the production of the methane according to step a).

4. The method as claimed in claim 1, wherein, for the energy generation according to step c) of the method, the hydrogen obtained is (1) converted in a fuel cell into electric energy, (2) used for heating, (3) used for cooling, (4) used as fuel for transport, or a combination of any one of (1) to (4).

5. The method as claimed in claim 1, characterized in that heat arising in the production of methane according to step a) and/or in the energy generation according to step c) is fed into a district heating network.

6. The method as claimed in claim 1, characterized in that the electric energy used in step a) is generated using renewable resources.

7. The method as claimed in claim 6, characterized in that the electric energy is generated by wind power or a solar plant.

8. The method as claimed in claim 1, characterized in that the step of production of the methane is carried out by a Sabatier reaction or by a hydrogenation reaction.

9. The method as claimed in claim 8, characterized in that the hydrogen, which is required in the Sabatier reaction or in the hydrogenation reaction, is obtained from water by electrolysis.

10. The method as claimed in claim 8, characterized in that the soot for carrying out the Sabatier reaction is burnt to form carbon dioxide or the soot is converted in the presence of oxygen and steam into synthesis gas, with the oxygen being obtained by electrolysis of water.

Description

(1) The invention will be described in more detail with the aid of the following drawings, the list of reference symbols and the claims.

THE FIGURES SHOW:

(2) FIG. 1 a scheme of the method for storing electric energy,

(3) FIG. 2 a scheme of a first embodiment of a power-methane conversion plant,

(4) FIG. 3 a scheme of a second embodiment of a power-methane conversion plant,

(5) FIG. 4 a scheme of a third embodiment of a power-methane conversion plant and

(6) FIG. 5 a scheme of a methane-power conversion plant.

EMBODIMENTS OF THE INVENTION

(7) In the following description of working examples of the invention, identical or similar components and elements are denoted by the same or similar reference symbols, with repeated description of these components or elements in individual cases being dispensed with. The figures depict the subject matter of the invention purely schematically.

(8) FIG. 1 shows a scheme of the proposed method for storing electric energy.

(9) FIG. 1 schematically shows a power-methane conversion plant 90 and a methane-power conversion plant 92. The power-methane conversion plant 90 takes electric energy from the power grid 10 when there is a surplus of electric power from renewable energy sources. The renewable energy can be, in particular, wind energy 12 or solar energy 14, but can also originate from other sources 16 such as hydroelectric power.

(10) The electric power is utilized in the power-methane conversion plant 90 to dissociate water H.sub.2O into hydrogen H.sub.2 and oxygen O.sub.2 by means of electrolysis 18. The oxygen O.sub.2 is reacted with soot C from a soot store 30 and water in a coal gasification unit 20 to give synthesis gas containing carbon dioxide CO.sub.2, carbon monoxide CO and hydrogen H.sub.2. As an alternative, it is possible to burn the soot C from the soot store 30 with introduction of oxygen O.sub.2 to form CO.sub.2. Both the combustion and the reaction to form synthesis gas liberate heat which is utilized for preheating the water H.sub.2O which is fed to the electrolysis 18. Likewise, the heat can be used for power generation, with the electric power produced likewise being able to be utilized, via a power line 36, for the electrolysis 18.

(11) The synthesis gas or carbon dioxide formed in the coal gasification unit 20 is subsequently introduced together with the hydrogen H.sub.2 from the electrolysis 18 into the methanation 22. There, methane gas is produced in a Sabatier reaction or a hydrogenation reaction. This is compressed and, after being freed of remaining carbon dioxide, fed into the public gas grid 24 which serves as gas store 26.

(12) When electric energy is required because no wind energy 12 or solar energy 14 is available at the time, methane gas is taken from the gas grid 24 and fed into the methane-power conversion plant 92. There, the methane gas is dissociated into hydrogen H.sub.2 and soot C by means of a hydrogen generator 28. The soot C is introduced into the store 30 where it is available for renewed conversion into methane. The hydrogen H.sub.2 from the hydrogen generator 28 is then supplied to a gas power station 32 which burns the hydrogen and generates electric power. The electric power generated is then again fed into the power grid 10.

(13) As an alternative or in addition, hydrogen 34 for fueling hydrogen-powered vehicles or for other purposes, for example for heating, can be produced by means of a further hydrogen generator 29.

(14) The carbon used is circulated in its entirety, being transported in solid form between the methane-power conversion plant 92, the carbon store 30 and the power-methane conversion plant 90. Complicated separation of carbon dioxide from combustion exhaust gases and transport of gaseous carbon dioxide are thus avoided.

(15) FIG. 2 shows a scheme of a first embodiment of a power-methane conversion plant.

(16) FIG. 2 schematically shows a power-methane conversion plant 90. The plant comprises five sections. In the first section 100, the electrolysis is carried out. Electric power is taken from the power grid 10 and supplied to a water electrolyzer 40. The electrolyzer 40 dissociates water H.sub.2O, which is heated to about 90 C. by means of process steam in a heat exchanger 60, into hydrogen H.sub.2 and oxygen O.sub.2. The electrolysis products are each temporarily stored in a hydrogen container 42 and an oxygen container 44. When sufficient amounts of hydrogen and oxygen are present, methane production is commenced.

(17) For this purpose, soot C is taken at a supply point 78 and introduced into a carbon store 30 in the second section 102. The soot C is taken from the carbon store 30 and fed to a dryer 46. The soot C is introduced together with oxygen O.sub.2 from the oxygen container 44 into a carbon gasification unit 48. The oxygen is preheated by means of a heat exchanger 60 in the second section 102.

(18) In the third section 104, the soot is converted into synthesis gas. The soot and the preheated oxygen react in the presence of steam at a temperature in the range from 900 to 1800 C., preferably from 1200 to 1800 C., in the carbon gasification unit 48 to form synthesis gas. The temperature in the reaction is particularly preferably about 1500 C. The heat liberated in the reaction of the soot to form synthesis gas can be withdrawn by means of a steam generator 52 and utilized as process steam. In the embodiment shown in FIG. 2, the process stream is conveyed via a heat line 80 to a heat exchanger 60 which heats water before entry into the electrolyzer 40. It is likewise possible to use part of the process stream for power generation in a power generator 62. The electric power generated can be fed into the power grid 10 and used for electrolysis.

(19) After gasification of the carbon, hydrogen from the electrolysis is added to the synthesis gas, with a ratio of carbon monoxide from the synthesis gas and hydrogen which is optimal for the hydrogenation reaction being set. Here, a stoichiometric (1:3) to slightly superstoichiometric ratio based on hydrogen is optimal.

(20) The gas mixture then flows into a hydrogenation plant 54 in the fourth section 106, where the synthesis gas is converted over a catalyst into methane according to the reaction CO+3H.sub.2=CH.sub.4+H.sub.2O. Once again, the heat liberated in the hydrogenation reaction can be withdrawn by means of a steam generator 52 and utilized as process steam. In the embodiment depicted, the heat is conveyed via a heat line 80 to a heat exchanger 60 which heats the oxygen before entry into the carbon gasification unit 48. A further heat exchanger 60 preheats the hydrogen before mixing with the synthesis gas. The methane gas is cooled by means of a heat exchanger 60.

(21) In the last section 108, the methane gas is passed through a carbon dioxide removal 56. In the carbon dioxide removal 56, CO.sub.2 remaining in the methane gas is separated off, for example by means of polyimide hollow fiber membranes, and recirculated to the methanation process. The CO.sub.2 is fed together with the oxygen O.sub.2 back to the carbon gasification unit 48. The methane gas is subsequently compressed by means of a methane compressor 58. Heat generated here is removed by means of a further heat exchanger 60 before the methane gas is fed into the public gas grid 24. Here, the gas grid 24 serves firstly as transport medium and secondly as gas store.

(22) Process heat which is not required for preheating the water, the hydrogen or the oxygen can be fed via a further heat exchanger 60 into a district heating network 76 and utilized further.

(23) FIG. 3 shows a scheme of a further embodiment of a power-methane conversion plant.

(24) FIG. 3 schematically shows a second embodiment of a power-methane conversion plant 90. The plant once again comprises five sections. In the first section 100, the electrolysis is carried out. Electric power is taken from the power grid 10 and supplied to a water electrolyzer 40. The electrolyzer 40 dissociates water H.sub.2O, which is preheated to about 90 C. by means of process steam in a heat exchanger 60, into hydrogen H.sub.2 and oxygen O.sub.2. The electrolysis products are each temporarily stored in a hydrogen container 42 and an oxygen container 44.

(25) When sufficient amounts of hydrogen and oxygen are present, methane production is commenced.

(26) For this purpose, soot C is taken at a supply point 78 and introduced into a carbon store 30 in the second section 102. The soot C is taken from the carbon store 30 and fed to a dryer 46. The soot C is introduced together with oxygen O.sub.2 from the oxygen container 44 into a carbon combustion unit 50.

(27) In the third section 105, the soot is burned together with the oxygen O.sub.2 from the electrolysis to form carbon dioxide CO.sub.2. The oxygen is preheated by means of a heat exchanger 60. The heat liberated in the combustion of the soot C can be withdrawn by means of a steam generator 52 and utilized as process steam. The process steam can be conveyed via a heat line 80 to the soot dryer 46. It is likewise possible to use part of the process steam for power generation in a power generator 62. The electric power generated can be fed into the power grid 10 and used for the electrolysis.

(28) In the fourth section 106, the hydrogen obtained in the electrolysis is then added to the CO.sub.2 produced, with a ratio of carbon dioxide to hydrogen which is optimal for the Sabatier reaction being set. A stoichiometric (1:4) to slightly superstoichiometric ratio based on hydrogen is optimal. The gas mixture then flows into a hydrogenation plant 54 where it is converted over a catalyst into methane. The heat liberated in the Sabatier reaction CO.sub.2+4H.sub.2CH.sub.4+2H.sub.2O can likewise be withdrawn by means of a steam generator 52 and utilized as process steam. In the embodiment shown in FIG. 3, the process steam is conveyed via a heat line 80 to the heat exchanger 60 in order to preheat water before entry into the electrolyzer 40. Before being mixed with the carbon dioxide, the hydrogen is heated by means of a further heat exchanger 60.

(29) In the last section 108 the methane gas is passed through a carbon dioxide removal 56. In the carbon dioxide removal 56, CO.sub.2 remaining in the methane gas is separated off, for example by means of polyimide hollow fiber membranes, and recirculated to the methanation process. The CO.sub.2 is conveyed together with the hydrogen H.sub.2 from the electrolysis back to the hydrogenation plant 54. The methane gas is subsequently compressed by means of a methane compressor 58. Heat generated here is removed by means of a further heat exchanger 60 before the methane gas is fed into the public gas grid 24.

(30) Process heat which is not required for preheating the water, the hydrogen or the oxygen can be fed via a further heat exchanger 60 into a district heating network 76 and utilized further.

(31) FIG. 4 shows a scheme of a third embodiment of a power-methane conversion plant.

(32) The power-methane conversion plant 90 shown schematically in FIG. 4 can, as in the two previous embodiments, be divided into five sections. In the first section 100, the electrolysis is carried out. The operating parameters and technical configuration are the same as in the two above-described working examples.

(33) The storage facility for soot is shown in section 102. Soot C is taken at a supply point 78 and introduced into a carbon store 30. From there, the soot C is taken off and transported to a hydrogasification plant 118. Drying of the soot C is not necessary.

(34) In the third section 117, soot is conveyed together with hydrogen and steam to the hydrogasification plant 118. Sand which has been preheated to a temperature in the range from 700 to 800 C. is fluidized there by means of the gas mixture. The sand represents a bed of solids. A pressure in the range from 2.0 to 3.0 MPa (20 to 30 bar) prevails in the apparatus. The H.sub.2/H.sub.2O/soot mixture is converted into methane CH.sub.4 and carbon dioxide CO.sub.2. The ratio of the products CH.sub.4:CO.sub.2 is about 1:1. The product gases and part of the sand bed are discharged from the apparatus. Sand is separated off in the cyclone 120 and fed to the regenerator 119. Since soot is not completely reacted in the hydrogasification plant 118, it is conveyed together with sand to the regenerator 119. In the regenerator 119, the soot is burned in the presence of oxygen from the oxygen container 44. Sand is likewise fluidized here. Regenerated sand is separated off in a further cyclone 120 and returned to the hydrogasification plant 118.

(35) The combustion gases are mixed with reaction products and additional hydrogen from the hydrogen container 42, so that an optimal ratio of carbon dioxide to hydrogen of 1:4 is set. The mixture is cooled to the optimal temperature for the subsequent methanation. In this embodiment, the waste heat is used for steam generation and generation of electric power. Electric power is employed internally for operation of the compressors 58 and also for the water electrolysis 40.

(36) In the fourth section 106, the conversion of the carbon dioxide into methane over a catalyst is carried out in a hydrogenation plant 54. The catalyst can be divided into a plurality of, for example two, sections. The heat of reaction is removed between the sections in order to increase the yield. As an alternative, a reactor cascade can be used for the same purpose. Part of the reaction gas mixture is for this purpose circulated by means of a blower 59. The steam H.sub.2O (g) produced in the downstream steam generator 52 is converted into electric power and utilized for the electrolysis in the electrolyzer 40.

(37) In the last section 108, water H.sub.2O (l) is firstly separated off from the gas mixture by condensation in a separator 81. As described above in respect of the first and second embodiments, the methane gas is subsequently prepared for introduction into the public gas grid 24.

(38) FIG. 5 shows a scheme of a methane-power conversion plant.

(39) A methane-power conversion plant 92 is shown in FIG. 5. In the first section 110 of the plant, methane gas is taken from the gas grid 24. The methane gas is preheated by means of a heat exchanger 60 and subsequently introduced into a plasma hydrogen generator 64. There, a plasma is generated by means of microwave radiation and dissociates the methane gas into carbon (soot) and hydrogen H.sub.2 at a temperature in the range from about 400 to 600 C.

(40) Hydrogen and carbon are taken off from the plasma hydrogen generator 64 and separated from one another at a soot filter 66 in the second section 112. The soot is agglomerated in a granulation device 68. This can, for example, be configured as a drum granulator. The soot is subsequently dried in a dryer 46 before being transported into a carbon store 30. From there, the soot can be loaded by means of a loading facility 79 into a suitable transport container and transported away. The return transport to the power-methane conversion plant can, for example, be effected by means of goods vehicles or railroad cars.

(41) The hydrogen which has been separated off is temporarily stored in a hydrogen container 42 in the third section 114 before being converted into electric power in the fourth section 116. For this purpose, the hydrogen H.sub.2 is mixed with air 82 and subsequently burned in a hydrogen turbine 72. A power generator 62 generates electric power from the hot combustion exhaust gases and this power can be fed into the power grid 10. Part of the electric energy generated is required by the conversion plant itself for operation of the plasma hydrogen generator 64 and is conveyed via the power line 36.

(42) The heat obtained in the combustion of the hydrogen can be utilized as process heat in a steam generator 52. The steam generated can then be utilized via a heat line 80 for, for example, preheating the methane gas. It is likewise possible to feed part of the heat into a district heating network 76.

(43) As an alternative, it is also possible to use a methane-power conversion plant which operates on the basis of a cyclic bromination-oxidation process. Here, heat is firstly generated by means of the cyclic bromination-oxidation process. This heat can be used for power generation and/or fed into a district heating network.

(44) In a further embodiment, a thermal process, e.g. the gas black Degussa process, can be used instead of the plasma process for dissociating the methane.

(45) The invention is not restricted to the working examples described here and the aspects emphasized therein. Rather, many modifications which are a matter of routine for a person skilled in the art are possible within the scope defined by the claims.