DE102015214592A1 - Process for producing a fuel gas and plant for producing a fuel gas with an electrolysis system for electrochemical carbon dioxide utilization - Google Patents

Abstract

Described is a biogas plant with an electrolysis system, e.g. an electrolyzer (1), with an anode (A), a cathode (K) and a membrane (M) for the separation of anolyte and catholyte circuit (AK, KK). The carbon dioxide (CO2) produced during biogas production is recycled to avoid emissions to the atmosphere. With the electrolyzer (1) and, for example, a connected reactor (2), the carbon dioxide (CO2) is processed into essential substances for fuel gas production, which are then added to the biogas and significantly increase its calorific value.

Description

The present invention relates to a production process for a fuel gas. Moreover, the present invention relates to a plant for producing a fuel gas.

State of the art

The gas mixture produced in bioreactors of biogas plants contains approximately 55% methane (CH ₄ ), around 45% carbon dioxide (CO ₂ ) and various traces such as hydrogen sulfide (H ₂ S) or amines. The biogas in this composition can not be fed into the natural gas grid. Compared to type H natural gas in 2011 from Russia, the carbon dioxide from the biogas must be almost completely removed and the lack of higher homologous gases such as ethane, propane or butane, which provide for the increase in heating or heating value must so far expensive to be compensated.

The necessary separation of the carbon dioxide from the biogas has the further disadvantage that its emission into the atmosphere makes a further contribution to climate damage by carbon dioxide.

Currently, about 80% of the world's energy needs are met by the burning of fossil fuels, whose combustion processes cause a worldwide emission of about 34,000 million tonnes of carbon dioxide into the atmosphere each year. Due to this release into the atmosphere, most of the carbon dioxide is disposed of, e.g. for a lignite-fired power plant, up to 50,000 tonnes per day. Carbon dioxide is one of the so-called greenhouse gases whose negative effects on the atmosphere and the climate are discussed. Since carbon dioxide is thermodynamically very low, it can be difficult to reduce to recyclable products, leaving the actual recycling of carbon dioxide in theory or academia.

Natural carbon dioxide degradation occurs, for example, through photosynthesis. In this process, carbon dioxide is converted into carbohydrates in a temporally and on a molecular level spatially divided into many steps. This process is not easily adaptable on an industrial scale. A copy of the natural photosynthesis process with large-scale photocatalysis is not yet sufficiently efficient. Even with less than 1%, the photosynthesis itself is not exactly efficient.

Consequently, it is technically necessary to propose a solution, such as biogas obtained from a biogas plant can be cost-effectively and ecologically brought to feed quality. In particular, the proposed solution should be able to manage only with volatile energy sources. It is an object of the invention to provide an improved fuel gas system and an improved manufacturing method for a regenerative fuel gas.

These objects of the present invention are achieved by a manufacturing method according to claim 1 and by a plant for producing fuel gas according to claim 11. Advantageous embodiments of the invention are the subject of the dependent claims.

Description of the invention

The fuel gas production method of the present invention comprises a first step of passing a catholyte in a catholyte circuit, introducing the catholyte and carbon dioxide into a cathode compartment and bringing them into contact with a cathode where at least a portion of the carbon dioxide is reduced to carbon monoxide, wherein for this first step, an electrolysis system is used for carbon dioxide utilization. It is generated in this electrolysis system, a first product gas mixture having carbon monoxide and hydrogen gas, wherein the first product gas mixture is then transferred to a reactor. The inventive production method for a fuel gas also comprises a second step, in which at least a portion of the first product gas mixture is reacted in the reactor to a second product gas mixture, which gaseous, in particular short-chain, hydrocarbons, in particular propane (C ₃ H ₈ ) and / or butane (C ₄ H ₁₀ ). In a third step, this second product gas mixture is then added to a supply of methane-containing gas (CH ₄ ). Short-chain hydrocarbons are to be understood as meaning aliphatic hydrocarbon compounds having chain lengths of up to seven carbon atoms.

A reactor is to be understood in particular as meaning a chemical reactor which is designed such that chemical processes can take place therein or chemical reactions can be carried out therein. It may be, for example, a stirred tank or a flow tube, which is suitable as a flow reactor. Depending on which type of reaction is to take place in the reactor, it may be a reactor which is highly heat-resistant, for example, for exothermic reactions or via which heat can be added or removed to a reaction. It may be a high-pressure reactor or the reactor may be adapted to various chambers, tubes, etc. a preferred chemical reaction sequence.

The production process according to the invention has the advantage of further processing carbon dioxide into a valuable material. At the same time, a product gas mixture is produced, whose short-chain hydrocarbons, already added in small amounts to a methane-containing gas volume, upgrade this to a fuel gas, as it can be fed into the natural gas grid.

In a particularly advantageous embodiment of the invention, the production method comprises a preceding step in which biomethane is produced from biomass in a bioreactor.

Under a bioreactor is, for example, a fermenter to understand how it is preferably used in biogas plants. In it, the biomass is degraded in an anaerobic process in several steps to biogas and a digestate. The fermenter is a container that is normally airtight and has, for example, an agitator and various measuring and control technology devices to control the process. For this fermentation process mostly biomass of any kind is suitable. As a rule, waste and renewable raw materials are fermented in bioreactors. The resulting biogas is a combustible gas. This contains first around 55% methane (CH ₄ ) and around 45% carbon dioxide (CO ₂ ) and various traces, such as hydrogen sulfide or amines.

In a particularly advantageous embodiment of the invention, the manufacturing process comprises a further preceding step, in the biogas produced as described, the carbon dioxide contained is withdrawn and this carbon dioxide is fed to the electrolysis system for carbon dioxide utilization.

In a particularly advantageous embodiment of the invention, the manufacturing process comprises a further preceding step: After the removal of the carbon dioxide from the biogas, the remaining biomethane is supplied to the supply of methane-containing gas. First, biogas is produced from the fermentation of biomass, this biogas is then removed from the carbon dioxide contained and this carbon dioxide is fed to the electrolysis system for carbon dioxide utilization.

By biomethane is meant a combustible gas mixture which has at least 96 mol% methane (CH ₄ ). Commonly spoken of biomethane, even if the Biomethangasgemisch still traces such as hydrogen sulfide (H ₂ S), but which are quantitatively below 0.01 mol%, predominantly in the ppm range. Where ppm is parts per million and 1 ppm is 0.0001%.

This variant of the manufacturing process has the advantage of being able to completely extract fuel gas from biomass. The coupling of the biomass fermentation to the electrolysis system and the utilization of the carbon dioxide content in the biogas to higher homologs, by means of which the biomethane in turn can be upgraded to a higher heating or heating value, makes this fuel gas production process independent of other energy sources.

The extraction of the carbon dioxide from the biogas can be carried out, for example, via amine scrubbing, by means of aminoethanol compounds or amino acids or alternatively via a pressure swing absorption with water and / or methanol.

In a preferred embodiment of the invention, the electrical energy for the carbon dioxide electrolysis, for example, taken from renewable electricity or energy sources in the manufacturing process. In this case, for example, a respective buffer storage for the carbon dioxide to be utilized as well as for the first product gas mixture is provided in the electrolysis system. Thus, the carbon dioxide utilization can be operated in times in which electric power from volatile energy sources available or, for example, so the carbon dioxide electrolysis even at times of Stromüberangebotes be coupled, regardless of which source. Preferably, the entire manufacturing process uses, where necessary, renewable sources of electricity.

In particular, the production process is operated in the first step, or set the electrolysis system so that the first product gas mixture in addition to carbon monoxide and hydrogen gas also has a proportion of carbon dioxide of at least 0.1 vol .-%. In particular, the proportion of carbon dioxide in the first product gas mixture is at least 1% by volume, advantageously between 1% by volume and 5% by volume, preferably at least 3% by volume. This has the advantage that the reduction reaction does not have to be optimized in all control parameters for a pure production of carbon monoxide and hydrogen gas, moreover, the carbon dioxide content in the subsequent conversion of the first product gas mixture in the second causes a high yield of the desired short-chain hydrocarbons. For the production of long-chain hydrocarbons via carbon monoxide and hydrogen gas as starting materials, the synthesis gas mixture would have to be free of carbon dioxide down to the ppm range. The Rectisol process, for example, a physical acid gas scrubbing, provides purified synthesis gas mixtures whose residual carbon dioxide content is between 2 ppm by volume and 3% by volume.

In a particularly advantageous variant of the production method, the constituents of the first product gas mixture are generated in a common electrolysis step. In particular, the current density at the cathode and / or the pressure and / or the temperature in the cathode space are used as control parameters for the composition of the first product gas mixture. The temperature is preferably varied between 5 ° C and 130 ° C, the pressure between atmospheric pressure and 40bar. The joint production has the advantage that only a single electrolyzer is needed. In aqueous electrolytes, at atmospheric pressure and room temperature, a maximum of 3 g carbon dioxide dissolve per 1000 g solution, ie 0.3 percent by weight. If the electrolyzer is operated at (too) high current density, the conversion of water to hydrogen gas takes place at the cathode as additional competition reduction reaction. Thus, in addition to the pressure, the current density forms a control parameter for generating the desired carbon monoxide-hydrogen gas mixture. In addition, the control parameters are selected so that in the first product gas mixture is also still a carbon dioxide content.

Alternatively, carbon monoxide and hydrogen can be produced in different electrolysis steps, respectively in different electrolysis units, and combined after production in the desired composition to form the first product gas mixture. The division into two electrolysis steps offers the advantage of optimally adapting the electrolysers to the respective reduction reaction and of being able to discharge excess carbon monoxide or hydrogen gas from the system for the subsequent process to the first product gas mixture in order to use it elsewhere. This alternative is of particular advantage for large-scale plants, where less attention is paid to the investment costs but much more emphasis is placed on energy efficiency. Because especially for the reduction reaction to hydrogen gas in the carbon dioxide electrolyzer energy-consuming high overvoltage is needed.

Preferably, in the second step of the production process, a Fischer-Tropsch synthesis is run, by means of which at least part of the product gas mixture in the reactor is converted into a second product gas mixture, which then comprises gaseous hydrocarbons, in particular propane and / or butane. In the described production method, this second product gas mixture can be supplied to the methane reservoir, for example directly, that is to say without an intermediate step for purification.

For this purpose, a Fischer-Tropsch reactor is typically used. Fischer-Tropsch synthesis, sometimes Fischer-Tropsch, is a large-scale coal liquefaction process. In this case, a heterogeneous catalytic conversion of synthesis gas, a carbon monoxide-hydrogen mixture, takes place in a wide range of gaseous and liquid hydrocarbons. The process offers the possibility of producing various types of coal and other carbon sources, liquid low-sulfur synthetic fuels, synthetic motor oils and hydrocarbons as a raw material base for the chemical industry. As a rule, a reactor suitable for the Fischer-Tropsch process thus has a feed for the coal or the synthesis gas, a steam outlet, a gas outlet, which preferably passes directly to a gas scrubber, a water jacket for cooling, a drive for a distributor as well as a rotary grate and an ash sluice.

In the Fischer-Tropsch synthesis, a variety of catalysts is used. The most commonly used are based on the transition metal cobalt, iron, nickel and ruthenium. As carrier find porous metal oxides with large specific surface areas such as kieselguhr, alumina, zeolites and titanium dioxide use. Since it is not in the present synthesis step to produce long-chain hydrocarbons or methanol, but is of particular interest on the short-chain hydrocarbons, can be used with less suitable or poisoned catalysts, as they occur when in the synthesis gas mixture too high a carbon dioxide content of more than 3 vol .-% is present.

The amounts of propane and butane to be produced in the reactor are relatively low, so that only less than 20% of the carbon dioxide present in the biogas after the removal of the carbon dioxide would have to be worked up at all. Thus, the production process can be operated, for example, even at higher capacity so that in the same system used an excess of liquefied gases is generated, which are then not fed to the methane, but can be separately discharged, stored or reused.

The invention thus has the further advantage of being able to simultaneously utilize the exhaust gas resulting from the biogas production in order to avoid the emission into the atmosphere, and to be able to further process it into essential substances for the production of fuel gas.

Systematic studies of the electrochemical reduction of carbon dioxide are still a relatively recent field of development. Only for a few years has there been an effort to develop an electrochemical system that can reduce an acceptable amount of carbon dioxide. Research on a laboratory scale has shown that it is preferable to use metals as catalysts for the electrolysis of carbon dioxide. From the publication Electrochemical CO2 reduction on metal electrodes by Y. Hori, published in: C. Vayenas, et al. (Eds.), Modern Aspects of Electrochemistry, Springer, New York, 2008, pp. 89-189 , Faraday efficiencies can be seen on different metal cathodes, see Table 1. Carbon dioxide, for example, almost exclusively reduced to carbon monoxide at silver, gold, zinc, palladium and gallium cathodes, produced on a copper cathode, a variety of hydrocarbons as reaction products.

For example, predominantly carbon monoxide and little hydrogen would be produced on a silver cathode. The reactions at the anode and cathode can be represented by the following reaction equations: Cathode: 2CO ₂ + 4 e ^- + 4H ⁺ → 2CO + 2H ₂ O Anode: 2H ₂ O → O ₂ + 4H ⁺ + 4e ^-

As can also be seen from Table 1, a large number of hydrocarbons are formed as reaction products, for example, on a copper cathode. Of particular economic interest is, for example, the electrochemical production of carbon monoxide, methane or ethylene, ethanol or monoethylene glycol. These are higher-energy products than carbon dioxide. Carbon monoxide: CO ₂ + 2e ^- + H ₂ O → CO + 2OH ^- ethylene: 2CO ₂ + 12e ^- + 8H ₂ O → C ₂ H ₄ + 12OH ^- Methane: CO ₂ + 8e ^- + 4H ₂ O → CH ₄ + 4OH ^- ethanol: 2CO ₂ + 12e ^- + 9H ₂ O → C ₂ H ₅ OH + 12OH ^- Monoethylene glycol: 2CO ₂ + 10e ^- + 8H ₂ O → HOC ₂ H ₄ OH + 10OH ^- electrode CH ₄ C ₂ H ₄ C ₂ H ₅ OH C ₃ H ₇ OH CO HCOO ^- H ₂ Total Cu 33.3 25.5 5.7 3.0 1.3 9.4 20.5 103.5 Au 0.0 0.0 0.0 0.0 87.1 0.7 10.2 98.0 Ag 0.0 0.0 0.0 0.0 81.5 0.8 12.4 94.6 Zn 0.0 0.0 0.0 0.0 79.4 6.1 9.9 95.4 Pd 2.9 0.0 0.0 0.0 28.3 2.8 26.2 60.2 ga 0.0 0.0 0.0 0.0 23.2 0.0 79.0 102.0 pb 0.0 0.0 0.0 0.0 0.0 97.4 5.0 102.4 hg 0.0 0.0 0.0 0.0 0.0 99.5 0.0 99.5 In 0.0 0.0 0.0 0.0 2.1 94.9 3.3 100.3 sn 0.0 0.0 0.0 0.0 7.1 88.4 4.6 100.1 CD 1.3 0.0 0.0 0.0 13.9 78.4 9.4 103.0 tl 0.0 0.0 0.0 0.0 0.0 95.1 6.2 101.3 Ni 1.8 0.1 0.0 0.0 0.0 1.4 88.9 92.4 Fe 0.0 0.0 0.0 0.0 0.0 0.0 94.8 94.8 Pt 0.0 0.0 0.0 0.0 0.0 0.1 95.7 95.8 Ti 0.0 0.0 0.0 0.0 0.0 0.0 99.7 99.7 Table 1:

The table shows Faraday efficiencies [%] of products produced by carbon dioxide reduction on various metal electrodes. The values given apply to a 0.1 M potassium bicarbonate solution as electrolyte and current densities below 10 mA / cm ² .

Silver-containing and / or copper-containing cathodes and / or catalysts are therefore preferably used in the electrolysis system. The catholyte preferably has water in the first step.

In a preferred variant of the production method, the reactor is operated in the second step of the method according to the invention so that the second product gas mixture in addition to propane and / or butane also has ethane and / or pentane. The simultaneous generation of several different higher homologs of methane is advantageous for the most accurate adaptation of the fuel gas composition to the currently in each case be fed into the natural gas network fuel gas or widened the number of possible catalysts.

In a particularly advantageous variant of the production process, an excess of the second product gas is generated in the second step and this or its constituents are discharged from the reactor for separate further processing and bottled, for example, in order to supply them to additional recycling. This has the advantage, in addition to the low proportion of higher homologs, which is necessary for the upgrading of methane to a feedable fuel gas, to be able to produce even more recyclables. This can make the manufacturing process and the equipment used more economical and thus add another economic aspect to the CO ₂ utilization and biomass utilization.

The invention also includes a plant for producing a fuel gas, which has an electrolysis system for carbon dioxide utilization. This electrolysis system comprises an electrolyzer having an anode in an anode compartment, a cathode in a cathode compartment, wherein the cathode compartment has at least one access for carbon dioxide and is configured to bring the incoming carbon dioxide into contact with the cathode where it is at least partially reduced to carbon monoxide wherein the system comprises a first gas line, which is connected to the electrolyzer and configured to remove a first product gas mixture from the catholyte circuit of the electrolyzer and transfer it to a reactor. In addition, the plant comprises a reactor which is designed to convert the first product gas mixture into a second product gas mixture and the plant comprises a methane reservoir, which is connected to the reactor via a second gas line so that the second product gas mixture can be supplied to the methane reservoir. The invention thus embodied has the advantage that carbon dioxide can be utilized electrochemically and can be converted into valuable substances which upgrade methane in a methane reservoir to a fuel gas. The methane reservoir can be connected to a bioreactor so that it can be fed from the bioreactor with biomethane. This has the advantage that in addition to the carbon dioxide utilization for the production of fuel gas biomethane can be obtained from biomass and can be further processed to Biobrenngas.

Alternatively or additionally, the methane reservoir can be connected to a synthesis reactor so that it can be fed from the synthesis reactor with synthetically produced methane. This has the advantage of being able to also combine synthetically produced methane with the advantageous fuel gas production process according to the invention with the carbon dioxide utilization and production step for gaseous short-chain hydrocarbons.

In the plant for producing a fuel gas, a Fischer-Tropsch reactor is preferably used as the reactor.

The system described can be modified by one or more additional reactors so that, in addition to the fuel gas for feeding into the natural gas network or alternatively other products can be produced.

The simplest way to use biogas to date is its direct power generation via an internal combustion engine and generator. Higher profits have so far been achieved by being cached and only at times of high electricity prices a re-conversion took place. The use in the natural gas network is actually the most sensible option, because it protects fossil resources. With the production method according to the invention and the described plant for the production of fuel gas can now be dispensed with the addition of previously fossil propane and butane and in a compact process that can be based entirely on renewable energies feed conditions for the fuel gas can be guaranteed.

Examples and embodiments of the present invention will be further exemplified with reference to FIGS 1 to 4 the attached drawing:

1 shows an arrangement of a biogas plant with a carbon dioxide electrolyzer,

2 shows a three-chamber structure of an electrolytic cell with gas diffusion electrode,

3 shows a PEM structure (polymer electrolyte membrane) of an electrolytic cell,

4 shows a greatly enlarged view of a gas diffusion electrode (GDE).

1 schematically shows an example of a plant in which, as described, in addition to a carbon dioxide reduction unit biogas is generated, which is then upgraded by means of higher homologous, which are provided from the carbon dioxide utilization, to a fuel gas with feed quality.

Top left in the 1 First, a power plant K _{reg is shown} symbolically, which preferably generates electric current I from a regenerative energy source such as solar, wind, or water energy. This current I, for example, at least the electrolysis system 1 made available. Additionally or alternatively, excess current from another source can also be used.

The electrolysis system 1 comprises a voltage source U for the electrolyzer comprising an anode A in an anode compartment AR, a cathode K in a cathode compartment KR and two separate electrolyte circuits, an anolyte circuit AK passing through the anode compartment AR, and a catholyte circuit KK passing through the cathode compartment KR is running. Anolyte and catholyte side are connected to each other via a membrane M, which extends between the anode AR and cathode chamber KR. Alternatively to the 2-chamber diagram of the electrolyzer in the 1 this can also be done, for example, in a 3-chamber design with gas diffusion electrode GDE as in 2 or in a polymer electrolyte membrane assembly PEM with porous electrodes as in 3 shown to be executed.

Both circuits AK, KK are each equipped with at least one pump P, but possibly also several pumps P in the course, which ensure a continuous flow of electrolyte through the electrolysis chambers AR, KR. In addition, new electrolysis educts are brought to the electrodes A, K by the pumping of the electrolyte and discharged electrolysis products from the electrolyzer. In the catholyte KK also an educt inlet EE and preferably an Eduktreservoir ER is provided, via which the carbon dioxide CO ₂ can be metered into the electrolyte. The electrolysis products and by-products can be taken from the cycles AK, KK. For this purpose, for example Gasabscheidebecken GA in both circuits AK, KK are provided. In the anolyte circuit AK, a by-product is withdrawn via a product outlet NPA, which may be, for example, oxygen gas O ₂ or chlorine gas Cl ₂ , depending on which electrolyte and which anode material is used. From the catholyte, a product mixture is preferably removed via the product outlet PA, which contains carbon monoxide CO, hydrogen gas H ₂ , and, for example, also a small residual fraction of unused carbon dioxide CO ₂ . This product gas mixture from the catalytic cycle KK is then illustrated by a follower arrow, a Fischer-Tropsch reactor 2 fed.

The Fischer-Tropsch reactor 2 has at least one vapor outlet DA and a product outlet for gaseous products PA. The product gas mixture from the CO ₂ electrolyzer 1 is via a synthesis gas inlet SGE in the Fischer-Tropsch reactor 2 initiated. Therein, propane and butane are preferably produced. Steam inlet and outlet DE, DA can also be used for heat regulation or alternatively for cooling. For example, any excess heat from other steps may be used. Since in fact only small amounts of propane and butane are needed for the upgrading to the feed gas, it is possible the vast majority of in the electrolysis plant 1 produced syngas mixture other facilities to provide, where the synthesis gas mixture can be further processed, for example, to methane. Conceivable is the coupling with a Sabatier plant, in which carbon dioxide CO _{2 is converted} with hydrogen H ₂ into methane CH ₄ and water: CO ₂ + 4H ₂ → CH ₄ + 2H ₂ O. CO + 3H ₂ → CH ₄ + H ₂ O

Even more efficient is the conversion of carbon monoxide CO with hydrogen H ₂ , as less energy is involved in the formation of water.

Accordingly, it is also for this further processing of the product gas mixture produced in the electrolysis plant 1 not absolutely necessary to extract an electrolysis product completely pure. Again, a residual amount of unconsumed carbon dioxide CO _{2 is} not a disadvantage.

That in the Fischer-Tropsch reactor 2 produced gas product now contains at least propane and butane. Long-chain hydrocarbons could also be produced by the Fischer-Tropsch process; however, the synthesis gas mixture used would have to be free of carbon dioxide CO ₂ and hydrogen gas H ₂ up to the PPM range. This would again very high demands on the electrolysis system 1 put. So here, for example, a residual carbon dioxide gas CO _{2 is accepted} in the synthesis gas mixture and the Fischer-Tropsch process are produced with so-called poisonous catalysts in high yields short-chain hydrocarbons such as propane and butane.

The product gas from the Fischer-Tropsch reactor 2 which contains butane C ₄ H ₁₀ and propane C ₃ H ₈ and ethane C ₂ H ₆ is finally from the biogas plant 3 obtained methane CH ₄ added to obtain a fuel gas mixture having a composition which is suitable for feeding into the natural gas network.

In bioreactors of biogas plants 3 a gas mixture is produced which has about 55% methane CH ₄ , about 45% carbon dioxide gas CO ₂ and various traces, such as hydrogen sulfide H ₂ S. This product mixture can not yet be fed into the natural gas grid. First of all, the carbon dioxide CO _{2 must} be removed therefrom, which can be carried out, for example, via amine scrubbing with aminoethanol compounds and amino acids or alternatively via pressure swing absorption with water and methanol. The thus separated carbon dioxide then becomes the carbon dioxide recovery unit, namely the electrolysis system 1 fed.

Especially in comparison with the characteristic values for currently typically used natural gas of type H, it is also clear that the biogas initially lacks the necessary higher homologous gases such as ethane, propane or butane for the calorific value, see table. Composition natural gas type H mol% carbon dioxide CO ₂ 0.17 nitrogen N ₂ 0.82 oxygen O ₂ <0.01 methane CH ₄ 96.99 Ethan C ₂ H ₆ 1.42 propane C ₃ H ₈ 0.43 Butane C ₄ H ₁₀ 0.07 pentanes C ₅ H ₁₂ 0.01 Hexane and higher KW C ₆ H ₁₄₊ <0.01 Gas characteristics Billing calorific value H _{s, n} kWh / m ³ 11.190 Calorific value (calculated) Hi _{, n} kWh / m ³ 10,090 Ratio H _{i, n} / H _{s, n} Hi _{, n} / .9017 Accounting standard density rho _n kg / m ³ .7420 Relative density (calculated) d Air = 1 .5740 Wobbe index W _sn kWh / m ³ 14,770 Wobbe index W _{i, n} kWh / m ³ 13.320 Methane number ( +2 ) (calculated) MZ kWh / m ³ 90 fuel Calorific value (in MJ / kg) Calorific value (in MJ / kg) Calorific value (in MJ / m ³ ) Calorific value (in MJ / m ³ ) Calorific value (in kWh / m ³ ) hydrogen 141.8 119.972 12.745 1,783 2,995 Carbon monoxide 10,103 10,103 12,633 12,633 3,509 Blast furnace gas 1.5-2.1 1.5-2.1 2.5-3.4 2.5-3.3 from 0.695 to 0.917 city gas 18.21 16.34 19 ... 20 17 ... 18 4.72 to 5.00 natural gas 36 ... 50 50,0,13 39.819 35.883 9.968 methane 55.498 50.013 39.819 35.883 9.968 Ethan 51.877 47.486 70.293 64.345 17.874 Ethylene (ethene) 50.283 47.146 63.414 59.457 16.516 Acetylene (ethyne) 49.912 48.222 58.473 56.493 15,693 propane 50.345 46.354 101.242 93.215 25,893 n-butane 49.5 45.715 134.061 123.81 34.392 i-butane 49.356 45.571 133.119 122.91 34.142

Since, as already mentioned, only relatively small amounts of ethane, propane and butane are necessary, it is also conceivable to fill in the amine scrubbing part of the carbon dioxide CO ₂ separated from the methane gas CH ₄ together with the water and the hydrogen sulphide H ₂ S.

In the 4 is still an enlarged schematic representation of the gas diffusion electrode GDE shown, as shown in the 2 is used:
From a gas reservoir GR behind the porous cathode K, the carbon dioxide CO ₂ pushes into the catholyte in the cathode space KR under elevated pressure. Arrows indicate the flow direction of the catholyte in the cathode space KR. The carbon dioxide CO ₂ is introduced into the gas diffusion electrode GDE via a carbon dioxide inlet CO ₂ -E. A laboratory monitor preferably works in a pressure range of 2 bar. The carbon dioxide CO ₂ is eg after a pressure swing absorption in the electrolyzer 1 fed. This can be operated for example as alkali electrolyzer.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited non-patent literature

• Electrochemical CO2 reduction on metal electrodes by Y. Hori, published in: C. Vayenas, et al. (Eds.), Modern Aspects of Electrochemistry, Springer, New York, 2008, pp. 89-189 [0027]

Claims (13)

Manufacturing method for a fuel gas - in which an electrolysis system ( 1 ) is used for carbon dioxide utilization, - in which in a first step, a catholyte is guided in a catholyte (KK), the catholyte and carbon dioxide (CO ₂ ) introduced into a cathode chamber (KR) and in contact with a cathode (K) where at least a portion of the carbon dioxide (CO ₂ ) is reduced to carbon monoxide (CO), - in the so in the electrolysis system ( 1 ) is produced a first product gas mixture comprising carbon monoxide (CO) and hydrogen gas (H2), and which in a reactor ( 2 ), in which, in a second step, at least part of the first product gas mixture in the reactor ( 2 ) is converted to a second product gas mixture having gaseous hydrocarbons, in particular propane (C ₃ H ₈ ) and / or butane (C ₄ H ₁₀ ), and wherein - in a third step, the second product gas mixture a supply of methane-containing gas (CH ₄ ) is added. A manufacturing method according to claim 1, wherein in a preliminary step in a bioreactor ( 3 ) Biomethane is produced from biomass. Production process according to claim 2, wherein in a further preceding step - first biogas is produced, - the biogas then contained carbon dioxide (CO ₂ ) withdrawn and this the electrolysis system ( 1 ) is supplied to the carbon dioxide recovery and in which - after the removal of the carbon dioxide (CO ₂ ) from the biogas remaining biomethane is supplied to the stock. Manufacturing method according to one of the preceding claims, in which the electrolysis system ( 1 ) is operated in the first step so that the first product gas mixture in addition to carbon monoxide (CO) and hydrogen gas (H2) also has a proportion of carbon dioxide (CO ₂ ) of at least 3 vol .-%. Manufacturing method according to one of the preceding claims, wherein the components of the first product gas mixture are produced in a common electrolysis step. A manufacturing method according to claim 5, wherein in the electrolysis step the current density at the cathode (K) and / or the pressure and / or the temperature in the cathode space (KR) are used as control parameters for the composition of the first product gas mixture. Production process according to one of the preceding claims 1 to 4, in which carbon monoxide (CO) and hydrogen (H ₂ ) are produced in different electrolysis steps. Production process according to one of the preceding claims, in which in the reactor ( 2 ) a Fischer-Tropsch synthesis is driven. Production process according to one of the preceding claims, in which the reactor ( 2 ) is operated in the second step so that the second product gas mixture in addition to propane (C ₃ H ₈ ) and / or butane (C ₄ H ₁₀ ) and ethane (C ₂ H ₆ ) and / or pentane (C ₅ H ₁₂ ) , Production process according to one of the preceding claims, in which an excess of the second product gas mixture is generated, which for separate further processing from the reactor ( 2 ) is discharged and / or bottled. Plant for the production of a fuel gas - with an electrolysis system ( 1 ) for carbon dioxide utilization, comprising an electrolyzer with an anode (A) in an anode compartment (AR), a cathode (K) in a cathode compartment (KR), wherein the cathode compartment (KR) has at least one access for carbon dioxide (CO ₂ ) and configured to bring the incoming carbon dioxide (CO ₂ ) into contact with the cathode (K), where it is at least partially reduced to carbon monoxide (CO), with a first gas line connected to and configured with the electrolyzer, a first one Product gas mixture from the catholyte (KK) of the electrolyzer to remove and into a reactor ( 2 ), With a reactor ( 2 ), which is configured to convert the first product gas mixture into a second product gas mixture and - with a methane reservoir, which via a second gas line so with the reactor ( 2 ), that the second product gas mixture can be supplied to the methane reservoir. Plant according to claim 11, wherein the methane reservoir is connected to a bioreactor so that it can be fed from the bioreactor with biomethane. Plant according to claim 11 or 12, wherein the methane reservoir is connected to a synthesis reactor, so that it can be fed from the synthesis reactor with synthetically produced methane.

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