DE102012200221A1 - Producing a methane-rich gas, useful as fuel for fuel cell, comprises producing synthesis gas from biomass, and catalytic conversion of the synthesis gas enriched in hydrogen to form a methane-rich wet gas - Google Patents

Abstract

Producing a methane-rich gas comprises (a) producing synthesis gas from biomass, and (b) catalytic conversion of the synthesis gas enriched in hydrogen to form a methane-rich wet gas.

Description

The present invention relates to a process for producing a methane-rich gas.

In order to meet the continuously increasing global energy demand, which increased by about 40% between 1990 and 2010, which is particularly the case for industrialized nations, great efforts are being made globally, on the one hand to increase efficiency in the exploitation of fossil fuels and, on the other hand, to avert this For example, from nuclear energy or systems based on fossil fuels to, for example, systems based on renewable resources or other renewable energy sources such as wind and solar.

In addition to the finite nature of available fossil resources such as oil or natural gas, there are environmental reasons (eg the greenhouse effect) and political / financial (eg a world-wide uneven distribution) reasons that make alternatives to these resources necessary. An example of this is the so-called synthetic natural gas ("synthetic natural gas" or "substitute natural gas"), a natural gas substitute that is produced on the basis of coal, especially lignite, or biomass via synthesis gas. The synthetic natural gas must correspond as closely as possible to the natural gas in its composition and its properties, so that the technical systems that are conventionally wholly or partially operated with natural gas, have to be converted as little as possible.

The production of synthetic natural gas from biomass is conventionally carried out in a multi-layered process which can be divided into the following four sub-processes: (a) conversion (ie gasification (under oxygen deficiency)) of the biomass to a synthesis gas, (b) purification of the synthesis gas thus produced, (c) catalytically converting the purified synthesis gas into a methane-rich gas; and (d) preparing the methane-rich gas so obtained, that is, drying (H ₂ O deposition) and CO ₂ separation.

Sub-process (a) can be formulated as follows: CH _m O _n + z H ₂ O a ₁ CO + a ₂ H ₂ + a ₃ CO ₂ + a ₄ H ₂ O + a ₅ CH ₄ + tars (1)

Equation (1) is not a reaction equation in the strict sense, since starting materials and products are not different (for example, H ₂ O can be found on both sides). Also, the coefficients z, a _{i are} not fixed quantities but variables which depend on each other, ie the proportions, on the reaction conditions, etc. Rather, equation (1) states that a given amount of biomass (CH _m O _n ) and water (H ₂ O) converts to the components on the right side of equation (1), both before and after the reaction ( 1) water is present; the individual reactions can not be taken from equation (1). For wood and typical biomass, a mean value of m = 1.44 and n = 0.66 can be given as a rough orientation.

During gasification, the sulfur, alkaline earth and chlorine components occurring in natural biomass are converted into gaseous compounds. Ash, coke and, in the case of fluidized bed gasification systems Bed material particles are present as aerosols in the gas stream. In sub-step (b), these particles, in particular those which prove to be problematic for the subsequent methanation, are removed and sulfur compounds and alkali compounds are separated off or precipitated.

The products of sub-process (a) after the gas purification step form the educts of sub-process (c): a ₁ · CO + a ₂ · H ₂ + a ₃ · CO ₂ + a ₄ · H ₂ O + a ₅ · CH ₄ → b ₁ · CH ₄ + b ₂ · H ₂ O + b ₃ · CO ₂ + b ₄ · H ₂ (2)

This equation too is not a reaction equation in the strict sense, for it is unknown which chemical elements that form the educts combine to which products. In other words, the gas mixture on the left side contains water as well as the gas mixture on the right side. The coefficients a _i , b _j are therefore again no fixed variables, but variables which depend on one another, ie on the proportions, on the reaction conditions, etc.

The crucial chemical reaction (in the strict sense) contained in equation (2) is the above-mentioned methanation, ie the production of methane according to one of the following concurrent equations: CO + 3H ₂ ↔ CH ₄ + H ₂ O (ΔH = -206 kJ / mol) (3) CO ₂ + 4 · H ₂ ↔ CH ₄ + 2 · H ₂ O (ΔH = -253 kJ / mol, sabatizing process) (4)

Another important chemical reaction is the so-called hydrogen shift reaction occurring simultaneously with the chemical reactions (3), (4): CO + H ₂ O ↔ CO ₂ + H ₂ (ΔH = -42 kJ / mol) (5)

The chemical reactions (1) - (3) are equilibrium reactions, as indicated by the double arrow "↔". This means that the equilibrium can be shifted to the right by addition of educts and addition of products according to the LeChatelier principle.

The reaction described by equation (2) therefore has the disadvantage that it is not completely in the sense of a stoichiometry, so that carbon dioxide (CO ₂ ) must first be separated from the raw gas before the "pure gas" - a methane-rich gas - as a substitute product for natural gas (synthetic natural gas) of the desired utilization can be supplied.

It is therefore an object of the present invention to modify the method described above in such a way that the methane content of the synthesis gas (which is therefore generally referred to as "methane-rich gas" according to the present invention) increases and the proportion of CO _{2 is} reduced, ideally optimal boundary conditions is equal to zero.

This object is solved by the features of claim 1. Further advantageous embodiments are defined in the subclaims.

According to the present invention (claim 1), a method of producing a methane-rich gas comprises the steps of (sub-processes) (a) generating synthesis gas from biomass and (b) catalytically converting the synthesis gas under hydrogen enrichment, thereby producing a humid methane-rich and low-carbon gas becomes. The main advantage of this process is that enrichment with hydrogen reduces CO ₂ emissions. In addition, the systems for the separation of carbon dioxide (CO ₂ ) can be designed smaller and thus more cost-effective.

According to an advantageous aspect of the present invention (claim 2), in step (b) exactly enough hydrogen is added so that the methanation taking place during the catalytic conversion proceeds stoichiometrically. This is the optimization of the method defined in claim 1, wherein advantageously can be dispensed with a device for the removal of the carbon dioxide entirely. The amount per unit time of the supplied hydrogen can either be determined experimentally on the basis of process data such as the type and amount of biomass to be gasified, the amounts of oxygen, water vapor, etc. used for the gasification, the temperature used for the conversion and the like Experience based, the temperature can be constant or can go through a predetermined profile. Alternatively, the amount per unit time of the supplied hydrogen may be regulated, as is the case according to a further advantageous aspect of the present invention (claim 12), according to which the enrichment of the carbon dioxide contained in the moist methane-rich gas is controlled variable.

According to an advantageous aspect of the present invention (claim 3), the hydrogen added in step (b) is regenerative hydrogen. Regenerative hydrogen is - nomen est omen - obtained through the use of renewable or renewable (primary) energy and therefore from the point of view of the energy industry as a secondary energy sources, renewable energy sources are sources that either renew themselves at short notice or their use does not contribute to the depletion of the source, such as wind energy, solar energy, etc. It is important that hydrogen can be used to store energy. An advantage of using hydrogen is the ability to generate it at times when large amounts of primary energy are present (energy peaks) and then stored until the time when energy is needed, for example, for purposes of the present invention or as a fuel a fuel cell.

According to an advantageous aspect of the present invention (claim 4), the regenerative hydrogen is obtained by electrolysis of water. Hydrogen is present in nature in a variety of organic and inorganic compounds. A very simple compound is water (H ₂ O), which is virtually unlimited and easy to handle and safe to handle. Therefore, it is beneficial to this To use compound as a starting material for the production of hydrogen, that is to produce the hydrogen electrolytically under conversion of regenerative electrical energy into chemical energy according to:

That is, the advantageous aspect of the present invention according to claim 4 defines the starting substance from which the hydrogen is recovered, while the advantageous aspect of the present invention according to claim 3 determines that the energy required for it is regenerative.

According to an advantageous aspect of the present invention (claim 5), the water is recovered by drying the synthesis gas and / or the wet methane-rich gas. Assuming an electrolytic production of hydrogen from water, it is not only procedurally and therefore economically, but also ecologically advantageous and useful to use for this purpose that water from the already required drying of the synthesis gas and / or the wet methane-rich gas is won. Under certain circumstances, however, it may also be favorable or even necessary to use additional, external sources.

According to an advantageous aspect of the present invention (claim 6), the synthesis gas is obtained by gasification of the biomass. This has the advantage that renewable raw materials are used. Basically equally possible, but disadvantageous in this respect, is the gasification of coal, as it was previously practiced for the production of so-called city gas.

According to an advantageous aspect of the present invention (claim 7), the gasifier for gasifying the biomass is a fluidized bed gasifier.

According to an advantageous aspect of the present invention (claim 8), the fluidized bed gasifier is an allothermally operating heat pipe reformer. A heat pipe reformer is ideally suited for the production of syngas, which should be methanated. The reason for this is the possibility of being able to influence the ratio of carbon monoxide to hydrogen over the temperature and / or pressure in the process in such a way that it is optimal for the methanation. Due to the pressure-charged fluidized bed, the synthesis gas u. U. not be further compressed, but can be fed directly to the natural gas network after a gas treatment. For heatpipe reformer technology is for example on the WO 00/77128 or the DE 10 2008 055 957 A1 directed.

According to an advantageous aspect of the present invention (claim 9), the oxygen produced in the electrolysis is used for the gasification of the biomass. The oxygen can be used to increase the temperature in the reformer and thus to produce less tars or to achieve an oxidative reduction of existing or produced tars in the synthesis gas. The process management is not strictly autothermal.

According to an advantageous aspect of the present invention (claim 10), the synthesis gas is purified before drying. In this case, a removal of those impurities takes place, which prove to be problematic for the following sub-processes of the method according to the invention. For this purpose, particles that are present in the gas must be separated; in addition, a separation of sulfur and alkali compounds takes place.

According to an advantageous aspect of the present invention (claim 11), the dried synthesis gas is buffered prior to enrichment with regenerative hydrogen.

The above advantages will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. In the drawings are:

1 a schematic representation of a conventional method for producing a methane-rich gas;

2 a schematic representation of the principle of a method for producing a methane-rich gas according to a preferred embodiment of the present invention; and

3 a schematic representation of a technical realization of a method for producing a methane-rich gas according to another preferred embodiment of the present invention.

This in 1 The schematic conventional process is a linear process chain consisting essentially of the following four subprocesses: (i) allothermal gasification, (ii) gas purification, (iii) methanation and shift, and (iv) gas treatment. The sub-processes (i) to (iv) correspond to the sub-processes (a) to (d) described above. Gasification distinguishes between autothermal and allothermal processes. In autothermal gasification, exothermic and endothermic reactions take place in parallel, so that the overall process is independent of external heat input; the exothermic reactions (burns) provide the energy required for (oxygen-reduced) gasification. In the allothermal gasification, an indirect heat supply through a heat exchanger such as in a heat pipe reactor. To carry out the in 1 Carburettor used in the process shown is an allothermal carburetor according to this definition, in which the starting materials biomass and water vapor are converted by the addition of heat from outside into a synthesis gas. The further sub-processes are already described above.

2 shows a corresponding representation according to the present invention, wherein the total process from the second sub-process (b) of the above-described conventional process initially differs in that the synthesis gas between the gas purification and the subsequent methanation (sub-process (c)) is supplied hydrogen. As a result, the chemical conditions in the synthesis gas are changed so that in the ideal case, the synthesis gas present after methanation is free of carbon dioxide and rich in methane (methane-rich gas) (see claim 2), and in the real case, at least the proportion of carbon dioxide in the synthesis gas is reduced in favor of the proportion of methane (see claim 1), wherein the extent of the reduction is a function of the amount of hydrogen supplied.

3 shows a concretization of in 2 schematically shown method in the sense of a more technical-detailed representation according to a further advantageous embodiment of the present invention. In 3 is a gas conversion path from a gasification reactor 10 via a gas purification device 12 , a first water separator 14 , a cache 16 , a methanation reactor 18 and a second water separator 20 , along which in the gasification reactor 10 formed synthesis gas is converted into a dry, methane-rich gas, shown as a solid line. Further, a first water path from the first water separator 14 to an electrolysis unit 22 and a second water path from the second water separator 20 to the electrolysis unit 22 each shown in phantom; Excess water can be removed separately (that of the second water separator 20 to the right in 3 laxative, dot-dash line). And finally, there is an oxygen path from the electrolysis unit 22 to the gasification reactor 10 shown in dashed lines. The gasification reactor is used 10 the production of the synthesis gas from biomass according to step (a) and the methanation reactor 18 the catalytic conversion of the synthesis gas enriched with hydrogen according to step (b) of claim 1 of the present invention. The necessary hydrogen is supplied by the electrolysis unit 22 supplied by the first and second water separators 12 . 20 supplied water electrolytically splits. The in this electrolytic splitting by the electrolysis unit 22 Oxygen generated in turn is via the oxygen path to the gasification reactor 10 fed. If one follows the Gasumwandlungsweg, so lies between the gasification reactor 10 and the gas purifier 12 a relatively moist, contaminated or crude syngas (A in 3 ), between the gas purifier 12 and the first water separator 14 a relatively humid, purified synthesis gas, between the first water separator 14 and the cache 16 a relatively dry, purified synthesis gas, after the supply of hydrogen, a hydrogen-enriched, relatively dry, purified synthesis gas, between the methanation reactor 18 and the second water separator, a relatively humid, methane-rich gas, and after the second water separator, a relatively dry, methane-rich gas as the final product (B in FIG 3 ), that with a pump 24 is transported further. The pump 24 in 3 is representative of all transport device of all fluid streams that are required for their transport.

As it is in 3 is shown, a portion of the relatively dry, purified synthesis gas directly to a gas engine 26 be fed, in turn, a generator 28 drives.

LIST OF REFERENCE NUMBERS

10

 gasification reactor

12

 Gas cleaning device

14

 First water separator

16

 cache

18

 methanation

20

 Second water separator

22

 electrolysis unit

24

 pump

26

 gas engine

28

 generator

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 00/77128 [0024]
• DE 102008055957 A1 [0024]

Claims (12)

Process for producing a methane-rich gas, comprising the steps of: (a) generating synthesis gas from biomass; and (b) catalytically converting the synthesis gas under hydrogen enrichment to produce a humid methane-rich gas. A method according to claim 1, characterized in that in step (b) just enough hydrogen is added so that the taking place during the catalytic conversion methanation proceeds stoichiometrically. A method according to claim 1 or 2, characterized in that the hydrogen added in step (b) is regenerative hydrogen. A method according to claim 3, characterized in that the regenerative hydrogen is obtained by electrolysis of water. A method according to claim 4, characterized in that the water is obtained by drying the synthesis gas and / or the wet methane-rich gas. A method according to claim 5, characterized in that the synthesis gas is obtained by gasification of the biomass. A method according to claim 6, characterized in that the gasifier for gasifying the biomass is a fluidized bed gasifier. A method according to claim 7, characterized in that the fluidized bed gasifier is an allothermally operating heat pipe reformer. A method according to claim 6 or 7, characterized in that the oxygen produced during the electrolysis is used for the gasification of the biomass. Method according to one of claims 5 to 9, characterized in that the synthesis gas is purified before drying. A method according to claim 10, characterized in that the dried synthesis gas is buffered before enrichment with regenerative hydrogen. Method according to one of claims 1 to 11, characterized in that the enrichment is regulated with hydrogen, wherein the carbon dioxide contained in the wet methane-rich gas is detected as a controlled variable.

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