DE10356276A1 - Method for recovery of carbon dioxide from biogas comprises compressing biogas and treating product with regenerated absorption stream so that carbon dioxide and trace materials are removed to enrich absorption stream - Google Patents

Abstract

Recovery of carbon dioxide from biogas comprises compressing biogas to produce a first process stream; feeding this to an absorption column and passing it through in counter-current to a regenerated absorption stream such that carbon dioxide is removed and the absorption stream is enriched with it, and passing enriched absorption stream to stripping stage to recover carbon dioxide stream and a carbon dioxide-depleted stream. Method for recovery of carbon dioxide from biogas containing methane, carbon dioxide and trace materials, especially hydrogen sulfide, comprises: compressing the biogas to produce a first process stream (3); feeding this to an absorption column (5) and passing it through in counter-current to a regenerated absorption stream (12) so that carbon dioxide and trace materials are removed and the absorption stream is enriched with them; and passing the enriched absorption stream (5b) to a stripping stage (8) to recover a carbon dioxide stream (11) and a carbon dioxide-depleted stream which is used as regenerated absorption stream (12). At least one of the process streams (3, 5b) or components (6a, 6b) of these obtained by enrichment with carbon dioxide is chemically or physically treated to remove hydrogen sulfide; or a third product stream is removed from the absorption column at a position between the inlet for the regenerated absorption stream and ! the inlet for the first process stream. Independent claims are included for: (1) apparatus for carrying out the method; and (2) apparatus for recovery of carbon dioxide from biogas containing methane, carbon dioxide and trace materials, especially hydrogen sulfide, comprising a compressor (2, 4), an absorption column, a stripping unit and a purification unit (7).

Description

In the wet pressure gas scrubbing at 6-10 bar operating pressure, a process for the physical separation of easily soluble impurities at 2-5 bar preferably and / or for chemical, preferably oxidative, separation of impurities with preferably hydrogen peroxide is integrated, so as to remove impurities from the gas phase to place in a pre- or post-treatment stage in the liquid, preferably aqueous phase during the treatment. Accompanying substances are, in particular, strongly odorous and sometimes toxic substances such as sulfides, hydrogen sulfide, organic S-containing compounds, amines, silicon-containing compounds or substances derived from CH ₄ . An advantage of the invention is that by integrating the separation into the treatment process, the proportionate operating costs for the carbon dioxide can be reduced and / or a carbon dioxide with high purity can be obtained. Advantageously, the quality of methane in the treated biogas can also be improved. The already in steady state reactive state of the components to be removed is utilized.

In order to fertilize greenhouses with carbon dioxide, natural gas is usually converted into electricity and the purified waste gas is led into greenhouses (see 2 ). In the purification of exhaust gases, combustion-derived nitrogen oxides (NOx), carbon monoxide (CO), sulfur dioxide (SO ₂ ), ethylene, etc. are removed. There is no experience with biogas (see 2 ). One possibility is to treat biogas to natural gas quality and forward it to the power plant (see 3 ). The biogas upgrading plant also releases CO ₂ as co-product. While relatively high purity methane, ie residues of non-hazardous carbon dioxide (1-2% by volume) and traces of toxic hydrogen sulfide (H ₂ S, less than 5 mg / m ³ ) can be produced, carbon dioxide is usually present in biogas disturbing and sometimes poisonous accompanying substances loaded.

Accompanying substances include inert added or entrained gases such as air, nitrogen, oxygen, but also components produced in the anaerobic fermentation process, such as hydrogen sulphide, the sparingly soluble carbon disulfide (CS ₂ ), the water-insoluble carbon sulfide (COS), Compounds of the mercaptan family (R-SH, in which 1 H is replaced by an aryl or alkyl group, for example, mercaptopropionic, thiophenol, mercaptoethanol and ethanethiol, methanethiol), benzene (C ₆ H ₆ ), toluene (C ₇ H ₈ ), ethylbenzene (C ₈ H ₁₀ ), xylene (C ₉ H ₁₂ ), cumene (C ₉ H ₁₂ ), diphenyl disulfide (C ₁₂ H ₁₀ S ₂ ), amines, aldehydes, terpenes (C ₁₀ , C ₁₅ , C ₂₀ , C ₂₅ , C ₃₀ , C ₄₀ ), organosilicon compounds (silanes, silanols and siloxanes), higher hydrocarbons and ammonia (NH ₃ ). The listing of all these substances does not mean that they are necessarily contained in the biogas. Their appearance is partly suspected and depends strongly on the composition of the substances to be exhausted, the load on the digester and the burden of the digested sludge. However, substances that lie below the detection limit can nevertheless accumulate as solids, liquids that are not soluble in water or in aqueous solution during prolonged operation.

The Contamination of carbon dioxide with these substances is that carbon dioxide along with these substances in the gas treatment plant adsorbed or absorbed, but not methane and then methane is obtained as an energetic product stream. During desorption of carbon dioxide to regenerate the adsorbent or absorbent the co-product carbon dioxide together with the interfering impurities free again. There are more process steps needed to do that To represent carbon dioxide so pure, i. Accompanying substances, in particular Hydrogen sulfide and / or mercaptans and / or carbon sulfur compounds and / or remove ammonia and / or others so far so that the accompanying substance in greenhouses harmful or used in the food industry.

These process steps usually take place before the biogas processing plant (see 1 ) or subsequently (see 3 ). Although cleaning before the treatment plant reduces the H ₂ S contents from approx. 1000 - 3000 ppm to approx. 50 to 300 ppm. However, lowering the concentration is not enough to meet the requirements for CO ₂ usability. Dry CO ₂ should, if possible, have a purity before liquefaction and bottling which exceeds 99% by volume, preferably 99.9% by volume, in order to be used in the food industry. A residual methane content of 1 to 4% by volume in CO ₂ does not interfere in the greenhouse and does not produce any explosive mixtures there either.

Methane is odorless and tasteless. The explosion limits are 5 to 15% by volume methane in air. However, it must be remembered that H ₂ S can still be smelled at concentrations in air of 8 μg / m ³ . At concentrations of 50-150 mg / m ³ in air, it has a sweetish odor, it kills the sense of smell. In water, the taste threshold is between 0.05 and 0.1 mg / liter. The threshold for odor and taste is about 0.1 mg / liter.

In upstream and downstream systems ver Biological systems (trickling filters or biofilters) are usually used which combine the advantage of low operating costs with the disadvantage of susceptibility to concentration collisions with H ₂ S, which can go as far as poisoning the filter. Biofilters are therefore combined with alkaline scrubbers. For chemical neutralization, 1 kg of H ₂ S requires 4.9 kg of NaOH. The purchase price for 1 kg of NaOH is approx. 5.3 EUR / kg. In biofilters, at> 16% oxygen and <80 ppm H ₂ S, the H ₂ S content can be reduced to <1 ppm H ₂ S. In trickling filters, 1/3 air dilution is required when the concentrated CO ₂ volumetric flow is loaded with 300 ppm H ₂ S. The possible reduction of the H ₂ S concentration to <10 ppm with this method is not sufficient for direct use in greenhouses. In addition, the advantage of high CO ₂ concentrations is made up for by the Luftzumischung again.

The desorption from the adsorbent or absorbent of the biogas treatment plant is carried out by relaxation of the working pressure between 6 to 10 bar to ambient pressure and subsequent application of negative pressure and / or at low pressure by stripping with a stripping gas, preferably with air (see 5 . 6 . 7 and 9 ). The concentration of carbon dioxide in the released carbon dioxide flow is much higher at negative pressure (about 90 to 96 vol%) than when stripping with air (10 to 35 vol%) (see 8th and 10 ). It should therefore be the goal in the regeneration of the adsorbent or absorbent to reduce the pressure at least to ambient pressure better still on negative pressure.

In particular, the carbon dioxide-containing gas may be subjected to one or more subsequent purification steps to either discharge it into the environment or to use it. So far, biogas upgrading plants were designed so that the CO ₂ -containing gas was discharged into the environment and had to pass depending on the H ₂ S content, a biofilter.

In order to remove the interfering impurities from the CO ₂ volumetric flow, they must be brought into a reactive state again after being released to ambient pressure. When operating at ambient pressure biological and chemical processes that use water as a solvent, increases compared to the operating pressure of 6-10 bar in the biogas processing plant residence time and contact time until enough H ₂ S has gone into solution. This has an influence on plant size and plant costs. When cleaning, it should be avoided to consume raw materials, which must be disposed of afterwards. This is also the case with activated carbon, especially when it is coated (eg with iodide) to remove hydrogen sulfide. Due to the high operating costs, activated carbon is only suitable for post-purification, ie removal of small amounts of H ₂ S. But even with biofilters, the packing material must be replaced periodically. Biofilters are also sensitive to concentration shocks and can be poisoned or acidified. The electricity consumption of installed drives in pumps and blowers must also be taken into account.

Further should be a constant Chemical use can be reduced to a minimum. degradation products of these chemical additives should not be harmful on the process of Reprocessing.

Absorption with alkaline washes with pure caustic soda or soda are known. Sodium hydroxide, however, causes high operating costs. At the high concentration of carbon dioxide such processes are not useful. Zinc oxide is sometimes used to absorb H ₂ S. Another preferred option is removal by oxidation. For this purpose, potassium permanganate, peracetic acid and in particular hydrogen peroxide H ₂ O ₂ are preferably used. The latter has the advantage that no harmful reaction products are left behind. Hydrogen peroxide is the preferred oxidizing agent. It reacts with hydrogen sulfide to form elemental colloidal sulfur and water. It would even be possible to separate and market the high-purity sulfur. At the same time, ammonia, mercaptans, phosphines, silicon compounds, etc. are oxidized.

The addition of H ₂ O ₂ into the sewerage network for the desulphurisation of wastewater is described in the DE 198 00 312 A1 recommended). Experiences with the addition of H ₂ O ₂ to drinking water in America showed that the H ₂ S concentration could not be lowered below 10 mg / l. Odor problems could not be eliminated even at high dosages of H ₂ O ₂ . H ₂ O ₂ has a very good effect on the very easily soluble in water NH ₃ , which can be removed quantitatively. Hydrogen sulphide H ₂ S, on the other hand, is only partially removed, due to the low solubility of H ₂ S in water. The H ₂ S concentration can not be lowered below the odor and taste thresholds of 0.1 mg / m 3. The addition of H ₂ O ₂ was recommended to remove odors from exhaust air caused by some of the above (www.h2o2.com).

It can be assumed that the oxidation of, for example, H ₂ S is catalyzed by H ₂ O _2, for example, by free iron (II) ions (Fe ²⁺ ) in a concentration of up to 2-3 mg / liter ( US 20020113017 A ). The H ₂ S oxidation with H ₂ O ₂ (1 mg / l) with iron (II) ions as catalyst is in the water of an inland lake (2 mg H ₂ S / l) at atmospheric pressure and 15-20 ° C after 5 min. 90% complete. It is known from the literature that iron chelate complexes can also be used as catalyst. Other processes employ a vanadium content of at least 10 ppm to desulfurize mineral acids ( DE 198 60 709 A1 ). With H ₂ S removal from phosphoric acid, the vanadium concentration is up to 200 ppm ( EP 198 70 612 EP 87 108 529 ).

The After treatment of the carbon dioxide volume flow generates operating costs of different origin, the exploitation of carbon dioxide in the Compared to conventionally commercially available carbon dioxide many times makes it uneconomic. For obvious reasons, the aftertreatment the carbon dioxide-containing volume flow in downstream systems more expensive than if the removal of the accompanying substances during the Treatment of the biogas in the treatment plant itself happens. The Accompaniments are then removed from the steady state. The additional Operating costs are caused by the consumption of hydrogen peroxide, apart from the fixed costs caused by the investment.

Wet pressure gas scrubbing ( 4 ) is often used for the treatment of biogas. It essentially consists of a raw gas separator 1 , a two-stage compressor 2 and 4 , a 14 meter high absorption column 5 , a CH ₄ stress column 6 , a stripping column 8th , a water pump 9 , the regenerated absorption water ( 12 ) and a dryer 14 for methane 14 , In the completely continuous process, a steady state is established. This equilibrium offers the possibility of fractionally separating accompanying substances because they are dissolved under operating conditions such as elevated pressure between 6 to 10 bar and temperature between 7 and 25 ° C. The separation of impurities can be achieved by physical or chemical means or a combination of both.

It is not known that in existing biogas upgrading plants Accompanying substances during the Treatment process to be separated as the invention suggests.

An advantage of the invention is that by integrating the separation into the treatment process, the proportionate operating cost of the carbon dioxide can be reduced. The already in steady state reactive state of the components to be removed is utilized. The separation of the interfering components with physical and / or chemical processes also has positive effects on the purity of the product gas methane after leaving the dryer 14 ,

In particular, hydrogen sulfide can be separated by physical means together with other readily soluble components and impurities with very little absorption of carbon dioxide at about 2 to 5 bar, preferably about 3 bar. This pressure occurs in the way of increasing the pressure to 6 to 10 bar, for example after the first stage of compression 2 , If the biogas in a separate, the column 5 upstream L absorption column 17 (please refer 5 and 9 ) at 2-5 bar with at the head of 17 added absorption water 12 ' is washed, substances with higher solubility than carbon dioxide, in particular ammonia and hydrogen sulfide, absorbed.

When the L absorption column 17 where L represents the higher solubility or the absorption of readily soluble substances, conditions corresponding to this pressure and temperature occur in the lower part of the absorption column 5 on. The absorption water, which is mainly loaded with slightly soluble substances, becomes the absorption column at certain points 5 deducted, which depend on pressure and temperature ( 6 ). In its place is either fresh or regenerated absorption water 12 '' fed to the absorption column. The process can be further developed so that additional substances along the absorption column 5 at different absorption levels, here: height levels, can be deducted.

That as well as the process stream 18 ( 5 and 9 ) with slightly soluble substances laden absorption water 5b and or 5c out 5 may be according to principles of the Claus process or as described below in the system 7 treated with H ₂ O ₂ and then as absorption water 12 reused and on the head 13 the absorption column 5 be added (see 7 . 9 and 6 ).

Chemically, hydrogen sulfide (H ₂ S) can be used as an alternative to physical removal or together with it along with other impurities and impurities after addition of H ₂ O ₂ to the system 7 be removed. The system 7 should be a controlled metering device for H ₂ O ₂ , a storage tank (holding tank), sensor for H ₂ S content and redox potential, a metering device for Fe-II salts, iron tubes for the continuous release of Fe ²⁺ ions, and one in the overflow built-in decanter for mud included. The removal of the by-products by oxidation has the advantage that then neither the product gas carbon dioxide nor the recirculated regenerated absorption water 12 are charged. The consumption of absorption water can be reduced. The chemical oxidation with H ₂ O ₂ is flexible. The dosage can be quickly adapted to changing concentrations of H ₂ S and other interfering substances. Also shock loads can be intercepted. Hydrogen peroxide can eg be metered according to the measured concentration of H ₂ S in the raw biogas and in the holding tank and the redox potential in the holding tank.

hydrogen peroxide can also reliably Growing with microorganisms throughout the treatment plant suppress, by occasionally in higher Cans are added as needed for oxidation.

The addition of peroxide into the absorption column 5 in the absence of physical separation, would be slightly soluble with the L-absorbing column 17 Although the best alternative to the removal of H ₂ S, because the quality of the product gas methane would be even better. However, it should be borne in mind that, depending on the reaction kinetics, charges of H ₂ S of about 0.01 to 1 kg / h can clog the packing material with the sulfur formed after a short time, if it is not ensured that the packing material is removed from the documents can be exempted. For this purpose, suitable devices may be nozzles, through which either treated methane or absorption water is sprayed at high pressure, loosen the packing material and rinse off deposits on the surface. Then must be on the ground in the swamp of 5 Mud can be deducted. However, the sulfur remains until after leaving the absorption column 5 colloidally in solution and does not settle on the packing material 5 still in the CHa-relaxation column 6 is the addition of H ₂ O ₂ in ( 5 ) makes sense. The stay tank in the system ( 7 ) serves to separate the sulfur-containing sludge. In the presence of the physical separation of slightly soluble substances with the L absorption column ( 17 ) or from the absorption column ( 5 ) as described above, the addition of H ₂ O ₂ in ( 5 ) optional. However, it should be avoided that free peroxide enters the treated methane gas stream.

If the addition of hydrogen peroxide is not in the absorption column 5 takes place, it can be loaded in the absorption water 5b and or 5c directly after the absorption column 5 or better after the intermediate first relaxation stage 6 be metered in any case in front of the stripping column 8th , as long as the pressure is not less than 2 bar and the accompanying substances are in a reactive state, ie dissolved. After the addition of H ₂ O ₂ , the absorption water enters the storage tank of the system 7 , Iron II ions (Fe ²⁺ ) may be present in the system 7 , preferably in or in front of the holding tank, preferably in a concentration of up to 2-3 mg / liter, if not due to the process in the absorption water 6b and / or 18 already exist. The residence time is chosen between 10 and 60 minutes, so that all oxidation reactions correspond to the reaction kinetics as completely as possible and the solids can be removed. The aim is to lower the H ₂ S concentration in the separated CO ₂ to below 0.1 mg / m ₃ , which can be considered as an odor threshold. In the holding tank, colloidal sulfur aggregates into larger particles prone to floating layer formation, if necessary with the addition of a flocculation aid. The residence tank can be equipped with stirrers and sedation and sedimentation compartments. The overflow can be passed through a device that completes the separation of suspended particles, eg a cyclone or lamella separator. The purification of the absorption water thus carried out can contribute to foam formation in the absorption column 5 to reduce.

The removal of the accompanying substances as described above makes sense especially in connection with a high CO ₂ concentration in the separated CO ₂ volume flow. As described above, the CO ₂ concentration is in the stripped CO ₂ volumetric flow 11 previously in conventionally operated treatment plants between 10 and 35% by volume. By incorporation of a CO ₂ recovery column 15 vertically above the stripping column 8th for relaxation of under a pressure of 3 to 6 bara standing in the system 7 desulfurized absorption water to atmospheric pressure to obtain a water vapor-saturated CO ₂ volumetric flow 16 with a CO ₂ concentration of 90 - 97% by volume. When the rich absorbent water enters the top of the CO ₂ flash column, a sufficient residence time is preferably provided by means of a manifold and various internals to maintain the expansion to preferably CO ₂ vapor equilibrium pressure slightly above atmospheric (0.02-0) , 1 barg). Advantageously, a level sensor stops at the bottom of the column 15 a liquid barrier for sealing upright. The liquid flows by gravity into the stripping column 8th , preferably on the head. The CO ₂ volumetric flow 11 contains 50 to 80% of the total CO ₂ , on average about 70%. The introduction of the CO ₂ stress voltage column 15 alone makes sense, even if the system 7 not installed (see 8th and 10 ).

The high CO ₂ concentration in the greenhouse allows accurate CO ₂ dosing near the leaf by means of perforated hoses laid on the floor and thus achieves particularly high added value, which can make a major contribution to offsetting the additional costs of biogas upgrading described above. The CO ₂ utilization by the plants is higher than when catalytically purified waste gas from biogas plants is injected into the greenhouse via the plants, which only has 8-10% CO _{2 by} volume. In any case, the stripping air ( 11 ) are used together with engine exhaust in greenhouses (see 3 ).

To get out of the stripping column 8th a CO ₂ volumetric flow with a similar high CO ₂ concentration as in the stream 16 to get the stripping column 8th procedurally be further developed. Instead of a fan, the pressure side stripping air 10 at the bottom of 8th A blower can suck in on the head of 8th be connected. This creates in 8th a vacuum which releases the CO ₂ still dissolved in the absorption water.

The in the figures contained values for physical quantities such as Pressure, temperature and concentration are each for themselves and in the specified Combinations only preferred values.

Insofar as number ranges are indicated for the components of the system variants shown in the figures, the respective upper limits and lower limits also per se are considered to be preferred. For example, the term "10-35 vol.% CO ₂ for the carbon dioxide stream 11 that the carbon dioxide flow 11 preferably has a carbon dioxide content of at least 10 vol.% And the upper limit specified is only a preferred upper limit.

Claims (23)

Process for the recovery of carbon dioxide from a methane, carbon dioxide and associated substances, in particular hydrogen sulfide, containing biogas, in which: a) the biogas is compressed to produce a first process stream ( 3 b) at least part of the first process stream ( 3 ) in an absorption column ( 5 ), which has a column head ( 13 ) and a column bottom, in countercurrent with a regenerated absorption current ( 12 ) depleted of carbon dioxide and impurities and the absorption current ( 12 ) are enriched in carbon dioxide and accompanying substances, c) the carbon dioxide and accompanying substances enriched absorption stream from the absorption column ( 5 ) as the third process stream ( 5b ; 5c ) is removed, d) from the third process stream ( 5b ; 5c ) in a stripping stage ( 8th ) a carbon dioxide-enriched carbon dioxide stream ( 11 ) and a carbon dioxide-depleted volumetric flow e) and from the carbon dioxide-depleted volumetric flow of the stripping stage ( 8th ) the regenerated absorption current ( 12 f) wherein at least one of the process streams ( 3 ; 5b ; 5c ) or a part derived therefrom by carbon dioxide enrichment ( 6b ; 6c ; 7a ) by means of chemical cleaning or physical separation hydrogen sulfide withdrawn or the third process stream ( 5c ) from the absorption column ( 5 ) is discharged at a location between a supply point for the absorption stream ( 12 ) and a feed point for the first process stream ( 3 ) or part ( 3 ' ) of the first process stream ( 3 ) is arranged. Method according to claim 1, characterized in that from the first process stream ( 3 ) in one of the absorption columns ( 5 ) upstream absorption column ( 17 ) a second process stream enriched in carbon dioxide and accompanying substances ( 3 ' ) and a fourth process stream enriched in carbon dioxide and adjuncts ( 18 ) and that at least a part of the second process stream ( 3 ' ) of the absorption column ( 5 ) is supplied to the third process stream ( 5b ; 5c ) to obtain. Method according to the preceding claim, characterized in that the first process stream ( 3 ) in the upstream absorption column ( 17 ) in countercurrent with a regenerated second absorption stream ( 12 ' ) to form the second process stream ( 3 ' ) depleted of carbon dioxide and impurities and the second absorption stream ( 12 ' ) to form the fourth process stream ( 18 ) are enriched in carbon dioxide and accompanying substances. Method according to the preceding claim, characterized in that the second absorption stream ( 12 ' ) from the carbon dioxide-depleted volume flow of the stripping step ( 8th ), preferably from the regenerated first absorption stream ( 12 ) is diverted. Method according to one of the three preceding claims, characterized in that the biogas in a first compression stage ( 2 ) and as the first process stream ( 3 ) of the upstream absorption column ( 17 ) and that the second process stream ( 3 ' ) in one of the upstream absorption columns ( 17 ) downstream second compression stage ( 4 ) and the absorption column ( 5 ) is supplied. Method according to one of the preceding claims, characterized in that in the case of the withdrawal of the third process stream ( 5c ) according to step f) additionally a fifth process stream ( 5b ) discharged from the column bottom and at least a portion of the fifth process stream ( 5b ) of the stripping stage ( 8th ) is supplied. Method according to one of the preceding claims, characterized in that in the case of the withdrawal of the third process stream ( 5b ) according to step f) of the absorption column ( 5 ) between the feeds a third absorption stream ( 12 '' ), the withdrawal of the third process stream ( 5c ) compensates at least in part, the third absorption stream ( 12 '' ) preferably at a height level below the point of withdrawal of the third process stream ( 5c ) is supplied. Method according to the preceding claim, characterized in that the third absorption stream ( 12 '' ) from the carbon dioxide-depleted volume flow of the stripping step ( 8th ), preferably from the regenerated first absorption stream ( 12 ) is diverted. Method according to one of claims 1 to 5, characterized in that the third process stream (5b) from the column bottom of the absorption column (5) 5 ) is discharged. Method according to one of the preceding claims, characterized in that the first process stream ( 3 ) or a part ( 3 ' ) of the first process stream ( 3 ) the column bottom and the absorption stream ( 12 ) the column head ( 14 ) of the absorption column ( 5 ). Method according to one of the preceding claims, characterized in that from the third process stream ( 5b ; 5c ) in one of the absorption columns ( 5 ) downstream CH ₄ recovery column ( 6 ) a methane-enriched volume flow ( 6a ) and a sixth process stream enriched in carbon dioxide and accompanying substances ( 6b ) and the sixth process stream ( 6b ) at least to a part of the stripping stage ( 8th ) is supplied. Method according to one of the preceding claims, characterized in that the third process stream ( 5b ; 5c ) or part of the third process stream ( 5b ; 5c ) in an oxidation system ( 7 ), an oxidizing agent (H ₂ O ₂ ), preferably H ₂ O ₂ , is added to reduce the concentration of one or more of the impurities by oxidation of the one or more impurities. Method according to the preceding claim, characterized in that the oxidizing agent (H ₂ O ₂ ) is added in a holding tank. Method according to one of the two preceding claims in combination with claim 11, characterized in that the oxidizing agent (H ₂ O ₂ ) the sixth process stream ( 6b ) or part of the sixth process stream ( 6b ) is added. Method according to Claim 2 in combination with one of the three preceding claims, characterized in that the oxidizing agent (H ₂ O ₂ ) belongs to the fourth process stream ( 18 ) or part of the fourth process stream ( 18 ) is added. Method according to one of the preceding claims, characterized in that the stripping step ( 8th ) a CO ₂ recovery column ( 15 ) and from at least one of the process streams ( 5b . 5c . 6b . 18 ) or a part of the at least one of the process streams ( 5b . 5c . 6b . 18 ) in the CO ₂ recovery column ( 15 ) by depressurizing a carbon dioxide-enriched volume flow ( 16 ) having a carbon dioxide concentration of at least 80% by volume, preferably at least 90% by volume, and a carbon dioxide-depleted volumetric flow ( 20 ) be won. Process according to the preceding claim, characterized in that the carbon dioxide-depleted volumetric flow ( 20 ) from the CO ₂ recovery column ( 15 ) of the stripping column ( 8th ) and from which the regenerated absorption water ( 12 . 12 ' . 12 '' ) or a part of it is won. Method according to one of the two preceding claims, characterized in that a process stream ( 7a ) which has a reduced concentration of the one or more of the impurities, which is at least one of the process streams, due to the chemical purification, preferably oxidation, wherein preferably only the process stream subjected to the chemical purification ( 7a ) of the CO ₂ recovery column ( 15 ) is supplied. Method according to one of the three preceding claims, characterized in that the carbon dioxide-enriched volume flow ( 16 ) from the CO ₂ recovery column ( 15 ) has a hydrogen sulphide content of 0.1 mg / m3 or less. Method according to one of the preceding claims, characterized in that the carbon dioxide stream ( 11 ) from the stripping stage ( 8th ) has a hydrogen sulfide content of 0.1 mg / m ³ or less. Method according to one of the preceding claims, characterized in that in the stripping step ( 8th ) Ambient pressure or preferably a negative pressure prevails. Plant for the implementation the method according to any one of the preceding claims. Plant for the production of carbon dioxide from a methane, carbon dioxide and impurities, in particular hydrogen sulphide, containing biogas, the plant comprising: a) a compressor ( 2 . 4 ), b) an absorption column ( 5 ), which at an upper feed point an absorption current ( 12 ) and at a lower feed point a process stream obtained from the biogas ( 3 ; 3 ' ), and the at least one discharge point has for a methane depleted and enriched in carbon dioxide and impurities process stream ( 5b ; 5b . 5c ), c) a stripping step ( 8th ) with a supply point for a process stream ( 6b . 18 ; 6c . 18 ; 7a ; 20 ), one to a stripping gas, a recycle point for a regenerated absorption stream and a carbon dioxide-enriched carbon dioxide discharge point ( 11 ), d) and a cleaning system ( 7 ) for a dry cleaning of a carbon dioxide-enriched process stream in the plant ( 6b ; 6b . 18 ).