DE102014111298A1 - Process for the production of methane - Google Patents

Abstract

The invention relates to a method for producing methane, comprising the steps of a) providing a sewage treatment plant 1 with at least one digester tower 3, wherein the digester tower 3 comprises at least one device for the supply of a raw sludge, at least one device for the removal of a digested sludge and at least one outlet for the b) provision of a bioreactor 14, wherein the bioreactor 14 at least one device for the supply of a resulting in the treatment plant 1 raw sludge and / or for the supply of the digested sludge from the digester, the digester sludge, at least one device for the supply of methane and carbon dioxide-containing biogas produced in the digestion tower of the sewage treatment plant, at least one device for the supply of hydrogen and at least one outlet for the methane-enriched gas produced in the bioreactor 14, c) producing methane and carbon dioxide-containing em digester gas in the digester 3 of the sewage treatment plant, d) transferring at least a portion of the resulting in the sewage treatment plant sludge and / or at least a portion of the digested sludge from the digestion plant sludge in the bioreactor 14, e) transferring the methane prepared in step c) g) producing methane-enriched gas in the bioreactor 14, wherein during the formation of the methane-enriched gas in the bioreactor 14 adjacent to the raw sludge fed in step d) and or digested sludge no further substrate is present, and h) removing the methane-enriched gas formed in the bioreactor 14 from the bioreactor.

Description

Technical area

The invention relates to a method for producing methane.

State of the art

In the context of the energy turnaround towards renewable energies, methane is of great importance as a chemical energy source with a wide range of uses in the areas of heat generation, fuels or power generation and represents a readily storable form of energy for existing infrastructure in the natural gas grid. In particular in energy conversion systems, which to convert the gas into chemical gaseous energy sources, methane appears to be a suitable energy carrier. Methane is generated from the starting materials carbon dioxide and hydrogen. The hydrogen is usually provided in these systems by electrolysis of water with an electrolyzer using excess flow or other convenient stream. The carbon dioxide required for methane production can come from a variety of sources, such as industrial or combustion gases, but preference is given to using climate-friendly CO ₂ from renewable sources such as biomass, such as biogas plants.

Compared to the chemically catalytic methanation, for example by the Sabatier method come to biological methanation processes in which the biomethane is formed by methanogenic microorganisms without expensive and sensitive catalysts and put less demands on reaction conditions such as temperature and pressure and purity the starting gases CO ₂ and H ₂ .

The WO 2008/094282 A1 describes a biological system for methane production from hydrogen and carbon dioxide using a microorganism culture in a culture medium containing at least one type of methanogenic microorganism. The carbon dioxide comes from an industrial process, while the hydrogen can be obtained, inter alia, by electrolysis using cheap excess current.

According to WO 2012/094583 A1 The biological methane production from CO ₂ and H _{2 is} carried out in a bioreactor using a special strain of the methanogenic microorganism Methanothermobacter thermoautotrophicus, which has a very slow growth rate and uses little of the carbon dioxide offered for biomass construction, but a high percentage of CO ₂ converted into methane.

The WO 2012/110257 A1 discloses a system for storing electrical energy in the form of methane, using electricity from renewable and non-renewable energies for hydrogen production. The hydrogen is introduced together with carbon dioxide into a reactor containing cultures of methanogenic microorganisms in a culture medium, which then produce methane.

In order for economic operation to be possible at all, the bioreactor should deliver high methane productivity relative to the reactor volume. The volume-specific methane formation rate (standard cubic meter of methane per cubic meter of reactor volume per day, Nm ³ / m ³ _RV d) should be at least 10 Nm ³ / m ³ _RV d, preferably at least 30 Nm ³ / m ³ _RV d, in particular of at least 50 Nm ³ / m ³ _RV d.

The technical processes and systems for biological methanation described in the prior art which are suitable for generating "corresponding methane formation rates" use continuously operated bioreactors which are inoculated with precultures of special methanogenic microorganisms. These precultures grow in suitable aqueous culture media with a special composition of salts, nutrients and trace elements.

The biogas formed in fermenters of biogas plants contains, in addition to methane and other gases such as oxygen or hydrogen sulfide, which are contained only in small concentrations, a proportion of carbon dioxide which is in the range of about 30 vol .-% to 55 vol .-%. A gas similar in composition to the biogas accumulates in digestion towers of sewage treatment plants and is referred to as a digester gas or sewage gas. As a rule, the methane content in the digester gas is slightly higher than in agricultural biogas plants.

In order to establish biological methanation on the market in the future, it is necessary, despite these approaches, to provide simple and cost-efficient systems that can be well integrated into existing structures and processes, with high methane yield achieved with the least possible effort and low costs shall be.

Presentation of the invention

The invention, as characterized in the claims, is based on the object to provide a method which is suitable under Using a CO ₂ -containing gas and using gaseous hydrogen to produce biomethane in an efficient manner.

This object is achieved by the method for producing methane according to claim 1. Further advantageous details, aspects and embodiments of the present invention will become apparent from the dependent claims, the description and the examples.

The present invention provides a method for producing methane. The method comprises the steps of a) providing a sewage treatment plant with at least one digestion tower, the digestion tower comprising at least one apparatus for supplying a raw slurry, at least one apparatus for removing a digested sludge and at least one outlet for the methane- and carbon dioxide-containing fermentation gas produced in the digestion tower b) providing a bioreactor, wherein the bioreactor at least one device for the supply of a resulting in the sewage treatment plant sludge and / or for the supply of digested sludge from the digester of sewage treatment plant, at least one device for the supply of the digester in the digester c) production of methane- and carbon dioxide-containing fermentation gas in the digestion tower of the sewage treatment plant, d) transferring, for example, methane- and carbon dioxide-containing fermentation gas, at least one device for the supply of hydrogen and at least one outlet for the resulting in the bioreactor methane-enriched gas at least part of the raw sludge accumulating in the sewage treatment plant and / or at least part of the digested sludge taken from the digester of the sewage treatment plant into the bioreactor, e) transferring the methane- and carbon dioxide-containing fermentation gas produced in step c) into the bioreactor, f) supplying hydrogen b) producing methane enriched gas in the bioreactor, wherein no further substrate is present during the formation of the methane enriched gas in the bioreactor in addition to the raw sludge and / or digested sludge supplied in step d), and h) removing the in the bioreactor formed methane-enriched gas from the bioreactor.

The inventive method is carried out with the aid of a wastewater treatment plant, which is supplemented by a separate bioreactor for biological methanation. This bioreactor is fed with the methane and carbon dioxide-containing biogas produced in the digestion tower of the sewage treatment plant and with externally generated hydrogen. As a substrate in the bioreactor only raw sludge and / or digested sludge from the sewage treatment plant is used. The bioreactor for biological methanation advantageously has a suitable system for introducing gas into the liquid reactor medium for the CO ₂ -containing gas and for the hydrogen. The methane-enriched gas formed is removed from the bioreactor via a product gas line.

With the method according to the invention, a biological methanation is carried out in the context of a sewage treatment plant, which has the advantage that in a wastewater treatment plant already produced from organically polluted wastewater CO ₂ -rich gas is present as CO ₂ source for the biological methanation. The CO ₂ content in the digester gas, which is actually produced as a waste product in a wastewater treatment plant, is upgraded to biomethane by biological methanation. In addition, it has been found that the bioreactor in which the methane-enriched gas is formed can be operated exclusively as a substrate with the raw sludge and / or digested sludge accumulating in the sewage treatment plant. This biological material, which is actually produced as "waste" of the wastewater treatment plant, is thus immediately sent for further utilization.

definitions:

Wastewater treatment plant: Wastewater treatment plant is a wastewater treatment plant. Municipal sewage treatment plants mainly process organically polluted wastewater from the sewage system, but in some cases also accept readily degradable organic waste. Industrial sewage treatment plants produce special, organically polluted wastewater in the respective industry.

Digester gas: In the following, digester gas or sewage gas is to be understood as meaning a gas which is formed in an anaerobic fermentation during the sludge stabilization in a digester column of raw sludge under the action of various microorganisms. It contains as its main components methane and carbon dioxide. In addition, it contains water vapor and optionally small amounts of hydrogen, nitrogen, oxygen, hydrogen sulfide, other trace components such as siloxanes or aromatic hydrocarbons and ammonia. Digester gas is referred to in the present application as "methane and carbon dioxide-containing biogas".

Raw digester gas: Raw digester biogas refers to the digester gas which originates directly from one or more anaerobic digestion towers of the sewage treatment plant and has not been further processed by a digester gas treatment plant.

Biomethane: By biomethane is meant below a methane, which is formed by the action of hydrogenotrophic methanogenic microorganisms in an anaerobic bioreactor with supply of hydrogen-containing and carbon dioxide-containing gas. The term biomethane is to be understood as a distinction from synthetic methane used in chemically catalytic methanation is formed. The biomethane produced in accordance with the present invention can also contain, in addition to CH ₄ , fractions from the educt gases introduced into the bioreactor for methanation, for example unreacted hydrogen and carbon dioxide, so that it does not have to be 100% methane. However, methane is the main constituent of biomethane. In the context of the present application, biomethane is also referred to as "methane-enriched gas".

Bio natural gas: Bio natural gas means a methane gas that can be fed into the natural gas grid according to the applicable guidelines (eg DVGW Guidelines G260, G262 in Germany) and biogenic directly from biomass (feedable biogas) or biological methanation using CO ₂ Origin (feedable biomethane).

Sewage sludge: Sewage sludge is produced during wastewater treatment and is a mixture of water and solids. One distinguishes between raw sludge and digested sludge. Raw sludge accumulates on sewage treatment plants as primary sludge in the mechanical treatment stage or as surplus sludge in the biological stage. Raw sludge generally has a very high water content with correspondingly low solids content. A stabilization of the sewage sludge is achieved by the biodegradable substance of the sewage sludge is reduced by microbial metabolic processes so far that odor emissions and other adverse effects on the environment are largely excluded. This treatment takes place in larger sewage treatment plants in digester towers.

Raw sludge: Raw sludge is untreated sewage sludge that has not yet been stabilized. Raw sludge accumulates on sewage treatment plants as primary sludge or excess sludge.

Primary sludge: Primary sludge accumulates as sewage sludge from the first mechanical treatment stage.

Excess sludge: Excess sludge or secondary sludge is produced in the biological purification of the wastewater in an aerobic stage and is rich in microorganisms. If necessary, tertiary or quaternary sludges may be obtained in further purification stages.

Digested sludge: After fouling in so-called digestion towers, the raw sludge is stabilized and is referred to as digested sludge, which only causes a slight odor nuisance. It contains only 1 to 5 percent dry matter and therefore still needs to be dehydrated. If a sludge separation takes place by mechanical dewatering, the sludge is separated into solid (high dry matter content) and liquid fractions (low dry matter content, sludge water, turbid water). Without sludge separation remains a sludge with a dry matter content (TS content), which lies between that of a solid and a liquid digested sludge component. This sludge must be deprived of water by biological dewatering or sewage sludge drying.

Fouling gas formation: Foul gas formation means the production of a gas with methane and carbon dioxide as essential constituents, which takes place in an anaerobically operated digester along with a degradation of biomass and in which a multiplicity of different microorganisms and different metabolic pathways (for example hydrolysis, acidogenesis, acetogenesis, Methane formation) are involved.

Methanization: Methanation is understood to mean methane formation starting from the gaseous substances hydrogen and carbon dioxide as educt gases. Biological methanation describes the formation of biomethane with the aid of hydrogenotrophic methanogenic microorganisms in an aqueous medium. It follows the chemical equation: 4H ₂ + CO ₂ → CH ₄ + 2H ₂ O.

Methanization Medium: Methanization medium refers to the reactor contents of the bioreactor suitable to carry out a biological methanation. The methanation medium includes a complex medium which is composed of raw sludge and / or digested sludge from the treatment plant and aqueous solutions for diluting it. It contains all the nutrients and trace elements necessary for the growth of corresponding hydrogenotrophic methanogenic microorganisms as well as the hydrogenotrophic methanogenic microorganisms themselves.

Preferably, the methane formation takes place in the bioreactor at a pressure which is higher than the atmospheric pressure of about 1 bar, as is present in digestion towers of conventional sewage treatment plants. Particularly preferably, the pressure in the bioreactor is 1 bar to 30 bar, and particularly preferably at a pressure of q2 to 16 bar. The increased pressure improves the solubility of the gases hydrogen and carbon dioxide in the gas introduction, which is particularly important in hydrogen, so that higher methane formation rates are possible. According to a preferred embodiment of the present invention, after step c) and before step e) of the process according to the invention, steps e01) produced methane and carbon dioxide-containing fermentation gas in a compressor and e02) compressing the methane and carbon dioxide-containing fermentation gas produced in step c) in the compressor to a pressure between 1 bar and 30 bar, preferably a pressure between 2 bar and 16 bar carried out as Step e) the step e) transferring the methane- and carbon dioxide-containing fermentation gas compressed in step e02) into the bioreactor is carried out. The methane- and carbon dioxide-containing digester gas is thus not transferred directly into the bioreactor in this case, but first passed into a compressor, in which then the pressure of the gas is increased to the desired value in the bioreactor.

As already described, the methane- and carbon dioxide-containing crude fermentation gas from the anaerobic digestion towers of the sewage treatment plant, which has a CO ₂ content of about 20% to 45%, serves as source of CO ₂ for the biological methanization in the bioreactor.

In existing sewage treatment plants, especially in Germany, it has not been customary to treat raw sewage gas in such a way that it can subsequently be fed into the gas grid as bio natural gas or used as biogas for the fuel market. In the context of the present invention, however, use of the biogas as a natural gas substitute comes to the fore, so that a digester gas processing plant for raw fermentation gas or methane-enriched fermentation gas makes sense as an additional plant component of a sewage treatment plant. Alternatively, the crude raffinate gas may also initially be supplied to a digester gas treatment plant in which a separation of the methane- and carbon dioxide-containing crude raffinate gas into a methane-rich raffinate gas and a weak-gas-rich, low-carbon dioxide gas take place. The lean gas is usually a waste product of the Faulgasaufbereitung dar. Especially when the Faulgasaufbereitung with the processing methods pressure swing adsorption, amine scrubbing or after a membrane separation process occurs, CO ₂ -rich weak gases as residual gases, the very high CO ₂ levels of over 80% up to 99 , 99% may have, so almost pure CO ₂ correspond. Advantageously, after step c) and before step e) of the process according to the invention, steps c1) transfer of the methane- and carbon dioxide-containing fermentation gas produced in step c) into a digester gas treatment plant and c2) separation of the methane- and carbon dioxide-containing biogas produced in step c) the digester gas treatment plant in a methane-rich fermentation gas and a carbon dioxide-rich lean gas performed and as step e), the step e) converting the carbon dioxide-rich lean gas obtained in step c2) into the bioreactor.

As already mentioned, the methane formation in the bioreactor takes place at a pressure which is higher than the atmospheric pressure of about 1 bar, as is the case in the digestion towers of conventional sewage treatment plants. For the reasons already mentioned, therefore, according to a preferred embodiment after step c2) and before step e) of the preferred embodiment described above, the steps c3) transferring the carbon dioxide-rich lean gas obtained in step c2) into a compressor and c4) compressing the gas in step c2) obtained carbon dioxide-rich lean gas in the compressor to a pressure between 1 bar and 30 bar, preferably a pressure between 2 bar and 16 bar performed and as step e), the step e) converting the compressed in step c4) carbon dioxide-rich lean gas into the bioreactor.

The hydrogen required for the biological methanation is preferably generated by an electrolyzer, which is integrated into the plant consisting of a sewage treatment plant and an additional bioreactor. Preferably, the hydrogen is generated in the electrolyzer using surplus electricity from the power grid by electrolysis from water. The provision of surplus electricity is provided by a control energy box so that the biological methanation plant can participate in the amount of the capacity of the corresponding electrolyzer in the control energy market. In this case, the supply of hydrogen is not continuous, but only at certain irregular time intervals, so that the supply of hydrogen into the bioreactor for biological methanation occurs discontinuously. Alternatively, hydrogen from other sources such as hydrolysis gas, synthesis gas, product gas from chemical reactions, H ₂ biological origin such as produced by algae H ₂ or H ₂ from photocatalysis for biological methanation can be used.

In order to achieve efficient biological methanation of CO ₂ and H ₂ to biomethane, the bioreactor for biological methanation is operated under conditions other than the conditions under which the methane- and carbon dioxide-containing biogas is produced in the digester towers of a sewage treatment plant. While methane production in the anaerobic digestion tower of the sewage treatment plant is operated as usual in the mesophilic range at temperatures up to 45 ° C., in the bioreactor for producing a methane-enriched gas by biological methanation for better methane yield, preferably in a thermophilic range at temperatures above 45 ° C methanized, especially at temperatures of 50 ° C to 100 ° C, more preferably at temperatures of 60 ° C to 75 ° C. According to a preferred embodiment of the present invention, therefore, the bioreactor on a tempering and the temperature control is operated such that the present in the bioreactor, raw sludge and / or digested sludge at a temperature T ≥ 45 ° C, preferred 50 ° C ≤ T ≤ 100 ° C, more preferably 60 ° C ≤ T ≤ 75 ° C is maintained.

The bioreactor for producing a methane enriched gas by biological methanation may be in the form of various reactor types such as a stirred tank reactor or column reactor. It should be made of pressure-resistant and temperature-resistant material, such as stainless steel. In comparison with the anaerobic digestion towers of a sewage treatment plant, the reactor volume of the bioreactor for biological methanization is significantly smaller, for example in a range of 1/10 to 1/100 of the volume of a digester of the sewage treatment plant. The bioreactor for producing a methane-enriched gas by biological methanation is preferably integrated as a retrofittable unit in an existing sewage treatment plant and is in principle transportable as an independent module.

The CO ₂ -containing gas from the treatment plant and the hydrogen for the biological methanation are introduced via supply lines in the bioreactor. Preferably, the two gases are already mixed prior to introduction into the reactor in a gas mixing chamber. Therefore, according to a preferred embodiment of the present invention, after step e02) or after step c4) and before step e) of the preferred embodiment described above, steps c5) transferring the carbon dioxide-rich lean gas compressed in step e02) or in step c4) into one Gas mixing chamber, c6) supplying hydrogen into the gas mixing chamber and c7) mixing at elevated pressure of the gases introduced into the gas mixing chamber in steps c5) and c6), wherein steps e) and f) are steps f1) transferring in step c7 ) gas mixture is carried into the bioreactor.

The volume ratio of the two educt gases fed to the bioreactor corresponds in principle to a ratio of H ₂ to CO ₂ of 4: 1. However, depending on the composition of the methane-enriched gas withdrawn from the bioreactor, this ratio can be varied in a range of about 3: 1 to 5: 1, more preferably in a range of 3.5: 1 to 4.5: 1, if to detect a residual or excess of H ₂ or CO ₂ in the methane-enriched gas. If a higher proportion of unreacted CO _{2 is} measured in the produced methane-enriched gas compared to the H ₂ content, then a stoichiometric ratio of H ₂ to CO ₂ of slightly more than 4: 1 is used. If a higher proportion of unreacted hydrogen is measured in comparison to the CO ₂ content, a stoichiometric ratio of H ₂ to CO ₂ of slightly less than 4: 1 is used, so that in the optimal case neither H ₂ nor CO ₂ in the product gas besides methane are included. Depending on the admission pressure of the H ₂ and CO ₂ -containing educt gases, appropriate compressors or pressure reducers are used to adapt the educt gases to the pressure prevailing in the bioreactor.

The feed of the H ₂ - and CO ₂ -containing educt gases takes place either directly via supply lines into the bioreactor, preferably in the lower region of the bioreactor or via a gas introduction into the substrate supply line for the bioreactor. Advantageously, therefore, the supply of hydrogen into the bioreactor and the supply of the methane and carbon dioxide-containing fermentation gas or the carbon dioxide-rich lean gas in the bioreactor, depending on the composition of the extracted from the bioreactor methane-enriched gas regulated so that the molar ratio of hydrogen to carbon dioxide when added to the bioreactor between 3: 1 and 5: 1, wherein an increased supply of hydrogen in the case of an increased proportion of carbon dioxide in the withdrawn from the bioreactor methane-enriched gas and an increased supply of methane and carbon dioxide-containing biogas and of carbon dioxide-rich lean gas in the case of an increased proportion of hydrogen in the withdrawn from the bioreactor methane-enriched gas.

Particularly suitable systems for introducing gas have been found to be dynamic mixers, multiphase pumps (e.g., Edur pumps), and agitators, particularly gassing agitators (e.g., Ekato) and column reactors. These gas injection systems can be combined with reactor-integrated gas fine distribution systems such as perforated plates, diffusers, sintered materials, membranes. For the gas introduction and distribution to work well, the viscosity of the reactor contents should not be too high.

It has unexpectedly been found that the raw sludge and / or digested sludge from the treatment plant in the methanation bioreactor can serve as a culture and nutrient medium for corresponding methanogenic microorganisms as well as a source of the appropriate hydrogenotrophic methanogenic microorganisms without major adaptations. Corresponding optional adaptations are limited to a dilution of the raw sludge and / or digested sludge from the sewage treatment plant by adding water or aqueous buffer or salt solutions with the aim that a certain TS content is achieved in the methanation medium of the bioreactor. The treatment plant supplies via the supplied CO ₂ -containing gas so both the CO ₂ source for the bioreactor for biological methanation and the culture and nutrient medium and the appropriate hydrogenotrophic methanogenic microorganisms, which convert the gaseously supplied H ₂ and CO ₂ to biomethane , Is in the bioreactor under the specified conditions of pressure and temperature gaseous H ₂ and CO _{2 added} over a period of time (several hours to several days), selective methanogenic microorganisms proliferate and begin biological methanation.

Unexpectedly, the digested sludge and / or raw sludge from a mesophilic-operated digester of a sewage treatment plant can also be used as an inoculum and methanization medium to thermophilically operate a bioreactor for biological methanation. Usually, materials from thermophilic fermenters are used as inocula for thermophilic fermenters. In order to take account of the changed conditions in the bioreactor for the biological methanation in relation to the conditions in the conventional digestion tower of a sewage treatment plant, the material from the sewage treatment plant is optionally adapted so that it is better suited as Methanisierungsmedium and thus in the bioreactor a correspondingly high volume specific Methane formation rate can be achieved. In particular, the content of dry matter is adjusted by adding water or aqueous salt or buffer solutions. According to a preferred embodiment, the bioreactor is supplied with water or an aqueous salt or buffer solution in an amount such that the content of organic dry matter in the bioreactor is from 0.5% to 6%, preferably from 1% to 4%, particularly preferably from 1 , 5% to 3.5%. In addition, trace elements for adapting the living conditions prevailing in the bioreactor to the biocenosis present in the bioreactor are particularly preferably fed to the bioreactor.

According to a particularly preferred embodiment of the present invention, the bioreactor in addition to the raw sludge and / or the digested sludge from the sewage treatment plant as substrate exclusively water or aqueous salt or buffer solutions to adjust the content of dry matter and / or trace elements to adjust the conditions prevailing in the bioreactor living conditions for the present in the bioreactor biocenosis and / or methanogenic microorganisms in the form of cultures or inoculation substrates as an inoculum.

In principle, any type of raw sludge and / or digested sludge from the sewage treatment plant are suitable as substrate for the bioreactor for biological methanation. Excess sludge proved to be particularly suitable among the raw sludges, although it comes from an aerobic stage of the treatment plant and the methanation takes place under anaerobic conditions. Particular preference is given to a liquid raw sludge and / or digested sludge constituent having a DM content (content of dry matter measured in% w / w) of from 0.5% to 7%, preferably from 1% to 5%, particularly preferably from 1.5 % to 3.5% used. Since the content of organic dry matter (oTS) in the raw sludge and digested sludge is about 50 to 80% of the TS content, the oTS content must be set correspondingly lower. Particularly favorable proved that in particular digested sludge and sometimes even excess sludge already incurred in the sewage treatment plant with a corresponding TS content. In addition, the liquid portion of the digested sludge after mechanical septic fluid separation and a TS content of about 0.5 to 1.5% proved to be suitable for use as a methanization medium. This mechanical sludge separation is achieved for example by a separation with a press screw or a corresponding sludge press. Therefore, after step c) and before step d) of the process according to the invention, steps d01) transferring at least part of the digested sludge arising in the sewage treatment plant into a sludge separator and d02) separating the sludge arising in the sewage sludge treatment plant in the sludge separator into a liquid sludge sludge constituent are particularly preferred and a solid digested sludge ingredient is carried out, and as step d), the step d) is carried out by transferring the liquid digested sludge ingredient obtained in step d02) into the bioreactor.

Since in the methanization per mole of CH ₄ each two moles of water are formed, the reactor contents are diluted in the biological methanation over time. Accordingly, an excessive reduction of the TS content is counteracted by a supply of raw sludge and / or digested sludge from the sewage treatment plant with a corresponding dry matter content.

In contrast to prior art culture media, the raw and / or digested sludges taken from the treatment plant and used as the methanization medium in the bioreactor and simultaneously serving as the inoculum are not autoclaved since they are hydrogenotrophic methanogenic microorganisms present in the bioreactor cause the biological methanation, at least in small numbers already included. The hydrogenotrophic methanogenic microorganisms are selectively enriched by the conditions in the bioreactor such as high temperature, elevated pressure and supply of only gaseous educts, while other microorganisms are depleted from the raw and / or digested sludge. The digested sludge must not be brought to anaerobic conditions with appropriate handling, since the digested sludge already comes from an anaerobic process. In addition, the raw and / or digested sludge does not need to be synthetic from a variety of different salts and Trace elements are produced and there is no need to add a reducing agent, which brings a significant overall simplification of the system with it.

Since the raw and / or digested sludge from the sewage treatment plant also serves as a source of the hydrogenotrophic methanogenic microorganisms, in an established bioreactor usually no cultures of methanogenic microorganisms must be added. In certain situations, such as start-up of the bioreactor after initial or refill, it may be advantageous to add methanogenic microorganisms in the form of cultures or inoculum (inoculum) to achieve a faster methane production rate more quickly. Suitable inoculum is, in addition to material derived from a corresponding bioreactor for biological methanation, which is operated with material from the treatment plant, also cultures of hydrogenotrophic methanogenic microorganisms, which are added in the form of pure cultures or mixtures of different hydrogenotrophic methanogenic microorganisms.

The replacement of the bioreactor contents can be carried out both continuously and by regular or irregular batchwise exchange of significant portions of the methanation medium. The exchange rate of the methanation medium in the bioreactor is relatively small compared to bioreactors with synthetic culture media. The bioreactor according to the present invention is used at exchange rates of less than 0.15 / d (15% exchange per day), preferably less than 0.10 / d, more preferably less than 0.05 / d or less than 0, 02 / d operated. The exchange rates apply to the entire bioreactor content, ie to the "culture medium" and the microorganisms contained therein. According to a preferred embodiment, a maximum of 15% of the total amount of sewage sludge present in the bioreactor is taken from the bioreactor per day, more preferably at most 10% of the total amount per day, more preferably at most 5% of the total amount per day, and most preferably at most 2 per day %. When using the liquid portion after the digested sludge separation with a TS content of about 0.5% to 1.5%, the exchange rates are in the range of about 10% per day, but also here no microorganisms must be supplied. When using excess sludge or digested sludge, the replacement rates are smaller.

A device for the retention or immobilization of the methanogenic microorganisms or a recurring or continuous addition of the methanogenic microorganisms is not necessary at these exchange rates, since the methanogenic microorganisms accumulate to the required extent under the given selective conditions. The methanization medium is introduced into the bioreactor from a feed tank, which is fed with raw sludge and / or digested sludge from the sewage treatment plant, via a feed device. Via a discharge device, part of the reactor contents is discharged into a spent methanization medium tank. Since the material is essentially biogenic residual material from the attached sewage treatment plant, it can subsequently be easily returned to the circulation of the sewage treatment plant, so that synergy effects from the combination of a sewage treatment plant with an affiliated separate biological methanization in the bioreactor not only by using CO ₂ from the digestion process for biological methanation, but by handling the same biomass material in both parts of the plant. This simplifies both the handling of the biomaterials by the operating staff, the procurement and disposal of the culture medium for the bioreactor, and the integration of a new plant section for biological methanation into an existing sewage treatment plant. Higher temperatures in the biological methanization also contribute to a sanitation of the material, so that disposal of Methanisierungsmedium is little problematic. Advantageously, therefore, after step h) of the process according to the invention, the steps h1) removal of the sewage sludge accumulating in the bioreactor from the bioreactor and h2) transferring the sewage sludge removed from the bioreactor into the sewage sludge disposal of the sewage treatment plant.

The bioreactor preferably has a system for controlling the temperature of the reactor contents. This is suitable to set the appropriate temperature for biological methanation by default. This is preferably a temperature above 45 ° C, in particular from 50 ° C to 100 ° C, particularly preferably from 60 ° C to 75 ° C. The temperature control system is suitable for both heating and cooling the reactor contents if the reactor contents become too hot during the exothermic methanation. For the heating of the reactor contents, waste heat from the electrolyzer can be used. When cooling the bioreactor, the waste heat can be used to control the temperature of the anaerobic digestion towers of the wastewater treatment plant.

The bioreactor is advantageously also equipped with a pressure-holding valve and correspondingly pressure-stable components, so that a reaction pressure higher than the atmospheric pressure, in particular an overpressure up to 30 bar, preferably an overpressure of up to 16 bar can be set.

According to a further, particularly preferred embodiment, the bioreactor with sensors equipped to measure temperature, pressure and pH. In addition, a system for pH control can be provided. It has been found that in the bioreactor operated according to the invention, especially at elevated pressure and elevated temperature, biological methanation takes place in a relatively wide pH range, namely in a range from about pH 5.5 to pH 8.5. Advantageously, the bioreactor is therefore supplied with acid or lye in an amount such that the pH in the bioreactor is from 5.5 to 8.5, preferably from 6.0 to 7.5.

For all gas flows from and to the bioreactor, both the gas composition and the gas flow (amount of gas per time) are measured.

A control engineering regulates and regulates both the inflows and outflows of Methanisierungsmedium and the gas inflows and outflows into the bioreactor, in particular the ratio of the gas flows of added hydrogen in relation to added CO ₂ -containing gas from the sewage treatment plant. In general, H _{2 is used} to CO ₂ in a ratio of 4: 1; However, a fine control takes place after the composition of the starting gas from the bioreactor. In order to achieve the highest possible biomethane content, neither too much hydrogen nor too much CO _{2 should} be measurable in the starting gas of the bioreactor. Accordingly, there is a fine control, in which the ratio of added H ₂ to CO ₂ of the ratio of 4: 1 may vary slightly up or down, in particular in the range of 3: 1 to 5: 1, in particular in the range of 3.5 : 1 to 4.5: 1.

A Biomethanleitung leads the methane-enriched gas formed in the bioreactor preferably at the upper end of the bioreactor and a further utilization to. As a rule, gas cooling and gas drying and optionally desulphurisation take place before further utilization. If the composition of the methane-enriched gas does not meet the requirements for a feedable gas, for example because too much unreacted CO ₂ or H _{2 is} contained, the methane-enriched gas is fed into the digester gas treatment plant. If the methane-enriched gas formed complies in its gas composition with feed quality, it is fed directly to the gas feed system at the respective site of the sewage treatment plant.

The amount of educt gases supplied depends primarily on the amount of available hydrogen and next to the amount of CO ₂ , which is available from the sewage treatment plant. When using surplus electricity from the power grid for water electrolysis H _{2 is} not continuously available, so that the bioreactor for biological methanation in on / off operation is used. This is not a problem in the bioreactor according to the present invention, since the hydrogenotrophic methanogenic microorganisms in the methanization medium also survive longer phases without supply of educt gases and start with a renewed supply of H ₂ and CO ₂ promptly with the biological methanation.

In addition to the feed-in of biomethane into the natural gas network, there is the possibility of being used as fuel, for example in the form of a bio-natural gas filling station or a material use. In particular, a use of biomethane as a fuel in municipal sewage treatment plants, because so preferably a municipal fleet can be operated on vehicles with a gas station on the premises. As a further possibility of utilization offers in times of too low electricity supply a reconversion of biomethane with simultaneous heat generation, for example via a CHP. In this case you need a buffer for the produced biomethane. A CHP is usually present anyway in a sewage treatment plant, because the treatment plants preferably already require significant amounts of electricity and heat for their own use (pumps for activated sludge basins, heating digestion towers, sewage sludge drying).

In the biological methanation plant according to the present invention, biomethane in a gas quality of more than 95% methane, in particular more than 97% methane, can be produced in the bioreactor. This is due to the specific reaction conditions in the bioreactor for biological methanation such as temperature, pressure, microorganism density of hydrogenotrophic methanogens in the wastewater from the sewage treatment plant as well as suitable H ₂ and CO ₂ gas introduction system also with volume specific methanation rates of at least 10 Nm ³ / m ³ _RV d or of at least 30 Nm ³ / m ³ _RV d or of at least 50 Nm3 / m ³ _RV d possible.

A decisive advantage of the method according to the invention in comparison to other methods known in the prior art is that the cost-effectiveness is increased, because already existing infrastructure and technology as well as on a sewage treatment plant already existing biomass materials (raw and digested sludge) and gases ( Crude gas, lean gas from digester gas treatment) can be used to increase the methane yield through a separate biological methanation device. The reaction conditions and the technical devices for introducing the educt gas H ₂ and CO ₂ differ in the bioreactor for biological methanation of the conditions in the conventional anaerobic digestion towers of a sewage treatment plant, so that significantly higher volume-specific methane formation rates can be achieved here. By the little costly provision of a methanation medium for the bioreactor for biological methanation using biomaterial from the treatment plant will reduce the cost of biological methanation and increase the acceptance of expanding a wastewater treatment plant to a plant with biological methanation.

Brief description of the drawings

To illustrate the invention and to illustrate its advantages, embodiments are given below. The exemplary embodiments will be explained in more detail in connection with the figures. It goes without saying that the statements made in connection with the exemplary embodiments are not intended to limit the invention. Show it:

1 Representation of an embodiment of the method according to the invention in the form of a block diagram;

2 Representation of another embodiment of the method according to the invention in the form of a block diagram.

Ways to carry out the invention

1 shows an embodiment of the method according to the invention in the form of a block diagram. With 1 is a conventional sewage treatment plant called. The bioreactor 14 having plant part is with 2 designated. Pipelines transporting liquid substances are shown by solid lines, pipelines for gases are shown in dashed lines.

Biogenic feedstocks are used as fermentation substrate in the anaerobic digester 3 a sewage treatment plant introduced and fermented there, with methane and carbon dioxide-containing biogas produced. The fermentation can be carried out in one stage in only one digestion tower or in several stages in several digestion towers. The digested sludge is after fermentation in a sludge storage 4 transferred. Fermentation processes in digested sludge storage also result in lower quantities of digester gas. The stabilized digested sludge from the sludge storage 4 is about a sludge separation 5 separated into a solid digested sludge component and a liquid digested sludge component. The liquid digested sludge ingredient is placed in a container 7 stored for liquid sludge, the solid digested sludge component accordingly in a warehouse 6 for solid sludge. The solid digested sludge component is usually dried further and then thermally utilized in waste incineration plants or Zementwerden. The methane- and carbon dioxide-containing digester gas from the digestion tower 3 as well as the sludge storage 4 is via a Rohfaulgasleitung 8th the digester gas processing plant 9 fed. In the digester gas processing plant 9 On the one hand, a treated digester gas with a high methane content and as a waste product CO ₂ -rich lean gas is generated. The treated digester gas is fed via a pipeline into the gas feed system 10 brought where it is for a feed into the natural gas network 11 is finally conditioned.

CO ₂ -rich lean gas from the digester gas treatment plant is via the lean gas line 12 a compressor 13 fed to the corresponding low-CO ₂ lean gas for biological methanation in the bioreactor 14 necessary pressure brings. The hydrogen required for biological methanation in the bioreactor is passed through an electrolyzer 15 generated. Since a large part of the common electrolyzers provides hydrogen under pressure, there is usually no further compressor before the hydrogen is introduced into the bioreactor 14 necessary. Depending on the used Einbringsystem (not shown) for the hydrogen and CO ₂ -rich reactant gases, these are before being fed into the injection system in a gas mixing chamber 16 in which two pipelines are merged, mixed or as separate gases directly into the separate bioreactor 14 supplied (not shown).

The electrolyzer 15 preferably draws the power necessary for the electrolysis of water as an excess current from the power grid. A control energy box 17 regulates when favorable electricity for water electrolysis is obtained. The bioreactor 14 is with a temperature control system 18 connected. On the one hand, the bioreactor must 14 can be heated to the advantageous for biological methanation higher temperature, on the other hand, the bioreactor 14 in times of methanation, give off heat or must be cooled, since the methane formation is exothermic. The waste heat of the electrolyzer 15 in the electrolysis of water can also for the temperature control 18 be used. In accordance with the invention, the bioreactor 14 operated with a medium which is taken directly from the adjacent treatment plant, preferably with the liquid digested sludge component. Liquid sludge is removed from the container 7 for liquid sludge in the storage tank 19 which serves as a reservoir for the methanization medium with which the bioreactor 14 operated for biological methanation.

To the changed conditions in the bioreactor 14 for the biological methanation against the conditions in a conventional digestion tower 3 a sewage treatment plant to take into account, the liquid sludge in the reservoir 19 be adapted so that he as Methanisierungsmedium is more suitable. In particular, the TS content can be adjusted by adding an aqueous medium, or an inoculum containing hydrogenotrophic methanogenic microorganisms can be added. The methanation medium is removed from the receiver 19 in the bioreactor 14 introduced for biological methanation via a pipeline and subsequently at elevated pressure and at elevated temperature, the educt gases H ₂ and CO _{2 are converted} to a methane-enriched gas.

The bioreactor 14 is equipped with sensors for temperature, pressure and pH measurement as well as a system for pH control. The bioreactor 14 for biological methanation is preferably continuously fed with methanization, but with a small compared to synthetic culture media exchange rate of less than 0.15 / d (15% exchange per day), in particular less than 0.10 / d, in particular less than 0.05 / d, in particular less than 0.02 / d.

Part of the contents of the reactor is transferred via a discharge device to a spent methanization medium tank 20 derived. Since the material is essentially biogenic residual material from the attached sewage treatment plant, it is subsequently returned to the circulation of the sewage treatment plant, in particular into the tank 7 for liquid sludge. In the bioreactor 14 formed methane-enriched gas is via the Biomethanleitung 21 either back to the digester gas treatment plant 9 if the composition of the gas does not meet the requirements for a feedable gas or directly for the gas feed system 10 supplied (not shown). If necessary, dewatering and cooling or desulfurization of the biomethane produced before the gas feed or introduction into the digester gas treatment plant is still necessary.

For all gas flows both the gas composition and the gas flow (amount of gas per time) are measured. A control technology controls and regulates both the inflows and outflows of methanation medium and the gas inflows and outflows into the bioreactor 14 , In particular, the ratio of the gas flows of added hydrogen from the electrolyzer 15 in relation to added CO ₂ -containing gas from the sewage treatment plant 1 , In general, H _{2 is used} to CO ₂ in a ratio of 4: 1; However, a fine control takes place after the composition of the starting gas from the bioreactor 14 in a ratio of 3: 1 to 5: 1, in particular in a ratio of 3.5: 1 to 4.5: 1. In order to achieve the highest possible biomethane content, neither too much hydrogen nor too much CO _{2 must be present} in the starting gas of the bioreactor 14 be measurable.

The amount of educt gases supplied in the embodiment described here depends primarily on the amount of hydrogen produced, which is limited by the presence of excess flow and indicated by the control energy box.

In 2 a further embodiment of the method according to the invention is shown in the form of a block diagram. Unlike the in 1 described plant here is methane and carbon dioxide-containing biogas through the Rohfaulgasleitung 22 the compressor 13 fed and together with H ₂ from the electrolyzer 15 in the gas mixing chamber 16 mixed in the ratio of about 1: 4 and then in the bioreactor 14 brought in. There, the biological methanization of the methane content of the methane and carbon dioxide-containing fermentation gas is increased in favor of a reduction of the CO ₂ content, so that a methane-enriched gas is formed. This will be via the Biometh instruction 21 back to the digester gas processing plant 9 provided that the gas quality does not comply with the feed-in requirements or directly into the gas feed system 10 if the gas quality is feedable (not shown).

Alternatively or in addition to the gas feed system 10 For example, the digester gas from the treatment plant and also the methane-enriched gas from the bioreactor can be burned in a CHP plant, generating electricity and heat. The electricity from the CHP is fed into the grid. In this variant, an additional gas storage must be available. In times of surplus electricity, the biomethane produced must not be converted into electricity via the CHP, because there is enough electricity in the grid anyway. The additionally produced biomethane must either be temporarily stored or fed into the gas grid. For this purpose, by switching off the power supply CHP increases the amount of control energy that can be provided. If there is too little power in the network, positive balancing energy can be provided as the CHP can then additionally utilize the previously generated biomethane from the gas buffer and thus produce more electricity. Since a sewage treatment plant has a high intrinsic demand for electricity and heat, alternative use of the generated electricity and heat within the sewage treatment plant is also possible with utilization of the additionally produced biomethane via a CHP.

LIST OF REFERENCE NUMBERS

1

sewage plant

2

Plant part with bioreactor

3

digester

4

Digested sludge storage

5

Digested sludge separation

6

Container for solid sludge ingredient

7

Container for liquid sludge ingredient

8th

Rohfaulgasleitung

9

Digester gas treatment plant

10

Gas feed

11

natural gas network

12

Lean gas line

13

compressor

14

bioreactor

15

electrolyzer

16

Gas mixing chamber

17

Regelenergiebox

18

Temperature control unit

19

storage container

20

Container for spent methanization medium

21

biomethane line

22

Rohfaulgasleitung

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 2008/094282 A1 [0004]
• WO 2012/094583 A1 [0005]
• WO 2012/110257 A1 [0006]

Claims (14)

Method for producing methane comprising the steps of a) providing a sewage treatment plant ( 1 ) with at least one digester ( 3 ), the digestion tower ( 3 ) - at least one device for the supply of a raw sludge, - at least one device for the removal of a digested sludge, - at least one outlet for the methane- and carbon dioxide-containing biogas produced in the digester, b) providing a bioreactor ( 14 ), the bioreactor ( 14 ) - at least one device for the supply of a in the sewage treatment plant ( 1 ) resulting raw sludge and / or for the supply of digested sludge taken from the digester of the sewage treatment plant, - at least one device for the supply of the digester in the digestion plant resulting methane and carbon dioxide-containing fermentation gas, - at least one device for the supply of hydrogen, - at least an outlet for that in the bioreactor ( 14 c) production of methane- and carbon dioxide-containing digester gas in the digestion tower ( 3 ) of the sewage treatment plant, d) transferring at least part of the raw sludge arising in the sewage treatment plant and / or at least part of the sludge taken from the digester of the sewage treatment plant into the bioreactor ( 14 ), e) transferring the methane- and carbon dioxide-containing fermentation gas produced in step c) into the bioreactor ( 14 f) supply of hydrogen into the bioreactor ( 14 g) producing methane-enriched gas in the bioreactor ( 14 ), during the formation of the methane-enriched gas in the bioreactor ( 14 ) in addition to the raw sludge and / or digested sludge supplied in step d), no further substrate is present, h) taking out the product in the bioreactor ( 14 ) formed methane-enriched gas from the bioreactor. Process according to Claim 1, characterized in that, after step c) and before step e), the steps e01) transfer of the methane- and carbon dioxide-containing fermentation gas produced in step c) into a compressor ( 13 ) and e02) compacting the methane and carbon dioxide-containing fermentation gas produced in step c) in the compressor ( 13 ) to a pressure between 1 bar and 30 bar, preferably a pressure between 2 bar and 16 in step e) the step e) transfer of the methane- and carbon dioxide-containing fermentation gas compressed in step e02) into the bioreactor ( 14 ) is carried out. Process according to Claim 1, characterized in that, after step c) and before step e), steps c1) transfer of the methane- and carbon dioxide-containing fermentation gas produced in step c) into a fermentation gas treatment plant ( 9 ) and c2) separating the methane and carbon dioxide-containing fermentation gas produced in step c) in the digester gas treatment plant ( 9 ) are carried out in a methane-rich fermentation gas and a carbon dioxide-rich lean gas and, as step e), the step e) transferring the carbon dioxide-rich lean gas obtained in step c2) into the bioreactor ( 14 ) is carried out. Process according to Claim 3, characterized in that, after step c2) and before step e), steps c3) transfer of the carbon dioxide-rich lean gas obtained in step c2) into a compressor ( 9 ) and c4) compressing the carbon dioxide-rich lean gas obtained in step c2) in the compressor ( 9 ) to a pressure of between 1 bar and 30 bar, preferably a pressure of between 2 bar and 16 bar, and as step e) the step e) transferring the compressed carbon dioxide-rich lean gas obtained in step c4) into the bioreactor ( 14 ) is carried out. Method according to one of claims 2 or 4, characterized in that after step e02) or after step c4) and before step e), the steps c5) converting the compressed carbon dioxide-rich lean gas obtained in step e02) or in step c4) into one Gas mixing chamber ( 16 ) c6) supply of hydrogen into the gas mixing chamber ( 16 ) and c7) mixing at elevated pressure of those in steps c5) and c6) in the gas mixing chamber ( 16 ), and as steps e) and f) the step f1) transferring the gas mixture obtained in step c7) into the bioreactor ( 14 ) is carried out. Method according to one of claims 1 to 5, characterized in that after step c) and before step d) steps d01) transferring at least a portion of the from the digester ( 3 ) of sewage sludge taken from the treatment plant into a sludge separator ( 5 ) and d02) separation of the sludge taken from the digester of the treatment plant in the Digestion Separator ( 5 ) are carried out in a liquid digested sludge component and a solid digested sludge component and, as step d), the step d) transferring the liquid digested sludge component obtained in step d02) into the bioreactor ( 14 ) is carried out. Method according to one of claims 1 to 6, characterized in that the bioreactor ( 14 ) has a temperature control device and the temperature control device is operated such that in the bioreactor ( 14 ) present raw sludge and / or digested sludge at a temperature T ≥ 45 ° C, preferably 50 ° C ≤ T ≤ 100 ° C, more preferably 60 ° C ≤ T ≤ 75 ° C. Method according to one of claims 1 to 7, characterized in that the supply of hydrogen into the bioreactor ( 14 ) and the supply of the methane and carbon dioxide-containing fermentation gas or of the carbon dioxide-rich lean gas into the bioreactor ( 14 ) depending on the composition of the bioreactor ( 14 ) is controlled so that the molar ratio of hydrogen to carbon dioxide upon addition to the bioreactor ( 14 ) is between 3: 1 and 5: 1, wherein an increased supply of hydrogen in the case of an increased proportion of carbon dioxide in the from the bioreactor ( 14 ) and an increased supply of methane and carbon dioxide-containing biogas or carbon dioxide-rich lean gas in the case of an increased proportion of hydrogen in the from the bioreactor ( 14 ) taken methane-enriched gas occurs. Method according to one of claims 1 to 8, characterized in that the bioreactor ( 14 ) Water or an aqueous salt or buffer solution is added in an amount such that the content of organic dry matter in the bioreactor ( 14 ) from 0.5% to 6%, preferably from 1% to 4%, particularly preferably from 1.5% to 3.5%. Method according to one of claims 1 to 9, characterized in that from the bioreactor ( 14 ) per day a maximum of 15% of the total amount in the bioreactor ( 14 ), preferably at most 10% of the total amount per day, more preferably at most 5% of the total amount per day, in particular preferably at most 2% per day. Method according to one of claims 1 to 10, characterized in that the bioreactor ( 14 ) is equipped with sensors for measuring temperature, pressure and pH. Method according to one of claims 1 to 11, characterized in that the bioreactor ( 14 ) Acid or lye is added in an amount such that the pH in the bioreactor is from 5.5 to 8.5, preferably from 6.0 to 7.5. Method according to one of claims 1 to 12, characterized in that the bioreactor ( 14 ) Trace elements for adapting the conditions prevailing in the bioreactor living conditions for the present in the bioreactor biocenosis are supplied. Method according to one of claims 1 to 13, characterized in that after step h) the steps h1) removing the in the bioreactor ( 14 ) sewage sludge from the bioreactor ( 14 ) and h2) transferring the from the bioreactor ( 14 ) are carried out in the sewage sludge disposal of sewage treatment plant.

Images