Title of Invention

A PROCESS FOR OBTAINING BIOGAS BY BIOLOGICAL TREATMENT OF A FERMENTATION MEDIUM IN A FERMENTATION REACTOR

Abstract

The present invention relates to a process for obtaining biogas by biological treatment of a fermentation medium in a fermentation reactor, in which an electron acceptor-containing medium, in particular oxygen containing gas or gas mixture, such as air and/or nitrate and/or nitrite, is fed into the fermentation medium to suppress the formation of unwanted hydrogen sulphide (H2S), characterized in that essentially all the fermentation medium contained in the fermentation reactor (2) is passed through a zone (5) into which the electron acceptor containing medium is fed, with a sufficient time of contact between the electron acceptor-containing medium and the fermentation medium being adjusted in order to suppress H2S formation in the fermentation medium at least to the extent that negligible amounts of H2S are present in the biogas.

Full Text

Description Process and apparatus for obtaining biogas The invention relates to a process for obtaining biogas by bioiogical treatment of a fermentation medium in a fermentation reactor, in which an electron acceptor-containing medium, in particular oxygen-containing gas or gas mixture, for example air and/or nitrate and/or nitrite, is fed into the fermentation medium to suppress the formation of unwanted hydrogen sulphide (H2S) , and to an apparatus for carrying out the process. To obtain biogas, substrates containing organic substances, for example sewage sludges, liquid manure or wet refuse, are kept and, where appropriate, circulated in a container with substantial exclusion of air. During this, the microorganisms contained in the substrates convert the organic substances partly into gaseous substances. This process is referred to as digestion and is normally carried out in bioraactors designed as digestion containers, which are also referred to as fermentation reactors. Depending on the nature of the substrates and of the bioreactor operation, slightly different compositions of the biogas result. The biogas typically contains about 7 0% by volume CH4 and about 30% by volume CO2 . If the substrates also have sulphur compounds, which is usually the case with sewage sludges and wet refuse, these are broken down by the microorganisms to give hydrogen sulphide (H2S), which is eventually found at a concentration of up to 1% by volume in the biogas. Since hydrogen sulphide has toxic and corrosive effects, the hydrogen sulphide content of the biogas must be reduced in order to avoid environmental damage and damage to secondary systems, for example lines and gas engines. In the state of the art, the hydrogen sulphide content of the biogas is reduced to a reasonable level in purification stages downstream of the bioreactor, which stages can be designed, for example, as scrubber, adsorption unit or biological desulphurization system. The need for an additional purification stage results in high capital costs and increases the space required for the complete biogas system. In addition, subsequent purification in the gas phase usually results in waste products, for example Fe sulphide pellets, flowers of sulphur, sulphuric acid etc., which must be disposed of. DD 226,552 Al discloses a process for reducing the hydrogen sulphide content in the biogas, in which iron hydroxide is added in suspended form to the sludge to be treated and is mixed with the latter. The intention of this is chemical binding of the hydrogen sulphide. It is also known to meter iron chloride into the fermentation medium to be treated. However, processes of this type operating with chemical metering are problematic because of the addition of chemicals to the fermentation medium. For example, the introduction of corrosive chloride ions may have adverse effects on the durability of the fermentation reactor. £P 0 143 149 Bl has also already proposed suppression of the formation of hydrogen sulphide in the bioreactor itself. This entails introducing with the fresh sludge or the water into the bioreactor an amount of oxygen such that the biogas which is formed has a residual oxygen content of 0.01 to 3.0% by volume. Elaborate control is necessary for this in order to ensure that, on the one hand, the hydrogen sulphide content in the biogas is adequately reduced and, on the other hand, the oxygen content in the bioreactor is not so high that the toxic effect of the oxygen on the methane-forming bacteria results in excessive interference with biogas formation. Previous solutions for suppressing H2S formation by inhibiting H2S-generating microflora are based on metering air into the fermentation feed or on introducing air at points into the fermentation reactor. Plant practice and our own investigations on this problem have shown that, because of the high rates of oxygen consumption, as a rule metering into the feed or at points is insufficient because the oxygen has been converted biochemically after only a short distance, and thus the interference with the H2S formers in the large-volume fermentation reactor is insuf¬ficient to achieve a significant effect. Attempts are made to remedy this by metering excess air, but this involves an increased safety risk, and the content of inert nitrogen, which is then high, in the biogas results in a reduction in caloric quality. It can furthermore be attempted to improve the feed into the entire reactor by appropriately surface-covering air distribution arrangements on the fermenter base. However, there are doubts about the long-term operating safety of such distribution arrangements for waste and sludge fermentation suspensions with a high dry matter content. The invention is based on the object of providing a process of the type mentioned at the outset, and an apparatus for carrying out the process, which ensure that the biogas leaving the fermentation reactor is substantially free of hydrogen sulphide and that there is negligible impairment of biogas formation, with avoidance of the described disadvan¬tages of the state of the art. This object is achieved by the process of the invention in that essentially all the fermentation medium present in the fermentation reactor is passed through a zone into which the electron acceptor-containing medium is fed, adjusting the time of contact between the electron acceptor-containing medium and the fermentation medium to be sufficient for suppression of H2S formation in the fermentation medium at least to the extent that negligible H2S contents are present in the biogas. The electron acceptor-containing medium which is expediently used is air or another oxygen-containing gas or gas mixture. Another possibility is to employ nitrite or nitrate as electron acceptor. A combination of diverse electron acceptors is also conceivable. The essence of the invention can most simply be described/ in the case of the use of air as electron acceptor-containing medium, in the following way: The problems of the state of the art which have been mentioned are solved by the invention in such a way that it is not attempted to have a substantially homogeneous distribution of air in the complete fermentation chamber; on the contrary/ the entire contents of the fermentation reactor are transported in a defined manner through an oxygen-containing zone with an adequate time of contact between gas and ferment¬ation medium. An industrially simple and residue-free suppression of H2S formation in the fermentation medium is achieved reproducibly in this way. The process according to the invention ensures operationally stable, in plant engineering terms, suppression of H2S formation in the fermentation medium. It is expedient for essentially the whole of the fermentation medium present in the fermentation reactor to be passed several times, preferably at least twice, an hour through the zone. In addition, the electron acceptor-containing medium is preferably metered into the zone in amounts such that the fermentation medium maintains adequate contact, during passage through the zone, with the electron acceptor-containing medium in order to suppress H2S formation in the fermentation medium. In a preferred embodiment of the invention, the time of contact between the electron acceptor-containing medium and the fermentation medium in the zone and/or the amount of the electron acceptor-containing medium metered into the zone is adjusted so that the electron acceptor-containing medium is broken down biochemically to such an extent that process- impairing amounts of electron acceptor-containing medium are no longer present in the biogas. In the case of use of air or an oxygen-containing gas or gas mixtures as electron acceptor-containing medium, this achieves biochemical breakdown of the oxygen to such an extent that process-impairing amounts of oxygen are no longer present in the biogas, This management of the process ensures that there is no impairment of the anaerobic treatment of the organic substances, and thus of the obtaining of biogas, in the fermentation reactor. When air is used as electron acceptor-containing medium, additionally the amount of air fed into the zone per unit time is preferably adjusted so that the nitrogen content resulting in the biogas from the nitrogen content of the air results in a negligible reduction in quality of the biogas in terms of caloric utilization. This makes it possible to utilize the biogas for example as fuel gas without restrictions. A particularly preferred embodiment of the invention provides for using the biogas formed in the fermentation reactor as propellant gas in order to transport the fermentation medium through the zone* If the zone is designed for example as conduit pipe, biogas is pumped into the interior of the conduit pipe. As a consequence of the reduction in the density of the mixture in the conduit pipe and of the buoyant force of the gas, the fermentation medium is transported upwards through the conduit pipe. It is expedient in this connection to adjust the hydraulic conditions by choosing the conduit pipe geometry and the injected biogas stream so that the entire contents of the fermentation reactor are pumped at least twice an hour through the conduit pipe. The invention furthermore relates to an apparatus for obtaining biogas having a fermentation reactor to receive fermentation medium and an offtake line for biogas. The stated object is achieved by the apparatus in that a conduit arrangement is disposed in the interior of the fermentation reactor, with at least one feed line for an electron acceptor-containing medium terminating in the conduit arrangement or in the vicinity of an open end of the conduit arrangement, and means are provided for transporting essentially all the fermentation medium contained in the fermentation reactor through the conduit arrangement. The fermentation reactor thus essentially takes the form of a loop reactor with internal loop in the form of a conduit arrangement, which can be designed, for example, as a conduit pipe. The conduit arrangement is preferably designed as a conduit pipe disposed essentially vertically and centrally in the ferment¬ation reactor. For the injection of biogas as propellant gas, a biogas branch line branching off the biogas offtake line preferably terminates in the conduit arrangement or in the vicinity of an open end of the conduit arrangement. In another preferred embodiment of the invention, the geometry of the conduit arrangement is chosen so that an adequate time of contact between the fermentation medium and the electron acceptor-containing medium is ensured in order essentially to suppress H2S formation in the fermentation medium. Furthermore, the geometry of the conduit arrangement, the size of the fermentation reactor and the dimensions of the biogas branch line are preferably chosen so that essentially all the fermentation medium contained in the fermentation reactor can be passed at least twice an hour through the conduit arrangement. Another embodiment of the apparatus according to the invention provides for the conduit arrangement being designed to be heatable. For this purpose, the conduit arrangement is expediently designed as conduit pipe which has a jacket. The jacket is provided with an inlet and an outlet for hot water. In this embodiment of the invention, several effects which make it possible to obtain essentially H2S-free biogas in a stable process are achieved simultaneously; Circulation of the fermentation medium through the inner loop, designed as conduit pipe, of the fermentation reactor homogenizes the entire contents of the fermentation reactor, Because of the metering in of an electron acceptor-containing medium, for example air, H2S formation in the fermentation reactor is simultaneously suppressed. In addition, the contents of the fermentation reactor are kept, by the heating of the conduit pipe, at an operating temperature which is optimal for the biological treatment of the fermentation medium. An additional effect is also achieved with the vertical, central alignment of the conduit pipe in the fermentation reactor: The fermentation medium sucked in at the lower end of the conduit pipe is preferably pumped by injection of biogas to the upper end of the conduit pipe and is ejected at the upper end of the conduit pipe vertically upwards into the surrounding ferment¬ation medium. The central surface gush thus brought about above the conduit pipe achieves destruction of a floating covering in the fermentation reactor, which in turn has beneficial effects on stability of the process . The invention provides a whole series of advantages: In contrast to processes in the state of the art, no , metering in of chemicals is necessary to suppress H2S. In addition, no disposal of waste products is required. The invention achieves particularly high stability of the process with industrially simple means. Modification of existing biogas plants is possible at minimal expense. In the simplest case it is merely necessary to instal air metering in a conduit pipe homogenization system employed in the fermentation reactor. The invention is to be explained in detail hereinafter by means of an example which is depicted diagrammatically in the figures- Figure 1 shows a flow diagram for a biogas plant with integrated H2S suppression Figure 2 shows a comparison of the operating results of fermentation reactors with and without air metering in the fermentation chamber Figure 1 depicts by way of example a plant for the fermentation of wet refuse. The wet refuse is prepared in pretreatment steps, which are not shown in the figure, to result in pulp and hydrolysate. The pulp and the hydrolysate are fed as fermentation medium through line 1 into the fermentation reactor 2. Methanization of the pulp and of the hydrolysate is carried out in the fermentation reactor 2 . For this purpose, the fermentation reactor 2 is kept under anaerobic conditions, and the contents of the fermentation reactor are circulated. The anaerobic biomass contained in the fermenting pulp and the hydrolysate converts the organic substances partly into CO2 and CH4. The resulting biogas is taken out of the fermentation reactor 2 through line 3. The liquid and/or solid portions of the fermented wet refuse are taken out of the fermentation reactor 2 through line 4, Since the pulp and the hydrolysate also contain sulphur compounds, without further measures there would also be formation of H2S which would eventually be found in the biogas. In order to minimize the unwanted H2S contents in the biogas, in contrast to the state of the art where it is attempted to have substantially homogeneous distribution of air in the entire ferment¬ation chamber, the entire contents of the fermentation reactor are transported in a defined way through an oxygen-containing zone 5 with a sufficient time of contact between oxygen-containing gas and fermentation medium. For this purpose, the fermentation reactor 2 is in the form of a loop reactor with interior loop in the form of a conduit pipe 5 which is disposed centrally and vertically and acts as oxygen-containing zone. Moreoverr biogas which is pumped into the lower part of the interior of the conduit pipe and is branched off from the biogas offtake line 2 through a biogas branch line 6 acts as propellant gas. As a consequence of the reduction in the density of the mixture in the conduit pipe 5 and of the buoyant force of the gas, the fermentation medium is transported upwards through the conduit pipe 5. In this case, the hydraulic conditions are adjusted by the choice of the conduit pipe geometry and of the injected biogas stream in such a way that Che entire contents of the fermentation reactor are pumped at least twice an hour through the conduit pipe 5. Air is metered by means of an air feed line 7 into the interior upflow of the conduit pipe 5 in ratios of amounts such that the fermentation medium has adequate contact with oxygen while passing through the conduit pipe 5 in order to limit H2S formation in its metabolic processes in the required manner. At the same time, the oxygen is broken down biochemically to such an extent that process-impairing amounts of oxygen are no longer present in the biogas. The air required for thxs can be minimized so that the nitrogen in the biogas does not result in a reduction in the quality of the gas for further caloric utilization. To maintain an operating temperature which is optimal for the biological treatment of the fermentation medium, the conduit pipe 5 is designed to be heatable. For this purpose, the conduit pipe 5 is provided with a double-walled jacket which has an inlet 8 and an outlet 9 for hot water. In addition, the temperature of the contents of the fermentation reactor can be controlled by means of a heat exchanger 19 through which hot water flows. The use of a central vertical conduit pipe 5 in the fermentation reactor 2 as oxygen-containing zone achieves several effects simultaneously: On the one hand, reliable H2S suppression is achieved because of the air metering in the conduit pipe 5, On the other hand, the contents of the fermentation reactor are homogenized by circulation through the interior loop of the fermentation reactor 2. In addition, the temperature of the ferment¬ation reactor is controlled by means of the hot water pumped through the jacket of the conduit pipe. Finally, because of the emergence of fermentation medium from the upper end of the conduit pipe 5, a central surface gush is produced above the conduit pipe 5 and results in destruction of a floating covering in the fermentation reactor 2. The described circulation of the fermentation medium is sufficient for reliable impairment of the H2S formers in the entire fermentation chamber in the required manner. This is not associated with impairment of methane formation from the fermentation reaction, as proved by the measurements depicted in Figure 2 . Figure 2 shows the operating results for a fermentation reactor with air metering into the fermentation chamber (reactor 1) compared with those of a conventional fermentation reactor without air metering (reactor 2) . We Claims 1, Process for obtaining biogas by biological treatment of a fermentation medium in a fermentation reactor, in which an electron acceptor-containing medium, in particular oxygen-containing gas or gas mixture, for example air and/or nitrate and/or nitrite, is fed into the fermentation medium to suppress the formation of unwanted hydrogen sulphide (H2S) , charac¬terized in that essentially all the fermentation medium contained in the fermentation reactor (2) is passed through a zone (5) into which the electron acceptor-containing medium is fed, with a sufficient time of contact between the electron acceptor-containing medium and the fermentation medium being adjusted in order to suppress H2S formation in the fermentation medium at least to the extent that negligible amounts of H2S are present in the biogas. 2, Process according to Claim 1, characterized in that in that essentially all the fermentation medium contained in the fermentation reactor (2) is passed several times, preferably at least twice, an hour through the zone (5). 3. Process according to Claim 1 or 2, character¬ised in that in that the electron acceptor-containing medium is metered into the zone (5) in amounts such that the fermentation medium maintains adequate contact with the electron acceptor-containing medium while passing through the zone (5) in order to suppress H2S formation in the fermentation medium. 4. Process according to any of Claims 1 to 3, characterized in that the time of contact between the electron acceptor-containing medium and the ferment¬ation medium in the zone (5) and/or the amount of the electron acceptor-containing medium metered into zone (5) is adjusted so that the electron acceptor-containing medium is broken down biochemically to such an extent that process-impairing amounts of the electron acceptor-containing medium are no longer present in the biogas. 5. Process according to any of Claims 1 to 4, characterized in that when air is used as the electon acceptor-containing medium the amount of air fed into 2one (5) per unit time is adjusted so that the nitrogen content resulting in the biogas from the nitrogen content of the air results in negligible reduction in the quality of the biogas in terms of caloric utilization. 6. process according to any of Claims 1 to 5, characterised in that the fermentation medium is transported through the zone (5) by means of a propellant gas which preferably consists of the biogas, 7. Apparatus for obtaining biogas having a fermentation reactor to receive fermentation medium and a biogas offtake line, characterized in that a conduit arrangement (5) is disposed in the interior of the fermentation reactor (2) , with at least one feed line (7) for an electron acceptor-containing medium terminating in the conduit arrangement (5) or in the vicinity of an open end of the conduit arrangement (5), and means are provided for transporting essentially all the fermentation medium contained in the fermentation reactor (2) through the conduit arrangement (5), 8. Apparatus according to Claim 7, characterized in that the conduit arrangement (5) is designed as a conduit pipe disposed essentially vertically and centrally in the fermentation reactor {2). 9. Apparatus according to Claim 7 or 8, charac¬terized in that a biogas branch line (6) branching off the biogas offtake line (3) termrnates in the conduit arrangement (5) or in the vicinity of an open end of the conduit arrangement (5). 10. Apparatus according to any of Claims 7 to 9, characterized in that the geometry of the conduit arrangement (5) is chosen so that a sufficient time of contact between the fermentation medium and the electron acceptor-containing medium is ensured in order essentially to suppress H2S formation in the ferment¬ation medium, 11. Apparatus according to any of Claims 7 to 10/ characterized in that the geometry of the conduit arrangement (5) , the size of the fermentation reactor (2) and the dimensions of the biogas branch line (6) are chosen so that essentially all the fermentation medium contained in the fermentation reactor (2) can be transported at least twice an hour through the conduit arrangement (5). 12- Process and apparatus for obtaining biogas, substantially as hereinabove described and i1lustrated with reference to the accompanying drawings.

Documents:

1336-mas-1998-abstract.pdf

1336-mas-1998-claims filed.pdf

1336-mas-1998-claims granted.pdf

1336-mas-1998-correspondnece-others.pdf

1336-mas-1998-correspondnece-po.pdf

1336-mas-1998-description(complete)filed.pdf

1336-mas-1998-description(complete)granted.pdf

1336-mas-1998-drawings.pdf

1336-mas-1998-form 1.pdf

1336-mas-1998-form 26.pdf

1336-mas-1998-form 3.pdf

1336-mas-1998-form 5.pdf

1336-mas-1998-other documents.pdf

1336-mas-1998-pct.pdf