PROCESS FOR PRODUCING A METHANE-ENRICHED GAS

Abstract

A process for the production of a methane-enriched gas including the steps of providing a bioreactor having at least one device for supplying a gas, and at least one outlet for removing the methane-enriched gas generated in the bioreactor; providing a device for determining the proportion of carbon dioxide in the methane-enriched gas removed from the bioreactor; specifying a target value S for the proportion of carbon dioxide in the methane-enriched gas removed from the bioreactor; supplying carbon dioxide-containing gas to the bioreactor; supplying hydrogen-containing gas to the bioreactor; forming methane-enriched gas in the bioreactor; removing the methane-enriched gas formed in the bioreactor from the bioreactor; determining an actual value for the proportion of carbon dioxide in the methane-enriched gas removed from the bioreactor; comparing the target value S with the determined actual value; regulating the quantity of supplied carbon dioxide-containing gas and/or regulating the quantity of supplied hydrogen-containing gas in a manner such that the determined actual value corresponds to the specified target value S, wherein the target value S specified for the proportion of carbon dioxide in the methane-enriched gas removed from the bioreactor satisfies the condition 0 vol% < S ≤ 5 vol%.

Claims

1-16. (canceled)

17. A process for the production of a methane-enriched gas, comprising the steps of: a) providing a bioreactor having: at least one device for supplying a gas, at least one outlet for removing the methane-enriched gas generated in the bioreactor, b) providing a device for determining the proportion of carbon dioxide in the methane-enriched gas removed from the bioreactor, c) specifying a target value S for the proportion of carbon dioxide in the methane-enriched gas removed from the bioreactor, d) supplying carbon dioxide-containing gas to the bioreactor, e) supplying hydrogen-containing gas to the bioreactor, f) forming methane-enriched gas in the bioreactor, g) removing the methane-enriched gas formed in the bioreactor from the bioreactor, h) determining an actual value for the proportion of carbon dioxide in the methane-enriched gas removed from the bioreactor, i) comparing the target value S specified in step c) with the actual value determined in step h), j) regulating the quantity of carbon dioxide-containing gas supplied in step d) and/or regulating the quantity of hydrogen-containing gas supplied in step e) in a manner such that the actual value determined in step h) corresponds to the target value S specified in step c), wherein the target value S specified in step c) for the proportion of carbon dioxide in the methane-enriched gas removed from the bioreactor satisfies the condition 0 vol% < S ≤ 5 vol%.

18. The process according to claim 17, wherein the target value S specified in step c) for the proportion of carbon dioxide in the methane-enriched gas removed from the bioreactor satisfies the condition 0 vol% < S ≤ 4 vol%.

19. The process according to claim 17, wherein the target value S specified in step c) for the proportion of carbon dioxide in the methane-enriched gas removed from the bioreactor satisfies the condition 0 vol% < S ≤ 2 vol%.

20. The process according to claim 17, wherein the target value S specified in step c) for the proportion of carbon dioxide in the methane-enriched gas removed from the bioreactor satisfies the condition 0 vol% < S ≤ 1.5 vol%.

21. The process according to claim 17, wherein the target value S specified in step c) for the proportion of carbon dioxide in the methane-enriched gas removed from the bioreactor satisfies the condition 0.1 vol% ≤ S ≤ 5 vol%.

22. The process according to claim 17, wherein the target value S specified in step c) for the proportion of carbon dioxide in the methane-enriched gas removed from the bioreactor satisfies the condition 0.1 vol% ≤ S ≤ 4 vol%.

23. The process according to claim 17, wherein the target value S specified in step c) for the proportion of carbon dioxide in the methane-enriched gas removed from the bioreactor satisfies the condition 0.1 vol% ≤ S ≤ 2 vol%.

24. The process according to claim 17, wherein the target value S specified in step c) for the proportion of carbon dioxide in the methane-enriched gas removed from the bioreactor satisfies the condition 0.1 vol% ≤ S ≤ 1.5 vol%.

25. The process according to claim 17, characterized in that the target value S specified in step c) for the proportion of carbon dioxide in the methane-enriched gas removed from the bioreactor satisfies the condition 0.5 vol% ≤ S ≤ 5 vol%.

26. The process according to claim 17, wherein the target value S specified in step c) for the proportion of carbon dioxide in the methane-enriched gas removed from the bioreactor satisfies the condition 0.5 vol% ≤ S ≤ 4 vol%.

27. The process according to claim 17, wherein the target value S specified in step c) for the proportion of carbon dioxide in the methane-enriched gas removed from the bioreactor satisfies the condition 0.5 vol% ≤ S ≤ 2 vol%.

28. The process according to claim 17, wherein the target value S specified in step c) for the proportion of carbon dioxide in the methane-enriched gas removed from the bioreactor satisfies the condition 0.5 vol% ≤ S ≤ 1.5 vol%.

29. The process according to anyone of claim 17, wherein at least two devices are provided for supplying gases are provided, wherein at least one device for supplying a carbon dioxide-containing gas and at least one device for supplying a hydrogen-containing gas are provided.

30. The process according to anyone of claim 17, wherein the device for supplying a carbon dioxide-containing gas and/or the device for supplying a hydrogen-containing gas is a device for regulating flow.

31. The process according to anyone of claim 17, wherein the regulation in step j) is carried out exclusively on the basis of the comparison carried out in step i).

32. The process according to anyone of claim 17, wherein the comparison in step i) and the regulation in step j) are carried out by means of a processor unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0068] The invention will now be described in more detail in connection with the drawings with the aid of exemplary embodiments. Obviously, the statements made with respect to the exemplary embodiments do not limit the invention. In the figures:

[0069] FIG. 1 shows the process in accordance with the invention in the form of a block diagram;

[0070] FIG. 2 shows, in the form of a graphical representation, measured values for the product gas composition and procedural parameters when implementing an embodiment of the process in accordance with the present invention;

[0071] FIG. 3 shows, in the form of a graphical representation, measured values for the product gas composition and procedural parameters when implementing a further embodiment of the process in accordance with the present invention;

[0072] FIGS. 4A, 4B, 4C show, in the form of a graphical representation, measured values for the product gas composition and procedural parameters when implementing a further embodiment of the process in accordance with the present invention;

[0073] FIG. 5 shows, in the form of a graphical representation, measured values for the product gas composition and procedural parameters when implementing a further embodiment of the process in accordance with the present invention.

WAYS OF CARRYING OUT THE INVENTION

[0074] Exemplary Embodiment 1 can be explained in more detail in association with FIG. 1. FIG. 1 shows the process in accordance with the invention in the form of a block diagram.

[0075] Firstly, a target value S is specified for the proportion of carbon dioxide in the methane-enriched gas removed from the bioreactor. The regulation of the quantity of carbon dioxide-containing gas supplied and/or the regulation of the quantity of hydrogen-containing gas supplied is carried out in accordance with the invention in a manner such that the measured actual value 7 of the proportion of CO.sub.2 in the product gas is as close as possible to the specified target value S or corresponds thereto.

[0076] The regulation of the quantity of the carbon dioxide-containing gas supplied and/or the regulation of the quantity of hydrogen-containing gas supplied is carried out exclusively on the basis of the comparison of the specified target value S with the measured actual value 7 of the proportion of carbon dioxide in the methane-enriched gas removed from the bioreactor. The control deviation 2 is calculated by producing the difference of the target value S minus the actual value 7.

[0077] An appropriately programmed control module which is usually installed on a processor unit functions as the controller 3, which generates feedback 8 of the measured actual value 7 to the starting point of the control loop and then carries out a comparison of the specified target value S with the measured actual value 7. The aim of this mode of operation of the controller is to reduce the control deviation 2 to zero. The regulation of the quantity of supplied carbon dioxide-containing gas and/or the regulation of the quantity of supplied hydrogen-containing gas is carried out in a manner such that the control module calculates an amended ratio of the educt gases H.sub.2 (or hydrogen-containing gas) and CO.sub.2 (or CO.sub.2 containing gas). This acts as the control variable 4. The ratio of the educt gases is adjusted by means of the metering of the educt gases as a reaction to the actual value 7, i.e. the CO.sub.2 content in the product gas.

[0078] The methanation reactor itself constitutes the control system 5 of the system. Any parameters which vary the composition of the product gas from the methanation can act as disturbances 6 acting on the control system 5, for example variations in the pH or the temperature in the bioreactor, but also variations in the composition of the reactor medium due to growth and metabolization of the microorganisms.

[0079] FIG. 2 shows, as a graphical representation, measured values for the product gas composition as well as further procedural parameters when implementing an embodiment of the process for biological methanation in accordance with the present invention. The test results demonstrate that when using a process in accordance with the present invention with target values for the CO.sub.2 content in the product gas in the range from 1 vol% to 5 vol% inclusive, a bioreactor can be operated in a stable manner, without carrying out a pH adjustment by measuring the pH in the reactor and appropriate addition of base or acid. In addition, the data show that without further gas treatment, a methane-enriched gas is obtained as the product gas which has a very high methane content of at least 93 vol% with a hydrogen concentration of less than 2.5 vol%.

[0080] The biological methanation was carried out in a bioreactor which contained digested sludge from a municipal sewage treatment plant as the methanation medium, to which individual trace elements and nutrients had been added. The biological methanation was carried out with the addition of a methanogenic microorganism from the genus Methanothermobacter. The production of the methane-enriched gas was carried out in a continuous stirred tank reactor with a 75 litre reactor content, at a temperature of 65° C. and a pressure of 7 bar. Over the entire time period, a methane-enriched gas with a methane formation rate (MFR) of 60 standard cubic metres of methane per cubic metre of reactor contents per day [Nm.sup.3/m.sup.3d)] was produced. Over the time period shown in FIG. 2, CO.sub.2 regulations with target values of 1 vol%, 2 vol%, 3 vol%, 4 vol% and 5 vol% of carbon dioxide in the product gas in an increasing order were carried out. The information on the time axis corresponds to the time in hours.

[0081] Graph A of FIG. 2 shows the supply of the educt gases H.sub.2 and CO.sub.2. This was carried out from gas bundles each with pure H.sub.2 and CO.sub.2 in pressurized gas cylinders. The respective gas volume flow in standard litres per hour [Nl/h] was adjusted using a mass flow regulator from Bronkhorst. The hydrogen volume flow (V_H2; broken line) was kept constant over the entire time period of the test and was pre-set to 750 Nl/h. The value for the supplied carbon dioxide volume flow (V_CO2; black line) was obtained after measuring the actual value and calculating the control deviation from the control variable output by the CO.sub.2 controller in connection with the specified hydrogen volume flow. Over the time period of the test shown, the increase in the target values for CO.sub.2 from 1 vol% to 5 vol% generated values for the carbon dioxide volume flow in the range from 166 Nl/h to 187 Nl/h. In the time period from 50 hours to 56 hours, because of an interruption to the hydrogen supply, no educt gases could be supplied, and so no methane-enriched gas could be produced.

[0082] Graph B of FIG. 2 shows the measured concentration of CO.sub.2 in the product gas (c_CO2; black line) in vol%. This corresponds to the actual value of the control variable. The target value specified in the respective time period for the process in accordance with the invention for carbon dioxide in the product gas (S_CO2) in vol% is shown as a broken line.

[0083] Graph C of FIG. 2 shows the measured values for the methane concentration in the product gas (c__CH4; black line) in vol%, as well as the hydrogen concentration in the product gas (c_H2; broken line) in vol%. All of the gas components of the product gas were measured with inline gas analysis devices from Emerson.

[0084] Graph D of FIG. 2 shows the measured values for the mass of the reactor contents (m; black line) in kilograms [kg] on the scale shown on the left (black values) as well as the measured pH values (pH; broken line) on the right hand scale. The mean reactor content was 75 kilogram. A portion of the reactor contents was removed at regular intervals and subsequently supplemented with fresh substrate or methanation medium so that a certain amount of exchange for fresh material was carried out. This can be seen in graph D of FIG. 4 as small downward deflections and a subsequent increase in the mass. The pH of the methanation medium was not measured with a pH probe directly in the bioreactor, but in the outflow from the reactor, each time directly after removing the reactor content to be exchanged. The relevant pH for the reactor contents corresponded in each case to the lowest measured value in an exchange cycle (the arrows show examples).

[0085] Over a time period of approximately 12 hours, respectively, the biological methanation was carried out in accordance with the process of the invention with CO.sub.2 regulation with specified target values of 1 vol%, 2 vol%, 3 vol%, 4 vol% and 5 vol% carbon dioxide in the product gas in an increasing series. With each of the target values, a stable methanation with good gas quality in the product gas without the addition of base or acid was possible. A methane formation rate of 60 standard cubic metres of methane per cubic metre of reactor contents per day was obtained. Each time, the predetermined target value for the CO.sub.2 concentration in the product gas was not obtained immediately after inputting the value, but only after a certain initial period. This behaviour is also desirable to a certain extent, because very rapid variations to the reaction conditions in biological systems are not generally advantageous because they can endanger the stability of processes.

[0086] In addition, a certain transient response with values over and under the predetermined target value was observed. However, this is typical control behaviour and depends to a certain extent on the regulation parameters which are set. In all cases, however, after a few hours, the specified target value reached a stable level and held. The hydrogen concentrations in the product gas were less than 2.5 vol% throughout. Up to a set concentration of 3 vol% CO.sub.2, the CH.sub.4 concentration was more than or equal to 95 vol%. With specified set concentrations of CO.sub.2 of 4 vol% or 5 vol%, the CH.sub.4 concentration was 93 vol% or more, because here, due to the specification by the controller, a correspondingly high CO.sub.2 content was present in the product gas. Even after interrupting the methanation because of an interruption of the hydrogen supply in the time period from hour 50 to hour 56, the methanation could be continued without delay with the same level of methane formation and equally good product gas quality. The pH, which was determined at regular intervals each time directly in the reactor contents discharged from the reactor, was in the range from approximately 8.1 to 7.5 over the entire time period of the test.

[0087] After regulating the CO.sub.2 target value to 5 vol%, the adjustment was dropped in one step to a target value of 1 vol% CO.sub.2. The transient responses had a larger amplitude than in the variations in steps of only 1 vol%, and the regulation by means of the control variable also led to the volume flow for the educt gas CO.sub.2 being briefly reduced to a value of 147 Nl/h, but here too, after a few hours the specified target value of 1 vol% CO.sub.2 was obtained. No problems arose in returning the bioreactor including its contents to the starting condition with a low CO.sub.2 content. In the product gas, with a target value for CO.sub.2 specified at 1 vol%, a methane concentration of 97 vol% and a residual hydrogen concentration of 2 vol% were obtained. As a rule, this meets the specified limits for a biomethane which can be fed into the grid.

[0088] FIG. 3 shows, in the form of a graph, measured values for the product gas composition as well as further procedural parameters when implementing an embodiment of the process for biological methanation in accordance with the present invention. The test results show that when applying a process in accordance with the present invention with target values for the CO.sub.2 content in the product gas in the range from 1 vol% to 0.1 vol%, a bioreactor can be operated in a stable manner without carrying out a pH adjustment by measurement of the pH in the reactor and the appropriate addition of base or acid. In addition, the data show that without further gas treatment, a methane-enriched gas can be obtained as a product gas which has a very high methane content of at least 97 vol%, with a hydrogen concentration of less than 2 vol%.

[0089] The exemplary embodiment shown in FIG. 3 is a continuation of the biological methanation described in FIG. 2. The test conditions as well as the representations of graphs A to D correspond to those of exemplary embodiment 2, with the exception that in the present case, CO.sub.2 target values of 1 vol% down to 0 vol% in steps of 1 vol%, 0.5 vol%, 0.2 vol%, 0.1 vol% and 0 vol% were specified.

[0090] This example was begun with a target value of 1 vol% CO.sub.2, measured in the product gas. This specified target value corresponded to the start value and the end value for Exemplary Embodiment 2. Down to a target value of 0.1 vol% CO.sub.2, biological methanation was possible. The values for the volume flow of carbon dioxide for the educt gas were in the range from 178 Nl/h down to 167 Nl/h, while the hydrogen volume flow was held constant at 750 Nl/h. The methane concentrations in the product gas were in the range between 97 vol% to 98.5 vol% with a hydrogen concentration of about 1.5 vol% to 1.75 vol%. The measured pH values in the outflow of methanation medium from the reactor were between pH 8.1 and pH 8.8. The regulation to a target value S of 0.1 vol% CO.sub.2 in the product gas was maintained for a time period of 20 hours.

[0091] Next, regulation was carried out such that no more CO.sub.2 could be measured in the product gas, by setting the target value for CO.sub.2 to zero. This specification for the CO.sub.2 regulation was maintained over a time period of 8 hours. As can be seen in graph A of FIG. 3, the volume flow for the educt gas carbon dioxide was reduced from 171 Nl/h to a concentration of 157 Nl/h. The pH rose to a value of almost 9. Approximately two and a half hours after specifying the target value of 0 vol% CO.sub.2 in the product gas, CO.sub.2 could no longer be measured in the product gas. After another 2 hours, the product gas quality deteriorated substantially. In the next 4 hours, the methane content dropped from a value of 98 vol% to a value of 81 vol%. At the same time, the proportion of hydrogen in the product gas rose from approximately 1.5 vol% to almost 17 vol%.

[0092] In addition, the hydrogen sulphide concentration in the product gas after 63 hours was observed to climb from a value of 50 ppm to a value of more than 120 ppm, along with an increase in the hydrogen concentration and a drop in the methane concentration. Similarly, an increase in the ammonia concentration was observed in the product gas. The same observations were repeated two more times when, during a period of the test, target values for the concentration of carbon dioxide in the product gas were set to zero. In this manner, it was shown that regulation in the sense that the carbon dioxide is completely consumed does not work with biological methanation, while a regulation to a target value for the CO.sub.2 in the product gas in the range from 0.1 vol% to 5 vol% using the process in accordance with the invention did work, and a good to very good quality methane-enriched gas was provided.

[0093] A further advantage of the process in accordance with the invention was found, in that surprisingly, a pH adjustment by adding acid or base with simultaneous continuous pH measurement in the bioreactor could be dispensed with.

[0094] In order to prevent a total “crash” of the biological methanation in the bioreactor, after 66 test hours, the target value for the CO.sub.2 in the product gas was raised again from 0 vol% to 1 vol%. While the carbon dioxide flow in the educt gas rose directly after the adjustment, the gas quality also rapidly improved again. It took a few hours for the pH to drop and for the CO.sub.2 content in the product gas to settle at 1 vol%, however the biological methanation could subsequently be carried out again and the bioreactor including its contents could be returned to an operable state.

[0095] FIGS. 4A, 4B, 4C and 4D show a further exemplary embodiment for the implementation of the process for biological methanation in accordance with the present invention. The graphical representation of the measured values for the product gas composition as well as further procedural parameters are produced in an analogous manner to that described in connection with FIGS. 2 and 3, with the sole difference being that in the lowest section of the figure, the methane formation rate (MFR) is provided instead of the pH. The regularly repeating upward and downward deflections in the target methane formation rate occur because in each case, when substrate was exchanged in the bioreactor, variations in the quantity of the reactor contents occurred over a specific period, as well as pressure variations, which resulted in temporary variations in the volume flow of the product gas, which lead to variations in the MFR. The pH was not measured in the reactor, but samples were repeatedly taken in order to measure the pH.

[0096] The biological methanation was carried out in a bioreactor which contained digested sludge from a municipal sewage treatment plant. However, in this example, no trace elements and nutrients were added; only a reducing agent was added to the methanation medium. The biological methanation was carried out without the addition of a methanogenic microorganism. The production of the methane-enriched gas was carried out in a continuous stirred tank reactor with a 60 litre reactor content at a temperature of 65° C. and a pressure of 7 bar.

[0097] In the time period shown in FIG. 4A, the target value for the CO.sub.2 concentration in the product gas was set at 1 vol% for the regulation. At the start of methanation, the gas volume flow for the educt gas hydrogen was raised in steps so that after 3 hours, methane-enriched gas could already be produced at a methane formation rate of 85 Nm.sup.3/(m.sup.3d). At that point in time, the proportion of CO.sub.2 in the product gas had already settled at the target value of 1 vol%. Over the entire time period of 30 hours of test time up to 150 hours, the methanation was stable at the same methane formation rate. A methane-enriched gas was produced with a methane fraction of approximately 94 vol% to 95 vol%, a hydrogen fraction of approximately 3.5 vol% and a carbon dioxide fraction of 1 vol%. The interfering signals which can be seen in the gas analysis values as irregular repeating vertical lines result from sampling. Beyond the 152 hour test point, the educt gas supply was increased so that after a further 2 to 3 hours, a methane formation rate of 153 Nm.sup.3/(m.sup.3d) was obtained. The target value for the CO.sub.2 adjustment was unchanged in this case. With the methane formation rate of 153 Nm.sup.3/(m.sup.3d), a gas quality of approximately 89 vol% methane and about 8 vol% hydrogen was obtained.

[0098] After pausing the educt gas supply, the methanation was continued at the same methane formation rate, but with a CO.sub.2 target value of 3 vol% being specified. This is shown in FIG. 4B. The volume flows for H.sub.2 and CO.sub.2 were specified so that from the outset, a methane formation rate of 153 Nm.sup.3/(m.sup.3d) was obtained. The oscillations in the CO.sub.2 values in the product gas at 3 vol% lasted about 4 hours.

[0099] The methane-enriched gas contained about 87 vol% of methane and about 8 vol% of hydrogen as well as 3 vol% of carbon dioxide. Immediately thereafter, the methane formation rate was raised to a value of 200 Nm.sup.3/(m.sup.3d), keeping the CO.sub.2 target value the same. This was possible without problems. The gas quality deteriorated somewhat at the high methane formation rate to values of about 84 vol% methane and about 11 vol% hydrogen.

[0100] After a lengthy methanation pause, the methanation was continued with a methane formation rate of 200 Nm.sup.3/(m.sup.3d), wherein the CO.sub.2 target value was set at a value of 2 %. This can be seen in FIG. 4C. At the start of methanation, at 18 test hours, it took 8 hours for the stable CO.sub.2 value of 2 vol% to be established. The gas quality for the product gas improved to approximately 87.5 vol% methane and 8.5 vol% hydrogen. After an interruption of 6 hours to the educt gas supply, a very rapid restart was carried out under the same conditions, wherein the CO.sub.2 target value of 2 vol% was also obtained from the outset.

[0101] The Exemplary Embodiment described in FIGS. 4A to 4D shows that in a further methanation medium, even at high methane formation rates of up to 200 Nm.sup.3/(m.sup.3d), biological methanation in accordance with the process of the present invention was possible at different specified target values for the carbon dioxide in the product gas, even when the methane-enriched gas still did not reach an infeed quality directly after leaving the bioreactor. By means of a subsequent gas treatment with separation of the residual hydrogen, methane-enriched gas produced in this manner can be converted into a bionatural gas or SNG. In this example as well, there was no pH adjustment by the addition of acid or base. The pH measured in the samples from the bioreactor was at values between a pH of 7.6 and 8.4 throughout the test period, which was well suited to methane formation. In addition, the measured values for the hydrogen sulphide content or ammonia content in the product gas were insignificant.

[0102] FIG. 5 shows a further exemplary embodiment for carrying out the process for biological methanation in accordance with the present invention. The graphical representation of the measured values for the product gas composition as well as further procedural parameters are analogous to those shown in connection with FIGS. 4A to 4D.

[0103] The biological methanation was carried out in a bioreactor which contained a synthetic culture medium as the methanation medium, as used in the prior art for hydrogenotrophic methanogenic archaea (see, for example, WO 2008/094282 A1 or WO 2012/110257 A1). The medium was inoculated with a methanogenic strain from the order Methanobacteriales. The production of the methane-enriched gas was carried out in a continuous stirred tank reactor with a 60 litre reactor content at a temperature of 65° C. and under a pressure of 7 bar.

[0104] In the time period shown in FIG. 5, the target value for the CO.sub.2 concentration in the product gas was set at 1 vol% for the regulation. At time point zero, the biological methanation was started up. In this regard, the gas volume flow for the educt gas hydrogen was raised in steps so that after 4 hours, methane-enriched gas could already be produced at a methane formation rate of 150 Nm.sup.3/(m.sup.3d). After a further 5 hours, the CO.sub.2 fraction in the product gas had settled at the target value of 1 vol%. Up to a 37 hours of test, a methane-enriched gas with a methane content of 90 vol% to 92 vol% was produced, with a hydrogen content of 7 vol% to 8.5 vol%. Beyond the 37 hour test point, the educt gas supply was increased so that a short time later, a methane formation rate of 200 Nm.sup.3/(m.sup.3d) was obtained. The target value for the CO.sub.2 regulation was unchanged.

[0105] The methanation was very stable over the entire remaining test period, wherein the gas quality over the test period improved slightly. A methane-enriched gas was produced in which the methane fraction of approximately 85 vol% increased slightly to 89 vol%, while the hydrogen fraction dropped from approximately 13.5 vol% to approximately 10 vol% and the carbon dioxide fraction oscillated around the target value of 1 vol%. The targeted gas qualities in the product gas in this exemplary embodiment with a system in which a synthetic culture medium was used with defined methanogens as the reactor content agreed very well with the values obtained in Exemplary Embodiment 4, in which digested sludge was used as the substrate, with comparable methane formation rates, and no defined methanogenic microorganism was added.

[0106] After the start of methanation, the pH in the synthetic culture medium first had to stabilize somewhat, but from approximately 30 hours, a small quantity of caustic soda was also added at regular intervals in order to keep the pH to a value of between 6.6 and 7.2, during which the biological methanation functioned smoothly, although only after the CO.sub.2 fraction in the product gas had been regulated as described in the present invention.

TABLE-US-00001 List of Reference Signs S target value (reference variable) 2 control deviation (target value - actual value) 3 controller 4 control variable 5 control system 6 disturbance 7 actual value (control variable) 8 feedback