METHOD TO USE INDUSTRIAL CO2 CONTAINING GAS FOR THE PRODUCTION OF A METHANE ENRICHED GAS COMPOSITION

Abstract

The present invention refers to a method using CO.sub.2 containing emissions or waste gas for the production of methane enriched gas compositions.

Claims

1. Method to produce methane from CO.sub.2 containing gases in a bioreactor comprising: a. culturing methanogenic microorganism in a continuous process; b. providing CO.sub.2 containing gases, comprising H.sub.2S and/or O.sub.2; c. controlling and, optionally, regulating the pH value continuously to be at a given value; d. optionally, feeding the culture of methanogenic microorganisms with additional H.sub.2 in a stoichiometric ratio of CO.sub.2:H.sub.2 between 1:0.6 to 1:5; e. collecting methane or a methane enriched gas composition.

2. Method according to claim 1, characterized in that the step of culturing the methanogenic microorganism comprises: i. keeping said methanogenic microorganism in a suitable liquid culture medium providing a nitrogen source and salts; ii. keeping the culture conditions anaerobic or facultatively anaerobic; iii. optionally stirring the culture; iv. removing metabolic water from the culture continuously; and v. keeping the temperatures in a range from 32° C. and 90° C. or 32° C. and 85° C. at atmospheric pressure.

3. Method according to any of the previous claims 1 to 2, characterized in that the methanogenic microorganism is cultured in the presence of additionally added sulfide, preferably in the form of Na.sub.2S and/or ammonium, preferably in the form of NH.sub.4OH.

4. Method according to any of the previous claims 1 to 3, characterized in that the methanogenic microorganism are cultured up to a density of microorganisms in the culture measured as OD.sub.610 being at least 14 and up to 60 and corresponding to a dry weight of the microorganisms in the culture of at least 2.5 g/L and up to 20 g/L.

5. Method according to any of the previous claims 1 to 4, characterized in that the methanogenic microorganism culture is continuously stabilized and/or regulated to be kept at a given value of below or at pH 10, of below or at pH 9, of below or at pH 8 or at pH 7 by adding suitable amounts of an acid and/or base; and/or that the methanogenic microorganism culture is continuously stabilized and/or regulated to keep the given pH value by dosing suitable amounts of NaOH or NH.sub.4OH and HCl or H.sub.2SO.sub.4 to the culture.

6. Method according to any of the previous claims 1 to 5, characterized in that at least one methanogenic microorganism is selected from the group of Archaea or archaebacteria comprising of Methanobacterium, Methanobrevibacter, Methanothermobacter, Methanococcus, Methanosarcina, Methonopyrus or mixtures thereof.

7. Method according to any of the previous claims 1 to 6, characterized in that the methanogenic microorganism is anaerobic and/or oxygen tolerant.

8. Method according to any of the previous claims 1 to 7, characterized in that in the suitable liquid culture medium of step i. is a moderately saline environment, wherein the concentration of the chloride anion is in the range of 12 mmol/L to 300 mmol/L; and/or wherein the concentration of NaCl is in the range of 0.4 g/L to 12 g/L, preferably in the range from 3 g/L to 6 g/L and more preferably around 5.6 g/L.

9. Method according to any of the previous claims 1 to 8, characterized in that the methanogenic microorganism is selected from the group of naturally selected or natively apt microorganisms, adapted halophilic microorganisms and genetically engineered microorganisms, all capable of living and thriving in moderately saline environment.

10. Method according to any of the previous claims 1 to 9, characterized in that the CO.sub.2 containing gases is the carbon source for the methane production and derives from CO.sub.2 rich emissions and/or waste gas emissions.

11. Method according to any of the previous claims 1 to 10, characterized in that the CO.sub.2 containing gas comprises at least 20% CO.sub.2 and/or the CO.sub.2 containing gas comprises up to 5.000 mg/L of H.sub.2S and/or the CO.sub.2 containing gas comprises up to, but not more than 5% O.sub.2.

12. Method according to any of the previous claims 1 to 11, characterized in that the entire procedure or at least one step is carried out under atmospheric pressure conditions and/or under pressurized conditions with up to 16 or up to 420 bar.

13. Method according to any of the previous claims, characterized in that the collected methane is essentially free of solid contaminants, such as e.g. foams, small solid particles, like dust and dirt particulates, in suspension, greases, or gaseous contaminants, such as e.g. water vapour, hydrogen sulphide, siloxanes, ammonia and halogen compounds (chloride, fluoride), volatile organic compounds (VOCs), such as e.g. limonene and other terpenes, and other trace contaminants.

14. A method to produce methane from CO.sub.2 or CO.sub.2 containing gases in a bioreactor comprising: a. culturing a methanogenic microorganism with CO.sub.2 and H.sub.2 at a first pH, followed by b. continuous culturing said microorganism at an second pH; c. controlling and, optionally, regulating the pH value continuously to be at a given value; d. collecting methane or a methane enriched gas composition.

15. The method according to claim 14, wherein the first pH value ranges from pH 5.5 to 7.0 to induce rapid replication of the methanogenic microorganism and wherein the second pH value ranges from pH 7.1 to 10 to increase methane production compared to the methane production of step a.

Description

DESCRIPTION OF THE FIGURES

[0125] FIG. 1: Methanation reestablishment following pH regulation in cultures of Methanothermobacter thermautotrophicus, after the addition of H.sub.2S, according to examples 1 and 2. It can be observed that the collapse of the culture (400 h) could not be avoided by solely reducing the amount of H.sub.2S (300 h). The pH regulation plays therefore a relevant role in the reestablishment of methanation and increase of biomass concentration.

[0126] FIG. 2: Consequences for methanation following addition of up to 24000 mg/L of S.sup.2− using cultures of Methanothermobacter thermautotrophicus. The data show the surprising performance in terms of metabolic activity of the culture after the addition of very large quantity of contaminants (Na.sub.2S), owing to the implementation of pH regulation.

[0127] FIG. 3: CO.sub.2 conversion rate and hydrogen sulfide content and oxygen content present in the process feed gas using cultures of Methanothermobacter thermautotrophicus.

[0128] FIG. 4: Stability of methanation upon switching feeding gas from CO.sub.2 to Sulfix II, treated geothermal gas, containing oxygen and hydrogen sulfide. The aerobic manipulation of the methanogenic microorganisms using cultures of Methanothermobacter thermautotrophicus contributes to the efficiency of the method.

[0129] FIG. 5: Persistence of methanation using only treated geothermal gas, according to example 5 using cultures of Methanothermobacter thermautotrophicus. The data show the surprising persistence of methanation even when a very low stoichiometric ratio of H.sub.2 to CO.sub.2 was maintained and additional contaminants were added to the culture, according to an embodiment of the method of the present invention.

[0130] FIG. 6: Consequences for methanation using Methanobrevibacter arboriphilus as biocatalyst following addition of 24 mg/L of S.sup.2− (black filled circles), 121 mg/L of S.sup.2− (grey filled circles) up to 12.700 mg/L of S.sup.2− (unfilled circles). Initial pH values are indicated. Initial methanation was set to 100% and further values were normalized to this value. Each measurement point at a given time of a graph is indicated by a circle and represents the value of one repetition or the average value of two independent repetitions. The data show both the surprising performance in terms of metabolic activity of the culture after the addition of contaminants (Na.sub.2S) when the pH regulation is implemented and the clear performance decrease when no pH regulation is implemented.

[0131] FIG. 7: Consequences for methanation using Methanothermus fervidus as biocatalyst following addition of 24 mg/L of S.sup.2− (black filled circles), 121 mg/L of S.sup.2− (grey filled circles) up to 12.700 mg/L of S.sup.2− (unfilled circles). Initial pH values are indicated. Initial methanation was set to 100% and further values were normalized to this value. Each measurement point at a given time of a graph is indicated by a circle and represents the value of one repetition or the average value of two independent repetitions. The data show both the surprising performance in terms of metabolic activity of the cultures after the addition of contaminants (Na.sub.2S) when the pH regulation is implemented and the clear performance decrease when no pH regulation is implemented.

[0132] FIG. 8: Average CO2 conversion rate of a Methanobrevibacter arboriphilus culture at a first pH value (pH<7.2) and after shifting the pH to an alkaline second pH value (pH<7.45) higher than the first pH value.

[0133] The following examples illustrate viable ways of carrying out the described method as intended, without the intent of limiting the invention to said examples.

EXAMPLE 1

[0134] Simulation of Hydrogen Sulfide Effect on Standard Methanation Process Using Cultures of Methanothermobacter thermautotrophicus

[0135] At the beginning of the experiment the biomethanation process using methanogenic archaea (cultures of Methanothermobacter thermautotrophicus) was at constant performance conditions (stable biomass concentration of 10 to 12 g/L and volumetric methane productivity of approx. 40 L.sub.CH4/L.sub.reactor/day). Under such standard conditions the feeding gas input (H.sub.2 and CO.sub.2), the biomass of the methanogenic archaea (biocatalysator) as well as pH value (between pH 8.0 and 8.5-dependent on the amount of dissolved CO.sub.2) were stable.

[0136] For the start of the experiment at a defined point in time (see FIG. 1 at 189 hours run time) 2000 ppm H.sub.2S were continuously added into the hydrogen and carbon dioxide gas supplied. Initially the methane conversion rate was at the expected level when 60 ppm H.sub.2S were added (˜90% conversion), but dropped to a value below the detection limit within 192 hours after addition of 2000 ppm H.sub.2S. The culture also turned black upon addition of 2000 ppm H.sub.2S, which indicated the formation of non-dissolvable metal-sulfide complexes, as they are known for the media components nickel, iron and cobalt.

[0137] The addition of 2000 ppm H.sub.2S lead thus to a sudden and steady decrease in both the biomass concentration and the volumetric methane productivity, of about 50% and 100% respectively. This “collapse” could also not be recovered, by reducing the H.sub.2S concentration to only 60 ppm H.sub.2S—a value, which is known to be usually tolerated by the process (311 hours run time in FIG. 1).

[0138] Without being bound by the hypothesis, we speculated that the addition of these relatively high amounts of H.sub.2S lead to the formation of insoluble sulfide salts with certain metals present in the culture medium. This hypothesis was supported by the black coloring of the biocatalyst suspension upon addition of H.sub.2S, it is known, that e.g. iron creates a black, non-soluble salt with sulfide. If these metals are not dissolved but precipitated, they are not available to the biocatalyst and this limitation will cause an effect—as observed—of decrease in biocatalyst biomass concentration and total loss of methanation productivity.

EXAMPLE 2

[0139] pH Regulation Enables Tolerance of High Concentrations of Sulfide (S.sup.2−) Up to 24000 Mg/L Using Cultures of Methanothermobacter thermautotrophicus

[0140] In FIG. 2 a graph reporting the methanation activity of a culture of Methanothermobacter thermautotrophicus according to the method of the present invention is shown, as measured in H.sub.2 reduction (or conversion). The results of five experiments are reported in the graph, corresponding to the 1-5 in the table and identified by the symbols in the legend, both shown on the right.

[0141] In the first 2 experiments, a concentration of 12000 mg/L of sulfide (S.sup.2−) is administered, in the form of Na.sub.2S, to the culture, while providing pH regulation, within the framework described in the method of the present invention. It can be seen in the graph that a high conversion of H.sub.2 is achieved in presence of said concentration of 12000 mg/L of sulfide, and such results are steadily maintained for the duration of the experiment.

[0142] Experiment 3 shows the results on the conversion of H.sub.2 of administering 24000 mg/L of sulfide (S.sup.2−) to the culture without pH regulation: the pH of the culture drifts to highly alkaline (>12) and the conversion lowers dramatically, followed by a slow rise towards normalization.

[0143] Experiments 4 and 5 show the results on the conversion of H.sub.2 of administering 24000 mg/L of sulfide (S.sup.2−) to the culture with pH regulation: the pH of the culture is actively maintained at values within the framework described in the method of the present invention, resulting in an unexpectedly and surprisingly high H.sub.2 conversion and steady and continuous performance of the culture in presence of said concentration of 24000 mg/L of sulfide for the duration of the experiment.

[0144] From the comparison of the results of experiments 3 with those of experiments 4 and 5 it can be inferred that the culture undergoing pH regulation according to the method of the present invention can not only withstand high concentrations of sulfide without loss of performance or continuity of the methanation process, but also improve on its own performance.

EXAMPLE 3

[0145] Improving the Process by Regulating the pH Value Using Cultures of Methanothermobacter thermautotrophicus

[0146] To maintain the methanation process performance in the test set up upon addition of H.sub.2S, the inventors introduced a further process step of effectively controlling the pH value and thereby insured that during the entire process a pH around 7 was obtained. Under these process conditions using cultures of Methanothermobacter thermautotrophicus, the formation of HS.sup.− and S.sup.2− was found to be much lower and also the formation of the insoluble salts seemed to be reduced.

[0147] For this, the process culture—after the experimental set up of Example 1—was diluted with fresh culture medium by a factor of 1:2 (381 h run time in FIG. 1). In this experiment the dilution approach was used, because it is an easy way to decrease the concentration of unwanted components, such as the previously precipitated metal salts, in the biocatalyst suspension, without having to completely restart the process. Additionally, the pH value was set to a range of 7 (ca. ±0.3). In this starting phase the H.sub.2S supplied was still 60 ppm.

[0148] Under the diluted and pH stabilized culture conditions, the biocatalyst culture reinitiated both methane productivity and biomass production (FIG. 1; run time 450 h).

[0149] At 501 h run time, the H.sub.2S supply was set to 2000 ppm again. With the new pH setting, the negative, previously observed effect of H.sub.2S addition on the culture performance and growth could be resolved.

[0150] The pH fluctuation between approx. 500 and 630 h is due to the fact, that the pH dosing had to be manually adjusted and the new dosing rate sufficient to maintain pH 7 had to first be determined. Non-sufficient supply of pH-controlling base caused the pH value to drop to the pH values of around pH 6, which also showed in a loss of methane productivity.

EXAMPLE 4

[0151] Hydrogen Sulfide Tolerance at Concentrations Up to 16000 ppm of H.sub.2S Using Cultures of Methanothermobacter thermautotrophicus

[0152] With the pH control strategy according to Example 3, also much higher concentrations—e.g. up to 16000 ppm H.sub.2S—could be continuously added into the test system and fed together with the hydrogen and carbon dioxide gas supplied to the biomethanation process.

[0153] For this, initially the methane conversion rate in cultures of Methanothermobacter thermautotrophicus was stabilized at the expected level when 60 ppm H.sub.2S were added (˜90% conversion) but dropped to a value below the detection limit within 192 hrs after addition of 2000 ppm H.sub.2S.

[0154] The methanation in the presence of high levels of H.sub.2S was only to be stabilized, when the inventors restarted the process and—manually—decreased the pH value of the culture, which normally would be between 8 and 8.5, down by at least one log to a pH-value of around 7.

[0155] This modification—surprisingly—allowed to run the methanation process with addition of up to 16000 ppm H.sub.2S, without any relevant change in the methane productivity.

EXAMPLE 5

[0156] Hydrogen Sulfide Tolerance of the Methanation Process Under the Influence of Additional Oxygen Using Cultures of Methanothermobacter thermautotrophicus

[0157] The results according to Example 4 were reproduced in an experimental set up using cultures of Methanothermobacter thermautotrophicus where up to 12000 ppm H.sub.2S were added. Additionally, O.sub.2 was added in addition to H.sub.2S in this experiment, in steps of 1000 ppm/day and up to a final concentration of 5000 and 7000 ppm/day.

[0158] The simultaneous addition of H.sub.2S and O.sub.2 showed no significant effect on the conversion efficiency (remaining at 90%). This result is the basis and nicely proves the unexpected advantage of the pH control according to the present invention, even when using “real” or at least “treated” geothermal gas in the methanation process.

EXAMPLE 6

[0159] Methanation Process with Pretreated Geothermal Gas Using Cultures of Methanothermobacter thermautotrophicus

[0160] Raw non-condensable fraction of geothermal gas can contain quite high amounts of H.sub.2S, in the range of up to 10 times more than the typically stated 30000 ppm H.sub.2S. As in some industrial settings these high amounts are decreased to 30000 ppm by pretreating the raw geothermal gas in the so-called Sulfix process (e.g. at the Hellisheiði geothermal power plant, Iceland), in this experimental set the pretreated gas was used for experiments using cultures of Methanothermobacter thermautotrophicus. This gas is named “treated geothermal gas”.

[0161] It is to note that the treated geothermal gas typically contains oxygen (O.sub.2) at concentrations of up to approx. 2% which is a situation that is created by a compressor and would normally not be part of the natural environment of methanogenic archaea.

[0162] The long-term effect of such high H.sub.2S or O.sub.2 levels, and especially the combination of both, has never been systematically investigated hitherto. However, the tolerance of methanation process according to this invention towards these parameters is a prerequisite to allow usage of the hydrogen (H.sub.2) and carbon dioxide (CO.sub.2) present in the non-condensable fraction of treated geothermal gas for methane production.

[0163] The typical composition of the treated geothermal gas is shown in Table 1:

TABLE-US-00001 TABLE 1 Typical composition of treated geothermal gas. Gas composition of treated geothermal gas Composition (vol %) H.sub.2S 3.3% CO.sub.2 51.5% H.sub.2 36.1% N.sub.2 6.6% O.sub.2 1.1% CH.sub.4 1.0% H.sub.2O 0.3% Sum 100.0%

[0164] The process was initiated with pure CO.sub.2 and electrolytic H.sub.2 to start the culture before switching to geothermal gas for 3 days. Subsequently, the “pure” CO.sub.2 was replaced by CO.sub.2 from treated geothermal gas and flow rates were adjusted accordingly, in order to maintain the total inlet flows of 0.4 L/min and a stoichiometry of 4.3:1. Consequently, the flow of treated geothermal gas was 0.125 L/min corresponding to a CO.sub.2 flow of approximately 0.065 L/min (53% CO.sub.2 content in the treated geothermal gas). By dilution with additional electrolytic H.sub.2, the other gases such as H.sub.2S, O.sub.2 and N.sub.2 at the inlet were 0.6 vol. %, 0.4 vol. % and 2.4 vol. %, respectively (FIG. 4). As illustrated in FIG. 4, the switch from CO.sub.2 to treated geothermal gas caused a slight drop of the conversion of CO.sub.2 into CH.sub.4 from 98% to 87%, which was quickly recovered within a few hours and resulted again in a conversion rate above 95%.

EXAMPLE 7

[0165] Methanation Process with Treated Geothermal Gas Only Using Cultures of Methanothermobacter thermautotrophicus

[0166] One test should be especially mentioned here, since the experiment, where the addition of electrolytic H.sub.2 was completely turned off, was particularly successful.

[0167] For this, the process—as described above—was established solely on treated geothermal gas using cultures of Methanothermobacter thermautotrophicus. This did allow running the methanation process without an electrolyzer installed on solely (treated) geothermal gas.

[0168] The treated geothermal gas had a ratio of H.sub.2:CO.sub.2 of ca. 0.67:1 thus far below the often believed optimum ratio 4:1. Consequently, incomplete CO.sub.2 conversion was expected. However, the process being adapted with a pH-control did run extremely successfully on the pretreated gas solely.

[0169] The “total” CO.sub.2 conversion was around 13.6%, which corresponds to ca. 80% of the theoretical maximum conversion rate (of ca. 16.2%), given this stoichiometry. This very surprising effect demonstrated that the methanation process did operate successfully, even if only treated geothermal gas was used as process feed (FIG. 5).

EXAMPLE 8

[0170] Methanation at high concentrations of sulfide using cultures of Methanobrevibacter arboriphilus

[0171] In FIG. 6 three graphs reporting the methanation activity of cultures of Methanobrevibacter arboriphilus with and without applying the pH controlling and regulating method of the present invention is shown, as measured in H.sub.2 reduction (or conversion). Three different sulfide concentrations were tested each represents a single graph. Average optical density (OD.sub.610) of the cultures was 3 to 5.

[0172] In a first experiment (black filled circles), a concentration of 24 mg/L of sulfide (S.sup.2−) was administered in the form of Na.sub.2S, to the culture of the Methanobacteria species, while providing pH regulation, within the framework described in the method of the present invention. It can be seen in the corresponding graphs of FIG. 6 that a high methane formation is achieved in presence of said concentration of 24 mg/L of sulfide regulated at an initial pH value of 7.2, and such results are steadily maintained for the duration of the experiment.

[0173] Similar results have been observed by using cultures of Methanobrevibacter arboriphilus in a second experiment (grey filled circles), when a 5-fold higher concentration of 121 mg/L of sulfide is administered while providing pH regulation at an initial pH value of around 7.2: These conditions resulted in an unexpectedly and surprisingly high methanation and continuous performance of the culture in presence of said concentration of 121 mg/L of sulfide.

[0174] In a third experiment the methanation was analyzed when administering 12.700 mg/L of sulfide, respectively, to the culture without any pH regulation. FIG. 6 shows the results at an initial pH value of 8.7, i.e. around pH value 9 (cf. unfilled circles). In this scenario without pH regulation the initial pH increased to a pH value of above 9.0 and consequently the methanation lowers dramatically during the duration of the experiment.

EXAMPLE 9

[0175] Methanation at High Concentrations of Sulfide Using Cultures of Methanothermus fervidus

[0176] Comparable as in FIG. 6, in FIG. 7 three graphs reporting the methanation activity of cultures of Methanothermus fervidus with and without applying the pH controlling and regulating method of the present invention is shown, as measured in H.sub.2 reduction (or conversion). Three different sulfide concentrations were tested each represents a single graph. Average optical density (OD.sub.610) of the cultures was 1.5 to 2.5.

[0177] The results of the first experiment (black filled circles) given a concentration of 24 mg/L of sulfide (S.sup.2−) and of the second experiment (grey filled circles) given a concentration of 121 mg/L of sulfide showed similar results as in the case when using Methanobrevibacter arboriphilus as can be seen in FIG. 7. I.e., these concentrations of sulfide resulted in a high methane formation, and such results are steadily maintained for the duration of the experiment.

[0178] In the third experiment, where the methanation was analyzed when administering 12.700 mg/L of sulfide, respectively, to the culture without any pH regulation similar results as in the case of the third experiment using cultures of Methanobrevibacter arboriphilus were received. FIG. 7 shows the results (cf. unfilled circles). In this scenario without pH regulation the initial pH of 8.7 increased to a pH value of above 9.0 and consequently the methanation lowers dramatically during the duration of the experiment.

[0179] From the comparison of the results of experiments 1 and 2 with those of experiment 3 of Methanobrevibacter arboriphilus and Methanothermus fervidus it can be inferred that the cultures of the two further and distinct microorganism genera undergoing pH regulation according to the method of the present invention consistently can withstand high concentrations of sulfide without loss of performance or continuity of the methanation process.

EXAMPLE 10

Improved Cultivation Method

[0180] In the attempt to further improve the methanation, the inventors also improved the cultivation techniques of methanogenic microorganisms. In brief, a Methanobrevibacter arboriphilus culture was grown at steady conditions for roughly 400 hours at a pH<7.2 (average 7.1). Under these conditions, an average (CO.sub.2) conversion of 23% was observed.

[0181] After this initial starting the cultivation, the pH was increased to be constantly above 7.45 (average 7.6). The surprising effects on methanation over time is depicted in FIG. 8. This pH shift to an increased pH resulted in an average (CO.sub.2) conversion of 35% and was thus unexpectedly improved to an average methanation rate advantageously ca. 50% higher than at the respective lower pH condition (cf. FIG. 8).

[0182] In a similar experiment, where a Methanobrevibacter arboriphilus culture was first grown at steady conditions at a pH<6.5 showed a comparable unexpected and beneficial increase in methanation after the pH was shifted to be constantly above 7.45 (data not shown).

[0183] From these experiments the inventors of the present invention concluded—without being bound by theory that—against any prediction of the state of the art knowledge a shift to increased pH values had a beneficial effect on the metabolism and methanation performance of methanogenic microorganisms as shown in the Examples of the present invention for different Methanobacteria species.

REFERENCES

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