A gaseous fermentation method for improving biogas quality by using zeolitic imidazolate framework-8 (ZIF-8) material

Abstract

The present application belongs to the field of environmental protection and new energy technology, and relates to a gaseous fermentation method for improving biogas quality by using zeolite imidazolate framework structure ZIF-8 material. The ZIF-8 material for improving the biological conversion efficiency of CO2 to CH4 is prepared, and has the effect of selectively adsorbing carbon dioxide and hydrogen. When applied to gaseous fermentation, the quality of the obtained methane can be effectively improved. The ZIF-8 material prepared by the present application is applied to an anaerobic gaseous fermentation system, and only a small amount of addition can promote the biological methanation process of CO2 in the biogas, increase the proportion of methane in the biogas, improve the quality of the biogas, and make the biogas reach the standard of direct use.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of environmental protection and new energy technology, and particularly relates to a gaseous fermentation method for improving biogas quality by using zeolite imidazolate framework structure ZIF-8 material. BACKGROUND

[0002] Methane production by anaerobic digestion is a widely used organic waste resource and energy processing technology. In an anaerobic digestion reactor, organic matter in municipal sludge, kitchen waste and livestock manure is utilized by microorganisms to produce biogas mainly composed of methane and CO2. Since the methane proportion of biogas produced by traditional anaerobic digestion reactors is usually only 50%-70%, which is far lower than the content standard that can be directly used as natural gas products, and the CO2 proportion of 30%-50% cannot be effectively utilized, on the one hand, the calorific value of biogas is reduced, and on the other hand, a large amount of carbon emissions is formed. Therefore, how to further convert CO2 in biogas into methane so that its proportion reaches more than 90% to improve the quality of biogas is a problem that needs to be solved by those skilled in the art.

[0003] In the gaseous fermentation system, the methane in the biogas produced by the traditional anaerobic digestion facility can be further purified by the metabolic mode of microorganisms to improve its quality, so that it can reach the utilization standard, and at the same time, carbon emissions are reduced. Hydrogenotrophic methanogens use hydrogen as an electron donor and CO2 as a carbon source to convert CO2 into methane through biochemical reactions. This process is mild and has low energy consumption, and compared with the chemical conversion process, the cost can be greatly reduced, but at the same time, it also faces problems such as long reaction time (low CO2 methanation efficiency, long period required to reach high methane proportion), and unstable biogas upgrading effect. Therefore, how to regulate and strengthen the gaseous fermentation process to improve the quality of biogas has become the core of technical research in the field. SUMMARY

[0004] The purpose of the present application is to provide a gaseous fermentation method for improving the quality of biogas by using zeolite imidazolate framework structure ZIF-8 material, to solve the problems existing in the prior art and improve the conversion efficiency of CO2 to methane in biogas.

[0005] To achieve the above purpose, the present application provides the following solutions:

[0006] One of the technical solutions of the present application: a preparation method of ZIF-8 material for improving the biological conversion efficiency of CO2 to CH4 is provided, and the steps include:

[0007] Mixing Hmim (2-methylimidazole) methanol solution and Zn(NO3)2·6H2O methanol solution to obtain a mixed solution;

[0008] After the mixed solution is stirred and reacted at 5 DEG C, the precipitated product is collected by centrifugation, washed and dried to obtain the ZIF-8 material.

[0009] Further, the molar ratio of Hmim to methanol in the Hmim methanol solution is 1:26-32.

[0010] Further, the molar ratio of Zn(NO3)2.6H2O to methanol in the Zn(NO3)2.6H2O methanol solution is 1:240-270.

[0011] Further, the molar ratio of Zn(NO3)2.6H2O to Hmim in the mixed solution is 1:5-10.

[0012] Further, the stirring speed is 200 rpm, and the time is 1 h.

[0013] Further, the washing is washing the precipitated product with methanol at least once by centrifugation.

[0014] Further, the drying temperature is 60 DEG C, and the time is 12 h.

[0015] The second technical scheme of the present application provides a ZIF-8 material for improving the biological conversion efficiency of CO2 to CH4, which is prepared by the above method.

[0016] The third technical scheme of the present application provides an application of the above ZIF-8 material for improving the biological conversion efficiency of CO2 to CH4 in anaerobic methane production gaseous fermentation to improve the methane quality in biogas.

[0017] The fourth technical scheme of the present application provides an application of the above ZIF-8 material for improving the biological conversion efficiency of CO2 to CH4 in gaseous fermentation to improve the anaerobic methane production efficiency.

[0018] The fifth technical scheme of the present application provides a method for improving the methane quality in biogas in gaseous fermentation, which comprises the following steps:

[0019] S1, adding inoculum and carbon-free nutrient solution for growth of hydrogen-utilizing methanogen in a gaseous fermentation reactor;

[0020] S2, adding the above ZIF-8 material for improving the biological conversion efficiency of CO2 to CH4 into the gaseous fermentation reactor;

[0021] S3, purging the gaseous fermentation reactor with H2 to ensure an anaerobic environment in the gaseous fermentation reactor;

[0022] S4, then injecting biogas and H2 into the gaseous fermentation reactor, and performing gaseous fermentation reaction on the gaseous fermentation reactor at 35-37 DEG C.

[0023] Further, the CO2 in the biogas accounts for 20% to 50%, and the methane accounts for 50% to 80%.

[0024] Further, the inoculum is the discharge of a traditional anaerobic digestion reactor.

[0025] The discharge of the traditional anaerobic digestion reactor is anaerobic digestion sludge, which contains a high proportion of organic matter, important elements such as nitrogen and phosphorus, and a variety of microbial communities such as Firmicutes (acid-producing bacteria), Bacteroidetes (hydrogen-producing and acetic acid-producing bacteria), Synergistetes (synergistic acetic acid-oxidizing bacteria), Methanobacterium (methanogenic bacteria), etc.

[0026] Further, the volume ratio of the inoculum to the carbon-free nutrient solution is 1 to 3:1, and the sum of the volumes of the inoculum and the carbon-free nutrient solution accounts for 25% to 40% of the total volume of the gaseous fermentation reactor.

[0027] Further, the components of the carbon-free nutrient solution for the growth of hydrogen-utilizing methanogenic bacteria include: 380 to 420 mg / L of KH2PO4, 400 to 450 mg / L of Na2HPO4, 300 to 420 mg / L of Na2S·9H2O, 280 to 310 mg / L of NH4Cl, 250 to 350 mg / L of NaCl, 95 to 120 mg / L of CaCl2·2H2O, 90 to 110 mg / L of MgCl2·6H2O, 2 to 5 mg / L of MnCl2·4H2O, 1 to 3 mg / L of FeCl2·4H2O, and 0.8 to 2.0 mg / L of EDTA.

[0028] Further, the volume ratio of H2 to CO2 in the biogas is 3 to 5:1.

[0029] Further, the amount of ZIF-8 material added for improving the biological conversion efficiency of CO2 to CH4 is 0.25 to 0.75 g / L.

[0030] Here, the amount of ZIF-8 material added is based on the total volume of the inoculum and the carbon-free nutrient solution for the growth of hydrogen-utilizing methanogenic bacteria in step S1, i.e., when the total volume of the inoculum and the carbon-free nutrient solution for the growth of hydrogen-utilizing methanogenic bacteria is 1 L, the amount of ZIF-8 material added is 0.25 to 0.75 g.

[0031] The zeolitic imidazolate framework (ZIF-8) material used in the application belongs to a kind of metal organic framework (MOF) material, and the MOF is a kind of crystalline porous material, which has a large specific surface area and high porosity. The pore size of the ZIF-8 material is between the pore sizes of CO2 H2 and methane Therefore, the ZIF-8 material has good adsorption effect on CO2 and H2, but poor adsorption capacity on methane, so as to realize good gas separation. Under the action of the ZIF-8 material, CO2 and H2 in the headspace part of the gaseous fermentation reactor are more easily pulled into the liquid phase part and utilized by methanogens growing on the surface of the ZIF-8 to be converted into methane; methane has poor solubility in water and adsorption capacity on the ZIF-8, so as to be released from the liquid phase part into the headspace part in time, and after a period of reaction, the concentration of methane in the headspace part can reach more than 90%.

[0032] The application discloses the following technical effects:

[0033] By controlling the molar ratio of Hmin and Zn, the application improves the crystallinity, yield, specific surface area and t-plot micropore volume of the ZIF-8 material, and the larger specific surface area can provide more adsorption sites to enhance the adsorption performance.

[0034] The ZIF-8 material prepared by the application is a kind of porous MOF material, which has the functions of selectively adsorbing CO2 and H2, so as to realize the separation of CO2 and methane, and therefore, CO2 in the headspace part of the gaseous fermentation system can be pulled into the liquid part, so that the hydrogen-utilizing methanogens can be fully utilized, and the produced methane can be released from the liquid part in time, so as to quickly realize biogas purification.

[0035] The ZIF-8 material prepared by the application is applied to an anaerobic gaseous fermentation system, and only a small amount of addition can promote the biological methanation process of CO2 in the biogas, so as to increase the proportion of methane in the biogas, improve the quality of the biogas, and make the biogas reach the standard of direct use. BRIEF DESCRIPTION OF DRAWINGS

[0036] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed in the embodiments. Obviously, the drawings described below only constitute some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained from these drawings without creative effort.

[0037] Figure 1 XRD diffraction pattern of the ZIF-8 material prepared for Example 1;

[0038] Figure 2 CV curves of the fermentation broth in the gaseous fermentation system under the condition that different amounts of the ZIF-8 material prepared in Example 1 are added in Examples 2-4 and Comparative Example 1;

[0039] Figure 3 Figures showing the changes of the acetic acid concentration, coenzyme F420 activity and electron transfer activity in the fermentation broth with the increase of the ZIF-8 material concentration after the ZIF-8 material is added in Examples 2-4 and Comparative Example 1, wherein a is the acetic acid concentration change figure, b is the coenzyme F420 activity change figure, and c is the electron transfer activity change figure;

[0040] Figure 4 Methane proportion comparison figure in three cycles in Examples 2-4 and Comparative Example 1. DETAILED DESCRIPTION

[0041] The various illustrative embodiments of the present application will now be described in detail below. This detailed description is not intended to limit the present application, but rather to provide a more full understanding of certain aspects, features and embodiments of the present application.

[0042] It should be understood that the terms used in the present application merely describe particular embodiments, and are not intended to limit the present application. In addition, for the numerical ranges in the present application, it should be understood that each intermediate value between the upper limit and the lower limit of the range is also specifically disclosed. Each smaller range within any stated range or within any stated intermediate value, as well as any other stated value or intermediate value within the stated range, is also encompassed within the present application. The upper and lower limits of these smaller ranges can be independently included or excluded from the range.

[0043] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present application pertains. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application, preferred methods and materials are described. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials in connection with which the documents are cited. In the case of conflict between the present specification and any document incorporated by reference, the present specification will control.

[0044] Many modifications and variations of the specific embodiments of the application can be practiced in accordance with the teachings of the description of the application, which are to be considered as illustrative only. Other embodiments of the application will readily suggest themselves to those skilled in the art from the disclosure of the application. The disclosure of the application and examples is to be considered exemplary only.

[0045] As used herein, the terms "comprise", "comprising", "include", "including", "have", "having" and the like are to be construed in an open, non- limiting sense, as opposed to a closed or limiting sense.

[0046] Unless otherwise specified, the room temperature and normal temperature referred to in the specific embodiments of the application both mean 20-30°C.

[0047] The inoculum used in the specific embodiments of the application is the effluent of a conventional anaerobic digestion reactor, specifically anaerobic digestion sludge, which contains a high proportion of organic matter, and also contains various important elements such as nitrogen and phosphorus, and also contains many different types of microorganisms with different functions, such as Firmicutes (acid-producing bacteria), Bacteroidetes (hydrogen-producing and acetic acid-producing bacteria), Syntrophica (syntrophic acetic acid-oxidizing bacteria), Methanobacterium (methanogenic bacteria), etc. The principle of the gaseous fermentation reaction is to inject hydrogen and biogas into the anaerobic reactor, and through the synergistic action of various microorganisms such as methanogenic bacteria and syntrophic bacteria, H2 and CO2 in the biogas are converted into CH4, so as to increase the CH4 content in the biogas and achieve biogas purification. Therefore, based on the principle of the gaseous fermentation reaction, the biogas purification process (the methane content is increased from 60% to more than 90%) and the CO2 methanation process can be equally replaced, without affecting the technical effects of the application.

[0048] The components of the carbon-free nutrient solution for the growth of hydrogen-utilizing methanogenic bacteria used in the specific embodiments of the application include: 380-420 mg / L of KH2PO4, 400-450 mg / L of Na2HPO4, 300-420 mg / L of Na2S·9H2O, 280-310 mg / L of NH4Cl, 250-350 mg / L of NaCl, 95-120 mg / L of CaCl2·2H2O, 90-110 mg / L of MgCl2·6H2O, 2-5 mg / L of MnCl2·4H2O, 1-3 mg / L of FeCl2·4H2O, and 0.8-2.0 mg / L of EDTA.

[0049] Example 1

[0050] The preparation steps of the ZIF-8 material for improving the biological conversion efficiency of CO2 to CH4 are as follows:

[0051] S1, 3.005 g (10 mmol) of Zn(NO3)2·6H2O was dissolved in 100 mL of MeOH (methanol), denoted as solution A, the molar ratio of Zn(NO3)2·6H2O and methanol was 1:247;

[0052] S2, 6.702 g (80 mmol) of Hmim was dissolved in 100 mL of MeOH, denoted as solution B, the molar ratio of Hmim and methanol was 1:31;

[0053] S3, solution B was poured into solution A at 5°C, stirred at 200 rpm for 1 h, centrifuged, and the solid was separated from the liquid to obtain a white precipitate;

[0054] S4, the obtained white precipitate was washed with MeOH by centrifugation three times, and then dried at 60°C for 12 h to obtain the ZIF-8 material for improving the biological conversion efficiency of CO2 to CH4.

[0055] Figure 1 The XRD diffraction pattern of the ZIF-8 material prepared in Example 1 showed that the ZIF-8 was basically composed of Zn 2+ The imidazolate ions are connected to each other by coordination bonds, and the structure is similar to that of zeolite, which has a three-dimensional pore structure, so it also has good thermal stability and chemical stability, and can maintain a crystalline state even when exposed to harsh conditions.

[0056] Example 2

[0057] The steps for improving the quality of methane in biogas are as follows:

[0058] S1, inoculum (traditional anaerobic digestion reactor effluent) and carbon-free nutrient solution for growth of hydrogen-utilizing methanogens were sequentially added, wherein the volume ratio of inoculum and carbon-free nutrient solution for growth of hydrogen-utilizing methanogens was 3:1, and the sum of the volumes of inoculum and carbon-free nutrient solution for growth of hydrogen-utilizing methanogens was one-third of the total volume of the gaseous fermentation reactor;

[0059] S2, the ZIF-8 material for improving the biological conversion efficiency of CO2 to CH4 prepared in Example 1 was added to the gaseous fermentation reactor at a dosage of 0.25 g / L, and the reactor was sealed with a butyl rubber plug and an aluminum cap;

[0060] S3, the gaseous fermentation reactor was purged with H2 to ensure an anaerobic environment in the gaseous fermentation reactor;

[0061] S4, then biogas (CO2 and CH4 volume ratio of 4:6) and H2 were injected into the gaseous fermentation reactor at a volume ratio of 10:16;

[0062] S5, then the gaseous fermentation reactor was subjected to gaseous fermentation reaction in a 35℃ shaker;

[0063] S6, the reaction was carried out in 5-day cycles, at the beginning of each cycle, biogas and hydrogen were re-injected, but no ZIF-8 material was added, and the gaseous fermentation reactor was numbered as E_0.25.

[0064] At the end of the first cycle, the proportion of methane in the headspace of the gaseous fermentation reactor reached 93.0%, and at the end of the third cycle, the proportion of methane in the headspace of the gaseous fermentation reactor reached 94.2%.

[0065] Example 3

[0066] Compared with Example 2, the only difference is that the ZIF-8 material for improving the biological conversion efficiency of CO2 to CH4 is added to the gaseous fermentation reactor at a dosage of 0.50 g / L, and the gaseous fermentation reactor is numbered as E_0.50.

[0067] At the end of the first cycle, the proportion of methane in the headspace of the gaseous fermentation reactor reached 93.5%, and at the end of the third cycle, the proportion of methane in the headspace of the gaseous fermentation reactor reached 94.5%.

[0068] Example 4

[0069] Compared with Example 2, the only difference is that the ZIF-8 material for improving the biological conversion efficiency of CO2 to CH4 is added to the gaseous fermentation reactor at a dosage of 0.75 g / L, and the gaseous fermentation reactor is numbered as E_0.75.

[0070] At the end of the first cycle, the proportion of methane in the headspace of the gaseous fermentation reactor reached 96.0%, and at the end of the third cycle, the proportion of methane in the headspace of the gaseous fermentation reactor reached 98.5%.

[0071] Comparative Example 1

[0072] Compared with Example 2, the only difference is that no ZIF-8 is added in step S2.

[0073] At the end of the first cycle, the proportion of methane in the headspace of the gaseous fermentation reactor reached 87.3%, and at the end of the third cycle, the proportion of methane in the headspace of the gaseous fermentation reactor reached 91.0%, and the gaseous fermentation reactor was numbered as Control.

[0074] Comparative Example 2

[0075] Compared with Example 2, the only difference is that the dosage of ZIF-8 material for improving the biological conversion efficiency of CO2 to CH4 in step S2 is 0.1 g / L.

[0076] At the end of the first cycle, the proportion of methane in the headspace portion of the gaseous fermentation reactor reached 86.4%, and at the end of the third cycle, the proportion of methane in the headspace portion of the gaseous fermentation reactor reached 90.8%.

[0077] Comparative Example 3

[0078] Compared with Example 2, the only difference is that the amount of ZIF-8 material used for improving the biological conversion efficiency of CO2 to CH4 in step S2 is 1 g / L.

[0079] At the end of the first cycle, the proportion of methane in the headspace portion of the gaseous fermentation reactor reached 87.2%, and at the end of the third cycle, the proportion of methane in the headspace portion of the gaseous fermentation reactor reached 89.3%.

[0080] Test Example

[0081] Figure 2 The CV curves of the fermentation liquid in the gaseous fermentation system under the condition of adding different amounts of ZIF-8 material prepared in Example 1 in Examples 2-4 and Comparative Example 1 are shown in the following figure: Figure 2 It can be seen that the redox peak of the CV curve increases with the increase of the amount of ZIF-8 material, and the redox peak corresponds to the reduction of CO2 and the oxidation of organic acid, respectively, which indicates that the addition of ZIF-8 material can indeed strengthen interspecies electron transfer to improve microbial metabolic activity.

[0082] Figure 3 The changes of the concentration of acetic acid, coenzyme F420 activity and electron transfer activity in the fermentation liquid after the addition of ZIF-8 material in Examples 2-4 and Comparative Example 1 with the increase of the concentration of ZIF-8 material are shown in the following figure, wherein a is the change figure of the concentration of acetic acid in the fermentation liquid, b is the change figure of the coenzyme F420 activity, and c is the change figure of the electron transfer activity. Figure 3 It can be seen that the addition of ZIF-8 material prepared in Example 1 greatly promotes the generation of acetic acid, which indicates that in the gaseous fermentation system, in addition to the methanation of CO2, there may also be homoacetogenesis and mutual acetate oxidation processes; the increase of coenzyme F420 to a certain extent also means that the methanation process of CO2 is enhanced; and the increase of ETS content further reflects the enhancement of interspecies electron transfer of microorganisms.

[0083] Figure 4 The comparison figure of the proportion of methane in three cycles in Examples 2-4 and Comparative Example 1 is shown in the following figure: Figure 4 It can be seen that in each batch experiment, the proportion of methane in the biogas of the experimental groups of Examples 2-3 to which ZIF-8 material is added can reach more than 90%, which is higher than that of the control group (Comparative Example 1) without adding ZIF-8 material, achieving the goal of upgrading biogas, and further indicating the cyclic stability of the ZIF-8 material prepared by the present application.

[0084] The above-described embodiments are merely intended to describe the preferred modes of the present application, and are not intended to limit the scope of the present application. Various modifications and improvements of the present application made by those skilled in the art based on the above-described embodiments should fall within the scope of the present application defined by the claims.

Claims

1. A method for improving the quality of methane in biogas in a gaseous fermentation, characterized by the steps of The application relates to a method for improving the biological conversion efficiency of CO2 to CH4. S1, adding inoculum and carbon-free nutrient solution for growth of hydrogen-utilizing methanogens in a gaseous fermentation reactor; S2, adding ZIF-8 material for improving the biological conversion efficiency of CO2 to CH4 into the gaseous fermentation reactor; S3, purging the gaseous fermentation reactor with H2 to ensure an anaerobic environment in the gaseous fermentation reactor; S4, then injecting biogas and H2 into the gaseous fermentation reactor, and performing gaseous fermentation reaction on the gaseous fermentation reactor at 35-37 DEG C; The inoculum is a discharge of a traditional anaerobic digestion reactor; The discharge of the traditional anaerobic digestion reactor is anaerobic digestion sludge; The preparation steps of the ZIF-8 material for improving the biological conversion efficiency of CO2 to CH4 include: Mixing 2-methyl imidazole methanol solution and Zn (NO3) 2.6H2O methanol solution to obtain a mixed solution; After stirring reaction of the mixed solution at 5 DEG C, the precipitate is collected by centrifugation, washed and dried to obtain the ZIF-8 material.

2. The method of claim 1, wherein, The molar ratio of 2-methyl imidazole to methanol in the 2-methyl imidazole methanol solution is 1:26-32; the molar ratio of Zn (NO3) 2.6H2O to methanol in the Zn (NO3) 2.6H2O methanol solution is 1:240-270; and the molar ratio of Zn (NO3) 2.6H2O to 2-methyl imidazole in the mixed solution is 1:5-10.

3. The method of claim 1, wherein, The stirring speed is 200 rpm, and the stirring time is 1 h; the washing is centrifugal washing of the precipitate with methanol at least once; and the drying temperature is 60 DEG C, and the drying time is 12 h.

4. The method of claim 1, wherein, The CO2 accounts for 20%-50% of the biogas, and the methane accounts for 50%-80%.

5. The method of claim 1, wherein, The volume ratio of the inoculum to the carbon-free nutrient solution is 1-3:1, and the sum of the volumes of the inoculum and the carbon-free nutrient solution accounts for 25%-40% of the total volume of the gaseous fermentation reactor.

6. The method of claim 1, wherein, The volume ratio of H2 to CO2 in the biogas is 3-5:1; and the adding amount of the ZIF-8 material for improving the biological conversion efficiency of CO2 to CH4 is 0.25-0.75 g / L.

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