A method for improving the methane production and purity of anaerobic fermentation of straw-like raw materials

Abstract

The present invention discloses a method for improving the methane production and purity in the anaerobic fermentation of straw-like raw materials, which comprises the following steps: S1. Pretreatment; S2. Inoculating the pretreated initial liquid material with hydrogen-producing and acid-producing microorganisms, and carrying out anaerobic fermentation by means of magnetic field strengthening and external circulation; S31. Collecting the fermentation gas in step S2, preparing nano-bubble water, then adding the pretreated initial liquid material, inoculating hydrogenophilic methanogenic microorganisms, and carrying out anaerobic fermentation to produce methane; S32. Collecting the fermentation liquid in step S2, inoculating acetic acidophilic methanogenic microorganisms, and carrying out anaerobic fermentation to produce methane by means of magnetic field strengthening and external circulation. This application separately uses acid-producing substrates and hydrogen to separate the methanogenesis stage into two independent pathways of acidophilic methanogenesis and hydrogenophilic methanogenesis. Combining magnetic field strengthening and nano-bubble technology, it improves the organic matter degradation effect, hydrogen-producing and acid-producing effect, as well as the utilization efficiency of hydrogen and acid-producing substrates, increases the methane production and purity, and shortens the entire fermentation cycle.

Description

Technical Field

[0001] The present invention belongs to the technical field of biological fermentation, and particularly relates to a method for improving the methane production and purity of straw-like raw materials in anaerobic fermentation. Background Art

[0002] With the increasing scarcity of fossil energy and the gradual advancement of the carbon peak goal, the development and utilization of renewable clean energy are strongly advocated. The anaerobic fermentation technology using straw-like biomass as raw materials converts biomass energy into biogas through microorganisms, which can not only utilize organic waste to alleviate environmental pollution, but also realize the substitution of traditional energy, and is of great significance for the development of renewable clean energy. However, due to problems such as low gas production rate, long fermentation cycle, and low utilization rate of fermentation substrates, the development of biogas in China is still relatively backward. Therefore, the development of a low-cost, high-efficiency and high-value anaerobic fermentation process for straw-like biomass has become an urgent need under China's sustainable development strategy.

[0003] In the anaerobic fermentation process, the organic matter conversion process generally includes three stages: hydrolysis acidification, hydrogen production and acetic acid production, and methanation. Among them, the hydrolysis acidification stage: in an anaerobic environment, macromolecular organic matter is decomposed into small-molecular organic matter, such as amino acids, monosaccharides and fatty acids, etc. under the action of hydrolase; the hydrogen production and acetic acid production stage: in this stage, small-molecular organic matter is converted into acetic acid and hydrogen under the action of hydrogen-producing and acetic acid-producing bacteria. At the same time, part of the acetic acid will also be further converted into methane; the methanation stage: in the methanation stage, acetic acid and hydrogen are converted into methane and carbon dioxide under the action of methanogens.

[0004] In traditional technologies, the above three stages are all completed in the same reactor (generally referred to as single-stage reaction hydrogen production), resulting in low raw material utilization rate, low methane production and purity. In recent years, some studies have separated the stages before methanation (hydrolysis acidification and hydrogen production and acetic acid production stages) and the methanation stage in different reactors (generally referred to as two-stage fermentation hydrogen production). The methanation stage uses the hydrogen-producing fermentation tail liquor of the previous stage to produce methane, which can not only shorten the fermentation cycle, but also significantly improve the biomass conversion rate. And on the basis of two-stage anaerobic fermentation, strengthening the hydrogenotrophic methanogenesis pathway and converting the acid-producing waste gas (H 2 and CO 2 ) into CH 4 is an effective means to increase the total energy output in the anaerobic fermentation process. However, in actual production, due to the low hydrogen production rate and low heat and mass transfer efficiency, the benefit of directly using hydrogen to increase methane production in the fermentation process is limited. Therefore, finding an economical and efficient method for enhancing fermentation hydrogen production, improving the heat and mass transfer efficiency in the hydrogenotrophic methanogenesis pathway, and reasonably matching it with the two-stage anaerobic fermentation process to establish an economical and efficient anaerobic fermentation system for straw-like biomass is the key to promoting the development of biogas in China. Summary of the Invention

[0005] The object of the present invention is to provide a method for improving the methane production and purity in the anaerobic fermentation of straw-like raw materials to solve the deficiencies of the prior art. This method mainly strengthens the methane generation pathway through magnetic fields and nano-bubbles. The fermentation cycle of this system is short, the gas production rate and energy conversion efficiency are high, and the operation is efficient.

[0006] The object of the present invention is achieved by the following technical solutions:

[0007] A method for improving the methane production and purity in the anaerobic fermentation of straw-like raw materials, comprising the following steps:

[0008] S1. Pretreatment: Aerobically degrade the straw-like raw materials by inoculating aerobic microorganisms to obtain an initial liquid material;

[0009] S2. Hydrogen and acid production fermentation: Inoculate the initial liquid material with hydrogen and acid-producing microorganisms, and carry out anaerobic fermentation to produce hydrogen and acidic substrates by means of magnetic field enhancement and external circulation reflux of the reaction liquid material; control the pH of the fermentation broth to be 6.0 - 6.2, and the acidic substrate is mainly acetic acid;

[0010] S3. Methane production fermentation: including,

[0011] S31. Hydrogenotrophic methane production fermentation: Collect the gas generated in the fermentation in step S2, and prepare nano-bubble water; then use the nano-bubble water, add the pretreated initial liquid material, inoculate hydrogenotrophic methane-producing microorganisms, and carry out anaerobic fermentation to produce methane; control the pH of the fermentation broth to be 7.0 - 7.2;

[0012] S32. Acetotrophic methane production fermentation: Collect the fermentation broth generated in the fermentation in step S2, adjust the pH to 7.0 - 7.2, inoculate acetic acid-type methane-producing microorganisms, and carry out anaerobic fermentation to produce methane by means of magnetic field enhancement and external circulation reflux of the reaction liquid material, and control the pH of the fermentation broth to be 7.0 - 7.2.

[0013] Preferably, when carrying out the aerobic degradation in step S1, the moisture content of the straw-like raw materials is adjusted to 50% - 70% by using the biogas slurry after aeration; after the aerobic degradation is completed, the mass percentage content of the total solid matter in the raw materials is further adjusted to 6% - 8% to obtain the initial liquid material;

[0014] The biogas slurry is the liquid obtained by solid-liquid separation of the materials after fermentation in step S3.

[0015] Preferably, the inoculum in step S1 is sludge;

[0016] Preferably, the inoculum in step S2 is the sludge after heat treatment to inactivate methanogens;

[0017] The inoculum in step S3 is the sludge after heat treatment to inactivate methanogens;

[0018] In step S31, the inoculum is sludge in which hydrogenotrophic methanogenic microorganisms after domestication are the dominant flora.

[0019] In step S32, the inoculum is sludge in which acetotrophic methanogenic microorganisms after domestication are the dominant flora.

[0020] Preferably, the fermentation temperature in step S2, step S31 and step S32 is 30°C to 35°C.

[0021] Preferably, the magnetic field enhancement is carried out during the external circulation process.

[0022] Preferably, in the initial stage of fermentation in step S2, by detecting the concentration of VFAs in the fermentation broth in real time, with acetic acid as the main target product, the magnetic field intensity and reflux parameters are regulated to determine the optimal magnetic field intensity and reflux parameters.

[0023] Preferably, the magnetic field intensity in step S2 does not exceed 100 mT, and the external circulation reflux ratio is 50% to 55%.

[0024] Preferably, the feeding amount of the nano-bubble water in step S31 accounts for 8% to 12% of the total feeding mass.

[0025] Preferably, in the initial stage of fermentation in step S32, by detecting the methane production in real time, the magnetic field intensity and reflux parameters are regulated to determine the optimal magnetic field intensity and reflux parameters.

[0026] Preferably, the magnetic field intensity in step S32 does not exceed 100 mT; the external circulation reflux ratio is 50% to 55%.

[0027] In this application, firstly, the magnetic field enhancement is used to increase the bacterial population abundance, and strengthen the degradation effect of organic waste and the hydrogen production and methane production effects during the anaerobic fermentation process. In addition, the external circulation of the feed liquid enables the microorganisms to be fully mixed with the substrate, which is beneficial to the growth and reproduction of the microorganisms. Moreover, the synergistic effect of the magnetic field and the external circulation not only increases and regulates the target product in the hydrogen production stage, making it mainly acetic acid, which is beneficial to subsequent methane production.

[0028] On the other hand, in this application, acid-producing substrates and acid-producing waste gases are respectively used for methane fermentation, which increases the material utilization rate. The introduction of nano-bubbles improves the heat and mass transfer efficiency between the bacterial strains and the acid-producing waste gases, and solves the problem of low gas-liquid mass transfer efficiency in the traditional hydrogenotrophic methanogenesis process. Moreover, through the hydrogenotrophic methanogenesis pathway, the conversion and utilization of hydrogen and carbon dioxide in the methane production stage are strengthened, the methane purification cost is reduced, and the biomass conversion rate is increased to a greater extent during the fermentation process compared with a single process, the methane production is increased, and in-situ biogas purification is realized.

[0029] This application has beneficial technical effects of strong adaptability, short processing cycle, and good economic benefits compared with the prior art, and is applicable to medium and large-scale biogas projects to increase methane production and achieve in-situ biogas purification. Detailed implementation manners

[0030] The present invention provides a method for improving the methane production and purity of anaerobic fermentation of straw-like raw materials, comprising the following steps:

[0031] S1. Pretreatment: inoculating aerobic microorganisms to the straw-like raw materials for aerobic degradation to obtain an initial liquid material.

[0032] The fermentation raw materials of this application are preferably straw-like raw materials such as corn, rice, and wheat straws. As those skilled in the art can understand, the straw-like raw materials should be crushed before pretreatment, preferably crushed to a particle size less than 20 mm. During the aerobic degradation process, the moisture content is adjusted to 50% - 70%, and after the aerobic degradation is completed, the mass percentage of the total solid substances in the raw materials is adjusted to 6% - 8% to obtain the initial liquid material.

[0033] In the pretreatment, the adjustment of the moisture content of the materials preferably uses the biogas slurry after aeration. The biogas slurry is preferably the liquid separated from the fermentation materials in step S3 (see details later), which can reduce the water consumption of the whole system. In addition, the biogas slurry contains certain nutrients, and the dissolved oxygen content in the liquid increases after aeration, which is beneficial to aerobic degradation.

[0034] The main purposes of the pretreatment are: one is to facilitate the full contact between the straw and microorganisms, so as to minimize the time required for straw degradation during the fermentation process; the second is to preliminarily aerobically degrade the macromolecular substances such as straw cellulose into small molecular substances such as glucose and organic acids, shortening the time of subsequent hydrogen production fermentation. The third is to adjust the moisture content of the straw after the aerobic degradation is completed, so that the mass percentage of the total solid substances in the initial liquid material is 6% - 8%, meeting the requirements of subsequent fermentation.

[0035] During the pretreatment process, affected by the conversion of organic matter by aerobic microorganisms into organic acids, the pH of the liquid material will decrease. However, at the same time, a part of the microorganisms will further decompose and transform the organic acids, causing the pH of the liquid material to rise. Since the decomposition rate of the organic acids is always less than the rate of converting organic matter into organic acids, during the pretreatment period, a pH test paper is used to measure the pH of the liquid material. When the pH rises, it is considered that the pretreatment is completed. Generally, the pretreatment time is about 10 days.

[0036] In this step, the sludge from the sewage treatment plant can be inoculated. The sludge contains aerobic microorganisms and anaerobic microorganisms, and the aerobic microorganisms among them can grow and reproduce under aerobic conditions to degrade the straw.

[0037] During the traditional anaerobic fermentation process of straw-like substances, the hydrolysis rate is slow. Through pretreatment, the anaerobic fermentation hydrolysis time can be greatly reduced, shortening the entire fermentation cycle.

[0038] S2. Hydrogen and acid production fermentation: The initial feed liquid is inoculated with hydrogen- and acid-producing microorganisms, and anaerobic fermentation is carried out to produce hydrogen and acidic substrates by means of magnetic field strengthening and external circulation reflux of the reaction feed liquid. The pH of the fermentation broth is controlled to be 6.0 - 6.2, and acetic acid is the main component in the acidic substrate.

[0039] The fermentation pH has an important impact on the growth of hydrogen- and acid-producing microorganisms. Therefore, the pH of the fermentation broth is controlled between 6.0 and 6.2. Extreme pH values will inhibit the growth of hydrogen- and acid-producing microorganisms. Under normal fermentation conditions, the fermentation pH generally remains between 6.0 and 6.2. However, during the new feed period or abnormal fermentation period, the pH may fluctuate to a certain extent. When pH > 6.2, it can be adjusted by adding a certain amount of acid such as citric acid. When pH < 6.0, it can be adjusted by adding a certain amount of base.

[0040] Acetic acid can be better utilized by subsequent acetic acid-utilizing methanogens, thereby promoting methane production. Therefore, by controlling acetic acid as the main component in the acidic substrate, the subsequent fermentation for methane production can be promoted.

[0041] Preferably, medium-temperature fermentation is adopted in this application, and the fermentation temperature is 30°C - 35°C.

[0042] After pretreatment, this application carries out fermentation in two stages: hydrogen and acid production and methane production to shorten the fermentation cycle and improve the biomass conversion rate. The hydrogen and acid production fermentation in the first stage uses the pretreated initial feed liquid and adopts the methods of magnetic field strengthening and external circulation. Among them, the magnetic field will produce different biological effects on different organisms, and its effect mainly depends on the magnetic field intensity. Appropriate magnetic field intensity has an obvious enrichment effect on hydrogen-producing acetic acid bacteria such as Synergistetes bacteria, can promote their growth and reproduction, increase their abundance, and increase the production of acetic acid and hydrogen during the acid production process. Too low magnetic field intensity cannot achieve the desired promotion effect, while too high magnetic field intensity will cause inhibitory effects. The return of biogas slurry can make the microorganisms and substrates fully mixed, which is beneficial to the growth and reproduction of microorganisms. In addition, applying magnetic field and external circulation can also enhance the activity of enzymes produced by microorganisms during the fermentation process, thereby further promoting the reduction of straw substances and improving the hydrogen and acetic acid production effects.

[0043] Therefore, preferably, in the initial stage of fermentation, the concentration of VFAs (volatile fatty acids) in the fermentation broth can be detected in real time, with acetic acid as the main target product, to regulate the magnetic field intensity and reflux parameters, and determine the optimal magnetic field intensity and reflux parameters corresponding to the fermentation, so as to improve the fermentation effect. When abnormalities or large changes occur during the fermentation process such as the replacement of fermentation raw materials, the optimal magnetic field intensity and reflux parameters can be re-determined.

[0044] Preferably, the magnetic field strength does not exceed 100 mT, more preferably 60 - 80 mT, and the external circulation reflux ratio is controlled at 50% - 55%. Generally, better fermentation effects can be achieved within this range.

[0045] Preferably, the magnetic field strengthening is carried out during the external circulation process, which can effectively reduce the floor area and layout range of the magnetic field while applying the magnetic field to the reactor liquid.

[0046] The fermenting microorganisms in this stage can use the sewage plant sludge after heat treatment to inactivate methanogens. The sludge contains all the microorganisms required for the entire anaerobic stage. By heat treatment, methanogens can be inactivated, leaving only the strains required for the hydrogen production stage. The heat treatment temperature is preferably 70 - 90 °C, and the treatment time is preferably 20 - 40 min.

[0047] S3. Methanogenic fermentation: including,

[0048] S31. Hydrogenotrophic methanogenic fermentation: Collect the gas generated in the fermentation of step S2 and prepare nano - bubble water; the preparation pressure of the nano - bubble water is preferably 0.4 - 0.6 Mpa; then use the nano - bubble water, add the pretreated initial feed liquid, inoculate hydrogenotrophic methanogenic microorganisms, and carry out anaerobic fermentation to produce methane, controlling the fermentation pH to be 7.0 - 7.2. The addition of the pretreated initial feed liquid ensures the survival of the other strains, and the other strains can be used to further produce hydrogen and CO 2 from the pretreated initial feed liquid. Taking hydrogenotrophic methanogenic microorganisms as the dominant strains, it can promote the utilization of CO 2 in the reactor through the reaction of CO 2 + 4H 4 = CH 2 + 4H 2 O, which not only increases the methane production but also improves the methane concentration.

[0049] This stage also preferably uses mesophilic fermentation, and the fermentation temperature is specifically 30 °C - 35 °C; the feeding amount of the nano - bubble water is preferably 8% - 12% of the total feeding mass, and the rest is the pretreated initial feed liquid.

[0050] The microorganisms in this stage are preferably the sewage plant sludge with hydrogenotrophic methanogenic microorganisms as the dominant flora after domestication.

[0051] S32. Acetotrophic methanogenic fermentation: Collect the fermentation broth generated in the fermentation of step S2, adjust the pH to 7.0 - 7.2, inoculate acetic - acid - type methanogenic microorganisms, and carry out anaerobic fermentation to produce methane by means of magnetic field strengthening and external circulation reflux of the reaction liquid, controlling the pH of the fermentation broth to be 7.0 - 7.2. Different from S31, at this time, acetic - acid - producing methanogenic microorganisms in the reactor are the dominant strains, mainly through CH 3 COOH = CH 4 + CO2 React to produce methane. In this step, no pre-treated initial feed liquid is added, which can avoid introducing H 2 resulting in an increase in hydrogen partial pressure and inhibiting the aceticlastic methanogenesis pathway.

[0052] This stage preferably also adopts mesophilic fermentation, and the fermentation temperature is specifically 30°C to 35°C.

[0053] The microorganisms in this stage are preferably sewage plant sludge with domesticated aceticlastic methanogenic microorganisms as the dominant flora.

[0054] Similar to step S2, appropriate magnetic field intensity and external circulation reflux parameters can promote the growth and reproduction of aceticlastic methanogens and improve the methane production rate. Therefore, at the initial stage of fermentation, the magnetic field intensity and reflux parameters can be regulated by real-time detection of methane production to determine the optimal magnetic field intensity and reflux parameters. Preferably, the magnetic field intensity in S32 does not exceed 100 mT, and more preferably 40 - 60 mT, and the external circulation reflux ratio is 50% - 55%.

[0055] In the second stage of this application, the fermentation gas and fermentation substrate in the first stage are respectively used for hydrogenotrophic methanogenesis fermentation and acidophilic methanogenesis fermentation. Compared with the prior art in which the fermentation gas and fermentation substrate are fermented together, the utilization rates of hydrogen and acidic substrates are higher, and the methane production and purity obtained are higher. Among them, hydrogenotrophic methanogenesis fermentation is enhanced by using nanobubbles. First, the fermentation gas containing hydrogen in the first stage is made into nanobubbles, and then methanogenesis fermentation is carried out. Nanobubbles can improve the heat and mass transfer efficiency between microorganisms and raw materials, solve the problem of low gas-liquid mass transfer efficiency in the traditional hydrogenotrophic methanogenesis process, and improve methane production and purity; acidophilic methanogenesis fermentation is enhanced by using a magnetic field, and its function is based on the biological magnetic effect to regulate the population abundance of acidophilic methanogens and promote the activity of corresponding enzymes.

[0056] Therefore, based on the two-stage fermentation methanogenesis in the prior art, this application separately uses the acid-producing substrate and hydrogen to separate the methanogenesis stage into two independent pathways of acidophilic methanogenesis and hydrogenotrophic methanogenesis to produce methane. Combining the magnetic field strengthening and nanobubble technologies, it improves the organic matter degradation effect, hydrogen and acid production effect, and the utilization efficiency of hydrogen and acid-producing substrates, increases methane production and purity, and shortens the entire fermentation cycle.

[0057] The reactor used for fermentation in this application preferably adopts a CSTR dark fermentation reactor.

[0058] The biogas produced after fermentation in steps S31 and S32 is collected uniformly. After purification, it can not only be used for gas production but also for system heating; the materials after fermentation in steps S31 and S32 are collected uniformly. The biogas slurry after solid-liquid separation can be used as the water source of the fermentation system after aeration, such as for supplementing water in the pretreatment stage or the later fermentation stage; the biogas residue can be used for fertilizer production after composting, thus promoting the cleanliness and energy flow optimization in the anaerobic fermentation process.

[0059] Example 1

[0060] S1. Collect corn straw and crush it to an average particle size of less than 20 mm.

[0061] S2. Adjust the moisture content of the crushed corn straw to 60% with the aerated biogas slurry, and inoculate the sludge from the sewage treatment plant for aerobic degradation. After adding the biogas slurry, the moisture content of the straw is adjusted to 60%. The initial pretreatment time is set to 10 d. When the pH of the material liquid rises, the degradation ends. Then, water is added to adjust the mass percentage of the total solid matter in the material liquid to 7% to obtain the initial material liquid. The change of the pH of the material during the aerobic degradation process is shown in Table 1. At the 10th d, the pH of the material rises significantly, and the aerobic degradation pretreatment ends.

[0062] Table 1 Change of the pH of the solution during aerobic degradation with time

[0063] Time 1 day 2 days 3 days 4 days 5 days 6 days 7 days 8 days pH 6.8 6.3 6.1 5.8 5.8 5.4 5.6 5.6 Time 9 days 10 days pH 5.8 6.2

[0064] S3. Feed the initial material liquid into the first CSTR reactor, and inoculate the sludge from the sewage treatment plant obtained after heat treatment at 80 °C for 30 min (at this time, the methanogens in the original sludge are basically inactivated). Heat the reactor and maintain the heating temperature at about 35 °C. The external circulation method is used for hydrogen-producing and acid-producing anaerobic fermentation. A magnetic field is added to the external circulation to strengthen the external circulation liquid. During the initial stage of fermentation, regularly detect the concentration of VFAs in the fermentation broth in the reactor. With acetic acid as the main target product, adjust the magnetic field intensity and reflux parameters. Determine that the magnetic field intensity in this example is 70 mT and the external circulation reflux ratio is 52%. Regularly detect the pH value of the fermentation broth in the reactor. When pH > 6.2, add an appropriate amount of citric acid solution; when pH < 6.0, add sodium bicarbonate solution. The hydraulic retention time in this stage is 4 d.

[0065] S4. Collect the fermentation gas in step S3, purify it with water and enter the nano-bubble generator at a low flow rate to prepare nano-bubble water at 0.5 Mpa.

[0066] S5. Feed the nano-bubble water into the second CSTR reactor from the bottom, then add the pretreated corn straw liquid, inoculate the activated sludge obtained by acclimating the sewage plant sludge (with hydrogenotrophic methanogens as the dominant strain), and carry out hydrogenotrophic methanogenic anaerobic fermentation at 35 °C. The feeding amount of the nano-bubble water is 10% of the total feeding amount, adjust the fermentation pH to 7.0 - 7.2, and the hydraulic retention time is 30 d.

[0067] S6. Collect the fermentation broth from step S3, adjust its pH to 7.1 with sodium bicarbonate, and then transport it to the third CSTR reactor from the top, inoculating the activated sludge obtained by acclimating the sewage plant sludge (with aceticlastic methanogens as the dominant strain). Carry out aceticlastic methanogenic anaerobic fermentation at 35 °C by the external circulation method, add a magnetic field to strengthen the external circulation liquid in the external circulation, regularly detect the methane production at the initial stage of fermentation, adjust the magnetic field intensity and reflux parameters, determine that the external circulation reflux ratio of this example is controlled at 52%, the magnetic field intensity is 50 mT, and adjust the fermentation pH to 7.0 - 7.2. The hydraulic retention time in this stage is 15 d;

[0068] S7. Collect the fermentation gases from steps S5 and S6, and detect that the methane content in the biogas is about 58% and the methane production is about 234 mL / g VS. Collect the fermentation materials from steps S5 and S6, separate them into biogas residue and biogas slurry by solid-liquid separation, and the biogas slurry is circulated and transported to step S1 for pretreatment after aeration.

[0069] In industrial production, using the traditional single-stage and two-stage fermentation methods, the methane content in the biogas is generally about 50%, and the methane production is 210 mL / g VS. Using the method of this application, the methane content and production are increased. On the other hand, after the pretreatment of this application, the subsequent fermentation stages of acid production and hydrogen production and aceticlastic methanogenic fermentation only take about 19 d (hydraulic retention time of 4 d in the acid production and hydrogen production stage + hydraulic retention time of 15 d in the aceticlastic methanogenic fermentation stage), greatly shortening the fermentation cycle (in this application, methanogenesis mainly relies on aceticlastic methanogens, and the hydrogenotrophic methanogenic stage further improves the methane production and purity).

[0070] Although the preferred embodiments of the present invention have been described, those skilled in the art can make additional changes and modifications once they know the basic creative concept. Therefore, the appended claims are intended to be construed to include the preferred embodiments as well as all changes and modifications falling within the scope of the present invention. Obviously, those skilled in the art can make various changes and variations to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention also intends to include these modifications and variations.

Claims

1. A method for improving the methane yield and purity of anaerobic fermentation of straw raw materials, characterized in that: The following steps are involved: S1. Pretreatment: Inoculate aerobic microorganisms into the straw raw material for aerobic degradation to obtain an initial feed solution; S2. Hydrogen-acid production fermentation: The initial feed liquid is inoculated with hydrogen-acid production microorganisms, and anaerobic fermentation is performed to produce hydrogen and acidic substrates by means of magnetic field enhancement and external circulation of the reaction liquid; the pH of the fermentation liquid is controlled to be 6.0-6.2, and acetic acid is mainly present in the acidic substrate; S3.Methanogenic fermentation: include, S31. Hydrogenophilic methanogenic fermentation: collecting the gas produced by the fermentation in step S2 to prepare nanobubble water; then using the nanobubble water, adding the pretreated initial feed liquid, inoculating hydrogenophilic methanogenic microorganisms, and performing anaerobic fermentation to produce methane; controlling the pH of the fermentation liquid to 7.0~7.2; S32. Acidophilic methanogenic fermentation: The fermentation broth produced in step S2 was collected, the pH was adjusted to 7.0-7.2, acetophilic methanogenic microorganisms were inoculated, and anaerobic fermentation and methanogenesis were performed by magnetic field enhancement and external circulation of the reaction liquid, and the pH of the fermentation broth was controlled to 7.0-7.2; Step S1: the inoculated material is sludge; The inoculated material in step S2 is sludge after heat treatment and inactivation of methanogens; Step S31: the inoculated material is sludge with domesticated hydrogenophilic methanogenic microorganisms as the dominant flora; In step S32, the inoculated material is sludge in which acetophilic methanogenic microorganisms are the dominant flora after being domesticated; The magnetic field strengthening in step S2 and step S32 is performed during the external circulation process; the magnetic field intensity does not exceed 100mT, and the external circulation reflux ratio is 50% to 55%.

2. The method for improving the methane yield and purity of anaerobic fermentation of straw raw materials according to claim 1, characterized in that: Step S1: during the aerobic degradation, the aerated biogas slurry is used to adjust the moisture content of the straw raw material to 50% to 70%; after the aerobic degradation is completed, the total solid matter mass percentage in the raw material is adjusted to 6% to 8% to obtain the initial feed liquid; The biogas slurry is the liquid obtained by solid-liquid separation of the material after fermentation in step S3.

3. The method for improving the methane yield and purity of anaerobic fermentation of straw raw materials according to claim 1, characterized in that: The fermentation temperature in step S2, step S31 and step S32 is 30°C to 35°C.

4. The method for improving the methane yield and purity of anaerobic fermentation of straw raw materials according to claim 1, characterized in that: In step S2, at the initial stage of fermentation, the concentration of VFAs in the fermentation broth is detected in real time, acetic acid is taken as the main target product, and the magnetic field strength and reflux parameters are adjusted to determine the optimal magnetic field strength and reflux parameters.

5. The method for improving the methane yield and purity of anaerobic fermentation of straw raw materials as claimed in claim 1, characterized in that: The nano bubble water feed amount in step S31 accounts for 8% to 12% of the total feed mass.

6. The method for improving the methane yield and purity of anaerobic fermentation of straw raw materials according to claim 1, characterized in that: In step S32, at the initial stage of fermentation, the methane production is detected in real time, and the magnetic field strength and reflux parameters are adjusted to determine the optimal magnetic field strength and reflux parameters.