A system and method for purifying biogas based on fenton iron anaerobic digestion

Abstract

The application discloses a system and method for purifying biogas based on Fenton iron sludge anaerobic digestion, and belongs to the technical field of solid waste resource utilization in wastewater treatment processes. The application utilizes the fact that Fenton iron sludge can produce fermentation iron sludge rich in Fe(II) through dissimilatory iron reduction in an anaerobic environment, and biogas to be purified is introduced into the fermentation iron sludge, so that CO2 in the biogas penetrates into the liquid phase and reacts with Fe(II) to produce FeCO3, while the CO2 is fixed in the form of carbonate, and the biogas is purified and the CO2 impurities are removed. Meanwhile, FeCO3 as a cheap iron salt can be recycled in the process of industrial production, such as participating in sewage coagulation and improving sludge activity, and can also be used as a soil conditioner and an iron fertilizer to improve soil structure and be used in agricultural production, so that the problem of resource utilization of Fenton iron sludge is solved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of solid waste resource utilization in wastewater treatment process, and relates to a system and method for purifying biogas based on Fenton iron mud anaerobic digestion. BACKGROUND

[0002] As a kind of renewable energy, the main component of biogas, methane (CH4), accounts for about 50% to 70% of biogas, has high calorific value and good combustion performance, and can play an important role in replacing fossil energy. The main impurity carbon dioxide (CO2) in the biogas produced in the anaerobic fermentation process of sewage treatment plant will reduce the combustion value of biogas, and the trace impurities such as hydrogen sulfide in it will corrode the equipment during combustion. In order to improve the energy utilization efficiency of the sewage treatment industry, it is necessary to remove the impurities in the biogas by biogas purification and purification to prepare high-quality biomethane. The method of absorbing and mineralizing CO2 into carbonate is considered as a promising technology, but the current mainstream CO2 fixation technologies such as chemical absorption, physical adsorption and membrane separation have problems such as high cost, high energy consumption and potential environmental pollution.

[0003] As a kind of advanced oxidation technology (AOP), the classical homogeneous Fenton catalytic oxidation process plays an important role in treating refractory organic matter in wastewater. Fenton iron mud, as a by-product of the technology, is mainly composed of iron hydroxide, which can adsorb and flocculate a large amount of organic pollutants. However, the current mainstream Fenton iron mud treatment technology has problems such as poor economy and easy to cause secondary pollution, so it is of great significance to find a cheap way to resource utilization of Fenton iron mud for environmental protection and resource recycling.

[0004] CN114920433A discloses a method for resource utilization of chemical iron sludge generated in antibiotic wastewater treatment, and utilizes Fenton iron sludge to prepare an iron oxide columnar adsorbent for dry desulfurization. CN116212924A discloses a multi-site catalyst made of Fenton iron sludge, a preparation method and application thereof. The Fenton iron sludge is directly pyrolyzed to obtain a multi-site catalyst made of Fenton iron sludge by adjusting the pyrolysis temperature and time. CN116589080A discloses a method for preparing a biofilm filler using waste sludge. The raw material for preparing the filler is 60%-70% of a high polymer matrix, 10%-35% of residual sludge and 5%-10% of Fenton iron sludge. The preparation method is as follows: collecting the dewatered residual sludge and Fenton iron sludge in the sewage treatment process, drying and crushing them for use, mixing the high polymer matrix and the dried and sieved sludge according to a certain proportion, and then adding them into a double-screw extruder and extruding them into a filler mold. CN118416889A discloses a resource treatment method of Fenton iron sludge and an iron-containing biochar prepared by the method. The Fenton iron sludge is a Fenton reaction generated iron-containing sludge. The Fenton iron sludge is mixed with activated sludge. The sludge to be treated is sequentially subjected to acid treatment, alkali treatment and high-temperature pyrolysis treatment to obtain a biochar with uniform iron element distribution and high specific surface area. The above-mentioned utilization methods of Fenton iron sludge generally have defects such as high energy consumption, complex process and low utilization rate of iron resources.

[0005] At present, various forms of iron salts are widely used in the urban water treatment industry, and the demand is large. However, the production of iron salts at present comes from by-products of the metallurgical process, thereby increasing the transportation cost of iron salts and generating carbon footprint. Compared with the iron salts applied in the traditional water treatment industry, ferrous carbonate has been proved to be a suitable substitute for FeCl2 and FeCl3.

[0006] It is urgent to develop a treatment system and method to combine biogas in purified wastewater treatment process and consumption of Fenton iron sludge, to produce ferrous carbonate in situ in water plant, to promote the recycling of iron resources in water plant, and to reduce related carbon emissions. SUMMARY

[0007] The application aims to provide a system and method for purifying biogas based on Fenton iron sludge anaerobic digestion, which utilizes the fact that Fenton iron sludge can produce Fe(II)-rich fermentation iron sludge through dissimilatory iron reduction in an anaerobic environment, and the biogas to be purified is introduced into the fermentation iron sludge, so that CO2 in the biogas penetrates into the liquid phase and reacts with Fe(II) to produce FeCO3, thereby achieving the purification of biogas and the fixation of CO2 in the form of carbonate, and FeCO3 can be recycled in the process of industrial production, such as participating in sewage coagulation and improving sludge activity, and can also be used as a soil conditioner and iron fertilizer to improve soil structure and be used in agricultural production, thereby solving the problem of resource utilization of Fenton iron sludge.

[0008] The primary aspect of the application is to provide a system for purifying biogas based on Fenton iron sludge anaerobic digestion, which comprises an anaerobic fermentation device, a biogas storage tank, a biogas purification device, a methane storage tank and a FeCO3 storage tank; the anaerobic fermentation device is provided with a microwave treatment device, an anaerobic reaction chamber, a feed inlet and a discharge outlet, and the anaerobic reaction chamber is inoculated with anaerobic digestion sludge; the biogas storage tank is provided with a gas inlet and a gas outlet; the biogas purification device is provided with multiple stages of trays, a feed inlet, a gas inlet, a discharge outlet and a gas outlet; the methane storage tank is provided with a gas inlet and a gas outlet; and the FeCO3 storage tank is provided with a feed inlet and a discharge outlet; the discharge outlet of the anaerobic fermentation device and the feed inlet of the biogas purification device are connected by a pipeline, the gas outlet of the biogas storage tank and the gas inlet of the biogas purification device are connected by a pipeline, the gas outlet of the biogas purification device and the gas inlet of the methane storage tank are connected by a pipeline, and the discharge outlet of the biogas purification device and the feed inlet of the FeCO3 storage tank are connected by a pipeline.

[0009] Further, a pretreatment device is arranged in front of the gas inlet of the biogas storage tank to perform desulfurization and dehydration treatment on the biogas.

[0010] Further, the microwave treatment device is arranged outside the anaerobic reaction chamber, and a stirrer is arranged inside the anaerobic reaction chamber. The microwave treatment device performs microwave pretreatment on the Fenton iron sludge in the anaerobic reaction chamber, directly heats water molecules and other polar molecules in the sludge under the action of electromagnetic waves, uniformly and rapidly increases the temperature, makes the gas in the sludge expand and rupture, effectively destroys the gas barrier in the sludge and the sludge agglomerate structure, and improves the sludge fluidity and the anaerobic treatment effect. In addition, after the microwave treatment, the temperature in the anaerobic chamber is increased to a certain extent, and the cell wall of the inoculated anaerobic digestion sludge is broken, so that the organic matter in the sludge can more fully participate in the anaerobic digestion, thereby further improving the anaerobic reaction efficiency. The stirrer stirs the Fenton iron sludge during the anaerobic digestion process to improve the mass transfer efficiency.

[0011] Further, the biogas purification device has multiple feeding ports and multiple discharging ports, which are arranged at different heights of the biogas purification device, so that the fermentation iron slurry is graded into and output from the biogas purification device; and the bottom of the biogas purification device is provided with a microporous aeration device, which is used for contacting the biogas with the fermentation iron slurry after the biogas is compressed into small bubbles.

[0012] Further, the multiple-stage tray is a metal bubble cap tray, which is used for uniformly dispersing the carbon dioxide, which is not completely reacted with the bottom layer of the fermentation iron slurry, into the fermentation iron slurry of the upper layer, so that the carbon dioxide in the biogas is fully contacted with the ferrous iron, and the biogas is efficiently purified.

[0013] Another aspect of the present application provides a method for purifying biogas based on Fenton iron sludge anaerobic digestion, which comprises the following steps:

[0014] S1: adding the Fenton iron sludge into an anaerobic reaction chamber, inoculating anaerobic digestion sludge, and pre-treating the sludge by a microwave treatment device to obtain the fermentation iron slurry by taking the Fenton iron sludge as a culture medium;

[0015] S2: grading the fermentation iron slurry and feeding it into a biogas purification device, and feeding the biogas to be purified into the biogas purification device from the bottom by a microporous aeration device, so that the biogas to be purified is contacted with the multiple layers of the fermentation iron slurry for full reaction;

[0016] S3: after the reaction is completed, collecting the methane gas purified by the biogas purification device by a gas collecting device and inputting the methane gas into a methane storage tank; and collecting the generated FeCO3 solid by a separation device and inputting the FeCO3 solid into a FeCO3 storage tank.

[0017] Further, in the step S1, the mass ratio of the anaerobic digestion sludge to the Fenton iron sludge is (0.05-0.5) : 1, and the fermentation iron slurry is obtained by adding an appropriate amount of water into the anaerobic reaction chamber and mixing the water with the fermentation iron slurry; the fermentation conditions are as follows: the temperature is 20-30 DEG C, the stirring speed is 50-200 rpm, the pH value is 4-8, and the fermentation time is 5-20 days.

[0018] Further, in the step S2, the reaction time is 1-5 hours, and the reaction temperature is room temperature.

[0019] Further, in the step S2, the biogas to be purified is fed into the biogas purification device from the bottom by the microporous aeration device and fully contacts with the fermentation iron slurry.

[0020] Further, in the step S2, the biogas to be purified is compressed into fine bubbles with a diameter of 3-5 mm by the microporous aeration device, so as to maximize the gas-liquid contact area and make the biogas to be purified fully contact with the fermentation iron slurry.

[0021] Further, in step S2, the number of layers of the multi-layered fermentation iron sludge is at least 3 layers.

[0022] Technical principle of the present application: Fenton iron sludge undergoes dissimilatory iron reduction in the anaerobic digestion process, anaerobic iron-reducing bacteria use Fe(III) in Fenton iron sludge as terminal electron acceptor, and obtain energy by coupling the reduction of Fe(III) with the oxidation of organic matter in Fenton iron sludge to further grow and metabolize. Through this process, the main iron component of Fenton iron sludge is transformed from Fe(III) to Fe(II), producing fermentation iron sludge. The fermentation iron sludge is directly contacted with biogas, allowing CO2 in the biogas to penetrate into the liquid phase and react with Fe(II) in the fermentation iron sludge to produce FeCO3 (CO2+H2O→H2CO3; H2CO3→2H + +CO3 2+ ; Fe 2+ +CO3 2+ →FeCO3), thereby removing CO2 in the biogas.

[0023] Compared with the prior art, the present application has the following beneficial technical effects: The present application uses fermentation iron sludge to purify biogas, and uses the large amount of Fe(II) in the fermentation iron sludge to efficiently and stably chemically react and absorb CO2 in the biogas to be purified, thereby reducing the content of carbon dioxide in the biogas and increasing the content of methane, achieving efficient and low-cost biogas purification. The present application takes advantage of the feature of Fenton iron sludge that it can undergo mild dissimilatory iron reduction reaction to prepare a green and low-cost Fe(II) source to absorb CO2 in the biogas, thereby solving the problem of high cost of fixing CO2 in the traditional chemical absorption method. The FeCO3 iron salt obtained by the present application can be used in industrial and agricultural production, such as being used as a coagulant or improving sludge activity in the water treatment industry, or being used as a soil conditioner or iron fertilizer to improve soil structure in agriculture, so that Fenton iron sludge can be recycled and utilized. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 A system structure schematic diagram for purifying biogas based on Fenton iron sludge anaerobic digestion is provided.

[0025] Figure 2 A method flowchart for purifying biogas based on Fenton iron sludge anaerobic digestion is provided.

[0026] Figure reference numerals: 1-Anaerobic fermentation device; 11-Anaerobic fermentation feed inlet; 12-Anaerobic fermentation discharge outlet; 13-Anaerobic fermentation gas outlet; 14-Microwave treatment device; 2-Biogas purification device; 21-Biogas purification gas inlet; 22-Biogas purification feed inlet; 23-Biogas purification discharge outlet; 24-Biogas purification gas outlet; 3-Biogas storage tank; 31-Biogas storage tank inlet; 32-Biogas storage tank outlet; 4-FeCO3 storage tank; 41-FeCO3 storage tank feed inlet; 42-FeCO3 storage tank discharge outlet; 5-Methane storage tank; 51-Methane storage tank inlet; 52-Methane storage tank outlet; 6-Pump. Detailed Implementation

[0027] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the protection scope of the present invention. Example 1

[0028] A system for anaerobic digestion and biogas purification based on Fenton iron sludge, such as Figure 1 As shown, the system includes an anaerobic fermentation unit 1, a biogas storage tank 3, a biogas purification unit 2, a methane storage tank 5, and a FeCO3 storage tank 4. The anaerobic fermentation unit 1 is equipped with an inlet 11, an outlet 12, a gas outlet 13, and a microwave processing device 14. Anaerobic digested sludge is inoculated into the anaerobic fermentation unit 1. The biogas storage tank 3 is equipped with an inlet 31 and an outlet 32. The biogas purification unit 2 is equipped with an inlet 21, an inlet 22, an outlet 23, and an outlet 24. The methane storage tank 5 is equipped with an inlet 51 and an outlet 24. The FeCO3 storage tank 4 is equipped with an inlet 41 and an outlet 42. The outlet 12 of the anaerobic fermentation device and the inlet 22 of the biogas purification device are connected by a pipe and a pump 6. The outlet 32 ​​of the biogas storage tank and the inlet 21 of the biogas purification device are connected by a pipe and a pump 6. The outlet 24 of the biogas purification device and the inlet 51 of the methane storage tank are connected by a pipe and a pump 6. The outlet 23 of the biogas purification device and the inlet 41 of the FeCO3 storage tank are connected by a pipe and a pump 6. Example 2

[0029] The biogas purification system of Example 1 includes the following steps:

[0030] S1 Fenton iron sludge is added into the anaerobic fermentation device, inoculated with anaerobic digestion sludge, mixed with an appropriate amount of water to form a slurry, and then subjected to microwave pretreatment through a microwave treatment device, and then fermented and cultured to obtain fermented iron sludge by taking the Fenton iron sludge as a culture medium. The mass ratio of the anaerobic digestion sludge to the Fenton iron sludge is 0.05:1, the fermentation temperature is 20°C, the stirring speed is 50 rpm, and the pH value is 4, and the fermentation time is 20 days.

[0031] S2 The fermented iron sludge is transported into the biogas purification device, and the biogas (CO2 volume content is 25%) generated in the anaerobic fermentation process of the sewage treatment plant is introduced from the bottom of the biogas purification device through a microporous aeration device. The fermented iron sludge is fully contacted and reacted with carbon dioxide at room temperature, and the reaction time is 5 hours.

[0032] S3 After the reaction is completed, the methane gas (CO2 volume content is 1%) purified from the biogas is collected by a gas collection device and input into a methane storage tank; the generated FeCO3 solid is separated and collected by a centrifuge and input into a FeCO3 storage tank, and the iron recovery rate is 98%. Example 3

[0033] The system of Example 1 is used to purify biogas, including the following steps:

[0034] S1 Fenton iron sludge is added into the anaerobic fermentation device, inoculated with anaerobic digestion sludge, mixed with an appropriate amount of water to form a slurry, and then subjected to microwave pretreatment through a microwave treatment device, and then fermented and cultured to obtain fermented iron sludge by taking the Fenton iron sludge as a culture medium. The mass ratio of the anaerobic digestion sludge to the Fenton iron sludge is 0.5:1, the fermentation temperature is 30°C, the stirring speed is 200 rpm, and the pH value is 8, and the fermentation time is 5 days.

[0035] S2 The fermented iron sludge is transported into the biogas purification device, and the biogas (CO2 volume content is 25%) generated in the anaerobic fermentation process of the sewage treatment plant is introduced from the bottom of the biogas purification device through a microporous aeration device. The fermented iron sludge is fully contacted and reacted with carbon dioxide at room temperature, and the reaction time is 1 hour.

[0036] S3 After the reaction is completed, the methane gas (CO2 volume content is 3%) purified from the biogas is collected by a gas collection device and input into a methane storage tank; the generated FeCO3 solid is separated and collected by a centrifuge and input into a FeCO3 storage tank, and the iron recovery rate is 95%. Example 4

[0037] The system of Example 1 is used to purify biogas, including the following steps:

[0038] S1 Fenton iron sludge is added into the anaerobic fermentation device, anaerobic digestion sludge is inoculated, and appropriate water is added to mix the fermentation iron sludge into slurry. The slurry is subjected to microwave pretreatment, and the fermentation iron sludge is obtained by fermentation culture using the Fenton iron sludge as culture medium. The mass ratio of the anaerobic digestion sludge to the Fenton iron sludge is 0.25:1, the fermentation temperature is 25℃, the stirring speed is 150 rpm, the pH value is 7, and the fermentation time is 15 days.

[0039] S2 The fermentation iron sludge is transported into the biogas purification device, and the biogas (25% of CO2 volume content) generated in the anaerobic fermentation process of the sewage treatment plant is introduced from the bottom of the biogas purification device through the microporous aeration device and fully reacts with the fermentation iron sludge at room temperature. The reaction time is 3 hours.

[0040] S3 After the reaction is completed, the methane gas (1.5% of CO2 volume content) purified by the biogas purification device is collected by the gas collection device and input into the methane storage tank. The generated FeCO3 solid is separated and collected by the centrifuge and input into the FeCO3 storage tank. The iron recovery rate is 97%.

[0041] Although the embodiments of the present application have been shown and described above, it should be understood that the above embodiments are exemplary and should not be construed as limiting the present application, and those of ordinary skill in the art can make changes, modifications, replacements and variations to the above embodiments without departing from the principles and purposes of the present application within the scope of the present application. The protection scope of the present application is defined by the claims and equivalent technical solutions thereof.

Claims

1. A system for purifying biogas based on Fenton iron sludge anaerobic digestion, comprising an anaerobic fermentation device, a biogas storage tank, a biogas purification device, a methane storage tank and a FeCO 3 storage tank; the anaerobic fermentation device is provided with a microwave treatment device, an anaerobic reaction chamber, a feed inlet and a discharge outlet, Fenton iron sludge is added to the anaerobic reaction chamber and inoculated with anaerobic digestion sludge, and the Fenton iron sludge is subjected to dissimilatory iron reduction during anaerobic digestion; the biogas storage tank is provided with a gas inlet and a gas outlet; the biogas purification device is provided with multiple stages of trays, a feed inlet, a gas inlet, a discharge outlet and a gas outlet; the methane storage tank is provided with a gas inlet and a gas outlet; and the FeCO 3 storage tank is provided with a feed inlet and a discharge outlet; the discharge outlet of the anaerobic fermentation device and the feed inlet of the biogas purification device are connected by a pipeline, the gas outlet of the biogas storage tank and the gas inlet of the biogas purification device are connected by a pipeline, the gas outlet of the biogas purification device and the gas inlet of the methane storage tank are connected by a pipeline, and the discharge outlet of the biogas purification device and the feed inlet of the FeCO 3 storage tank are connected by a pipeline.

2. The system of claim 1, wherein, A pretreatment device is arranged in front of the gas inlet of the biogas storage tank to perform desulfurization and dehydration treatment on the biogas.

3. The system of claim 1, wherein, The microwave treatment device is arranged outside the anaerobic reaction chamber, and a stirrer is arranged inside the anaerobic reaction chamber.

4. The system of claim 1, wherein, The biogas purification device is provided with multiple feed inlets and multiple discharge outlets, which are arranged at different heights to allow the fermentation iron sludge to be fed into and discharged from the biogas purification device in stages; and a microporous aeration device is arranged at the bottom of the biogas purification device to compress the biogas into small bubbles and contact the biogas with the fermentation iron sludge.

5. The system of claim 1, wherein, The multiple stages of trays are metal bubble cap trays.

6. A method of purifying biogas using the system according to any one of claims 1 to 5, characterized in that, The system comprises the following steps: S1. Fenton iron sludge is added to the anaerobic fermentation device, inoculated with anaerobic digestion sludge, and subjected to dissimilatory iron reduction during anaerobic digestion, and the Fenton iron sludge is used as a culture medium to obtain fermentation iron sludge; S2. The fermentation iron sludge is fed into the biogas purification device in stages, the biogas to be purified is introduced into the biogas purification device from the bottom through the microporous aeration device, and the biogas to be purified is contacted with multiple layers of fermentation iron sludge for sufficient reaction; S3. After the reaction is completed, the methane gas purified by the biogas purification device is collected by a gas collecting device and input into the methane storage tank, and the generated FeCO 3 solid is collected by a separation device and input into the FeCO 3 storage tank.

7. The method of claim 6, wherein, In step S1, the mass ratio of anaerobic digestion sludge to Fenton iron sludge is (0.05-0.5) : 1, and the fermentation iron sludge is obtained by mixing a proper amount of water with the fermentation iron sludge to form a slurry and then fermenting the slurry; the fermentation conditions are a temperature of 20-30℃, a stirring speed of 50-200 rpm, a pH value of 4-8, and a fermentation time of 5-20 d.

8. The method of claim 6, wherein, In step S2, the reaction time is 1-5 h, and the reaction temperature is room temperature.

9. The method of claim 6, wherein, In step S2, the biogas to be purified is compressed into fine bubbles with a diameter of 3-5 mm by the microporous aeration device at the bottom of the biogas purification device, and then fully contacts with the fermentation iron sludge.

10. The method of claim 6, wherein, In step S2, the number of layers of the multiple layers of fermentation iron sludge is at least 3.

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