Method for producing biogas

Abstract

To efficiently remove hydrogen sulfide while reducing an environmental load in reducing hydrogen sulfide in biogas generated in anaerobic treatment of organic waste such as biomass, and to provide a method for removing hydrogen sulfide in such biogas.SOLUTION: The environmental load is reduced by adding the iron-based hydroxide or the slurry containing the iron-based hydroxide to the organic waste before or during the anaerobic treatment of the organic waste generated from biomass or the like, and the hydrogen sulfide can be efficiently removed from the generated biogas. In particular, as the iron-based hydroxide, iron-based neutralization sludge obtained from pickling wastewater in a steel product manufacturing process in a steel mill can be utilized.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing biogas generated by anaerobic treatment of organic matter, and more particularly to a method for removing hydrogen sulfide from biogas. [Background technology]

[0002] Currently, in light of global environmental conservation efforts such as the Sustainable Development Goals (SDGs), the use of biomass (renewable organic resources derived from living organisms, excluding fossil fuels) is attracting attention as an energy source that does not rely on fossil fuels. For example, there is a demand to recycle waste containing organic matter (organic waste), such as biomass from food residues, kitchen garbage, and livestock manure. One method for utilizing this organic waste is to generate biogas through anaerobic treatment using anaerobic microorganisms such as methane fermentation, which can then be collected and used. For example, technology such as burning biogas to generate electricity is widely adopted as a clean technology that can reduce environmental impact.

[0003] An example of the configuration of an anaerobic treatment plant that produces biogas is shown below. · Raw material acceptance: Accepts organic resources (organic waste) such as food waste and leftover food. Pre-treatment: Removal of foreign matter from organic resources and crushing them into small pieces. Anaerobic treatment: Decomposing organic matter using anaerobic microorganisms such as those involved in methane fermentation produces gas (biogas). Because the biogas produced contains hydrogen sulfide, for environmental reasons, anaerobic treatment tanks are often sealed to prevent leakage into the external environment. Gas purification: Removal of hydrogen sulfide from biogas. Gas storage tank: Transported through a sealed route and stored in a sealed container such as a tank. Energy recovery: For example, biogas can be burned and steam recovered in a boiler for use in generating electricity.

[0004] On the other hand, biogas generated by anaerobic digestion often contains not only methane and carbon dioxide but also hydrogen sulfide, which is generated by sulfur compounds (sulfides) contained in organic waste. Methane and carbon dioxide are greenhouse gases that contribute to global warming, and hydrogen sulfide causes foul odors and corrosion in the surrounding environment. Therefore, it is undesirable to directly release biogas and its exhaust gases into the atmosphere. Hydrogen sulfide, in particular, not only has a foul odor and remains in the biogas combustion exhaust, but also poses health risks such as olfactory abnormalities, eye damage, respiratory disorders, and pulmonary edema. Furthermore, it can cause corrosion of metal products such as generators and boilers, hindering the stable operation of such equipment. Therefore, removing hydrogen sulfide from biogas is essential from the perspectives of health, hygiene, environmental conservation, and equipment maintenance.

[0005] Methods for removing hydrogen sulfide from biogas include dry desulfurization, wet desulfurization, and biological desulfurization. Dry desulfurization is a method of removing hydrogen sulfide from biogas by passing the biogas through a tower filled with an iron-based desulfurization agent (also known as an iron-based coagulant), causing the iron ions and hydrogen sulfide to react and solidify into iron sulfide (FeS).

[0006] Wet desulfurization is a method of removing hydrogen sulfide from biogas by passing the biogas containing hydrogen sulfide through an alkaline absorption solution such as NaOH, which solidifies the biogas into sulfides such as sodium sulfide.

[0007] Biological desulfurization is a method of aerobic decomposition and removal of hydrogen sulfide using sulfur-oxidizing bacteria. A hybrid hydrogen sulfide removal method has been proposed that combines biological desulfurization with dry desulfurization or wet desulfurization rather than using it alone (for example, Patent Document 3).

[0008] Furthermore, methods for removing hydrogen sulfide from biogas using by-products from wastewater treatment after anaerobic biomass treatment have also been proposed. For example, it has been proposed to biologically nitrify and denitrify the wastewater after anaerobic treatment, introduce biogas into the resulting wastewater (nitrified liquid), and detoxify the hydrogen sulfide in the biogas by reacting it with the dissolved oxygen in the nitrified liquid (Patent Document 1).

[0009] It has also been proposed to add an iron-based coagulant such as ferric chloride or ferric sulfide to wastewater after anaerobic treatment of biomass, solidify and coagulate the sulfur (S) in the wastewater as iron sulfide (FeS), return a portion of the concentrated sludge after coagulation to the anaerobic treatment tank, and remove hydrogen sulfide from the biogas using the iron-based coagulant contained in the concentrated sludge (Patent Document 2). [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-24980 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-207944 [Patent Document 3] Japanese Patent Application Laid-Open No. 2011-184656 Summary of the Invention [Problem to be solved by the invention]

[0011] Dry desulfurization involves contacting biogas with an iron-based desulfurization agent, and because this involves contact between gas and a solid, there is a limit to the contact area, and the poor contact reactivity on the outer surface of the solid makes it difficult to increase the efficiency of hydrogen sulfide removal.

[0012] In the wet desulfurization method, biogas is passed through an alkaline absorption solution to react and separate sulfur (S) through gas-liquid contact. However, just like the dry method, there is a problem that not only is there a limit to the contact area, but the gas-liquid contact reactivity is also poor, making it difficult to increase the efficiency of hydrogen sulfide removal.

[0013] Although biological desulfurization uses sulfur-oxidizing bacteria to aerobicly decompose hydrogen sulfide, sulfuric acid is produced, which requires neutralization during wastewater treatment. Furthermore, hybrid desulfurization methods can be combined with dry or wet desulfurization methods to increase the efficiency of hydrogen sulfide removal, but the problem of increased wastewater treatment load remains.

[0014] Patent Document 2 proposes iron-based flocculants such as ferric sulfate, ferric chloride, ferrous sulfate, and ferrous chloride. However, when using iron chloride-based flocculants such as ferric chloride, the chlorine content generates hydrogen chloride (hydrochloric acid), which is mixed into the biogas, resulting in a high environmental impact (treatment load for environmental conservation) and the risk of causing corrosion of equipment and hindering stable operation. Furthermore, when using iron sulfate-based flocculants such as ferric sulfate, the sulfate ions contained in them limit the desulfurization effect, and hydrogen sulfide may be generated during the anaerobic treatment process, reducing the efficiency of hydrogen sulfide removal from the biogas. Furthermore, the generation of sulfuric acid in the biogas increases the environmental impact.

[0015] In view of these problems, the present invention aims to provide a method for producing biogas that efficiently removes hydrogen sulfide while reducing the environmental load when reducing the amount of hydrogen sulfide in biogas generated during anaerobic treatment of organic waste such as biomass. [Means for solving the problem]

[0016] [a] The inventors first searched for a substance to replace the conventional ferric sulfate and ferric chloride as a desulfurization agent. As a result, they discovered that iron-based hydroxides such as ferric hydroxide can be used as an iron ion source for immobilizing sulfur (S). By using iron-based hydroxides, the environmental impact can be significantly reduced because sulfuric acid and hydrochloric acid are not generated as reaction by-products. Furthermore, they found that the absence of corrosive hydrochloric acid and sulfuric acid improves the maintainability of equipment. H2SFe2S3H2O

[0017] [stomach] Next, we conducted research and development into improving reaction efficiency. We discovered that reactivity could be improved by mixing a slurry of iron hydroxide as a desulfurization agent with organic waste, rather than by bringing the gas into contact with a solid desulfurization agent (flocculant) or liquid alkaline absorption liquid, as in conventional dry and wet methods. Therefore, it is effective to mix a slurry of iron hydroxide with organic waste before or during anaerobic treatment of the organic waste. We found that this makes it possible to efficiently remove sulfur (S) from the biogas generated from the organic waste.

[0018] The present invention was made based on the above findings, and the gist of the present invention is as follows. [1] A biogas production method for producing biogas from organic waste by anaerobic treatment, A method for producing biogas, comprising adding an iron-based hydroxide to the organic waste before or during the anaerobic treatment. [2] The method for producing biogas according to [1] above, wherein the iron-based hydroxide is added to the organic waste in the form of a slurry containing the iron-based hydroxide. [3] The method for producing biogas according to [1] or [2], wherein at least a portion of the iron-based hydroxide is obtained from sludge containing the iron-based hydroxide. [4] The method for producing biogas according to any one of [1] to [3], wherein the iron-based hydroxide is at least one selected from the group consisting of ferric hydroxide, iron oxide hydroxide, and a mixture of iron oxide hydroxide and iron oxide hydrate. [5] The method for producing biogas according to any one of [1] to [4], further comprising adding one or more selected from ferrous sulfate, ferric sulfate, ferrous chloride, ferric chloride, and polysilica iron to the organic waste. [6] The method for producing biogas according to any one of [1] to [5] above, wherein the organic waste is obtained from biomass. [Effects of the Invention]

[0019] According to the present invention, when organic waste generated from biomass or the like is subjected to anaerobic treatment to produce biogas, the generation of sulfuric acid and hydrochloric acid is suppressed, thereby reducing the environmental burden and enabling the efficient removal and reduction of hydrogen sulfide in the biogas. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a conceptual diagram showing an example of the flow of a biogas production method according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention will be described below based on one embodiment of the present invention (hereinafter simply referred to as the present invention). However, the present invention is not limited to the embodiment described here.

[0022] Organic waste is not limited to any particular composition, as long as it contains organic matter. Furthermore, although it is called organic waste, it is not limited to waste. However, from the perspective of the recent SDGs, there is a demand to gasify (biogasify) waste containing organic matter generated from biomass such as food waste, kitchen garbage, and livestock manure as raw materials and make effective use of it. For this reason, anything containing organic matter that can be used to generate biogas will be called organic waste. There are also no particular restrictions on the type of biomass. It can be plant-based or animal-based, but these are excluded if they have turned into fossil resources (oil, coal, etc.).

[0023] Anaerobic treatment of organic waste is a process in which organic matter is decomposed into methane, carbon dioxide, etc. through the metabolic action of anaerobic microorganisms, such as methane fermentation, and there are no particular restrictions on the type of anaerobic microorganism or the treatment method. Anaerobic treatment of organic waste generates biogas containing methane, etc., and there is a need to remove and reduce the hydrogen sulfide contained in the biogas.

[0024] [Iron hydroxide] By using iron hydroxides such as ferric hydroxide as a desulfurization agent and reacting the hydrogen sulfide (HS) in biogas with iron ions, sulfur (S) can be immobilized and hydrogen sulfide can be removed. Furthermore, unlike conventional desulfurization agents such as ferric sulfate and ferric chloride, this agent does not produce sulfuric acid or hydrochloric acid as reaction by-products, significantly reducing the environmental burden and preventing corrosion of equipment.

[0025] Iron-based hydroxides are hydroxides of iron, and in the case of trivalent iron hydroxides, this refers to, for example, ferric hydroxide (Fe(OH)3), iron oxide hydroxide (FeO(OH)), and a mixture of iron oxide hydroxide and iron oxide hydrate (Fe2O3·H2O), while in the case of divalent iron hydroxides, this refers to, for example, ferrous hydroxide (Fe(OH)2). The iron-based hydroxides are preferably one or more selected from ferric hydroxide, iron oxide hydroxide, and a mixture of iron oxide hydroxide and iron oxide hydrate, or one or more selected from these with ferrous hydroxide added. Note that ferric hydroxide (Fe(OH)3) itself is unstable and usually exists in the form of iron oxide hydroxide (FeO(OH)) or a mixture of iron oxide hydroxide and iron oxide hydrate (Fe2O3·H2O).

[0026] By adding iron hydroxide to organic waste, the hydrogen sulfide in the biogas generated from the organic waste reacts with the iron hydroxide, decomposing the hydrogen sulfide to produce iron sulfide. Because iron sulfide is a solid, this reaction immobilizes the sulfur in the biogas and removes the hydrogen sulfide. For example, the reaction between iron oxide hydroxide and hydrogen sulfide is as follows: 2FeO(OH)+3H2S→Fe2S3+4H2O

[0027] The form of the iron-based hydroxide when added to organic waste is not particularly limited. It can be powdered iron-based hydroxide, or water can be added to form a slurry (slurry containing iron-based hydroxide). Adding a slurry containing iron-based hydroxide to organic waste can efficiently react the hydrogen sulfide in the biogas generated from the organic waste with the iron-based hydroxide. The mechanism by which the reaction is promoted by slurrying is unclear. However, adding a slurry containing iron-based hydroxide results in solid-liquid contact with the organic waste, and the contact covers the surface of the organic waste. This allows the biogas generated within the organic waste to come into contact with the iron-based oxide evenly and efficiently, where the hydrogen sulfide in the biogas reacts with the iron-based hydroxide. This is thought to enable the efficient decomposition and removal of hydrogen sulfide in the biogas.

[0028] In order to allow as much biogas generated from organic waste to react with iron-based hydroxide as possible, it is recommended to mix iron-based hydroxide with the organic waste before or during anaerobic treatment of the organic waste. In order to allow as much biogas as possible to react, it is desirable to add and mix it with the organic waste before biogas is generated, i.e., before anaerobic treatment. In particular, in the case of a slurry containing iron-based hydroxide, it is desirable to cover the surface of the organic waste with the slurry containing iron-based hydroxide before biogas is generated. When adding a slurry containing iron-based hydroxide during anaerobic treatment, it is desirable to add it as early as possible in the anaerobic treatment (preferably immediately after the start).

[0029] When forming an iron-based hydroxide into a slurry, if the water content of the slurry is low, the iron-based hydroxide will not be uniformly redispersed, so the water content should be 60% or more by weight. Preferably, it should be 65% or more, or 70% or more. If the water content of the slurry is high, deliquescence will occur during storage and transportation, and the reactivity with hydrogen sulfide will decrease, so the water content should be 85% or less. Preferably, it should be 83% or less, or 80% or less.

[0030] [Iron-based neutralized sludge] Iron hydroxides are found in large amounts in sludge (iron-based neutralized sludge) obtained by oxidizing and neutralizing wastewater containing ferrous sulfate or ferrous chloride, which is generated during the surface treatment of iron materials with sulfuric acid or hydrochloric acid, followed by dehydration. For example, iron-based neutralized sludge with a high iron hydroxide content can be obtained by oxidizing and neutralizing wastewater (wastewater after acid cleaning) from steel plates and other products generated during the steelmaking process, followed by dehydration. This iron-based neutralized sludge generated during steel manufacturing is generated in large quantities, and is usually disposed of as waste. This is useful because it can be effectively utilized, reducing environmental impact. The effective utilization of iron-based neutralized sludge generated during steel manufacturing is welcomed not only from the perspectives of the environment and SDGs, but also from economic perspectives. It is desirable that at least a portion of the iron hydroxides added to organic waste be the sludge containing iron hydroxides described above (iron-based neutralized sludge).

[0031] The sludge produced after oxidation and neutralization of pickling wastewater from steelmaking processes can be used directly without dehydration. However, considering the burden of transportation and storage (physical burden and economic burden due to the large volume of waste), it is preferable to use it as iron-based neutralized sludge after first dehydrating it. Then, when adding it to organic waste, the water content can be adjusted to create a slurry of iron-based neutralized sludge with the specified water content (i.e., a slurry containing iron-based hydroxides).

[0032] Depending on the oxidation and neutralization conditions, this iron-based neutralized sludge may contain ferric hydroxide (Fe(OH)3), iron oxide hydroxide (FeO(OH)), a mixture of iron oxide hydroxide and iron oxide hydrate, as well as hydrous iron oxides such as goethite (α-FeOOH), akaganite (β-FeOOH), and lepidocrocite (γ-FeOOH). The reaction of hydrous iron oxide with hydrogen sulfide has the same effect as that of iron oxide hydroxide. Table 1 shows an example of the composition of iron-based neutralized sludge obtained by neutralizing pickling wastewater from the steelmaking process, holding it at 110°C for two hours, and drying it.

[0033] [Table 1]

[0034] In the anaerobic treatment process, iron-based coagulants, such as ferrous chloride, ferric chloride, ferrous sulfate, ferric sulfate, and polysilica iron, may be added. However, when chloride-based coagulants are used, the chloride ions contained in them generate hydrogen chloride (hydrochloric acid), which can cause corrosion of the equipment. When iron sulfate-based coagulants are used, they contain sulfate ions, which can become a source of hydrogen sulfide during anaerobic treatment, potentially increasing the amount of hydrogen sulfide in the biogas. Therefore, it is recommended that iron chloride-based and iron sulfate-based coagulants be used in combination with iron hydroxide as a supplement to the iron hydroxide.

[0035] An example of the flow of a method for producing biogas in organic waste treatment using the present invention is shown in Figure 1. Organic waste 1 generated from biomass or the like is fed into an adjustment tank 2, where the moisture content is adjusted as necessary. Next, the organic waste 3 with the moisture content adjusted is transferred to an anaerobic treatment tank 4. In the anaerobic treatment tank 4, anaerobic treatment such as fermentation (e.g., methane fermentation) by anaerobic microorganisms is carried out, generating biogas 8. Note that the adjustment tank 2 and the anaerobic treatment tank 4 may be a single tank that combines the functions of both.

[0036] Furthermore, in an iron-based hydroxide storage tank 16 that stores iron-based hydroxide (for example, iron-based neutralized sludge obtained from a steelmaking process), the water content is adjusted as necessary to prepare a slurry 14 containing iron-based hydroxide. The slurry containing iron-based hydroxide is supplied to either or both of the adjustment tank 2 and the anaerobic treatment tank 4, and added to the organic waste. At this time, a kneading device may be provided in the adjustment tank 2 and the anaerobic treatment tank 4, and the organic waste and the slurry containing iron-based hydroxide may be kneaded. Furthermore, an iron-based coagulant 15 may be added to either or both of the adjustment tank 2 and the anaerobic treatment tank 4, as necessary.

[0037] Biogas 8 generated in anaerobic treatment tank 4 may be passed through biological desulfurization equipment 9 or dry desulfurization equipment 10 to remove remaining hydrogen sulfide as needed. Purified biogas 11 from which hydrogen sulfide has been removed is stored in gas holder 12 and distributed to a generator, boiler 13, or other appropriate destination. Meanwhile, digested liquid (organic waste residue) 5, which is the residue from anaerobically treated biogas in anaerobic treatment tank 4, is temporarily stored in digested liquid tank 6 and is eventually transferred to digested liquid treatment equipment 7, where it is disposed of after being treated to be environmentally sound. This series of facilities will enable biogas to be produced from organic waste generated from biomass and other sources, and used as a new form of energy. [Example]

[0038] An example of the present invention will be described. Biomass waste, such as food waste and kitchen garbage, was prepared as organic waste by finely crushing it. The organic waste was divided into six parts, and three sets of two were prepared, for a total of six organic wastes. One of the sets was left untreated and biogas was generated by anaerobic treatment, while the other was added with a desulfurizing agent and biogas was generated by anaerobic treatment.

[0039] Iron-based neutralized sludge obtained from pickling wastewater in the steel product manufacturing process at a steelworks (steel manufacturing process) was divided into two parts, one of which was prepared as a slurry containing iron-based hydroxides after adjusting the water content (Test 1), and the other was prepared as sludge containing iron-based hydroxides as is (Test 2).The components and moisture content of the iron-based neutralized sludge excluding water are shown in Table 2.

[0040] [Test 1] First, one of the prepared organic wastes was placed in a test tank, and then transferred to an anaerobic treatment tank without adding anything. Biogas was generated, and the hydrogen sulfide concentration in the biogas was measured. Next, one of the other organic wastes was placed in the test tank, and then a slurry of iron-based neutralized sludge (a slurry containing iron-based hydroxides) was added and mixed in. The organic waste mixed with the slurry containing iron-based hydroxides was then transferred to an anaerobic treatment tank, where biogas was generated through anaerobic treatment, and the hydrogen sulfide concentration in the biogas was measured.

[0041] [Test 2] Similarly, one of the other sets of organic wastes was put into the test tank, and then transferred to the anaerobic treatment tank without adding anything, and biogas was generated, and the hydrogen sulfide concentration in the biogas was measured. Next, one of the other organic wastes was placed in the test tank, and then iron-based neutralized sludge (sludge containing iron hydroxide) was added and mixed in. The organic waste mixed with the sludge containing iron hydroxide was then transferred to an anaerobic treatment tank, where biogas was generated through anaerobic treatment, and the hydrogen sulfide concentration in the biogas was measured.

[0042] [Test 3] As a comparative example, a conventional iron-based coagulant (ferric chloride) was tested as a desulfurization agent. Similarly, one set of organic waste was treated with no other organic waste and biogas was generated by anaerobic treatment, while the other organic waste was added with ferric chloride in the test tank, then transferred to the anaerobic treatment tank, where biogas was generated by anaerobic treatment, and the hydrogen sulfide concentration in the biogas was measured.

[0043] The test results are shown in Table 3. It was confirmed that both Tests 1 and 2, which are examples of the present invention, showed improved hydrogen sulfide removal rates compared to Test 3, which is a comparative example, in which a conventional iron chloride-based coagulant was used.

[0044] [Table 2]

[0045] [Table 3] [Industrial Applicability]

[0046] The present invention can be used in industries that handle organic matter, particularly biomass. Because it can efficiently remove hydrogen sulfide when generating gas (biogas) from organic matter such as biomass, the present invention can be used in a wide variety of industrial fields. [Explanation of symbols]

[0047] 1. Organic waste 2 Adjustment tank 3. Organic waste 4. Anaerobic treatment tank 5 Digestive juices 6 Digestive fluid tank 7. Digestive fluid treatment equipment 8. Biogas 9 Biological desulfurization equipment 10 Dry desulfurization equipment 11. Refined biogas 12 Gas holder 13 Generator or boiler 14 Slurry containing iron hydroxide 15 Iron-based flocculants 16 Iron hydroxide reservoir

Claims

1. A biogas production method for producing biogas from organic waste by anaerobic treatment, A method for producing biogas, comprising adding an iron-based hydroxide to the organic waste before or during the anaerobic treatment.

2. 2. The method for producing biogas according to claim 1, wherein the iron-based hydroxide is added to the organic waste in the form of a slurry containing the iron-based hydroxide.

3. The method for producing biogas according to claim 1 or 2, wherein at least a portion of the iron-based hydroxide is obtained from sludge containing the iron-based hydroxide.

4. 3. The method for producing biogas according to claim 1, wherein the iron-based hydroxide is at least one selected from the group consisting of ferric hydroxide, iron oxide hydroxide, and a mixture of iron oxide hydroxide and iron oxide hydrate.

5. 3. The method for producing biogas according to claim 1 or 2, further comprising adding to the organic waste one or more selected from the group consisting of ferrous sulfate, ferric sulfate, ferrous chloride, ferric chloride, and polysilica iron.

6. The method for producing biogas according to claim 1 or 2, wherein the organic waste is obtained from biomass.

7. 4. The method for producing biogas according to claim 3, wherein the iron-based hydroxide is at least one selected from the group consisting of ferric hydroxide, iron oxide hydroxide, and a mixture of iron oxide hydroxide and iron oxide hydrate.

8. 4. The method for producing biogas according to claim 3, further comprising adding to the organic waste one or more selected from the group consisting of ferrous sulfate, ferric sulfate, ferrous chloride, ferric chloride, and polysilica iron.

9. The method for producing biogas according to claim 3, wherein the organic waste is obtained from biomass.

10. The method for producing biogas according to claim 4, wherein the organic waste is obtained from biomass.

11. The method for producing biogas according to claim 5, wherein the organic waste is obtained from biomass.

12. The method for producing biogas according to claim 6, wherein the organic waste is obtained from biomass.

13. The method for producing biogas according to claim 7, wherein the organic waste is obtained from biomass.

14. The method for producing biogas according to claim 8, wherein the organic waste is obtained from biomass.

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