Methane gas production method

Abstract

To provide a technique capable of making methane fermentation efficient and reducing a concentration of phosphoric acid contained in digestive juice.SOLUTION: A methane gas production method includes a step of performing methane fermentation by bringing an iron steel slug and an organic waste into contact with each other under an anaerobic condition.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a method for producing methane gas.

Background Art

[0002] In the realization of a decarbonized society, energy production such as biomass power generation and biogas production using biomass is one of the essential efforts. Conventionally, methane fermentation has been used as one of the biogas production technologies. As a conventional proposal regarding methane fermentation, for example, Patent Document 1 discloses an anaerobic treatment step of anaerobically treating organic substances to form anaerobic treatment sludge, an aeration treatment step of bringing an oxygen-containing gas into contact with the anaerobic treatment sludge to form aeration treatment sludge, a flocculation step of injecting a flocculant into the aeration treatment sludge to form flocculated sludge, a dehydration step of dehydrating the flocculated sludge, and a nitrification step of nitrifying ammonia contained in the dewatered separation liquid from the dehydration step. In the anaerobic treatment step before or during the anaerobic treatment step, an iron compound is injected into the organic substances. In the aeration treatment step, the oxidation-reduction potential (oxidation-reduction potential based on a silver / silver chloride electrode) of the aeration treatment sludge is controlled to -100 mV or less, and iron(II) contained in the anaerobic treatment sludge is oxidized by bringing the anaerobic treatment sludge containing the iron compound into contact with the oxygen-containing gas. The oxidized iron(III) in the flocculation step is used as a flocculant, and the exhaust gas from the nitrification step is reused as the oxygen-containing gas in the aeration treatment step. A method for treating organic substances is described.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, the proposal described in Patent Document 1 is a method of oxidizing iron to produce an oxide and decomposing an organic substance using the oxide. Therefore, the inventor of the present application has found that there is room for improvement in terms of improving the efficiency of methane fermentation and reducing the concentration of phosphoric acid contained in the digested liquid after methane fermentation. Accordingly, a technique capable of improving the efficiency of methane fermentation and reducing the concentration of phosphoric acid contained in the digested liquid is required.

Means for Solving the Problems

[0005] The present disclosure can be realized in the following forms.

[0006] (1) According to one aspect of the present disclosure, a method for producing methane gas is provided. This production method includes a step of performing methane fermentation by bringing steel slag and organic waste into contact under anaerobic conditions. According to the method for producing methane gas of this aspect, since the iron component contained in the steel slag can be dissolved to efficiently enhance the reducibility, the methane fermentation can be made efficient. Furthermore, phosphorus in the digested liquid can be fixed by calcium and iron contained in the steel slag, and as a result, the phosphoric acid concentration of the digested liquid can be reduced. Therefore, the efficiency of methane fermentation can be improved and the phosphoric acid concentration contained in the digested liquid can be reduced.

[0007] (2) In the method for producing methane gas described in (1) above, the steel slag may contain blast furnace slag. According to the method for producing methane gas of this aspect, since blast furnace slag having a relatively low phosphorus content is used, an increase in the phosphoric acid concentration contained in the digested liquid can be suppressed.

[0008] (3) In the method for producing methane gas described in (1) or (2) above, in the step, the mass ratio of the steel slag to the organic waste (steel slag / organic waste) may be 0.5 or more and 4.0 or less. According to the method for producing methane gas of this aspect, a decrease in the production efficiency of methane gas can be suppressed.

[0009] (4) In the method for producing methane gas according to any one of (1) to (3) above, in the above step, the steel slag may be used as the upper layer and the organic waste may be used as the lower layer. According to the method for producing methane gas in this form, the steel slag after methane fermentation can be easily reused.

[0010] (5) In the method for producing methane gas according to any one of (1) to (4) above, the above step may be performed for a period of 7 days or more and 20 days or less. According to the method for producing methane gas in this form, a decrease in the production efficiency of methane gas can be suppressed.

[0011] (6) In the method for producing methane gas according to any one of (1) to (5) above, the above step may be performed under the conditions of 30 °C or more and 45 °C or less. According to the method for producing methane gas in this form, the energy consumption required for heating can be suppressed.

[0012] (7) In the method for producing methane gas according to any one of (1) to (6) above, the phosphoric acid concentration of the digested liquid generated by the above step may be 0.1 mg / L or less. According to the method for producing methane gas in this form, the phosphoric acid concentration contained in the digested liquid after methane fermentation can be reduced, so that the treatment of the digested liquid can be simplified.

[0013] (8) In the method for producing methane gas according to any one of (1) to (7) above, the COD of the digested liquid generated by the above step may be 100 mg / L or less. According to the method for producing methane gas in this form, the concentration of organic substances contained in the digested liquid after methane fermentation can be reduced, so that the treatment of the digested liquid can be further simplified.

[0014] Note that the present disclosure can be realized in various forms. For example, it can be realized in the forms of a method for producing biogas, a biogas production system, a biogas production apparatus, a methane gas production system, a methane gas production apparatus, a methane fermentation treatment method for organic waste, a methane fermentation treatment system for organic waste, a methane fermentation treatment apparatus for organic waste, etc.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Modes for Carrying Out the Invention

[0016] FIG. 1 is a process diagram showing the procedure of the method for producing methane gas as one embodiment of the present disclosure. In the method for producing methane gas in the present disclosure, first, steel slag and organic waste are prepared (step P110). Then, methane fermentation is carried out by bringing the steel slag and the organic waste into contact under anaerobic conditions (step P120).

[0017] In the present disclosure, the “steel slag” means slag generated in the steel manufacturing process and contains iron. The steel slag is not particularly limited. For example, it may be blast furnace slag or steelmaking slag. However, from the viewpoint of suppressing an increase in the phosphoric acid concentration contained in the digested liquid, it is preferable to include blast furnace slag having a relatively low phosphorus content. The blast furnace slag is not particularly limited, and examples thereof include granulated blast furnace slag and slowly cooled blast furnace slag. The steelmaking slag is not particularly limited, and examples thereof include converter slag and electric furnace slag. As the steel slag, only one kind may be used, or two or more kinds may be used in combination. According to the method for producing methane gas of the present disclosure, since the steel slag as a by-product generated in the steel manufacturing process is used, the by-product can be effectively utilized. Therefore, compared with the method of using iron instead of steel slag as in the present application, it can contribute to the effective utilization of resources and is an environmentally friendly method.

[0018] Organic waste corresponds to biomass resources. The organic waste is not particularly limited, and examples thereof include food waste, livestock excrement, sludge, plant waste, and the like. From the viewpoint of the production efficiency of methane gas, it is preferable that the organic waste is easily decomposable, such as food waste or food waste. As the organic waste, only one kind may be used, or two or more kinds may be used in combination. According to the method for producing methane gas of the present disclosure, since organic waste is used, the waste can be effectively utilized.

[0019] The step of performing methane fermentation (step P120) is performed under anaerobic conditions. Therefore, it is preferably performed under conditions controlled so that oxygen is not present. The microorganisms in methane fermentation are also called methane-producing bacteria and are not particularly limited, but are preferably the microorganisms originally contained in the raw organic waste. By using the microorganisms contained in the organic waste, it is possible to omit the separate addition of methane-producing bacteria. In addition to the microorganisms contained in the organic waste, or instead of the microorganisms contained in the organic waste, other methane-producing bacteria may be added.

[0020] The form of bringing the steel slag into contact with the organic waste is not particularly limited. For example, the steel slag and the organic waste may be mixed, or the steel slag and the organic waste may be brought into contact without mixing. From the viewpoint of easily reusing the steel slag after methane fermentation, it is preferable not to mix the steel slag and the organic waste. As a form of bringing the steel slag into contact with the organic waste without mixing, for example, the organic waste may be used as the upper layer and the steel slag may be used as the lower layer. However, from the viewpoint of facilitating the reuse of the steel slag after methane fermentation, it is particularly preferable to use the steel slag as the upper layer and the organic waste as the lower layer.

[0021] Although the mass ratio of steel slag to organic waste (steel slag / organic waste) in the step of performing methane fermentation (step P120) is not particularly limited, from the viewpoint of suppressing a decrease in the production efficiency of methane gas, it is preferably 0.5 or more and 4.0 or less, more preferably 0.8 or more and 3.5 or less, still more preferably 1.0 or more and 3.0 or less, and even more preferably 1.5 or more and 2.5 or less.

[0022] The treatment temperature in the step of performing methane fermentation (step P120) is not particularly limited as long as it is within the temperature range in which microorganisms can perform methane fermentation. From the viewpoint of suppressing a decrease in the production efficiency of methane gas, the step of performing methane fermentation (step P120) is preferably carried out under the conditions of 25°C or more and 60°C or less, more preferably under the conditions of 30°C or more and 60°C or less, still more preferably under the conditions of 30°C or more and 45°C or less or 50°C or more and 60°C or less, and even more preferably under the conditions of 33°C or more and 37°C or less or 53°C or more and 57°C or less. Further, from the viewpoint of suppressing the energy required for heating, the step of performing methane fermentation (step P120) is even more preferably carried out under the conditions of 30°C or more and 45°C or less, and particularly preferably carried out under the conditions of 33°C or more and 37°C or less.

[0023] Although the period of the step of performing methane fermentation (step P120) is not particularly limited, from the viewpoint of suppressing a decrease in the production efficiency of methane gas, it is preferably carried out for a period of 5 days or more and 30 days or less, more preferably for a period of 6 days or more and 25 days or less, and still more preferably for a period of 7 days or more and 20 days or less.

[0024] In the method for producing methane gas according to the present disclosure, since methane fermentation is carried out using steel slag, the reducibility can be efficiently increased. As a result, the generation rate of methane gas at the initial stage of methane fermentation can be increased, so that the fermentation period can be shortened. Therefore, the efficiency of methane fermentation can be improved. Further, according to the method for producing methane gas in the present disclosure, it is possible to suppress the complication of the methane gas production apparatus and the production process, so that the production efficiency of methane gas can be increased with a simple configuration.

[0025] By methane fermentation, biogas containing methane gas is generated. The biogas may contain gases such as carbon dioxide, ammonia, and hydrogen sulfide in addition to methane gas. The method for producing methane gas of the present disclosure may further include a step of recovering the generated methane gas or biogas. The method for recovering the gas is not particularly limited, and examples thereof include a method of separating and recovering the gas by providing a gas-solid-liquid separator above the fermentation tank. In the method for producing methane gas according to the present disclosure, as a result of increasing the production efficiency of methane, the concentration of methane gas contained in the biogas can be increased. The biogas obtained by methane fermentation may be used as methane gas as it is, or may be used after further increasing the methane concentration. The method for further increasing the methane concentration is not particularly limited, and examples thereof include a method of dissolving the biogas in water and selectively separating methane using the difference in solubility in water.

[0026] In methane fermentation, biogas is generated, and digested liquid as fermentation residue is also generated. The method for producing methane gas in the present disclosure may further include a step of separating and recovering the digested liquid and the steel slag. The method for separating and recovering the digested liquid and the steel slag is not particularly limited, and examples thereof include a method of providing a drain port (valve) at a position directly above the steel slag layer.

[0027] Generally, the digested liquid produced by methane fermentation contains a large amount of organic matter and nutrients. Therefore, while the digested liquid can be used as liquid fertilizer, it may also be discharged into rivers etc. after undergoing wastewater treatment. Generally, in wastewater treatment, chemicals such as flocculants are used, which leads to problems such as the complication of the treatment process and the increase in treatment costs. According to the method for producing methane gas of the present disclosure, as shown in the examples described later, the concentration of phosphoric acid contained in the digested liquid after methane fermentation can be reduced, so that the treatment of the digested liquid can be simplified. The mechanism by which the concentration of phosphoric acid contained in the digested liquid can be reduced is not clear, but the following estimated mechanism is presumed. That is, it is considered that calcium and iron contained in steel slag can fix phosphorus in the digested liquid, resulting in a decrease in the phosphoric acid concentration of the digested liquid. Also, according to the method for producing methane gas of the present disclosure, the concentration of organic matter contained in the digested liquid after methane fermentation can also be reduced, so that the treatment of the digested liquid can be further simplified. Further, according to the method for producing methane gas of the present disclosure, since steel slag is used, the concentration of iron ions contained in the digested liquid can be increased. Here, iron ions are considered to have the effect of fixing hydrogen sulfide. Therefore, it is expected that according to the method for producing methane gas of the present disclosure, the concentration of hydrogen sulfide contained in the digested liquid can be reduced.

[0028] According to another aspect of the present disclosure, a method for methane fermentation treatment of organic waste is provided. This method for methane fermentation treatment of organic waste brings steel slag into contact with organic waste. According to this method for methane fermentation treatment of organic waste, calcium and iron contained in steel slag can fix phosphorus in the digested liquid, and as a result, the phosphoric acid concentration of the digested liquid can be decreased.

Examples

[0029] Hereinafter, the present invention will be described more specifically by way of examples, but the present invention is not limited to the following examples.

[0030] (1) Method Figure 2 is an explanatory diagram showing the schematic configuration of the apparatus 10 used in the examples. 100 g of food waste 30 was put into a 1.5 L plastic container 20 having the dimensions shown in Figure 2, 200 g of blast furnace slag 40 was put thereon, and then 1 L of tap water 50 was put in. As the food waste 30, a mixture obtained by adding rice husks to crushed vegetables which are food residues from restaurants was used. A gas pack 80 was connected to the lid 60 of the container with a plastic tube 70. The apparatus 10 was left standing in an incubator (not shown) and maintained at 35°C to perform mesophilic fermentation. The amount of gas and gas components in the gas pack 80 were measured periodically. For the analysis of gas components, a gas chromatograph (manufactured by Shimadzu Corporation, GC-2014ATF) was used. After 25 days, the concentration of phosphate ions, iron ions, and chemical oxygen demand (COD) contained in the digested solution were measured. For the measurement of the phosphate ion concentration, Pack Test Phosphate (WAK-PO4, DPM2-PO4-C, both manufactured by Kyoritsu Chemical-Check Laboratory Co., Ltd., Pack Test is a registered trademark) was used. For the measurement of the iron ion concentration, Pack Test Iron Ion (WAK-Fe(D), DPM2-Fe-D, both manufactured by Kyoritsu Chemical-Check Laboratory Co., Ltd., Pack Test is a registered trademark) was used. For the measurement of COD, Pack Test COD (WAK-COD-2, DPM-MTSP, both manufactured by Kyoritsu Chemical-Check Laboratory Co., Ltd., Pack Test is a registered trademark) was used.

[0031] (2) Results Figure 3 is an explanatory diagram showing the amount of biogas produced. In Figure 3, the horizontal axis represents the fermentation period (days), and the vertical axis represents the amount of biogas produced (L). For example, to produce 3 L of biogas, a fermentation period of 24 days was required under the condition without steel slag, while a fermentation period of 15 days was required under the condition with steel slag. Therefore, it was found that by using steel slag, the fermentation period can be shortened by 9 days (0.625 times). Also, after 15 days from the start of fermentation, the biogas production rates under both conditions are similar and increase at a similar gradient. In contrast, when comparing the biogas production amounts during the initial 15 days of fermentation, it was found that the biogas production amount is about three times more under the condition with steel slag than under the condition without steel slag. From the above results, it was shown that biogas can be efficiently produced by using steel slag.

[0032] Figure 4 is an explanatory diagram showing the proportion of methane gas contained in biogas. In Figure 4, the horizontal axis represents the fermentation period (days), and the vertical axis represents the proportion of methane (%) contained in biogas. When using steel slag, it became clear that the proportion of methane contained in biogas is higher and the production efficiency of methane gas is higher compared to the case without using steel slag. Methanogenesis and its efficiency greatly depend on the reducibility of the solution. It is considered that the stronger the reducibility, the more active the activity of methanogenic bacteria and the better the efficiency of methanogenesis. In this example, the improvement in fermentation efficiency is considered to be due to the fact that steel slag developed reducibility early.

[0033] Figure 5 is an explanatory diagram showing the concentration of substances contained in the digestive fluid. In Figure 5, for the case with and without using steel slag, the concentration (mg / L) of phosphate ions (PO4 3- ) and COD (mg / L) in each digestive fluid, and iron ions (Fe 2+ and Fe 3+) and the concentration of phosphate ions (mg / L) are shown. By using steel slag, the COD decreased from 360 mg / L to 70 mg / L, and the phosphate ion concentration decreased from 43 mg / L to 0.03 mg / L. Therefore, the COD removal rate by using steel slag in methane fermentation was approximately 80%, and the phosphate removal rate was 99.9% or more. Also, by using steel slag, the concentration of iron ions increased from 3.28 mg / L to 14 mg / L, and it was found that the iron ions increased by about 4.3 times. Generally, steel slag contains several percent of iron, and the increase in iron ions indicates that the iron contained in the steel slag is dissolved (Fe → Fe 2+ + 2e - ). This dissolution is considered to contribute to the improvement of the efficiency of methane fermentation. Here, generally, it is known that hydrogen sulfide is generated in methane fermentation. The increase in iron ions accompanying the use of steel slag is considered to contribute to the removal of hydrogen sulfide contained in the digested liquid.

[0034] The present invention is not limited to the above-described embodiments, and can be realized in various configurations without departing from the gist thereof. For example, the embodiments and examples corresponding to the technical features in each form described in the summary of the invention can be appropriately replaced or combined in order to solve some or all of the above-described problems, or to achieve some or all of the above-described effects. Also, if the technical feature is not described as essential in this specification, it can be appropriately deleted.

Explanation of Reference Numerals

[0035] 10... device, 20... plastic container, 30... food waste, 40... blast furnace slag, 50... tap water, 60... lid, 70... plastic tube, 80... gas pack

Claims

1. A method for producing methane gas, comprising: The method includes a step of contacting steel slag with organic waste under anaerobic conditions to perform methane fermentation. A method for producing methane gas.

2. The method for producing methane gas according to claim 1, The iron and steel slag includes blast furnace slag. A method for producing methane gas.

3. 3. The method for producing methane gas according to claim 1 or 2, In the step, the mass ratio of the steel slag to the organic waste (steel slag / organic waste) is 0.5 or more and 4.0 or less. A method for producing methane gas.

4. 3. The method for producing methane gas according to claim 1 or 2, In the above step, the steel slag is an upper layer and the organic waste is a lower layer. A method for producing methane gas.

5. 3. The method for producing methane gas according to claim 1 or 2, The process is carried out for a period of at least 7 days and not more than 20 days. A method for producing methane gas.

6. 3. The method for producing methane gas according to claim 1 or 2, The step is carried out under conditions of 30°C or higher and 45°C or lower. A method for producing methane gas.

7. 3. The method for producing methane gas according to claim 1 or 2, The phosphoric acid concentration of the digestion liquid produced by the process is 0.1 mg / L or less. A method for producing methane gas.

8. 3. The method for producing methane gas according to claim 1 or 2, The COD of the digested liquid produced by the process is 100 mg / L or less. A method for producing methane gas.

Images