JP7679938B2 - Fuel gas production apparatus and fuel gas production method - Google Patents

Description

The present invention relates to a fuel gas production apparatus and a fuel gas production method.

Lignocellulosic biomass is an abundant source of organic carbon on Earth that constitutes the cell walls of plant cells, i.e., the main component of plant fiber, and is therefore attracting attention as an alternative energy resource to fossil fuels such as petroleum. In addition, because lignocellulosic biomass is a difficult-to-decompose organic waste, the decomposition stage requires a great deal of energy, cost, and time. For this reason, the expansion of its use as an energy resource has been stagnant.

A technology has been known that produces biogas by decomposing lignocellulosic biomass using ruminal microorganisms present in the first stomach fluid (rumen fluid) of ruminants to produce volatile fatty acids such as acetic acid, and then carrying out methane fermentation using the volatile fatty acids as a substrate (e.g., Non-Patent Document 1).

Non-Patent Document 1 discloses a two-phase process in which a methanogenesis tank (second reactor) is connected to an acidogenesis tank (first reactor) for decomposing cellulose to produce volatile fatty acids. Specifically, it discloses that in the acidogenesis tank, cellulose is decomposed by the action of ruminal microorganisms to produce volatile fatty acids, and a liquid phase rich in volatile fatty acids is supplied to a methanogenesis tank using a membrane separation device, where methane fermentation is carried out using the volatile fatty acids as a substrate to produce methane gas.

Hubub J. Gijzen, et al., "High-Rate two-phase process for the anaerobic degradation of cellulose, employing rumen microorganisms for an efficient acidogenesis," Biotechnology and Bioengineering, Vol. 31, pp. 418-425 (1988)

However, in the two-phase process described in Non-Patent Document 1, solid-liquid separation is performed for the sake of adjusting the solid retention time (SRT), so the solid concentration (TS) of the treatment liquid in the acid generation tank needs to be 1.0% by weight or less. This requires a large amount of medium and dilution water, which causes the problem that methane generation equipment is limited to huge capacity methane fermentation tanks or UASB and EGSB systems that can handle high loads, and even if an attempt is made to apply this to existing methane fermentation tanks, the tank capacity is insufficient, making it impossible to apply. In addition, when a methane fermentation tank using the UASB or EGSB method is applied, the suspension containing a large amount of suspended solids (high SS concentration) in the acid generation tank is not treated, and only the membrane filtrate water separated from the solid-liquid is treated, so the suspension is discharged outside the system, and a large amount of solid waste is generated.

The present invention aims to provide a fuel gas production device and a fuel gas production method for producing biomass-derived fuel gas that can reduce the amount of liquid medium, such as artificial medium (artificial saliva), used to adjust the solids concentration (TS) in the treatment liquid in which organic waste is decomposed, and the amount of dilution water used, while also reducing the amount of solid waste discharged.

In order to solve the above problems, the fuel gas production apparatus of the present invention is a fuel gas production apparatus for producing fuel gas derived from biomass, and includes a culture tank for culturing ruminal microorganisms, a pretreatment tank for mixing the culture solution of the ruminal microorganisms obtained in the culture tank with organic waste and treating the organic waste to convert it into easily degradable organic waste and volatile fatty acids, and a fermentation tank for fermenting the easily degradable organic waste and volatile fatty acids obtained in the pretreatment tank as substrates.

According to the above configuration, ruminal microorganisms (lignocellulose-decomposing bacteria) that decompose organic waste can be cultured in the culture tank and stably supplied to the downstream pretreatment tank. Therefore, when operating the fuel gas production device, the frequency of collecting ruminal fluid from ruminants and transporting it to the facility where the fuel gas production device is installed is reduced, thereby reducing costs. In addition, since the pretreatment tank is specialized for treating organic waste and producing easily decomposable organic waste such as partial decomposition products of organic waste and volatile fatty acids, the amount of organic waste supplied to the pretreatment tank can be set higher than usual. Furthermore, since the pretreatment liquid in the pretreatment tank does not need to be separated into solid and liquid and is supplied to the fermentation tank connected downstream, a complete mixing type fermentation (anaerobic digestion) type fermentation tank that treats solid (slurry-like) waste, which is widely used both domestically and internationally, can be adopted. Unlike UASB, which only processes membrane-filtered water from pretreatment liquid that has been separated into solid and liquid in an upstream pretreatment tank, fermentation tanks using the complete mixing fermentation method can also process suspensions, which reduces the amount of liquid culture medium such as artificial medium (artificial saliva) and dilution water used. In addition, since fermentation is carried out using the easily decomposable organic waste contained in the suspension of the pretreatment liquid as a substrate, it is possible to reduce the amount of solid waste discharged and to expect an increase in the production of fuel gas. Furthermore, since the processing carried out in each tank, namely the culture tank, pretreatment tank, and fermentation tank, is clear, operation management and troubleshooting are made easier.

The fuel gas production apparatus of the present invention may also be configured to supply the organic waste to the culture tank and the pretreatment tank at a predetermined ratio.

According to the above configuration, by supplying organic waste to the culture tank and the pretreatment tank at a predetermined ratio, ruminal microorganisms (lignocellulose-decomposing bacteria) with high decomposition activity for organic waste can be cultivated in the culture tank, and easily decomposable organic waste such as partial decomposition products of organic waste and volatile fatty acids can be efficiently produced in the pretreatment tank.

The fuel gas production apparatus of the present invention may also be configured to supply a pretreatment liquid containing the easily decomposable organic waste and the volatile fatty acids obtained in the pretreatment tank to the culture tank.

According to the above configuration, the pretreatment liquid in the pretreatment tank contains undecomposed or partially decomposed lignocellulose contained in the organic waste as well as substances that serve as nitrogen and phosphorus sources, which serve as nutrient sources for the lignocellulose-decomposing bacteria. Therefore, by supplying the pretreatment liquid to the culture tank, the growth and proliferation of the lignocellulose-decomposing bacteria in the culture tank is further promoted, and further hydrolysis increases the production of volatile fatty acids, etc.

The fuel gas production apparatus of the present invention may further include a solid-liquid separation means for separating the culture liquid into solid and liquid in the culture tank.

According to the above configuration, by providing a solid-liquid separation means in the culture tank, the culture liquid in the culture tank can be separated into a filtrate containing a high concentration of volatile fatty acids that are unfavorable for the growth and proliferation of lignocellulose-decomposing bacteria, and a suspension containing suspended matter to which many lignocellulose-decomposing bacteria are attached. This makes it possible to individually control the hydraulic retention time (HRT) and the solid retention time (SRT), and the filtrate containing a large amount of volatile fatty acids is discharged from the culture tank, promoting the growth and proliferation of the lignocellulose-decomposing bacteria contained in the suspension.

The fuel gas production apparatus of the present invention may also be configured to supply easily decomposable organic waste to the fermenter.

According to the above configuration, easily decomposable organic waste is supplied to the fermentation tank and not to the pretreatment tank, so the pretreatment tank can be made compact. As a result, the amount of liquid culture medium such as artificial culture medium (artificial saliva) and dilution water used can be reduced.

In order to solve the above problems, the fuel gas production method of the present invention is a fuel gas production method for producing biomass-derived fuel gas, and includes a culture process for culturing ruminal microorganisms, a pretreatment process for mixing the culture solution of the ruminal microorganisms obtained in the culture process with organic waste and treating the organic waste to convert it into easily degradable organic waste and volatile fatty acids, and a fermentation process for fermenting the easily degradable organic waste and the volatile fatty acids obtained in the pretreatment process as substrates, and the culture process further includes a solid-liquid separation process for separating the culture solution into solid and liquid.

According to the above configuration, in the culture step, ruminal microorganisms (lignocellulose-decomposing bacteria) that decompose organic waste can be cultured and stably supplied to the pretreatment tank in which the pretreatment step is carried out. Therefore, when carrying out the fuel gas production method, the frequency of collecting ruminal fluid from ruminants and transporting it to the facility in which the fuel gas production method is carried out is reduced, thereby reducing costs. In addition, since the pretreatment tank in which the pretreatment step is carried out is specialized for the functions of easily decomposable organic waste such as partial decomposition products of organic waste and the production of volatile fatty acids, the amount of organic waste supplied to the pretreatment tank can be set higher than usual. Furthermore, since the pretreatment liquid in the pretreatment tank does not need to be separated into solid and liquid and is supplied to the downstream fermentation tank, the fermentation step can be carried out by adopting a fermentation tank of a complete mixing type fermentation (anaerobic digestion) method that treats solid (slurry-like) waste, which is widely used both domestically and overseas. Unlike UASBs and the like, which can only treat membrane-filtered water of solid-liquid separated pretreatment liquid, fermentation tanks using the complete mixing fermentation method can also treat suspensions, so that the amount of liquid medium such as artificial medium (artificial saliva) and dilution water used can be reduced. In addition, since fermentation is performed using easily decomposable organic waste contained in the suspension of the pretreatment liquid as a substrate, the amount of solid waste discharged can be reduced and an increase in the amount of fuel gas generated can be expected. Furthermore, since the processes performed in each process, namely the culture process, the pretreatment process, and the fermentation process, are clearly defined, operation management and troubleshooting can be easily performed. In addition, by providing a solid-liquid separation process for separating the culture solution of ruminal microorganisms into a solid-liquid separation solution and a suspension, the culture solution in the culture tank can be separated into a filtrate containing a high concentration of volatile fatty acids that are unfavorable for the growth and proliferation of lignocellulose-decomposing bacteria, and a suspension containing suspended matter to which many lignocellulose-decomposing bacteria are attached. This makes it possible to control hydraulic retention time (HRT) and solid retention time (SRT) separately, and the filtrate containing a large amount of volatile fatty acids is discharged from the culture tank, promoting the growth and proliferation of the lignocellulose-decomposing bacteria contained in the suspension.

In the fuel gas production method of the present invention, the fermentation process may include at least one of a methane fermentation process and a hydrogen fermentation process.

According to the above configuration, the fermentation process includes at least one of a methane fermentation process and a hydrogen fermentation process, so that methane gas or hydrogen gas that can be used as fuel gas can be produced according to the purpose.

According to the present invention, it is possible to reduce the amount of liquid medium such as artificial medium (artificial saliva) used to adjust the solids concentration (TS) in the treatment liquid in which organic waste is decomposed, and the amount of dilution water used, and to reduce the amount of solid waste discharged.

1 is a diagram showing a schematic configuration of a fuel gas production apparatus according to a first embodiment. FIG. 2 is a diagram showing a schematic configuration of a culture tank of the fuel gas production apparatus according to the first embodiment. FIG. 5 is a diagram showing a schematic configuration of a fuel gas production apparatus according to a second embodiment. FIG. 11 is a diagram showing a schematic configuration of a fuel gas production apparatus according to a third embodiment. FIG. 1 is a diagram showing a conventional fuel gas production apparatus. FIG. 2 is an explanatory diagram of a first example of supplying organic waste to the fuel gas production apparatus according to the first embodiment. FIG. 11 is an explanatory diagram of Example 2 in which organic waste is supplied to a two-phase fuel gas production apparatus.

The following describes in detail the embodiments and drawings relating to the fuel gas production apparatus for producing biomass-derived fuel gas and the fuel gas production method for producing biomass-derived fuel gas according to the present invention. However, the present invention is not intended to be limited to the configurations shown in the embodiments and drawings described below.

First Embodiment
Hereinafter, a fuel gas production apparatus 100 and a fuel gas production method according to the first embodiment will be described with reference to Figures 1 and 2. Figure 1 is a diagram showing a schematic configuration of the fuel gas production apparatus 100 according to the first embodiment. Figure 2 is a diagram showing a schematic configuration of a culture tank 10 of the fuel gas production apparatus 100 according to the first embodiment.

The fuel gas production device 100 shown in FIG. 1 is a device that produces fuel gas such as biomethane gas and biohydrogen using organic waste B as a raw material. The fuel gas production device 100 is a three-phase system consisting of a culture tank 10 for culturing ruminal microorganisms, a pretreatment tank 20 that uses ruminal microorganisms (lignocellulose-decomposing bacteria) that treat the organic waste B to convert (decompose) the organic waste B into a form that is easily usable in the downstream fermentation tank 30 (easily decomposable organic waste and volatile fatty acids), and a fermentation tank 30 that ferments the easily decomposable organic waste and volatile fatty acids, which are the decomposition products of the organic waste B, as substrates to produce fuel gas.

(culture tank)
Rumen microorganisms present in ruminal fluid collected from ruminants such as cows are cultured in the culture tank 10. When a liquid medium M such as an artificial medium (artificial saliva) and organic waste B are supplied to the culture tank 10, the ruminal microorganisms (lignocellulose-decomposing bacteria) grow by decomposing and metabolizing the organic waste B, and produce decomposition products of the organic waste B (easily decomposable organic waste such as partial decomposition products of lignocellulose, and volatile fatty acids (VFAs) such as acetic acid, propionic acid, butyric acid, and valeric acid). The culture solution of the lignocellulose-decomposing bacteria in the culture tank 10 is stably supplied to the downstream pretreatment tank 20.

The conditions in the culture tank 10 are preferably set to simulate the physiological state of the first stomach (rumen) of ruminants such as cows. In addition, it is preferable that the hydraulic retention time (HRT) and solid retention time (SRT) are arbitrarily controlled by the solid-liquid separation means 40 so that the proliferation of the lignocellulose-decomposing bacteria is sufficient. Since the lignocellulose-decomposing bacteria proliferate by adhering to the surface of solid matter in the culture solution, it is preferable that the operation is controlled so that the SRT is longer than the HRT.

(Pretreatment tank)
The pretreatment tank 20 is supplied with a culture solution of lignocellulose-decomposing bacteria from the upstream culture tank 10, and furthermore, organic waste B is introduced and mixed. In the pretreatment tank 20, the organic waste B is decomposed by the action of the lignocellulose-decomposing bacteria, and decomposition products of the low-molecular-weight organic waste B (easily decomposable organic waste such as partial decomposition products of lignocellulose and volatile fatty acids (VFAs) such as acetic acid, propionic acid, butyric acid, and valeric acid) are produced. Note that it is sufficient that at least one selected from easily decomposable organic waste and volatile fatty acids is produced in the pretreatment tank 20. As with the culture tank 10, the pretreatment tank 20 is preferably set to a condition simulating the physiological state of the first stomach (rumen) of a ruminant such as a cow.

In the fuel gas production apparatus 100, it is preferable to supply organic waste B to the culture tank 10 and the pretreatment tank 20 at a predetermined ratio (for example, 1:10). In the culture tank 10, ruminal microorganisms (lignocellulose-decomposing bacteria) that have high decomposition activity for organic waste B can be cultured, and in the pretreatment tank 20, organic waste B is efficiently treated.

(Fermentation tank)
When the pretreatment liquid containing a large amount of easily decomposable organic waste and volatile fatty acids is supplied from the pretreatment tank 20 to the fermentation tank 30, a fermentation reaction using the easily decomposable organic waste and volatile fatty acids as substrates (raw materials) proceeds, and fuel gas G is generated. The substrate supplied to the fermentation tank 30 may contain at least one selected from easily decomposable organic waste and volatile fatty acids. The fermentation reaction performed in the fermentation tank 30 may be methane fermentation contributing to the production of biomethane gas, or hydrogen fermentation contributing to the production of biohydrogen. For example, it may be hydrogen/methane two-stage fermentation in which hydrogen fermentation is performed as a first stage and then methane fermentation is performed as a second stage. The fermentation residue R discharged from the fermentation tank 30 is concentrated and can be used as fertilizer, building material, etc.

As described above, the fuel gas production apparatus 100 according to this embodiment has the advantage that the roles of each tank are separate: (1) the culture tank 10 for culturing ruminal microorganisms, (2) the pretreatment tank 20 for treating organic waste B to produce easily degradable organic waste and volatile fatty acids, and (3) the fermentation tank 30 for carrying out a fermentation reaction using easily degradable organic waste and volatile fatty acids as substrates.

(solid-liquid separation means)
FIG. 2 is a diagram showing a schematic configuration of the culture tank 10 of the fuel gas production apparatus 100 according to the first embodiment, and a filtration membrane 41 is shown as a solid-liquid separation means 40 installed in the culture tank 10. The solid-liquid separation means 40 separates the culture liquid into a filtrate and a suspension. As shown in FIG. 2, in the culture tank 10, the culture liquid is stirred by the stirrer 50, and the filtrate separated into solid and liquid by the filtration membrane 41 and the suspension containing undecomposed or partially decomposed suspended matter are sent to the downstream pretreatment tank 20 through the filtrate sending pipe 11 and the suspension sending pipe 12, respectively. The hydraulic retention time (HRT) and solid retention time (SRT) of the culture liquid are individually controlled by appropriately adjusting the amount of the filtrate and suspension of the culture liquid drawn out from the culture tank 10 and sent to the pretreatment tank 20 by the pump P. Since the filtrate and suspension of the culture liquid contain a large number of lignocellulose-decomposing bacteria, when they are supplied to the downstream pretreatment tank 20, the decomposition of the organic waste B is efficiently performed. In addition, in Fig. 2, a membrane separation method using a filtration membrane 41 is illustrated as the solid-liquid separation means 40, but separation by gravity sedimentation separation method or flotation separation method may also be used (not shown). In addition, when a membrane separation method is performed as the solid-liquid separation means 40, as shown in Fig. 2, the filtration membrane 41 may be installed so that the inside of the filtration membrane 41 is negative pressure with the entire filtration membrane 41 immersed in the culture solution, but in order to reduce the power of the pump P of the filtrate delivery pipe 11 and reduce clogging of the filtration membrane 41 by suspended matter, the filtration membrane 41 may be installed so that the inside and outside of the filtration membrane 41 are equal pressure with the upper end of the filtration membrane 41 immersed in the culture solution so as to be located above the liquid level (not shown).

Second Embodiment
3 is a diagram showing a schematic configuration of a fuel gas production apparatus 101 according to the second embodiment. The fuel gas production apparatus 101 according to the second embodiment further includes a pretreatment liquid supply pipe 21 for supplying the pretreatment liquid obtained in the pretreatment tank 20 to the culture tank 10, by mixing the organic waste B with dilution water W and feeding the mixture into the pretreatment tank 20 in addition to the fuel gas production apparatus 100 according to the first embodiment. The pretreatment liquid contains undecomposed or partially decomposed lignocellulose and substances that serve as nitrogen and phosphorus sources. Therefore, by supplying the pretreatment liquid to the culture tank 10, the growth and proliferation of lignocellulose-decomposing bacteria in the culture liquid is further promoted, and further hydrolysis improves the production of volatile fatty acids and easily decomposable organic waste with a small molecular weight.

Third Embodiment
FIG. 4 is a diagram showing a schematic configuration 102 of a fuel gas production apparatus according to a third embodiment. The fuel gas production apparatus 102 according to the third embodiment further includes a path for supplying easily decomposable organic waste C to the fermenter in addition to the fuel gas production apparatus 100 according to the first embodiment. Examples of easily decomposable organic waste C include livestock manure, sludge, and food waste. In the fuel gas production apparatus 102 according to the third embodiment, the easily decomposable organic waste C is supplied to the fermenter 30 and not to the pretreatment tank 20, so that the pretreatment tank 20 can be made compact. Accordingly, the amount of liquid medium M such as an artificial medium (artificial saliva) and dilution water W used can be reduced.

Figure 5 shows a fuel gas production system 200 configured with a conventional two-phase process. In the conventional fuel gas production system 200, the cultivation of ruminal microorganisms and the decomposition of organic waste B are carried out in a single culture tank/pretreatment tank 220. The pretreatment liquid in the culture tank/pretreatment tank 220 is separated into solid and liquid by a filtration membrane 41 (solid-liquid separation means 40), and the filtrate is sent to the downstream UASB 230 through the filtrate delivery pipe 11. In the UASB 230, methane fermentation is carried out using the volatile fatty acids contained in the filtrate as a substrate.

In order for the solid-liquid separation of the pretreatment liquid in the culture tank and pretreatment tank 220 to be carried out without any problems, it is necessary to adjust the solid concentration (TS) of the organic waste B supplied to the culture tank and pretreatment tank 220 to be approximately 1.0% by weight or less. Since a large amount of liquid medium M such as an artificial medium (artificial saliva) or dilution water W is required to adjust the solid concentration (TS), the methane fermentation tank connected downstream of the culture tank and pretreatment tank 220 is limited to UASB and EGSB, which are capable of handling high loads. However, when the SS concentration of the pretreatment liquid supplied from the upstream culture tank and pretreatment tank 220 exceeds 500 mg/L (1000 mg/L in the case of EGSB), it adversely affects the settling of the granular sludge present in the methane fermentation tank. Therefore, only the filtrate obtained by solid-liquid separation of the pretreatment liquid by the filtration membrane 41 is supplied to the UASB 230. As a result, UASB230 is unable to process the pretreatment liquid (suspension) which contains a large amount of undecomposed suspended matter, and a large amount of solid waste S is discharged.

(Fuel gas production method)
The fuel gas production method of the present invention will be described in detail below. The fuel gas in the present invention includes biomethane gas produced by methane fermentation and biohydrogen produced by hydrogen fermentation. Biogas is composed of about 60% methane and about 40% carbon dioxide, and may be used as fuel as it is, or may be used as high-concentration methane gas after removing carbon dioxide.

[Organic waste]
Examples of organic waste that can be used as a raw material for fuel gases such as biomethane gas and biohydrogen include unused agricultural and forestry waste such as forest thinnings, rice straw, rice husks, bagasse, thatch, and aquatic plants, as well as lignocellulosic industrial waste such as vegetable waste, tea leaves, coffee grounds, soybean pulp, shochu dregs, construction waste, waste paper, waste paper, and urban waste, and lignocellulosic biomass such as biomass resource crops such as Erianthus and Giant Miscanthus. In addition, paper cut into pieces by a shredder is incinerated because the fibers are broken and it is difficult to recycle, but such cut paper can also be used as a raw material. Furthermore, only one type of the organic waste listed above may be used as a raw material, or multiple types may be mixed together to use as a raw material.

Lignocellulose, the main component of the cell wall of plant cells, is composed of hemicellulose, cellulose, and lignin tightly bound together. In the ruminal fluid present in the first stomach (rumen) of ruminants such as cows, there are many ruminal microorganisms that produce enzymes that decompose lignocellulose. In the fuel gas production method of the present invention, in the culture step, ruminal microorganisms (lignocellulose-decomposing bacteria) that decompose organic waste are cultured, in the pretreatment step, the organic waste is decomposed using the action of the lignocellulose-decomposing bacteria to produce easily decomposable organic waste, volatile fatty acids, etc., and in the fermentation step, a fermentation reaction is carried out using the easily decomposable organic waste, volatile fatty acids, etc. as fermentation substrates (raw materials), thereby making it possible to efficiently produce fuel gases such as biomethane gas and biohydrogen. In the fermentation step, if the conditions are such that methane-producing bacteria are predominant, methane fermentation is carried out and biomethane gas is produced. On the other hand, in the fermentation step, if the conditions are such that hydrogen-producing bacteria are predominant, hydrogen fermentation is carried out and biohydrogen is produced. Depending on the use and purpose of the fuel gas, the fermentation process may be a methane fermentation process or a hydrogen fermentation process. In addition, a two-stage fermentation process of hydrogen and methane fermentation may be performed, in which a hydrogen fermentation process is performed as a pre-reaction of the methane fermentation process. In this case, methane fermentation may be performed after hydrogen fermentation by controlling the pH, HRT, etc. in one fermentation tank, or the hydrogen fermentation tank and the methane fermentation tank may be separated, and hydrogen fermentation and methane fermentation may be performed in separate tanks.

[Rumen microorganisms]
Rumen microorganisms are anaerobic bacteria that exist in ruminal fluid, which is a digestive fluid present in the first stomach (rumen) of ruminants. Ruminants include cows, sheep, goats, deer, camels, llamas, etc. For example, the first stomach of an adult cow has a volume of 150 to 200 L, and many lignocellulose-decomposing bacteria, hemicellulose-decomposing bacteria, lignin-decomposing bacteria, starch-decomposing bacteria, methanogens, hydrogen-producing bacteria, etc. live in the ruminal fluid. Lignocellulose-decomposing bacteria can produce enzymes such as cellulase that decompose lignocellulose (fibrous material). Therefore, when ruminants ingest fibrous material such as grass, the lignocellulose-decomposing bacteria decompose the lignocellulose, producing volatile fatty acids (VFAs) such as acetic acid, propionic acid, butyric acid, and valeric acid, which serve as an energy source for ruminants. Hydrogen-producing bacteria produce acetic acid and hydrogen using substrates such as propionic acid, butyric acid, and valeric acid produced by lignocellulose-decomposing bacteria.Methanogenic bacteria produce methane using substrates such as acetic acid or hydrogen and carbon dioxide produced by lignocellulose-decomposing or hydrogen-producing bacteria.

(Culture process)
In the culturing step, ruminal microorganisms (lignocellulose-decomposing bacteria) having high lignocellulose decomposition activity are cultured. In the culturing step, the temperature, oxidation-reduction potential (ORP), hydraulic retention time (HRT) or solid retention time (SRT), and ammonium nitrogen concentration of the medium are adjusted using the quantity of at least one of three types of lignocellulose decomposing bacteria, for example, Fibrobacter succinogenes, Ruminococcus albus, and Prevotella ruminicola, as an indicator, to obtain a culture solution of ruminal microorganisms (lignocellulose decomposing bacteria) having high lignocellulose decomposition activity. The quantification of the number of bacteria of these bacteria is not particularly limited, but is preferably carried out by a quantitative PCR method, which can rapidly and accurately measure the number of bacteria of a specific bacterium. When the number of bacteria of lignocellulose decomposing bacteria is quantified using the quantitative PCR method, genomic DNA is extracted from the culture solution and purified. Since the culture solution of ruminal microorganisms can be cultured in large quantities or subcultured, the culture solution of ruminal microorganisms in which the number of bacteria has been adjusted can be used for lignocellulose decomposition as necessary.

As a medium for the culture solution of ruminal microorganisms, a natural medium whose base material is derived from natural products may be used, or a synthetic medium in which all the various nutrients necessary for the growth of ruminal microorganisms are composed of chemicals may be used. In particular, it is preferable to contain a carbon source such as cellulose or hemicellulose, in addition to a nitrogen source such as ammonium salt, which is a nutrient source useful for ruminal microorganisms, a phosphorus source such as phosphate, etc. In addition, since the pretreatment liquid contains undecomposed or partially decomposed lignocellulose contained in organic waste, and substances that serve as nitrogen and phosphorus sources as nutrient sources for lignocellulose-decomposing bacteria, the pretreatment liquid may be supplied to the culture solution as shown in FIG. 3. In addition, when volatile fatty acids (VFAs) such as acetic acid are produced in the pretreatment process following the culture process, the pH of the pretreatment liquid decreases and the osmotic pressure increases, which changes the number and function of lignocellulose-decomposing bacteria. Therefore, it is preferable to add a buffering agent to the medium to mitigate the change in pH and the increase in osmotic pressure. As a buffering agent, it is preferable to use a liquid medium such as an artificial medium (artificial saliva) that imitates the saliva of ruminants, which has a high buffering capacity. Examples of buffering agents include sodium chloride, sodium bicarbonate, phosphate, and potassium chloride. The pH of the medium is adjusted to 6.0 to 7.5, more preferably 6.5 to 7.0, by adding an acid or alkali as necessary. The water used for the medium is preferably tap water or groundwater. The ammonium nitrogen concentration of the medium is controlled by adding water to 50 to 2,000 mg/L, more preferably 60 to 500 mg/L.

To make the culture process anaerobic like the environment in the rumen, the oxidation-reduction potential (ORP) of the culture solution is adjusted to -100mv or less, more preferably -200mv or less, and even more preferably about -250mv. Since ruminal microorganisms (lignocellulose-decomposing bacteria) are anaerobic bacteria, when the ORP becomes higher than a predetermined value (when it approaches an aerobic state), measures such as injecting an inert gas such as nitrogen gas or carbon dioxide gas, adding reducing substances such as cysteine, L-ascorbic acid, sodium sulfide, ascorbic acid, methionine, thioglycol, and DTT, or adding organic matter to consume oxygen through the action of facultative anaerobic bacteria may be taken. The culture process may also be carried out in a closed system under a nitrogen or carbon dioxide atmosphere.

In culturing ruminal microorganisms in the culture tank 10, it is important that the solid retention time (SRT) is longer than the hydraulic retention time (HRT). That is, lignocellulose-decomposing bacteria grow by attaching to the surface of solids, and therefore, if the SRT is short, they are discharged from the system together with the solids. On the other hand, if the HRT is longer than necessary, it will cause negative factors such as a decrease in pH due to the produced volatile fatty acids (VFA) and inhibition of microbial growth. Specifically, the HRT is adjusted to 8 to 36 hours, preferably 10 to 24 hours, and the SRT is adjusted to 24 hours or more, preferably 48 to 168 hours, and more preferably 72 to 168 hours. In addition, the temperature of the reaction system in the culture process is controlled using a temperature sensor or the like to be 35 to 42°C, more preferably 37 to 40°C. The organic waste B fed into the culture tank 10 is adjusted so that the solids concentration (TS) in the culture solution is 0.1 to 1.5% by weight, more preferably 0.5 to 1.0% by weight. If the solids concentration (TS) is 0.1% by weight or less, there is a risk that the growth and proliferation of the lignocellulose-decomposing bacteria will not be promoted. On the other hand, if it exceeds 1.5% by weight, there is a risk that solid-liquid separation of the culture solution will not be performed properly.

(Solid-liquid separation process)
In the culture step, solid-liquid separation of the culture solution of the ruminal microorganism in the culture tank 10 may be performed by a filtration membrane 41 shown in Fig. 2, or may be performed by a gravity sedimentation method or a flotation concentration method (not shown). The culture tank 10 shown in Fig. 2 is provided with a filtrate delivery pipe 11 and a suspension delivery pipe 12, but when performing solid-liquid separation by the gravity sedimentation separation method or the flotation separation method, the culture solution is discharged while being stirred when a liquid phase (suspension) with a high SS concentration is discharged outside the tank, and the liquid phase with a low SS concentration is discharged by stopping stirring and leaving it to stand for a while, so that only one liquid delivery pipe may be used.

(Pretreatment process)
In the pretreatment process, the lignocellulose contained in the organic waste is partially decomposed by lignin-decomposing enzymes produced by lignocellulose-decomposing bacteria, loosening the strong structure of the lignocellulose, and then decomposed into cellulose and hemicellulose by endoglucanase, exoglucanase, xylanase, etc., respectively, and converted into hexose (six monosaccharides) such as glucose. Furthermore, pyruvic acid and the like are produced from hexose (six monosaccharides) such as glucose, and as the reaction proceeds, volatile fatty acids (VFAs) such as acetic acid, propionic acid, butyric acid, and valeric acid are produced. Hydrogen and carbon dioxide are also produced during the metabolic process. In the pretreatment process, it is not necessary for lignocellulose to be decomposed and metabolized to hexose (six monosaccharides) and volatile fatty acids (VFAs). It is sufficient for the refractory lignocellulose to be decomposed to a form that is easily utilized in the downstream fermentation process, for example, to the extent of oligosaccharides. In the pretreatment step, the solids concentration (TS) of the organic waste B supplied to the pretreatment tank 20 is adjusted to 1.0 to 10.5% by weight, more preferably 0.5 to 10.0% by weight. If the solids concentration (TS) is 1.0% by weight or less, there is a risk that the production of easily decomposable organic waste and volatile fatty acids, which serve as substrates for fermentation, will decrease. On the other hand, if it exceeds 10.5% by weight, there is a risk that it will become difficult to discharge the pretreatment liquid by pump.

In order to make the pretreatment process anaerobic like the environment in the rumen of a ruminant, the oxidation-reduction potential (ORP) of the pretreatment liquid is adjusted to -100 mv or less, more preferably -200 mv or less, and even more preferably about -250 mv. Since ruminal microorganisms (lignocellulose-decomposing bacteria) are anaerobic bacteria, when the ORP becomes higher than a predetermined value (when it approaches an aerobic state), measures such as injecting an inert gas such as nitrogen gas or carbon dioxide gas, adding reducing substances such as cysteine, L-ascorbic acid, sodium sulfide, ascorbic acid, methionine, thioglycol, and DTT, or adding organic matter to consume oxygen through the action of facultative anaerobic bacteria, may be taken. The pretreatment process may be carried out in a closed system under a nitrogen or carbon dioxide atmosphere.

In the pretreatment process, since the HRT and SRT are individually controlled in the upstream culture process, there is no need to individually control the HRT and SRT, and pretreatment (hydrolysis, acid production) is performed in a completely mixed system. In other words, the pretreatment liquid in the pretreatment tank 20 where the pretreatment process is carried out is not separated into a filtrate and a suspension, so the HRT and SRT are the same. It is preferable that both the HRT and SRT of the pretreatment liquid are 4 to 36 hours. In addition, the temperature of the reaction system in the pretreatment process is controlled using a temperature sensor or the like to be 35 to 42°C, more preferably 37 to 40°C. The reaction in the pretreatment process may be carried out statically or with stirring, but it is preferable to carry out the reaction with stirring in order to speed up the progress of the pretreatment process.

(Fermentation process)
In the fermentation step, a fuel gas is produced using the easily decomposable organic waste and volatile fatty acids produced in the pretreatment step as substrates. Here, examples of the fuel gas include biogas containing biomethane gas produced by methane fermentation as a main component, and biohydrogen produced by hydrogen fermentation. In the fermentation step, whether methane fermentation or hydrogen fermentation is performed can be adjusted by controlling the temperature and pH in the fermenter 30. For example, when the temperature in the fermenter 30 is about 37° C. (or 55° C.) and the pH is 7.0 to 8.0, methane fermentation is mainly performed because methane-producing bacteria are predominant. On the other hand, when the temperature in the fermenter 30 is about 55° C. and the pH is 4.0 to 5.0, hydrogen-producing bacteria are predominant, and hydrogen fermentation is mainly performed. These fermentation steps may be performed alone, or two-stage fermentation may be performed, such as a hydrogen/methane fermentation step. The methane fermentation step and the hydrogen fermentation step will be described below.

(Methane fermentation process)
In the methane fermentation process, methane fermentation is carried out using volatile fatty acids such as acetic acid, hydrogen and carbon dioxide as substrates. As in the pretreatment process, the methane fermentation process is carried out under anaerobic conditions. The composition of the biogas produced in the methane fermentation process is 60-70% methane, 30-40% carbon dioxide, and other trace amounts of nitrogen, oxygen, hydrogen sulfide, water, etc.

Methane fermentation may be either wet methane fermentation or dry methane fermentation. Also, either mesophilic methane fermentation or high-temperature methane fermentation may be used. In the case of mesophilic fermentation, it is carried out for 20 to 30 days, more preferably 23 to 27 days, and even more preferably about 25 days, and in the case of high-temperature fermentation, it is carried out for 10 to 20 days, more preferably 13 to 17 days, and even more preferably about 15 days. In the case of mesophilic fermentation, methane fermentation is carried out at 20 to 40°C, more preferably 30 to 37°C or lower, and even more preferably about 37°C, and in the case of high-temperature fermentation, it is carried out at 40 to 60°C, more preferably 50 to 55°C, and even more preferably about 55°C. Also, methane fermentation is carried out at a pH of 6.8 to 7.6, and more preferably a pH of 7.0 to 8.0.

(Hydrogen fermentation process)
In the hydrogen fermentation process, hydrogen fermentation is carried out using volatile fatty acids such as propionic acid, butyric acid, and valeric acid as substrates. The hydrogen fermentation process is carried out under anaerobic conditions, similar to the pretreatment process using lignocellulose-decomposing bacteria. In the hydrogen fermentation process, biohydrogen and carbon dioxide are produced in a ratio of about 1:1, and organic acids such as acetic acid are produced.

Hydrogen fermentation is carried out at 30 to 60°C, more preferably 50 to 60°C, and even more preferably 55°C. Hydrogen fermentation is also carried out at a pH of 4.0 to 6.5, and more preferably a pH of 4.0 to 5.0.

In the hydrogen fermentation process, acetic acid, hydrogen, and carbon dioxide, which are substrates for methane fermentation, are produced. Therefore, a two-stage fermentation of hydrogen and methane fermentation may be performed after the hydrogen fermentation process. Hydrogen-producing bacteria grow faster than methane-producing bacteria, so the bacteria can be retained even if the HRT in the tank is set to 2 to 3 hours. In the case of two-stage fermentation of hydrogen and methane fermentation, hydrogen fermentation takes 1 to 2 days and methane fermentation takes 7 to 10 days, which is shorter than the number of fermentation days when methane fermentation is performed alone. Furthermore, in the case of two-stage fermentation of hydrogen and methane fermentation, a part of the methane fermentation liquid discharged from the methane fermentation process may be returned to the hydrogen fermentation process. In the hydrogen fermentation process, the pH decreases due to the production of volatile organic acids, which may decrease the activity of the hydrogen-producing bacteria and reduce the amount of hydrogen generated. However, by returning the methane fermentation liquid to the hydrogen fermentation process, the decreased pH can be neutralized or the decrease in pH can be prevented, making it possible to suppress the decrease in activity of the hydrogen-producing bacteria and maintain the amount of hydrogen generated.

(Case Study)
The following is an example of treating lignocellulosic waste discharged from food factory A using a three-phase process and a two-phase process. As a fermentation tank, a UASB is not used, but an existing complete mixing type methane fermentation tank for solid biomass is used.

<Conditions for consideration>
・Amount of organic waste discharged: 2.2 tons per day (2.2t-DW/d)
- Volume of existing methane fermentation tank: 200 ^m3
- Planned HRT of existing methane fermentation tank: 10 days - Solid concentration (TS) of biomass in culture tank (culture tank and pretreatment tank): Approximately 1.1% by weight
Solid concentration of biomass in pretreatment tank (TS): about 10.0% by weight
HRT of culture tank or culture tank/pretreatment tank: about 14 hours SRT of culture tank or culture tank/pretreatment tank: about 62 hours

The SRT of the culture tank or the culture tank/pretreatment tank can be calculated by the following formula (1), where dV is the amount of liquid culture medium such as artificial culture medium (artificial ^saliva ) or dilution water supplied per day ( ^m3 /d), fV is the amount of filtrate discharged per day (m3 ^/ d), and FV is the tank volume of the culture tank (m3). Reference was made to Gijzen et al., "Continuous cultivation of rumen microorganisms, a system with possible application to the anaerobic degradation of lignocellulosic waste materials," Vol. 25, pp. 155-162 (1986).

The HRT of a methane fermentation tank is calculated by dividing the tank volume (m ³ ) of the methane fermentation tank by the daily supply amount (m ³ /d) dV of liquid medium such as artificial medium (artificial saliva) or dilution water.

(Case 1)
6 is an explanatory diagram of Case 1 in which organic waste is supplied to the fuel gas production apparatus according to the first embodiment. In Case 1, a three-phase process consisting of a culture tank 10, a pretreatment tank 20, and a methane fermentation tank 30 was considered.
Of the 2.2 t-DW/d of lignocellulosic biomass discharged from the food factory A, 0.22 t-DW/d is used as a substrate (medium component) for culturing ruminal microorganisms in the culture tank. In order to ensure sufficient solid-liquid separation in the culture tank, the supply amount (dV) of liquid medium such as artificial medium (artificial saliva) or dilution water is 20 m ³ /d, and the solid concentration (TS) in the culture liquid is 1.1 wt%. In addition, in order to ensure sufficient proliferation of ruminal microorganisms in the culture tank, for example, if the SRT is 64 hours, the HRT is about 11 hours, and the discharge amount (fV) of the filtrate is 17 m ³ /d, the tank volume (FV) of the culture tank is 8 m ³ according to the above formula 1. The culture tank is operated and controlled so that the pH is approximately 6.9, the oxidation-reduction potential (ORP) is -250 mV or less, and the temperature is 39°C.
Of the 2.2 t-DW/d of lignocellulosic biomass discharged from food factory A, the remaining 1.98 t-DW/d of the 0.22 t-DW/d input to the culture tank is supplied to the pretreatment tank, where it is mixed with 17 m ³ /d of filtrate and 3 m ³ /d of suspension discharged from the culture tank. The optimal value of the HRT (same as SRT because it is a complete mixing type) of the pretreatment tank varies depending on the type of organic waste to be treated, but as shown in Figure 6, if it is set to 12 hours, a pretreatment tank with a tank volume of 10 m ³ can be used. (If the HRT is set to 6 hours, a tank volume of 5 m ³ can be used, and if the HRT is set to 24 hours, a pretreatment tank with a tank volume of 20 m ³ can be used.) The pretreatment tank is operated and controlled to have a pH of approximately 6.9, an oxidation-reduction potential (ORP) of -250 mV or less, and a temperature of 39°C, just like the culture tank.
20 ^m3 /d of pretreated liquid treated in the pretreatment tank is supplied to a methane fermentation tank (tank volume 200 ^m3 ). The HRT in the methane fermentation tank is calculated by dividing the tank volume of the methane fermentation tank, 200 ^m3 , by the daily supply amount of liquid medium such as artificial medium (artificial saliva) or dilution water, 20 ^m3 /d (dV), and is therefore 10 days [200 ( ^m3 )/20 ( ^m3 /d)]. As this is the same number of days as the planned HRT of the existing methane fermentation tank, 10 days, it is clear that a sufficient HRT can be secured for the methane fermentation tank in Case 1 to function.

(Case 2)
7 is an explanatory diagram of Case 2 in which lignocellulosic biomass is supplied to a two-phase fuel gas production apparatus. In Case 2, a two-phase process consisting of a culture tank/pretreatment tank 320 and a methane fermentation tank 30 was considered.
When 2.2 t-DW/d of lignocellulosic biomass discharged from the food factory is fed to the culture tank/pretreatment tank so that the solid concentration (TS) is 1.1% by weight so that solid-liquid separation using membrane filtration in the culture tank/pretreatment tank can be performed well, 200 ^m3 of liquid medium such as artificial medium (artificial saliva) is supplied to the culture tank/pretreatment tank per day. In other words, 200 ^m3 of treated water is supplied to the downstream methane fermentation tank through the culture tank/pretreatment tank. The HRT in the methane fermentation tank is the value obtained by dividing the tank volume of the methane fermentation tank, 200 ^m3 , by the supply amount of liquid medium such as artificial medium (artificial saliva) or dilution water per day, 200 ^m3 /d (dV), and is therefore [200 ( ^m3 )/200 ( ^m3 /d)] per day. Since this is significantly shorter than the planned HRT of 10 days for the existing methane fermentation tank, it is clear that a sufficient HRT cannot be secured for the methane fermentation tank in Case 2 to function.

As described above, by adopting a three-phase process consisting of a culture tank for ruminal microorganisms, a pretreatment tank specialized in the functions of hydrolysis and acid production of organic waste, and a methane fermentation tank, it is possible to increase the concentration of organic waste treated in the pretreatment tank, which in turn reduces the amount of pretreatment liquid supplied to the methane fermentation tank, and it has become clear that it is possible to ensure sufficient HRT for existing complete mixing-type methane fermentation tanks to function.

The fuel gas production device and fuel gas production method of the present invention can be used to decompose organic waste, including industrial waste such as urban waste, such as waste paper and other waste paper, and lignocellulosic industrial waste, such as food waste, agricultural and forestry waste, and construction waste, and to generate fuel gas such as methane or hydrogen using the decomposition product as a raw material.

10: Culture tank 11: Filtrate delivery pipe 12: Suspension delivery pipe 20: Pretreatment tank 21: Pretreatment liquid supply pipe 30: Fermentation tank 40: Solid-liquid separation means 41: Filtration membrane 50: Agitator 100, 101, 102: Fuel gas production device B: Organic waste C: Easily decomposable organic waste G: Fuel gas M: Liquid medium such as artificial medium (artificial saliva) W: Dilution water S: Solid waste T: Treated water P: Pump R: Fermentation residue

Claims (6)

A fuel gas production apparatus for producing a fuel gas derived from biomass,
A culture tank for culturing ruminal microorganisms;
A pretreatment tank in which the culture solution of the rumen microorganisms obtained in the culture tank is mixed with organic waste and the organic waste is treated to convert it into easily decomposable organic waste and volatile fatty acids;
a fermenter for performing fermentation using the easily decomposable organic waste and the volatile fatty acids obtained in the pretreatment tank as substrates ,
The culture tank is provided with a solid-liquid separation means for separating the culture solution into a filtrate and a suspension,
a filtrate delivery pipe for supplying the filtrate to the pretreatment tank, and a suspension delivery pipe for supplying the suspension to the pretreatment tank . The fuel gas production device according to claim 1, characterized in that the organic waste is supplied to the culture tank and the pretreatment tank at a predetermined ratio. The fuel gas production device according to claim 1 or 2, characterized in that the pretreatment liquid containing the easily decomposable organic waste and the volatile fatty acids obtained in the pretreatment tank is supplied to the culture tank. 4. The fuel gas production device according to claim 1 , wherein the device is configured to supply easily decomposable organic waste to the fermenter. A fuel gas production method for producing a biomass-derived fuel gas, comprising:
A culturing step of culturing ruminal microorganisms;
A pretreatment process in which the culture solution of the rumen microorganism obtained in the culture process is mixed with organic waste, and the organic waste is treated to convert it into easily decomposable organic waste and volatile fatty acids;
a fermentation step in which the decomposable organic waste obtained in the pretreatment step and the volatile fatty acids are fermented as substrates,
The culture step further includes a solid-liquid separation step of separating the culture solution into a filtrate and a suspension ,
A fuel gas production method comprising : a filtrate delivery step of supplying the filtrate to the pretreatment step; and a suspension delivery step of supplying the suspension to the pretreatment step . 6. The method for producing fuel gas according to claim 5 , wherein the fermentation process includes at least one of a methane fermentation process and a hydrogen fermentation process.

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