AU2022433034A1 - Methane generation system and methane generation method - Google Patents

Abstract

This methane generation system (1) comprises an alkali treatment unit (20) for bringing a lignocellulose biomass and an alkali solution into contact with one another to generate an alkali treatment solution containing a first solid component and a first liquid component, a saccharification unit (40) for decomposing the cellulose and/or hemicellulose derived from the lignocellulose biomass contained in the alkali treatment solution using saccharifying bacteria to generate a saccharified solution containing a second liquid component, an acid generation unit (60) for generating an organic acid from a mixed solution containing the first liquid component and the second liquid component using acidogenic bacteria, and a methane generation unit (70) for generating methane from the organic acid using methanogens.

Description

DESCRIPTION TITLE OF THE INVENTION: METHANE GENERATION SYSTEM AND METHANE GENERATION METHOD TECHNICAL FIELD

[0001]

The present disclosure relates to a methane generation system and a methane

generation method.

BACKGROUND ART

[0002]

In recent years, the use of renewable energy such as biomass, instead of fossil

resources such as natural gas, has been promoted in order to reduce the increase in the carbon

dioxide concentration in the atmosphere. In addition, a method is known in which a

bioalcohol is generated as a biofuel by microbial fermentation of a monosaccharide obtained

through hydrolysis of cellulose and hemicellulose in biomass.

[0003] Patent Literature 1 discloses a method for generating a sugar including a step of wet

pulverizing herbaceous biomass, a step of bringing the pulverized biomass into contact with

a basic compound, and a step of performing a saccharification treatment on alkali-treated

biomass using an enzyme. Patent Literature 1 also discloses that an alcohol can be generated

by fermenting sugar.

CITATION LIST PATENT LITERATURE

[0004]

Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2015

70822

SUMMARY OF THE INVENTION TECHNICAL PROBLEM

[0005]

In the conventional method, biomass is subjected to a saccharification treatment

using an enzyme. However, common enzymes are expensive. In contrast, a saccharification

treatment may be performed using saccharifying bacteria instead of enzymes. However,

when generating an alcohol, it is necessary to use microorganisms such as yeast and use a

monosaccharide as a substrate for alcoholic fermentation. Accordingly, when saccharifying

bacteria are used, a monosaccharide generated from biomass may be decomposed by

saccharifying bacteria, and the amount of the substrate for alcoholic fermentation may be

reduced. Accordingly, when a saccharification liquid is generated using saccharifying

bacteria, the amount of the alcohol recovered as a biofuel from biomass may be reduced. In

contrast, methane is used as a raw material for various chemical syntheses in addition to

biofuels.

[0006] Thus, an object of the present disclosure is to provide a methane generation system

and a methane generation method capable of efficiently generating methane from

lignocellulosic biomass using microorganisms.

SOLUTION TO PROBLEM

[0007]

A methane generation system according to the present disclosure includes an alkaline

treatment unit that brings lignocellulosic biomass into contact with an alkaline liquid to

generate an alkaline treatment liquid including a first solid component and a first liquid

component, a saccharification unit that generates a saccharification liquid including a second

liquid component by decomposing, using saccharifying bacteria, at least one of cellulose or

hemicellulose derived from the lignocellulosic biomass and included in the first solid

component, an acid generation unit that generates, using acidogenic bacteria, an organic acid

from a mixed liquid including the first liquid component and the second liquid component,

and a methane generation unit that generates methane from the organic acid using

methanogenic bacteria.

[0008]

The methane generation unit may be supplied with biogas generated by the

saccharifying bacteria.

[0009] The methane generation system may include a first solid-liquid separation unit that

separates the alkaline treatment liquid into a first solid component and the first liquid

component, the first solid component may be supplied to the saccharification unit, and the

first liquid component may be supplied to the acid generation unit.

[0010]

The first solid-liquid separation unit may include a first screen and a first ejection

part that ejects the alkaline treatment liquid onto the first screen, and may separate a liquid

component which has passed through the first screen, as the first liquid component, and a

solid component remaining on the first screen, as the first solid component.

[0011]

The methane generation system may include a grinding unit that grinds the

lignocellulosic biomass, and the alkaline treatment unit may bring the lignocellulosic

biomass ground by the grinding unit into contact with the alkaline liquid to generate the

alkaline treatment liquid.

[0012]

The methane generation system may include a second solid-liquid separation unit

that separates the saccharification liquid into a second solid component and the second liquid

component, the second solid component may be supplied to at least one of the grinding unit

or the saccharification unit, and the second liquid component may be supplied to the acid

generation unit.

[0013]

The second solid-liquid separation unit may include a second screen and a second

ejection part that ejects the saccharification liquid onto the second screen, and may separate

a liquid component which has passed through the second screen, as the second liquid

component, and a solid component remaining on the second screen, as the second solid

component.

[0014]

The acid generation unit may include a mixing part that mixes the first liquid

component and the second liquid component to generate the mixed liquid and decomposes

at least one selected from the group consisting of cellulose, hemicellulose, and an

oligosaccharide included in the first liquid component, using saccharifying bacteria which

has flowed from the saccharification unit, and a housing part that houses the acidogenic

bacteria and generates the organic acid from the mixed liquid supplied from the mixing part.

[0015]

The acid generation unit may include a first flow path through which the first liquid

component passes, a second flow path through which the second liquid component passes,

and a housing part that is arranged between the first flow path and the second flow path,

houses the acidogenic bacteria, and generates the organic acid from the mixed liquid

generated by mixing the first liquid component supplied from the first flow path and the

second liquid component supplied from the second flow path.

[0016]

The methane generation unit may be supplied with biogas generated by the

acidogenic bacteria.

[0017]

The lignocellulosic biomass may include food waste.

[0018]

A methane generation method according to the present disclosure includes a step of

bringing lignocellulosic biomass into contact with an alkaline liquid to generate an alkaline

treatment liquid including a first solid component and a first liquid component, a step of

generating a saccharification liquid including a second liquid component by decomposing,

using saccharifying bacteria, at least one of cellulose or hemicellulose derived from the

lignocellulosic biomass and included in the first solid component, a step of generating, using

acidogenic bacteria, an organic acid from a mixed liquid including the first liquid component

and the second liquid component, and a step of generating methane from the organic acid

using methanogenic bacteria.

ADVANTAGEOUS EFFECTS OF INVENTION

[0019]

The present disclosure provides a methane generation system and a methane

generation method capable of efficiently generating methane from lignocellulosic biomass

using microorganisms.

BRIEF DESCRIPTION OF DRAWINGS

[0020]

[FIG. 1] FIG. 1 is a schematic diagram illustrating a methane generation system according

to one embodiment.

[FIG. 2] FIG. 2 is a schematic diagram illustrating a first solid-liquid separation unit or a

second solid-liquid separation unit according to one embodiment.

[FIG. 3] FIG. 3 is a schematic diagram illustrating an acid generation unit according to one

embodiment.

DESCRIPTION OF EMBODIMENTS

[0021]

Some exemplary embodiments will be described below with reference to the

drawings. Note that dimensional ratios in the drawings are exaggerated for convenience of

the description and are sometimes different from actual ratios.

[0022]

As illustrated in FIG. 1, a methane generation system 1 according to the present

embodiment includes a grinding unit 10, an alkaline treatment unit 20, a first solid-liquid

separation unit 30, a saccharification unit 40, a second solid-liquid separation unit 50, an

acid generation unit 60, a methane generation unit 70, and an aerobic treatment unit 80. Note

that the solid line in FIG. 1 indicates a liquid flow path through which a liquid component

flows, the dash-dot line indicates a solid flow path through which a solid component flows,

and the dashed line indicates a gas flow path through which a gas component flows.

[0023]

The grinding unit 10 grinds lignocellulosic biomass. By grinding lignocellulosic

biomass in the grinding unit 10, the vascular bundle and parenchyma of the lignocellulosic

biomass can be destroyed. Accordingly, the treatment efficiency of the lignocellulosic

biomass can be improved in the process after the grinding unit 10. The shape of the lignocellulosic biomass before the treatment in the grinding unit 10 may be, for example, powder, particles, fibers, chips, plates, or flakes.

[0024]

The lignocellulosic biomass is biomass including lignocellulose. The lignocellulose

includes at least one selected from the group consisting of cellulose, hemicellulose, and

lignin. The lignocellulosic biomass may include at least one selected from the group

consisting of grass and woody biomass, its processing products, and its waste. The grass and

woody biomass may include at least one of herbaceous biomass or woody biomass. The

herbaceous biomass may include at least one selected from the group consisting of oil palm,

rice, wheat, banana, sugar cane, corn, cassava, sago palm, nipa palm, yam, sorghum, and

potato. The woody biomass may include at least one selected from the group consisting of

cedar, cypress, pine, eucalyptus, and beech.

[0025]

The lignocellulosic biomass may include food waste. Some of the food waste is

treated through incineration and landfilling, but by using them as biomass, garbage can be

reduced and resources can be effectively utilized. Food waste may include industrial waste,

general waste, and mixtures thereof. General waste may include general business waste,

general household waste, and a mixture thereof. Industrial waste means items obtained

secondarily in the course of food manufacturing, processing, or cooking that cannot be used

for human consumption. Examples of industrial waste include food waste from food

processing plants. General business waste means food waste from business that is discarded

after the food has been served for human consumption or without being served for human

consumption. Examples of general business waste include food waste from restaurants.

General household waste means food waste from household that is discarded after the food

has been served for human consumption or without being served for human consumption.

Examples of general household waste include food waste such as kitchen waste from

household. Specifically, food waste may include at least one selected from the group

consisting of beer barley dregs, tea dregs, coffee husks, soybean dregs, bran, fruit dregs, and

vegetable dregs.

[0026]

The grinding unit 10 may include a wet grinding machine or a dry grinding machine.

When the grinding unit 10 is a wet grinding machine, the lignocellulosic biomass can be

efficiently micronized. Especially, when the lignocellulosic biomass includes food waste,

the food waste can be efficiently made even smaller.

[0027]

The grinding unit 10 may include a whetstone. The material of the whetstone may be

a non-porous whetstone. The grinding unit 10 may include a stone mill grinding machine. A

stone mill grinding machine can finely grind lignocellulosic biomass. In addition, a stone

mill grinding machine can operate at a lower power than a machine such as a ball mill. The

grinding unit 10 may be a wet stone mill grinding machine. The whetstone may include an

upper grinder and a lower grinder. The upper grinder and the lower grinder are arranged

opposite each other across a clearance, and either one of the upper grinder or the lower

grinder may be rotatably provided. The upper grinder and the lower grinder may have a

circular shape with an opening in the center. When lignocellulosic biomass is supplied

through an opening in the center of the circle, the lignocellulosic biomass may be discharged

as ground matter from the outer peripheral edge of the circle while being ground in the

clearance by rotation of at least one of the upper grinder or the lower grinder.

[0028]

The setting value of the clearance in the wet stone mill grinding machine may be

within a range of -200 m to +100 m. When the clearance setting value is in this range, the particle size can be made smaller while the reduction in the processing speed of

lignocellulosic biomass is suppressed. The rotational speed of the wet stone mill grinding

machine may be within a range of 1,500 rpm to 2,500 rpm. When the rotational speed is in

this range, the particle size can be also made smaller while the reduction in the processing

speed of lignocellulosic biomass is suppressed. The average particle size of the

lignocellulosic biomass after grinding may be 12 m or less, or 11 m or less. The average

particle size of the lignocellulosic biomass after grinding may be 1 m or more. Note that

the average particle size is a median diameter based on the number measured using a laser

diffraction scattering method.

[0029]

The alkaline treatment unit 20 brings lignocellulosic biomass into contact with an

alkaline liquid to generate an alkaline treatment liquid. In the present embodiment, the

alkaline treatment unit 20 brings lignocellulosic biomass which has been ground in the

grinding unit 10 into contact with an alkaline liquid to generate an alkaline treatment liquid.

By bringing the lignocellulosic biomass into contact with the alkaline liquid, proteins and

lipids of the lignocellulosic biomass are dissolved in the alkaline liquid. Thus, cellulose and

hemicellulose in the lignocellulosic biomass are exposed, and an enzyme generated by the

saccharifying bacteria in the saccharification unit 40 is easily accessible to the cellulose and

hemicellulose. Consequently, the decomposition efficiency of the cellulose and

hemicellulose can be improved. In particular, since food waste often includes a large amount

of proteins and lipids and there is a high possibility that access to cellulose and hemicellulose

by an enzyme is inhibited, the effect of the alkaline treatment is considered to be large.

[0030] In addition, by performing alkaline treatment before saccharification treatment, the

decay of lignocellulosic biomass can be suppressed due to the bactericidal effect of the

alkaline liquid. Thus, the contamination of the saccharification unit 40 with various bacteria

can be suppressed. In particular, since food waste often includes a large amount of proteins

and lipids, the contribution of the bactericidal effect of the alkaline liquid increases. By

sterilization with the alkaline liquid, the properties of lignocellulosic biomass can be

maintained and stable saccharification treatment can be performed.

[0031]

The alkaline liquid may be an aqueous solution in which at least one selected from

the group consisting of an alkali metal hydroxide, an alkali metal carbonate, an alkali metal

hydrogen carbonate, an alkali earth metal hydroxide, an alkali earth metal carbonate, and

ammonia is dissolved. The alkali metal may be sodium or potassium. The alkaline earth

metal may be calcium or magnesium. Among these, the alkaline liquid is preferably a sodium

hydroxide aqueous solution. The amount of sodium hydroxide contained in the alkaline

liquid may be within a range of 0.045 g to 0.20 g relative to 1 g of the dry solid fraction of

lignocellulosic biomass. The alkaline liquid may be added in such a manner that the dry solid

fraction of lignocellulosic biomass is within a range of 1% to 5% by weight.

[0032]

In the alkaline treatment, which is the contact between the lignocellulosic biomass and the alkaline liquid, the lignocellulosic biomass may be immersed in the alkaline liquid, or the alkaline liquid may be poured over the lignocellulosic biomass. The temperature of the alkaline treatment may be within a range of 60 °C to 80 °C. When the alkaline treatment temperature is in such a temperature range, proteins and lipids dissolution, and sterilization can be performed simultaneously. The time of alkaline treatment may be within a range of 12 hours or more and 20 hours or less. By setting the time of alkaline treatment to 12 hours or more, the amount of remaining dissolved proteins and lipids can be reduced. Further, by setting the time of alkaline treatment time to 20 hours or less, the treatment cost for the dissolution rate of proteins and lipids can be reduced. Note that the reaction temperature of the alkaline treatment is preferably 60 °C or more, and the alkaline treatment time is preferably 12 hours or more continuously. By performing the alkaline treatment under such conditions, the sterilization of spore-forming bacteria resistant to alkaline liquids can be promoted.

[0033] The alkaline treatment liquid includes a first solid component and a first liquid component. In the alkaline treatment unit 20, proteins and lipids in the lignocellulosic biomass are dissolved in the alkaline liquid by contact between the lignocellulosic biomass and the alkaline liquid. Thus, the first solid component includes at least one of cellulose or hemicellulose. The first liquid component includes at least one of proteins or lipids. Further, the first liquid component includes at least one of a monosaccharide or an oligosaccharide. The alkaline treatment liquid generated in the alkaline treatment unit 20 is supplied to the first solid-liquid separation unit 30.

[0034] The first solid-liquid separation unit 30 separates the alkaline treatment liquid into a first solid component and a first liquid component. The separation method at the first solid liquid separation unit 30 is not particularly limited, and the first solid component and the first liquid component may be separated using a centrifugal separation method, a precipitation method, or the like. However, in the centrifugal separation method, the power required to drive the apparatus is large, and the driving cost may be high. In the precipitation method using a precipitation tank, when the particle size of the solid component is small or the ionization repulsion between particles of the solid component is large, the solid component is difficult to precipitate, and the solid-liquid separation may not be performed efficiently. In addition, when the first liquid component includes a large amount of sugar, the viscosity of the first liquid component may increase, and the first solid component may become difficult to precipitate. Accordingly, the first solid-liquid separation unit 30 may separate the first solid component and the first liquid component using a screen.

[0035]

As illustrated in FIG. 2, the first solid-liquid separation unit 30 may include a first

screen 31 and a first ejection part 32 that ejects an alkaline treatment liquid onto the first

screen 31. The first solid-liquid separation unit 30 may separate a liquid component that has

passed through the first screen 31 as the first liquid component and a solid component

remaining on the first screen 31 as the first solid component. In the separation method using

the first screen 31, since the alkaline treatment liquid is ejected onto the first screen 31 for

solid-liquid separation, the driving cost can be reduced and the treatment efficiency can be

increased compared with the centrifugal separation method and the precipitation method.

[0036] The first ejection part 32 may be arranged at a first end which is one end of the first

screen 31. The first ejection part 32 may be arranged to eject the alkaline treatment liquid

substantially parallel to the plane of the first screen 31 at the first end.

[0037]

The first screen 31 may have a flat shape, but is curved in an arc shape. With this

configuration, when the first ejection part 32 ejects the alkaline treatment liquid toward the

first screen 31, the solid component remains on the first screen 31 and flows down as

illustrated with the dash-dot line, and the liquid component penetrates the first screen 31 as

illustrated with the solid line. Thus, the first solid component and thefirst liquid component

can be separated more efficiently. The first screen 31 may include multiple wires, and wires

extending in a straight line in one direction may be arranged in an arc shape in a direction

perpendicular to the one direction and may each be arranged across multiple holes or slits.

The wires may be metal wedge wires having a wedge-shaped cross section as viewed from

the extending direction.

[0038]

The pore size or slit width of the first screen 31 may be within a range of 5 m to

100 tm. When the pore size or slit width is 5 m or more, clogging is less likely to occur

and the processing speed of solid-liquid separation is high. The pore size or slit width may

be 20 m or more. When the pore size or slit width is 100 m or less, most of the cellulose

and hemicellulose can be supplied to the saccharification unit 40, and the lignocellulosic

biomass can be effectively utilized. The pore size or slit width may be 75 m or less. Note

that the slit width means the width of the portion where the distance between adjacent wires

is the smallest. The pore size means the diameter where the size of a line segment passing

through the center is the smallest.

[0039] The first ejection part 32 may include a nozzle, and the alkaline treatment liquid may

be ejected from the nozzle toward the first screen 31 at high pressure. The discharge pressure

of the alkaline treatment liquid at the first ejection part 32 may be within a range of 0.2 MPa

or more and 0.3 MPa or less.

[0040]

The methane generation system 1 may include a single first solid-liquid separation

unit 30 or multiple first solid-liquid separation units 30. In this case, among the multiple first

solid-liquid separation units 30, the pore size or slit width of the first screen 31 at the first

stage may be set larger than the pore size or slit width of the first screen 31 at the second

stage. This can improve the separation efficiency of the alkaline treatment liquid. For

example, the pore size or slit width of the first screen 31 at the first stage may be set to 75

jam, and the pore size or slit width of the first screen 31 at the second stage may be set to 40

jam.

[0041]

The first liquid component separated by the first solid-liquid separation unit 30 is

supplied to the acid generation unit 60. The first liquid component includes a

monosaccharide, proteins, and lipids. These can serve as substrates when acidogenic bacteria

generate an organic acid. In contrast, the first solid component separated in the first solid

liquid separation unit 30 is supplied to the saccharification unit 40. The first solid component

includes at least one of cellulose or hemicellulose. Thus, by supplying the first solid

component to the saccharification unit 40, at least one of cellulose or hemicellulose can be decomposed in the saccharification unit 40 to generate a saccharification liquid. The first solid component also includes lignin.

[0042]

By adding water to the first solid component separated in the first solid-liquid

separation unit 30, a diluted solution may be generated. An acid such as hydrochloric acid is

added to the diluted solution to neutralize it to a pH suitable for processing in the

saccharification unit 40. The amount of the dry solid fraction of the first solid component

contained in the diluted solution may be within a range of 1% by weight or more and 4% by

weight or less. The amount of the dry solid fraction contained may be 1.5% by weight or

more. The amount of the dry solid fraction contained may be 2.5% by weight or less.

[0043]

The saccharification unit 40 decomposes, using saccharifying bacteria, at least one

of cellulose or hemicellulose derived from lignocellulosic biomass included in the first solid

component to generate a saccharification liquid. The saccharifying bacteria secrete an

enzyme that decomposes at least one of cellulose or hemicellulose. Thus, by performing the

saccharification treatment using saccharifying bacteria, a saccharification liquid can be

generated without using expensive commercial enzymes for saccharification. At least one

of cellulose or hemicellulose is decomposed by the secreted enzyme, and a saccharification

liquid including at least one of a monosaccharide or an oligosaccharide in which 2 to 10

monosaccharides are bonded is generated. The monosaccharide may include at least one of

pentose or hexose.

[0044]

The saccharifying bacteria may be a single strain or a group of bacteria including

multiple strains. Examples of the strain include a strain of the genus Paenibacillus. Such a

strain is particularly suitable for saccharification of food waste. The culture temperature of

the strain is preferably medium temperature or higher and more preferably within a range of

40 °C or more and 50 °C or less. The culture period in the saccharification unit 40 may be

two days or more. The saccharifying bacteria may include anaerobic cellulolytic bacteria in

addition to a strain of the genus Paenibacillus. It is preferable that the anaerobic cellulolytic

bacteria can be cultured at a culture temperature similar to that of the strain. Examples of

such anaerobic cellulolytic bacteria include Ruminiclostridium josui, Ruminiclostridium cellulolyticum, Ruminiclostridium herbifermentans, Ruminiclostridium papyrosolvens, and

Acetivibrio clariflavus.

[0045]

The saccharifying bacteria can grow using a sugar obtained by decomposing

cellulose and hemicellulose. If the saccharification liquid including the saccharifying

bacteria is not withdrawn at a rate higher than the growth rate of the saccharifying bacteria,

the concentration of the bacteria in the saccharification unit 40 can be maintained without

adding additional saccharifying bacteria. Accordingly, the saccharification treatment can be

continuously performed at a lower cost than that using a commercial enzyme. In addition,

the commercial enzyme may be decomposed by a proteolytic enzyme generated by various

bacteria. In contrast, in the case of the saccharification treatment using saccharifying bacteria,

the saccharifying bacteria can suppress the increase of various bacteria. Thus, in the case of

the saccharification using saccharifying bacteria, the equipment cost for reducing the

contamination of various bacteria can be reduced compared with the case using the

commercial enzyme. In particular, when the culture temperature of the saccharifying bacteria

is set to a medium temperature or higher, the growth of various bacteria abundant in the

natural world can be suppressed, and the contamination risk can be further reduced.

[0046]

In addition, the saccharifying bacteria may decompose a monosaccharide and

generate an organic acid. In the case of alcoholic fermentation, it is necessary to use a

monosaccharide as a substrate, but in the methane generation system 1 according to the

present embodiment, methane is generated. The organic acid can be used as a substrate for

methane fermentation. Thus, even when a monosaccharide is decomposed by saccharifying

bacteria and an organic acid is generated, methane can be generated.

[0047]

The saccharification liquid generated by saccharifying bacteria includes a second

solid component and a second liquid component. The second solid component includes

cellulose, hemicellulose, proteins, and lipids that remain dissolved in the saccharification

liquid. The second liquid component includes at least one of a monosaccharide or an

oligosaccharide. The saccharification liquid generated in the saccharification unit 40 is

supplied to the second solid-liquid separation unit 50.

[0048]

The second solid-liquid separation unit 50 separates the saccharification liquid into

the second solid component and the second liquid component. The structure of the second

solid-liquid separation unit 50 may be the same as that of the first solid-liquid separation unit

30. That is, the second solid-liquid separation unit 50 may separate the second solid

component and the second liquid component using a centrifugal separation method, a

precipitation method, or a separation method using a screen.

[0049]

As illustrated in FIG. 2, the second solid-liquid separation unit 50 may include a

second screen 51 and a second ejection part 52 that ejects the saccharification liquid onto the

second screen 51. The second solid-liquid separation unit 50 may separate a liquid

component that has passed through the second screen 51 as the second liquid component and

a solid component remaining on the second screen 51 as the second solid component. In the

separation method using the second screen 51, since the saccharification liquid is ejected

onto the second screen 51 for solid-liquid separation, the driving cost can be reduced and the

precipitation efficiency can be increased compared with the centrifugation method and the

precipitation method. The second solid-liquid separation unit 50 employed can be similar to

the first solid-liquid separation unit 30. Thus, the second screen 51 and the second ejection

part 52 employed may be similar to the first screen 31 and the first ejection part 32,

respectively.

[0050]

Note that the second liquid component may include a solid component having a small

particle size. In particular, when the saccharifying bacteria grown in the saccharification unit

40 is a several m in size, they may be separated into the second liquid component as

suspended matter. However, when the culture environment changes due to a decrease in

temperature or the like, the saccharifying bacteria dissolve and are decomposed by

acidogenic bacteria or methanogenic bacteria. In addition, proteins and lipids that are not

dissolved in the alkaline liquid to remain as suspended matter are also decomposed by

acidogenic bacteria. Thus, the second liquid component including the suspended matter

moves to the acid generation unit 60, leading to an increase in the methanation efficiency.

Note that since the residue with completely solubilized cellulose includes a large amount of

lignin, it may be used as a raw material for bioplastics.

[0051]

The second solid component may be supplied to at least one of the grinding unit 10

or the saccharification unit 40. When the second solid component is returned to the grinding

unit 10, cellulose and hemicellulose which have not been fully decomposed in the

saccharification unit 40 are ground again and the crystallinity decreases, and thus further

saccharification can be expected in the saccharification unit 40. In addition, when the second

solid component is returned to the saccharification unit 40, a large amount of saccharifying

bacteria can be maintained, and thus the saccharification efficiency may be further improved.

The second solid component may be reused by conversion to feed or the like, incinerated to

recover thermal energy, or disposed of by landfilling or the like. Since the second solid

component includes a large amount of lignin, it may be used as a raw material for bioplastics.

In contrast, the second liquid component is supplied to the acid generation unit 60.

[0052]

The acid generation unit 60 generates, using the acidogenic bacteria, an organic acid

from a mixed liquid including the first liquid component and the second liquid component

which has been included in the saccharification liquid. The first liquid component includes

an oligosaccharide, proteins, and lipids dissolved in the alkaline liquid in the alkaline

treatment unit 20. The first liquid component may include suspended matter that has not

been dissolved in the alkaline liquid and remains as small fine particles. The second liquid

component may contain an oligosaccharide left without being decomposed cellulose and

hemicellulose into a monosaccharide. The oligosaccharide, proteins, and lipids included in

the first liquid component and the second liquid component may not be sufficiently

decomposed by methanogenic bacteria alone.

[0053]

In contrast, acidogenic bacteria can generate an organic acid through anaerobic

fermentation using an oligosaccharide, proteins, and lipids. By generating an organic acid as

a substrate for methane fermentation in the acid generation unit 60, the amount of methane

generated in the methane generation unit 70 can be improved. In addition, proteins and lipids

are not easily oxidized to carbon dioxide by acidogenic bacteria, and proteins and lipids are not easily decomposed excessively. Thus, the target organic acid can be supplied to the methane generation unit 70, and the amount of methane generated in the methane generation unit 70 can be improved.

[0054]

As illustrated in FIG. 3, the acid generation unit 60 may include a first flow path 61,

a second flow path 62, a mixing part 63, and a housing part 64. The first flow path 61 is

connected to the alkaline treatment unit 20 via the first solid-liquid separation unit 30. The

first liquid component passes through the first flow path 61. The second flow path 62 is

connected to the saccharification unit 40 through the second solid-liquid separation unit 50.

The second liquid component passes through the second flow path 62.

[0055]

The mixing part 63 mixes the first liquid component supplied from the first flow path

61 and the second liquid component supplied from the second flow path 62 to generate a

mixed liquid. Since the alkaline treatment liquid passing through the first flow path 61 is

treated with an alkaline liquid, the pH is large. In contrast, the pH of the saccharification

liquid passing through the second flow path 62 is reduced due to an acidic substance

generated by saccharifying bacteria. Thus, by mixing the first liquid component and the

second liquid component in the mixing part 63, the pH can be adjusted without adding an

excessive amount of a pH adjuster. Accordingly, the neutralization treatment of alkali can

be reduced, and the environmental load can be reduced. The mixing part 63 may have a

stirring part, and the first liquid component and the second liquid component may be stirred

using the stirring part. The mixed liquid is not particularly limited as long as it has a pH

suitable for acidogenic bacteria, but it may be neutral or may have a pH of 6 to 8. When the

mixed liquid is neutral only by mixing the first and second liquid components, no pH adjuster

is required. The pH adjuster may be hydrochloric acid or acidic factory wastewater. The

residence time of the mixed liquid in the mixing part 63 may be within a range of 1 day or

more and 5 days or less. The mixed liquid generated in the mixing part 63 is supplied to the

housing part 64.

[0056]

The housing part 64 houses acidogenic bacteria. The acidogenic bacteria may include

multiple groups of bacteria. It is sufficient that the temperature in the housing part 64 is a temperature suitable for the acidogenic bacteria, and it may be, for example, within a range of 30 °C or more and 40 °C or less. The culture period is not particularly limited but may be within a range of 1 day or more and 3 days or less. The housing part 64 may have a stirring part, and the mixed liquid supplied from the mixing part 63 may be stirred using the stirring part.

[0057]

The acid generation unit 60 may include the mixing part 63 and the housing part 64.

The mixing part 63 may generate a mixed liquid by mixing the first liquid component and

the second liquid component and decompose at least one selected from the group consisting

of cellulose, hemicellulose, and an oligosaccharide included in the first liquid component,

using saccharifying bacteria which has flowed from the saccharification unit 40. The housing

part 64 houses acidogenic bacteria and generates an organic acid from the mixed liquid

supplied from the mixing part 63.

[0058]

The first liquid component may include an oligosaccharide that has not been

decomposed to a monosaccharide. The first liquid component may also include some

cellulose and hemicellulose which have dissolved in the alkaline liquid. In contrast, the

second liquid component may include saccharifying bacteria. The saccharifying bacteria can

decompose not only cellulose and hemicellulose but also oligosaccharides. Thus, by mixing

the first liquid component and the second liquid component in the mixing part 63, the

saccharifying bacteria included in the second liquid component can decompose cellulose,

hemicellulose, and an oligosaccharide included in the first liquid component. A decomposed

low-molecular-weight substance can be used as a substrate or a nutrient for acidogenic

bacteria or methanogenic bacteria. Thus, the temperature of the mixing part 63 is preferably

suitable for saccharifying bacteria, and may be, for example, within a range of 40 °C or more

and 50 °C or less.

[0059]

The acid generation unit 60 may include the first flow path 61 through which the first

liquid component passes and the second flow path 62 through which the second liquid

component passes. The acid generation unit 60 may include the housing part 64 that is

arranged between the first flow path 61 and the second flow path 62, houses acidogenic bacteria, and generates an organic acid from a mixed liquid generated by mixing the first liquid component supplied from the first flow path 61 and the second liquid component supplied from the second flow path 62.

[0060] By arranging the housing part 64 between the first flow path 61 and the second flow path 62, the first liquid component flowing in the first flow path 61 and the second liquid component flowing in the second flow path 62 are subjected to heat exchange with the mixed liquid in the housing part 64. Thus, the mixed liquid in the housing part 64 can be heated by the first liquid component flowing in the first flow path 61 and the second liquid component flowing at a temperature in the second flow path 62. For example, when the temperature of the first liquid component is 70 °C and the temperature of the second liquid component is 45 °C, the temperature of the housing part 64 can be maintained at about 35 °C. By arranging the housing part 64 between the first flow path 61 and the second flow path 62, the culture temperature of the acidogenic bacteria is optimized, and the energy required for heating can be reduced.

[0061] Part of the first flow path 61 and the second flow path 62 may be formed by the outer wall of the acid generation unit 60. The temperature of at least one of the first liquid component passing through the first flow path 61 or the second liquid component passing through the second flow path 62 may be higher than the optimum temperature of the acidogenic bacteria. Thus, by forming part of the first flow path 61 and the second flow path 62 using the outer wall of the acid generation unit 60, thefirst liquid component flowing in the first flow path 61 and the second liquid component flowing in the second flow path 62 can be subjected to heat exchange with air outside the acid generation unit 60. Accordingly, the first liquid component flowing in the first flow path 61 and the second liquid component flowing in the second flow path 62 can be efficiently cooled.

[0062] The methane generation unit 70 generates methane from an organic acid using methanogenic bacteria. The methane obtained in the methane generation unit 70 can be used as an energy source for power plants, boilers, and the like. In the methane generation system 1 according to the present embodiment, since methane is generated in the methane generation unit 70, it is not necessary to concentrate the fermented alcohol in a distillation column, such as for bio-alcohol. Accordingly, lignocellulosic biomass can be effectively utilized with less energy.

[0063] The methane generation unit 70 discharges biogas including methane. The methane

generated can be recovered and used as a biofuel derived from lignocellulosic biomass.

Methane can also be used as a raw material for various chemical syntheses. In contrast, a

liquid component is also discharged from the methane generation unit 70. Since the liquid

component discharged from the methane generation unit 70 includes acidogenic bacteria, at

least part of the liquid discharged from the methane generation unit 70 may be transferred to

the housing part 64 of the acid generation unit 60. By planting the acidogenic bacteria in the

housing part 64, the generation of an organic acid can be promoted.

[0064]

The methanogenic bacteria may include high-temperature methanogenic bacteria

active at high temperatures, such as within a range of 50 °C to 60 °C, and may include

mesophilic methanogenic bacteria active at moderate temperatures, such as within a range

of 35 °C to 38 °C. When high-temperature methanogenic bacteria are used, the methane

fermentation time can be shortened. When low-temperature methane bacteria are used, the

temperature required for heating the fermenter can be reduced.

[0065]

The methane generation unit 70 may be supplied with biogas generated by

saccharifying bacteria. The first solid component supplied to the saccharification unit 40

includes proteins and lipids left undissolved in the alkaline liquid. Since saccharifying

bacteria are used in the saccharification unit 40, proteins and lipids are decomposed under

anaerobic conditions to generate biogas including at least one of an organic acid or hydrogen.

Volatilization of organic acids causes odor. Thus, by supplying the biogas generated in the

saccharification unit 40 to the methane generation unit 70, the organic acid is not directly

discharged to the atmosphere, and thus the odor can be suppressed. The methanogenic

bacteria can generate methane from the organic acid, hydrogen, and carbon dioxide. Thus,

by supplying the biogas generated in the saccharification unit 40 to the methane generation

unit 70, the recovery rate of methane can be improved.

[0066] The methane generation unit 70 may be supplied with biogas generated by

acidogenic bacteria. The biogas may be supplied to the methane generation unit 70 through

a gas flow path, and the liquid including an organic acid generated in the acid generation

unit 60 may be supplied to the methane generation unit 70 through a liquid flow path. The

mixed liquid in the acid generation unit 60 includes proteins and lipids. The acidogenic

bacteria decompose proteins and lipids to generate biogas including an organic acid,

hydrogen, and carbon dioxide. Accordingly, by supplying the biogas generated in the acid

generation unit 60 to the methane generation unit 70, the organic acid is not directly released

to the atmosphere, and thus the odor can be suppressed. The methanogenic bacteria can

generate methane from the organic acid, hydrogen, and carbon dioxide. Thus, by supplying

the biogas generated in the acid generation unit 60 to the methane generation unit 70, the

recovery rate of methane can be improved.

[0067]

The biogas supplied from at least one of the saccharification unit 40 or the acid

generation unit 60 may be supplied from the lower part of the methane generation unit 70 by

aeration. When the biogas is made to pass through the mixed liquid in the methane generation

unit 70, the biogas dissolves in the mixed liquid. The organic acid, hydrogen, and carbon

dioxide dissolved in the mixed liquid are consumed by methanogenic bacteria to generate

methane. The organic acid, hydrogen, and carbon dioxide in the mixed liquid are consumed

by methanogenic bacteria and their concentration decreases, and thus the biogas can continue

to dissolve in the mixed liquid even when the biogas is continuously made to pass through

the mixed liquid.

[0068] Methane fermentation using hydrogen is performed in a reaction such as 4H2 + C02

-> CH4 + 2H20. Accordingly, the alkali concentration of the alkaline liquid may be reduced

in the alkaline treatment unit 20 to suppress the dissolution of proteins and lipids.

Consequently, the amount of proteins and lipids decomposed in the saccharification unit 40

can be increased, and the amount of hydrogen generated in the saccharification unit 40 and

supplied to the methane generation unit 70 can be increased. The methane generation unit

70 may be supplied with carbon dioxide. Accordingly, the conversion efficiency of methane generated from hydrogen can be improved. The carbon dioxide supplied to the methane generation unit 70 may be, for example, carbon dioxide derived from fossil fuel. This can reduce the amount of carbon dioxide released into the atmosphere.

[0069] Methane fermentation using an organic acid generates carbon dioxide. Thus, at least

part of the gas including carbon dioxide discharged from the methane generation unit 70

may be supplied to the methane generation unit 70. Accordingly, methane can be generated

by reacting carbon dioxide with hydrogen. Consequently, the amount of carbon dioxide

discharged from the methane generation unit 70 into the atmosphere can be reduced and the

amount of methane generated can be increased.

[0070]

The aerobic treatment unit 80 can treat the liquid component discharged from the

methane generation unit 70 with a known activated sludge treatment method using aerobic

microorganisms. Thus, the organic matter remaining in the liquid component can be

decomposed into carbon dioxide, water, and the like, and it becomes possible to discharge

them into rivers, oceans, and the like. When the liquid component discharged from the

methane generation unit 70 is discharged into the sewage, the aerobic treatment unit 80 may

not be provided.

[0071]

Note that the grinding unit 10, the alkaline treatment unit 20, the first solid-liquid

separation unit 30, the saccharification unit 40, the second solid-liquid separation unit 50,

the acid generation unit 60, and the methane generation unit 70 may each be a batch type in

which the supply and discharge of the raw material are repeated as one unit, or may be a

continuous type in which the supply and discharge of the raw material are simultaneously

performed continuously.

[0072]

As described above, the methane generation system 1 according to the present

embodiment includes the alkaline treatment unit 20, the saccharification unit 40, the acid

generation unit 60, and the methane generation unit 70. The alkaline treatment unit 20 brings

lignocellulosic biomass into contact with an alkaline liquid to generate an alkaline treatment

liquid including a first solid component and a first liquid component. The saccharification unit 40 decomposes, using saccharifying bacteria, at least one of cellulose or hemicellulose derived from the lignocellulosic biomass and included in the first solid component to generate a saccharification liquid including a second liquid component. The acid generation unit 60 generates, using acidogenic bacteria, an organic acid from a mixed liquid including the first liquid component and the second liquid component. The methane generation unit 70 generates methane from the organic acid using methanogenic bacteria.

[0073]

The methane generation method according to the present embodiment includes a step

of bringing lignocellulosic biomass into contact with an alkaline liquid to generate an

alkaline treatment liquid including a first solid component and a first liquid component. The

method includes a step of generating a saccharification liquid including a second liquid

component by decomposing, using saccharifying bacteria, at least one of cellulose or

hemicellulose derived from lignocellulosic biomass and included in the first solid component.

The method includes a step of generating, using acidogenic bacteria, an organic acid from a

mixed liquid including the first liquid component and the second liquid component. The

method includes a step of generating methane from the organic acid using methanogenic

bacteria.

[0074]

In the alkaline treatment unit 20, a monosaccharide, proteins, and lipids in the

lignocellulosic biomass are dissolved in the alkaline liquid. Accordingly, an enzyme

generated by saccharifying bacteria is easily accessible to cellulose and hemicellulose, and

the decomposition efficiency of the cellulose and hemicellulose is improved.

[0075]

The monosaccharide, proteins, and lipids included in the first liquid component and

the second liquid component can be used as a substrate for acidogenic bacteria. Acidogenic

bacteria and methanogenic bacteria can also use, as a substrate, a low-molecular-weight

substance whose monosaccharide has been decomposed by saccharifying bacteria. Thus, the

lignocellulosic biomass can be effectively utilized to generate methane. Consequently, the

methane generation system 1 and the methane generation method according to the present

embodiment, can efficiently generate methane from lignocellulosic biomass using

microorganisms.

[0076]

The entire contents of Japanese Patent Application No. 2022-004297 (filed January

14, 2022) are incorporated herein by reference.

[0077]

Although several embodiments have been described, modifications and variations

thereof are possible based on the above-described disclosure. All components of the above

embodiments and all features described in the claims may be individually extracted and

combined as long as they do not conflict with each other.

[0078]

The present disclosure can contribute, for example, to goal 7 "Ensure access to

affordable, reliable, sustainable and modern energy for all", and goal 12 "Ensure sustainable

consumption and production patterns".

REFERENCE SIGNS LIST

[0079]

1 Methane generation system

10 Grinding unit

20 Alkaline treatment unit

30 First solid-liquid separation unit

31 First screen

32 First ejection part

40 Saccharification unit

50 Second solid-liquid separation unit

51 Second screen

52 Second ejection part

60 Acid generation unit

61 First flow path

62 Second flow path

63 Mixing part

64 Housing part

70 Methane generation unit

Claims (12)

1. [Claim 1]

   A methane generation system, comprising:

   an alkaline treatment unit that brings lignocellulosic biomass into contact with an

   alkaline liquid to generate an alkaline treatment liquid including a first solid component and

   a first liquid component;

   a saccharification unit that generates a saccharification liquid including a second

   liquid component by decomposing, using saccharifying bacteria, at least one of cellulose or

   hemicellulose derived from the lignocellulosic biomass and included in the first solid

   component;

   an acid generation unit that generates, using acidogenic bacteria, an organic acid

   from a mixed liquid including the first liquid component and the second liquid component;

   and

   a methane generation unit that generates methane from the organic acid using

   methanogenic bacteria.
2. [Claim 2]

   The methane generation system according to claim 1, wherein the methane

   generation unit is supplied with biogas generated by the saccharifying bacteria.
3. [Claim 3]

   The methane generation system according to claim 1 or 2, further comprising:

   a first solid-liquid separation unit that separates the alkaline treatment liquid into the

   first solid component and the first liquid component, wherein

   the first solid component is supplied to the saccharification unit, and

   the first liquid component is supplied to the acid generation unit.
4. [Claim 4]

   The methane generation system according to claim 3, wherein the first solid-liquid

   separation unit: includes a first screen and a first ejection part that ejects the alkaline

   treatment liquid onto the first screen; and separates a liquid component which has passed

   through the first screen, as the first liquid component, and a solid component remaining on

   the first screen, as the first solid component.
5. [Claim 5]

   The methane generation system according to any one of claims 1 to 4, further

   comprising:

   a grinding unit that grinds the lignocellulosic biomass, wherein

   the alkaline treatment unit brings the lignocellulosic biomass ground by the grinding

   unit into contact with the alkaline liquid to generate the alkaline treatment liquid.
6. [Claim 6]

   The methane generation system according to claim 5, further comprising:

   a second solid-liquid separation unit that separates the saccharification liquid into a

   second solid component and the second liquid component, wherein

   the second solid component is supplied to at least one of the grinding unit or the

   saccharification unit, and

   the second liquid component is supplied to the acid generation unit.
7. [Claim 7]

   The methane generation system according to claim 6, wherein

   the second solid-liquid separation unit: includes a second screen and a second

   ejection part that ejects the saccharification liquid onto the second screen; and separates a

   liquid component which has passed through the second screen, as the second liquid

   component, and a solid component remaining on the second screen, as the second solid

   component.
8. [Claim 8]

   The methane generation system according to any one of claims 1 to 7, wherein

   the acid generation unit includes:

   a mixing part that mixes the first liquid component and the second liquid component

   to generate the mixed liquid and decomposes at least one selected from the group consisting

   of cellulose, hemicellulose, and an oligosaccharide included in the first liquid component,

   using the saccharifying bacteria which has flowed from the saccharification unit; and

   a housing part that houses the acidogenic bacteria and generates the organic acid

   from the mixed liquid supplied from the mixing part.
9. [Claim 9]

   The methane generation system according to any one of claims 1 to 8, wherein the acid generation unit includes: a first flow path through which the first liquid component passes; a second flow path through which the second liquid component passes; and a housing part that is arranged between the first flow path and the second flow path, houses the acidogenic bacteria, and generates the organic acid from the mixed liquid generated by mixing the first liquid component supplied from the first flow path and the second liquid component supplied from the second flow path.
10. [Claim 10] The methane generation system according to any one of claims 1 to 9, wherein the methane generation unit is supplied with biogas generated by the acidogenic bacteria.
11. [Claim 11] The methane generation system according to any one of claims 1 to 10, wherein the lignocellulosic biomass includes food waste.
12. [Claim 12] A methane generation method, comprising: a step of bringing lignocellulosic biomass into contact with an alkaline liquid to generate an alkaline treatment liquid including a first solid component and a first liquid component; a step of generating a saccharification liquid including a second liquid component by decomposing, using saccharifying bacteria, at least one of cellulose or hemicellulose derived from the lignocellulosic biomass and included in the first solid component; a step of generating, using acidogenic bacteria, an organic acid from a mixed liquid including the first liquid component and the second liquid component; and a step of generating methane from the organic acid using methanogenic bacteria.