WO2010046914A1

WO2010046914A1 - Process for continuous production of biogas from biomass

Info

Publication number: WO2010046914A1
Application number: PCT/IN2009/000548
Authority: WO
Inventors: Nitant Vishnu Mate; Santosh Raghunath Gondhalekar; Arvind Purushottam Joshi
Original assignee: Kirloskar Integrated Technologies Ltd.; Gangotree Eco Technologies Pvt. Ltd
Priority date: 2008-10-06
Filing date: 2009-10-05
Publication date: 2010-04-29

Abstract

The invention presented is a method for improving biogas generation by segregation of the two stages i.e. acidogenesis and methanogenesis by means of two separate reactors and semi- permeable membrane units. The semi- permeable membrane unit permits the migration and concentration equilibration of low molecular size fatty acids but does not permit the migration of high molecular weight undigested, semi- digested feed as also of microbial cells. The transportation of volatile fatty acids is brought about by circulating the acid filtrate and the slurry containing the methanogenic microorganisms through two different semi permeable tubes placed inside a balancing tank thus permitting concentration equilibrium of volatile fatty acids with the contents of the methanogenic reactor without exchanging the two microbial populations. The overall effect is faster conversion of feed into biogas without undue loss of the methanogens or accumulation of unwanted metabolites.

Description

FIELD OF INVENTION:

The present invention relates to a process for production of biogas from biomass. More particularly it relates to a process in which the acidogenic and methanogenic micro- organisms are retained in their respective digesters and the transfer of volatile fatty acids (VFA' s) in the acidogenic digester to the methanogenic digester is brought about by passing the two streams through semi permeable passages in a balancing tank using the principle of concentration equilibration.

BACKGROUND OF INVENTION:

Generation of biogas from biomass has been one of the options for getting over the progressively worsening energy crisis. Traditionally, this route has been only used for waste utilization or effluent treatment. Attention is continuously being given to find other routes to convert biomass into energy, such as, biodiesel, ethanol and biomass gasification.

PRIOR ART:

1. Patent Application US 4022665 discloses "Two phase anaerobic digestion."

An improved two phase anaerobic digestion process in which an initial phase continually receives an organic feed for short detention times of less than two days under conditions which efficiently liquefy and breakdown the feed to lower molecular weight acids and other intermediates for conversion to methane. A succeeding phase is operated to treat the lower molecular weight acids and intermediates for detention times of about two to about seven days under conditions which efficiently lead to production of methane. The feed is loaded in the first phase at rates from about 1 to about 10 pounds of total organics per cubic foot per day; and the products from the initial phase are loaded in the succeeding phase at rates of about 0.1 to about 0.5 pounds total organics per cubic foot per day.

Drawbacks:

The outflow of the fluid through the second digester drains the slow multiplying microbial population in this digester making it necessary to increase its size so that a longer retention time ensures microbial multiplication at the same rate as its depletion.

2. Patent Application US 4696746 discloses "Two phase anaerobic digestion."

A two phase anaerobic digestion process for production of methane from organic carbonaceous material in which an active acid forming microbial population is maintained in a first acid forming digestion phase and an active methane forming microbial population is maintained in a second methane forming digestion phase, the liquid effluent from the acid forming digestion phase being passed to a first methane forming digester and gaseous product from the acid forming digestion phase being passed to a second separated methane forming digester for production of methane in the first and second methane forming digesters of the methane forming digestion phase. The two separated methane digestion phases provide increased overall methane production.

Drawbacks:

Microbial population keeps exiting through the outflow of the methanogenic digester necessitating large retention times and hence large digesters to avoid a wash out of the slow multiplying methanogens. This limits the speed of the process. 3. Indian Patent Application: 210309 discloses "A three stage biomethanation process."

A three step biomethanation process, to convert starch or sugary agricultural feed stock into a methane rich gas mixture for facilitating generation of biogas to be used as kitchen fuels, electrical power generation or transportation from renewable biomass in a cost-effective manner forms the basis of this patent. The process brings about conversion of starch-rich or sugar-rich biomass into methane through three stages, namely, hydrolysis, acidogenesis and methane formation. The invention deploys enzymes/physical/microbial parameters to hasten the hydrolytic reaction) of Carbohydrates in the first stage. It also deploys microbial consortia that have been enriched for a targeted feed in order to speed up the process of this conversion. The formation of alcohol from starch is prevented, as a result of the consortia deployed, the conversion efficiency is substantially improved and the retention time is also reduced to cut the capital cost of the plant.

Drawbacks:

Though the process is divided into three stages, it is observed that no attempts have been made to retain the microbial consortia in their respective digesters. This limits the process speed as every time the new bacteria need to be generated.

4. Patent Application: US 5529692, discloses "Method and apparatus for anaerobic biological hydrolysis and for subsequent biomethanization."

A method for the biological processing of organic substances and more particularly for anaerobic biological hydrolysis for subsequent biomethanization, with pH value control, in the case of which the dissolved and/or undissolved organic substances supplied to a first reactor are at least subjected to slight acidification in such reactor, the major part of the undissolved, at least partly acidified organic substances taken from the first reactor are supplied to a second reactor for the performance of at least one solids hydrolysis step and the main part of the dissolved, at least partly acidified organic substances from the first reactor and from the second reactor are supplied to a third reactor for the performance of at least one methanization step.

Drawbacks:

As there is no separation of microorganisms for acid digester as well as for methane digester, the consortia get lost; every time they get drained out through the output slurry. This limits the work force for conversion of biomass into methane and thus limits the speed of the process.

5. Patent Application US: 4781836 discloses "Method for biomethanation."

An improved process for the biomethanation of an organic substrate includes treating the substrate in a first reactor to form organic acid anions, passing an aqueous preparation containing dissolved organic anions through an anion exchanger so that the organic acid anions are adsorbed and separated from the remainder of the aqueous preparation, desorbing the organic acid anions and passing the desorbed acids to a second reactor containing methanogenic bacteria which convert the acids to methane. In a preferred embodiment, a bicarbonate solution is produced in the second reactor and it is used to desorb the organic acid anions and regenerate the anion exchanger into the bicarbonate form.

Drawbacks:

In this case where introduction of an ion exchange column overcomes such a loss of microbial population, the feed streams have to be interrupted when the ion exchange resins get saturated with volatile fatty acids, as also for recharging of the ion exchanger. There is a buildup of non- ionic toxins and metabolites in the methanogenic digester.

Deficiencies of prior art:

1. Most designs of biogas plants running in a continuous mode have exit streams that continuously drain microbial cells from the digesters.

2. Whereas the acidogenic micro- organisms are able to make up the cell loss through fast multiplication, the methanogenic micro- organisms cannot do so, being slow multipliers.

3. Most digester failures result from depletion of methanogenic micro- organisms.

4. In case 5 where introduction of an ion exchange column overcomes such a loss of microbial population, the feed streams have to be interrupted when the ion exchange resins get saturated with volatile fatty acids as also when the columns are to be reloaded.

5. There is no mechanism available to drain off unwanted metabolites.

OBJECT OF INVENTION:

Object of the present invention is to provide a method and apparatus for continuous production of biogas to avoid digester failure because of depletion of methanogenic micro- organisms.

Another object of present invention is to provide a method and apparatus for a continuous, fast and efficient process for biomass conversion.

SUMMARY OF INVENTION:

The present invention encompasses a process for continuous production of biogas from biomass in two separate digesters separated by a set of two semi permeable devices that are placed inside a balancing tank. The balancing tank allows acids formed in the acidogenic reactor to pass through and enter into the methanogenic reactor through a process of concentration equilibrium. Thus microbes present in the two slurries are retained in their corresponding digesters after passing through the balancing tank.

The selective transfer of volatile fatty acids from the acidogenic reactor to the methanogenic reactor without permitting either the larger molecules or microbial cells is brought about by a concentration gradient through the two semi-permeable channels placed inside the balancing tank.

The concentration of volatile fatty acids (VFA) in the acidogenic stream is much higher in acid filtrate semi permeable tube compared to that in the stream carrying the methanogenic slurry. The volatile fatty acids migrate across the membrane into water in order to bring about concentration equilibrium. As the concentration of volatile fatty acids in the balancing tank rises, another concentration gradient is formed between the water in the balancing tank and the channel carrying the methanogenic stream. The volatile fatty acids thus continuously get transported into the methanogenic digester. These then get acted upon by methanogens and get converted into methane.

Any metabolites accumulating in the methanogenic digester get transported to the acidogenic digester by a reverse concentration gradient. The fluid in the methanogenic digester does not physically exit the digester and a loss of slow multiplying methanogens is thus prevented.

The efficiency of the system can be enhanced by increasing the concentration of volatile fatty acids (i.e. by removal of water using the reverse osmosis process in the acid digester stream), by increasing the surface area of the interfacing membrane or by creating turbulence in the moving streams etc. It can also be improved by manipulating stream pressures and creating a back and forth movement of fluid across the membranes without any net migration of fluids.

Other features, advantages, and objects of the present invention will become more apparent and be more readily understood from the following detailed description, which should be read in conjunction with the accompanying drawing.

STATEMENT OF THE INVENTION

According to the present invention there is provided an apparatus for continues production of biogas from various types of biomass, the selective permeation method comprising;

an acidogenic reactor for mixing pulverized biomass with fresh water and acidogenic microorganisms to form an acidogenic slurry;

a methanogenic reactor for conversion of volatile fatty acids to methane;

a filter unit capable of separating volatile fatty acid solution and slurry from the acidogenic mix received from acidogenic reactor;

an acid filtrate tank for storing the acid received form the filter unit;

a balancing tank having first and second semi-permeable membrane tubes enclosed therein,

a first peristaltic pump for pumping the volatile fatty acid solution from the acid filtrate tank through the first semi-permeable membrane tubes of balancing tank and back to the acid filtrate tank; and a second peristaltic pump for pumping the methanogenic slurry from the methanogenic reactor through the second semi-permeable membrane tubes of balancing tank and back to the methanogenic reactor,

wherein water acting as a buffering means maintained around the first and second semi-permeable membrane tubes, thereby enabling passage of acid out of the membrane and permitting the migration of volatile fatty acids from the acid solution to the water in the balancing tank and then to methanogenic slurry without exchanging microbial populations using the principle of concentration equilibrium.

Typically, wherein the balancing tank comprises;

a chamber for enclosing;

a first semi-permeable tube enclosed in the chamber, for passage of acid solution there from;

a second semi-permeable tube enclosed in the chamber and in juxtaposition with respect to first semi-permeable tube, for passage of acidogenic slurry therefrom;

a first inlet and a first outlet openings for circulating the acid solution through the first semi-permeable openings; and

a second inlet and a second outlet openings for circulating the methanogenic slurry through the second semi-permeable openings,

wherein water is maintained in the balancing tank and around the first semipermeable tube and second semi-permeable tube for providing a passage for volatile fatty acids & maintaining the concentration gradient. Typically, wherein the acidogenic reactor comprises a stirrer for stirring the acidogenic slurry; and

a heater for heating the acidogenic reactor for maintaining the temperature of the acidogenic reactor between 30 to 45 degree centigrade.

According to the present invention there is also provided a selective permeation method comprising steps of;

feeding an acidogenic reactor with a suitable pulverized biomass, fresh water and acidogenic microorganisms;

filtering the acidogenic slurry by filter unit for separating acid solution there from;

collecting the acid solution in the acid filtrate tank for circulating there from;

pumping the acid solution by first peristaltic pump from the acid filtrate tank through the balancing tank and collecting back the acid solution to the acid filtrate tank;

simultaneously, storing the methanogenic slurry in the methanogenic reactor through ;

pumping the methanogenic slurry by second peristaltic pump from the acid filtrate reactor for circulation; and

filtering the acid solution and methanogenic slurry by a filter having one or more of semi-permeable membrane tubes enclosed in a chamber,

wherein the acid solution is allowed to pass through the first semi-permeable membrane tubes and is collected back in the acid filtrate tank, the methanogenic slurry is allowed to through the second semi-permeable membrane tube and collected back in the methanogenic reactor, water acting as a buffering means maintained around the first and second semi-permeable membrane tubes, thereby enabling passage of acid out of the membrane and permitting the migration of volatile fatty acids from the acidogenic slurry to the water in the balancing tank and then to methanogenic slurry without exchanging microbial populations using the principle of concentration equilibrium.

Typically, wherein organic matter used as feed is suitably treated using chemical, physical, or enzymatic means to convert it into suitable small units to facilitate acidogenesis.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a schematic view of an apparatus for continues production of biogas according to the present invention.

Part list

1 Acidogenic Reactor

2 Filtration Unit

3 Acid Filtrate Tank

4 Peristaltic pump for acid solution

5 Balancing tank through which two semi permeable membrane channels pass

6 Peristaltic pump for Methanogenic slurry

7 Methanogenic Reactor DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:

The foregoing objects of the invention are accomplished and the problems and shortcomings associated with prior art techniques and approaches are overcome by the present invention as described below in the preferred embodiment.

Embodiments of the invention are discussed below with reference to the Figure 1. However, those skilled in the art will readily appreciate that the detailed description given herein with respect to the figure is for explanatory purposes and may be embodied in various forms as the invention extends beyond these limited embodiments. Therefore, specific details disclosed herein are not to be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching one skilled in the art to employ the present invention in virtually any appropriately detailed system, structure or matter.

The present invention is best understood by reference to the detailed figure and description set forth herein. The following description should be read in conjunction with accompanying figure 1.

Figure 1 shows a schematic view of an apparatus for continues production of biogas according to the present invention. The acidogenic and methanogenic stages of the overall process are carried out in two separate reactors, such as acidogenic reactor (1) and methanogenic reactors (7).

At regular intervals when acidogenesis is complete, a portion of the slurry is filtered by means of a filtration unit (2) and transferred to an acid filtrate tank (3) and acidogenic reactor (1) is topped up with fresh feed. The filtration unit (2) is capable of separating slurry and acid solution from acidogenic reactor. In an embodiment, the organic matter used as feed is re-treated using chemical, physical, or enzymatic means to convert it into suitable small units to facilitate acidogenesis.

A balancing tank (5) includes first and second semi-permeable membrane tubes. The acid solution is circulated through first semi-permeable tube, and the methanogenic slurry through second semi-permeable tube. Further, water is maintained around the first and second semi-permeable tubes for balancing. The balancing tank (5) is placed between the above mentioned acid filtrate reactor (3) and the methanogenic reactor (7).

The contents of acid tank no. 3 and the methanogenic reactor (7) are then circulated through the respective semi-permeable tubes of the balancing tank (5) by means of the two peristaltic pumps and returned to the tank respective reactors. In an embodiment, the apparatus is shown to include two peristaltic pumps, such as first peristaltic pump (4) and second peristaltic pump (6). The first peristaltic pump (4) pumps the acid solution from the acid filtrate tank (3) through the first semipermeable membrane of the balancing tank (5) and back to the acid filtrate reactor (3). The second peristaltic pump (6) pumps the methanogenic slurry from the methanogenic reactor (7) through the second semi-permeable membrane tube of the balancing tank (5). The water maintained around the first and second semi-permeable tubes is used to channel the volatile fatty acids (VFA' s) from acid solution to the methanogenic slurry.

In an embodiment, the balancing tank (5) includes a chamber, first and second semipermeable tubes, first inlet and first outlet, and second inlet and second outlet. The plurality of semi-permeable tubes is enclosed in the chamber. The first inlet and first outlet, and second inlet and second outlet are configured on the chamber. The acid solution from the acid filtrate tank (3) is passed thought the first inlet to enter into the first semi-permeable membrane tube of the balancing tank (5). The first outlet is provided for egressing the used acid solution out of the first semi-permeable membrane tube of the balancing tank (5). Ok

Further, the methanogenic slurry from the methanogenic reactor (7) is passed thought the second inlet to enter into the chamber and flows through the second semipermeable membrane tube of the balancing tank (5). The second outlet is provided for egressing the used methanogenic slurry through second semi-permeable membrane tube of the balancing tank (5).

The present invention also includes a method for continuous production of biogas from various types of biomass, a method comprising steps of feeding an acidogenic reactor (1) with purlverised feed, fresh water and acidogenic micro-organisms. Further, filtering the acidogenic slurry by filter unit (2) for separating acid solution from the slurry. Thereafter, collecting the acid solution in the acid filtrate tank (3) for circulating therefrom. Then pumping the acid solution by first peristaltic pump (4) from the acid filtrate tank through the balancing tank (5) and collecting back the acid solution to the acid filtrate tank.

Simultaneously, pumping the methanogenic slurry by second peristaltic pump from the methanogenic reactor for circulation. Care is taken to ensure that the pressures inside the two re-circulating stream and the balancing tank match to ensure that there is no migration of liquids in and out of the semi- permeable surfaces.

Therefore, the acid solution is allowed to pass through the first semi-permeable membrane tubes and is collected back in the acid filtrate tank (3). The methanogenic slurry is allowed to pass through the second semi-permeable membrane tubes and collected back in the methanogenic reactor (7). The water acting as a buffering means, is maintained around the first and second semi-permeable membrane tubes for enabling passage of acid and permitting the migration of volatile fatty acids from the acid solution to the water in the balancing and then to methanogenic slurry without exchanging microbial populations using the principle of concentration equilibrium.

The volatile fatty acid concentration of the stream, on entering the methanogenic reactor (7) brings about a fast conversion of these acids into methane by the progressively increasing numbers of methanogens.

The overall effect is to bring about a much faster conversion of feed into biogas without undue loss of the methanogens.

Suitably treated fresh feed may be mixed up with fresh water/ return stream to the acidogenic tank (1) for further acidogenesis.

The return stream to the acidogenic tank (1) may be drained to the extent fresh water is used to keep the volume of the recirculating stream constant and to allow for wash off of accumulating metabolites.

The embodiments of the invention as described above and the methods disclosed herein will suggest further modification and alterations to those skilled in the art. Such further modifications and alterations may be made without departing from the spirit and scope of the invention.

The invention is further described with the help of following illustration.

Illustration 1:

In the following illustration acetic acid is used for conversion into biogas.

The tank 7 is filled with an active methanogenic culture obtained from a biogas plant in routine operation. 20,000 ppm acetic acid is circulated through acid filtrate semi permeable tube and methane filtrate is circulated through methane filtrate semi permeable tube. The two streams are started using the two peristaltic pumps 4 and 6. The pressures in the circulating streams and the balancing tank and matched to ensure that there is no migration of fluids across the membrane surface.

Gas generation is observed after 8 hours.

The experiment was repeated daily for 8 days and showed conversion of acetic acid to methane in a reproducible manner

ADVANTAGES OF PRESENT INVENTION:

The method gets over the problems faced in the prior art of suboptimal conditions of a single digester and the risk of digester failure.

As the micro organisms are retained in their respective digesters, the process speed is improved. This results in reduction of capital cost over the conventional digesters, for getting the same output.

Any unwanted metabolites formed in the methanogenic digester are removed by a reverse concentration gradient.

The process of biogas production is continuous i.e without any break.

The invention is thus to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the following claims.

Claims

We Claim:

1. An apparatus for continuous production of biogas from various types of biomass, the apparatus comprising; an acidogenic reactor for mixing pulverized feed with fresh water and acidogenic culture; a methanogenic reactor for conversion of volatile fatty acids to methane therein; a filter unit capable of separating acid solution from acidogenic reactor; an acid filtrate tank capable of storing the acid solution received from the filter unit; a balancing tank having first and second semi-permeable membrane tubes enclosed therein, a first peristaltic pump for pumping the acid solution from the acid filtrate reactor through the first semi-permeable membrane tube of balancing tank and back to the acid filtrate reactor; and a second peristaltic pump for pumping the methanogenic slurry from the methanogenic reactor through the second semi-permeable membrane tubes of balancing tank back to the methanogenic reactor, wherein water acting as a buffering means maintained around the first and second semi-permeable membrane tubes, thereby enabling passage of volatile fatty acids out of the membrane and permitting the migration of volatile fatty acids from the acidogenic slurry to the water in the balancing tank and then to methanogenic slurry without exchanging microbial populations using the principle of concentration equilibrium.

2. The apparatus as claimed in claim 1, wherein the balancing tank comprises; a chamber for enclosing; a first semi-permeable tube enclosed in the chamber, for passage of acid solution therefrom; a second semi-permeable tube enclosed in the chamber in juxtaposition with respect to first semi-permeable tube, for passage of methanogenic slurry therefrom; a first inlet and a first outlet openings for circulating the acid solution through the first semi-permeable openings; and a second inlet and a second outlet openings for circulating the methanogenic slurry through the second semi-permeable openings, wherein water is maintained in the balancing tank around the first semipermeable tube and second semi-permeable tube to enable concentration equilibrium of volatile fatty acids, pH metabolites etc.

3. The apparatus as claimed in claim 1, wherein the acidogenic reactor comprises a stirrer for stirring the contents; and

4. The apparatus as claimed in claim 1, wherein organic matter used as feed is pre-treated using chemical, physical, or enzymatic means to convert it into suitable small units to facilitate acidogenesis.

5. A method for continuous production of biogas from various types of biomass, the method comprising steps of; feeding an acidogenic reactor with a biomass, fresh water and acidogenic microorganisms; filtering the acidogenic slurry by filter unit for separating acid solution and slurry therefrom; collecting the acid solution in the acid filtrate tank for circulating therefrom; pumping the acid solution by first peristaltic pump from the acid filtrate tank through the balancing tank and collecting back the acid solution to the acid filtrate reactor; pumping the methanogenic slurry by second peristaltic pump from the methanogenic reactor for circulation; and permeating the acid solution through plurality of semi-permeable membrane tubes enclosed in a chamber, wherein the acid solution is allowed to pass through the first semi-permeable membrane tube and is collected back in the acid filtrate tank, the methanogenic slurry is allowed to pass through the second semi-permeable membrane tube and collected back in the methanogenic reactor, water acting as a means to facilitate transport maintained around the first and second semi-permeable membrane tubes, thereby enabling passage of volatile fatty acids out of the membrane and permitting the migration of volatile fatty acids from the acid solution to the water for balancing and then to methanogenic slurry without exchanging microbial populations using the principle of concentration equilibrium.

6. The method of claim 1, wherein organic matter used as feed is pre-treated using chemical, physical, or enzymatic means to convert it into suitable small units to facilitate acidogenesis.