FIELD OF INVENTION:

The present invention relates to the pretreatment of agricultural residues, used as feeds in the biogas plants for generating biogas.

BACKGROUND OF THE INVENTION:

With the supply of fossil fuels getting progressively scarce, there is an increasing trend to generate energy from renewable sources. Several approaches have been tried for this purpose such as conversion of vegetable oils to produce bio-diesel, conversion of corn or other starch rich feeds to produce bio-ethanol, utilization of biomass by gasification technique and the like. Use of this biomass has created some side effects in the energy market. The large scale diversion of corn for ethanol production has been viewed as a major factor leading to reduced availability of foods and hence, an increase in world food prices.

Generation of biogas from biomass has been mainly used as a means of waste utilization or of effluent treatment. This route has recently been found attractive as a commercial source of energy and a good deal of work is reported.

A variety of grasses and agricultural residues have been tried in attempts to convert these into methane as a source of energy. One of the most popular feeds tried is maize whole plant. It has been used in European experiments in the form of silage to ensure round the year availability. As a practice, maize is fed in the form of whole plant-leaves, stems, grain, and cob included. A screw type device is commonly used for introducing the biomass into the biogas plant from its bottom. Special agitators are used to break the scum and ensure a continuous contact of the bio-matter with microbial consortia that serially convert the degradable matter into biogas. After a predetermined residence time, the unconverted biomass exits the digester from the bottom without disturbing the anaerobic conditions.

The biomass is partially processed before feeding into the digester. Mostly, the whole plant is cut into pieces and silaged. One of the major problems is that the feed in question tends to float in water and presents difficulty in its conversion to biogas. For operating a continuous or a continuous batch-fed biogas plant, the feed needs to enter the digester from its bottom to keep the system in anaerobic condition. The floating matter needs special designs for ensuring this condition. The material tends to form a scum once entered.

The present invention, in which a combination of physical and mechanical means, such as size reduction and agitation have been made to overcome this problem, helps create pumpable slurry of biomass that can enter the biogas plant from its bottom inlet and reduces or eliminates scum formation. Thus, the difficulties experienced in prior art are overcome.

PRIORART:

1) Emerging trends in small scale biogas plants for agro residues and biomass feedstock - A case study from India.

By H N Chanakya and Rene Moletta at

'Centre for Sustainable Technologies (formerly ASTRA Centre), Indian Institute of Science, Bangalore- 560012, India, chanakva@astra.iisc.ernet.in &

'Laboratory for Environmental Biotechnology', INRA, Narbonne, 11100, France, moletta@ensam.inra.fr

Abstract: A case study is presented on emerging trends in small scale biogas plants for agri-residues and biomass. Work done at five different Indian institutions was reviewed. Data on a number of plant designs that can use biomass in place of cow dung is presented.

A specific reference is made to continuously floating biomass in the biogas digester.

Drawback:

Floating biomass has been identified as a major drawback for operating the plants in a continuous manner. No appropriate method is suggested to settle the biomass in the digester.

2) Patent Application WO/2007/059688-discloses" A method and an apparatus to produce biogas."

A method and an apparatus to produce biogas with garbage and sewage and plant are provided, whose advantage is continuous production on a scale. The processing comprises the following steps: loading the material to be treated and basic substance into a liquid permeable container; sending the container into a sieving tank to sieve and screen; sending the container into a pretreatment tank for heating and sterilizing; sending the container into a sealed biogas generating tank for fermentation; extracting the container from the tank after finishing the fermentation and sending the new container filled with the material to be treated; opening the extracted container to separate the residue therein.

Drawback:

No pretreatment is done to the feed. When agricultural residues are used, it is necessary to be able to handle huge amounts of biomass. The process described circumvents the problem by mechanically moving small packets of enclosed biomass through the reactor. Method is difficult to upscale.

3) Patent Application WO/2005/061114 discloses "A method and plant for pre- treatment of source separated wet organic waste."

Method and plant for pre-treatment of source separated wet organic waste from house holds, institutional house holds etc., comprising shredding of the waste in a coarse shredder unit, allowing the shredded material to fall down to a fine dissolver unit where more heavy objects are removed, adding a process liquid until a desired content of solids of 6 to 12 % is achieved, shredding the mixture

with a rapid rotating paddle mechanism, transferring the mixture to a separation unit for plastics, and transportation of the remainder mixture into a plant for production of biogas and decomposing remainder.

Drawback:

The method is used specifically for the organic waste from households and not for agricultural residues as such. The organic material is only shredded and screw conveyers are used for transportation. The feed material is not prone to formation of a scum and no mechanism is proposed for either preparing a pumpable slurry or for de-flotation of scum.

4) Patent Application WO 2006017991 discloses -"Stepped sequential treatment method for municipal domestic refuse."

The present invention provides a treatment method of municipal domestic refuse. In the method the organic matter processes an anaerobic fermentation; the obtained methane can be helpful to burning to generate electricity. The biogas residue from the anaerobic fermentation can be used as a culture material for growing edible mushrooms. The residue discharged from edible mushrooms can be used to cultivate earthworm. Besides the organics, the other substance of the municipal domestic refuse will be incinerated to generate electricity. The present invention realizes a comprehensive utilization of waste resource.

Drawback:

The process is for municipal or household waste disposal that does not present difficulties in slurrification or of scum formation. There is no special pretreatment for the feedstock for producing biogas. The output of one stage is utilized for the next sequential stage.

5) Patent Application WO/2004/076082 discloses "Process for treatment of an organic waste material."

The present invention relates to a process for treatment of an organic waste material, the process comprising the steps of subjecting said organic waste material to a heat treatment process, where the bio-availability of a resulting hydrolyzed product is increased for later treatment, and of subjecting said hydrolyzed product to at least one anaerobic fermentation process whereby biogas is produced and an organic residue is formed, wherein said process furthermore comprises a first step of providing said organic waste material in a solid form to said heat treatment process. The present invention furthermore relates to a process for treatment of an organic waste material wherein the aforementioned process furthermore comprises a fourth and a fifth step of separating said organic residue into a liquid phase and a solid phase, and combusting said solid phase of said organic residue at a maximum temperature of 900 ⁰ C resulting in liberation of thermal energy and concentrated combustion residue, and the formation of flue gasses.

Drawback:

The method suggests only 'heating' as the part of the pre treatment. No attempt is made to convert floating agricultural produce to pumpable slurry. The process is not aimed at preventing scum accumulation.

Other Drawbacks in Prior art:

The processes have large retention time for feeds into the digester. It ranges from 20 to 40 days. The longer retention time increases the capital cost accordingly.

The biomass floating in the digester has to be regularly pushed down in the body of the digester to ensure a proper contact. Even after deploying specially designed agitators, the contact with the digester medium is not appropriate.

Plants designed for feeds that can be readily fed in the form of slurry cannot be used for agricultural residue as feeds.

When scum formation and accumulation continues at the surface of the digester, the conversion efficiency goes down.

OBJECT OF THE INVENTION:

Object of the present invention is to provide a method which uses physical and mechanical means to change floating nature of the agricultural residue to a bottom settling sludge and enables to form slurry that can be pumped in easily.

Another object of the present invention is to provide a method that ensures an appropriate contact between the biomass and the microbial culture medium.

Yet another object of the present invention is to provide a method that ensures any scum formed on the surface is made to sink, whereby there is continuous contact of the biological matter forming the scum with the microbial consortia.

Yet another object of the present invention is to provide a method which reduces the capital cost of the equipment.

SUMMARY OF INVENTION:

The present invention describes physical and mechanical methods for changing the floating nature of agricultural residues to bottom settling sludge to form slurry which can be pumped in easily.

The invention ensures that scum formed on the surface is made to sink by continuous contact of the biological matter with microbial consortia.

STATEMENT OF THE INEVNTION

According to the present invention there is provided a method for treatment of agricultural residue into a pumpable slurry, method comprises steps of; reducing the particle size of the agricultural residue to form a pumpable slurry;

introducing the pumpable slurry to the base of a digester using a mud pump or sludge pump; regulated stirring the floating slurry for removing tiny air bubbles therefrom, thereby resulting in sinking of the floatated agricultural residue for ensuring a continuous good contact between the agricultural residue and the microbial culture medium.

Other features, advantages, and objects of the present invention will become more apparent and be more readily understood from the following detailed description and illustrations set forth herein.

DETAILED DESCRIPTION OF PRESENT INVENTION:

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 described in the present embodiment. However, those skilled in the art will readily appreciate that the detailed description given herein 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 describes a process that uses physical and mechanical methods to change floating nature of the agricultural residue to a bottom settling sludge and enables formation of slurry that can be pumped easily. This is ensured in the first place by reducing the particle size of the agricultural produce suitably so that the organic matters form pumpable slurry. The slurry thus formed can be easily introduced to the base of the digester using a mud pump or sludge pump and the digester does not need any special design, for inlets and outlets or screw mechanisms for the slurry.

In the early stages of acidogenesis of the organic matter, there is some gas formation that floatates the organic particles as is observed in normal froth floatation. At this stage the tiny gas bubbles floatating the organic matter are dislodged by periodical agitation that results in sinking of the floatated organic matter. This ensures a continuous good contact between the biomass and the microbial culture medium. Exit of the biomass from the plant is simple as well.

It will be further apparent that a number of additional changes and modification may be made without departing from the spirit and scope of the present invention. Therefore, the invention should not be limited except by the claims.

The invention is further described with the help of following illustrations:

Illustration 1

In the following illustration sorghum is used as a substrate.

Given sorghum sample is pulverized into particle size less than 6mm.

20 gm of sample that has passed through a 6 mm sieve is mixed with 500 ml of water and 100 ml of acid culture (dung solution) is added to this mixture. On stirring for 4 hours the biomass settles down to slurry that can be pumped through the bottom inlet of a standard biogas plant.

During the course of the next 20 hours a portion of the biomass is lifted to the surface by the gas bubbles generated during acidogenesis. The contents of the reactors are stirred at 250 rpm for a minute. This results in total disappearance of the scum and on excellent contact of the biomass with the microbial consortia in the reactor. ^"

Illustration 2

In the following illustration Napier Grass is used as a substrate

Given Napier grass sample is pulverized into particle size less than 6mm

20 gm of sample that has passed through a 6 mm sieve is mixed with 500 ml of water and 100 ml of acid culture (dung solution) is added to this mixture. On stirring for 4 hours the biomass settles down to slurry that can be pumped through the bottom inlet of a standard biogas plant.

During the course of the next 20 hours a portion of the biomass is lifted to the surface by the gas bubbles generated during acidogenesis. The contents of the reactors are stirred at 250 rpm for 30 minutes. This results in total disappearance of the scum and on excellent contact of the biomass with the microbial consortia in the reactor.

Illustration 3

In the following illustration sorghum is used as a substrate.

Given sorghum sample is pulverized into different particle sizes of 1.6 mm, 6 mm and 2 cm.

20 gm of each particle size sample is mixed with 500 ml of water and 100 ml of acid culture (dung solution) is added to this mixture. The reactors are subjected to continuous mechanical stirring at approximately 250 rpm. Floating layer and settled layer of substrate from each reactor are noted at appropriate time interval.

Details of difference between floating layers of 1.6 mm, 6 mm and 2 cm particle size samples at different time intervals of experiment is seen in the following table, thus confirming the said process.

2 cm particle size sample shows a floating layer of 3.7 cm at 0 hour of experiment. Even after continuous stirring at 250 rpm floating layer from this reactor does not become zero. On addition of dung culture to the same reactor acidogenesis is accelerated with good gas generation which causes some particles to float at the surface after 24 hrs of experiment.

1.6 mm particle size sample shows a floating layer of 2.2 cm at 0 hour of experiment. After 1 hour of continuous stirring at 250 rpm same sample shows a floating layer of 0 cm.

6 mm particle size sample shows a floating layer of 1.7 cm at 0 hour of experiment which became 0 cm after 4 hours of continuous mechanical stirring at 250 rpm.

Addition of 100 ml dung solution to both the reactors at 8 hours of experiment accelerates the acidogenic process. At 24 hours of experiment (at 1440 minutes), gas generation is observed due to acidogenesis which makes 6 mm particle size sample float at the surface. This floating layer is broken in a minute using

mechanical stirring. The 1.6 mm particle size sample does not form any scum as compared to 6 mm particle size sample.

ADVANTAGES OF THE PRESENT INVENTION:

Any conventional digester can be used for biogas generation. No specific design such as screw conveyers is required.

Once the agricultural residue is properly pretreated, the further processing of the microbial digestion is simplified.

The proper pre-treatment effectively results in reduced retention time of the feedstock that leads to a cost-saving of capital, as the biogas plant size gets reduced.