Description:Field of the invention

The present invention relates to a field of biogas production and more particularly relates to the process and an apparatus for producing biogas from Lignocellulosic biomass such as agricultural crop residue like rice straw, wheat straw, maize straw, soybean trash and like, or the energy crops like Napier grass, bamboo grass and the like.

Background of the invention

Biogas production by anaerobic digestion is an attractive approach for producing clean fuel biomethane and reducing the environmental pollution. However, many fluctuations in quality and quantity of biogas production occur during the biochemical processes.

Crop residue such as rice straw, wheat straw, maize straw or even bamboo is composed of high quantity of cellulose & hemicellulose, which is tough material for production of biogas. In conventional biogas production process, this Lignocellulosic biomass is pretreated chemically, mechanically or biologically, and fed to the digester having anerobic conditions. The chemical or biological or mechanical pretreatment requires lot of resources. Specifically, biogas is produced by microorganisms under anaerobic conditions, that decomposes cellulose and hemicellulose to produce carbon dioxide and methane. The cellulose and hemicellulose are first converted to glucose in the hydrolysis step, and the glucose is further decomposed to acetic acid by acidogenic and acetogenic bacteria. The acetic acid undergoes further decomposition to produce biogas that is a mixture of carbon dioxide and methane.

In the prior art technology of biogas generation, it is considered that anaerobic digestion of Lignocellulosic biomass is a green technology since it provides a better alternative for waste utilization as well as for reducing greenhouse gas emissions.

Further, in prior art processes of biogas production, most of the bio methanation plants, operate at mesophilic range of temperature of about 35 to 40 degree centigrade, while few operate at thermophilic range temperature of about 52 to 56 degree centigrade. Each temperature range has some advantages as well as some disadvantages, like higher energy consumption in thermophilic range, while lower bacterial activity for lower temperature range.

However, biogas production using Lignocellulosic biomass including crop residue like rice straw, wheat straw, maize straw is not economically viable as the biogas produced per ton of dry matter of rice straw is about 60 to 120 kg of biomethane. Further, there is no consistency in quality & quantity of biogas production in conventional biogas production process.

Accordingly, there is a need of a process and an apparatus for producing biogas from Lignocellulosic biomass including crop residue which overcomes the above-mentioned drawbacks of the prior art.

Objective of the invention

An object of the present invention is to increase quantity of production of biogas from Lignocellulosic biomass, including the crop residue.

Another object of the present invention is to eliminate the chemical & biological pretreatment of the Lignocellulosic biomass, that otherwise is carried out before the feedstock is fed to the digester,

Yet another object of the present invention is to eliminate chemical & biological pretreatment thereby reducing time as well as the natural resources, that are required for the bio methanation.

Summary of the invention

Accordingly, the present invention provides a process for producing biogas from Lignocellulosic biomass such as crop residue. The process comprises shredding the Lignocellulosic biomass and adding the Lignocellulosic biomass into a digester. Specifically, the digester includes a liquid microbial culture maintained therein, in a form of a slurry.

The process further comprises pumping out 10% to 50% of the slurry from the digester in a first tank and oxygenating the slurry of the first tank to enrich the slurry between 2 mg/lit to 20 mg/liter level of dissolved oxygen therein.

Finally, the process includes recirculating the oxygenated slurry back into the digester, wherein the quantity of biogas produced in the digester is increased between 10% to 80%, due to the addition of dissolved oxygen in the slurry.

Brief description of drawings.

Figure 1 shows a flowchart of a process for producing biogas from lignocellulosic biomass, in accordance with the present invention.

Detailed description of the invention

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

The present invention provides a process and an apparatus for producing biogas from Lignocellulosic biomass such as crop residue in a cost effective, efficient and user-friendly manner. The process and apparatus produce more quantity of biogas from the same quantity of biomass, as that compared to the conventional biogas production processes.

In one aspect, the present invention provides a process (100) for producing biogas from Lignocellulosic biomass such as crop residue. In an embodiment, the biomass includes but not limited to rice straw, wheat straw, cotton straw, soybean trash, sugarcane bagasse or similar crop residues, as well as energy crops like Napier grass, bamboo grass or similar cultivated energy crops. The process (100) is shown in flowchart of figure 1, in accordance with an embodiment of the present invention.

At first step (10), the process (100) for producing biogas, in accordance to the present invention, comprises shredding of the crop residue. In an embodiment, the crop residue is shredded in a mechanical shredder equipment. Shredding increases the surface area of the Lignocellulosic biomass. This increases the exposure of cellulose and hemi cellulose to the microbial consortium, making it easy for the bacteria to access it and digest.

At second step (20), the process (100) comprises adding the shredded crop residue into a digester. Specifically, the digester includes a liquid microbial culture maintained therein at predefined temperature. Specifically, the shredded Lignocellulosic biomass is added in the digester, mixed using the agitators inside the digesters, to form a part of the digester slurry.

In an embodiment, the slurry of the digester comprises of consortium of bacteria, including the hydrolytic type, acidogenic type, acetogenic and methanogenic bacteria. It also comprises of the required macro and micro nutrients including nitrogen, phosphorus, potassium, sulphur, calcium, magnesium, and also the trace nutrients required for the metabolism of the bacteria. Most of the nutrients are available from the plant biomass that the bacteria consume. Excluding the consortium of bacteria and all types of nutrients, the digester slurry is mostly composed of water, which is about 90% in quantity.

In an embodiment, the proportion of quantity of solid biomass fed in the slurry of the digester is 1 kg to 20 kg of solid Lignocellulosic biomass, per cubic meter of the digester slurry.

In an embodiment, the slurry of the digester is maintained at 52 degrees centigrade to 56 degrees centigrade.

In an embodiment, the slurry comprises of macro nutrients like Nitrogen (N) Phosphorus (P) & Potassium (K) where the absolute quantity of Nitrogen (N) is between 1,500 to 3,000 ppm per liter of digester slurry.

At third step (30), the process (100) comprises pumping out of the digester slurry, from 10% to 50% of the total quantity, per day, from the digester and put it in the first tank, called oxygenation tank.

At fourth step (40), the process (100) comprises oxygenating the slurry of the first tank, to enrich it with oxygen, between 2 mg/lit to 20 mg/liter of dissolved oxygen level therein. In an embodiment, the slurry of the first tank is oxygenated for about 1 hr to 3 hrs., till the dissolved oxygen value reaches the desired quantity. In an embodiment, the oxygen from the air or liquid oxygen is used to oxygenate the slurry.

At fifth step (50), the process (100) comprises recirculating the oxygenated slurry back into the digester.

In preferred embodiment, the quantity of biogas produced in the digester is increased by about 10% to 80%, per day, over the conventional prior art processes, because of additional effect of oxygenation, which is from 2 mg/lit to 10 mg/lit, per day, that is calculated within the total digester slurry volume.

Specifically, the biogas from the digester is collected in the balloon, then purified to enrich methane to above 90% purity, by removing the unwanted gases from the biogas, and the purified methane gas is compressed and used at step (60).

In another aspect, the present invention provides an apparatus for producing biogas from Lignocellulosic biomass. The apparatus is described in conjunction with process described in first aspect of the present invention. Specifically, the biomass is selected from a group consisting of but not limiting to grass, rice straw, wheat straw, Napier grass, bamboo and the like. However, any other digestible biomass can be used as per other alternative embodiments of the present invention. The apparatus comprises a shredder, a digester and an oxygenation tank.

The shredder is a mechanical shredder known in the art which reduces the size of the biomass into small pieces which are easy for handling purpose. Further, the shredded biomass increases surface area thereof which increases the accessibility of food for the bacteria, for improved digestion.

The digester includes a liquid microbial culture maintained therein. The digester receives shredded biomass from the shredder. In an embodiment, the digester is a continuous stirred tank reactor. To maintain the Total Solids (TS) of the digester slurry constant, between 8% to 12%, every day part of the slurry is passed through the Solid Liquid Separator equipment, where the permeate is put back into the digester and the digestate is removed from the system. The additional advantage of the process is that the microbial count is almost unaltered. Here, the digester slurry is not discarded from the digester, but only the digestate solids are removed from the system.

The oxygenating tank is operably connected to the digester. The slurry from the digester, in quantity of 10% to 50% of the total volume of the slurry in the digester, is taken out and put in the oxygenation tank. In the oxygenation tank, the slurry is oxygenated by purging the oxygen extracted from the air or using the stored liquid oxygen. Specifically, the slurry is oxygenated to reach the dissolved oxygen level between 2 mg/lit to 20 mg/liter, therein.

Once the slurry is oxygenated, the slurry is recirculated back into the digester. The oxygen supports in increasing the bacterial count of acidogenic bacteria, as well as provide energy to enhance hydrolysis of Lignocellulosic biomass.

In prior art, when 1 ton of solid shredded Lignocellulosic biomass, including rice straw is fed to the digester, the approximate biomethane produced is between 60 kg to 120 kg, which is without oxygenating the slurry.

In accordance with the present invention, when 1 ton of solid shredded Lignocellulosic biomass, including rice straw is fed to the digester, and digester slurry is oxygenated, by 2 mg/lit to 10 mg/lit, equivalent dissolved oxygen level, then the approximate biomethane produced would be 130 to 200 kg, which is about 10% to 80% higher than the prior art processes.

Accordingly, the present invention provides that oxygenating the slurry of the digester increases the production of biogas. This is because the oxygen energizes the ATP molecules that triggers the hydrolysis of the cellulose & hemi cellulose biomass. Also, the oxygen promotes bacterial growth of acidogenic bacteria, that later produces more enzymes, viz., cellulase enzyme to hydrolyze the cellulose molecule. As more glucose is produced due to better hydrolysis, the higher biogas quantity gets produced.

Example

The 50 kg rice straw is shredded using the mechanical shredder equipment and added to the CSTR (continuous stirred-tank reactor) type digester, per day, having 5,000 liters of microbial slurry, therein. The microbial slurry with shredded rice straw is stirred with the interval of 10 minutes every hour, using a stirrer. The temperature of the digester is maintained between 52 to 56 degrees Celsius. The digester slurry is enriched with nutrients, with Urea as source of Nitrogen, so that the absolute value of Nitrogen in the slurry is between 1,500 mg/lit to 3,000 mg/lit. The total solids within the slurry are maintained between 8% to 12% with that of the Lignocellulosic biomass.

Further, 1,500 liters (i.e. 30%) of slurry is pumped out, every day, from the digester and fed into the first tank, called oxygenation tank, and slurry is purged with oxygen till the dissolve oxygen level reaches 20 mg/lit, using the 90% to 95% pure oxygen from air. It takes about 2 hours to reach the desired level of dissolved oxygen. The oxygenated slurry is pumped back into the digester. The oxygenation & feeding of rice straw is carried out every day. Also, the slurry is passed through the Solid Liquid Separator, every day, where the permeate is put back into the digester and the digestate, the filter cake is removed out of the system. The total solids content of the digester slurry is maintained between 8% to 12%. Due to the effect of oxygenation of the slurry, it is observed that the biogas production is increased by about 50%, as compared to the conventional prior art processes. Conventionally, 1 ton of solid rice straw fed per day, produces biomethane between 60 kg to 120 kg, per day, where as in the present inventive process, the biomethane produced from 1 ton of solid rice straw is between 130 kg to 200 kg, per day.

Advantages of the invention

1. The process and apparatus of the present invention increases production of biogas by 10% to 80% higher than the biogas produced from the conventional prior art processes.

2. The process and apparatus of the present invention demonstrates that biogas production can be increased even without any chemical or biological pretreatment of Lignocellulosic biomass. The daily operational cost is substantially low.

The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above learnings. The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, and to thereby enable others skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omissions and substitutions of equivalents are contemplated as circumstances may suggest or render expedient, but such omissions and substitutions are intended to cover the application or implementation without departing from the spirit or scope of the claims of the present invention.

, Claims:We Claim:

1. A process for producing biogas from Lignocellulosic biomass such as crop residue, the process comprising steps of:
shredding the Lignocellulosic biomass;
adding the Lignocellulosic biomass into a digester, the digester a liquid microbial culture maintained therein, in the form of a slurry;
pumping out 10 to 50% of the slurry from the digester in a first tank;
oxygenating the slurry of the first tank to enrich the slurry between 2 mg/lit to 20 mg/liter level of dissolved oxygen therein; and
recirculating the oxygenated slurry back into the digester, wherein the quantity of biogas produced in the digester is increased between 10% to 80%, due to the addition of dissolved oxygen in the slurry.

2. The process as claimed in claim 1, wherein the biomass is any one selected from Napier grass, Bamboo rice straw, wheat straw, cotton straw, soybean trash, organic vegetable waste, animal waste, and food waste. ,

3. The process as claimed in claim 1, wherein the slurry of the first tank is oxygenated within 1 hr. to 3 hrs,

4. The process as claimed in claim 1, wherein the proportion of biomass fed to slurry in the digester, the Solid Loading Rate (SLR) to be between 1 kg to 10 kg per cubic meter of slurry, per day.

5. The process as claimed in claim 1, wherein the digester is maintained at thermophilic range of temperature, of about 52 degree Celsius to 56 degree Celsius;

6. The process as claimed in claim 1, wherein the slurry of the digester is having the absolute quantity of Nitrogen content between 1,500 mg/lit to 3,000 mg/lit;.

7. The process as claimed in claim 1, wherein the total solid content of the digester slurry to be between 6% to 12% comprised of the Lignocellulosic biomass.

8. The process as claimed in claim 1, whereas the shredded Lignocellulosic biomass with the particle size ranging from length or breadth of 2 mm to 20 mm and with the third-dimension depth to be less than 2 mm.

9. The process as claimed in claim 1, whereas the biomethane content in the biogas produced, from Lignocellulosic biomass is between 48% to 54%.

10. The process as claimed in claim 1, whereas the ratio of the biogas production volume to the digester slurry volume, to be between 2.0 to 10.0, per day, and measured in cubic meter of biogas produced to cubic meter of digester slurry volume.

11. The process as claimed in claim 1, wherein the digester used is continuous stirred-tank reactor (CSTR).