Biogas theorry

Biogas is one of the renewable sources of energy. It can be used for domestic and farm use.

What is biogas? It mainly comprises of hydro-carbon which is combustible and can produce heat and energy when burnt. Bio-gas is produced through a bio-chemical process in which certain types of bacteria convert the biological wastes into useful bio-gas. Since the useful gas originates from biological process, it has been termed as bio-gas. Methane gas is the main constituent of biogas.

Biogas production process The process of bio-gas production is anaerobic in nature and takes place in two stages. The two stages have been termed as acid formation stage and methane formation stage. In the acid formation stage, the bio-degradable complex organic compounds present in the waste materials are acted upon by a group of acid forming bacteria present in the dung. Since the organic acids are the main products in this stage, it is known as acid forming stage. In the second stage, groups of methanogenic bacteria act upon the organic acids to produce methane gas.

Raw materials for biogas production Although, cattle dung has been recognized as the chief raw material for bio-gas plants, other materials like night-soil, poultry litter and agricultural wastes can also be used.

Advantages of biogas production It is a eco-friendly fuel The required raw materials for biogas production are available abundantly in villages It not only produces biogas, but also gives us nutrient rich slurry that can be used for crop production It prevents the health hazards of smoke in poorly ventilated rural households that use dung cake and fire-wood for cooking It helps to keep the environment clean, as there would be no open heap of dung or other waste materials that attract flies, insects and infections Availability of biogas would reduce the use of firewood and hence trees could be saved

Components of the bio-gas production plant There are two major models - fixed dome type and floating drum type Both the above types have the following components (i) Digester : This is the fermentation tank. It is built partially or fully underground. It is generally cylindrical in shape and made up of bricks and cement mortars. (ii) Gas holder: This component is meant for holding the gas after it leaves the digester. It may be a floating drum or a fixed dome on the basis of which the plants are broadly classified. The gas connection is taken from the top of this holder to the gas burners or for any other purposes by suitable pipelines. (iii) Slurry mixing tank: This is a tank in which the dung is mixed with water and fed to the digester through an inlet pipe. (iv)  Outlet tank and slurry pit: An outlet tank is usually provided in a fixed dome type of plant from where slurry in directly taken to the field or to a slurry pit. In case of a floating drum plant, the slurry is taken to a pit where it can be dried or taken to the field for direct applications.

Points to be considered for construction of a biogas plant Site selection: While selecting a site for a bio-gas plant, following aspects should be considered: The land should be leveled and at a higher elevation than the surroundings to avoid water stagnation Soil should not be too loose and should have a bearing strength of 2 kg/cm2 It should be nearer to the intended place of gas use (eg. home or farm). It should also be nearer to the cattle shed/ stable for easy handling of raw materials. The water table should not be very high. Adequate supply of water should be there at the plant site. The plant should get clear sunshine during most part of the day. The plant site should be well ventilated. A minimum distance of 1.5m should be kept between the plant and any wall or foundation.    It should be away from any tree to prevent root interference. It should be at least 15m away from any well used for drinking water purpose. Availability of raw materials : The size of the biogas plant is to be decided based on availability of raw material. It is generally said that, average cattle yield is about 10 kg dung per day.  For eg. the average gas production from dung may be taken as 40 lit/kg. of fresh dung. The total dung required for production of 3 m3 biogas is   3/0.04= 75 kgs. Hence, a minimum of 4 cattle is required to generate the required quantity of cow dung.

Useful resources

Average maximum biogas production from different feed stocks Sl. No. Feed Stock Litre /kg of dry matter % Methane content 1. Dung 350* 60 2. Night-soil 400 65 3. Poultry manure 440 65 4. Dry leaf 450 44 5. Sugar cane Trash 750 45 6. Maize straw 800 46 7. Straw Powder 930 46 * Average gas production from dung may be taken as 40 lit/kg. of fresh dung when no temperature control is provided in the plant. One Cu. m gas is equivalent to 1000 litres.

Average dung yield Sl. No. Living Beings Quantity of Dung / Night Soil produced (kg/day) 1. Cow, Heifer 10.0 2. Bullock 14.0 3. Buffalo 15.0 4. Young bovine 5.0 5. Horse 14.0 6. Horse, young 6.0 7. Pigs, over 8 score 2.5 8. Pigs, under 8 score 1.0 9. Ewes, rams and goats 1.0 11. Lambs 0.5 12. Duck 0.1 13. 10 hens 0.4 14. Human beings 0.4 Note : For free grazing animals the availability of dung may be taken as 50 per cent of the amount given in the table

Requirement of cattle for various sizes of biogas plants Plant Size in m 3 Minimum number of cattle required 2 3 3 4 4 6 6 10 8 15 25 45

Carbon to Nitrogen Ratio of various materials Sl. No. Material Nitrogen Content (%) Ratio of Carbon to Nitrogen 1. Urine 15.18 8:1 2. Cow dung 1.7 25:1 3. Poultry manure 6.3 N.A.* 4. Night soil 5.5-6.5 8:1 5. Grass 4.0 12:1 6. Sheep waste 3.75 N.A. * 7. Mustard straw 1.5 20:1 8. Potato tops 1.5 25:1 9. Wheat straw 0.3 128:1 *  N.A.:- Data Not Available

Calorific values of commonly used fuels Commonly used fuels Calorific values in Kilo calories Thermal efficiency Bio-gas 4713/M3 60% Dung cake 2093/Kg 11% Firewood 4978/Kg 17.3% Diesel (HSD) 10550/Kg 66% Kerosene 10850/Kg 50% Petrol 11100/Kg ---

 

Equivalent quantity of fuel for 1 m3 of biogas Name of the fuel Kero-sene Fire-wood Cow-dung cakes Char-coal Soft coke Butane Furn-anceOil Coal gas Electricity Equivalent quantities to  1 m3 of Bio-gas 0.620 3.474 kg 12.296 kg 1.458 kg 1.605 kg 0.433 kg 0.4171 1.177 m3 4.698 kWh

Bio-Gas Requirements Sl. No. Use Quantity requirement 1. Cooking 336 - 430 1/ day / person 2. Gas Stove 330 1/ hr /5 cm burner 470 1/hr/10 cm burner 640 1/hr/15 cm burner 3. Burner Gas Lamp 126 1/lamp of lighting equivalent to 100 watt filament lamp. 70 1/hr/1 mantle lamp 140 1/hr/2 mantle lamp 1691/lir/3 mantle lamp 4. Dual fuel engine 425 1/hp/hr

A format for cost estimation for establishing biogas plants Model :                                                                      Capacity : Sr. No. Item Quantity Rate/Unit Quantity Cost 1. Earth Work 2. Bricks 3. Cement 4. Sand 5. Morrum/ Stones 6. Skilled labour days for construction of plant 7. Unskilled labour days for plant construction 8. A.C. Pipes (when required) 9. Gas holder 10. Pipes & fittings with. sizes 11. Gas burner/chullah 12. Gas lamp (when required) 13. Any other item, if  required with specific details 14. Transportation charges *  The rates should be as per the State Government schedule of  rates or approved district schedule of  rates.

Technical guidelines for establishment of biogas plants i. Digester Design The recommendation of KVIC is to have a digester volume of 2.75 times the volume of gas produced per day. KVIC recommendation for the depth of the plant is between 4 to 6 m according to the size but for economical use of building materials, a depth to diameter ratio between 1.0 to 1.3 are considered ideal for all types of plants. In a floating drum plant, a continuous ledge is built into the digester at a depth 10 cm. shorter than the height of the gas drum to prevent the gas holder from going down when no gas is left in it. It helps in preventing the gas inlet being choked. It also guides the gas bubbles rising from the side of the plants into the gas bolder. In some plants slurry is fed at the bottom and removed at the top. When the digester diameter exceeds 1.6 m, a parti­tion wall is provided in the digester to prevent short circuiting of slurry flow and increasing its retention period. In case of fixed dome plants, the volume of digester comes to between 1.5 times to 2.75 times the gas produced per day. Here, the higher the plant capacity, the lesser becomes the ratio of digester volume to gas produced per day. ii.  Gas Holder Design The design of a gas holder is influenced by the digester dia­meter and distribution of gas use during the day. For domestic plants, the gas holder capacity is kept at 60 per cent of a day's gas production and in case of laboratories, it is kept at 70 per cent of the day's gas production.  In a floating drum plant, the gas holder diameter is 15 cm. less than the diameter of the digester and accordingly the other dimensions are decided. The gas holder can be given a rotary movement around its guide to break the scum formation at the top. In a fixed dome plant the dome angle is kept between 17° and 21° and it gives a pressure upto 100 cm. of water. Due to higher pressure, the diameter of gas pipelines can be re­duced and the gas can be taken to greater distance. In this plant, care should be taken to provide an earth pressure equivalent to 100 cm of  water column from the top of the dome. Always use 'A' class bricks in the domes for better stability. iii.  Inlet Tank Before the dung is fed into the plant, it is mixed with water in a tank to give a solid content of 7.5 per cent to 10 per cent in the slurry. This tank also helps in removing grass and other floating materials from the raw materials to prevent excessive scum formation in the plant. This tank is connected to the digester by an asbestos cement pipe. The floor of the mixing tank is given a slope opposite to the direction of inlet pipe to help heavy inorganic solid particles to settle and get separated from the slurry.

Nutrient status of Biogas slurry     N P2O5 K2O Bio-gas slurry    1.4    1.0 0.8    Farm Yard Manure (FYM)    0.5    0.2 0.5 Town Compost    1.5    1.0 1.5

Do's and Dont's for floating Drum Plant

Shakthi-surabhi - biogas plant for households Shakthi-Surabhi is a kitchen waste based biogas plant. It works on similar principles of a traditional biogas plant, but has been modified to suit urban requirements also. The unit consists of an inlet waste feed pipe, a digester, gas holder, water jacket, a gas delivery system and an outlet pipe. It is developed by the Vivekananda Kendra, Natural Resources Development Project (Vknardep), Kanyakumari, Tamil Nadu. In what way does this unit promise to be a better alternative to the conventional biogas plants? Cattle dung is a major input for the conventional plants. And everyday the dung should be mixed as slurry and poured into the gas tank. But for Shakthi-surabhi, cattle dung is required for initial charging. Later on, kitchen and other wastes (leftover cooked food (veg and non-veg), vegetable wastes, material from flour mills, non edible oil seed cakes (neem, jatropha etc)) alone are sufficient for producing the required gas. The unit comes in two attractive colours in capacities from 500 to 1,500 litres. It is easy to fix or relocate and can be installed either at the backyard (if it is an independent house) or in the terrace or sunshade in flat structures. Required feed materials Performance About 5 kg of waste is required for a 1 cubic metre plant which is equal to 0.43 kg of LPG. It is estimated that 100 cubic metres of biogas could produce 5 KW of energy to meet a 20-hour power requirement of a house The process is hygienic and is devoid of odour and flies. The unit also helps in controlling climate change effects and arrests green house gases, and the digested outlet slurry of the unit acts as good organic manure