What is Biogas?How does a bio gas plant work?

BIOGAS

What is Biogas?

Bio gas is a good fuel. Have you thought how this is foamed? Biomass like animal excreta, vegetable wastes and weeds undergo decomposition in the absence of oxygen in a bio gas plant and form a mixture of gases. This mixture is the bio gas. Its main constituent is methane. This is used as a fuel for cooking and lighting.

 How does a bio gas plant work?

AEROBIC AND ANAEROBIC BIO-CONVERSION PROCESS

 

There is mainly threw aerobic and anaerobic bio-conversion process for the biomass energy applications: There are:

Bio products: Converting biomass into chemicals for making products that typically are made from petroleum

Bio fuels: Converting biomass into liquid fuels for transportation.

Bio power: Burning biomass directly, or converting it into a gaseous fuel or oil, to generate electricity.

Bio products: Whatever products we can make from fossil fuels, we can make using biomass. These bio products, or bio based products, are not only made from renewable sources, they also often require less energy to produce than petroleum-based products.

Researchers have discovered that the process for making bio fuels releasing the sugars that make up starch and cellulose in plants also can be used to make antifreeze, plastics, glues, artificial sweeteners, and gel for toothpaste.

Other important building blocks for bio products include carbon monoxide and hydrogen. When biomass is heated with a small amount of oxygen present, these two gases are produced in abundance. Scientists call this mixture biosynthesis gas. Biosynthesis gas can be used to make plastics and acids, which can be used in making photographic films, textiles, and synthetic fabrics.

When biomass is heated in the absence of oxygen, it forms pyrolysis oil. A chemical called phenol can be extracted from pyrolysis oil. Phenol is used to make wood adhesives, molded plastic, and foam insulation.

 

Bio fuels:  Unlike other renewable energy sources, biomass can be converted directly into Liquid fuels, bio fuels. For our transportation needs (cars, trucks, buses, airplanes, and trains). The two most common types of bio fuels are ethanol and bio diesel.

Ethanol is an alcohol, the same found in beer and wine. It is made by fermenting any biomass high in carbohydrates (starches, sugars, or cellulose) through a process similar to brewing beer. Ethanol is mostly used as a fuel additive to cut down a vehicle's carbon monoxide and other smog-causing emissions. But flexible fuel vehicles, which run on mixtures of gasoline and up to 85% ethanol, are now available.

Bio diesel is made by combining alcohol (usually methanol) with vegetable oil, animal fat, or recycled cooking greases. It can be used as an additive to reduce vehicle emissions (typically 20%) or in its pure form as a renewable alternative fuel for diesel engines.

Other bio fuels include methanol and reformulated gasoline components. Methanol, commonly called wood alcohol, is currently produced from natural gas, but could also be produced from biomass.

There are a number of ways to convert biomass to methanol, but the most likely approach is pas tion. Gasification involves vaporizing the biomass at high temperatures, then removing impurities for the hot gas and passing it through a catalyst, which converts it into methanol.

Most reformulated gasoline components produced from biomass are pollution reducing additives, such as methyl tertiary butty ether (MTBE) and ethyl tertiary butty ether (ETBE).

 

Bio power:    Bio power, or biomass power, is the use of biomass to generate electricity. There are six major types of biopower systems: direct fired, cofiring, gasification, anaerobic digestion, pyrolysis and small, modular.

Most of the biopower plants in the world use direct fired systems. They burn bioenergy feedstock’s directly to produce steam. This steam is usually captured by a turbine, and a generator then converts it into electricity. In some industries, the steam from the power plant is also used for manufacturing processes or to heat buildings. These are known as combined heat and power facilities. For instance, wood waste is often used to produce both electricity and steam at paper mills.

Many coal fired power plants can use cofiring systems to significantly reduce emissions, especially sulfur dioxide emissions. Coal firing involves using bioenergy feedstock’s as a supplementary energy source in high efficiency boilers.

Gasification systems use high temperatures and an oxygen starved environment to convert biomass into a gas (a mixture of hydrogen, carbon monoxide, and methane). The gas fuels what's called a gas turbine, which is very much like a jet engine, only it turns an electric generator instead of propelling a jet.

The decay of biomass produces gas methane that can be used as an energy source. In landfills, wells can be drilled to release the methane from the decaying organic matter. Then pipes from each well carry the gas to a central point where it is filtered and cleaned before burning. Methane also can be produced from biomass through a process called anaerobic digestion. Anaerobic digestion involves using bacteria to decompose organic matter in the absence of oxygen.

Methane can be used as an energy source in many ways. Most facilities burn it in a boiler to produce steam for electricity generation or for industrial processes. Two new ways include the use of micro turbines and fuel cells. Micro turbines have outputs of 25 to 500 kilowatts. About the size of a refrigerator, they can be used where there are space limitations for power production. Methane can also be used as the "fuel" in a fuel cell. Fuel cells work much like batteries but never need recharging, producing electricity as long as there's fuel.

In addition to gas, liquid fuels can be produced from biomass through a process called pyrolysis. Pyrolysis occurs when biomass is heated in the absence of oxygen. The biomass then turns into a liquid called pyrolysis oil, which can be burned like petroleum to generate electricity. A biopower system than uses pyrolysis oil is being commercialized.

Several biopower technologies can be used in small, modular systems. A small, modular system generates electricity at a capacity of 5 megawatts or less. This system is designed for use at the Silla town level or even at the consumer level. For example, some farmers use the waste from their lives to provide their farms with electricity. Not only do these systems provide renewable energy, they help farmers and ranchers meet environmental regulations.

Small, modular systems also have potential as distributed energy resources. Distributed as resources refer to a variety of small, modular power generating technologies that can be combined improve the operation of the electricity delivery system.

 

RAW MATERIALS

 

All types of organic wastes which can form slurry are suitable for producing biogas by the process of anaerobic digestion in a biogas plant. Wood and sugar biogases are difficult and time consuming with this process and incineration may be preferred. The choice of raw material (in feed) is based on availability of the waste. The biogas plant is designed to suit particular type of in feed.

Biogas production taken different time period depending upon raw material; temperature; process adopted etc.

The biomass used as a raw material can be classified into the following categories.

Waste                                         Cultivated and Harvested

Agricultural wastes                  Agricultural energy crops

Rural animal wastes                Aquatic crops

Poultry waste                               

Butchery waste

Urban waste (garbage)            Forest crops

Aquatic wastes

Forest wastes

Coconut husk waste

Industrial wastes

 

Others are poultry waste, piggery waste, sheep, goat, cow, horse dung, Slaughter house waste, coconut shell, husk, waste garbage, fruit skins and leftovers.

The waste is generated periodically and can be converted into useful biogas. The problem of waste disposal is solved as the sludge is used as manure.

Waste --------- Bio gas Plant----------- Bio gas

                                    |

                         Sluge Manure

The cultivated or harvested biomass is specially grown on land or in sea/lake for obtaining raw materials for bio gas production.

 

PROPERTIES OF BIO GAS

 

Main properties of bio gas are:

1. Comparatively simple and can be produced easily.

2. Burns without smoke and without leaving ash as residues.

3. Household wastes and bio-wastes can be disposed of usefully and in a health manner.

4. Reduces the use of wood and to a certain extent prevents deforestation

5. The slurry from the biogas plant is excellent manure.

 

BIO GAS PLANT TECHNOLOGY

The important parts of biogas plant are:

1. The tank where biomass undergoes decomposition (digester)

2. The tank where biomass is mixed with water (mixing tank)

3. The tank where slurry of biomass is collected (out flow tank)

4. Arrangement to store gas.

Due to the action of bacteria in the absence of oxygen, biogas is produced in the plant Thin collected in the tank. In the gasholder type plant, the cylinder rises up as the gas fills the tank and storage capacity increases. The gas storage capacity of dome type will be less than that of gasholder type. Residue of biomass (slurry) can be used as good manure.

Biogas plants are built in several sizes, small (0.5 m®/day) to very large 2500 m®/day). Accord. Ingly, the configurations are simpler to complex.

Biogas plants are classified into following main types.

--Continuous type or batch type.

--Drum type and dome type.

There are various configurations within these types.

 

CONTINUOUS TYPE

Continuous type biogas plant delivers the biogas continuously and is fed with the biomass regularly. Continuous type biogas plant is of two types.

 

(A) SINGLE STAGE CONTINUOUS TYPE BIO GAS PLANT

In such a plant Phase-I (acid formation) and Phase-II (machination) are carried out in the sale chamber without barrier. Such plants are simple, economical, easy to operate and control. These pla are generally preferred for small and medium size biogas plants. Single stage plants have lesser rale gas production than the two stage plant.

 

(B) TWO STATE CONTINUOUS TYPE BIOGAS PLANT

In such a plant the Phase-I (acid formation) and Phase-II (methane formation) take plo separate chambers. The plant produces more biogas in the given time than the single stage plant.. ever, the process is complex and the plant is costlier, difficult to operate and maintain. Two stage P is preferred for larger biogas plant systems.

 

BATCH TYPE BIO GAS PLANT

The in feed biomass is fed in batches with large time interval between two consecutive batches. One batch of biomass indeed is given sufficient retention time in the digester (30 to 50 days). After completion of the digestion, the residue is emptied and the fresh charge is fed. The fresh biomass charge may be subjected to aeration or nitrogenation after feeding and then the digester covers are closed for the digestion process. Thereafter, the Biogas is derived from the digester after 10 to 15 days. Fermentation continues for 30 to 50 days.

 

Salient Features:

 

1. Batch type biogas plant delivers gas intermittently and dis-continuously.

2. Batch type biogas plant may have several digesters (reactors) which are fed in a sequential manner and discharged in a sequential manner to obtain the output biogas continuously.

3. Batch type biogas plants have longer digestion time and are therefore more suitable for materials which are difficult for anaerobic digestion (e.g. harder, fibrous biomass).

4. Batch type biogas plant needs initial seeding to start the anaerobic fermentation.

5. Batch type biogas plant needs larger volume of the digester to accommodate large volume of the batch. Hence initial cost is higher.

6. Operation and maintenance is relatively more complex. Batch type biomass plants need well organized and planned feeding. Such plants are preferred by European farmers. Such plants are not yet popular in India.

 

FIXED DOME TYPE DIGESTER

 

In the fixed dome type digester biogas plant, the digester and gas-collector (gas dome) are enclosed in the same chamber. This type of construction is suitable for batch type biogas plant. The digester is conveniently built at or below ground level in comparatively cooler zone. The construction of the digester is with locally available materials like, bricks, and terra-cotta. The pressure inside the digester increases as the biogas is liberated. The biogas gets collected in the upper portion of the digester in a dome shaped cavity. The outlet pipe is provided at the top of the fixed dome. Alternatively the gas collector (gas holder) is a separately installed chamber. The digester tank and gas collector chamber are separated by a water seal tank.

      The arrangement of a separate gas collector is preferred as the tapping of gas from the gas holder does not affect the pressure and the digestion process in the main digester. The water seal tank prevents the return of the gas from the gas collector to the digester chamber.

                     An additional displacement chamber may be provided for providing space to the displacement Slurry in the digester due to gas pressure in the upper dome of the fixed type digester. The fixed dome type digester can be fed on daily basis with small quantities of the slurry. The excess slurry in the digester gets accommodated in the displacement chamber. The level of the slurry in the main digester and the displacement collector can vary in accordance with the pressure and volume of the biogas in the fixed type of dome. The pressure in the fixed dome and the displacement gas collector are almost the same as they are connected by the outlet from the main digester.

 

Floating Gas Holder Type.

 

In this design a dome made floats above the slurry in the disaster. In the Fig. 2.4, the disaster tank is of cylindrical masonry construction. The floating dome is of fabricated steel construction. The dome guide shaft provides the axial guide to the floating dome. As the gas is collected in it. The sliding bearing provides smooth sliding surface and guide to the floating dome the gas generated in the slurry gets collected in the dome and the dome arises. The water seal tank provides separation between the gas in the dome and the outlet gas.