biogas and biomass

8/8/2019 Biogas and Biomass

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Biogas & Biomass


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What is biogas?

Biogas is a clean fuel produced through anaerobic

digestion of several organic wastes like agricultural,

animal, domestic and industrial. Made from organic

waste matter after it is decomposed, biogas istremendously used as fuel. A relatively clean burning,

colorless, and odorless gas, biogas is composed of

methane (CH4), carbon dioxide (CO2), and some traces of

nitrogen, ammonia (NH3), sulfur dioxide (SO2), hydrogen

sulfide (H2S), and hydrogen, depending on the feedstockused.


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It typically refers to a gas produced by

the biological breakdown of organic

matter in the absence of O2. The

average composition of biogas is CH450-60%, CO2 30-40%, H2 5-10%, N2 2.6%

and H2S traces.


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Manufacturing ProcessBiogas is produced in a no "oxygen is present"

environment or in an anaerobic environment, whencertain bacteria decompose organic material. The whole

process is referred to as anaerobic digestion (AD).

AD effectively treats the organic fraction of waste which

has many benefits. The process not only leads to a

healthy and clean environment, but also produces arenewable energy source like methane. The by-product

produced during the process is a solid residue which is

high-grade manure. In a biogas plant, biomass like

vegetable wastes, animal excreta, and weeds undergodecomposition in the absence of oxygen and form a

mixture of gases. This mixture is the biogas which is used

as a fuel for cooking and lighting.


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Principle of biogas productionStep I: Solubilization and hydrolysis : The complex

organic substances are solubilized by enzymatic

hydrolysis. This solubilized substance acts as a substrate

for second step.

Step II: Acidogenesis : Soluble organic compounds are

converted into organic acids with gases like CO2, H2 and

H2S. Two groups of bacteria are involved in this step :(a) Fermentative hydrolytic bacteria : E. coli, Bacteroides

etc. attack on complex organic compounds (like fats,

proteins, carbohydrates) and convert them into butyric,

lactic, propanoic acids and alcohols.(b)Acetogenic bacteria :Desulphovibrio convert the

organic acids into acetic acid, hydrogen and CO2.


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Step III Methanogenesis

The products acid and CO2 in step II act as a substrate for this

step and methanogenic bacteria like Methanococcus,

Methanothrixetc. convert them into methane.

CO2 + 4H2 ------> CH4 + 2H2O

CH3COOH -------> CH4 + CO2Complex organic substrate in the presence of fermentative

hydrolytic bacteria (E. coli, Bacillus clostridium) areconverted to butyric acid, propanoic acid and lactic acid.

These acids on being treated with acetogenic bacteria yield

acetic acid, hydrogen and CO2. These products when treated

with methanogenic bacteria(Methanococcus, Methanothrix)give methane.


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Biogas plants are biogas generating systems. Biogas

formed after decomposition of organic wastes in

biogas plants is piped or transported to homes for

cooking and heating, and used on a large scale in

many countries of the world today.

The Biogas plants are considered to be the cheap

sources of energy in rural areas. Biogas is also used for

running engines for shaft and electrical power

generation with little or no pollution.


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Types of Biogas Plants :

1. Floating gas-holder type2. Fixed dome type


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Floating Gas-holder Type Biogas plant

A digester tank or a well is made out of concrete and it called

the digester tank, which has two parts: the inlet and theoutlet. The inlet is from where slurry is transported to the

tank, which has a cylindrical dome made of stainless steel.

This dome floats on the slurry and collects the gas

generated. That is why such a biogas plant is known as

floating gas holder type. Fermentation of the slurry takes for

about 50 days. The pressure inside the cylindrical dome

increases when more gas is formed by bacterial

fermentation. The gas is then transported out through outlet

pipe. The decomposed matter moves into the next chamberin tank. By using the outlet pipe this is then removed to the

overflow tank, which is used as manure for cultivation

purposes. The optimum digestion takes place between a pH

of 7.0 to 7.4 at a temperature of 20-40

C.


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Fixed Dome Type Biogas plant

Here also, a well and a dome are made out of

concrete, which is called the digester tank. Since the

dome is fixed, this gas plant is known as fixed dometype. The manufacturing process is similar to the

floating holder type bio gas plant, where the slurry

expands and overflows into the overflow tank.


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Fixed dome type bio-gas plant


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Components of Biogas PlantsInlet pipe: The slurry is moved into the digester throughthe inlet pipe/tank.

Mixing tank:The feed material like dung is gathered inthe mixing tank. Using sufficient water, the material is

thoroughly mixed till a homogeneous slurry is formed.

Digester:Inside the digester, the slurry is fermented.

Biogas is produced through bacterial action.Gas holder or gas storage dome: The biogas thus formed

gets collected in the gas holder. It holds the gas till the time

it is transported for consumption.

Outlet pipe:The slurry is discharged into the outlet tank.This is done through the outlet pipe or the opening in thedigester.

Gas Pipeline:The gas pipeline carries the gas to theutilization point like a stove or lamp.


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Biogas PlantConstructionImportant criteria for biogas plant construction are:

1)The amount of waste material available for processing.

2)The amount of gas required for a specific use.

Application Areas1. Cooking: One of the most common use of biogas is for

cooking in a specially designed burner. A biogas plant with a

capacity of of 2m3 is enough for providing cooking fuel to afamily of four to five.

2. Lighting: Another use of biogas is lighting gas lamps. The

biogas required to power a 100 candle lamp (60 W) is 0.13

m3 per hour.

3. Power generation: This gas is also used to operate a dual-

fuel engine. It can replace up to 75% of the diesel.


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Benefits of Biogas Plants

1. A non-polluting and renewable source of energyis created in biogas plants.

2. It is an excellent way of energy conversion.

3. Biogas plants produce enriched organic manure.

This can be used as fertilizers.4. Biogas as a gas provides improvement in the

environment, and sanitation and hygiene.

5. The biogas plants provide a source for

decentralized power generation.6. Most important of all, such plants provide

employment generation in the rural areas.


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Biomass is natural biological matter material which canbe used to obtain energy e.g. dead trees, unused crops,

wood chips, lawn cuttings, animal waste etc. Biomass is

mainly plant or plant derived material where the energy is

accumulated due to photosynthesis which is once again

dependent upon sunlight. When such materials are burnt

energy produced is known as biomass energy.

It is the best source of alternative energy as it solves theproblem of solid waste management. It is a renewable

energy source based on carbon cycle.

On combustion the carbon goes back into the atmosphere

as CO2. Biomass can be directly converted into fuel e.g.

wood can be converted into charcoal and cattle dung can be

converted into bio-gas. Several plants including

miscanthus, hemp, willow, sugarcane and palm-oil are

grown for bio-mass.


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Anaerobic decomposition of organic wastes by certainmicro-organisms produces bio-gas and a nutrient rich

compost (a very good fertilizer). Biomass is converted

into liquid fuels like C2H5OH and CH3OH. Paddy husk is

converted into solid smokeless fuels. Biomass used forproducing energy is called biofuel. They can bebroadly divided as solid (dried manure, wood),

liquid(biodiesel, vegetable oil) or gas fuel derived

from recently dead biological material.


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Conversion of biomass into liquid fuels like ethanol or

methanol which can be used as a fuel. The liquid fuels can be

easily transported in tanks.

Ethanol can be produced by fermentation of biomass(molasses) -

Liquor molasses is the dark, brown coloured mother liquor

left after crystallization of sucrose from concentrated

sugarcane juice. Molasses(50-60% cane sugar) on dilution

with water yield 8-10% solution of sugar which on

acidification with dil. H2SO4 gives acidified molasses free

from undesirable bacteria.

On fermentation at 30C for 2-3 days. In the presence of

yeast and NH3NO3/(NH4)3PO4 yields 5-10% ethanol.On fractional distillation rectified spirit (90-95% ethyl

alcohol) is obtained. This on digestion with lime for 2 days

and distillation gives power alcohol (absolute alcohol).


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biogas and biomass

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