MODULE -4

BIOMASS ENERGY
Session 25
Biomass is the organic material from plants and animals that can be used as a fuel source
Biomass can be burned directly for heat or converted to liquid and gaseous fuels through
various processes.

9.1 BIOMASS PRODUCTION

Organic substances exist in a wide variety, ranging from living organisms to dead matter. These
organic materials are primarily composed of carbon (C), combined with elements such as
hydrogen (H), oxygen (O), nitrogen (N), and sulphur (S). Together, these elements form
various organic compounds including carbohydrates, proteins, and lipids.

In nature, microorganisms (MOs) play a vital role in breaking down complex carbon
compounds. Through digestion processes—such as aerobic and anaerobic decomposition—
these microorganisms convert complex organic matter into simpler substances. This biological
transformation is essential for biomass production, enabling the conversion of waste and
natural materials into usable energy and nutrients.

9.1.1 Direct method

Raw materials that can be used to produce biomass energy are available throughout the world
in the following forms:
1. Forest wood and wastes
2. Agricultural crops and residues
3. Residential food wastes
4. Industrial wastes
5. Human and animal wastes
6. Energy crops

Raw biomass has a low energy density based on their physical forms and moisture contents
and their direct use are burning them to produce heat for cooking. The twin problems of
traditional biomass use for cooking and heating are the energy inefficiency and excessive
pollution

9.1.2 Indirect method

1. Thermo-electrical conversion: The direct combustion of biomass material in the

boiler produces steam that is used either to drive a turbine coupled with an electrical
generator to produce electricity or to provide heat for residential and industrial system
2. Biomass conversion to fuel: Biomass conversion processes can be classified under two
main types: (a) Thermo-chemical conversion includes processes such as destructive
distillation, pyrolysis, and gasification. (b) Biological conversion includes processes
such as fermentation and anaerobic digestion
Gasification produces a synthesis gas with usable energy content by heating the biomass with
less oxygen than needed for complete combustion.
Pyrolysis yields bio-oil by rapidly heating the biomass in the absence of oxygen.
Anaerobic digestion produces a renewable natural gas (methane gas) when organic matter is
decomposed by bacteria in the absence of oxygen. As a result, it is often advantageous to
convert this waste into more readily usable fuel form like producer gas. Hence, it is the
attractiveness of gasification. The efficiency of a direct combustion or biomass gasification
system is influenced by several factors such as including biomass moisture content, combustion
air distribution and amounts (excess air), operating temperature and pressure, and flue gas
(exhaust) temperature.

9.2 ENERGY PLANTATION

An interesting approach for the large-scale planned use of wood is the ‘energy plantation’
approach. energy plantation means growing select species of trees and shrubs which are
harvestable in a comparably shorter time and are specifically meant for fuel. The fuel
wood may be used either directly in wood burning stoves and boilers or processed into
methanol, ethanol, and producer gas.
It has been suggested that electrical power be produced by the energy plantation approach, the
wood grown in this manner being used as a fuel for the boilers of a conventional power plant.
The technology of biomass-based electric power plants is well established in the USA and
Europe and there are over 500 such plants use wood, wood waste, and various types of
agricultural waste.

9.3 BIOMASS GASIFICATION

Gasification is conversion of solid Hydrocarboneous fuels (wood/ wood-waste, agricultural

residues, chicken manure, coal, Municipal Waste etc.) into a combustible gas mixture called
Producer Gas/ Syn Gas. The Gasifier is essentially a chemical reactor where various physical
and chemical processes take place and break the solid fuel down into Producer Gases. Four
distinct processes take place in a gasifier:

1. Drying of the fuel: The moisture in the feed comes out in this zone in the form of water
vapor. Drying takes place in the upper most portion of the Gasifier, through heat transferred
from the high temperature combustion zone.

2. Pyrolysis This is chemical decomposition of organic materials by heating in absence of

oxygen at temperatures above 200 degrees C. During pyrolysis, volatiles are released (in the
form of gas) and char is produced.

3. Combustion This is where controlled oxygen is given to the fuel and oxidation/ burning
occurs. Heat and energy are released.

4. Reduction The combustion products mainly CO2 (Carbon-di-Oxide) and H2O (water
vapor) get reduced in the presence of high temperature carbon to finally give CO (Carbon-
mono-oxide) and H2 (Hydrogen).
GASIFIER AND THEIR CLASSIFICATIONS
Biomass gasifier may be considered as a chemical reactor in which biomass goes through
several complex physical and chemical processes and producer or syngas is produced and
recovered.
There are two distinct types of gasifiers:
1. Fixed bed gasifier: In this gasifier, biomass fuels move either counter current or concurrent
to the flow of gasification medium (steam, air, or oxygen) as the fuel is converted to fuel gas.
They are relatively simple to operate and have reduced erosion.
Since there is an interaction of air or oxygen and biomass in the gasifier, they are classified
according to the way air or oxygen is introduced in it.
(a) Downdraft gasifiers: In the downdraft gasifier, the air is passed from the layers in the
downdraft direction. Single throat gasifiers are mainly used for stationary applications, whereas
double throat gasifier is used for varying loads as well as automotive purposes.
(b) Updraft gasifiers: Updraft gasifier has air passing through the biomass from bottom and
the combustible gases come out from the top of the gasifier.
(c) Cross draft gasifiers: It is a very simple gasifier and is highly suitable for small outputs.
With slight variation, almost all the gasifiers fall in the abovementioned categories.

2. Fluidized bed gasifier: In fluidized bed gasifier, an inert material (such as sand, ash, or char)
is utilized to make bed and that acts as a heat transfer medium

CHEMISTRY OF REACTION PROCESS IN GASIFICATION

1. Drying Zone

• What happens? Water (moisture) in the fuel evaporates.

• Temperature range: Up to 200°C
• Other effects: Some organic acids are released, which can cause corrosion in
gasifiers.

2. Pyrolysis Zone (Breaking fuel without oxygen)

• What happens? Fuel starts to break down, and gases/tar are released.
• Temperature stages:
o 200°C – 280°C: Releases carbon dioxide, acetic acid, and water.
o 280°C – 500°C: Main pyrolysis happens. Produces tar, gases, and a little
methanol.
o 500°C – 700°C: Small amount of hydrogen gas is produced.

3. Combustion Zone (Oxidation)

• What happens? Carbon in fuel burns with oxygen to produce carbon dioxide and
heat.
• Reaction:
C + O2 → CO2 + Heat
• Why it is important: The heat from this zone powers the rest of the gasifier.

4. Reduction Zone

• What happens? Hot gases react with carbon, but no oxygen is present.
• Main reactions:
1. CO₂ reacts with carbon to make CO (carbon monoxide):
C + CO2 + Heat → 2CO (Endothermic – uses heat)
 2. Water reacts with carbon to make CO and H₂ (hydrogen):
C + H2O + Heat → CO + H2 (Endothermic)
3. CO reacts with water to make CO₂ and H₂:
CO + H2O → CO2 + H2 + Heat (Exothermic – gives heat)
• Temperature: Around 800°C to 1,000°C
• Importance: Produces flammable gases (CO and H₂) used as fuel.

Review questions:
1. Define biomass
2. Define biomass gasification
3. Types of gasifiers.

Session 26

9.7 UPDRAFT GASIFIERS

The updraft fixed bed ("counter-current") gasifier consists of a fixed bed of carbonaceous fuel
(e.g. coal or biomass) through which the "gasification agent" (steam, oxygen, and/or air) flows
in counter-current configuration. The ash is either removed dry or as a slag. The updraft gasifier
consists of a top fed fuel bed through which the "gasification agent" (steam, oxygen, and/or
air) flows in from the bottom and exits through the top as gas. Updraft gasifiers are thermally
efficient because the ascending gases pyrolyze and dry the incoming biomass, transferring heat
so that the exiting gases leave very cool.

The updraft gasifier has been the standard of coal gasification for 150 years and it is also
popular in biomass cook stoves.
A downdraft gasifier is a co-current reactor where air enters the gasifier at a certain height
below the top. The product gas flows downward (giving the name downdraft) and leaves
through a bed of hot ash Since it passes through the high-temperature zone of hot ash, the tar
in the product gas finds favourable conditions for cracking. For this reason, a downdraft
gasifier, of all types, has the lowest tar production rate.
Downdraft gasifiers are widely used in the following applications:
1. Continuous baking ovens (bread, biscuits, and paint)
2. Batch type baking oven (rotary oven for bread)
3. Dryers and curing (tea, coffee, mosquito coil, and paper drying)
4. Boilers
5. Thermal fluid heaters
6. Annealing furnaces
7. Direct fired rotary kilns
8. Internal combustion engines

Cross draft gasifiers:

The gasifier is a vertical cylindrical vessel of varying cross section. The biomass is fed in at
the top at regular intervals of time and is converted through a series of processes into producer
gas and ash, as it moves down slowly through various zones of the gasifier. The crucial
difference is that the air will be entering the gasifier from the side of the reactor, rather than
from the top or the bottom

FLUIDIZED BED GASIFICATION

• Air, Oxygen, or Steam Inlet (Bottom):
• Purpose: Supplies the fluidizing gas (air, oxygen, or steam).
• Effect: This gas flows upwards through the distributor plate, making the solid
particles (fuel) behave like a fluid – hence, “fluidized bed.”
• Distributor Plate:
• Function: Evenly distributes the incoming air/steam across the bed.
• Importance: Helps maintain uniform fluidization and temperature.
• Fluidized Bed:

• The fuel particles are suspended and well-mixed by the upward flow of air/steam.
• Reactions like drying, pyrolysis, combustion, and reduction happen here due to
high temperatures.

• Fuel Feed (Side Entry):

• Solid fuel (e.g., biomass) is fed into the bed.

• It reacts with the hot gases and converts into syngas (CO + H₂).

• Ash Outlet:

• Where unreacted solids and residue (ash) are removed from the system.

• Gas Outlet (Top):

• The produced gas (called producer gas or syngas) exits from the top.
• Contains flammable gases like CO, H₂, and some CO₂.

• Cyclone Separator:

• Purpose: Removes fine particles from the gas.

• These fines can be recirculated back into the bed to ensure complete conversion and
minimize waste
Advantages and Benefits
Advantages
1. Reduced cost of boiler or dryer operation by using wood and/or bark wastes rather than gas
or oil.
2. Reduced cost for additional steaming capacity when compared to new wood and or barkfired
boilers.
3. Reduced dependency on external fuel sources for propane, natural gas, and oil.
Benefits
1. High overall efficiency: High efficiency in the range of 70%–90% can be achieved.
2. Fuel flexibility: The fluidized bed gasifiers have fuel flexibility and operate satisfactorily
with highly variable feed materials. Ranging from coal, shredded wood and bark to sawdust
fines, or lump wood with particle sizes of less than 4–6 cm.
3. Highly reliable: The fluidized bed gasifier neither have moving grates nor other moving
parts in the high temperature regions of the bed and hence they are highly reliable.
4. Low purchase and installation costs: Air flow used in the gasifiers is comparatively low,
and hence size of gasifier is small and compact. These permit systems to be completely shop
fabricated and assembled on skids, thereby reducing purchase price and installed costs.
5. Flexible operations: Fuel gas product of fluidized bed gasifier is easily applied to a variety
of industrial processes including boilers, dry kilns, veneer dryers, or several pieces of
equipment at once. Thus, they provide flexible operations.
6. Low emissions: They are very low emission gasifiers and do not require exhaust clean up
devices.

Uses of Biomass Gasifier

A biomass gasifier converts solid biomass (like wood, crop waste, etc.) into a gas (called
producer gas) that can be used in many ways:

Thermal Applications (Heat-based uses)

The gas can be used directly for:

• Cooking
• Drying
• Boiling water
• Generating steam

⚙️ Mechanical & Electrical Power

The gas can also run internal combustion engines to produce:

• Mechanical power (for machines)

• Electric power

🧼 Cleaning the Gas

Before using the gas in engines, it must be cleaned to remove:

• Dust (particulates)
• Tar

Cleaning system includes:

1. Cyclone separator
2. Scrubber
3. Filter
 Use in Engines

1. Spark-Ignition Engines (Petrol Engines):

o Can run entirely on producer gas.
o Mixes gas with air, sucks it in, and ignites it.
2. Compression-Ignition Engines (Diesel Engines):
o Works in dual-fuel mode (both diesel and producer gas).
o Only 25% diesel is used compared to full-diesel mode.
o Diesel starts the ignition, and then the gas-air mix burns.

Biomass Gasification in India

• India is a leading country in biomass gasification.

• Systems are available for:
o Thermal output: From 60,000 to 5,000,000 kJ/h
o Electric output: From 3 kW to 500 kW

Example of a large system:

• Produces 1,450 kW heat or 500 kW electricity

• Uses wood blocks at 500 kg/h
• Produces 1,250 m³/h of gas

🧼 Liquid Fuels from Biomass

1. Methanol Production:

• Made by gasifying wood/straw and converting the gas into methanol using chemical
reactions.

2. Ethanol Production:

• Made by fermenting crops like:

o Sugarcane
o Maize
o Cassava
o Tapioca
• Ethanol + Petrol = Alternate fuel for vehicles
• Countries like Brazil use it widely due to surplus land.
• India has limited land, so large-scale ethanol production is difficult.

Photosynthesis and Fuel

 • Plants and algae absorb CO₂ and release oxygen using sunlight.
• This process produces carbohydrates, which are the base for biomass.
• Even fossil fuels come from ancient plants that stored energy from the sun.

GASIFIER BIOMASS FEED CHARACTERISTICS

What is a Gasifier?

A gasifier is a machine that turns biomass (like wood, crop waste, or organic garbage) into a
gas that can be used for energy

But here is the catch:

Even though many gasifier makers say their machines can work with any biomass, that is not
exactly true. Each gasifier works best with specific types of biomass. You cannot just
throw anything in and expect it to work well.

Important Characteristics of Biomass Fuel:

These features determine how well a gasifier works:

1. Energy Content & Bulk Density

o Fuel with more energy and higher density (compactness) lasts longer and
produces more power.
2. Moisture Content (Water in fuel)
o Too much water is bad. It wastes heat and clogs filters.
o Ideal moisture content: Less than 20%
3. Dust Content
o All biomass creates dust.
o Too much dust can block engines, so filters are needed.
o Less dust = less maintenance.
4. Tar Content
o Tar is a sticky substance that can block engine parts.
o It is hard to remove during gasification, so it must be cleaned later with filters
and coolers.
5. Ash and Slagging
o After burning, the leftover minerals become ash.
o Sometimes ash melts into slag (a hard clump), which blocks fuel flow.
o Two solutions:
▪ Low temp operation (use steam or water)
▪ High temp operation (melt and drain the slag out)

Best Fuels for Gasifiers:

• Charcoal – No tar, low ash. Very clean, used in WW2.

Downside: Wastes 50% of energy during its production from wood.
• Wood – Also reliable if it’s dry and clean.
Biomass Feed (Fuel)

The major biomass sources presently used are as follows:

1. Sugarcane and corn, wheat, sugar beet, sweet sorghum, and cassava to produce bioethanol.
2. Rapeseed, sunflower seeds, soybean, canola, peanuts, jatropha, coconut, and palm oil for
biodiesel production.
3. Wide range of cellulosic materials (such as grassy crops, woody plants, by-products from
the forestry and agricultural sector including wood residues, stems, and stalks and municipal
wastes constitute the so-called second generation of feedstock).
4. Wastes and residues constitute a large source of biomass. These include solid and liquid
municipal wastes, manure, lumber, and pulp mill wastes, and forest and agricultural residues.
Review questions:
1. What is updraft gasifier
2. What is downdraft gasifier
3. What is fluidized bed gasification.

Session 27

APPLICATION OF BIOMASS GASIFIER

1. Motive power
2. Direct heat application
3. Electrical power generation
4. Chemical production

COOLING AND CLEANING OF GASIFIER

For efficient and effective use of gas for numerous applications, it should be cleaned of tar and
dust, free from moisture content and cooled. Therefore, cooling and cleaning of the gas is one
of the most important processes in the whole gasification system. The failure or the success of
producer gas units depends completely on their ability to provide a clean and cool gas to the
engines or for burners. The temperature of gas coming out of generator is normally between
300°C and 500°C. The energy density of gas can be increased primarily by cooling it.
Normally, there are three types of filters used for cleaning of gas which is schematically a
downdraft gasification system with cleaning and cooling train.

They are classified as dry, moist, and wet.

1. Cyclone filters: Cyclone dust collectors can be used as pre-separators to reduce the dust
load reaching a final, more efficient filter. They can act as protective devices to remove large
hot particles from the gas or air stream to prevent damage to the filter media/material.
2. Wet scrubber: Even after cyclone filtering, the gas still contains fine dust, particles, and tar.
It is further cleaned by passing through a wet scrubber where gas is washed by water in Counter
current mode. The scrubber also acts like a cooler, from where the gas goes to cloth filter for
final cleaning.

3. Cloth filters: It is a fine filter in quite a few gasification systems, the hot gases are
passed through the cloth filter, and then only do they go to the cooler.
 Schematic diagram of producer gas plant

Biogas Energy
Biogas a renewable fuel that is produced when organic matter, such as food or animal
waste, is broken down by microorganisms in the absence of oxygen. This process is called
anaerobic digestion. For this to take place, the waste material needs to be enclosed in an
environment where there is no oxygen
10.1 INTRODUCTION
Anaerobic digestion is a process that breaks down organic matter into simpler chemical
components in the absence of oxygen. This process has proved to be very effective to treat
organic wastes for minimizing environmental pollution. The common organic wastes are listed
as follows:
1. Sewage sludge
2. Organic farm wastes
3. Municipal solid wastes
4. Organic industrial and commercial wastes
5. Forests and agricultural wastes
The digestion process itself takes place in digester, which is classified in terms of temperature,
water content of feedstock and the number of stages (single or multi-stage). The by-products
of anaerobic digestion, namely biogas and digestate, can be used to create a source of income.

10.2 BIOGAS AND ITS COMPOSITION

The main component of biogas is methane (CH4) which is popularly known as biogas, gobar
gas, clear gas, etc. it is clean non-polluting and low-cost gas.

Review questions:
1. List application of biomass gasifier.
2. Define biogas
 3. List the composition of biogas

Session 28
10.3 ANAEROBIC DIGESTION
It is a biological process that produces a gas (commonly known as biogas) in the absence
of oxygen and has major components of methane (CH4) and carbon dioxide (CO2).
Anaerobic digestion of methane gas production is a series of processes in which microorganism
break down biodegradable material in the absence of oxygen which completes through
following steps:
1. In the first step, the organic matter (e.g. plants residues, human and animal wastes and
residues) is decomposed (hydrolysis) to break down the organic material into usable-sized
molecules such as sugar.
2. Conversion of decomposed matter into organic acids is the second step.
3. Finally, organic acids are converted to biogas (methane gas).
10.3.1 Process Stages of Anaerobic Digestion
The biological and chemical stages of anaerobic digestion are shown in Figure 10.1. These are
divided into the following four main stages:
1. Hydrolysis
2. Acedogenesis
3. Acetogenesis
4. Methanogenesis

The four main stages are explained as follows.

10.3.1.1 Hydrolysis

Complex organic materials like carbohydrates, fats, and proteins are broken down into simpler
forms (e.g. sugars, amino acids, fatty acids). This makes them easier for microbes to digest in
the next stages.
.10.3.1.2 Acidogenesis

The simple molecules from hydrolysis are further broken down by fermentative bacteria.
They produce volatile fatty acids, hydrogen, CO₂, ammonia, and other compounds.
.
10.3.1.3 Acetogenesis

The products of acidogenesis are converted into acetic acid, hydrogen, and carbon dioxide.
These are the key substances used in the final stage to make biogas.

10.3.1.4 Methanogenesis

Methanogenic bacteria convert acetic acid and hydrogen into methane (CH₄) and carbon
dioxide (CO₂).
This stage produces the actual biogas used for energy.
.
A simplified generic chemical equation for the overall processes outlined earlier is as follows:
C6H12O6 → 3CO2 + 3CH4 (Glucose (a sugar) is broken down into methane and carbon
dioxide.)
The remaining indigestible material cannot be used by microbes and any dead bacterial remains
constitute the digestate.

10.4 BIOGAS PRODUCTION

Anaerobic processes either occur naturally or created in a controlled environment, namely a
biogas plant in which organic wastes are put in an airtight container called digester to perform
anaerobic digestion process.

10.4.1 Construction Parts of Biogas Plants

Figure 10.2 shows various parts of typical biogas plant. It is a brick and cement structure having
the following five sections:
1. Mixing tank
2. Digester tank
3. Dome or gas holder
4. Inlet chamber
5. Outlet chamber
10.4.1.1 Mixing Tank
It is the first part of biogas plants located above the ground level in which the water and cow
dung are mixed in equal proportions (the ratio of 1:1) to form the slurry that is fed into the inlet
chamber.
10.4.1.2 Digester Tank
It is a deep underground well-like structure and is divided into two chambers by a partition
wall in between. It is the most important part of the cow dung biogas plants where all the
important chemical processes or fermentation of cow dung and production of biogas takes
place. The digester is also called as fermentation tank. It is cylindrical in shape and made up of
bricks, sand, and cement built underground over the solid foundation. Two openings are
provided on the opposite sides and at the specified height of digester for inflow of fresh cow
dung slurry and outflow of used slurry as manure.
The two long cement pipes are used as follows:
1. Inlet pipe opening into the inlet chamber for inputting the slurry in digester tank.
2. Outlet pipe opening into the overflow tank (outlet chamber) for the removal of spent slurry
from the digester tank. A separator is also placed in the middle of digester tank to improve
effective fermentations of feedstock.
10.4.1.3 Dome or Gas Holder
The hemispherical top portion of the digester is called dome. It has fixed height in which all
the gas generated within the digester is collected. The gas collected in the dome exerts pressure
on the slurry in the digester. The dome or gas holder is made either fixed dome or floating
dome type. Cement and bricks are used in the construction of fixed dome, and it is constructed
using approximately at the ground surface.
Floating dome type is an inverted steel drum resting on the digester above the ground surface.
The drum floats over the digester and moves up and down with biogas pressure.
10.4.1.4 Inlet Chamber
The cow dung slurry is supplied to the digester of the biogas plant via inlet chamber, which is
made at the ground level so that the slurry can be poured easily. It has bell mouth sort of shape
and is made up of bricks, cement, and sand.
10.4.1.5 Outlet Chamber
The digested slurry from the biogas plants is removed through the outlet chamber. The opening
of the outlet chamber is also at the ground level. The slurry from the outlet chamber flows to
the pit made especially for this purpose.
10.4.1.6 Gas Outlet Pipe and Valve
The gas holder has an outlet at the top which could be connected to gas stoves for cooking or
gas-lighting equipment or any other purpose. Flow of the gas from the dome via gas pipe can
be controlled by valve. The gas taken from the pipe can be transferred to the point of use.
10.4.1.7 Foundation
The foundation forms the base of the digester where the most important processes of biogas
plant occur. It is made up of cement, concrete, and bricks strong enough so that it should be
able to provide stable foundation for the digester walls and be able to sustain the full load of
slurry filled in it. The foundation should be waterproof so that there is no percolation and
leakage of water.

10.4.2 Working of Biogas Plant

1. Cattle dung and water mixed in equal proportion and poured into inlet chamber.
2. Through inlet pipe slurry enters the fermentation tank, digestion takes place biogas is formed.
Around 60days anaerobic bacteria will ferment the slurry.
3. This gas is accumulated in gas holder. Outlet pipe will be closed due to pressure
in holder it pushes slurry to outlet chamber.
4. The biogas creates a bubble in inlet and outlet chamber during escape by understanding this
gas valve to be opened for various application.
5. As the slurry is removed fresh slurry to be added. Size of plant decides the
capacity of plant.
10.4.3 Types of Biogas Plants
Fixed dome and floating dome construction are the two types of biogas plants. Based on these
types, several biogas plant models are developed.
10.4.3.1 Fixed Dome Type
Schematic of a fixed dome biogas plant is given in Figure 10.3. It consists of following parts
1. Mixing tank: In mixing tank, the water and cattle dung are mixed thoroughly in the ratio of
1:1 to form the slurry.
2. Inlet chamber: The mixing tank opens underground into a sloping inlet chamber.
3. Digester: Digester is a huge tank with a dome type ceiling. The ceiling of the digester has an
outlet with a valve for the supply of biogas. The inlet chamber opens from below into the
digester tank. The digester opens from below into an outlet chamber which is opened from the
top into a small overflow tank.

10.4.3.1.1 Working Principle

The various forms of organic biodegradable biomass are collected and mixed with equal
amount of water properly in the mixing tank to form slurry. The slurry is fed into the
digester tank through inlet chamber and pipe, and the digester is partially filled by about half
of its height. The feeding of slurry is then discontinued for about 60 days when anaerobic
bacteria present in the slurry decomposes or ferments the biomass in the presence of water.
Biogas is then formed and starts accumulating in the upper dome area of the biogas plants, and
the pressure is exerted on the spent slurry to force it flow into the outlet chamber. Finally, the
spent slurry overflows into the overflow tank from where it is manually removed and used as
manure for agricultural crops and plants. Gas control valve at the top of dome is opened
partially or fully to supply required gas for applications. A functioning plant is fed continuously
with the prepared slurry to obtain a continuous supply of biogas
10.4.3.1.2 Advantages of fixed dome-type biogas plant are as follows:
1. The costs of a fixed dome biogas plant are relatively low as compared to floating dome type.
2. It is simple in construction as no movable dome exists.
3. It is made up of concrete, bricks, and cements and long life of the plant (20 years or more)
can be expected.
4. Underground and almost ground surface dome construction saves space and protect from
physical damage to the plant.
5. The anaerobic digestion processes in the digester are little influenced by temperature
fluctuation in day and night.
10.4.3.1.3 Disadvantages of biogas plant are as follows:
1. Porosity and cracks in plant walls is the major drawbacks.
2. Maintenance is rather difficult.
10.4.3.2 Floating Type
The floating gas holder type of biogas plant is shown in Figure 10.4. The construction and
working principle of this biogas plants is similar to fixed dome type except that gas holder
tank is made up of steel and placed on the top of digester circular tank and is movable up
and down also shown in Figure

10.4.3.2.1 Advantages Floating dome-type biogas plant has the following advantages:
1. Very efficient
2. Simple maintenance scheduling possible
10.4.3.2.2 Disadvantages Floating dome-type biogas plant has the following disadvantages:
1. Expensive
2. Steel drum may rust
3. Requires regular maintenance
BENEFITS OF BIOGAS
1. Production of energy- the calorific value of biogas is 6KWh/m3. small and medium units
are used for the purpose of cooking and lighting purpose. Large unit is used for power
generation.
2. Transformation of organic waste into high quality organic fertilizer- also called as
fertilizer producer, the fertilizer comes out has 3 times more nitrogen compared to open air
digestion. Since it is closed type nitrogen is preserved in it no chance of escaping.
3. Health benefits of biogas plant and improvement of hygienic condition- respiratory
diseases, illness, eye infection, asthma are avoided where the plant is been established. hygienic
cooking is possible and many harmful organic organisms are killed inside the plant.
4. Reduction of workload: work load for women is reduced in collecting wood from distance
places, cleaning and firing it. Home remains free from smoke and dust.
5. Environmental advantages: protection of soil, air, forest, and water i.e helps in
deforestation.
6. Global environmental benefits of biogas technology- capturing CH4 will reduce global
warming. CO2emmision is also reduced

FACTORS AFFECTING THE SELECTION OF A MODEL OF A BIOGAS PLANT

1. Cost: the initials and maintenance cost to be low as possible
2. Simplicity in design: The design should be simple for ease of operation. As rate of skilled
person using the plant is less.
3. Durability: longer lifespan of plant is required where in people will adopt the technology
and use.
4. Suitability for use with available raw materials: based on raw material used it should be
designed.
5. Input and output use frequency: input and output design must be in particular size and
potential as it frequently used.

Review questions:
1. Define anaerobic digest
2. Benefits of biogas
3. Types of biogas plant
Session 29
BIOGAS PLANT FEEDS AND THEIR CHARACTERISTICS
Any type of biodegradable material can be used as a source but for economical reason few are
preferred. Cattle dung is mostly used as input. Based on input gas production will vary.
Carbon nitrogen ratio (C\N) ratio-ratio of carbon and nitrogen present is a organic matter. If
ration is ranging from 20-30 then it is considered as optimum for anaerobic digestion. A
pH value should not exceed 8.5 if it increases it shows toxic effect. C/N ratio of cattle dung is
24 and human is 8. These may be mixed to bring it into optimum condition.

Advantage:
1. clean fuel
2. No residue, smoke, dust are produced
3. · Non polluting
 4. Health and Economical benefits.
5. Provide nutrient rich manure

Limitations:
1. high initial cost
2. availability of raw material continuously
3. social acceptability.
4. Maintenance and repair cost is high

Uses:
1. Used as domestic fuel
2. Fuel for motive power
3. Used for electricity generation

Tidal Energy
Tides are periodic rises and falls of large bodies of water. Gravity is one major force that creates
tides. In 1687, Sir Isaac Newton explained that ocean tides result from the gravitational
attraction of the sun and moon on the oceans of the earth. Spring tides are especially strong
tides that occur when the earth, the sun, and the moon are in a line. The gravitational forces of
the moon and the sun both contribute to the tides. Spring tides occur during the full moon and
the new moon. Neap tides are especially weak tides. They occur when the gravitational forces
of the moon and the sun are perpendicular to one another with respect to the earth. Neap tides
occur during quarter moons.

TIDAL ENERGY RESOURCE

Tides are the waves caused due to the gravitational pull of the moon and the sun (although
its pull is very low). The rise of seawater is called high tide and fall in seawater is called low
tide and this process of rising and receding of water waves happen twice a day and cause
enormous movement of water. Thus, enormous rising and falling movement of water is called
tidal energy, which is a large source of energy and can be harnessed in many coastal areas of
the world. Tidal dams are built near shores for this purpose in which water flows during high
tide and water flows out of dam during low tides. Thus, the head created results in turning the
turbine coupled to electrical generator.
Tidal energy has been developed on a commercial scale among the various forms of energy
contained in the oceans. When the moon, the earth, and the sun are positioned close to a
straight line, the highest tides called spring tides occur. When the earth, moon, and sun are
at right angles to each other (moon quadrature), the lowest tides called neap tides occur. The
water mass moved by the moon’s gravitational pull when moon is very close to ocean and
results in dramatic rises of the water level (tide cycle). The tide starts receding as the moon
continues its travel further over the land, away from the ocean, reducing its gravitational
influence on the ocean waters (ebb cycle).

TIDAL ENERGY AVAILABILITY

Gravitational forces between the moon, the sun, and the earth cause the rhythmic rise and fall
of ocean waters throughout the world. Those result in tide waves. The moon exerts more than
twice as great a force on the tides as the sun due to its much closer position to the earth. As a
result, the tide closely follows the moon during its rotation around the earth, creating diurnal
tide and ebb cycles at any ocean surface. The amplitude or height of the tide wave is very small
in the open ocean where it measures several centimetres in the centre of the wave distributed
over hundreds of kilometres. However, the tide can increase dramatically when it reaches
continental shelves, bringing huge masses of water into narrow bays, and river estuaries along
a coastline. For instance, the tides in the Bay of Fundy in Canada are the greatest in the world,
with amplitude between 16 and 17 m near shore. Table 11.1 gives ranges of amplitude for some
locations with large tides.
Table 11.2 provides glimpses of few potential sites for tidal power generation.

Review questions:
1. Advantages and dis advantages of biogas
2. Define tidal energy.
3. Tidal energy resources.
Session 30

TIDAL POWER GENERATION IN INDIA

Important site location and estimated power potential of a few Indian tidal energy plant is given
in Table 11.3.

Nevertheless, the possibility of developing tidal power scheme in India may be examined in
the following all aspects:
1. Economic aspects of tidal power schemes when compared to the conventional schemes.
2. Problems associated with the construction and operation of plant.
3. Problems related to the hydraulic balance of the system to minimize the fluctuation
in the power output.
4. Environmental effects of the schemes

ENERGY AVAILABILITY IN TIDES

Tidal Stream Generator

Review questions:
1. Formula for tidal energy
2. Formula for tidal power
3. Tidal energy availability

Session 31
TIDAL POWER BASIN
The basin system is the most practical method of harnessing tidal energy. It is created by
enclosing a portion of sea behind erected dams. The dam includes a sluice that is opened to
allow the tide to flow into the basin during tide rise periods and the sluice is then closed. When
the sea level drops, traditional hydropower technologies (water is allowed to run through hydro
turbines) are used to generate electricity from the elevated water in the basin.

Single-basin System

Single water reservoir is closed off by constructing dam or barrage. Sluice (gate), large
enough to admit the water during tide so that the loss of head is small, is provided in the dam.
The single-basin system has two configurations, namely:
1. One-way single-basin system: The basin is filled by seawater passing through the sluice gate
during the high tide period. When the water level in the basin is higher than the sea level at low
tide period, then power is generated by emptying the basin water through turbine generators.
This type of systems can allow power generation only for about 5 h and is followed by the
refilling of the basin. Power is generated till the level of falling tides coincides with the level
of the next rising tide.
2. Two-way single basin: This system allows power generation from the water moving from
the sea to the basin, and then, at low tide, moving back to the sea. This process requires bigger
and more expensive turbine.
Single-basin system has the drawbacks of intermittent power supply and harnessing of only
about 50% of available tidal energy
Two-basin Systems

The two basins close to each other, operate alternatively. One basin generates power when
the tide is rising (basin getting filled up) and the other basin generates power while the
tide is falling (basin getting emptied). The two basins may have a common power house or
may have separate power house for each basin. In both the cases, the power can be generated
continuously. The system could be thought of as a combination of two single-basin systems, in
which one is generating power during tiding cycle, and the other is generating power during
emptying.

Co-operating Two-basin Systems

This scheme consists of two basins at different elevation connected through the turbine. The
sluices in the high- and low-level basin communicate with seawater directly, as shown in Figure
11.3. The high-level basin sluices are called the inlet sluices and the low level as outlet sluices.
The basic operation of the scheme is as follows:
1. The rising tide fills the high-level basin through the sluiceways.
2. When the falling seawater level is equal to the water level in the high-level basin, the
sluiceways are closed to prevent the outflowing high-level basin water back to the sea.
3. The water from high-level basin is then allowed to flow through the turbine generators to
the low-level basin.
4. When the falling seawater level becomes lower than the rising water level in the low-level
basin, the sluiceways are opened to allow water to flow into the sea from the low-level basin.
This process continues until the water level in the low-level basin equals to the rising sea level.
Then, the sluiceways are closed to prevent the filling of low-level basin from the seawater.

Figure 11.4 gives another schematic diagram of co-coordinating two-basin tidal power stations.
With two basins, one is filled at high tide and the other is emptied at low tide. Turbines are
placed between the basins and between the basin and the sea. These two basin systems allow
continuous power generation. However, they are very expensive to construct due to the cost of
the extra length.

TURBINES FOR TIDAL POWER

The bulb-type turbine shown in Figure 11.5 consists of a steel shell completely enclosing the
generator that is coupled to the turbine runner.

The turbine is mounted in a tube within the structure of the barrage, and the whole machine
being always submerged. When the power demand on the system is low during the rising tides,
the unit operates as a pump to transfer water from sea to the basin. When the load on this system
is high, the unit will work as a generator, and deliver the stored energy that is a valuable
additional input to the system.
Review questions:
1. Types of tidal powder basin
2. How two basin s work
3. How bulb type turbine work.

Session 32
ADVANTAGES AND DISADVANTAGES OF TIDAL POWER

The following are the advantages of tidal power:

1. About two-third of earth’s surface is covered by water, there is scope to generate tidal energy
on large scale
2. Techniques to predict the rise and fall of tides as they follow cyclic fashion and prediction
of energy availability is well established.
3. The energy density of tidal energy is relatively higher than other renewable energy sources.
4. Tidal energy is a clean source of energy and does not require much land or other resources
as in harnessing energy from other sources.
5. It is an inexhaustible source of energy.
6. It is an environment friendly energy and does not produce greenhouse effects.
7. Efficiency of tidal power generation is far greater when compared to coal, solar, or wind
energy. Its efficiency is around 80%.
8. Even though capital investment of construction of tidal power is high, running and
maintenance costs are relatively low.
9. The life of tidal energy power plant is very long.
The following are the disadvantages of tidal power:
1. Capital investment for construction of tidal power plant is high.
2. Only a very few ideal locations for construction of plant are available and they too are
localized to coastal regions.
3. Unpredictable intensity of sea waves can cause damage to power generating units.
4. Aquatic life is influenced adversely and can disrupt the migration of fish.
5. The energy generated is not much as high and low tides occur only twice a day and
continuous energy production is not possible.
6. The actual generation is for a short period of time. The tides only happen twice a day so
electricity can be produced only for that time, approximately for 12 h and 25 min.
7. This technology is still not cost effective and more technological advancements are required
to make it commercially viable
Review questions:
1. Advantages of tidal energy
2. Dis advantages of tidal energy.