Module 4

Chapter 9: Biomass Energy

1. Introduction

 Solar energy is stored in trees and plants through photosynthesis.

 This stored energy can be converted into liquid fuels like ethanol, suitable for internal
combustion engines.

 Ethanol can also be produced on a large scale from cellulose.

 As energy costs rise, more focused research is expected in biological energy systems.

 The goal is to ensure that energy gained via intensive plant-based systems exceeds the
energy lost during conversion into fuels (ethanol, methane).

2. Importance of Sugarcane in India

 Sugarcane fermentation to produce ethanol is highly favorable.

 It's well-suited to Indian climatic conditions.

 O ers a marginal net energy gain that is viable for sustainable energy production.

3. Applications of Biomass

Biomass is used for:

 Heating

 Electric power generation

 Combined heat and power (CHP) systems

4. Biomass Conversion Methods

Several techniques exist to convert biomass into usable energy:

Method Description

Direct Burning Biomass is burned to generate electricity.

Gasification Converts biomass into synthesis gas (syngas).

Anaerobic Digestion Produces methane gas from biomass without oxygen.

5. Key Issues for Biomass Implementation

Before implementing biomass production systems, the following issues must be carefully
studied:

1. Vegetation Selection

o Identify plant species suitable for intensive energy plantations.

 o Define criteria for selecting bio-generating crops.

2. Land Availability

o Evaluate the type and quantity of land available for energy crop cultivation.

3. Harvesting Methods

o Plan e ective harvesting strategies for conceptual plantations.

4. Techno-Economic Analysis

o Compare two options:

a. Burning crops directly for electricity.
b. Converting crops to clean gaseous fuels (like methane or low BTU gas), either:

 On the farm, or

 At centralized market locations.

9.1 – Biomass Production

1. Origin and Definition

 The sun is the primary source of all renewable energy, including biomass.

 Through photosynthesis, sunlight energy is stored in plants, algae, and some bacteria.

 The simplified chemical reaction of photosynthesis:

6𝐶𝑂2 + 6𝐻2𝑂 + 𝑠𝑜𝑙𝑎𝑟 𝑒𝑛𝑒𝑟𝑔𝑦 → 𝐶6𝐻12𝑂6 + 6𝑂2

Biomass refers to organic material produced by photosynthesis and includes:

o Trees, agricultural crops, grass, algae

o Residues from processing

o Animal and human waste

2. Sources of Biomass

 Biomass can be obtained from:

o Forests and wood residues

o Agricultural land

o Arid and wastelands

o Planned or unplanned collection

o Human and animal waste

3. Advantages of Biomass

 Biomass is:

o Widely available
 o Controllable

o Often more accessible than other renewables

 Can be converted to energy via direct or indirect methods

9.1.1 – Direct Methods of Biomass Energy Use

A. Raw Biomass Sources

1. Forest wood and residues

2. Agricultural crops and by-products

3. Residential food waste

4. Industrial organic waste

5. Animal and human waste

6. Dedicated energy crops

B. Challenges in Direct Use

 Low energy density due to moisture and physical form

 Used traditionally for cooking and heating by burning

 Problems:

o Energy ine iciency

o Pollution

o Di iculties in transportation and storage

 Pre-processing and conversion technologies are needed to improve e iciency

9.1.2 – Indirect Methods of Biomass Energy Use

A. Electricity and Heat Production (Thermo-electrical Conversion)

 Biomass is combusted in boilers to produce steam → drives turbines → generates

electricity

 Also used for residential/industrial heating

 Equipment is expensive, and energy recovery is low

 However, improved technology ensures lower emissions than fossil fuels

B. Biomass Conversion to Fuels

i. Thermo-chemical Conversion

 Destructive distillation
  Pyrolysis: Heating biomass in absence of oxygen → produces bio-oil

 Gasification: Limited oxygen supply → produces syngas (usable energy gas)

ii. Biological Conversion

 Fermentation: Converts biomass (e.g., sugarcane) to ethanol

 Anaerobic digestion: Organic matter decomposed by bacteria (no oxygen) → produces

methane

C. Advantages of Indirect Methods

 Cleaner fuels like ethanol, biogas, and syngas can be created

 Gasification is attractive for producing usable gas (producer gas)

 E iciency depends on factors like:

o Biomass moisture content

o Combustion air control

o Temperature and pressure

o Exhaust gas heat

9.2 – Energy Plantation

1. Definition and Concept

 Energy plantation refers to the planned cultivation of specific tree species on a large
scale to ensure a continuous supply of wood as a fuel source.

 These plantations are designed to produce biomass for cooking, heating, or electric
power generation.

2. Tree Species Used

 Trees are selected based on fast growth, ability to coppice, and suitability to local
conditions.

Common Trees for Energy Plantation:

 Global examples: Pine, cottonwood, hybrid poplar, sweetgum, eucalyptus

 India-specific: Eucalyptus, Babool, Casuarinas

3. Advantages of Energy Plantations

 Coppicing ability: Trees regrow after cutting, allowing for repeated harvests.

 Supported by genetic improvement programs for better yield and disease resistance.

 Combines fast growth with high wood production.

 Decades of experience (past 40–50 years) have led to:

 o Improved soil preparation

o E ective planting and cultivation techniques

o Better species-soil-climate matching

o Advances in biogenetics, pest, disease, and fire control

4. Energy Plantation for Power Generation

 Biomass (wood) from plantations can be used as boiler fuel in conventional power
plants to generate electricity.

 This method is already well established in the USA and Europe with over 500 biomass
power plants.

o They use wood, wood waste, and agricultural residues.

Example Calculation:

 Assuming 1% photosynthetic conversion e iciency:

o A 1,000 MW power plant would need about 1,000 km² of plantation area.

o This area need not replace agricultural land.

5. Environmental Considerations

 Energy plantations should not lead to monoculture, as this can:

o Weaken ecological balance

o Reduce biodiversity

 Careful planning is needed to maintain ecological sustainability while promoting

renewable energy production.

9.3 – Biomass Gasification

1. Definition

Biomass gasification is the partial combustion of solid biomass (wood, crop residues) into a
combustible gas mixture known as producer gas.

2. Chemical Reaction

The generalized reaction:

Biomass + air → CO + CO₂ + CH₄ + H₂ + N₂ + H₂O (vapour)

 The gas produced is incomplete combustion, meaning less oxygen is used than
required for full combustion.

 This gas mixture is called producer gas, which contains:

o Carbon monoxide (CO)

o Carbon dioxide (CO₂)

 o Methane (CH₄)

o Hydrogen (H₂)

o Nitrogen (N₂)

3. Applications of Producer Gas

1. Running internal combustion engines

2. Substitute for furnace oil in heat-based industries

3. Production of methanol, a clean fuel and valuable chemical feedstock

4. Gasification Process Stages

1. Drying – Removal of moisture from biomass

2. Pyrolysis – Decomposition of biomass in absence of oxygen

3. Combustion – Generates the heat required for the other steps

4. Cracking – Breaks down tar molecules into lighter gases

5. Reduction – Converts CO₂ and H₂O back to CO and H₂

5. Types of Gasification

(a) Low-Temperature Gasification

 Temperature range: 750°C to 1100°C

 Produces gas with higher hydrocarbons

 Used for:

o Steam production

o Electricity generation

o Combined Heat and Power (CHP) units

(b) High-Temperature Gasification

 Temperature range: 1200°C to 1600°C

 Produces syngas (CO + H₂)

 Can be converted into synthetic diesel suitable for diesel engines

6. Properties of Producer Gas

 Composition varies with biomass type and gasifier design

 Nitrogen forms a large part (50–60%), diluting energy content

 Using oxygen instead of air improves calorific value but adds cost

 Gas yield: ~2.5 m³ of producer gas per 1 kg biomass

  Air needed: ~1.5 m³ of air/kg biomass for partial combustion
(Complete combustion needs ~4.5 m³ of air)

7. E iciency Calculation

Formula:
𝐸𝑛𝑒𝑟𝑔𝑦 𝑓𝑟𝑜𝑚 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑟 𝑔𝑎𝑠
𝜂𝑔𝑠 =
𝐸𝑛𝑒𝑟𝑔𝑦 𝑖𝑛 1 𝑘𝑔 𝑜𝑓 𝑏𝑖𝑜𝑚𝑎𝑠𝑠
Example:

 1 kg wood → 2.5 m³ of gas

 Gas calorific value: 5.4 MJ/m³

 Wood calorific value: 19.8 MJ/kg

5.4
𝜂𝑔𝑠 = 2.5 × ≈ 68.18%
19.8
 Typical e iciency: 60% – 70%

8. Average Temperature of Producer Gas

 Normal range: 300°C – 400°C

 Can rise to ~500°C if partial combustion of gas occurs

 Solution: Increase air supply to prevent gas burning inside gasifier

9. Example Gas Compositions (from Table 9.1)

Biomass Feedstock Gasifier CO (%) H₂ (%) CH₄ (%) Calorific Value (MJ/m³)

Charcoal Downdraft 28–31 5–10 1–2 4.60–5.65

Wood (12–20% MC) Downdraft 17–22 16–20 2–3 5.00–5.86

Coconut Shells Downdraft 19–24 10–15 11–15 7.20

Charcoal Updraft 30 19.7 3.6 5.98

9.4 – Theory of Gasification

1. Definition

 Gasification is a special form of pyrolysis, where biomass is thermally decomposed

(destructive distillation) in the presence of limited oxygen.

 The process operates at around 1,000°C in a reactor known as a gasifier.

 The aim is partial combustion, which yields charcoal, tars, oils, and combustible
gases rather than full oxidation products.

2. Combustion vs. Gasification

Aspect Complete Combustion Gasification (Partial Combustion)

Oxygen In excess Limited

Products CO₂, H₂O, N₂, unused O₂ CO, H₂, CH₄, tars, dust

Energy release High heat Less heat, more gas

3. Gasification Process Steps

1. Biomass → Charcoal (via pyrolysis)

2. Charcoal + CO₂/H₂O → CO + H₂

o These reactions occur in the glowing (hot) zone of the gasifier.

o This zone promotes reduction reactions that produce energy-rich gases.

4. Key Reactions in Gasification

 C + H₂O → CO + H₂ (Water–gas reaction)

 C + CO₂ → 2CO (Boudouard reaction)

These reactions are endothermic (require heat), which is supplied by the partial combustion
of biomass.

5. Gasifier Design Objective

The design must ensure:

 E icient conversion of biomass to charcoal

 Proper temperature and residence time to promote CO and H₂ production

 Removal or control of by-products like tar and dust

6. Typical Composition of Producer Gas

Gas Volume (%)

CO 20–22%

H₂ 15–18%

CH₄ 2–4%

CO₂ 9–11%

N₂ 50–54%

Note: High nitrogen content comes from the use of air as the gasifying agent.

7. Gasification Output

 Useful products:

o Carbon monoxide (CO)

 o Hydrogen (H₂)

o Methane (CH₄)

 Unwanted by-products:

o Tar (condensable hydrocarbons)

o Dust (particulate matter)

(Figure 9.1)

Inputs:

 Biomass + limited air → fed into Gasifier

Gasifier Output:

 CO, H₂, CH₄ (useful)

 Tar and dust (undesirable)

9.5 – Gasifiers and Their Classifications

Definition

A biomass gasifier is a chemical reactor where biomass is converted into producer gas or
syngas via thermochemical processes like drying, pyrolysis, combustion, and reduction.

Main Types of Gasifiers

1. Fixed Bed Gasifiers

 Biomass moves against or along the flow of the gasifying agent (air/oxygen/steam).

 Simple design, low erosion, used widely.

 Subtypes:

Type Description Air Flow Fuel Flow Key Use

Air flows from top to bottom IC engines, power

Downdraft Downward Downward
through fuel bed generation
Type Description Air Flow Fuel Flow Key Use

Air enters at bottom, gas exits Thermal use, tolerant to

Updraft Upward Downward
from top fuel variability

Air enters from the side, gas Small systems, heat +

Crossdraft Horizontal Horizontal
exits opposite side power

Figure: Fixed Bed Gasifier Types

Comparison Table: Fixed Bed Gasifiers

Feature Updraft Downdraft Crossdraft

Fuel Flexibility High (coal, briquettes) Limited (wood, charcoal) Low (dry wood, coke)

Gas Quality Low (more tar) High (cleaner gas) High

Applications Thermal only Power + thermal Heat + power

Fuel-Gas Flow Opposite Same direction Lateral

Fuel Tolerance Tolerant to ash, moisture Sensitive Requires clean fuel

Updraft: Best for diverse, low-cost fuels

Downdraft: Best for clean gas, engine use
Crossdraft: Best for small-scale, fast-response systems

2. Fluidized Bed Gasifiers

 Use sand, ash, or char as a fluidized medium.

 Biomass is suspended in a hot, bubbling bed to ensure uniform temperature and

e icient gasification.

 Advantages:

o Handles wide range of fuels

o High e iciency and heat transfer

o Better scalability and fuel flexibility

Selection Factors

 Fuel type and form

  Ash and moisture content

 Desired gas quality

 Application (power vs. thermal)

9.6 – Chemistry of Reaction Process in Gasification

Overview

Biomass gasification involves four main zones in the gasifier. Each zone features distinct
chemical reactions and temperature ranges, transforming solid fuel into a mixture of
combustible gases known as producer gas.

1. Drying Zone

 Purpose: Removes moisture from biomass

 Temperature: Up to 200°C

 Output: Water vapor and organic acids (e.g., acetic acid)

 Note: Organic acids can cause corrosion inside gasifiers

2. Pyrolysis Zone

 Temperature Range: Approximately 200°C–700°C

 Key Products: Tar, volatile gases, and light hydrocarbons

Temperature (°C) Products/Events

200–280 CO₂, acetic acid, water vapor

280–500 Tar formation, CH₃OH, CO₂

500–700 Small quantity of hydrogen gas

 Pyrolysis is the thermal decomposition of biomass in the absence of oxygen

3. Combustion (Oxidation) Zone

 Main Reaction:

𝐶 + 𝑂₂ → 𝐶𝑂₂ + ℎ𝑒𝑎𝑡
 Purpose: Supplies heat for the reduction and pyrolysis zones

 E ect: Increases internal temperature of the gasifier

4. Reduction Zone

 Condition: Oxygen-deprived environment

 Primary Reactions:
Reaction Type Chemical Equation Notes

Boudouard Reaction 𝐶 + 𝐶𝑂₂ + ℎ𝑒𝑎𝑡 → 2𝐶𝑂 Endothermic, produces CO

Water–Gas Reaction 𝐶 + 𝐻₂𝑂 + ℎ𝑒𝑎𝑡 → 𝐶𝑂 + 𝐻₂ Increases fuel content

Water–Gas Shift 𝐶𝑂 + 𝐻₂𝑂 → 𝐶𝑂₂ + 𝐻₂ + ℎ𝑒𝑎𝑡 Exothermic, lowers heating value

 Temperature Range: Approximately 800°C–1,000°C

 If temperature drops below 800°C, gas quality decreases

Gas Properties

 Heating Value: Around 4,000 to 5,000 kJ/m³

 Main Components: CO, H₂, CH₄ (combustible); CO₂, N₂ (non-combustible)

 Conversion E iciency: About 75%

𝐻𝑒𝑎𝑡 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 𝑖𝑛 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑟 𝑔𝑎𝑠
𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =
𝐻𝑒𝑎𝑡 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 𝑖𝑛 𝑖𝑛𝑝𝑢𝑡 𝑏𝑖𝑜𝑚𝑎𝑠𝑠

9.7 – Updraft Gasifiers

The updraft gasifier (also called a counter-current gasifier) is one of the oldest and simplest
types of gasifiers.

 Air (or steam) is introduced at the bottom.

 Fuel (biomass) is fed from the top.

 Gases flow upwards, while the solid fuel moves downwards.

 Combustion occurs near the grate at the bottom, followed by reduction, pyrolysis,
and drying zones moving upward.

This counter-current flow results in e icient heat transfer as the rising hot gases preheat and
pyrolyze the incoming biomass.

Gasifier Zones and Temperatures

Zone Function Approx. Temperature

Drying Zone Evaporates moisture ~200°C

Pyrolysis Zone Releases tar, volatiles ~400–600°C

Reduction Zone Produces CO, H₂ from char ~950°C

Combustion Zone Burns part of the fuel to supply heat ~2300°C (local hot spot)

Ash Pit Collects residual ash —

Syngas Characteristics

 The product gas (syngas) exits at the top.

 It contains high amounts of tar, which limits its direct use in internal combustion
engines or turbines unless cleaned or cracked.

 Best used where high flame temperature is desired and moderate dust content is
acceptable.

Advantages

 Simple design and operation

 Good heat recovery due to counter-current flow

 High thermal e iciency for heat-only applications

Limitations

 High tar content in output gas

 Not suitable for flu y or low-density fuels

 Slagging issues with high-ash biomass

Typical Applications

1. Packaged boilers

2. Thermal fluid heaters

3. Aluminium melting/annealing furnaces

4. Industrial fryers and roasters

9.8 – Downdraft Gasifiers

The downdraft gasifier is a co-current gasifier where:

 Biomass feed and gasification air both move downward through the gasifier.

 Air is introduced at or just above the oxidation (combustion) zone.

 The producer gas exits at the bottom of the gasifier, as shown in the schematic.
Working Principle

1. Biomass feed enters from the top.

2. As it descends:

o It is dried in the drying zone.

o Volatile matter is removed in the pyrolysis zone.

o The oxidation zone combusts part of the char to generate heat.

o In the reduction zone, hot gases react with the remaining char to form CO and
H₂.

3. Gasification air is injected into a narrow throat section, creating high-temperature

conditions that crack tar and oils, resulting in low tar gas output.

Advantages

 Low tar content in syngas (suitable for engines)

 Fast start-up time (5–10 minutes)

 Good for medium-to-small scale thermal and power applications

Limitations

 Sensitive to fuel moisture (preferably below 20%)

 Char throat design can be prone to clogging if not properly maintained

 Not suitable for high-ash fuels

Typical Applications

1. Continuous baking ovens (bread, biscuits, paint)

2. Rotary ovens (batch baking)

3. Dryers and curing units (tea, co ee, mosquito coil, paper)

4. Boilers

5. Thermal fluid heaters

6. Annealing furnaces

7. Direct fired rotary kilns

 8. Internal combustion (IC) engines

9.9 – Cross-Draft Gasifier

A cross-draft gasifier is a fixed-bed gasifier where the air enters from one side, and the
producer gas exits from the opposite side. Its design separates the ash bin, combustion
zone, and reduction zone, unlike updraft or downdraft types.

Working Principle

1. Biomass (e.g., charcoal, wood, coke) is fed from the top at regular intervals.

2. As fuel descends:

o It goes through drying, pyrolysis, combustion, and reduction zones.

3. Air is introduced laterally (from the side), and gas is drawn horizontally across the
bed.

4. The producer gas exits from the opposite side, not from the top or bottom as in other
designs.

Key Features

 Operates best with dry air blast and low ash fuels (e.g., charcoal, wood).

 Not suitable for high ash or high moisture biomass.

 High temperatures are achieved in the combustion zone.

 Produces gas with:

o High CO content

o Low H₂ and CH₄ content

esign Characteristics

 Typically a vertical cylindrical vessel.

 Zones are spatially separated, which improves control but limits fuel flexibility.

 Compact and suitable for small-scale, on-demand applications.

Advantages

 Simple design
  Rapid response and quick start-up

 Produces high-temperature flame (useful in heat-intensive processes)

Limitations

 Limited to low ash, dry fuels

 Lower hydrogen yield

 Not ideal for large-scale or variable load operations

9.10 – Fluidized Bed Gasification

Fluidized bed gasification is a high-e iciency, flexible, and clean biomass conversion
technology. It is widely used for prepared biomass and waste fuels such as:

 Wood waste

 Bark

 Agricultural residues

 Refuse Derived Fuel (RDF)

It produces a low-emission fuel gas suitable for boilers, dryers, kilns, and other industrial heat
applications.

Working Principle

 A bed of inert particles (sand, ash, or char) is fluidized using a stream of air, oxygen, or
steam from below.

 When the bed reaches a target temperature, fuel is introduced from the bottom.

 The fuel mixes rapidly with the hot bed material and is instantly pyrolyzed and
gasified.

 There are no distinct zones like in fixed-bed gasifiers; instead, drying, pyrolysis, and
gasification happen simultaneously.

 Tar cracking and gas-phase reactions further enhance gas quality.

Component Function

Fluidized bed Heat transfer medium and reaction zone

Distributor plate Controls reactive gas flow

Fuel input Introduces biomass into bed

Cyclone Removes particulates from syngas

Ash outlet Collects and removes ash

Recirculation system Reintroduces fine particles back into the reactor

9.10.1 – Advantages and Benefits

9.10.1.1 Advantages
1. Cost-e ective operation: Reduces dependence on expensive fossil fuels.

2. Lower capital costs: Especially for steam generation compared to new boilers.

3. Energy independence: Reduces reliance on LPG, natural gas, and oil.

9.10.1.2 Benefits
Benefit Description

Thermal e iciency ranges from 70% to 90%, depending on fuel moisture

High E iciency
and ash.

Can handle varied biomass types and particle sizes (e.g., sawdust,
Fuel Flexibility
bark, chips).

High Reliability No moving parts in high-temp zones → fewer breakdowns.

Low Installation
Compact design enables modular skid-mounted systems.
Cost

Flexible Operation Can supply gas to multiple devices simultaneously.

Low Emissions Clean-burning process, often no exhaust treatment required.

Applications

Fluidized bed gasifiers are suitable for:

 Boilers

 Veneer dryers

 Rotary kilns

 Multiple simultaneous industrial applications

9.11 – Use of Biomass Gasifier
Applications

Biomass gasifiers convert solid biomass into producer gas, which can be used for:

Thermal Applications

 Cooking

 Water heating

 Steam generation

 Drying and space heating

Power Generation

 Internal Combustion (IC) Engines

o Spark-Ignition Engines: Can run solely on producer gas (with air).

o Compression-Ignition Engines (Diesel Engines): Operate in dual-fuel mode

(25% diesel + producer gas).

Note: Before being used in engines, the gas must be cleaned using:

 Cyclone separator

 Scrubber

 Filter unit

Performance & Capacity in India

India is a global leader in biomass gasification technology, with facilities for testing and
standardization.

Application Output Range

Thermal 60,000 – 5 × 10⁶ kJ/h

Electrical 3 – 500 kW

Largest System Example:

 Thermal Output: 5 × 10⁶ kJ/h (≈ 1,450 kW)

 Electric Output: 500 kW

 Fuel Used: Wood blocks (25–100 mm, 500 kg/h)

 Gas Output: 1,250 m³/h

 Engine Type: Compression-ignition (dual-fuel, uses only 25% diesel)

9.11.1 – Liquid Fuels from Biomass
Methanol Production

 Source: Wood or straw

 Process: Gasification → Chemical synthesis

 Application: Substitute fuel

Ethanol Production

 Source: Sugarcane, maize, cassava, tapioca

 Process: Fermentation

 Usage: Ethanol–petrol blend for automotive fuel

Limitations for India:

Large-scale ethanol production demands extensive agricultural land, which is limited in India
(unlike Brazil, which produces ethanol on a large scale).

Photosynthesis and Energy

Photosynthesis plays a central role in:

 Carbon cycling and oxygen production

 Formation of fossil fuels (from ancient photosynthetic organisms)

 Two types:

o Oxygenic photosynthesis: Produces oxygen (plants, algae, cyanobacteria)

o Anoxygenic photosynthesis: Does not produce oxygen (some bacteria)

9.12 – Gasifier Biomass Feed Characteristics

There is no universal gasifier. Each gasifier design must be tailored to the specific
characteristics of the biomass feed.

Key Biomass Feed Parameters Influencing Gasifier Design:

1. Energy content

2. Bulk density

3. Moisture content

4. Dust content

5. Tar content

6. Ash and slagging behavior

9.12.1 Energy Content & Bulk Density
 Higher energy content + higher bulk density → longer operation from a single fuel
charge.

 Results in smaller gasifier volume for same energy output.

9.12.2 Moisture Content

 Moisture increases heat loss, cooling load, and pressure drop.

 Desirable: < 20% moisture content

 Requires pre-treatment (drying) for wet biomass.

9.12.3 Dust Content

 Clogs engine and filters.

 High dust → frequent filter maintenance.

 Gasifier design must minimize dust production.

9.12.4 Tar Content

 Tar clogs carburettors and valves → causes malfunction.

 ~200 complex chemical compounds in tar.

 Di icult to eliminate during gasification; thus removed using filters and coolers.

 More research needed in in-gasifier tar cracking.

9.12.5 Ash and Slagging Characteristics

 Ash = oxidized mineral residue post-combustion.

 Problems:

1. Slag formation → clogs fuel flow.

2. Ash covers ignition points → lowers reactivity.

➤ Slagging Mitigation Methods:

Type Method

Low-temperature Steam or water injection

High-temperature Melt ash and tap molten slag

Reliable Biomass Fuels

 1. Charcoal

o Tar-free, low ash, reliable

o Loses ~50% of wood’s original energy in production

2. Wood

o Widely tested and compatible with many gasifiers

9.12.6 Common Biomass Feedstocks

1. For Bioethanol

 Sugarcane, corn, wheat, sugar beet, sweet sorghum, cassava

2. For Biodiesel

 Rapeseed, sunflower, soybean, canola, jatropha, coconut, palm oil

3. Second-Generation Feedstocks (Cellulosic)

 Woody plants, grassy crops, forest/agricultural residues, municipal waste

4. Waste-Based Biomass

 Manure, sludge, pulp mill waste, food industry wastewater

 Used in anaerobic digestion for methane production

9.13 – Applications of Biomass Gasifiers

Biomass gasifiers produce producer gas that can be used for multiple purposes in energy,
agriculture, industry, and chemical production.

1. Motive Power Applications

Used to generate mechanical (shaft) power for industrial/agricultural machinery:

 (a) Diesel engines (dual-fuel or 100% gas mode)

 (b) Water pumps

 (c) Agricultural vehicles (tractors, harvesters)

 (d) High-e iciency Stirling engines

2. Direct Heat Applications

Gasifier heat is used directly in thermal processes:

 (a) Crop/Food Drying:

E.g., large cardamom, ginger, rubber, tea
Temp range: 85°C–125°C
  (b) Tile & Pottery Baking:
Temp range: 800°C–900°C

 (c) Metal Melting (non-ferrous):

Temp range: 700°C–1000°C

 (d) Boiler fuel for producing steam/hot water in:

o Silk reeling

o Dyeing

o Turmeric boiling

o Cooking

o Jaggery (gur) making

3. Electrical Power Generation

 Generation of electricity from few kW to hundreds of kW

 For local use or grid integration

 Ideal for o -grid rural electrification and small-scale energy independence

4. Chemical Production

Producer gas can be a chemical feedstock for:

 Methanol

 Formic acid

9.14 – Cooling and Cleaning of Producer Gas

For safe and e icient use of producer gas (e.g., in engines or burners), it must be cooled and
cleaned to remove:

 Dust

 Tar

 Moisture

Why Cooling and Cleaning is Necessary

  Gas exits the gasifier at 300°C to 500°C.

 Cooling increases energy density and protects equipment.

 Cleaning ensures tar-free, moisture-free, and dust-free gas, which is crucial for
engine performance and burner life.

Cooling System

 Most systems use gas-to-air heat exchangers with natural (free) convection.

 Some coolers also perform partial scrubbing (removal of moisture + tar).

 Ideal condition: Ambient temperature gas, free from tar and water.

Gas Cleaning System

Three types of filters are used in sequence:

1. Cyclone Filter

 First stage of cleaning.

 Removes large particles (above 5 µm).

 Especially e ective since 60–65% of particles in producer gas are above 60 µm.

2. Wet Scrubber

 Second stage.

 Removes fine dust, particles, and tar.

 Uses counter-current water flow to clean gas.

 Also cools the gas.

3. Cloth Filter (e.g., Fiberglass filter)

 Final cleaning stage.

 Removes remaining fine particles.

 Must be used before condensation (i.e., when gas is still hot and above dew point), or
water will clog the filter.

Chapter 10 – Biogas Energy

10.1 Introduction
Anaerobic Digestion: Core Process

 Anaerobic digestion is the breakdown of organic matter in the absence of oxygen.

 It produces:

o Biogas (a renewable energy source)

 o Manure (used as an organic fertilizer)

Purpose & Benefits

 Waste treatment: Reduces environmental pollution by decomposing organic waste

safely.

 Energy generation: Converts waste into usable biogas.

 Soil enrichment: Produces organic manure, improving agricultural productivity.

 Income & comfort: Supports rural livelihoods and clean energy access.

Digesters: Classification Criteria

Anaerobic digesters are classified based on:

1. Feedstock type (e.g., human waste, animal dung, crop residues)

2. Water content (dry or slurry-based)

3. Operating temperature (mesophilic or thermophilic)

4. Digestion stages (single-stage or multi-stage)

Common Feedstocks

Organic materials used include:

 Human and animal excreta

 Agricultural and forest wastes

 Sewage sludge

 Municipal solid waste

 Organic farm waste

 Organic commercial and industrial waste

Applications

 Domestic use: Cooking, lighting, heating

 Industrial use: Waste management, energy generation

 Agriculture: Fertilizer from digested slurry

10.2 Biogas and Its Composition

 Biogas is a clean, low-cost, and non-polluting fuel primarily composed of methane
(CH₄), ranging from 50% to 70%.

 The main source of methane is the anaerobic digestion of organic waste in the absence
of oxygen.

Key Properties:
  Methane is highly inflammable and provides 9,000 kcal/m³ of energy.

 It is also referred to by names such as:

o Sewerage gas

o Marsh gas

o Gobar gas

o Bio-energy

Combustion Characteristics:

 Ignition temperature: 650°C–750°C

 Burning e iciency: ~60% in conventional biogas stoves

 Calorific value: ~20 MJ/m³

 Biogas is ~20% lighter than air, and burns with a clear blue flame.

Energy Equivalence (1,000 ft³ of biogas ≈):

 600 ft³ of natural gas

 4.6 gallons of diesel

 5.2 gallons of gasoline

 6.4 gallons of butane

Typical Composition (Table 10.1):

S. No. Substance Symbol Percentage

1 Methane CH₄ 50–70%

2 Carbon dioxide CO₂ 30–40%

3 Hydrogen H₂ 5–10%

4 Nitrogen N₂ 1–2%

5 Water vapour H₂O 0.2–0.3%

6 Hydrogen sulphide H₂S Trace

Domestic Utility:

 A family with 4 cows/bu aloes can produce ~175 ft³/day of biogas, su icient for
cooking and lighting.

 Benefits:

o Cleaner indoor air (no smoke from traditional biomass)

o Easier utensil cleaning

 o Produces enriched organic manure as a by-product

10.3 Anaerobic Digestion

Definition:

 Anaerobic digestion is a biological process that breaks down organic matter in the
absence of oxygen, producing biogas, primarily methane (CH₄) and carbon dioxide
(CO₂).

Biogas Sources:

 Organic feedstock includes plant residues, animal/human waste, agricultural and

food waste.

10.3.1 Process Stages of Anaerobic Digestion

Anaerobic digestion takes place in four key stages:

1. Hydrolysis

 Purpose: Breaks down complex organic molecules (e.g. carbohydrates, proteins, fats)
into simple sugars, fatty acids, and amino acids.

 Result: Molecules become soluble and accessible to bacteria.

 Key Products: Sugars, amino acids, fatty acids, hydrogen, acetate.

2. Acidogenesis

 Purpose: Fermentative bacteria further break down the products of hydrolysis.

 Key Products:

o Volatile fatty acids (VFAs)

o Ammonia (NH₃)

o Carbon dioxide (CO₂)

o Hydrogen sulphide (H₂S)

3. Acetogenesis

 Purpose: Converts VFAs and alcohols into acetic acid, hydrogen, and carbon dioxide.
  Prepares the substrates required for the next stage (methanogenesis).

4. Methanogenesis

 Purpose: Final stage where methanogenic bacteria convert:

o Acetic acid, hydrogen, and CO₂ into methane (CH₄) and CO₂.

 Sensitivity: Methanogenesis is highly sensitive to pH changes.

Overall Reaction Equation:

𝐶6𝐻12𝑂6 → 3𝐶𝑂2 + 3𝐶𝐻4

(Glucose to carbon dioxide and methane)

By-product: Digestate

 Digestate is the non-digestible residue and includes dead microbial cells and
undigested organic matter.

 It is rich in nutrients and often used as organic fertilizer.

10.4 Biogas Production

Principle:

 Biogas is produced through the biodegradation of organic material (like cow dung)
under anaerobic (oxygen-free) conditions.

 This process occurs naturally or in a biogas plant, where anaerobic digestion is carried
out in a sealed digester.

10.4.1 Construction Parts of a Biogas Plant

A typical biogas plant consists of the following seven major components, as shown in the
structure (Figure 10.2):

1. Mixing Tank

 Located above ground.

 Function: Mixes cow dung and water in 1:1 ratio to form slurry.

 The slurry is then fed into the digester via the inlet chamber.
2. Digester Tank (Fermentation Tank)

 Underground cylindrical chamber made of brick, cement, and sand.

 Divided internally into two chambers by a partition wall.

 Main function: Anaerobic digestion of slurry to produce biogas.

 Two pipe connections:

o Inlet pipe: For fresh slurry input.

o Outlet pipe: For spent slurry (manure) removal.

 Contains a separator to improve fermentation e iciency.

3. Dome or Gas Holder

 Hemispherical top part of the digester.

 Collects biogas produced during fermentation.

 Two types:

o Fixed dome: Made of bricks and cement.

o Floating dome: Steel drum that moves up/down with gas pressure.

4. Inlet Chamber

 Located at ground level.

 Shaped like a bell mouth.

 Function: Receives slurry from the mixing tank and directs it into the digester.

5. Outlet Chamber

 Located at ground level, opposite to inlet chamber.

 Function: Releases digested slurry (manure) from the digester into a separate
collection pit.

6. Gas Outlet Pipe and Valve

 Located at the top of the dome.

 Connects to cooking appliances, lights, or other gas-using devices.

 Valve: Controls flow of gas.

7. Foundation

 Base of the digester made of concrete, bricks, and waterproofing material.

 Provides structural stability and prevents leakage.

10.4.2 Working of a Biogas Plant
The biogas plant operates on anaerobic digestion inside a sealed digester. The process
involves the following sequential steps:

1. Slurry Preparation

 Cattle dung and water are mixed in 1:1 ratio in the mixing tank to form slurry.

 The slurry is poured into the digester through the inlet chamber until it fills up to the
cylindrical level.

2. Anaerobic Digestion

 Inside the digester, the slurry undergoes anaerobic fermentation.

 Over time, biogas is produced as a result of microbial breakdown.

3. Gas Collection

 The biogas accumulates in the dome (gas holder) at the top of the digester.

 As gas builds up, pressure increases, pushing slurry into the inlet and outlet
chambers.

 The slurry level in the digester drops, and it rises in the outlet chamber.

4. Pressure Regulation

 If the gas valve remains closed, pressure increases further.

 Eventually, the gas may escape through inlet/outlet chambers, causing bubbling and
froth formation.

5. Gas Quantity Estimation

 The rise of slurry level in the inlet and outlet chambers indicates the amount of gas
generated.

6. Gas Usage

 Opening the gas valve (fully or partially) allows the gas to be drawn out for applications
like cooking or lighting.

 As gas is withdrawn, pressure drops, and slurry returns to its original level in the
digester.

7. Continuous Operation

 Used slurry is expelled via the outlet chamber.

 Fresh slurry must be added regularly to maintain continuous biogas production.

 The size of the digester determines the plant’s gas generation capacity.
10.4.3 Types of Biogas Plants
Biogas plants are mainly classified into two types:

 Fixed Dome Type

 Floating Dome Type

10.4.3.1 Fixed Dome Type

Structure (see Fig. 10.3)

 Mixing Tank: Slurry (cow dung + water) mixed in 1:1 ratio.

 Inlet Chamber: Sloped passage underground, guides slurry into digester.

 Digester Tank: Dome-shaped structure where anaerobic digestion occurs.

 Gas Outlet: At the top of the dome, equipped with a control valve.

 Outlet Chamber: Collects spent slurry.

 Overflow Tank: Stores excess slurry exiting from outlet.

Working Principle

1. Slurry is fed through the inlet into the digester and left undisturbed (~60 days).

2. Anaerobic bacteria decompose biomass, producing biogas (mainly methane).

3. Gas accumulates in the dome, and pressure pushes the digested slurry into the outlet
chamber and finally into the overflow tank.

4. Gas is withdrawn via the gas control valve for use.

5. Continuous operation is maintained by regular feeding of slurry.

Advantages

1. Lower cost than floating dome type.

2. Simple design with no moving parts.

3. Durable: Constructed with bricks and cement (lifespan >20 years).

4. Space-e icient: Built underground, resistant to external damage.

5. Stable performance: Less a ected by daily temperature fluctuations.

Disadvantages
 1. Risk of porosity and cracks in structure.

2. Di icult to maintain or repair once installed.

10.4.3.2 Floating Dome Type

Structure (see Fig. 10.4 & Fig. 10.2)

 Similar to fixed dome type, but:

 Gas Holder: A movable steel drum that floats over the digester.

 Moves up/down based on gas accumulation.

Working Principle

 Functions similar to fixed dome type.

 Gas collects in the floating drum, which rises as gas accumulates and falls as gas is
consumed.

 Pressure from the drum pushes digested slurry out via outlet chamber.

Advantages

1. High e iciency in gas collection.

2. Easy maintenance of gas holder (movable and accessible).

Disadvantages

1. Costlier than fixed dome due to steel components.

2. Steel drum is prone to rust and corrosion.

3. Requires frequent maintenance to ensure mobility and gas tightness.

10.4.4 Di erent Models of Biogas Plants

Biogas plants come in a variety of designs and capacities, typically ranging from 2 to 180
m³/day. In India, around 3 million small-capacity biogas plants have been installed, though
social acceptability and other issues have limited widespread adoption.

Key Points

 KVIC (Khadi and Village Industries Commission) model is one of the most common
and traditional designs (see Figures 10.2 & 10.4).
  A 2 m³/day KVIC unit costs around ₹15,000.

 Other designs range from ₹10,000 to ₹25,000 for similar capacities.

 Community biogas plants are promoted for better waste utilization and operational
control (temperature, scum, pH, etc.).

10.4.4.1 Types of Fixed Dome Biogas Plants

Model Origin / Notes

1. Chinese Fixed - Dome is arch-shaped. - Digester is cylindrical with round top and
Dome bottom. - Millions built in China.

- First fixed dome model in India. - Cracking and gas leakage issues. - No
2. Janata Model
longer in use.

3. Deenbandhu - Improved version of Janata. - Crack-resistant and cost-e ective. - Uses

Model a hemispherical digester.

4. CAMARTEC - Developed in Tanzania. - Simplified hemispherical dome on a rigid

Model foundation ring.

10.4.4.2 Types of Floating Drum Biogas Plants

Model Characteristics

- Oldest and most popular floating drum model in India. - Traditional steel
1. KVIC Model
drum design.

2. Pragati Model - Features a hemispherical digester.

3. Ganesh
- Uses angular steel and plastic foil.
Model

4. Arati Biogas
- Low-cost design using plastic water tanks or fiberglass drums.
Model

- Combines the durability of fixed dome with stability of floating drum. -

5. BORDA Model
Includes water jacket for long life.

10.5 BENEFITS OF BIOGAS

Biogas energy systems o er a wide array of economic, environmental, and health-related
benefits for individuals, communities, and the planet.

1. Energy Production (Heat, Light, Electricity)

 Calorific value: ~6 kWh/m³ (≈ 0.5 litres of diesel oil).

 Used as a substitute for conventional fuels in cooking and heating.

  Small/medium plants (≤ 6 m³/day): used mainly for cooking and lighting.

 Larger plants: can power engines or electricity generators.

2. Organic Waste Management and Fertilizer Production

 Converts manure and biomass into high-quality fertilizer.

 Digestate is 3x richer in nitrogen than open-air compost.

 Nitrogen is preserved in the digester, not lost as ammonia like in open composting.

 Biogas plants do not add nitrogen, they preserve existing nutrients e iciently.

3. Health and Hygiene Benefits

 Biogas reduces respiratory illnesses (asthma, eye infections, etc.).

 Homes using biogas have cleaner indoor air due to smoke-free cooking.

 Pathogens and parasites (e.g., cholera, hookworm, typhoid bacteria) are destroyed in
the digester.

 Promotes nutritious cooking: improves food digestibility and hygiene.

4. Reduced Workload for Women

 Saves time and e ort in:

o Collecting firewood

o Cleaning smoke-stained utensils

 Improves home environment:

o No smoke

o Less dust

o Reduces need for firewood storage space

5. Environmental Protection (Local Scale)

 Protects forests by reducing wood fuel demand.

 Improves soil quality with better organic fertilizers.

 Reduces overgrazing through improved fodder availability.

 Conserves water and air quality by controlling pollution sources.

6. Global Environmental Benefits

 Renewable energy source that reduces:

o Fossil fuel dependence

o Carbon dioxide emissions

o Methane emissions (a potent greenhouse gas) by capturing it.

  Biogas helps in mitigating global warming and climate change e ects.

10.6 FACTORS AFFECTING THE SELECTION OF A BIOGAS PLANT

MODEL
Choosing the appropriate model of a biogas plant depends on technical, economic,
environmental, and operational factors. The key considerations are:

1. Cost

 Initial (construction) and maintenance costs must be as low as possible.

 The cost should be economical per unit of gas produced.

 A ordability is essential for both individual users and society at large.

2. Simplicity in Design

 Design must be:

o Simple to construct

o Easy to operate and maintain

 This is crucial in areas with:

o Low literacy

o Limited access to skilled labor

 Simpler designs enhance user adoption and reliability.

3. Durability

 Long lifespan is important, especially in:

o Remote or rural areas

o Regions with low motivation or awareness

 Durable construction o sets higher initial investment by providing:

o Long-term performance

o Reduced need for frequent repairs

4. Compatibility with Available Inputs

 Design must suit the type of feedstock:

o For agricultural waste (like rice/maize straw), batch or discontinuous feed

systems are preferred.

o For animal dung or kitchen waste, continuous or semi-continuous systems

are suitable.

 Plant model must ensure e icient digestion of the specific waste available.
5. Usage Frequency (Inputs and Outputs)

 Frequency of biogas use (daily, occasional) influences:

o Size and capacity of the gas holder

o Storage requirements

 Input feeding pattern (regular or occasional) a ects:

o Choice of feeding mechanism (batch vs. continuous)

o Sizing of slurry tanks and digesters

10.7 BIOGAS PLANT FEEDS AND THEIR CHARACTERISTICS

Biogas plants can utilize any biodegradable organic material, but cost, availability, and gas
yield make some materials more suitable than others.

Key Points

 Cattle dung is the most commonly used feedstock due to abundance and suitability.

 Economic value comes from:

o Biogas (energy)

o Slurry (organic fertilizer)

o Avoiding landfill/disposal costs

10.7.1 Carbon/Nitrogen (C/N) Ratio

 C/N Ratio a ects methanogenic activity and gas production e iciency:

o Optimal range: 20–30

o Too high C/N → Nitrogen deficiency → Incomplete digestion → Low gas

o Too low C/N → Ammonia buildup → High pH (>8.5) → Toxic to microbes

Examples of C/N Ratios (Table 10.3):

Raw Material C/N Ratio

Duck/ Human dung 8

Chicken dung 10

Goat/ Pig dung 12–18

Cow/ Bu alo dung 24

Raw Material C/N Ratio

Water hyacinth 25

Elephant dung 43

Maize straw 60

Rice straw 70

Wheat straw 90

Sawdust >200

Cattle dung (C/N ~24) is ideal. High-C/N materials (e.g., straw) should be mixed with low-C/N
materials (e.g., human waste) to balance the ratio.

Biogas Potential by Feedstock (Table 10.2):

Type of Dung Gas per kg (m³)

Cattle 0.023–0.040

Pig 0.040–0.059

Poultry (Chicken) 0.065–0.116

Human 0.020–0.028

10.7.2 Advantages of Biogas Feeds

1. Clean fuel with high calorific value

2. Smoke-, residue-, and dust-free

3. Non-polluting; provides health benefits

4. Generates nutrient-rich manure (N & P)

5. Cost-e ective with long-term use

10.7.3 Limitations
1. High initial installation cost

2. Irregular availability of organic material

3. Social resistance to human/animal waste reuse

4. Requires periodic maintenance

10.7.4 Uses of Biogas
1. Domestic fuel (cooking, lighting)

2. Fuel for motive power

3. Electricity generation

Here is a well-organized summary of Chapter 11: Tidal Energy, focusing on section 11.1 –
General:

Chapter 11: Tidal Energy

11.1 General Overview
Oceans o er several renewable energy opportunities through the movement and temperature
of water. These can be harnessed in three primary ways:

1. Tidal Energy

 Involves storing seawater during high tides in a reservoir behind a dam constructed
onshore.

 During low tide, the potential energy of the stored water is used to drive turbines and
generate electricity, similar to a hydropower plant.

 Key requirement: A minimum tidal range (di erence between high and low tides) of at
least 4 meters is essential for economic viability.

2. Wave Energy

 Uses the kinetic energy of ocean waves to rotate underwater turbines, functioning like
a submerged windmill.

 Converts dynamic wave motion into electrical power.

3. Ocean Thermal Energy

 Relies on the temperature gradient between warm surface water and cold deep-sea
water.

 This temperature di erence is utilized to generate electricity using a thermal cycle,

similar to geothermal energy systems.

11.2 Tidal Energy Resource

What Causes Tides?

 Gravitational pull of the moon (primary) and the sun (secondary) causes the rise and
fall of sea levels:
 o High tide: Water rises.

o Low tide: Water recedes.

 This cycle occurs twice a day and causes enormous water movement.

Tidal Energy Defined

 Tidal energy is the kinetic and potential energy from the periodic rise and fall of
ocean water.

 This energy can be harnessed in coastal areas using tidal dams.

Working Principle

 Tidal dams are built near the shore.

 During high tide, seawater flows into the dam reservoir.

 During low tide, water flows out, creating a head (di erence in water levels).

 This head is used to rotate turbines, which drive electric generators.

Types of Tides

Type of Tide Occurs When E ect on Tide Height

Spring Tide Earth, Moon, and Sun are in a straight line Highest tidal rise

Neap Tide Earth, Moon, and Sun form a right angle Lowest tidal range

idal Cycle

 When the moon is near the ocean, gravitational pull is strong → high tide (rising).

 As the moon moves away, pull weakens → low tide (receding or ebb tide).

11.3 Tidal Energy Availability

Cause of Tides

 Tides result from gravitational interactions between:

o Moon (primary influence)

o Sun (secondary, weaker due to distance)

o Earth

 The moon causes diurnal tide and ebb cycles, as it moves around the earth.

Tide Behavior

 In the open ocean, tide wave amplitude is small (only a few cm).

 When tides approach continental shelves or narrow bays, the amplitude increases
drastically, creating large tides.
Examples of Large Tides

Country Site Highest Tide Range (m)

Canada Bay of Fundy 16.2

France Port of Granville 14.7

England Severn Estuary 14.5

India Gulf of Cambay (Gujarat) 11

India Gulf of Kutch (Gujarat) 8

India Sundarban (Ganges Delta, W. Bengal) 5

Russia Penzhinskaya Guba 13.4

Russia Bay of Mezen (White Sea) 10

Tidal Project Feasibility Factors

 Highly site-specific: Needs significant tidal range (preferably >4m).

 Favorable basin topography is essential for cost-e ective construction.

 No fossil fuels used, making it clean energy.

Challenges / Concerns

 Environmental impacts include:

o High silt formation (as tidal flow is blocked)

o Disruption to marine ecosystems

 Tidal projects tend to have greater ecological impact than wave energy projects.

Reliability

 Highly predictable compared to solar or wind energy.

 Tidal patterns are known long in advance.

Potential Tidal Power Sites Globally

Country Site Avg. Tide Height (m) Basin Area (km²) Power Potential (MW)

USA Cook Inlet 4.35 3100 18,000

Russia Mezen 5.66 2640 15,000

Russia Tugur 5.38 1080 6,790

UK Severn 8.3 490 6,000

Country Site Avg. Tide Height (m) Basin Area (km²) Power Potential (MW)

UK Mersey 8.4 60 700

Argentina San-Jose 6.0 780 7,000

Australia Walcoti 8.4 260 1,750

Australia Secure 8.4 130 570

Korea Carolina Bay 4.7 90 480

11.4 Tidal Power Generation in India

Overview

India, with its long coastline, estuaries, and gulfs, possesses several regions with strong tidal
ranges, suitable for tidal power generation. These areas are highly favorable for converting
tidal energy into electricity using tidal turbines.

Major Indian Tidal Energy Sites

Site Location Tide Heights (m) Estimated Power Potential (MW)

Gulf of Cambay, Gujarat 11 (6.7 average) 7,000

Gulf of Kutch, Gujarat 8 (5.23 average) 12,000

Ganges Delta, Sundarban, West Bengal 5 (2.97 average) 8,000

Note: These locations have tidal ranges large enough to drive power turbines e iciently.

Current Developments

 Several government bodies and organizations are actively engaged in constructing

tidal power plants at the identified sites.

 The Gulf of Cambay is considered to have the largest tidal energy reservoir in India.

Other Promising Exploration Zones

 Western Coast:

o Gulf of Kutch (Mandva)

o Gulf of Cambay (Hazira)

o Maharashtra (Janjira and Dharmata)

 Eastern Coast:

o Hooghly Delta

o Chhatarpur
 o Puri (Odisha)

Key Considerations for Tidal Project Implementation

1. Economic Viability:

o Compare the cost-e ectiveness of tidal power vs conventional power

schemes.

2. Construction and Operation Challenges:

o Assess technical di iculties in setting up and maintaining tidal plants.

3. Hydraulic System Stability:

o Ensure power output is stable despite tidal fluctuations.

4. Environmental Impact:

o Study e ects on marine life, siltation, and coastal ecosystems.

11.5 Leading Countries in Tidal Power Plant Installation

Several countries have invested in tidal energy projects, with varying installed capacities
based on their coastal geography and technological infrastructure.

Installed Tidal Power Capacities – Table

Country Site Location Installed Capacity (MW)

France La Rance 240 MW (24 bulb-type turbines × 10 MW each)

UK The Severn Barrage 8,560 MW (214 turbines × 40 MW each)

Russia Kislaya Guba 0.4 MW

Canada Annapolis 18 MW

China — 3.9 MW

11.6 Energy Availability in Tides

Tides carry both potential energy (due to height di erence) and kinetic energy (due to water
movement). Tidal power systems harness either or both forms.

11.6.1 Potential Energy of Tides

 Potential energy (E) is due to the vertical rise and fall of water.

Equation:
1
𝐸= 𝜌𝑔𝐴ℎ
2
Where:

o E = energy (Joules or Watt-hours)

o ρ = seawater density (~1025 kg/m³)

o g = acceleration due to gravity (9.8 m/s²)

o A = area under consideration (m²)

o h = tidal amplitude (m)

 Simplified form per square meter of sea surface:

𝐸 = 1.4ℎ 𝑊ℎ 𝑜𝑟 𝐸 = 5.04ℎ 𝑘𝐽
 Only a fraction of this energy is usable due to environmental limits (typically 20–60%,
called significant extraction factor).

11.6.2 Kinetic Energy of Tides

 Kinetic Energy (KE) is the energy of moving water mass:

Equation:
1
𝐾𝐸 = 𝑚𝑉
2
Where:

o m = mass of water (kg)

o V = water flow velocity (m/s)

 This energy is captured using tidal stream generators (also called tidal energy
converters or TECs).

11.6.3 Power Output from Potential Energy

 For a dam-based tidal plant, average power over a tidal cycle is:

𝑃 = 0.226𝐴𝐻
Where:

o P = power in watts

o A = area of the reservoir (m²)

o H = tidal range (m)

 Assumes:

o 𝜌 = 1040 𝑘𝑔/𝑚^3
o Tidal period T=6 hours
11.6.4 Power Output from Tidal Stream Generator
 TECs function like underwater wind turbines, using water flow to spin blades.

Equation:
1
𝑃 = 𝜌𝐴ℎ𝑇𝑉
2
Where:

o P = power (W)

o ρ = seawater density

o A = swept/capture area

o hT = overall e iciency (water to wire)

o V = flow velocity (m/s)

Key point: Tidal energy is more predictable than wind or solar, but depends on location-
specific factors like water depth, flow velocity, and tidal range.

11.7 Tidal Power Basin

The tidal basin system is one of the most practical methods for harnessing tidal energy. A
basin is formed by building a dam (barrage) to enclose part of the sea.

Working Principle:

 High tide: Water enters the basin through sluice gates.

 Low tide: When the sea level falls, water from the basin is released through turbines to
generate electricity.

 The amount of power generated is proportional to the square of the head (height
di erence), as per Equation 11.7:

𝑃 = 0.226𝐴𝐻
Due to variation in tidal range, tidal basin systems have been developed in di erent
configurations to maximize power generation.

11.7.1 Single-Basin System

This is the simplest and most common arrangement.

Structure:
Types:

1. One-Way Single-Basin System:

o Water fills the basin during high tide through sluice gates.

o Power is generated during low tide when water is released from basin to sea
through turbines.

o Generates power for ~5 hours per tidal cycle.

o Disadvantage: Long intervals with no power generation.

2. Two-Way Single-Basin System:

o Power is generated both during incoming and outgoing tidal flows.

o Requires reversible, more expensive turbines.

o Provides more continuous power, but with higher cost.

Limitations of Single-Basin System:

 Intermittent power supply (not continuous).

 Can utilize only ~50% of the total tidal energy.

Diagram Reference (Figure 11.1):

11.7.2 Two-Basin Systems

The two-basin tidal power system is an enhancement over the single-basin system. It aims to
provide a more continuous and steady power output.

Diagram (Figure 11.2):

  Depicts two basins side-by-side.

 Each has its own sluices and power house.

 The sea flows in and out of the basins, enabling alternating power generation.

Working Principle:

 Two basins operate in alternating cycles:

o One basin generates power during tide rise (filling).

o The other basin generates power during tide fall (emptying).

 Turbines and sluice gates are used to control flow and generate electricity.

 Powerhouses may be common or separate for each basin.

Advantages:

 Provides more continuous power compared to a single-basin system.

 Allows for better regulation of turbine output by adjusting flow valves.

Limitations:

 It cannot completely eliminate fluctuations between spring tides (higher) and neap
tides (lower).

 Only partially solves the problem of inconsistent tidal energy output.

Solution: Combined Operation with Pumped Storage or Steam Plant

1. Tidal + Pumped Storage Plant:

o Surplus tidal energy is used to pump water into an upper reservoir.

o During low tide output, water from the reservoir is released to generate
additional power.

o Technically feasible but costly due to high infrastructure and storage capacity
needs.

2. Tidal + Steam Plant via Grid:

o Tidal energy reduces the load on steam plants when available.

o Saves fuel and reduces wear on steam generators.

o Requires matching installed capacities, making it expensive overall.

11.7.3 Co-operating Two-Basin Systems

This type of tidal power system uses two basins at di erent elevations and allows for nearly
continuous power generation by smart water flow management.

Working Principle:
  Two basins:

o High-level basin (connected to sea via inlet sluices)

o Low-level basin (connected to sea via outlet sluices)

 The basins are connected through turbines, which generate electricity as water flows
from the higher basin to the lower one.

Operational Cycle:

1. Rising Tide:

o Seawater enters the high-level basin through inlet sluices.

2. Peak Tide:

o When sea level equals high-basin level, inlet sluices are closed to prevent
backflow.

3. Power Generation:

o Water flows from high-level to low-level basin through turbine generators,

producing electricity.

4. Low Tide:

o When sea level is lower than the low-level basin, outlet sluices are opened.

o Water flows from low-level basin to the sea.

5. Start of New Cycle:

o As sea level rises again to match low-level basin, the low-basin sluices are
closed.

o When it reaches high-basin level, high-basin sluices are reopened, repeating

the cycle.

Advantages:

 Allows continuous or near-continuous power generation.

 More e icient than single-basin systems.

 Avoids the direct impact of fluctuating tidal levels.

Limitations:

 High construction cost due to the need for:

o Two separate basins.

o Extra sluices and turbines.

o Longer dykes or embankments.

Figure 11.3 Description (Co-operating System):

  High- and low-level basins side-by-side.

 Powerhouse with turbines in between.

 Inlet sluices allow seawater into high-level basin.

 Outlet sluices release water from low-level basin to sea.

Figure 11.4 – Coordinating Two-Basin System (Alternative Layout):

 Shows Basin A and Basin B arranged to:

o Fill one basin at high tide.

o Empty the other at low tide.

 Turbines placed between the basins and sea to generate power during both phases.

11.8 Turbines for Tidal Power

Tidal turbines must maintain high e iciency under rapidly varying water head conditions.
Di erent types of turbines are used to handle this challenge.

Types of Suitable Turbines:

1. Kaplan Turbine:

o Adjustable blades

o Performs well under varying heads

o Suitable for tidal applications

2. Propeller Turbine:
 o Similar to Kaplan but with less complexity

o Blade angle can be adjusted for falling water

o Good for capturing energy during ebb tides

3. Bulb-type Reversible Horizontal Turbine:

o Invented by French engineers

o Functions both as a turbine (during high demand) and a pump (during low
demand)

o Used in La Rance Power Station, France

11.8.1 Bulb-type Turbine – Details:

 Design:

o Compact unit with generator enclosed in a steel shell

o Mounted in the flow passage (submerged)

o Includes runner vanes, metal casing, and draft tube

 Working:

o During rising tide (low demand): Acts as a pump (sea → basin)

o During falling tide (high demand): Works as a generator (basin → sea)

 Advantages:

o Reversible operation (dual mode: pump and generator)

o Compact design

 Drawbacks:

o Hard to maintain due to submerged placement

o Water flows around the generator, complicating servicing

11.8.2 Commercial Status of Tidal Stream Devices (as of 2009):
Company Class Technology Country Year Stage

Aqua Marine Power Tidal Horizontal Axis Turbine UK 2007 Prototype

Verdant Power Tidal Horizontal Axis Turbine US 2000 Commercial

Marine Current Turbines Tidal Horizontal Axis Turbine UK 2000 Commercial

SMD Hydrovision Tidal Horizontal Axis Turbine UK 2003 Prototype

Open-Hydro Tidal Open Centre Turbine Ireland 2006 Pre-commercial

Hammerfest Strom Tidal Horizontal Axis Turbine Norway 2007 Pilot

11.9 Advantages and Disadvantages of Tidal Power

Advantages:

1. Large Potential:

o About ⅔ of Earth is covered by water, giving large-scale scope for tidal energy.

2. Predictable Source:

o Tides follow a cyclic, predictable pattern, enabling accurate energy

forecasting.

3. High Energy Density:

o Tidal energy has higher energy density compared to other renewables.

4. Eco-Friendly:

o Clean energy source with minimal land usage and no emissions.

5. Inexhaustible:

o Tidal energy is a renewable and endless resource.

6. No Greenhouse Gases:

o Produces no pollution or CO₂ emissions.

7. High E iciency:

o E iciency ~80%, much higher than coal (~30–35%), wind (~30–45%), and solar
(~15–20%).

8. Low Operational Costs:

o High initial investment, but low running and maintenance costs.

9. Long Lifespan:

o Tidal plants have a long operational life (several decades).

Disadvantages:

1. High Initial Cost:

o Capital cost is very high due to dam/barrage construction.

2. Limited Suitable Sites:

o Few ideal locations, restricted to certain coastal areas.

3. Vulnerability to Wave Damage:

o Unpredictable sea conditions can damage turbines and equipment.

4. Impact on Aquatic Life:

o Can disrupt fish migration and a ect marine biodiversity.

5. Intermittent Generation:

o Tides occur only twice a day, so power is not continuous.

6. Short Generation Window:

o Actual power generation is possible for only about 12 hours 25 minutes per day.

7. Still Expensive:

o Not yet commercially viable without further technological improvement.

11.10 Problems in Exploiting Tidal Energy

Geographical & Transmission Challenges

1. Remote Generation Sites:

o Tidal energy is produced far from consumption areas, requiring costly and
complex transmission infrastructure.

2. Limited Suitable Locations:

o Requires specific geographic and tidal conditions:

 Mean tidal amplitude ≥ 7 meters

 Semi-diurnal tides (2 high & low tides/day)

o Only a few global sites meet these conditions.

3. Ocean-Only Construction:

o Plants can only be built on coastlines, making them irrelevant for inland
communities.

Technical & Operational Issues

4. Intermittent Power Supply:

o Power is generated only ~10 hours/day when tides are moving.

 o Often mismatched with electricity demand.

5. Low Energy Output:

o Current designs do not produce large amounts of electricity.

6. High Cost:

o Very expensive to construct tidal plants and barrages.

o Also slow to build, increasing total investment risk.

Environmental Impact

7. Ecosystem Disruption:

o Alters natural flow, sedimentation, and local vegetation.

o Can lead to:

 Erosion

 Winter icing

 Reduced flushing of bays

8. Wildlife Threats:

o Fish migration blocked (e.g., salmon killed by turbines)

o Bird habitats a ected (mudflats no longer exposed during low tide)

9. Fish Ladder Limitations:

o Installed to help fish migration, but never 100% e ective.

10. Tidal Level Changes:

 Can impact:

o Navigation

o Recreation

o Shoreline flooding

o Marine life

11. Barrages Block Outlets:

 May require locks for navigation—costly and slow to operate.

12. Long-Term Environmental Impact:

 Estuaries a ected miles upstream/downstream.