UNIT-5: BIO-MASS & OTEC

• Bio-mass: Availability of bio-mass and its conversion theory

• Ocean Thermal Energy Conversion (OTEC): Availability, theory and working principle,
performance and limitations.
• Wave and Tidal Wave: Principle of working, performance and limitations.
• Waste Recycling Plants.
Biomass resources
Biomass resources for energy production encompass a wide spectrum of materials ranging from silviculture (forest), agriculture
(field), aquaculture (fresh and sea water) and industrial and social activities that produce organic waste residues (food
processing, urban refuse etc.). The following are the resources of biomass.
(i) Forests: Forests, natural as well as cultivated, serve as a source of fuelwood, charcoal and producer gas. Forest waste and
residues from forest-processing industries can be utilized at the mill itself. Forest resource is consumed, not just for firewood but
also for sawn timber, paper making and other industrial purposes.
(ii) Agricultural Residues: Crop residues such as straw, rice husk, coconut shell, groundnut shell, sugar cane bagasse are gasified
to obtain producer gas. Alternatively, these are converted to fuel pellets or briquettes and used as solid fuel.
(iii) Energy Crops: Certain cultivated plants produce raw material for biofuels. The greatest potential for energy farming occurs in
tropical countries, especially those with adequate rainfall and soil conditions. There are some plants which produce raw material
as follows: (a) Sugar plants (b) Starch plants (c) Oil producing plants
(iv) Aquatic Plants: Some water plants grow faster than land-based plants and provide raw materials for producing biogas or
ethanol. These are water hyacinth, kelp, seaweed an algae.
(v) Urban Waste: Urban waste is of two types: (a) Municipal Solid Waste (MSW or garbage) and (b) sewage (liquid waste).
Energy from MSW can be obtained from direct combustion or as a landfill gas. Sewage can be used to produce biogas after
some processing.
Usable forms of biomass:
Biomass is organic material that reacts with oxygen in combustion and natural metabolic processes to release heat. Sometimes, it
is used as such in its original form but more often it is transformed into modern energy forms such as liquid and gaseous fuels,
electricity, and processes heat to provide energy services needed by rural and urban populations and also by industry.
(i) Fuelwood (Virgin Wood): Wood is the most obvious and oldest source of biomass energy. This was the main source of energy
used by mankind for centuries. Direct combustion is the simplest way to obtain heat energy. Its energy density is 16-20 MJ/kg. It
can also be convertedto more useful forms such as charcoal or producer gas. About half of the world population depends on
(ii)Charcoal: Charcoal is a clean (smokeless), dry, solid fuel, black in colour. It has 75-80% carbon content and has energy density
of about 30 MJ/kg. It is obtained by the carbonisation process of woody biomass to achieve higher energy density per unit
mass, thus making it more economical to transport. It can be used as fuel in domestic environment as it burns without smoke.
In the industrial sector it is used in specialised applications where specific fuel characteristics are required, such as high carbon
and low sulphur content. Chemical grade charcoal has many uses in laboratory and industrial chemical processes. It is also used
for making high-quality steel. It is in common use in many developing countries such as Brazil.
(iii) Fuel Pellets and Briquettes: Crop residues such as straw, rice husk and waste wood are pressed to form lumps, known as
fuel pellets or briquettes and used as solid fuel, The purpose is to reduce moisture content and increase the energy density of
biomass making it more feasible for long distance transportation.
(iv) Bio-diesel: Some vegetable oils, edible as well as non-edible, can be used (after some chemical processing) in pure form or
blended with petroleum diesel as fuel in a compression ignition (diesel) engine. Bio-diesel is simple to use, biodegradable, non-
toxic, and essentially free of sulphur and aromatics.
(v) Bio-ethanol: Ethanol (C,H,OH) is a colourless liquid biofuel. Its boiling point is 78°C and energy density is 26.9 MJ/kg. It can
be derived from wet biomass containing sugars (e.g., sugarcane, sugarbeet, sweet sorghum), starches (grains and tubers such as
potato, cassava) or cellulose (woody matter). The main constituents of woody matter are legnin (fibrous part) and cellulose
(juicy part). Ethanol is largely produced from sugarcane (maize in the USA).
(vi)Biogas: Organic wastes from plants, animals and humans contain enough energy to contribute significantly to energy supply
in many areas, particularly the rural regions of developing countries. Aquatic biomass can also be used. Biogas is produced in a
biogas ferment or digester. It is used for cooking, lighting (using mantle lamps), heating and operating small IC engines. It is
unlikely to be used for mobile vehicles on a large scale because of low pressure and high inert fraction. Use of biogas is
widespread in rural China and India. Two-third of China's rural families use biogas as their primary fuel.
(vii) Producer Gas: Woody matter such as crop residue, wood chips, bagasse (fibrous residue of sugarcane after juice
extraction), rice husk and coconut shells, can be transformed to producer gas (also known as synthesis gas, syn gas, wood gas,
and water gas or blue gas) by a method known as gasification of solid fuel.
Advantages of biomass:
1.It is a renewable source of energy.
2. It contains condensed carbon, hydrogen and oxygen molecules.
3. Ethanol from waste can be used as a bio fuel.
4. Waste disposal problem is reduced.
5. Rural development is encouraged,
6. The pollutant emissions from combustion of biomass are usually lower than those from fossil fuels.
7. Commercial use of biomass may avoid or reduce the problems of waste disposal in other industries.
8. Use of biogas plant, apart from supplying clean gas, also leads to improved sanitation, better hygienic condition in rural areas.
9. The nitrogen rich bio-digested slurry and sludge from a biogas plant serves as a very good soil condition and improves the
fertility of the soil.
10. The varying capacity can be installed and any capacity can be operated.
disadvantages of biomass:
1.Lower thermal contents than fossil fuel,
2. High moisture contents.
3. It is a dispersed and land intensive source.
4. It is often of low energy density.
5. Not feasible to set up at all locations.
6. The capacity is determined by availability of biomass and not suitable for varying loads.
7. It is also labour-intensive and the cost of collecting large quantities for commercial application is significant.
8. High transportation cost.
9. Feeding difficulty in the system.
Biomass conversion theory:
1. Physical Method: The simplest form of physical conversion of biomass is through compression of combustible material: Its
density is increased by reducing the volume by compression through the processes called pelletization and briquetting.
(a) Pelletization: Pelletization is a process in which waste wood is pulverized, dried and forced under pressure through an
extrusion device. The extracted mass is in the form of pellets (rod; 5 to 10 mm dia and 12 mm long), facilitating its use in
steam power plants and gasification system. Pelletization reduces the moisture to about 7 to 10 per cent and increases the
heat value of the biomass.
(b) Briquetting: Briquetting is the process of making small size compressed blocks (briquettes) to get more surface area per unit
weight of biomass by adding suitable binder and is an old well known technique.it includes
(i) Moisture removal
(ii) densification
2. Direct Combustion (Incineration): In this process the biomass is burned in the presence of oxygen and produces heat, light and
products. Any biomass (wood, crop residues, waste, animal dung in the form of dung cake etc.) having density equivalent to
wood, if burnt in open (chulhas), gives heat with thermal efficiency of 3-4%.
3. Thermochemical: The basic thermochemical process to convert biomass into a more valuable and/or convenient product is
known as pyrolysis. Biomass is heated either in absence of oxygen or by partial combustion of some of the biomass in restricted
air or oxygen supply. Pyrolysis can process all forms of organic materials including rubber and plastics, which cannot be handled
by other methods. The products are three types of fuels—usually, a gas mixture (H,, CO, CO2, CH, and N2), an oil-like liquid (a
water-soluble phase including acetic acid, acetone, methanol and a non-aqueous phase including oil and tar) and a nearly pure
carbon char. The distribution of these products depends upon the type of feedstock, the temperature and pressure during the
process and its duration and the heating rate.
4. Biochemical: There are two types of biochemical conversion process of biomass. The fuel is used for the purpose of heating,
electricity generating and fuel cell etc.
(a) Ethanol (Alcoholic) fermentation: The ethanol fuel has a considerable potential substitute for oil and is a growing market in
future. Ethanol is not confined to road transport but has many other applications, e.g., cogeneration, domestic appliances,
chemical applications, aviation fuel. The 10% ethanol (C,H,OH) is produced by fermentation of sugar (C,H,206) and is
separated from mixture by distillation and used as gasohol (petrol containing up to 26% ethanol). Alcoholic fermentation is
the decomposition in the absence of air of simple hexose sugars (sugars containing six carbon atoms per molecule, i.e.,
C,H,206) in aqueous solution by the action of an enzyme (a natural catalyst) present in yeast, in acidic conditions (pH value of
4 to 5). Thus,The general chemical equation for ethanol fermentation is:
C6H12O6 (glucose) → 2 C2H5OH (ethanol) + 2 CO2 (carbon dioxide)

Anaerobic Fermentation (Anaerobic Digestion): This process converts decaying wet biomass and animal wastes into biogas
through the decomposition process by the action of anaerobic bacteria (bacteria that live and grow in absence of oxygen).
Carbon present in biomass may be ultimately divided between fully oxidized CO, and fully reduced CH4.
Biogas is also known as marsh gas, wet gas, swamp gas and also as gobar gas in India. It contains mainly methane (55 to 65%)
and carbon dioxide (30 to 40%). It also contains a very small percentage of ammonia, hydrogen sulphide and nitrogen. It is a slow
burning gas having a calorific value in between 5000 to 5500 kcal/kg. Biogas is also known as sewer gas, in India. This is the
mixture of gas produced by methanogenic bacteria while acting upon biodegradable materials in an anaerobic condition. Biogas
is about 20 per cent lighter than air and has an ignition temperature in the range of 650°C to 750°C. It is an odourless and
colourless gas that burns with clear blue flame similar to that of LPG gas. Its calorific value is 20 MJ/m and burns with 60%
efficiency in a conventional biogas stove. The biogas is alike natural gas, with methane a main common fuel. The natural gas is
created naturally while biogas is created in biodigesters
Biogas is produced from wet biomass with about 90-95% water content by the action of anaerobic bacteria. Part of the carbon is
oxidized and another part reduced to produce CO2, and CH4 These bacteria live and grow without oxygen. They derive the
needed oxygen by decomposing biomass. The process is favoured by wet, warm and dark conditions. The airtight equipment
used for conversion is known as a biogas plant or digester, which is constructed and controlled to favour methane production.
The conversion process is known as anaerobic fermentation (or biodigestion). Nutrients such as soluble nitrogen compounds
remain available in solution and provide excellent fertilizer and humus. The energy available from the combustion of biogas is 60-
90% of the input dry matter heat of combustion. Thus, the energy-conversion efficiency of the process is 60-90%.
Various stages of anaerobic digestion process.
Stage I: First of all, the original organic matter containing complex compounds, e.g., carbohydrates, protein and fats is broken
down through the influence of water (known as hydrolysis) to simple water-soluble compounds. The polymers (large molecules)
are reduced to monomers (basic molecules). The process takes about a day at 25°C in an active digester.
Stage II: The micro-organisms of anaerobic and facultative (that can live and grow with or without oxygen) groups, together
known as acid formers, produce mainly acetic and propionic acids. This stage also takes about one day at 25°C. Much of CO2, is
released in this stage.
Stage III: Anaerobic bacteria, also known as methane formers, slowly digest the products available from the second stage to
produce methane, carbon dioxide, a small amount of hydrogen and a trace amount of other gases. The process takes about two
Biogas plant:
The biogas plant is a physical structure, commonly known as biodigester. Since various chemical and microbiological reactions
take place in the biodigester, it is also known as bio-reactor or anaerobic reactor. The central part of a plant is an enclosed tank
known as the digester and the ratio of diameter to its length is kept 1.5:1.0. The main function of this structure is to provide
anaerobic condition within it. As a chamber, it should be air and water tight. It can be made of various construction materials and
in different shapes and sizes. Construction of this structure forms a major part of the investment cost. This is an airtight tank
filled with the organic waste, and which can be emptied out to remove slurry with some means. The various designs available
depend on the type of organic waste to be used as raw material, the temperature and the materials available for construction.
The biogas plant has to start with dung (7% solid) then by mixing vegetables etc. Table 9.3 gives the requirements of average
daily input with the size of Biodigesters plant. In well run digesters, each tonne of input will produce 200-400 m of biogas with
50% - 75% methane.
A biogas batch plant system refers to a type of anaerobic digestion system designed to produce biogas through the
decomposition of organic materials in batches. Biogas is a renewable energy source that primarily consists of methane and
carbon dioxide, generated through the microbial breakdown of organic matter in the absence of oxygen.
Here's a general overview of how a biogas batch plant system works:
Feedstock Preparation: Organic materials such as agricultural residues, crop waste, food scraps, or animal manure serve as
feedstock for the biogas batch plant. These materials need to be properly prepared by shredding or chopping to increase their
surface area, which facilitates the microbial digestion process.
Loading Batch Tanks: The prepared feedstock is loaded into batch tanks or reactors. These tanks are sealed to create anaerobic
(oxygen-free) conditions necessary for the activity of methanogenic bacteria that produce methane.
Anaerobic Digestion: The anaerobic digestion process takes place within the batch tanks. Microorganisms break down the
organic matter in the absence of oxygen, producing biogas as a byproduct. This process typically involves multiple stages,
including hydrolysis, acidogenesis, acetogenesis, and methanogenesis.
Hydrolysis: Complex organic compounds are broken down into simpler compounds.
Acidogenesis: Simple compounds are converted into volatile fatty acids.
Acetogenesis: Volatile fatty acids are converted into acetate and hydrogen.
Methanogenesis: Methane is produced from acetate and hydrogen.
Biogas Collection: The biogas produced during the anaerobic digestion process is collected and stored. It primarily consists of
methane (CH₄) and carbon dioxide (CO₂), with trace amounts of other gases.
Utilization of Biogas: The collected biogas can be used as a renewable energy source for various applications. Common uses
include electricity generation, heating, and cooking. Biogas can also be upgraded to biomethane for injection into natural gas
pipelines or used as a transportation fuel.
Residue Management: After the anaerobic digestion process, the remaining slurry or digestate is rich in nutrients and can be
used as a fertilizer. Proper management of the digestate is essential to avoid environmental impacts.
Batch systems have the advantage of simplicity and ease of control, making them suitable for smaller-scale operations or
Floating drum type biogas plant (khadi village industries commission model (KVIC))
A floating drum type biogas plant is a specific design of a fixed-dome biogas digester that is often used for small-scale,
decentralized biogas production. It's a popular choice in rural areas for households or communities where there is a sufficient
supply of organic waste, such as animal manure or kitchen waste. The floating drum biogas plant consists of several key
components:
Digester Tank: The main part of the biogas plant is the digester tank, which is typically constructed using brick and mortar or
other materials that can withstand the corrosive nature of the digester environment. The tank is partially buried in the ground
and has a dome-shaped top.
Floating Drum: The key feature of this design is the floating drum, which is a gas-tight, dome-shaped container that "floats" on
the surface of the slurry inside the digester. The drum is usually made of steel
or reinforced concrete and is
sealed to trap the biogas produced
during the anaerobic digestion
process.
Inlet Pipe,
Gas Outlet Pipe,
Overflow Pipe: Excess liquid
generated during the digestion
process can overflow from the
digester and is typically channeled
to a storage facility or used as
fertilizer.
 Deenbandhu biogas plant
KVIC design of bio digester

Janata model Manipal model

ocean thermal energy conversion
The conversion of solar energy stored as heat in the ocean into electrical energy is known as ocean thermal energy conversion
(OTEC). Ocean thermal energy conversion is a technology that converts solar radiation into electric energy. It uses the ocean's
temperature gradient (the ocean's layers of water have different temperatures) to generate the power based on second law of
thermodynamics. As long as the temperature between the warm surface water and the cold deep water differs by about 20°C, an
OTEC system can produce a significant amount of power, with little impact on the surrounding environment. the theoretical
efficiency of conversion is nearly 2-3% only. Ocean Thermal Energy Conversion (OTEC) is a new technology, needed to be
harnessed especially in India where the coastline is about 6000 km. Basically, the OTEC converts the thermal energy, available
due to temperature difference between the warm surface water and the cold deep water, into electricity. Power from the OTEC is
renewable and eco-friendly. An OTEC plant can operate in remote islands and sea shore continuously. It is very low grade solar
thermal energy, so the efficiency of energy recovery is quite low.
The main objective of ocean thermal energy conversion is to utilize the solar energy trapped by the ocean into usable energy.
This kind of energy is found in tropical oceans where the 20°C difference between top surface and at the depth is sufficient to
provide thermal energy continuously, to be utilized for useful work., The principle of OTEC is that there is a temperature
difference between water at the bottom of the sea and the water at the top. This temperature difference can be used to operate
a heat engine. Most of the radiation is being absorbed at the surface layer of water and become lighter. The mixing between hot
water and cold water is prevented because no thermal convection occurs between hot and cold water layer. This means that the
surface layer will act as a source and cold layer act as a sink. Therefore, it is essential to connect the reversible heat engine
between source and cold sink to produce work, that can be converted into required applications. The absorption of solar
radiation in the water varies and can be expressed by Lambert's law
OTEC stands for Ocean Thermal Energy Conversion, and it is a method for generating electricity by utilizing the temperature
difference between warm surface seawater and cold deep seawater..Here's an overview of the theory and working of OTEC:
1. Temperature Difference in the Ocean: The surface of the ocean is heated by the sun, resulting in warm surface water.
At deeper levels, the water is much colder.
2. Working Fluid and System Types: OTEC systems typically use a working fluid with a low boiling point, such as ammonia or a
mixture of ammonia and water. There are three main types of OTEC systems: closed-cycle, open-cycle, and hybrid systems.
3. Closed-Cycle OTEC: Warm surface seawater is used to vaporize the working fluid with a low boiling point (like ammonia) in a
heat exchanger. The vaporized fluid is then used to drive a turbine connected to a generator, producing electricity.
Cold seawater from deeper levels is used to condense the vapor back into a liquid state.
4. Open-Cycle OTEC: Warm surface seawater is directly evaporated in a low-pressure chamber, creating steam.The steam is then
used to drive a turbine connected to a generator.Cold seawater from deeper levels is used to condense the steam back into
water, which is then pumped back to the ocean.
5. Hybrid OTEC: Combines elements of both closed-cycle and open-cycle systems to optimize efficiency.
6. Efficiency and Environmental Impact: OTEC systems can achieve higher efficiencies in tropical regions where the temperature
difference between surface and deep seawater is significant. OTEC is considered a clean and renewable energy source as it
doesn't rely on fuel combustion and produces minimal greenhouse gas emissions.
7. Applications: OTEC can be used for various applications, including electricity generation, desalination of seawater, and
providing cooling for onshore facilities.
8. Challenges and Considerations: OTEC systems face engineering challenges related to materials that can withstand the
corrosive nature of seawater.The economics of OTEC are influenced by factors such as the cost of constructing and maintaining
offshore facilities.
9. Global Potential: OTEC has the potential to provide a significant and continuous source of renewable energy, especially in
tropical regions where temperature differences are most pronounced. Overall, while OTEC has great potential as a renewable
energy source, it is not yet widely implemented on a large scale. Research and development continue to address technical and
economic challenges to make OTEC more viable and competitive in the energy market.
Advantages of Ocean Thermal Energy Conversion (OTEC):
Renewable and Sustainable:
OTEC utilizes the temperature difference between warm surface seawater and cold deep seawater, making it a renewable
and sustainable energy source.
Constant Energy Production:
OTEC systems can potentially operate continuously as long as there is a sufficient temperature gradient, providing a stable
and reliable source of electricity.
Clean Energy:
OTEC produces electricity without burning fossil fuels or emitting greenhouse gases, making it an environmentally friendly
and low-impact energy source.
Multipurpose Applications:
OTEC can be integrated with other technologies, such as
desalination processes or providing cooling for onshore facilities,
increasing its versatility.
Abundant Resource in Tropical Regions:
Tropical regions with warm surface waters and deep cold waters
are ideal locations for OTEC, and these areas often have a high
potential for energy production.
Reduced Dependency on Fossil Fuels:
OTEC has the potential to reduce reliance on traditional energy
sources, particularly in coastal regions with access to suitable
ocean conditions.
Limitations and Challenges of OTEC:
High Initial Costs:
The construction and deployment of OTEC facilities involve high initial capital costs, which can be a significant barrier to
widespread adoption.
Engineering Challenges:
Designing and constructing OTEC systems require materials that can withstand the harsh and corrosive marine environment,
posing engineering challenges.
Location-Specific:
OTEC is most efficient in tropical regions with a substantial temperature difference between surface and deep seawater. This
geographical limitation restricts its applicability to specific locations.
Energy Transmission:
In many cases, OTEC facilities are located offshore, and transmitting the generated electricity to onshore locations may
involve the use of undersea cables, which can be technically challenging and expensive.
Limited Efficiency: OTEC systems typically have lower efficiency compared to some other renewable energy sources, such as solar
or wind power.
Environmental Impact:While OTEC is considered a clean energy source, the discharge of nutrient-rich deep seawater near the
surface could potentially affect marine ecosystems, and careful monitoring and mitigation measures are necessary.
Competition with Other Renewable Sources:
OTEC competes with other well-established and rapidly advancing renewable energy technologies, such as solar and wind
power, which may be more economically viable in certain regions.
Lack of Commercial-Scale Implementation:
As of my last knowledge update in January 2022, there were limited examples of large-scale commercial OTEC
implementations, indicating that the technology has not yet achieved widespread adoption.
Wave energy : Wave energy is a form of renewable energy that can be harnessed from the motion of the waves. There are
several methods of harnessing wave energy that involve placing electricity generators on the surface of the ocean.
Oscillation water column
An oscillating water column is a partially submerged, hollow structure. It is open to the sea below the waterline, enclosing a
column of air on top of a column of water. Waves cause the water column to rise and fall, which in turn compresses and
decompresses the air column. This trapped air is allowed to flow to and from the atmosphere via a turbine, which usually can
rotate regardless of the direction of the airflow. The rotation of the turbine is used to generate electricity.
Hose pump :The device is a heaving floating body that converts kinetic energy of vertical water motion into electricity via a
hose pump system. First experiences with power generating buoys, dating back to the 1970s, were carried on by Budal and
Falnes
The Pelamis Wave energy Converter (WEC) is an innovative concept for extracting energy from ocean waves and converting it
into a useful product such as electricity, direct hydraulic pressure or potable water. The system is a semi-submerged, articulated
structure composed of cylindrical sections linked by hinged joints.
Advantages Marine energy is a renewable, clean source of energy, only requiring water's natural movement to generate power.
Marine energy resources are abundant throughout the United States. The country is home to miles of ocean coastline and river
resources, posing incredible potential for capitalizing on this resource.
Disadvantages: The biggest disadvantage of obtaining energy from waves is location, only power plants and cities near the
ocean will directly benefit from its potential. Generating energy from waves can be dangerous for some nearby species.
Machinery can alter the seabed, change habitats near the coast, and generate noise pollution.