Diploma in Mechanical Engg. Babasaheb Phadtare Polytechnic, kalamb-walchandnagar Sub- Power plant engineering Unit-Waste heat recovery, co geration and tri-generation. By- Prof. Kokare Amol Yashwant

1. Unit No. 4 Waste Heat Recovery,
Cogeneration and Tri-generation
Department of Mechanical Engineering Prof. Kokare A.Y.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Subject- Power Plant Engineering

2. Unit No. 4
Waste Heat Recovery,
Cogeneration and Tri-generation
Course Outcome (CO):
Measure waste heat recovery in a
typical thermal power plants.

3. Waste Heat Recovery
• Introduction
A waste heat recovery unit (WHRU) isan energy recovery heat exchanger that
transfers heat from process outputs at high temperature to another part of the
process for some purpose, usually increased efficiency. The WHRU is a tool
involved in cogeneration. Waste heat may be extracted from sources such as hot
flue gases from a diesel generator, steam from cooling towers, or even waste
water from cooling processes such as in steel cooling.
Waste heat is heat, which is generated in a process by way of fuel combustion
or chemical reaction, and then “dumped” into the environment even though it
could still be reused for some useful and economic purpose. The essential
quality of heat is not the amount but rather its “value”. The strategy of how
to recover this heat depends in part on the temperature of the waste
• Waste heat found in the exhaust gas of various processes or even from the exhaust
stream of a conditioning unit can be used to preheat the incoming gas. This is one
of the basic methods for recovery of waste heat. Many steel making plants use this
process as an economic method to increase the production of the plant with lower
fuel demand.

4. Classification of WHRS on Temperature
Range Bases
High Temperature Heat Recovery
The following Table 8.2 gives temperatures of waste gases from industrial process equipment in the high temperature
range.All of these results from direct fuel fired processes.
Medium Temperature Heat Recovery
The following Table 8.3 gives the temperatures of waste gases from process equipment in the medium temperature range.
Most of the waste heat in this temperature range comes from the exhaust of directly fired processunits.
Low Temperature Heat Recovery
The following Table 8.4 lists some heat sources in the low temperature range. In this range it is usually not practical to
extract work from the source, though steam production may not be completely excluded if there is a need for low-
pressure steam. Low temperature waste heat may be useful in a supplementary way for preheating purposes.

5. Classification of WHRS on Temperature
Range Bases
Low Temperature Heat Recovery
The following Table 8.4 lists some heat sources
in the low temperature range. In this range it is
usually not practical to extract work from the
source, though steam production may not be
completely excluded if there is a need for low-
pressure steam. Low temperature waste heat
may be useful in a supplementary way for
preheating purposes.

6. Benefits of Waste Heat Recovery
• Direct Benefits:
Recovery of waste heat has a direct effect on the efficiency of
the process. This is reflected by reduction in the utility
consumption & costs, and process cost.
• Indirect Benefits:
1. Reduction in pollution: A number of toxic combustible wastes such
as carbon monoxide gas, sour gas, carbon black off gases, oil sludge,
Acrylonitrile and other plastic chemicals etc., releasing to atmosphere
if/when burnt in the incinerators serves dual purpose i.e. recovers
heat and reduces the environmental pollution levels.
2. Reduction in equipment sizes: Waste heat recovery reduces the fuel
consumption, which leads to reduction in the flue gas produced. This
results in reduction in equipment sizes of all flue gas handling
equipment's such as fans, stacks, ducts, burners, etc.

7. Benefits of Waste Heat Recovery
3. To reduce operating costs: By recycling and reusing the waste
heat energy, there will be less consumption of fuel. reduce the
operating cost of industry.
4. To make huge savings and earn high profit: By switching to the
most energy-efficient technology available, companies can make
huge savings and significantly reduce environmental impact. In
other words, industry will gain higher profit.
5. Increased demand: In the "World Energy Outlook" report the
International Energy Agency (IEA) predicts world energy demand
to increase by 45% over the next 20 years.
6. To challenge and face higher energy prices: Day by day, prices
of fossil fuels and other sources of energy are increasing.
Therefore, if energy is not saved and conserved by adopting
various technologies including the waste heat recovery method.

8. • There are many different commercial recovery units for the
transferring of energy from hot medium space to lower
one:
• Recuperators: This name is given to different types of
heat exchanger that the exhaust gases are passed through,
consisting of metal tubes that carry the inlet gas and thus
preheating the gas before entering the process. The heat
wheel is an example which operates on the same principle
as a solar air conditioning unit.
• Regenerators: This is an industrial unit that reuses the
same stream after processing. In this type of heat
recovery, the heat is regenerated and reused in the
process.
Classification of WHRS on basis of
Type of Equipment's

9. • Heat pipe exchanger: They have the ability to transfer
heat hundred times more than copper. Heat pipes are
mainly known in renewable energy technology as being
used in evacuated tube collectors. The heat pipe is mainly
used in space, process or air heating, in waste heat from a
process is being transferred to the surrounding due to its
transfer mechanism.
• Thermal Wheel or rotary heat exchanger: consists of a
circular honeycomb matrix of heat absorbing material,
which is slowly rotated within the supply and exhaust air
streams of an air handling system.
• Economizer: In case of process boilers, waste heat in the
exhaust gas is passed along a recuperators that carries the
inlet fluid for the boiler and thus decreases thermal energy
intake of the inlet fluid

10. CLASSIFICATION OF WASTE HEAT
1. High grade heat above 300°C:
The high grade waste heat carried away by flue gases can be
recovered with the help of properly designed heat transfer
equipment. The use of high grade waste heat is the result of
technical necessity rather than economical reasons.
Source: High grade waste heat is in the form of flue gases
and can be readily recovered through heat transfer
equipment's.
2. Low grade heat below 300°C:
Low grade waste heat is usually in the form of process steam
and drain waters.
Source: Low grade heat is in the form of process steam and
rain water, commonly found in many o industrial plants like
food processing chemical industries.

11. Sr.
No
Source of Waste
Heat
Quality
1. Heat in flue gases. High grade. The higher the temperature, the greater the
potential value for heat energy.
2. Heat in vapour
streams
High grade. The higher the temperature, the greater the
potential value for heat energy. When these vapour
streams are condensed, latent heat is also recoverable.
3. Convective and
radiant heat lost
Law grade if recovered, it may be used for space heating or
air preheating
4. Heat losses in cooling
water
Low grade If recovered, these heat losses offer useful gains
when heat is exchanged with incoming fresh water
5. Heat losses in
providing chilled
water . or in the
disposal of chilled
water
High grade, if it can be utilized to reduced demand for
refrigeration Low grade, if refrigeration unit is used as a
form of heat pump

12. Heat Recovery Methods 1. Recuperator
• Recuperators is a special purpose counter-flow
energy recovery heat exchanger positioned within
the supply and exhaust air streams of an air
handling system, or in the exhaust gases of an
industrial process, in order to recover the waste
heat. Generally, they are used to extract heat from
the exhaust and use it to preheat air entering the
combustion system.
• In a recuperator, heat exchange takes place between
the flue gases and the air through metallic or
ceramic walls. Duct or tubes carry the air for
combustion to be pre-heated, the other side contains
the waste heat stream. A recuperator for recovering
waste heat from flue gases is shown in Figure. The
inner tube carries the hot exhaust gases while the
external annulus carries the combustion air from
the atmosphere to the air inlets of the furnace
burners.

13. waste heat recovery boiler(WHRB)
Waste Heat Recovery Boiler (WHRB)

14. Principle of Working: . "Waste heat recovery boilers are those boilers,
which either uses waste heat in the gases coming out of diesel engine
or gas turbines at high temperature or uses the waste fuel in the
incinerators. Incinerator is an apparatus for burning waste fuel at high
temperatures.
Use of waste heat recovery better is the most convenient and widely
used 1 low installation cost 2. Compact in se 3. No operating problems
Waste Heat Recovery Boiler (WHRB)

15. LATENT HEAT RECOVERY METHOD
1. Waste heat recovery boilers especially designed for
capturing latent heat: They are found in any size.
2. Working fluids having less boiling point temperature than
water: Latent heat recovery from low temperature streams
for example, refrigerants, brine solutions) is possible by the
use of working fluids having boiling point less than water.
ADVANTAGES OF SENSIBLE HEAT RECOVERY
Advantages of sensible heat recovered are,
1. Sensible heat recovered from the waste flue gases can be
used either for winter air conditioning or preheating the air,
which is required for combustion of fuel.
2. Sensible heat recovered from the hot gases leaving the
metal furnaces can be used for heating the steel scraps.

16. ADVANTAGES OF WASTE HEAT RECOVERY FOR INDUSTRIAL
SECTOR
1. Waste heat recovery increases to the overall efficiency of
the industrial process.
2. Waste heat recovery reduces fuel consumption and so
decreases both the cost of fuel and energy consumption.
3. Waste heat recovery reduces harmful emissions
containing carbon dioxide, oxides of nitrogen etc.
4. Waste heat recovery helps to reduce the equipment size.
Due to reduced size of equipment, fuel consumption
reduces. It also reduces the requirements for handling the
fuel, pumps, filters, fan etc.

17. BENEFITS OR ADVANTAGES OR USES OF WASTE HEAT
RECOVERED
Direct Benefits of Waste Heat Recovery,
1. To heat the air, which is used for the purpose of
combustion.
2. To heat the air, which is used in winter air conditioning.
3. In case of metal furnaces used for melting steel scrap,
waste heat can be used for heating the air up to 300°C,
4. Waste heat of low temperature range (0 to 120°C) can be
used for the production of bio-fuels by growing algae farms
or can be used in green houses or in eco-industrial parts.
5. High grade waste heat (more than 650°C) can be used for
generation of electricity or mechanical work.

18. APPLICATIONS OF WASTE HEAT RECOVERED
Common applications of waste heat recovered are:
1. Pre-heating combustion air for boilers, ovens and
furnaces.
2. Pre-heating fresh air used to ventilate the building.
3. Hot water generation including pre-heating boiler feed
water.
4. Direct steam generation for process or power generation.
5. Space heating 6. Drying.
Special applications of waste heat recovered are:
1. Animal shelters. 2. Aqua-cultural uses.
3. Green houses. 4. Agricultural uses.
5. Process heating

19. ANIMAL SHELTERS
• The growth rate of some animals is strongly influenced by
atmospheric temperature.
• Proper control of temperature using waste heat can
increase the productivity as well as it can decrease the fuel
consumed to create artificial temperature.
This is particularly more effective for farms of small
animals. For example: Poultry farm.

20. AQUA CULTURAL USES
An artificial pond, where temperature is controlled with the help of
waste heat recovered from the exhaust steam of thermal power plant
• It is observed that the fish yield increases to 1000 kg per acre per
year as compared to 50 to 300 kg per acre per year in a pond with
phosphorous fertilizer.
• In addition to temperature dissolved oxygen and nutritional
adequacy are the other factors Lake or pond responsible for their
growth rate.

21. GREEN HOUSES
A greenhouse (also called a glasshouse) is a building, in which, plants
are grown. The waste heat can be used to produce green house
climate, where air temperature and relative humidity required for
different vegetables can be controlled. The favorable conditions for
the vegetables like tomatoes, onions and egg plants are 20 to 75oC
during day time and 15 to 70°C during night time, which can be
controlled easily by means of green house climates.

22. PROCESS HEATING:
Some of the applications of process heating in industries
are listed below.
1. Preheating of air required for combustion in boiler
furnace.
2. Reheating of fresh air for hot air driers.
3. Waste heat recovered from furnace can be used as heat
source for oven,
4. Waste heat recovered from the flue gases can be used
for preheating of boiler feed water in economizer.
5. Drying, curing and baking ovens.
6. Heating, ventilating and air conditioning system.

23. Thermal wheel/ Heat Wheel
• A thermal wheel, also known as a rotary heat exchanger, or rotary air-to-air enthalpy
wheel, or heat recovery wheel, is a type of energy recovery heat exchanger positioned
within the supply and exhaust air streams of an air-handling system or in the exhaust gases
of an industrial process, in order to recover the heat energy. Other variants
include enthalpy wheels and desiccant wheels. A cooling-specific thermal wheelis
sometimes referred to as a Kyoto wheel.

24. Cogeneration & Trigeneration
Dnyan, Kala, Krida and Krishi Prathisthan’s
Department of Mechanical Engineering Prof. Kokare A.Y.

25. COGENERATION
.Cogeneration or Combined Heat and Power (CHP) may be defined
as, "the sequential generation of two different forms of useful energy
from a single primary energy source, typically mechanical energy and
thermal energy

26. NEED FOR COGENERATION
A cogeneration plant produces both, electrical power and process
heat simultaneous
1. Process heat.
There are several industries such as paper mills textile mills, chemical
industry, processing units sugar factories rice mills oil production and
refining, heating or and so on, where saturated steam at the desired
temperature is required for heating, drying etc. For constant
temperature heating (or drying).
2. Electrical power: Apart from the process heat the factory also
needs power to drive various machines, for lighting and other
purposes. Therefore, two separate units were required for generating
steam of two qualities.
But, having two separate units for process heat and power is
wasteful. Therefore, the idea of cogeneration came into existence.
Dnyan, Kala, Krida and Krishi Prathisthan’s

27. Principle and Advantages of Cogeneration
• Cogeneration or Combined Heat and Power (CHP) is defined as the sequential
generation of two different forms of useful energy from a single primary energy
source, typically mechanical energy and thermal energy. Mechanical energy may be
used either to drive an alternator for producing electricity, or rotating equipment such as
motor, compressor, pump or fan for delivering various services. Thermal energy can be
used either for direct process applications or for indirectly producing steam, hot water,
hot air for dryer or chilled water for process cooling.
• Cogeneration provides a wide range of technologies for application in various domains of
• economic activities. The overall efficiency of energy use in cogeneration mode can be
up to 85 per cent and above in some cases.
• Along with the saving of fossil fuels, cogeneration also allows to reduce the emission of
greenhouse gases (particularly CO2 emission). The production of electricity being on-site,
the burden on the utility network is reduced and the transmission line losses
eliminated. Cogeneration makes sense from both macro and micro perspectives.
• At the macro level, it allows a part of the financial burden of the national power utility to
be shared by the private sector; in addition, indigenous energy sources are conserved.
• At the micro level, the overall energy bill of the users can be reduced, particularly when
there is a simultaneous need for both power and heat at the site, and a rational energy
tariff is practiced in the country
Dnyan, Kala, Krida and Krishi Prathisthan’s
Department of Mechanical Engineering Prof. Kokare A.Y.

28. Dnyan, Kala, Krida and Krishi Prathisthan’s
Department of Mechanical Engineering Prof. Kokare A.Y.

29. Dnyan, Kala, Krida and Krishi Prathisthan’s
Department of Mechanical Engineering Prof. Kokare A.Y.

30. Dnyan, Kala, Krida and Krishi Prathisthan’s
Department of Mechanical Engineering Prof. Kokare A.Y.

31. Classification Of Cogeneration Systems
1. Topping cycle or Topping system or Topping cycle CHP plant.
2. Bottoming cycle or Bottoming or Bottoming cycle CHP plant.
TOPPING CYCLE
topping cycle, the fuel supplied is first used to produce power (i.e.
electricity) and then thermal energy (process heat). The thermal
energy (waste heat) produced is a by-product of the thermodynamic
cycle, which is used to satisfy process heat or other thermal
requirements, such as heating of water and buildings.
In a topping cycle, cogeneration plant generates electrical or
mechanical power first followed by the heat recovery boiler to create
low pressure process steam or to drive a secondary steam turbine.
TYPES OF TOPPING CYCLE COGENERATION SYSTEMS
1. Combined cycle topping system, 2. Gas turbine topping system,
3. Steam turbine topping system, 4 . Heat recovery topping system.

32. Combined Cycle Topping System
It produces mechanical energy. This mechanical energy drives an
electric generator, which generates electrical energy (electricity).
Exhaust gases released by gas turbine Steam contain large amount of
heat. Therefore, exhaust gas can be either used to provide heat to
various domestic and industrial applications or they can be sent to a
heat recovery system (steam generator or boiler) to generate steam,
which may be further used to drive a secondary steam turbine.

33. Gas Turbine Topping System
In Gas turbine CHP Plant, gas turbine is used to drive a synchronous
generator to produce energy (electricity), The exhaust gases leaving
the gas turbine are sent to a heat recovery boiler, where heat
contained by exhaust gases can generate steam or it can be used as
process heat.
Advantages of Gas Turbine Topping Compressor
1. Good fuel efficiency. 2. Simple plant 3. Less civil construction cost,
4. Less impact on environment. 5. High flexibility in operation.

34. Steam Turbine Topping System
Steam turbine CHP plant is used to generate electrical energy
(electricity) using a steam turbine and an electric generator. The
exhaust steam leaving the steam turbine is High pressure steam then
used as low-pressure process steam to heat water for various
purposes.
Advantages of Steam Turbine Topping System:
1. Simple in construction, 2. Easy to operate.
3. Suitable for low quality fuel

35. Bottoming System
In a bottoming cycle, the primary fuel is utilized for generating high
temperature thermal energy. The heat rejected from the process
(waste heat) is used to generate electrical power through a waste
heat recovery boiler and a turbine coupled with electric generator

36. Factors Influencing The Choice Of Type Of
Cogeneration Power Plant
1. Base electrical load matching
2. Electrical Load Matching
3. Base Thermal Load Matching
4. Thermal Load Matching
5. Quality of Thermal Energy Needed
6. Fuels Available
7. System Reliability
8. Local Environmental Regulation

37. Advantages and Disadvantages of Cogeneration Plant (CHP)
1 Cogeneration helps to improve the efficiency of the plant
2. Lower emissions to the environment, particularly
carbon dioxide (CO), the main greenhouse gas.
3. In addition to carbon dioxide (CO), cogeneration
reduces emissions of particulate matter, nitrous oxides,
sulphur dioxide, mercury etc, which would otherwise lead
to increased pollution
4. Cogeneration reduces cost of production and improve
productivity
5. Cogeneration is more economical as compared to
conventional power plant
6. Cogeneration reduces the manufacturing price and
enhances output.

38. Advantages and Disadvantages of Cogeneration Plant (CHP)
7. Cogeneration helps to conserve utilization of water as
well as the cost of water.
8. Cogeneration optimizes the energy supply to all types
of consumers
9. Increased efficiency of energy conversion and use
cogeneration is the most effective and efficient form of
power generation
10. Cogeneration results in large cost savings
11. Cogeneration has increased competitiveness
amongst the energy generation companies and forced
or limit the energy prices,
12. Cogeneration has increased employment
opportunities.

39. Advantages and Disadvantages of Cogeneration Plant (CHP)
13. Enhancing operational efficiency to lower overhead
costs.
14. Reducing energy waste, thereby increasing energy
efficiency, utility grid for energy demands,
15. Being an alternative source of energy generation,
cogeneration reduces the dependency of electric
16. Cogeneration allows companies to replace aging
infrastructure
Disadvantages of Cogeneration Plant (CHP):
1. High capital cost,
2. Moderate efficiency, if it runs at part load instead of
full load.

40. Dnyan, Kala, Krida and Krishi Prathisthan’s
Department of Mechanical Engineering Prof. Kokare A.Y.

41. Dnyan, Kala, Krida and Krishi Prathisthan’s
Department of Mechanical Engineering Prof. Kokare A.Y.
TRI-GENERATION
Trigeneration can be defined as, "the simultaneous process of
cooling, heating and power generation from only one fuel input".

43. Dnyan, Kala, Krida and Krishi Prathisthan’s
Department of Mechanical Engineering Prof. Kokare A.Y.

44. Dnyan, Kala, Krida and Krishi Prathisthan’s
Department of Mechanical Engineering Prof. Kokare A.Y.

45. Dnyan, Kala, Krida and Krishi Prathisthan’s
Department of Mechanical Engineering Prof. Kokare A.Y.

46. Dnyan, Kala, Krida and Krishi Prathisthan’s
Department of Mechanical Engineering Prof. Kokare A.Y.

47. Need of Tri-generation Plant (CCHP)
1. Trigeneration power plant does maximum utilization
of primary fuel.
2. Trigeneration satisfies the need of electricity, process
heat and cooling simultaneously.
3. Instead of using three separate units for generation
of electricity, process heat and cooling, use of a single
trigeneration plant is much economical.
4. In conventional method of power plants, 3 separate
units will require 3 primary fuels or working fluids for
generation of electricity, process heat and cooling,
whereas, a single trigeneration plant uses a single
primary fuel to generate 3 energies (i.e. electricity,
process heat and cooling) simultaneously.

48. Need of Tri-generation Plant (CCHP)
5.Trigeneration gives lower emissions to the
environment, particularly carbon dioxide (CO2), which is
the main greenhouse gas.
6. In addition to carbon dioxide (CO), trigeneration also
reduces the emissions of particulate matter, nitrous
oxides, sulphur dioxide, mercury etc., which results in
reduced pollution.
7. Efficiency of trigeneration power plant is 90%,
whereas, efficiencies or conventional power plant and
cogeneration are 35% and 80% respectively.
8. Transmission line losses are reduced to greater
extent.

49. Sr.
No
Comparative
Point
Trigeneration Cogeneration
1. Definition simultaneous process of
cooling, heating and power
from ne fuel
Source.
Cogeneration is sequential
generation of two cooling,
heating from fuel.
2. Alternative
name
Combined Heating, Cooling
and
Power (CCHP).
Combined Heat and Power
(CHP)
3. Forms of
energies
Electricity, Heating and
Cooling.
Electricity and Heating
4. Efficiency 90%. 80%.
5. Absorption
refrigeration
system
Needed Not Needed

50. Advantages of Tri-generation Plant (CCHP)
1. Low payback period
2. Protection against electricity cost and outages
3. Replacing thermal energy
4. Energy efficiency
5. Environmentally sustainable
6. Savings on energy costs
7. Less emissions of greenhouse gases
8. Back-up power to the site
9. Independence from the grid
10. Energy prices are rising
11. Low maintenance cost
12. Sale of electricity
13. Increased power reliability

51. Disadvantages and Application of Tri-generation
DISADVANTAGES OF TRIGENERATION
1. High capital cost. 2. More research work is needed.
3. Intense planning is required for designing tri-generation
systems for projects.
4. Applicability differs with each project.
APPLICATIONS OF TRIGENERATION
1. Data centers. 2. Food processing industries.
3. Manufacturing units. 4. Colleges and universities.
5. Military complexes. 6. Schools, colleges.
7. Office buildings. 8. Shopping centers and
Supermarkets. 9. Manufacturing plants.
10. Refrigerated warehouses, 11. Theatres.
12. Airports. 13. Golf/country clubs.
14. Casinos. 15. Resorts.