Lecture 12_PPE_Unit 4: Waste Heat Recovery, Cogeneration & Trigeneration

Program: Diploma (Mechanical)
Class: TY (ME) Semester: V
Course: Power Plant Engineering
Code: 22566Code: 22566
LECTURE 12:
Unit: 4. Waste Heat Recovery

02
wwwwwwwwwwww....ssssaaaannnnddddiiiippppffffoooouuuunnnnddddaaaattttiiiioooonnnn....oooorrrrggggMechanical Engineering Department, Sandip Polytechnic, NashikMechanical Engineering Department, Sandip Polytechnic, NashikMechanical Engineering Department, Sandip Polytechnic, NashikMechanical Engineering Department, Sandip Polytechnic, Nashik
Name of theName of theName of theName of the Trainer : Prof. Rushikesh Deoram SonarTrainer : Prof. Rushikesh Deoram SonarTrainer : Prof. Rushikesh Deoram SonarTrainer : Prof. Rushikesh Deoram Sonar
Years ofYears ofYears ofYears of Experience : 10Experience : 10Experience : 10Experience : 10
DomainDomainDomainDomain Expertise : Mechanical EngineeringExpertise : Mechanical EngineeringExpertise : Mechanical EngineeringExpertise : Mechanical Engineering
Qualification: M.E. (Design Engineering)Qualification: M.E. (Design Engineering)Qualification: M.E. (Design Engineering)Qualification: M.E. (Design Engineering)
Contact Details:Contact Details:Contact Details:Contact Details:
+91 9890481959+91 9890481959+91 9890481959+91 9890481959
rushikesh.sonar@sandippolytechnic.orgrushikesh.sonar@sandippolytechnic.orgrushikesh.sonar@sandippolytechnic.orgrushikesh.sonar@sandippolytechnic.org

03Unit IV: Waste Heat Recovery
UNIT OUTCOMES (UOs) :-
4a. Explain the need of Waste Heat Recovery of the given Thermal Power Plants.
4b. Explain with sketches working principle of Co-generation and Tri-generation in the given
Thermal Power Plant.

TOPICS COVERED IN PREVIOUS LECTURE :-
3.1 Steam power plant :3.1 Steam power plant :3.1 Steam power plant :3.1 Steam power plant : Classification, General arrangement, operating principle, advantages and
limitations, maintenance.
3.2 Gas power plant:3.2 Gas power plant:3.2 Gas power plant:3.2 Gas power plant: Introduction, components, advantages and limitations, maintenance.
4444....1111 :::: Waste Heat Recovery : IntroductionIntroductionIntroductionIntroduction
TOPICS TO BE COVERED IN THIS LECTURE :-
4444....1111 :::: Waste Heat Recovery : IntroductionIntroductionIntroductionIntroduction
4444....1111....1111:::: NeedNeedNeedNeed of Waste Heat recovery
4444....1111....2222:::: OpportunitiesOpportunitiesOpportunitiesOpportunities of Waste Heat recovery
4444....1111....3333:::: PresentPresentPresentPresent PracticesPracticesPracticesPractices of Waste Heat recovery

4.1: WASTE HEAT RECOVERY: Introduction
A Waste Heat Recovery Unit (WHRU) is an energy recovery heat exchanger that transfers heat from process
outputs at high temperature to another part of the process for some purpose, usually to increase the
efficiency.
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.
“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”.
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.
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.

4.1: WASTE HEAT RECOVERY: IN A NUTSHELL
• “Dumped” heat that can still
be reused
• “Value” (quality) more
important than quantity
• Waste heat recovery saves
fuel and increases thermal
efficiency
• An effective way to increase
energy efficiency is to
• “Dumped” heat that can still
be reused
• “Value” (quality) more
important than quantity
• Waste heat recovery saves
fuel and increases thermal
efficiency
• An effective way to increase
recover waste heat
recover waste heat

4.1: WASTE HEAT RECOVERY: SOURCE AND QUALITY OF WASTE HEAT
S. No Source of Waste Heat Quality of Waste Heat
1 Heat in flue gases The higher the temperature, the greater the potential value for
heat recovery
2 Heat in vapour streams As above but when condensed, latent heat also recoverable
3 Convective & radiant heat lost
from exterior of equipment
Low grade – if collected may be used for space heating or air
preheats
4 Heat losses in cooling water Low grade – useful gains if heat is exchanged with incoming
fresh water
5 Heat losses in providing
chilled water or in the disposal
of chilled water
1.High grade if it can be utilized to reduce demand for
refrigeration
2.Low grade if refrigeration unit used as a form of Heat pump
6 Heat stored in products
leaving the process
Quality depends upon temperature
7 Heat in gaseous & liquid
effluents leaving process
Poor if heavily contaminated & thus requiring alloy heat
exchanger

Waste Heat Recovery Systems (WHRS)
Low Temperature
Heat Recovery
High Temperature
Heat Recovery
Medium Temperature
Heat Recovery
4.1: WASTE HEAT RECOVERY: CLASSIFICATION BASED ON TEMPERATURE RANGES
Heat comes from
direct fuel fired
processes.
Heat comes from the
exhaust of directly fired
process units.
Low temperature waste heat may
be useful in a supplementary way
for preheating purposes.

4.1: WASTE HEAT RECOVERY: A) High Temperature Heat Recovery
Types of Devices Temperature (0C)
Nickel refining furnace 1370 – 1650
Aluminium refining furnace 650 –760
Zinc refining furnace 760 – 1100
Copper refining furnace 760 – 815
Steel heating furnace 925 – 1050
Steel heating furnace 925 – 1050
Copper reverberatory furnace 900 – 1100
Open hearth furnace 650 – 700
Cement kiln (Dry process) 620 – 730
Glass melting furnace 1000 – 1550
Hydrogen plants 650 – 1000
Solid waste incinerators 650 – 1000
Fume incinerators 650 – 1450
Table: Typical waste heat temperature at high temperature range from various sources

4.1: WASTE HEAT RECOVERY: B) Medium Temperature Heat Recovery
Types of Devices Temperature (0C)
Steam boiler exhaust 230 – 480
Gas turbine exhaust 370 – 540
Reciprocating engine exhaust 315 – 600
Reciprocating engine exhaust (turbo
charged)
230 – 370
Heat treatment furnace 425 – 650
Drying & baking ovens 230 – 600
Catalytic crackers 425 – 650
Annealing furnace cooling systems 425 – 650
Table: Typical waste heat temperature at Medium temperature range from
various sources

4.1: WASTE HEAT RECOVERY: C) Low Temperature Heat Recovery
Heat Source Temperature 0C
Process steam condensate 55-88
Cooling water from: Furnace doors 32-55
Injection molding machines 32-88
Annealing furnaces 66-230
Forming dies 27-88
Air compressors 27-50
Pumps 27-88
Internal combustion engines 66-120
Air conditioning and refrigeration condensers 32–43
Liquid still condensers 32-88
Drying, baking and curing ovens 93-230
Hot processed liquids 32-232
Hot processed solids 93-232

4.1: WASTE HEAT RECOVERY: Types of Commercial Equipments
1.Recuperators
Heat exchange occurs between flue gases and the air through metallic/ceramic walls.
Ducts/tubes carry combustion air to be preheated in the combustion chamber; the other side
contains waste heat stream.
Inlet air from
atmosphere
Types of Recuperators:-
1. Convective Recuperator
2. Metallic Recuperator
Outside
ducting
Tune plate
Preheated
air
Centre tube plate
Exhaust gas
from process
2. Metallic Recuperator
3. Hybrid Recuperator
4. Ceramic Recuperator
5. Self-recuperative Burners

4.1: WASTE HEAT RECOVERY: Types of Recuperators
1. Convective Recuperator:
• Hot gas through number of parallel small diameter tubes.
• Tubes can be baffled twice or thrice to allow gas to pass over them again.
• Baffling increases heat exchange but more expensive exchanger is needed.

2. Metallic Recuperator:
• The radiation recuperator consists of two
concentric lengths of metal tubing.
• 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.
to the air inlets of the furnace burners.
• The radiation heat transfer is most
effective at high temperature--usually
above 1,400°F (760°C)

3. Hybrid Recuperator:
• For maximum effectiveness of heat transfer, hybrid
recuperators are used.
• These are combinations of radiation and convective
designs, with a high-temperature radiation section
followed by a convective section.
• These are more expensive than simple metallic
• These are more expensive than simple metallic
radiation recuperators, but are less bulky.

4. Ceramic Recuperator:
• They have been developed to overcome the temperature limitations of metal recuperators as
metal recuperators are normally used for temperatures from 1,600°–1,800°F (870° to 980°C)
• New Design have following features:-
Reduced leakage rates.
Air pre-heat temperature < 700°C
Air pre-heat temperature < 700°C
Cannot be used for gases that contain particulates, corrosive gases, and condensable vapors.
Requires high maintenance due to potential for air leaks.

5. Self-recuperative Burners:
• A special class of recuperators, known as self-recuperative burners, is now offered by several
burner suppliers.
• In this system, the recuperator is integrated with the burner itself, so there is no need to have
hot air piping from the recuperator to the burners resulting in substantial cost advantage.
• The self-recuperative burners cannot give the same heat transfer rate or heat transfer
efficiency; hence the fuel savings are limited to 30%–60%

2.Regenerators
They are widely used in glass and steel melting furnaces to recover heat from high-temperature
exhaust gases, normally above 2,500°F (1,370°C).
They are made from high-temperature refractory bricks or
specially designed ceramic shapes.
The efficiency of the regenerator depends on the size of the
regenerator; the time span between reversals; and the thickness,
regenerator; the time span between reversals; and the thickness,
conductivity, and heat storage ratio of the brick.
Accumulation of dust and slag on the surfaces reduce efficiency of
heat transfer.

3. Heat Wheels / Thermal Wheels / Rotary Heat exchangers
Applications in low- to medium-temperature waste heat recovery systems, usually limited to about
600°F (315°C).
The wheel itself is a sizable porous disk, fabricated with
material having a fairly high heat capacity.
It rotates between two side-by-side ducts: one is a cold gas
duct, the other a hot gas duct. The axis of the disk is located
duct, the other a hot gas duct. The axis of the disk is located
parallel and on the partition between the two ducts.

4. Heat Pipe Exchangers (HPHE)
It is a thermal energy absorbing and transferring system that has no moving parts and therefore
requires minimal maintenance.
• Transfer up to 100 times more thermal energy than
copper.
• It Consist of Three elements:
Sealed container
Capillary wick structure
Capillary wick structure
Working fluid
• Works with evaporation and condensation.
• The heat pipe is mainly used in space, process or
air heating.

4. Heat Pipe Exchangers (HPHE)
Typical applications
• Process to space heating: Transfers thermal energy from
process exhaust for building heating
• Process to process: Transfers recovered waste thermal
energy from the process exhaust to the
incoming process air
• HVAC applications: Cooling and heating by recovering
• HVAC applications: Cooling and heating by recovering
thermal energy

5. Economizers
Used with a boiler system to pre-heat the boiler feed water with the flue gas heat or, in an air pre-
heater, to pre-heat the combustion air.
• 1% fuel savings if
• 60 °C rise of feed water temp through
economizer.
• 200 °C rise in combustion air temp
through air preheater.

4.1.1: NEED OF WASTE HEAT RECOVERY
1. Recovery of waste heat has a direct effect on the efficiency of the process.
2. This is reflected by reduction in the utility consumption, costs and process cost.
3. Reduction in pollution : A number of toxic combustible wastes such as carbon monoxide gas, sour gas, etc, releasing to
atmosphere if/when burnt in the incinerators serves dual purpose i.e. recovers heat and reduces the environmental pollution
levels.
4. Reduction in equipment sizes : Waste heat recovery reduces the fuel consumption, which leads to reduction in the flue gas
4. 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.
5. Reduction in auxiliary energy consumption: Reduction in equipment sizes gives additional benefits in the form of reduction in
auxiliary energy consumption like electricity for fans, pumps etc.
6. 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.

4.1.2: OPPORTUNITIES OF WASTE HEAT RECOVERY: Iron and Steel Industry
Waste Heat Source Heat Recovery Opportunities
Blast furnace gas (BFG) Maintaining heating value to reduce or eliminate use of supplementary fuel, such as
natural gas.
Coke oven gas (COG) Use of sensible heat and chemical heat of COG—elimination of need to cool or
quench the COG as discharged from the coke oven batteries.
Steam from liquid steel refining
area
Recovery of steam heat and condensate return.
area
Hot coke discharged from coke
ovens
Recovery of high-grade (temperature) sensible heat from hot coke.
Hot products discharged from
various furnaces
Recovery of heat from high-temperature (1,300°–2,200°F) (700°–1,200°C) steel
products.
Contd…….

4.1.2: OPPORTUNITIES OF WASTE HEAT RECOVERY: Iron and Steel Industry
Waste Heat Source Heat Recovery Opportunities
Waste heat from recuperator or a regenerative burner
system used on various heating or heat treating furnaces
Recovery of sensible and latent heat from fly-use gases.
Low-grade heat available in the form of hot water used
in cooling systems for various operations (e.g., caster,
reheat furnaces, and roll cooling)
Recovery of heat from low-temperature water (usually less
than 125°F or 52°C).
reheat furnaces, and roll cooling)
Radiation – convection heat loss from furnace walls Recovery of heat for use within the plant.
EAF exhaust gases Recovery of chemical and sensible heat.

4.1.3: PRESENT PRACTICES OF WASTE HEAT RECOVERY
Many sectors of industry have very good potential for Waste heat recovery. Industrial units in following
sectors have WHRS installed and used effectively:
 Aluminum – Primary Production
 Aluminum – Recycling and Secondary Melting
 Steel – Integrated Steel Mill
 Steel – Mini Mill or EAF Mill
 Glass – Fiberglass Manufacturing
 Glass – Fiberglass Manufacturing
 Chemicals and Petroleum Refining – Major Operations
 Paper – Paper Mill
 Food – Food (Snack) Manufacturing
 Cement – Dry Process and Shaft Furnaces
 Coatings – Vinyl Coating Mill

In this lesson, We have learnedIn this lesson, We have learnedIn this lesson, We have learnedIn this lesson, We have learned
4.1 Basics of Waste Heat Recovery and WHRS
4.1.2 Need of Waste Heat Recovery
SUMMARY
4.1.2 Need of Waste Heat Recovery
4.1.3 Opportunities and Present Practices of Waste Heat Recovery

Our Next Video Lecture Topic
4.2 Cogeneration or Combined Heat And Power (CHP)
4.2.1 Need of Cogeneration
4.2.2 Opportunities of Co-generation
4.2.3 Present Practices of Co-generation
4.3 Trigeneration or Combined Cooling, Heat And Power (CCHP)
Till Then Stay Connected,
THANK YOU
4.3 Trigeneration or Combined Cooling, Heat And Power (CCHP)
4.3.1 Need of Trigeneration
4.3.2 Opportunities of Trigeneration
4.3.3 Present Practices of Trigeneration

Lecture 12_PPE_Unit 4: Waste Heat Recovery, Cogeneration & Trigeneration

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