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Industrial waste heat is the energy that is generated in industrial processes which is not put into any practical use and is lost, wasted and dumped into the environment. Recovering the waste heat can be conducted through various waste heat recovery technologies to provide valuable energy sources and reduce the overall energy consumption. In this paper, a comprehensive review is made of waste heat recovery methodologies and state of the art technologies used for industrial processes. By considering the heat recovery opportunities for energy optimisation in the steel and iron, food, and ceramic industries, a revision of the current practices and procedures is assessed. The research is conducted on the operation and performance of the commonly used technologies such as recuperators, regenerators, including furnace regenerators and rotary regenerators or heat wheels, passive air preheaters, regenerative and recuperative burners, plate heat exchangers and economisers and units such as waste heat boilers and run around coil (RAC). Techniques are considered such as direct contact condensation recovery, indirect contact condensation recovery, transport membrane condensation and the use of units such as heat pumps, heat recovery steam generators (HRSGs), heat pipe systems, Organic Rankine cycles, including the Kalina cycle, that recover and exchange waste heat with potential energy content. Furthermore, the uses of new emerging technologies for direct heat to power conversion such as thermoelectric, piezoelectric, thermionic, and thermo photo voltaic (TPV) power generation techniques are also explored and reviewed. In this regard, the functionality of all technologies and usage of each technique with respect to their advantages and disadvantages is evaluated and described.
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Thermal Science and Engineering Progress
journal homepage: www.elsevier.com/locate/tsep
Waste heat recovery technologies and applications
Hussam Jouhara
, Navid Khordehgah, Sulaiman Almahmoud, Bertrand Delpech,
Amisha Chauhan, Savvas A. Tassou
Institute of Energy Futures, College of Engineering, Design and Physical Sciences, Brunel University London, UB8 3PH, UK
ABSTRACT
Industrial waste heat is the energy that is generated in industrial processes which is not put into any practical use
and is lost, wasted and dumped into the environment. Recovering the waste heat can be conducted through
various waste heat recovery technologies to provide valuable energy sources and reduce the overall energy
consumption. In this paper, a comprehensive review is made of waste heat recovery methodologies and state of
the art technologies used for industrial processes. By considering the heat recovery opportunities for energy
optimisation in the steel and iron, food, and ceramic industries, a revision of the current practices and proce-
dures is assessed. The research is conducted on the operation and performance of the commonly used tech-
nologies such as recuperators, regenerators, including furnace regenerators and rotary regenerators or heat
wheels, passive air preheaters, regenerative and recuperative burners, plate heat exchangers and economisers
and units such as waste heat boilers and run around coil (RAC). Techniques are considered such as direct contact
condensation recovery, indirect contact condensation recovery, transport membrane condensation and the use of
units such as heat pumps, heat recovery steam generators (HRSGs), heat pipe systems, Organic Rankine cycles,
including the Kalina cycle, that recover and exchange waste heat with potential energy content. Furthermore,
the uses of new emerging technologies for direct heat to power conversion such as thermoelectric, piezoelectric,
thermionic, and thermo photo voltaic (TPV) power generation techniques are also explored and reviewed. In this
regard, the functionality of all technologies and usage of each technique with respect to their advantages and
disadvantages is evaluated and described.
1. Introduction
With the growing trend of increases in fuel prices over the past
decades as well the rising concern regarding global warming, en-
gineering industries are challenged with the task of reducing green-
house gas emissions and improving the eciency of their sites.
In this regard, the use of waste heat recovery systems in industrial
processes has been key as one of the major areas of research to reduce
fuel consumption, lower harmful emissions and improve production
eciency.
Industrial waste heat is the energy that is generated in industrial
processes which is not put into any practical use and is wasted or
dumped into the environment. Sources of waste heat mostly include
heat loss transferred through conduction, convection and radiation
from industrial products, equipment and processes and heat discharged
from combustion processes [1]. Heat loss can be classied into high
temperature, medium temperature and low temperature grades. Waste
Heat Recovery (WHR) systems are introduced for each range of waste
heat to allow the most optimum eciency of waste heat recovery to be
obtained.
High temperature WHR consists of recovering waste heat at tem-
peratures greater than 400 °C, the medium temperature range is
100400 °C and the low temperature range is for temperatures less than
100 °C [2]. Usually most of the waste heat in the high temperature
range comes from direct combustion processes, in the medium range
from the exhaust of combustion units and in the low temperature range
from parts, products and the equipment of process units [2].
It is estimated that the UK industrial sector consumes as much as
17% of the overall UK economys energy consumption and generates
about 32% of the UKs heat-related CO
2
emissions. From this value and
as can be seen from Fig. 1, 72% of the UK industrial demand is from
industrial thermal processes of which 31% is classied as low tem-
perature process heat [3] and almost 20% of that or 40 TWh/yr is es-
timated to have potential for industrial waste heat recovery [4].Itis
found that the most energy consuming industries in the UK are cement,
ceramic, iron and steel, reneries, glassmaking, chemicals, paper and
pulp and food and drink. These industries together contribute about
£50 bn/yr to the UKs economy [4]. This indicates that improving
https://doi.org/10.1016/j.tsep.2018.04.017
Received 2 January 2018; Received in revised form 25 April 2018; Accepted 26 April 2018
Corresponding author.
E-mail address: hussam.jouhara@brunel.ac.uk (H. Jouhara).
Thermal Science and Engineering Progress 6 (2018) 268–289
2451-9049/ © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
T
energy eciency through waste heat recovery models can help UK
businesses to reduce the operating costs of their businesses, improve the
energy eciency of their sites and reduce the UKs industrial CO
2
emissions.
2. Waste heat recovery systems
Waste heat recovery methods include capturing and transferring the
waste heat from a process with a gas or liquid back to the system as an
extra energy source [5]. The energy source can be used to create ad-
ditional heat or to generate electrical and mechanical power [6].
Waste heat can be rejected at any temperature; conventionally, the
higher the temperature, the higher the quality of the waste heat and the
easier optimisation of the waste heat recovery process. It is therefore
important to discover the maximum amount of recoverable heat of the
highest potential from a process and to ensure the achievement of the
maximum eciency from a waste heat recovery system [7].
The quantity or the amount of available waste heat can be calcu-
lated using the equation shown below.
× ×
Q
VρC TΔ
P(1)
where, Q(J) is the heat content, Vis the owrate of the substance (m
3
/
s), ρis density of the ue gas (kg/m
3
), C
p
is the specic heat of the
substance (J/kg.K) and ΔTis the dierence in substance temperature
(K) between the nal highest temperature in the outlet (T
out
) and the
initial temperature in the inlet (T
in
) of system.
Depending on the type and source of waste heat and in order to
justify which waste heat recovery system can be used, it is essential to
investigate the amount and grade of heat recoverable from the process.
There are many dierent heat recovery technologies available
which are used for capturing and recovering the waste heat and they
mainly consist of energy recovery heat exchangers in the form of a
waste heat recovery unit. These units mainly comprise common waste
heat recovery systems such as air preheaters including recuperators,
regenerators, including furnace regenerators and rotary regenerators or
heat wheels and run around coil, regenerative and recuperative bur-
ners, heat pipe heat exchangers, plate heat exchangers, economisers,
waste heat boilers and direct electrical conversion devices. These units
all work by the same principle to capture, recover and exchange heat
with a potential energy content in a process.
2.1. Regenerative and recuperative burners
Regenerative and recuperative burners optimise energy eciency
by incorporating heat exchanger surfaces to capture and use the waste
heat from the hot ue gas from the combustion process [8]. Typically,
regenerative devices consist of two burners with separate control
valves, which are connected to the furnace and alternately heat the
combustion air entering the furnace. The system works by guiding the
exhaust gases from the furnace into a case which contains refractory
material such as aluminium oxide [9]. The exhaust gas heats up the
aluminium oxide media and the heat energy from the exhaust is re-
covered and stored. When the media is fully heated, the direction of the
ue gas is reversed, with the stored heat being transferred to the inlet
air entering the burner and the burner with hot media starts ring.
Combustion air from the hot media then heats up the cooler media and
the process starts again. Through this technique, the regenerative
burner can save the fuel needed to heat the air and this improves the
eciency of combustion [10] (see Fig. 2).
Burners that incorporate recuperative systems are also used com-
mercially. A recuperative burner has heat exchanger surfaces as part of
the burner design, which capture energy from the heated gas that
passes through the body of the burner [12]. The burner uses the energy
of waste gas from the exhaust to preheat the combustion air before it
gets mixed with the fuel. The burners consist of an internal heat ex-
changer with various features such as grooves, counter current ow and
ns, which are used to establish thermal contact between the waste
exhaust gases and the combustion air coming from the supply pipe
[13]. The design works by collecting the both the exhaust gas and waste
heat from the body of the burner nozzle, and using them both to
transfer heat into the combustion air. This air preheating results in an
improved eciency of combustion and thus more heat from the nozzle.
It should be noted that the burner and the nozzle are inserted into
Nomenclature
Symbols
Cp specic heat (J/kg.K)
hspecic enthalpy (J/kg)
Qheat content (J)
qheat (J)
sspecic entropy (kJ/kg.K)
Ttemperature (K)
Vow rate (m
3
/s)
wwork (J)
ρdensity (kg/m
3
)
Acronyms
BOF Basic Oxygen Furnace
CHP Combined Heat and Power
CRC Clausius-Rankine Cycle
HRSG Heat Recovery Steam Generator
IC Internal Combustion
ORC Organic Rankine Cycle
RCA Spinning Disk Atomiser
SDA Rotating Cup Atomiser
WHR Waste Heat Recovery
RAC run around coil
Subscripts and superscripts
CO carbon monoxide
CO
2
carbon dioxide
CP Critical Pressure
H
2
Molecular Hydrogen
Fe pig iron
FeO iron oxide
Tb Average Temperature
Tc Temperature Cold
ΔTTemperature Dierence
Fig. 1. Energy consumption in the UK manufacturing industry [3].
H. Jouhara et al. Thermal Science and Engineering Progress 6 (2018) 268–289
269
the furnace body and the waste heat is transferred to the burner by
convection from the exhaust gases. Osaka Gas [14] demonstrates that
for a furnace with a temperature of 1000 °C the air can be preheated to
at least 500 °C, indicating a considerable improvement of thermal ef-
ciency (see Fig. 3).
2.2. Economisers
Economisers or nned tube heat exchangers that recover low
medium waste heat are mainly used for heating liquids. The system
consists of tubes that is covered by metallic ns to maximise the surface
area of heat absorption and the heat transfer rate [15].
The system is located in the duct carrying the exiting exhaust gases
and it absorbs the waste heat by letting the hot gases pass through
dierent sections covered by the nned tubes. Liquid is passed through
the tubes and it captures heat from the nned tubes. The hot liquid is
then fed back to the system, maximising and improving the thermal
eciency [16]. Based on a study conducted by Spirax Sarco [17],itis
shown that if an economiser is used for a boiler system, it can increase
the eciency by 1% for every 5 °C reduction of ue gas temperature.
This indicates that the fuel consumption of the system can be reduced
by 510% with a payback period of less than 2 years [18]. Economisers
recover the waste heat and improve the eciency of a system by pre-
heating the uid in the system such as the feedwater in a steam gen-
erator or a boiler, so less energy is required to achieve the boiling
temperature. In another study by Maxxtec [19], it is noted that re-
gardless of the design of the system, if the temperature of the ue gas is
reduced by 140 °C, the fuel consumption can be reduced by 7%.
It is investigated that several dierent types of economisers are
available for dierent applications but they have the same functionality
[20]. These designs include nned tubes, coiled tubes, non-condensing
and condensing economisers. The condensing and non-condensing
Fig. 2. Regenerative burner mechanism [11].
Fig. 3. Recuperative burner structure [14].
H. Jouhara et al. Thermal Science and Engineering Progress 6 (2018) 268–289
270
types are mainly used to improve the eciency of boiler systems,
whereas the other types are commonly used in thermal power plants
and large processing units to recover waste heat from the ue gas.
Having mentioned that, Vandagri[21] investigates, economisers
that are used for low-temperature heat recovery namely as deep
economisers are also available that are made out of advanced materials
such as Teon, carbon and stainless-steel tubes and can withstand the
acidic condensate deposition on the surface of the heat exchanger.
Glass-tubed economisers are on the hand used for gas to gas heat re-
covery and for low to medium temperature applications [22].
2.3. Waste heat boilers
Waste heat boilers consists of several water tubes that are placed in
parallel to each other and in the direction of the heat leaving the
system. The system is suitable to recover heat from medium high
temperature exhaust gases and is used to generate steam as an output.
The steam can then be used for power generation or directed back to
the system for energy recovery [23].
For example, as J+G[24] reports, in a coal power plant the heat
generated from the combustion process after leaving the combustion
chamber has a temperature of up to 1000 °C. The use of a waste heat
boiler in this case allows the recovery and utilisation of the heat of the
ue gas to vaporise a uid and produce steam that can be used for
energy generation through turbines and generators.
The pressure and the rate of steam production mainly depends on
the temperature of the waste heat. If the waste heat is not sucient for
the system to produce the required amount of steam, an auxiliary
burner unit or an after burner in the exhaust gases can be added to the
system to compensate for that [25].
As Turner [26] reports, waste heat boilers can also be coupled with
other waste heat recovery equipment such as afterburners, preheaters
and nned-tubed evaporators to improve eciency by preheating the
feed water and produce superheated steam if required (see Fig. 4).
2.4. Air preheaters
Air preheaters are mainly used for exhaust-to-air heat recovery and
for low to medium temperature applications. This system is particularly
useful where cross contamination in the process must be prevented.
Such applications can include gas turbine exhausts and heat recovery
from furnaces, ovens, and steam boilers [27].
Air preheating can be based on two dierent designs, the plate type
and the heat pipe type. The plate type consists of parallel plates that are
placed perpendicular towards the incoming cold air inlet. Hot exhaust
air is fed into the channels between the plates, transferring heat to the
plates and creating hot channels, through which the cold air is passed.
The heat pipe type on the other hand consists of a bundle of several
sealed pipes placed in parallel to each other in a container. The con-
tainer is split into two sections accommodating cold and hot air, inlet
and outlet. The pipes inside the container accommodate a working uid
which when faced with the hot waste gas at one end of the pipes,
evaporates and moves towards the other end of the pipe where cold air
is passing [28]. This results in heat being absorbed at the hot section of
the pipe, which is transferred to the cold section, heating the cold
moving air over the pipes. The working uid then condenses and moves
towards the hot section of the pipe, repeating the cycle [29].
As Nicholson [30] explain, there are mainly three commonly used
types of air preheaters which are classied as regenerators, including
rotary regenerators, run around coil, and recuperative. These technol-
ogies all function with the same principle as air preheaters, however,
have dierent congurations and used for dierent purposes (see
Fig. 5).
2.4.1. Recuperators
Recuperators are a form of heat exchanger units that are usually
made out of metallic or ceramic materials, depending on their appli-
cation, and they are used to recover waste exhaust gases at medium to
high temperature [32].
In this technology, the hot exhaust gases are passed through a series
of metal tubes or ducts that carry the inlet air from atmosphere. This
result in the recuperator preheating the inlet gas which then re-enters
the system. The energy that is now available in the system can therefore
be described as the energy which does not have to be supplied by the
fuel, meaning that a decrease in energy demand and production costs is
achieved [33].
Metallic recuperators are used for applications with low medium
temperatures, while heat recovery in high temperature application is
better suited to ceramic recuperators. Recuperators can be said to
mainly transfer heat to the inlet gas based on convection, radiation or a
combination of radiation and convection. A radiation recuperator
consists of metallic tubes around the inner shelf where hot exhaust
gases pass through. The cold incoming air is then fed to the tubes
around the hot shelf and heat is radiated to the wall of the tubes (see
Fig. 6).
The tubes transfer the heat to the cold air, which is then delivered to
the furnace burners. On the other hand, the convective recuperator
exchanges heat by passing hot exhaust gases through relatively small
diameter tubes that are placed in a larger shelf. The cold air is passed
through the large shelf, picking up heat from the small hot tubes inside
the shelf that is heated by the waste gas.
A combination of radiant and convective recuperators provides
another possibility which can maximise heat transfer eectiveness. In
this technology, hot exhaust gas is fed into a larger shelf and then split
into smaller diameter tubes. Cold air is fed into and around the shelf,
and this results in a quantitative improvement in heat transfer [35] (see
Fig. 7).
2.4.2. Regenerators
Regenerators transfer heat from the hot gas duct to the cold gas duct
through storing the waste heat in a high heat capacity material. The
Fig. 4. Schematic of a waste heat boiler incorporating parallel water tubes [26].
H. Jouhara et al. Thermal Science and Engineering Progress 6 (2018) 268–289
271
system consists of a chamber which is used as a link between the hot air
duct and the cold air duct that takes the heat energy from the hot side,
stores and delivers it to the cold side. For instance, regenerative fur-
naces consist of two brick chambers through which hot and cold air
exchange heat. As hot combustion gases pass through the brick
chamber, heat from the hot ue gas is absorbed, stored and delivered to
the cold airow when it is passed through the chamber. The ow of the
preheated gas is then injected into the ow going to the combustion
chamber, decreasing the amount of energy needed to heat the system.
Two chambers are used so that, one is transferring heat to the ow
entering the system, the other one is absorbing heat. The direction of
inlet ow is changed frequently to allow a constant heat transfer rate to
be obtained [36].
Regenerators are suitable for high temperature applications such as
glass furnaces and coke ovens and they have been historically used with
open-hearth steel furnaces. Regenerators are particularly suitable for
applications with dirty exhausts, however, they can be very large in size
and have very high capital costs, which is a disadvantage with this
technology [37].
2.4.3. Rotary regenerators
Rotary regenerators work in a similar manner to xed regenerators,
however, in this technology, heat is transferred through a porous
thermal wheel between the hot and cold ows. In this system, two
parallel ducts containing hot and cold ows are placed across a rotary
disk or heat wheel which is made out of a high thermal capacity ma-
terial. The heat wheel takes and stores heat from the ow coming
through the hot duct, rotates and delivers it to the ow coming through
the cold duct. Rotary regenerators are used for low medium tem-
perature applications and could potentially oer a very high overall
heat transfer eciency [38] (see Fig. 8).
The reason heat wheels are not suitable for high temperature ap-
plications is due to the structural stresses and the possibility of large
expansion and deformations that can be caused by high temperature
dierences between the two ducts [39]. Having said that, heat wheels
made out of ceramic materials can be used for high temperature ap-
plications.
As heat wheels are mainly made out of porous material, cross con-
tamination therefore be cannot prevented. This can be a major
Fig. 5. Air preheater layout showing air movement [31].
Fig. 6. Diagram of metallic recuperator [34].
Fig. 7. Combined radiation and convective type recuperator [33].
Fig. 8. Schematic diagram of heat wheel [33].
H. Jouhara et al. Thermal Science and Engineering Progress 6 (2018) 268–289
272
disadvantage especially when cross contamination between the two
ducts must be prevented. However, this is shown to be advantageous for
applications where recovering humidity and moisture from the outlet
duct is required [38].
2.4.4. Run around coil (RAC)
This system as can be seen from Fig. 9 and according to Toolan [40],
consists of a pair of coiled heat exchangers that are connected to each
other by a run around coil that is lled with a uid such as a water or
glycol or a mixture of both [41]. The liquid in the coil takes the waste
heat that is captured by the primary recuperator from the exhaust gas of
a process and transports it to the secondary recuperator where it would
be mixed with the supply air. The possibility of exchanging heat be-
tween the two air streams is due to the liquid round coil system, which
are connected to each other by a pumped pipework.
This unit is used when the sources of heat are too far from each
other to use a direct recuperator and when cross contamination be-
tween the two ow sources due to moister, corrosive gases, toxic and
biological contaminates needs to be prevented. This system is found to
have a very low eectiveness when compared to a direct recuperator
and needs a pump to operate, which requires additional energy input
and maintenance [43]. Having said that and as Carbon Trust [44] dis-
covers, the eectiveness and eciency of this technology can be im-
proved by using a secondary heat source as shown in Fig. 10.
2.5. Plate heat exchanger
Plate heat exchangers are used to transfer heat from one uid to
another when cross contamination needs to be avoided. A plate heat
exchanger is made out of several thin metal plates that are stacked or
brazed in parallel to each other and formed into a hollow metallic shell.
Each plate usually consists of dierent pressed patterns that are sur-
rounded with gaskets to control the uid ow and produce turbulence
for better heat transfer [45]. The gaskets are arranged in such a way
that allows only one type of uid to ow through one gap, while the
other uid gets directed through the adjacent gap [46]. As can be seen
from Fig. 11, between every two consecutive plates a space or passage
has been implemented that makes the hot and cold uids ow along
and through the plane of the plates.
This way, the hot and cold uids pass through each section of the
heat exchanger passing over the front and back of plates alternatively,
exchanging heat and not getting contaminated with each other. The
other advantage plate heat exchangers oer in comparison with similar
types of heat exchanger such as the conventional shell and tube heat
exchangers is the fact the hot and cold uids are exposed to a larger
surface area per unit volume and a larger heat transfer coecient [48].
It is reported that there are mainly three types of plate heat ex-
changer, arranged in either single-pass or multi-pass arrangements, as
shown in Figs. 12 and 13 [49,50]. The plates of plate heat exchangers
can either be gasketed, brazed or be welded together [51,52].Ina
gasket plate heat exchanger, a gasket usually made out of a polymer
material is placed between the plates that works as a seal and separator
around the edges of the plates [53]. The plates are placed and clamped
together in a frame with the use of tightening bolts and two thicker
pressure plates on each side.
The design therefore allows the heat exchanger to be dismantled for
cleaning or be optimised to have a bigger or smaller capacity by re-
moving or adding additional plates [54]. The use of gaskets with this
design brings the advantage of resistance to thermal fatigue and sudden
pressure variation as it gives exibility to the plate pack. This is ideal
for applications that constantly go through thermal cycling by being
exposed to variations of temperature [55]. Having said that, the use of
gaskets is restricted by the operating temperature and pressure of the
cycle which is a disadvantage [56]. Gasket plate heat exchangers are
nonetheless proven to give ecient and eective heat transfer with a
recovery rate of up to 90% [57].
In a brazed plate heat exchanger, all the plates are brazed together
by using copper or Nickel in a vacuum furnace [59]. The design unlike
the gasket heat exchanger oers more resistance to higher pressure and
temperature ranges and is relatively cheap to maintain [60]. However,
as it is brazed, it cannot be dismantled which means issues can be raised
when cleaning or modifying the size is required. As the design also has a
more rigid construction than the gasket type, it is more susceptible to
thermal stress and any sudden or frequent variation in temperature and
load can lead to fatigue and the failure of the structure. IITD [61]
concludes therefore, that brazed heat exchangers are mainly used for
applications where temperature variation is slow and applications
where thermal expansion is gradual, such as with thermal oils.
Welded plate heat exchangers are reported to have more exibility
and resistance when it comes to thermal cycling and pressure variations
as compared to brazed heat exchangers [55]. This advantage is
achieved through the use of laser welding techniques that hold the plate
pack together by welded seams [62]. This type of heat exchanger is
shown to have higher temperature and pressure operating limits and
because of this they are found to be suitable for heavy duty applications
[63]. Nonetheless, similar to the brazed heat exchangers, they cannot
be dismantled and modied in size.
With the use of a plate heat exchanger in an experiment, Cipollone
[64] demonstrated that the performance of an evaporator used for a
heat recovery steam generator can be improved when a plate heat ex-
changer is used as the component to superheat the working uid of the
system.
2.6. Heat pipe systems
As can be seen from Fig. 14, a heat pipe is a device which can
transfer heat from one place to another with the help of condensation
and vaporisation of a working uid. A heat pipe consists of a sealed
container, a wick structure, and a small amount of working uid such as
water, acetone, methanol, ammonia or sodium that is in equilibrium
with its own vapour [65]. A heat pipe can be divided into three dif-
ferent sections: the evaporator section, the adiabatic transport section
and the condenser section.
When heat is applied to one end of the pipe, it is conducted through
the pipe wall and wick structure and the working uid inside the pipe
evaporates. As a result, a vapour pressure is generated which drives the
vapour through the adiabatic transport section to the other end of the
pipe. The vapour then condenses by losing the latent heat of vapor-
isation through the wick structure and wall of the pipe to the heat sink.
The vapour ow then turns into liquid and is absorbed by the wick
structure. The capillary pressure that is created by the menisci in the
wick structure drives the liquid back to the hot end of the pipe and the
Fig. 9. Schematic of run around coil system [42].
H. Jouhara et al. Thermal Science and Engineering Progress 6 (2018) 268–289
273
cycle repeats [66].
It can be shown that heat pipes have very high eective thermal
conductivities. Solid conductors such as aluminium, copper, graphite,
and diamond have thermal conductivities ranging from 250 to 1500 W/
m K whereas heat pipes have eective thermal conductivities in the
range from 5000200,000 W/m K [67].
Heat pipes are constructed from a range of dierent materials such
as aluminium, copper, titanium, Monel, stainless steel, Inconel and
tungsten. The choice of the material used for heat pipes largely depends
on the application temperature range and the compatibility of the
material with the working uid [68].
As mentioned earlier, the heat pipe wick structure aids the transport
of the working liquid from the condenser back to the evaporator.
Various materials and techniques are used to construct the heat pipe
wick structure, however as PSC [70] reports, groove, screen/woven and
sintered powder metal structures are the most common. It is also re-
ported that heat pipes referred to as thermosyphons are also available;
they have no wick structure and work only with the aid of gravity.
These heat pipes cannot be used in a horizontal orientation and should
be placed vertically. Having mentioned this, heat pipes with a wick
structure can operate in both horizontal and vertical orientations and
do not have such a limitation.
As shown in Fig. 15, screen mesh structure wicks are usually made
out of copper or stainless materials and are expanded against the pipe
wall to form the wick structure. Heat pipes made with this structure are
capable of transporting the working uid both horizontally and verti-
cally and also against gravity at a very slight angle from the horizontal
[70]. Grooved wick structures on the other hand consist of raised dents
that are made perpendicular to the pipe surface by extrusion or
threading processes commonly out of copper or aluminium materials.
Heat pipes made with this type of structure can operate in gravity aided
and horizontal orientations and similar to screen wick structures can
transport liquid at a slight angle from horizontal [71]. In contrast, as
ATS [72] showed in the conducted experiments, sintered copper
powder structures are capable of transporting the working uid against
gravity vertically and also horizontally with not much limitation. This
type of wick structure is made usually from copper powder particles
that are fused together to form a sintered wick structure.
Fig. 10. Run around coil system compromising a secondary heat source [44].
Fig. 11. Schematic of a plate heat exchanger [47].
Fig. 12. Single-pass conguration plate heat exchanger [58].
Fig. 13. Multi-pass conguration plate heat exchanger [58].
Fig. 14. Schematic of a heat pipe [69].
Fig. 15. Common wick types of a heat pipe [73].
H. Jouhara et al. Thermal Science and Engineering Progress 6 (2018) 268–289
274
The type of working uid used in a heat pipe largely depends on the
temperature range of the application for which the heat pipe is being
used. For example, as explained by Faghri [65] and ACT [74], for low
temperature applications in the temperature range of 200550 K, am-
monia, acetone, Freonrefrigerants and water are used. Most appli-
cations of heat pipes usually fall within this temperature range and
water is reported to be the mostly used uid as it is cheap, has good
thermo-physical properties and also is safe to handle [75]. Heat pipes in
general have a high thermal conductivity, which results in a minimal
temperature drop for transferring heat over long distances, long life that
requires no maintenance, as they incorporate passive operation and no
moving parts which can wear out and they have lower operation costs
when compared to the other types of heat exchangers [76].
2.6.1. Pulsating heat pipes
Pulsating heat pipes or PHPs are passive closed two-phase heat
transfer devices that are similar to conventional heat pipes and are
capable of transporting heat without the requirement of any additional
power input. As shown in Fig. 16, the system consists of a narrow
meandering long tube that is lled with a working uid. The PHP can
be in the form of either open-loop or closed-loop conguration and
operate by the oscillatory ow of liquid slugs and vapour plugs. As can
be seen from Fig. 16, in the closed-loop conguration, both ends of the
tube are connected to each other, whereas, for the open-loop cong-
uration, one end of the tube is welded and pinched o, while the other
end is open and connected to a charging valve [77,78]. The main dif-
ference between PHPs and heat pipes is the fact that there is no wick
structure to deliver the condensate to the condenser and heat transfer is
entirely achieved by the oscillatory ow [79].Holley and Faghri [80]
developed a PHP with a sintered copper wick structure and demon-
strated that the working uid is better distributed throughout the pipe
which results in more nucleation sites for boiling and as a result the
uctuation in local temperature is reduced.
For instance in an experiment, [82] investigated and proved that
with the use of closed-loop oscillating heat pipe, the quantity of using
fuel in pottery kilns can be reduced and energy thrift can be achieved.
In this experiment, a closed-loop oscillating heat pipe that was made
out of copper capillary tube and lled with R123 and water was used as
a heat exchanger to recover the waste heat from pottery kilns.
2.7. Heat recovery steam generator (HRSG)
The heat recovery steam generator (HRSG) is a complex system used
to recover the waste heat from the exhaust of a power generation plant.
It consists of several heat recovery sections such as an evaporator, super
heater, economiser and steam drum, which are very large in size. By
looking at the conguration of a HRSG in Fig. 17, it can be pointed out
that the superheater is placed in the path of the hottest gas upstream of
the evaporator and the economiser is placed downstream of the eva-
porator in coolest gas.
Typically, HRSGs comprise a triple pressure system, this being high
pressure, reheat or intermediate pressure and low pressure [84]. The
system can also recover the waste heat from the exhaust of manu-
facturing processes to improve overall eciencies by generating steam
that can be used for process heating in the factory or for driving a steam
turbine to generate electricity. It is reported that with the use of HRSG
for steam production, a system eciency of as high as 7585% can be
achieved [85].
The system contains an evaporator section and a steam drum for
converting water to steam. The steam is then superheated as its tem-
perature is increased beyond the saturation point. As can be seen from
Fig. 18, the evaporator is located between the economiser and the su-
perheater with the steam drum on top of it.
In the evaporator, the steam for the turbine is generated which is
then delivered to the steam drum and the superheater. As shown in
Fig. 18, in the steam drum the steam and water mixture is separated
from the saturated steam as the feedwater is delivered to the eva-
porator.
The steam is separated in two steps through a combination of
gravity and mechanical work before it gets delivered to the superheater.
This heats the steam beyond the saturation temperature, i.e. generating
superheated steam. The economiser on the other hand, preheats the
feedwater to the evaporator, thus improving the eciency of steam
generation. The steam generated in the process is then sent to a ther-
modynamic cycle such to generate power and improve the eciency of
the plant.
2.8. Thermodynamic cycles used for waste heat recovery
As Costiuc et al. [87] explain, through the use of thermodynamic
cycles, heat recovery from waste sources can be directly conducted to
obtain electrical energy and improve energy eciency of a process. In
this regard, Nemati et al. [88] through a comparative thermodynamic
analysis of Organic Rankine Cycle and Kalina Cycle suggested that the
use of thermodynamic cycles that employ organic working uids en-
ables a cost eective and promising way of energy recovery from
moderate grades of waste heat sources. In this chapter therefore, the
usage and functionality of the mentioned thermodynamic cycles for
WHR will be reviewed.
2.8.1. Organic Rankine cycle
The Organic Rankine Cycle works on the principle of the Clausius-
Rankine cycle, however, the system uses organic substances with low
boiling points and high vapour pressures as the working uid to gen-
erate power instead of water or steam [89]. It has been shown that the
use of an organic uid as the working uid makes the system suitable
for utilising low grade waste heat and for power generation using en-
ergy sources such as geothermal [90], biomass [91], and solar appli-
cations [92].
The Clausius-Rankine Cycle or CRC is introduced as the ideal vapour
power cycle and is described as the elementary operating cycle for all
power plants that use an operating uid such as water to generate
electricity [93]. A typical Rankine cycle consists of a pump, a con-
denser, an evaporator and a generator. Fuel is burned in the evaporator
and the water as the working uid is heated to generate superheated
steam. This is then directed to the turbine to generate power and then
passed through the condenser, losing heat and turning back into its
Fig. 16. Schematic of a pulsating heat pipe [81].
H. Jouhara et al. Thermal Science and Engineering Progress 6 (2018) 268–289
275
liquid state. The liquid water is then pumped into to the evaporator and
the cycle is repeated.
When compared to the CRC, typically an Organic Rankine cycle
(ORC) system consists of a heat exchanger which is connected to an
evaporator and a preheater in a cycle, and a recuperator that is linked
to a condenser [94]. This way, when waste heat travels from the source
and passes over the heat exchanger, the heat exchanger will heat the
intermediate uid which then cycles through the evaporator and pre-
heater. The organic uid is then heated by intermediate uid causing it
to vaporise and becomes superheated vapour.
The vaporised organic uid then passes with high enthalpy through
the turbine and the vapour expands causing the turbine to spin and
generate electricity [95]. The vapour then exits the turbine and passes
over the recuperator to reduce the temperature and preheat the organic
uid at a later stage.
At the condenser, air or water from a cooling tower or the en-
vironment condenses the organic vapour back into a uid. Once the
uid reaches the pump, the system is pressurised to the required level
and the uid will then pass again to the recuperator where it is reheated
and the cycle restarts [96] (see Fig. 19).
Mamun [98] showed an ORC oers many advantages when com-
pared to conventional steam turbine for waste heat recovery. Stefanou
et al. [99] demonstrated that with the use of an ORC along with a waste
heat recovery steam generator unit in a steel mill, a net eciency of
almost 22% can be achieved.
The design and performance of an ORC system nonetheless depends
on the selection of the working uid and its specications in terms of
thermodynamics and environmental and safety criteria [100]. This
therefore indicates that the selection of the optimum working uid is an
important task when considering the use of ORC for waste heat re-
covery processes. For instance, Douvartzides and Karmalis [101] con-
sidered 37 dierent working uid substances and demonstrated that by
appropriately selecting the working uid and the operation for the
cycle, the overall eciency of a plant can be increased by almost 6%
and the fuel consumption can be reduced by 13%.
2.8.2. Kalina cycle
Similar to Organic Rankine cycle, the Kalina cycle is a variant of
Rankine cycle that uses the working uid in a closed cycle to generate
electricity. This system however, commonly uses a mixture of water and
ammonia as the working uid [102] in a process that usually consists of
a recuperator and separator in addition to other components of a
Rankine cycle to generate steam and power (see Fig. 20).
The dierence between the Kalina cycle and cycles that use a single
uid to operate is in the fact that the temperature does not remain
constant during boiling and this is shown to result in a greater e-
ciency for the cycle [104]. In a single-uid cycle, the working uid is
uniformly heated to the evaporating temperature at which a constant
supercritical or superheated steam is generated. However, for a binary
mixture working uid such as in the Kalina cycle, the temperature of
each uid is increased separately during evaporation as a result of each
uid having a dierent boiling point. This will result in a better thermal
matching with the evaporator and condenser as the source of the
cooling medium does not need to satisfy a particular working uid in
the process [105] (see Fig. 21).
This can be also clearly illustrated in the T, s diagrams shown below
which indicates that for the Kalina cycle, the average hear rejection
temperature (Tc) is lower and the average heat addition temperature
(Tb) is higher when compared to a Rankine cycle. This and according to
Eq. (2) derived for Carnot eciency (η
Carnot
), will result in a higher
thermal eciency [106].
=−
η
TT1/
Carnot cb (2)
Based on above explanation, Wang [107] by developing a mathe-
matical model from the waste heat recovered through a waste heat
boiler demonstrated that the Kalina cycle system shows a better per-
formance when compared to ORC.
On the other hand, Milewski [108] studied the concept of WHR
based on the ORC and Kalina cycle in the steel industry and discovered
that the Kalina cycle oers a better result when the recovered heat is of
medium-high grade nature. In this study, the ORC was a competitor to
the Kalina cycle when the recovered heat was below 200 °C.
Fig. 17. Heat Recovery Steam Generator (HRSG) [83].
Fig. 18. Typical heat recovery steam generator components [86].
Fig. 19. Schematic of a Typical Organic Rankine cycle [97].
H. Jouhara et al. Thermal Science and Engineering Progress 6 (2018) 268–289
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2.9. Heat pumps
A heat pump is a thermodynamic device which takes and transfers
heat from a heat source and to a heat sink using a small amount of
energy [109]. Heat pumps collect heat from air, water, or ground and
are categorised as air-to-air, water source and geothermal heat pumps.
Heat pumps can be used as an ecient alternative to furnaces and air
conditioners to cool or heat an environment [110]. Having mention
that, Chua [111] explains that heat pump systems can also be used to
oer economical and ecient alternative of recovering heat from var-
ious sources to improve overall energy eciency. In this sight and as
McMullan [112] describes the heat pump has become an important
component in the context of WHR and energy ecient processes.
A heat pump works with the same principle as refrigerators and air-
conditioners, however, employs a refrigerant cycle to produce hot air
and/or water by extracting heat from a heat source and passing that to
an evaporator to heat the refrigerant at low pressure. This is then de-
livered to a compressor to produce high pressure and temperature gas
that can be delivered to a heat exchanger (condenser) [113] (see
Fig. 22).
Baradey [115] discusses that heat pump in particular are good for
low-temperature WHR, as they give the capability to upgrade waste
heat to a higher temperature and quality. This was for instance de-
monstrated in a study, where, from a heat source of 4560 °C, the heat
pump delivered almost 2.511 times more useful energy comparing to
other WHR systems used for the equal heat input [116].
Through reclaiming waste heat that is dissipated into the environ-
ment and upgrading it by the means of a heat pump, a resulting useful
heat can be generated and used directly for the process to reduce the
energy intake and improve the overall eciency of the system (see
Fig. 23).
2.10. Direct electrical conversion devices
Systems are also available that produce electricity directly from
waste heat and eliminate the need for converting heat to mechanical
energy to produce electrical energy. These technologies include the use
of thermoelectric, piezoelectric, thermionic, and thermo photo voltaic
(TPV) devices for electricity generation [118].Khalid et al. [119]
mentions that these technologies are not widely used in industry,
however, a few have undergone prototype testing and have oered
promising results. Below the technologies that were mentioned as direct
electrical devices are explained.
Fig. 20. Conguration of a Kalina cycle consisting a Recuperator and Separator
[103].
Fig. 21. Comparison of Rankine and Kalina cycles [106].
Fig. 22. Heat pump working diagram [114].
Fig. 23. Heat pump diagram in the context of WHR [117].
H. Jouhara et al. Thermal Science and Engineering Progress 6 (2018) 268–289
277
2.10.1. Thermoelectric generation
Thermoelectric devices are made out of semiconductor materials
that generate electrical current when they face a temperature dier-
ential between two surfaces [120]. Discovered in 1821 by Thomas Jo-
hann Seebeck, the system works based on a principle called Seeback
eect, which as can be seen from Fig. 24, is described as the generation
of electrical current (I) between two semiconductors when the mate-
rials are subject to a hot (T
H
)and cold source (T
C
)[121]. The system
has a very low eciency of 25%, however, as Caillat et al. [122] re-
ports, recent advances in nanotechnology have allowed electrical gen-
eration eciencies of 15% or greater to be achieved. Based on a study
conducted by Hendricks and Choate,[123], it is explained that advanced
thermoelectric packages can appropriately be used to produce elec-
tricity and obtain major energy savings from the waste heat dissipated
from medium-high temperature range applications, for instance glass or
metal furnaces [124,125]. Nonetheless, Hendricks and Choate [123]
reports that maintaining a large temperature dierence and obtaining
high heat transfer rates across the two rather thin surfaces of the device
are issues and challenges at present which require advances in heat
transfer systems and materials. Having mentioned that, Remeli et al.
[126] in an experiment demonstrated that using the combination of
heat pipes and thermo-electric generators can lead to further utilising
power generation for industrial processes. In their project, a bench type
containing thermoelectric generators was modelled and fabricated
which through testing with a counter ow air duct heat exchanger,
indicated an increase in the ratio of mass ow rate of the upper duct to
lower duct. A higher mass ow rate ratio proved a higher power output
from the system and an increase of the overall system performance.
2.10.2. Piezoelectric power generation
Piezoelectric Power Generation (PEPG) is a process of converting
low temperature heat energy directly to electricity [128]. Piezoelectric
devices for heat recovery are made out of thin-lm membranes and they
work by converting ambient vibration such as oscillatory gas expansion
into electricity [129131]. There are technical challenges and dis-
advantages associated with these devices that limit their use for heat
recovery, namely, low eciency, high internal impedance, the need for
long term durability and very high cost [123,132]. Having mentioned
that, the main issue with the use of PEPG devices are associated with
the high cost of manufacturing these devices as well as the way the
systems must be designed to enable power generation, reliability and
stability [133].
2.10.3. Thermionic generator
Thermionic devices operate in a similar manner to thermoelectric
devices in the sense that they produce electric current through tem-
perature dierence between two media without the use of any moving
objects, however, they operate through thermionic emission [134].In
this technology and as can be seen from Fig. 25, a temperature dier-
ence between a hot cathode (Emitter) and a cooler anode (Collector)
generates a ow of electrons between metal and metal oxide surfaces
through a vacuum in an interelectrode space to generate electricity
[135]. The functionality of this technology is shown to be limited to
high temperature applications and be inecient, however, several at-
tempts have been made to improve their eciency and enable their use
for low temperature applications [136] (see Fig. 26).
2.10.4. Thermo photo voltaic (TPV) generator
These devices are used to directly convert radiant energy into
electricity similar to the functionality of solar panels [138]. These
systems are shown to potentially enable a new method of waste heat
recovery and they use an emitter, a radiation lter and a Photo Voltaic
(PV) cell to produce electricity from a heat source [139]. The system
employs an emitter which, when heated by the heat source, emits
electromagnetic radiation. The PV cell then converts the radiation to
electrical energy and the spectral lter ensures that only the radiation
waves with the correct wavelength matching the PV cell pass through.
The eciency of a TPV device is investigated to range from 1% to 20%,
depending on the radiation and heat transfer radiated from the emitter
and the arrangement of the generator [140,141].
For instance, in a study Utla and Onal [142] demonstrated that by
applying TPV cells systems with an eciency of 7.3% as a WHR method
in an iron and steel production plant, the energy eciency of the plant
can be improved by almost 189971 MJ annually. Nonetheless, it has
been found that PV cells have a limited operating temperature range
and their eciency decreases as the cell temperature increases [143].
Having said that, high eciency PV cells that can withstand high
temperature ranges are also available, however, they are expensive and
increase system costs [140,141].
3. Low-temperature waste heat recovery challenges and
opportunities
Recovering waste heat is more feasible and easier when tempera-
tures are in the medium high range [6]. Having said that, there are
vast opportunities for recovering waste heat in the low temperature
range as most industrial waste heat is in this category, as demonstrated
by Haddad et al. [145] and shown in Fig. 27.
Nevertheless, recovering low temperature waste heat is found to be
more challenging than medium high temperature waste heat. The
reason for this is mainly because of the problems associated with the
method of collecting the waste heat [146].
For instance, water vapour exists in low temperature exhaust gases
and it tends to cool down, mix with other particles and deposit corro-
sive solids onto heat exchanger surfaces [25]. Heat exchanger surfaces
therefore have to be cleaned or replaced on a regular basis to maintain
the functionality of the heat exchanger, which can be uneconomical.
The use of advanced materials that minimise corrosion and reduce the
need for regular maintenance in this regard should therefore be con-
sidered [147].
In addition, as the heat transfer rate is low when recovering low-
temperature waste heat, large heat exchangers may be required to
achieve optimum heat transfer. This is mainly because convective heat
transfer rates are a function of temperature dierence between two
locations and the area through which heat is transferred [148]. Having
said this, the cost of equipment used to recover heat for low tempera-
ture applications may be less as lower waste heat temperatures allow
the use of less expensive materials [149].
Nonetheless, the main challenge with the low temperature waste
heat recovery can be nding a use for the recovered heat. Potential uses
for low temperature waste heat can include using a heat pump to im-
prove and increase heat to a higher temperature and the use of the
waste heat to produce domestic hot water, space heating and process
heating [147,150].
The method of recovering low-temperature waste heat include
cooling exhaust gases below dew point temperatures. The dew point
temperature is the temperature at which a moist gas mixture begins to
Fig. 24. Principle of the thermoelectric generation [127].
H. Jouhara et al. Thermal Science and Engineering Progress 6 (2018) 268–289
278
condense and becomes saturated as it is cooled down at a constant
pressure [151]. This way, the waste heat from a low temperature pro-
cess is captured, transferred through a heat exchanger and used to
preheat a process or for low-temperature power generation. In addition
to the technologies introduced in the previous chapter, techniques that
employ units such as transport membrane condensers, direct and in-
direct contact condensation recovery are also describe to be used for
low-temperature WHR [152].
3.1. Direct and indirect contact condensation recovery
A direct contact condensation recovery unit works by mixing the
waste exhaust gas with cooled water to produce hot water for domestic
and preheating uses. As can be seen from Fig. 28, the system uses a
direct mixture heat exchanger which includes a water dispenser, ex-
haust inlet, exhaust outlet and hot water outlet. The ue exhaust gas
and water move in counter-ow directions from the bottom and the top
of the heat exchanger, respectively. This results in waste heat being
transferred to the cool water, which is then stored at the bottom of the
heat exchanger. The hot water can then be fed and provide heat to an
external system [153]. The heat exchanger can also be used to transfer
heat from immiscible liquid liquid and solid liquid or solid-gas
[150]. A disadvantage with this system due to absence of a separating
wall is the fact that particles from the ue gas can be mixed with the
water, which may require ltering before exiting the heat exchanger
[150,154].
Indirect contact condensation recovery on the other hand, consists
of a shell and tube heat exchanger made from advanced materials such
as Teon, glass and stainless steel to minimise corrosion from acidic
Fig. 25. Schematic of a thermionic generator [137].
Fig. 26. Operating principle of a TPV device [144].
Fig. 27. Low-temperature categories.
Fig. 28. Schematic of a direct contact condensation recovery unit [155].
H. Jouhara et al. Thermal Science and Engineering Progress 6 (2018) 268–289
279
condensate deposition. The system transfers heat from the exhaust
gases to the cool water that is owing through the pipes of the heat
exchanger. This system provides the advantage of eliminating cross
contamination of the ue gas and water and can be designed to work as
alter of a process [26].
3.2. Transport membrane condenser
Similar to direct contact condensation recovery systems, transport
membrane condenser units produce hot water from water vapour from
ue gas streams. As can be seen from Fig. 29, the system works by
extracting and delivering the hot water back into the system feed water
directly from the exhaust gas at a temperature above the dew point
through a capillary condensation channel [146]. This way, unlike direct
contact condensation recovery systems, the water is extracted through a
membrane channel rather than directly from the ue gas and so the
recovered water is not contaminated and does not require ltering.
4. Summary table
The table below shows a summary of all the technologies in-
vestigated in this paper including their temperature range, benets and
limitations:
5. Waste heat recovery opportunities in industry
Dierent industrial processes consume dierent amounts of energy
and produce dierent quantities and qualities of waste heat. To take
advantage of the potential of industrial waste heat, it is therefore es-
sential to look into and analyse the industrial processes used in large
energy consuming industries and to investigate what suitable waste
heat recovery methods can be applied to the systems of each sector.
As mentioned before and indicated by McKenna and Norman [157],
the largest amounts of industrial waste heat in the UK are mainly as-
sociated with cement, ceramics, iron and steel, reneries, glassmaking,
chemicals, paper and pulp, and food and drink industries. When con-
sidering waste heat recovery options for industrial processes, it is im-
portant to examine the source and the usefulness of the waste heat
produced and discover which waste heat recovery method is the most
suitable.
In this paper, the iron and steel, ceramic and food industries were
selected to investigate how optimising energy management through the
use of waste heat recovery systems could be achieved in each sector.
The reason for selecting the mentioned industries to conduct further
investigations for the application of WHR is give an indication how and
what WHR technologies can be applied to dierent industrial and
production processes that demonstrate all waste heat temperature
ranges (low-high).
5.1. Waste heat in iron and steel industry
Iron and steel production is a resource and energy intensive process
which involves extensive amounts of heat and raw material. Waste heat
recovery in the iron and steel industry includes recovering heat dis-
sipated from high-temperature sources such as furnaces used for sinter,
coke, iron, and steel production, which is investigated to account for
roughly 8% of the overall industrial energy consumption in the UK [4].
A common method of waste heat recovery in the iron and steel in-
dustry is from clean streams of gases from production processes. For
instance, Jouhara et al. [158] in an experiment demonstrated that for a
heat source of 450 °C, the use of a Flat Heat Pipe (FHP) in the wire
cooling line of a steel production facility can oer a recovery of heat up
to 15.6 (kW). In this experiment, an innovative FHP model was con-
structed at the dimensions of 1 m height × 1 m width and tested at the
hottest point of the cooling zone of the production line. The model was
charged with water at a ow rate of 0.38 kg/s and hot gases dispersing
from the process impacted on a at heat pipe panel which was inclined
at an angle to the horizontal.
In another study demonstrated by [159] and with the use of waste
heat boiler, a waste heat recovery system was produced that can re-
cover sensible heat from hot air emitted by the cooling process of sinter
coolers located downstream of sinter machines. The system generated
approximately 280 MW of power, increasing the overall eciency of
the plant by almost 6%.
Heat recovery plant for contaminated and dirty exhaust gases from
coke ovens, blast furnaces, oxygen furnaces and electric arc furnaces
are also available, yet implemented less, due to the limitations and high
capital costs of current methods [160]. For instance, Mandil [161] re-
ports that the procedure for producing coke in coke ovens extracts gas
with high quality waste heat from the exit of the coke oven and the
(a) TMC Unit placed in a Duct (water
passing through pipes).
(b) TMC Unit placed in a Duct (flue gas
passing though pipes).
Fig. 29. Shows dierent transport membrane condenser units [156].
H. Jouhara et al. Thermal Science and Engineering Progress 6 (2018) 268–289
280
Table 1
Summary table of WHR technologies.
Technologies Temperature Range
Used
Benets Limitations
Regenerative Burners High Saving fuel by preheating the combustion air and improving
the eciency of combustion.
The system requires additional components such as a pair of
heat exchange media and several control valves to function,
which can be complex.
Recuperative Burners High Both the exhaust gas and waste heat from the body of the
burner nozzle are capture and more heat from the nozzle is
generated.
The burner and the nozzle needs to be inserted into the
furnace body, which may require installation and
modication of the furnace.
Economisers Low Medium The system maximises the thermal eciency of a system by
recovering low-medium temperature heat from the waste
ue gas for heating/preheating liquids entering a system.
The system may need to be made out of advanced materials to
withstand the acidic condensate deposition, which can be
expensive.
Waste Heat Boilers Medium High The system is suitable to recover heat from medium high
temperature exhaust gases and is used to generate steam as
an output.
An additional unit such as an auxiliary burner or an after
burner might be needed in the system if the waste heat is not
sucient to produce the required amount of steam.
Recuperators Low High The technology is used for applications with low high
temperatures and is used to decrease energy demand by
preheating the inlet air into a system.
To maximise heat transfer eectiveness of the system, designs
that are more complicated may need to be developed.
Regenerators Medium High The technology is suitable to recover waste heat from high
temperature applications such as furnaces and coke ovens
and for applications with dirty exhausts.
The system can be very large in size and have very high capital
costs.
Rotary Regenerators Low Medium Rotary regenerators are used for low medium temperature
applications and could potentially oer a very high overall
heat transfer eciency.
The system is not suitable for high temperature applications
due to the structural stresses and the possibility of
deformations that can be caused by high temperature
Run around coil (RAC) Medium High This unit is used when the sources of heat are too far from
each other to use a direct recuperator and when cross
contamination between the two ow sources needs to be
prevented
This system is found to have a very low eectiveness when
compared to a direct recuperator and needs a pump to
operate, which requires additional energy input and
maintenance
Heat Recovery Steam
Generator (HRSG)
High The system can be used to recover the waste heat from the
exhaust of a power generation or manufacturing plant to
signicantly improve overall eciencies by generating
steam that can be used for process heating in the factory or
power generation.
The system requires several components to function and may
require an additional burner to improve the quality of the
recovered waste heat. On the other hand, the system is very
bulky and require on site construction.
Plate Heat Exchanger Medium High Plate heat exchangers have high temperature and pressure
operating limits and are used to transfer heat from one uid
to another when cross contamination needs to be avoided.
Parameters such as frequent variation in temperature and load
must be studied and based on that suitable heat exchanger
design must be chosen to avoid failure of the structure of the
heat exchanger for the application
Heat Pipe Systems Medium High Heat pipes have very high eective thermal conductivities,
which results in a minimal temperature drop for transferring
heat over long distances and long life that requires no
maintenance, as they incorporate passive operation. They
have lower operation costs when compared to the other
types of heat exchangers.
To achieve an optimum performance from the heat exchanger,
appropriate design, material, working uid and wick type
based on the application and temperature range of the waste
heat must be studied and chosen.
Thermoelectric Generation Medium High The system produces electricity directly from waste heat and
eliminate the need for converting heat to mechanical energy
to produce electrical energy.
The system has a very low eciency of 25%, however, recent
advances in nanotechnology have allowed electrical
generation eciencies of 15% or greater to be achieved
Piezoelectric Power
Generation
Low The system can be used for low-temperature waste heat
recovery and works by converting ambient vibration such as
oscillatory gas expansion directly into electricity.
The system are found to have a low eciency, high internal
impedance, need for long term durability and very high cost.
Thermionic Generator High The device is used for high temperature waste heat recovery
and works by producing electric current through
temperature dierence between two media without the use
of any moving objects
The functionality of this technology is shown to be limited to
high temperature applications and be inecient, however,
several attempts have been made to improve their eciency
and enable their use for low temperature applications
Thermo Photo Voltaic
(TPV) Generator
Low High These devices are used to directly convert radiant energy into
electricity and oer a better eciency when compared to
other direct electrical conversion devices.
The device is found to have a limited operating temperature
range and their eciency decreases as the temperature
increases. Having said that, high eciency PV cells that can
withstand high temperature ranges are also available,
however, they are expensive and increase system costs.
Heat Pump Low Medium Heat pumps transfers heat from a heat source to a heat sink
using a small amount of energy and can be used to oer
economical and ecient alternative of recovering heat from
various sources to improve overall energy eciency. Heat
pumps in particular are good for low-temperature WHR, as
they give the capability to upgrade waste heat to a higher
temperature and quality.
In order to use this system, the method of capturing the waste
heat based on its source and grade must rstly be analysed
and in that respect, appropriate heat exchanger and system
installation needs to be set up.
Direct Contact
Condensation
Recovery
Medium High the system uses a direct mixture heat exchanger without a
separating wall and can be used to transfer heat from
immiscible liquid liquid and solid liquid or solid-gas.
Due to absence of a separating wall in this heat exchanger,
particles from the ue gas can be mixed with the water, which
may require ltering before exiting the heat exchanger.
Inirect Contact
Condensation
Recovery
Medium High The system provides the advantage of eliminating cross
contamination of the ue gas and water and can be designed
to work as a lter of a process.
The system consists of a heat exchanger which in order to
minimise corrosion from acidic condensate should be made
from advanced materials and can be expensive.
Transport Membrane
Condenser
Medium High The system works by extracting and delivering the hot water
back into the system feed water directly from the exhaust gas
through a capillary condensation channel. This way, the
water is extracted through a membrane channel rather than
directly from the ue gas and so the recovered water is not
contaminated and does not require ltering.
The system employs a capillary condensation channel, which
in order to minimise corrosion from acidic condensate, may
need to be made from advanced materials and can be
expensive.
H. Jouhara et al. Thermal Science and Engineering Progress 6 (2018) 268–289
281
oven chamber. The heat content of the waste gas extracted from the exit
of the oven chamber is not usually recovered and goes to waste as it
contains a high level of tars and other materials that if not ltered will
deposit on the heat exchanger surfaces used for waste heat recovery.
Therefore, various technologies are employed to lter coke oven
exhaust particles from the oven chamber outlet so that the waste heat
can be recovered and used. The processes include ltering and re-
moving tars and other materials from the coke oven gas and using the
remaining recycled (clean) gas to generate an additional heat source
and preheat the coke oven using regenerators investigated in Section
2.4.2 [162]. Having said that, the hot gas that is extracted from the
oven ue can also provide a good heat source and can be further re-
covered with the use of heat pipes [163].
Blast furnaces on the other hand contain several auxiliary blast
stoves that provide heat through ue gases to convert iron oxide (FeO)
into pig iron (Fe) [164]. In blast furnaces, waste heat is mainly re-
covered from the combustion exhausts and is transferred to re-
generators to be reused in the system for reheating the blast furnace and
preheating the combustion air.
The pig iron purication process is usually conducted in the basic
oxygen furnace (BOF) where oxygen is injected to the hot metal and
scrap and uxes added to control metal erosion. The main waste heat
recovery methods used for this process include semi wet and wet open
combustion and supressed combustion techniques. The open combus-
tion system includes the use of a waste heat boiler to recover waste heat
that is produced as a result of the reaction of oxygen in the furnace gas
duct. On the other hand, in the supressed combustion technique, the
combustion gas is collected to be used as fuel through reducing air
inltration and inhibit combustion of the CO and by adding a skirt to
the outlet converter mouth [165,166].
According to Quader et al. [167], approximately 30% of steel is
produced through the electric arc furnace route (EAF) by melting re-
cycled steel scrap. Similar to the supressed combustion technique, the
waste heat recovery method used in electric arc furnaces aims to cap-
ture and collect ammable by-product gases such as CO to provide
additional heat for the system. The electric smelting technique used in
electric arc furnaces is reported to be one of the most common manu-
facturing methods in steel production from recycled scrap. In this
method, electric arc furnaces that employ carbon electrodes are used to
melt recyclable steel scrap and cut-os from product manufacture and
consumers. During furnace operation in this system, several emission
gases and pollutants are released with a temperature ranging between
1300 and 1900 °C [168]. These waste gases can be recovered and be
used to preheat the scrap materials that are placed in the furnace. Using
preheaters in this method results in the carbon electrodes requiring less
energy to melt the scrap and allows reduced electricity consumption for
the plant.
Heat recovery from solid product streams such as slags, hot cokes,
cast steels and hot steels is also demonstrated to have signicant po-
tential. For instance, the waste heat from a coke oven can be recovered
through coke dry quenching and wet quenching. The process of coke
dry quenching involves catching the hot incandescent coke in a cooling
Table 2
Comparison of dierent WHR methods.
Author Process Description Observations and Remarks
[173,174] Solid slag impingement
process
In this process, the stream of liquid slag is fragmented and
turned into granules and particles through striking the stream
into previously solidied particles. The recycled particles with
the slag granules are then fed to a uidised bed, where heat
recovery is conducted.
The heat is recovered through adjusting the product temperature
to 500800 °C by controlling the ratio of recycled to liquid slag in
the uidised bed. This process generated steam with a
temperature up to 250 °C and a heat recovery rate up to about
65%.
[175177] Mechanical stirring
process
This process includes striking and crushing the molten slag with
the use of rotating blades or moving sticks. Through this action,
heat is recovered in a container by radiation and conduction to
water pipes. Then the crushed particles are discharged into a
uidised bed to recover additional energy.
With the use of a waste heat boiler in the nal stage of the
process a recovery eciency of up to 59% through this method
can be achieved.
[176179] Rotating drum process Through this process, a stream of molten slag is broken up into
particles as it is poured onto a rotating drum. The particles are
then fed into a uidised bed where heat is recovered.
This process has been tested in full scale and has been proven to
recover 5060% of the slag heat into a hot airow.
[180183] Air blast method In this method, the slag is heated to obtain the optimum
viscosity and ow rate which is then delivered to a channel
where a high pressure air nozzle breaks down the slag stream
into particles. The system uses two waste heat boilers for
recovery to generate steam.
The rst boiler recovers heat through radiation and convection
from the slag ying droplets, whereas the second boiler which is
located at the bottom of the unit recovers heat from cooler slag
granules. An energy balance study has shown that about 41% of
heat is recovered by steam and another 39% could potentially be
recovered from the exhaust stream at 500 °C
[184,185] Rotating cup atomiser
(RCA) process
The process contains a high speed rotating cup which generates
a centrifugal force and surface tension that can cause the
breakup of the high temperature slag into particles. In this
process, high temperature slag is gradually poured into the
rotating cup and at the same time air is blown to recover heat
from the hot particles.
The process produces hot air and solid granules which are then
dropped into a uidised bed for further heat recovery. This
process has been tested commercially and has proven to recover
59% of the slag heat and cool down the slag particles to 250 °C
[172,186,187] Spinning disk (SDA)
atomiser processes
The spinning disk (SDA) atomiser works with the same
principle as the rotating cup atomiser (RCA) process but with
the dierence that the slag ow is poured onto a high speed
rotating disk rather than a cup. The slag particles are then
dropped into a collection uidised or packed bed where further
heat recovery is obtained.
Through blowing an air ow onto the slag particles, primary heat
recovery is achieved which is then further improved at the
second recovery stage. The heat generated from these operations
is indicated to be at a temperature above 600 °C which can be
used for steam generation or process preheating.
[171,188,189] Methane reforming
reaction process
In this process, reactive gases are employed to cool down the
molten slag particles and transfer the waste heat into chemical
energy. This in return will conduct an intensive heat exchange
from the molten slag to the gas mixture and produce H
2
and CO,
which can be used as fuel. This means that the high temperature
from the slag is stored as chemical energy
Through a proposed process design, hot slag is poured onto a
rotating cup and then deposited into a packed bed where a
mixture of methane and water are injected and CO + H
2
is
generated. The system is estimated to cool down the slag
temperature to 150 °C and recover 51% of the slag heat. Heat
from the slag deposit can be further recovered at the bottom of
the unit to improve the total recovery to almost 83%
[190,191] Direct use for making high
value-added product
In this process the sensible heat from the slag is converted
though several chemical processes and into high value added
mineral wool which can be used for thermal insulation.
The heat from the slag was used as a heat source and the
recovery rate was discovered to be up to 70%.
H. Jouhara et al. Thermal Science and Engineering Progress 6 (2018) 268–289
282
chamber and by passing an inert gas over the coke to recover and de-
liver the waste heat loss to a waste heat boiler [169]. This process can
also be conducted through wet quenching where heat is transferred to
cool water that is sprayed over the hot coke. Wet quenching is also used
to recover waste heat from hot slag, however as Shan et al. [170] re-
ports, this process is found to be an inecient method of waste heat
recovery as it consumes a large amount of water, fails to recover the
sensible heat and is less environmental friendly. Having said that, it is
reported that other technologies that employ chemical techniques have
also been developed that oer more ecient waste heat recovery.
As Sun [171] explains, the recovery from slag is possible in three
dierent forms: recovery as hot air or from steam, conversion of the
waste heat to fuel through chemical reaction, and the use of thermo-
electric power generation. In regard to thermal energy recovery, Zhang
[172] explains that recovery is conducted through dry granulation in-
cluding mechanical crushing methods such as the solid slag impinge-
ment process, the mechanical stirring process and the rotating drum
process. Other techniques such as the air blast method, the centrifugal
granulated method such as the spinning disk (SDA) and Rotating cup
(RCA) atomiser processes are also available and have been studied as
shown in Table 1 below. Chemical methods on the other hand include
the use of the methane reforming reaction process and direct use for
making high value-added products (see Table 2).
5.2. Waste heat in food industry
The food industry is estimated to account for about 26% of the EUs
total energy consumption and to be the UKs fourth highest industrial
energy user [192,193]. Most of the waste heat produced in the food
industry is classied as low-medium temperature [194]. Having said
that, the amount of available waste heat in the food industry depends
largely on the type of process in question and widely varies from sector
to sector. This is mainly due to the fact that dierent industries use
dierent processes for production and this indicates that the actual
amount of useful waste heat can only be determined by conducting a
comprehensive audit for the energy usage of processes. Based on the
study conducted by Feldman [39], it is claimed that there are, however,
general opportunities for waste heat recovery in the food industry that
can be discussed in this paper.
It is estimated that depending on the process, energy wastage is
between 10% and 45%. Potentially, the main sources of the waste heat
are associated with heating and refrigeration systems, hot streams of
water or air used in production and heat from processing operations
[195].
In the red meat processing industry, for example, the source of
waste heat can be classied into recovery from refrigeration systems,
meat processing and by-product rendering [196]. For instance, in a
slaughter house, the refrigeration of carcasses is the most energy in-
tensive process. On the other hand, if by-product rendering takes place,
this can be a major energy user. The clean-up operation that uses large
amount of hot water can also be nominated as a major energy consumer
and also processes such as scalding, singeing and hair removal can use
an extensive amount of energy [197].
For instance, in hog singeing operations where heat is used to dry
out the hog carcasses, the majority of heat is released into the atmo-
sphere. Waste heat recovery can be used largely for this operation to
provide a more ecient production by supplying the energy require-
ment for the dehairing and scalding processes. For instance, based on
the study conducted by Ashraet al. [198] and Environment Agency
[199] it is explained that singeing operations produce a waste ue gas
of up to 800 °C that through the use of waste heat recovery equipment
such as economisers can be utilised for boilers and to pre-heat the
feedwater to produce hot water. On the other hand, recovery of heat
from overow hot water from the hot scalding process with the use of
automatically operated scalding chambers is also achievable [197,200].
The production of processed meat is more energy demanding than
slaughtering meat as it involves operations such as cooking, cooling,
smoking, etc. As Fritzson and Berntsson [201] reports, the majority of
heat loss in food processing operations is associated with refrigeration
and curing of the product. Based on the type of operation, waste heat
sources can include heat from condensers, waste water, smoking vents
and cooker exhaust [202]. Again, recovery from these sources must be
studied based on individual cases as for some waste heat sources, such
as waste water and cooker exhaust, recovery may be dicult and not
economical because of the grease and food waste products in the ex-
haust.
On the other hand, it has been shown that in poultry processing,
where bird meat is prepared and processed, the largest quantity of
energy and energy loss is associated with the scalding, cooling and
freezing processes. Shupe and Whitehead [203] reports that, for instance,
when overow from scalders and chillers occur, heat recovery can ea-
sily be conducted by collecting and returning the energy back to the
scalder or chiller systems. Heat can be recovered from refrigeration
condenser systems and be used to preheat the boiler that is used for
processing wash water [204]. The operation of obtaining heat from a
refrigerant condenser can be conducted through the use of a de-su-
perheater, which can be installed between the compressor and con-
denser to recover heat in a temperature range of 6090 °C [205].
Heat rejected by the pasteurisation process and refrigeration con-
densers on the other hand are the main source of waste heat in dairy
processing plants [206,207]. Nevertheless, the waste heat from the
dryer exhaust can also be a potential waste heat source that can be used
to preheat supply air for the spray dryers [208]. Having mentioned that,
the heat from the pasteurisation and milk cooling processes can be
recovered and be used to preheat cold milk in the regenerator through
the use of heat exchangers such as economisers or CO
2
heat pumps
[209]. The heat from refrigeration condensers is used to produce hot
water for clean-up, preheat boiler feed water, or heat culture tanks for
some operations [207,209].Singh and Dasgupta [209] showed that with
the use of a heat pump with an internal heat exchanger for combined
heat recovery and hot water generation, the total fuel cost for pro-
duction can be reduced by nearly 46% with a payback period of ap-
proximately 40 months.
In another study conducted by Bowater [210] and with the use of
heat pumps, the energy eciency of a large meat production plant was
improved. In this study, heat pumps were used to recover heat from the
refrigeration condensers of the plant to produce heating and hot water
to a temperature of 65 °C, indicating a possible daily energy saving of
up to £530.
Similar to the dairy industry, in the egg processing industry waste
heat can be recovered from the pasteurisation process and can be used
through a regenerator to preheat the cold egg product. The waste heat
from the refrigeration systems, hot waste water from egg washing and
exhaust air from egg drying process can also be recovered to preheat
boiler feed water, heat egg wash water and preheat the dryer air [211].
In freezing and canning processes the main operations are conducted in
fruit and vegetable processing [39]. In freezing operations, the major
waste heat is dissipated from the refrigeration system condenser. The
waste heat is derived from hot refrigerant and is easily recoverable
[212]. On the other hand, in canning operations the major waste heat is
reported to come from wastewater and retort vents. Waste heat re-
covered from these operations in fruit and vegetable processing can be
used for water heating, can washing, blancher makeup water, plant
clean up and boiler feed water [195].
In biscuit manufacture and bakeries the major waste heat sources
are from ue gases coming from the cooking ovens, fryers, pan washers
and boilers. Hot water can then be produced from the recovered waste
heat for use in clean-up. The recovery of heat from the cooking oven
exhausts is also a possibility for other uses [207,208]. For instance,
[213] used a thermo-acoustic heat engine (TAHE) to recover the low
grade waste heat that is dissipated from the exhaust gas of cooking
ovens in biscuit manufacture. The technology works with a prime
H. Jouhara et al. Thermal Science and Engineering Progress 6 (2018) 268–289
283
mover that does not have any mechanical parts and the engine consists
of two heat exchangers and a stack of parallel plates that are contained
in a cylindrical casing and it converts thermal energy to acoustic en-
ergy. The investigation concluded that by recovering waste heat with a
temperature of 150 °C, an output of 1030 W of acoustic power with a
thermal engine eciency of 5.4% can be obtained.
In another study Mukherjee et al. [214]demonstrated that with the
use of air-preheaters, 4% savings can be achieved in the oven fuel
consumption. In an experiment, heat was recovered from the exhaust
ow of an industrial baking oven and the primary air supply of the
burner was increased to 105 °C. This study indicated that the increase in
the primary air temperature can result to a saving of at least £4,200 of
running cost.
5.3. Waste heat in the ceramic industry
The ceramic industry is one of the most energy-intensive industries
in the UK [215]. As stated by the British Ceramic Confederation Energy
Policy, ceramic manufacturers should be concerned about the en-
vironmental and economical impacts of their businesses and take steps
in that regard to protect the worlds resources and reduce their carbon
footprint [216]. With respect to this, the use of waste heat recovery
units in the ceramic industry has been identied as an eective way of
achieving this goal and improving industrial energy eciency [217].
In order to discover the waste heat potential and identify how the
recovered waste heat can be used, it is important to identify the sources
of available heat in the production process and investigate the eec-
tiveness of waste heat recovery technologies with these sources [215].
As UNIDO & ECC [218] reports, electrical energy and chemical energy
in the form of fuel are the main types of energy used in the ceramic
industry. The electrical energy is used to power the motors of the
production equipment and machines and the chemical energy in the
form of fuel is used to provide thermal energy to heat the kilns and
furnaces. The ceramic production process typically consists of ve
stages. In the rst stage, the raw material and additives are ground and
mixed to produce material slurry. The material slurry is then then fed
into a drying tower where it gets dried and converted to powder so it
can be pressed to a shape and form unred ceramic. This then passes to
another drying operation through a hot chamber where controlled heat
allows the product water content to be reduced before the material
enters a kiln and is red to form blank ceramic. The product is then sent
to a polisher to achieve a smooth surface. As Peng et al. [219] explains,
the two energy consuming operations producing the most emissions are
the drying and ring operations. It is reported by Delpech et al. [220]
that the ring stage is the largest consumer of energy in the ceramic
production process and this contributes to almost 50% of energy loss
through the ue gas and the cooling gas exhaust.
In this stage of production, the ceramic structural integrity such as
mechanical strength, abrasion resistance, dimensional stability and re-
sistance to water, chemical and heat is increased by heating up the
product to a temperature between 750 °C and 1800 °C. The chart below
illustrates a breakdown of the main thermal energy consumption in
ceramic manufacture [221] (see Fig. 30).
Many dierent waste heat recovery technologies in this regard have
been investigated and introduced to accommodate and recover heat
from the drying and ring processes. For instance, as Delpech et al.
[220] explains, the best available techniques for recovery in the
ceramic industry include recovery of excess heat from roller kilns by the
use of cogeneration (or combined heat and power), Organic Rankine
Cycles to generate electricity and the use of heat pipe systems.
5.3.1. Recovery of excess heat from roller kilns
It is investigate that for the recovery of excess heat from roller kilns,
some processes employ heat exchangers to recover heat from the kiln
exhaust and preheat the combustion air entering the system [222].
Nonetheless, it is noted that because the combustion gases, possible
corrosion problems for the heat exchanger can occur. Having said that,
the heat from the cooling zones of tunnel kilns can be recovered to
preheat the dryer or used through the mean of other methods men-
tioned such as CHP or ORC to generate heat and electricity for the
process and plant.
Through this method, the generated electricity can be utilised in the
oven to power the air and exhaust fans and the generated heat from the
process can be used to heat equipment that dries the ceramics
[223,224]. For instance in an experimental study of an ORC (Organic
Rankine Cycle) for low grade waste heat recovery in a ceramic industry
[225] proved that by recovering 200300 °C from Kiln gases, the
maximum cycle eciencies can reach a gross electrical eciency of
12.5% with a net electrical eciency of 11%. Nonetheless, on the other
hand, Mezquita [226] demonstrated in a study that through the re-
covery and diluting the ue gas stacks by using an oxidiser with am-
bient air, the working uid temperature can be raised to 105 °C, esti-
mating a potential energy savings up to 17.3%. The technique was
discovered to lead to an energy saving of 685 kW and annual cost
savings of more than 190 kwithout the requirement of any special
investment.
Nevertheless it is argued that transporting the heat from the source
to use may be a challenge and in this regard suitable heat insulation
maybe required. Having mentioned that, signicant energy savings
have been achieved with the use of new technologies such as the use of
thermal oil to transfer heat from the afterburner to the dryer [227] (see
Fig. 31).
6. Conclusion
In conclusion, industrial waste heat is the energy lost in industrial
processes to the environment. Waste heat recovery in industry covers
methods of collection and re-use of the lost heat of industrial processes
that can then be used to provide useful energy and reduce the overall
energy consumption. Heat loss is mainly classied into high tempera-
ture, medium temperature and low temperature grades and waste heat
recovery systems are correspondingly introduced for each range of
waste heat. The selection of heat recovery methods and techniques
largely depends on key factors such as the quality, quantity and the
nature of heat source in terms of suitability and eectiveness. The
identication of the waste sources is an important aspect when looking
into waste heat recovery methods for industrial processes in order to
achieve optimum results and eciency. In this regard, a comprehensive
review is presented for waste heat recovery methodologies and state of
the art technologies used in industrial processes.
It was investigated that, there are many dierent heat recovery
technologies available for capturing the waste heat and they mainly
consist of energy recovery heat exchangers in the form of a waste heat
recovery unit. These units mainly comprise common waste heat re-
covery systems and all work by the same principle to capture, recover
and exchange heat with a potential energy content in a process.
Fig. 30. Thermal energy sources in ceramic industry.
H. Jouhara et al. Thermal Science and Engineering Progress 6 (2018) 268–289
284
It was discovered that major heat recovery equipment are mainly
categorised based on the temperature range and the type of uid being
recovered in the process and each has a dierent usage. For instance, it
was studied, regenerative and recuperative burners optimise energy
eciency by incorporating heat exchanger surfaces to capture and use
the waste heat from the hot ue gas from the combustion process,
whereas, economisers recover low medium waste heat and are mainly
used for heating liquids. On the other hand, it was explored that waste
heat boilers are suitable to recover heat from medium high tem-
perature exhaust gases and are mainly used to generate steam for power
generation or energy recovery.
Systems such as air preheaters were found to be useful for exhaust-
to-air heat recovery and for low to medium temperature applications.
This system was revealed to be particularly useful where cross con-
tamination in the process must be prevented. Nonetheless systems that
incorporate several heat recovery systems such as the heat recovery
steam generator was also reviewed and it was discovered that this
complex system recovers the waste heat and employs a thermodynamic
cycle to generate power and improve the eciency of a power or
manufacturing plant.
The working principles of thermodynamic cycles that are mainly
used for waste heat recovery such as the Organic Rankine Cycle and the
Kalina cycle were also studied and it was determined that the Kalina
cycle oers a better result when the recovered heat is the medium-high
grade. However, the ORC was a competitor when the recovered heat
was in the low-medium range. Nevertheless, the functionality of plate
heat exchanger and heat pipe systems were also looked at and it was
learned that these heat exchangers can be used to transfer heat from
any temperature range and from one source to another when cross
contamination needs to be avoided.
The functionalities of direct electrical conversion devices were also
explored and it was discovered that these systems produce electricity
directly from waste heat and eliminate the need for converting heat to
mechanical energy to produce electrical energy. Nonetheless, due to the
limitations these technologies oer, they are not widely used in in-
dustry. The working principles of heat pumps were also studies and it
was learned that this technology is in particular good for low-tem-
perature waste heat recovery, as it gives the capability to upgrade waste
heat to a higher temperature and quality.
By considering the heat recovery opportunities for energy optimi-
sation in the steel and iron, food, and ceramic industries, current
practices and procedures were assessed and reviewed. For instance, by
looking into the waste heat recovery potential for the iron and steel
industry, it was revealed that the sources of waste heat are mainly
within the range of medium-high temperature but challenges and lim-
itations related to recovery methods exist due to the presence of dirty
and low quality waste heat. In this regard, new and innovative tech-
nologies and techniques are employed to recover the waste heat from
dierent sources in iron and steel production processes.
The investigated technologies and techniques include, the use of
Heat Pipes to recover heat from the cooling line; regenerators to recover
the waste heat from the exits of coke ovens, oven chambers, and blast
furnaces; semi wet and wet open combustion and supressed combustion
techniques for recovery from the basic oxygen furnaces; capturing and
using ammable by-product gases and waste heat through preheaters in
electrical arc furnaces; heat recovery through coke dry quenching and
wet quenching and recovery as hot air steam or conversion of the waste
heat to fuel through chemical reactions from hot slag.
Waste heat recovery opportunities in food industry were also in-
vestigated and it was discovered that the potential sources of waste heat
in this industry are mainly associated with heating and refrigeration
systems, hot streams of water or air and heat from processing opera-
tions. Heat recovery from these sources must be studied on an in-
dividual case basis as some waste heat sources such as waste water and
cooker exhaust recovery may be dicult and uneconomical to utilise
because of grease and food waste products in the exhaust.
Having said that, technologies such as thermoacoustic heat engines
(TAHEs), economisers, automatic scalding chambers, heat pumps and
de-superheaters are considered to be alternative methods of optimising
energy management in the processes of the food industry.
In the ceramics industry, the primary sources of available waste
heat come from kilns and furnaces mainly through the drying and ring
operations. The available techniques for waste heat recovery in this
industry include recovery of excess heat from roller kilns though co-
generation or combined heat and power, the Organic Rankine Cycles to
generate electricity and heat pipe systems.
The functionality of each system has been analysed and it has been
shown that recovery of excess heat from roller kilns can be carried out
by accumulating the waste heat from the cooling zones of the kiln
tunnel and using that as a heat source to preheat the dryer or through
the means of other mentioned methods such as CHP or ORC to generate
heat and electricity for the plant.
To sum up, this paper indicates that improving energy eciency by
utilising waste heat recovery in industrial processes is achievable based
on dierent approaches and with the use of dierent state of the art
technologies. However, in order to obtain the most optimum eciency
for a system through waste heat recovery, the type of process in ques-
tion should be always examined and analysed and then a method of
waste heat recovery for optimising energy eciency should be as-
signed.
Conict of interest
None.
Acknowledgements
The presented work is the result and contribution of the following
Fig. 31. Schematic of a Ceramic Kiln [228].
H. Jouhara et al. Thermal Science and Engineering Progress 6 (2018) 268–289
285
projects: Horizon 2020 Project Design for Resource and Energy e-
ciency in CerAMic Kilns- DREAM Project (Grant No: 723641), Horizon
2020 Industrial THERMal energy recovery conversion and management
(Grant No: 680599) and ESPRC (Grant No: EP/P004636/1).
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... The heat generated in the IC engine can be categorized as a high temperature where the temperature is above 400 °C. The higher the temperature range, the higher the quality of the waste heat and it is easier for utilization of the waste heat recovery process [6]. ...
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Problems of sustainable development and environmental protection pose a challenge to humanity unprecedented in scope and complexity. Whether and how the problems are resolved have significant implications for human and ecological well-being. Accordingly, this work proposes an electricity and freshwater cogeneration system using solar energy and natural gas dual sources. The proposed system is a combination of a heliostat field with a gas turbine cycle as the top systems and thermal vapor compression-multi effect desalination, steam Rankine cycle, organic Rankine cycle, and thermoelectric generator as subsystems. For performance analysis, energy, exergy, exergoeconomic, economic, and environmental analyses were performed. To optimize the system, a coupled model of the support vector regression, multi-objective grey wolf optimization algorithm, multi-objective Grasshopper optimization algorithm, and two different decision-making methods were suggested. According to obtained results, the best swirling number and pressure ratio were 0.95 and 7, respectively. Moreover, the exergy efficiency, total product cost rate, and CO2 emission were chosen as the best optimization scenario, which led to 45.6% of exergy efficiency, 2.716 $/GJ of total product cost rate, and 30.26 kg/s of freshwater. Moreover, the total exergy destruction rate decreased from 15153 kW to 14820 kW after the optimization of the system.