Low Quality Waste Heat Recovery
• Waste Heat recovery is the collection of the
heat created as an undesired by-product of
the operation of the piece of an equipment
or machinery to fulfill a desired purpose
• Fossil fuels are the world’s dominant energy
supply. As these resources are nonrenewable, we must utilize it prudently.
Hence Waste Heat Recovery is the need of
• It is estimated that about 20-50% of the
Industrial Heat is lost as waste heat in the
form of hot exhaust gases, cooling
water, heat lost from the hot equipment
surfaces and heated products.
• There are 3 essential components in Waste
heat recovery as shown in the figure-
Sources of Waste Heat
• Waste heat losses arises from equipment inefficiencies and from
thermodynamic limitations on the equipments and processes.
Factors to be considered for Waste Heat Recovery
• Quantity of Waste Heat:- It is the measure of how much energy is
contained in a waste stream. It is given by Q = m*Cp*ΔT.
• Waste Heat Temperature/Quality:- It is a key factor which is used to
determine the waste heat recovery feasibility. For effective utilization of
waste heat, it must be kept in mind that as the heat transfer takes place
from a hot body to a cold body, the temperature of the waste heat source
must be higher than the heat sink.
Depending upon these two factors, the thermodynamic assessment of any
power plant or other equipment can be done.
• Suppose a system is at 1000 K & the atmosphere is at 300 K, then the
exergy (maximum useful work) of the system is 70%.
• 30% of the energy available is not useful, as it is unavailable energy and
cannot be converted to work.
This is a very simplistic diagram of a coal-fired power plant.
•Here, there are two major sources of waste
1. Flue gases/stack gases
2. The steam after having passed through the
The temperature in both the cases is around
100-150 0C. But the atmospheric temperature
is around 25 – 30 0C. Hence the exergy
available is around (150 – 30) = 120 0C
•This is a schematic diagram of a
coal-fired power plant.
•First of all, coal is pulverized in a
mill, and it is mixed with the air
from the atmosphere, which is
then sent to the boiler where it
is heated at a very high
temperature to obtain the flue
•These flue gases come in
contact with the very pure water
which is passed through the
section of water, which is then
converted into steam. Now, this
steam drives the
turbine, generates the electricity.
The steam is sent back to the
condenser to be condensed back
Qualitative thermodynamic assessment
with an aging boiler
• The Heat balance calculations we
perform are based on the first
law of thermodynamics.
• The shortcoming of this analysis
is that it does not take into
account properties of the system
environment, or degradation of
energy quality through dissipative
processes. In short it doesn’t take
into consideration, the
irreversibility of the process.
• Hence exergy
assessment is done which is
based on the second law of
Ways and Means to use the Waste Heat
Passive Air Preheaters
Waste Heat Boilers
• Heat Pumps
Most of these changes have been
employed by the power plants. Air
has to be preheated and the steam
from the HP turbine has to be sent
through the boiler. As in the
regenerative Rankine cycle, even the
feed water is preheated.
Now these Heat Exchangers and economizers
are employed to make certain design changes
in the plant. Now Power plants are based on
the Rankine Cycle. We can have following
cycles to replace it:1) Regenerative Rankine Cycle 2) Rankine Cycle
with Reheat 3) Organic Rankine Cycle
But does this really solve our problem?
• These Recuperators, Regenerators, Air Preheaters, Economizers, Waste
Heat Boilers work in the High to Medium Temperature Range.
• What we are really concerned is about the low temperature range at
which the waste is directly thrown into the atmosphere. So these
equipments won’t come in use then.
• The real difficulties in using the Low Temperature Waste Heat Recovery:1. Corrosion on H.E. surface : As the water vapor in the exhaust gas cools,
some droplets will condense on the H.E. surface. These are highly
corrosive in nature.
2. Large H.E. Surface required : As the heat transfer is dependent on the
H.E. area as well as the temperature gradient, due to lower temperature
gradient in low temperature waste heat, large heat transfer area is
3. Find a use for the low temperature heat
Organic Rankine Cycle
• We use an organic fluid which has a boiling point much lower to that of the
temperature of the condensate water at 150 0C.
• The relatively low temperature heat is converted into useful work, which in
turn is converted into electricity.
• The major requirement in this process is that the organic vapor is cooled and
condensed by air or water at a low temperature (typically near 30° C).
Carbon Capturing using Waste Heat : A Future Potential
Coal-fired power plant is a major source of Carbon Dioxide, and because majority
of the power plants in India are Coal-based, it is a major source for global
Waste heat can be employed for Carbon Capture
Solvents such as MEA, DEA are used to absorb the CO2 from the flue gases
generated in the boiler where the solvent is passed from the top, flue gases from
The CO2 so captured would then be passed through a heater (utilizing the waste
heat), which would separate it into solvent and CO2.
The solvent is recycled, whereas CO2 is stored underground.
•Waste Heat Recovery : Technology and Opportunities in U.S. Industry
prepared by U.S. Department of Energy
•Exergy Analysis and Efficiency improvement of a Coal-fired Thermal Power
Plant in Queensland by R. Mahamud, M.M.K. Khan, M.G. Rasul and M.G.
•For Carbon Capture, http://www.greentechmedia.com/articles/read/couldwaste-heat-be-the-secret-for-carbon-capture
• A Power Point Presentation on Exergy Analysis
This Presentation has been prepared by :Vismay Harani
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