ABSTRACT
As fuel prices continue to
escalate the relevance of efficient energy is apparent to companies everywhere,
from the smallest concern to the largest multinational. The methods and
techniques adopted to improve energy utilization will vary depending on
circumstance, but the basic principle of reducing energy costs relative to
productivity will be the same. As such, field of energy conservation calls for
a new insight into the newer sources of
energy besides the conventional sources that can be employed in various industries
as well as in domestic applications.
One such source is ‘Waste
heat in various industrial processes’. This
paper presents an overview of various waste heat recovery systems that are
available & a case study on ‘recuperator ‘as a waste heat recovery system.
The recuperator under consideration has been installed upon the billet
reheating furnace in the rolling mill section of ‘Ferrous Alloys Corporation
(FACOR)’ – a steel company situated at M.I.D.C. Hingna, Nagpur.
The case study proves the effectiveness
of various waste heat recovery systems in general & recuperators in
particular as non conventional sources of energy. This leads to lower
consumption of fuel. Lower consumption of fuel not only increases the productivity
of any thermal plant but also helps in reducing pollution levels caused for a
given level of the plant output.
Introduction
Waste heat:
Waste heat is heat, which is generated
in a process by way of fuel combustion or chemical reaction, and then “dumped”
into the environment even though it could still be reused for some useful and
economic purpose. The essential quality of heat is not the amount but rather
its” value”. The strategy of how to recover this heat depends in part on the
temperature of the waste heat gases and the economics involved.
Quality of heat:
Depending upon the type of
process, waste heat can be rejected at virtually any temperature from that of
chilled cooling water to high temperature waste gases from an industrial
furnace or kiln. Usually higher the temperature, higher the quality and more
cost effective is the heat recovery.
Classification:
High Temperature Heat
Recovery:
As the name suggests these systems
are used where heat is being discarded at high temperatures (650 0C
& above)i. e. the quality of heat is very good. Such rejection of high
quality heat generally results from
direct fuel fired processes.
Medium Temperature Heat
Recovery:
Most of the waste heat in this temperature
range comes from the exhaust of directly fired process units. The range for
medium temperature waste heat is generally considered to be from 200 0C
to 650 0C.
Low
Temperature Heat Recovery:
This category includes any
system where heat is being discarded at temperatures below 200 0C.In
this range it is usually not practical to extract work from the source, though
steam production may not be completely excluded if there is a need for
low-pressure steam. Low temperature waste heat may be useful in a supplementary
way for preheating purposes.
Commercial
Waste Heat Recovery Devices
Recuperators
In a recuperator, heat exchange
takes place between the flue gases and the air through metallic or ceramic
walls. Duct or tubes carry the air for combustion to be pre-heated, the other
side contains the waste heat stream. A recuperator for recovering waste heat
from flue gases is shown in figure.
Regenerator
The Regeneration, which
is preferable for large capacities, has been very widely used in glass and
steel melting furnaces. It consists of a two way flow passage for the fluids.
For one cycle the flow takes place in one direction so that heat in the flu gases
is absorbed by the fire bricks on the exhaust side. In the second cycle, the
direction of flow of gases is reversed so that the incomig air is preheated as
it passes over the hot fire bricks and gives the exhaust heat to the fire
bricks on the other side which is now acting as the exhaust side.
Heat Wheels:
A
heat wheel is finding increasing applications in low to medium temperature
waste heat recovery systems. Figure 8.6 is a sketch illustrating the
application of a heat wheel. It is a sizable porous disk, fabricated with
material having a fairly high heat capacity, which rotates between two
side-by-side ducts: one a cold gas duct, the other a hot gas duct. The axis of
the disk is located parallel to, and on the partition between, the two ducts.
As the disk slowly rotates, sensible heat (moisture that contains latent heat)
is transferred to the disk by the hot air and, as the disk rotates, from the
disk to the cold air. The overall efficiency of sensible heat transfer for this
kind of regenerator can be as high as 85 percent.
Heat Pipe:
A heat pipe can transfer
up to 100 times more thermal energy than copper, the best-known conductor. In
other words, heat pipe is a thermal energy absorbing and transferring system
and have no moving parts and hence require minimum maintenance.
The Heat Pipe comprises of
three elements - a sealed container, a capillary wick structure and a working fluid. The
capillary wick structure is integrally fabricated into the interior surface of the container tube and
sealed under vacuum. The Heat Pipe comprises of three elements - a sealed
container, a capillary wick structure and a working fluid. The
capillary wick structure is integrally fabricated into the interior surface of the container tube and
sealed under vacuum.
Typical Application
The heat pipes are used in
following industrial applications:
Process
to Space Heating: The heat pipe heat exchanger transfers the thermal energy from
process exhaust for building heating.
Process
to Process: The heat pipe heat exchangers recover waste thermal energy from
the process exhaust and transfer this energy to the incoming process air.
HVAC Applications:
Cooling:
Heat pipe heat exchangers precools the building make up air in summer and thus
reduces the total tons of refrigeration, apart from the operational saving of
the cooling system.
Heating: The above process is reversed during winter to preheat the make
up air.
Economiser:
In case of boiler system, economizer
can be provided to utilize the flue gas heat for preheating the boiler feed
water. On the other hand, in an air pre-heater, the waste heat is used to heat
combustion air. In both the cases, there is a corresponding reduction in the
fuel requirements of the boiler..
Shell and Tube Heat
Exchanger:
When the medium containing
waste heat is a liquid or a vapor which heats another liquid, then the shell
and tube heat exchanger must be used since both paths must be sealed to contain
the pressures of their respective fluids. The shell contains the tube bundle,
and usually internal baffles, to direct the fluid in the shell over the tubes
in multiple passes.
Plate heat exchanger:
Plate heat exchanger consists of a series of separate
parallel plates forming thin flow pass. Each plate is separated from the next
by gaskets and the hot stream passes in parallel through alternative plates
whilst the liquid to be heated passes in parallel between the hot plates. To
improve heat transfer the plates are corrugated.
Hot liquid passing through a
bottom port in the head is permitted to pass upwards between every second plate
while cold liquid at the top of the head is permitted to pass downwards between
the odd plates. When the directions of hot & cold fluids are opposite, the
arrangement is described as counter current. A plate heat exchanger is shown in
figure.
Waste Heat Boilers:
Waste heat boilers are
ordinarily water tube boilers in which the hot exhaust gases from gas turbines,
incinerators, etc., pass over a number of parallel tubes containing water. The
water is vaporized in the tubes and collected in a steam drum from which it is
drawn off for use as heating or processing steam.
THE CASE STUDY
Energy performance assessment of
recuperator
Data available: 340tubes X 43mm OD X
1250mm
1] Heat duty: Qf = mf Cp [Ti-To]
= ρV Cpf [Ti-To]
= 1.19x9583x1226.5
[650-400]
= 941.4kw
2] Capacity ratio R= (Ti-To)/ (to-ti)
= (650-400)/
(300-30)
= 0.925
3] Effectiveness S = (to-ti)/ (Ti-ti)
= 0.4354
4] LMTD = θi- θo/ (log (θi/ θo)
= 359.9°C
5] Overall Heat Transfer Coefficient
[OHTC]
OHTC = U= Qf/(A x ΔT)
= 0.0965kw/m2 K
Energy performance
assessment of furnace
WITHOUT RECUPERATOR
Data available: % of
excess air= 100%
To=
650°C
Calorific value of fuel (LDO) =10700 kcal/kg
Cost of fuel = Rs.16 per kg
Oil consumed= 60lit/tonne
Production of firm= 4000tonne/month
1] Theoretical air required to burn
1kg of oil= 14kg
2] Total air supplied = Theoretical
air ( 1+ excess air/100)
= 28 kg/kg of
oil
3] Sensible heat loss = m Cp ΔT
Where m = actual mass of air supplied / kg of fuel + mass of fuel
= 28+1
= 29 kg/kg of oil
Q = 29 X 0.29 (650-30)
= 5214.2 kcal/kg of oil
Heat lost = 48.73%
4] Heat utilized = C.V.- heat lost
= 10700 – 5214.2
= 5485.8 kcal/kg of
oil
5]
Cost of oil/annum = 60 X4000 X12 X16 = rs. 46,080,000
WITH
RECUPERATOR
Data available: To = 400°C
% of excess air= 100%
Calorific value of fuel (LDO) =10700 kcal/kg
Cost of fuel = Rs.16 per kg
Oil consumed= 40lit/tonne
Production of firm= 4000tonne/month
1] Theoretical air required to burn
1kg of oil= 14kg
2] Total air supplied = Theoretical
air (1+ excess air/100)
= 28 kg/kg of
oil
3] Sensible heat loss = m Cp ΔT
Where m = actual mass of air supplied / kg of fuel + mass of fuel
= 28+1
= 29 kg/kg of oil
Q = 29 x 0.25 [400 -30]
= 2735.17 kcal/kg of oil
Heat lost = 25.56%
4] Amount of heat recovered = Heat without recuperator - Heat with
recuperator
= 5214.2 – 2735.17
= 2479.03 kcal/kg of oil
5] % of heat recovered = (% heat lost with recuperator) /
(% heat lost with out recuperator)
= (25.56/ 48.73) x 100
= 52.45%
6] Heat utilized = C.V.- heat lost
= 10700 – 2735.17
= 7964.83 kcal/kg of
oil
7]
Heat obtained per unit expenditure = 7964.83 ÷ 16
= 497.81 kcal/re.
8] Effective increase in heat available per unit expenditure =
497.81 – 342.86
= 154.95 kcal/re.
9]
Cost of oil/annum = 40 X4000 X12 X16 = rs. 30,720,000
Savings in oil costs= 46080000-3072000
= rs.4308000
|
RESULT:
1] For every rupee that is
spent upon the fuel, an additional
154.95 kcal of heat is available.
2] Annually Rs. 4,308,000 are saved towards
fuel costs.
CONCLUSION
Thus we see that due to the use
of recuperator the fuel requirement for a given operation is greatly reduced as
a result of which, following benefits are obtained:
Direct Benefits:
Recovery of waste heat has a
direct effect on the efficiency of the process. This is reflected by reduction
in the utility consumption & costs, and process cost.
Indirect Benefits:
a) Reduction in
pollution:
A number of toxic combustible wastes such as
carbon monoxide gas, sour gas, carbon black off gases, oil sludge, etc, releasing to atmosphere if/when burnt in
the incinerators serves dual purpose i.e. recovers heat and reduces the
environmental pollution levels.
b) Reduction in
equipment sizes:
Waste heat recovery reduces the fuel
consumption, which leads to reduction in the flue gas produced. This results in
reduction in equipment sizes of all flue gas handling equipments such as fans,
stacks, ducts, burners, etc.
c) Reduction in
auxiliary energy consumption:
Reduction in equipment sizes gives additional
benefits in the form of reduction in auxiliary energy consumption like
electricity for fans, pumps etc.
