1
. Introduction
Waste heat is defined as the residual heat after heat recovery within a process, heat recovery between several
processing units on a site, and residual heat rejected to cooling water and air from a site utility system. According to
IPCC (2010 data) Industry is responsible for 32% of worldwide greenhouse gas emissions, i.e. about 16 billion tonnes
of CO2 equivalent, including indirect emissions due to electricity used in industrial plants. According to the
International Energy Agency, energy/ waste heat recovery together with recycling is expected to bring 9% of the
necessary emissions reduction. Waste heat recovery (WHR) can significantly improve the site energy efficiency
which has been identified as the most cost-effective measure for carbon dioxide mitigation especially in the short and
medium term.
1
.1 Opportunities to achieve a more competitive, secure and sustainable energy system and to meet its long-
term 2050 greenhouse gas reductions target
a 40% cut in greenhouse gas emissions compared to 1990 levels;
at least a 32% share of renewable energy consumption, with an upward revisions clause for 2023;
indicative target for an improvement in energy efficiency at EU level of at least 32.5%, following on from
the existing 20% target for 2020.
In this context, efficient recovery of waste heat in the form of power, heat and chilling can contribute in three ways:
(
(
i)
ii)
supplying power and heat in the respective grid;
balancing the grid (both power and thermal) against the fluctuation caused by increased share of variable
renewables in the energy mix;
(
iii)
increasing energy efficiency and hence the energy security of waste heat generating site through saving
of primary energy.
1
.2 Barriers of WHR adoption
Although there are huge potential and numerous benefits of WHR, there are some barriers that impact the economy
and effectiveness of heat recovery equipment and impede its wider installation.
Barrier 1: Lack of justification: The lack of accurate evaluation of cost benefit of the WHR project does not build
confidence.
Barrier 2: Long payback period: Costs of heat recovery equipment, auxiliary systems, and design services lead to
long payback periods in certain applications.
Barrier 3: Composition of exhaust stream: Many exhaust streams are contaminated or containing fouling
particulates of some form. Resulting issues can be clogging and abrasion of devices, and corrosion or deposit
formation when temperatures are reduced below dew points of certain components when heat is extracted from a
stream. These aspects can limit the possibility to recover heat from some sources, or significantly increase the
associated costs.
Barrier 4: Economy of Scale: Equipment costs favour large-scale heat recovery systems and create challenges for
small-scale operations.
Barrier 5: Intermittent Character of waste heat source: This might cause fatigue in the heat exchange materials
due to thermal cycling. Moreover, the intermittent character of the recoverable energy flows and its variations during
normal operating conditions might not ensure energy efficient performance.
Barrier 6: Lack of awareness and initiatives: Industrial engineers often do not have the time or knowledge to
explore all waste heat recovery methods and are “unsure” or “uninformed” of the benefits of WHR. Novel methods
of WHR are often ignored due to lack of understanding.
Barrier 7: Lack of space: In some cases, spaces for installing WHR devices are very limited. This becomes more
complicated when additional energy storage needs to be installed while recovering waste heat from intermittent
sources.
Barrier 8: Expensive consultancy service: Expensive consultancy is required from start of potential WHR projects.
It demotivates WHR initiatives.
1
.3 Features of the tool
This tool is designed addressing the above stated barriers of WHR adoption. The main features of the tool are:
It will characterise waste heat stream i.e., the exhaust flue gas processing minimum numbers of user inputs;
It will provide dimensions of WHR devices such as heat pipe heat exchanger (HPHE) and thermal storages
for a specific WHR application;
It will provide stand alone solution where user will select the capacity of HPHE and the tool will give
different options for thermal energy storage;
It will provide multiple options of WHR solution depending on the flue gas temperature and the amount of
heat to be recovered;
It will provide informed investment decision support on the marginal investment (on WHR technology)
through evaluating (for each of the options for WHR solution) different financial parameters such as payback
period, net present value (NPV), internal rate of return (IRR), return on investment (ROI) etc.;
To account the uncertainty of different variables such as retrofitting factor, discount rate etc., it will perform
sensitivity analysis to determine the impact of these factors on payback period, NPV, IRR, ROI;
It will evaluate environmental performance for each of the options for WHR solution;
2
. Functional modules of the tool
There are five modules which are:
1
2
3
4
5
. Flue Gas Characterization
. Waste heat utilisation
. WHR solution
. Cost performance
. Environmental performance
User will provide data related to waste heat source through module 1 i.e., “flue Gas Characterization” module. Results
of module 1 and user data will be processed and fed to other modules to formulate different waste heat recovery
solutions along with the respective cost and environmental performances.
2
.1 Module 1- Flue Gas Characterization
Processing the user data, this module will predict the quality and quantity of the available waste heat and properties
of flue gas exiting the furnace. Using the temperature, flow measurement and the stream composition data, waste
heat stream will be characterised in terms of quantity and quality. For instance, if a waste heat stream contains ‘n’
th
1
number components, then the heat content in the I component can be expressed as:
1
Waste Heat Recovery: Technology and Opportunities in U.S. Industry; Prepared by BCS, Incorporated March 2008
th
i
Where h (t) is the enthalpy of the I species at the specified temperature T, r is the reference temperature (either
ambient temperature or the minimum allowed temperature of WHR), and Cp,I is the specific heat capacity of the
2
species as a function of temperature. Cp can be expressed as:
p
Where, T in Kelvin, C in kJ/kmol, K
After adding the enthalpies of all the constituents, the heat content of the waste stream will be determined. To
calculate the heat content of the flue gas, all the constituent gases (such as N
considered as ideal gas.
2
, H
2
, CO
2
, CO, SO
2
, H
2
O
(g) etc.) will be
Quantity
The key information of the results is outlined in the following table:
Item
Unit
Average density of flue gas (ambient to flue gas temperature)
Average density of flue gas (max allow temp of flue gas)
kg/m3
kg/m3
kW
Waste heat energy flow rate through the flue gas (w.r.t. ambient temperature)
Actual recoverable waste heat energy flow rate (w.r.t. maximum allowable temperature*) kW
Average specific heat capacity of flue gas (ambient to flue gas temperature)
Average specific heat capacity of flue gas (ambient to flue gas temperature)
kJ/kg.C
3
kJ/m .C
kJ/kg.C
Average specific heat capacity of flue gas (maximum allowable temperature* of the flue
gas to flue gas temperature)
Average specific heat capacity of flue gas (maximum allowable temperature* of the flue
gas to flue gas temperature)
3
kJ/m .C
*
Maximum allowable temperature of the flue gas = Acid dew point temperature + safety margin
The tool will offer three options to characterise flue gas. Based on the available information, user can select any of
the three options.
Option 1 (Flue Gas Characterization)
2
Source: B.G. Kyle, 1984, Chemical and Process Thermodynamics
Mandatory input (option 1)
3
1
2
3
4
5
6
. Natural gas (Fuel) flow rate, m /s
. O concentration in exhaust, %
. Flue gas temperature, C
. Furnace operating cycle
. Furnace ON time
2
o
. Furnace OFF time
Optional input (option 1)
1
2
3
4
. Natural gas composition
o
. Acid dew point temperature, C
o
. Ambient temperature, C
3
. Sp. Heat capacity of flue gas, kJ/m .C
Information flow diagram within option 1:
The logic and algorithm developed under the assumption of complete combustion. Other assumptions made under
option 1 of flue Gas Characterization (The corresponding values can be edited through admin panel) are listed in
the following table
Item
Unit Quantity Comments
o
o
Acid dew point temperature, C
Ambient temperature, C
Safety margin for acid dew point temperature, C
Pressure of the gaseous substance
C
C
C
150 If corresponding user input is null
20 If corresponding user input is null
20 Internal design variable
o
o
o
o
atm
1 Internal design variable
Option 2 (Flue Gas Characterization)
Mandatory input (option 2)
3
1
2
3
4
. Flue gas flow rate, m /s or kg/s
o
. Flue gas temperature, C
. Furnace operating cycle
. Flue gas composition, %vol or %mass
(
The default list of flue gas constituents)
Flue gas constituents
%Vol or %mass
N2
O2
CO
CO2
H2O
NO
N2O
NO2
SO2
SO3
C6H6
HCl
5
6
. Furnace ON time
. Furnace OFF time
Optional input (option 2)
o
. Acid dew point temperature, C
o
1
2
. Ambient temperature, C
Information flow diagram within option 2: