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Theoretical, Practical, and Zero-emission Exergy Recovery in Energy Conversion Processes
Part I: Definition and importance of the exergy concept
1An introduction to the exergy
concept
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Outline Energy versus exergy: Carnot engine Exergy definition Physical and chemical source of exergy: driving force EROI A simple example Parts of My research
Case study 1: Natural gas treatment and transport Case study 2: UCG process with spontaneous CO2 capture
and sequestration Case study 3: Geothermal energy
2An introduction to the exergy
concept
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Energy and its quality
Nicolas Leonard Sadi Carnot (1796-1832)
Cycle and reversibility
3An introduction to the exergy
concept
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James Joule (1818-1889)
James Watt (1736-1819)
Sadi Carnot (1796-1832)
4An introduction to the exergy
concept
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Carnot Cycle
An introduction to the exergy concept 5
The Carnot engine cycleRef: Van Ness, H. C., Understanding Thermodynamics
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T-S Diagram of Carnot Cycle
Cendel Y. A., Boles M. A., “Thermodynamics; an engineering approach”, 5 th edition6
An introduction to the exergy concept
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Energy or Exergy
Heat Source @ 900 K
Heat Source @ 500 K
Power Cycle,Carnot Engine
Qin
Qout,1
Qin
Power Cycle,Carnot Engine
Qout,2
Heat Sink @ 300 K
Work Work
Wnet = (1-300/900)Qin
= 0.67 Qin Wnet = (1-300/500)Qin
= 0.4 Qin
7An introduction to the exergy
concept
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Quantitative Definition
Control volume
Q 0
P0, T0, S0, H0 P, T, S, H
Environment (Q0 enters the environment at P0 and T0)
Environment (Q0 enters the environment at P0 and T0)
First Law:W = Q0 + ∆H
Second Law:Sgen = ∆S + ∆S0 ∆S0 =Q0/T0
8An introduction to the exergy
concept
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Quantitative Definition
W = ∆H – T0 ∆S + T0 Sgen ∆H = H – H0
∆S = S – S0
Wmin = H – H0 – T0 (S – S0) + T0 Sgen
and
Gives:
Isentropic path (Sgen = 0)
Wmin = (H – T0 S) – (H0 – T0 S0)
and
Gives:
Definitions:Availability:
BT, P = HT, P – T0 ST, P
Exergy:ExT, P = BT, P – BT0, P0
9An introduction to the exergy
concept
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Driving Force (∆)
Potential energy (∆Z) Kinetic energy (∆V) Chemical energy (∆x) Heat (∆T) Mechanical energy (∆P)
An introduction to the exergy concept 10
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Chemical Exergy
11An introduction to the exergy
concept
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Chemical Exergy
Standard chemical exergy of elements has been reported in the literature based on the average composition of earth’s crust, oceans, and the atmosphere
Ex0ch, j = ∆fG0
j, T0 + ∑ vi Ex0ch, i
∑ vi ei Cj
Ex0ch, i is the standard chemical exergy of element i
∆fG0j, T0 is the standard Gibbs free energy of formation of
component j
12An introduction to the exergy
concept
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Theoretical, Practical, and Zero-emission Exergy Recovery in Energy Conversion Processes
Part II: Formulation and Case studies
13An introduction to the exergy
concept
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EROEI
Energy Return On Energy Investment Definition:
How much of the energy available in a “natural resource” can be extracted per unit energy consumed in the recovery process, e.g., upstream operation, transportation, etc.
Shortcomings: Does not consider the energy quality, i.e. exergy Highly depends on the current state of technology
and its development
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EROEI
How much of the energy available in a natural resource can be extracted per unit energy consumed in the recovery process, e.g., upstream operation, transportation, etc.
An introduction to the exergy concept 15
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Data?
Translate the recorded economical data to energy data using commercial databases
$$$$$$
Energy
ConversionFactor
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What happens?
CaCO3 + Q ↔ CaO + CO2 Q(calculated) = 3074 kJ/kg CaO
?!!
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Process Flow Diagram
An introduction to the exergy concept 18
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Theoretical, Practical, and zero-emission exergy recovery Theoretical exergy recovery
Based on the reversible processes shows the maximum possible exergy recovery with the following assumptions: No mechanical exergy loss in the process equipments No cumulative exergy consumptions (CExC) in the material and energy
streams
Practical exergy recovery Based on the irreversible processes and considering the current
technology, uses the following assumptions: Mechanical efficiency of process equipments Cumulative exergy consumption (CExC) in the material and energy streams
Zero-emission exergy recovery Considering the exergy consumption in cleaning the environmental
footprints of fossil fuel life cycle in addition to the practical recovery
An introduction to the exergy concept 19
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Calculation of theoretical, practical and sustainable exergy consumption
An introduction to the exergy concept 20
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A simple example
An introduction to the exergy concept 21
W = Wisentropic / ηc
Exch
ηpp
Exel = ηpp Exch
ηed
Exp = Wisentropic / ηc / ηed / ηpp
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Natural Gas
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Results for Natural Gas
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An introduction to the exergy concept 24
Underground Coal Gasification (UCG)- Un-minable coal resources: less than 15% of the known coal resources can be extracted using conventional mining methods-Environmental issues of mining-Low quality coal seams
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UCG with mineral injection
An introduction to the exergy concept 25
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Chemical reactions
An introduction to the exergy concept 26
Combustion C + O2 CO2 + 393.77 kJ/mol
Gasification Global reaction
C + 2 H2O + CaO CaCO3 + 2 H2 + 87.9 kJ/mol Boudouard reaction
C + CO2 2 CO – 172.58 kJ/mol Shift reaction
CO + H2O CO2 + H2 – 41.98 kJ/mol Methanation
C + 2 H2 CH4 + 74.90 kJ/mol
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Equilibrium relations
An introduction to the exergy concept 27
∏i
y i φi vi , j= P
Po − v
j
K j
yi: gas phase mole fraction
P0: standard pressure (1 bar)
P: system pressure
Kj: equilibrium constant of reaction j
vi,j: stoichiometric coefficient of component i in reaction j
Φi: fugacity coefficient of component i in a gas mixture
exp
−∑i
v i , jGio
RT≡K j
ΔGo
RT=
ΔG0o−ΔH 0
o
RT 0
ΔH 0
o
RT
1T∫T 0
T ΔCPo
RdT−∫
T0
T ΔC Po
RTdT
∑i
v i , j H 0io≡ΔH 0
o
∑i
v i , jC Pi
o ¿ ΔCPo
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UCG process flow diagram (1)
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UCG process flow diagram (2)
An introduction to the exergy concept 29
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Application in UCG process
An introduction to the exergy concept 30
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Exergetic recovery of UCG (1)
An introduction to the exergy concept 31
2 3 4 5 6 7 8 9- 4 0 . 0
- 2 0 . 0
0 . 0
2 0 . 0
4 0 . 0
6 0 . 0
8 0 . 0
1 0 0 . 0
T h e o r e t i c a lP r a c t i c a lZ e r o - e m i s s i o n
W a t e r t o o x y g e n m o l a r r a t i o
Co
al e
xerg
y re
cove
ry (
%)
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Sustainable exergetic recovery of fossil fuels
An introduction to the exergy concept 38