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CONCEPT OF CHEMICAL

EXERGYExergy : Max. work that can be

obtained from any form of

energy( e.g. in a flowingstream)

Environment can interact with systems

in three ways:

Thermal interaction

Mechanical Interaction

Chemical interaction

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CONCEPT OF CHEMICAL

EXERGYEnvironmental State & Dead State

Mech & Thermaleqlbm. With

environment

{Thermo-mechanical exergy}

M+T+Chemicaleqlbm.

{Total exergy}

Ex : CO2flowing in a pipe at To, P0.

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CONCEPT OF CHEMICAL

EXERGYChemical Exergy :Max amount of work obtainable when

the substance under consideration

is brought from environmental stateto Dead State by processes involving

heat transfer & exchange of matter

only with the environment.

or work obtainable due to diff. in

the chemical potential

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3 kinds of

ref.

substances

(i) Gaseous

components of

atmosphere

(ii)Solid substances

from the Earths crust

(iii)Ionic-Nonionic

substances from

oceans

Reference Substances for exch.

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for ref. substances

chxe Diff in chem. Potentials of the

substance in stream & that inthe environment

e.g. for gaseous phase, say CO2, atTo, Powe would visualise a

system for extracting chxe

Reference Substances for exch.

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Chemical Exergy

Wsh S.P.M.

To, Po

1 2 Environ-

ment

TER

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Chemical Exergy

chxe Work output in isothermal exp.

2121 gg

2

1

f

fnRT

Making ideal gas assumption

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Chemical Exergy

ecooco

och

x ynRTP

PnRTe

2

2

Ideal gas Mixture

?ch

xe

mol. fraction ofCO2 in

environment

i

i

e

ii

ch yyyRTe )}/{ln(0

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Total Exergy

ch

xx egz

V

sThsThe 2)()(

2

0000

Flow Process

Non-flow Process

ch

xegzV

sTvPusTvPu 2

)()(2

0000000

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Chemical Exergy of fuels

For substances not present in

environment consider an

idealised reaction of thesubstance with other substances

(usually ref. substances for

which exergies are known) so

that the final products also are

reference substances

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Chemical Exergy of fuels

ex. Hydrocarbon fuel.

O2

CaHb

Q

Wsh

H2O (liq)

CO2

(T0, P0)T0

P0

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Chemical Exergy of fuels

Assuming no irreversibility

chx

chxsh

chx

chfuelx OHcoo

eb

eaWeb

ae22max2 24

,

OHb

aCOOb

aHCba 222

24

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As already provedrsh

GW max,

&e

i

ch

x ynRTe i

2

4/

22

2

be

oH

ae

co

baeo

r

ch

x

yy

ynRTGefue l

Chemical Exergy of fuels

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Standard Environment for Calculating the

Chemical Exergy of Hydrocarbon Fuels

0.7567

0.2035

0.03030.0003

0.0092

N2

02

H20C02

Other

101.325 kPa298.15 K

Mole

Fractionxi

Substance

(All in GasPhase)

Pressure,P0Temperature

T0

Composition

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Example

Solution : The reaction equation is

02020 2224 HCCH

200

2

0

0

22

2

HC

TRee

e

yy

yln

2422 000 22 gggge CHHCCh

x

Calculate the chemical exergy in methane gas at

250C and 101.325 kPa.

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SOLUTION

2035.020

ey

0003.020 C

ey

0303.002 H

e

yAt 250C and 101.325 kPa

molkgkJgC /630,39420

molkgkJgH /730,22802 molkgkJgCH /844,504

020

g

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SOLUTION

730,2282630,394 Chxe

0844,50

2

2

0303.00003.0

2035.0ln15.2983143.8

molkgkJ550,29250,801

molkgkJ800,830