Lecture 7.5

Chemical Exergy

CONCEPT OF CHEMICAL EXERGY

Exergy : Max. work that can be obtained from any form of energy( e.g. in a flowing stream)

Environment can interact with systems in three ways:

Thermal interactionMechanical InteractionChemical interaction

CONCEPT OF CHEMICAL EXERGY

Environmental State & Dead State

Mech & Thermal eqlbm. With environment {Thermo-mechanical exergy}

M+T+Chemical eqlbm.

{Total exergy}

Ex : CO2 flowing in a pipe at To, P0.

CONCEPT OF CHEMICAL EXERGY

Chemical Exergy :

Max amount of work obtainable when the substance under consideration is brought from environmental state to Dead State by processes involving heat transfer & exchange of matter only with the environment.

or work obtainable due to diff. in the chemical potential

3 kinds of ref.

substances

(i) Gaseous components of atmosphere

(ii)Solid substances from the Earth’s crust

(iii)Ionic-Nonionic substances from oceans

Reference Substances for exch.

for ref. substances

chxe Diff in chem. Potentials of the

substance in stream & that in the environment

e.g. for gaseous phase, say CO2, at To, Po we would visualise a system for extracting ch

xe

Reference Substances for exch.

Chemical Exergy

Wsh S.P.M.

To, Po

1 2 Environ-ment

TER

Chemical Exergy

chxe Work output in isothermal exp.

2121 gg

2

1

f

fnRT

Making ideal gas assumption

Chemical Exergy

ecoo

co

ochx ynRT

P

PnRTe

2

2

Ideal gas Mixture

?chxe

mol. fraction of CO2 in environment

i

ieii

ch yyyRTe )}/{ln(0

Total Exergy

chxx egz

VsThsThe

2)()(

2

0000

Flow Process

Non-flow Process

chxegz

VsTvPusTvPu

2)()(

2

0000000

Chemical Exergy of fuels

For substances not present in environment consider an idealised reaction of the substance with other substances (usually ref. substances for which exergies are known) so that the final products also are reference substances

Chemical Exergy of fuels

ex. Hydrocarbon fuel.

O2

Ca Hb

Q

Wsh

H2O (liq)

CO2 (T0, P0)

T0

P0

Chemical Exergy of fuels

Assuming no irreversibility

chx

chxsh

chx

chfuelx OHcoo

eb

eaWeb

ae22max2 24,

OHb

aCOOb

aHC ba 222 24

As already proved rsh GW max,

&ei

chx ynRTei

2

4/

22

2

beoH

ae

co

baeo

rchx

yy

ynRTGe

fuel

Chemical Exergy of fuels

Standard Environment for Calculating the Chemical Exergy of Hydrocarbon Fuels

0.7567

0.2035

0.0303

0.0003

0.0092

N2

02

H20

C02

Other

101.325 kPa298.15 K

Mole Fraction

xi

Substance (All in Gas Phase)

Pressure,P0TemperatureT0

Composition

Example

Solution : The reaction equation is

02020 2224 HCCH

200

20

0

22

2

HC

TRee

e

yy

yln

2422 000 22 gggge CHHC

Chx

Calculate the chemical exergy in methane gas at 250C and 101.325 kPa.

SOLUTION

2035.020 ey0003.0

20 C

ey

0303.002H

ey

At 250C and 101.325 kPa

molkgkJgC /630,39420

molkgkJgH /730,22802

molkgkJgCH /844,504

0

20 g

SOLUTION

730,2282630,394 Chxe

0844,50

2

2

0303.00003.0

2035.0ln15.2983143.8

molkgkJ550,29250,801

molkgkJ800,830

End of Lecture