1. Lecture 7.5 Chemical Exergy

2. 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 interaction Mechanical Interaction Chemical interaction

3. CONCEPT OF CHEMICAL EXERGY Environmental State & Dead State Mech & Thermal eqlbm. With environment {Thermo- mechanical exergy} M+T+Chemical eqlbm. {Total exergy} Ex : CO 2 flowing in a pipe at T o, P 0.

4. 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

5. 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 e x ch.

6. for ref. substances Diff in chem. Potentials of the substance in stream & that in the environment e.g. for gaseous phase, say CO 2, at T o, P o we would visualise a system for extracting

7. Chemical Exergy W sh S.P.M. T o, P o 1 2 Environ- ment TER

8. Chemical Exergy Work output in isothermal exp. Making ideal gas assumption

9. Ideal gas Mixture mol. fraction of CO 2 in environment

10. Total Exergy Flow Process Non-flow Process

11. 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

12. ex. Hydrocarbon fuel. O2O2 C a H b Q W sh H 2 O (liq) CO 2 (T 0, P 0 ) T0P0T0P0

13. Chemical Exergy of fuels Assuming no irreversibility

14. As already proved & Chemical Exergy of fuels

15. Standard Environment for Calculating the Chemical Exergy of Hydrocarbon Fuels 0.7567 0.2035 0.0303 0.0003 0.0092 N 2 0 2 H 2 0 C0 2 Other 101.325 kPa298.15 K Mole Fraction x i Substance (All in Gas Phase) Pressure,P 0 Temperature T 0 Composition

16. Example Solution : The reaction equation is Calculate the chemical exergy in methane gas at 25 0 C and 101.325 kPa.

17. SOLUTION At 25 0 C and 101.325 kPa

19. End of Lecture