1. FIRST LAW ANALYSIS OF COMBUSTION SYSTEMS
P M V Subbarao
Professor
Mechanical Engineering Department
Efficient Use of Resource thruSufficient Supply of Oxygen…..

2. Phenomenological Modeling of Combustion
Engineering Objective of Combustion:
To Create Maximum Possible Temperature through conversion of microscopic potential energy into microscopic kinetic energy.
Thermodynamic Strategy for conversion:
Constant volume combustion
Constant pressure combustion

3. Classification of Combustion Systems
External Combustion Systems: Transfer the thermal energy liberated due to combustion to surroundings.
Process Heat Utilization Surroundings.
Power generating Surroundings.
Air is the source of oxygen.
Internal Combustion Systems: Thermal energy liberated due to combustion is sued generate Mechanical Power.
Air is a working fluid and sources of oxygen.

4. Steam Power Plant: An External Combustion system in SSSF
Wout
Qin
Qout
Win

5. Furnace in A Steam Power Plant

6. First Law Analysis of External Combustion System: SSSF
Thermal power plants running on Ranke Cycles use an external combustions system known as Coal (fuel) Fired Steam generator.
First Law Analysis of a Combustion System (SSSF) in molar form :

7. First Law Analysis of A Furnace
First Law Analysis of a Furnace (SSSF) in molar form :

8. Reciprocating IC Engine : An Internal Combust System : Control Mass

9. Turbojet Engine: : An Internal Combust System : Control Volume

11. A Rocket on the Way to Orbit

12. First Law Analysis of An Internal Combustion System
First Law Analysis of a Furnace (SSSF) in molar form :

13. How to use the Calorific Value of Fuel in Design of Combustion Systems???

14. Enthalpy of Combustion
Combustion is an exothermic reaction which releases a large amount of energy.
This energy change is redefined as the enthalpy of combustion.
The enthalpy change associated with a combustion reaction when the reactants and products are in their respective standard states is called the standard enthalpy of combustion.
The standard heats of combustion of several compounds are the amount of energy that can be transferred as heat from the reactor to surroundings and is equal to -∆Hc.

15. Change in Enthalpy standard states? solid Na? liquid gas Hg? N2?
There are many forms (or expressions) of change in enthalpy used in engineering.
Hvaporization, Hfusion, Hcombination & Hformation.
The most useful is standard enthalpy of formation (H°f):
the enthalpy change that accompanies the formation of 1 mole of a compound from elements in their standard states.
standard states?
solid
liquid gas
Na?
Hg? N2?

16. Enthalpy of Formation
Enthalpies of formation can be used to calculate enthalpy of combustion.
Enthalpy of formation: enthalpy to form 1 mole of any substance.
Standard Enthalpy of Formation:
Enthalpy change when 1 kmol of species is formed in its Standard State at a Specified Temperature from the most stable forms of its constituent elements in their standard forms (at the same temperature).

17. State of Zero Enthalpy
The enthalpy of formation of any elemental substance in its most stable form is always zero.
The Form favored in Equilibrium at 1 Atmosphere and specified temp. (usually K)
Carbon as graphite; oxygen as O2 gas; copper as an elemental solid (metal).

18. Standard Enthalpy of Formation For CO2
∆Hf0 CO2(gas) = Standard Enthalpy of combustion/reaction.
C (s, graphite) + O2 (g) → CO2(g)
Nomenclature:
∆ Hf at 25 0C & 1 atm

19. Standard Enthalpy of Combustion
Hess's Law: states that regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes.
This law is a manifestation that enthalpy is a state.
If we know the Hf of all reactants & products, we can calculate enthalpy of formation of any compound.
C3H8 + 5O2  3CO2 + 4H2O
(gas) (gas) (gas) (liquid)
Expressed via Hess’s Law:
C3H8 --> 3C + 4H2
H1 = -Hf [C3H8(g)]
3C + 3O2 --> 3CO2
H2 = 3Hf [CO2(g)]
4H2 + 2O2 --> 4H2O
H3 = 4Hf [H2O(l)]
C3H8 + 5O2 --> 3CO2 + 4H2O
H°c = H1 + H2 + H3

20. Enthalpies of combustion of fuels
Industrial fuels:
fuel value (MJ/kg)
Coal
Oil 45
Natural gas 49
Gasoline 48 H

21. First Law Analysis of A General Combustion System
First Law Analysis of a Furnace (SSSF) in molar form :

22. Variation of Specific Heat of Ideal Gases
Air
1.05
-0.365
0.85
-0.39
Methane
1.2
3.25
0.75
-0.71
CO2
0.45
1.67
-1.27
0.39
Steam
1.79
0.107
0.586
-0.20 O2
0.88
0.54
-0.33 N2
1.11
-0.48
0.96
-0.42

23. g cp cv

24. Properties of Fuels C0 C1 C2 C3 C4 Fuel Methane -0.29149 26.327
1.5656
Propane
74.339
8.0543
Isooctane
181.62
20.402
Gasoline
256.63
64.750
0.5808
Diesel
246.97
32.329
0.0518

25. First Order Models for Variable Specific Heats
For general ideal gases:
ap = – 1.1 kJ/kg.K
k1 = 1.32610-4 – 3.39510-4 kJ/kg.K2