1. Lecture 36
Combustion Reactions

2. Combustion products to the turbine
Combustion Processes
Why do mechanical engineers have to know about combustion? Consider a combustion chamber in a gas turbine cycle,
Our current model
What really happens
Fuel input
Air from compressor
Air to turbine
Air from compressor
Combustion products to the turbine
How is this analyzed??

3. Combustion Fuels Combustion Reaction
Stored chemical energy
Combustion Reaction
Transforms the chemical energy stored in the fuel to thermal energy (heat)
Goals of this section of the course
Understand combustion chemistry
Use combustion chemistry to determine the heat released during a combustion process
Heat of reaction

4. Combustion The combustion of a fuel requires oxygen, Fuel
In the most general sense, a fossil fuel makeup is,
The Greek letters signify the atomic composition of the fuel.
For example ...

5. Combustion
Oxidant
The oxidant must contain oxygen. The most abundant ‘free’ source is atmospheric air. By molar percent, atmospheric air is considered to be ...
For every mole of oxygen involved in a combustion reaction, there are 79/21 = 3.76 moles of nitrogen.

6. Combustion Products (for fuels with no sulfur content)
Complete Combustion: CO2, H2O, and N2
Incomplete Combustion: CO2, H2O, N2, CO, NOx
Combustion with Excess Oxygen: CO2, H2O, N2, and O2
NOTE: Fuels containing sulfur have the potential of introducing sulfuric acid into the product stream.

7. Combustion Terminology
Theoretical or Stoichiometric Air
The amount of air required for complete combustion of the fuel
Determined by balancing the combustion reaction
Excess or Percent Theoretical Air
The amount of air actually used in the combustion process relative to the stoichiometric value
Can cause incomplete combustion or excess oxygen

8. Combustion Terminology
In many combustion processes, one of the parameters we are interested in is how much air (or oxygen) is required per unit quantity (moles or mass) of fuel.
Air-Fuel and Fuel-Air Ratios
Equivalence Ratio

9. Equivalence Ratio and Products
Stoichiometric (F=1)
CO2, H2O, N2
Lean (F < 1 with T < 1800 R)
CO2, H2O, N2, O2
Rich (F > 1 with T < 1800 R)
CO2, H2O, N2, O2, CO, H2
Rich (F > 1 with T > 1800 R)
CO2, H2O, N2, O2, CO, H2, H, O, OH, N, C(s), NO2, CH4
ME 322
Advanced courses ME 422 & 433

10. Stoichiometric (Complete) Combustion
Stoichiometric Combustion of a General Fuel in Air
Atomic Balances
5 equations
5 unknowns
(n0 through n4)

11. Lean Combustion Lean Combustion of a General Fuel in Excess Air
PTA = Percent Theoretical Air
expressed as a decimal
6 equations
6 unknowns
(x0 through x5)
Requires a stoichiometric
balance first (to get n0)

12. Example – Octane Combustion
Given: Gasoline (modeled as octane - C8H18) burns completely in 150% theoretical air (or 50% excess air).
Find:
the A/F ratios (mass and molar)
the equivalence ratio
the dew point of the products of combustion at assuming that the products are at 1 atm

13. Example – Octane Combustion
In order to calculate the air-fuel ratios and the equivalence ratio, we need to know how much air is used in the combustion reaction. This is determined by balancing the combustion reaction.
In order to determine the dew point of the products, we need to know the molar composition of the products. This is also determined by balancing the combustion reaction.
Everything depends on the correct balance of the combustion reaction!

14. Example – Octane Combustion
Solution strategy ...
1. Balance the stoichiometric reaction to get n0
2. Balance the reaction with 150% theoretical air
3. Calculate the required (A/F) ratios, the equivalence ratio, and the dew point temperature of the products

15. Example – Octane Combustion
Stoichiometric Reaction

16. Example – Octane Combustion
Combustion in 150% theoretical air

17. Example – Octane Combustion
The molar (A/F) ratio can now be found ...

18. Example – Octane Combustion
The mass-based (A/F) ratio can be found knowing the molecular masses of the air and the fuel,
The molecular mass of the air is,
The molecular mass of the fuel is,

19. Example – Octane Combustion
Now, the mass-based (A/F) can be found ...
Once the (A/F) ratios are determined, the equivalence ratio can be found,

20. Example – Octane Combustion
The dew point of the products is the temperature where the water vapor condenses,
Tdp = Tsat at Pw (partial pressure of the water vapor)

21. Example – Problem 15.42
Given: Combustion exhaust with 9.1% CO2, 8.9% CO,
82% N2, and no O2
Find:
fuel model CnHm
mass percent of carbon and hydrogen in fuel
molar air/fuel ratio and percent theoretical air (PTA)
dew point temperature at .106 MPa

22. Example – Problem 15.42
STEP 1: Write balance equation using ORSAT data
CnHm + a(O N2)  9.1 CO CO + bH2O + 82 N2
STEP 2: Solve for unknowns and write fuel model CnHm
n = ? m = ? a=? b=?

23. Example – Problem 15.42
STEP 3: Compute molar mass of fuel & mass composition
Mfuel = 18lbmolC/lbmolfuel*(12lbm/lbmolC) lbmolH/lbmolfuel*(1lbm/lbmolH) = 249 lbm/lbmolfuel
C: 18*(12)/249  87% H: 33*(1)/249  13%

24. Example – Problem 15.42 STEP 4: Calculate molar air/fuel ratio
21.8 * ( ) /1 = moles air / mole fuel
STEP 5: Write equation for stoichiometric combustion
C18H (O N2)  18 CO H2O N2
STEP 6: Find theoretical air %TA = (21.8 / 26.75) * 100 = 83%

25. Example – Problem 15.42 STEP 7: Find dew point of combustion products
# moles of H2O (in original equation) = 16.5
# moles of other combustion products = 100 # moles of all products (in original equation) = 116.5
Pw = .106 * (16.5/116.5) = .015 Mpa Tsat (.015 MPa) = 54 C