7/24/2019 ChE 322N - Lesson 3 (Thermal Efficiency of Combustion)

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Thermal efficiency of combustion November 23 and 25, 2015

measures the fraction of the total heat from the fuel that can used to generate power.

Calorific value losses

• 

Due to incomplete combustion

Products of incomplete combustion other than the gases found in the fuel

Carbon Monoxide: 282.99 MJ/kmol

Hydrogen Gas: 285.84 MJ/kmol

Methane Gas: 890.36 MJ/kmol

Carbon: 393.51 MJ/kmol

• 

Due to sensible heat

  =  +  + 2 + −2 

 =  = [  +  + 2 + −2] 

∆  = 

=    [  +  + 2 + −2]

 

∆ =

 

=

  �(

0) +

2

 (

2

  02) +

3

 (

3

  03) +

(1

0 

 1

)

 

∆  = 

=  �+

2 ( + 0) +

32 + 0 + 02 +

0 ( 0) 

Theoretical flame temperature:  is the maximum attainable temperature by the combustion

 products when the combustion reaction goes to completion and is carried out adiabatically.

 Adiabatic flame temperature:  is the maximum attainable temperature by the combustion

 products when the combustion is carried out adiabatically.

 = ∆  +

2 ( + 0) +

32 + 0 + 02+ 

0+ 0 

• 

Due to uncondensed water vapor resulting from the combustion of the fuel

Heat of vaporization of water: 44.04 MJ/kmol

7/24/2019 ChE 322N - Lesson 3 (Thermal Efficiency of Combustion)

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Sample Problem 1: A blast furnace gas analyzing 27% CO, 12% CO2, 2% H2, 5% CH4 and 54% N2 at 25°C,

98 kPa and saturated with water vapor is burned in 15% excess air. Air is supplied at 30°C, 762 torrs and

also saturated with water. 90% of the total carbon is the fuel in the fuel is converted to CO2, the rest to

CO. Molar ratio of H2 to CO is 1:4in the stack gas, which leaves at 200°C and 750 torrs. Calculate:

a. 

Orsat Analysis of the stack gas

b. 

Air fuel ratio by volume

c. 

Moles stack gas/mole fuel

d. 

Gross Calorific Value of the fuel in KJ/m3 

e. 

Adiabatic flame temperature

f. 

% calorific value loss due to unburnt combustible

g. 

% calorific value loss due to uncondensed water

h. 

% calorific value loss due to sensible heat

i. 

Thermal efficiency of combustion

 j. 

Dew Point of the stack gas

k. 

Relative saturation of the stack gasl. 

Percent saturation of the stack gas

Sample Problem 2: Calculate the theoretical flame temperature when ethylene at 25°C is burned with:

a. 

The theoretical amount of air at 25°C

b. 

25% excess air at 25°C

c. 

50% excess air at 25°C

d. 

100% excess air at 25°C

e. 

50% excess air preheated to 500°C