lect11-combustion_process

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CHE 31. INTRODUCTION TO CHEMICAL ENGINEERING CALCULATIONS
Lecture 11
Combustion Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
LECTURE 11. Combustion Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
2
A Combustion Process
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
3
Chemical Reactiions Associated with Combustion Processes
C + O2 ========> CO2
C + 0.5O2 ========> CO
2H + 0.5O2 ========> H2O
S + O2 ========> SO2
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
4
Terms Associated with Combustion Processes
Orsat Analysis
Refers to the type of gas analysis which eliminates water as 
a component (dry-free basis). If water is included in the 
report, it is termed wet-basis analysis.
Theoretical Air
The amount of air required for complete combustion of C, 
H, and S. It does not depend on how much material is 
actually but what can be burned.
Excess Air
The amount of air in excess of that required for complete 
combustion. The % excess air is the same as % excess O2.
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
5
Example 11-1. Theoretical and Stoichiometric Air
In a given process, 100 kmol of carbon is burned in a 
furnace. It has been found that 20% of the carbon 
undergoes incomplete combustion resulting to CO 
production. 
The rest of the carbon undergoes complete combustion. 
Determine the amount of air required (in kmol) if 50% 
excess O2 must be satisfied.
Relevant Reactions:
C + O2 ========> CO2
C + 0.5O2 ========> CO
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
6
Example 11-1. Theoretical and Stoichiometric Air
Calculate for theoretical O2 needed:
Assume that all the carbon is burned completely to CO2.
100 kmol C (1/1) = 100 kmol O2
It is not correct to do the following:
C  CO2: 100 kmol C (0.80)(1/1) = 80 kmol O2
C  CO: 100 kmol C (0.20)(0.5/1) = 10 kmol O2
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
7
Example 11-1. Theoretical and Stoichiometric Air
Total O2 required stoichiometrically based on the actual 
process:
Stoichiometric O2 = (80 + 10) kmol = 90 kmol
Theoretical O2 is based not on what is stoichiometrically
needed according to what is actually burned.
Theoretical Air = (100 kmol)(1/0.21) = 476.2 kmol
And the actual air supplied:
Actual Air = 476.2 kmol (1.5) = 714.3 kmol
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
8
Example 11-2. Combustion of Propane (C3H8)
Fuels for motor vehicles other than gasoline are being eyed 
because they generate lower levels of pollutants than does 
gasoline.
Compressed propane (C3H8) has been suggested as a source 
of economic power for vehicles. Suppose that in a test, 20 
kg of C3H8 is burned with 400 kg of air to produce 44 kg of 
CO2 and 12 kg of CO.
Calculate the percent excess air.
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
9
Example 11-2. Combustion of Propane (C3H8)
Write the overall combustion reaction for the fuel assuming 
it is burned completely:
C3H8 + 5O2 ========> 3CO2 + 4H2O
For 20 kg of C3H8, the theoretical O2 required is:
3 8 2
3 8 2
3 8 3 8
1kmol C H 5O20kg C H =2.27kmol O
44.09kgC H 1C H
  
  
  
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
10
Example 11-2. Combustion of Propane (C3H8)
The actual O2 supplied is
2
2
1kmol air 1air400kgair =2.90kmol O
29kgair 0.21O
  
  
  
The percent excess air (or O2) is
2 2
2
2.90kmol O -2.27kmol O%excessair = ×100=28%
2.27kmol O
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
11
Example 11-3. Combustion of Methane (CH4)
Generation of methane-rich biogas is a way to avoid high 
waste-disposal costs, and burning it can meet up to 60% of 
the operating costs for such waste-to-energy plants.
Consider the complete combustion of 16.0 kg of methane 
(CH4) in biogas with 300 kg of air. Determine the % excess 
of air, and the total moles and composition of the flue gas.
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
12
Example 11-3. Combustion of Methane (CH4)
Degrees of Freedom Analysis: Atomic Balance
Unit: Reactor
unknowns (P,x1,x2,x3,x4) +5
independent atomic specie(s)
independent nonreactive molecular specie(s)
other equations:
Degrees of freedom 0
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
13
Example 11-3. Combustion of Methane (CH4)
Write the atomic species balances (mole basis):
(1) C: 16 kg CH4 (1/16)(1) = Px1
(2) H: 16 kg CH4 (1/16)(4) = Px4
(3) O: 300 kg Air (1/29)(0.21)(2) = 2Px2 + 2Px1 + Px4
(4) N: 300 kg Air (1/29)(0.79)(2) = 2Px3
(5) x: x1 + x2 + x3 + x4 = 1
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
14
Example 11-3. Combustion of Methane (CH4)
Simplifying the equations
(1) C: 1 = Px1
(2) H: 4 = Px4
(3) O: 4.34 = 2Px2 + 2Px1 + Px4
(4) N: 16.34 = 2Px3
(5) x: x1 + x2 + x3 + x4 = 1
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
15
Example 11-3. Combustion of Methane (CH4)
If composition of flue gas is expressed in terms of actual 
number of moles (n’s) instead of mole fractions (x’s)
C: 1 = n1
H: 4 = n4
O: 4.34 = 2n2 + 2n1 + n4
N: 16.34 = 2n3
n: n1 + n2 + n3 + n4 = P
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
16
Example 11-3. Combustion of Methane (CH4)
Solving for the n’s and P:
n1 = 1 kmol CO2 n2 = 0.17 kmol O2
n3 = 2 kmol H2O n4 = 8.18 kmol N2
P = 11.35 kmol
Solving for the mole fractions:
x1 = (1/11.35) = 0.09 kmol CO2/kmol P
x2 = (0.17/11.35) = 0.01 kmol O2/kmol P
x3 = (8.18/11.35) = 0.72 kmol N2/kmol P
x4 = (2/11.35) = 0.18 kmol H2O/kmol P
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
17
Example 11-3. Combustion of Methane (CH4)
Solving for % excess air:
Write the overall combustion reaction for the fuel assuming 
it is burned completely:
CH4 + 2O2 ========> CO2 + 2H2O
For 16 kg of C3H8, thetheoretical air required is:
4 2
4
4 8 2
1kmolCH 2O 1Air 29kg Air16kg CH = 276kg Air
16kg CH 1CH 0.21O 1kmol Air
     
     
     
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
18
Example 11-3. Combustion of Methane (CH4)
Solving for % excess air:
Overall combustion reaction for the CH4:
CH4 + 2O2 ========> CO2 + 2H2O
For 16 kg of C3H8, the theoretical air required is:
4 2
4 8 2
1kmolCH 2O 1Air 29kg Air16kgCH4 = 276kg Air
16kgCH 1CH 0.21O 1kmolAir
300kg Air - 276kg Air%excessair = ×100 = 8.7%
276kg Air
     
     
     
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
19
Example 11-4. Combustion of Coal
A local utility burns coal having the following composition 
on a dry basis:
Component Percent
C 83.05
H 4.45
O 3.36
N 1.08
S 0.70
Ash 7.36
Total 100.00
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
20
Example 11-4. Combustion of Coal
The average Orsat analysis of the flue gas during a 24-hr 
test was:
Component Percent
CO2 + SO2 15.4
CO 0.0
O2 4.0
N2 80.6
Total 100.00
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
21
Example 11-4. Combustion of Coal
Moisture in the fuel was 3.90% and the air on the average 
contained 0.0048 lbm H2O/lbm dry air. 
The refuse showed 14.0% combined elements as in the coal 
(i.e. C, H, O, N, S) and the remainder being ash. It may be 
assumed that these combined elements occur in the same 
proportions as they do in the coal.
Estimate the amount of amount of flue-gas (dry basis), 
amount of water coming out of the process, and the 
%excess air.
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
22
Example 11-4. Combustion of Coal
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
23
Example 11-4. Combustion of Coal
Basis: 100 lbm of coal
Ash Balance: 0.0736(100 lbm) = 0.86R
R = 8.56 lbm
Combustible elements in refuse 
0.14(8.56 lbm) = 1.20 lbm
Assuming the combustible elements (C, H, O, N, S) occur in 
the same proportions as they do in the coal, the quantities 
of the combustibles in R on an ash-free basis are:
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
24
Example 11-4. Combustion of Coal
Component
mass 
(lbm)
ash-free
mass %
Amt. in R
(lbm)
Amt. in R
(lbmol)
C 83.05 89.65 1.076 0.0897
H 4.45 4.80 0.058 0.0537
O 3.36 3.63 0.0436 0.0027
N 1.08 1.17 0.014 0.0010
S 0.70 0.76 0.009 0.0003
Total 92.64 100.00 1.200 0.1474
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
25
Example 11-4. Combustion of Coal
Find the lbmol of H and O due to water in coal:
H: 100 lbm (3.9/96.1)(1/18)(2/1) = 0.451 lbmol H
O: 100 lbm (3.9/96.1)(1/18)(1/1) = 0.225 lbmol O
Find the mole fraction of H and O due to moisture in air:
H: 0.0048 lbm H2O/lbm DA (29/18)(2/1) = 0.0154
O: 0.0048 lbm H2O/lbm DA (29/18)(1/1) = 0.0077
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
26
Example 11-4. Combustion of Coal
Solve A, W, and P using (C+S), H, and N balances
(C+S) Balance (mole basis):
(83.05/12) + (0.70/32) = P(0.154) + 0.0897 + 0.0003
H Balance (mole basis):
(4.45/1) + 0.451 + 0.0154A = 2W + 0.0537
N Balance (mole basis):
(1.08/14) + 2(0.79A) = 2P(0.806) + 2(0.001)
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
27
Example 11-4. Combustion of Coal
Solving the balance equations gives
P = 44.5 lbmol
A = 45.4 lbmol
W = 2.77 lbmol
Determine the theoretical air required to burn completely 
all the C, H, and S in the coal.
C: (83.05/12)(1/1) = 6.92 lbmol O2
H: (4.45/1)(1/4) = 1.11 lbmol O2
S: (0.70/32)(1/1) = 0.022 lbmol O2
Total O2 required = (6.92 + 1.11 + 0.022) = 8.052 lbmol O2
LECTURE 10. Solving Material Balance Problems Involving Reactive Processes
Prof. Manolito E Bambase Jr. Department of Chemical Engineering. University of the Philippines Los Baños 
SLIDE
28
Example 11-4. Combustion of Coal
Since there is already O present in the coal, this amount is 
subtracted from the theoretical requirement.
O2 in coal = (3.36/16)(1/2) = 0.105 lbmol O2
Corrected O2 required = (8.052 – 0.105) = 7.947 lbmol O2
Actual O2 supplied = 45.35 (0.21) = 9.524 lbmol O2
And the % excess air is calculated as:
2 2
2
9.524kmol O - 7.947kmol O%excessair = ×100 = 19.8%
7.947 kmol O