• Support PF! Buy your school textbooks, materials and every day products Here!

Methane Combustion and Chemical Equilibrium

  • #1
This may belong in the Chemistry section but this is a problem from my Thermodynamics class, so I'll post it here and then move to the Chem section if need be.

Homework Statement



Methane (CH4) is the principal constituent of natural gas and is often used to represent natural gas. Consider the operation of a natural gas fired water heater operating with 15% excess air. Assuming the fuel to be CH4 and the products of the reaction to be CO2, H2O, N2, O2 , and CO, calculate the concentration (percent by volume) of each product if the temperature in the products is 1000K.

Homework Equations



CH4 + (a)(m)(O2 + 3.76N2 → bCO2 + cH2O + dN2 + eO2 + fCO

2CO2 [itex]\Leftrightarrow[/itex] 2CO + O2

where:
a=moles for stoichiometric combustion
m=excess air ratio (1.15 in this case)
b,c,d,e,f = moles of products

The Attempt at a Solution



To balance equation:

From combustion reaction:
C: 1 = b+f
H: 4 = 2; c=2
O: 2*a*1.15 = c+2e+f
N: 3.76*1.15*a*2 = 2d

From dissociation reaction:
2b = 2f + e (2 CO2 makes 2CO and 1O2)

I still only have 5 equations and 6 unknowns.

There is also the equilibrium constant K that we are given a table of:

[URL]http://upload.wikimedia.org/wikipedia/en/math/a/e/1/ae1ea56f2e557025e57022730f141861.png[/URL]

[URL]http://upload.wikimedia.org/wikipedia/en/math/2/b/1/2b116a1f13940176246dd9e08a0c0be3.png[/URL]

I know how to solve the problem when CO is not created. It becomes a much simpler problem. However, when CO enters the problem, I end up with not enough equations.

Thanks for your help!
 
Last edited by a moderator:

Answers and Replies

  • #2
Borek
Mentor
28,301
2,685
Are you given K value? Look to me like it can be just a stoichiometry problem, where you are to assume reaction proceeded to the end.

Alternatively, the only equilibrium to be taken into account is 2CO + O2 <-> 2CO2
 
  • #3
Yes, we are given a K value. However, we haven't been taught this type of Chemistry yet, so I'm having difficulty solving the K equation.

I know that the equation for 2CO + O2 <-> 2CO2 is used to find concentrations of the substances, but I'm not exactly sure how...

Edit: K = 3.68 x 10^-21

After I set up K = ((Y_CO)^2 x (Y_O2))/ (Y_CO2)^2 I get stuck on solving for the terms. I assume Y_CO2 = 1 because it is the only term on the right side. However, I am unsure on how to find Y_CO and Y_O2.
 
Last edited:
  • #4
Borek
Mentor
28,301
2,685

Related Threads for: Methane Combustion and Chemical Equilibrium

  • Last Post
Replies
10
Views
6K
Replies
8
Views
8K
Replies
1
Views
2K
Replies
0
Views
8K
Replies
5
Views
3K
  • Last Post
Replies
1
Views
1K
Top