Fuel gas

[Haariskhan]

I'm really stuck on this question:

 

A fuel gas consists of methane and ethane in unknown proportions is burned in a furnace with air. Combustion is known to be complete and an analysis of the flue gas on a dry basis gives 85.99% N2 and 10.88% CO2.

Determine

a) composition of the fuel gas,

b) the percentage excess air used.

 

My basis I've chosen(Don't know if this is an appropriate basis) is 1 kmol of the fuel gas. I've let x=kmol CH4 and 1-x=kmol C2H6. From there I wrote equations for CO2 formed and theoretical O2 needed, but now I am extremely stuck.

 

Can someone give me a method of approaching these sorts of questions? I don't even know if what I have done above is right.

[pavelcherepan]

I would try to solve it differently.

 

When you burn 1 mol of CH4 completely you get 1 mol of co2 and 2 mol of co2 when you burn butane.

 

Normal co2 concentration is .4% then you get the first equation you need to solve:

 

x + 2y = (10.88-0.4)

 

Then you're also down on oxygen. You use 2 mol of o2 to completely burn methane and 3.5 mol to completely burn 1 mol of butane. Normal o2 concentration is 22% then:

 

2x + 3.5y = (22-(100-85.99-10.88))

 

Solving thaw two equations you get molar concentrations of methane and buthane in original gas mix.

 

I get 1.22 mol of butane and 8.04 mol of methane.

 

Actually, this is all wrong. Nitrogen concentration is 86% meaning that all ratios have changed.

Edited January 11, 2016 by pavelcherepan