[ian123]

I just want to ask question about this as I'm not sure what I to do next:
The product gas from a combustion reaction has the following dry basis molar composition:
CO2 80.35%
CO 4.73%
SO2 0.03%
O2 14.89%
Pure oxygen is fed to the furnace in 10% excess of that required for complete combustion. There is no oxygen in the fuel (fixed or gaseous). Calculate the elemental composition of the fuel (mole % of elements C,H & S).Base calculations on 100 moles of dry outlet gas.
 
Take SO2 to be the fully combusted form of S.

Here is my working so far:
'O'in='O'out
2*'O2'in=2*'CO2'out+1*'CO'out+2*'SO2'out+2*'O2'out+1*'H2O'out
 
2*'O2'in=2*80.35+1*4.73+2*0.03+2*14.89+1*'H2O'out

'C'in='C'out
'C'in='CO2'out+'CO'out=80.35+4.73=85.08mols

'S'in='S'out
'S'in=1*'SO2'out=1*0.03=0.03mols

0.1=(O2feed-O2theoretical)/(O2theoretical)

Complete combustion
S+O2->SO2              0.03mols O2
C+O2->CO2             85.08mols O2
H2+1/2O2->H2O     x mols       O2

Incomplete Combustion
C+1/2O2->CO

Thanks

[mjc123]

Hello again.

Can I suggest again that you start by writing a balanced equation for the combustion (the one you suggested was not balanced). If you have x moles of C, how many moles of CO₂ do you get? How many moles of O₂ does this consume? Likewise with H₂O and SO₂.

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[ian123]

2CxHySz+(2x+(1/2)y+2z)O2->2xCO2+yH2O+2zSO2

Or this
C+O2->CO2
C+1/2O2->CO
H2+1/2O2->H2O
S+O2->SO2

[Arkcon]

So, we know complete combustion happens, even though there is some CO product.  You have 100 moles of product, and it is CO₂ 80.35% and CO 4.73%, so how much carbon is that?  Likewise for sulfur.  The balance is hydrogen.

[mjc123]

OK; it will probably be easier if you divide through by 2, so
C_xH_yS_z + (x + y/4 +z)O₂  xCO₂ + y/2H₂O + zSO₂
Let us write A = x + y/4 + z. Now if we have 10% excess of oxygen, it becomes
C_xH_yS_z + 1.1A O₂  xCO₂ + y/2H₂O + zSO₂ + 0.1A O₂
Can you write the equation for the incomplete combustion case where each mole of C generates β mol CO and (1-β) mol CO₂?

[Enthalpy]

Not knowing the amount of water produced, I believe the proportion of hydrogen in the fuel is impossible to determine.

[mjc123]

No it isn't. You can get an expression for the amount of oxygen left over, from which (knowing x, z and β) you can find y.