[Snobbibjorn]

I have an assignment where i need to calculate how much air is needed to completely combust 100kg of a dry fuel with the masspercentages:
C 75%
H 6.2%
S 2.8%
O 12%
and some minerals that do not react.

it is specified that the combustion of the fuel only creates CO₂, H₂O and SO₂
we consider the air to be made of 21% O and 79% N.
the answer is to be multiplied with an excess air coefficient of λ=1.8.

I have tried some different calculation methods but none of them give me the solution stated in the book.

The book has 62.4 Kmol air as the solution.

I hope someone is able to help me as i am quite stuck with the assigenment.

[Hunter2]

Convert Percentage into mass each element, then convert into moles.
According combustion reaction convert to the moles of necessary oxygen. Subtract the oxygen what is in the compound. Convert to moles of air.

[Snobbibjorn]

I think that is what i have done: https://ibb.co/KsYXBhy

it just comes out as 74.2Kmol compared to the 64.2 written in my book, maybe a spelling error, or have i missed something?

[Hunter2]

Consider the mass values are given in C, H, S ,O not given in H2  or O2
Develop each combustion reaction for single atomar element.

The combustion uses O2

For example H + 0.25 O2 => 0.5 H2O

Also O is 0.5 O2

Molar mass H is 1.0079 g/mol

recalculate again.
Carbon and sulfur are correct.

[Snobbibjorn]

ahh makes sense, thanks for the help