Simple combustion problems may occur on the PE exam. At the core of most combustion problems is a chemical reaction between a simple fuel in the form of CxHy and an oxidizer which for the case of the PE exam is usually air. Most problems involve determining reaction coefficients for complete combustion. Once these coefficients are determined, they can be used to calculate common parameters like excess air or total mass of fuel, etc. The most basic form of the combustion reaction for a simple hydrocarbon involves stoichiometric combustion with pure oxygen:
For complete combustion, the reaction coefficients work out to be:
For stoichiometric combustion with air, nitrogen must be added to the reaction. It should be assumed that air consists of 21% O2 and 79% N2 which can be shown in the general case as:
It is important to note that nitrogen is an inert gas and therefore does not combust in the combustion process but should be included in the equation if present to avoid any calculation errors.
If not given in the problem statement, the correct chemical formula needs to be referenced in order to determine combustion reaction coefficients. Table C1-1 gives the formulas for some basic, common hydrocarbon fuels:
Some important defenitions to know while considering combustion type problems are:
Mole of gas: The quantity of any gas whose mass is equal to its molecular weight. I.e. 32 grams of oxygen, 44 grams of carbon dioxide, etc. Multiplying the moles of a substance by its mole weight will give you the actual weight of that substance. For example if there are 2 moles of air and air has a mole weight of 29.9 lb/lbmol, then there are total 29.9 lb/lbmol X 2 mol = 59.8 lbs of air.
Stoichiometric air: also known as theoretical air which is the minimum amount of air required for the complete combustion of a fuel.
Excess air: the amount of air in addition to the air required for complete combustion.