Stoichiometry without the chemistry lecture
How much air a fuel needs, explained by counting.
How much air a fuel needs to burn completely is decided by simple counting, since every fuel molecule needs a fixed, specific number of oxygen molecules to react fully, and supplying more or fewer than that number changes what is left over once the burning has finished.
Counting the oxygen a fuel molecule needs
Burning a hydrocarbon completely turns every carbon atom in it into carbon dioxide and every hydrogen atom into water, and each of those products needs a fixed number of oxygen atoms. Count the atoms in the fuel molecule and the oxygen requirement follows. Methane, the main part of natural gas, has one carbon and four hydrogens, so each molecule needs two oxygen molecules: one for the carbon dioxide and one shared across the two water molecules.
Air is only about a fifth oxygen, the rest mostly nitrogen, so those two oxygen molecules arrive wrapped in roughly eight molecules of nitrogen that do nothing but soak up heat. That is why each volume of methane needs close to ten volumes of air, and why a flame in ordinary air never runs as hot as the same fuel burning in pure oxygen. Propane, with three carbons and eight hydrogens, needs five oxygen molecules per molecule and so about twenty-four volumes of air, which is why switching a burner from natural gas to propane changes its correct air setting without anything about its geometry being touched.
This exact match, with nothing spare on either side, is the stoichiometric ratio. It is set by the fuel's formula, so flame size, burner design and delivery have no effect on it, and tuning a burner is entirely a matter of air supply.
Forks, knives and glasses
Setting a table where each place needs one fork, one knife, and one glass makes the same counting obvious. Eight forks, six knives, and ten glasses on the sideboard make six complete settings, because the scarcest item sets the limit. The two spare forks and four spare glasses sit there unused.
A fuel-air mixture works the same way. Whichever ingredient runs out first, fuel or oxygen, sets how much complete combustion can happen, and the leftover of the other sits unreacted. Buying a hundred more forks produces no extra place settings once the knives have run out, and pumping more air past a flame whose fuel is already fully burned burns nothing more.
A burner running rich, with more fuel than the air can react, is short of knives: the leftover fuel shows up as the yellow, sooty flame of unburned carbon. A burner running lean is carrying spare glasses: it burns cleanly but spends some of its heat warming surplus air. A caterer who orders a few extra forks to cover breakages is doing what a burner tuned slightly lean does on purpose, keeping a small known surplus of the forgiving ingredient so the evening never runs short.
Tuning a burner by the count
Tuning a burner becomes a matter of working out how much air a given fuel flow needs and adjusting the air supply toward that figure, whatever flame looks most impressive by eye. A large, vigorous flame can simply mean more unburned fuel passing through, much as a sideboard crowded with forks says nothing about how well the party is catered. Some equipment skips the eye altogether, using an exhaust sensor to report how much oxygen is left over and adjusting the air supply automatically as conditions drift.