Step 1: Understanding the Concept:
Combustion is an exothermic chemical reaction between a fuel and an oxidant.
Step 2: Key Formula or Approach:
Theoretical Air (Stoichiometric Air): The exact minimum amount of air required to provide enough oxygen for the complete oxidation of all the combustible elements in the fuel.
Complete Combustion: All carbon turns to $CO_2$, hydrogen to $H_2O$, and sulfur to $SO_2$.
Step 3: Detailed Explanation:
Complete combustion occurs when there is sufficient oxygen to react with all the fuel. While industrial burners often use "excess air" to ensure no fuel is wasted, the \textit{definition} of complete combustion is satisfied when exactly the theoretical (stoichiometric) amount of air is provided. If there is less than this amount, "incomplete combustion" occurs, producing carbon monoxide ($CO$) and soot.
Step 4: Final Answer:
Complete combustion occurs when exactly theoretical air is supplied.