In a perfect furnace, the combustion air flow would be matched to the fuel flow to give each fuel molecule the exact amount of oxygen needed to cause complete combustion.
However, in the real world, combustion does not proceed in a perfect manner.
Unburned fuel (usually CO and H2) discharged from the system represents a heating value loss (as well as a safety hazard).
Since combustibles are undesirable in the offgas, while the presence of unreacted oxygen there presents minimal safety and environmental concerns, the first principle of combustion management is to provide more oxygen than is theoretically needed to ensure that the fuel burns.
As you can see in the graph traditionally the mixture has to be ‘Rich’ in fuel to gain the most power and this is because the burn is naturally incomplete so we cannot extract all the available power in the combustion of the fuel.
By applying the first principle of combustion there is less power and although never actually 100%, there is the best natural burn of the fuel available.