Furnace Technology
In practical operation of any furnace, zero oxygen percentage or theoretical CO₂ percentage in flue gas is rarely achieved, because of

Use of excessive positive draft in the furnace
Use of non-preheated combustion air
Imperfect mixing of fuel & air and infiltration of air
Use of pulverised solid fuels

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Furnace Technology
Turn down ratio of a burner

Should be 1:1 for a batch type furnace.
Is normally much more for a continuous furnace as compared to a batch furnace
Is the ratio of maximum to minimum permissible heat input rates
Should be as low as possible i.e., 1 : 2

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