Concept:
Flue gas analysis is done to determine the percentage of gases such as \(CO_2\), \(O_2\), and \(CO\) in combustion gases.
Step 1: Flue gas is produced during combustion of fuels.
Step 2: Its analysis helps in checking combustion efficiency.
Step 3: Orsat apparatus is used for volumetric analysis of flue gases.
Step 4: Bomb calorimeter measures calorific value of fuel, not flue gas composition.
Therefore, the apparatus used to analyze flue gas is:
\[
\boxed{\text{Orsat apparatus}}
\]