Final answer:
An air preheater is used to ensure that fuel particles can come in closer contact with oxygen for complete combustion in a furnace.
Step-by-step explanation:
Ensuring that fuel particles can come in closer contact with the oxygen for complete combustion is achieved by air preheater. An air preheater is a device that preheats the combustion air supplied to a furnace by recovering the heat from the flue gas.
This preheated air allows for more efficient combustion by increasing the temperature of the fuel particles, promoting better mixing with oxygen, and enhancing the overall combustion process.