What condition must be met to ensure complete combustion of fuel in a furnace?

To ensure complete combustion of fuel in a furnace, it is essential that the temperature in the furnace reaches a sufficiently high level to ignite the fuel as it enters. The ignition temperature is the minimum temperature required to initiate combustion. When fuel ignites and reaches this temperature, it can react with oxygen in the air to produce energy in the form of heat and light, as well as byproducts such as carbon dioxide and water vapor.

If the temperature is not high enough to ignite the fuel, incomplete combustion will occur, which can lead to the production of undesirable emissions such as carbon monoxide and unburnt hydrocarbons, and can also decrease the efficiency of the system. Proper control of the furnace temperature is therefore critical for optimal fuel efficiency and environmental compliance.

The options suggesting that a low temperature is needed, that the temperature does not affect combustion, or that a specific humidity level is required do not align with the fundamental principle that sufficient heat is necessary for fuel ignition and complete combustion.