The furnace, steam drum, burner, water-cooled wall, superheater, economizer, air preheater, framework, and furnace wall in the boiler body of a steam generator constitute the core components for steam production, collectively referred to as the boiler proper. The two most critical components in the boiler proper are the furnace and the steam drum.

Furnace: Also known as the combustion chamber, it is the space where fuel is burned. A furnace that burns solid fuel on a grate in a bed of fire is called a stoker-fired furnace, also known as a bed furnace; a furnace that injects liquid, gaseous, or pulverized solid fuel into a fire chamber for combustion is called a chamber-fired furnace, also known as a fire chamber furnace; a furnace where air suspends coal particles for fluidized bed combustion, suitable for burning low-grade fuels, is called a fluidized bed furnace; a cylindrical furnace that uses an air stream to make coal particles rotate at high speed and burn intensely is called a cyclone furnace.

The cross-section of a furnace is generally square or rectangular. Since fuel burns in the furnace to form flames and high-temperature flue gas, the furnace walls surrounding it are constructed from high-temperature-resistant and insulating materials. Water-cooled wall tubes are often installed on the inner surface of the furnace walls, which not only protect the walls from being damaged by fire but also absorb a significant amount of radiant heat from the flames and high-temperature flue gas. The structure, shape, volume, and height of the furnace must ensure complete fuel combustion and reduce the flue gas temperature at the furnace outlet below the temperature at which slag begins to condense.

When the temperature inside the furnace exceeds the ash melting point, the ash becomes molten. Molten slag particles adhere to the water-cooled wall tubes or other components when they come into contact with them. As the adhered ash particles accumulate, they form slag blocks, a process known as slagging. Slagging reduces the heat transfer efficiency of the boiler's heating surfaces. In severe cases, it can block the flue gas flow passages, affecting the safe and economical operation of the boiler.

Furnace design must fully consider the characteristics of the fuel used. Each boiler should ideally burn the fuel it was designed for. When fuels with significantly different characteristics are used, the economic efficiency and reliability of boiler operation may be compromised.