The process of converting fuel into heat in a boiler essentially involves two core steps: fuel combustion releasing heat and heat transfer. The entire process is not complicated and can be explained in plain language.
The first step is fuel combustion releasing heat. Whether it’s natural gas, coal, diesel, or biomass, these fuels inherently contain chemical energy. When mixed with sufficient air and ignited within the furnace, they undergo a vigorous oxidation reaction—commonly known as combustion. During this process, the fuel’s chemical energy is converted into thermal energy, releasing intense heat. The furnace temperature can soar above 1000°C, simultaneously producing high-temperature flue gas and a small amount of ash residue.
The second step is heat transfer. The heat released by fuel combustion is not directly absorbed by the working fluid (water or thermal oil). Instead, it must be transferred through the boiler’s heat transfer surfaces. Heat transfer surfaces act as the boiler’s “thermal bridges,” primarily comprising components like water walls, convection tubes, and economizers. The high-temperature flames and flue gases transfer heat to these surfaces through radiation and convection. The surfaces then conduct this heat to the internal working fluid.
For instance, the high-temperature flames within the furnace directly radiate heat to the water-cooled walls, where the water absorbs the heat and gradually warms up. Meanwhile, as the high-temperature flue gases flow through the convection tubes and economizer, they transfer residual heat to the working fluid via convection, further elevating its temperature. In this manner, the chemical energy of the fuel is successfully converted into thermal energy for the working fluid through the two steps of combustion and heat transfer, ready for industrial use.
