A hot blast refers to preheated air that is blown into a blast furnace. The technique is used in metallurgical operations such as steelmaking to increase the efficiency of the refining process. It works by recycling waste gases that would otherwise need to be vented. Burning these waste gases to preheat the air reduces the amount of fuel needed in the blast furnace and reduces the cost of operations.
Metallurgical processing typically makes use of a blast furnace to convert raw solids such as ore to purified liquid metals. High carbon fuel, known as coke, is used to heat the ore in order to reduce it to its constituent components so that purified metal may be separated out. Hot gases are produced as a by-product of this process.
There are two methods of dealing with waste gases. One option is to clean them of any environmentally hazardous substances and vent them out of the system. This results in substantial waste of energy, however. Instead, waste gases may be routed to a special stove where they are burned to preheat air that is subsequently fed to the main blast furnace.
Three or four hot blast stoves are typically aligned along the air intake to the main blast furnace. At any given time, one stove is generally in use while the others are being brought up to temperature. Unheated air, known as the cold blast, is driven with a turbo blower through the stove, where it is heated.
Once heated, this air exits the stove as the hot blast and is routed into the blast furnace main line. If needed to regulate temperature, cold air may be added by way of a mixing line that is connected to the main. A valve controls the release of the cold blast.
Prior to entering the main chamber of the blast furnace, the hot blast enters a pipe encircling the structure. This pipe has several equally spaced nozzles from which the hot blast is injected into the furnace. These nozzles are known as tuyeres, and there may be 15 to 40 of them around the circumference of the blast furnace, depending on its size.
The hot blast in a blast furnace, sometimes supplemented with natural gas, combusts along with the coke to achieve the necessary temperature to process the ore. The preheating of this air improves combustion efficiency and reduces the amount of fuel required in the main blast furnace. It may also speed up production of the final processed metal. This produces substantial cost savings by recycling energy that would otherwise be wasted.