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A flue gas stack is an industrial structure similar to a chimney. Gases that are byproducts of industrial processes are released into the air at heights where they can be dispersed high up above the ground. Pollution levels of carbon monoxides, nitrogen oxides, and sulfur dioxides can be controlled according to environmental policies and government regulations. Some stacks can be up to 1,300 feet (396 meters) tall.
Flue gas is directed out of the chimney by air pressure. Air rising through the flue gas stack is hotter and less dense than air outside the structure, so its pressure is lower. The air pressure being higher outside forces the air and gases upward and out the top of the chimney, in a process referred to as natural draft or stack effect. This process can be combined with other industrial processes such as flue gas desulphurization, in which sulfur dioxide is altered by absorption, neutralization, or oxidation and converted into a solid compound. The solid waste is removed by equipment from the gas stream before rising into the flue gas stack.
Factors that go into designing a flue gas stack include its height and diameter, the amount of combustion air required, and the temperature of the gas after it is outside the combustion zone. The composition and density of the gas, and the frictional resistance of the gas with the stack materials, are also important design considerations. It is also important to consider how much heat is lost from the gas inside the tube and the elevation above sea level.
Before a flue gas stack can be built, all of these factors must be fit into mathematical calculations. Organizations such as the American Society of Mechanical Engineers (ASME) as well as agencies affiliated with national governments have established guidelines on how to calculate the design of flue gas stacks. Computers are also used to model what happens to the air once it leaves the chimney, using a technique known as air dispersion modeling. Only when engineers know they have created a design that complies with local standards can they proceed with the building phase.
Acid rain sometimes falls around a flue gas stack and in the local area surrounding it. Equipment can be installed inside the stack to remove particulates and acids. Another important step in designing the finished product is to build a scale model and observe how steam droplets react in the structure and as they enter into the surrounding air.