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in Ekibastuz, Kazakhstan, the tallest of its kind in the world () Diagram of 25 tallest flue gas stacks worldwide ]] A flue-gas stack, also known as a smoke stack, chimney stack or simply as a stack, is a type of chimney, a vertical pipe, channel or similar structure through which combustion product gases called are exhausted to the outside air. Flue gases are produced when coal, oil, natural gas, wood or any other fuel is combustion in an industrial furnace, a power station steam-generating boiler, or other large combustion device. Flue gas is usually composed of carbon dioxide (CO2) and water vapor, as well as nitrogen and excess oxygen remaining from the intake combustion air. It also contains a small percentage of pollutants such as particulate matter, carbon monoxide, nitrogen oxides and sulfur oxides. The flue gas stacks are often quite tall, up to , to increase the stack effect and dispersion of pollutants.
When the flue gases are exhausted from stoves, ovens, fireplaces, heating furnaces and boilers, or other small sources within residential abodes, restaurants, hotels, or other public buildings and small commercial enterprises, their flue gas stacks are referred to as chimneys.
The powerful association between industrial chimneys and the characteristic smoke-filled landscapes of the industrial revolution was due to the universal application of the steam engine for most manufacturing processes. The chimney is part of a steam-generating boiler, and its evolution is closely linked to increases in the power of the steam engine. The chimneys of Thomas Newcomen’s steam engine were incorporated into the walls of the engine house. The taller, freestanding industrial chimneys that appeared in the early 19th century were related to the changes in boiler design associated with James Watt’s "double-powered" engines, and they continued to grow in stature throughout the Victorian period. Decorative embellishments are a feature of many industrial chimneys from the 1860s, with over-sailing caps and patterned brickwork.
The invention of fan-assisted forced draft in the early 20th century removed the industrial chimney's original function, that of drawing air into the steam-generating boilers or other furnaces. With the replacement of the steam engine as a prime mover, first by diesel engines and then by electric motors, the early industrial chimneys began to disappear from the industrial landscape. Building materials changed from stone and brick to steel and later reinforced concrete, and the height of the industrial chimney was determined by the need to disperse combustion flue gases to comply with governmental air pollution control regulations.
The equation below provides an approximation of the pressure difference, Δ P, (between the bottom and the top of the flue gas stack) that is created by the draft: Natural Ventilation Lecture 2
where:
The above equation is an approximation because it assumes that the molar mass of the flue gas and the outside air are equal and that the pressure drop through the flue gas stack is quite small. Both assumptions are fairly good but not exactly accurate.
where:
Also, this equation is only valid when the resistance to the draft flow is caused by a single orifice characterized by the discharge coefficient C. In many, if not most situations, the resistance is primarily imposed by the flue stack itself. In these cases, the resistance is proportional to the stack height H. This causes a cancellation of the H in the above equation predicting Q to be invariant with respect to the flue height.
Designing chimneys and stacks to provide the correct amount of natural draft involves a great many factors such as:
The calculation of many of the above design factors requires trial-and-error reiterative methods.
Government agencies in most countries have specific codes which govern how such design calculations must be performed. Many non-governmental organizations also have codes governing the design of chimneys and stacks (notably, the ASME codes).
A great many power plants are equipped with facilities for the removal of sulfur dioxide (i.e., flue-gas desulfurization), nitrogen oxides (i.e., selective catalytic reduction, exhaust gas recirculation, thermal deNOx, or low NOx burners) and particulate matter (i.e., electrostatic precipitators). At such power plants, it is possible to use a cooling tower as a flue gas stack. Examples can be seen in Germany at the Power Station Staudinger Grosskrotzenburg and at the Rostock Power Station. Power plants without flue gas purification would experience serious corrosion in such stacks.
In the United States and a number of other countries, atmospheric dispersion modeling www.air-dispersion.com studies are required to determine the flue gas stack height needed to comply with the local air pollution regulations. The United States also limits the maximum height of a flue gas stack to what is known as the "Good Engineering Practice" (GEP) stack height. Guideline for Determination of Good Engineering Practice Stack Height (Technical Support Document for the Stack Height Regulations), Revised (1985), EPA Publication No. EPA–450/4–80–023R, U.S. Environmental Protection Agency (NTIS No. PB 85–225241)Lawson, Jr., R.E. and W.H. Snyder (1983). Determination of Good Engineering Practice Stack Height: A Demonstration Study for a Power Plant, EPA Publication No. EPA–600/3–83–024. U.S. Environmental Protection Agency (NTIS No. PB 83–207407) In the case of existing flue gas stacks that exceed the GEP stack height, any air pollution dispersion modelling studies for such stacks must use the GEP stack height rather than the actual stack height.
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