Stacks at power generating stations are much more than tall towers used as a landmark for locating power plants. They are a necessary and essential piece of equipment needed to add buoyancy to the exhaust gases as they exit to the atmosphere. Their height and diameter are selected to match the specific requirements of each plant, and they are usually a low-maintenance item.
Stacks may be low maintenance, but they are not no maintenance. The cost of preventing corrosion may be as little as $10,000, but the cost of repair or replacement could be many times that or even put your plant out of commission until the stack problem is corrected. It pays to invest in stack liner protection (Figure 8).
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| 8. An unprotected stack. The protective coating on the stack liner must be selected based on the actual flue gas temperature entering the stack. A well-applied coating system can prevent catastrophic corrosion that is expensive to repair and may require a long outage. Courtesy: Bril Inc. |
At a typical 800-MW power plant, the exit gas temperature leaving the boiler is approximately 700F. The flue gas then passes through the economizer and the selective catalytic reduction system to remove the nitric oxides in the gas. The gas then passes through a heat exchanger or air preheater that transfers the thermal energy from the exhaust gases to the incoming boiler combustion air to improve the overall unit combustion efficiency. The flue gas temperature leaving the air preheater will be reduced to around 350F before next passing through a series of air quality equipment steps to remove SO2, particulates, and perhaps mercury until the gas reaches the induced draft fan on a balanced draft unit. This fan pulls the flue gases from the boiler through this equipment and sends it to the stack. On the upstream side of the boiler, a forced draft fan provides combustion air to the boiler combustors, pulverizers, and other equipment.
A typical small industrial facility like a hospital or school has boilers that are much smaller (75 MW or less capacity). Flue gas leaving these boilers is much lower in temperature (typically between 351F and 500F), and the unit may not be outfitted with an air preheater. These small facilities may have a heat exchanger such as an economizer but not an air heater. The economizer extracts heat from the flue gas to heat the water needed in the boiler. The gas leaving the economizer is usually above 350F, but, as is often the case with the smaller boiler systems, the flue gas goes directly to the stack.
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