Stacks at power generating stations are much more than tall towers used as landmarks for locating power plants. They are essential equipment needed to send the exhaust flue gas into the atmosphere. Their height and diameter are designed specifically for the plant and are usually a low- or no-maintenance item. The cost of preventing corrosion may be as little as $10,000—compared with the cost of repair or replacement, which could be double or triple that figure, as well as the costs associated with putting a plant out of commission until a stack problem is corrected. Clearly, it pays to protect stack linings.

Typical Power Plant Layout

At a typical power plant, the exit gas temperature leaving the boiler is approximately 700°F on an 800-megawatt capacity boiler. The flue gas is then passed through a large box called a selective catalytic reducer to remove nitric oxides. The gas then passes through a heat exchanger, or “air heater,” which exchanges the heat from the flue gas to heat the air used for combustion in the boiler. The flue gas leaving the air heater is around 350°F. It passes through a series of air pollution equipment to remove mercury and particulates until it gets to the induced draft fan, which pulls the flue gas through the air heater to the stack.

A typical small industrial facility like a hospital or school has boilers that are much smaller (75 megawatt or lower capacity), where the flue gas leaving the boiler is much cooler (between 351°F and 500°F). These small facilities may have a heat exchanger, such as an economizer, but not an air heater. An economizer extracts heat from the flue gas to heat the water needed in the boiler. The gas leaving the economizer is usually above 350°F, but because air pollution requirements are not currently as stringent in the industrial sector as in the power sector, the flue gas can go straight to the stack.

Corrosion Protection for Stacks

The rules for determining what a stack requires for protection against corrosion are based on the temperature of the flue gas entering the stack at full load.

1. 130°F to 350°F
   At these low temperatures, it is recommended that the stack interior be painted or sprayed with a high, solid-type, non-asphaltic, mastic-type coating, 3/16-inch to 1/4 inch wet thickness. This type of coating will prolong the life of the stack interior by protecting against hot sulfuric, hydrochloric, and hydrofluoric acid solutions, as well as vapors present in the flue gas. The application of this mastic is usually done by painters, not bricklayers, and must be done on a dry, clean surface. For existing stacks, once it has been determined that the stack requires a mastic coating, the entire interior stack surface must be sandblasted to a near-white condition, per code SSPC-SP-10. This will ensure that the mastic material will adhere.
2. 351°F to 400°F
   At these mid-range temperatures, the stack interior does not require an internal coating of mastic protection, but it does require external insulation and lagging to prevent condensation on the outside.
3. 401°F to 850°F
   It has been found that if the gas temperature is in this range, no internal or external protection is required to prevent corrosion.
4. 851°F and above
   At these elevated temperatures, the internal lining of the stack must be protected with refractory. The refractory material should match the chemistry of the acids within the flue gas. The refractory is typically made of three parts acid-resistant aggregate, one part lumnite cement. It is pneumatically or gun applied, usually 2 inches thick, through a reinforcing material such as road-mesh or chicken-wire mesh. The reinforcing mesh is held to the stack interior using stand-offs such as slab spacers, t-slot studs, or studs and nuts.

   Attention should be paid to exit flue gas temperature, especially if a different size economizer or air heater is added to the back-end of the boiler, changing the exit flue gas temperature to the stack. This change could affect the level of stack protection required to prevent corrosion.

   Stacks rarely need repair and are low-maintenance items that require attention only if they have corrosion issues. Protecting them from corrosion could save astronomical costs for repair or replacement. For example, to install a mastic coating (see item 1 above) on a 7-foot diameter, 96-foot-high existing stack operating at 300°F will require scaffolding, sand blasting, and priming and coating with the mastic, which will take approximately 6 days. Compare that to the cost of replacement: More than $250,000 for the new stack, which may take 30 days. This does not include revenue lost because the boiler cannot run without a stack.

   The bottom line: Protecting stacks from corrosion has a minimal cost compared to replacement or repair. Don’t forget stacks when making changes to the boiler.