When it comes to the construction of utility-grade chimneys, specifying a fiberglass-reinforced plastic (FRP) or nickel alloy-clad lining system is the current trend for structures operating downstream of a wet flue gas cleaning process. In the case of units operating downstream of a dry environment, carbon steel (often with an acid-resistant coating) or low-grade, stainless steel lining systems are more common. Still, specifiers must be familiar with brick-lined chimneys because of their prevalence in the marketplace and the necessity of understanding the structures in order to maintain and retrofit them properly.
Chimneys begin to deteriorate exponentially from the moment they are built, even before being put into service. While signs are often hidden during the structure's early years, decline accelerates rapidly and often catches building owners and facility managers off-guard. Due to the changing climate and deregulation in the energy industry, routine maintenance has been relegated to the backburner. Many facility managers simply have not spent the money needed to upgrade or replace power plants, and infrastructure investment has declined significantly over the last five years. (Some industrial plants have also delayed repairs and maintenance because of the economy.) However, with proper attention, most structures can be repaired or retrofitted and continue to serve for years beyond their intended life spans.
Performing structural repairs is something many facility managers do not wish to contemplate due to complexity and expense. Procedures can involve exposing the reinforcing steel by removing damaged, unstable concrete, and then sandblasting the steel and concrete substrate to remove foreign matter. (This must be completed before applying new concrete and protective coatings.)
With metal stacks, often designed for heights under 61 m (200 ft), the steel is welded and exterior stiffeners are added. Weld joints can suffer from small cracks caused by wind loads and thermal stresses during the life of the stack. Once a crack begins, corrosion soon follows-often in locations hidden from the eye. For all chimney structures, regular inspections and maintenance are necessities-but so is specifying the proper materials in the first place.
As a general rule, the trend today for construction of utility grade chimneys that will operate downstream of a wet flue gas cleaning process is to specify fiber-reinforced plastic lining or a high nickel alloy clad steel lining system. In the case of units that will operate downstream of a dry environment, carbon steel - often with an acid-resistant coating on it -- or low-grade stainless steel lining systems are common. However, specifers and designers must be familiar with brick lined chimneys because of their prevelance in the marketplace and the necessity of understanding the structures in order to maintain and retrofit them properly. It is difficult to point to a single document for reference since it depends on the materials used, however, chimney repair experts typically refer to guidelines and documents from the American Society of Civil Engineers, ASTM, American Concrete Institute and the International Building Code.
Causes of chimney deterioration
Whether it is a simple, unlined steel stack or a complex chimney with a reinforced concrete shell surrounding one or more flues, many factors can contribute to deterioration. A chimney's sheer height, slender form, and unique function create an environment quite different from other structures-one where degradation is accelerated and hazards magnified.
Flue gas
Combustion of sulfur-containing coal and oil produces corrosive components that attack the calcium silicate hydrates strengthening the concrete. Additionally, gases bleeding into the annular space between the liner and chimney shell condense and form an acidic liquid-when it reaches the reinforcing steel, the result can be corrosion-induced cracks, delamination, and/or spalling.
Holes in liner
When the liner begins to deteriorate and the chimney is directly exposed to hot gases, a second form of deterioration can occur. Metal liners have either an exterior liner insulation (maintaining a "hot side" temperature above the dew point of acid gases) or an interior acid-resistant coating. Should the insulation become damaged or saturated (or the original acid-resistant coating fail), holes develop in the liner and allow the process gas to come into direct contact with the chimney shell. When this happens, the stresses are both chemical and thermal, and the concrete will crack and spall.
For example, recent damage to a 183-m (600-ft) fiberglass-reinforced plastic (FRP) liner at a regional energy company occurred because carryover from the absorber caused ash deposits to collect in the elbow, overloading the support structure. The new design required changing the top-supported 90-degree elbow section to a base-mounted, roll-clad alloy T-section to transition the flue gas from the scrubber to the stack.
Carbonation
Concrete's pH level declines as it is exposed to carbon dioxide-this reduces the protection it offers to the embedded steel reinforcing. In many chimneys, carbonation proceeds at an annual rate of approximately 1 mm (0.04 in.). While this may seem insignificant, in 25 to 30 years, as much as 38 mm (1.5 in.) of concrete will have been carbonated. Since the outer curtain ring of reinforcing steel is embedded beneath only 38 mm of concrete, it can be left exposed to corrosive elements.
When reinforcing steel rusts, it undergoes a volumetric change (sometimes expanding 12 to 14 times in size), causing the concrete to spall and fall off. Carbonation occurs more rapidly at cracks and construction joints, and where heat is applied. Some facilities use wet scrubbers to remove sulfur compounds from flue gases, but the scrubber residue combines with water to form sulfuric acid, which is extremely corrosive to most metals.
Weather
Beyond the effects of operations on chimneys and stacks, it is important to consider the influence of wind, rain, and fluctuating ambient temperatures. While structures exposed to dramatic seasonal fluctuations are at a greater risk, no facility is immune to the effects of environmental forces.
When wind velocity is greater than zero, an airflow pattern quickly develops around the stack. Aerodynamic volume displacement increases wind velocity, producing a zone of negative pressure on the leeward side and drawing in the flue gas. Since the external column surface temperature is much lower than that of the emerging flue gas, condensation occurs on the leeward side, leaving yellow, brown, or black deposits. These deposits retain corrosive moisture which, over time, results in erosion of the concrete surface. The application of a suitable coating to the affected concrete surface can stop this type of damage.
Construction techniques
Another source for deterioration lies in the techniques used for chimney construction. Jump form construction is typically used in the erection of reinforced concrete chimney columns as high as 213.4 m (700 ft). This involves casting in place individual sections, typically 2.3 m (7.5 ft) in height, until the specified column height is reached. The resulting cold joints between the individual column sections are referred to as construction joints. (No bond is created between the column sections.)
In many cases, defects in the wall (usually at construction joints) result from inadequate vibration of the concrete during placement. With continued concrete curing/shrinkage, the construction joints continue to open. This, along with the deterioration of sealing grout, creates an open path for corrosive elements to attack reinforcing steel embedded in the concrete wall. Once the reinforcing steel is corroded, the structural integrity of the concrete column is compromised.
Coatings to the rescue
Specifiers can select from various specialized coatings, able to withstand a range of chemical and thermal conditions. Concrete chimneys are coated for three reasons-protection against environmental elements, defense against process gases, and compliance with Federal Aviation Administration (FAA) requirements (i.e. flashing lights for nighttime and warning paint for daytime).
Selecting the most appropriate coatings requires consultation with a chimney expert. Not all chimney repair professionals are equal. It is crucial that your chimney repair consultant has expertise and experience not only in construction, but also design, inspection and maintenance.] For example, an acrylic-based aviation warning paint sufficiently provides proper visibility for seven to 10 years. However, this paint does not always provide proper resistive qualities, nor does it always adequately seal construction joints and other defects in the column surface that can lead to future deterioration and costly repairs. In such cases, a chemical-resistant epoxy coating should be applied to the upper chimney zones.
More than just the chimney's exterior should be considered when specifying coatings. For example, in older generation chimneys, flue gasses run hot and the liners-typically brick-are installed within inches of the column's interior. In this type of chimney, much of the heat carried in the flue gas is transmitted to the concrete column, resulting in the formation of condensation on internal surfaces. It is imperative for the concrete to "breathe," which can be accomplished through an external coating that allows the permeation of the moisture vapor from the interior. When the coating prevents breathing, the trapped moisture corrodes the reinforcing steel. For this purpose, a breathable, elastomeric, acrylic-based coating is often a good solution, provided it is compatible with the heavier-bodied material used to seal construction joints and other defects. Coating protection can also apply to the chimney flue or liner. (Operating conditions should be researched carefully.)
Unfortunately, some specifiers solely consider the process gas temperature as it exits the boiler or the scrubber. Using only the normal online temperature can be a mistake, as the insulated steel liner surface temperature (normally above the dew point), may not behave as expected. Temperatures inside the chimney liner fluctuate-especially during startup or shut-down operations-increasing the probability of acid condensation. Other temperatures that could occur along with the chemical composition of the flue gas must be considered before a selection is made.
Alternate coatings for metal stacks have a higher heat resistance for internal/external surfaces and are made from vinylester and phenolic compounds, rather than acrylics and enamels. Another coating "metalizes" the stack's interior by applying a pressurized molten zinc or aluminum alloy, enhancing cathodic protection during high-temperature operations.
Problems and solutions with brick liners
Many structures built in the 1970s through the mid-1980s are equipped with freestanding, acid-resistant brick linings. Although some of these independent brick-lined chimneys operate dry-and therefore encounter minimal problems-many operate downstream of old-generation, wet flue gas desulfurization (FGD) systems, still using bypassed flue gas for reheat.
Initially, the acid-resistant brick liner was touted as an ideal alternative to carbon steel-the latter being a material susceptible to the harsh acidic environment found downstream of the wet scrubbers. However, after two to three decades of service, the majority of brick liners have developed a lean or deflected position as a result of operating wet. Studies indicate numerous complex factors can influence this deflection, but there is very rarely a common contributor. Only two common factors were identified in all leaning liner cases:
- The use of bypass gas for reheat.
- The use of red shale brick in the liner construction.
Other varied factors that can influence liner lean include the quantity/composition of moisture deposited on the liner impingement area, coal composition, flue gas reheat methods, and fluctuating gas temperatures. While some chimneys can operate for years without any problems, the effects are quick for others, resulting in deflection in as little as a year.
One variable reason for the liner leaning is the impact of the wet flue gas' varying temperatures exiting the main scrubber duct and then hitting the liner wall at 180 degrees from the breech opening-a fairly large area commonly referred to as the "impingement zone." Over time, irreversible moisture expansion of brick and/or mortar occurs, which causes lateral deflection of the liner at upper elevations.
During periods of high wind loading, column movement could lead to contact with the stationary brick liner, which may cause partial failure. Dislodged bricks can fall inside the annulus, knocking out or damaging emissions monitoring equipment, and resulting in an unscheduled shutdown.
Solutions for correcting the leaning liner condition are limited and can be expensive to implement. When the liner has moved dangerously close to the column interior or internal platforms, a counterweight system incorporating heavy concrete weights connected to the liner with cables and pulleys can pull the liner to an acceptable position within the column. However, the counterweight system should be used only as a last resort, as the additional weight imposed on an isolated portion of the liner can result in structural damage to its base.
When the liner's deflection has been identified, but is not yet dangerous, studies have shown the installation of a target wall can help. This addition at the base of the liner helps prevent temperature fluctuations or transport of moisture into the liner wall, alleviating additional lean of the liner. Regular inspections are essential to monitoring any deflection problems. Without these inspections, deflection may increase rapidly, requiring drastic action once detected.
In addition to deflection, brick liners are also frequently damaged by corrosion of the liner-reinforcing system. The liners incorporate circumferential steel bands that limit the expansion of the vertical cracks caused by exposure to a thermal gradient. Corrosion damage to these bands occurs when moisture passes through the liner wall from the interior. The condensation formed on internal liner surfaces is forced through the liner wall as a vapor via thermal conductivity. Depending greatly on ambient conditions, crystalline accumulations develop as the vapors condense on the cooler liner exterior. As they feed on moisture, the acidic crystals grow-damage is slow but constant-and often result in corrosion of liner bands. At locations where bands are exposed to liquid flowing through defects in the liner wall, corrosion damage progresses rapidly. In essence, any liner penetration points (i.e. cracks, test ports, areas around a flexible breaching seal) can provide an area for condensation or carryover water to flow through the liner wall.
Often, corrosion damage results in failure of the affected liner bands. Without the entire liner band reinforcing system being 100-percent effective, existing vertical cracks in the liner expand in width and length. In extreme cases, serious structural damage to the liner can occur.
Most brick-lined chimneys operating in a wet environment incorporate an annulus pressurization system. Although this does not eliminate moisture migration through the wall, it reduces rapid damage by preventing the passage of flowing water. (Keeping the annulus pressurization fans in good working condition is important, as is careful monitoring of the system to maintain the liner bands and reinforcements to full capacity.)
Additionally, certain materials are available to seal the internal liner surface to prevent the passage of moisture through the wall. However, these materials tend to be time-consuming and expensive to install, so some renovation projects involve completely replacing the existing carbon steel liner bands with corrosion-resistant bands.
Overall, a lack of maintenance or incorrect repair procedures can result in near catastrophic conditions for brick liners. Such was the case for a power generation company in the Southwest. The brick liners in two 122-m (400-ft) tall chimneys were designed with two bypass breechings and one scrubber breeching. Hot, dry flue gasses hit one side of the internal liner wall, and cooler wet flue gas hit the opposite wall, imposing tremendous stress on the liner walls and causing numerous vertical cracks.
Corrosion, caused by liquid flowing through cracks in the wall, destroyed the liner-reinforcing bands, while existing vertical cracks continued to expand. Movement of the liner wall (i.e. downward shifting) displaced lintel beams over the bypass breech openings. In one of the liners, a 9.1-m (30-ft) tall, 4-m (13-ft) wide pie-shaped area below the scrubber duct (held in place by extremely strained liner reinforcing bands) was leaning outward and was in danger of falling out of the liner wall. Bricks in the area of the breech openings had disintegrated. The two 102-mm (4-in.) inside rows of brick in the 406-mm (16-in.) thick liner wall were removed from a core sample as rubble.
The contractor stabilized the structurally deficient liners by placing a reinforced concrete sheath around the bottom 47.2 m (155 ft) of the liners to an elevation above the breech openings. Dead loads on the deficient liner wall were transferred to the sheath through hundreds of steel dowels. At the same time, areas of disintegrated bricks were removed from the internal liner wall surface, and the resulting cavities were filled with structural gunite to the original thickness of the liner wall. In the vertical area of the sheath, a sacrificial insulating lining system protected internal liner surfaces.
The cost of replacing the brick liners would have been several million dollars and would have taken several months. However, with proper inspections and maintenance, the extensive repairs required to stabilize the liners likely would have rendered the repairs unnecessary.
Preparing for the future
Responding to regulations and dealing with aging/ignored chimneys is a big task for the industry. With the advent of power plant owners are required to add flue gas cleaning equipment, which in most cases results in new chimneys. Additionally, older plants are being decommissioned and have to be replaced.
There also is a massive need in the general market for chimney retrofit, due to both aging facilities as well as delayed maintenance. For example, brick-lined chimneys operating in a partially wet scrubbed environment (i.e. older chimneys operating downstream of wet FGD systems) require retrofit services because linings have been exposed to harsh conditions. Extensive repairs are likely to allow the chimneys to operate from a structural standpoint.
Authors
Jim Naylor, Vice President of Sales and Marketing for Pullman Power. Naylor is a member of the American Society of Civil Engineers and the Cooling Tower Institute.
Kenny Kendall, Special Projects Coordinator for Pullman Power, has been with Pullman Power, LLC since 1988 but has been involved in the inspection, repair and modifications of industrial chimneys for 35 years. He has worked on projects throughout the United States, Canada, Europe and South America. In his current role, he works with clients to establish inspection and preventative maintenance programs for their chimneys as well as assists them with developing repair or modification procedures when required.