Many structures built in the 1970s through the mid-1980s were equipped with free-standing, acid-resistant brick linings. Although some of these independent brick-lined chimneys operate dry and encounter minimal problems, many work downstream of old-generation wet FGD systems still using bypassed flue gas for reheat.
At its inception, the acid-resistant brick liner was touted as the answer to one of this type of chimney's biggest maintenance problems – liners made from carbon steel that were quickly damaged by the harsh acidic environment found downstream of the wet scrubbers.
Unfortunately, though, the new brick liners were not a panacea. After two or three decades of service, the majority of these liners have developed a lean or deflected position as a result of operating wet. Studies indicate that 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 and the application of red shale brick in the liner construction. Other varied factors that can influence liner lean include the quantity and composition of moisture deposited on the liner impingement area, composition of the coal used, flue gas reheat methods, and fluctuating flue gas temperatures. While some chimneys can operate for years without any problems, the effects are quick for others, resulting in deflection in just one year.
One variable reason for liner leaning is the impact of differing temperatures of wet flue gas exiting the main scrubber duct and then hitting the liner wall at 180° F from the breech opening, a fairly large area commonly referred to as the impingement zone. Over time, moisture expansion – irreversible growth – of brick and/or mortar occurs. Expansion of the liner wall in the impingement area causes lateral deflection at upper elevations.
The significance of the liner being in a deflected position is that during periods of high wind loading, movement of the column could result in contact with the stationary brick liner. The possible resulting contact between the column or platform and the liner – usually at the top of the structure –could cause partial failure of the liner. Dislodged bricks may fall inside the annulus, knocking out or damaging emissions monitoring equipment, which would result in an unscheduled shutdown.
Solving the problem
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 because the additional weight imposed on an isolated portion of the liner can result in structural damage to its base.
If deflection of the liner has been identified but not yet dangerous, studies have indicated that installing a target wall – an area in the liner that helps prevent temperature fluctuations or transport of moisture in the flue gas into the liner wall – halts additional lean of the liner. Placing a target wall at the base of the liner helps prevent temperature fluctuations or transport of moisture into the wall, which helps alleviate additional lean of the liner. Regular inspections are essential to monitoring any liner deflection problems. Without these regular inspections, deflection may increase rapidly, requiring drastic action once detected.
In addition to deflection, brick liners are commonly damaged by corrosion of the liner-reinforcing system. Brick liners incorporate a system of circumferential steel bands to limit the expansion of vertical cracks that inherently develop, particularly in thicker areas of the liner wall that are susceptible to a greater thermal gradient.