By Douglas J. Smith, IEng
LIKE MANY POWER plants constructed in the early 1970s, Louisville Gas & Electric’s (LG&E) 2,000 MW Mill Creek coal-fired power plant was required to add a wet scrubbing systems to comply with EPA pollution control guidelines. As part of the wet scrubber installation a re-heater system was also installed. The re-heaters are designed for full flow heating of the treated flue gas prior to its release into the atmosphere.
Unfortunately, the re-heater proved inefficient and reduced the plant’s energy production by approximately three percent. In addition, the re-heater system resulted in continual maintenance problems from leaking tubes, and the flue gas left a sulfuric acid residue as it passed through the stack. Over a period of time this residue caused severe deterioration of the carbon steel linings in the plant’s three 630-ft stacks.
After evaluating the problem Mill Creek decided to remove the re-heater system, replace the stack’s old titanium linings with “wall paper” linings made from C-276, a high nickel alloy, and install a new liquid collection system. Because C-276 does not rust or deteriorate in environments where the acidity level is 2.5-pH, the material is ideal for stacks subjected to sulfuric acid contamination.
Pullman Power, LLC, a Structural Group company, was awarded the contract to retrofit the stack linings and Alden Dynaflow received a contract to design and model the liquid collection system.
Retrofit Posed Many Challenges
Regardless of the retrofit strategy selected, projects of this type have typically required an outage time of 12 to 16 weeks, far greater than a normal 4
to 8-week planned maintenance outage. Besides the cost of the retrofit, the potential lost revenue from plant downtime is also significant.
LG&E requested Pullman Power to come up with a design that would substantially reduce the outage time. The end result was a design that would cut the time for wallpapering by 50 percent. Together with optimizing the retrofit construction, the Unit 1 retrofit took only six weeks to complete.
During the first week of the outage, the unit was allowed to cool down and the fly ash and acidic deposits that had accumulated were removed. To protect the stack a primer was applied to the carbon steel surface.
Since the wallpaper retrofit procedures are very time consuming, Pullman Power installed two separate sets of scaffolding, one at the top of the stack and another at the 200 ft. level. A safety barricade, between the upper and lower scaffolding, enabled two crews to work simultaneously within the stack.
Using Mill Creek’s historical operating data for the mist eliminators and absorbers and the stack’s design criteria, Alden Dynaflow conducted air quality modeling of the system. The modeling showed that the 18 to19 ft. diameter of the existing carbon steel liner would be too narrow to protect the environment from flue gas pollutants once the re-heater system was removed. At a flue gas velocity of 85 to 100 ft/second, the stack would also experience flow reversal and subsequent formation of an acidic liquid.
The problem was resolved by constructing a chamber in the bottom 100 ft of the stack so as to increase the diameter of the lower chamber to 22 ft and thus reduce the velocity of the flue gas. By removing the lower portion of the existing liner at the 220 ft level, and installing a new 100 ft. long liner, Dynaflow’s model showed that this would provide an acceptable level of liquid collection and protection from acids in the flue gas.
In order to meet the aggressive completion schedule set for Unit 1, it was necessary to construct the liner in advance of the scheduled outage. This was accomplished by building the new liner around the old one prior to the outage. Unfortunately, during installation of the new liner a portion of the existing liner began to buckle.
An investigation of the problem revealed that the existing carbon steel lining had deteriorated more than originally estimated. In some places the thickness was less than 1/8-inch. As a consequence, the existing liner had to be reinforced by adding vertical structural supports to the exterior side of the liner. Although the additional structural repair was unexpected the work did not interfere with the outage schedule. In parallel with the work at the bottom of the stack, the C-276 high nickel alloy, 1/16-inch thick, was being applied at the top of the stack.
Installing new stack liners at Louisville Gas & Electric’s Mill Creek coal-fired power plant. Photograph courtesy of Structural Group
Every 40 feet, the lining was 100 percent seal-welded to the carbon steel stack. As a result, any leaks will be contained within a 40-ft. section. During the final two weeks of the retrofit project, the liquid collection system, the gutters, and the sealed FRP piping, which collect and transports the wet flue gas to the annular space and down to the collection tank, was installed. Retrofit of Unit 1 was completed in December 2002.
Because of the reduction in outage time, the projected three percent improvement in capacity, and the reduction in future maintenance costs, the relining retrofit is expected to have a payback of three years.