View looking down from chimney (Unit 2 scrubbers at the bottom) |
As most of you know, I have just finished an outage where the #3 steam boiler and steam turbine / generator were overhauled. During this outage I was tasked with the change out of the stack liner (which is the metal tube inside of the concrete chimney), replacing the scrubber tower outlet dampers, lining the scrubber outlet ducts (before the dampers) with an inconell alloy called Hastelloy and given the ASTM designation of C-276. It is a nickel alloy and is very resistant to chemical attack. The chimney is 385 feet tall with the stack liners sticking up an extra 15 feet or 400 feet above the ground. The liners are 16 foot 7 inches in diameter and are 360 foot tall (they start 40 feet up in the chimney) |
| The flue gas contains sulfur. In the scrubbers the sulfur is taken out of the flue gas and removed in a slurry. This mixing with limestone and water forms calcium carbonate to precipitate out, but makes the flue gas very wet. The moisture from the water is carried over out of the scrubbers and into the flue gas. This makes the flue stream contaminated with sulfuric acid. This acid eats the carbon or stainless steels that make up the duct work, dampers, and stack. | 
Unit #3 stack starting to be lowered to the ground |
ST#3 stack almost to the ground |
Unit 3 stack being cut into pieces - notice the lifting lugs |
| Coatings have been applied to the metals of the duct work system, but these coatings are a temporary solution and every outage (every two years) they have to be hydro blasted off (with 40,000 psi water) and the re-applied. This cleaning and re-application process not only takes lots of time, but also costs lots of money. These two (mirror image) boilers were built in the mid 1970's and have two scrubber towers each. The purpose of the two scrubber towers is to be able to use one with one out of service and maintenance can be performed. When the EPA made some additional pollution control rulings in 2001 these scrubbers have to both be operated all the time to remove enough sulfur. This additional tower makes the flue gas cooler and wetter, thus more acid attacking the steel. |
New liner being lifted up - notice the lifting rods and cables going down the center to hold up the floating scaffolding inside during construction |
New liner in construction. Liner assembly cart on left, floating scaffolding in side new liner suspended above where new can will roll into. |
The chimney contains two stacks (one for unit 2 and one for unit 3). The stack was removed by placing hydraulic jacks on the chimney roof, and then running some steel jacking rods down the liner and attaching the rods onto some lifting lugs (seen here). Then the jacks lift the rods off of stop blocks and lower them the 20 inch stroke of the jack to another stop block. This process is repeated over and over until the heights are reached. The stack was lowered down the chimney until it was inches above the floor. Then 10 feet were cut off and the newly cut can was cut into thirds to get out of the chimney. |
| The new stack was assembled in the opposite fashion. The new stack was brought in in thirds and then assembled on an assembly cart. This cart was then rolled into position under the liner and welded to it. The new stack was made from a high strength carbon steel (A-588) with a 1/16th inch wall paper of hastelloy (C-276) on the inside. Then once the carbon steel was welded together a 6" wide strip of hastelloy was welded over the joint giving a complete hastelloy interior of the new stack. Once the stack was assembled and put together, it was then raised into place in the same fashion that it was removed. |
Hole where Unit 3 liner was |
New liner with insulation on the exterior |
Liner assembly cart and floating scaffolding |
Old liner being removed |
The new stack liner was then insulated with a 2 inch fiberglass insulation to minimize the flue gas from cooling off to the point where it condenses any more water prior to exiting the stack. Due to the prices in the metals market recently, the price for the hastelloy is very volatile. Since it is mostly made of nickel (90% +), it is tied to the price of nickel. When we purchased it, the price of the hastelloy was $20/pound compare that to steel at $100/ton. This doesn't sound like much, but it is very expensive. Some of the pipe that we installed for the interior bracing in the duct work that was made of C-276 was $40/inch (that was for a 4" schedule 40 piece of pipe). |
Old damper being removed |
New damper being lifted into position |
| The two outlet ducts have dampers on them that were also replaced and the new dampers that were installed were also made from C-276. Even though we have to run both scrubbers almost all of the time, we can purchase low sulfur coal (think expensive premium coal) and run one scrubber while we take the other tower out of service. This is an expensive option, but it is cheaper than taking the unit down to do the maintenance. The new dampers weighed in at 18,000 pounds each, and required the use of a 275 ton crane to set them in position (due to the height and distance away from the crane that the dampers had to be placed). In some of the photos below you can see the interior of the duct work and the pressure washing of the coating off the walls so that we could weld on the hastelloy. I don't remember how high it is to the top of the scrubber towers, but it is 165 stairs to the top, then you can climb a 10 rung ladder to the top of the outlet ducts. |
New damper being lifted into positon with a 275 ton crane |
Looking down at crane and U3 scrubber towers and new dampers |
New dampers and seal air fan skids ontop of scrubber outlet ducts |
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Before Outlet duct with coating washed off walls and steel strips being installed |
After New hastelloy wallpaper and internal trusses installed in outlet ducts |
| In the photos below, you can see the breaching duct (this is the duct that breeches the chimney and caries the flue gas into the stack). The breaching duct floor was lined with the hastelloy. We would have liked to line the entire duct with it, but due to the enormous expense, only some of it was done during this outage with the remaining to be done during the next outage (in two more years). You can also see of the interior gutters that were placed in the breaching duct where it opens into the stack. These gutters are to collect any water that is traveling along the walls and collect it into an exterior drain. |
Breaching duct gutters in opening into new stack |
New hastelloy installed on floor and bottom of walls in breaching duct |