DTE Energy-owned Monroe Power Plant, located in Monroe, Mich., is the second largest coal-fired power plant in the United States with four generating units producing a total of 3,300 megawatts of electricity. The plant was built in the 1970s and has the not-so-great distinction of being fifth on a list of the 100 most polluting coal plants, according to data gathered for a study from the EPA’s Toxics Release Inventory.
But despite this unfortunate ranking in 2009, Monroe Power Plant has been in the process of installing a new emissions control system for nearly 10 years.
Part of that process has included demolition of two of the original 800-ft. exhaust stacks to be replaced by new 580-ft. stacks. The new stacks will support the flue-gas desulfurization (scrubbers or FGDs) emissions control system that works to reduce sulfur dioxide and other emissions; and are able to handle the moisture being added to the exhaust by the emissions reductions process.
“Today with the improvement of emission control, many existent power plants are replacing old stacks with scrubbers to control the emission of sulfur dioxide from the coal being burnt,” explains Francois Villeneuve, vice president, Engineering and Special Projects, Fraco Products, Ltd. “After the new system is put in place and the new stack is operating, the task of demolishing, or rather dismantling the old stack begins.”
Most likely when you think of demolition, images of imploding buildings come to mind; however, demolition of a smokestack is less Hollywood action sequence and more meticulous process (at least most of the time).
The process begins with the lining of the stack, which usually features steel, being torn down through various methods. After that is complete comes the concrete demolition and the mast climbers time to shine.
“Explosives are only suitable when there are no other structures around. In the case of stacks, usually they are close to other plant every buildings and structures still in operation, so debris has to be controlled. Typically, and in the case of the Monroe Power Plant, the stacks are dismantled from the top down using various systems—mast climbers included,” explains Villeneuve.
Once the lining of the stack is removed, the dismantling begins by pushing the rebar, concrete, and other extraneous materials inside the remaining empty space, which functions like a debris chute. Waste is being removed from the bottom of the stack through a hole which is emptied from time to time to accommodate for more debris.
“Mast climbers are used by workers to access the work area and to position the demolition equipment and robots necessary to tear down the stack. The platform is adjusted around the structure to ensure nothing falls down. If debris does fall on the platform, it's shoveled back inside the stack,” explains Villeneuve.
Equipment such as hydraulic hammers, crushers, or shears are used to cut and drop blocks of concrete down—a process that can actually be done by hand but would take a significant amount of manpower and time. The sizes of blocks are determined by the vibrations the power plant can withstand without shutting down.
Villeneuve explains: “There are many control sensors around a power plant, and when there is too much vibration, the fans and equipment needed for operation shut down. To cushion those blocks of concrete, a bed, anywhere from four to six feet deep, of sand, or coal in this case, is created at the bottom of the stack to absorb the debris coming down.”
Fraco began dismantling the stacks in 2011 and completed the project in summer 2012. The mast climbers they equipped their project team with were Fraco’s high-capacity, high traveling speed, self-contained ACT-8 gas-powered units. Although the mast climbers were doing work at a power plant, gaspowered units were selected to eliminate reliance on electricity and reduce the amount of cables traveling up 800 ft.
“A mast climber is the right type of equipment to elevate personnel and provides a safe working area; in addition, if something breaks off the stack, it stays contained on the platform,” says Villeneuve. “Having an enclosed platform that can carry significant loads—the remote control machines alone are around 2,500 lbs. each—and considering that you’re tearing down what is supporting you from the top-down, makes the mast climber most suitable for this type of work.”
Overcoming the Challenges
Projects in any industry have their hurdles to get over. At the Monroe Power Plant, the initial challenge to face was the fact the mast climbers intended for the project were designed for 550 ft. not 800 ft. Villeneuve discusses the team’s solution: “Our first challenge to surmount was figuring out how to reach that height. So we developed a new load bearing tie system to hold the masts and redistribute the load of mast and equipment onto the structure. The load bearing ties resemble an overhang base, and were designed to tolerate a certain deflection and to redistribute efforts. It is made from a five foot section mast and bolted to a predetermined position on the stacks.”
The next hurdle the project team faced was that the area around the bottom of the stack was cluttered with piping. The mast climbers had to be installed either between or above the piping systems. This required some units to be installed in the air, and have access points on many different levels, considering there wasn’t always enough space at the bottom for the platform to come down.
“Then there is the fact that when you tear down a stack, you take down the mast climber as you go along. Equipment travels up and down, and once every thing is installed, you bring down the mast ties and equipment,” says Villeneuve. “So we had two units installed on one mast—one being an access unit.”
The stacks tapered diameter made for interesting days on the job, because when the diameter of the stacks decreased with height, the eight installed units that the project team could utilize to 600 ft., became four units from that point, up to 800 ft.— with the same load capacity needed. The units leaned as close to the structure as possible, and a conveyor belt of sorts made from carpet or rubber, and resting on planking, was placed in front of the mast climbers to account for the space between the equipment and stack. “If you keep the mast climber straight, it would be too far away from the structure; you have to enclose it as near to the structure as possible. The belt is cut to adjust around the structure, and according to the changing diameter, it has to be readjusted—but these are necessary steps to maintain a safe jobsite,” affirms Villeneuve.
And then there's the unforeseen element. In the case of the Monroe project the unexpected discovery concerned the strength of the structure—a critical element to installing a mast climber. Revealed when the time had come to build the special components for the project, was that instead of a typical two layers of rebar supporting the concrete structure, there was only one. “We basically found out the structure was not what we thought it was after winning the job and viewing rough drawings. When you’re pushing from the outside of a structure, it is critical that the structure be strong enough to apply the load,” explains Villeneuve. “With just the one layer of rebar, the concrete was not strong enough to tolerate the punching forces, so along with the customer, the solution devised was to spread the load by using custom-made rolled H-beams. We simply weren’t expecting that issue.”
In the end, the team was able to meet the challenges and complete the project. New innovations for equipment that came from this project will make the next job just a bit easier.
You Live, You Learn
Considering the many stack projects Fraco has been involved in all over the world, Villeneuve says: “What we’ve learned is that with projects such as these, premises must be reevaluated every time. Engineers at a power plant require a lot of information and have to verify everything. You need to be prepared to answer a lot of questions, because everything is done—and should be done—by the book to ensure a safe, successful project.”
On any project, whatever the scope, it is vastly important for project team members to communicate thoroughly and openly, in order to learn from one another, avoid startling challenges, and ensure safety on the jobsite. “We have to understand what the contractor wants to accomplish so that our equipment solutions fit their needs. There is often too much time wasted trying to figure that out in order to proceed with a project. But it goes both ways—we have to explain clearly our limitations in order to find solutions,” concludes Villeneuve.