The purpose of a tall stack in a thermal power plant is to discharge the emissions as high as possible so that dispersion takes place in a very large area. As the dispersion area increases, the concentration of the dust or emission at the ground level is less and does less damage to humans and vegetation.
The stack in coal fired power plants can be tall in the range of 200 to 300 M. The height depends on the size of unit, local regulations, population densities, wind and other atmospheric conditions. A Kazakhstan stack at 420 M tall is reportedly the tallest in the world.
The stack has an outer RCC shell and a steel inner lining for the flue gas path. The outer shell also sometimes called the windshield provides the strength. The inner lining protects the concrete layer from corrosive flue gases. Connection of multiple units to a single shell will have multiple inner flue gas liners.
Slip form method
The most common method adopted for the construction of an RCC outer shell is by using the ‘slip form method’. In this method the form work for the concrete literally slips up cm by cm to produce an integrated concrete column. Continuous pouring of concrete ensures a joint free construction. At a rate of 0.3 meters per hour, this is a considerable fast construction method.
The construction of a tall stack is complicated because of many reasons.
- The work has to be done at great height.
- The form work has to move continuously upward.
- The diameter of the form work has to change continuously.
- The gap between the inner and the outer form also has to change to accommodate the change in thickness of the shell.
A complicated system of jacks and radial screws makes this possible in the “slip form method".
Radial steel yokes, which have an inverted “U" at both ends, support the slip form shutters all around the shell. Walers strengthen and keep the shutters in place. The working deck and scaffolding is supported from the walers and yokes. The yokes are supported on hydraulic jacks, which move on jack rods embedded in the concrete. As the concrete pouring progresses the jacks lifts up the yokes, together with the decks and slip form. Screwed on pieces extend the jack rod as the work progresses. The entire system of decks and shutters appear as if floating on top of the stack.
Radial screws adjust the diameter and the width of the pouring. Sufficient gap between the top of the yoke and the slip form allows installation of reinforcement steel.
A laser alignment system allows the operator to control verticality and avoid twisting.
Follow this link to a video on this method of construction. https://www.youtube.com/user/mmcg968#p/a/u/1/lxGvgQRX-o8
Construction of other vertical structures like silos and core structures in high-rise buildings also use this method.
Jump Form Method
Another but similar method that is in use for this type of construction is the “jump form method." In this method the form work jumps up to the next layer after the bottom layer is cast. This is suitable for stacks, which have lining – like refractory lining in the inside of the shell.The concrete is not continousas in the slip form method.