Modification of the Dolna Odra Unit 5 – 8 chimney
Built in 2002, the Unit 5-8 chimney originally contained a sectional flue of interlocking, acid resistant brick. The brick flue had a diameter of 8,7 metres and it was externally insulated with 50mm thick thermal insulation.
Situation before modification
The Unit 7 & 8 FGD system is connected to the common flue gas duct that runs between units 5 through 8 and the chimney. This FGD system has a rotary gas-to-gas-reheater (GGH) which increases the temperature of the desulfurized gas to about 80°C.
The Unit 5 & 6 FGD system will also be connected to the common Unit 5-8 flue gas duct. It will not be equipped with any reheat. The Unit 5 & 6 FGD will be commissioned in 2012. Around that time, the Unit 7 & 8 FGD will be modernized and its GGH will be removed. The Unit 5-8 chimney will then become a wet stack.
The 8.7 metre diameter of the brick flue would result in a wet stack gas flow velocity of well over 23 m/s. Such a flow velocity would likely result in excessive carryover of liquid droplets from the top of the chimney.
To make the chimney suitable for safe wet stack operation, the owner decided to “open up” the chimney by removing the brick and installing PennguardTM directly onto the concrete windshield. This design change results in a chimney top diameter of 12.6 metres and a very safe, gas flow velocity of 11-12 m/s.
Situation after modification
EPC contractor Rafako selected Uniserv SA as the contractor for the design and execution of the chimney modification works. Uniserv’s scope of work included the construction and demolition of a 60 m high temporary stack.
Following completion of the temporary stack, the contractor removed the brick flue and its concrete supports. Platforms for monitoring equipment and maintenance were installed on the outside of the concrete shell.
Following grit blasting of the concrete surface, spot repairs and application of PenntrowelTM Epoxy Primer, the 54 mm PennguardTM lining was installed onto a surface area of 7,575 m² by teams of up to 18 brick layers in a total of 72 shifts. All of this installation work was performed under supervision of Hadek Quality Assurance Inspectors.
How low gas flow velocities will minimize the risk of “spitting” in the Dolna Odra Unit 5 – 8 chimney
The operation of FGD “wet stacks”, without any reheat of the desulfurized flue gas stream, can be very economical. At the same time however, it creates a risk of “spitting”; droplets of acid condensate will be blown out of the top of the stack and create a nuisance problem for the power station and the surrounding area.
One of the most important design parameters that will determine the risk of spitting is the gas flow velocity within the chimney. Once this velocity exceeds a critical limit, the liquid film on the chimney wall will be entrained into the gas stream as droplets.
Maximum stack liner gas velocities for 4 materials.
Source: Alden Research Laboratory Inc.
Figure 1 shows the maximum flue gas velocities for different chimney wall materials, as recommended by specialist consultants Alden Research Laboratory from the United States. The approach chosen for the Dolna Odra Unit 5 – 8 chimney can be considered especially safe. After the removal of the brick flue and the application of the PennguardTM lining onto the concrete windshield, the effective diameter of the chimney varies from 13.8 m at the flue gas entry level to 12.6 m at the top.
In the lower 44 metres of the chimney, the maximum gas flow velocity for wet stack operation will be between 9.35 and 11.3 m/s. In the upper, cylindrical part of the chimney, the maximum gas flow velocity for wet stack operation will be at around 12 m/s, which represents a safety margin of over 30 % below Alden’s recommended design velocity limit of 18.3 m/s (60 f/s).