A highly corrosive flue gas
Before the desulphurisation process, the flue gas has an SO₂ content of 18.000 mg/Nm³. The FGD absorbers remove more than 90 % of this SO₂. Then, as the cleaned gas comes out of the absorber into the outlet duct it is mixed with a much smaller stream of hot air coming from the air preheaters and its temperature is raised from a water saturated 60 °C to a much drier 75-85°C.
Even after a highly efficient desulphurisation, the flue gas is still extremely corrosive to mild steel. The reason is that SO₃, which causes the formation of acidic condensate in the ductwork, is only partially removed from the gas stream by the wet FGD process. The average SO₃ content for the unscrubbed flue gas in Teruel Power Station is 140 mg/Nm³ and much of this will “slip” through the absorbers into the outlet ducts.
The reheating of the scrubbed flue gas by 15-25°C will raise the temperature of the gas stream above its water dewpoint of 60 °C, but it can never raise the temperature above the acid dewpoint.
Typically, the acid dewpoint of a flue gas will lie between 120 and 160 °C, depending on the SO₃ and water content of the flue gas.
The reheating system will evaporate most of the moisture present in the scrubbed gas stream. Only minor quantities of condensate will remain, but this condensate would be very corrosive to steel and so a highly resistant internal lining is required. The operating conditions in the mixing chamber, where the reheating takes place, are especially corrosive.
The advantages of PennguardTM
The selection of a PennguardTM
lining for the Teruel FGD outlet ducts was based on its long term reliability and its very low maintenance requirement and also on some of its practical properties.
First of all, a number of power stations worldwide that fire high sulfur fuels have gathered positive long term experience using PennguardTM linings in FGD outlet ducts and chimneys. The operating experience shows that even under highly acidic conditions, PennguardTM linings offer an estimated service life of 20 years and require very little maintenance.
Furthermore, PennguardTM offers a very practical advantage. A PennguardTM lining provides excellent thermal insulation and keeps the external surface of a flue gas duct at a cool, nearly ambient temperature. In the Teruel FGD project, the use of a PennguardTM lining therefore eliminated the need for external insulation, leading to a savings in engineering, construction and future maintenance costs.
Some details about the application
The application of PennguardTM linings to the three large outlet ducts was performed between September and December of 1998. After mounting of the ducts onto their support construction, they were first grit blasted and cleaned, after which the PennguardTM Block Primer was spray applied to the steel surface. Construction details such as flanges of expansion joints and manholes were then protected by 2 mm thick strips of a special corrosion resistant alloy. Special design details had been prepared for this in the planning phase of the project.
The PennguardTM lining itself was installed by a group of 6-8 Spanish brick layers and the average production rate was around 1,5 m² per applicator per hour. An experienced team of 2 quality inspectors and a site manager, all from Hadek, were present on the project site to train and support the applicators and to monitor the quality of the installed linings.
As the “finishing touch”, all floor areas (i.e. the lower 1/6 of these round ducts) received additional protection from a 30 mm thick layer of Tufchem Silicate Concrete. This special acid resistant concrete is part of the PennguardTM product line. It will protect the PennguardTM lining on the floor from mechanical damage that could potentially occur during cleaning work or other activities.