Wet limestone flue gas desulphurisation (FGD) systems have been selected for their efficiency and low running cost by almost all power companies in India for removing sulphur dioxide from flue gas and complying with the emission control notification of the Ministry of Environment, Forest and Climate Change.
The clean flue gas streaming out of an FGD system is 100 per cent water saturated and condenses in the outlet duct connecting to the chimney and in the chimney itself. The condensate is a diluted sulphuric acid (1 per cent), which is highly corrosive for the outlet duct and the chimney.
In the recent past, when there were no FGD systems installed, the flue gas would be dry and so there was no chance of corrosion of the steel liners. In India, for wet flue gas FGD projects, most contractors prefer to use borosilicate glass block linings inside steel flues while major power companies give options for metallic solutions such as titanium and high nickel alloy “C 276”.
All three options have been in use worldwide in outlet ducts and chimneys downstream of wet FGD systems. The key factors that can help the procurer to choose between them are as follows:
- Installation: It is a fact that titanium has excellent long-term corrosion resistance, but there is the risk of exposure of the mild steel flue to wet flue gases through the welding porosities caused during steel flue construction.
- Price: Although C 276 has excellent corrosion resistance, its price is much higher than that of the other two options. Borosilicate glass block linings and titanium-clad plates are clearly more economical.
- Maintenance: All three options are good and provide maintenance-free service for more than 20 years. It has been found that many power stations in the US have been using borosilicate linings for more than 25 years. The leading supplier of borosilicate linings claims to have gathered a successful operating experience exceeding 30 years.
- Design: In order to prevent stack liquid discharge (“spitting”), chimney flues have to be dimensioned to ensure that the flue gas velocity is limited to the maximum allowable velocity for the chosen material of construction. According to the “EPRI Revised Wet Stack Design Guide”, chimneys lined with certain borosilicates can handle higher velocities than those lined with titanium or high nickel alloy. Consequently, these chimneys can be built with slightly smaller internal flues. One supplier of borosilicate linings has developed a special lining pattern that can handle velocities greater than those listed in the EPRI guide.
- Life cycle cost: If carefully designed and executed, all the three options can function with minimum or no maintenance for 25 years. It is recommended that any supplier of these materials should be carefully evaluated for its track record and product quality, so as to avoid the risk of shortened lining life expectancy and unplanned maintenance cost and outages.
The main factor to be considered in material selection for lining of FGD wet stacks is its chemical resistance to high humidity and the acidic operating environment. Below are some of the major testing parameters for checking if the lining material is suitable to achieve the desired performance in such an environment for a service life of up to 30 years.
- Chemical resistance to acid
- Wet stack surface properties
- Thermal shock resistance
- Health and safety properties during installation
- Fire and earthquake resistance
Although major power companies have allowed construction contractors to choose from all three options (borosilicate, titanium and high nickel alloy) for chimney linings, some private companies are specifying only borosilicate, thereby leading to the perception that only one technology option exists. Globally, it is quite common for companies to allow all three, each of which has been proven effective in actual use. Despite recent import restrictions on products originating from select countries, all three options are still valid and none of these can be considered better than the other.