Alloy C276 and 22 in Flue Gas Desulphurisation Plants
Flue Gas Desulphurisation plants are designed to last the life expectancy of the coal fired power stations. A number of large coal fired power stations have extended operational lives up to 40 years and most energy providers require a life time guarantee for construction materials used to build the FGD plant.
Exhaust gases from the coal fired boiler enter the base of the absorber tower. The gases rise through multiple trays in the absorber tower where they are sprayed with wet calcium carbonate (wet limestone). The acidic elements in the gases are neutralised by reacting with the calcium carbonate. However the process does not completely eliminate 100% of the sulphur dioxide, sulphur trioxide, chlorine, chlorides and fluorides in the exhaust stream. Net effect is that at lower temperatures the remaining acidic gases are formed below the acid dew point in the outlet ductwork and chimney stack. Hence the design of these plants inevitably has to account for extremely high risk of crevice & pitting corrosion in the outlet ducts and stack. As a result of this alloy C276 and alloy 22 are commonly specified as materials for the outlet duct lining, dampers, mist eliminators, spray nozzles and pipe work at the top of the absorber tower. Alloy 22 with 6% more chromium than C276 has been increasingly used in FGD plants where the environment is more oxidizing and contains high levels of free chlorides.
NeoNickel has supplied multiple packages of both alloy C276 and alloy 22 sheet and plate for use in lining wet scrubbing systems in Flue Gas Desulphurisation Systems. The type of coal burned in coal fired gas stations has a major impact on the acid content of the flue gases. C276 and alloy 22 materials are specified extensively in FGD systems particularly those which utilise high efficiency wet limestone processing to remove both sulphur dioxide (SO₂), sulphur trioxide (SO₃), chlorine (Cl₂), chlorides and fluorides from the exhaust gases generated by coal fired power stations. In principle, FGD plants are added to coal fired power stations to eliminate the effect of acid rain. Acid rain is formed when the exhaust gases (mentioned above) enter the atmosphere and combine with water vapour to form either sulphurous, sulphuric or hydrochloric acid. Acid rain has a devastating effect on forestry and the landscape.
Chemical reaction of the formation of acid rain:
SO₂ + H₂O (water vapour/rain) → H₂SO₃ (sulphurous acid/acid rain)
SO₃ + H₂O (water vapour/rain) → H₂SO₄ (sulphuric acid/acid rain)
2Cl₂ + 2H₂0 (water vapour/rain) → 4HCL + O2 (Hydrochloric Acid/acid rain)