Treatment of Flue Gas Desulphurisation Effluent with Freeze Crystallization

Kusile was the first power station of Eskom to implement a wet flue gas desulfurization system for the control of SO2 and other greenhouse gasses.The waste water from the flue gas desulfurization (FGD) plant contained a high total dissolved solids (TDS) concentration of 50 000 mg/L due to: (i) absorption of SO2, chloride and boron from the coal to the FGD waste water, (ii) dissolution of sodium, potassium, magnesium, manganese and calcium form the limestone when reacted with the acid gasses from the combustion chamber.A large portion of the calcium sulphate in solution precipitated as gypsum due to its limited solubility.The study focused on disposal, electrical and chemical cost of three FGD waste water management methods.It was found that the cost for a 4 800MW plant will amount to R9.7 million per month for disposal at a toxic waste disposal site, R11.41 million per month when pre-treated with lime/sodium carbonate followed by evaporation and R0.96 million per month when treated with Freeze crystallization.Freeze crystallization was found to be the most cost effective as it did not need chemical pre-treatment and it consumes less energy than evaporation (330kJ/kg in the case of freeze versus 2 260kJ/kg in the case of evaporation).Energy usage was found to be 100 kWh/kg ice.The TDS of the Feed amounted to 50 000 mg/L compared to less than 3 000 mg/L in the melted ice.