An incident occurred in October 2022 when a section of the Kusile Unit 1 flue gas desulphurisation (FGD) duct collapsed, which led to the shutdown of three units - Unit 1, Unit 2, and Unit 3 - due to shared chimney facilities. As a result, the Department of Environment, Forestry and Fisheries (DEFF) granted Eskom an exemption to temporarily bypass pollution regulations at Kusile,allowing the station to operate without desulphurisation from November 2023 until the permanent stacks are repaired and restored in 2024. This exemption granted the station a year-long waiver from desulphurisation. Each unit operates at a capacity of 800 MW. Assuming the Flue Gas Desulphurisation (FGD) system operates with 90% SO₂ removal efficiency using limestone (CaCO₃) as the sorbent with a stoichiometric ratio of 1.2:1 (CaCO₃:SO₂). The slurry in the wet FGD system contains 10% solids by weight, and the slurry density is 1000 kg/m³. The coal has a heating value of 24 MJ/kg, and the plant operates at 45% efficiency. The FGD system produces gypsum (CaSO₄·2H₂O) as a byproduct, which can be sold for construction purposes. • Limestone (CaCO₃) reaction: CaCO₃ + SO₂ + ½O₂ → CaSO₄ + CO₂ • CaCO₃ + SO₂ + ½O₂ + 2H₂O → CaSO₄·2H₂O + CO₂ • Molecular weights: CaCO₃ = 100 g/mol, SO₂ = 64 g/mol, S: 32 g/mol, CaSO₄·2H₂O = 172g/mol. a) Calculate the coal consumption rate of each unit (in tonnes/hour) anand determine the uncontrolled SO₂ emission rate (in tonnes/hour) during the time of exemption if low-grade (3.5% sulphur) coal was supplied. b) Estimate the flue gas flow rate (in m³/hour), and the SO₂ concentration in the flue gas (in ppm) before FGD. Assuming 1 kg of coal produces 10 m³ of flue gas at standard conditions (25°C, 1 atm) and ideal gas behaviour (1 mole of gas occupies 24.45 L at 25°C, 1 atm). c) Determine the volume of slurry (in m³/hour) required to achieve 90% SO₂ removal and the amount of gypsum produced (in tonnes/hour) from the FGD reaction. d) Estimate the annual cost of limestone (in Rand) if the plant operates 8500 hours/year, and limestone costs R400/tonne. e) Critically evaluate the use of a wet FGD system for Kusile, considering the 3% sulphur content and potential operational challenges (e.g., scaling, corrosion). Suggest one alternative SO₂ control strategy and justify its feasibility. f) If high-grade (1% sulphur) coal was used, estimate the reduction in the uncontrolled flue gas emissions in % and reduced annual limestone cost in R/year.
