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A fluidized bed combustion system having an output of 35 MW at 80% efficiency when using a coal of heating value 26 MJ/kg with a sulphur content of 3.6% requires a particular limestone to be fed to it at a calcium-sulphur molar ratio of 3.0 so as to limit emissions of SO2 adequately. The limestone used contains 85% CaCO3. The required flow rate of limestone will be

A fluidized bed combustion system having an output of 35 MW at 80% efficiency when using a coal of heating value 26 MJ/kg with a sulphur content of 3.6% requires a particular limestone to be fed to it at a calcium-sulphur molar ratio of 3.0 so as to limit emissions of SO2 adequately. The limestone used contains 85% CaCO3. The required flow rate of limestone will be Correct Answer 2405 kg/h 

Calculation:

Given

Output = 35 MW; η = 80%

⇒ Heat supplied = 35/0.80 = 43.75 MW;

Heating value of coal = 26 MJ/kg

⇒ Mass flow rate of fuel supplied = 43.75/26 = 1.6827 kg/s;

Given coal has sulphur content of 3.6 %

⇒ Mass flow rate of sulphur = 1.6827 × 0.036 = 0.0605 kg/s;

Moles of sulphur = 0.0605/32 = 1.89 × 10-3 kmol/s;

A particular limestone to be fed to it at a calcium-sulphur molar ratio of 3.0

Moles of calcium (CaCO3) required = 3 × 1.89 × 10-3 kmol/s = 5.679 × 10-3 kmol/s;

Mass of CaCO3 required = 0.5679 kg/s;

Mass of limestone required = 0.5679/0.85 = 0.668 kg/s = 2405 kg/hr;

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