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Qu. A steam power plant has 20% overall efficiency. To generate each unit of electricity 900gm of coal is required. Calculate the calorific value of coal.

Sol.                             CV(Calorific Value) = ?

900gm = 0.9 kg                   

Heat content by 0.9kg of coal = 0.9 x CV

                                    Efficiency  η = output/input= 860kCal/Heat input

                                      0.20 = 860/(0.9 x CV)

CV= 860/(0.9 x 0.2)

CV = 4777.78  kCal/kg

 

 


Qu. A 250MWsteam power plant consumes 160T/hr(tonns per hour) with calorific value of coal is 3300kCal/kg. Calculate the overall efficiency.

Sol.                             Efficiency  η = ?

CV(Calorific value) = 3300kCal/kg

Coal flow Q = 160T/hr

Plant capacity MW = 250              

Heat content by 0.9kg of coal = 0.9 x CV

                                    UHR(unit heat rate) = (Q x CV)/MW

                                                 = (160 x 3300)/250

                                                 = 2112 kCal/kWh

Efficiency  η = output/input = 860kCal/unit heat rate

             η = 860/2112 = 0.40719

      η = 40.719%

 

 


Qu. Calculate the amount of coal required in TPH(Tonns per hour) for a 600MW, 37.5% efficient thermal power plant if the calorific value of coal is 3150kCal/kg.

Sol.                             CV(Calorific value) = 3150kCal/kg

Coal flow Q = ?

Efficiency  η = 37.5% = 0.375

Plant capacity = 600 MW  

                                    UHR(unit heat rate)= 860/η

                        UHR = 860/0.375 =2293.33 kCal/kWh

                                    UHR = (Q x CV)/MW

                       Q = (MW x UHR)/CV

                                    Q = (600 x 2293.33)/3150 = 436.8 T/hr

 

 


Qu. Calculate the GCV(gross calorific value) of coal for which the following are the percentage obtained by ultimate analysis.

Moisture       18%

Ash                 8%

Carbon           59%

Oxygen           8.4%

Hydrogen      3.7%

Nitrogen        1.2%

Sulphur         1.7%

Sol.                        GCV is calculated by Dulong’s formula :

                        GCV = (33820 x C) + 144000( H – O/8) + (9.204 x S)

In the above formula, we can see that we need only percentage of C, H, O and S to calculate the GCV.

Now put the values in the Dulong’s formula,

                        GCV = (33820 x 59) + 144000 x (3.7 – (8.4/8)) + (9.204 x 1.7)

                                 = 2377 kJ/kg

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