Steam Generators take heat produced by combustion reactions to usable electrical
ID: 718189 • Letter: S
Question
Steam Generators take heat produced by combustion reactions to usable electrical energy; however, there are inefficiencies between the conversion of heat energy to shaft work energy and between the conversion of shaft energy to electrical energy.
If a powerplant needs produce 500 GJ/day of electrical energy, what is the flowrate of coal necessary if the heating value (energy released from combustion) of coal is 33412 kJ/kg? The combustion occurs at 2000 K, the efficiency of the conversion of shaft work to electrical energy is 35%, and the cold heat reservoir operates at 500 K.
Explanation / Answer
Two steps are followed
Step 1
Chemical reaction produces H = 33412 kJ/kg coal
Step 2
Electrical energy produced = 500 GJ/day x 10^6 kJ/GJ
= 5*10^8 kJ/day
For the step 2
Efficiency = 35%
Energy input at step 2 = 5*10^8/0.35
E = 1428 x 10^9 kJ/day
Mass flow of coal required = E/H
= (1428 x 10^9 kJ/day) / (33412 kJ/kg coal)
= 42756 kg/day