A steam power cycle is connected to a furnace burning bagasse from a sugar milland is producing 200 MW of power from its turbine. The cycle has a thermal efficiencyof 30 %. Heat is rejected from the cycle to a condenser which is cooled with water froma river. For the purpose of this exercise, ignore the work associated with the pump.i. Draw an overall diagram showing the major energy flows.ii. What is the heat input for this cycle?iii. How much heat is rejected through heat transfer to the river water?iv. Bagasse has a HHV of 17 MJ/kg. If 90% of the heat from combustion istransferred to the boiler, what rate of bagasse needs to be burned (in kg/s)?
Question
A steam power cycle is connected to a furnace burning bagasse from a sugar milland is producing 200 MW of power from its turbine. The cycle has a thermal efficiencyof 30 %. Heat is rejected from the cycle to a condenser which is cooled with water froma river. For the purpose of this exercise, ignore the work associated with the pump.i. Draw an overall diagram showing the major energy flows.ii. What is the heat input for this cycle?iii. How much heat is rejected through heat transfer to the river water?iv. Bagasse has a HHV of 17 MJ/kg. If 90% of the heat from combustion istransferred to the boiler, what rate of bagasse needs to be burned (in kg/s)?
Solution
i. The overall diagram would show the following major energy flows:
- Bagasse from the sugar mill is burned in the furnace.
- The heat from the furnace is transferred to the boiler, which produces steam.
- The steam drives the turbine, producing 200 MW of power.
- The steam is then condensed and the heat is rejected to the river water.
- The condensed water is then returned to the boiler, completing the cycle.
ii. The heat input for this cycle can be calculated using the formula for thermal efficiency, which is the ratio of the work output to the heat input. Rearranging the formula gives us:
Heat input = Work output / Thermal efficiency Heat input = 200 MW / 0.30 = 666.67 MW
iii. The heat rejected through heat transfer to the river water can be calculated by subtracting the work output from the heat input:
Heat rejected = Heat input - Work output Heat rejected = 666.67 MW - 200 MW = 466.67 MW
iv. The rate of bagasse that needs to be burned can be calculated by dividing the heat input by the heat content of bagasse and the efficiency of the boiler:
Rate of bagasse burned = Heat input / (HHV of bagasse * Efficiency of boiler) Rate of bagasse burned = 666.67 MW / (17 MJ/kg * 0.90) = 43.63 kg/s
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