5. Flue gas from a sugar mill furnace burning bagasse fibre in air contains 8.35

Question

5. Flue gas from a sugar mill furnace burning bagasse fibre in air contains 8.35% CO2, 15.5% H2O, 0.55% 02 and 75.6%N2. 130kmol/min of the flue gas exits the furnace at 1300°C and 1atm and is used to generate steam in an adiabatic boiler (i.e. well-insulated heat exchanger). The flue gas leaves the boiler at 350°C and 1atm. Saturated liquid water at 75°C enters the adiabatic boiler and is heated by the flue gas to its exit conditions: saturated steam at 4.8bar pressure. Calculate the mass flowrate of water/steam leaving the boiler.

Explanation / Answer

Specific heats

CO2: 1.2 KJ/kg/K = 1.2*44=52.8 J/mol/K

H2O: 2.4 KJ/Kg/K = 2.4*18 = 43.2 J/mol/K

O2: 1.1 KJ/Kg/K= 35.2 J/mol/K

N2: 1.1 KJ/kg/K = 30.8 J/mol/ K

Total heat released in cooling flue gas from 1300 C to 350 C= (8.35*52.8+15.5*43.2+.55*35.2+75.6*30.8)/100*130*(1300-350)=4271025 KJ/min

Enthalpy of water at 75 C saturated = 313 KJ/kg

Enthalpy of steam at 4.8 bar saturated = 2746 KJ/kg

=>4271025=flowrate of water*(2746-313)

=> flowrate of water = 1755 Kg/minute

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