The desired gas temperature exiting the flame mixing chamber and entering the reaction chamber of a thermal incinerator is 750oC. The polluted airflow rate is 500 m3/min at 77oC. The fuel gas (methane) is available at 20oC and the burner draws in supplemental air at 25oC at the rate of 14 kg air/kg fuel gas.

a) If heat losses amount to 10% of the heat generation, calculate the required flow rate of methane. The heat of combustion of methane is 48,000 kJ/kg. Assume that the contaminants in the polluted air stream have negligible heating value.   

b) A catalytic incinerator is investigated to replace the thermal incinerator. Because of the lower required temperature, the fuel gas flow rate drops to 0.7 kg/min. Calculate the temperature of the gas at the entrance of the reaction chamber for the catalytic incinerator. Also, estimate the energy saving, in MJ/day, due to reduced fuel use. Assume 24 hr/day operation.

specific heat capacity

1.0 kJ/kg.K (polluted air; supplemental air)

2.2 kJ/kg.K (methane)

density of air

1.2 kg/m3 (at 25oC)

assumed linearly proportional to temperature (via the ideal gas law)