Question

1.) A fuel gas containing 95mol% methane and the balance ethane is burned completely in a furnace with 25% excess air (79mol% N2, 21mol% O2). The stack gas leaves the furnace at 900°C and is cooled to 450°C in a waste heat boiler. Assume a pressure of 1atm throughout the process.

(a) Draw a fully labeled flow diagram, and calculate the amount of heat (in kJ) that must be transferred from the gas in the waste heat boiler to achieve the cooling. Use a basis of 100mol of fuel gas. (Use Table B.8 for specific enthalpy values)

The waste heat boiler is a heat exchanger, where the stack gas is cooled via a stream of boiler feedwater at 40°C. This stream of liquid water is converted to steam in the process to be used in other applications.

(b) For the same basis as (a), how much saturated steam at 50 bar can be produced? Assume that the specific enthalpy of feedwater is the same as that of saturated water at the same temperature.

(c) Show that the assumption used in (b) is valid. (Hint: What is the difference between the specific enthalpy of the subcooled feedwater and saturated water at 40°C?)

(d) If a desired flow rate of steam of 1250kg/h is desired for use in other applications, what is the required feed rate of the fuel gas (in kmol/h)? What is the volumetric flow rate (in m3/s) of the gas leaving the boiler?

Answer 1