2. Biogas is produced through anaerobic digestion of waste food. It has a

composition of 80% methane and 20% carbon dioxide (by volume). The

biogas is burned stoichiometrically in air (of composition 21% oxygen and

79% nitrogen by volume) with the flame used to heat a tube containing flowing

water. The biogas has an initial temperature of 25°C and the combustion

gases exit from the burner at 102°C.

a) Write a stoichiometric equation for the combustion of methane in air, and

then adapt it for burning the biogas in air, assuming products are carbon

dioxide, water, nitrogen only.

[5 marks]

b) Calculate the mass flowrate of air for the complete combustion of 1 kg/s of

biogas. Molecular weights of appropriate elements and compounds can be

found on page 9.

[5 marks]

c) From considering the combustion of the biogas, show that an energy

transfer takes place of 35.5 kJ per kg of biogas that is burned.

[5 marks]

d) After heating, in the burner, the hot water passes through a shell and tube

heat exchanger and is used to heat an oil flow. The water from the shell

and tube heat exchanger passes directly back to the burner at 20°C.

The flowrate of biogas into the combustor is 50 kg/s.

Water enters the burner with a flow rate of 7kg/s at a temperature of 20°C.

Oil enters the shell and tube heat exchanger at a flow rate of 37.5 kg/s at a

temperature of -5°C.

i. Sketch the arrangement

ii.

Calculate the surface area of the heat exchanger, given an overall

heat transfer coefficient of 750 W/m² K

Take heat capacities as water 4.2 kJ/kgK oil 1.6 kJ/kgk

[10 marks]

Ref: ME20214G74-2