A fuel gas consists of 75% butane (C4H10), 10% propane (C3H8)and 15% butene (C4H8) by volume. It is to be fed to the combustionchamber in 10% excess air at 25ºC, where it is completely burnt tocarbon dioxide and water. The flue gases produced are to be used togenerate 5 bar steam from water at 90ºC.
(a) Write balanced equations for the combustion of eachcomponent of the fuel gas.
(b) Explain the need for excess air.
(c) Determine the actual fuel:air ratio (i) by volume (ii) bymass.
(d) Calculate: (i) the net calorific value (CV) per m3 of thefuel/air mix at 25ºC (ii) the net calorific value (CV) per kmol ofthe fuel/air mix at 25ºC.
(e) Determine the composition of the flue gases by volume(assuming the inlet air is dry): (i) on a wet basis (ii) on a drybasis.
(f) Determine the maximum flame temperature.
(g) State how varying the amount of excess air may affect theflame temperature.
(h) Determine the ‘furnace efficiency’ if the flue gases leavethe boiler at 300ºC.
(i) If 5% of the heat available for steam production is lost tothe atmosphere, determine the amount of steam raised per hour whenthe total flow of flue gases is 1400 kmol h-1.
(j) Determine the dew point temperature assuming that the fluegas pressure is 1.00 bar and the inlet air: (i) is dry (ii)contains 0.8 kg water per kmol of air at the temperature of theinlet air.
(k) If the flue gases exiting the boiler are used to preheat thewater fed to the boiler from a temperature of 28ºC to 90ºC andassuming: – a mean specific heat capacity for water over thistemperature range to be 4.2 kJ kg-1 K-1 – a mean molar heatcapacity for the flue gases up to 300ºC to be 31 kJ kmol-1 K-1 -10% of the heat required to heat the water is lost in the heatexchanger – all water entering the system is converted to steamdetermine the final outlet temperature of the flue gas and state ifthe dew point will be reached in both of the cases given in part(j).
(l) Give two advantages of preheating the water in this way andone disadvantage.
(m) Give two reasons why the presence of any sulphur in the fuelmix would be undesirable.
Data: Net calorific value (MJ m-3) at 25ºC of:
Butane (C4H10) = 111.7 MJ m-3
Butene (C4H8) = 105.2 MJ m-3
Propane (C3H8) = 85.8 MJ m-3
Air is 21% oxygen, 79% nitrogen by volume and 23.3% oxygen and76.7% nitrogen by mass.
Atomic mass of C = 12, O = 16, N=14 and H = 1.
. . .