HEAT TRANSFER AND COMBUSTION

You will need graph paper, a calculator, a copy of Appendix 1 from lesson HTC - 4 - 2 and access to steam tables to answer this TMA.

1. A fuel gas consists of 75% butane (C₄H₁₀), 10% propane (C₃H₈) and 15% butene (C₄H₈) by volume.

It is to be fed to the combustion chamber in 10% excess air at 25ºC, where it is completely burnt to carbon dioxide and water. The flue gases produced are to be used to generate 5 bar steam from water at 90ºC.

With the aid of the data at the end of the question, steam tables and the enthalpy table given in the Appendix of lesson HTC - 4 - 2:

(a) Write balanced equations for the combustion of each component of the fuel gas.

(b) Explain the need for excess air.

(c) Determine the actual fuel:air ratio

(i) by volume
(ii) by mass.

(d) Calculate:

(i) the net calorific value (CV) per m³ of the fuel/air mix at 25ºC
(ii) the net calorific value (CV) per kmol of the 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 dry basis.

(f) Determine the maximum flame temperature.

(g) State how varying the amount of excess air may affect the flame temperature.

(h) Determine the ‘furnace efficiency' if the flue gases leave the boiler at 300ºC.

(i) If 5% of the heat available for steam production is lost to the atmosphere, determine the amount of steam raised per hour when the total flow of flue gases is 1400 kmol h-1.

(j) Determine the dew point temperature assuming that the flue gas 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 the inlet air.

(k) If the flue gases exiting the boiler are used to preheat the water fed to the boiler from a temperature of 28ºC to 90ºC and assuming:
- a mean specific heat capacity for water over this temperature range to be 4.2 kJ kg^-1 K^-1
- a mean molar heat capacity 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 heat exchanger
- all water entering the system is converted to steam

determine the final outlet temperature of the flue gas and state if the 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 and one disadvantage.

(m) Give two reasons why the presence of any sulphur in the fuel mix 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 and 76.7% nitrogen by mass.
Atomic mass of C = 12, O = 16, N=14 and H = 1.