1. A fuel gas has the following volumetric composition:
CO2 4%
CO 6%
CH4 30%
H2 50%
O2 2%
N2 8%
It is burned to produce a flue gas which contains on a dry basis by
volume:
CO2 8%
O2 5.4%
N2 86.6%
Calculate:
(i) the theoretical air supply
(ii) the actual air supply
(iii) the % excess air.
2. A fuel consists of carbon and hydrogen only. When completely burned in air the flue gas on a dry basis has the following analysis:
CO2 12%
O2 5%
N2 83%
The temperature of the flue gases produced is 1600°C and these are fed to a boiler used to raise steam. 15% of the heat available at 1600°C is lost to the surroundings and the flue gases leave the boiler at 300°C.
Calculate:
(i) the amount of air supplied
(ii) the composition of the fuel
(iii) the amount of steam raised per hour if the enthalpy change from water feed to steam product is 2490 kJ kg-1 and the flow of wet flue
gas = 100 kmol h-1.
3. A power plant burns 100% methane (CH4), supplied at 25°C and 1 atmosphere pressure. 20% excess air is supplied at the same conditions.
(a) Write a balanced equation for the reaction.
(b) Calculate the heat of combustion of methane at 25°C and 1 atmosphere from the data given below.
(c) Assuming complete combustion of the methane to carbon dioxide and water and no heat loss to the surroundings, confirm that the
theoretical flame temperature will be approximately 2070 K.
(d) State, without calculation, the effect on flame temperature of using:
(i) less excess air
(ii) more excess air.
Data:
Standard heat of formation of CH4 (g) = -74 900 kJ kmol-1
Standard heat of formation of H2O(g) = -242 100 kJ kmol-1
Standard heat of formation of CO2 (g) = -393 000 kJ kmol-1
For enthalpy values use the table in lesson MAEB - 4 - 3.
4. In the production of chlorosilanes, chloromethane gas (CH3Cl) is produced as a by-product. This needs to be removed and treated to prevent potential hazardous material entering the atmosphere. This is achieved by continuously dissolving and reacting it with the
stoichiometric quantity of potassium hydroxide (KOH) solution in a continuous scrubbing tower according to the reaction:
CH3Cl(g) + KOH(aq) → KCl(aq) + CH3OH(l)3 (g) ?Hr° = - 167.7 kJ mol-1
It is important that the reaction temperature is maintained below 22°C to ensure maximum removal of the chloromethane. Cooling water at 10°C is available and the maximum rise in water temperature should be 5°C.
Calculate:
(i) the amount of KOH solution required per hour
(ii) the adiabatic temperature of the reaction (no cooling is used)
(iii) the minimum amount of cooling water required to maintain the temperature at 22°C, if no heat is lost to the surroundings.