Gas properties, combustion and utilisation
What this domain tests
Expect calculations and explanations linking gas composition to calorific value, air requirement, flame behaviour and appliance performance. You should be able to move between gross and net calorific value, stoichiometry, excess air and Wobbe index.
Key properties
- Town gas in Hong Kong is rich in hydrogen (roughly half by volume), with methane, carbon dioxide and small amounts of carbon monoxide. Its gross calorific value is around 17-18 MJ/m3 and its relative density is well below air.
- LPG is supplied as a propane/butane mixture. Propane has a gross calorific value near 95.7 MJ/m3 of vapour and butane near 124-126 MJ/m3; LPG vapour is heavier than air.
- Gross calorific value includes latent heat recovery in the products; net calorific value does not. Condensing appliances can use the difference, but appliance ratings are usually quoted on gross CV in Hong Kong.
- Wobbe index = calorific value divided by the square root of relative density. It predicts interchangeable heat input at a fixed injector pressure. A gas with a different Wobbe index will not burn correctly without injector changes.
Combustion and excess air
Stoichiometric methane combustion, CH4 + 2O2 -> CO2 + 2H2O, needs 2/0.21 = 9.52 m3 of air per m3 of methane. Propane, C3H8 + 5O2 -> 3CO2 + 4H2O, needs 5/0.21 = 23.8 m3 of air per m3 of propane. Primary aeration controls flame stability; too little air gives yellow tipping and carbon monoxide, too much gives flame lift and lower efficiency.
Worked example
A flue-gas analyser reads 6% dry oxygen on a natural-gas appliance. Excess air ratio = 21/(21 - 6) = 1.4, so excess air is 40%. If the appliance is rated at 100 kW gross and the measured gross efficiency is 82%, input = 100/0.82 = 122 kW and stack loss is about 22 kW. Reducing excess air to 20% would lower stack loss, but only if CO remains low and the flame remains stable.
Traps that cost marks
- Mixing gross and net CV in one efficiency calculation.
- Calculating air requirement from mass instead of volume without converting correctly.
- Assuming a gas with a similar calorific value is interchangeable while ignoring Wobbe index and density.
- Forgetting that LPG vapour is heavier than air and town gas is lighter.
- Treating CO as an efficiency issue only; it is first an incomplete-combustion and safety issue.