CCN1 Combustion Principles & Flue Systems 1 — Questions and Answers
Question 1: What are the three requirements for complete combustion of natural gas?
- Sufficient fuel, sufficient oxygen (air), and adequate temperature (ignition temperature maintained) (Correct answer)
- Fuel, spark ignition, and water vapour
- Fuel, nitrogen, and a catalyst
- Fuel, flue gas, and a pilot light
Correct answer: Sufficient fuel, sufficient oxygen (air), and adequate temperature (ignition temperature maintained)
Complete combustion requires three elements: sufficient fuel (gas), sufficient oxygen from the air supply, and maintenance of a temperature above the ignition threshold. Deficiency in any of these results in incomplete combustion.
Question 2: What is the primary product of complete combustion of natural gas?
- Carbon dioxide (CO2) and water vapour (H2O) (Correct answer)
- Carbon monoxide (CO) and soot
- Nitrogen dioxide (NO2) and steam
- Hydrogen sulphide (H2S) and water
Correct answer: Carbon dioxide (CO2) and water vapour (H2O)
Complete combustion of natural gas (methane, CH4) produces carbon dioxide and water vapour as the primary products: CH4 + 2O2 → CO2 + 2H2O. These are not immediately toxic in normal concentrations.
Question 3: Why is carbon monoxide (CO) produced during incomplete combustion?
- Insufficient oxygen supply causes partial oxidation of carbon in the fuel, producing CO instead of CO2 (Correct answer)
- Carbon monoxide is always produced in equal amounts to CO2
- CO is produced by the pilot light flame only
- CO is a normal by-product of all gas combustion processes
Correct answer: Insufficient oxygen supply causes partial oxidation of carbon in the fuel, producing CO instead of CO2
When there is insufficient oxygen for complete combustion, carbon atoms are only partially oxidised, producing carbon monoxide (CO) rather than carbon dioxide (CO2). CO is highly toxic even at low concentrations.
Question 4: What is the Wobbe Index and why is it significant for gas appliances?
- A measure of the interchangeability of gases — appliances are calibrated for gas with a specific Wobbe Index, and using gas outside this range causes inefficient combustion (Correct answer)
- A measure of gas pressure at the meter
- The calorific value of natural gas in kJ/m3
- The maximum safe gas consumption of an appliance
Correct answer: A measure of the interchangeability of gases — appliances are calibrated for gas with a specific Wobbe Index, and using gas outside this range causes inefficient combustion
The Wobbe Index quantifies the interchangeability of gas types. Appliances are designed for gas within a specific Wobbe Index range. Using gas outside this range disrupts the air-to-gas ratio, causing incomplete combustion or appliance malfunction.
Question 5: What is the stoichiometric air-to-gas ratio for natural gas combustion?
- Approximately 10 parts air to 1 part gas by volume (Correct answer)
- 1 part air to 1 part gas
- 20 parts air to 1 part gas
- 5 parts air to 1 part gas
Correct answer: Approximately 10 parts air to 1 part gas by volume
The stoichiometric (theoretically perfect) combustion of natural gas requires approximately 10 volumes of air per volume of gas. In practice, a small excess of air is provided to ensure complete combustion.
Question 6: What is 'vitiation' of the air supply in the context of gas appliance operation?
- Reduction of available oxygen in the room air due to combustion or inadequate ventilation, leading to incomplete combustion and CO production (Correct answer)
- Contamination of the gas supply with air
- A type of flue blockage causing back-draughting
- Vibration of the burner caused by incorrect gas pressure
Correct answer: Reduction of available oxygen in the room air due to combustion or inadequate ventilation, leading to incomplete combustion and CO production
Vitiation occurs when combustion or poor ventilation reduces the oxygen content of room air below the level required for complete combustion. The appliance then produces increasing amounts of CO as oxygen depletes.
CCN1 Combustion Principles & Flue Systems 1
What are the three requirements for complete combustion of natural gas?