CCN1 Combustion Principles & Flue Systems

Question 1

What are the three requirements for complete combustion of natural gas?

Accepted Answer

Sufficient fuel, sufficient oxygen (air), and adequate temperature (ignition temperature maintained)

Answer

Fuel, spark ignition, and water vapour

Answer

Fuel, nitrogen, and a catalyst

Answer

Fuel, flue gas, and a pilot light

Question 2

What is the primary product of complete combustion of natural gas?

Accepted Answer

Carbon dioxide (CO2) and water vapour (H2O)

Answer

Carbon monoxide (CO) and soot

Answer

Nitrogen dioxide (NO2) and steam

Answer

Hydrogen sulphide (H2S) and water

Question 3

Why is carbon monoxide (CO) produced during incomplete combustion?

Accepted Answer

Insufficient oxygen supply causes partial oxidation of carbon in the fuel, producing CO instead of CO2

Answer

Carbon monoxide is always produced in equal amounts to CO2

Answer

CO is produced by the pilot light flame only

Answer

CO is a normal by-product of all gas combustion processes

Question 4

What is the Wobbe Index and why is it significant for gas appliances?

Accepted 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

Answer

A measure of gas pressure at the meter

Answer

The calorific value of natural gas in kJ/m3

Answer

The maximum safe gas consumption of an appliance

Question 5

What is the stoichiometric air-to-gas ratio for natural gas combustion?

Accepted Answer

Approximately 10 parts air to 1 part gas by volume

Answer

1 part air to 1 part gas

Answer

20 parts air to 1 part gas

Answer

5 parts air to 1 part gas

Question 6

What is 'vitiation' of the air supply in the context of gas appliance operation?

Accepted Answer

Reduction of available oxygen in the room air due to combustion or inadequate ventilation, leading to incomplete combustion and CO production

Answer

Contamination of the gas supply with air

Answer

A type of flue blockage causing back-draughting

Answer

Vibration of the burner caused by incorrect gas pressure