Technical Standards and Safety Authority (TSSA) G3 Practice Exam

What are the major constituents of flue gases after the proper combustion of natural gas in air?

1. CO, H2O, N2, excess air, and heat

2. H2O, N2, primary air, and heat

3. CO2, H2O, N2, O2, and heat

4. O2, CO, N2, H2O, and heat

The correct answer is: CO2, H2O, N2, O2, and heat

The major constituents of flue gases after the proper combustion of natural gas in air primarily include carbon dioxide (CO2), water vapor (H2O), nitrogen (N2), oxygen (O2), and heat. During the combustion process of natural gas, which is primarily composed of methane (CH4), the hydrocarbons react with oxygen from the air to produce carbon dioxide and water, generating heat as a byproduct. Under ideal combustion conditions, the reaction can be expressed as follows: CH4 + 2 O2 → CO2 + 2 H2O + heat In this ideal scenario, nitrogen is also present in the air and doesn't react but is carried along in the flue gas. Excess oxygen could also be present depending on the efficiency of the combustion process, but typically it is in smaller amounts compared to the main products. This composition—primarily CO2 and H2O, along with nitrogen and residual oxygen—correctly represents the expected constituents of flue gases after complete and proper combustion of natural gas. The inclusion of heat indicates that the combustion reaction is exothermic, which is why it is generated as a product. Understanding this process is crucial for optimizing combustion efficiency and controlling emissions in heating systems.