What gas is primarily produced during combustion in a boiler?

The primary gas produced during combustion in a boiler is carbon dioxide (CO2). Combustion is the chemical reaction that occurs when a fuel ignites in the presence of oxygen, resulting in the release of energy in the form of heat. During this process, carbon in the fuel reacts with oxygen from the air to form carbon dioxide. This is the most common outcome for complete combustion, where there is enough oxygen present to fully oxidize the fuel.

While carbon monoxide can also be produced during incomplete combustion—when there is insufficient oxygen—this is not the primary product of well-functioning boilers operating under ideal conditions. Similarly, oxygen, being a reactant in the combustion process, is consumed rather than produced. Lastly, nitrogen is present in the air used for combustion, but it does not participate in the chemical reactions that produce energy and is not a product of the combustion itself. Therefore, carbon dioxide is the correct response, as it reflects the typical outcome of efficient combustion in a boiler.