During combustion in a boiler, which gas is produced as a by-product?

During the combustion process in a boiler, the primary by-product produced is carbon dioxide. This occurs when hydrocarbon fuels (such as natural gas, oil, or coal) burn in the presence of oxygen. The combustion reaction can be simplified to showing that carbon from the fuel combines with oxygen from the air to produce carbon dioxide and heat.

The formation of carbon dioxide is a key indicator of complete combustion, which is essential for maximizing efficiency and minimizing pollutants. If combustion is incomplete, other by-products, such as carbon monoxide, may be formed, but the focus of this question is on the primary by-product of complete combustion.

While other substances like oxygen, hydrogen, and ash are associated with combustion, they do not represent the main gaseous by-product typically measured when analyzing boiler emissions. Oxygen is consumed during the process, hydrogen may be released as water vapor depending on the fuel, and ash is a solid residue from burning certain fuels but is not a gas. Thus, carbon dioxide is the most accurate choice reflecting the outcome of combustion in a boiler.