If perfect combustion occurs in a natural gas furnace, what percentage of CO2 will be present in the flue gases?

When natural gas undergoes perfect combustion, it is converted into carbon dioxide (CO2) and water (H2O) with minimal or no production of carbon monoxide (CO) or other harmful byproducts. In such an ideal situation, the composition of the flue gases will typically contain a significant amount of carbon dioxide because natural gas is primarily made up of methane (CH4).

The percentage of CO2 that results from the complete combustion of natural gas is generally around 6% by volume when conditions are optimal, such as maintaining the correct fuel-to-air ratio and achieving thorough mixing in the combustion chamber. This aligns with the choice indicating that 2% to 5.5% of CO2 would be present; however, in a perfectly functioning system, it can approach higher values, often close to or exceeding this range.

If a furnace is optimized for perfect combustion, it will indeed yield CO2 concentrations that can reasonably fall between 2% to 5.5%. This highlights the efficiency of the furnace under ideal conditions. In practice, while some systems might lower the percentage through various inefficiencies, the ideal combustion scenario supports the notion that the combustion products will yield sufficient CO2 within that specified range.