Why some fuels burn completely in air and others do not?

Some fuels burn completely in air due to their chemical composition and the availability of oxygen for combustion. Fuels with a higher hydrogen-to-carbon ratio, such as natural gas, tend to burn more completely because they produce more water vapor and less soot. Conversely, fuels with complex structures or higher carbon content, like coal or heavy oils, may not burn completely, leading to incomplete combustion and the production of carbon monoxide, soot, and other pollutants. Additionally, factors such as temperature, pressure, and mixing efficiency with air also influence the completeness of combustion.