Combustion is the exothermic chemical reaction (a reaction in which heat is given off) of hydrogen and carbon atoms contained in fuels with oxygen. In this reaction, the carbon is oxidized to form carbon dioxide (CO2) or, if insufficient oxygen is available, carbon monoxide (CO). If there is an excess of oxygen present, non-reacted O2 will be in the products of combustion. Excess O2 makes heating inefficient, thus requiring more gas for the same results. In addition, excess air also allows for the formation of pollutants such as Nitrous Oxide (NO) and Nitrogen Dioxide (NO2). Together, NO and NO2 make up what is referred to as NOx.

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