Fuels
Combustion describes the exothermic reaction between a fuel and an oxidizer, most often oxygen from air. Normally, the reactive equilibrium lies almost completely on the product side (∼99.99%,), so that for computations it can be assumed that all fuel is consumed, as long as enough oxygen is present.
However, by the Le Chatelier principle, the reaction will be less complete when the product temperature is high: in very hot combustion one might have to account for the law of mass action.
Many fuels are hydrocarbons, CxHy , of different compositions. Of these, the best known are methane (natural gas) CH4, propane C3H8, n-octane C8H18, n-dodecane C12H26, and hydrogen H2. Octane is the main ingredient in gasoline which is a mixture of various hydrocarbons. Similarly, Diesel fuel is a mixture of heavier hydrocarbons such as dodecane. Other fuels contain additional elements as well, e.g. methyl alcohol CH3OH, ethyl alcohol C2H5OH, or coal which is mainly carbon (C) and other elements (S, O, H, N, ash) in varying amounts.
The basic reactions occurring are the formation of water and carbon dioxide
Carbon monoxide is an odorless and highly toxic gas that is a fuel itself (CO + 1 O2 CO2). Combustion processes take place in many reaction steps, which involve the splitting of larger molecules into smaller units, and the formation of new species. The formation of H2O and CO is relatively fast, while the formation of CO2 is slow, hence CO is formed first, and then reacts to CO2 later.