Combustibles,fuels and their combustion products: A view through carbon isotopes

Stable (i.e. non-radioactive) carbon-isotope composition (δ¹³C) in fuels has been extensively used as an indicator of the processes leading to the generation of their parent crude-oil. With the example of those used in Paris (France), this preliminary study isotopically characterizes fuels and combustibles, as well as the isotopic relations existing with their combustion by-products, i.e. gases (CO₂) and particles (bulk carbon). Results show that δ¹³C in fuels is clearly related to their physical state, with natural gas being strongly depleted in ¹³C while coal yields the highest δ¹³C, and liquid fuels display intermediate values. This relation is also valid for combustion gases, although δ¹³C values of combustion particles form a homogeneous range within which no clear distinction is observed. Combustion processes are accompanied by carbon-isotope fractionation (noted Δ¹³C) resulting from the combustion being incomplete. Carbon-isotope fractionation is strictly negative (Δ¹³C = ?1.3‰) during the formation of combustion gases, but generally positive in particle formation even if values close to zero are observed. Using simple mixing equations for describing the closed system formed by fuel, CO₂ and carbonaceous particles, we discuss the carbon budget for spark-ignition (unleaded gasoline) and diesel engines. Stable carbon isotopes corroborate the already-proved superior efficiency of diesel combustion mode compared with spark ignition, as carbon is preferentially transformed into CO₂.