Carbon formation from methane in combustion products

Abstract

The rates of deposition of carbon on alumina surfaces and on soot particles, have been measured in a pilot scale tubular reactor in which cold methane was mixed with combustion products at 1920°K. A hard grey metallic film of carbon, quite free of soot, was deposited on alumina surfaces for initial methane concentrations between 12 and 24 per cent. An induction period of slow growth rate, before a film covered the surface completely, was followed by a constant growth rate. Measured growth rates were from 0·06 × 10 −6 to 1·43 × 10 −6 g/cm 2 sec of carbon on alumina at 1270°K to 1450°K, and from 0·1 × 10 −4 to 1·14 × 10 −4 g/cm 2 sec on soot particles at 1370°K to 1700°K. Methane decomposition rates were much higher than predicted by the unimolecular mechanism indicating a predominance of radical reactions. Carbon deposition rates were related to the mole fraction, χ, of hydrocarbons in the gas which bear more than three carbon atoms per molecule, by, m ̇ f = 1·0 × 10 2 n.χ . exp (−42,300/ RT f ), g/cm 2sec for carbon film, m ̇ s = 4·6 × 10 3 nχ exp (− 46,100/ RT g ), g/cm 2 sec for soot. A precoat of soot increased the growth rate of film carbon by 1·8 to 7·8 times yielding a hard adherent dull brown film