Soot formation in ethylene and propane diffusion flames

Abstract

Abstract Small samples of gas have been withdrawn from the luminous regions of the diffusion flames of ethylene and propane burning on a Wolfhard Parker burner and these have been analysed by gas chromatography. In general, the parent hydrocarbon is rapidly decomposed on approaching and within the luminous region, acetylene being the major hydrocarbon product found. The concentration of acetylene falls rapidly near the interface with the oxidation zone suggesting that the main hydrocarbon oxidised in the latter is acetylene. The rates of soot collection, the uncorrected temperatures in the flames and the concentration of stable hydrocarbon species have also been measured when oxygen, hydrogen and acetylene respectively were added to the fuel stream. Sooting rates and uncorrected flame temperatures have also been measured when the ‘oxygen index’ of the ‘air’ supply was varied, and it has been found that the sooting rate for both fuels increases with an increase in ‘oxygen index,’ the rate of increase falling off, however, at higher values. Oxygen, when introduced into the fuel, leads to increased temperatures and sooting rates in the case of ethylene and to increased temperatures but decreased sooting rates in the case of propane. The sooting rates of flames of ethylene with acetylene or oxygen as additives increase linearly with the area under the acetylene concentration profile in the luminous zone (measured at a height of ∼ 5 mm above the burner). This area is proportional to the amount of acetylene present in the luminous zone at this height, which corresponds roughly with the onset of luminosity. Acetylene thus appears to be the major stable species via which soot is formed in such flames. Propane flames with oxygen as additive to the fuel, however, behave differently in that although there is an increase in temperature and in the amount of acetylene in the luminous zone the sooting rate decreases. This discrepancy remains unexplained and requires further investigation.