Liquid flame: combustion of metal suspensions in liquid sulfur

Thermodynamic calculations show that some metals can react with sulfur without the formation of gaseous products at normal pressure and yet demonstrate sufficiently high flame temperatures to support the propagation of stable flames. For example, a stoichiometric ternary mixture of iron, manganese, and sulfur demonstrates gasless combustion at an equimolar concentration of iron and of manganese with an adiabatic flame temperature of about 2000 °C. Differential thermal analysis of the mixture shows no exothermic reactions below 280 °C. Therefore, sulfur in the mixture can be safely melted (m.p. 119 °C), converting a powder blend into a liquid suspension that is free from gas bubbles. Symmetrical cylindrical flames in shallow pools of suspensions of Fe and Mn powders in liquid sulfur and combustion of the same liquid mixtures in preheated narrow steel tubes have been studied to determine flame propagation speeds as a function of mixture composition. It was found that, contrary to the behavior of the calculated flame temperature, flame speed decreases with the increase of the manganese content in the mixture and is not affected by mixture dilution with the combustion product. Direct measurements of the flame temperatures by thermocouples indicated a weak dependence of the peak flame temperature on mixture composition and revealed a two-stage flame structure. The existence of the two distinct reaction zones in the mixture of two reactive metals with sulfur is in accordance with qualitative theoretical predictions by the theory of flame with parallel reactions existing in the literature. According to theory, the reaction with the higher flame speed in a corresponding binary single-metal–sulfur mixture will form the leading stage of the complex flame front and will govern the flame propagation speed in the ternary composition. The speed of flame propagation in pure Fe–S mixture is almost three times higher than the flame speed in Mn–S mixture. As a result, the iron–sulfur reaction dominates the flame propagation mechanism in Fe–Mn–S suspension.