Molecular-Beam Sampling Study of Extinguishment of Methane-Air Flames by Dry Chemicals

Abstract

Flame inhibition by dry-chemical powders was studied by adding various powders to a diffusion flame and measuring the composition and temperature profiles using a molecular-beam mass-spectrometer sampling system. The diffusion flame was a methane/oxygen flame held near the stagnation plane of an opposed-jet burner. The downward-flowing jet contained a mixture of air and powder, the upward-flowing jet a mixture of methane and nitrogen. Powders of Al₂O₃, NaHCO₃ K.HCO₃, NH₄H₂PO₄ and KCI were used. Particle diameters were in the range from 38 to 43 μm. In order to obtain significant inhibition without extinguishing the flame, powder feeding rates of 3 mg/litergas were used for NaHCO₃, KHCO₃and NH₄H₂PO₄; 2 mg/litre gas for KCI. Arbitrarily, a feeding rate of 2 mg/litergas was used also for Al₂O₃. Concentrations of the major species (CH₄, O₂, N₂, H₂O, and CO₂) were measured using the mass spectrometer; the temperature was measured using the time-of-flight technique.

The feasibility of molecular-beam mass-spectrometer sampling from a flame containing powder was demonstrated. The observed greater reduction of [CO2] than [H₂O] when adding either KHCO₃ or KCI is interpreted to be consistent with scavenging of H and/or OH radicals via

as suggested by Friedman and Levy.

For the powder feeding rates used here, the several powders rank, in temperature-reduction effectiveness, in the order: Al₂O₃ (least effective), NaHCO₃, KHCO₃, KCI, NH₂H₄PO₂ (most effective). They rank, in reaction-inhibition effectiveness, in the order: Al₂O₃ (least effective), NaHCO₃, KHCO₃, NH₄H₂PO₄, and KCI (most effective). Since the concentration of KCI was only 2/3 that of NH₄H₂PO₄, it is concluded that, of the five powders examined, KCI has the greatest overall effectiveness in temperature reduction and reaction inhibition.