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

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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.