Combustion of Mg and composite Mg·S powders in different oxidizers

• Published in: Combustion and flame Vol. 195; pp. 292 - 302
• Main Authors: Liu, Xinhang, Schoenitz, Mirko, Dreizin, Edward L.
• Format: Journal Article
• Language: English
• Published: New York Elsevier Inc 01-09-2018; Elsevier BV
• Subjects: Burn rate; Burn-in; Carbon dioxide; Carbon dioxide lasers; Combustion products; Diffusion rate; Emission analysis; Fluidized bed combustion; Hydrocarbons; Laser beam heating; Laser ignition; Magnesium; Magnesium oxide; Mechanical milling; Metal combustion; Oxidation; Oxidizing agents; Particle combustion; Particulate composites; Photomultiplier tubes; Reaction kinetics; Smoke; Spherical powders; Sulfur; Surface kinetics; Burn rate; Metal combustion; Particle combustion; Laser ignition
• ISSN: 0010-2180; 1556-2921
• DOI: 10.1016/j.combustflame.2018.03.036

Micron-sized, spherical magnesium powders were ignited by a CO2 laser beam and by injecting them in the products of air–C2H2 and air–H2 flames. The same experiments were performed with composite Mg·S powders prepared by mechanical milling magnesium and elemental sulfur powders. The non-spherical Mg powder used to prepare composites was also explored in selected combustion tests. Flow conditions were varied in experiments performed in air with all materials. The combustion products were collected for particles burning in air; the products were studied using electron microscopy. Optical emission produced by burning particles was recorded using filtered photomultipliers. The emission pulses were processed to recover the particle burn times and their temperatures. Fine Mg particles burn in air very rapidly, with the burn times under 1 ms for particles finer than ca. 10 µm. The apparent trend describing burn time as a function of the particle size for such particles is t∼d0.5. The particles burn without generating a detectable standoff flame zone or producing smoke; combustion products are particles of MgO with dimensions comparable to those of the starting Mg powder particles. Both the particles burn times and their measured flame temperatures decrease slightly when particles are carried by faster air flows. The present experimental results is interpreted qualitatively assuming that the reaction occurs at or very near the boiling Mg surface and its rate is affected by both surface kinetics and the inward diffusion of oxygen. It is further proposed that the fine, solid MgO particles form either directly on surface of Mg droplet or in its immediate vicinity. Deposition of MgO crystals on liquid Mg causes little change in the particle burn rate. Combustion of Mg in air–C2H2 and air–H2 flames occurs much slower than in air. Combustion of composite Mg·S particles follows a two-step process. In the first step, sulfur is evaporated. When the particles are heated by a CO2 laser beam, rapid evaporation of sulfur leads to a sudden change in the particle velocity. Once sulfur is removed, the particles burn similarly to the pure Mg.