Observations on the Combustion of Polymers (Plastics): From Single Particles to Groups of Particles

Abstract

This investigation aimed at examining the combustion behavior of polymer particles in groups and at comparing it to that of single particles. Combustion observations were subsequently related to measurements on emissions of organic pollutants, such as PAHs and soot, which were previously measured in this laboratory under similar conditions. The following plastics (polymers), commonly found in municipal waste streams, were burned in powder form: poly(styrene) (PS), poly(ethylene) (PE), polyvinyl chloride) PVC, and poly(methyl methacry-late) (PMMA). Particles, 125-212um, were introduced to a drop-tube furnace, at steady-flow conditions, and burned in cylindrical streams (clouds), in 1250 K air. The particle mass loading in the furnace was varied to achieve either dilute clouds, where isolated (single) particle combustion occurred, or dense clouds where interactive particle combustion look place. Information on the combustion characteristics and flame temperatures of the particles was obtained by cinematography and three-color pyrometry. Results revealed striking differences in the combustion behavior of the four polymers burning in particle clouds. PVC particles burned with bright individual flames, resisting the formation of group flames, even at high particle number densities. In dense clouds, envelope flame diameters were generally smaller and their durations were shorter than in single particle combustion. Poly(ethylene) particles in very dilute clouds burned individually, surrounded by low-sooting, faint envelope flames. When particles burned in close proximity to each other, they abruptly exhibited localized flash-vaporization and formed highly-luminous, large, group flames, “puffs”. At high particle number densities the entire stream was engulfed in a cooler flame containing finely-dispersed soot. Poly(styrene) particles burned mostly as discrete entities at relatively low flowrates, and the progression to partial group flames was gradual as the particle mass loading increased. PS particles consistently formed flames with long wakes which, upon extinction formed strings of highly-agglomerated soot. Flame temperatures were lower in group combustion than in individual particle combustion. While single particle envelope flames were previously recorded to be in the neighborhood of 2000 K, those of group flames were found to be only a few hundred degrees above the gas temperature, at globally fuel-rich conditions.