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Abstract
This is a study on the combustion characteristics of poly (styrene) (PS) particles either plain, or with various degrees of crosslinking in the range of 5-25%. Effects of crosslinking were examined by producing and burningindividual particles in the size range of 47—63 /an, while effects of particle size were examined by burning particles of the same degree of crosslinking (8%) in the size range of 47-350 jira. Measurements during combustion of styrene monomer droplets were also conducted. Combustion took place in a laminar, drop tube furnace at thermochemical conditions pertinent to municipal waste incinerators, i.e., in air at high heating rates and gas temperatures ranging from 1050 to 1400 K. Combustion of single particles/drops was monitored by three-color pyrometry and high-speed cinematography, to obtain temporal information on the radiant intensity, size, temperature and soot content of the flame, as well as the total burnout time of the particles. Plain (uncrosslinked) PS particles, in the size range of 47-63 urn, formed constant diameter envelope flames, somewhat larger than the maximum fame diameter of similar size crosslinked particles. As the degree of crosslinking increased, the total burnout time also increased and the instantaneous flame diameter decreased throughout the combustion period of these particles. Heterogeneous (char) combustion was observed in crosslinked particles only, upon extinction of the volatile flames. At high degrees of crosslinking, indications of a second wave of pyrolysis, pronounced in the plain PS, ceased to be present. Comparisons between low heating rate TGA experiments and high heating rate combustion experiments for plain PS indicated that the pyrolysis (decomposition) mechanisms depend on the heating rate. The maximum flame diameters were 10 to 25 times larger than the initial particle diameters and the flame/char temperatures ranged from 1800 to 2200 K. Average burning rates were estimated to be in the range of 0.01 to 0.14 mg/s. For particles in the range of 150- 340 μm the average mass burning rate as well as the maximum flame diameter were found to be proportional to the initial particle diameter, which suggests that the combustion was diffusion controlled. Smaller particles (47-63μm) were found to burn faster than what the above trend would predict, probably due to enhanced diffusion of oxygen to the flame. The monomer droplets burned with elongated and, overall, larger and hotter flames than those of similar size polymer particles. The furnace wall temperature was found to affect the burnout time and the ignition delay of the particles, but not the flame temperature, which was, instead, influenced by the particle size. Instantaneous soot volume fractions in the flame were found to change throughout combustion, averaging in the range of 4 x 10−4 to 4 x 10−5.