Closure

Results of equilibrium thermochemical calculations for the thermal destruction of nonplastic and plastic materials show the effect of material composition on the flame temperature, particulate emission, metals, dioxins, and product gas composition. The effect of waste composition has greater influence on adiabatic flame temperature, combustion air requirement, and the evolution of products and intermediate species. The combustion of waste in air produces higher flame temperature for 100% plastic than for nonplastic and mixtures. The 100% plastic requires lower number of moles of oxidant than 100% nonplastic and mixtures. Plastic produces HCl and H2S with concentration levels ranging from 1000 to 10,000 ppm. Emission of NO and NO2 from 100% nonplastic showed an increase with increase in moles of air while that from 100% plastic a slight decrease with increase in moles of air. The higher theoretical flame temperatures predicted with plastic waste corresponds to lower waste feed rate requirement of plastic at constant furnace temperature. This resulted in higher excess air operation with plastic waste and hence lower equivalence ratio. The gas residence time calculated for all the samples was found to be about 1 s. Variation of residence time more or less follows the same trend as excess air for all the samples.

The experimental data and calculated combustion parameters show different burning characteristics of plastic and nonplastic. Temperature dependency of

CO emission is in good agreement with the theoretical calculations. Higher CO emission was observed for both (100 and 85%) nonplastics than for 100% plastic. Incomplete combustion occurs with the paper burning because of lower heat release than plastic. Concentrations of dioxin vary between the various samples tested. Dioxin and furan levels were found to be higher from nonplastic/plastic samples than from 100% nonplastic. Nonplastic waste yields lowest levels of dioxins and furans. Out of four samples of various 50/50 plastic/nonplastic mixtures, PCDD/PCDF emission levels are lowest for the sample without polyvinyl chloride (sample 5). Metals in fly ash and bottom ash did not reveal any specific pattern for their variation; however, in general plastic increases metals emission. The presence of lead was found in all samples after combustion, although its source is not well understood. The highest amount of chlorine was found in a 100% plastic sample. Zinc and arsenic are present only in plastic samples. Sulfur is only detected in trace quantities from 100% nonplastic sample. Bottom ash analysis proved the presence of trace metal elements such as vanadium, barium, indium, magnesium, and manganese in ultra trace levels. The results provided here give the insights on the design guidelines for the thermal destruction of plastic containing wastes.

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