A study on toxic organic emissions from batch combustion of styrene.

Abstract

Results from a laboratory-scale investigation on batch combustion of styrene are reported herein. Limited quantities of waste styrene monomer are incinerated, however this monomer is, also, the primary pyrolyzate during combustion of waste polystyrene, the second most abundant polymer produced worldwide. Thus, its combustion-generated emissions are of importance to the operation of hazardous waste incinerators and municipal waste-to-energy powerplants. This work focuses on emissions of polycyclic aromatic hydrocarbons (PAHs), particulates, as well as carbon monoxide. To investigate methods for minimizing such emissions, batch combustion of the monomer was conducted in a two-stage muffle furnace. An additional air mixing chamber was installed between the two stages. Small quantities of the liquid monomer were inserted in the primary furnace which served as a gasifier/burner. The furnace temperature was in the range of 300-1000 degrees C and diffusion flames were formed under most conditions. Upon mixing with additional air, combustion of unburned gaseous fuel and primary reaction products continued in the secondary furnace (afterburner), which was kept at a constant temperature of either 1000 or 800 degrees C. Using this technique, conditions that minimize emissions were explored and theoretical investigations on the fate of pollutants in the secondary furnace were undertaken. Results revealed that combustion of styrene, which is a highly volatile fuel, occurred with the formation of flames that were often non-anchored, unsteady and unstable. Emissions of organic pollutants, soot and CO were more intense than in the case of the polystyrene combustion, studied previously under identical conditions, due to the additional depolymerization/pyrolysis steps therein. The emissions from the secondary furnace exceeded those of the primary furnace, consistent with the fact that a very significant fraction of the fuel conversion occurred in the secondary chamber. Clear trends in the emissions of PAHs and soot, products of incomplete combustion, with the temperature of the primary furnace (gasifier) were observed. Emissions were drastically reduced with lowering the gasifier temperature. While final cumulative emissions of PAHs and soot accounted for more than one third of the mass of the fuel at high temperatures, their concentrations at the exit of the afterburner were negligible when the primary furnace was operated at 300 degrees C under pyrolytic conditions. In the latter case air was added to the afterburner. Numerical modeling based on a complex reaction network was used for the description of the primary furnace as well as of the afterburner. Kinetic analysis showed acetylene and benzene to be key species in the growth of PAHs. Formation of PAHs in the afterburner, found experimentally, was reproduced by the model using a plug-flow assumption.