Emissions of batch combustion of waste tire chips: The afterburner effect

A laboratory investigation was performed on the emissions from batch combustion of waste tire chips in fixed beds to identify techniques and conditions that minimize toxic emissions. Tire derived fuel (TDF), in the form of waste tire chips (1 cm), was burned in a two-stage combustor. Batches of tire chips were introduced to the primary furnace where gasification and oxidative pyrolysis took place. The gaseous effluent of this furnace was mixed with streams of additional air and, subsequently, it was channeled into the secondary furnace (afterburner) where further oxidation took place. The arrangement of two furnaces in series allows for independent temperature control; varying the temperature in the primary furnace influences the type and the flux of pyrolysates. The additional-air mixing section between the two furnaces allows for mostly heterogeneous and fuel-lean combustion in the afterburner. Results showed that both the operating temperature of the primary furnace, in the range of 500-1000 degreesC, and the existence of the afterburner had marked influences on the emissions of pollutants. Results showed that for this fuel use of combustion staging, with an additional-air mixing section, had a very beneficial effect. It drastically reduced the emissions of CO (by factors of 3-10), the particulates (by factors of 2-5) and the cumulative PAH (by factors of 2-3). Many health-hazardous PAH components were practically eliminated. Overall oxidizing conditions prevailed and the minimum oxygen mole fraction never fell below 2% in the effluent of either furnace. The operating primary furnace temperature (pyrolysis temperature) also proved to be important, with temperatures at the low side of the 500-1000 degreesC range producing fewer pollutants, upon treatment in the afterburner.