IMPACT OF TIRE-DERIVED FUEL ON THE CHEMICAL COMPOSITION OF COAL-COMBUSTION PRODUCTS

Every year in the U.S., millions of scrap tires are discarded. Conventionally, these tires are disposed of in landfills. However, this disposal method not only contributes to landfill crowding, but also poses a potential threat to human safety and health. Tires are buoyant when buried underground, so over time, whole tires migrate upwards through landfill piles, thus harming landfill covers. If tires are not buried, they are often stockpiled or illegally dumped. These piles provide a breeding ground for mosquitoes and rodents, and are also prone to fire due to their tendency to retain heat.

To help alleviate our nation’s current scrap tire problem, various solutions for reusing tires have been, and are currently being, developed. Today, approximately 20 percent of tires are reused. One use of scrap tires involves burning tire chips as a source of energy. Tire-derived fuel (TDF) is being explored as a means of reducing fossil fuel use. TDF produces more energy through combustion than coal incineration, so, for this reason, it is considered a favorable option. However, the potentially harmful effects of burning these tires are yet to be fully discovered.

The Purdue University power plant conducted an experiment with two different sets of fuel combusted at the same conditions (»1500 °C): pure coal and a mixture of 95 wt% coal plus 5 wt% TDF. Compared to pure coal, the TDF-coal mixture is considerably richer in Zn (133±34 vs. 32±43 ppm), and also has a higher S content (1.76±0.91 vs. 0.57±0.55 wt%). The bulk Zn concentrations in combustion products derived from TDF are 238, 3850, and 61500 ppm in bottom ash, mechanical separator ash, and electrostatic precipitator ash, respectively. These concentrations are high when compared to the pure coal values of 68, 106, and 3450 ppm. This increase may lead to higher Zn concentrations in both atmospheric emissions and leachates from ash disposal sites.