NOx and SO2 Emissions From Pulverized Coal and Waste Tire: The Role of Devolatilization and Char Combustion Phases

In this basic experimental study the NOx and SO2 emissions of waste tires and coal are contrasted. Experiments were conducted in a laboratory-scale drop-tube furnace by fluidizing powders of coal and tire crumb (≈ 200μm) at different gas temperatures (1400K-1600K). Emissions of SO2, H2S, CO2, CO, NO and NO2 were monitored at the exit of the furnace. Pyrometric and cinematographic observations revealed that the coal particles exhibited distinct volatile and char combustion phases, while tire particles exhibited a primary volatile phase followed by a char combustion phase, which was accompanied by simultaneous burning of heavier pyrolyzates. The total recorded combustion times were considerably shorter for tire particles primarily because of their lower density and lower char yield. At a gas temperature of 1450 K, primary volatile flame temperatures for both coal and tire were in the neighborhood of 2200-2400 K, while char temperatures were 2000-2100 K for the tire and 1850-2000 K for the coal particles. Mass burning experiments, under oxidizing conditions, showed that the SO2 and CO2 emissions of the pulverized bituminous coal used were comparable to those of the tire crumb. However, the amount of NOx released by coal was four times that released by the tire and in either case it did not vary with gas temperatures in the range of 1400 to 1600K. Co-firing of pulverized coal and tires resulted in NOx emissions proportional to the weight average of the blends. The nitrogen released during the volatile and char combustion phases of coal and tire crumb was proportional to the mass burned during each phase. Sulfur, however, was preferentially released during the devolatilization phase. The CO2 released during the devolatilization and combustion stages was, as expected, directly proportional to the amount of carbon in the volatiles and the char, respectively.