Comparative Study on the Combustion and Emissions of Waste Tire Crumb and Pulverized Coal

Abstract

Comparative results are presented on the combustion and toxic emissions (SO2, NOx, CO, and PAHs) from a pulverized bituminous coal and ground waste automobile tires. The combustion behavior of single particles in the range of 60−212 μm was monitored pyrometricaly and cinematographically, at a gas tem perature of 1150 °C and a heating rate of ≈105 °C/s. Tire particles burned faster (by a factor of 2−3) than similar size coal. Coal particles experienced distinct devolatilization and char combustion stages. Upon intense volatile combustion, the chars of tire particles burned accompanied by secondary devolatilization that aided the burnout of their carbon black residue. No soot appeared to escape the volatile flame envelope. The SO2 emissions from the combustion of steady-flow clouds (aerosols) of tire and coal particles were comparable (within a factor of 2), the NOx emissions of tires were lower than those of coal by a factor of 3−4, reflecting their lower fuel nitrogen content. Insignificant amounts of CO were detected in the effluent of both fuels at fuel-lean or stoichiometric conditions. CO sharply increased in fuel-rich combustion, while NOx emissions decreased. No polynuclear aromatic hydrocarbon (PAH) emissions were detected in the effluent of fuel-lean and stoichiometric combustion of clouds of coal particles at a furnace gas residence time of 0.75 s. Even at mildly fuel-rich conditions, 1.0 < φ < 1.8, only small amounts of PAHs were detected in the effluent of burning coal particles. At higher equivalence ratios, φ, PAH emissions increased sharply. Combustion of clouds of tire particles produced no detectable PAHs at φ < 0.6, but as stoichiometry was approached or exceeded, the PAH emissions increased substantially. Thus, the onset of detectable PAH emissions occurred at lower φ for tires than for coal. Beyond that point, the PAH emissions of both fuels increased exponentially in fuel-rich combustion, asymptotically reaching the pyrolytic gas-phase PAH emissions in N2. Individual gas-phase PAH emissions of tires were 1.5−2 times higher than those of coal at comparable fuel-rich φ and were 1.5−10 times higher under pyrolytic conditions, where maximum amounts were recorded. Similar trends were observed in the condensed-phase PAH emissions of both fuels; however, the highest yield was obtained under oxygen-starved oxidative environments. Differences in the PAH emission tendencies of the two fuels were amplified when combustion took place in fixed beds. Coal burned relatively clean, while tire crumb emitted copious amounts of smoke and PAHs, with individual compounds dramatically exceeding (by 10−100 times) those from burning pulverized coal.