Comparison of the combustion behaviour of pulverized waste tyres and coal

Abstract Comparative combustion studies were performed on particles obtained from pulverized bituminous coal and waste automobile tyres (rubber). Particle size cuts of 75–90 and 180–212 μm were burned in a thermogravimetric analyser, at low heating rates, and in an electrically heated drop-tube furnace, at high heating rates. The combustion of individual particles in the drop-tube furnace was observed with three-colour pyrometry, to obtain time-temperature histories and with high-speed cinematography to record flame particle size histories. Combustion was conducted at a gas temperature of 1450 K, in air. Upon pyrolysis, the phenomena of melting, swelling and formation of large blowholes were observed only in the case of the coal particles. The tyre particles formed chars with rough surfaces and smaller blowholes. Separate volatile and char combustion phases were detected for the coal particles studied. Tyre particles experienced an intense primary volatile combustion phase, followed by a phase of simultaneous secondary volatile combustion, of lesser intensity, and char combustion. During the initial volatile phase combustion, the peak flame temperatures were comparable for both materials, in the range 2200–2400 K. The secondary volatile/char combustion phase, observed for the type particles, was cooler, i.e. 2000–2100 K. The coal chars burned with temperatures of 1850–2000 K. Combustion was diffusionally controlled (regime III) for coal chars of both sizes and for tyre chars of the larger size cut only. Char burnout times were considerably shorter for tyre particles than coal, which can be attributed to the secondary devolatilization and the lower density of the former.