Devolatilization and Combustion of Tire Rubber and Pine Wood in a Pilot Scale Rotary Kiln

Cement production is highly energy intensive and requires large quantities of fuels. For both economical and environmental reasons, there is an increasing tendency for utilization of alternative fuels in the cement industry, examples being tire derived fuels, waste wood, or different types of industrial waste. In this study, devolatilization and combustion of large particles of tire rubber and pine wood with equivalent diameters of 10 mm to 26 mm are investigated in a pilot scale rotary kiln able to simulate the process conditions present in the material inlet end of cement rotary kilns. Investigated temperatures varied from 700 to 1000 °C, and oxygen concentrations varied from 5% v/v O2 to 21% v/v O2. The devolatilization time of tire rubber and pine wood were found to mainly depend on temperature and particle size and were within 40 to 170 s. Rate limiting parameters for char oxidation of tire rubber and pine wood were found to be bulk oxygen concentration, mass transfer rate of oxygen, raw material fill degree, raw material characteristics, and temperature. Kiln rotational speed only had a minor effect on the char oxidation when the raw material bed was in a rolling motion. Initial fuel particle size also influenced the char conversion time for pine wood char but had no influence on tire char conversion time, because the tire rubber crackled into several smaller char fragments immediately after devolatilization. The char conversion times were from 40 to 480 s for tire char and from 30 to 1300 s for pine wood char, depending on the conditions. Models for devolatilization and char oxidation of tire rubber and pine wood are validated against experimental results. ■ INTRODUCTION The cement industry is responsible for approximately 2% of the world’s primary energy consumption. Fuel consumption accounts for about 30−40% of the total cement clinker production costs. Due to competition in the cement market, increasing fossil fuel prices, and environmental concerns, cement producers have increased the utilization of alternative fuels as a substitute for fossil fuels in order to achieve the most economic fuel mix. In this context, “alternative fuels” cover all nonfossil fuels and waste from other industries. Popular alternative fuels in the cement industry are tire-derived fuels (TDF), biomass residues, and different commercial and industrial wastes. It is attractive to utilize coarse, solid alternative fuel particles directly in the material inlet end of cement rotary kilns in order to save expenses for shredding of the fuels to smaller particles and to increase fuel flexibility of the system. High temperatures in the rotary kiln and material retention times of several minutes provide good conditions for fuel burnout. Combustion of solid fuels in the material inlet end may, however, have negative impact on the process stability of the kiln system in the form of increased tendency for deposit build-ups. The mechanism behind formation of deposit build-ups is described elsewhere. The influence of alternative fuel utilization in the material inlet end of the rotary kiln on process stability often limits how large quantities of fuel are possible to use in this way. Little information is available in the literature about combustion of large fuel particles under the conditions in the material inlet end of rotary kilns, and it is possible that a systematic investigation can optimize the utilization of alternative fuels in the material inlet end of rotary kilns. Figure 1 shows utilization of large fuel particles in the form of whole tires in the material inlet end of the rotary kiln. The raw material bed temperature is typically around 900 °C and increases toward 1450 °C in the burning zone. The gas temperature is around 1000−1200 °C in the material inlet end and may be above 2000 °C in the burner flame in the burning zone. Bulk oxygen concentration in the material inlet end is typically quite low, 2−7% v/v. Solids and gas residence times in the rotary kiln are typically 15−30 min and 5−10 s, respectively. Devolatilization of large TDF particles have been studied by several researchers. In addition, studies have been made with micro thermogravimetric analysis (micro-TGA) on small TDF samples to determine reaction kinetics and composition of oil, gas, and char. Devolatilization times of cylindrical tire rubber particles with diameters from 7 to 22 mm and heights of 35 mm at 840 °C in an inert atmosphere are reported to be 75 to 300 s when increasing the particle diameter from 7.5 to 22 mm. Devolatilization times of tire rubber particles with thicknesses in the range of 6−12 mm in air and in the temperature interval from 700 to 1000 °C were found to be 30−100 s depending on thickness and temperature. Generally, results reported in literature about devolatilization of large TDF particles show clear tendencies for devolatilization time to increase with increasing particle size and decrease with increasing temperature and/or oxygen concentration. Received: September 8, 2011 Revised: December 20, 2011 Published: December 20, 2011 Article