Improvements in Pyrolysis of Wastes in an Externally Heated Rotary Kiln

(Experimental Study on Heat Transfer Enhancement)

Abstract

The effects of rotating speed and internal structure on the performance of an externally heated rotary kiln for waste pyrolysis were investigated. A newly developed method was adopted to evaluate the overall heat transfer coefficient k_m-w from the inner wall to the wastes for this purpose. The experimental results revealed that k_m-w monotonically increased with the number of lifters and their height. When six lifters 200 mm in height were attached to the inner wall of the kiln, the mean value of k_m-w increased from 38.6 W/m²K to 45.3 W/m²K at 2.7 rpm. In addition, k_m-w increased to 50.1 W/m²K when the rotating speed was increased to 4.0 rpm. In the water vaporization phase during the course of the pyrolysis process, the height of the lifters had a significant influence on k_m-w. However, the number of lifters had a significant impact on k_m-w in the pyrolysis phase of the plastic-based wastes. According to measurements, a 10 % increase in k_m-w could be obtained when installing lifters to attain a ratio of lifter height H_l to the thickness of the waste layer H_w larger than 0.45 or when arc length between two lifters L_l to the arc length of the interface between the wastes and the kiln wall L_w was larger than 1.