Improvements in Pyrolysis of Wastes in an Externally Heated Rotary Kiln (Experimental Study on Heat Transfer Enhancement)

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/m(2)K to 45.3 W/m(2)K at 2.7 rpm. In addition, k(m-w) increased to 50.1 W/m(2)K 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.