The simulation of particle movement and heat transfer in rotary kiln for plastic waste pyrolysis

Abstract

The wide application of massive plastic products in our daily life leads to the rapid generation of plastic wastes. The end-of-life management of plastic wastes has become a serious public concern. Converting plastic waste to energy products is a promising solution to both energy shortage and environment pollution. Plastic waste could be converted to pyrolysis gas, pyrolytic oil and pyrolytic char after pyrolysis in rotary kiln. The loading of heat carrier in the rotary kiln has an important impact on the particle movement and heat transfer, hence affecting the distribution of gas, oil and char in the pyrolysis products. This article used quartz sand as the heat carrier to pyrolyze plastic waste, and the heat carrier loading increased from 5% to 15%. With the increase of heat carrier loading, the yield of oil and char increased, but the gas yield decreased. By the analysis of particle movement and heat transfer in EDEM, it was found that loading more heat carrier could increase both the translational velocity and angular velocity of particles. The translational velocity showed an almost linear correlation to the loading of heat carriers. A noticeable phenomenon is that the velocity profile of 5% heat carrier loading was instable with time. This is because low heat carrier loading such as 5% could not impose sufficient friction on the kiln wall, and the heat carrier bed slipped on the kiln wall from time to time. On the contrary, the slipping move did not occur in cases with heat carrier loading more than 10% because of sufficient friction on the kiln wall. At these higher heat carrier loadings, the only way to release of gravitational potential is the rolling of particles from the top position along the bed surface. This rolling mode of particle movement certainly caused high angular velocity of particles, and increased the frequency of particle contacts. With 5% heat carrier loading, the average angular velocity of particles was one magnitude lower due to the fact that particles were slipping as a whole rather than each particle rolling from the top to the bottom. Owing to the difference of particle movement at different heat carrier loadings, the heat transfer was also affected by the heat carrier loadings. The cases with more heat carrier could definitely enhance the physical contact between particles, which facilitated the heat transfer from heat carrier to plastic particles. The temperature profile from EDEM showed that the plastic waste particles were heated at a higher heating rate if more heat carrier was in the rotary kiln. Therefore, loading more heat carriers accelerated volatilization process, and reduced the retention time of volatiles. This led to the increment of oil production and decrement of gas production. In addition, the increased loading of heat carrier could maintain the temperature of particle bed at higher level to enhance the carbonation reaction of volatiles, which increased the yield of pyrolytic char. The DEM simulation provided the detailed information of particle’s movement and heat transfer, which could be used as an effective tool to interpret the experimental observation and optimize the process parameters.