Insight into the charging methods effects during clean recycling of plastic by co-pyrolysis with low-rank coal

Co-pyrolysis of waste plastic and low-rank coal provides a promising method for high-efficiency utilization of them. Previous studies about co-pyrolysis of plastic and low-rank coal mainly concentrated on evenly physical mixture (PM) of feedstock. It has been found that the uneven distribution of materials in the reactor could be caused by the apparent differences in density and property between plastic and coal. In order to investigate the effects of interaction strength between coal and plastic on the thermal behavior and properties of products, a set of co-pyrolysis experiments with different charging methods of one Chinese low-rank coal and high density polyethylene (HDPE) were carried out. To be specific, the charging methods of PM and layered placement (LP), which include charging coal on top of plastic (CT), plastic in the middle of coal like a hamburger (HB) and plastic on top of coal (PT). Results show that, with the same plastic addition ratio, the weight loss of LP co-pyrolysis has almost no reduction compared with that of PM and only one peak is observed from DTG curves instead of two peaks as shown in the theoretical calculated (Cal) curve. It means that there is still an obvious interaction between coal and plastic during co-pyrolysis. Compared to the charge of PM, the tar yield of LP co-pyrolysis decreases slightly while enhances the char yield. The properties of char samples of LP show no noticeable differences as compared that of PM. In term of tar products, co-pyrolysis with HDPE increases the H/C atomic ratio of tar from 1.286 to 1.480 compared to coal pyrolysis alone. However, charging methods make little difference in H/C atom ratio of tar in co-pyrolysis reactions, which is around 1.50. On the other hand, the content of heavy components in tar obtained from LP co-pyrolysis reduces obviously, which is 18.06%, 22.83% and 26.76% for CT, HB and PT, respectively, compared to 36.60% for that of PM. Simultaneously, LP co-pyrolysis can effectively reduce the molecular weight of tar. Additionally, compared to the PM case, the content of H2 and light hydrocarbons (CnHm with n < 5) of LP co-pyrolysis decrease from 10.70% to 7.59% and from 24.56% to 21.67%, respectively. This results from the more effective utilization of H element in the feedstock, especially with the charge of CT. Thus, charging the coal on the top of plastic is a promising method of improving the quality of tar.