Effect of the temperature in the nature and extension of the primary and secondary reactions in the thermal and HZSM-5 catalytic pyrolysis of HDPE

Abstract Catalytic pyrolysis of plastic wastes is a promising recycling alternative to the disposal methods currently used for this type of residues (i.e. land filling or energetic valorisation). In order to optimize the yields of compounds obtained with this treatment, the knowledge of the parameters’ influence on the degradation process is of great interest. Pyrolysis temperature and volatiles residence time are the most influential variables in the process, since they affect the primary as well as the secondary reactions. In this work, the pyrolysis temperature effect on the primary and secondary reactions of the thermal and HZSM-5 catalyzed pyrolysis of HDPE is reported and evaluated, in the range 500–800 °C. For this purpose, two different equipments have been used, i.e. a flash coil pyrolyzer (pyroprobe 1000), that allows us to study the primary products, since the extent of secondary reactions can be neglected, and a fluidized bed reactor, where the extent of the secondary reactions is significant. The results of the present study show that 1-hexene is the major product obtained when primary thermal cracking reactions are mainly taking place. However, the major compounds obtained when thermal secondary reactions are present in larger extension are propene at low temperatures and ethene at high temperatures. In catalytic pyrolysis, the effect of HZSM-5 is clearly evident at all temperatures evaluated, increasing the volatile yields in both equipments used. The influence of this catalyst is more significant in the primary cracking reactions showing an increase of the volatile compounds with the degradation temperature. In this case, propene is the main volatile product obtained reaching a yield of 30.6% at the highest temperature evaluated. When the presence of the secondary reactions is evident, using a fluidized bed reactor, the combined effect of generation and possible cracking reactions leads to a low-dependent product distribution on the degradation temperature, propene being the main volatile compound in the range of temperatures studied. It has been observed that branched hydrocarbons are formed mainly from secondary reactions and are quickly destroyed by increasing the temperature. In the zeolite catalyzed pyrolysis the differences between the yields obtained in both equipments are lower than in the thermal case.