The pyrolysis and gasification of high density polyethylene in a batch reactor

Solid organic waste contaminated with radionuclides is a challenging wasteform in its raw state. NRCN, as the national repository for radioactive waste in Israel, accepts all such waste from industry, hospitals, universities and from the Israeli nuclear research centers. Organic waste is characterized by its high volume to weight ratio, together with the hazard of its radiolysis and consequently, the generation of combustible and explosive gases (e.g. H2 and light hydrocarbons). Thus, minimizing waste volumes together with stabilizing the wasteform are of high significance in the treatment of contaminated polymers. In this study, the thermal decomposition process is aimed at maximizing the room temperature gas yield, and minimizing the oil and wax fraction that will be harder to entrain in a large scale system. High density polyethylene (HDPE) is a thermoplastic polyolefin characterized by its high strength and diverse utilities. In the current research, we investigated the thermal decomposition of HDPE using thermogravimetric analysis (TGA) at the milligram scale; whereas a laboratory scale thermal decomposition system was utilized for experiments at the gram scale. The TGA method enables the study of the thermal decomposition parameters, whereas the macro-scale experimental system enables the study of the decomposition products (e.g. char, wax, oil and gas). Polymer decomposition was studied under variable atmospheres, ranging from pure pyrolysis, under a nitrogen atmosphere, to gasification under 88% nitrogen and 12% oxygen atmosphere in the macro-scale system (with a similar total gas flow rate). Only pyrolytic conditions were applied in the TGA experiments. Decomposition temperatures ranged between 450 and 600°C for HDPE in the macro-scale system. TGA experiments focused on narrower temperature scale between 420 and 480°C. An exponential increase in the decomposition rate of HDPE was detected with the increase in the experimental temperature in the TGA experiments. The rate of decomposition at 420°C was 0.79 ± 0.074%/min; this value has increased by two orders of magnitude to 49.71 ± 1.1%/min at 480°C. The activation energy calculated using the Arrhenius equations is 298 KJ/mol. In the macro-scale system, at 450°C, under pyrolytic conditions, only 22% of the raw material was converted to gas; the remaining mass was converted to wax and oil with only minor formation of char. On the other hand experiments conducted at 525 and 600°C yielded similar amounts of gas and oil, where 63-65% of the experimental charge were converted to gas, 27-29% condensed as wax at temperatures higher then 180°C and only 5-7% condensed as oil and wax at lower temperature. The formation of over 30% of wax and oil is expected to be problematic in the operation of a large scale decomposition system. Thus, thermal decomposition of HDPE was attempted under gasification conditions with 3, 7 and 12% of O₂ in the atmosphere. The experiments were conducted at 525°C. The proportion of produced gas ranged between 54% at 3% O₂ up to 61% with 12% of O₂ in the atmosphere. None of the oxidized environments have produced more gas compared to the pyrolytic conditions, whereas an increase in the fraction of the low condensation temperature gases was noted. To conclude, neither pyrolysis nor gasification leads to high enough gas fraction at room temperature to be considered as a single state thermal treatment for contaminated solid organic material.