Enhanced pyrolysis of oil sludge and polymer waste in sub and supercritical water: Production of low-carbon syngas, and liquid hydrocarbons using bimetallic catalyst based on nickel-cobalt

Accumulation of waste of plastic materials, formation of the oil sludge and hydrocarbons spills from household users and petroleum industries processes in nature, constitute a major danger for the environment whence, research of new effective techniques to expand the possibilities of the wastes recycling, is crucial. However, this paper focuses on the synthesis of bimetallic catalyst (BMC) based on Ni-Co supported on Al₂O₃ for the pyrolysis of polyethylene (PE), polyethylene terephthalate (PET) and oil sludge (OS) samples in sub and supercritical water (sub- and SCW) for fuels and low carbon syngas production. Experiments were performed in a batch reactor at the temperatures and pressure ranges 350 – 410°C and 213.7 – 268.9 bars for 1 h. Characterization of the BMC and pyrolysis products was done by XRD, XRF, SEM-EDX, TGA, FTIR, GC-MS, GC and EPR methods, respectively. The findings reveal that the yield of the total gases, including unsaturated gases, increase with the pyrolysis temperature, and are composed by C₁–C₄, C₂=C₄, H₂, CO and CO₂. At a temperature of 380°C it is found that, the pyrolysis of the low density PE and that of PET-II, in the absence of BMC significantly generated the C₂=C₄, CO and CO₂ up to 6.2, 33.14 and 48.34 mol.%, through a secondary cracking and decarboxylation of PET, respectively. It is found that, the composition of the resulting liquid product from pyrolysis of PET, is mostly composed of Benzoic acids, Biphenyl and that, the Benzoic acid catalyzes the reaction of CO₂ formation itself, in the absence of BMC. Thus, use of BMC reduced the rate of CO and CO₂ by 2.80 and 3.5 times while that of C₁, H₂ and ΣC₂-C₄ increased to maximal of 45 mol.%, 24.63 mol.% and 44.59 mol.%, respectively. Moreover, a high conversion rate of 58.46 wt% was achieved from the PE-I, and that of liquid hydrocarbons of 47.24 wt% observed for PE-IV, at 380°C in the presence of BMC. The results revealed that most of H₂ source is essentially based on the water gas shift reaction and Sabatier's. Around 43–50 wt% of water was involved in the pyrolysis of PE, and that 0.4 % of Dodecadien-1-ol and 5 % of Decanedioic acid were detected only in the liquid product using the BMC. Overall, the use of BMC for pyrolysis of samples in SCW water is beneficial for the production of H₂ and C₁ with the reduction of CO and CO₂ emissions.