Co-upcycling of sludge and LDPE to aromatics via catalytic pyrolysis over Ni/HZSM-5@biochar

High ash content in sludge corresponds to high concentration of heteroatoms (N and O), causing elevated energy consumption and reduced product quality during single-feed sludge pyrolysis. Plastics, characterized by their high H/C ratio and high calorific value, are ideal candidates for co-pyrolysis with sludge, as they can enhance bio-oil quality by increasing hydrocarbon content and reducing oxygen content. To sustainably convert sludge to high-value bio-oil, a nickel-modified HZSM-5 zeolite supported on wheat straw biochar catalyst (Ni/HZSM-5@BC) was employed for the synergistic catalytic pyrolysis of sludge and LDPE. Compared with non-catalytic pyrolysis, the catalyst with a Ni loading of 3 wt% increased the feedstock conversion rate from 70.7% to 89.1%, and reduced the selectivities of oxygenates and nitrogen-containing compounds from 65.9% and 2.8%, respectively, to 0. Meanwhile, the selectivity of aromatic compounds reached 100%, with a BTX selectivity of 88.4%. This catalyst also exhibited good stability after six pyrolysis cycles. Analyses including XRD, SEM, TEM, FT-IR, Raman spectroscopy, and TG indicated that the catalyst structure remained largely unchanged, with HZSM-5 remaining stably coated on the biochar and Ni remaining well dispersed within the hierarchical micropore-mesopore structure. Furthermore, the catalyst exhibited a low coke yield (3.72 wt%) that increased only modestly with cycling (an increase of 0.52 wt% after six pyrolysis cycles). Experiments with sludge model compounds further clarified the synergistic interactions between sludge and LDPE and supported the proposed catalytic co-pyrolysis mechanism over Ni/HZSM-5@BC.