Effects of Low-Pressure Steam and Wood Biochar on Product Distribution During HDPE Pyrolysis

Mild pyrolysis of high-density polyethylene (HDPE) can produce wax-rich intermediates, but selectivity is governed by cracking severity. This study examined the effects of in situ low-pressure steam and wood biochar on HDPE product distribution in a stirred batch reactor. Screening at 350–410 °C identified 390 °C as a transition between heavy condensed material and lighter products; maximum total reactor pressures during the focused experiments were 8.8–20.5 bar. At 390 °C, neat HDPE gave 83.0 wt.% condensed product and a 17.0 wt.% apparent gas fraction calculated by difference. Water (3.2 or 9.1 wt.%) and biochar (HDPE:biochar = 2:1) applied separately reduced the apparent gas fraction and increased condensed-product recovery to 91.3–93.9 wt.%. The higher water loading favored the >C40 TGA volatility interval, whereas biochar did not selectively enrich C20–C40. Their combination showed an apparent non-additive redistribution: the apparent >C40 fraction decreased to 22.3 wt.%, while C20–C40, <C20, and gas reached 46.1, 17.1, and 14.4 wt.%, respectively. 1H NMR showed restoration of the alkene-region proton contribution relative to the water-only and biochar-only conditions. Semi-quantitative GC-MS analysis of the detectable fraction was consistent with a shift toward lower-carbon-number hydrocarbons under the combined condition. These results are consistent with low-pressure steam modifying the effect of wood biochar and indicate that HDPE product distribution can be tuned under comparatively mild pressure.