The cement industry’s shift toward carbon neutrality is expanding the use of alternative fuels such as waste synthetic resin (WSR). Because WSR particles are approximately 500 times larger than pulverized coal, their thermal inertia delays devolatilization and repositions heat release relative to the sintering zone. Using three-dimensional steady-state RANS CFD at approximately 21% thermal substitution, we use the devolatilization-completion distance (xdev) as a primary diagnostic proxy for sintering-zone heat-release confinement. The particle-size effect was decomposed into a single-diameter axis (SD: 5–25 mm)andanoversize-tail axis (Rosin–Rammler distributions, PSD: upper limits 20–35 mm). Across these cases, D90, the 90th-percentile diameter of the fed distribution, acts as a first-order coarse-tail scale for organizing xdev, while the upper-tail shape and the population of the largest particles provide secondary corrections. Under the present modeled conditions, xdev generally falls within the sintering zone or near its rear boundary (within the 0.5 m post-processing resolution) when D90 is near or below approximately 20 mm, placing this value as a boundary-sensitive coarse-tail scale rather than a sharp confinement threshold; the centerline CO-rich region can persist downstream, further reducing the rear margin at the 20 mm level, with the SD single-diameter baseline at d = 15 mm (SD3) providing the safer mechanistic reference condition. The double-peak flame structure observed in monodisperse (SD) cases reflects the single-diameter idealization: for the studied dm = 15 mm PSD conditions it is smoothed into a single broad peak, with SD3 showing an approximately 86 °C higher peak than PSD1. These results indicate that managing coarse-tail metrics such as D90, rather than relying only on nominal upper-limit or arithmetic-mean diameters, is more directly connected to sintering-zone heat-release localization.