Influence of Waste Synthetic Resin Particle Size on Flame-Structure Transition and Sintering-Zone Heat-Release Localization in a Cement Kiln Main Burner: A CFD Analysis

The cement industry is shifting toward alternative fuels such as waste synthetic resin (WSR), but co-firing WSR at the main burner is difficult. WSR particles are approximately 500 times larger than pulverized-coal particles, so they delay devolatilization and reposition heat release relative to the sintering zone. Three-dimensional steady-state Reynolds-averaged Navier–Stokes (RANS) computational fluid dynamics (CFD) simulations were performed at approximately 21% thermal substitution. The primary diagnostic proxy for sintering-zone heat-release confinement was the devolatilization-completion location (xdev), defined as the last sampled axial location at which the volatile-release quantity exceeds 5% of its own maximum. The particle-size effect was decomposed into a single-diameter axis (SD: 5–25 mm) and an oversize-tail axis based on Rosin–Rammler particle size distributions (PSD: upper limits 20–35 mm). The 90th-percentile diameter of the injected distribution, D90, acts as a first-order coarse-tail scale that organizes xdev, while the upper-tail shape provides secondary corrections. When D90 is near or below approximately 20 mm, xdev generally falls within the sintering zone or near its rear boundary, identifying this value as a boundary-sensitive scale rather than a sharp threshold. The centerline CO-rich region can persist downstream and further reduce the rear margin, so the single-diameter condition of d=15 mm provides a more conservative mechanistic reference condition. Monodisperse (SD) cases up to 15 mm exhibit a dual-peak flame structure that smooths into a single broad peak under PSD conditions, with the 15 mm single-diameter peak approximately 84 °C higher than that of the distributed condition with the same 15 mm characteristic diameter, a spread parameter of n=1.2, and a 20 mm upper limit. These results indicate that managing D90, rather than nominal upper-limit or arithmetic-mean diameters, is more directly connected to sintering-zone heat-release localization. All results were obtained at a single thermal substitution rate and without experimental validation, so they are reported as conditional, screening-level trends rather than as a universally validated fuel specification.