Three-dimensional CFD simulation of non-premixed combustion in a full-scale cement rotary kiln: effects of primary air ratio on NOx and CO emissions

Cement rotary kilns face critical challenges in balancing carbon monoxide ( 𝐶 𝑂 ) and nitrogen oxide ( 𝑁 ⁢ 𝑂 𝑥 ) emissions. To rigorously investigate burner aerodynamics without the prohibitive computational costs of multiphase reactions, this research employs a phase-decoupling approach, modeling the clinker bed as a dynamic heat sink. A comprehensive 3D computational fluid dynamics (CFD) simulation of methane non-premixed combustion is developed for a 1 0 0 0 t p d industrial rotary kiln. The coupled effects of ± 2 0 % variations in primary air (PA) momentum on flame morphology, macro-mixing dynamics, and pollutant kinetics are evaluated using the Realizable 𝑘 − ∊ model, probability density function (PDF) approach, discrete ordinates (DO) radiation, and extended Zeldovich mechanism. Results reveal that a + 2 0 % increase in PA momentum enhances shear-layer turbulence and establishes a robust internal recirculation zone (IRZ). This accelerated macro-mixing reduces the methane burnout length from 8 . 1 m to 7 . 2 m and suppresses mixing-limited exit 𝐶 𝑂 mass fractions from 2 . 9 9 % to 1 . 4 0 % . Crucially, the enhanced flow advection reduces species residence time within the high-temperature zone ( 𝑇 > 1 8 0 0 K ). This “aerodynamic quenching” mechanism restricts the slow Zeldovich kinetics, substantially decreasing the peak 𝑁 ⁢ 𝑂 𝑥 mass fraction from 3 . 5 × 1 0 − 3 to 2 . 2 × 1 0 − 3 . Conversely, a 2 0 % momentum reduction weakens localized mixing, elevating both 𝑁 ⁢ 𝑂 𝑥 and 𝐶 𝑂 ( 4 . 7 8 % ) emissions. Despite the simultaneous emission reduction benefits in the high-momentum scenario, it concentrates severe convective and radiative heat fluxes onto the refractory lining ( 5 – 1 1 m ), jeopardizing structural integrity. Ultimately, the baseline condition offers the most viable thermodynamic balance, demonstrating that tailored staged combustion is required to safely break the 𝑁 ⁢ 𝑂 𝑥 - 𝐶 𝑂 trade-off.