CFD–DEM Analysis of Mechanically Induced Airflow and Near-Field CO2/CH4 Transport During Compost Turning

Mechanical compost turning produces short-lived greenhouse-gas concentration peaks, yet the physical mechanism linking particle agitation to near-field gas transport remains poorly understood. A two-stage numerical framework combining the discrete element method (DEM) and computational fluid dynamics (CFD) was developed to investigate this process during industrial windrow turning. In Stage 1, a coupled CFD–DEM model resolved rotor-driven particle motion and the resulting mechanically generated airflow. The time-averaged airflow field was then transferred to a Volume-of-Fluid multiphase model to simulate CO2 and CH4 transport. The framework was evaluated against field measurements obtained from a commercially operated eWender windrow turner. Although turning produced a visually prominent plume, measured and simulated near-field air velocities remained predominantly below 1 m/s, indicating a low-velocity transport regime rather than strong atmospheric convection. CO2 and CH4 concentrations exhibited short transient bursts during active turning, while measured CO2 concentrations remained predominantly within approximately 1500–4000 ppm. Simulated air velocities and CO2 concentrations fell within the corresponding measured ranges. The results indicate that mechanically generated airflow contributes mainly to local gas redistribution and near-field concentration enhancement rather than sustained or far-reaching transport. Accordingly, the visually prominent plume and short-duration concentration peaks observed during turning should not be interpreted alone as evidence of proportionally increased total greenhouse-gas emissions.