NUMERICAL-MODEL FOR THE DYNAMIC SIMULATION OF A LARGE-SCALE COMPOSTING SYSTEM

A numerical model simulating airflow pattern, heat and mass transfer a nd degradation in the two dimensional cross-section of a deep bed comp osting vessel was developed. The model accounts for compressibility of the material and predicts spatial and temporal changes in state varia bles. The model was validated at a commercial facility that composts a mix of biosolids, bark and sawdust. Simulations were performed to qua ntify the effects of (1) initial moisture level, (2) depth of bed, (3) ambient air temperature, (4) cooling air recirculation, (5) material degradability and (6) blockage of plenum, on cost of aeration and spat ial homogeneity of degradation within the vessel. Results show that co st of aeration is lowest when the material is at an initial moisture l evel of 55% and the bed depth is 3.5 m. Energy required per unit of dr y matter degraded decreases as the ambient temperature increases. The increased aeration requirement when cooling air was recirculated was q uantified, and shows that overall energy requirements are reduced by r ecirculating ail: Aeration energy requirements and system throughput w ere compared under different operating parameters.