Optimizing Heat Extraction from Compost

Abstract The need for renewable sources of energy has fueled interest in harvesting the heat produced by composting. Pilot-scale compost reactors were built with in-vessel heat exchangers to test the effect of heat extraction on the composting process. Water was passed through copper tubes embedded in the compost-filled barrels and the temperatures of the compost and the inlet and outlet water were monitored. More heat could be extracted with higher water flow rates. Compost temperatures were especially sensitive to the water flow rate during the thermophilic stage. The data from this experiment was then used to update a computational model of the composting process. COMSOL Multiphysics™ (v. 5.2, COMSOL AB, Stockholm, Sweden) was used to create a three-dimensional, finite-element simulation of mass and energy balances in the compost barrels. The model was validated against empirical data from the experiment. Simulated and empirical data were in general agreement from the start of composting until peak thermophilic temperatures, at which point they diverged, likely due to inappropriate heat transfer boundary conditions in the model. This work is a step in the development of empirically validated computational tools for the optimal design of compost heat extraction systems.