This study evaluates the common process and set-up design of a static compost bioreactor for heat recovery. A technology, which fits the goal of a sustainable, growing bioeconomy which combines the utilization of compost heat and compost material. Interest on this technology has been growing the last years but precise data of pilot scale reactors is rare. Data is required to adjust the process for custom needs and further technical development. Therefore, lignin-cellulose based biomass was composted in unaerated cylindrical compost reactors size 20 to 70 m3 for 140 days. The biomass comes with C:N ratio of about 25:1, water content of 43-48 %, organic matter content of 40.6 % d.m. and calorific value of 8.3 MJ/kg d.m. Spatial distribution of temperature and gas concentration (oxygen, carbon dioxide, methane) within the reactor shows methane production of the anaerobic core area. Maximum thermal power of 5.2 kW from a 63 m3 reactor with average temperature of heating flow about 40 °C was reached. Maximum recovered heating power of 4.8 MJ/kg d.m. was calculated for an operation of 6 month. This corresponds to 50 % of the measured calorific value. Biggest influence factors detected on the recovered heating power of the pilot scale reactor has been the size of reactor, the set up quality and the control of heat exchanger. The spatial correlation between heat production and aerobic digestion suggests a technical development in terms of aeration.

Composting experiments with heat recovery reveal spatial non-uniformity in parameters such as temperature, oxygen concentration and substrate degradation. In order to recover heat from static compost piles via integrated heat exchanger there is the need to investigate the temperature distribution for placing the heat exchangers and the interaction between heat recovery, substrate degradation and oxygen concentration to ensure quality of composting process. This study introduces a spatial model to predict the variation in controlling parameters such as temperature, oxygen concentration, substrate degradation and airflow patterns in static compost piles with heat recovery using Finite element method (FEM) in COMSOL Multiphysics ® Version 5.3. The developed two-dimensional axisymmetric numerical model considers the compaction effects and is validated to real case pilot-scale compost pile experiments with passive aeration. Strong matching with the real case experiment was achieved. The spatial model demonstrated that the compaction effect is extremely important for realistic modeling because it affects airflow, temperature distribution, oxygen consumption and substrate degradation in a compost pile. Heat recovery did not disrupt the composting process. Case studies revealed strong influence of convective heat loss through the edges and a 10 % improvement of heat recovery rate with ground insulation. The simulation indicates that an optimized placing of heat recovery pipes could increase the average heat extraction by 10-40 %.