Possibilities of heat recovery from waste composting process
|This dissertation examines the possibilities of the heat recovery from the waste composting process. Due to the microbial activity, the composting mass can reach high temperature (70oC and higher), so the hypothesis is that is possible to “extract” heat from the mass. The rationale behind this hypothesis is the experience of the waste management company – Donarbon Ltd., which shows that the compost can stay hot for a period of a couple of months.
The key research question is to what extent is it possible to recover heat without interfering with the composting process.
The dissertation is based on the experimental results that were obtained with collaboration with Donarbon Ltd. The experiments were conducted in prototype scale (a 30 m3 container). They consist of running the composting process, with parameters adjusted according to literature review, experience of the employees and the experimental results itself. While the microbial activity is going on, the temperature is measured by 1.5 m long stainless steel thermo probes that are inserted into the compost mass. The other important feature of the experimental set up is the heat transfer system, which is based on the water that is running through the container.
Until now, four experiments have been conducted. The first two experiments did not have a water system installed. They have showed that the aeration has to be optimized as it is creating a dehydrating environment, and it is a limiting factor if the oxygen input is based on diffusion mechanisms.
Two latter experiments have had the running water system installed. They showed that the heat could be extracted from the composting process in the non-uniform manner. The temperature difference of the water on the inlet and outlet varied from one degree to the highest 20 degrees. The experiments showed that the layout of the system, as well as the material is very important feature in setting the heat transfer.
The next milestone is to make detailed energy balances for heating the water, to see if the process is feasible. The energy that could be obtained from the composting is the same as the energy that would be gained by burning it, but the time scale is different. The energy obtained from the composting process is to be compared with getting heat from conventional sources.