Paolo Zampieri - Implementing a numerical model of a thermocompost system

This is a summary of a presentation during the 3rd International Biomeiler Conference in Leipzig in 2018.
This presentation has been transcribed and summarised by Arie van Ziel, please contact me if you'd like to extend on the text or edit something.

This research is a preliminary work of one year and Carlos [ ... ] is continuing to extend the project.

The first step was to compute the heat transfer coefficient, there might be an error, so it’s gonna be checked again.

After this the kinetics of the composting proces had to be modelled. The idea is that it tells something about how the bacterial composition and the biological proces involved.

Then the physics of the process is modelled in a standardised way with the first two processes as input data.

The model is based on a simple biomeiler setup as build from Andrea. The data was stable for a long period and then the system started pumping and every 10 minutes a measurement was taken. The outlet temperatures where lowered by about 10 degrees before sending the water back in around ± 40 degrees Celsius.

From this data a model for the heat transfer to the pipes was made. This was tricky because there is a film condensation on the pipes and the phase change makes for complex calculations.

The model was extended to a continuous system by using a finite element method to calculate the changes over time. From this the behaviour over the length of the pipes could be modelled. And from this the model could be checked with the outlet temperatures measured in the real life situation. After reaching the end of the tube in ±15 minutes there is a sudden temperature drop in the outlet temperature, because it takes some time for the water in the tube to heat up over the whole length.

Next was modelling the composting reaction. For this the most simple basic chemical reaction was used to calculate the system. In reality it is a far more complex combination of micro-processes, but for this model it was to much to take all those into account. The summerized version gives a relatively trustworthy image of the proces.

The constants in the calculation were taken from a active aerated setup of general composting from a professor in Milano.

The assumptions for the whole system where a temperature insulated setup with cold 20°C relatively dry air coming in and saturated air coming out with a temperature similar to the reaction temperature.

From this a Matlab algoritm was made to find the constants for the composting proces. By running this algoritm it could be checked with the experimental data. It was very hard to find a match, so it was decided to focus on the startup phase (the first 10 days). Only for the temperature parameter it was possible to find a constant, for aeration and humidity not a consistent result could be obtained. This is why the current model should not be used for quantitative analysis.

About the physical phenomena there was a distinction assumed between the solid and gasseous phase inside the compost. Within these two phases there is also assumed a transfer between water vapour and fluid water within the solid part. To model the heat balance between all pehnomena 8 formulas were used. 2 for fluid dynamics, 2 for heat transport and 4 for the movement and development of micro-organism species in both the gasseous and solid points in the compost.

Comsol is a multiphysics simulator and therefor was used to model the system. It was modelled in a 2D field, which could be rotated around a central symetry axis.

To see how the chimney effect would work the outer layer was modelled in the same way as the current biomeiler setup in Leipzig.

The first results show a steady state after about 30 hours in terms of the air flow. The chimney effect increases over time and is very much dependent on the porosity of the material. This porosity should be measured more in detail in reality to check the airspeeds that are possible.

After some simulations of the basic setup the tubes where included as points with stable heat sinks. They are only visible in the simulation of the temperature difference between the temperature of the solid mass and the air temperature and not found in just the air temperature simulation.

The drying rate in the pile showed a very uniform drying, even though the image shows a high colour difference. The starting moisture content was about 16000 mol/m3 and in 48h (2d) is dropped to about 14000 mol/m3. The influence of the heatsinks is also so little that it is barely visible in the graphs. The overal drying rate is strictly correlated to the saturation of the air inside the compost.

The CO2 concentration reaches a steady state after only 6 hour, but the results don’t seem realistic because the concentration is then similar to the external conditions in the atmosphere. This is due to the high air flow rates that are reached because of the large temperature differences.

Because there is no oxygen dependence in the model yet, so modelling of anaerobic conditions is not possible at the moment.

Conclusions are that the permeability of porous media seems much higher then in reality. This should be tested in reality, which can relatively easily be done. Due to this and other small shortcomings in the experimental input data the whole process is much faster then in reality.

The qualitative results however are very interesting and the start has been made to have a modelling method for the biomeiler. With future research, as happening at the moment in several places, also the quantitative results can be improved easily. The applications are then very wide, so it was worth the effort to spend a full year on developing this model.