Selfheating and Cooling of Digesters

When I started constructing and commissioning the first high-loaded biogas digesters (digester load 8-10 kg oDM/cbm/d) in 2005, an unexpected phenomenon appeared – the digesters warmed up by themselves in summer without activated heating. The real problem was the too fast increase of temperature, leading to a breakdown of the biology. In a few weeks the planned digester temperature of 39 °C increased to 43 °C, and the digester biology suddenly became unstable.

At a more detailed examination of the ongoing biochemical processes in the digester, it is found, that this warming-up is initiated by the digester biology. In plants with a very high proportion of liquid manure, also with the fermentation of industrial waste water and with the sewage sludge digestion, this effect does not matter, as on the one hand the substrate has a low energy density (biogas potential), so that at the degradation of the organic fraction comparably less heat is released and on the other hand the substrate is so cold and diluted, that the less released heat is insufficient to generate a considerable measurable warming-up.

High energy density in silage enhances heating
With the fermentation of silage, the situation is completely different. The energy density is about ten times higher than with liquid manure, this means, per ton silage ten times more heat is released and this heat must be absorbed by the more than ten times smaller remaining mass from the fermentation. Thereby self-heating develops to an important factor for the stable operation of a biogas plant, in particular, when the heating cannot be controlled.

To control the increase of temperature, in several biogas plants I know, digester cooling systems are integrated. Depending on the philosophy of the operator, temperatures of 39, 42 or 44 °C are set, and retained in summer by means of the digester cooling. In my experience two basic errors lie herein.

Controlling of the digester temperature
First, the absolute level of the digester temperature is not important. Therefore it makes no sense to retain for example 42 °C or 44 °C with major efforts, a considerable higher temperature could also be chosen. The second error lies in the assumption, that only a cooling system would be able to control the digester temperature.

A much more economically sensible way to control and stabilize the digester temperature at a strong self-heating lies in raising the digester temperature from the start to a level, which reliably prevents an increase in the summer months.

When creating a heat balance for the digester, one could find, that at a higher digester temperature also a proportional higher quantity of heat is discharged with the digester output. This additional heat discharge must be so high, that the temperature in the digester does not increase further in midsummer. The only disadvantage of this method is, that slightly more heat is needed for heating in winter. But as most biogas plants have surplus heat anyway, this can be ignored in terms of economy.

The optimum digester temperature depends on the fermentation system, the load of the digester and the climatic conditions. Plants with silage monofermentation in Germany with a load till 10 kg/cbm/d prevent the temperature problem in summer, if they are operated at a digester temperature of 48 to 52 °C. With a digester temperature of 52 °C they are on the safe side regarding the uncontrolled self-heating in Germany. You can better use the cooling system for cooling the biogas or for supporting the CHP heat exchanger in summer. This is more sensible.

If you have questions, please leave a comment or contact me. I will collect the questions and answere in a special article.