Nele Müller - Deutsches Biomasseforschungszentrum DBFZ

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.

Already ten years ago Nele got to know the biomeiler, three years ago she met Heiner Cuhls and started working more on the technique. Last year she did her master thesis at DBFZ, they do several kinds of biomass processing.

Nele has a pragmatic point of view, very much science based.

Basic explanation of science behind heat sources.

1st part: Oxidation of cellulose, aerobic part is much larger then the anaerobic part.

2nd part: actinomycetes, degradation of lignin

3rd part: formation of humus acids

Explaining aeration of compost piles.

There is always a chimney effect, but without help there is an anaerobic core.

With passive bottom aeration, the pile can only be up to 1m high.

Currently in Leipzig there is a perforated chimney system in the biomeilers with passive aeration, which can temporarily be extended with a active pump on both sides and/or the top.

Wittenberg project with a beautiful picture of the church of Luther and all 11 biomeilers.

There were 4 sizes, 25-70m3 in size. The location of all where very close and in a lower area with not much direct sun and wind. The layout was based on the artists plan to prevent continual sight lines.

Classical ‘Heiner’-layout with 3 layers of horizontal tubes. permanent temperature measurement with internal electronic sensors. Every biomeiler had it’s own heating system cycle, with a filter, pump and flow sensor. 12 hours on at night, 12 hours off during the day. The little bed of exotic plants was using the heat. The total amount of heating was much more than the heat use of the plant bed.


We always do a standard check of the biomass in terms of water content, organic matter, c:n ratio and particle size, caloric value.

After only 6 months the biomeilers where taken apart, so we got results of an intermediate situation.

From outside to inside: light brown border area, white ring and black core area. Where the most oxygen was there was high degradation, but in the middle not. The C:N ration in the core was higher, so there are several theories about what caused this. Lack of oxygen is the most probable, but this will be investigated later.

The white ring was a fungi creating a ring/ball around the middle area. Probably because of the ...?

Comparison of different temperature and gas measurements

On different biomeilers, heights and sides there were similar results on the gas profile.

This could be because of similar wind and shading.

With ‘cake’ diagrams it’s possible to explain the difference in gas content.

It is clear that because of the chimney effect there is a lot of methane and CO2 exiting from the biomeiler. The data is also checked with the outside measurements.

In the highest point is 30-40% volume of methane! [this caused a lot of debate during the conference]

In terms of temperature, at the start mainly the top and the side is hot, relating directly to the oxygen content into the biomeiler. Also it relates to the density of the material. Also keep in mind that the temperature you measure is always a product of the heat transport within the system.

Temperature box plot clearly show the result of the size difference of the biomeilers. Also the ones where heat is not taken out, are clearly visible. Also in measurements of each biomeiler separately it is visible that there is a heat storage effect. Actually by taking out heat, the bacterial activity increases by preventing too high temperatures. The total composting process can be extended by using the heat as compared to normal composting. Heat distribution inside the biomeiler is however more equally spreading if the heat is not used.

The interesting thing about this is that looks like it is possible to steer the kinds of micro-organisms inside the system.

If anyone makes claims about high temperatures in combination with high power output, it should be supported by data to prevent it being turned into ‘urban legend’.

Statistically you can see if there are any correlations. Some interesting results seem to be:

  • The way of heat exchanging is very much influencing the proces. 8 l/min. flow, and 14° delta T. Minimum temperature should be at least 45°C.
  • The quality of doing the total construction, and changing the density of the biomeiler by compression during the build up.
  • It is good to start the heat taking only after the initial temperature peak.

Many phenomena are very clearly explained by the data, but many are not yet, so we should measure a lot more.

Even though the heat and vapour are transported through the biomeiler, many bacteria will stay in the same spot.

Power measurements are sadly not available, because there is only two months of data. There are some estimates, but they are not completely certain.

About the current project in Leipzig 2018

The goal is to create a system which produces an amount of heat useable for a household.

The biomeilers that were build directly after chipping the wood, were heating up much faster than the ones build later from the same material.

Factor list divided into can (25) or cannot (30) influence. Created hypothesis about which changes will happen if changing one of those factors.

Some factors are not researched at all yet, so the focus is on this ‘blind spot’.

The current setup is 6 biomeilers which we will see on Sunday.

One of the subjects is the control of the passive airflow through the biomeilers. So four biomeilers will be completely packed in plastic foil with a controllable valve in the ‘chimney’.

They tried to make the startup the same as much as possible and the changes will be made in the type of flow regulation from the heat exchangers.

There is not yet an optimum calculated between the surface area of the heat exchanger and the volume of biomass.

[More on the Leipzig project will follow shortly]