Drying wood chips through fermentation

Fermentation as a natural dryer

When woody plants are chipped and stacked in a heap, fermentation begins naturally. This fermentation generates a significant rise in temperature — up to 70–80 °C at the core of the heap, due to intense bacterial activity. Not only does this heat help drive out moisture, but the bacteria responsible for the fermentation actually consume water to live. This is how green wood chips can be dried quickly, making them suitable for use in an automatic boiler without resort to other drying methods — such as the long wait required when drying logs.

This technique offers both a time advantage — you can chip immediately after felling without waiting for the wood to dry in log form, which can take months or even years — and a practical advantage, since chipping green wood requires less power and generates less noise, dust and equipment wear than chipping dry wood. It also allows the whole tree to be valorised, from trunk to branch tips.

The fermentation drying protocol

To achieve good results, a few simple rules must be followed during harvesting and storage.

1. 1
   Chip immediately after felling

   It is essential to chip the wood as quickly as possible after felling — ideally cutting and chipping in a continuous operation. This clears the working area for greater safety, avoids tangles of branches that complicate chipping, and above all allows fermentation to begin immediately and vigorously on freshly cut material.

2. 2
   Store at 1 metre height for 1 month

   During the active fermentation phase, the heap height must be kept to approximately 1 metre. If the heap is too large, the heat produced at the core — which can reach 70–80 °C — can cause internal charring and material loss, among other problems. A height-limited heap avoids this runaway effect and delivers optimal drying.

3. 3
   After one month: build the permanent storage heap

   After one month, active fermentation slows and the temperature drops. The chips can then be re-stacked to much greater heights, with no risk of charring. The slower fermentation that continues gradually dries the chips further, until a moisture content below 20% is achieved in three to five months.

Granulometry and drying quality

Chip granulometry is doubly critical in this process: it determines both the quality of fermentation drying, and the performance of subsequent use in an automatic boiler.

Ferment to dry, or ferment to heat?

The fermentation described above is a short, dry fermentation — the goal is to drive out moisture, for eventual use in a chip boiler. But the same reaction can be managed differently: by watering the heap regularly, fermentation is maintained over time and heat can be extracted by contact — hot water, greenhouse heating, etc — while simultaneously producing a high-value organic soil amendment.

This is the approach explored by Jean Pain from 1975 onwards, described in detail in The Methods of Jean Pain. The two uses are not mutually exclusive: the choice between them depends essentially on the volume of material available and the objective.