The digestate to be dried is spread very evenly over the top stage.

The 25 m long drying tunnel has a capacity of 10 t per day.

The dried digestate with a dry matter content of 85 to 90 percent leaves the drying tunnel.

There is often not sufficient land for spreading digestate.

In the production of electricity and heat using biomass for biogas, a digestate is produced which, like slurry, has a dry matter content of 5 to 15 percent.  Sufficient storage capacity must be available for this digestate and there must be adequate land available for spreading it as a fertiliser.  This can be a problem in regions with intensive livestock production where all the existing land is required for livestock wastes.

Optimal use of nutrients from biogas units

Until now, the coarse solids were separated from the digestate using an auger press and dried.  In this process, the dried material accounts for only 10 to 15 percent the total digestate volume leaving a large volume of the liquid fraction that must be spread.  Ludger Möllenbeck from Füchtorf, North Rhine-Westphalia, whose biogas unit went on the electricity network in 2002 is familiar with this problem.  The farmer has been involved in biogas for more than 20 years.  “I gave my first talk about biogas as far back as 1984”. Those who know Ludger Möllenbeck recognise him as an expert who has many ideas and who knows how to realise these through suitable partners.  There remains a problem because biogas digestate is more difficult to separate than slurry and the liquid phase is rich in ammonia (NH3).  To address this, the technical college Münster (“Fachhochschule”) has guided a demonstration project on the theme “Optimised use of nutrients from biogas units through ammonia stripping and digestate processing”.  This project was supported by the Federal Ministry for Food, Agriculture and Consumer Protection.

What happens to the volatile ammonia?

Drying of the digestate using heat from the combined heat and power (CHP) system

This led to consideration of how to dry the ammonia free digestate.  Ludger Möllenbeck had the idea to use a trying tunnel.  Drying tunnels for manure are not new, until now they are used mainly for drying poultry manure.  The drying of digestate from biogas units is a completely new application.

In cooperation with the Big Dutchman company, a 25 m long drying tunnel with ten stages was constructed.  To handle digestate with a dry matter content of approx. 8%,  untreated digestate is mixed with dried digestate before loading onto the highest stage of the tunnel.  Two spreading augers ensure even spreading and mixing of the digestate.  The digestate goes through the tunnel on conveyor belts and is continuously taken from the tunnel in a dry condition.  By drawing warm air through the tunnel using negative pressure, heat from the CHP system is used to dry the digestate.  The perforated conveyor belts ensure the warm air penetrates the digestate.

The drying material spends two days in the tunnel.  At the end, the material has a dry matter content of 85 to 90 percent and can be used as a valuable potassium and phosphate fertiliser.

Conclusion

The drying of biogas digestate opens up new opportunities, particularly in regions with intensive animal production.  The dried digestate can be transported economically and therefore used on distant land.

The drying of digestate not only contributes to environmental protection through processing organic residues, it can also raise the economic returns from the biogas unit because water is evaporated from the digestate removing the need for separation.

Dipl.-Ing. agr. Janett Peschel
Big Dutchman International GmbH