Project meeting of STEP at the site of the company ATS - Agro Trading & Solutions in Hardegsen (Photo: DBFZ)

Biogas Biomass technologies Validation market potential Biomass furnace

Project ID 03KB099

CLEANPELLET - Development of methods for production of digestate-based pellets with low emission combustion properties for use in small-scale furnaces

Combustion (Photo: DBFZ)


  • 01.09.2014 – 31.08.2017


Fraunhofer Institute for Ceramic Technologies and Systems IKTS
Biomass Technologies and Membrane Process Engineering
Winterbergstraße 28
01277 Dresden

Björn Schwarz – project manager
Telephone: +49 (0)351 2553-7745

Dr. Burkhardt Faßauer – direct contact person
Telephone: +49 (0)351 2553-7667


03KB099C - Senzyme GmbH

03KB099D - DBFZ Deutsches Biomasseforschungszentrum gemeinnützige GmbH


Associated partners

Cooperation partners


03KB099 - IWE-Ingenieurgesellschaft für Wasser und Entsorgung mbH


Results only available in German.


Within Europe Germany is the country with the largest abundance of lumber. Nevertheless, studies predict a Europe-wide supply shortage in wood of about 290 million m³/a in 2030. The increasing energetic use of wood pellets in decentralized small-scale furnaces is considered as one of the main reasons. Therefore the project CLEANPELLET aims to develop an alternative to pure wood pellets on the basis of specifically modified fermentation residues from biogas production. Presently such fuels are not permitted in Germany. Due to their different ingredients they cause problems during combustion, like:

  • Odour and pollutant emissions (aromatic, sulfur and nitrogen compounds)
  • High dust emission by reason of high ash content 
  • High slagging tendency due to a low ash melting point
  • Poor initial ignition


Aim of CLEANPELLET is the production of digestate-based pellets with low emission combustion. Therefore combinations of physical, thermal and biologic-enzymatic processes are to be tested for their capability of separating the substances, causing the problems mentioned above, from the solid digestate fraction. Furthermore studies to improve the pelletizing of digestates in laboratory and pilot plant scale are performed. Next to the usage of additives the effects of variations in process conditions on the fiber structure are also examined. Finally the produced pellets are tested for their combustion characteristics and a legal classification will be performed.


Within CLEANPELLET Fraunhofer IKTS focusing the conditioning of biogas digestate for further use as a low emission fuel in small-scale furnaces. Several digestate samples of wet and dry fermentation plants will be analyzed regarding their material composition. Based on this results multiple physical, thermal and biologic-enzymatic processes are valuated with regard to their capability of cleansing the solid fraction of problematic ingredients like K, Na, S, N and ash. For this purpose a test rig in pilot plant scale will be constructed. In addition the process will be analyzed via process tomography for further optimization. An attempt is to be made to recover valuable materials from the process liquid with the help of ceramic membranes. Finally, a balancing and both legal and market-based classification of the entire process is performed by the IWE mbH as a subcontractor of Fraunhofer IKTS.


  • Research on pelletizing behavior of produced modified/not modified solid digestate
  • Derivation of process windows for conditioning reactor
  • Manufacturing and build-up of a pilot plant-scale conditioning reactor
  • Operation/optimization of the reactors
  • Continuous production of pellets in pilot-plant scale
  • Analysis of additives


Senzyme GmbH will provide the project partners with enzyme blends for the enzymatic digestion of solid biogas residue. At first, they will develop a high-throughput screening to identify the most suitable enzyme blends. The relevant parameters in this experiment are: degradation of dry matter, variation of particle size, water holding capacity, composition of saccharides and the release of ions.

Senzyme GmbH offers a large quantity of products with diverse enzyme spectra which can be combined at will. Because of the high number of possible variations Senzyme GmbH have to work with Experimental Design for development and evaluation in their trials. During these tests they are also looking for correlations between effects and enzyme spectrum. Further studies on enzyme dosage and enzyme stability will follow and the most promising candidates will be presented to the co-operation partners. On the basis of their experiences Senzyme GmbH will further optimize the enzyme blends.


The DBFZ will analyze the produced pellets for their combustion properties, their relevant legal framework and their economic viability. Combustion tests will be conducted in a small-scale boiler (<100kW) and the emissions will be monitored. Through these tests the overall suitability of the pellets for application in small-scale boilers will be assessed and predictions for application in medium-scale plants will be made. Additionally, in order to assess and compare energetic and technological properties, the DBFZ will conduct pelletizing tests as a reference scenario to the densification technology of LEHMANN-UMT GmbH.



  • Studies on conditioning of fermentation residues in laboratory and pilot plant scale
  • Process tomographic evaluation of the conditioning process
  • Studies on the separation of contaminants/materials recovery by means of ceramic membranes
  • Balancing and legal/market-based classification of the entire process


  • Digestate compact and conditioning
  • Solid-fluid separation


  • Development of a high-throughput screening
  • Selection of suitable enzyme blends for an optimized degradation of lignin-rich digester residue


  • Laboratory analysis of pellets to assess their chemical and physical-mechanical characteristics as well as combustion properties
  • Combustion test and emission measurements on pellets in a small-scale boiler
  • Assessment of relevant legal framework of the pellets and their economic viability


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