Automated Manufacture of Fertilizing Granules from Burnt Wood Ash 
Thomas Svantesson 
External reviewer: Prof. Tore Hägglund, Dep. of Automatic Control, LTH

This work considers control of an ash transformation process, which transforms wood ash produced at district heating plants into fertilizing granules. The manufactured granules are recycled back to the forest grounds, as a fertilizer, or as a tool to reduce the acidification in the forest soil at the spreading area. Other areas of application are, for example, structural fill and substitute for cement in ready-mix concrete. 

The ash transformation process includes mixing, size reduction, granulation, drying, sorting and packing. Furthermore, the transformation process uses ETEC-dolomite that acts as a binding agent and therefore improves the strength of the produced granules. Due to the dolomite, the lime effect of the produced fertilizers is increased, whereas the leaching speed of the wood ash included heavy-metals is decreased. A robust machine is developed and controlled by an industrial control system in order to enable continuous and automatic manufacture. At present, the units for mixing and size reduction are fully implemented, built to comply with the industrial requirements for continuous operation. The remaining stages are controlled to a certain extent, but are still based on the earlier prototype. 

Mixing ash/dolomite/water in order to obtain granular material is one method to stabilize wood ashes. The main problem is predicting the quantity of water to be added since the necessary amount varies with the wood ash quality. The implemented controller is therefore able to determine this optimal amount without any measure of the wood ash quality, as for example the ash carbon content. However, the produced granules do not benefit from high carbon content. Therefore, two potential on-line methods for carbons in ash monitoring are presented but not implemented due to financial reasons.