Ashing of Hydrothermally Carbonized Fish Sludge : Co-combustion of hydrochar from fish sludge with agricultural residue

Sammanfattning

Norway is the world’s largest producer of aquatic salmon and the expanding aquaculture industry is generating substantial amounts of fish sludge as a byproduct. Although it contains valuable nutrients such as phosphorus, it is currently largely discharged into the sea leading to nutrient loss and environmental impacts. Fish sludge is a particularly challenging waste stream due to its high moisture, making handling and treatment difficult. Hydrothermal carbonization (HTC) is well suited for processing such wet biomass. Interreg Sweden-Norway project “Circular synergies between land and sea” coordinated by Research institute of Sweden is investigating HTC-treatment as a pathway to upgrade fish sludge to a higher valuable product while retaining nutrients such as phosphorus. The aim of this thesis was to investigate the potential for nutrient recovery with a focus on phosphorus, from combusted and co-combusted HTC treated fish sludge with potassium-rich agricultural residue and to evaluate the fuel behavior during combustion. Hydrochar was produced at pilot-scale from fish sludge at RISE Processum, Örnsköldsvik. The produced hydrochars were subjected to thermogravimetric analysis (TGA) to investigate their combustion behavior and co-combustion performance with a potassium-rich agricultural fuel, wheat straw and sunflower husk. The resulting ashes were analyzed using X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) to evaluate phosphorus phase transformation and interaction with potassium. Comparisons were made between hydrochars produced from freshwater and saltwater fish sludge, as well as between their corresponding blends with agricultural residue. The results show that co-combustion of hydrochar with wheat straw improves the combustion performance while co-combustion with sunflower husk generally reduces the combustion performance. Wheat straw provided the most consistent improvements in ignition, burnout, combustibility and combustion stability. Ash analyses revealed that phosphorus in pure hydrochar ashes was mainly present as Mg-substituted whitlockite and hydroxylapatite, phases associated with low solubility and therefore limited short-term availability for plant. Co-combustion promoted the formation of alkali-substituted phosphate phases associated with higher solubility such as K-Ca-phosphates and K-Mg-substituted whitlockite which is associated with higher solubility and potentially improved plant availability. In summary this thesis demonstrates that co-combustion of hydrochar derived from fish sludge with potassium-rich agricultural residues can enhance both combustion performance and the fertilizer potential of the resulting ash showing an alternative pathway for hydrochar.