Co-pyrolysis of fish with pruning waste for biochar production as an amendment for composting in the biorefinery scenario

The circular economy model encourages waste from linear process chains as feedstocks to recover bioproducts. To this end, biorefineries are gaining increased attention as they produce valuable products from biomasses by utilising different conversion processes [1]. Fishery industry residues can make suitable feedstock in coastal areas for implementing biorefineries as global production is expected to reach 204 million tonnes (Mt) by 2030 due to growing demands [2]. This leads to massive amounts of leftovers accounting for 30–70% of original fish, depending on processing requirements [3]. Companies pay discarding fees for the safe disposal of these leftovers. A company in Ancona, Italy, whose discards production accounts for 26 t.y-1, incurs annually 5720 € in disposal costs. However, being composed of a substantial amount of proteins, minerals, lipids, polysaccharides, vitamins etc., and recovering these biomolecules may not only close the loop in the fish industry but also mitigate environmental issues related to the seafood industry. Different valuable products can be recovered depending on treatment processes (composting, pyrolysis, enzymatic hydrolysis). This study aims to co-pyrolysis fish waste (FW) with pruning waste (PW) to obtain biochar as an amendment for downstream composite biochar compost to obtain compost soil amendment for application in the agricultural sector.

Different feedstock (FW, GW, PW shellfish, microalgae, enzymatic hydrolysis residues) were characterised using TGA analysis to evaluate their potential for biochar production. TGA analysis of FW showed around 67.2% weight loss in the organic fraction and low inorganic content, however, up to 80% moisture content. Although FW would make suitable feedstock for biochar production, their wet nature would require extensive energy demands for drying. Alternatively, co-pyrolysis with other feedstocks such as lignocellulosic green waste (GW) and PW biomass can overcome this drawback, as they are commonly used for thermochemical processes and TGA characterisation showed about 51.4% and 60.1% weight loss in organic fraction for GW and PW respectively. Lab scale pyrolysis tests simulating slow pyrolysis resulted in 39.8±8% biochar for 100% GW, whereas 36.5% biochar was obtained for 50: 50% (FW: GW). Similarly, 37±1.5% for 30: 70% (FW: GW) while only 26.9% biochar db was obtained for 100% FW. Biochar characterisation showed a FW: GW ratio higher than 30:70% exceeds the maximum limit for nickel concentration set by EU reg. (2019/1009).

To confirm the replicability and process scale-up, pilot-scale pyrolysis tests were performed using lab scale parameters: Temperature of 400℃, a heating rate of 5.33℃/min, at different residence times (RT) of 40, 50 and 57 minutes. Both fish and pruning waste were pretreated (ground and dried at 80℃ for 24 h) to reduce moisture content and particle size. Results showed that with increasing RT, biochar content increased, attributable to secondary reactions for biochar production. 55.9% biochar was obtained at a RT of 57 minutes. Whereas a decreasing trend was observed with reducing RT of 50 and 40 minutes (49.6% and 49.9% respectively). While biooil content showed an increasing trend (12.5-17.9%). Biochar and bio-oil are currently being characterised for physio-chemical properties and heavy metals analysis. Besides, the pH and EC of biochar were within the required limit set by Italian legislation (D.Lgs. 75/2010). More tests are ongoing to confirm to further optimise the pyrolysis process as well as syngas collection and characterisation. In conclusion, pilot tests show promising results for potential upscaling of the co-pyrolysis process for FW and PW for biochar production and subsequent addition to the downstream composite composting process to produce soil improvers for the agricultural sector.