Energy Potential of Pellets Produced from Green Waste Compost

Green-waste compost may represent an alternative feedstock for the production of densified solid fuels; however, its energetic use is limited by its high mineral content and variable composition. The aim of this study was to evaluate the fuel and mechanical properties of pellets produced from green-waste compost and conifer sawdust. The compost was obtained from waste generated during the maintenance of urban green areas and was collected during two periods in 2025. For each collection period, five variants were prepared, including pellets made from 100% compost and mixtures containing 20–50% conifer sawdust. Nitrogen, carbon, and hydrogen contents, ash content, volatile matter, lower heating value (LHV), maximum compression force, and the energy required to reach the maximum force were determined. The composition of the feedstock mixture and the compost collection period significantly affected the properties of the pellets. Carbon content ranged from 31.40 to 38.56%, ash content from 26.20 to 42.56%, and volatile matter from 44.63 to 58.08%. The LHV ranged from 10.13 to 14.21 MJ·kg−1. The highest LHV, maximum compression force, and compression energy were obtained for variant P1C50, containing 50% compost and 50% conifer sawdust from the first collection period. Pellets produced from compost collected during the first period generally exhibited lower ash content, higher LHV, and greater mechanical resistance than the corresponding variants produced from compost collected during the second period. The formulations containing 50% sawdust generally showed the most favorable energetic and individual-pellet compression results; however, the responses at intermediate sawdust proportions were non-monotonic and batch-specific, and high ash content remained the main limitation.