Co-hydrothermal carbonization of pomelo peel and gasifier coke for production of fuel pellets: Complementarity of feedstocks and synergistic effects

The production of nitrogen fertilizer in China generates a substantial quantity of gasifier coke (GFC) as an industrial by-product, posing challenges for its effective recycling. In this study, a combination of co-hydrothermal carbonization (co-HTC) and pelleting technology was employed to convert pomelo peel (PP) and GFC into fuel pellets. Notably, these two feedstocks, characterized by significantly distinct physicochemical properties, exhibited beneficial complementarity, mitigating the limitations of using a single feedstock for fuel preparation. The severe caking and non-uniformity of PP hydrochar resulted in significant flaws in the pellets prepared through direct blending with GFC. The co-HTC technology significantly alleviated the poor processability of PP hydrochar and the low energy density of PP hydrochar pellet while greatly improving the flammability of GFC and the mechanical strength of GFC pellet. The results demonstrated a significant enhancement in uniformity of hydrochar after co-HTC, leading to a reduction in compression energy consumption to 6.16-7.62 J/g and an increase in pellet energy density to 30.60-35.08 GJ/m3. The ignition temperature and the activation energy of GFC were significantly reduced. The co-HTC technology effectively addressed the challenge of GFC pelletizing by achieving fuel pellets with a compressive strength of 11.89 MPa and a drop resistance of 99.8 %. Interestingly, the co-HTC process showed significant synergistic effects, increasing the energy yield of PP hydrochar to 73.38-92.9 % and effectively reducing the ash and sulfur content in GFC to 4.99-11.92 % and 0.35-0.55 %, respectively. The most pronounced synergistic effects were observed when blending PP and GFC in a 5:5 ratio. The complementarity of feedstocks and the synergistic effects of co-HTC converge to facilitate the conversion of biomass and industrial waste into high-performance fuel pellets.