Green coal-based fuel produced via hydrothermal carbonization of disposable tableware in seawater medium

To address the need for solid waste resource utilization and promote sustainable development, this study employed hydrothermal carbonization (HTC) in a seawater medium to convert disposable tableware into green coal (hydrochar). Our results revealed that the energy yields of hydrochar produced at 220 °C in freshwater and seawater were 69.13% and 70.22%, respectively, with the seawater medium yielding 1.09% higher than freshwater. Elevating hydrothermal temperatures enhanced fixed carbon content, reduced volatile matter, and intensified coalification. Seawater's catalytic role promoted structural evolution, evidenced by heightened aromatization, diminished oxygen-containing functional groups, and stabilized carbon skeletons. Furthermore, HTC in seawater significantly improved hydrochar's thermal stability and combustion performance: Inorganic salts elevated the combustion energy barrier while reducing activation energy. Critically, seawater-mediated hydrochar curtailed emissions of CO2, NO2, and SO2, while mitigating slagging and scaling risks on heated surfaces. The resultant green coal outperformed lignite in fuel quality, exhibiting a higher heating value, lower ash content, and lower sulfur content, despite having a lower fixed carbon content than lignite. This work not only addresses dual crises of energy security and waste management but also advances carbon-neutral technologies by integrating resource recovery with environmental stewardship.