Toward a More Sustainable Paper Industry: The Contribution of the Hydrothermal Carbonization for Solid Residues

The paper industry has historically relied on virgin raw materials as the primary resource for production. Moreover, it requires high energy and water consumption and use of chemicals like chlorine. In response to these environmental challenges, the paper industry is progressively moving toward more sustainable practices aligned with circular economy's principles. However, the shift to recycling introduces new challenges, particularly the generation of new solid typologies of residues, such as pulper rejects and fine screen debris. This study investigates hydrothermal carbonization (HTC) as a circular solution for transforming such two solid residue streams into an effective solid fuel. More specifically, it verifies the technical feasibility of such technology on a laboratory scale. Moreover, the energy potential and environmental impacts of the obtained fuel are evaluated. Finally, it is checked whether the obtained hydrochar is compliant with the EU directive regarding Solid Recovered Fuel (SRF). Our findings confirm that HTC is an effective solution for the transformation of investigated solid residues. Moreover, all obtained hydrochars have a higher level of high heating value when compared with the original inputs (up to 83,6%). In this respect, the operating severity (250 °C) and liquid-to-solid ratio emerged as primary levers to raise the heating value while curbing halogens, mercury, and SRF-critical inorganics. Finally, the obtained hydrochars meet EU legislation in terms of energy and environmental performance, reaching even SRF Class 1 levels. This in turn confirms that HTC process parameters and feedstock mix can be properly tuned to deliver SRF-grade solids with stable, high-quality combustion behavior.