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

## TL;DR

This paper explores using hydrothermal carbonization to convert paper industry waste into sustainable fuel that meets EU standards.

## Contribution

The study demonstrates the technical feasibility and benefits of using HTC to transform pulper rejects and fine screen debris into high-quality solid fuel.

## Key findings

- Hydrothermal carbonization increases the heating value of solid residues by up to 83.6%.
- HTC reduces halogens, mercury, and critical inorganics while meeting EU SRF standards.
- Optimal parameters like temperature and liquid-to-solid ratio enhance fuel quality and environmental performance.

## Abstract

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.