Torrefaction of kraft pulp mills sludges

Area of Science:

• Biomass energy conversion
• Waste-to-energy technologies
• Chemical engineering

Background:

• Primary and secondary sludge from pulp mills are biomass waste streams.
• Current disposal methods include landfilling, representing a missed resource opportunity.
• Torrefaction is a thermochemical process for biomass energy enhancement.

Purpose of the Study:

• To assess the technical feasibility of torrefying pulp mill sludge (primary, secondary, and mixed).
• To evaluate the impact of varying torrefaction temperatures and residence times on sludge properties.
• To determine the potential for sludge-derived torrefied biomass in energy generation.

Main Methods:

• Sludge samples (primary, secondary, mixed) were subjected to torrefaction.
• Process parameters included temperatures of 260, 290, and 320°C.
• Residence times varied at 20, 40, and 60 minutes.

Main Results:

• Increased torrefaction temperature and residence time improved physical and chemical properties.
• Heating value increased due to elimination of non-energetic compounds and carbon concentration.
• Moisture reduction led to higher lower heating values; energy yield and density also increased.
• Optimal conditions (320°C, 60 min) yielded significant heating value increases (76% PS, 27% SS, 41% MIX).

Conclusions:

• Torrefaction is a viable method to convert pulp mill sludge into valuable energy biomass.
• Optimized torrefaction parameters enhance energy content and density, making sludge suitable for energy generation.
• This process offers a sustainable alternative to landfilling, promoting a circular economy in the pulp and paper industry.