Integrating torrefaction of pulp industry sludge with anaerobic digestion to produce bioenergy and biochemicals: Techno-economic and environmental feasibility analysis

Abstract

In order to improve the economic competitiveness of bioenergy carriers and biochemicals, they must be produced from low-cost or no-cost feedstock and at a reduced operational expenses. In that regard, this study focused on understanding the techno-economic feasibility of using pulp sludge as a low-cost alternative feedstock to forestry biomass in torrefaction. Economic feasibility of further integrating pulp sludge torrefaction with anaerobic digestion to produce energy, biomethane and volatile fatty acids (VFA) was also studied. The operational expenses were around 8.2 and 2.04 M€ and the minimum selling price of torrefied pellets was 407 and 189 €/t for forestry biomass torrefaction and pulp sludge torrefaction respectively. In case of integrated approaches, VFA production showed higher economic feasibility with a torrefied pellets selling price of 163 €/t compared with biomethane production (213 €/t). The biomethane and VFA selling price can be reduced by 90 and 64% compared with current market price at torrefied pellets selling price of 260 €/t. Sensitivity analysis revealed that, moisture content of the sludge is the main influencing parameter on the overall economic feasibility of the pulp sludge torrefaction. The environmental analysis showed that pulp sludge torrefaction has higher emissions compared with forestry biomass torrefaction.

Keywords

• Pulp sludge torrefaction
• Torrefaction condensate
• Anaerobic digestion
• Volatile fatty acids
• Techno-economic analysis
• Environmental feasibility