Comparative cradle-to-gate life cycle assessment of wood pellet production with torrefaction

Abstract

Torrefaction is a thermal pre-treatment process for upgrading raw biomass into a more energy dense fuel. Torrefied biomass is combined with a densification process to increase its bulk density similar to conventional wood-pelleting production. This paper identifies the significant environmental impacts associated with production and delivery of these two fuels, using cradle-to-gate life cycle assessment. A feedstock of Scots Pine is modelled for a localised torrefaction/wood pellet plant located in Norway, with the products from each facility delivered to a power station in the UK.Results show that the relative benefits of torrefaction over wood-pellets are higher on per MJ delivered basis due to the higher calorific value of the fuel. The climate change and fossil depletion impacts for torrefied pellets modelled were lower than wood pellets, using an assumption that the drying requirement of the reactor was 3.0MJ/kg water removed for both cases. Sensitivity analysis of the model indicated that the relative impact improvement of the torrefied pellet case compared to wood pellets is strongly dependent on the biomass drying requirement and the proportion of total process heat supplied by the re-circulated torrefaction gas. Land requirements for torrefied pellets are higher due to the mass losses in production.