Impact of supercritical extraction on solid fuel wood pellet properties and off-gassing during storage

Abstract

Biofuel pellets derived from wood raw material are an important carbon neutral source of energy. Their storage and bulk transportation can lead to serious hazards as a result of off-gassing (CO, CO₂, CH₄ combined with serious oxygen (O₂) depletion). Herein, supercritical carbon dioxide extraction (scCO₂) has been demonstrated as an efficient tool in significantly reducing these emissions from wood pellets. 84% of the lipids and resin acids have been removed from the sawdust prior to pelletisation. Crucially, this work reports the first off-gassing measurements associated with lipid autoxidation in wood pellets post scCO₂ extraction of the sawdust. These off-gassing processes were considerably reduced for scCO₂-extracted sawdust pellets, when compared to pellets prepared from virgin untreated sawdust (reference pellets). Significant reductions in the levels of CO, CO₂ and CH₄ (85, 85, and 94% respectively) were observed. A slight reduction in O₂ concentration (20% to 19.3%) was observed for the scCO₂-pellets, while an 8% decrease (reduction in O₂ concentration from 20% to 12%) was observed for the reference. The results support a connection between the amount of lipids/resin acids and intermediate products of autoxidation (i.e. 71% reduction in aldehydes) and the off-gassing of CO, CO₂ and CH₄. Finally, there was low impact on the production, durability, calorific values and density of scCO₂ pellets compared to the reference pellets. This work demonstrated that scCO₂ extraction is effective as a pre-treatment technology for wood based pelletised fuels, considerably reducing the risks associated with off-gassing and oxygen depletion, while also highlighting potential chemicals and biofuels which could be generated from extracts.