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

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 (O-2) depletion). Herein, supercritical carbon dioxide extraction (scCO(2)) 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 scCO(2) extraction of the sawdust. These off-gassing processes were considerably reduced for scCO(2)-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 O-2 concentration (20% to 19.3%) was observed for the scCO(2)-pellets, while an 8% decrease (reduction in O-2 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 scCO(2) pellets compared to the reference pellets. This work demonstrated that scCO(2) 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.