Supercritical CO2 extraction of woody and herbaceous biomass of perennial industrial crops reduces off-gassing during pellet storage

Abstract

Lignocellulosic biowastes processed by supercritical fluid extraction (SFE) can become a valuable energy source in the era of bioeconomy development. However, stored biomass, including pelleted biomass, undergoes off-gassing, which could pose a threat to human health and life. The aim of this study was to examine CO, CO₂, and volatile organic compound (VOC) emissions from stored pellets produced from woody and herbaceous biomass of five species of perennial industrial crops (PICs): Salix viminalis (SV), S. purpurea (SP), Populus nigra × maximowiczii (PNM), Miscanthus×giganteus (MG), and Helianthus salicifolius (HS). The analyzed pellets were produced from biomass that was or was not subjected to supercritical CO₂ extraction. Headspace concentrations of CO, CO₂, VOCs, and O₂ were measured in closed glass bottles over a period of 42 days. The content of lipophilic extractives and UV-Vis spectra were also analyzed. Pellets made from non-extracted HS biomass (HS-ne) were characterized by the highest concentrations of CO, CO₂, and VOC, which peaked on storage day 42 (3600, 23,500 and 250 ppm, respectively). In HS-ne pellets, CO, CO₂, and VOC emission factors (EFs) reached 172, 1791, and 23 μg kg^–1 day^–1, respectively. Biomass pretreatment significantly reduced off-gas emissions. In all pellets, the SFE process decreased EFs for CO, CO₂, and VOC by 30 %, 33 %, and 69 %, respectively, and an even greater reduction was noted in HS pellets (56–87 %). In turn, the lowest EF values were observed for PNM (31 μg CO kg^–1 day^–1 on average), SV (351 μg CO₂ kg^–1 day^–1), and SV and SP pellets (0.6 and 0.8 μg VOC kg^–1 day^–1, respectively). A better understanding of off-gassing processes in pellets made from PIC biomass, including biomass subjected to SFE, is key to developing effective strategies for their safe storage, transportation, and use.