How does extraction of biologically active substances with supercritical carbon dioxide affect lignocellulosic biomass properties?

In the bio-based economy concept, any biomass should not be used directly for energy purposes without considering the possibility of using it for bioproducts with higher added value. Extractable phytochemicals found in lignocellulosic biomass of perennial industrial crops (PIC) are used in the pharmaceutical, cosmetics, chemical, food and feed industries. Therefore, these compounds should be obtained first, and only the so-called PIC extraction biomass should be used, for example, for the production of bioenergy. An efficient low-energy and environmentally friendly method of extracting phytochemicals from the plant biomass is supercritical carbon dioxide (scCO₂) extraction. The aim of the study was to assess the thermophysical properties and chemical composition of 19 types of PIC biomass previously subjected to two types of scCO₂ extraction compared to the biomass before extraction. The biomass after pure scCO₂ and scCO₂/H₂O (1 wt%) extraction contained less ash (by 4.9 and 11.3%), carbon (by 3.1 and 7.8%), hydrogen (by 5.8 and 8.9%), nitrogen (by 4.5 and 6.8%), sulfur (by 3.8 and 3.8%), lignin (by 5.4 and 1.1%), hemicellulose (by 14.4%) and more chlorine (by 4.7 and 15.3%) compared to the biomass before extraction. In addition, the biomass after extraction with pure scCO₂ contained more cellulose and was characterized by lower moisture content and higher heating value and lower heating value. However, the biomass after scCO₂/H₂O extraction contained more substances soluble in cold and hot water. Generally, the bark of four short-rotation coppice (SRC) species was characterized by a high content of nitrogen, sulfur, substances soluble in cold and hot water, other soluble substances and at the same time low cellulose content (26–28% d.m.). In turn, the wood of all SRC species was characterized by a high cellulose content (51–56% d.m.). However, all herbaceous crops and grasses contained more chlorine, hemicellulose and generally less lignin compared to the SRC biomass. To assess the practical suitability of the PIC biomass extraction for the production of bioenergy (solid, liquid or gaseous biofuels), further research is needed.