# Green and Efficient Processing of Wood with Supercritical CO2: A Review
## 5 Wood Thermochemical Conversion
### Bio-Oil Purification  
Bio-oil can be obtained by pyrolysis and liquefaction of biomass. Bio-oil is a complex mixture of more than 400 different compounds containing 25% water [114]. Large amounts of oxygen-containing compounds need to be removed to increase their volatility and thermal stability for bio-oil used as fuel [115]. Compared with expensive organic solvents, scCO\({}_{2}\) is a good medium for extracting heat-sensitive components such as bio-oil [116] because of its lower critical value. Moreover, different types of compounds can be selectively extracted by changing the pressure, so that there are no chemical residues after extraction [117]. Higher purity and better quality fuels of biomass thermal conversion can be extracted with scCO\({}_{2}\).  
[PERSON] et al. [118] used scCO\({}_{2}\) to separate water from wheat-hemlock bio-oil, which was easily dissolved in scCO\({}_{2}\). The high percentage of water (45.0 wt%) was separated at 10-30 MPa and 40 \({}^{\circ}\)C. [PERSON] and [PERSON] [119] extracted pine wood (from North America) bio-oil with scCO\({}_{2}\), and reached the maximum extract yield of 14.3 wt% at 80 \({}^{\circ}\)C and 30 MPa, the water content of bio-oil was 25.4 wt%-28.0 wt%, and the extraction rate was 26.5% at 20 MPa and 60 \({}^{\circ}\)C with activated carbon as the catalyst. [PERSON] et al. [120] conducted scCO\({}_{2}\) extraction from wheat-sawdust bio-oil, and the first three fractions were tested at 45 \({}^{\circ}\)C and 25 MPa, then pressure was increased to 30 MPa for the fourth fraction. This method separates water from bio-oil with higher moisture content and low calorific value. The calorific value increased to 30.0-44.5 KJ/Kg. In the first three fractions, the value of pyrenoids and oxygenated benzenoids were high, while the values of fatty acids and high molecular mass alcohols were high in the fourth fraction.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**References** & **Wood Species** & **Liquefaction Media** & **Catalyst** & **Conditions** & **Bio-Oil Yield** \\ \hline \multirow{2}{*}{[109]} & \multirow{2}{*}{Palm kernel (_Elais guinensis_ Jacq.) shell} & \multirow{2}{*}{H\({}_{2}\)O} & — & 300 \({}^{\circ}\)C; 30 MPa & 3.00–6.59 wt\% \\  & & & ScCO\({}_{2}\) & 300 \({}^{\circ}\)C; 25 MPa & 12.03 wt\% \\  & & ScCO\({}_{2}\) & — & 8 wt\% \\  & & & K\({}_{2}\)CO\({}_{3}\) & 300 \({}^{\circ}\)C; 11 MPa & 29.3 wt\% \\
[110] & White pine (_Pinus strobus_ L.) sawdust & Acetone & K\({}_{2}\)CO\({}_{3}\) & 300 \({}^{\circ}\)C; 45 MPa & 27.9 wt\% \\  & Ethanol & K\({}_{2}\)CO\({}_{3}\) & 300 \({}^{\circ}\)C; 73 MPa & 30.8 wt\% \\  & & H\({}_{2}\)O & K\({}_{2}\)CO\({}_{3}\) & 300 \({}^{\circ}\)C; 79 MPa & 17.3 wt\% \\
[111] & Oak wood (_Quercus puhescens_) & H\({}_{2}\)O & K\({}_{2}\)CO\({}_{3}\) & 320 \({}^{\circ}\)C & 27 wt\% \\  & & Fe & 320 \({}^{\circ}\)C & 32 wt\% \\
[112] & Aspen wood (_Populus tremula_ L.) & H\({}_{2}\)O & – & 17 wt\% \\  & & NiMo & 350 \({}^{\circ}\)C; 15 MPa & 70.3 wt\% \\
[113] & Silver birch (_Betula_ sp.) & Supercritical ethanol & — & 19 wt\% \\  & & & beta zeolite & 234 \({}^{\circ}\)C & 25 wt\% \\ \hline \hline \end{tabular}
\end{table}
Table 4: Wood liquefaction yield by using different liquefaction media.