# Green and Efficient Processing of Wood with Supercritical CO2: A Review
## 5 Wood Thermochemical Conversion
### Wood Liquidation  
Hydrothermal liquefaction is a common method of wood liquefaction, which is carried out with water under a subcritical state [103]. The hydrothermal is attractive when operating conditions are near the critical point of water, this is because: the mass transfer resistance is greatly reduced under supercritical state, there is no need for a drying process, energy is saved, and the change to temperature and pressure of water significantly promotes the physical properties changes, thereby improving the effective separation of products and by-products [103]. However, hydrothermal liquefaction needs to reach high temperatures, resulting in specialized materials to deal with this. Moreover, catalysts for hydrothermal liquefaction are necessary, which can possibly cause blockage of the reactor [104].  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**References** & **Wood Species** & **Gasification Agent** & **Catalyst** & **Conditions** & **Syngas** & **Conversion** \\ \hline
[21] & _Eucalyptus grandis_ & Supercritical water & — & 400 \({}^{\circ}\)C-450 \({}^{\circ}\)C & Gas mixture of & 64–73\% \\  & Wood chips oak and beech & & NiFe\({}_{2}\)O\({}_{4}\) & & H\({}_{2}\) and CH\({}_{4}\) & 83–95.5\% \\
[100] & _(supplied by J. Rettemair \& & CO\({}_{2}\)_ & dolomitic & 850 \({}^{\circ}\)C & Gas mixture of & 89\% \\  & _Solme GmbH_ ) & & limestone & CO and CH\({}_{4}\) & & \\
[101] & The wood residues of the & & — & 500–600 \({}^{\circ}\)C & Gas mixture of & 46.9–73.2\% \\  & pine tree & & K\({}_{2}\)CO\({}_{3}\) & 19.8 MPa-43 MPa & H\({}_{2}\), CH\({}_{4}\) and CO\({}_{2}\) & 59.1–80.9\% \\
[104] & _Populus albu_ L. & Supercritical water & — & 400–600 \({}^{\circ}\)C & Gas mixture of & 45.2–70.4\% \\  & & K\({}_{2}\)CO\({}_{3}\) & & H\({}_{2}\) and CH\({}_{4}\) & 47.6–81.0\% \\
[106] & The wood residues of the & ScCO\({}_{2}\) & — & 650 \({}^{\circ}\)C–800 \({}^{\circ}\)C; 30 MPa & CO & 43.2–65.8\% \\  & fir tree & & K\({}_{2}\)CO\({}_{3}\) & & 56.7–77.5\% \\ \hline \hline \end{tabular}
\end{table}
Table 3: Wood synthesis gas conversion rates by using different gasification agent.  
As a green solvent, when compared with supercritical water, the critical temperature of scCO\({}_{2}\) is much lower, thereby the expensive high temperature equipment is avoided. ScCO\({}_{2}\) can not only be directly used as a liquefaction solvent, but also can be used as a liquefaction catalyst. [PERSON] et al. [105] added scCO\({}_{2}\) to the solvent water of a biomass hydrothermal reaction, which can promote the in situ formation and dissociation of carbonic acid in the reaction of water and CO\({}_{2}\) under high temperature and pressure. This forms a natural catalyst, thus avoiding harmful inorganic acids (hydrochloric acid and sulfuric acid) that harm the environment [107; 108; 109]. The results showed that the yield of bio-oil was improved with scCO\({}_{2}\) as a catalyst at 300 \({}^{\circ}\)C. Additionally, in a liquefaction reaction at 300-330 \({}^{\circ}\)C, the property of the liquefaction medium can be changed by adding scCO\({}_{2}\) due to the generation and dissociation of carbonic acid in water. [PERSON] et al. [110] liquefied pine using different solvents at 300 \({}^{\circ}\)C, including scCO\({}_{2}\), acetone, ethanol, and H\({}_{2}\)O. Under the help of catalysis of K\({}_{2}\)CO\({}_{3}\), the results showed that scCO\({}_{2}\) and organic solvents had a similar bio-oil yield, while the bio-oil yield using H\({}_{2}\)O as the solvent was relatively low. Table 4 shows the detailed yield of wood liquefaction in different liquefaction media. However, tree species influence the results to some extent because of the differences in their internal structure.