Green and Efﬁcient Processing of Wood with Supercritical CO 2 : A Review

: Wood processing is a crucial step of wood utilization, but the adding of environmentally hazardous feedstocks and the use of unreasonable technology allow it to harm the environment and human health. Supercritical CO 2 (scCO 2 ) is a non-toxic, odorless, and safe solvent, which is widely used in studies and industrial production, but there is no review summarizing wood processing with scCO 2 . The unique structure and chemical properties of wood combined with scCO 2 technology produce positive results. In this paper, wood processing with scCO 2 is summarized, including wood impregnation, wood drying, wood thermochemical conversion, and wood extraction. The green and efﬁcient characteristics of wood processing with scCO 2 are explained in detail for researchers, engineers, and investors to provide a clean wood processing method. Further study is needed to reduce its energy consumption and commercialize it eventually. scCO 2 as the transfer medium can effectively improve the extraction rate compared with conventional process, and the temperature and pressure are easy to achieve [40].

The use of toxic organic feedstocks and underdeveloped technology in wood processing causes serious pollution to the environment and increases risk of human diseases. The chemical methods (such as acetylation [11], silanization [12], and etherification [13]) improve the dimensional stability of wood by reducing the number of hydroxyl groups. The dimensional stability and strength property are improved after treatment with thermosetting resin as chemical modifiers [14,15]. However, the use of such fossil-based chemical modifiers contradicts environmental protection and causes environmental degradation in the process of use and production [16]. Heat treatment can improve the stability of wood, but the primary organic aerosols (POA) released during the treatment, due to the high treatment temperature, and POA exposure in the atmosphere cause unhealthy effects on people, such as cardiopulmonary diseases [17]. At the same time, volatile organic compounds (VOCs) are discharged from the wood and released directly into the air. VOCs can react with nitrogen oxides in the air to produce ozone, which can increase the risk of respiratory diseases in humans [18]. The organic solvents used in wood extraction and impregnation include ethanol [19], diethyl ether [20], chloroform [21], and hexane [21]. These solvents result in a negative influence on the environment and humans [22].
Green wood processing is expected. Wood processing pollution can be mitigated by reducing the use of toxic organic feedstocks and improving technology. Many wood Green wood processing is expected. Wood processing pollution can be mitigated b reducing the use of toxic organic feedstocks and improving technology. Many wood pro cessing technologies suffer from low efficiency [23][24][25]. The wood processing yield im proved using scCO2, a green solvent, by increasing pressure [26,27]. These are the tw most important advantages of scCO2 in wood processing: efficiency and environmentally friendly.
ScCO2 is a recognized green and clean solvent and is easy to reuse. The techniqu with scCO2 takes advantage of excellent solubility to promote the efficiency of wood pro cessing; however, the application in wood processing with scCO2 has not been summa rized. In this paper, the application in wood processing with scCO2 is summarized in de tail, including wood impregnation, wood drying, wood thermochemical conversion, and wood extraction. Combined with the properties of scCO2, the green and efficiency ad vantages in wood processing are explained. This paper aims to provide an efficient and green wood processing technology which treats wood using a green solvent of scCO2 t researchers, engineers, and investors, and to strengthen the clean wood production and to achieve sustainability.

Characteristics of Supercritical CO2
CO2 is an emission by large-scale industrial sources including fossil-fuel power sta tions [28], building cement [29], steelworks [30], etc. and a by-product that is generated during the production of ammonia, ethanol, and natural gas in refineries, making it easily sourced [31].
ScCO2 is used due to its low critical temperature (31.1 °C) and pressure (7.39 MPa [32]. Figure 1 [32] shows CO2 reaches a supercritical state by heating above its critical tem perature and pressurizing above its critical pressure. In the supercritical state, the densit is like that of a liquid and the viscosity like that of a gas, which gives it good solubility and transfer performance [33][34][35]. ScCO2 has been used as a medium and solvent in pet rochemical engineering [36], the food industry [37], the pharmaceutical industry [38], and cellular material [39]. These studies have shown that using scCO2 as the transfer medium can effectively improve the extraction rate compared with conventional process, and th temperature and pressure are easy to achieve [40].  ScCO 2 is susceptible to pressure near the critical point, allowing its density to be easily changed by changing the pressure of the control system, which affects its solubility [41]. CO 2 experiences significant thermal physical fluctuations near its supercritical point, which also helps enhance convection and heat transfer [42,43]. Table 1 [44,45] shows the dissolving capacity and transfer characteristics of supercritical fluids relative to liquids and gases. These characteristics indicate that scCO 2 can be of great use for various wood processing applications. Furthermore, scCO 2 , a low polar solvent, is easy to recover as a low polarity molecule without additional cleaning steps [46]. The polarity of scCO 2 can be adjusted by adding a co-solvent, such as ethyl lactate, ethyl acetate, or ethanol [47], thus improving the yield of the polar compounds [48].

Wood Impregnation
Wood impregnation is a process to force wood modifying agents-preservatives, fire retardant, and dye-into wood structures by increasing pressure in the vessel [49]. Wood impregnation with scCO 2 is more efficient compared with conventional wood impregnation [50][51][52][53]. Applications of wood impregnation with scCO 2 that are summarized in this section include wood preservation with biocides, wood inflaming retarding, wood dyeing, and wood acetylation. An overview of wood impregnation application with scCO 2 shows in Table 2.
The liquid permeability of wood can be improved with scCO 2 treatment, which is one of reasons for impregnation efficiency improving under this method. Wood liquid permeability is a physical property that describes the fluid through the connected voids of wood structure under a static or capillary pressure gradient [54]. The high permeability of wood can increases the migration rate of liquid in wood [50]. This was first demonstrated in 1995 using scCO 2 -either alone or with entrainment (methanol)-and was used to improve the permeability of Douglas-fir (Psuedotsuga menziseii (Mirb.) Franco) heartwood samples [55]. The high-pressure difference of scCO 2 treatment will destroy the wood structure and form new water and other liquid transfer channels, thus increasing the liquid flow in the wood. Matsunaga et al. [56] pretreated sugi (Cryptomeria japonica D. Don) with scCO 2 and showed that pits were broken in the pretreated sample with a scanning electron microscope (SEM), which created new water and other liquid transfer channels.  [60] Wood dyeing Facilitating dye uptake at moderate temperatures Improving recycling rates of both CO 2 and dyes [25,61] Wood acetylation Acetylation reagent penetrate into the core of wood [62]

Wood Preservation with Biocides
Wood easily decays and discolors in humid environments. Wood preservation is used to improve its antibacterial capacity and its resistance to wood-decaying fungi, woodbluing fungi, molds, and wood-damaging insects, which extends the service life of wood products [63][64][65]. The one of primary wood preservation methods is to add preservatives in the wood through impregnation [66]. However, it is difficult to impregnate the inner part of the wood with preservatives by conventional impregnation due to the large fluid resistance and poor permeability of wood [67]. At present, wood impregnation is treated by water-based or hydrocarbon solvents [57]. The uses of water-based solvents in wood impregnation leads to volume expansion of wood and dimensional stability cannot be restored immediately [57]. The use of hydrocarbon solvents has a negative effect on the environment due to VOCs emission.
The idea of wood preservation with scCO 2 was carried out due to properties of CO 2 in a supercritical state that was mentioned before. Cookson et al. [57] exploited the timber impregnation with the sufficient solubility (2.32 g/L~5.07 g/L) of the insecticide permethrin using scCO 2 in 50 • C and 15 MPa. Even though the maximum solubility (30.07 g/L) was achieved at 30 MPa, the lowest effective pressure is always adopted in the business, which can reduce the cost. A methanol co-solvent improves the solubility of permethrin in scCO 2 . After decompression, the insecticide loses its solubility and deposits within the wood, thus protecting wood. There is a higher insecticide retention value and better protection under scCO 2 compared to a light organic solvent. Kjellow et al. [50] used organic fungicides (tebuconazole, propiconazole, and IPBC) for wood impregnation using different pressures and temperatures of scCO 2 . They concluded that the proportion of fungicide in the wood and CO 2 had a significant effect on the fungicide deposition. The fungicide solubility increased with the density of the CO 2 . Fungicide deposition occurred due to the loss of fungicide solubility during decompression and because a large amount of fungicide was adsorbed by the wood during impregnation.
There are vast differences in impregnation efficiency between softwood and hardwood due to large differences in wood structure. Moreover, artificial boards also have particular impregnation characteristics under scCO 2 treatment. Muin et al. [68] and Muin and Tsunoda [58] studied five kinds of wood-based composite materials (medium-density fiberboard (MDF), hardwood plywood, softwood plywood, particleboard, and oriented strand board) with 3-iodo-2-propynyl butylcarbamate (IPBC) preservative under scCO 2 treatment. The results showed that IPBC improved the preservation of these wood-based composite materials. Furthermore, the treatment largely depended on the temperature, pressure, fungicide dosage, and wood-based composite material [69]. Particleboard, MDF, and hardwood plywood achieved the best preservation at 55 • C and 11.7 MPa, while softwood, plywood, and OSB showed the best performance at 35 • C and 9.81 MPa.
Kang et al. [70] treated Douglas-fir and radiata pine (Pinus radiata D. Don) with cyproconazole using scCO 2 impregnation, and the results showed that treatment conditions led to different distributions of fungicides. Most studies indicate fungicide can move in the wood structure by decompression in the vessel, because pressure gradient force preservatives loaded the water to flow. However, decompression can easily lead to water passage blockage, which results in the uneven deposition of fungicides, making it difficult to process wood [55,71]. In the study [70], they adopted temperature-induced biocide deposition at the end of the cycle, and the results showed that this method reduced the consumption of fungicide and more efficiently retained fungicides with minimal line clogging. Kang et al. [59] studied radiata (Pinus radiata D. Don) pine lumber specimen impregnation with subcritical CO 2 and scCO 2 . The results showed that compared with scCO 2 , subcritical CO 2 treatment produces a higher retention rate and more uniform distribution. Besides, subcritical CO 2 treatment has economic advantages over scCO 2 , such as lower investment costs due to low pressure and lower energy consumption due to feedstock heating at low temperature.

Flame Retardancy of Wood
Wood is a porous biomass material that is composed of cellulose, hemicellulose, and lignin, as well as a large number of polar groups containing oxygen atom, such as hydroxyl and methoxyl [72]. These components of wood indicate that it is flammable. For safety purposes, wood is commonly treated with flame retardants by impregnation of the flame retardants into the porous wood structure using a vacuum compression technique or by coating onto the wood surface [73]. The flame retardancy of wood is not only related to the characters of flame retardants, but also the distribution of flame retardants in wood [74]. Eastman et al. [60] prepared poplar-silicone resin composite materials through scCO 2 extraction, using ethanol as an auxiliary solvent. The results showed that the addition of silicone improved the refractory properties, mechanical properties, because silicon resin has a high solubility in scCO 2 [60]. Tsioptsias and Panayiotou [75] added silicon-based polymer coatings to spruce (Picea abies (L.) Karst) and beech (Fagus sylvatica L.) samples that were impregnated with scCO 2 and the results showed that the flame retardancy of the treated wood was improved.

Wood Dyeing
Wood dyeing can eliminate color differences and improve defects in timber such as uneven color, tarnish, and color changes [76]. Wood dyeing methods include coloring matter dyeing, chemical dyeing, biological dyeing, hot chemical dyeing, and structural color [77]. It is challenging for conventional hydrothermal dyeing techniques to achieve satisfactory dyeing results due to poor permeability of some parts of wood. However, the use of scCO 2 , especially when a co-solvent is added, can significantly improve the dyeing effect. Jaxel et al. [61] used scCO 2 as the dyeing carrier to impregnate different tree species with the dye Blue 134. Lipophilic dyes are easily solubilized in scCO 2 . They also added the commonly used paper sizing agent AKD to improve the dispersive capacity of the dye, which helped the dye permeate through the wood using scCO 2 . An anthraquinone derivative was also synthesized to improve the solubility of DB 134 in scCO 2 and achieved a significant dyeing effect. Jaxel et al. [62] studied wood dyeing without the addition of an organic co-solvent, and the water-saturated scCO 2 better accessibility of cell wall by expansion.

Wood Acetylation
Wood acetylation is a wood modification method used to improve its dimensional stability and durability by reducing the content of -OH groups. It is accomplished by esterifying hydroxyl groups of wood with an acetylation reagent [78]. Treating wood with scCO 2 improves the permeability of the wood, allowing acetylation agents to penetrate the wood better.
Matsunaga et al. [79] acetylated Japanese cedar using scCO 2 fluid by acetic anhydride. The results showed that, compared to treatment without the use of CO 2 , using scCO 2 in the acetylation had a higher bulking effect which increased the stability of wood. Acetic anhydride becomes a uniphase when in scCO 2 above 90 • C and responds rapidly in the wood to speed up acetylation. In a subsequent study, Matsunagaet et al. acetylated four species of wood of Paraserianthes falcate (Albizia falcataria Linn. Fosberg.), Alstonia macrophylla (Alstonia macrophylla wall. ex G. Don), Pinus caribaea (Pinus caribaea Morelet), and Hevea brasiliensis (Hevea brasiliensis (Willd. ex A. Juss) Muell. Arg.) with acetic anhydride in scCO 2 . After 8 h of treatment, anti-swelling efficiency values increased by more than 60%, and the effect of macrophylla was the best.

Wood Drying
The conventional kiln drying with a heating device is the most common drying method in the wood drying industry, which is commonly carried out by boilers powered with fossil fuel [80]. This results in excessive greenhouse gas and POA emissions, which have a negative impact on the environment [17]. Compared with conventional drying, scCO 2 drying reduces the harm of greenhouse gases emission to the environment because CO 2 is a feedstock instead of a fossil fuel. However, the high pressure of scCO 2 increases power consumption.
Several researchers have studied wood dewatering using scCO 2 [80][81][82][83]. CO 2 is circulated between the supercritical state and the gas phase in wood by changing the scCO 2 pressure. The large pressure difference increases the liquid permeability of wood and the CO 2 bubbles carry the water out of the wood [81]. The water removal without changing the microstructure of the wood during scCO 2 treatment due to its excellent solubility and transfer characteristics [81,82].
Wood drying with scCO 2 has a high rate of dewatering. Franich et al. [81] cycled CO 2 between the gas phase and supercritical state to dewater radiata pine sapwood samples with sizes of 18 mm × 18 mm × 100 mm under different pressures and temperatures. The longest drying time in one cycle lasted for 16 min and the shortest was for 2 min. The results showed that the moisture content of lumber was quickly reduced in this drying method. Compared with the conventional drying method, the wood drying rate was increased with scCO 2 due to the pressure difference in several cycles of pressurization-depressurization. Gabitov et al. [84] dried wood with scCO 2 , and 87% of the water was removed in the first drying cycle. Dawson et al. [83] dried radiata pine sapwood using scCO 2 , and after seven cycles the moisture content decreased from 174% to 39%.
Collapse is an abnormal shrinkage occurring above the fiber saturation point (FSP), which is caused by the capillary forces in the cell due to the rapid removal of free water from the lumen. Collapse is accompanied with an internal crack and surface cracking, thus wood strength is decreased [84]. The collapse of wood can be reduced by using scCO 2 drying due to fact that the water is pushed out from the cell lumen directly [85]. Additionally, the negative capillary pressure of the wood cell wall decreased in a supercritical state, which has little liquid-gas interfacial tension [62]. Dawson et al. [85] studied the collapse of Eucalyptus nitens (Deane and Maiden) with scCO 2 pretreatment compared to conventional drying, and the results showed that this pre-drying method reduced the collapse rate by 75%. Dawson and Pearson [23] dried softwood and hardwood samples with scCO 2 and found that the shrinkage of wood after scCO 2 drying was much smaller compared with conventional drying, and wood cracks were significantly reduced.
Moisture distribution and migration are fundamental processes that occur during wood drying. Behr et al. [86] studied the distribution of CO 2 and water with scCO 2 in green radiata pine (Pinus radiata D. Don) sapwood by 13 C nuclear magnetic resonance (NMR) spectroscopy and proton magnetic resonance imaging (MRI). The results showed that CO 2 first entered the latewood and then diffused into the adjacent earlywood, while less CO 2 entered the earlywood directly. Meder et al. [87] observed the wood drying process using scCO 2 combined with forced ventilation through MRI and found that after scCO 2 drying the 1 H MRI signal intensity was weakened, indicating the removal of water. Newman et al. [88] used NMR to study wood drying with scCO 2 , and the experimental results showed that the scCO 2 soluble extractives were removed to open new water pathways in earlywood and the water removal by diffusing into scCO 2 only occurred when the volume of scCO 2 was larger than the volume of wood. As the pressure drops, the CO 2 bubbles expand to help with wood dewatering. Franich et al. [89] observed the wood scCO 2 drying process in real-time using MRI and 13C NMR. CO 2 diffused and bound to free water when the pressure increased during drying. The way in which water combines with CO 2 is different when the pressures changed.
However, the development of wood drying with scCO 2 remains an ongoing challenge due to the final moisture content near the FSP, which means the dewatered wood using scCO 2 cannot be utilized directly. The scCO 2 technology combined with other drying technology is more suitable for actual wood drying.

Wood Thermochemical Conversion
Bio-fuels are a sustainable and effective alternative to fossil fuels, and reduce environmental pollution compared to fossil fuels [90]. Biomass fuels include biomass ethanol, bio-oil, etc. Bio-oil can be produced by thermochemical conversion of wood, residual crops, and pulp mill fertilizers. Wood thermochemical processes include liquefaction, pyrolysis, and gasification [91]. Wood gasification uses a gasification agent to convert raw wood materials into a product of gas under the condition of low oxygen content and high temperature (800 • C-1400 • C). The wood gasification product of gas mainly contains H 2 , CO, and CH 4 and is used for fuel and power generation [92,93]. Wood pyrolysis and liquefaction techniques are similar, but the processes are different. Pyrolysis is the thermal degradation of wood under the condition of 400-700 • C without oxygen, and the pyrolysis temperature is usually lower than the gasification. Bio-oil and Bio-crude are the wood pyrolysis products [94]. Wood liquefaction is performed on wood under the conditions of low temperature (250 • C-400 • C), high pressure, and with a solvent. The wood liquefaction products can produce polyurethane, epoxy resin, adhesives, etc. [95].

Wood Gasification
Currently, there are many studies using CO 2 gas as the gasification atmosphere of wood [93,[96][97][98]. CO 2 gas is a kind of a clean medium and it also has the following advantages: control of the ratio of H 2 /CO through the input of CO 2 , save on the cost of steam and oxygen, and increase the production of syngas [91,99,100]. It can be seen that compared with scCO 2 , CO 2 gas is a lower cost choice as a wood gasification medium. This is because gasification operation can be realized without high pressure (critical pressure of CO 2 is 7.39 MPa) only under the condition of a catalyst [98], and the high pressure increases the operation cost. At present, the most commonly used supercritical fluid medium for wood gasification is supercritical water [22,[101][102][103][104] for economic reasons. A few scholars are still studying scCO 2 wood gasification [105], and the specific value of syngas conversion rates are shown in Table 3. It can be seen from the table that compared with the gasification agent gas CO 2 and supercritical water, wood gasification using scCO 2 has a higher syngas conversion rate without catalyst addition, but the operation cost is higher.

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].
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. Chan 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 • C. Additionally, in a liquefication reaction at 300-330 • 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. Wang et al. [110] liquefied pine using different solvents at 300 • 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.

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 biooil [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 .
Naik 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 • C. Feng and Meier [119] extracted pine wood (from North America) bio-oil with scCO 2 , and reached the maximum extract yield of 14.3 wt% at 80 • 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 • C with activated carbon as the catalyst. Rout et al. [120] conducted scCO 2 extraction from wheat-sawdust bio-oil, and the first three fractions were tested at 45 • 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.

Natural Component of Wood
Wood extraction with scCO 2 is an effective method to obtain various high-value chemical products. ScCO 2 extraction has a simple process, fast mass transfer rate, high extraction efficiency, and is a green separation technology [121]. ScCO 2 is a low-polar solvent, in which the solubility of polar compound is very low, and the extraction efficiency can be improved by adding a polar co-solvent, such as ethanol or ethyl acetate [48]. However, excessive addition of organic solvents will affect the environment. A brief overview of wood extraction with scCO 2 is shown in Table 5.
Candeia (Eremanthus erythropappus) wood is a Brazilian tree species with a high commercial value, and the chemical components of its oil can be used to make cosmetics and drugs, especially the active ingredient, s-bisabolol, which has sedative, anti-allergy, and anti-bacterial characteristics [122]. Santos et al. [123] used scCO 2 to extract candeia wood oil at different temperatures and pressures. The results showed that the highest extraction rate of mahogany oil was 1.42% and the residue was 0.41% at 70 • C and 240 bar. Queiroz and Cajaiba [124] extracted chemical substances from candeia wood using scCO 2 with ethanol. The extraction rate of this method was higher than steam distillation. Souza et al. [125] conducted dynamic scCO 2 extraction of candeia wood, and the kinetic curve showed that supercritical extraction and phase behavior were the main factors influencing the extraction efficiency.
Pine extract is important in the fields of food, medicine, and health [126]. Sarikaki et al. [127] extracted Pinus brutia bark under different scCO 2 extraction conditions by adding 3% ethanol and performing ultrasonication. The results showed that 46.8% (-)-catechin was extracted, and almost 100% (-)-catechin gallate and (-)-epicatechin were extracted. Conde et al. [128] extracted low-molar-mass phenolics and lipophilic compounds from maritime pine (Pinus pinaster) wood with scCO 2 . The maritime pine wood extracts had strong antibacterial effects. Ethanol was used as a co-solvent, and the products extracted accounted for 4.1 wt% of the dried wood. Braga et al. [129] used scCO 2 to extract maritime pine bark, and achieved a maximum extraction of 84%, which was rich in catechin and epicatechin. The extract content was improved after the addition of ethanol solvent during extraction.
An aromatic essential oil can be extracted from Eucalyptus robusta. This oil is secreted by eucalyptus leaves and can kill insects, repel mosquitoes, reduce inflammation, and is used for sterilization [130,131]. Zhou et al. [132] analyzed the extraction conditions for volatile oils from Eucalyptus grandis × Eucalyptus urophylla by scCO 2 . The extraction temperature was 80 • C, the extraction pressure was 40 MPa, and the extraction time was 8 h, reaching a maximum extraction rate of 7.86%. Chemical components were analyzed by gas chromatography-mass spectrometry. The content of eucalyptol and α-pinene were 45.57% and 24.78%, respectively. Santos et al. [133] used supercritical CO 2 to extract eucalyptus globulus bark, with added ethanol, and showed an optimal extraction rate and high selectivity for flavonoids. Patinha et al. [134] treated Eucalyptus grandis × Eucalyptus globulus bark with dichloromethane for 7 h, and then used scCO 2 to extract lipophilic extractives from eucalyptus bark after drying. They detected lipophilic extractives by gas chromatography-mass spectrometry, with beta-sitosterol in the bark interior and triterpene in the bark exterior.
In the latest studies, some scholars have used scCO 2 to extract cedarwood oil from eastern red cedar (Juniperus virginiana L.) to protect the wood from termites and fungi. ScCO 2 has also been used to extract an ingredient from Pinus pinaster that is used to resist pests, and a volatile oil from Aquilaria sinensis with a high economic value [135][136][137]. Table 5. Wood liquefaction yield by using different liquefaction media.

CCA Wood Detoxification
The service life of wood can be prolonged by preservation; however, widely used preservatives commonly contain CCA (Cu, Cr, and As). After being treated with CCA, wood is toxic, especially when it is burned or buried. This has terrible effects on the environment, and wood treated with CCA must be disposed of properly [138]. ScCO 2 is a green solvent and can be used to efficiently extract toxic substances from CCA wood. El-Fatah et al. [139] used scCO 2 to extract CCA wood, and the best extraction effect was achieved when the temperature was 59.85 • C and the pressure was 24 MPa. The extraction rates of Cu, Cr, and As were 63.5%, 28.6%, and 31.3%, respectively. Wang and Chiu [140] used scCO 2 to treat CCA wood and showed that this extraction method could significantly reduce the production of acidic and organic solvent wastes, as well as effectively reduce the cost of solvent extraction.

Conclusions
This paper presents a summary of the applications in wood processing with scCO 2 . In wood impregnation and extraction, scCO 2 is a solvent with strong solubility in lowpolar compounds (such as preservative, flame retardant, and the natural composition of wood) and the polarity can be changed by adding a polar solvent. The uses of a non-toxic solvent, scCO2, instead of organic solvents reduce VOCs emissions. The great pressure difference destroys the pits membrane and increases the permeability of wood, making it easier to impregnate wood with modifiers and the composition in the wood (natural composition and CCA) is also easier to extract. In wood drying, this method reduced CO 2 emissions compared with conventional wood drying. The drying rate is increased due to CO 2 bubbles carrying water out from the wood during depressurization when the scCO 2 transformed into the gas phase. Additionally, collapse in wood is reduced, which is a severe drying defect occurring easily in conventional drying, but because the scCO 2 is a kind of fluid with little interfacial tension between gas and liquid it reduced the negative cell wall pressure caused by water escaping from the wood. In wood thermochemical conversion, scCO 2 is more of a liquefaction medium than gasification, due to the critical conditions of scCO 2 being more suited for the temperature and pressure of liquefaction. For gasification, supercritical water and CO 2 gas are better low-cost green mediums. The efficiency of wood liquefaction with scCO 2 is similar to the organic medium, and it can be improved by adding a catalyst. However, the scCO 2 has an environmental advantage over organic solvents. ScCO 2 wood processing is a potential technique that has the aforementioned advantages, but it will take a long time to be commercially viable because of the high cost of high-pressure apparatuses and high-power consumption. This obstacle still needs active research and development to minimize the cost and energy consumption. Moreover, the specific recycling utilization needs to be further studied to show the result of scCO 2 wood processing in business. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.

Data Availability Statement:
No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest:
The authors declare no conflict of interest.