Supercritical CO2 (sCO2) dewaters coal through two mechanisms. For bituminous coal of 9.5% average moisture content the water is dissolved out of the coal and the concentration of water in the bulk sCO2 approximates its solubility limit. For lignite (54% average moisture content) the water extracted by the sCO2 exceeds its solubility limit in sCO2, which implies that a part of the water is displaced from, rather than dissolved out of the coal. Thus, there is a clear distinction between wet coals where most of the water is displaced, as compared to drier coals where the water is dissolved out. Hence, sCO2 dewatering will be especially efficient for high-moisture coals where there is significant free water available for displacement. Precedent for this process is available from the dewatering of wood flakes.
The commercial or industrial applications of 3D printing or additive manufacturing are continuously increasing in diverse areas mainly in rapid prototyping. 3D printing has become part of a novel industrial growth area where simplification of assembly, waste minimization, and mass customization are important, such as aerospace, orthopedic and medical research, defense, and jewelry. There has been continuous growth or improvement in additive manufacturing, which includes the type of materials used, metamaterials, and advancements in the printers or the software. 3D printing has explored the areas where materials have been manufactured which are several times lightweight, high strength compared to traditional parts, and also resulted in a reduction in CO2 emissions. Biodegradability and sustainability are the major concern for any industry. The price of conventional thermoplastic filaments is one of the main sources of revenue and profitability for the industry. In addition to its relatively high price, some of the concerns in its wide use are the moisture resistance and VOC emissions, including iso-butanol and methyl-methacrylate (MMA) during 3D printing. These emissions cause voids in the structure which compromises the mechanical strength of the 3D-printed objects. Additives have been added with thermoplastics, such as diatoms and biodegradable materials, such as ceramics, biomaterials, graphene, carbon fibers, binders for metals, sand, and plaster to reduce the cost and VOC emissions. The cost of these additives is relatively less than the thermoplastic filaments. There has been tremendous innovative growth in the field of additive manufacturing, including solutions such as 3D-printed houses and titanium drones. The addition of additives opens the new potential applications in new arising technology, especially in robotics like behavior, mechanisms respond to user demands which are known as 4D printing where new dimension has been added to 3D printing. It is a process where a 3D-printed object transforms itself into another structure over the influence of external energy input, such as temperature, light, or other environmental stimuli. 4D printing is simply referred to as 3D printing transforming over time. 4D printing is an all-new emerging area in the field of additive manufacturing which has diverse applications in biomedical, defense, robotics, etc.
Supercritical CO2 (sCO2) removes both water and extractives from wood chips and flakes at 60 °C. The water appears to be mostly displaced by sCO2 because its nominal concentration in sCO2 exceeds its solubility limit. SEM imaging and contact angle measurements show no major differences in surface properties between sCO2-treated and thermally dried flakes, which suggests that their interaction with resin should be similar. An economic analysis for the removal of water and extractives from pine flakes for the manufacture of oriented strand board shows that sCO2 treatment is potentially much more cost-effective than thermal drying from both capital and operational perspectives. The main reasons are that the water is removed by displacement rather than through evaporation, environmental control costs are drastically reduced, and the extractives removed represent a value stream instead of pollutants whose emissions need to be controlled. Because the sCO2 is largely recirculated, the process is greener than conventional direct dryers that generate CO2 from the combustion of wood fines used as a fuel.
