BENEFICIATION OF COAL USING SUPERCRITICAL WATER AND CARBON DIOXIDE EXTRACTION

This work explores the use of carbon dioxide, water, and their mixtures as solvent for the precombustion beneficiation of raw coal without using any toxic mineral acids in the temperature range of 200–400 °C. The fluid polarity, ionic constant, and supercritical point can be adjusted by H2O/CO2 ratio and temperature. Adding carbon dioxide to hydrothermal fluid also increases the ionization by forming carbonic acid. Extractions with supercritical fluids have several benefits including enhanced mass transport, ease of separation and recycle, wide range of extractive capability and tunability, better inherent safety, and in the case of carbon dioxide and water–low cost. A semi-continuous extraction system was designed and built in which pressure, temperature and the relative flow rates of CO2 and H2O can be controlled. Coal powder is kept in a packed bed and the extraction is carried out at 143 bar pressure. Using sulfur as a model heteroatom, extractive efficiency is examined as a function of the temperature, fluid composition, fluid flow, and extraction time. The results indicate that carbon dioxide, water, and supercritical water-carbon dioxide (ScWC) all can effectively extract about 50% of total sulfur from bituminous coal in 1 h. Extraction above 350 °C decreased effectiveness, and extraction above the supercritical point of pure water caused hydrothermal carbonization. ScWC extraction may provide necessary control to prevent organic dissolution while removing sulfur.

The extraction process was further developed to introduce ultrasound energy to enhance mass transfer from solid coal particles to the fluid phase. At the high temperature and pressure conditions as noted above, the introduction of ultrasound was successful and tested for coal extraction. The degree of mass transfer enhancement can be controlled by the intensity of the ultrasound. Such an enhancement, opens up possibility of relaxing the requirement on the fine particle size of the coal. We found that the conversion of pyrite was nearly complete for the best extracted samples with organic sulfur mostly untouched, indicating that the mass transport even without ultrasound was fairly good.
In the base design of the extractor, the fluid entry and exit points are both on the top cover plate of the vessel. Here scWC fluid enters at the top and then leaves from the top after extraction, so it is possible that not all the coal solids are efficiently contacted with the fresh fluid coming in.
To overcome this possible channeling effect, we have further enhanced the extractor design. The xiv apparatus is designed to allow for the fluid to enter at the top, go through the pack bed of coal, and then exit from the bottom carrying extracted components. The design was tested successfully.

Coal Use and Industry
Despite recent emergence of natural gas, nuclear, solar, and wind power, coal remains an important energy source to the United States and the world. For example, in 2018,27.4% of the United States energy demand was met by burning coal, while for the world the number was even higher, about 40%. According to proven reserves, the world has about 150 years' worth of energy from coal combustion, but only 50 years' worth of energy from oil or natural gas. Furthermore, the United States owns 28% of the world's coal reserves compared to only 2.3% of the world's oil reserves. (EIA International Energy Outlook 2017  Despite this reserve of coal, the production of coal has been decreasing since 2008 due to the rise of natural gas combined cycle generators (Odetayo 2018, De Gouw 2014, fracking (Weinhold, 2012), and concerns over atmospheric and local air pollution (Seinfeld, 2016). Although coal had dominated U.S. electric power production for many years, natural gas generation has recently overtaken coal based power production. Stringent air quality standards set by the EPA and MATS compliance have placed restrictions on the particulates, sulfur, and NOx emissions of coal fired power plants due to the realization that serious local and global health concerns can arise from their unabated release (Sebor, 2014). Even before decreases in coal fired generation between 2006-2015, sulfur dioxide emissions from coal were decreasing due to an increase in mandates to clean combustion exhaust. Even in 2015 after a 25% reduction in coal fired generation and widespread adoption of exhaust cleaning methods, coal dominated the sulfur dioxide emissions in the U.S. The closure of large capital-intensive coal fired generation systems was forced by natural gas' boom in production, the ease of handling compared to coal, and the difference in sulfur emissions. Despite the current energy climate of the U.S. leaning toward natural gas, coal is still the more prominent source for electric power generation around the globe. The petroleum rich Middle East and South and Central America have little coal consumption and can be expected to remain that way. But as Europe and North America decrease their reliance on coal moving forward, the Asian Pacific region has substantially increased coal consumption in the last several decades.
Africa may also see a substantial increase in energy consumption in the coming decades that would look similar to the increase in coal consumption in the Asia Pacific due to the wealth of coal in Africa (Pollet 2015). Liquid and solid hydrocarbons are both finite resources that will deplete in time. Solid hydrocarbon coals are sourced from decayed and compacted plant matter, as opposed to petroleum and natural gas which are the product of compacted animal remains (Teichmüller 1966). While eons of crustaceans may contribute to substantial liquid hydrocarbon reserves in the oceans, the surface has been more heavily populated with flora and thus coals. Another 100 years may pass before liquid hydrocarbons become hard to find, but coal should be around for another half millennium. With the world's population trending ever higher, all sources of energy are important now and into the future because access to energy is so closely tied to quality of life.

Coal Structure and Chemistry
Raw coal is ranked by its carbon content into four major categoriespeat, lignite, bituminous coal, and anthracite. Peat and lignite are low rank coals which have not been exposed to enough pressure for enough time to develop into higher rank coals. Bituminous coals are the most commonly used coals for large scale electric production, but lignite is also popular.
Anthracite is the highest rank coal and is often very dense. The moisture, volatile matter, and oxygen content of coal decrease with rank, and the carbon content increases with rank. Figure 6: Coal Rank (Meshram 2015) Coal also contains ash and hazardous aerosolized pollutants. Generally higher rank coals contain less ash and sulfur, but mines and regions may vary greatly in ash content and sulfur compared to others. While North American coals are usually fairly clean and of high rank, Chinese coals are often high in ash content (Yan 2003, He 2016, and Indian coals are known for high sulfur content (Ken 2019, Baruah 2007, Chabukdhara 2016. The ash contains metals and metal oxides ranging from the lightest metals to the heaviest. The most common elements by mass are magnesium, sodium, aluminum, silicon, iron, and calcium (Gabler 1982). Heavy elements are present in ppm concentrations . When combusted, the lighter ash metals will be swept up with the flue gas and the heavier metals will form slag that impedes unit operations (Baruah 2010). Although some heavy metals are scarce and production is limited to China, light and heavy ash has been collected for centuries and stored in landfills due to the levels of toxic metals that cannot be separated economically from the valuable metals (Humphries 2010, Dai 2016, Lin 2017.
As opposed to the ash content that remains as a solid after combustion, more volatile elements will form noxious vapors that are harder to contain and destructive to the environment.
Sulfur, mercury, and chlorine are elements which form hazardous aerosolized pollutants. While mercury and chlorine are often present at ppm levels, sulfur is often present at concentrations of 1-5 mass%. Sulfur in raw coal is present in three formspyrite, sulfate, and organic (Calkins 1994). Pyritic sulfur is sulfur that is bound to iron, either as FeS or FeS2. Sulfate form is not commonly found in nature, but can be formed upon oxidative processing. For the organic form, sulfur can be bonded to the carbon structure in a variety of ways (Davidson 1994). Organic sulfur is found in coal as mercaptans (RSH), disulfides (RS-S-R'), sulfides (R-S-R'), and thiophenes (heterocyclic). Organic sulfur is thus integrated into the carbon structure and difficult to remove.
Generally, raw coals may contain a similar amount of sulfur in pyrite and organic forms. Upon combustion, the sulfur in raw coal will form sulfur dioxide, which is toxic to humans if released to the atmosphere and corrosive to plant equipment. Emission of other highly active molecules, such as chlorine and mercury, also contributes to deterioration of the atmospheric environment (Akers 1996, Yan 2002). Other forms of sulfur possible in less oxidative conditions include hydrogen sulfide and elemental sulfur.  Figure 6 demonstrates a modern coal fired combustion scheme. Particulate matter from ash combustion is separated from flue gas using electrostatic filters . Limiting NOx production is achieved by looping of reaction gases and effective flame control (Belošević 2017).
Wet flue gas desulphurization (WFGD) units have become the industry standard to limit the sulfur dioxide emissions, with state of the art installations capturing 90-95% of sulfur and a variable amount of chlorine and mercury (Scala 2008, Van Otten 2011. In WFGD, flue gas is passed through concentrated aqueous lime solutions that adsorb SOx vapors. The resultant slurry can be used to form calcium sulfate dehydrate (i.e., gypsum) but will mostly accumulate as solid waste as elemental sulfur or calcium sulfate. Separating particulates and sulfur from flue gas requires the processing of large volumes of flue gas and leads to a significant thermal load on the plants (Wei 2017). Treatment of waste in air, water, and solids is a concerning issue that has large impacts and costs that are hard to calculate. Improper solid and liquid waste management can lead to failures in the short term requiring remediation efforts (Rivera 2017, Santamarina 2019), but continued air pollution has become a top international concern for the long term (Gupta 2010). The future of allowable and untaxable air pollution is unclear. Thus, current combustion of raw coal leads to a plethora of issues concerning environmental safety, materials handling, and combustion efficiency.

Pre-combustion extractions
Pre-combustion extractions of raw coal have been an intensive area of research for some time with a few promising methods for removing all of the sulfur and ash content reported recently.
Results are summarized in Table 1. The most effective precombustion cleaning methods are classified as biological, physical, or chemical. For biologic coal beneficiation, a bacteria that consumes sulfur at a high rate is allowed to digest the coal. The most effective biologic digestion was shown to remove 72% of the total sulfur content using Bascillus Subtilis (El-Midany et al., 2018). Unfortunately, the beneficiation using microbes is slow, costly, and difficult to maintain or scale up.
Physical methods include gravity separation, froth flotation, oil agglomeration, and magnetic separation. In these techniques, a difference in the physical characteristics between mineral matter and coal matter is exploited. In gravity separation, small cyclones enhance the gravity field in an aqueous medium, forcing light hydrophobic coal particles to the top and causing mineral matter to sink. Gravity separation is more effective at cleaning coarse coals, but is ineffective at removing finely assimilated pyrite. Froth flotation takes advantage of the differences in buoyancy as well, but instead uses surfactants to increase removal efficiency. Oil agglomeration techniques operate very similarly, but use vegetable oils as medium instead of water. Magnetic separation relies on the fact that coal matter is a weakly diamagnetic, but the sulfur and ash components are paramagnetic and iron is strongly paramagnetic. In pilot scale tests, magnetic separation was shown to achieve 40 wt.% total sulfur removal (80 wt.% pyrite removed) and 35 wt.% ash removal (Uslu et al., 2004). While physical remediation of coal is not suitable for chemically bonded minerals and requires energy intensive grinding, industrial implementations of physical coal cleaning methods can be found because of their ease of scalability. Chemical beneficiation of raw coal is considered the most promising method to effectively remove all sulfur forms. Acids and alkaline solutions can be used sequentially and in various mixtures to produce coal with both low ash and low sulfur. Some commercial ash-free coal methods requiring heated oil as solvent may exist, but the process is not economically feasible.
Most of these extraction schemes are inspired from high-value materials; unfortunately, coal is a low value commodity and these methods which call for even dilute acids and microwave radiation cannot be performed economically on an industrial scale. In addition, the residual acids and caustics in the coal, even in small amounts, will be combusted upon use, which creates new environmental challenges. Furthermore, the hydrocarbon bonds should not be broken down so as to retain heating value. While hydrothermal extractions have been used to clean coal, they usually decrease the heating value by a considerable amount due to organic dissolution and fractionation into gas and liquid phase (Timpe et al. 2001). Recently, a research group did show that carbonic acid could be used to enhance aqueous extraction of coal by bubbling CO2 through stagnant hot water, but the experimental temperatures and pressures were very mild (Gao et al. 2017;Ding et al. 2019).

Two-stage, Acid-Base Leaching
The vast majority of efforts on coal beneficiation has been with harsh acid leaching. Vasilakos et all showed in 1983 that pyrite portions of coal can be extracted with chlorinated water at room temperature. They also tested the effect of various solvents other than water and found that extracting in carbon tetrachloride prevented loss of thermal value of coal seen with water as solvent. (Vasilakos et al., 1983) In 2000, Balaz reported reducing total sulfur content of coal from 1.9wt% to 1.1 wt% by treating 20 grams coal with 200 mL of 5% NaOH at 90C for 30-120 minutes. Arsenic level was also decreased by 95%. ( Baláž et al., 2001).  (Meshram, 2015)

Thermal Treatment
While thermal treatment in oxygen atmosphere will lead to combustion, thermal treatment in inert atmosphere such as nitrogen has been studied in efforts to liquify and gasify coal.
Pressurized CO2 atmosphere is more oxidative than nitrogen atmosphere, but much less oxidative than oxygen or air atmosphere. While coal behavior in high temperature CO2 in unknown, studies of combustion, gasification/liquefaction, oxyfuel combustion, and pyrolysis can give some understanding of the coal's carbon and ash behavior and separation in high temperature oxidative environments.
Traditionally, the only pre-combustion extraction performed on coal at a large scale is the production of coke to produce iron. Coke is an upgraded form of coal that has had sulfur, ash, and volatile matter removed by baking in a coke oven in the absence of oxygen at temperatures up to 800℃. Although this process is not economical for power production, the production of steel in a blast furnace requires a solid fuel source that will burn at the correct temperature, usually about 1250 ℃, and add little impurity to the steel. Thus coal is stripped of ash and sulfur using high temperature and then allowed to carbonize to achieve the correct thermal character. Carbonization of coal to coke may take up to 48 hours. Coke and iron ore are packed into a blast furnace. The blast furnace continually consumes ore and coke mix fed from the top and collects liquid steel at the bottom. The industrial operation of coke ovens and blast furnaces are known to be highly toxic to workers as the oven is not a sealed, pressurized vessel, but rather an oven made of brick with leaks.
Further upgrading of coal into carbon materials can be achieved with an Acheson furnace to produce synthetic graphite for carbon filters and battery electrodes on up to high quality graphite. Coke is usually produced by the consumers, but price is generally $300-$500/tonne.
Amorphous carbon may cost up to $1000/tonne, and high grade graphite may cost up to $10,000/tonne. Natural graphite is mined, and usually costs about 1/3 the price of synthetic graphite. Natural and synthetic graphite compete, with synthetic carbon chosen by the producers of specialty products and natural graphite chosen by price driven industries.
Coal gasification is the other industrial extraction method for coal. Large scale production of 'town gas' is reported in Germany as early as the mid 1800's using biomass gasification. In gasification schemes, the feedstock is converted at 1200-1600℃ at pressure of 2-8MPa in an atmosphere at promotes the production of syngas. Being above the ash fusion temperature, the molten ash will drain down the walls of the reactor. The syngas must be cooled and treated. Thus, the ash and sulfur are separated by volatilizing the carbon content in a hot, non-combustion step.
The process is not energy efficient or economic, but can create clean easily handled fuel.

Hydrothermal Leaching
While washing coal with water at room temperature is not effective due to its hydrophobic carbon structure, treating the coal with pressurized and superheated water can effectively remove coal substituents.

Hydrothermal Carbonization
Hydrothermal treatment is also used to convert low grade biomass such as cellulose, wood, and even sewage with temperatures in the range of 180-220 ℃. The major processes are dehydration and decarboxylation resulting in an increase in carbon content and calorific value. Additionally, the carbonized biomass has lower hydrogen and oxygen content resulting in something close to a low rank coal. Polymerization of the biomass can be explained by the elimination of carboxyl and hydroxyl groups creating unsaturated hydrocarbons which polymerize through stepwise aldol condensation (Kabyemela, Nelson).

Ultrasound Treatment
Ultrasound waves can be added to solution to enhance the mass transport of the fluid. A piezoelectric crystal is attached to a horn that is placed in the vessel near the solids. Crystals may also by placed on the exterior of vessels after some calculation, but thick steel will dampen ultrasound waves. Traditionally, the formation of bubbles leads to increased mixing and increased ion activity caused by the collapse of the bubbles. In supercritical solution though, two phases cannot exist and bubbles will not form. Still, the addition of pulsed ultrasound waves are expected to further increase the mixing and mass transport of the supercritical fluid which may be important for penetrating the solid matrix of coal. Most ultrasound studies focus on the use of basic hydroxide solutions that enhance the formation of hydrogen peroxide, which performs the critical steps in chemical extraction of sulfur and ash.
Saikia et al. tested alkali mixture of 1M:1M KOH and NaOH with 20 kHz ultrasonic energy for 3 hours and found that the ash content and pyritic content was decreased up to 85% but were less effective at removing sulphate or total sulfur (~15%). Effect of ultrasonication on leaching is not well understood. Ultrasound cause mechanical vibrations resulting in cavitation with increased chemical reactivity. In addition, shearing can crack the surface and increase the liquid uptake in the solid matrix by capillary action. The applied ultrasound energy was not enough to break the carbon-sulfur bonds, but that the highly reactive species created by ultrasound in water such as OH radicals, H2O2, and ozone oxidize sulfur compounds to sulphoxides, sulphones, and sulphates. (Saikia et al., 2014) Ambedkar et al. also examined the acid demineralization of coal using ultrasonication.
They utilized 2 N HNO3 with 3 vol% H2O2 with 20 kHz sonication at 500W for 30 minutes and observed 87% sulfur removal. It was concluded that particle breakage due to cavitation is dominant in low-frequency ultrasound, whereas streaming phenomenon is dominant in highfrequency ultrasound. Sonication increases the sulfur removal. (Ambedkar et al., 2011)  In 1987, Slegeir first suggested use of carbon dioxide and water as a treatment for coal.
They treated batches of coal-water slurry pressurized to 800-1200 psi with CO2 at 30-80℃, and found that the treatment produced coal with decrease in moisture and ash content and increase in grindability. (reference not found) and water in an autoclave and pressurizing the cell with carbon dioxide. At CO2 partial pressures of 0.1, 0.5, 1.0, and 7.0 MPa the pH was estimated to be 3.75, 3.52, 3.38, and 3.12, respectively.
Subsequent thermal extraction with 0.1 mL/min crude derived organic solvents (not specified) was carried out under 1.0 MPa nitrogen atmosphere and 320-400 o C to produce ash-free coal (HyperCoal). It was shown that the pretreatment in CO2 atmosphere increased the hypercoal extraction yield ~10% by increasing removal of calcium and magnesium. (Masaki et al., 2005) In 2017 and 2019, Gao et al. used low-pressure carbonic acid solutions to leach coal. In a typical experiment, 1 gram of coal slurry suspended in 50 grams of water was bubbled by 200 mL/min CO2 for 24 hours. The pH was measured to be 5.6. The CO2 bubbling allowed for the removal of organic forms of sodium and calcium that were not accessible by leaching in water alone. They were able to remove up to 31 wt% of calcium and 71 wt% of sodium. The removal of total ash was found to be 9.4-19.2%. Small differences in efficiency were observed at 25, 40, and 60 ℃; with 60℃ giving the better efficiency. (Gao et al., 2017;Ding et al., 2019) In 2006, Iwai et al. reported on upgrading low rank coal with supercritical carbon dioxide at 15 MPa and 40 o C to remove iron and calcium. Experimenting with chelating agents methanol, ethanol, and acetyl acetone they found that recovery rate of iron and calcium were not high but that the acetyl acetone increased recovery rate of iron substantiatally because the acetyl acetone solution had water in it. They remarked that water played a critical role in their extraction. Good recovery rates for calcium were only achieved by adding acetic acid with ethanol to the sCO2.
Recovery was shown to increase with increasing carbon dioxide flowrate and decreasing particle size. (Iwai et al., 2006) In 2006, Anitescu reviewed the many extractions attempted on soil samples including supercritical water treatment and supercritical carbon dioxide in order to remove organic toxins PCB and PAH. They found that raising the temperature of water to 400 o C decreased the polarity of water such that extractions were effective. Also, scCO2 extractions were found to increase in effectiveness by raising the temperature to 200 o C. (Anitescu et al., 2006) Although carbonic acid was shown to be effective by several groups at removing lighter metals from coal such as Na, Mg, and Ca for prevention of slagging. Most other ash content was not accessible at the experimental temperatures and pressures used so far. Also, various groups have demonstrated that water enhances supercritical carbon dioxide extractions. Increasing temperature to extract with supercritical water (hydrothermal leaching) has also been shown to be effective, but extractions using heat, water, and pressurized CO2 all together are experimentally difficult and have not been reported so far. In this work, we would like to examine if combining the three would allow for the extractive conditions to be strong enough to remove higher metals such as iron and sulfur, but weak enough to leave the carbon in the solid form.
The key hypothesis of this work is that by utilizing mixtures of supercritical CO2 and H2O at high temperature and pressure conditions, inorganic elements can be selectively extracted from coal. The CO2-H2O ratio can be used to fine-tune the extraction efficiency. Ionization provided by the formed carbonic acid along with the high thermal energy can cause reaction of the inorganic content, and the products are then solubilized and efficiently carried away by the high density of the fluid. We are interested in exploring the extraction in the range of 200 -400 o C with pressure in the range of 2000-3000 psi which has not been studied before.

Key Motivation for This Work
Inspired by previous successful extraction efforts but keeping in mind the necessary thermo-economic restrictions for scale up on coal, we propose an extraction using supercritical mixtures of water and carbon dioxide. The mixture will form in-situ carbonic acid capable of leaching heteroatoms from coal but also will be finely tunable to prevent loss of hydrocarbon heating value. Also, supercritical extractions have the benefit of simple separation upon completion of extraction using fractionation at decreasing pressures. Power plants would be able to use carbon dioxide generated from combustion, otherwise emitted, and use waste heat to perform economical and environmentally protective extractions on raw coal. Because sulfur comprises the majority of hazardous aerosolized pollutants by mass, sulfur is studied as a model heteroatom before and after extraction in this work.

Mechanism of Extraction
Extractions involve the separation of components. Chemical reactions between solvent and extraction target assist in separating the target by forming intermediate complexes that are more soluble in the solvent. The solvent flow also aids in then physically separating the target molecule from its previous bond into the fluid bulk. The three major groups of extractions are cation exchangers, solvating extractants, and anion exchangers. Cation exchangers can be either acidic extractants or chelating extractants. Organic acids such as sulfonic, carboxylic, phosphoric, and phosphonic acid are acid cation exchanging extractants (Qiu 1993). Acid cation exchange is highly pH dependent. Chelating extractants, such as hydroxyoximes, contain two functional groups that form bidenate complexes with metal ions and are used industrially for copped extraction (Flett 1979). Solvating extractions contain electron rich oxygen or sulfur atoms that can bind to metal atoms, forming an anionic coordinating ligand (Mooiman 1991). This extraction relies on the formation of an uncharged ion pair or complex in the aqueous phase. Examples of solvating extractants are phosphoric esters, alkyl sulfides, and ketones. Anion exchangers such, as quaternary ammonium salts and ranges of amines, are useful for separating metals present in electrolytic solutions as neutral ion pairs or anionic species (IDE 1985).
Also important to extraction is the physical separation caused by the force of the solvent.
Pressure washing is a form of extraction where a target, usually dirt, is separated from a sturdier substrate, like driveways or the sides of a building. With pressurized solutions, a similar effect is created when the pressure is decreased and a sharp flow of solvent toward the outlet is created.
For large scale use, the extraction must a few simple but difficult to satisfy requirements.
First, the solvent mixture must function efficiently in terms of economic constraint. In other words, the cost of the extractant solution cannot exceed the value added to the mass treated. The extracting solvent should be cheap, and the value added to the solid substantial. Second, the solvent must be regenerated in a stripping reaction at a high rate. The solvent should not bond so tightly to the target that the target cannot be removed at some other physical condition allowing for solvent regeneration. Also, the solvent should not attack the substrate so violently that it bonds irreversibly to the substrate, and sent for combustion in the case of coal. Third, the extraction should provide maximum safety to plant personnel and equipment. The inherent danger of handling more caustic and harsh extracting solutions at high flowrates and enthalpy may very easily be preventative to scaleup.

Supercritical Fluid Extraction
Supercritical fluid extractions have several benefits including enhanced mass transport, ease of separation and recycle, wide range of extractive capability and tunability, and better inherent safety. A fluid is supercritical when it is compressed beyond its critical pressure and heated beyond its critical temperature, and the fluid density can be adjusted by changing pressure and temperature. The diffusivity of the supercritical fluids is higher than that of liquid solvents, and can be easily varied. For typical conditions, diffusivity in supercritical fluids is of the order of 10 -3 cm 2 /s as compared to 10 -1 for gases and 10 -5 for liquids. The viscosity of typical supercritical fluids is of the order of 10 -4 g/cm-s which is similar to that of gases and about 100 fold lower than that of liquids. The combination of the high diffusivity and low viscosity provide rapid equilibration of the fluid to the mixture to be extracted, hence extraction can be reached close to the thermodynamic limits. In addition, the supercritical temperature can be manipulated by changing the pressure, which can be done to ensure that the fluid mix is supercritical at the intended extraction temperature.
For many extraction applications in food and pharmaceutical industries, carbon dioxide is the supercritical fluid of choice because it is non-flammable, non-toxic, inexpensive, and has mild critical temperature (31.1 o C and 74 bar). Hence, much of the attention has been given to supercritical carbon dioxide for practical extraction applications. However, scCO2 is too mild for extraction of hard minerals from materials such as coal.
One the other hand, water is supercritical at >374 °C temperature and >221 bar pressure.
Supercritical water has liquid-like density and gas-like transport properties, and behaves very differently than it does at room temperature. For example, it is highly non-polar, permitting complete solubilization of the most organic compounds and gases. The resulting single-phase mixture does not have many of the conventional transport limitations that are encountered in multi-phase reactors. The physiochemical properties of water, such as viscosity, ion product, density, and heat capacity, also change dramatically in the supercritical region with only a small change in the temperature or pressure, resulting in a substantial increase in the rates of chemical reactions (Gupta 2005). For example, Figure 1 shows how density, dielectric constant and ionic product of water vary with temperature at 240 bar. From Figure 10, it is interesting to see that the dielectric behavior of 200 o C water is similar to that of ambient methanol, 300 o C water is similar to ambient acetone, 370 o C water is similar to methylene chloride, and 500 o C water is similar to ambient hexane (Gupta, 2005). Hence, for our application of coal beneficiation, subcritical conditions (e.g., 300 o C) may provide a conducive solvent atmosphere where various ionic species are effectively solubilized in the fluid and removed.

Chapter 2: Reaction and Phase Equilibria in ScWC Extraction
The proposed extraction utilizes favorable reaction and thermodynamic phase equilibria environment in the supercritical water and carbon dioxide (ScWC). There are three noticeable elements to the ScWC medium: (a) ionization due to reactions among H2O and CO2 molecules, (b) reactions with the coal matrix, and (c) phase equilibrium which determines the phase density (i.e., vapor, liquid, supercritical) important for the solublization of the extracted components.

Ionization Reactions in Supercritical water and carbon dioxide (ScWC)
Self-ionization of water to form H+ and OH-ions and reaction of water with CO2 leads to the following ionization reactions: Equilibrium data for reaction 1a: The ionization of carbonic acid to produce HCO3and H + ions has been reported as: Equilibrium data for reaction 2a: The dissociation of carbonic acid to produce CO3 -and H + ions has been reported as Equilibrium data for reaction 2b: The hydrolysis of carbonate to produce HCO3-and OH-is reported as Equilibrium data for reaction 3: The ionization of water to produce H+ and OH-is reported as:

Reactions with the coal components
In the proposed ScWC process, water molecule dissociates to form hydrogen ion and hydroxyl ion that both participate in further extraction reactions. At room temperature, the ions are in small concentration, and the dissociation increased with temperature upto about 300 o C and then it rapidly drops, as shown in Figure 1 (Kritzer et al. 2001 The CO2-H2O mixture is acidic which will help to break down complex mineral structure and provide access to metal ions. Many complex reactions have been hypothesized regarding the 6 7 5 specific forms of sulfur. Although this paper does not attempt to study these reactions, a comprehensive analysis of the reaction network can further help understand the molecular mechanism involved so that a rational design for industrial level extraction plants can be made.

Phase Equilibria of CO2-H2O mixtures
Carbon dioxide has critical point of 31.1 o C and 74 bar which is mild in process conditions, whereas water has high critical point 374 °C and 221 bar which is high in process conditions. A mixture of the two, depending upon the relative composition, the mixture can exit as a single liquid phase, single vapor pressure, two vapor-liquid phases, or a single supercritical phase. Such a variation in the physical behavior gives rise to a degree of choice and control on what is required for a given application. For example, the liquid phase can stabilize the ions hence, ionization is present. On the other hand, vapor phase, in general, cannot stabilize the ions, hence the ionization is suppressed.
To further understand the behavior, we have carried out chemical reaction equilibria and fluid phase equilibria simultaneous calculations using the commercial process simulator Apsen+ and the results for selected CO2/H2O ratios and process conditions are shown below. The 20 19 34 calculations show that the ionic content is strong with temperature up to 275℃. At 300℃ and beyond, the fluid becomes a gas and no longer has the density to stabilize ions. This calculation is does not account for supercritical fluid, but does highlight an important phenomenon. If the energy of the supercritical fluid is further increased, the effectiveness of the fluid as an ionic solution will decrease because the density of the fluid will decrease and the fluid will act more closely to a gas.
The liquid concentration calculations below may be helpful to groups considering carbon dioxide   The above calculations provide a map of the ionic conditions fluid densities observed as a function of CO2/H2O ratio and temperature. For simplicity, we have kept the pressure for calculation as 143 bar. Also, the liquid phase properties are not too impacted by the pressure. It is also noted that high fluid density is needed to stabilize the ions. In the proposed ScWC process, it is expected that both ionic and non-ionic extractions will play a prominent role.

Chapter 3. Experimental Work
For the experimental work to test out the proposed extraction process, a new apparatus was constructed. Design of the apparatus and it development over time is described below. The concept was tested on a commercial coal obtained from mine in Pennsylvania.

Materials
Supercritical fluid grade CO2 and deionized water and their mixtures were used as extraction fluids. The pulverized bituminous coal with particle size 250-1250 micron was obtained from Rosebud Mining Company (Kittanning, PA) identified as Clymer Blend CPA#101315.
Nanopure filtered water was used for all testing. High purity SFE grade CO2 tank with dip tube was provided by Airgas. Coal standard 2683c was acquired from NIST. For analytical work, 70% environmental grade nitric acid from GFS Chemicals was used for ICP-OES. Boric acid was purchased from the American Borate Company, and pure sodium sulfate and sodium hydroxide were purchased from Sigma Aldrich.

Coal Extraction Apparatus
A semi-continuous coal extractor was designed and built as shown in Figure 3, in which coal powder is packed in the extraction vessel and the extraction fluid is pumped continuously. International was used as a flow vessel to extract coal solids. The vessel volume is 250 mL and material of construction is S316 stainless steel. The vessel itself is rated to a maximum continuous working temperature of 400 ℃, maximum temperature of 450 ℃, and a maximum working pressure of 200 bar. A Supercritical 24 constant flow/constant pressure dual-piston pump was used to pressurize the vessel with CO2. An Eldex 1SMP pump (0-5 mL/min, <200 bar) was used to pump water. The two streams meet at a tee joint that immediately feeds to a check valve at the vessel inlet. The vessel was heated with a Rama 1000W heating band controlled by a temperature controller. A Swagelok KPB1P0A422P20000 backpressure regulator was used to control the pressure of the outlet. Gas exiting from the regulator was fed to room temperature separator to allow aqueous suspension leachate to separate from gaseous CO2.

Typical Experimental Procedure
In a typical extraction, the cooling/chilling unit on the Supercritical 24 pump were turned on and allowed to cool for 20 minutes. The vessel was loaded with about 10-50 grams of pulverized coal and covered with porous glass wool to suppress carryover of the coal particles by the extraction fluid. The vessel was tightened and the backpressure regulator left slightly open. If the extraction called for water, the Eldex pump would be then be primed, turned off, and connected to the vessel inlet. The CO2 tank was then opened and the pump turned on. The regulator was then closed all the way and checked for zero outlet flow. The vessel was allowed to fully pressurize to 153 bar with only CO2 over the course of about 25 minutes at which point the backpressure regulator was opened and a steady flow established. The temperature controller was then turned on and set to 75º C. If the extraction called for water, the Eldex pump would also be turned on.
Once the vessel reached about 50º C, the setpoint was increased to 175º C and then again to desired set point. Heating to desired set temperature would take about 25 minutes. Extraction time is then considered to be time zero once the vessel has reached the desired temperature. Upon completion of an extraction, both pumps and the heating would be turned off and the CO2 tank closed. The regulator was loosened to completely depressurize the vessel and then closed to allow the evacuated vessel to cool overnight. Usually, the vessel temperature would increase past reaction temperature at the beginning of cooling as incoming fluid was shut off and no longer provided a heat duty. Coal solids were weighed before and after extraction. Other than solids embedded in the glass wool used a filter, total coal mass did not seem to change during extraction. Liquid and solid samples would be collected the next day and stored for analysis. Figure 19: Temperature profile for a typical experiment. Data for experiment E102219 for which the extraction is considered to start at 143 minute timescale. At the end of the experiment forced cooling is achieved with low pressure carbon dioxide flow before ambient cooling.

Reactor Development and Troubleshooting
The Columbia vessel and the carbon dioxide pump were left in the lab from a previous student who had used the reactor in a batch setup with a valve at the inlet to the reactor that could be closed when the vessel had been charged with fluid. In order to build a flow type reactor, the pressure gauge and the rupture disc were consolidated to one of the three openings to the vessel.
New fittings were used to update the other two openings into inlet and outlet flows. A backpressure regulator was installed on the outlet to maintain pressure in the vessel. The first struggle with developing the system was actually purchasing incorrectly a front pressure regulator as opposed to back pressure. The backpressure regulator maintains the pressure behind the regulator whereas the font pressure regulator maintains pressure downstream of the regulator. Different suppliers may specify one but not the other, which lead to ordering a so-called regulator, which was actually a front pressure regulator.
With the correct regulator, the pump was experimented with to become familiar with the vessel pressurizing. It took a couple of months to realize that the pump was not working correctly and needed to be serviced. Without understanding how the pressure should respond to good flow of carbon dioxide, the system would be allowed to pressurize to the liquid carbon dioxide level but the pressure would not build beyond. The pump was sent to the manufacturer for servicing.
Servicing the pump involves removing the hood, disconnecting the Peltier element on the front that acts to cool the pump heads, removing the pump casing and check valves, and washing the valves and heads. Very small buildups of contaminant would cause the pump to stop working as the pump heads were only several millimeters diameter. Upon getting the pump back from the manufacturer, the pump worked for a couple weeks and then seemed to be stalled again. In speaking with the service technician, it was discovered that they had reinstalled old seals after cleaning the internal surfaces. The pump manual recommends replacing seals whenever servicing the pump. The manufacturer offered to service the pump again free of charge, but instead the pump has been serviced in house since. Servicing the pump mainly involves thoroughly cleaning the inlet and outlet check valves and replacing seals. It was also decided that the vessel should have a check valve at the inlet to ensure that no solids or fluid from the reactor could back contaminate the pump. In order to ensure that the carbon dioxide is liquid for pumping, the inlet carbon dioxide to the pump is cooled. Without cooling, the fluid will cavitate and phase shift in the pump instead of being pushed through the check valves. The pump contains an electric cooling Peltier device, but we thought that more cooling may be necessary. A ThermoScientific ACCEL 500 was purchased that could cool refrigerant down to -40 ℃. A large loop was made with the ½" ID plastic tubing for the cooling unit and filled with ethylene glycol. The steel line from the carbon dioxide tank to the pump was then fed into the plastic tubing. Finally, this was the last adjustment necessary to produce good pumping of carbon dioxide. Figure 24. Photograph of the cooling heat exchanger to liquefy CO2.
In terms of trouble pumping carbon dioxide, we also found that the grade of carbon dioxide tank may be important. At the beginning of the setup, we used a low grade of carbon dioxide and had trouble in concert with a pump that I did not realize was not working. The tank was labeled as having a dip tube, but that did not seem to be the case. At the same time that the pump came back from servicing, we got a new carbon dioxide tank that was high purity supercritical fluid grade that was specifically manufactured for us with a dip tube. The dip tube ensures that fluid coming from the tank is liquid from the bottom of the tank, and not vapor from the top of the tank. With the newly cleaned and working pump, we did not return to lower grades of carbon dioxide though we probably could have. Experiments have consumed 5 large tanks in the past two years.
With pressurizing understood, we moved to heating the vessel. A new heating jacket was purchased toward the beginning of experiments and has been used with a temperature controller.
The setpoint on the controller needed to be increased in stages. The controller would often fail to recognize enough change in temperature and stop itself for 'loop break.' For this reason, it was important to insulate the vessel well. A large sheet of glass wool was purchased for high temperature insulation that is also used as filter inside the vessel. Adding temperature in stages, we found that at 300 ℃ the regulator would stop working correctly because the fluid had reached temperature at the backpressure regulator that would melt the Teflon seal inside. Thus, a cooling coil made from steel tubing was added between the vessel and the regulator that could sit in water.
Within three turns of coil, the steel goes from reactor temperature to something I can rest my finger on by evaporating water in the bath.

Figure 2511 Burned out regulator seals from the back pressure regulator
Containment of solids has been the other main experimental challenge. The vessel has a glass containment piece to pack coal into, but the temperature probe and sonicating horn fit down into the glass so forming a good seal is tricky. The best way to contain solids in the vessel was to pack two or three layers of glass wool overtop the coal and try to fit the glass wool snuggly around the horn and thermocouple. Often the vessel would still leak some solids, especially if carbon dioxide pressure decreased sharply and caused fluid to rush to the outlet. For this reason, an inline filter was necessary and installed before the regulator. The filter would need to be disassembled and cleaned with each experiment, and regulators were disassembled and cleaned about once every month or if they became clogged which would prevent the regulator from operating properly. If the regulator became clogged during an experiment, the regulator could be completely closed, and then reopened. Sometimes this would crush the solid enough to get to through, but sometimes the system would build pressure and not be able to get through the regulator even when the regulator was opened. This led to a lot of large depressurizations and scrapped experiments.

Chapter 4. Analysis
Coal was analyzed using the instruments available in chemical engineering, chemistry and nano-characterization center at VCU. For the test that could not be run at VCU, we have gotten commercial testing done from Standard Laboratories, Inc. (South Charleston, WV) which specializes in coal analysis. ASTM methods of standard analysis are listed for coal and followed where applicable.

Moisture Content
Moisture content could be measured by weighing one gram of coal and drying in an oven.
Typically, coal would be dried overnight in our vacuum oven at 80℃ and -100 psig. The coal would then be weighed after drying, and the difference attributed to moisture present in the coal assuming complete drying.

Elemental (Ultimate) Analysis
Ultimate analysis is the list by mass of the individual elements in coal. Ultimate analysis is usually obtained by using an elemental analyzer, which pyrolyzes 0.5 -1.0 mg of substance and then sends vapor through FTIR analysis. With well calibrated spectra and very accurate flow controls, the typical elemental analyzer is able to give carbon, hydrogen, and nitrogen percentages. Some instruments are also able to give sulfur percentages. The balance then can be assumed to be oxygen or oxides. Dr. Bukavekas has a CHN elemental analyzer in the Biology Department, and Old Dominion had a CHNS elemental analyzer.

Heating Value
Heating value of the coal can be experimentally calculated with a bomb calorimeter.
Lacking one at VCU, a calculation using the percentages from an ultimate analysis can be used.

Ash Content
Light and heavy metals present in the coal will be left after combustion. To calculate ash percentage, procedure calls for one gram of coal to be loaded into a furnace with flow of air and heated to 750 ℃ for 4 hours, or until no further change in mass. After the carbon, nitrogen, and hydrogen burn off, the remaining solids can be weighed and used to calculate the ash percentage in the coal.

Sulfur Forms
Sulfur can be in the form of pyrite, organic, or sulfate. Total sulfur is found using elemental analysis (ultimate analysis) using pyrolysis and FTIR of vapors. Pyrite and sulfate are found using wet chemistry methods. ASTM methods for pyrite and sulfate quantification are rudimentary and can be influenced by coal structure and the presence of a wide array of interfering elements in coal ash. Standard Laboratories updated methods for pyrite and sulfate quantification were helpful for obtaining data that would have been hard for us to reproduce well. The organic sulfur, containing the most diverse set of bonds, is then calculated as the difference between the total sulfur and the pyrite and sulfate portions.

Chlorine
Commercially analyzed using an ASTM standard and wet chemistry methods.

Mercury
Commercially analyzed using an ASTM standard and wet chemistry methods. ICP-MS also can be used for mercury counts.
Thermogravimetric Analysis TGA was used to understand the strength of the coal bonds with respect to temperature.
In a nitrogen atmosphere, a coal sample of about 3 mg would be slowly heated to 800℃.
Typically, the coal will lose mass in three phases. First, moisture vaporizes around 300. In the second phase, volatile sulfur content will decompose, and then finally the volatile carbon portion will evaporate. Thus, the shape of the figures and the final mass remaining are useful tools for analyzing coal.

ICP-OES
Agilent ICP-OES in the Chemisty Instrumentation Lab used to analyze the content of aqueous leachates. A calibration curve was created for sulfur using known dissolved amounts of sodium sulfate in 2% nitric acid. Although this method was used to confirm that sulfur existed in the aqueous leachate, we did not pursue closing vigorous testing of leachates because a significant portion of sulfur may be extracted as gas, but also, a significant portion of the sulfur in the aqueous leachate may evaporate or form salts before the ability to analyze.

Raman Shift
Raman was used to understand changes to the carbon structure that may have been caused by extraction. Also, because the coal contains a wide variety of elements, the influence of the sample's fluorescence is a weak indicator of ash extraction. Spectra for raw coal had higher counts and more angled baseline than extracted coal due to higher fluorescence of ash metals.
For carbon, two characteristic carbon peaks appear at 1350 and 1600 cm -1 . With baseline subtracted, the peak heights and ratios are the same between extracted and raw coals. A slight broadening of the peak at 1350 cm -1 is noticeable with two shoulder peaks becoming more prominent.

XRD
A PANalytic XRD in VCU's Nano Characterization Center was used to understand the crystallinity of carbon in coal samples before and after extraction. XRD is also helpful for determining the presence of certain minerals, though they did not interest us.

BET
BET was used to determine if the surface area of the coals were greatly affected by extraction. Although raw coal could not be analyzed because of the volatile sulfur portion, the extracted coals could be properly offgassed and analyzed.

ICP-MS
ICP-MS was the most widely used method for us to quantitively analyze our coal in house. This technique allowed us to look directly at the coal mass before and after extraction instead of measuring resultant gas, liquid, and precipitated solids. Carbon, sulfur, and almost all ash metals could be assayed using ICP-MS. Mercury could be analyzed, but not chlorine. Coal would be dried, pressed into a pellet with boric acid as filler, and then ablated using a laser.
Abalated mass would then be swept into a spectrometer and counted.
Approximately 32 mg of dried coal and 593 mg boric acid would be weighed, mixed, and ground together with a high-energy ball mill for 60 seconds with three small balls and three large balls. Then, the mixture would be taken the Chemistry and pressed using a Carver press and an 11 mm die. 3 tons of applied force for 2 minutes was used to fashion pellets.
A 1550W laser (8 J/cm 2 ) would be focused on the pellet and used to ablate and ionize the pellet surface at low pressure in the presence of plasma gas. Ions are swept by nebulizer gas into the spectroscopy unit and produce counts for each element. A laser spot size of 75 microns was used to ablate 5 mm of pellet to 7.0 mm depth while rastering at 75 micron/s. Line length could be adjusted to obtain an appropriate number of counts to build calibration curves. The experimental error in the sulfur analysis is expected to be 5%.
Pellets were fashioned with various amounts of a NIST coal standard (sulfur, 2 wt.%) in order to create calibration curves with good linearity. From these, we could calculate the total mass of sulfur in a given sample from the sulfur counts. We found that isotopes sulfur 33 and sulfur 34 were both useful, but that sulfur 34 would have an order of magnitude more counts. Although most samples had similar counts for carbon, samples that had been hydrothermally carbonized showed lower carbon counts. For these samples(15-E062519 and 23-E102219), the carbon was more resistant to ablation, and the sulfur values were calculated with respect to the amount of carbon that was actually ablated.

Chapter 5. Case Study I -Testing flow ratios and temperatures on bituminous coal
The first test of the proposed technology was done on the bituminous coal. Experiments were performed at various temperatures with CO2 flow, H2O flow (hydrothermal), and combined flow (ScWC) in order to asses the effectiveness of supercritical carbon dioxide as solvent and to compare combined flow extraction to more typical hydrothermal extractions. Because fluid characteristics can vary complexly with CO2-H2O ratio, a variety of temperatures were explored ranging from 200 -400 ℃, and sulfur remaining was used as a measure of effectiveness. In our first study, we explored various mixtures and worked to constantly expand our experimental capability, increasing in temperature and flow rate. A master list of all the experiments conducted is given below: The coal mass extracted decreased over time as we came to understand the amount needed for analysis but changes are negligible. The fluid path of our vessel does not directly cross through the loaded solids, so it is likely that the solvent to coal ratios are all higher than necessary.
Further testing will be need to understand the mass transfer characteristics in a well design flow path. For the same reason, in the present study, the differences in time are not as important as changes in temperature and fluid composition.
The coal feedstock was analyzed independently by Standard Laboratories using prescribed ASTM methods. The analysis is shown in the Table 2. It showed a 1.14 wt% sulfur content using ASTM D2492. Our in-house ICP-MS analysis gave a similar sulfur content of 1.15 wt.%, but due to high variation, the coal was mixed more thoroughly and a raw value of 1.37 wt% was obtained thereafter. The raw values were assumed to hold constant for all experiments in case study 1, but future experiments have individual raw samples. A total of 17 extraction runs were carried out using the above bituminous coal at a constant pressure of 143 bar with varying extraction time, temperature, water flow rate and CO2 flow rate, as shown in Table 3. The sulfur content was tracked by ICP-MS analysis in the extracted coal as a measure of extraction effectiveness. Standard Labs results are also presented when available.
Solids recovered after extraction were usually within 2% of the mass loaded after accounting for loss of moisture, except for extractions with pure water at temperatures around 350 ℃ where about 10% of the mass was lost. The exact masses recovered were difficult to obtain as some coal particles entangled in the glass wool used as filter. In future studies, the glass wool will be weighed before the experiment and then after collection and drying to assess the change in mass of the coal during extraction. Figure 35: Sulfur remaining in coal after extraction using hydrothermal, ScWC, and CO2.
Data in Figure 5 shows that carbon dioxide, ScWC, and hydrothermal extraction are all similarly capable of extracting 50% sulfur. Extraction with carbon dioxide was most effective at 350 ℃, while extraction with water seemed to be most effective around 300 ℃ where the ionic concentration was highest. As expected, the hydrothermal extraction was more aggressive at lower temperatures than the carbon dioxide, and the mixture was somewhere in between. Combined flow may provide a synergy that increases extraction but more accurate assessment of the raw coal would be necessary. ScWC, or combined flow, does seem to inhibit organic dissolution that would be expected and was seen from strictly hydrothermal extraction. Although a limited testing was done with flowrate, increasing the flowrate of carbon dioxide extraction at 350℃ from 5.82 to 11.64 g/min did result in reducing sulfur content from 0.92 to 0.73 wt.%. While we studied only a single-step desulfurization reactions, analysis of the sulfur forms indicated that extraction with pure carbon dioxide at 200℃ caused pyritic sulfur to be transformed into sulfates that may be easily washed out with warm water. While raw coal contained only 0.02% sulfate and 0.50% pyritic sulfur, E120218 contained 0.16% sulfate and 0.28% pyritic sulfur. In future studies, each extracted mass of coal will be washed with warm water to ensure that the sulfate formed is removed. Sulfur forms analysis was also performed for E061819, the best CO2 extracted coal. Results showed that the total sulfur content was 0.74 wt%, in excellent accordance with ICP-MS calculated value of 0.73 wt%. The pyritic, sulfate, and organic portions were 0.01, 0.03, and 0.64 wt% respectively. Good conversion of pyrite indicates that the mass transfer from solid to fluid was strong, but the raw level of organic sulfur indicates that organic sulfur was totally inaccessible by CO2 at 350℃.
In the case of the combined and hydrothermal extractions at 395 o C, the powder coal feedstock turned into a porous-fused mass by a process called hydrothermal carbonization ( Figure   6). BET analysis showed that solid product surface area was low, with surface area of 14 m 2 /g and average pore radius of 9.9 Å, probably similar to raw coal, which could not be analyzed due to volatile sulfur content. The coal after the extraction at lower temperatures was not fused. Opaque yellow product was typical for all successful extractions. Ash content was evaluated for the three best extracted coals E061819 (CO2 only), E071019 (ScWC), and E072419 (H2O only). Raw coal was found to have 6.75 % ash, in good accordance with SL data. Neither carbon dioxide nor water seemed to affect the ash content, with E061819 testing at 7.05% and E072419 testing at 6.42%. ScWC extraction, on the other hand, decreased the ash content to 3.77%, a 45% reduction in ash.
Extracted coal from E102219 was further sent out for proximate analysis to the Standard Laboratories. The results are compared with the coal feedstock in  Data from ICP-MS analysis of samples from various experiments is given below: Observing low carbon 12 counts seems to be a way to assess whether or not the coal has carbonized. The more graphitic carbon is more resistant to laser ablation. The first four experiments are standards, and afterword, the amount of carbon ablated is fairly consistent, with three exceptions: experiment E042519(label 12), E070319 (label 15), and E102219 (label 23). All three had water that became supercritical as the experiment was shutdown or during operation. In a supercritical state the polarity of the fluid will be very low and the carbon framework may be affected. Surprisingly though, E042519 seemed to experience C12 Counts per Sample some carbonization despite not having any water present during the experiment. Furthermore, carbonization was not noted during the extraction of coal with carbon dioxide at higher temperature in experiment E092619. Experiments after E042519 were cooled with low pressure carbon dioxide flow in order to prevent the vessel from overheating at the beginning of shutdown, when the low enthalpy fluid inlet is stopped. Therefore, carbonization of our coal seems to occur in CO2 near 350 ℃, and in water above 375℃, and may prefer natural cooling as opposed to forced cooling.
The sulfur counts were first converted to sulfur value for the pellet using the calibration curve created. Next the overall weight of sulfur in the pellet would be calculated using the specific weight of the pellet formulated. From the overall weight of sulfur in the pellet the overall weight of sulfur in the coal was determined by assuming no sulfur was present in the filler, which was boric acid. Because some samples were resistant to ablation due to carbonization, the sulfur value was scaled up by the ratio of the typical carbon ablated divided by the specific carbon ablated. Typical counts for carbon were about 70,000,000 so E070319 and E102219 were scaled by 2.08 and 2.47 to values of 0.83 and 0.87 wt% sulfur, which correspond well with independent analysis To further understand the characteristics of the extracted coal from E102219 experiment, x-ray diffraction and thermo-gravimetric analyses were carried out and the results are compared to raw coal. XRD spectra for coal contain two main peaks for carbon at 2θ of 25° and 26.5°. Raw coal has a more prominent peak at 25° with a smaller peak at 26.5°. For E102219, the higher intensity indicates a more ordered crystalline structure as compared to raw coal. Also, the peak at 26.5° has become larger than the peak at 25°. The 26.5° peak is associated with synthetic graphite, and the peak at 25° can be attributed to carbon that is not graphitic. Thermogravimetric analysis was carried out in nitrogen atmosphere and shows the difference in volatility between raw and extracted coal. By 800 ℃, the raw sample has lost 23.2% of mass, compared to only 7.7% mass for E1022. Although we extracted only to 395℃, E102219 is considerably less volatile in the range of 400 -800℃.
Raman spectra for raw sample have higher counts and a more angled baseline due to fluorescence of metals contained in the raw coal ( Figure 8). This fluorescence is not noticed in the extracted sample from E102219, which has a nearly flat baseline and lower noise, indicating a lack of fluorescence generating metals. Two characteristic carbon peaks appear at 1350 and 1600 cm -1 . With baseline subtracted, the peak heights and ratios are the same. A slight broadening of the peak at 1350 cm -1 is noticeable with two shoulder peaks becoming more prominent. crosslinking volatile carbon into fixed carbon would increase the heating value. Importantly, the combined extraction seems to crosslink carbons that would be extracted and lost by hydrothermal extraction. 8. What is the impact of the extraction treatment on the flow-ability of the cleaned coal?

Key Scientific Observations
To use an extraction tower with automated semi-continuous hopper feed, the coal should not be allowed to carbonize so that coal flows smoothly into the combustion unit. To this end, the temperature should be kept lower than 350℃, which is where sulfur extraction efficiency is higher anyway. Controlling the cooling rate of the coal after extraction may also be an effective way to prevent carbonization and thus clogging.
9. How does the particle size and surface area influence the kinetics?
We were not able to study different particle sizes, but decreasing the particle size will increase the extraction efficiency by increasing mass transport capability.
Getting below the crushed size that we use would require more energy intensive grinding whose cost would start to rival the marginal benefit of faster kinetics.

Chapter 6. Case Study II -Adding ultrasound energy to improve extraction of Bituminous Coal
Ultrasound waves can be added to solution to enhance the mass transport of the fluid. A piezoelectric crystal is attached to a horn that is placed in the vessel near the solids. Crystals may also by placed on the exterior of vessels after some calculation, but thick steel will dampen ultrasound waves. Traditionally, the formation of bubbles leads to increased mixing and increased ion activity caused by the collapse of the bubbles. In supercritical solution though, two phases cannot exist and bubbles will not form. Still, the addition of pulsed ultrasound waves are expected to further increase the mixing and mass transport of the supercritical fluid which may be important for penetrating the solid matrix of coal. Most ultrasound studies focus on the use of basic hydroxide solutions that enhance the formation of hydrogen peroxide, which performs the critical steps in chemical extraction of sulfur and ash. Study is ased on experiments E061220, E062420, and E062520 which are all run for one hour at optimum temperature for the given fluid mixture on 10 grams of coal. E061220 was run with only water at 315℃, E062420 was run with combined flow at 335℃, and E062520 was run with only CO2 at 350℃. The mass of the glass wool filter is measured before the experiment and after collecting and drying so that the mass of the coal recovered is now accurate enough to calculate the organic dissolution. In addition, individual raw samples were collected instead of assuming a flat raw value. Coal was also washed after extraction with 90 ℃ water to remove any sulfur that had been oxidized to sulfate form. Samples will be ground, pelletized, and analyzed by ICPMS and for ash content using the tube furnace. Experiments were performed successfully at 2100 psi without leaks. Ultrasound energy was pulsed for one second on, one second off at 120 Watts. An aututuner keeps frequency between 20-25 kHz. An increase in temperature was noticed when ultrasound was turned on, but the temperature controller effectively kept temperature at the experiment setpoint. After accounting for coal collected in the glass filter, organic dissolution could be approximated. The mass of coal collected after extraction and drying for E061220, E062420, and E062520 were 9.32, 8.74, and 8.55 grams. Thus, organic dissolution did not seem to occur appreciably for hydrothermal treatment as compared to CO2 treatment or combined flow.
Ash content analysis indicates that the extractions with ultrasound were not particularly effective. All three extracted samples seemed to test at levels equal to raw coal. For E061220, E062420, and E062520 -8.38, 8.30, and 7.66% ash. With more vigorous mixing the ash content of later samples was a bit higher (around 8%) than earlier studied raw (around 6.5%). There is also a possibility that some of the glass wool may have broken off and/or dissolved followed by precipitated on solid coal. This could increase the ash content of the coal.
ICP-MS also indicated that samples were not extracted in terms of the sulfur content.
Raw sample had sulfur counts of 256,000 compared to 230,000 for water only 211,000 for combined flow and 199,000 for only CO2, corresponding to 1.08 wt% sulfur still remaining in the extracted sample.
The increased energy from the ultrasound decreased effectiveness compared to runs without ultrasound at the same temperature. This decrease in extraction can probably be explained by a critical decrease in fluid density. Although the mixtures were still supercritical, they behaved more like gases due to the increase in energy and mixing from the ultrasound pulses. The carbon dioxide was less affected and produced the best results with ultrasound because the carbon dioxide is already well superheated and increases in the fluid enthalpy will no longer result in drastic change to density.
In future work, the ultrasound should be tested below 300 ℃ to discover if the coal can be extracted with higher solvent and thermal efficiency compared to higher temperature without ultrasound.

Chapter 7. Industrial Applications
The technology developed in this work can potential contribute to the energy industry in a variety of way in addressing current challenges. Some of the applications are summarized below: Reduction in the emission of pollutants The proposed supercritical water-carbon dioxide (ScWC) extraction technology has potential to help industry in mitigating hazardous aerosol pollutant emission by reducing the need to treat flue gas and increasing the use of flue gas streams for energy recovery. Successfully removing sulfur and other hazardous molecules from the raw coal before combustion would eliminate/reduce pollution of ash, limestone slurry, and air. While many post combustion efforts prove to trade one form of pollution for another, our proposed pre-combustion extraction process seeks to reduce waste in all three phases. In the traditional combustion, treating the flue gas instead of raw coal results in a substantial volume of gas to be processed. This also hinders the ability to further harvest thermal energy from the stream; small improvements in the thermal efficiency of large power plants can have drastic overall economic effects. Pollutant-free flue gas stream may also allow for direct compression of carbon dioxide for sequestration and use. The presence of sulfur and mercury is the main reason for coal processing for power generation to become an expensive proposition. Removing them pre-combustion also offers the potential to use coal for other beneficiated products where these two elements are not desired in the products. The use of CO2 and H2O mixture has potential for industrial applications in power plants as both CO2 and H2O are available on site. The proposed process has potential to provide new synergy with Brayton cycle power generation, oxyfuel combustion, biomass cofiring, and enhanced oil recovery technologies or sequestration.

Figure 43: Carbon Dioxide Process Synergy for Clean Coal Fired Electricity Production
Increase in the power production efficiency To avoid the substantial volume of nitrogen to be separated from the combustion exhaust, oxyfuel combustion could be used whereby the inlet air is separated into pure nitrogen and oxygen with a cryogenic separations unit. Without the volume of nitrogen, carbon dioxide as a diluent would also be fed to the combustion unit in order to control the combustion compositionenvelope and temperature of combustion. Without nitrogen to separate, the combustion effluent will be a high percentage carbon dioxide with some water.
The power generation unit could incorporate Brayton cycles where carbon dioxide is used as the working fluid instead of water. These plants have been in development for some time as a next generation technology aiming for several percentage points in thermal energy efficiency at large scale power plants. The power generation unit, like the extraction unit, will not consume carbon dioxide but rather use it as a working fluid.
The extraction unit would involve one large solid-supercritical fluid extraction tower with hoppers for loading and unloading. The extraction effluent containing sulfur and metals would then be used in a pressure fractionation scheme using the water, carbon dioxide, and entrainers to condense out elements from high mass to low mass in sequential flash tanks. Heavier metals in aqueous suspension would be produced by relieving a small amount of enthalpy from the effluent in the first flash tank, while lighter metals would remain dissolved and travel with the vapor to the next flash tank. The pressure and temperature of the flash tanks can be set according to the number of tanks possible or the desired separation. In this way, valuable and scarce heavy metals can be targeted and concentrated instead of accumulating in ash piles. Carbon dioxide would be cleaned of sulfur and regenerated through a process like the Claus process, whereby vaporous sulfur is burned to form solid elemental sulfur and the heat recycled.

Recovery of rare earth elements
Rare earth elements are heavy metals that are used in small quantities in the construction of advanced technological devices. The U.S. and the world have long relied on China for rare earth elements due to a heavily concentrated mud pile in China. Recent discovery of another trove in Japan was announced in 2012, but the site is far underground and will take some years to produce. China even has the ability to use these rare elements in trade wars, as the production is not economical at the lower concentrations of the same element mixes found in coal.
Carboxyhydrothermal processing may provide the U.S. with the ability to access and concentrate the vast stores of rare earth elements seen as waste in coal ash piles.

Upgradation of biomass for use in power plants
Due to the flexibility of design with temperature and flow ratio, the carboxyhydrothermal extraction could handle various ranks of feedstock. The process would also be useful for upgrading lower grade biomass for cofiring. Often these biomass feedstocks have high amounts of ash that can destabilize the combustion parameters, so only a small portion can be added.
Despite low calorific value compared to hydrocarbons, biomass waste cofiring is beginning to be mandated in order to improve sustainability. The proposed extraction would be able to demineralize biomass sources to the necessary degree, and then add value to the biomass through hydrothermal carbonization.
Having these technologies work together will create synergy through the collective handling of carbon dioxide and cost-effective separation, but most importantly; carbon dioxide handling will allow for sequestration instead of release to the atmosphere. Because carbon

Chapter 8. Conclusions
Supercritical water-CO2 (ScWC) and supercritical CO2 extractions are proposed for cleaning of coal before combustion. Coal beneficiation is demonstrated in 200-400 o C range at 143 bar using a semi-continuous process without needing any mineral acid or organic solvents by studying the sulfur removal. A high pressure chemical reactor scheme was developed in order to perform extractions. An accurate ICP-MS technique was developed to quickly track sulfur removal of our process.
Carbon dioxide extraction at 350 ℃ is effective similar to the typical hydrothermal extraction which was most effective at 290℃. Extraction with carbon dioxide caused more difficult-to-remove pyritic and organic sulfurs to become sulfate by 200℃. Extracted solids contained about 50% less sulfur than raw coal for 1-hour extractions using CO2 at 350 ℃, water at 290 ℃ or combined H2O-CO2 at 320 ℃. Thus, the extractive strength of the fluid can be tuned by adjusting the temperature and H2O/CO2 flow ratio. Furthermore, the ash content was considerably affected by ScWC extraction, with combined extraction producing 50% ash reduction in one hour.
We demonstrated a new reaction of sulfur with carbon dioxide whereby carbon dioxide is used to donate oxygen atoms to a solid matrix in forming carbon monoxide. This reaction is similar to previous reaction studied with nitric acid whereby elemental sulfur and sulfur dioxide are produced. Expensive organic solvent, then, may be replaced by use of an abundant waste product.
In studying the mass transport of the fluids, we found that adding ultrasound did not increase the effectiveness of extraction at previous optimized temperatures. While the carbon dioxide extracted sample still contained 50% sulfur, the pyritic sulfur was almost completely reacted and the organic portion was completely unreacted. This indicates that the mass transport into the coal matrix was already good, but the temperature was not sufficient to target the organic sulfur bond at all.
Extraction with pure water above the supercritical point of pure water caused hydrothermal carbonization of coal solids where volatile carbons were crosslinked into fixed carbons. Some carbonization occurred even with strictly carbon dioxide extraction. Previous reports of carbonization convert a biomass waste into a low rank coal, but here we show that carbonization can occur at higher temperatures to produce higher rank coals from middle rank coals.
Carbonization caused the coal mass to fuse upon drying and may be interesting for further carbon upgrading, but should be avoided for coal meant to be combusted. Hence, it is preferable to operate at 320 o C or below to obtain a free-flowing extracted coal.
These studies provide several meaningful scientific discoveries in the area of thermodynamic solution manipulation and extraction for biomass conversion and beneficiation.
Despite answering several important questions, many new questions have arisen that should be studied in future work.

Chapter 9. Suggested Future Research
To further developed the proposed technology, following future work is proposed: Case Study III -Test ScWC for high sulfur coal The technology should be further tested if high sulfur coal can be treated. If successful, the ScWC can help reduce the environmental pollution in many countries that rely on high sulfure coal. One example is high sulfur coals in India and China which are rapidly expanding their coal use. The high sulfur content coal (~4%) can be studied using the combined flow technique. Coal should be loaded into a reactor extension that allows for more direct fluid-solid contact than loading into the reactor vessel. Coal should be extracted for 1, 3, and 6 hours with combined flow at 335 ℃. The temperature may be set at 355℃ to account for cooling between the vessel and the extension. The organic dissolution, sulfur forms, TGA, and ash content should be studied as previously for the ultrasound experiment.
Case Study IV -Test ScWC to produce ultra-clean coal and coke The ScWC has potential to produce ultra-clean coal and coke from the low-sulfur coals in the United States. The product can then have high value applications in the metals and batteries industries. The bituminous coal should be loaded into the vessel extension and extracted for 8 hours with combined flow at 335℃. The coal should then be unloaded, and washed. A small portion should be removed for analysis. The extracted coal should then be added to the reactor vessel and hydrothermally carbonized at 400℃ for 48 hours.
Step one should produce a fairly clean coal, and step two should increase the value of the carbon structure. Ash and sulfur should be studied with the tube furnace and ICP-MS, and the carbon content will be studied using XRD, TGA, RAMAN, and heating value will be measured by Standard Labs.