Recovery of carbon fibres and production of high quality fuel gas from the chemical recycling of carbon fibre reinforced plastic wastes

A solvolysis process to depolymerize the resin fraction of carbon fibre reinforced plastic waste to recover carbon fibre, followed by hydrothermal gasification of the liquid residual product to produce fuel gas was investigated using batch reactors. The depolymerisation reactions were carried out in ethylene glycol and ethylene glycol/water mixtures at near-critical conditions of the two solvents. With ethylene glycol alone the highest resin removal of 92.1% was achieved at 400 °C. The addition of water to ethylene glycol led to higher resin removals compared to ethylene glycol alone. With an ethylene glycol/water ratio of 5, at 400 °C, resin removal was 97.6%, whereas it was 95.2% when this ratio was 3, at the same temperature. The mechanical properties of the recovered carbon fibre were tested and showed minimal difference in strength compared to the virgin carbon fibre. The product liquid, containing organic resin degradation products was then subjected to catalytic supercritical water gasification at 500 °C and 24 MPa in the presence of NaOH and Ru/Al₂O₃ as catalysts, respectively. Up to 60 mol.% of H₂ gas was produced with NaOH as catalyst, and 53.7 mol.% CH₄ gas was produced in the presence of Ru/Al₂O₃.     

Simultaneous effect of temperature and pressure on catalytic hydrothermal gasification of glucose

Gasification of glucose in near- and supercritical water was investigated at temperature and pressure ranges from 400 to 600 °C and 20 to 42.5 MPa with a reaction time of 1 h. Hydrothermal gasification of glucose was performed in the absence and presence of catalyst (K₂CO₃) in a batch reactor. The influences of temperature and pressure in the supercritical regimes of water, catalyst were examined in relation to the yield and composition of the gases and aqueous products. The product gases were analyzed by gas chromatography, and the aqueous products were analyzed by high performance liquid chromatography. The gases produced were carbon dioxide, methane, hydrogen, carbon monoxide, and C₂–C₄ hydrocarbons and there was significant production of aqueous products and residue. The aqueous products composed of oxygenated compounds, including carboxylic acids (glycolic acid, formic acid, acetic acid), furfurals (furfural, 5-hydroxymethyl furfural, 5-methyl furfural), phenols (phenol, methyl phenols, hydroxy phenols, methoxy phenols), aldehydes (formaldehyde, acetaldehyde, acetone, propionaldehyde), ketones (3-methyl-2-cyclo-pentene-1-one, 2-cyclo-pentene-1-one) and their alkylated derivatives. Carbon gasification efficiencies were improved by addition of K₂CO₃ into the reacting system. Carbon gasification efficiency reached maximum (94%) at 600 °C and 20 MPa. The yield of hydrogen among gaseous products increased with increasing temperature and decreasing pressure.     

Products,pathways, and kinetics for reactions of indole under supercritical water gasification conditions

Indole is commonly reported as a product from hydrothermal processing of algal biomass. The reactions of indole in supercritical water were investigated between 550 and 700 °C in quartz, mini-batch reactors. The indole disappearance rate followed first-order kinetics, and the activation energy was 155 ± 10 kJ/mol. Methane and hydrogen were the most abundant gaseous products under most of the conditions tested, whereas benzene was the most abundant liquid-phase product. Hydrogen and carbon gasification efficiencies (HGE and CGE) exhibited values up to 79% and 20%, respectively. The influence of water density on the yields of H₂, CH₄, and C₂H₆ was negligible at densities above 0.081 g/ml, but the CO₂ yield increased with water density whereas the CO yield decreased. The yield of CH₄ increased significantly as the initial indole concentration increased. The collective results, which showed how the yields of numerous intermediate reaction products responded to changes in the process variables, permitted advancement of a potential reaction network.     

Corrosion of ceramics for vinasse gasification in supercritical water

Supercritical water gasification (SCWG) is a very efficient process to convert wet biomass into energetic gases. Unfortunately, SCWG reactor may strongly corrode due to the addition of temperature, pressure and the presence of corrosive species. In the present paper, the corrosion of various ceramic materials in subcritical and supercritical water (SCW) gasification process was studied in a batch reactor. We compare the corrosion in distillated water and the corrosion in sugar beet slurry that will be gasified under supercritical conditions. The experimental temperatures were 350 °C and 550 °C and the pressure was 25 MPa. Technical ceramics (SiC, alumina, Y stabilized zirconia, Si₃N₄, BN, aluminosilicate, cordierite-mullite) show poor capability to sustain corrosion whereas graphite and glassy carbon are the highest performance materials in our working conditions.     

Improvement in hydrogen production from hard-shell nut residues by catalytic hydrothermal gasification

The hydrothermal gasification of some hard-shell nut residues (hazelnut, walnut and almond shells) was performed in a batch type reactor at temperature and pressure ranges of 300–600 °C and 88–405 bar, respectively. The biomass samples were converted into gaseous product (hydrogen, carbon dioxide, methane, carbon monoxide and C₂–C₄ compounds), aqueous product (carboxylic acids, furfurals, phenols, aldehydes and ketones) and solid products after hydrothermal gasification. Hydrogen production was improved by using natural mineral catalysts (Trona, Dolomite and Borax). The activity of selected natural mineral catalysts in hydrothermal gasification can be ordered as being Trona [Na₃(CO₃)(HCO₃)·2H₂O] > Borax [Na₂B₄O₇·10H₂O] > Dolomite [CaMg(CO₃)₂]. The most effective catalyst was found to be Trona at 600 °C leading enhancement in hydrogen yields (mol H₂/kg C in biomass) for hazelnut, walnut and almond shells as 82.4%, 74.1% and 42.4%, respectively.     

Catalytic gasification of lignin with Ni/Al2O3–SiO2 in sub/supercritical water

Lignin has been gasified with a Ni/Al₂O₃–SiO₂ catalyst in sub/supercritical water (SCW) to produce gaseous fuels. XRD pattern at 6θ angle shows characteristic peaks of crystalline NiO, NiSi, and AlNi₃, suggesting that Al₂O₃–SiO₂ not only offers high surface area (122 m² g) for Ni, but also changes the crystal morphology of the metal. 9 mmol/g of H₂ and 3.5 mmol/g of CH₄ were produced at the conditions that 5.0 wt% alkaline lignin plus 1 g/g Ni/Al₂O₃–SiO₂ operating for 30 min at 550 °C. A kinetic model was also developed, and the activation energies of gas and char formation were calculated to be 36.68 ± 0.22 and 9.0 ± 2.4 kJ/mol, respectively. Although the loss of activity surface area during reuse caused slight activity reduction in Ni/Al₂O₃–SiO₂, the catalyst system still possessed high catalytic activity in generating H₂ and CH₄. It is noted that sulfur linkage could be hydrolyzed to hydrogen sulfide in the gasification process of alkaline lignin. The stable chemical states of Ni/Al₂O₃–SiO₂ grants its insensitivity to sulfur, suggesting that Ni/Al₂O₃–SiO₂ should be economically promising for sub/supercritical water gasification of biomass in the presence of sulfur.     

Hydrogen production from glycerol by reforming in supercritical water over Ru/Al2O3 catalyst

Supercritical water is a promising medium for the reforming of hydrocarbons and alcohols for the production of hydrogen at high pressures in a short reaction time. Water serves both as a dense solvent as well as a reactant. In this work, hydrogen is produced from glycerol by supercritical water reforming over a Ru/Al₂O₃ catalyst with low methane and carbon monoxide formation. Experiments were conducted in a tubular fixed-bed flow reactor over a temperature range of 700–800 °C, feed concentrations up to 40 wt% glycerol, all at short reaction time of less than 5 s. Glycerol was completely gasified to hydrogen, carbon dioxide, and methane along with small amounts of carbon monoxide. At dilute feed concentrations, near-theoretical yield of 7 mol of hydrogen/mol of glycerol was obtained, which decreases with an increase in the feed concentration. Based on a kinetic model for glycerol reforming, an activation energy of 55.9 kJ/mol was observed.     

Effect of steam and sodium hydroxide for the production of hydrogen on gasification of dehydrochlorinated poly(vinyl chloride)

Dehydrochlorinated poly(vinyl chloride) (PVC) and activated carbon were pyrolyzed with sodium hydroxide in a flow of steam and nitrogen at 3.0 MPa and 560–660 °C. In both cases, hydrogen and sodium carbonate were the main products, and methane, ethane, and carbon dioxide were minor products. The gasification rate increased with partial steam pressure, and the reaction order with respect to steam partial pressure was 0.69. For both dehydrochlorinated PVC and activated carbon, the gasification rate increased with the NaOH/C molar ratio. However, the rate became saturated at NaOH/C ratios higher than 2.0. The activation energy of gasification of dehydrochlorinated PVC or activated carbon was 178 kJ/mol, assuming first-order reaction rate. These experimental results indicate that hydrogen was produced from the reaction: C+2NaOH+H₂O→Na₂CO₃+2H₂.     

Experimental investigation of role of steam in entrained flow coal gasification

Experimental investigations of the influence of excess oxygen coefficient, H₂O/coal mass ratio using high-temperature steam, mean mass diameter of pulverized coal and coal size fraction on basic characteristics of coal gasification were performed. Experiments were carried out on a laboratory scale (0.09 m i.d. × 1.5 m high) coal gasification apparatus with lignite type of coal. Influence of steam was realized through comparison of results obtained from experiments with (H₂O/coal = 0.287 kg kg⁻¹) and without steam addition (H₂O/coal = 0.024 kg kg⁻¹). High values of carbon conversion, obtained both for finely ground and for coarse pulverized coal points to the easiness of lignite gasification, i.e. to its high suitability for gasification.     

Continuous hydrothermal gasification of glycerol mixtures: Effect of glycerol and its degradation products on the continuous salt separation and the enhancing effect of K3PO4 on the glycerol degradation

At the Paul Scherrer Institut (PSI) a continuous process for the catalytic hydrothermal gasification of wet biomass to synthetic natural gas (SNG) has been developed. The catalytic reactor is operated at temperatures of 400–450 °C and pressures of 25–30 MPa. Salts contained in the biomass and released during the liquefaction step are continuously withdrawn in the supercritical salt separation step and recovered as a concentrated brine upstream of the catalytic reactor. The recovered salts may be recycled as valuable nutrients or fertilizers after a certain work-up.Salt management was identified as critical issue in many different hydrothermal processes such as supercritical water oxidation (SCWO) and in catalyzed or non-catalyzed gasification technologies in near- and supercritical water. In this article we focus on the influence of organics, in this case glycerol and its hydrothermal degradation products, on the continuous salt separation performance. In the presence of organics higher temperatures are needed in the salt separator for an efficient salt separation and recovery due to a higher overall fluid density in the presence of glycerol compared to the density of pure water at the same conditions. Increasing temperatures in the salt separator lead to an increased degradation and, in particular, gasification of the glycerol. The salt studied, i.e. K₃PO₄, catalyzed the gasification of the glycerol to CO, H₂, CO₂, and CH₄ as well as the water gas shift reaction. Due to the increased glycerol gasification at 460 °C in the salt separator, the fluid mixture density was lowered to similar values of pure water under the same conditions. Hence, at the fluid temperature of 460 °C in the salt separator the same salt separation performance was observed for water–K₃PO₄ and for an aqueous mixture of 20 wt.% glycerol with K₃PO₄.     

Low temperature catalytic steam gasification of HyperCoal to produce H2 and synthesis gas

HyperCoal is an ultra clean coal with ash content <0.05 wt%. Catalytic steam gasification of HyperCoal was carried out with K₂CO₃ at 775–650 °C for production of H₂ rich gas and synthesis gas. The catalytic gasification of HyperCoal showed nearly four times higher gasification rate than raw coal. The major gases evolved were H₂: 63 vol%, CO: 6 vol% and CO₂: 30 vol%. Catalyst was recycled for four times without any significant rate loss. The partial pressure of steam was varied from 0.5 atm to 0.05 atm in order to investigate the effect of steam pressure on H₂/CO ratio. The H₂/CO ratio decreased from 9.5 at 0.5 atm to 1.9 at 0.05 atm. No significant decrease in gasification rate was observed due to change in partial pressure of steam. Gasification rate decreased with decreasing temperature and become very slow at 650 °C. The preliminary results showed that HyperCoal, an ash less coal, could be a potential hydrocarbon resource for H₂ and synthesis gas production at low temperature by catalytic steam gasification process.     

Cosolvent-modified supercritical carbon dioxide extraction of phenolic compounds from bamboo leaves (Sasa palmata)

This study highlights the possibility of supercritical carbon dioxide for extracting phenolic compounds from bamboo leaves that have shown antioxidant and anticancer activities. The CO₂ extraction solvent was modified by adding ethanol–water mixture cosolvent of different concentrations to allow extraction of both polar and non-polar compounds. Conventional Soxhlet extraction was also done to investigate the advantages of supercritical extraction over the conventional extraction method. For addition of 5% (mol) of a 25:75 (mol:mol) ethanol–water mixture solvent to CO₂, the highest amount of polyphenols (7.31 ± 0.06 mg/g bamboo leaves in catechin equivalents) and radical scavenging activity (3.65 ± 0.05 mg/g bamboo leaves in BHA equivalents) at 20 MPa and 95 °C, could be obtained among the mixture cosolvents studied. For Soxhlet extraction with a 25:75 (mol:mol) ethanol–water mixture, 1.48 times the amount of phenolic compounds (10.85 ± 0.52 mg/g bamboo leaves in catechin equivalents), could be isolated compared with the supercritical extraction method, however, the radical scavenging activity (3.30 ± 0.05 mg/g bamboo leaves in BHA equivalents) was 0.90 times lower than the extract obtained from the supercritical extraction method. The seven major antioxidative compounds identified from the SC-CO₂ extraction method were: (1) dl-alanine, (2) gluconic acid, (3) phosphoric acid, (4) ß-siosterol, (5) β-amyrene, (6) α-amyrin acetate and (7) friedelin.     

NaCl–H2O-assisted preparation of SrTiO3 nanoparticles by solid state reaction at low temperature

An environmentally friendly NaCl–H₂O system was developed to synthesize monodisperse strontium titanate (SrTiO₃) nanoparticles from commercially available raw materials (SrCO₃ and rutile) by solid state reactions. The formation rate of SrTiO₃ was accelerated by the addition of NaCl and water vapor. Single phase SrTiO₃ was obtained by calcination at 700 °C for 2 h in water vapor (H₂O flow rate of 2.0 mL/min) by the addition of 50 wt% NaCl, although 900 °C and 750 °C for 2 h were required to complete the reaction by calcinations in air and air by the addition of 50 wt% NaCl, respectively. The results demonstrate that both NaCl and H₂O played vital roles to accelerate the formation of SrTiO₃ nanoparticles at relatively low temperature. On the basis of experiments and analysis, a rational growth mechanism has been proposed and discussed.     

Coke removal from deactivated Co–Ni steam reforming catalyst using different gasifying agents: An analysis of the gas–solid reaction kinetics

The reactivity of various gases, namely; O₂, air, CO₂, H₂ and N₂, with carbon deposited on alumina-supported Co–Ni catalyst during propane reforming in a fluidized bed reactor at 773–973 K using relatively low feed steam:carbon ratio (0.8–1.5) has been investigated in a thermogravimetric analysis unit. Analysis of the transient solid weight loss revealed that carbon removal mechanism is dependent on the type of gasifying agent. Carbon gasification kinetics using O₂ and air followed the Avrami-Erofeev (A2) model while data for both CO₂ and H₂ were captured by the geometrical (contracting area, R2) model. However, carbon gasification with inert N₂ proceeded at much slower rate (about 10 times lower than air) and was adequately fitted by the one-dimensional diffusion (D1) model. Specific reaction rates from these phenomenological models were also linearly correlated with the catalyst carbon content with reactivity coefficient of the gasifying agent decreasing in the order, O₂ > air > CO₂ > H₂ > N₂. In order to minimize energy consumption during catalyst regeneration, reduce greenhouse gas emissions and reduce catalyst sintering, it would be desirable to employ a mixture of air and CO₂ as the carbon gasifying agent to take advantage of the coupled exothermic (air oxidation) and endothermic (reverse Boudouard reaction involving CO₂ and carbon) nature taking place during the carbon removal operation.     

Phase behavior of 1,3,5-tri-tert-butylbenzene–carbon dioxide binary system

1,3,5-tri-tert-butylbenzene (TTBB) is solid at ambient conditions, and has substantial solubility in liquid and supercritical carbon dioxide. We present the phase behavior of TTBB–CO₂ binary system at temperatures between 298 and 328 K and at pressures up to 20 MPa. Phase diagrams showing the liquid–vapor, solid–liquid and solid–vapor equilibrium envelopes are constructed by pressure–volume–temperature measurements in a variable-volume sapphire cell. TTBB is highly soluble in CO₂ over a wide range of compositions. Single-phase states are achieved at moderate pressures, even with very high TTBB concentrations. For example, at 328 K, a binary system containing TTBB at a concentration of 95% by weight forms a single-phase above 2.04 MPa. TTBB exhibits a significant melting-point depression in the presence of CO₂, 45 K at 3.11 MPa, where the normal melting point of 343 K is reduced to 298 K. With its high solubility in carbon dioxide, TTBB has potential uses as a binder or template in materials forming processes using dense carbon dioxide.     

High-pressure phase equilibria for the binary system carbon dioxide + dibenzofuran

The experimental solubility of dibenzofuran in near-critical and supercritical carbon dioxide and the solid–liquid–vapor (SLV) equilibrium line for the CO₂ + dibenzofuran system are reported. The built in-house static view cell apparatus used in these measurements is described. The solubility of naphthalene in supercritical CO₂ and the CO₂ + naphthalene SLV line are also determined in order to assess the reliability and accuracy of the measurement technique. The solubility of dibenzofuran in carbon dioxide is determined at 301.3, 309.0, 319.2, 328.7 and 338.2 K in the 6–30 MPa pressure range. Solubility data are correlated using the Chrastil model and the Peng–Robinson equation of state. This equation is also used to predict the CO₂ + dibenzofuran SLV line. Results show the feasibility of using supercritical CO₂ to extract dibenzofuran.     

Hydrothermal extraction and hydrothermal gasification process for brown coal conversion

A novel coal conversion process was proposed: the method combines “a hydrothermal extraction of brown coal (HT-Extraction)” and “a catalytic hydrothermal gasification of the extract (CHT-Gasification)” both of which are performed under the exactly same conditions of less than 350 °C and less than 20 MPa. Organic compounds in the aqueous phase, extracted from brown coal, was gasified using a novel Ni-supported carbon catalyst developed by the authors, producing combustible gas rich in CH₄ and H₂. Through this process performed at 350 °C and 18 MPa, an Australian brown coal was almost perfectly converted into 53% of upgraded coal, 23% of methane, and 24% of carbon dioxide on carbon basis. Simultaneously, 4.4 mol of hydrogen was generated from 100 mol of carbon of the coal. This process transferred 97% of energy involved in the raw coal to the products, indicating its effectiveness.     

Catalytic hydrogen production from 2-propanol in supercritical water: Comparison of some metal catalysts

The gasification of organics in supercritical water is a promising method for the direct production of hydrogen at high pressures, and in order to improve the hydrogen yield or selectivity, activities of various catalysts are evaluated. In this study, hydrogen production from 2-propanol over Ni/Al₂O₃ and Fe–Cr catalysts was investigated in supercritical water. The experiments were carried out in the temperature range of 400–600 °C and in the reaction time range of 10–30 s, under a pressure of 25 MPa. The hydrogen yields and selectivities of Ni/Al₂O₃ and Fe–Cr used in this study, and those of Pt/Al₂O₃ and Ru/Al₂O₃ used in our previous work were compared. The hydrogen contents of the gaseous products obtained by using Ni/Al₂O₃ and Fe–Cr were measured as 62 mol% and 70 mol%, respectively, at low temperatures and reaction times. However, the hydrogen yields remained in low levels when compared with that of Pt/Al₂O₃ used in previous study. Pt/Al₂O₃ was established to be the most effective and selective catalyst for hydrogen production. During the catalytic gasification of a 0.5 M solution of 2-propanol, hydrogen content up to 96 mol% and hydrogen yield of 1.05 mol/mol 2-propanol were obtained.     

Direct thermochemical liquefaction of microcrystalline cellulose by sub- and supercritical organic solvents

Direct thermochemical liquefaction of microcrystalline cellulose was carried out using sub- and supercritical solvents in a batch reactor. The decomposition efficiency of dodecane and m-xylene widely used in petrochemical industries was compared to that of methanol and 1,4-dioxane. At 400 °C, the conversion in methanol was the highest (92 wt% including gaseous products), but the operating pressure was too high. m-Xylene at the same temperature showed the conversion of 71.5 wt% with a lower number of products and milder pressure. Hydrogen contributed to the increase of the total conversion by 3–8% in m-xylene and methanol, compared with the results without adding any additional gas or with nitrogen pressurization. An acid-modified silica catalyst led to a significant increase of conversion in the supercritical methanol, but its effect was negligible in the aprotic solvents. The product compounds and property of solid residue depended on the supercritical solvents applied. The thermal-treated char after liquefaction in m-xylene, dodecane and 1,4-dioxane was an effective adsorbent for CO₂ adsorption, showing the level comparable with activated carbons.     

Phase equilibrium data of guaçatonga (Casearia sylvestris) extract + ethanol + CO2 system and encapsulation using a supercritical anti-solvent process

The aim of this research was to investigate the phase equilibrium behavior of a system containing guaçatonga extract + ethanol + CO₂ in order to help define the adequate conditions of temperature and pressure for the co-precipitation process, performed by means of supercritical anti-solvent (SAS) technique. Guaçatonga (Casearia sylvestris) is a native medicinal plant from Brazil, rich in valuable components such as β-caryophyllene, α-humulene and bicyclogermacrene. Phase equilibrium data were obtained by the static method using guaçatonga extract dissolved in ethanol (1:100, wt/wt), at temperatures ranging from 35 to 75 °C and CO₂ mass content from 60 to 90 wt%. It was noticed that the system exhibited solid–vapor–liquid, solid–liquid–liquid and solid–vapor–liquid–liquid transition types and a lower critical solution temperature behavior. Phase behavior study was considered for the definition of the SAS conditions applied for the encapsulation of guaçatonga extract in the biopolymer Pluronic F127. The conditions tested ranged from 80 to 140 bar at 45 °C. At 80 bar only segregated particles of extract and the biopolymer were detected, while at 110 and 140 bar an extract encapsulation was achieved.