CO2 sorption and transport behavior of ODPA-based polyetherimide polymer films

Plasticization phenomena can significantly reduce the performance of polymeric membranes in high-pressure applications. Polyetherimides (PEIs) are a promising group of membrane materials that combine relatively high CO₂/CH₄ selectivities with high chemical and thermal stability. In this work sorption, swelling, and mixed gas separation performance of 3,3′,4,4′-oxydiphthalic dianhydride (ODPA)-based PEI polymers, with 1, 2 or 3 para-aryloxy substitutions in the diamine moeiety, is investigated under conditions where commercial membranes suffer from plasticization. Particular focus is on the influence of the amount of para-aryloxy substitutions and the film thickness. Results are compared with those of commercially available polymeric membrane materials (sulphonated PEEK, a segmented block-co-polymer PEBAX and the polyimide Matrimid).The glassy polymers display increasing CO₂ sorption with increasing T_g. The larger extent of sorption results from a larger non-equilibrium excess free volume. Swelling of the polymers is induced by sorption of CO₂ molecules in the non-equilibrium free volume as well as from molecules dissolved in the matrix. Dilation of the polymer is similar for each molecule sorbed. Correspondingly, the partial molar volume of CO₂ is similar for molecules present in both regions.Mixed gas separation experiments with a 50/50% CO₂/CH₄ feed gas mixture showed high CO₂/CH₄ selectivities for the ODPA PEI films at elevated pressure. This shows that these materials could potentially be interesting for high-pressure gas separation applications, although additional gas permeation experiments using different feed gas compositions and thin films are required.     

European inter-laboratory comparison of high pressure CO2 sorption isotherms. I: Activated carbon

In order to assess and improve the quality of high-pressure sorption isotherms of carbon dioxide (CO₂) on coals, an inter-laboratory study (“Round Robin”) has been conducted among four European research laboratories. In a first round of measurements, excess sorption isotherms were determined on Filtrasorb 400 (F400) activated carbon at 318 K using the manometric (TU Delft and RWTH Aachen University) and the gravimetric (FP Mons and INERIS) method up to 16 MPa. The study shows that CO₂ sorption in the supercritical range can be determined accurately with both gravimetric and manometric equipment but requires thorough optimization of instrumentation and measuring as well as proper sample preparation procedures. For the characterization of the activated carbon F400, which we used as benchmark, we have determined a surface area of 1063 m² g⁻¹, and Dubinin-Radushkevich (DR) micropore volume of 0.51 cm³ g⁻¹. Additionally, we analysed the elementary near-surface composition by energy dispersive X-ray spectroscopy (EDX). To characterise the bulk composition of the F400 activated carbon, a proximate and ultimate analysis was performed.The observed excess sorption maxima around 5 MPa have values around 8.0 mol kg⁻¹, which are consistently higher (by upto 0.8 mol kg⁻¹) than literature data.     

Development of T type zeolite for separation of CO2 from CH4 in adsorption processes

Influence of synthesis parameters; silica sources, relative alkalinity and silicon module, were investigated on preparation of T type zeolite by hydrothermal method, using a two level factorial design. Crystallization time and reaction temperature were held constant at 7 days and 378 K, respectively. The synthesized products were characterized by XRD and SEM techniques. The results showed that increasing silicon module and decreasing relative alkalinity in the synthesis gel improved the product relative crystallinity. It was also observed that using colloidal silica as the silica source improved crystallinity and phase purity of T type zeolites. The prepared zeolite T with the highest relative crystallinity was examined in the batch adsorption experiments at three temperatures of 288, 298 and 308 K and various pressures from 0.1 up to 2 MPa to verify the ability of the material for selective adsorption and separation of CO₂ from CH₄. The adsorption capacities and isotherms of CO₂ and CH₄ were determined at the studied temperatures. The results showed that the highest ideal selectivity of CO₂/CH₄ could be achieved at atmospheric pressure and 308 K. The performance of the adsorbent was confirmed with breakthrough curves and breakthrough times resulted from dynamic adsorption experiments of the mixed gases.     

Development of new alumina-modified sorbents for CO2 sorption and regeneration at temperatures below 200 °C

A new regenerable alumina-modified sorbent was developed for CO₂ capture at temperatures below 200 °C. The CO₂ capture capacity of a potassium-based sorbent containing Al₂O₃ (KAlI) decreased during multiple CO₂ sorption (60 °C) and regeneration (200 °C) tests due to the formation of the KAl(CO₃)(OH)₂ phase, which could be converted into the original K₂CO₃ phase above 300 °C. However, the new regenerable potassium-based sorbent (Re-KAl(I)) maintained its CO₂ capture capacity during multiple tests even at a regeneration temperature of 130 °C. In particular, the CO₂ capture capacity of the Re-KAl(I)60 sorbent which was prepared by the impregnation of Al₂O₃ with 60 wt.% K₂CO₃ was about 128 mg CO₂/g sorbent. This excellent CO₂ capture capacity and regeneration property were due to the characteristics of the Re-KAl(I) sorbent producing only a KHCO₃ phase during CO₂ sorption, unlike the KAlI30 sorbent which formed the KHCO₃ and KAl(CO₃)(OH)₂ phases even at 60 °C. This result was explained through the structural effect of the support containing the KAl(CO₃)(OH)₂ phase which was prepared by impregnation of Al₂O₃ with K₂CO₃ in the presence of CO₂.     

Changes in pore structure of anthracite coal associated with CO2 sequestration process

Pore structure changing of coal during the CO₂ geo-sequestration is one of the key issues that affect the sequestration process significantly. To address this problem, the CO₂ sequestration process in an anthracite coal was replicated using a supercritical CO₂ (ScCO₂) reactor. Different coal grain sizes were exposed to ScCO₂ and water at around 40 °C and 9.8 MPa for 72 h. Helium pycnometer and mercury porosimetry provide the density, pore size distribution and porosity of the coal before and after the ScCO₂ treatment. The results show that after exposure to the ScCO₂-H₂O reaction, part of the carbonate minerals were dissolved and flushed away by water which made the true density increased as well as total pore volume and porosity most importantly in the micro-pore range. Hysteresis between mercury intrusion and extrusion was observed. Ink bottle shaped pores can be either damaged or created compared with the ScCO₂ treated coal samples. This suggests that the ScCO₂ treatment most likely increase the volumes of pores in anthracite coal, which also contributed to the increase in porosity of the treated samples. Therefore the CO₂ sequestration into coal appears to have the potential to increase significantly the anthracite microporosity which is very advantageous for CO₂ storage.     

Mechanism and kinetics of Al2O3 nanoparticles formation by reaction of bulk Al with H2O and CO2 at sub- and supercritical conditions

Oxidation of bulk samples of 〈Al〉 by water and H₂O/CO₂ mixture at sub- and supercritical conditions for uniform temperature increase and at the injection of H₂O (665 K, 23.1 MPa) and H₂O/CO₂ (723 K, 38.0 MPa) fluids into the reactor has been studied. Transition of 〈Al〉 into AlOOH and Al₂O₃ nanoparticles has been found out. Aluminum samples oxidized by H₂O and H₂O/CO₂ fluids at the injection mostly consist of large particles (300-500 nm) of α-Al₂O₃. Those oxidized for uniform temperature increase contain smaller particles (20-70 nm) of γ-Al₂O₃ as well. Mechanism of this phenomenon is explained by orientation of oxygen in H₂O polar molecules to the metal in the electric field of contact voltage at Al/AlOOH and Al/Al₂O₃ boundary. Addition of CO₂ to water resulted in CO, CH₄, CH₃OH and condensed carbon, increase in size of Al₂O₃ nanoparticles and significant decrease in time delay. In pure CO₂ 〈Al〉 oxidation resulted in oxide film. Using temperature and time dependences of gaseous reactant pressure and Redlich-Kwong state equation, kinetics of H₂ formation has been described and oxidation regularities determined. At aluminum oxidation by H₂O and H₂O/CO₂ fluids, self-heating of the samples followed by oxidation rate increase has been registered. The samples of oxidized aluminum have been studied with a transmission electronic microscope, a thermal analyzer and a device for specific surface measurement. The effect of oxidation conditions on the characteristics of synthesized nanoparticles has been found out.     

Calcination and sintering characteristics of limestone under O2/CO2 combustion atmosphere

The O₂/CO₂ coal combustion technology is an innovative combustion technology that can control CO₂, SO₂ and NO_x emissions simultaneously. Calcination and sintering characteristics of limestone under O₂/CO₂ atmosphere were investigated in this paper. The pore size, the specific pore volume and the specific surface area of CaO calcined were measured by N₂ adsorption method. The grain size of CaO calcined was determined by XRD analysis. The specific pore volume and the specific surface area of CaO calcined in O₂/CO₂ atmosphere are less than that of CaO calcined in air at the same temperature. And the pore diameter of CaO calcined in O₂/CO₂ atmosphere is larger than that in air. The specific pore volume and the specific surface area of CaO calcined in O₂/CO₂ atmosphere increase initially with temperature, and then decline as temperature exceeds 1000 °C. The peaks of the specific pore volume and the specific surface area appear at 1000 °C. The specific surface area decreases with increase in the grain size of CaO calcined. The correlations of the grain size with the specific surface area and the specific pore volume can be expressed as L = 744.67 + 464.64 lg(1 / S) and L = − 608.5 + 1342.42 lg(1 / ε), respectively. Sintering has influence on the pore structure of CaO calcined by means of influencing the grain size of CaO.     

Pulverized coal combustion in air and in O2/CO2 mixtures with NOx recycle

This paper presents experimental results of a 20 kW vertical combustor equipped with a single pf-burner on pulverised coal combustion in air and O₂/CO₂ mixtures with NO_x recycle. Experimental results on combustion performance and NO_x emissions of seven international bituminous coals in air and in O₂/CO₂ mixtures confirm the previous findings of the authors that the O₂ concentration in the O₂/CO₂ mixture has to be 30% or higher to produce matching temperature profiles to those of coal-air combustion while coal combustion in 30% O₂/70% CO₂ leads to better coal burnout and less NO_x emissions than coal combustion in air. Experimental results with NO_x recycle reveal that the reduction of the recycled NO depends on the combustion media, combustion mode (staging or non-staging) and recycling location. Generally, more NO is reduced with coal combustion in 30% O₂/70% CO₂ than with coal combustion in air. Up to 88 and 92% reductions of the recycled NO can be achieved with coal combustion in air and in 30% O₂/70% CO₂ respectively. More NO is reduced with oxidant staging than without oxidant staging when NO is recycled through the burner. Much more NO is reduced when NO recycled through the burner (from 65 to 92%) than when NO is recycled through the staging tertiary oxidant ports (from 33 to 54%). The concentration of the recycled NO has little influence on the reduction efficiency of the recycled NO with both combustion media—air and 30% O₂/70% CO₂.     

The performances of higher alcohol synthesis over nickel modified K2CO3/MoS2 catalyst

Ni modified K₂CO₃/MoS₂ catalyst was prepared and the performance of higher alcohol synthesis catalyst was investigated under the conditions: T = 280–340 °C, H₂/CO (molar radio) = 2.0, GHSV = 3000 h^− 1, and P = 10.0 MPa. Compared with conventional K₂CO₃/MoS₂ catalyst, Ni/K₂CO₃/MoS₂ catalyst showed higher activity and higher selectivity to C₂⁺OH. The optimum temperature range was 320–340 °C and the maximum space-time yield (STY) of alcohol 0.30 g/ml h was obtained at 320 °C. The selectivity to hydrocarbons over Ni/K₂CO₃/MoS₂ was higher, however, it was close to that of K₂CO₃/MoS₂ catalyst as the temperature increased. The results indicated that nickel was an efficient promoter to improve the activity and selectivity of K₂CO₃/MoS₂ catalyst.     

Theoretical study on interaction between CO2 and carbonyl compounds: Influence of CO2 on infrared spectroscopy and activity of CO

The interactions between CO₂ and carbonyl compounds at different CO₂ pressures have been studied both experimentally and theoretically. In situ high-pressure FTIR on carbonyl compounds, i.e., acetaldehyde, acetone, and crotonaldehyde, in supercritical CO₂ have been measured at various CO₂ pressures varying from 6 to 22 MPa. In order to get insights into the mechanism, theoretical study has been conducted concerning the effect of CO₂ on frequency shift of CO in acetaldehyde, acetone, benzaldehyde, crotonaldehyde and cinnamaldehyde at different CO₂ pressures. It has been shown that the experimental frequency shifts can be well simulated by the theoretical model calculations using particular structures, in which a carbonyl compound interacts with a few CO₂ molecules, depending on the carbonyl compounds examined, except for acetaldehyde.The interaction energies between CO₂ and those carbonyl compounds are also given. In addition, the effect of CO₂ on hydrogenation of crotonaldehyde and benzaldehyde has been discussed by means of the local softness (s⁺) calculated at CO₂ pressures of 0-22 MPa, which can explain the reactivity difference in the crotonaldehyde and benzaldehyde hydrogenations in supercritical CO₂.     

An economic feasibility study of O2/CO2 recycle combustion technology based on existing coal-fired power plants in China

In order to reduce the CO₂ emission from the coal-fired power plants, O₂/CO₂ recycle combustion (Oxy-combustion) technique has been proposed through combining a conventional combustion process with a cryogenic air separation process. The technique is capable of enriching CO₂ concentration and then allowing CO₂ sequestration in an efficient and energy-saving way. Taking into account the CO₂ taxation and CO₂ sale, the paper evaluates the economic feasibility of Oxy-combustion plants retrofitted from two typical existing conventional coal-fired power plants (with capacities of 2 × 300 MW and 2 × 600 MW, respectively) with Chinese data. The cost of electricity (COE) and the CO₂ avoidance cost (CAC) are also considered in the evaluation. The COE of the retrofitted Oxy-combustion plant is nearly the same as that of the corresponding conventional plant if the unit price of CO₂ sale reaches 17-22 $/t (different cases). The CAC of the retrofitted 2 × 300 MW Oxy-combustion plant is 1-3 $/t bigger than that of the retrofitted 2 × 600 MW Oxy-combustion plant. Supercritical plants are more economical and appropriate for Oxy-combustion retrofit. The result indicates that Oxy-combustion technique is not only feasible for CO₂ emission control based on existing power plants but is also cost-effective.     

Polymer melt rheology with high-pressure CO2 using a novel magnetically levitated sphere rheometer

A magnetically levitated sphere rheometer (MLSR) designed to measure viscosity of fluids exposed to high-pressure carbon dioxide has been developed. This device consists of a magnetic sphere submerged inside a test fluid within a high-pressure housing and levitated at a fixed point. The housing is constructed from an optically transparent sapphire tube. The cylindrical tube can be moved vertically to generate a shear flow around the levitated sphere. The difference in magnetic force required to levitate the sphere at rest and under fluid motion can be directly related to fluid viscosity. Rheological properties, specifically zero shear viscosities, of transparent high-pressure materials can be measured to a precision of about 5% and over a wide range of viscosities. In addition, operation at constant pressure, in concentration regimes from a pure polymer to an equilibrated polymer/supercritical fluid solution, and at shear rates over several orders of magnitude is possible, eliminating many of the disadvantages associated with other high-pressure rheometers. Experiments performed at different temperatures with a poly(dimethylsiloxane) melt at atmospheric pressure are compared with data from a commercial Couette rheometer to demonstrate device sensitivity and viability. Measurements of a PDMS melt plasticized by high-pressure CO₂ are performed to illustrate the utility of the new rheometer under high-pressure conditions. Experimental data are obtained at 30 °C, for pressures up to 20.7 MPa and CO₂ concentrations reaching 30 wt%. Viscosity reductions of nearly two orders of magnitude compared with the pure polymer viscosity at atmospheric pressure are observed. Additionally, the effects of pressure on a polymer/CO₂ system are directly investigated taking advantage of the constant pressure operation mode of the MLSR. This allows us, for the first time in experiments of polymers with supercritical fluids, to decouple the effects of CO₂ concentration and pressure in a single device.     

Separation and capture of carbon dioxide from CO2/H2 syngas mixture using semi-clathrate hydrates

The relation between anthropogenic emissions of CO₂ and its increased levels in the atmosphere with global warming and climate change has been well established and accepted. Major portion of carbon dioxide released to the atmosphere, originates from combustion of fossil fuels. Integrated gasification combined cycle (IGCC) offers a promising fossil fuel technology considered as a clean coal-based process for power generation particularly if accompanied by precombustion capture. The latter includes separation of carbon dioxide from a synthesis gas mixture containing 40 mol% CO₂ and 60 mol% H₂.A novel approach for capturing CO₂ from the above gas mixture is to use gas hydrate formation. This process is based on selective partition of CO₂ between hydrate phase and gas phase and has already been studied with promising results. However high-pressure requirement for hydrate formation is a major problem.We have used semiclathrate formation from tetrabutylammonium bromide (TBAB) to experimentally investigate CO₂ capture from a mixture containing 40.2 mol% of CO₂ and 59.8 mol% of H₂. The results shows that in one stage of gas hydrate formation and dissociation, CO₂ can be enriched from 40 mol% to 86 mol% while the concentration of CO₂ in equilibrium gas phase is reduced to 18%. While separation efficiency of processes based on hydrates and semi-clathrates are comparable, the presence of TBAB improves the operating conditions significantly. Furthermore, CO₂ concentration could be increased to 96 mol% by separating CO₂ in two stages.     

Microporous phenol-formaldehyde resin-based adsorbents for pre-combustion CO2 capture

Different types of phenolic resins were used as precursor materials to prepare adsorbents for the separation of CO₂ in pre-combustion processes. In order to obtain highly microporous carbons with suitable characteristics for the separation of CO₂ and H₂ under high pressure conditions, phenol-formaldehyde resins were synthesised under different conditions. Resol resins were obtained by using an alkaline environment while Novolac resins were synthesised in the presence of acid catalysts. In addition, two organic additives, ethylene glycol (E) and polyethylene glycol (PE) were included in the synthesis. The phenolic resins thus prepared were carbonised at different temperatures and then physically activated with CO₂. The carbons produced were characterised in terms of texture, chemical composition and surface chemistry. Maximum CO₂ adsorption capacities at atmospheric pressure were determined in a thermogravimetric analyser. Values of up to 10.8 wt.% were achieved. The high-pressure adsorption of CO₂ at room temperature was determined in a high-pressure magnetic suspension balance. The carbons tested showed enhanced CO₂ uptakes at high pressures (up to 44.7 wt.% at 25 bar). In addition, it was confirmed that capture capacities depend highly on the microporosity of the samples, the narrow micropores (pore widths of less than 0.7 nm) being the most active in CO₂ adsorption at atmospheric pressure. The results presented in this work suggest that phenol-formaldehyde resin-derived activated carbons, particularly those prepared with the addition of ethylene glycol, show great potential as adsorbents for pre-combustion CO₂ capture.     

Integrated coal pyrolysis with CO2 reforming of methane over Ni/MgO catalyst for improving tar yield

A new process to integrate coal pyrolysis with CO₂ reforming of methane over Ni/MgO catalyst was put forward for improving tar yield. And several Chinese coals were used to confirm the validity of the process. The experiments were performed in an atmospheric fixed-bed reactor containing upper catalyst layer and lower coal layer to investigate the effect of pyrolysis temperature, coal properties, Ni loading and reduction temperature of Ni/MgO catalysts on tar, water and char yields and CH₄ conversion at fixed conditions of 400 ml/min CH₄ flow rate, 1:1 CH₄/CO₂ ratio, 30 min holding time. The results indicated that higher tar yield can be obtained in the pyrolysis of all four coals investigated when coal pyrolysis was integrated with CO₂ reforming of methane. For PS coal, the tar, water and char yield is 33.5, 25.8 and 69.5 wt.%, respectively and the CH₄ conversion is 16.8%, at the pyrolysis temperature of 750 °C over 10 wt.% Ni/MgO catalyst reduced at 850 °C. The tar yield is 1.6 and 1.8 times as that in coal pyrolysis under H₂ and N₂, respectively.     

Development of an improved falling ball viscometer for high-pressure measurements with supercritical CO2

This study presents the development of an improved technique for viscosity measurements under high pressure. The apparatus is based on the principle of the falling ball viscometer, implemented in a high-pressure autoclave fitted with visualisation windows. The originality here is that the balls fall through a tube open at both ends with a diameter slightly greater than that of the balls, allowing a simplified modelling and numerical simulation. A numerical approach has been used for viscosity determination. Calculations have been made with COMSOL Multiphysics^® with the laminar Navier-Stokes model for Newtonian mixtures. It includes the specific hydrodynamic effects without the need for a calibration fluid. However, validation experiments were carried out at atmospheric pressure with dimethylsulfoxide (DMSO) at 298, 308 and 318 K and with cocoa butter at 313 and 353 K, with values of viscosity in the range from 1.4 to 45.4 mPa s. Comparative measurements with literature data have been conducted with cocoa butter saturated with carbon dioxide at 313 and 353 K and for pressures ranging from 0.1 to 25 MPa. At 313 K, viscosity varies from 45.4 mPa s to 3.1 mPa s while at 353 K it varies from 12.4 to 1.9 mPa s. For both isotherms tested, within the range 0-15 MPa, the higher the CO₂ dissolution in the cocoa butter, the lower the viscosity. However, this decrease in viscosity is more pronounced at the lowest temperature. Above 15 MPa the CO₂ dissolution effect on viscosity becomes insignificant, i.e. within the experimental error, due to a counter effect linked with the high hydrostatic pressure. Furthermore, the limits of use of this method have been determined. This technique is revealed as reliable and can therefore be used with other binary systems.     

Effect of subcritical CO2 on the structural and electrical properties of ORMOCERS-APE systems based on Zr and Al

Two series of hybrid inorganic-organic polymer electrolytes of the organically modified ceramics as polymer electrolytes (ORMOCERS-APE) type with formulas {Al[O(CH₂CH₂O)_8.7]_ρ/(LiClO₄)_z}_n (1.85 ≤ ρ ≤ 2.24, 0 ≤ z ≤ 1.06) and {Zr[O(CH₂CH₂O)_8.7]_ρ/(LiClO₄)_z}_n (1.80 ≤ ρ ≤ 1.99, 0 ≤ z ≤ 0.90) were treated with CO₂ in subcritical conditions (293 K and 5 MPa). The effect of CO₂ on the samples was investigated by using ESEM, thermal analysis (TG and DSC) and broad band dielectric spectroscopy (BDS).Both complexes {Al[O(CH₂CH₂O)_8.7]_ρ/(LiClO₄)_z}_n and {Zr[O(CH₂CH₂O)_8.7]_ρ/(LiClO₄)_z}_n after CO₂ treatment exhibited a change in the segmental relaxation with respect to the untreated samples. This phenomenon has been interpreted in terms of higher portion of free volume in the samples. The CO₂ treatment primarily lowered the conductivity of {Al[O(CH₂CH₂O)_8.7]_ρ/(LiClO₄)_z}_n complexes of about one order of magnitude, as opposed to {Zr[O(CH₂CH₂O)_8.7]_ρ/(LiClO₄)_z}_n complexes, where an increment of two orders of magnitude was observed. In both cases the conductivity of the treated and untreated materials versus the reciprocal absolute temperature presents the typical Vogel-Tamman-Fulcher (VTF) behavior. The different effects on the conductivities of the treated complexes are explained in terms of the modified anion-trapping ability of Al centers and in terms of the interactions of subcritical CO₂ with the host polymer and the salt. Insight about the conductivity mechanisms were provided by the study of the VTF parameters and the relaxation times determined from the Debye peaks of the imaginary resistivity, the imaginary permittivity and the correlated motion analysis.     

CO2 uptake of modified calcium-based sorbents in a pressurized carbonation-calcination looping

This study focuses on enhancing CO₂ uptake by modifying limestone with acetate solutions under pressurized carbonation condition. The multicycle tests were carried out in an atmospheric calcination/pressurized carbonation reactor system at different temperatures and pressures. The pore structure characteristics (BET and BJH) were measured as a supplement to the reaction studies. Compared with the raw limestone, the modified sorbent showed a great improvement in CO₂ uptake at the same reaction condition. The highest CO₂ uptake was obtained at 700 °C and 0.5 MPa, by 88.5% increase over the limestone at 0.1 MPa after 10 cycles. The structure characteristics of the sorbents on N₂ absorption and SEM confirm that compared with the modified sorbent, the effective pores of limestone are greatly driven off by sintering, which hinders the easy access of CO₂ molecules to the unreacted-active sites of CaO. The morphological and structural properties of the modified sorbent did not reveal significant differences after multiple cycles. This would explain its superior performance of CO₂ uptake under pressurized carbonation. Even after 10 cycles, the modified sorbent still achieved a CO₂ uptake of 0.88.     

Catalytic syngas production from greenhouse gasses: Performance comparison of Ru-Al2O3 and Rh-CeO2 catalysts

In this work, 3% Ru-Al₂O₃ and 2% Rh-CeO₂ catalysts were synthesized and tested for CH₄-CO₂ reforming activity using either CO₂-rich or CO₂-lean model biogas feed. Low carbon deposition was observed on both catalysts, which negligibly influenced catalytic activity. Catalyst deactivation during temperature programmed reaction was observed only with Ru-Al₂O₃, which was caused by metallic cluster sintering. Both catalysts exhibited good stability during the 70 h exposure to undiluted equimolar CH₄/CO₂ gas stream at 750 °C. By varying residence time in the reactor during CH₄-CO₂ reforming, very similar quantities of H₂ were consumed for water formation. Reverse water-gas shift (RWGS) reaction occurred to a very similar extent either with low or high WHSV values over both catalysts, revealing that product gas mixture contained near RWGS equilibrium composition, confirming the dominance of WGS reaction and showing that shortening the contact time would actually decrease the H₂/CO ratio in the syngas produced by CH₄-CO₂ reforming, as long as RWGS is quasi equilibrated. H₂/CO molar ratio in the produced syngas can be increased either by operating at higher temperatures, or by using a feed stream with CH₄/CO₂ ratio well above 1.     

CO2 sequestration in coals and enhanced coalbed methane recovery: New numerical approach

Mixed gases injection into a large coal sample for CO₂ sequestration in coals and enhanced coalbed methane recovery was investigated using a new numerical approach. A dynamic multi-component transport (DMCT) model was applied to simulate ternary gas (CH₄-CO₂-N₂) diffusion and flow behaviors for better understanding and prediction of gas injection enhanced coalbed methane (ECBM) recovery processes. Several cases were designed to analyze the effects of injection gas composition and pressure on gas displacement dynamics in a large coal sample. The calculated results suggest that mixed gas injections have similar profiles of methane recovery as pure N₂ injection, and mixtures of N₂ and CO₂ reduce the ultimate methane recovery compared to pure CO₂. The breakthrough time of pure CO₂ injection is longer than mixed gas injections. Injection gas composition has significant effect on produced gas composition.