Biological sequestration of carbon dioxide in geological formations

This paper presents the results of experimental investigation and analysis of challenges for utilizing enzyme bovine carbonic anhydrase for sequestration of CO₂ in saline formations. Several sets of controlled bench-top experiments were conducted, and results are presented in this paper, where effects of various parameters including pH, concentration of enzyme, and temperature on enhancing hydration and subsequent precipitation of CO₂ in the form of calcium carbonate were tested. A mathematical model describing the extent and rate of precipitation was developed upon analyzing the results of these tests. Subsequently, core flood tests were conducted where effect of enzyme on precipitation of CO₂ in Berea cores, and its impact on porosity and permeability of the porous media were investigated. These tests indicated that the pressure drop across cores was increased about 2-4 times, which is an indication of precipitation of CO₂ in the form of calcium carbonate in porous media. In addition to above tests, effect of timing and scheme of the injection on extent of CO₂ precipitation in porous media was tested. It was observed that co-injection of CO₂ and enzyme solution leads to higher pressure drop across the cores in tests reported here. Finally, the learning of above tests has been used to outline a series of potential challenges and propose solutions for effective utilization of enzyme bovine carbonic anhydrase for safe sequestration of CO₂ in saline formations.     

The mechanical behaviour of coal with respect to CO2 sequestration in deep coal seams

Carbon dioxide displays a strong affinity for coal due to its propensity to adsorb to the coal surface. The process of CO₂ adsorption on coal causes lowering of surface energy and, it is hypothesised that an associated decrease in surface film confinement results in a decrease in material tensile resistance. Following the results of work carried out on the mechanical influence of CO₂ on brown coal under in situ conditions [Viete DR, Ranjith PG. The effect of CO₂ on the geomechanical and permeability behaviour of brown coal: implications for coal seam CO₂ sequestration. Int J Coal Geol 2006;66(3):204–16], a theoretical explanation is proposed for the perceived lack of a weakening effect with the adsorption of CO₂ to coal at significant confining pressures. We propose that at significant hydrostatic stresses, resistance to failure is otherwise provided (by external confinement) and the effects of adsorptive weakening are concealed. Our model predicts that adsorptive weakening, fracturing under in situ stresses, and associated permeability increases are not an issue for coal seam CO₂ sequestration for sufficiently deep target seams. Lowering of the elastic modulus of coal upon introduction of CO₂ may proceed by means other than surface energy lowering and could well occur irrespective of the depth of sequestration. The effect of elastic modulus lowering under in situ conditions would be beneficial for the long-term retention of sequestered gases.     

Form coke reaction processes in carbon dioxide

Uncertainty in metallurgical coke supplies has prompted development of form coke from low quality coals and fines. Reaction rates have been measured and mechanisms identified that control carbonaceous briquette reaction rate in CO₂. Three briquette formulations were prepared, characterized and coked in an inert atmosphere at high temperature. A given weight of each formulation was then reacted in a packed bed with CO₂ at 1373 K for 0.5–2 h. Partially reacted briquettes contained a solid core with some internal reaction surrounded by a loosely adhering layer of carbon-containing ash. The reaction rate of briquettes with CO₂ was affected by diffusion of CO₂ through the bulk gas and the ash-carbon layer to the core surface, as well as CO₂–carbon reaction. Key variables governing briquette reaction rate included CO₂ mole fraction and briquette void fraction.     

Pilot plant study of two new solvents for post combustion carbon dioxide capture by reactive absorption and comparison to monoethanolamine

Application of new solvents will substantially contribute to the reduction of the energy demand for the post combustion capture of CO₂ from power plant flue gases. The present work describes tests of such new solvents in a gas-fired pilot plant, which comprises the complete absorption/desorption process (column diameters 0.125 m, absorber/desorber packing height 4.25/2.55 m, packing type: Sulzer BX 500, flue gas flow 30–100 kg/h, CO₂ partial pressure 35–135 mbar). Two new solvents CESAR1 (0.28 g/g 2-amino-2-methyl-1-propanol+0.17 g/g piperazine+0.55 g/g H₂O) and CESAR2 (0.32 g/g 1, 2-ethanediamine+0.68 g/g H₂O), which were developed in an EU-project, were systematically studied and compared to MEA (0.3 g/g monoethanolamine+0.7 g/g H₂O). The two new solvents and MEA were studied in the same way in the pilot plant and detailed results are reported for all solvents. In the present study the structured packing Sulzer BX 500 is used. The measurements are carried out at a constant CO₂ removal rate of 90% by an adjustment of the regeneration energy in the desorber for systematically varied solvent flow rates. An optimal solvent flow rate leading to a minimum energy requirement is found from these studies. Direct comparisons of such results can be misleading if there are differences in the kinetics of the different solvent systems. The influence of kinetic effects is experimentally studied by varying the flue gas flow rate at a constant ratio of solvent mass flow to flue gas mass flow and constant CO₂ removal rate. Results from these studies indicate similar kinetics for CESAR1, CESAR2 and MEA. The direct comparison of the pilot plant results for these solvents is therefore justified. Both CESAR1 and CESAR2 show improvements compared to MEA. The most promising is CESAR1 with a reduction of about 20% in the regeneration energy and 45% in the solvent flow rate.     

二氧化碳地质减排方法及其化工问题的分析

介绍了不同减排方法对二氧化碳的总处理量和二氧化碳保持时间及成本估算。通过比较,地质减排体现出明显的优势。从化学工程技术和应用基础理论层面上对二氧化碳地质减排需要解决的问题进行了分析,从中发现,高温高压地质条件下,二氧化碳-盐水体系及其在多孔介质中复杂多相平衡的热力学研究是二氧化碳地质减排的关键性课题。     

Synthesis of PCL/clay masterbatches in supercritical carbon dioxide

Pre-exfoliated nanoclays were prepared through a masterbatch process using supercritical carbon dioxide as solvent and poly(?-caprolactone) as organic matrix. In situ polymerization of ?-caprolactone in the presence of large amount of clay was conducted to obtain these easily dispersible nanoclays, collected as a dry and fine powder after reaction. Dispersion of these pre-exfoliated nanoclays in chlorinated polyethylene was also investigated. All the results confirm the specific advantages of supercritical CO₂ towards conventional solvents for filler modification.     

Carbon dioxide sequestration through novel use of ion exchange fibers (IX-fibers)

Electrical power generation and metal removal processes are practiced globally and share two common attributes that make them ideal candidates to be incorporated in a novel carbon dioxide sequestration scheme using ion exchange fibers (IX-fibers). First, the softening of boiler feed water used in power generation and the removal of metals from finishing wastewaters often employs the use of ion exchange for the purpose of selective separation. Second, both processes represent significant point source CO₂ emissions. This investigation demonstrated that using IX-fibers it is possible to sequester a portion of the CO₂ produced in these practices as carbonate alkalinity during the regeneration step of both the water softening and the trace heavy metal removal processes. Weak acid IX-fibers were used for hardness removal while hybrid cation exchange fibers (HCIX-F) loaded with hydrated Zr(IV) oxide (HZO) were used to remove toxic heavy metals such as zinc, cadmium and copper. IX-fibers offer the unique capability to use and consume CO₂ during the efficient regeneration of IX-fibers, whereas commercial ion exchange resins are not amenable to regeneration with CO₂. A much shorter intraparticle diffusion path length in cylindrical IX-fibers as compared to resin beads is the underlying reason for a highly efficient regeneration of the fibers. In addition to sequestering carbon dioxide, no hazardous or aggressive chemicals/brine solutions are present in the regenerant wastes as compared with traditional ion exchange processes.     

Infrared and carbon dioxide chemisorption study of Mo/ZrO2 catalysts

The interaction of Mo with zirconia has been investigated by infrared spectroscopy (IR) and carbon dioxide chemisorption. Quantitative analysis of the IR results indicated that Mo interacts preferentially with the most basic hydroxyl group (high frequency band at 3775 cm^–1). An approximately 79% decrease in the 3775 cm^–1 band is observed vs. 21% for the low frequency band at 3673 cm^–1, with increasing the Mo loading up to 1 wt%. The relative decrease of the IR band at 3775 cm^–1 was identical to that measured for the CO₂ uptake. The Mo cross-sections estimated from CO₂ chemisorption results were much higher than those typically reported for the Mo system. It was concluded that, as previously reported for the Mo/Al₂o₃ system, CO₂ chemisorption overestimates the surface coverage of Mo/ZrO₂ catalysts.     

Review on photocatalytic conversion of carbon dioxide to value-added compounds and renewable fuels by graphitic carbon nitride-based photocatalysts

Photocatalytic reduction of CO₂ is known as one of the most promising methods to produce valuable fuels and value-added compounds. To overcome selectivity and efficiency downsides, various photocatalysts have been designed and developed. This review discusses the state-of-the-art in photo-conversion of CO₂ over graphitic carbon nitride (g-C₃N₄)-based composites. The modification strategies to improve photocatalytic activity of g-C₃N₄ were classified into different categories and discussed as structural modifications, elemental doping, copolymerization, fabricating heterojunctions between g-C₃N₄ and other semiconductors, Z-scheme heterojunctions, noble metal/g-C₃N₄ photocatalysts, and design of ternary nanocomposites based on g-C₃N₄. Finally, perspectives and future research works in this field were also outlined.     

A multi-scale model for CO2 sequestration enhanced coalbed methane recovery

This paper presents a multi-scale model to simulate the multicomponent gas diffusion and flow in bulk coals for CO₂ sequestration enhanced coalbed methane recovery. The model is developed based on a bi-dispersed structure model by assuming that coal consists of microporous micro-particles, meso/macro-pores and open microfractures. The bi-disperse diffusion theory and the Maxwell-Stefan approach were incorporated in the model, providing an improved simulation of the CH₄-CO₂/CH₄-N₂ counter diffusion dynamics. In the model, the counter diffusion process is numerically coupled with the flow of the mixture gases occurring within macro-pores or fractures in coal so as to account for the interaction between diffusion and flow in gas transport through coals. The model was validated by both experimental data from literature and our CO₂ flush tests, and shows an excellent agreement with the experiments. The results reveal that the gas diffusivities, in particular the micro-pore diffusivities are strongly concentration-dependent.     

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.     

Analysis on the dyeing of polypropylene fibers in supercritical carbon dioxide

Polypropylene fibers were dyed in supercritical carbon dioxide system and the results were compared with those of fiber dyed in water system. Dye uptake value calculated by a UV spectrum indicated that polypropylene fiber dyeing was much better in carbon dioxide than in water. Optical microscopical analysis showed that dye molecules had diffused thoroughly into fiber in CO₂ because of the excellent compatibility between the dye and the CO₂. X-ray and birefrigence analysis demonstrated that plastification caused by the implementation of CO₂ made molecular chain more mobile and led to an increase in the dyeing of polypropylene fibers. Moreover a mechanical test and DSC analysis indicated that the fiber structure was not damaged when the fabric was dyed at 100 °C. Hence dyeing polypropylene using CO₂ as a transport medium was very feasible and worthy of further development.     

Electro-reduction of carbon dioxide to formate on lead electrode in aqueous medium

The electrochemical reduction of carbon dioxide on a lead electrode was studied in aqueous medium. Preliminary investigations carried out by cyclic voltammetry were used to determine the optimized conditions of electrolysis. They revealed that the CO₂ reduction process was enhanced at a pH value of 8.6 for the cathodic solution i.e. when the predominant form of CO₂ was hydrogenocarbonate ion. Long-term electrolysis was carried out using both potentiometry and amperometry methods in a filter-press cell in which the two compartments were separated by a cation-exchange membrane (Nafion^® 423). Formate was detected and quantified by chromatography as the exclusive organic compound produced with a high Faradaic yield (from 65% to 90%). This study also revealed that the operating temperature played a key role in the hydrogenation reaction of carbon dioxide into formate in aqueous medium.     

Interfacial tension of linear and branched PP in supercritical carbon dioxide

In this study, sessile drops are imaged in a high-pressure and high-temperature view chamber to determine the density and interfacial tension of linear polypropylene (LPP) and branched polypropylene (BPP) melts in supercritical carbon dioxide (CO₂). The pressure-volume-temperature (PVT) data of polyprophylene (PP)-CO₂ is investigated by monitoring the swelling changes of the polymer melt in supercritical CO₂. The density difference between the polymer/CO₂ mixture and the CO₂ is determined by combining the swelling results with the CO₂ solubility information in the polymer melt. Both the Sanchez-Lacombe (SL) and the Simha-Somcynsky (SS) equations-of-state (EOS) are applied to predict the density of the PP-CO₂ mixture, which is then compared to the density data obtained experimentally. The dependence of interfacial tension on the temperature and pressure of PP in supercritical CO₂ is investigated at temperatures from 180 °C to 220 °C and pressures up to 31 MPa. Effects of long-chain branching on the density and interfacial tension of PP-CO₂ mixtures are discussed.     

Reusable poly(allylamine)-based solid materials for carbon dioxide capture under continuous flow of ambient air

Poly(allylamine) (PAA) is prepared via free-radical polymerization and physically impregnated on fumed silica at various amine loadings. The PAA-silica composites were found to have significant potential as trace carbon dioxide adsorbents under ambient conditions. The sorbent materials are shown to have high adsorption capacities with desirable adsorption-desorption characteristics. The effects of temperature and humidity on adsorption capacity and kinetics were studied at near-ambient conditions. Sorbent regenerative ability was confirmed within around 8% change following subsequent adsorption-desorption cycles and thermogravimetric analysis.     

Supercritical carbon dioxide extraction of Baizhu: Experiments and modeling

In this work, supercritical CO₂ extraction has been carried out on a traditional Chinese herb of Baizhu under pressure of 15-45 MPa, temperature of 40-60 °C, mean powder size of 0.167-0.675 mm, and extraction time of up to 180 min. The maximum extraction yield obtained in 5 h is about 6.76 × 10⁻² g per gram raw materials at 60 °C and 45 MPa. The extraction process is correlated by means of five different mathematical models. The evaluation of these models against experimental data shows that among these models the Sovová model performs the best with an overall average absolute relative deviation of 1.62%, followed by Crank and Naik models, finally the Barton and Martínez models. From the Sovová model, the mass transfer coefficient in solid or fluid are obtained and they are varying in the ranges of 4.02-6.14 × 10⁻⁸ m/s and 0.88-2.87 × 10⁻⁹ m/s, respectively. These results suggest that solute diffusion in solid matrices and solute mass transfer in fluid are both important in affecting the supercritical CO₂ extraction process of Baizhu.     

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.     

Silver-coated ion exchange membrane electrode applied to electrochemical reduction of carbon dioxide

Silver-coated ion exchange membrane electrodes (solid polymer electrolyte, SPE) were prepared by electroless deposition of silver onto ion exchange membranes. The SPE electrodes were used for carbon dioxide (CO₂) reduction with 0.2 M K₂SO₄ as the electrolyte with a platinum plate (Pt) for the counterelectrode. In an SPE electrode system prepared from a cation exchange membrane (CEM), the surface of the SPE was partly ruptured during CO₂ reduction, and the reaction was rapidly suppressed. SPE electrodes made of an anion exchange membrane (SPE/AEM) sustained reduction of CO₂ to CO for more than 2 h, whereas, the electrode potential shifted negatively during the electrolysis. The reaction is controlled by the diffusion of CO₂ through the metal layer of the SPE electrode at high current density. Ultrasonic radiation, applied to the preparation of SPE/AEM, was effective to improve the electrode properties, enhancing the electrolysis current of CO₂ reduction. Observation by a scanning electron microscope (SEM) showed that the electrode metal layer became more porous by the ultrasonic radiation treatment. The partial current density of CO₂ reduction by SPE/AEM amounted to 60 mA cm⁻², i.e. three times the upper limit of the conventional electrolysis by a plate electrode. Application of SPE device may contribute to an advancement of CO₂ fixation at ambient temperature and pressure.     

Research progress in removal of trace carbon dioxide from closed spaces

In this paper, the removal of trace carbon dioxide from closed spaces through membrane process and biotransformation are introduced in detail. These methods include the microalgae photobioreactor, membrane microalgae photobioreactor, supported liquid membrane, membrane gas-liquid contactor, hydrogel membrane, and enzyme membrane bioreactor. The advantages and disadvantages of these methods are compared. It is found that higher CO₂ removal efficiency can be obtained in biotransformation and membrane process. However, a large volume and high energy consumption are needed in biotransformation, while the low permeability and stability must be solved in the membrane process.