Natural emissions of CO2 from the geosphere and their bearing on the geological storage of carbon dioxide

Carbon dioxide (CO₂) capture and storage has the potential to reduce CO₂ emissions from fossil fuel combustion. Although leakage from monitored CO₂ injection sites has been minimal to non-existent, experience from the natural gas storage industry suggests that, if it becomes a widely deployed technology, leaks may be expected from some storage sites. Natural occurrences of CO₂ in the geosphere, some of which have been exploited, provide insights into the types of emissions that might be expected from anthropogenic CO₂ storage sites. CO₂ emission sites are commonly found in clusters in CO₂-prone geological provinces: the most common natural emissions sites in sedimentary basins consist of carbonated springs and mofettes. These represent at worst only a local hazard. In volcanic and hydrothermal provinces, more energetic emissions may occur due to active supply from degassing magma. These include rare, sudden emissions from fissures and craters that have caused fatalities. It is unlikely that such provinces would be considered for CO₂ storage Major lake overturn events such as occurred at Lake Nyos in 1986 are considered highly unlikely to occur as a result of CO₂ storage, not least because CO₂ levels in lake waters can be monitored and remediated. Natural CO₂ fields indicate that under favourable conditions CO₂ can be retained in the subsurface for millions of years. The main risk from man-made CO₂ storage sites that does not have any close analogy in nature is considered to be a well blowout. A blowout that took place at a natural CO₂ field provides some indication of the likely hazard.     

Simulation of key parameters in CO2 injection for enhanced coalbed methane recovery in a deep well group

The use of CO₂-ECBM technology in deep coalbeds, by taking advantage of high CO₂ absorptivity, can displace more CH₄ as well as keeping long-term sequestration of carbon dioxide in large quantities. Our experiments in the block of northern Shizhuang show that the coalbed has an adsorption capacity for CO₂ which is two times of that for CH₄. With the pressure dropping, CH₄ (desorbs) more quickly than CO₂, and can be displaced effectively. Changes in reservoir physical properties caused by CO₂ injection mainly lie in permeability variations on account of matrix shrinkage and swelling during CO₂ adsorption and desorption. Usually, the permeability declines at first and increases rapidly as the reservoir pressure drops. Based on such variation patterns, geological and numerical models are established to analyze the influence of volumes, frequencies and modes of CO₂ injection on both methane production and recovery, as well as on CO₂ burying potentials, of both the well group and single wells. The modeling result shows that the gas recovery increases after 2-year CO₂ injection in the mode of 90-day injection, at a rate of 10–15 t/d, plus 90-day shut-in. Field tests indicate that the CO₂ adsorption capacity of the No. 3 coalbed is 8 t/d while the burying potential of the whole well group is about 12616t.     

Investigation of adsorption properties of geological materials for CO2 storage

The assessment for realistic CO₂‐adsorption capacities of different rocks is important for understanding the processes associated with CO₂ storage. This paper investigates the adsorption characteristics of rocks for CO₂ (limestone, sandstone, marl, claystone, clay, siltstone and metamorphic rock) by using a gravimetric method. The measurements were performed at 21°C with pressures from 1 up to 4 MPa. Sandstone (and clay with sand/sandstone) showed the largest adsorption capacity at 21°C. The highest amount of in situ CO₂ contents in measured samples was 21.4 kg/t. The CO₂‐adsorption capacities were lower than past results in different coal samples. The results indicate that adsorption of CO₂ into rocks may play an important role in storing CO₂ in subsurface rock. Copyright © 2012 John Wiley & Sons, Ltd.     

CO2 separation from the mixture of CO2/H2 using gas hydrates: Experimental and modeling

Hydrate formation is a new technique to separate hydrogen from carbon dioxide. In this way, modeling and prediction of gas hydrate kinetics is very important. Several experiments have been conducted to study the hydrate formation from pure carbon dioxide and mixture of hydrogen and carbon dioxide in a stirred reactor in different temperatures, pressures and compositions. The mass transfer approach model was used to predict the mass transfer coefficient for each experiment, and the dependency of temperature and pressure has been studied. It was observed that the mass transfer coefficient of CO₂ in the mixture is close to the pure system. The result of this work shows that the pure data on the kinetics for CO₂ hydrate formation is applicable for the case of CO₂ separation from the mixture of carbon dioxide and hydrogen.     

Hydrogen storage capacity at high pressure of raw and purified single wall carbon nanotubes produced with a solar reactor

Samples of single wall carbon nanotubes (SWNTs) were prepared using a solar reactor. Graphite targets containing different catalysts (Ni/Co, Ni/Y, Ni/Ce) allowed the synthesis of SWNTs soot in which nanotubes had different diameter distributions. Several consecutive stages of HCl treatment and thermal oxidation in air (HCl protocol) purified the samples. Another protocol involving HNO₃ treatment and H₂O₂ oxidation (HNO₃ protocol) was also used. Isotherms of hydrogen adsorption were volumetrically measured at 253 K under pressures below 6 MPa on raw and treated samples. The highest adsorption capacity (0.7  wt%) was measured on raw soot. HCl protocol clearly increases the BET surface area (_SBET)$(S_{\mathrm{BET}})$ and the microporous volume (_W0(_N2))$(W_0({\mathrm{N}}_2))$ measured by N₂ at 77 K of the treated samples with respect to the as-produced materials, whereas HNO₃ protocol decreases them. A correlation between textural properties and hydrogen storage capacities is discussed.     

Numerical simulation of hydrogen injection and withdrawal to and from a deep aquifer in NW Poland

Numeric modeling and the PetraSim program with a TOUGH2 deposit simulator have been applied to the evaluation of the viability of seasonal (cyclic) hydrogen storage in a deep aquifer, in the porous rocks of a well-recognized geological structure Suliszewo. The modeling was performed for one injection-and-withdrawal well located on the summit of the structure, under an assumption that the values of the fracturing pressure and capillary entry pressure will not be exceeded.Upconing seems to be the main obstacle in underground hydrogen storage. It was noted that the amount of recovered hydrogen increases in successive withdrawal cycles. It is shown that the management of large amounts of water during hydrogen withdrawal will be a serious environmental issue, important also for the cost-effectiveness of the underground storage. The obtained modeling results indicate that underground hydrogen storage in a deep aquifer may be performed with reasonable parameters of gas recovery.     

Effect of different reductants for palladium loading on hydrogen storage capacity of double-walled carbon nanotubes

The effects of different reductants for palladium loading on the hydrogen sorption characteristics of double-walled carbon nanotubes (DWCNTs) have been investigated. Pd nanoparticles were loaded on DWCNT surfaces for dissociation of H₂ into atomic hydrogen, which spills over to the defect sites on the DWCNTs. When we use different reductants, the reduction capabilities and other effects of the different reductants are different, which affects the hydrogen storage capacity of the DWCNTs. In this work, the amount of hydrogen storage capacity was determined (by AMC Gas Reactor Controller) to be 1.7, 2.0, 2.55, and 3.0 wt% for pristine DWCNTS and for 2.0%Pd/DWCNTs using H₂, l-ascorbic acid, and NaBH₄ as reductants, respectively. We found that the hydrogen storage capacity can be enhanced by loading with 2% Pd nanoparticles and selecting a suitable reductant. Furthermore, the sorption can be attributed to the chemical reaction between atomic hydrogen and the dangling bonds of the DWCNTs.     

Construction of porous Ni2P cocatalyst and its promotion effect on photocatalytic H2 production reaction and CO2 reduction

Due to superior light absorption abilities, porous materials are suitable to be served in photocatalytic reactions. In this study, porous Ni₂P is target-constructed from porous Ni(OH)₂ nanoflower. Promotion effect of the porous Ni₂P as cocatalyst is confirmed on photocatalytic performance of Ni₂P/CdS composite. The constructed porous Ni₂P/CdS photocatalyst shows much higher photocatalytic H₂ evolution rate (111.3 mmol h⁻¹ g⁻¹) from water and much higher CO (178.0 μmol h⁻¹ g⁻¹) and CH₄ (61.2 μmol h⁻¹ g⁻¹) evolution rates from CO₂ reduction than non-porous Ni₂P/CdS photocatalyst. Characterizations including UV-Vis diffuse reflectance, photoluminescence, transient photocurrent response, electrochemical impedance and electron paramagnetic resonance are conducted to verify the role of porous Ni₂P cocatalyst. The slow photon effect derived from porous structure Ni₂P is found to improve light path and increase the absorption utilization of light. The enhanced photocurrent intensity and the lowered resistance of porous Ni₂P/CdS due to the formed heterojunctions indicate much rapid isolation of photogenerated electron-hole pairs and rapid charge transfer of electrons. The higher signal of ⋅O₂- radicals is detected in porous Ni₂P/CdS than non-porous Ni₂P/CdS, which result in the remarkable photocatalyst activities of porous Ni₂P/CdS. Reaction mechanisms over Ni₂P/CdS photocatalyst are illustrated with a Z-scheme charge transfer path.     

Effect of thermal fluorination on the hydrogen storage capacity of multi-walled carbon nanotubes

Thermal fluorination of multi-walled carbon nanotubes (MWCNTs) was performed to improve their hydrogen storage capacity, based on three considerations. First, the surface of the MWCNTs was altered by thermal fluorination to create a pathway for the storage of hydrogen molecules inside the MWCNTs. These surface treatments increased the number of MWCNT defects through attack by fluorine radicals. The defects were identified using Raman peaks and TEM images. Second, thermal fluorination changed the pore structure by enlarging the specific surface area and the pore volume, which increased the number of hydrogen adsorption sites. Last, the induced fluorine groups enhanced the hydrogen storage capacity through attraction effects on the electron in the hydrogen molecules due to the high electronegativity of fluorine. In conclusion, thermal fluorination increased the hydrogen storage capacity of MWCNTs four-fold to 1.69 wt%.     

Hydrogen storage in saline aquifers: The role of cushion gas for injection and production

Hydrogen stored on a large scale in porous rocks helps alleviate the main drawbacks of intermittent renewable energy generation and will play a significant role as a fuel substitute to limit global warming. This study discusses the injection, storage and production of hydrogen in an open saline aquifer anticline using industry standard reservoir engineering software, and investigates the role of cushion gas, one of the main cost uncertainties of hydrogen storage in porous media.The results show that one well can inject and reproduce enough hydrogen in a saline aquifer anticline to cover 25% of the annual hydrogen energy required to decarbonise the domestic heating of East Anglia (UK). Cushion gas plays an important role and its injection in saline aquifers is dominated by brine displacement and accompanied by high pressures. The required ratio of cushion gas to working gas depends strongly on geological parameters including reservoir depth, the shape of the trap, and reservoir permeability, which are investigated in this study. Generally, deeper reservoirs with high permeability are favoured. The study shows that the volume of cushion gas directly determines the working gas injection and production performance. It is concluded that a thorough investigation into the cushion gas requirement, taking into account cushion gas costs as well as the cost-benefit of cushion gas in place, should be an integral part of a hydrogen storage development plan in saline aquifers.     

Microstructural and morphological investigations on Mg-Nb2O5-CNT nanocomposites processed by high-pressure torsion for hydrogen storage applications

A combined deformation process of high energy ball milling and subsequent high-pressure torsion method was applied to synthesize nanocrystalline magnesium powders catalyzed by Nb₂O₅ and/or multiwall carbon nanotubes. The effect of the different additives on the kinetics of the milled powders and the bulk disks produced by simultaneous uniaxial compression and severe shear deformation was examined in a Sieverts’-type apparatus. The microstructure and the morphology of the as-processed samples and the additives were characterized by X-ray diffraction and high-resolution transmission electron microscopy, respectively. Microstructural changes and morphological alterations after several absorption-desorption cycles were also studied. It was found that high-pressure torsion has significantly changed the texture of magnesium and the shape of carbon nanotubes. The combined use of Nb₂O₅ and carbon nanotubes was found to improve the desorption kinetics of Mg. Influence of the additives and processing methods on the evolution of the microstructure will also be demonstrated.     

The geo-database of caprock quality and deep saline aquifers distribution for geological storage of CO2 in Italy

One of the most promising options to stabilize and reduce the atmospheric concentration of greenhouse gases is Carbon Capture and Storage (CCS). This technique consists of separating CO₂ from other industrial flue gases and storing it in geological reservoirs, such as deep saline aquifers, depleted oil and/or gas fields, and unminable coal beds.A detailed reworking of all available Italian deep-drilling data was performed to identify potential storage reservoirs in deep saline aquifers. Data were organized into a GIS geo-database containing stratigraphic and fluid chemistry information as well as physiochemical characteristics of the geological formations. Caprock efficiency was evaluated via numerical parameterization of rock permeabilities, defining the “Caprock Quality Factor” (Fbp) for each well. The geo-database also includes strategic information such as the distribution of deep aquifers, seismogenic sources and areas, seismic events, Diffuse Degassing Structures, heat flow, thermal anomalies, and anthropogenic CO₂ sources.Results allow the definition of potentially suitable areas for future studies on CO₂ geological storage located in the fore-deep domains of the Alps and Apennines chains, where efficient marly-to-clayish caprocks lie above deep aquifers hosted in sands or limestones. Most of them are far form seismogenic sources and Diffuse Degassing Structures.     

Importance of clay-H2 interactions for large-scale underground hydrogen storage

Underground hydrogen storage is considered an option for large-scale green hydrogen storage. Among different geological storage types, depleted oil/gas fields and saline aquifers stand out. In these cases, hydrogen will be prevented from leaking back to the surface by a tight caprock seal. It is therefore essential to understand hydrogen interactions with shale-type caprocks. To this end, natural pure montmorillonite clay was exposed to hydrogen gas at different pressures (0–50 bar) and temperatures (77, 195, 303 K) to acquire data on its adsorption capacity related to UHS and caprock saturation. Montmorillonite was chosen because of its large specific surface area enabling quantification of the adsorption process. Hydrogen adsorption was successfully fitted with a Langmuir isotherm model and yielded small partition coefficients indicating that hydrogen does not preferentially adsorb to the clay surface. Adsorption on montmorillonite goes back to weak physisorption as inferred from minor negative changes in the enthalpy of reaction (−790 J/mol), derived from an Excel Solver approach to the van't Hoff equation. Based on own as well as literature values, adsorption capacities, which were originally reported as mol/kg or wt%, are recast as hydrogen volume adsorbed per specific surface area (μL/m²). The acquired range is surprisingly narrow, with values ranging from 3 to 6 μL/m², and indicates the normalised volume of hydrogen that can be expected to remain in the shale-type caprock after injected hydrogen migrated upwards through the porous reservoir. This ‘residual’ caprock saturation with hydrogen can be further restrained by considering the geothermal gradient and its effect on the molar volume of hydrogen. The experimental results presented here recommend injecting hydrogen deeper rather than shallower as pressure and temperature work in favour of increased storage volumes and decreased hydrogen loss through clay adsorption in the caprock.     

The surface and encapsulated storage of H2 on Ga12N12

Ga₁₂N₁₂ is a fullerene cage and particularly stable. The surface adsorption and encapsulated storage of H₂ on the Ga₁₂N₁₂ are thoroughly explored by applying a genetic algorithm combined with DFT calculations. The results reveal that one H₂ can form physical adsorption on the surface sites of Ga₁₂N₁₂ with ideal adsorption intensities (?0.227～-0.303 eV) and the mode of H₂ adsorption on a Ga atom is the most energy favored. Interestingly, the Ga₁₂N₁₂ can adsorb a maximum of 38H₂ molecules with mass density of 7.01 wt%. The electronic structure analysis indicates that the charge transferring between H₂ and Ga₁₂N₁₂ is observed. Meanwhile, H₂ is also effectively polarized, and thus the electrostatic interaction is improved. When H₂ is adsorbed on the Ga atom, weak covalent interactions are also induced. The encapsulation of H₂ in the Ga₁₂N₁₂ is an endothermic process and six H₂ molecules can be encapsulated in the Ga₁₂N₁₂ with the moderate encapsulation and releasing energy barrier of 3.286～4.508 eV and 0.390～3.008 eV, respectively. Our results suggest that Ga₁₂N₁₂ is regarded as a potential hydrogen storage material.     

Review on using the depleted gas reservoirs for the underground H2 storage: A case study in Niigata prefecture,Japan

Underground hydrogen storage (UHS) in depleted hydrocarbon reservoirs is a prospective choice to store enormous volumes of hydrogen (H₂). However, these subsurface formations must be able not only to store H₂ in an effective and secure manner, but also to produce the required volumes of H₂ upon demand. This paper first reviews the critical parameters to be considered for geological analysis and reservoir engineering evaluation of UHS. The formation depth, the interactions of rock-brine-H₂, the caprock (seal) and well integrity are the most prominent parameters as far as UHS is concerned. In respect of these critical parameters, tentative H₂ storage is screened from the existing gas storage fields in the Niigata prefecture of Japan, and it was revealed that the Sekihara gas field is a suitable candidate for UHS with a storage capacity of 2.06 × 10⁸ m³ and a depth of 1000 m. Then, a series of numerical simulations utilizing CMG software was conducted to find out the extent to which critical parameters alter H₂ storage capacity. The results demonstrated that this field, with a recovery factor of 82.7% in the sixth cycle of production is a prospective site for H₂ storage.     

The influence of the hydrogenation degree on selected properties of graphene as a material for reversible H2 storage

This article discusses the effect of hydrogenation of graphene (one-sided and two-side hydrogenation) in relation to the change in the physicochemical properties of graphane as a material capable of reversible H₂ storage. Therefore, the change of the system's energy was determined, differences in HOMO-LUMO molecular levels and the distribution of electrostatic potential as a function of its hydrogenation were simulated. At the same time, the mechanism of graphane reduction to graphene was discussed as a result of interaction with steam from the air. It has been shown that along with the increase in the degree of hydrogenation, the graphane changes its electrostatic potential from negative to positive, simultaneously pushing the negative charge to the edge of the graphene flake. This fact may have an impact on its further chemical reactions, which may significantly limit the sorption properties of graphene. Areas rich in negative charge will prefer chemical reactions with molecules of electrophilic properties, while positive areas with molecules of nucleophilic properties. This determines the elimination of the storage environment of graphene structures used as reversible sources of chemical bonding of hydrogen in order to increase their lifetime as well as sorption capacity.     

Synthesis of Mg2FeD6 under low pressure conditions for Mg2FeH6 hydrogen storage studies

Mg₂FeD₆ is successfully synthesised with various degrees of purity using reactive ball milling and annealing under low pressure deuterium conditions to a maximum of 10 bar. The deuteride of the low cost ternary metal hydride Mg₂FeH₆, is synthesised to enable further characterisation studies such as isotopic exchange behaviour. Both on laboratory and industrial scales, keeping the pressure low reduces the need for expensive compression systems and also minimises the quantity of gas necessary for use; therefore it is important to assess synthesis under these cost effective conditions. This is especially the case when using a specialised gas such as high purity deuterium. The maximum pressure chosen is 10 bar, to comply with the High Pressure Safety Act in Japan. This Safety Act limits the use of any gas including hydrogen and deuterium to 10 bar eliminating the use of traditional synthesis methods for Mg₂FeH₆ or Mg₂FeD₆ synthesis at high pressure (120 bar). Ball milling parameters such as milling times, ball to powder ratios as well as sintering times were altered to achieve improved Mg₂FeD₆ yields under these low pressure conditions.     

Carbon resistance of xNi/HTASAO5 catalyst for the production of H2 via CO2 reforming of methane

xNi/HTASAO5 catalysts (x = 2.5, 3.3, 4.4, 5.8, 8.2) were prepared for CO₂ reforming of methane. No crystalline nickel species formed on the catalysts with lower nickel content (≤4.4%), and large Ni⁰ crystallite formed on 5.8% (10 nm) and 8.2 wt%Ni/HTASAO5 (17 nm), whereas the surface concentration of Ce³⁺ decreased with Ni loading. The initial conversion of CH₄ increased from 29.5% to 46.9% with Ni loading. The xNi/HTASAO5 (x ≤ 4.4%) performed stably in the reaction due to the presence of dispersed Ni species and high surface Ce³⁺ content without coke formation, however, 5.8% and 8.2 wt%Ni/HTASAO5 exhibited decreased activity with time on stream, because of the formation of large Ni particles with lower surface Ce³⁺, leading to carbon accumulation. Thus, CH₄ conversion stabilized at about 43% and no carbon formed on 4.4 wt%Ni/HTASAO5 with optimum Ni loading.