Tar, among the most difficult contaminants to be removed, is the major technical barrier for the development and application of advanced biomass gasification technology. Catalytic cracking of tar, ...with the advantages of energy recovery from tar, quality increase of product gas and no secondary pollution, has become the main focus currently for tar removal. Among various types of catalysts, CaO based catalysts, including the naturally available calcined limestone/dolomite, the single-/multi-metal active components (Ni, Fe, Mg, etc.) and CaO composite catalysts, and the synthetic CaO based catalysts, have been demonstrated to be effective and economically viable for tar removal. Meanwhile, CaO is also being used in some novel CaO-looping gasification processes as a CO2 absorbent in recent years. Therefore, the CaO derived from different precursors, the in-situ and ex-situ applications of CaO catalysts and the tar removal under different operation conditions have been studied in this review. The tar cracking mechanism over CaO catalysts has been summarized. The single-/multi-metal doped CaO based catalysts for tar removal are also discussed. The use of CaO catalysts in some novel gasification processes and their tar removal efficiency have been also investigated. Finally, the main drawbacks and future perspectives of CaO based catalysts are discussed.
•The detailed mechanism of tar cracking over CaO surface was summarized.•In-/ex-situ use of CaO from different precursors at varied conditions were studied.•Introduction of other metals to form single-/multi-metal doped CaO catalysts was studied.•Tar removal efficiency of CaO catalysts in novel gasification process was clarified.•The main drawbacks and future perspectives of CaO based catalysts were given.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•A 3D convolutional neural network is able to create a functional relationship between pore morphology and the steady state solution of the Navier-Stokes equation for laminar flow.•Four geometric ...features extracted from the binary image are needed to make the model robust.•A model trained only with spherepacks is able to perform accurately in different domains including non-consolidated samples, synthetic heterogeneous geometries, and sandstone and carbonate x-ray images.
We present the PoreFlow-Net, a 3D convolutional neural network architecture that provides fast and accurate fluid flow predictions for 3D digital rock images. We trained our network to extract spatial relationships between the porous medium morphology and the fluid velocity field. Our workflow computes simple geometrical information from 3D binary images to train a deep neural network (the PoreFlow-Net) optimized to generalize the problem of flow through porous materials. Our results show that the extracted information is sufficient to obtain accurate flow field predictions in less than a second, without performing expensive numerical simulations providing a speed-up of several orders of magnitude. We also demonstrate that our model, trained with simple synthetic geometries, is able to provide accurate results in real samples spanning granular rocks, carbonates, and slightly consolidated media from a variety of subsurface formations, which highlights the ability of the model to generalize the porous media flow problem. The workflow presented here shows the successful application of a disruptive technology (physics-based training of machine learning models) to the digital rock physics community.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Limestone tropical forests represent a meaningful fraction of the land area in Central America (25%) and Southeast Asia (40%). These ecosystems are marked by high biological diversity, CO2 uptake ...capacity, and high pH soils, the latter making them fundamentally different from the majority of lowland tropical forest areas in the Amazon and Congo basins. Here, we examine the role of bedrock geology in determining biological nitrogen fixation (BNF) rates in volcanic (low pH) vs. limestone (high pH) tropical forests located in the Maya Mountains of Belize. We experimentally test how BNF in the leaf-litter responds to nitrogen, phosphorus, molybdenum, and iron additions across different parent materials. We find evidence for iron limitation of BNF rates in limestone forests during the wet but not dry season (response ratio 3.2 ± 0.2; P = 0.03). In contrast, BNF in low pH volcanic forest soil was stimulated by the trace-metal molybdenum during the dry season. The parent-material induced patterns of limitation track changes in siderophore activity and iron bioavailability among parent materials. These findings point to a new role for iron in regulating BNF in karst tropical soils, consistent with observations for other high pH systems such as the open ocean and calcareous agricultural ecosystems.
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BFBNIB, FZAB, GIS, IJS, INZLJ, KILJ, NLZOH, NMLJ, NUK, OILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK, ZRSKP
Understanding failure and strain localization in porous rock is of fundamental importance in rock physics. Confined compaction experiments on porous rocks have revealed a broad spectrum of failure ...modes. Techniques such as acoustic emission location and velocity tomography provide kinematic information on the partitioning of damage and localization of strain. Complementary observations on deformed samples using microscopy and microcomputed tomography (μCT) can also be used to image microscale damage and its distribution. Only by synthesizing such measurements on multiple scales could one infer the multiscale dynamics of compaction localization and similar rock failure phenomena. Located at the European Synchrotron Radiation Facility, the HADES rig allows direct in situ 3D imaging of the whole rock sample as it is triaxially compressed. The μCT data provide an integrated perspective of the spatiotemporal evolution of damage and strain localization on scales ranging from grain to continuum. We conducted an experiment on Leitha limestone (initial porosity of ∼22%) at a confining pressure of 20 MPa. With increasing differential stress, the sample strain hardened and two distinct yield points were identified in the stress-strain curve. The spatiotemporal evolution of local porosity and damage were analyzed at multiple scales. At a mesoscopic scale of 10 voxels (65 μm), the time-lapse μCT images reveal the strain partitioning associated with the first yield point and development of strain localization with the second. The latter development of five discrete compaction bands is the first unambiguous observation of such a bifurcation phenomenon in a porous carbonate rock, with geometric attributes comparable to compactions bands observed in porous sandstones. The μCT data on the voxel-scale elucidate in refined details the nucleation and propagation of discrete compaction bands under quasi-static loading, as well as the micromechanical processes, which in the past could only be inferred from a synthesis of kinematic observations of acoustic emissions activity and post-mortem observations of microstructure and damage.
•Strain localization was captured from the grain to continuum scale in a limestone.•Compaction bands can develop in a porous carbonate in dry conditions.•Inelastic compaction of a limestone is characterized by a double yielding behavior.•Time-dependent compaction was observed in a porous limestone.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Dissolution of carbonate minerals in karst aquifers has long been recognized to result from recharge of surface water undersaturated with respect to calcite from carbonic acid produced by hydration ...of dissolved atmospheric and respired CO2. However, dissolution also results from additional acids produced by reactions of redox sensitive solutes in the subsurface, which may represent a source of CO2 to Earth's atmosphere. Because the magnitude of dissolution by these additional acids is poorly constrained, we compare here fractions of dissolution from initial surface water undersaturation and subsurface redox reactions. Estimates are based on chemical mixing and geochemical (PHREEQC) modeling of time series measurements of water compositions at a spring vent that receives surface water during stream flooding and a stream sink-rise system in north-central Florida. During a single spring reversal, 9.2 × 105 kg of limestone dissolved. At the stream sink-rise system, where subsurface residence times are shorter than during the spring reversal, both limestone dissolution (102–104 kg) and precipitation (102–105 kg) occur as water flows through the conduits with residence times ranging from 10 to 70 h. At both sites, maximum calcite dissolution rates of ∼10 μM hr−1 occurs at subsurface residence times between 30 and 50 h. For subsurface residence time > ∼20–60 h, the models indicate that production of additional acid in the subsurface is required for ∼53 ± 7% of dissolution. Oxidation of organic carbon, ammonium, pyrite, iron, and/or manganese produce sufficient acid for additional dissolution, but dissolved oxygen is insufficient for these reactions, indicating some acidity is generated under anerobic conditions. Dissolution caused by subsurface reactions in our samples represents mobilization of 20 × 104-30 × 104 kg of CO2 via remineralization of organic carbon or carbonate dissolution by nitric and sulfuric acids. Acid produced by subsurface redox reactions during surface water-groundwater interactions, including non‑carbonic acids, are important in conduit development and carbon cycling in the carbonate critical zone.
•Surface water-groundwater exchange enhances carbonate dissolution.•Dissolution depends on residence time of recharged water in the subsurface.•Carbonic, sulfuric, and nitric acids from subsurface reactions aid dissolution.•Surface and subsurface acid sources can cause equal amounts of dissolution.•Dissolution by subsurface acid sources may release CO2 to the atmosphere.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In this study, the macro properties (residual compressive strength), meso properties (mesoscopic images), and micro properties (reaction products and pore structures) of paste specimens with various ...limestone and calcined clay contents at elevated temperatures (20, 300, 550, and 900 °C) are experimentally investigated. According to the experimental results, (1) the strengths of all samples increase at 300 °C, while those of the LC3 ternary blended pastes increase more significantly because of the formation of katoite and the further hydration of binders. After the treatments at 550 and 900 °C, the reduction in the strengths of the LC3 samples is greater than that of the plain paste. (2) With further increasing temperature, all samples generate more meso cracks. (3) At 900 °C, a large gehlenite crystalline phase is formed in the samples with calcined clay. In summary, the microscopic explanation for the macroscopic and mesoscopic properties of LC3 paste at elevated temperature is investigated.
•Macro–meso–micro properties of LC3 paste are investigated at elevated temperatures.•At 550 and 900 °C, the LC3 experiences a higher reduction in strength than plain paste.•The addition of calcined clay helps to form katoite, which improves the strength of LC3 at 300 °C.•As the temperature increases, all samples generate more meso cracks.•At 900 °C, gehlenite is formed in paste specimens containing calcined clay.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZRSKP
A fundamental question in rock physics is how the coupling of confining stress and pore pressure influences geophysical properties, which is manifested by the effective stress behavior of the porous ...rock. We investigated the effective stress behavior of four water‐saturated limestones with porosities ranging from 13% to 30%. Unlike previous experimental studies limited to the permeability, we also characterized the effective stress coefficients for pore volume change and bulk strain. The pore spaces of three of the limestones (two allochemical oolitic and one micritic) have significant fractions of macropores and micropores. In these three limestones with dual porosity the effective stress coefficients for permeability and pore volume change were observed to be consistently greater than 1, even though that for axial strain was less than 1. In a microscopically homogeneous assemblage, the effective stress coefficients for permeability, bulk volumetric strain, and pore volume change are predicted to be equal to or less than unity. Our data therefore show that these limestones cannot be modeled as microscopically homogeneous. Berryman (1992a, https://doi.org/10.1029/92JB01593) and Berryman (1992b, https://doi.org/10.1103/PhysRevA.46.3307) analyzed theoretically a rock made up of two porous constituents, and our experimental data are in agreement with inequalities he derived for effective stress coefficients of such an assemblage. For comparison, we studied the Leitha limestone that is made up predominately of macropores. Our data showed that all three effective stress coefficients in this case were less than unity, as predicted for a microscopically homogeneous assemblage.
Key Points
Our study is the first integrated investigation of the effective stress behaviors for both permeability and deformation in limestones
Four limestones with a diversity of microstructural attributes were investigated
For limestones with dual porosity effective stress coefficients for permeability and pore volume change were observed to be consistently greater than 1
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Abstract
Microfacies analysis and Paleoenvironment of Baba Formation were investigated in Anah area, Anbar Governorate, Western Iraq. Baba Formation was cropped out in Wadi Al-Khazgah section with 9 ...meters thick, consisting of white to creamy, chalky, coralline, high fracture Limestone. Five Microfacies types were recognized Coral Lime Boundstone,
Lepidocyclina
Lime Packstone, Nummulitids Lime Packstone, Miliolids Lime Packstone - Grainstone, and Coraline Algae Lime Wackestone described in 10 samples collected from one section (Wadi Al-Khazgah Al-Gharbi section). These Microfacies described on the basis of the presence of the Benthic Large Foraminifera (BLF) as Miliolids, Lepidocyclinide, Rotalide and skeletal fragments of brachiopod, coral, and algae shell. The paleoenvironment of Baba Formation was determined fore-Reef and Reef environments.
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