CO2-rich Atmosphere Research Articles

Glyphosate Resistance of C3 and C4 Weeds under Rising Atmospheric CO2.

The present paper reviews current knowledge on how changes of plant metabolism under elevated CO2 concentrations (e[CO2]) can affect the development of the glyphosate resistance of C3 and C4 weeds. Among the chemical herbicides, glyphosate, which is a non-selective and post-emergence herbicide, is currently the most widely used herbicide in global agriculture. As a consequence, glyphosate resistant weeds, particularly in major field crops, are a widespread problem and are becoming a significant challenge to future global food production. Of particular interest here it is known that the biochemical processes involved in photosynthetic pathways of C3 and C4 plants are different, which may have relevance to their competitive development under changing environmental conditions. It has already been shown that plant anatomical, morphological, and physiological changes under e[CO2] can be different, based on (i) the plant’s functional group, (ii) the available soil nutrients, and (iii) the governing water status. In this respect, C3 species are likely to have a major developmental advantage under a CO2 rich atmosphere, by being able to capitalize on the overall stimulatory effect of e[CO2]. For example, many tropical weed grass species fix CO2 from the atmosphere via the C4 photosynthetic pathway, which is a complex anatomical and biochemical variant of the C3 pathway. Thus, based on our current knowledge of CO2 fixing, it would appear obvious that the development of a glyphosate-resistant mechanism would be easier under an e[CO2] in C3 weeds which have a simpler photosynthetic pathway, than for C4 weeds. However, notwithstanding this logical argument, a better understanding of the biochemical, genetic, and molecular measures by which plants develop glyphosate resistance and how e[CO2] affects these measures will be important before attempting to innovate sustainable technology to manage the glyphosate-resistant evolution of weeds under e[CO2]. Such information will be of essential in managing weed control by herbicide use, and to thus ensure an increase in global food production in the event of increased atmospheric [CO2] levels.

Report of frozen equine embryos transfer in the Czech Republic

Several explanations have been proposed for the temporal differences in geologic processes associated with the modification of martian impact craters, which occurred throughout the Noachian, and the formation of valley networks, which occurred during the Noachian/Hesperian transition. Here we show that it could be a result of the changing nature of rainfall as the primordial atmospheric pressure on Mars waned through time. We calculate the terminal velocity and resulting kinetic energy from raindrops > 0.5 mm in diameter that would impact the surface of Mars in a CO2-rich atmosphere ranging in pressure from 0.5 to 10 bars. Our analyses indicate that the primordial atmosphere of Mars could not have exceeded ∼4.0 bars as raindrop sizes would have been limited to < 3 mm and surface erosion from rain splash and subsequent crater modification would not have occurred. At pressures between ∼3.0 and 4.0 bars, sediment transport from rain splash could occur, but surface runoff would have been limited, which could explain the modification of impact craters. Once atmospheric pressures waned to ∼1.5 bars, rainfall intensity could begin to exceed the infiltration capacity of most soils, which would be necessary to initiate martian valley network formation. Due to the lower gravity, a storm on Mars that occurred in a 1 bar atmosphere could generate raindrops with a maximum diameter of ∼7.3 mm compared to 6.5 mm on the Earth. However, rainfall from such a storm would be only be ∼70% as intense on Mars, primarily due to the lower martian gravity and resulting lower terminal velocities of the rain drops.

Carbon sequestration on Mars: COMMENT

Using data acquired from the Nili Fossae region of Mars by orbital remote sensing, Edwards and Ehlmann (2015) have demonstrated that CO2 was removed from the planet’s late Noachian–early Hesperian atmosphere by replacement (carbonation) of olivine-enriched basalts. Given the relatively low volumes (~20% at most) of carbonate present in the study area, and the apparent absence of carbonate-rich terranes of similar size elsewhere on Mars, they questioned whether carbonation alone could account for the loss of a putative thick CO2-rich atmosphere. The work of Edwards and Ehlmann is therefore a highly significant contribution to the long-running debate about the nature and evolution of the climate of early Mars. The authors noted that Martian meteorites contain carbonates, and suggested that further evaluation of these rocks could improve our understanding of the planet’s carbon cycle. Here we review published results from Martian meteorites, in particular the nakhlite group, which have already provided important insights into the mechanisms and time scales of mineralization and storage of CO2 in Mars’ crust. This evidence can help to discriminate between the various models for sequestration of Martian carbon that were discussed by Edwards and Ehlmann. The nakhlites were impact-ejected from Mars, probably from one location and in a single event. They are clinopyroxenites, and most contain olivine. The majority of these igneous rocks have been partially aqueously altered to produce a very fine-grained intergrowth of minerals including Fe-rich carbonates (up to ~1 vol%) and hydrous silicates (Gooding et al., 1991; Treiman et al., 1993; Changela and Bridges, 2011; Lee et al., 2013). These alteration products are collectively called ‘iddingsite.’ Water-rock interaction took place at some time between crystallization of the melt (ca. 1300 Ma) and impact-ejection (ca. 11 Ma; Eugster et al., 1997); the best current estimate is ca. 633 Ma (Borg and Drake, 2005). The high deuterium/hydrogen ratio of the iddingsite demonstrates that its parent aqueous fluids had equilibrated with Mars’ atmosphere (Hallis et al., 2012). Mineralogical and crystal-chemical records of atmosphere-water-rock interactions that are contained within the nakhlites support some of the conclusions of Edwards and Ehlmann, but also emphasize the importance of incorporating the meteorite record in any comprehensive model. For example, microstructural evidence demonstrates that the carbonates were formed by replacement of olivine and a plagioclase feldspar-rich mesostasis, whereas the volumetrically dominant augite has remained unaltered (Tomkinson et al., 2013). The differential susceptibility of silicates in these meteorites to carbonation helps to account for distinct contrasts between Nili Fossae rock units in their relative abundances of olivine and carbonate. The occurrence of carbonates within veins in the nakhlites (Tomkinson et al., 2013) is also consistent with the suggestion by Edwards and Ehlmann that fracturing had facilitated the movement of aqueous solutions through the Nili Fossae basalts. However, the partial replacement of nakhlite carbonates by phyllosilicates and Feoxyhydroxides (Tomkinson et al., 2013) demonstrates that this crustal store of carbon was not necessarily permanent. The age of the nakhlite carbonates shows that atmospheric CO2 was being mineralized and stored in the late Amazonian. Scarce 3900-m.y.old carbonates also occur in the Martian orthopyroxenite Allan Hills (ALH)84001 (Borg et al., 1999). The meteorite record therefore supports the ‘deep–diffuse sequestration’ model that was invoked by Edwards and Ehlmann to account for the carbonate that is ‘missing’ from the Martian crust on the assumption that its early atmosphere was thinned mainly by mineral sequestration. The deep-diffuse model proposes that low volumes of finely disseminated carbonate formed within the crust throughout much of Mars’ history, and as Edwards and Ehlmann note, these carbonates would be “undetectable by remote sensing.” Products of aqueous alteration are essentially absent from the shergottite meteorites, which sample younger than ca.600 Ma basalts from several different locations, thus indicating that the drivers of crustal sequestration had effectively ceased within the past few hundred million years. The record of reactions between the Martian atmosphere, hydrosphere, and lithosphere in our meteorite collections is manifestly very patchy as regards age spectrum and rock types (i.e., it is highly biased toward igneous rocks from younger terranes). Additionally, stresses accompanying impact ejection of these rocks, their interplanetary transfer and fall to Earth may militate against the sampling of heavily carbonated lithologies that are likely to be brittle and fractured. Nonetheless, information on the presence/absence and petrographic context of carbonate minerals in Martian meteorites provides very valuable information on the magnitude of CO2 sequestration, petrologic and microstructural controls on the carbonation reaction, and its temporal and geographical range. This record indicates to us that the deep-diffuse model remains viable, and it can be tested further as new meteorites from regions of Mars that have not been previously sampled become available for study.

Vanadium-lithium alumina a potential additive for coke oxidation by CO2 in the presence of O2 during FCC catalyst regeneration

The possibility of performing the regeneration of spent fluid catalytic cracking catalysts in rich CO2 atmosphere can simultaneously reduce greenhouse gas emissions and produce CO for sequential uses. Herein alumina modified by vanadium and lithium was revealed a remarkable catalyst (or additive) for this goal. High CO2 conversion was observed in the presence of helium (for instance at 720 and 800°C, 39 and 76% of CO2 was converted during the first 5min of reaction respectively) and it was reduced by less than 5% in the presence of oxygen. Moreover the catalyst works effectively regardless the coke type on spent catalyst, i.e. produced by means of hydrocarbon or bio feed cracking. This catalyst was tested as an additive and works properly up to one part to ten of spent USY. By means of either labeled CO2 or coke (both with carbon 13) it was shown that primary 13CO2 forms 13CO followed by oxygen mobility between alumina and USY promoting coke oxidation. 12CO2 formed in-situ similarly to 13CO2 readily reacts with coke producing CO. These effects improve the coke burning by CO2 and remarkably increase the CO/CO2 ratio even in the presence of O2. Finally these results strongly support that the FCC regeneration step can be kept under auto-thermal condition in CO2 rich atmosphere.

Modified atmosphere storage technique for the management of pulse beetle, Callosobruchus chinensis in Horse gram

The present study was aimed to investigate the effect of modified atmosphere with elevated levels of CO2 against pulse beetle, C. chinensis in stored horse gram. Seeds were artificially infested with C. chinensis and exposed to CO2 (30%, 40% and 50 %) and packed in air tight containers. Concentrations of 40% and 50% CO2 not only checked seed infestation but also checked the progeny production of pulse beetle and reduced the weight loss of seed compared to normal atmosphere. Storage of horse gram seeds in CO2 rich atmosphere also maintained seed quality without any detrimental effect on germination and seedling vigor up to six months of storage.

Climatic and geochemical implications of Archean pedogenic gypsum in the Moodies Group (∼3.2 Ga), Barberton Greenstone Belt, South Africa

Lithic sandstones of braided-fluvial to supratidal facies in the Paleoarchean Moodies Group (∼3.22Ga, Barberton Greenstone Belt, South Africa) include several regionally traceable units with common to abundant, in places rock-forming, nodular concretions of megaquartz pseudomorphs after gypsum, barite and calcite. Concretionary accumulations are stratiform and commonly associated with aqueously reworked, fine-grained, tuffaceous sediment of originally rhyodacitic composition and can grow to fist-sized agglomerates in crusts tens of m in lateral extent. Weathering of tuffaceous material and feldspar delivered alkali cations such as Ca, Ba, and K, while carbonates were likely supplied by silicate weathering of mafic to ultramafic volcanic rocks during exposure to a CO2-rich atmosphere. Sulfate ions were partly delivered by oxidative pyrite dissolution which may have included microbial and abiotic disproportionation of volcanic S or SO2. Concretionary growth apparently took place under pedogenic to early diagenetic conditions within unconsolidated granular sediment in the vadose zone, dominated by seasonal fluctuations of the groundwater level under evaporitic conditions. The concretions likely represent the oldest terrestrial evaporites known to date and form part of the oldest known compound paleosols. Their formation and composition constrain the local occurrence of sulfate in the Archean atmo- and hydrosphere, their interaction with the emerging biosphere, Archean weathering regime, local climate, and vadose-zone hydrodynamics.

Towards standardization: influence of CO2 rich atmosphere on degradation behavior of Fe-Mn-C alloy

Towards standardization: influence of CO2 rich atmosphere on degradation behavior of Fe-Mn-C alloy

An Approach To Enhance the CO2 Tolerance of Fluorite–Perovskite Dual-Phase Oxygen-Transporting Membrane

Most of the alkaline earth-containing perovskite-based oxygen-transporting membranes (OTMs) have insufficient tolerance toward CO2 that potentially limits their commercial applications, for example, oxy-fuel combustion processes with CO2 capture. One concern regarding the chemical potential of oxygen that may influence the CO2 tolerance of perovskites, however, is lacking effective investigations. In the present work, we demonstrate that the approach to increase the chemical potential of oxygen at the feed side contributes to stabilize the oxygen permeation fluxes of the fluorite–perovskite dual-phase OTM under CO2-rich atmosphere, and we further verify that oxygen can effectively act as a “buffer” to prevent the carbonate formation. Remarkably, we achieve high and stable oxygen permeation fluxes over 0.84 mL cm–2 min–1 during long-term operation at 900 °C with a 0.5 mm thickness 80 wt % Ce0.8Gd0.15Cu0.05O2−δ-20 wt % SrFeO3−δ (CGCO-SFO, nominal composition) dual-phase membrane using oxygen-enriched air as...

A spectral line list for water isotopologues in the 1100–4100 cm−1 region for application to CO2-rich planetary atmospheres

A line list of transitions for seven isotopologues of water vapor in the 1100–4100 wavenumber range has been constructed for application to atmospheres rich in CO2. The quantum transitions for H216O, H218O, H217O, HD16O, HD18O, and HD17O are from the 2012 HITRAN database (Rothman et al., 2013) and those for D216O are from the ab initio line list of Shirin et al. (2008). The database was constructed using the best available measured parameters and augmented with theoretical calculations. The half-width, γ, its temperature dependence, n, and the line shift, δ, are for CO2 as the colliding partner or self-collisions and are determined using the Modified Complex Robert–Bonamy (MCRB) theory. The resulting line list contains 282557 transitions and is available, in the HITRAN 2012 format, from the corresponding author or from the supplemental information of the journal.

FTIR imaging in diffusion studies: CO2 and H2O in a synthetic sector-zoned beryl

In this work we investigate the strongly inhomogeneous distribution of CO2 and H2O in a synthetic beryl having a peculiar hourglass zoning of Cr due to the crystal growth. The sample was treated at 800°C, 500 MPa, in a CO2-rich atmosphere. High-resolution FESEM images revealed that the hourglass boundary is not correlated to physical discontinuities, at least at the scale of tens of nanometers. Polarized FPA-FTIR imaging, on the other side, revealed that the chemical zoning acts as a fast pathway for carbon dioxide diffusion, a feature never observed so far in minerals. The hourglass zone boundary may be thus considered as a structural defect possibly due to the mismatch induced by the different growth rates of each sector. High-resolution synchrotron-light FTIR imaging, in addition, also allows enhancement of CO2 diffusion along the hourglass boundary to be distinguished from diffusion along fractures in the grain. Therefore, FTIR imaging provides evidence that different diffusion mechanisms may locally combine, suggesting that the distribution of the target molecules needs to be be carefully characterized in experimental studies. This piece of information is mandatory when the study is aimed at extracting diffusion coefficients from analytical profiles. Combination of TOF-SIMS and FPA data shows a significant depletion of type II H2O along the hourglass boundary, indicating that water diffusion could be controlled by the distribution of alkali cations within channels, coupled to a plug effect of CO2.

Speciation and diffusion profiles of H2O in water-poor beryl: comparison with cordierite

This paper reports on water speciation and diffusion in synthetic beryl samples treated in CO2-rich atmosphere, at 700 MPa and 700 and 800 °C, respectively. The study has been conducted by means of polarized FTIR (Fourier transform infrared) integrated with FPA (focal plane array) imaging. As expected, the infrared spectra show the presence of CO2 but also of minor H2O interpreted as resulting from moisture present in the starting materials used for the experiments. FPA-FTIR images show that H2O diffuses into the beryl matrix along the structural channels oriented parallel to [001]. Spectra collected along profiles parallel to the c-axis show subtle changes as a function of the distance from the crystal edge; these changes can be correlated to a progressive change in the H2O coordination environment in the channel, as a response to the varying H2O/alkali ratio. In particular, the data show that when 2H2O > Na+ apfu (atoms per formula unit), H2O can assume both type I and type II orientation; in the latter case, each Na cation coordinates two H2O[II] molecules (doubly coordinated H2O). If 2H2O < Na+ apfu, then H2O[II] molecules are singly coordinated to each Na cation. The same type of feature is observed and commented for the structurally related cordierite. Diffusion coefficients and activation energies have been also determined for both types of water molecules.

Oxygen transport membranes in a biomass/coal combined strategy for reducing CO2 emissions: Permeation study of selected membranes under different CO2-rich atmospheres

This contribution introduces how the integration of biomass as fuel in power plants would balance CO2 emissions and the related role of oxygen transport membranes (OTM) on it. CO2 capture techniques could be introduced to minimize CO2 emissions at the cost of a substantial energy penalty in the overall process. Among the different approaches, the use of pure O2 and/or N2-free oxidation gases for combustion and/or for gasification leads to promising energy efficiencies. Ceramic OTM membranes could be successfully integrated in such thermal processes, which enable to increase the net plant efficiency when CO2 capture is implemented. Further, this work reviews how selected ceramic materials and membrane architectures behave under CO2 containing atmospheres at high temperatures above 700°C. These conditions have been selected for checking the viability of these membrane compositions and configurations to fit in an oxy-co-gasification process, involving coal and biomass. The tested asymmetric membranes present competitive oxygen fluxes in the range 0.6–1.2mlmin−1cm−2 when using CO2 as (inlet) sweep gas at 850°C (optimal membrane operation conditions in oxy-fuel power plant) and stable oxygen production up to 100h of continuous operation in similar conditions. Specifically, La0.6Sr0.4Co0.2Fe0.8O3−δ and NiFe2O4–Ce0.8Tb0.2O2−δ composite materials showed the best results for oxygen permeation and time stability under CO2-rich atmospheres.

Communication: evidence of stable van der Waals CO2 clusters relevant to Venus atmosphere conditions.

Non-intrusive spectroscopic probing of weakly bound van der Waals complexes forming in gaseous carbon dioxide is generally performed at low pressures, for instance in supersonic jets, where the low temperature favors dimers, or in few-atmosphere samples, where the signature of dimers varying as the squared gas density is entangled with the dominating collision-induced absorption. We report experimental and theoretical results on CO2 dimers at very high pressures approaching the liquid phase. We observe that the shape of the CO2-dimer bands undergoes a distinctive line-mixing transformation, which reveals an unexpected stability of the dimers despite the collisions with the surrounding particles and negates the common belief that CO2 dimers are short-lived complexes. Our results furnish a deeper insight allowing a better modeling of CO2-rich atmospheres and provide also a new spectroscopic tool for studying the robustness of molecular clusters.

Introducing capnophilic lactic fermentation in a combined dark-photo fermentation process: a route to unparalleled H2 yields.

Two-stage process based on photofermentation of dark fermentation effluents is widely recognized as the most effective method for biological production of hydrogen from organic substrates. Recently, it was described an alternative mechanism, named capnophilic lactic fermentation, for sugar fermentation by the hyperthermophilic bacterium Thermotoga neapolitana in CO2-rich atmosphere. Here, we report the first application of this novel process to two-stage biological production of hydrogen. The microbial system based on T. neapolitana DSM 4359(T) and Rhodopseudomonas palustris 42OL gave 9.4 mol of hydrogen per mole of glucose consumed during the anaerobic process, which is the best production yield so far reported for conventional two-stage batch cultivations. The improvement of hydrogen yield correlates with the increase in lactic production during capnophilic lactic fermentation and takes also advantage of the introduction of original conditions for culturing both microorganisms in minimal media based on diluted sea water. The use of CO2 during the first step of the combined process establishes a novel strategy for biohydrogen technology. Moreover, this study opens the way to cost reduction and use of salt-rich waste as feedstock.

Initiation of plate tectonics from post‐magma ocean thermochemical convection

AbstractLeading theories for the presence of plate tectonics on Earth typically appeal to the role of present day conditions in promoting rheological weakening of the lithosphere. However, it is unknown whether the conditions of the early Earth were favorable for plate tectonics, or any form of subduction, and thus, how subduction begins is unclear. Using physical models based on grain‐damage, a grainsize‐feedback mechanism capable of producing plate‐like mantle convection, we demonstrate that subduction was possible on the Hadean Earth (hereafter referred to as proto‐subduction or proto‐plate tectonics), that proto‐subduction differed from modern day plate tectonics, and that it could initiate rapidly. Scaling laws for convection with grain‐damage show that though either higher mantle temperatures or higher surface temperatures lead to slower plates, proto‐subduction, with plate speeds of ≈1.75 cm/yr, can still be maintained in the Hadean, even with a CO2 rich primordial atmosphere. Furthermore, when the mantle potential temperature is high (e.g., above ≈2000 K), the mode of subduction switches to a “sluggish subduction” style, where downwellings are drip like and plate boundaries are diffuse. Finally, numerical models of post‐magma ocean mantle convection demonstrate that proto‐plate tectonics likely initiates within ∼100 Myr of magma ocean solidification, consistent with evidence from Hadean zircons. After the initiation of proto‐subduction, non‐plate‐tectonic “sluggish subduction” prevails, giving way to modern style plate tectonics as both the mantle interior and climate cool. Hadean proto‐subduction may hasten the onset of modern plate tectonics by drawing excess CO2 out of the atmosphere and cooling the climate.

Bacterial Contamination Levels of Autogenous Bone Particles Collected by 3 Different Techniques for Harvesting Intraoral Bone Grafts

The aim of this study was to compare levels of bacterial contamination of autogenous bone collected when using low-speed drilling, a back-action chisel, and a bone filter. Bone tissue samples were taken from 31 patients who underwent surgical extraction of their third lower molars. Before surgical removal of the molar, bone particles were collected by a low-speed drill or a back-action chisel. Then, a stringent aspiration protocol was applied during the ostectomy to collect particulate bone by a bone filter. Processing of samples commenced immediately by incubation in an anaerobic or a CO2-rich atmosphere. The number of colony-forming units (CFUs) was determined at 48 hours of culture. No significant difference in the number of CFUs per milliliter was observed between the low-speed drilling group and the back-action chisel group in the anaerobic or CO2-rich condition (P = .34). However, significantly more micro-organisms were found in the bone filter group than in the low-speed drilling group or the back-action chisel group in the anaerobic and CO2-rich conditions (P < .001). Particulate bone harvested with low-speed drilling or a back-action chisel is safer for use as an autograft than are bone particles collected with a bone filter. These results suggest that bone obtained from low-speed drilling is safe and straightforward to harvest and could be the method of choice for collecting particulate bone. Further research is needed to lower the bacterial contamination levels of autogenous bone particles used as graft material.

Outstanding hydrogen permeation through CO2-stable dual-phase ceramic membranes

Mixed electronic- and protonic-conducting composites made up of physical mixtures of La5.5WO11.25−δ–La0.87Sr0.13CrO3−δ (LWO–LSC) have been evaluated as H2 separation membranes for operation at temperatures greater than 550 °C. The mixture of these two ion-conducting phases led to non-linear synergetic effects; i.e. unexpected enhancement of the total conductivity and well-balanced ambipolar conductivity, resulting in appealing H2 permeation fluxes through robust ceramic membranes. The preparation, primary characterization, H2 permeation and stability studies of various composites is presented. Mixing LWO and LSC phases makes it possible (1) to improve the LSC sintering behavior and to achieve very high membrane densities and (2) to obtain compounds with high total conductivity, higher than that shown for LWO and LSC, separately. The highest permeation rate is achieved for the 50 vol%-LWO–LSC membrane, though other composite compositions showed higher total conductivity. Moreover, the influence on the H2 permeation of the composite composition, the humidification of gas streams, temperature and the use of various catalytic coatings on the membrane surface is evaluated. The nature of the transport mechanism is investigated by the permeation studies using deuterium tracers. The H2 permeation rates reported in this investigation for a 370 μm thick 50 vol%-LWO–LSC membrane, e.g. 0.15 mL min−1 cm−2 at 700 °C, are the highest reported values, up to date, for any bulk mixed protonic-electronic membranes. The H2 permeation magnitude achieved at moderate temperatures along with the proven stability in CO2-rich atmospheres are firm steps towards the future application of this type of membrane for industrial processes.

Screening in situ bone and teeth preservation by ATR-FTIR mapping

Bones and teeth are highly hierarchically structured and hererogeneous materials, and post mortem processes can reinforce this heterogeneity. It is therefore important to consider this heterogeneity to better understand diagenetic processes. In this study, ATR-FTIR mapping was applied to several heated and un-heated archaeological samples, and to similar modern references in order to test the potential of this method. ATR-FTIR mapping can provide spatially resolved information on alteration state of mineral and organic matter. This technique allowed to describe the spatial distribution of organic and mineral matter preservation in unheated Palaeolithic bones (Bize-Tournal, France) characterized by a better preservation in the centre of the cortical bone. Spatial variations in the chemical composition of an archaeological heated bone (Abri Pataud, France) compared to a modern reference suggested taphonomical uptake of carbonate in the most external part. This pattern could correspond to a process of re-carbonatation of the calcined mineral matter in the outermost part of the sample due to combustion in a CO2 rich atmosphere. FTIR-ATR is a powerful tool that allows for identifying and characterizing local heterogeneities in bone preservation. This technique open new prospects to reconstruct the taphonomical history of ancient samples.

Accuracy of the single-film model in the prediction of coal char conversion rates under oxy-fuel and conventional combustion conditions

The accuracy of the single-film model in the calculation of char burning rates and particle temperatures in conventional and oxy-fuel combustion conditions has been investigated taking as a reference the results obtained with a model with detailed gas-phase kinetics. Both models have been applied to subbituminous coal and anthracite char particles in different sizes (60–1000μm), in N2 and CO2-rich atmospheres with a low and a high oxygen concentration. For 120μm char particles, the models’ predictions have been also compared over an ample range of ambient gas temperatures (1373–1823K) and oxygen concentrations (4–50%). High particle diameters, bulk gas temperatures and oxygen concentrations result in CO-flames closer to the particle surface; however, the specific effects of the flame strongly depend on the coal rank and combustion conditions. In general, the single-film model accuracy is quite high in terms of burning rate predictions, except for fine anthracite particles burning at moderate bulk gas temperatures and high O2 concentrations; greater deviations have been found regarding particle temperatures, even in cases where the corresponding combustion rates are very similar. The oxy-combustion of the reactive coal char is found to be faster than its conventional combustion, except for the finest particles; although qualitatively similar, the difference in rates is overestimated if the single-film model is considered.

Calcium-looping for post-combustion CO2 capture. On the adverse effect of sorbent regeneration under CO2

The multicyclic carbonation/calcination (c/c) of CaO solid particles at high temperature is at the basis of the recently emerged Calcium-looping (CaL) technology, which has been shown to be potentially suitable for achieving high and sustainable post-combustion CO2 capture efficiency. Despite the success of pilot plant projects at the MWth scale, a matter of concern for scaling-up the CaL technology to a commercial level (to the GWth scale) is that the CaO carbonation reactivity can be recovered only partially when the sorbent is regenerated by calcination at high temperatures (around 950°C) as required by the CO2 high concentration in the calciner. In order to reactivate the sorbent, a novel CaL concept has been proposed wherein a recarbonator reactor operated at high temperature/high CO2 concentration leads to further carbonation of the solids before entering into the calciner for regeneration. Multicyclic thermogravimetric analysis (TGA) tests demonstrate the feasibility of recarbonation to reactivate the sorbent regenerated at high calcination temperatures yet at unrealistically low CO2 partial pressure mainly because of technical limitations concerning low heating/cooling rates. We report results from multicyclic c/c and carbonation/recarbonation/calcination (c/r/c) TGA tests at high heating/coling rates and in which the sorbent is regenerated in a dry atmosphere at high CO2 partial pressure. It is shown that at these conditions there is a drastic drop of CaO conversion to a very small residual value in just a few cycles. Moreover, the introduction of a recarbonation stage has actually an adverse effect. Arguably, CaCO3 decomposition in a CO2 rich atmosphere is ruled by CO2 dynamic adsorption/desorption in reactive CaO (111) surfaces as suggested by theoretical studies, which would preclude the growth of the regenerated CaO crystal structure along these reactive surfaces, and this effect would be intensified by recarbonation. Nevertheless, the presence of H2O in the calciner, which is also adsorbed/desorbed dynamically in CaO reactive planes, would shield CO2 adsorption/desorption thus mitigating the deeply detrimental effect of CO2 on the carbonation reactivity of the regenerated CaO structure. Oxy-combustion, which produces a significant amount of H2O, is currently used in pilot-scale plants to raise the temperature in the calciner. Auxiliary techniques are being explored to help heating the partially carbonated solids since oxyxombustion represents an important penalty to the CaL technology. Our study suggests that steam injection would be necessary in a dry calciner environment to avoid a sharp loss of CaO conversion if the sorbent is regenerated at high CO2 partial pressure.