98,489 research outputs found
Process engineering and development of post-combustion CO2 separation from fuels using limestone in CaO-looping cycle
No full textGlobal CO2 emissions produced by energy-related processes, mainly power plants, have increased rapidly in recent decades; and are widely accepted as the dominant contributor to the greenhouse gas (GHG) effect and consequent climate changes. Among countermeasures against the emissions, CO2 capture and storage (CCS) is receiving much attention. Capture of CO2 is the core step of CCS as it contributes around 75% of the overall cost, and may increase the production costs of electricity by over 50%. The reduction in capture costs is one of the most challenging issues in application of CCS to the energy industry. Using limestone in CaO-looping cycles is a promising capture technology to provide a cost-effective separation process to remove CO2 content from power plants operations. Limestone has the advantage of being relatively abundant and cheap, and that has already been widely used as a sorbent for sulphur capture. However, this technology suffers from a critical challenge caused by the decay in the sorbent capture capacity during cyclic carbonation/calcination, which results in the need for more sorbent make-up; hence a reduction in cost efficiency of the technology. The performance of sorbent influenced by several operating and reaction conditions. Therefore, much research involves investigation of influencing factors and different methods to reduce the sorbent deactivation. Cont/d
On evolution of bed material waves in alluvial rivers
No full textField and laboratory measurements have shown distinct characteristics of bed sediment waves under differing conditions, whilst their theoretical interpretation has emerged to be equivocal. This note aims to clarify the interpretation of evolution of bed material waves. The complete set of governing equations for the flow-sediment-morphology system is deduced to demonstrate its universally hyperbolic nature, irrespective of the sediment transport functions implemented to close the equations. The hyperbolic nature can admit not only attenuating bed material waves, but also shock-like waves that are not unusual in the real world. It is suggested that the theory of dispersion/diffusion is not universally appropriate for evolution of bed material waves
Mathematical modelling of alluvial rivers: reality and myth. Part 2: special issues, 2002
No full textThe last half a century has seen more and more developments and applications of mathematical models for fluvial flow, sediment transport and morphological evolution. However, the quality of this modelling practice has emerged as a crucial issue for concern, which is widely viewed as the key that could unlock the full potential of computational fluvial hydraulics. The major factors affecting the modelling quality comprise: ( a ) poor assumptions in model formulations; ( b ) simplified numerical solution procedure; ( c ) the implementation of sediment relationships of questionable validity; and ( d ) the problematic use of model calibration and verification as assertions of model veracity. An overview of mathematical models for alluvial rivers is provided in this and the companion paper ‘Part 1: General review’. This paper is the second part, dealing with three special issues of mathematical river models. First, turbulence closure models are highlighted, particularly with respect to the role of sediment in modulating turbulence and its implications for adapting turbulence closure models for fluvial sediment-laden flows. Second, the bottom boundary conditions are discussed in detail as one of the main sources of model uncertainty. And third, the commonly used calibration and verification/validation methodology in mathematical river modelling is addressed. It is argued that model calibration can be subjective, verification is impossible because models are not closed systems, and validation does not necessarily establish model truth. Confirmation of observations by models only supports model probability, rather than demonstrating model veracity. It is vital for model developers and end-users to keep aware of what mathematical river models can realistically reflect, and therefore avoid misleading decision-making. Additionally, some strategies are proposed which can improve the practice of mathematical river modelling
The Neutrophil/Lymphocyte Ratio Could Predict Noninvasive Mechanical Ventilation Failure in Patients with Acute Exacerbation of Chronic Obstructive Pulmonary Disease: A Retrospective Observational Study [Corrigendum]
No full textSun W, Luo Z, Jin J, Cao Z, Ma Y. Int J Chron Obstruct Pulmon Dis. 2021;16:2267–2277.
The authors have advised the author list on page 2267 is incorrect. The text “Yingmin Ma2” should read “Yingmin Ma1,2”.
The authors apologize for this error
Melt densities in the CaO-FeO-Fe2O3-SiO2 system and the compositional dependence of the partial molar volume of ferric iron in silicate melts
No full textThe densities of 10 melts in the CaO-FeO-Fe2O3-SiO2 system were determined in equilibrium with air, in the temperature range of 1200 to 1550°C, using the double-bob Archimedean technique. Melt compositions range from 6 to 58 wt% SiO2, 14 to 76 wt% Fe2O3 and 10 to 46 wt% CaO. The ferric-ferrous ratios of glasses drop-quenched from loop fusion equilibration experiments were determined by 57Fe Mössbauer spectroscopy.
Melt densities range from 2.689 to 3.618 gm/cm3 with a mean standard deviation from replicate experiments of 0.15%. Least-squares regressions of molar volume versus molar composition have been performed and the root mean squared deviation shows that a linear combination of partial molar volumes for the oxide components (CaO, FeO, Fe2O3 and SiO2) cannot describe the data set within experimental error. Instead, the inclusion of excess terms in CaFe3+ and CaSi (product terms using the oxides) is required to yield a fit that describes the experimental data within error. The nonlinear compositional-dependence of the molar volumes of melts in this system can be explained by structural considerations of the roles of Ca and Fe3+.
The volume behavior of melts in this system is significantly different from that in the Na2O-FeO-Fe2O3-SiO2 system, consistent with the proposal that a proportion of Fe3+ in melts in the CaO-FeO-Fe2O3-SiO2 system is not tetrahedrally-coordinated by oxygen, which is supported by differences in 57Fe Mössbauer spectra of glasses. Specifically, this study confirms that the 57Fe Mössbauer spectra exhibit an area asymmetry and higher values of isomer shift of the ferric doublet that vary systematically with composition and temperature (this study; Dingwell and Virgo, 1987, 1988). These observations are consistent with a number of other lines of evidence (e.g., homogeneous redox equilibria, Dickenson and Hess, 1986; viscosity, Dingwell and Virgo, 1987,1988). Two species of ferric iron, varying in proportions with temperature, composition and redox state, are sufficient to describe the above observations.
The presence of more than one coordination geometry for Fe3+ in low pressure silicate melts has several implications for igneous petrogenesis. The possible effects on compressibility, the pressure dependence of the redox ratio, and redox enthalpy are briefly noted
Location of greenspace matters: a new approach to investigating the effect of the greenspace spatial pattern on urban heat environment
No full textContext: Although many prior efforts have found that both spatial composition and configuration of greenspaces significantly affect the urban heat environment globally, the spatially heterogeneous effects of greenspace spatial patterns on the urban heat environment remain poorly understood for urban spaces. Objectives: We proposed a spatially explicit approach to investigate the spatially heterogeneous cooling effects of greenspaces and map the relative contributions of the greenspace spatial patterns to the characterization of the urban heat environment. Methods: The proposed approach integrated the best subsets regression method, geographically weighted regression (GWR), and hierarchical partitioning analysis. Two cities in southeastern China were selected to test our model. Landsat 5 image obtained in the summer was used to estimate the land surface temperature (LST) and greenspace spatial patterns were extracted from 0.5-m aerial images. Results: The results revealed that LST of Guangzhou can be well predicted by the percent cover (PER), the number of patches (NP), the area-weighted mean of the patch area (AREA_AM), and the area-weighted mean of the perimeter-area fractal dimension (FRAC_MN), while that of Shenzhen can be predicted by PER, NP, AREA_AM and the mean of the related circumscribing circle (CIRCLE_MN). The inclusion of additional landscape metrics did not yield significantly higher accuracies. The dominant landscape metrics of greenspace that determine the LST varied spatially across the two cities, with the PER accounting for the greatest variation. Conclusion: The results of our work demonstrate that the location of greenspace is a significant factor affecting the urban heat environment. The proposed approach provides a new understanding of the interaction between the greenspace spatial patterns and urban heat environments, providing useful information for tailoring greenspace planning policies for specific local sites
(Z)-1,2-Bis(4-nitrophenyl)ethene. Corrigendum
No full textThe name of the first author in the paper by Chen & Cao [Acta Cryst. (2007), E63, o3999] is corrected
Further perspectives on the evolution of bed material waves in alluvial channels
No full textIn one-dimensional mathematical models of fluvial flow, sediment transport and morphological evolution, the governing equations based on mass and momentum conservation laws constitute a hyperbolic system. Succinctly, the hyperbolic nature excludes dispersion or diffusion operators, which is well known in the context of differential equations. There is no doubt that the so-called ‘dispersion’ argument for bed material wave evolution is questionable, as we have explicitly asserted. Surprisingly, in a recent communication, the authors of the ‘dispersion’ argument suggest that dispersion is not precluded in hyperbolic systems. We provide herein further perspectives to help explain that the dispersion argument is neither appropriate nor necessary for interpreting bed material wave evolution. Also the continuity equations involved are addressed to prompt wider understanding of their significance. In particular, the continuity equation of the water–sediment mixture proposed by the authors of the ‘dispersion’ argument is proved to be incorrect, and inevitably their reasoning based on it is problematic. <br/
Effetto della velocità di deformazione sulla propagazione della cricca per fatica nell'Inconel X 750 ad elevata temperatura
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