Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Chemical looping offers a versatile platform to convert fuels and oxidizers in a clean and efficient manner. Central to this technology are metal oxide materials that can oxidize fuels, affording a reduced material that can be reoxidized to close the loop. Recent years have seen substantial advances in the design, formulation and manufacture of these oxygen carrier materials and their incorporation into chemical looping reactors for the production of various chemicals. This Review describes the mechanisms by which oxygen carriers undergo redox reactions and how these carriers can be incorporated into robust chemical looping reactors. One promising technology for modern energy and chemical conversions is chemical looping, central to which are redox cycles of metal oxides. This Review describes chemical looping schemes and the mechanisms by which metal oxide particles enable these technologies.

Metal oxide redox chemistry for chemical looping processes

As a promising approach for carbon dioxide capture, chemical looping combustion has been extensively investigated for more than two decades. However, the chemical looping strategy can be and has been extended well beyond carbon capture. In fact, significant impacts on emission reduction, energy conservation, and value-creation can be anticipated from chemical looping beyond combustion (CLBC). This article aims to demonstrate the versatility and transformational benefits of CLBC. Specifically, we focus on the use of oxygen carriers or redox catalysts for chemical production – a $4 trillion industry that consumes 40.9 quadrillion BTU of energy. Compared to state-of-the-art chemical production technologies, we illustrate that chemical looping offers significant opportunities for process intensification and exergy loss minimization. In many cases, an order of magnitude reduction in energy consumption and CO2 emission can be realized without the needs for carbon dioxide capture. In addition to providing various CLBC examples, this article elaborates on generalized design principles for CLBC, potential benefits and pitfalls, as well as redox catalyst selection, design, optimization, and redox reaction mechanism.

/pdf/chemical-looping-beyond-combustion-a-perspective-2ohy8fd521.pdf

Chemical looping beyond combustion – a perspective

Hydrogen is an attractive energy carrier due to its potentially high energy efficiency and low generation of pollutants, which can be used for transportation and stationary power generation. However, hydrogen is not readily available in sufficient quantities and the production cost is still high. Steam methane reforming (SMR) process is now the most widely used technology for H 2 production, but this process is complex and cannot get thorough carbon capture. Hydrogen production using chemical looping technology has received a great deal of attention in recent years because it can produce hydrogen with higher process efficiency and can capture carbon dioxide. Many researchers have carried out intensive research work on the hydrogen production processes using chemical looping technology. Based on the previous studies stated in the literature, the authors try to give an overview on the recent advances of two categories, chemical looping reforming (CLR) and chemical looping hydrogen production (CLH) processes. Besides, the characteristics of the processes are pointed out based on the comparison with the conventional SMR process. The existing technical problems and the aspects of future research of each approach are also summarized.

Review of hydrogen production using chemical-looping technology

Global warming due to the accumulation of atmospheric CO2 has received widespread attention in recent years. Although various CO2 capture technologies have been proposed, using the captured CO2 from power plants is increasingly popular because of concerns with regard to the safety of underground and ocean CO2 storage. Various techniques related to utilization of CO2 from the exhausted gas of power plants are discussed in this article. The existing and under-development technologies for CO2 utilization in the world are briefly reviewed. Two categories, direct utilization of CO2 and conversion of CO2 to chemicals and energy products, are used to classify different forms of CO2 utilization. Regarding the direct utilization of CO2, in addition to its use in soft drinks, welding, foaming, and propellants, as well as the use of supercritical CO2 as a solvent, CO2 capture via photosynthesis to directly fix carbon into microalgae has also attracted the attention of researchers. The conversion of CO2 into chemicals and energy products via this approach is a promising way to not only reduce the CO2 emissions, but also generate more economic value. Since CO2 is just a source of carbon without hydrogen, a clean, sustainable and cheap source of hydrogen should be developed. This article reviews the literature on the production of biofuel from microalgae cultivated using captured CO2, the conversion of CO2 with hydrogen to chemicals and energy products, and sustainable and clean sources of hydrogen, in order to demonstrate the potential of CO2 utilization.

/pdf/a-review-co2-utilization-2zmls8w4n8.pdf

A Review: CO2 Utilization

A novel process scheme for hydrogen production from coal with in situ CO2 capture, known as the coal-direct chemical looping (CDCL) gasification process, is discussed in this article. The CDCL process utilizes an iron oxide based oxygen carrier as a chemical looping medium to indirectly gasify coal into separate streams of H2 and CO2. ASPEN Plus reactor simulation models based on both thermodynamic equilibrium limitations and kinetic limitations are developed to analyze individual CDCL reactors. Process simulations are subsequently performed to estimate the performance of the CDCL process under various mass and energy management schemes. Reactor modeling results indicate that a moving bed reducer can effectively convert coal while reducing the oxygen carrier. The reduced oxygen carrier can in turn be oxidized by steam to produce hydrogen in a moving bed oxidizer. The fates of pollutants as well as the effects of various process operating parameters such as carbon and iron oxide conversions are also evalua...

Coal-Direct Chemical Looping Gasification for Hydrogen Production: Reactor Modeling and Process Simulation

We report a perovskite promoted iron oxide as a highly effective redox catalyst in a hybrid solar-redox scheme for partial oxidation and water-splitting of methane. In contrast to previously reported ferrite materials, which typically exhibit 20% or lower steam to hydrogen conversion, La0.8Sr0.2FeO3−δ (LSF) promoted Fe3O4 is capable of converting more than 67% steam with high redox stability. Both experiments and a defect model indicate that the synergistic effect of reduced LSF and metallic iron phases is attributable to the exceptional steam conversion. To further enhance such a synergistic effect, a layered reverse-flow reactor concept is proposed. Using this concept, over 77% steam to hydrogen conversion is achieved at 930 °C, which is 15% higher than the maximum conversion predicted by the second law for unpromoted iron (oxides). When applied to the hybrid solar-redox scheme for liquid fuels and hydrogen co-generation, significant improvements in the energy conversion efficiency can be achieved with reduced CO2 emissions.

http://www.rsc.org/suppdata/ee/c4/c4ee03431g/c4ee03431g1.pdf

Perovskite promoted iron oxide for hybrid water-splitting and syngas generation with exceptional conversion

The feasibility of a hybrid solar-redox process, which converts solar energy and methane into separate streams of liquid fuels and hydrogen through the assistance of an oxygen carrier, is investigated via both experiments and simulations. Fixed and fluidized-bed experiments are conducted to evaluate the redox performances of an oxygen carrier composed of iron oxide promoted with a mixed ionic–electronic conductor (MIEC) support. Over 95% conversion in the methane oxidation step and 60% steam to hydrogen conversion in the water-splitting step are observed. Aspen Plus® simulation based on experimental data and a comprehensive set of assumptions estimates the overall process efficiency to be 64.2–65.3% on a higher heating value (HHV) basis. Through the integration of solar energy, methane to fuel conversion efficiency can approach 100%. The proposed process has the potential to produce transportation fuels and hydrogen at high efficiency with reduced carbon footprint.

A hybrid solar-redox scheme for liquid fuel and hydrogen coproduction

Perovskite-structured AMnxB1−xO3 (A = Ca or Ba; B = Fe or Ni) redox catalysts for partial oxidation of methane

A rationalized strategy to optimize transition-metal-oxide-based redox catalysts for water splitting and syngas generation through a hybrid solar-redox process is proposed and validated. Monometallic transition metal oxides do not possess desirable properties for water splitting; however, density functional theory calculations indicate that the redox properties of perovskite-structured BaMnxFe1−xO3−δ can be varied by changing the B-site cation compositions. Specifically, BaMn0.5Fe0.5O3−δ is projected to be suitable for the hybrid solar-redox process. Experimental studies confirm such predictions, demonstrating 90 % steam-to-hydrogen conversion in water splitting and over 90 % syngas yield in the methane partial-oxidation step after repeated redox cycles. Compared to state-of-the-art solar-thermal water-splitting catalysts, the rationally designed redox catalyst reported is capable of splitting water at a significantly lower temperature and with ten-fold increase in steam-to-hydrogen conversion. Process simulations indicate the potential to operate the hybrid solar-redox process at a higher efficiency than state-of-the-art hydrogen and liquid-fuel production processes with 70 % lower CO2 emissions for hydrogen production

Iron‐Doped BaMnO3 for Hybrid Water Splitting and Syngas Generation

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