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In order to improve the energy efficiency of hydrogen storage for power generation, in this work, an improved Graz cycle (R-Graz cycle) was proposed, which can realize more efficient hydrogen to electricity conversion. The modeling and optimization program of the cycle was developed based on MATLAB, and the thermodynamic properties of the working fluid were obtained by REFPROP. The results show that the R-Graz cycle can reach a net efficiency of 70.98% at the design point, which is about 0.5% higher than that of the Graz cycle with the same input parameters, and much higher than that of the modern gas-steam combined cycle. With the same amount of hydrogen input, the R-Graz cycle can produce 0.34% more electricity than the Graz cycle. At the design point, the levelized cost of electricity (LCOE) of the R-Graz cycle is 156.23 $/MWh, which is 0.22% lower than that of the Graz cycle, and the net present value is also higher. Analysis on the influence of key parameters shows that increasing the inlet temperature and pressure of the high-temperature gas turbine and the inlet temperature of the high-pressure steam turbine as well as reducing the condenser pressure are conducive to improving the R-Graz cycle efficiency, among which the influence of the inlet pressure of the high-temperature gas turbine and condenser pressure is particularly significant. This study also presents the impact of these key parameters on the LCOE. This research can effectively increase the conversion efficiency of electricity-hydrogen-electricity mode, and provides a new path for hydrogen utilization. • A revised Graz cycle for power generation based on hydrogen storage is proposed. • The net efficiency of proposed cycle is higher than 70%. • Total efficiency of about 50% can be achieved in the hydrogen storage scenario. • The economy of revised cycle is better than that of Graz cycle on LCOE and NPV. [ABSTRACT FROM AUTHOR]

The structure and thermoacoustic instability (TI) of premixed CH 4 /H 2 /air swirling flames were experimentally investigated for a range of hydrogen fraction (η H2) up to 80% under different equivalence ratio (Φ) and swirl number (S) conditions. It is shown that the onset of TI is enhanced when increasing either η H2 , S , or Φ. The dominant frequency of TI increases dramatically with η H2. The higher dominant frequency in the hydrogen-enriched flames can be attributed to a shorter flame length which results in a reduced flame convection time. It is observed that the unstable flames are always accompanied by the appearance of outer recirculation zone (ORZ) flame. Therefore, the flame kernel residing in the ORZ can be an indicator of the occurrence of TI. The flame front of thermoacoustic unstable flames was observed to be more wrinkled, e.g., with larger mean absolute curvature (κ abs) and local flame surface area ratio (δ Σ max). Importantly, the phase-locked analysis shows that κ abs and δ Σ max can be modified at different oscillation phases, and their maximum and minimum values are simultaneously achieved at phase angles θ of about 0° and 180°, respectively. Variations of κ abs and δ Σ max are in phase with the heat release rate, indicating a strong correlation between the TI and flame structure modification; however, they show a phase lag of about 72° behind the pressure in this work. These results are vital when understanding and predicting the TI based on the flame structure, especially when adopting a visual detection method of the instability. [Display omitted] • Thermoacoustic instability of swirling lean premixed CH 4 /H 2 /air flames is studied. • The dominant frequency increases significantly with hydrogen fraction. • Appearance of the ORZ flame is highly related to the instability. • The flame front is more wrinkled in unstable flames. • Flame curvature and surface area oscillate in phase with heat release rate. [ABSTRACT FROM AUTHOR]

An Eulerian Monte-Carlo approach, the so-called Eulerian stochastic fields (ESF) method is implemented and evaluated for simulation of non-premixed hydrogen/air combustion in supersonic flows. The ESF method is integrated into a compressible flow large eddy simulation (LES) solver, and validated on a supersonic combustor with a strut as flame-holder. Comparison with experimental data and with results from a well-stirred reactor (WSR) model demonstrates the advantage of the LES-ESF method for simulation of local-extinction and re-ignition phenomena. The hydrogen/air flame structure and the stabilization of the combustion process in the supersonic combustor are analysed based on the present LES-ESF method. Oscillation of the recirculation zones is found to be the dominant mechanism for the local-extinction/re-ignition and the flame stabilization under the present condition. [ABSTRACT FROM AUTHOR]

This paper investigated methane/air flame characteristics with hydrogen addition in micro confined combustion space experimentally and computationally. The focus is on the effect of hydrogen addition on the methane/air flame stabilization, the onset of flame with repetitive extinction and ignition (FREI), and the global flame quenching in decreasing continuously combustion space. Furthermore, the effects of hydrogen addition on the flame temperature and the local equivalence ratio distribution were analyzed systematically using numerical simulations. In addition, the effects of hydrogen addition on the concentrations of OH and H radicals, and the critical scalar dissipation rate of local flame extinction were discussed. With a higher hydrogen ratio, the mixing is faster, and the flame is smaller. When the micro confined space is narrower, the heat loss to the combustor walls has a higher impact on the flames. The flames with higher hydrogen ratios have therefore lower peak flame temperatures and lower concentrations of H and OH radicals. The results show that hydrogen addition can effectively widen the stable combustion range of methane/air flames in the micro confined space by about 20% when the hydrogen addition ratio reaches 50%. The frequency and the maximum propagation velocity of FREI flames can be increased as well. The quenching distance of methane/hydrogen/air flames decreases nearly linearly with the increase of hydrogen ratio. This is attributed to the higher critical scalar dissipation rate of local flame extinction in flames with a higher hydrogen ratio. • Flame characteristics of CH 4 /Air with H 2 addition in the micro space were studied. • Adding H 2 can broaden the flame stabilization zone and reduce the quenching distance. • H 2 enhanced flame stabilization is due to the increased SDR of local flame extinction. • Hydrogen addition leads to a faster mixing of the fuel/air mixture. [ABSTRACT FROM AUTHOR]

Three-dimensional direct numerical simulation with detailed chemical kinetics of lean premixed CH 4 /air and H 2 /air flames at high Karlovitz numbers (Ka ∼ 1800) is carried out. It is found that the high intensity turbulence along with differential diffusion result in a much more rapid transport of H radicals from the reaction zone to the low temperature unburned mixtures (∼500 K) than that in laminar flamelets. The enhanced concentration of H radicals in the low temperature zone drastically increases the reaction rates of exothermic chain terminating reactions (e.g., H + O 2 +M = HO 2 + M in lean H 2 /air flames), which results in a significantly enhanced heat release rate at low temperatures. This effect is observed in both CH 4 /air and H 2 /air flames and locally, the heat release rate in the low temperature zone can exceed the peak heat release rate of a laminar flamelet. The effects of chemical kinetics and transport properties on the H 2 /air flame are investigated, from which it is concluded that the enhanced heat release rate in the low temperature zone is a convection–diffusion-reaction phenomenon, and to obtain it, detailed chemistry is essential and detailed transport is important. [ABSTRACT FROM AUTHOR]

Methane and hydrogen-enriched (25 vol% and 50 vol% H 2 -enriched CH 4) methane/air premixed flames were investigated in a gas turbine model combustor under atmospheric conditions. The flame operability ranges were mapped at different Reynold numbers (Re), showing the dependence on Re and H 2 concentrations. The effects of equivalence ratio (Φ), Re, and H 2 enrichment on flame structure were examined employing OH-PLIF measurement. For CH 4 /air cases, the flame was stabilized with an M shape; while for H 2 -enriched cases, the flame transitions to a П shape above a specific Φ. This transition was observed to influence significantly the flashback limits. The flame shape transition is most likely a result of H 2 enrichment, occurring due to the increase in flame speed, higher resistance of the flame to the strain rate, and change in the inner recirculation zone. Flow fields of CH 4 /air flames were compared between low and high Re cases employing high-speed PIV. The flashback events, led by two mechanisms (combustion-induced vortex breakdown, CIVB, and boundary-layer flashback, BLF), were observed and recorded using high-speed OH chemiluminescence imaging. It was found that the CIVB flashback occurred only for CH 4 flames with M shape, whereas the BLF occurs for all H 2 -enriched flames with П shape. • The premixed H 2 /CH 4 /air flames were studied in a burner with offset-cone swirler. • The effects of H 2 on flame structure and stability have been demonstrated. • Flame shape transition (M to П) occurred at certain Φ only for H 2 -enriched cases. • The П-shaped flames were found less prone to flashback. • Flashback driven by different mechanisms were observed for M- and П-shaped flames. [ABSTRACT FROM AUTHOR]

Hydrogen fuel cell electric vehicle (FCEV) can reduce air pollution as well as achieve efficient use of hydrogen energy. Farther travel distance requires larger hydrogen storage pressure, thereby imposing more demanding working conditions on the pressure reducing system. In this paper, a multi-stage high pressure reducing valve (MSHPRV) for hydrogen decompression in FCEV is proposed, and the effects of different structural parameters on its internal flow characteristics are investigated to achieve a better hydrogen decompression process. Results show that compared with perforated plate, multi-stage perforated sleeves and valve core hold the dominant position in hydrogen throttling process. Larger multi-stage perforated sleeve diameter, perforated plate diameter and pressure ratio relate to larger hydrogen kinetic energy, turbulence vortex and energy consumption. However, with the increase of perforated plate stage and perforated plate radius, the turbulent intensity and energy consumption inside MSHPRV decreases correspondingly. This study can provide some technical supports for achieving hydrogen decompression in FCEV when facing harsh working conditions, or help with dealing energy conversion during decompression process. • Demanding conditions are imposed on the pressure reducing system of hydrogen fuel cell electric vehicle. • Multi-stage high pressure reducing valve for hydrogen decompression is proposed. • Effects of different structural parameters on the internal flow characteristics are investigated. • A better hydrogen decompression process is achieved with the parametric results. [ABSTRACT FROM AUTHOR]

Abstract A comprehensive coarse grain model (CGM) is applied to simulation of biomass steam gasification in bubbling fluidized bed reactor. The CGM was evaluated by comparing the hydrodynamic behavior and heat transfer prediction with the results predicted using the discrete element method (DEM) and experimental data in a lab-scale fluidized bed furnace. CGM shows good performance and the computational time is significantly shorter than the DEM approach. The CGM is used to study the effects of different operating temperature and steam/biomass (S/B) ratio on the gasification process and product gas composition. The results show that higher temperature enhances the production of CO, and higher S/B ratio improves the production of H 2 , while it suppresses the production of CO. For the main product H 2 , the minimum relative error of CGM in comparison with experiment is 1%, the maximum relative error is less than 4%. For the total gas yield and H 2 gas yield, the maximum relative errors are less than 7%. The predicted concentration of different product gases is in good agreement with experimental data. CGM is shown to provide reliable prediction of the gasification process in fluidized bed furnace with considerably reduced computational time. Highlights • Computational time of Coarse Grain Method significantly shorter than that of Discrete Element Method. • In simulation of biomass gasification, maximal relative error of H 2 predicted by CGM is less than 4%. • Higher gasification temperature in favor of production of CO. • Higher gasification steam/biomass ratio in favor of production of H 2. [ABSTRACT FROM AUTHOR]

In this study, we report the facile fabrication of Sn-doped hematite film via mid-situ and ex-situ doping methods for efficient photoelectrochemical (PEC) performances. The morphology of Sn-doped Fe 2 O 3 films was varied with Sn precursor in the mid-situ doping process. The addition of SnCl 2 rendered a smooth-surfaced and well-distributed nanorod morphology, but SnCl 4 gave a deformed nanorod structure covered with layered coalescence of SnO 2 particles. The former demonstrated much higher photoelectrochemical performances as photoanodes than the latter. The photocurrent can be further improved by surface modification with SnCl 4 through spin-coating method. The effects of Sn precursors on the morphology, surface characteristics and the PEC properties of the photoanodes are investigated. [ABSTRACT FROM AUTHOR]

It is shown that ReaxFF molecular dynamics simulations (MDSs) can be used to study the high temperature gasification of glucose in supercritical water under microwave heating. At the early stage of gasification, yields of gas products have great difference under microwave and conventional heating models. It is considered that the difference results from the non-thermal effect of microwave. Therefore, simplified pathway model of free radical reactions is further constructed to study the influences of external electric fields on the yields of hydrogen gas, carbon monoxide and intermediate formaldehyde. Rotation characteristics of water and carbon monoxide electric dipoles are also studied at a high temperature. To analyze the impacts of external electric fields on the water-gas shift reactions, collision theory is introduced and several simplified collision models are constructed and detailed. Based on our studies, for the first time, it is concluded that using microwaves as heating source is feasible to the gasification of glucose in supercritical water, where no obvious thermal runaway is found. Moreover, the external microwave can increase the production of hydrogen gas through promoting the yields of glucose decomposed into formaldehyde and hydrogen free radical. [ABSTRACT FROM AUTHOR]

Solar hydrogen production via photoelectrochemical water splitting using hematite thin films is a promising approach for the chemical storage of solar energy. Nanorod structured hematite photoanodes provide a direct path for electron collection and hole transport, which have been attracting growing interest. In this study, efficient hematite thin films were synthesized though dissolution and recrystallization of FeOOH nanorods in phosphate solutions. It is found that the morphology and surface characteristic, and PEC photoelectrochemical properties of the hematite films are strongly affected by the aging conditions. The phosphate (NaH 2 PO 4 ) mediated hematite film exhibits 2.1 times higher photocurrent density than that of bare α -Fe 2 O 3 . The influences of phosphate solutions including Na 3 PO 4 , Na 2 HPO 4 and NaH 2 PO 4 , pH values, and aging time on the morphology regulation, isoelectric point, and PEC photoelectrochemical properties of the hematite films are investigated. [ABSTRACT FROM AUTHOR]

Ultrathin hematite films have been prepared on FTO using a separated two-phase hydrolysis–solvothermal reaction. The film thickness can be controlled by the number of deposition cycles. The Photoanode based on hematite film with 7 deposition cycles generated a photocurrent of 0.61 mA cm −2 at +1.5 V vs. RHE. The photocurrent can be improved by coating a layer of reducing graphene oxide (RGO) through spin coating. The enhanced performance is attributed to the promoting effects of RGO which acted as hole collector and transporter in the electrodes. This research demonstrates that graphene can be used as an alternative co-catalyst to enhance photoelectrochemical water splitting activity of ultrathin hematite film. The as-prepared photoanodes were notably stable under water oxidation conditions. [ABSTRACT FROM AUTHOR]

It's the first comparative transcriptome study of Tetraselmis subcordiformis' H 2 production with carbonyl cyanide m-chlorophenylhydrazone (CCCP) by de novo RNA-seq. The global analysis of the annotated genes and pathways unveiled it a good candidate for biofuels. Focusing on H 2 production, the genes coding active center subunit (HydA) and assembly subunits (HydEF and HydG) of hydrogenase proportionally expressed during the induction but stopped after illuminated; on contrast, the gene of the active centre stabilization subunit (Hyd3) highly expressed during the H 2 evolution, indicating its key role in the prolonged H 2 production. The results also showed genes of HCP4, ppGpp phosphohydrolase and RpoD relating to the hydrogenase expression regulation. On the energy aspect, genes of CF-ATPases and ferredoxin-NADP reductase in chloroplast down-regulated while Mito-ATPase in mitochondria and pyruvate metabolism pathway up-regulated during the H 2 production, consisting with our previous reports on physiological level. [ABSTRACT FROM AUTHOR]

Ultrathin hematite (α-Fe 2 O 3 ) film deposited on a TiO 2 underlayer as a photoanode for photoelectrochemical water splitting was described. The TiO 2 underlayer was coated on conductive fluorine-doped tin oxide (FTO) glass by spin coating. The hematite films were formed layer-by-layer by repeating the separated two-phase hydrolysis-solvothermal reaction of iron(III) acetylacetonate and aqueous ammonia. A photocurrent density of 0.683 mA cm −2 at +1.5 V vs. RHE (reversible hydrogen electrode) was obtained under visible light (>420 nm, 100 mW cm −2 ) illumination. The TiO 2 underlayer plays an important role in the formation of hematite film, acting as an intermediary to alleviate the dead layer effect and as a support of large surface areas to coat greater amounts of Fe 2 O 3 . The as-prepared photoanodes are notably stable and highly efficient for photoelectrochemical water splitting under visible light. This study provides a facile synthesis process for the controlled production of highly active ultrathin hematite film and a simple route for photocurrent enhancement using several photoanodes in tandem. [ABSTRACT FROM AUTHOR]

Abstract: Tetraselmis subcordiformis (formerly called Platymonas subcordiformis), a marine green alga, was previously demonstrated to photo-biologically produce hydrogen when treated with CCCP (Carbonyl Cyanide m-Chlorophenylhydrazone). The current results of our studies showed that there was a peak yield of hydrogen by T. subcordiformis at the stationary stage of its algal growth curve, suggesting that starvation of an essential element induced biochemical changes that enhanced hydrogen production by T. subcordiformis. Further investigation indicated that among nitrogen, sulphur and phosphorus (which are major components in the cultivation medium), nitrogen deprivation in the medium reduced the protein content inside the cells of T. subcordiformis, but increased the carbohydrate content over 4 times, resulting in 5.5 times increase in the hydrogen yield. [Copyright &y& Elsevier]