Ni-based catalysts for reforming of methane with CO2

Ni catalysts supported on different carriers like δ,θ-Al₂O₃, MgAl₂O₄, SiO₂–Al₂O₃ and ZrO₂–Al₂O₃ were prepared. The solids were characterized by chemical analysis, N₂ adsorption–desorption isotherms, X-ray powder diffraction, UV–vis diffuse reflectance spectroscopy, temperature-programmed reduction, high-resolution transmission electron microscopy and temperature-programmed oxidation. The catalytic properties of the samples were evaluated in the reaction of reforming of methane with CO₂ at 923 K. It was shown that this kind of support greatly affects the structure and catalytic performance of the catalysts. Ni catalyst supported on MgAl₂O₄ showed the highest activity and stability due to the presence of small well dispersed Ni particles with size of 5.1 nm. It was shown that the lowest activity of Ni catalyst supported on SiO₂–Al₂O₃ oxide was caused by the agglomeration of nickel particles and formation of filamentous carbon under reaction conditions detected by the high resolution transmission electron microscopy.     

A novel TiO2/PVC layer for use in a photoelectrochemical cell

Titanium dioxide (TiO₂) has been widely used with UV light to degrade organic waste contaminants. Immobilised layers of TiO₂ on electrode surfaces have shown enhanced activity when appropriate potentials have been applied. In this work, it is shown that a novel immobilised layer of TiO₂ on an electrode, a TiO₂/poly(vinylchloride) composite cast from THF, mineralises acetone or starch when exposed to a xenon arc light only if the electrode is connected to a Pt electrode where concomitant reduction of oxygen occurs. When an isolated electrode with an immobilised TiO₂ layer is exposed to UV light in a solution of starch or acetone, no decrease in acetone or starch concentration is observed.     

Thin film Pt/TiO2 catalysts for the polymer electrolyte fuel cell

Thin film Pt/TiO₂ catalysts are evaluated in a polymer electrolyte electrochemical cell. Individual thin films of Pt and TiO₂, and bilayers of them, were deposited directly on Nafion membranes by thermal evaporation with varying deposition order and thickness (Pt loadings of 3–6 μg cm⁻²). Structural and chemical characterization was performed by transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Oxygen reduction reaction (ORR) polarization plots show that the presence of a thin TiO₂ layer between the platinum and the Nafion increases the performance compared to a Pt film deposited directly on Nafion. Based on the TEM analysis, we attribute this improvement to a better dispersion of Pt on TiO₂ compared to on Nafion and in addition, substantial proton conduction through the thin TiO₂ layer. It is also shown that deposition order and the film thickness affects the performance.     

Japanese potential of CO2 sequestration in coal seams

As a reduction strategy for global warming by green-house gases underground storage or sequestration of CO₂ into coal beds or seams has been studied by the Japanese government and some associated organizations.     

Hydrogen storage in carbon fibers activated with supercritical CO2: Models and the importance of porosity

In this work we studied the adsorption of H₂ at 77 K and 0.0–0.12 MPa onto carbon fibers activated with supercritical CO₂ (ACFs) and with different burn-offs (10–53%). The highest amount of H₂ stored was 2.45 wt% in an ACF with a burn-off of 51% at 0.12 MPa. The measured isotherms were analyzed using an equilibrium model derived by analogy with a multiple-site Langmuir-type adsorption model. The different equilibria correspond to adsorption in pores of different sizes. The experimental results fitted a model with two different adsorption sites satisfactorily, allowing such sites to be related to the microporous structure of the ACFs. Thus, a high-energy adsorbent–adsorbate interaction site, associated with very small micropores, accessible only to very small molecules such as H₂, and another lower-energy site associated with larger pores can be proposed. The model also predicts the adsorption behavior under equilibrium conditions at higher pressures, allowing the maximum adsorption capacity of the ACFs to be determined. The results show that the ACFs adsorb most of the H₂ molecules at low equilibrium pressures, and that they become almost saturated at pressures around 1.0 MPa. The maximum H₂ storage capacity in these ACFs lies between 1.50 and 3.15 wt%.     

Silicon modified ultrafiltration-based proton-conductive membranes with improved performance for H2/Cl2 fuel cell application

Ultrafiltration (UF)-based proton-conductive membranes, which comprised nanosize SiO₂, polyethersulfone and aqueous acid absorbed, as an alternative to traditional ion exchange membranes, were first proposed and successfully prepared for H₂/Cl₂ fuel cell. Various membranes were prepared with different weight fractions of SiO₂ nanoparticles. The effect of silicon content on the performance of membranes was characterized. The ionic conductivity of a membrane doped with 3 M hydrochloric acid increased with the silicon content and reached 0.150 S cm⁻¹ at 15 wt.% SiO₂. A non-optimized H₂/Cl₂ fuel cell assembled with the modified UF membrane (115 μm thick) exhibited better performance than that with Nafion 115 membrane. It demonstrated that 12.67% and 55.03% improved at 10 wt.% and 15 wt.% SiO₂, respectively. The study provides an effective way to fabricate high performance porous membranes for H₂/Cl₂ fuel cell application.     

Technologies for increasing CO2 concentration in exhaust gas from natural gas-fired power production with post-combustion, amine-based CO2 capture

Enhanced CO₂ concentration in exhaust gas is regarded as a potentially effective method to reduce the high electrical efficiency penalty caused by CO₂ chemical absorption in post-combustion capture systems. The present work evaluates the effect of increasing CO₂ concentration in the exhaust gas of gas turbine based power plant by four different methods: exhaust gas recirculation (EGR), humidification (EvGT), supplementary firing (SFC) and external firing (EFC). Efforts have been focused on the impacts on cycle efficiency, combustion, gas turbine components, and cost. The results show that the combined cycle with EGR has the capability to change the molar fraction of CO₂ with the largest range, from 3.8 mol% to at least 10 mol%, and with the highest electrical efficiency. The EvGT cycle has relatively low additional cost impact as it does not require any bottoming cycle. The externally fired method was found to have the minimum impacts on both combustion and turbomachinery.     

An integrated power generation system combining solid oxide fuel cell and oxy-fuel combustion for high performance and CO2 capture

An integrated power generation system combining solid oxide fuel cell (SOFC) and oxy-fuel combustion technology is proposed. The system is revised from a pressurized SOFC-gas turbine hybrid system to capture CO₂ almost completely while maintaining high efficiency. The system consists of SOFC, gas turbine, oxy-combustion bottoming cycle, and CO₂ capture and compression process. An ion transport membrane (ITM) is used to separate oxygen from the cathode exit air. The fuel cell operates at an elevated pressure to facilitate the use of the ITM, which requires high pressure and temperature. The remaining fuel at the SOFC anode exit is completely burned with oxygen at the oxy-combustor. Almost all of the CO₂ generated during the reforming process of the SOFC and at the oxy-fuel combustor is extracted from the condenser of the oxy-combustion cycle. The oxygen-depleted high pressure air from the SOFC cathode expands at the gas turbine. Therefore, the expander of the oxy-combustion cycle and the gas turbine provides additional power output. The two major design variables (steam expander inlet temperature and condenser pressure) of the oxy-fuel combustion system are determined through parametric analysis. There exists an optimal condenser pressure (below atmospheric pressure) in terms of global energy efficiency considering both the system power output and CO₂ compression power consumption. It was shown that the integrated system can be designed to have almost equivalent system efficiency as the simple SOFC-gas turbine hybrid system. With the voltage of 0.752 V at the SOFC operating at 900 °C and 8 bar, system efficiency over 69.2% is predicted. Efficiency penalty due to the CO₂ capture and compression up to 150 bar is around 6.1%.     

Critical quality prediction for saturated flow boiling of CO2 in horizontal small diameter tubes

The dryout for flow boiling carbon dioxide (CO₂) in horizontal small diameter tubes is investigated through experiment and theoretical modeling. Tests are conducted in conditions where the saturation temperature is 0, 5, and 10 °C, heat flux is 7.2-48.1 kW/m² and mass flux is 500-3000 kg/m² s. The dryout phenomena of CO₂ are similar with those of water in many respects, while the effects of mass flux on dryout show differences among them. The dryout of CO₂ is predicted by a theoretical dryout model, which is developed and verified with steam-water data. Two entrainment mechanisms of interface deformation and bubble bursting are considered in the model and dryout is determined when the liquid film thickness is less than the critical liquid film thickness, the criteria film thickness of dryout. The present model well predicts the experimental critical qualities except when mass flux is relatively high, at which the deposition of liquid droplet on the liquid film and the occurrence of dryout patches become very significant.     

LaNi4.5Al0.5 alloy doped with Au used as anodic materials in a borohydride fuel cell

Fuel cells using borohydride as the fuel have received much attention because of their high thermodynamic cell voltage. Using rare-earth hydrogen storage alloys as the anodic catalyst materials instead of noble metals showed high catalytic activity both in the electrochemical oxidation and the hydrolysis of borohydride. In this work, we doped Au to modify the surface structure of LaNi_4.5Al_0.5 alloy by a self-reduction reaction method. The surface of the alloy particles was evenly covered with Au after treatment. The largest discharge current density increased from about 150 mA cm⁻² (discharge to −0.6 V versus Hg/HgO electrode) with the parent alloy to 250 mA cm⁻² with the Au-doped alloy. This finding suggested that the electrochemical catalytic activity of the alloy was enhanced after modification with Au. Fuel utilization also increased after modification with Au.     

Development of a new forced convection heat transfer correlation for CO2 in both heating and cooling modes at supercritical pressures

Experimental and numerical investigations on forced convection heat transfer of carbon dioxide at supercritical pressures in a prototypic printed circuit heat exchanger under both cooling and heating conditions have been performed in this present study. The experiment test section has nine semi-circular channels with a hydraulic diameter of 1.16 mm and a length of 0.5 m. Primary operational parameters include inlet pressure of 7.5–10 MPa, mass fluxes of 326 kg/m² s and 762 kg/m² s, inlet temperatures from 10 °C to 90 °C and the average heat flux was 30 kW/m². Beyond reproducing the regular experimental cases, numerical modeling also implemented higher heat fluxes of 60 kW/m² and 90 kW/m² in order to investigate the effect of heat flux. Good agreement was found between the experiments and FLUENT simulations using an SST k–w model with the near-wall region being completely resolved. The distinctive behavior of convection heat transfer at supercritical pressures between heating and cooling modes was systematically analyzed. A more physically reasonable property-averaging technique, Probability Density Function (PDF)-based time-averaged property, was developed to account for the effect of nonlinear dependency of properties on instantaneous local temperature. Furthermore, experimental and computational data were compared to empirical predictions by the Dittus–Boelter and Jackson correlations. The results showed that Dittus–Boelter correlation has better precision for the average value of the predicted heat transfer coefficient but cannot take account of the effect of heat flux. In contrast, the Jackson correlation, with property ratio correction terms to account for the distribution of the properties in the radial direction, could predict the distinction of heat transfer characteristics under heating and cooling conditions. However, it overestimates the average value of heat transfer coefficient in the whole range of the experiment conditions. Finally, a new correlation evaluated by PDF-based time-averaged properties for forced convection heat transfer of CO₂ in both heating and cooling mode at supercritical pressures was developed. Comparison of experimental and computational data with the prediction results by the new developed correlation reveals that it works quite well; i.e., more than 90% data in either heating or cooling mode with various heat fluxes are predicted within an accuracy of ±25%.     

Nanoporous SnO2 electrodes for dye-sensitized solar cells: improved cell performance by the synthesis of 18 nm SnO2 colloids

This paper reports on the synthesis of SnO₂ 18 nm diameter colloidal suspension for the fabrication of nanoporous electrodes. The new suspension allows the fabrication of thick and homogeneous electrodes by simple one layer spreading; in contrast to the successive spin coating of the commonly used commercial suspension that results in a thin inhomogeneous electrode. When used in dye-sensitized solar cells, the new electrodes increase the light-to-energy conversion efficiency by a factor of 2.1 in comparison with standard commercial suspension based electrodes. The improvement is mostly the result of an increase of the photocurrent. This increase is attributed to the better electrolyte migration, and presumably also to an increase of the photoinjected electron diffusion rate in the electrode.     

Decomposition analysis and mitigation strategies of CO2 emissions from energy consumption in South Korea

Energy-related CO₂ emissions in South Korea have increased substantially, outpacing those of Organisation for Economic Co-operation and Development (OECD) countries since 1990. To mitigate CO₂ emissions in South Korea, we need to understand the main contributing factors to rising CO₂ levels as part of the effort toward developing targeted policies. This paper aims to analyze the specific trends and influencing factors that have caused changes in emissions patterns in South Korea over a 15-year period. To this end, we employed the Log Mean Divisia index method with five energy consumption sectors and seven sub-sectors in terms of fuel mix (FM), energy intensity (EI), structural change (SC) and economic growth (EG). The results showed that EG was a dominant explanation for the increase in CO₂ emissions in all of the sectors. The results also demonstrated that FM causes CO₂ reduction across the array of sectors with the exception of the energy supply sector. CO₂ reduction as a function of SC was also observed in manufacturing, services and residential sectors. Furthermore, EI was an important driver of CO₂ reduction in most sectors except for several manufacturing sub-sectors. Based on these findings, it appears that South Korea should implement climate change policies that consider the specific influential factors associated with increasing CO₂ emissions in each sector.     

Improvement in dye-sensitized solar cells employing TiO2 electrodes coated with Al2O3 by reactive direct current magnetron sputtering

A novel surface modification method was carried out by reactive dc magnetron sputtering to fabricate TiO₂ electrodes coated with Al₂O₃ for improving the performance of dye-sensitized solar cells (DSSCs). The Al₂O₃-coated TiO₂ electrodes had been characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), UV–vis spectrophotometer, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The study results revealed that the modification to TiO₂ increases dye absorption amount, reduces trap sites on TiO₂, and suppresses interfacial recombination. The impact of sputtering time on photoelectric performance of DSSCs was investigated. Sputtering Al₂O₃ for 4 min on 5-μm thick TiO₂ greatly improves all cell parameters, resulting in enhancing the conversion efficiency from 3.93% to 5.91%. Further increasing sputtering time decreases conversion efficiency.     

Parametric optimization design for supercritical CO2 power cycle using genetic algorithm and artificial neural network

Supercritical CO₂ power cycle shows a high potential to recover low-grade waste heat due to its better temperature glide matching between heat source and working fluid in the heat recovery vapor generator (HRVG). Parametric analysis and exergy analysis are conducted to examine the effects of thermodynamic parameters on the cycle performance and exergy destruction in each component. The thermodynamic parameters of the supercritical CO₂ power cycle is optimized with exergy efficiency as an objective function by means of genetic algorithm (GA) under the given waste heat condition. An artificial neural network (ANN) with the multi-layer feed-forward network type and back-propagation training is used to achieve parametric optimization design rapidly. It is shown that the key thermodynamic parameters, such as turbine inlet pressure, turbine inlet temperature and environment temperature have significant effects on the performance of the supercritical CO₂ power cycle and exergy destruction in each component. It is also shown that the optimum thermodynamic parameters of supercritical CO₂ power cycle can be predicted with good accuracy using artificial neural network under variable waste heat conditions.     

Empirical study on the environmental Kuznets curve for CO2 in France: The role of nuclear energy

This paper attempts to estimate the environmental Kuznets curve (EKC) in the case of France by taking the role of nuclear energy in electricity production into account. We adopt the autoregressive distributed lag (ARDL) approach to cointegration as the estimation method. Additionally, we examine the stability of the estimated models and investigate the Granger causality relationships between the variables in the system. The results from our estimation provide evidence supporting the EKC hypothesis, and the estimated models are shown to be stable over the sample period. The uni-direction running from other variables to CO₂ emissions are confirmed from the casualty tests. Specifically, the uni-directional causality relationship running from nuclear energy to CO₂ emissions statistically provides evidence on the important role of nuclear energy in reducing CO₂ emissions.     

Stabilized zirconia-based sensor utilizing SnO2-based sensing electrode with an integrated Cr2O3 catalyst layer for sensitive and selective detection of hydrogen

A highly selective hydrogen (H₂) sensor has been successfully developed by using an yttria-stabilized zirconia (YSZ)-based mixed-potential-type sensor utilizing SnO₂ (+30 wt.% YSZ) sensing electrode (SE) with an intermediate Al₂O₃ barrier layer which was coated with a catalyst layer of Cr₂O₃. The sensor utilizing SnO₂ (+30 wt.% YSZ)-SE was found to be capable of detecting H₂ and propene (C₃H₆) sensitively at 550 °C. In order to enhance the selectivity towards H₂, a selective C₃H₆ oxidation catalyst was employed to minimize unwanted responses caused by interfering gases. Among the examined metal oxides, Cr₂O₃ facilitated the selective oxidation of C₃H₆. However, the addition or lamination of Cr₂O₃ to SnO₂ (+30 wt.% YSZ)-SE was found to diminish the sensing responses to all examined gases. Therefore, an intermediate layer of Al₂O₃ was sandwiched between the SE layer and the catalyst layer to prevent the penetration of Cr₂O₃ particles into the SE layer. The sensor using SnO₂ (+30 wt.% YSZ)-SE coated with a catalyst layer of Cr₂O₃ as well as an intermediate layer of Al₂O₃ exhibited a sensitive response toward H₂, with only minor responses toward other examined gases at 550 °C under humid conditions (21 vol.% O₂ and 1.35 vol.% H₂O in N₂ balance). A linear relationship was observed between sensitivity and H₂ concentration in the range of 20–800 ppm on a logarithmic scale. The results of sensing performance evaluation and polarization curve measurements indicate that the sensing mechanism is based on the mixed-potential model.     

Atomic layer deposition assisted Pt-SnO2 hybrid catalysts on nitrogen-doped CNTs with enhanced electrocatalytic activities for low temperature fuel cells

Nitrogen doped carbon nanotubes (CN_x) of a high nitrogen concentration were synthesized directly on carbon paper as the skeleton of a 3D composite electrode. Ultra-fine SnO₂ nanoparticles about 1.5 nm were deposited on CN_x with atomic layer deposition (ALD) technique. Pt nanoparticles from 1.5 to 4 nm were deposited on CN_x/carbon paper and SnO₂/CN_x/carbon paper with ethylene glycol reduction method. Three dimensional Pt/CN_x/carbon paper and Pt-SnO₂/CN_x/carbon paper composite electrodes were obtained, respectively. They were characterized over oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) for low temperature fuel cells. With similar sizes of Pt nanoparticles, the electrochemical surface area (ECSA) of Pt-SnO₂/CN_x/carbon paper is larger than that of Pt/CN_x/carbon paper. Pre-deposited SnO₂ nanoparticles promote the electrocatalytic activity of Pt toward ORR, carbon monoxide (CO) stripping and MOR. The underlying mechanisms for the enhanced activities are discussed.     

Numerical simulation of density-driven natural convection in porous media with application for CO2 injection projects

In this paper we investigate the mass transfer of CO₂ injected into a homogenous (sub)-surface porous formation saturated with a liquid. In almost all cases of practical interest CO₂ is present on top of the liquid. Therefore, we perform our analysis to a porous medium that is impermeable from sides and that is exposed to CO₂ at the top. For this configuration density-driven natural convection enhances the mass transfer rate of CO₂ into the initially stagnant liquid. The analysis is done numerically using mass and momentum conservation laws and diffusion of CO₂ into the liquid. The effects of aspect ratio and the Rayleigh number, which is dependent on the characteristics of the porous medium and fluid properties, are studied. This configuration leads to an unstable flow process. Numerical computations do not show natural convection effects for homogeneous initial conditions. Therefore a sinusoidal perturbation is added for the initial top boundary condition. It is found that the mass transfer increases and concentration front moves faster with increasing Rayleigh number. The results of this paper have implications in enhanced oil recovery and CO₂ sequestration in aquifers.     

Boiling coal in water: Hydrogen production and power generation system with zero net CO2 emission based on coal and supercritical water gasification

Coal is the single most important fuel for power generation today. Nowadays, most coal is consumed by means of “burning coal in air” and pollutants such as NO_x, SO_x, CO₂, PM2.5 etc. are inevitably formed and mixed with excessive amount of inner gases, so the pollutant emission reduction system is complicated and the cost is high. IGCC is promising because coal is gasified before utilization. However, the coal gasifier mostly operates in gas environments, so special equipments are needed for the purification of the raw gas and CO₂ emission reduction. Coal and supercritical water gasification process is another promising way to convert coal efficiently and cleanly to H₂ and pure CO₂. The gasification process is referred to as “boiling coal in water” and pollutants containing S and N deposit as solid residual and can be discharged from the gasifier. A novel thermodynamics cycle power generation system was proposed by us in State Key Laboratory of Multiphase Flow in Power Engineering (SKLMFPE) of Xi'an jiaotong University (XJTU), which is based on coal and supercritical water gasification and multi-staged steam turbine reheated by hydrogen combustion. It is characterized by its high coal-electricity efficiency, zero net CO₂ emission and no pollutants. A series of experimental devices from quartz tube system to a pilot scale have been established to realize the complete gasification of coal in SKLMFPE. It proved the prospects of coal and supercritical water gasification process and the novel thermodynamics cycle power generation system.