Nobuyoshi Nakagawa

Catalytic activity of Ni–YSZ–CeO2 anode for the steam reforming of methane in a direct internal-reforming solid oxide fuel cell

As one of the key technologies in the development of a direct internal-reforming solid oxide fuel cell, catalytic activity and stability of a Ni–YSZ–CeO2 anode on a zirconia electrolyte for the steam reforming of methane was investigated by experiments using a differential fuel cell reactor. The effects of the partial pressure of CH4, H2O and H2, and temperature as well as the electrochemical oxidation on the catalytic activity were analyzed. It was found that the catalytic activity of the Ni–YSZ–CeO2 anode was higher than that of the Ni–YSZ reported especially at low temperature. A deterioration of the catalytic activity of the anode was observed at low PH2 and high PH2O atmosphere, and also at high current densities. This might be caused by the oxidation of the Ni surface by H2O in the reaction gas and that produced by the anodic reaction. A rate equation for a fractional function for the steam reforming on open circuit was also proposed.

Characterization of gas fluidization regimes using pressure fluctuations

Flow regimes varying from bubbling fluidization to pneumatic transport were characterized by analyzing the differential pressure fluctuations. Two kinds of particles, typifying those from group A and group B, were employed in the experiments. It is found that, under all the operating conditions and at various axial positions, the standard deviation of the differential pressure fluctuations almost coincides, if the solids holdup is the same. For the two particles employed, the dimensionless standard deviations normalized by the maximum standard deviation at Uc showed a unique relationship with the solids holdup, giving a favorable characteristic that allows the determination of the boundaries between the flow regimes of gas-solids fluidization. Based on the experimental results, we find approximate values of: ϵsb > ϵs > 0.35, for bubbling fluidization; 0.15 < ϵs < 0.35, for turbulent fluidization; 0.05 < ϵs < 0.15 for fast fluidization; and ϵs < 0.05 for pneumatic transport.

Distinction between upward and downward flows in circulating fluidized beds

Abstract A simple momentum probe was used to distinguish between upward and downward flow regions in risers with diameters of 66, 97 and 150 mm. Experiments indicated that a typical core-annulus flow structure existed only above the inflection point corresponding to the axial voidage profile. In the lower dense region, due to extensive solids turbulence and mixing, no clear core-annulus flow structure was observed. In the core-annulus flow region, the dimensionless core radius increased gradually with increasing distance from the inflection point. The variations in the thickness of the downflow region with gas velocity, riser diameter and solids circulation rate were investigated. Analysis of the results indicated that the dimensionless core radii almost coincide for different operating conditions and axial positions, but at the same cross-sectional averaged solids holdup. An empirical correlation was therefore developed to describe the variation of core radius, which showed a favorable agreement between predicted and experimental data obtained by present and literature studies.

Flow structure in a fast fluidized bed

Flow structures were characterized in a 97 mm diameter by 3.0 m high circulating fluidized bed based on fluctuation behaviors of solids momentum. Measurements of time series of local solids momentum were carried out along the radial direction at both upper and lower parts of the bed for different operating conditions. By examining the probability distribution, standard deviations and power spectra, flow behavior in the upper dilute region was found to be much different from that in the bottom dense region. The results suggest that a core-annular structure does not always mean the flow with downflow of solids near the bed wall, it also includes the flow in which solids travel, on the time average, upwards rapidly in the dilute core region and much more slowly in the dense annular region. By changing the experimental conditions, both flow patterns were identified based on the measured signals from different radial positions. To further understand the completed hydrodynamics of gas-solids flow in the fast fluidized beds, a novel approach, based on the concept of fractals, was also adopted. Attempts were made to characterize the local flow properties by using the fractal dimension.

Contribution of the Internal Active Three‐Phase Zone of Ni‐Zirconia Cermet Anodes on the Electrode Performance of SOFCs

An extension of the effective reaction zone of the Ni-zirconia cermet anode in a solid oxide fuel cell was investigated. The reaction zone, the active three-phase zone (ATZ), of the cermet electrode consists of the zone that is formed on the flat interface between the solid electrolyte and the cermet layer, ATZ{sub 2D}, and another that extends three-dimensionally into the cermet layer, ATZ{sub 3D}, from the cermet/electrolyte interface. Anode performances of four types of cermet electrodes with zirconia powders of different ionic conductivities, but with a very similar microstructure, were compared, and the magnitude ratio of ATZ{sub 3D} and ATZ{sub 2D} was evaluated from the relationships between the performance and the ionic conductivity of zirconia in the cermet. For the anode of Ni-YSZ (yttria-stabilized zirconia, 8 mol % Y{sub 2}O{sub 3}) cermet on partially stabilized zirconia (3 mol % Y{sub 2}O{sub 3}) electrolyte, it was found that the contribution of ATZ{sub 3D} was almost 2.5 times larger than that of ATZ{sub 2D} at 1273 K, H{sub 2}-H{sub 2}O 2.4%.

Evaluation of the Effective Reaction Zone at Ni ( NiO ) / Zirconia Anode by Using an Electrode with a Novel Structure

The effect of nickel film thickness and the thickness of a porous ceramic layer, covering the Ni film, on anode polarization of a solid oxide electrolyte fuel cell was measured at around 1273 K. The electrode impedance was found to be dependent on the thickness of the porous layer but not on that of Ni film. DC polarization was affected by the thickness of neither film at relatively high partial pressure of hydrogen. The interface resistance, R i , obtained from the impedance plot consisted of a reaction resistance, R 1 , and a diffusion resistance, R 2 , caused by the porous ceramic layer. R 1 was proportional to P -1/2 H2O but was not affected by P H2 . These results suggested that the rate-determining process is an activation one and it takes place at very thin zone of the Ni film within 1 μm from the electrode/electrolyte interface. P H2 and P H2O dependencies of the current-voltage relation were similar to that obtained from a Ni patterned electrode and a Ni-YSZ cermet electrode by other researchers.

Catalytic activity of baker's yeast in a mediatorless microbial fuel cell

The catalytic activity of bakers yeast, Saccharomyces cerevisiae, as a biocatalyst was investigated in a mediatorless microbial fuel cell. The yeast cells that adhered on the anode surface were the active biocatalyst for glucose oxidation in a mediatorless biofuel cell, suggesting that the electron transfer took place through the surface confined species. The species in the anolyte solution including the dispersed yeast cells did not take a part in the electron transfer and thus in the power generation.

Hydrodynamic dependence on riser diameter for different particles in circulating fluidized beds

With two different kinds of particles and three different circulating fluidized (CFB) bed risers, the riser diameter influences on bed density for different particles were investigated by measuring the total pressure drop across the whole riser and the axial profile of apparent voidage. The risers had the same bed height of 3.0 m but different inner diameters of 66, 97 and 150 mm, respectively. Particles of FCC and silica sand were used, which belong to types A and B of Geldart classification, respectively. It was found that riser diameter has opposite influences for Geldart A and B particles on the total pressure drop across the whole riser and the differential pressure gradient (apparent solids concentration) at a specified bed elevation. With increasing riser diameter, these two pressure drops increase for group B but decrease for group A particles, consistent with several other literature measurements in pilot-scale risers. As a result of these different dependencies of bed density on riser diameter, the saturation-carrying capacity, i.e., the solids circulation rate at the dilute suspension collapse, was also found to vary differently with riser diameter for different particles. That is, the increase in riser diameter increases the saturation-carrying capacity for Geldart A but decreases it for Geldart B particles.

Anodic Polarization Related to the Ionic Conductivity of Zirconia at Ni-Zirconia/Zirconia Electrodes

To clarify the extension of the triple phase boundary (TPB), which directly affects the electrode polarization behavior, in the cermet layer of the Ni zirconia anode, we studied the anodic polarization properties of four types of Ni-zirconia cermets (Ni-0YZC Ni-3YZC, and Ni-8YZC) on three types of zirconia electrolytes (3YZE, 5YZE, and 8YZE). Based on impedance spectroscopy and current interruption measurements, the overall interfacial conductivity, I/R i was analyzed by dividing it into two increments conductivity due to the TPB at the surface of the electrolyte, (I/R i ) 2D , and that due to the active TPB inside the cermet. (I/R i ) 3D . It was found that (I/R i ) 2D shows a slight dependence, on the order of about 0.3, on the conductivity of the electrolyte, On the other hand. (I/R i ) 3D depended on the ionic conductivity of the zirconia in the cermet on the order of about 1.3, These results were explained by the theory deduced for the case that the active TPB expanded into the cermet layer.

Hydropyrolysis of coal in a powder-particle fluidized bed

Abstract Coal was pyrolysed in hydrogen in a powder-particle fluidized bed at atmospheric pressure. The coal powder was continuously fed into the bed, in which CoMo Al 2 O 3 catalyst particles or silica sand particles were fluidized. The effect of silica sand on the secondary reactions of volatile matter was quite small, whereas CoMo Al 2 O 3 showed high activity for the cracking of tarry materials. In the presence of CoMo Al 2 O 3 , a hydrocarbon liquid with a narrow product distribution was obtained; the main components were light aromatic hydrocarbons such as BTX and naphthalene. The product composition was quite sensitive to the pyrolysis temperature. The yields of light aromatic hydrocarbons with CoMo Al 2 O 3 increased with temperature to 590 °C and then sharply decreased above 600 °C. Maximum yields of light aromatic hydrocarbons, 7.2wt% (BTX 5.8 wt%, naphthalene 1.4 wt%), ~30 times those with silica sand, were obtained at 590 °C with a bed height of 10 cm. The yields of hydrocarbon gases increased with temperature; the CH4 yield was 32 wt% at 650 °C.