Kinetic study on nonisothermal dehydrogenation of TiH2 powders

The nonisothermal dehydrogenation of TiH₂ powders was studied using thermogravimetry and differential scanning calorimetry. The reaction model was established by estimating the activation energy. The results show the nonisothermal dehydrogenation occurred in a four-step process. The hydrogen released from the TiH_1.5∼2$\text{Ti}{\mathrm{H}}_{1.5\sim 2}$ phase in the first step, which led to the decrease of activation energy. The second step was derived from the formation of βH${\beta }_H$ in δ$\delta$ phase and the reaction model was Phase boundary reaction. In the third step, the hydrogen started to release from the βH${\beta }_H$ phase, and then the βH→αH${\beta }_{\mathrm{H}}\to {\alpha }_{\mathrm{H}}$ phase transformation happened. So the activation energy Eα$E_{\alpha }$ underwent a decrease followed by a quick increase. The fourth step corresponded to the formation of αH${\alpha }_H$ in βH${\beta }_H$ phase, and the slight oxidation resulted in the small fluctuation of activation energy.     

Kinetic characteristics of sodium borohydride formation when sodium meta-borate reacts with magnesium and hydrogen

Sodium borohydride is attracting considerable interests as a hydrogen storage medium. In this paper, we investigated the effects of hydrogen pressure, reaction temperature and transition metal addition on sodium borohydride synthesis by the reaction of sodium meta-borate with Mg and _H2${\mathrm{H}}_2$. It was found that higher _H2${\mathrm{H}}_2$ pressure was beneficial to _NaBH4${\mathrm{NaBH}}_4$ formation. The increase in reaction temperature first improved _NaBH4${\mathrm{NaBH}}_4$ formation kinetics but then impeded it when the temperature was raised to near the melting point of Mg. It was also found that some additions of transition metals such as Ni, Fe and Co in the _NaBO2+Mg+_H2${\mathrm{NaBO}}_2+\mathrm{Mg}+{\mathrm{H}}_2$ system promoted the _NaBH4${\mathrm{NaBH}}_4$ formation, but Cu addition showed little effect. The activation energy of the _NaBH4${\mathrm{NaBH}}_4$ formation from Mg, _NaBO2${\mathrm{NaBO}}_2$ and _H2${\mathrm{H}}_2$ was estimated to be 156.3 kJ/mol _NaBH4${\mathrm{NaBH}}_4$ according to Ozawa analysis method.     

H2 production with ultra-low CO selectivity via photocatalytic reforming of methanol on Au/TiO2 catalyst

_H2${\mathrm{H}}_2$ with ultra-low CO concentration was produced via photocatalytic reforming of methanol on Au/_TiO2$\mathrm{Au}/{\mathrm{TiO}}_2$ catalyst. The rate of _H2${\mathrm{H}}_2$ production is greatly increased when the gold particle size is reduced from 10 to smaller than 3 nm. The concentration of CO in _H2${\mathrm{H}}_2$ decreases with reducing the gold particle size of the catalyst. It is suggested that the by-product CO is mostly produced via decomposition of the intermediate formic acid species derived from methanol. The smaller gold particles possibly switch the HCOOH decomposition reaction mainly to _H2${\mathrm{H}}_2$ and _CO2${\mathrm{CO}}_2$ products while suppress the CO and _H2${\mathrm{H}}_2$O products. In addition, some CO may be oxidized to _CO2${\mathrm{CO}}_2$ by photogenerated oxidizing species at the perimeter interface between the small gold particles and _TiO2${\mathrm{TiO}}_2$ under photocatalytic condition.     

Second law analysis of forced convection in a circular duct for non-Newtonian fluids

The second law characteristics of fluid flow and heat transfer inside a circular duct under fully developed forced convection for non-Newtonian fluids are presented. Heat flux is kept constant at the duct wall. Analytical expressions for dimensionless entropy generation number (_NS$N_{\mathrm{S}}$), irreversibility distribution ratio (Φ  ), and Bejan number (Be$\mathit{Be}$) are obtained as functions of dimensionless radius (R$R$), Peclet number (Pe$\mathit{Pe}$), modified Eckert number (Ec$\mathit{Ec}$), Prandtl number (Pr), dimensionless temperature difference (Ω  ), and fluid index (m$m$ or n$n$). Spatial distributions of local and average entropy generation number, irreversibility ratio, and Bejan number are presented graphically. For a particular value of fluid index, n=1$n=1$ (or m=2$m=2$), the general entropy generation number expression for a non-Newtonian power-law fluid reduces to the expression for Newtonian fluid as expected. Furthermore, entropy generation minimization is applied to calculate an optimum fluid index (_nEGM$n_{\mathrm{EGM}}$). A correlation is proposed that calculates _nEGM$n_{\mathrm{EGM}}$as a function of group parameter (Ec×Pr/Ω$\mathit{Ec}\times \mathit{Pr}/\Omega$) and Peclet number (Pe$\mathit{Pe}$) within ±5% accuracy. Finally, for some selected fluid indices, the governing equations are solved numerically in order to obtain Nusselt number. It is observed that the numerically obtained asymptotic Nusselt number shows excellent agreement with the analytically obtained Nusselt number.     

CO2 reforming of CH4 by combination of thermal plasma and catalyst

Experiments on synthesis gas preparation from dry reforming of methane by carbon dioxide with thermal plasma only and cooperation of thermal plasma with commercial catalysts have been performed. In all experiments, nitrogen gas was used as the plasma gas to form a high-temperature jet injected into a tube reactor. A mixture of _CH4${\mathrm{CH}}_4$ and _CO2${\mathrm{CO}}_2$ was fed vertically into the jet. Both kinds of experiments were conducted in the same conditions, such as total flux of feed gases, the molar ratio of _CH4/_CO2${\mathrm{CH}}_4/{\mathrm{CO}}_2$, and the plasma power except with or without catalysts in the tube reactor. Higher conversion of _CH4${\mathrm{CH}}_4$ and _CO2${\mathrm{CO}}_2$, higher selectivity of _H2${\mathrm{H}}_2$ and CO, and higher specific energy of the process were achieved by thermal plasma with catalysts. For example, the conversions of _CH4${\mathrm{CH}}_4$ and _CO2${\mathrm{CO}}_2$ were high to 96.33% and 84.63%, and the selectivies of CO and _H2${\mathrm{H}}_2$ were also high to 91.99% and 74.23%, respectively. Both were 10–20%$10–20\%$ higher than those by thermal plasma only.     

Experimental investigation on the effect of natural gas composition on performance of autothermal reforming

This paper presents experimental study on catalytic autothermal reforming (ATR) of natural gas (NG) for hydrogen (_H2${\mathrm{H}}_2$) production over sulfide nickel catalyst supported on gamma alumina. The experiments are conducted on a cylindrical reactor of 30 mm in diameter and 200 mm in length with “simulated” NG of different composition under thermal-neutral conditions and fed with different molar air to fuel ratio (A/F$A/F$) and molar water to fuel ratio (W/F)$(W/F)$. The results showed that reforming performance is significantly dependent on A/F$A/F$, W/F$W/F$ and concentration of _C2+${\mathrm{C}}_{2+}$ hydrocarbons in inlet fuel. Fuels containing higher _C2+${\mathrm{C}}_{2+}$ hydrocarbons concentration have optimum performance in terms of more _H2${\mathrm{H}}_2$ at higher A/F$A/F$ and W/F$W/F$ but lower conversion efficiency. Good performance for ATR of fuel containing 15%–20% _C2H6${\mathrm{C}}_2{\mathrm{H}}_6$ can be achieved at A/F=5–7$A/F=5–7$ and W/F=4–6$W/F=4–6$, much higher than that for optimum performance of ATR of methane (A/F=3,W/F=2–2.5$A/F=3,W/F=2–2.5$). _CO2${\mathrm{CO}}_2$ in the inlet fuel does not have significant effect on the reversed water–gas shift reaction. Its effect on reforming performance is mainly due to the dilution of inlet fuel and products.     

Intake charge dilution effects on control of nitric oxide emission in a hydrogen fueled SI engine

Though hydrogen fueled spark ignition engine can operate at high thermal efficiency with almost zero emission of HC and CO, the high level of _NOx${\mathrm{NO}}_x$ poses problems. The high combustion temperature and lean mixtures used are the reasons. In this work, the effect of _N2${\mathrm{N}}_2$, _CO2${\mathrm{CO}}_2$ and hot EGR gas as diluents in the intake charge to suppress NO emission in a manifold injected hydrogen fueled SI engine was studied. Nitrogen as a diluent is not so effective at low loads while inducting smaller amounts, but very effective at higher loads where the mixture becomes richer and the dilution effect (oxygen depletion) is significant. On other hand, carbon dioxide is a good diluent with relatively better thermal effect and diluent effect and effectively controls NO emission at all output regions. However this is at the expense of thermal efficiency. Recirculating hot exhaust gas which contains both _N2${\mathrm{N}}_2$ and steam comes in between _N2${\mathrm{N}}_2$ and _CO2${\mathrm{CO}}_2$ in terms of its effectiveness. On the whole _N2${\mathrm{N}}_2$ is the most effective as it has minimum impact on thermal efficiency for a given level of permissible NO emission. Thus it is felt that cold EGR could be a good option. In all cases, a good control system is necessary to supply correct quantity of diluent.     

Hydrogen injection as additional fuel in gas turbine combustor. Evaluation of effects

Industrial gas turbines fuelled by fossil fuels have been used widely in power generation and combined heat and power for many years. However they have to meet severe _NOx${\mathrm{NO}}_x$, CO and _CO2${\mathrm{CO}}_2$ (greenhouse effect) emissions legislation in many countries. This paper reports a study on injection of small quantities of hydrogen in a hydrocarbon fuelled burner like additionally fuel to reduce the pollutants emissions. Hydrogen is injected in the primary zone, premixed with the air. Using this injection together lean primary zone, it is possible to reduce the _NOx${\mathrm{NO}}_x$ level while CO an HC levels remains approximately constant.     

Evaluation of conversion efficiency of light to hydrogen energy by Anabaena variabilis

Cyanobacteria provide an efficient system for producing _H2${\mathrm{H}}_2$ from water using solar energy. The energy conversion efficiency can be defined by the ratio of _H2${\mathrm{H}}_2$ produced to the light energy absorbed. An IR and opalescent plate method was used to measure the light energy absorbed. Since cyanobacteria absorb light in the visible range but not in the infrared range, the net amount of light energy absorbed by the cells can be estimated by measuring the IR and visible light intensities transmitted through the biochamber. A rectangular biochamber was used for measuring the conversion efficiency from light energy to _H2${\mathrm{H}}_2$ energy. A quantum meter and radiometer were used to measure the light intensity transmitted through the chamber. Anabaena variabilis was cultured in a BG11 medium with 3.6 mM NaNO₃${}_3$ and the light intensity was 40–50 μmol/m²/s$\mathrm{\mu }\mathrm{mol}/{\mathrm{m}}^2/\mathrm{s}$ in the growth phase and 120–140 μmol/m²/s$\mathrm{\mu }\mathrm{mol}/{\mathrm{m}}^2/\mathrm{s}$ in the _H2${\mathrm{H}}_2$ production phase. The maximum _H2${\mathrm{H}}_2$ production was 50 ml for 40 h and cell density was 1.2 g/l. The _H2${\mathrm{H}}_2$ production rate was 4.1 ml _H2/g${\mathrm{H}}_2/\mathrm{g}$ dry cell weight/h. Based on the light absorbed in the _H2${\mathrm{H}}_2$ production phase, the energy conversion efficiency from light to _H2${\mathrm{H}}_2$ was 1.5% on average and 3.9% at the maximum. Based on the light energy absorbed in the cell growth and _H2${\mathrm{H}}_2$ production phases, the energy conversion efficiency was 1.1% on average.     

Demonstration of sustained hydrogen photoproduction by immobilized,sulfur-deprived Chlamydomonas reinhardtii cells

It was demonstrated that immobilized, sulfur-deprived algal cultures can photoproduce H₂${}_2$. After identifying the optimal material and procedures for immobilization of Chlamyodomonas reinhardtii   at high cell density, we examined the effect of liquid mixing, sulfate content, acetate levels and light intensity on the H₂${}_2$-production activity of the culture. Our results indicate that (a) liquid mixing is important to provide homogeneous conditions for the immobilized culture; (b) sulfur deprivation is necessary for hydrogen production by immobilized cultures; and (c) high light intensity decreases H₂${}_2$ production. The maximum total volume of H₂${}_2$ produced by the system (160 ml of reactor volume) was 380 ml over 23 days, and the highest rate of H₂${}_2$ production observed was 45 ml day^-1${}^{-1}$. Cell immobilization significantly increased the duration of the H₂${}_2$-photoproduction phase (up to 4 weeks), maintained specific rates of H₂${}_2$ photoproduction similar to those of suspension cultures and showed potential for large increases in H₂${}_2$ production.     

Changes of hydrogen storage properties of MgH2 induced by heavy ion irradiation

In order to understand the influence of defect zones on desorption behavior of _MgH2${\mathrm{MgH}}_2$, Xe 120 keV ion irradiation of this material has been performed. DSC, SEM measurements, and SRIM calculations have been used to characterize induced modifications and its influence on the hydrogen desorption behavior of _MgH2${\mathrm{MgH}}_2$. We have demonstrated that the near-surface area of _MgH2${\mathrm{MgH}}_2$ plays the crucial role in hydrogen desorption kinetics. DSC analysis provides clear picture of vacancies influence on H diffusion and desorption in _MgH2${\mathrm{MgH}}_2$, and points out that there is possibility to control the thermodynamic parameters by controlled ion bombardment.