Hydrogenation Mechanisms of Mg during Reaction Ball
Magnesium hydrides are considered as the potential hydrogen storage materials. However, high work temperature, slow reaction kinetics and hard activation process limit the practical application of Mg-based hydrides system. Recently, the reaction ball milling (RBM) technique has been successfully introduced to prepare hydrogen storage materials [1, 2]. It combines the courses of sample preparation, activation and hydrogenation into one step. Pure Mg milled under the hydrogen atmosphere was studied by Gennari [3]. In his study, the metastable-MgH 2 was generated by the RBM process. A reduction of particle and crystallite size and an increase of specific surface were observed. Unfortunately, only half of Mg was transformed into hydrides and the crystallite size was still very big. Moreover, it took many hours to finish the hydriding process. Some researchers tried to improve the reaction efficiency between Mg and H 2 by addition of some second phases, such as FeTi 1.2 [4], TiO 2 [5], and amorphous TiMn 1.5 [6]. Due to the excellent kinetic properties, the reaction efficiency was greatly improved. However, few investigations have been directed to the mechanisms of hydriding process in the RBM process, especially with the existence of catalyst. Further investigation is much needed. 2 In this work, Ti 37.5 V 25 Cr 37.5 (bcc structure) alloy was used as catalyst due to good hydriding properties and high hydrogen storage capacity [7]. Ti 37.5 V 25 Cr 37.5 alloy was prepared using arc melting with electromagnetic stirring (EMS) in an argon atmosphere. The starting materials were of a purity of 99.9% and the alloy was turned and melted for four times to improve the homogeneity. Then the as-cast alloy was crushed into powder, which passed through a 0.5mm screen. Finally, the powder of the as-cast alloy (30wt. %) was mixed with Mg (purity>99%, 100 mesh) and then mechanically milled in a SPEX8000 machine. A cylindrical stainless steel vial was used with the stainless steel balls and the ball to powder ratio is 20:1. The vial was sealed by an O-ring and connected to a gas reservoir under an initial pressure of 1 MPa, as shown in Fig. 1 [8, 9]. The total volume of the system was 260 cm 3 and about 2.7 g material could be produced. The milling was carried out for the duration of 5 h. During the milling, small changes in the hydrogen pressure representing absorption of hydrogen by the sample were monitored …
Hydrogenation mechanisms of Mg during reaction ball milling
Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, Shenyang 110016, Peoples R China. Univ Sydney, Sch Aerosp Mech & Mechatron Engn, Ctr Adv Mat Technol, Sydney, NSW 2006, Australia.;Hu, YQ (reprint author), Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, Shenyang 110016, Peoples R China;huyeqi@imr.ac.cn
Full-scale measurements of SO2 gas phase concentrations and slurry compositions in a wet flue gas desulphurisation spray absorber
Abstract Two measurement campaigns were carried out at ENERGI E2's Asnaes Power plant, unit 5. The unit has a capacity of 620 MW e and is equipped with a wet flue gas desulphurisation (FGD) plant employing a counter-current spray absorber with five spray levels. In the first campaign, the power plant was firing Orimulsion ® with 2.85 wt% S resulting in a flue gas concentration of SO 2 exceeding 2000 ppmv. In the second campaign, the fuel applied was a low-S blended coal and the SO 2 concentration in the raw gas was around 400 ppmv. A novel probe for in situ sampling of gas phase concentrations in wet FGD spray absorbers was developed and applied for measuring axial profiles of the SO 2 gas phase concentrations in the absorber. The expected decrease in SO 2 concentrations along the height of the absorber was found in the spray section (from height 26.5 to 36.2 m) whereas the SO 2 concentration above the holding tank and below the gas inlet was quite low probably due to long local residence times in the region. Horizontal variations, due to somewhat different flow conditions near the column wall were investigated and the SO 2 concentrations were found to be higher near the wall. Measurements at different gross loads showed that the SO 2 gas phase concentration at a given position inside the absorber was roughly linearly related to the L/G ratio in the measuring interval. Turning off one of the lower spray levels, while burning coal with low S content, did not lower the overall removal efficiency of the absorber. However, the SO 2 gas phase concentration inside the lower part of the absorber was increased by a factor of 2–3. Measurements of slurry pH at different positions showed a decrease of approximately 0.5 units from the upper to the lower part of the absorber. The full-scale measurements provide a detailed set of experimental data for validation of mathematical models of a wet FGD spray absorber.