Jun-ichiro Yagi

Storage of thermal energy for effective use of waste heat from industries

Abstract Energy saving is one of the most effective strategies to protect the global environmental conditions. At present, considerable amount of waste heat is emitted from metallurgical and chemical industries, which can be used not only for municipal purposes but also for industries if recovered. In this paper, fundamental studies on heat transfer was conducted for developing a heat storage process by latent heat for recovering the high temperature waste heat over 500 K. Heat transfer experiments were attempted for a single encapsulated phase change materials and for a packed bed. Six different materials were tested as PCM from the points of view of high energy density storage, chemical stability, non-toxicity and cost performance. Those were two inorganic and four metallic materials. A single capsule containing PCM was heated for heat storage and then cooled for heat release in nitrogen gas stream where convective heat transfer was predominant. The metal PCMs were found to be excellent for heat storage because of uniform temperature in the capsule. Heat transfer simulation was also conducted for a packed bed process of spherical capsules providing the fundamental informations on the optimal design. As a result, concurrent flow for heat storage and release showed better result for effective use of storaged heat than counter-current flow. Exergy efficiency was also evaluated.

Multi-dimensional transient mathematical simulator of blast furnace process based on multi-fluid and kinetic theories

The ironmaking blast furnace is regarded as one of the biggest and most complex industrial reactors, because it includes various materials like gas, lump granular materials, liquids and powders and more than 30 major reactions and phase changes in a single reaction vessel. The mathematical simulator of this process developed in this study used the multi-fluid treatment as its framework, since the motions of these materials were governed by different flow mechanisms. The rates of the interactions among the phases and the chemical reactions were evaluated based on kinetic theories. The model successfully reproduced the fields of velocity, temperature and reaction in the furnace and its validity was confirmed. The simulator was also applied to a novel operation, namely the top gas recycling combined with the carbon-composite iron-ore charging and the waste plastics injection, and the advantages in furnace efficiency and environmental load were quantitatively indicated.

Activation behaviors of Mg2NiH4 at different hydrogen pressures in hydriding combustion synthesis

Abstract For the industrialization of the process of hydriding combustion synthesis of Mg2NiH4, the activation behaviors of Mg2NiH4 at different hydrogen pressures were measured by means of DSC (differential scanning calorimeter) and XRD (X-ray diffraction). The profiles of heat flow of DSC were very different at 0.5, 1.0, 2.0 and 4.0MPa hydrogen pressures within three cycles of temperature scanning at a rate of 0.1 K / s in hydriding combustion synthesis of Mg2NiH4. It is apparent that an activation behavior existed in the hydriding combustion synthesis since the height of DSC peak from the hydriding reaction of Mg2NiH4 increased with the cycling of temperature scanning. The pressure of hydrogen strongly affected this kind of an activation. The peak height increased as hydrogen pressure increased. It reached maximum values after the second cycle of temperature scanning at 4.0MPa of hydrogen pressure and after the third cycle at 2.0MPa. The patterns of XRD revealed that pure products of Mg2NiH4 were obtained after the third cycle of temperature scanning at 2.0MPa of hydrogen pressure and after the second cycle at 4.0MPa. These results are significant for developing the industrial process of hydriding combustion synthesis of hydrogen storage alloy Mg2NiH4 within only one temperature scanning without any activation process.

Reaction mechanism of hydriding combustion synthesis of Mg2NiH4

Abstract A previous study suggested that the process of combustion synthesis of Mg 2 NiH 4 from the compact of magnesium and nickel mixture in pressurized hydrogen cannot be expressed by a single reaction formula such as 2Mg+Ni+2H 2 =Mg 2 NiH 4 . In this paper, our attention is addressed to studying the reaction mechanism during the process of hydriding combustion synthesis of Mg 2 NiH 4 by means of a differential scanning calorimeter (DSC) and an X-ray diffractometer (XRD). The results show that this process is consisted of seven distinguished reactions: (1) Mg+H 2 →MgH 2 ; (2) MgH 2 →Mg+H 2 ; (3) 2Mg+Ni→Mg 2 Ni; (4) 2Mg+Ni→Mg 2 Ni (L); (5) Mg 2 Ni+0.15H 2 →Mg 2 NiH 0.3 ; (6) Mg 2 Ni+2H 2 →Mg 2 NiH 4 (HT); and (7) Mg 2 NiH 4 (HT)→Mg 2 NiH 4 (LT).

Activity and capacity of hydrogen storage alloy Mg2NiH4 produced by hydriding combustion synthesis

Abstract A high activity and a large capacity of hydrogen storage alloy Mg 2 NiH 4 produced by hydriding combustion synthesis were investigated by means of differential scanning calorimeter (DSC) and pressure-composition isotherms (PCT). The results showed that the product of Mg 2 NiH 4 from the hydriding combustion synthesis has enough activity to the hydriding reaction and the amount of hydrogen absorbed by the product reached the maximum (3.4–3.6%) mass near the theoretical value just after synthesis without any activation process. This kind of activity and capacity of hydrogen storage are not only stable, but also tolerant of high temperature of 850 K. PCT results give some phenomenon of three phases existing in isotherms. The relationships between the equilibrium plateau pressure and the temperature were Log P (0.1 MPa)=−3525/ T +6.667 for hydriding and Log P (0.1 MPa)=−3724/ T +6.883 for dehydriding.

Hydrogen storage alloy of Mg2NiH4 hydride produced by hydriding combustion synthesis from powder of mixture metal

As previous results, we reported that hydrogen storage alloy of Mg2NiH4 was produced by process of hydriding combustion synthesis in laboratory scale and regardless of the compressive pressure of the compacts from 140 MPa, 280 MPa, 550 MPa to 1.10 GPa. In present work, at first, we study the effect of the hydrogen atmosphere on the synthesis of Mg2NiH4 by this process. It is confirmed that the hydrogen atmosphere in the heating period plays an important role to the synthesis of Mg2NiH4 in the cooling period due to the expansion of the compact in the heating period resulted from the hydriding reaction of Mg+H2→MgH2. Then, we use powder of the mixture metal of magnesium and nickel as raw material directly and use lower pressures of hydrogen from 2.0 MPa, 1.0 MPa and to 0.5 MPa to produce Mg2NiH4. It is found that pure Mg2NiH4 from X-ray diffraction (XRD) analysis results can be produced at only 1.0 MPa hydrogen atmosphere when powder of mixture metal was used, which is much lower than that of 4.0 MPa when compact of mixture metal was used.

Reaction rate of combustion synthesis of an intermetallic compound

Abstract The reaction kinetics of the combustion synthesis of Mg 2 Ni from a powder mixture of magnesium and nickel has been investigated. The physical and chemical changes of the samples during the combustion synthesis were followed by scanning electron microscopy (SEM), electron probe X-ray microanalysis (EPMA), X-ray diffraction (XRD), and differential scanning calorimetry (DSC) to derive a suitable rate equation. The observations revealed that the synthesis of Mg 2 Ni progresses homogeneously rather than topochemically, and not through an intermediate phase, with acceleration by liquid generation of the eutectoid phase. The result was that the most reasonable expression is a second-order irreversible equation, k (1− f ) 2 , with an activation energy of 165 kJ/mol.

Hydriding and dehydriding behavior of the product in hydriding combustion synthesis of Mg2NiH4

Abstract The hydriding and dehydriding behavior of the product of hydriding combustion synthesis of Mg 2 NiH 4 was investigated by TG-DTA. During seven heating (dehydriding) and cooling (hydriding) cycles at a rate of 0.1 K/s, the TG-DTA curves of the combustion synthesis product were measured in 1.0 MPa hydrogen atmosphere. Behavior such as activation within the first four hydriding and dehydriding cycles was observed. The hydriding and dehydriding reactions gradually became more rapid from the first to the fourth cycle, and became almost constant after the fourth cycle. This behavior is likely an activation process. It is much easier with the present hydriding combustion synthesis than with the conventional ingot process. The splitting and overlapping of the peaks of the DTA curves for the dehydriding reaction provide information on the existence of two pairs of modifications of Mg 2 NiH 4 .

In situ X-ray diffraction study of the hydriding combustion synthesis of Mg2NiH4

Abstract An in situ X-ray diffraction study of the hydriding combustion synthesis of Mg 2 NiH 4 directly from the compact of magnesium and nickel mixture in a pressurized hydrogen atmosphere has been conducted. During two cycles of heating (hydriding) and cooling (dehydriding) in a temperature range from 300 to 823 K, the X-ray diffraction patterns were obtained in a pressure of 1.0 MPa at eleven points of the temperature. The obtained X-ray diffraction patterns reveal nine reactions existing in two cycles of heating and cooling. In the first cycle, (1) Mg+H 2 →MgH 2 , partially, (2) MgH 2 →Mg+H 2 , (3) 2Mg+Ni→Mg 2 Ni, in the heating period, and (4) Mg 2 Ni+2H 2 →Mg 2 NiH 4 (HT), partially, (5) Mg 2 NiH 4 (HT)→Mg 2 NiH 4 (LT), in the cooling period. In the second cycle, (6) Mg 2 NiH 4 (LT)→Mg 2 NiH 4 (HT), (7) Mg 2 NiH 4 (HT)→Mg 2 Ni+2H 2 , in the heating period, and (8) Mg 2 Ni+2H 2 →Mg 2 NiH 4 (HT), partially, (9) Mg 2 NiH 4 (HT)→Mg 2 NiH 4 (LT), in the cooling period. The X-ray diffraction intensity of Mg 2 Ni after the second cycle of hydriding decreases to 1/6 times of that after the first cycle of hydriding. In contrast, the X-ray diffraction intensity of Mg 2 NiH 4 after the second cycle of hydriding is six times of that after the first cycle of hydriding, although only one single phase of Mg 2 Ni exists at 823 K before hydriding either the first cycle or the second cycle.

Numerical Simulation of Innovative Operation of Blast Furnace Based on Multi-Fluid Model

A multi-fluid blast furnace model was simply introduced and was used to simulate several innovative iron-making operations. The simulation results show that injecting hydrogen bearing materials, especially injecting natural gas and plastics, the hydrogen reduction is enhanced, and the furnace performance is improved simultaneously. Total heat input shows obvious decrease due to the decrease of heat consumption in direct reduction, solution loss and silicon transfer reactions. If carbon composite agglomerates are charged into the furnace, the temperature of thermal reserve zone will obviously decrease, and the reduction of iron-bearing burden materials will be retarded. However, the efficiency of blast furnace is improved just due to the decrease in heat requirements for solution loss, sinter reduction, and silicon transfer reactions, and less heat loss through top gas and furnace wall. Finally, the model is used to investigate the performance of blast furnace under the condition of top gas recycling together with plastics injection, cold oxygen blasting and carbon composite agglomerate charging. The lower furnace temperature, extremely accelerated reduction rate, drastically decreased CO2 emission and remarkably enhanced heat efficiency were obtained by using the innovative operations, and the blast furnace operation with superhigh efficiency can be realized.