Hydrogen is a secondary energy that can be easily converted through water from carbonaceous resources such as coal and natural gas, various high temperature heat sources and electricity. It concerns not only a direct use of hydrogen but also an effective energy storage and an energy conservation. These advantages of the feature of hydrogen enable us to establish a hydrogen energy society. Besides of fossil fuels, hydrogen, electricity, and synthetic fuel are utilized as secondary energy in the right place for the right place, and efficiently supply and operate the heat and power necessary for our daily life. This complements the shortcomings of the electrical system which can be said to be the most important in modern society and contributes to the stabilization of electricity supply and demand balance by renewable energy such as solar power and wind power. In this article, features of hydrogen energy system for a low carbon society are introduced to exhibit the requirements concerning production, storage/transportation and utilization. Also, the background of various initiatives aimed at establishing a hydrogen society, the situation of research and development, Japan's basic hydrogen strategy and international trends are described to demonstrate prospects of future trends of technological developments.
Recently, renewable energy has been increasingly introduced for electricity generation in order to reduce greenhouse effect gas. However, the amount of electricity generation by renewable energy fluctuates according to climate, weather conditions and time of day. In addition, area where electricity can be generated by renewable energy eccentrically located. Therefore, it is necessary for us to find the way of transport and storage of the renewable energy. As the way of carrying and storing renewable energy, transformation from renewable energy to hydrogen is proposed. With the support of the New Energy and Industrial Technology Development Organization (NEDO) of Japan, Mitsubishi Heavy Industries (MHI) group is developing the natural gas/hydrogen-mixed firing and hydrogen-firing technologies for dry low NOx large-scale gas turbine. MHI group has successfully tested the hydrogen-mixed firing technology for gas turbines, which is targeted to be in operation in 2025 or later. The basic design of a new combustor for a hydrogen-fired combustor is also in progress with the aim of operation in 2030 or later.
In December 2014, Toyota launched the world's first commercially available fuel cell vehicle “MIRAI” and also started the demonstration program of the New Fuel Cell Bus “SORA” in 2018 for mid-long range transportation usage. The New FCV adopted the world's first FC system without an external humidifier, and overcame this challenge by improving water management within the fuel cell itself and through technical innovation of the control logic.
The hydrogen sensors are important items supporting the hydrogen safety at every phase of hydrogen production, transportation, storage, filling and consumption. Currently, there are three kinds gas sensors which a hot wire semiconductor type, a catalytic combustion type and a thermal conductive type to the hydrogen detection, and each detection range is different. For example, in the hydrogen station, a hot wire semiconductor type and a catalytic combustion type gas sensor are practically used. The thermal conductive type gas sensor is used as hydrogen concentration meter of hydrogen diffusion test. These three kinds of sensors already have many market results. However, the further improvement of the sensors is still necessary about power consumption, response time and durability to support safety of the future hydrogen energy society. So we had developed the catalytic combustion type hydrogen sensor in the shape of the miniature beads. It is using the optimized Pd-Pt/Al2O3 catalyst and the Pt micro-heater coil. Both warm-up time and response time of this sensor achieved less than 1 second by downsizing the element to 200 μm diameter. Furthermore, we have been joint development with Japan Aerospace Exploration Agency the novel hydrogen sensor which can detect trace amounts of hydrogen in vacuum or anoxia conditions. This sensor uses the cerium oxide which is nonstoichiometric oxide for hydrogen sensing material. The sensor resistance depending on hydrogen and oxygen partial pressure ratios at any pressure from 10-5Pa to atmospheric pressure. This sensor may use to the safety management of the reusable observation rockets. In addition, it may apply to air / fuel sensor. We assume that these gas sensor technologies contribute to hydrogen safety.
Hydrogen flame was visualized by imaging at 309 nm and about 10 μm. The ultra violet light at 309 nm was the emission spectrum of the OH radical, and the far infrared light at about 10 μm was the heat ray of the water vapor formed during combustion. The ultra violet image was amplified by an image intensifier. These images were converted the binarized-images using a threshold level of a brightness value. The spatial area that the ultra violet and far infrared light are emitted was extracted, and the flame image was displayed on the background image which was obtained simultaneously. The use of this method allowed imaging of hydrogen flame in outdoor, daylight conditions, up to a distance of 30 m. In addition, a hydrogen flame visualization method by imaging near infrared light was developed. Hydrogen flame was visualized by imaging at 950 nm, which coincides with the principal peak in the emission spectrum of the H2O molecule. The background image at 900 nm was obtained simultaneously, and the spatial region corresponding to the flame image was extracted by using differential imaging. The use of this method allowed imaging of hydrogen flame in outdoor, daylight conditions, up to a distance of 20 m.
A high-luminosity-light-collection system for highly spatial detection of chemiluminescence of radical species in flames has been developed. The system, multi-color integrated Cassegrain receiving optics (MICRO) is based upon a Cassegrain-type configuration, which implies that it employs only reflective components (in combination with an optical fiber for light collection). It provides therefore spherical- free and chromatic-aberration-free detection, which is of importance for high-spatial-resolution measurements and for the simultaneous monitoring of signals in different wavelength regions from a given spatial volume. The effective light-collection volume has been estimated to be only 1.6 mm×0.2 mm×0.2 mm by ray-tracing techniques, which is more than three orders of magnitude smaller than that provided by a corresponding simple single-lens system and comparable to that of laser-based techniques, e.g. Doppler velocimetry. The system is also easily aligned since the active probe volume can be visualized by sending in visible light through the system in the reverse direction.
To examine temperature of the flamelet of turbulent premixed flames, chemiluminescence intensities of OH, CH and C2 radicals obtained from unstrained methane-air and propane-air premixed flames have been examined using the newly developed chemiluminescence spectroscopy system. It has been shown that the chemiluminescence intensity ratio of 515nm/470nm bands of C2 obtained both from methane-air and propane-air unstrained flames uniquely depends on the temperature of unstrained methane-air and propane-air flames. To establish a novel technique which enables to estimates the temperature of the near-extinction flamelet of turbulent premixed flames, the relation between the temperature and the chemiluminescence intensity ratio of 515.5nm/470.5nm bands of C2 radical of strained flames has been sought by using a counter-flow burner. The unique relation between the flame temperature and the chemiluminescence intensity ratio of 515.5nm/470.5nm bands of C2 radical has been found to exist for the propane-air strained flame. Therefore, it can be concluded that the temperature of flamelets of the propane-air turbulent premixed flame can be estimated by using the relation between the temperature of the strained flame and the chemiluminescence intensity ratio of 515.5nm/470.5nm bands of C2 radical obtained in the present study.
The simulation of turbulent combustion has been of more importance to develop practical combustors. The flamelet (or tabulated chemistry) approach is one of the techniques to estimate combustion reaction rate and to describe turbulent combustion field. This approach can reduce computational cost in comparison with the reduced reaction schemes because the combustion reaction rate is just looked up from the pre-calculated database. The difficulty of the flamelet approach comes from the variety of how to make this pre-calculated database. In the present study, we reviewed previous researches in terms of the flamelet approach and summarized the ideas to make the database for the flamelet approach. First, the fundamental combustion reaction models such as the detailed reaction model and skeletal model were introduced. And then, how to generate the dataset for the database in flamelet, flamelet progress variable, and flamelet generated manifolds models were mentioned. Moreover, how to process the dataset into the database for the turbulent combustion simulation was pointed out. Finally, apart from making the database, the treatment of the database in the computational fluid dynamics was described briefly.
Carbon black consists of a number of carbon nanoparticles. To investigate the growth mechanism of carbon black, its morphology of nanoparticles with a comparable size is important. The particle size distributions of carbon black and nascent soot from ethylene or acetylene pyrolysis were analyzed by a scanning mobility particle sizer. Carbon black from pyrolysis was also classified by a differential mobility analyzer and collected by a thermophoretic sampling. The morphologies of size-graded carbon black were observed by scanning electron microscope in different experimental conditions. Particle size distribution showed that acetylene promoted soot formation more than that of ethylene at 1600–1800 K. From the results of image analysis, the morphologies of carbon black were complex with an increase in the mobility diameter of carbon black in long residence time because the number of particles increases with increasing mobility diameter. On the other hand, in short residence time, particle nuclei collided with aggregates, and the morphologies became simple. The temperature at which the ratio of complex morphologies was high depended on mobility diameter and/or residence time.