In order to contribute to the realization of a carbon neutral society by creating a hydrogen society, it is essential to build a supply chain that integrates the production, storage and use of hydrogen, and to find an energy carrier technology that can be effective in storing and transporting hydrogen. At AIST, mainly in the Department of Energy and Environment, we conduct a wide range of research and development work, from hydrogen production, storage, transport and use to evaluation and safety technology. In this paper, I am reporting on our efforts on hydrogen and energy carrier technology including hydrogen production, ammonia production and synthetic fuel production technology.

Towards carbon neutrality by 2050, an utilization of ammonia not only as hydrogen energy carrier but also as carbon free fuel is anticipated. The Green Growth Strategy Through Achieving Carbon Neutrality in 2050 which consists of 14 important targets were formulated in December 2020 and updated in June 2021. Fuel ammonia plays an important role in the strategy. In this manuscript, recent progress on ammonia combustion study obtained at the High Speed Reacting Flow laboratory, Institute of Fluid Science, Tohoku University were summarized. To clarify fundamental combustion characteristics of ammonia flame, laminar burning velocities were experimentally obtained up to 0.5 MPa and 500 K from spherically propagating flames. Also, product gas characteristics of ammonia/hydrogen/air flames stabilized in a stagnation flow were investigated experimentally and numerically. It was observed that large amount of N₂O was produced for the equivalence ratio of 0.6, and important reactions for N₂O production and reduction were identified. Results of liquid ammonia spray were also described briefly. The detailed spray characteristics were observed using 2-phase Structured Laser Illumination Planar Imaging (2p-SLIPI) and clarified that the spray characteristics of liquid ammonia spray were different from those of water spray. Then, the experiments of ammonia spray combustion were performed using a laboratory-scale swirl combustor. The stability map and product gas characteristics of liquid ammonia spray flames were obtained.

Combustion experiments in a 2.4 MW-class multi-burner furnace equipped with three coal burners vertically were carried out to investigate the effects of ammonia (NH₃) injection positions, NH₃ injection nozzle shapes, and operating conditions on the co-combustion characteristics of pulverized coal and NH₃ in a pulverized coal boiler. At an NH₃ co-firing rate of 20% at a lower heating value, the nitrogen oxides (NOx) concentration in exhaust gas was the lowest in the case of the NH₃ injection at the lower burner, where the residence time of NOx in the reduction atmosphere is longer, compared with NH₃ injection at all stages and the upper burner. For the NH₃ injection nozzle shape, the nozzle, which injects NH₃ simultaneously in the axial and radial directions of the burner, has the lowest unburned content in the ash collected by the electrostatic precipitator. When the nozzle described above is used for NH₃ injection at the lower burner at an NH₃ co-firing rate of 20% and the two-staged combustion rate of 35%, NOx concentration in exhaust gas and unburned content in the fly ash can be achieved at a level equivalent to those of pulverized coal. Under the above conditions, NH₃ concentration in exhaust gas was below the lower limit of detection, and nitrous oxide (N₂O) concentration in exhaust gas was the same level as that of pulverized coal combustion.

This paper describes the multi-dimensional flamelet modeling and the application to a combustion simulation by means of a large-eddy simulation (LES) on an oxy-fuel combustion system. The two-dimensional flamelet formulation for a three-feed combustion system can be simplified with the three types of the quasi-two-dimensional flamelet formulations (Q2DF) by post-processing of the conventional one-dimensional flamelet solutions. The Q2DF models are derived based on the treatments regarding the third stream; the diluent stream is mixed with the oxidizer stream and/or with the fuel stream, giving rise to the three models called Q2DF1, Q2DF2, and Q2DF3. LES with the three models were performed to analyze the characteristics of each model. It was found that the differences among the three models became apparent as the mixture fraction of the inactive third stream (Z₃) evolved larger, otherwise the three models gave almost the same accuracy. In the larger Z₃ region, its scalar dissipation rate played an important role and the mixing state (premixed or non-premixed) of the third stream with the other streams should be carefully considered. In addition, in the latter part of this paper, the three-dimensional flamelet modeling for coal-ammonia co-combustion is also described.

Combustion research using ammonia as a fuel for low-carbon energy is relatively new. In order to achieve carbon neutrality, low-carbon energy (ammonia and hydrogen) must be widely introduced not only in the commercial power generation field, but also in the industrial, consumer, and transportation fields. However, the social implementation of ammonia fuel is not clear. In this article, a low-carbon power generation system is proposed from the viewpoint of expanding the application of ammonia. The unit operations of the system: ammonia reforming catalyst, selective non-catalytic reactor, and pure hydrogen production technology are described.
Ammonia has the inherent challenge of burning very slowly compared to hydrocarbon fuels such as methane. The laminar combustion rate equivalent to that of CH₄ can be obtained with gas mixture of H₂/NH₃ = 0.5. Combustion of fuels containing NH₃ generates NOx. The reaction temperature range of SNCR can be lowered by adding H₂ to the exhaust gas. The added H₂ produces OH, which in turn produces NH₂, and then NH₂ promotes the de-NOx reaction.
A plasma membrane reactor has been developed to produce pure hydrogen for fuel cells. The reactor can separate pure hydrogen at lower temperatures than conventional membrane reactors.

From the perspective of long-term stable energy supply, Bioenergy is the only renewable energy alternative fuel to coal coke in the steel industry. This presenting development aims to achieve a carbon-neutral society in short/medium strategy as the primary energy that can be the only means of the long-term stockpile to avoid resource depletion. The development of bioenergy as primary energy is an urgent issue. This report describes our efforts to develop solid biofuels from the perspective of coal depletion within 200 years and the development of combustion science to accomplish a carbon-neutral society in the steel field. Especially, stockpiling is necessary from the viewpoint of a stable energy supply in the future. This article will also describe how solid biofuels utilization based on the green casting concept.

Nowadays the demand for energy resource in the world has been becoming large and this trend will continue in the near future. Nevertheless, consumptions of fossil fuels should be decreased to achieve carbon neutral society until 2050. Under the circumstances, utilization of various biomass resources is gathering attention. Therefore, combustion behaviors of biomass resources need to be elucidated in detail for highly efficient use of them as an alternative to fossil fuels. The purpose of this study is to measure the combustion rate of biomass under isothermal conditions, which is close to the combustion conditions of an actual industrial furnace, and then to clarify the reaction kinetics dividing volatile emission and char combustion steps. The combustion behaviors of black pine pellets, which was a typical biomass fuel, were elucidated in the isothermal combustion experiment. A butch furnace equipped with an electronic balance was used to examine the effects of the experimental temperature and the oxygen concentration in the atmospheric gas. Furthermore, based on experimental results, kinetic analysis was performed to determine the appropriate model among Volumetric model, Shrinking core model, and Random pore model.

This article outlines the history of GHG reduction in international aviation, the scheme of GHG reduction certification(CORSIA), the quality standard for safety use of SAF(ASTM), manufacturing technologies, and combustion behavior of SAF. In COP3 held in Kyoto in 1997, it was resolved that ICAO addresses GHG reduction in international aviation sector. ICAO has established the CORSIA and has started pilot operation with voluntary participation from 2021. CORSIA contains certification of degree of GHG emission in using various type of SAFs and GHG reduction by airlines. CORSIA defined three technical measures to achieving GHG reduction. Among three measures, SAF is assumed to contribute the most to reduction of GHG. COSIA includes certification scheme of GHG reduction accounting and sustainability criteria of SAF (CEF certification). ASTM certification is another certification(standard) scheme for commercial usage of SAF, which is required at the time of in starting CEF certification. In ASTM, SAF should be used within 50% of blending ratio (depending of the SAF type) against Jet A-1 or Jet A at present. In Apr. 2021, the work group has started for establishment of standard of 100% SAF specification. When considering 100% SAF standards, it is important to evaluate the impact of SAF on jet engine's components and combustion. A report on the behavior of HEFA-SPK in a small gas turbine engine was introduced. Some findings are described in the literature in terms of differences in combustion behavior such as emission.

Plasma technologies are considered a promising solution to answer challenges in combustion including the ignition and stabilization of lean flames, the control of thermoacoustic instabilities, the reduction of pollutant emissions, or the development of CO₂-free combustion chambers with hydrogen or ammonia. In this article, we present an overview of illustrative experiments of plasma technologies for flame ignition and stabilization with a particular attention to Nanosecond Repetitively Pulsed discharges. We also discuss the fundamental thermal, chemical, and hydrodynamic processes of these discharges, as a basis to guide the design of plasma-assisted combustion systems and the development of predictive models.

In this article, possibility of dielectric barrier discharge (DBD) for combustion control on internal combustion engines such as knocking mitigation was discussed with brief introduction of the DBD. The characteristics of the DBD among the non-thermal equilibrium plasmas and the reason for choosing surface DBD for the application were explained. The flow induced by the DBD, the method of evaluating the energy consumption, and the chemical species formed by the discharge in the fuel/air premixture were described. DBD generation in the fuel/air premixture produces alkylperoxide as important intermediate chemical species instead of ozone. To demonstrate the application of DBD in engine combustion, rapid compression and expansion machine (RCEM) was used to generate end-gas autoignition simulating knocking phenomena. The flame was propagated in the rectangular channel in the combustion chamber of the RCEM filled with the high temperature and high pressure premixture, and the DBD plug was located in the end-gas region. It was found that the strength of the knocking can be mitigated by generating DBD in advance with short duration. The mitigation phenomena was considered to be non-uniform distribution of the chemical species generated by DBD in the end-gas region leading the non-uniform progress of low temperature oxidation reaction, preventing the formation of strong pressure wave.

Dielectric barrier discharges can produce non-thermal plasma, which has been applied to ignition, plasma assisted combustion and fuel reforming. The authors have been investigating the ignition characteristics of radio-frequency dielectric barrier discharges at high pressure. In this paper, fundamentals of the dielectric barrier discharge and the energy measurement are explained approximately. After that, experimental results for ignition and combustion of the radio frequency barrier discharge are introduced with a plug electrode, focusing on the effects of pressure, the applied voltage, the alternate frequency, and the equivalence ratio. Detailed and time-resolved spectroscopies for dielectric barrier discharges are also shown under various conditions. From spectra of N₂ second positive system, vibrational and rotational temperatures were calculated by fitting measured spectra to the theoretical spectra. Results indicated that local heating and discharge memory effects promote the ignition under an optimized condition. In addition, behaviors of the plasma and initial kernel formations with a pin-to-hemisphere electrodes for dielectric barrier discharge are explained. Optimizing the parameters improve ignition performance of the dielectric barrier discharge.

Smoldering of a futon, a Japanese style mattress, is significantly affected by the blend ratio of polyester and cotton fibers comprising the wad that is stuffed in the futon; it was found that the smoldering is inhibited when a relatively small amount of polyester fibers is added to the wad. To understand this effect of blend ratio, experiments are conducted in this study with bare wads of cotton and polyester fibers having different blend ratios. A thin wad is laid on a hot plate with the intention of compensating the heat loss and letting the smoldering occur within the visible range in the wad so that the behavior of it can be observed from the surface. A spot of its surface is heated locally with a lamp and the temperature is measured by means of a thermographic camera for detecting the ignition. After it ignites, the smoldering behavior is recorded by a video camera or a microscope. It is found that the average of the ignition delay time increases as the polyester ratio increases, while data scatter widely depending on each sample. All the wad samples containing 25wt.% of polyester ignite just as those of 100wt.% cotton fibers, but the travel distances of the smoldering spread become distinctly shorter. These results indicate that the inhibition of the smoldering with small amount of polyester is originated from not the propensity of ignition but the propensity of smoldering spread. The microscopic observation reveals a vast movement of fibers in the preheating zone, which is caused by the melting and deformation of polyester fibers; such a movement may influence the propensity of the smoldering spread.

The effect of the frequency of discharges on the ignition delay time was investigated using a zero-dimensional reaction analysis. Assuming that the active species are produced instantaneously by nanosecond pulsed repetitive discharges, the reaction pathways for the consumption of O atoms produced by the discharges at different frequency of discharges are compared. Results showed that the time required for the ignition became shortest at a frequency of 20 kHz. The decrease in the reaction rate of between CH₂O and O atoms can be responsible for the increase in the time required for ignition in the high frequency of discharges. The CH₂O concentration during the multiple discharges was determined by the competition between the interval of the discharges and the time scale of the reactions related CH₃O.