Our research focuses on CO2 emission reduction of energy conversion systems in terms of exergy engineering.

In order to reduce the energy requirement for CO2 separation, a number of separation materials have been developed. Therefore, it is necessary for the evaluation of CO2 separation performance to apply a simplified prediction model. This study aimed to establish a model through the validation of Lab-scale tests and to apply the model for techno-economic analysis (TEA) of a chemical absorption plant that has large solution reservoirs and operates according to electricity prices at a cement plant and waste-to-energy plant.
In the Lab-scale tests, it was important to consider the energy balance in the preheater and stripper column. By modifying the energy used for stripping CO2 in the tests, the results showed good agreement with literature values. Also, when reproducing the experimental results with the simplified model, the scale difference between Lab-scale tests and larger systems was taken into account by adjusting model parameters, and the model was able to reproduce the test results with reasonable accuracy.
In the TEA, the annual operating schedule of a chemical absorption plant was determined by solving an operating-cost minimization optimization problem using General Algebraic Modeling System (GAMS). When design margins are introduced, expansion of heat pump capacity is preferred. This is because the decrease in operational expenditure resulting from increased power sales can outweigh the increase in capital expenditure. Sensitivity analysis under multiple conditions showed that the total cost is most strongly influenced by the amine regeneration heat requirement, followed by the installed capacity of the heat pump.

Among CO2 capture technologies essential for achieving carbon neutrality by 2050, this study focuses on the chemical absorption method, which separates and captures CO2 using a basic aqueous solution called an amine.
In chemical absorption, the amine solution degrades over several thousand operating hours. Since evaluating degradation resistance in new amine development requires a similar or even longer time, the development of an accelerated test method that can reproduce the same degradation mode in a shorter time is desirable.
In this study, with the aim of establishing an accelerated degradation testing method, mixed amine solutions of AMP (2-amino-2-methyl-1-propanol) and PZ (piperazine), which are regarded as next-generation standard solvents, were investigated. Operating conditions that contribute to degradation were systematically organized, and both accelerated degradation tests with intensified conditions and non-accelerated (normal) tests were conducted. Changes in liquid composition, liquid properties, and CO₂ separation and capture performance were evaluated.
As a result, under non-accelerated conditions, simulation of degradation was generally confirmed compared to previous studies. Furthermore, under accelerated 1 conditions, which increased the oxygen concentration and residence time in the flow path from the bottom of the absorber to the top of the regenerator (preheating section), accelerated degradation was confirmed. However, under accelerated 2 conditions, which increased the oxygen concentration and liquid flow rate, trends differing from those of non-accelerated and accelerated 1 conditions were observed in the vapor-liquid equilibrium characteristics and separation and recovery performance, making accelerated 2 unsuitable as an accelerated degradation test method.
When stress was defined as the amount of oxygen consumed for amine degradation minus the total amount of amine liquid, the accelerated degradation effect due to stress and the accelerated degradation effect due to liquid composition were generally consistent. The ratio of oxygen consumption in the absorber liquid reservoir and preheating section calculated during the stress calculation process showed that the preheating section accounted for a larger proportion than in accelerated 2, which may be the reason for the different trends.
Assuming that oxygen consumption in the absorber liquid reservoir is predominantly caused by metal ions, increasing the residence time in the stripper sump by raising the liquid level, and thereby increasing the liquid flow rate to approximately 4/3 times, is expected to achieve an accelerated degradation effect 1.16 times greater than that of accelerated 1 conditions. These results suggest that increasing the liquid flow rate may be an effective approach for accelerating degradation.
In this study, a testing method that accelerates degradation by increasing the oxygen concentration and the residence time in the preheating section was established. In addition, the acceleration of degradation through an increase in liquid flow rate may also be effective.

In the transition to carbon neutrality, decarbonization options must be compatible with existing energy infrastructure. Synthetic methane that can be transported through city-gas pipelines is attractive and aligns with Power-to-X routes. Many carbon-recycling schemes store carbon as liquid methanol, typically via syngas targeting CO:H2 = 1:2. This study proposes Electrochemical Partial Oxidation (EPOx) in a solid oxide fuel cell (SOFC) to couple oxygen separation and partial oxidation with low exergy loss, enabling single-step co-production of syngas and electricity for pipeline-grade blends and retrofit operation. To assess high-utilization operation with diluted methane, simulations and experiments were conducted on in-cell syngas formation. Because H2 diffuses faster than CH4 or CO, downstream regions can preferentially oxidize H2 instead of sustaining EPOx and related chemistry (including the CO water–gas shift). A conventional steady-state 1D SOFC model was extended to include explicit H2 consumption. The model predicts preferential H2 use near the outlet, reducing the syngas ratio to about CO:H2 = 1:1.8 and lowering heat absorption relative to EPOx, thereby increasing exergy losses. Experimentally, no open-circuit voltage was obtained and a dense layer formed in the porous anode. Two anode-substrate forming methods were compared: gel casting enabled more isotropic shrinkage and shorter fabrication time, while the doctor-blade method improved surface smoothness and substrate strength. Mercury porosimetry indicated gel casting produced higher porosity and larger pores.

This research investigates the effects of water conditions on the kinetics of CO₂ mineralization using basic oxygen furnace (BOF) steelmaking slag, with particular emphasis on the non-steady evolution of surface moisture states of powder particles under evaporative conditions and their governing influence on mineralization behavior. Previous studies have shown that an optimal range of water addition exists, which enhances reactant dissolution while avoiding excessive inhibition of gaseous CO₂ mass transfer. To address the rapid evaporation of surface moisture and the predominance of dry conditions after single spraying, a continuous spray–evaporation cycling operation was introduced. The results demonstrate that shortening the spray interval significantly enhances the CO₂ mineralization rate by extending the duration of an aqueous phase favorable for mineralization reactions.
Based on these findings, the sprayed slag powder was treated as a wet powder system, and an evaporation model based on wet powder drying theory was developed. The drying process was divided into three stages: surface free-water film evaporation, wet-core/dry-crust controlled drying, and sensible heating. Numerical predictions of the surface water film lifetime, water-film pH evolution, and surface temperature variations indicate that evaporation and drying behavior under water addition conditions is primarily governed by the lifetime of the surface water film formed after spraying.
Furthermore, the surface water film exists only for a short period, resulting in a limited time window for effective CO₂ mineralization. Within this window, evaporative concentration and a transient increase in system alkalinity cause Ca²⁺ dissolution to be concentrated in the early stage after spraying, appearing as a transient pH peak. Increasing the frequency of this transient process through spray–evaporation cycling is expected to further enhance the overall CO₂ mineralization rate.

Carbon recycling technology that permanently fixes CO₂ in mineral form is attracting attention as a strategy to mitigate global warming. This study aimed to scale up a CO₂ mineralization process utilizing Mg²⁺ in brine discharged from seawater desalination plants, focusing on improving valuable material recovery, product purity, and energy efficiency. Using a 20 TPD demonstration facility on Osaki, operations were generalized through equilibrium modeling with PHREEQC combined with laboratory validation. Incorporation of the saturation index enabled accurate simulation of solid–liquid equilibria in desulfurization and co-saturation steps and prediction of optimal operating conditions based on solution composition. One of the key challenges identified was that the Osaki process achieved a Mg2+ recovery rate of only 92.5 wt%, along with SO₄²⁻ contamination in the main product. To overcome this problem, a modified process was proposed in which electrodialysis-derived NaOH is added to precipitate Mg as Mg(OH)₂, thereby improving recovery and purity. The operations of Mg(OH)₂ precipitation, thermal decomposition, and subsequent CO₂ mineralization were established, confirming the feasibility of producing WMaCS concrete. Material balances and electricity demand were compared using demonstration data. The Mg(OH)₂ process yielded up to 76.0 kg of WMaCS per 20 t of seawater, while the Osaki process offers the advantage of zero liquid discharge (ZLD). The NaOH addition process reduced total power consumption by 74.8%, with electrodialysis accounting for only 16.5% of total electricity use. By quantitatively organizing the trade-offs of each process, it has become possible to propose the optimal process according to the conditions.