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

Expectations are rising for the conversion and utilization of C1 chemistry to methane using carbon recycling core materials as energy storage in the large-scale introduction of variable renewable energy (VRE). Gaseous methane is difficult to store in large quantities, and methanol, which is liquid at room temperature, is suitable for storage, but direct conversion from methane is difficult, and conversion via 2:1 Syngas is common. Research on partial oxidation using a catalyst is progressing as a means of conversion to syngas, but the reaction requires pure oxygen, and the problem is that the overall energy efficiency deteriorates due to the power required for its production. Therefore, we have proposed a unique electrochemical method using a solid oxide fuel cell (SOFC) as a membrane separation reactor for oxygen separation + partial oxidation. Electrochemical Partial Oxidation (EPOx) does not need to pressurize the air, and has a kind of membrane separation function that allows only oxide ions to completely permeate from the cathode to the anode in an ideal state without cross-leakage. With EPOx, the difference from the standard exergy of 2H2 + CO of 741 kJ/mol (consideration of mixing) can be extracted as power. In this study, in order to realize co-production to obtain 2H2+CO suitable for synthesizing methanol from methane while generating power in one step, we investigated the catalyst and the method of stacking the catalyst on the SOFC cell with an eye to implementation.

The iron and steelmaking industry is the largest source of CO2 emissions in Japan, and various steelmaking processes are proposed to achieve carbon neutrality in 2050. Among those processes, oxygen blast furnace is suited for carbon recycling steelworks. In oxygen blast furnace, pure oxygen is blown from tuyere instead of hot blast. This feature causes nitrogen-less condition of top gas, which leads to energy saving by omission of gas separation process. In addition, more tuyere reductants can be blown because of higher oxygen concentration in raceway area. In this research, carbon recycling steelworks with oxygen blast furnace using methane, methanol and DME as tuyere reductant is analyzed. Coke rate and blast rate are calculated through energy balance and wuestite reduction equilibrium on corrected Rist diagram, which applied the corrections and additions of calculation on the latest literature. After coke rate and blast rate are determined, all input and output flows of oxygen blast furnace are obtained from the simulation model of oxygen blast furnace on Aspen Plus. In this model, shaft gas injection unit, which blows combusted top gas into shaft area in order to prevent insufficient preheating of solid materials, is introduced. The simulation models of synthesis of methane, methanol and DME are also created to obtain required hydrogen and energy. Without carbon recycling, only methane blowing condition has CO2 reduction effect. On the other hand, with carbon recycling, all blowing conditions attain CO2 reduction. The maximum reduction value compared to the existing steelworks is 24% under the condition of methane blowing with carbon recycling. Due to the large exergy losses in water electrolysis and synthesis process, exergy effective ratio decrease to 32–35% under carbon recycling conditions.

In response to the recent global warming problem, the development of carbon cycle ironmaking system technology is underway, which is expected to significantly reduce CO2 emissions, which are considered to be the main cause of global warming. The CO2 absorbent is Lithium Silicate: Li4SiO4 (LS), a reusable absorbing ceramic that absorbs CO2 at 500~650 ºC and releases it at 800 ºC or higher. CO2 capture reactors filled with LS are desired to have a short absorption completion time, a large absorption volume density, and a low pressure drop. This year, we predicted the performance of the reactor with a honeycomb shape that is effective for improving these performances and verified them experimentally. Based on the results, we attempted to design a system and reactor that can achieve the CO2 reduction targeted by the steel industry.
A portion of the extruded honeycomb-shaped LS was cut out and the coefficients in the reaction rate equation were obtained using thermogravimetric analysis. Numerical calculations were performed using the coupled conservation of chemical species, mass, momentum, and energy equations.
In the case of a two- column temperature swing system for absorption and regeneration, the CO2 absorption reaction proceeds while the absorbed gas of 400 ºC cools the LS from 800 ºC. The reaction rate is small at high temperatures because the difference between the equilibrium CO2 concentration at that temperature and the local CO2 concentration, or driving force, is small. Therefore, the chemical reaction is the rate-limiting step in the CO2 absorption process, and the CO2 capture ratio is only about 30%. The performance of a three-column switching system, in which a cooling process to the LS absorption temperature range by air circulation is added after regeneration, was predicted to achieve the CO2 capture ratio of more than 90%, indicating that a 30% reduction in CO2 emissions from the blast furnace is possible.

The iron and steelmaking industry (ISM) is a major source of CO2 emissions in Japan, and even after the spread of low-carbon iron and steelmaking processes, reducing CO2 emissions from lime calcination (about 10%) will be difficult. Therefore, we propose a method to mineralize the emitted CO2 reacting the solid by-product (slag) from ISM with gas phase CO2. CO2 mineralization using slag can reduce CO2 emissions from lime calcination. In addition, CO2 mineralization can also generate profits because the carbonate produced by CO2 mineralization can be used as aggregate. On the other hand, the slow rate of CO2 mineralization using slag is a technical challenge. We have proposed CO2 mineralization process using ultra-slow cooling slag, which consists of three processes: (1) ultra-slow cooling of molten slag, (2) grinding of solidified slag, and (3) CO2 mineralization by gas-solid reaction. In this study, process design was conducted for the ultra-slow cooling process and the CO2 mineralization process.
Regarding (1), solidification tests using electric arc furnace slag under different cooling conditions were conducted to search cooling conditions for enhanced CO2 mineralization rate. Mineral phases and CO2 mineralization rate of electric arc furnace slag were evaluated based on heat transfer analysis and prediction of mineral phases in relation to the cooling rate. As a result, formation of mineral phases suitable for CO2 mineralization was confirmed, and their formation conditions were investigated.
Regarding (3), for the purpose of designing a conveyor reactor with a humidifying hood which is considered for installation in ISM, we aimed to construct a numerical model to simulate CO2 mineralization behavior under stacked slag powder conditions. CO2 mineralization behavior under stacked CaSiO3 conditions were predicted using the model and compared it with experimental data to investigate the rate-limiting conditions for CO2 mineralization under stacked powder conditions.

Carbon recycling technology, which reuses CO2 as a resource, is attracting attention as early diffusion of atmospheric sequestration technology for CO2, which is the main cause of global warming, is required. Our laboratory has so far developed a process to mineralize CO2 using the separated and recovered Mg while co-producing valuables such as gypsum, salt, and hydrochloric acid from concentrated seawater derived from a desalination plant. However, the detailed continuous operation conditions have not been established. In this study, we established a method for separating high-purity Mg that does not depend on the seawater composition and a method for improving the rate of CO2 mineralization among the unit operations, and developed a method for effectively using the resulting magnesium carbonate as concrete.
This carbon recycling technology consists of several operations. Seawater mainly consists of Mg2+, Ca2+, Na+, K+, Cl-, SO42- ions and a large amount of H2O. In the first operation, we developed a method to selectively recover Mg from seawater in an energy-saving manner through concentration by pressurized evaporation and cooling, and high-purification operation that applies the common ion effect. Next, the recovered Mg was heated at a low temperature to generate MgO that can mineralize CO2. Finally, CO2 was mineralized as carbonate by gas-solid catalytic reaction between MgO and CO2 gas.
In this research, we also developed a method to increase strength and semi-permanently fix concrete by adding a certain amount of carbonate to concrete aggregate.