(685e) First Operation of 5 kWCH4 Local Carbon Recycling System: Product Gas Quality of CO2 Methanation

An industrial sector makes up more than one-third of global CO₂ emission and therefore efforts for industrial CO₂ mitigation are to be further intensified and accelerated. Companies and organizations with awareness of its importance and urgency have launched initiatives, declaring their strategies as well as specific and challenging targets [1-3]. Application of on-site renewable energy, e.g., solar and wind, in industrial factories is one of the most promising approaches to reduce CO₂ emission significantly. However, its fluctuating nature in power necessitates energy storage within the premises when surplus power arises. Also, due to spatial constrains of factories, its storage media needs to have high volumetric energy density.

This study explored a local carbon recycling system (LCRS, Figure 1). The process begins by generating electricity from on-site solar cells and/or wind turbines. Thereby H₂ is produced, electrolyzing water. Concurrently, CO₂ is captured from flue gas of furnaces. Then CH₄ is synthesized by CO₂ methanation reaction (CO₂ + 4H₂ ↔ CH₄ + 2H₂O). The product gas is subsequently stored in a pressurized tank and, as is necessary, returned to furnaces as a fuel. The system realizes energy storage with CH₄ which has a high volumetric energy density, also enabling to utilize presently existing natural-gas furnaces with minimum retrofitting.

LCRS needs to reduce operating expense by enhancing its energy efficiency, which requires the load reduction of auxiliaries such as vacuum pumps, compressors and heaters. The previous studies [4,5] developed a two-staged methanation reactor which realizes high CO₂ conversion (≧99%) at low pressure of 200 kPa (G) and thermally self-sustained conditions. The reactor can recover the redundant heat in the process via thermal oil at the same time. A CO₂ separator was also developed in another study [6]. The separator repeatedly switching physical adsorption and desorption of CO₂, swinging temperature and pressure. It features exploitation of methanation heat for temperature swing and H₂ sweeping for reducing vacuum pump load.

This study built up a 5 kW_CH4 LCRS integrating the previously developed methanation reactor and CO₂ separator, and conducted its first operation. While a burner, which emulates a furnace in a factory, burnt the product gas of methanation, the CO₂ in the flue gas was continuously captured and converted to CH₄. The LCRS was started up from a cold state, sequentially implementing the followings: (1) heating of the reactor and the separator, (2) flue gas provision to the separator and feed gas provision to the reactor, and (3) provision of the methanation product gas to the burner. The result shows that the system can complete all the startup procedure in 53 minutes and realize the methanation product gas quality of more than 93% CH₄ concentration at steady state.

[1] Toyota Motor Corporation. “Challenge 3 Plant Zero CO₂ Emissions Challenge”, retrieved Feb. 2, 2020, from https://global.toyota/en/sustainability/esg/challenge2050/challenge3/.

[2] Denso Corporation. “EcoVision”, retrieved Feb. 2, 2020, from https://www.denso.com/global/en/csr/environment-report/ecovision/

[3] Toyota Industries Corporation. “Reduction in the volume of CO₂ Emissions” retrieved Feb. 2, 2020, from https://www.toyota-industries.com/csr/environment/process/co2/.

[4] Sayama, Shogo, et al. “Energy Efficiency Improvement of CO₂ Methanation Process by Using a Thermally Self-Sustained Two-Stage Reactor: Preliminary Evaluation of a Reactor Concept”, Kagaku Kogaku Ronbunshu, 2020, 46(3).

[5] Sayama, Shogo, et al. “Local Carbon Recycling System: Performance Evaluation of a Thermally Self-sustained Two-staged Methanation Reactor”, SCEJ 85th Annual Meeting, 2020.

[6] Kunitomi, Seiichi, et al. “Local Carbon Recycling System: Energy Consumption Reduction of CO₂ Adsorber”, SCEJ 85th Annual Meeting, 2020.