Design of Ecological CO2 Enrichment System for Greenhouse Production using TBAB + CO2 Semi-Clathrate Hydrate

CO2 enrichment in greenhouse production: Towards a sustainable approach

As the unique source of carbon in the atmosphere, carbon dioxide (CO2) exerts a strong impact on crop yield and quality. However, CO2 deficiency in greenhouses during the daytime often limits crop productivity. Crucially, climate warming, caused by increased atmospheric CO2, urges global efforts to implement carbon reduction and neutrality, which also bring challenges to current CO2 enrichment systems applied in greenhouses. Thus, there is a timely need to develop cost-effective and environmentally friendly CO2 enrichment technologies as a sustainable approach to promoting agricultural production and alleviating environmental burdens simultaneously. Here we review several common technologies of CO2 enrichment in greenhouse production, and their characteristics and limitations. Some control strategies of CO2 enrichment in distribution, period, and concentration are also discussed. We further introduce promising directions for future CO2 enrichment including 1) agro-industrial symbiosis system (AIS); 2) interdisciplinary application of carbon capture and utilization (CCU); and 3) optimization of CO2 assimilation in C3 crops via biotechnologies. This review aims to provide perspectives on efficient CO2 utilization in greenhouse production.

CO2 Capture from Flue Gas Based on Tetra-n-butylammonium Fluoride Hydrates at Near Ambient Temperature

Semiclathrate hydrates of tetra-n-butylammonium fluoride (TBAF) are potential CO₂ capture media because they can capture CO₂ at near ambient temperature under moderate pressure such as below 1 MPa. In addition to other semiclathrate hydrates, CO₂ capture properties of TBAF hydrates may vary with formation conditions such as aqueous composition and pressure because of their complex hydrate structures. In this study, we investigated CO₂ capture properties of TBAF hydrates for simulated flue gas, that is, CO₂ + N₂ gas, by the gas separation test with three different parameters for each pressure and aqueous composition of TBAF in mass fraction (w _TBAF). The CO₂ capture amount in TBAF hydrates with w _TBAF = 0.10 was smaller than that obtained with w _TBAF = 0.20 and 0.30. The results found that gas pressure greatly changed the CO₂ capture amount in TBAF hydrates, and the aqueous composition highly affected CO₂ selectivity. The crystal morphology and single-crystal structure analyses suggested that polymorphism of TBAF hydrates with congruent aqueous solution may lower both the CO₂ capture amount and selectivity. Our present results proposed that an aqueous solution with w _TBAF = 0.20 is advantageous for the CO₂ capture from flue gas compared to near congruent solutions of TBAF hydrates (w _TBAF = 0.30) and dilute solution (w _TBAF = 0.10).

Structure-driven CO2 selectivity and gas capacity of ionic clathrate hydrates

Ionic clathrate hydrates can selectively capture small gas molecules such as CO₂, N₂, CH₄ and H₂. We investigated CO₂ + N₂ mixed gas separation properties of ionic clathrate hydrates formed with tetra-n-butylammonium bromide (TBAB), tetra-n-butylammonium chloride (TBAC), tetra-n-butylphosphonium bromide (TBPB) and tetra-n-butylphosphonium chloride (TBPC). The results showed that CO₂ selectivity of TBAC hydrates was remarkably higher than those of the other hydrates despite less gas capacity of TBAC hydrates. The TBAB hydrates also showed irregularly high CO₂ selectivity at a low pressure. X-ray diffraction and Raman spectroscopic analyses clarified that TBAC stably formed the tetragonal hydrate structure, and TBPB and TBPC formed the orthorhombic hydrate structure. The TBAB hydrates showed polymorphic phases which may consist of the both orthorhombic and tetragonal hydrate structures. These results showed that the tetragonal hydrate captured CO₂ more efficiently than the orthorhombic hydrate, while the orthorhombic hydrate has the largest gas capacity among the basic four structures of ionic clathrate hydrates. The present study suggests new potential for improving gas capacity and selectivity of ionic clathrate hydrates by choosing suitable ionic guest substances for guest gas components.