Biomass is a promising renewable energy resource despite its low energy density, high moisture content and complex ash components. The use of biomass in energy production is considered to be approximately carbon neutral, and if it is combined with carbon capture technology, the overall energy conversion may even be negative in terms of net CO₂ emission, which is known as BECCS (bioenergy with carbon capture and storage). The initial development of BECCS technologies often proposes the installation of a CO₂ capture unit downstream of the conventional thermochemical conversion processes, which comprise combustion, pyrolysis or gasification. Although these approaches would benefit from the adaptation of already well developed energy conversion processes and CO₂ capture technologies, they are limited in terms of materials and energy integration as well as systems engineering, which could lead to truly disruptive technologies for BECCS. Recently, a new generation of transformative energy conversion technologies including chemical looping have been developed. In particular, chemical looping employs solid looping materials and it uniquely allows inherent capture of CO₂ during the conversion of fuels. In this review, the benefits, challenges, and prospects of biomass-based chemical looping technologies in various configurations have been discussed in-depth to provide important insight into the development of innovative BECCS technologies based on chemical looping.

Early transitional metal carbides are promising catalysts for hydrogenation of CO₂. Here, a two-dimensional (2D) multilayered 2D-Mo₂C material is prepared from Mo₂CT_x of the MXene family. Surface termination groups T_x (O, OH, and F) are reductively de-functionalized in Mo₂CT_x (500 °C, pure H₂) avoiding the formation of a 3D carbide structure. CO₂ hydrogenation studies show that the activity and product selectivity (CO, CH₄, C₂–C₅ alkanes, methanol, and dimethyl ether) of Mo₂CT_x and 2D-Mo₂C are controlled by the surface coverage of T_x groups that are tunable by the H₂ pretreatment conditions. 2D-Mo₂C contains no T_x groups and outperforms Mo₂CT_x, β-Mo₂C, or the industrial Cu-ZnO-Al₂O₃ catalyst in CO₂ hydrogenation (evaluated by CO weight time yield at 430 °C and 1 bar). We show that the lack of surface termination groups drives the selectivity and activity of Mo-terminated carbidic surfaces in CO₂ hydrogenation.

The temperature and pressure of the hydrothermal process occurring in a batch reactor are typically coupled. Herein, we develop a decoupled temperature and pressure hydrothermal system that can heat the cellulose at a constant pressure, thus lowering the degradation temperature of cellulose significantly and enabling the fast production of carbon sub-micron spheres. Carbon sub-micron spheres can be produced without any isothermal time, much faster compared to the conventional hydrothermal process. High-pressure water can help to cleave the hydrogen bonds in cellulose and facilitate dehydration reactions, thus promoting cellulose carbonization at low temperatures. A life cycle assessment based on a conceptual biorefinery design reveals that this technology leads to a substantial reduction in carbon emissions when hydrochar replacing fuel or used for soil amendment. Overall, the decoupled temperature and pressure hydrothermal treatment in this study provides a promising method to produce sustainable carbon materials from cellulose with a carbon-negative effect.