Biochar from agricultural by-products: From environmental burden to a sustainable CO₂ capture solution
This achievement is the result of the international cooperation project “Application of Residual Ash from Biomass Gasification as CO₂ Adsorbent” (code: QTJP01.02/22-24), led by Dr. Nguyen Hong Nam from the University of Science and Technology of Hanoi – Vietnam Academy of Science and Technology on the Vietnamese side, and Associate Professor, Doctor Toshiki Tsubota from Kyushu Institute of Technology (KIT), Japan, as the partner project leader.
Dr. Nguyen Hong Nam (third from left) with the research team in the laboratory
Vietnam is an agricultural country generating about 75 million tonnes of agricultural by-products annually, such as rice straw, bagasse, rice husks, and cassava stems. However, only about 40% of this volume is collected and used; most of the remainder is burned or discharged into the environment, causing pollution and wasting valuable resources. In recent years, biomass gasification technology has emerged as a potential solution, converting agricultural residues into syngas for electricity generation, heat production, or upgrading into clean fuel - suitable for small and medium-scale operations. Nevertheless, this process still generates a significant amount of ash residue that is not effectively treated, increasing operating costs and posing potential environmental risks.
Speaking about the significance of the project, Dr. Nguyen Hong Nam said:“After the gasification process, a large amount of solid ash waste remains due to the properties of biomass and the limitations of current technology. If not properly managed, this waste becomes an environmental burden, raising operational costs and undermining the sustainability of the technology. Therefore, our research focuses on transforming post-gasification biochar into an environmentally friendly, low-cost CO₂ adsorbent.”
Dr. Nguyen Hong Nam at the biomass gasification workshop in Mini-Hoa Lac, University of Science and Technology of Hanoi
In this study, the team adjusted a range of gasification conditions to operate a 20 kW-scale system, originally designed for wood fuel, using three common Vietnamese agricultural by-products: bagasse, macadamia shells, and rice straw. After gasification, the resulting ash and biochar were collected and analysed using advanced instruments to determine their physicochemical properties and material structure.
Residual biochar separated from the gasification system
The results showed that biochar from macadamia shells and bagasse had high specific surface areas (939 m²/g and 749 m²/g, respectively), along with dense networks of micropores and ultramicropores — ideal “tiny cavities” for trapping CO₂ molecules. Laboratory tests demonstrated that the CO₂ adsorption capacities of macadamia shell and bagasse biochars reached 2.8 mmol/g and 2.4 mmol/g, respectively - comparable to many activated carbons that require costly activation processes.
When tested under flue gas conditions (40°C, 15% CO₂ concentration), macadamia shell and bagasse biochars still maintained good adsorption performance (1.8 mmol/g and 1.6 mmol/g).
CO₂ adsorption performance of biochar under optimal and flue gas conditions
Biochar derived from bagasse and macadamia shells also exhibited high CO₂/N₂ selectivity, a particularly important feature for industrial exhaust treatment. Even after 30 reuse cycles, and under humid conditions, the materials retained over 90% of their original adsorption efficiency.
Commenting on the research outcomes, Dr. Nguyen Hong Nam stated “Our findings open new possibilities for using waste biochar in multiple fields, offering dual environmental and economic benefits. Once saturated with CO₂, the material can be applied in industry — notably in cement mineralisation. When a small portion of CO₂-loaded biochar is blended into fresh cement, the CO₂ is gradually released and reacts with Ca(OH)₂ in the cement to form CaCO₃, enhancing the material’s stability, density, and long-term strength. The Korea Institute of Geoscience and Mineral Resources has already contacted our team, expressing interest in collaborating on this research direction.”
The researchers have published their findings in two international SCI-E papers, two Scopus-indexed papers, and two domestic specialised journals. In addition, the project supported the training of two engineers and provided opportunities for three students and graduate trainees to study and exchange experiences in Japan. These initial successes confirm the practical application potential of the research and open prospects for future development. The team will continue to evaluate the material’s performance and durability under long-term application conditions, as well as to study surface modification methods to optimise CO₂ adsorption capacity. The team’s goal is to develop a highly effective and economical solution, contributing to affirming the sustainability and real-world applicability of this technology.
Translated by Tuyet Nhung
Link to Vietnamese version
