Enhanced carbon capture capacity in biochar from pyrolysis of Litchi branches with soil (Ferrasols) slurry

Biochar is one of the most important carbon-rich minerals with a porous structure in the carbon sequestration, immobilization of metal and organic contaminants, as well as soil fertility improvement. However, its large-scale use is very limited in the agriculture and environment, due to the high cost of production and transportation from agricultural biowaste to plant. In this study, a new technology was explored to directly convert from agricultural biowaste to biochar in the field. The local applications had significantly reduced the costs of biochar. Inspired by nature, biochar production was also proposed in the field, where only agricultural biowaste, water, and fire were required for biomass carbonization and charcoal formation; Specifically, Litchi branches sourced from subtropical regions were utilized as feedstock to explore an on-site production of biochar. Soil (Ferralsols) slurry was applied as a coating and quenching agent to create an oxygen-limited environment during fire-water coupled carbonization of the feedstock. This self-limiting oxidation approach was used to minimize the expenses of production, transportation, and utilization. Biochar that was produced with soil slurry coating and quenching also displayed the highest carbon content (83.5%) and carbon capture capacity (83.9%), exceeding unconverted biomass and biochar produced without coating by 16.7% and 37.8%, respectively. The burning process of Litchi branches was divided into three stages: 1) Surface was charred immediately but with an unburned core; 2) Surface was grayed out, while the core was in a self-ignition state with high temperature and limited oxygen, and the dark red char fell to the ground; and 3) The dark red char gradually burned out to be ash. Spraying water on the dark red char was used to prevent the occurrence of the 3rd stage, thus favoring the formation of biochar instead of ash. Furthermore, the soil coating likely acted as a barrier, thus reducing carbon monoxide or carbon dioxide release during combustion. Additionally, the soil integration was facilitated to form the mineral-carbon composite for the carbon capture. Scanning electron microscopy and energy spectroscopy analysis show that the regular structure of the carbon skeleton was observed after coating and quenching the iron-aluminum slurry. The iron-aluminum minerals were also loaded on the surface. A novel approach to carbonization accelerated a paradigm shift in biochar production from a sophisticated stationary facility to a simple way for practical use in the field. Low cost greatly contributed to the agricultural and environmental application of biochar. The economically viable combination of fire and soil (Ferralsols) slurry coupled carbonization can be expected to provide valuable insights for biochar adoption and carbon neutrality. In conclusion, the readily available agricultural residues and soil-based coatings can be integrated to mitigate the environmental impact of biochar production, in order to enhance the efficacy as a carbon capture. These findings can offer significant implications for the broader adoption of biochar as a sustainable solution, in order to promote climate and soil health.