Adopting green absorbent for CO2 capture and agricultural utilization: Biogas slurry and biomass ash case

Previously, the once-through CO₂ chemical absorption process by biogas slurry was experimentally verified to offer the unique advantages like low energy consumption, cost-effectiveness, and feasibility of CO₂ fixation in plants. However, this technology also faces some challenges and limitations, including a low CO₂ absorption rate and performance. To improve the effectiveness and reliability of this innovative carbon capture, utilization, and storage (CCUS) technology, this study proposes a novel method to enhance the CO₂ absorption performance without affecting agricultural applications of CO₂ by mixing biogas slurry with biomass ash as the green CO₂ absorbent. The results indicate that when the solid-liquid mass ratio of biomass ash to biogas slurry is 5:10, the CO₂ loading of the biomass ash and biogas slurry mixture (BA-BS) reaches 936.7 ± 59.1 mmol/kg. Furthermore, the pH of the BA-BS remains stable at 6.9, meeting the rhizosphere pH requirements for plant cultivation. The CO₂ absorption of the BA-BS liquid phase, referred to as improved biogas slurry (IBS), reaches its maximum at 230.4 ± 3.5 mmol/L, which is 126.8% higher than that of the unimproved biogas slurry. The nitrogen content in the BA-BS solid phase, calling improved biomass ash (IBA), also reaches its maximum at 4.24 ± 0.74 mg/g, thereby expanding the agricultural utilization of biomass ash. The most reasonable and effective way of utilizing CO₂-rich mixed biogas slurry and biomass ash involves use IBA as the base fertilizer for tomato cultivation, supplemented later with IBS to promote growth. This optimal application allows for substantial utilization of CO₂, introduced into the tomato cultivation environment by IBA and IBS. The carbon fixation of a single tomato has improved by 108.2%. This study thus provides a feasible solution for high-value negative carbonization of biogas slurry and biomass ash.