Co-combustion of brewery spent grain and coal: optimization strategies and synergistic effects

Brewery spent grain (BSG), rich in lignocellulose and protein, shows promise for waste-to-energy utilization via coal co-combustion. However, physicochemical incompatibilities necessitate optimized methods. This study integrated hydrothermal carbonization (HTC) into BSG/coal co-combustion, and three scenarios were systematically evaluated: raw BSG-coal blends, BSG-derived hydrochar and coal, and co-hydrochar from BSG and coal. Lignite demonstrated optimal synergy with untreated BSG (1:1 ratio), achieving superior combustion (index: 2.23 × 10−7) and interaction (0.46) metrics. HTC pretreatment of BSG at 210 °C enabled efficient lignite co-combustion, reducing burnout temperature to 621 °C while elevating combustion index to 3.73 × 10−7. Furthermore, co-HTC of BSG/lignite (1:3 ratio) at 210 °C maximized synergistic benefits, achieving the highest coefficients for energy recovery efficiency (1.09 %), caloric value (0.5 %), nitrogen reduction efficiency (16.12 %), and nitrogen content (−9.16 %). Among all scenarios, co-hydrochar combustion demonstrated optimal performance with low burnout temperature (627 °C), minimal activation energy (23.75 MJ kg−1), and suppressed NH3/NO/H2O emissions. These findings highlight HTC-assisted co-combustion as a sustainable solution for BSG valorization, offering operational advantages including cost-effective pretreatment, NOx mitigation, and combustion stability. The optimized protocols support scalable integration of bioresources into coal-fired systems, advancing circular economy strategies in energy production while addressing industrial biowaste challenges.