Numerical research on coal-biomass co-firing in a 1000 MW tangential-fired tower boiler for flexible operation under wide range of load conditions

• Numerical model for coal-biomass co-firing in a 1000 MW boiler is developed; • Biomass injection position and co-firing ratio are analyzed under a wide load range; • Stable operation is maintained when biomass is injected via standby burners; • Bottom-burner biomass injection improves combustion stability under medium–low loads; • Recommended co-firing ratios are 10%-15% for higher loads and 15%-20% for lower loads. Biomass co-firing is an important technical pathway to reduce carbon emissions from coal-fired power plants. Given that coal-fired power plants are required to frequently and deeply adjust the load for the accommodation of renewable energy, it is crucial to understand the combustion characteristics with biomass co-firing under different load conditions. Therefore, this paper establishes a numerical model of a 1000 MW tangential-fired tower boiler in case that biomass is injected via standby burners. Simulations of coal-biomass co-firing characteristics are then carried out under different load conditions. The distribution characteristics of flue gas temperature, major products concentration and burnout ratio under different biomass injection positions and co-firing ratios are investigated. Results show that: 1) Under co-firing conditions, the furnace can maintain normal and stable operation. Compared with pure coal cases, the flue gas temperature at the furnace outlet decreases by up to 76 K, and the wall heat flux decreases, resulting in a decrease in heat transfer to the water-steam system; the total burnout ratio is improved by 0.1–1.7 percentage points and reaching a maximum of 99.9%; 2) Biomass injection via bottom burners proves to be more effective, especially under medium–low loads. Compared to biomass injection via top burners, it improves temperature distribution, reduces NO emissions by up to 55.6 mg/m 3 and increases the total burnout ratio by 0.1–1.5 percentage points; 3) An increased co-firing ratio from 10% to 20% improves the uniformity of temperature distribution and decreases NO emissions, but sacrifices the total burnout ratio. Especially under lower loads, combustion stability is significantly enhanced; whereas under medium–high loads, the total burnout ratio deteriorates by up to 1.4 percentage points. The study findings offer valuable insights into coal-biomass co-firing in the boiler for flexible operation under a wide load range.