Abstract: The global pursuit of "dual carbon" goals and increasing energy demand has made clean and low-carbon technologies for coal-fired boilers a research hotspot. Biomass energy, characterized by its renewability, abundant reserves, environmental friendliness, and zero carbon emissions over its entire lifecycle, has emerged as a significant alternative to fossil fuels. Co-firing biomass with coal can significantly reduce fossil fuel consumption and greenhouse gas emissions while effectively utilizing agricultural and forestry residues. Currently, research on biomass co-firing in coal-fired power station boilers mostly focuses on 600~660 MW opposed-fired boilers, with less attention on 300~350 MW tangentially fired boilers. The complex combustion characteristics of this boiler type and unclear biomass co-firing issues have hindered the large-scale application of biomass energy. To address this, Fluent software was employed to simulate a 350 MW tangentially fired coal-fired boiler from a certain plant. Under constant total fuel calorific value and primary and secondary air ratios, eight different co-firing conditions were simulated. The aim was to reveal the influence patterns of different co-firing methods on the boiler's temperature field, flue gas component field, pollutant emissions, and ignition performance. This provides technical guidance and support for the large-scale utilization of biomass energy and the achievement of "dual carbon" goals. The research results indicate that good tangentially fired combustion can be achieved with biomass co-fired either in the same mill or different mills. Specifically, co-firing in the lower mills increases the main combustion zone temperature by 45.59 K to 60.23 K and the exhaust gas temperature by 2.49 K to 6.31 K. Co-firing in the upper mills has a minor impact on the main combustion zone temperature, ranging from -5.32 K to 26.87 K, but significantly increases the exhaust gas temperature by 14.64 K to 17.75 K. The amount of biomass co-fired is positively correlated with improved ignition performance, effectively shortening the ignition distance by up to 0.48m and reducing the ignition temperature by up to 44.69 K. Furthermore, appropriate biomass co-firing can effectively reduce NO_x emissions, but excessive co-firing ratios can lead to increased thermal NO_x generation, offsetting the emission reduction effect. Co-firing in the upper mills yields the best NO_x reduction, with NO_x emissions decreasing by 91.34 mg/m³ when 100% biomass is co-fired in Mill D. Biomass co-firing also increases the CO concentration in the main combustion zone of the furnace, with a positive correlation between the increment and the amount of biomass co-fired. The average increase in CO volume fraction ranges from 1020.68×10⁻⁶ to 6380.13×10⁻⁶, but the CO concentration at the furnace outlet can be reduced to zero. This study provides technical guidance and support for the large-scale utilization of biomass energy and the realization of the "dual carbon" goals. It reveals critical mechanisms governing combustion characteristics, improved ignition performance, NO_x emission control, and CO concentration variations in four-corner tangential coal-fired boilers after biomass co-firing. The findings establish a theoretical foundation and offer practical guidance for implementing biomass co-firing technologies in coal-fired power plants.