With abundant biomass reserves, environmentally friendly and low carbon emission intensity, black particles with higher energy density and better combustion performance can be obtained after treatment, which are suitable for mixing and firing of coal-fired boilers and can promote the early realization of the goal of ”double carbon”. In this paper, by means of numerical simulation, the combustion condition of mixed black particles in a 350 MW quadrangle tangential circular boiler under 100% and 20% load is studied, and the influence law of mixed black particles on the boiler temperature field, flue-gas component field, ignition distance and NO_x emission is obtained. The results show that the temperature of the main combustion zone in the furnace increases by 2∼27°C under 100% load. The concentration of O₂ decreased by about 0.5%∼1%. The increment of CO volume fraction is about 2450×10⁻⁶∼3780×10⁻⁶, and the increment of CO concentration is positively correlated with the mixing amount of black particles. The concentration reduction of NO_x is between 160 and 207 mg/m³. The more black particle mixing amount of a single burner, the more NO_x reduction; The ignition distance can be reduced to about 0.43 m at most. Compared with 100% load, the mixing of black particles under 20% load can increase the temperature of the burner nozzle section by about 7∼105°C. The concentration of O₂ decreased by 0.4%∼1.8%. The increment of the volume fraction of CO is about −150×10⁻⁶∼547×10⁻⁶. The concentration of NO_x at the nozzle section of each burner is between 141 and 164 mg/m³, and the low load can reduce the ignition distance of about 0.8 m. The mixing of black particles can improve the flue gas temperature in the furnace, increase the concentration of CO in the furnace, reduce the concentration of O₂ in the furnace, reduce the concentration of NO_x, and reduce the ignition distance of the fuel jet, which is conducive to the combustion stability of the boiler.