Why do the direct flow burners used in industrial boilers adopt a four corner arrangement?

Why do the direct flow burners used in industrial boilers adopt a four corner arrangement?
Due to the small diffusion angle of the airflow emitted by a single nozzle of the DC burner, the velocity attenuation is slow, and the range is relatively long. However, high-temperature flue gas can only mix around the airflow, causing the coal powder around the airflow to ignite first and then gradually expand towards the center of the airflow. Therefore, the ignition is delayed, the flame travel is long, and the ignition conditions are not ideal.
When using a four corner arrangement, the four airflow forms an imaginary tangential circle with a diameter of about 600-800mm in the center of the furnace. This tangential combustion method can ignite the airflow from adjacent burners and help ignite the airflow. At the same time, the airflow sprays into the furnace, generating strong rotation. Under the action of centrifugal force, the airflow expands around, forming a negative pressure in the center of the furnace, causing high-temperature flue gas to flow back to the root of the airflow from top to bottom, further improving the ignition conditions of the airflow. Due to the strong rotation of the airflow in the center of the furnace, the mixture of coal powder and air is strong, accelerating combustion and forming a high-temperature fireball in the center of the furnace. Moreover, the rotation and rise of the airflow extend the burning time of coal powder in the furnace, improving the degree of gas flow filling in the furnace.