When the capacity of the coal powder industrial boiler is below 40t/h, from the perspective of reducing NOx emissions, it is more appropriate to arrange a single burner at the top or bottom of the boiler. This ensures that the flame of the burner is symmetrical around the center of the furnace, allowing the air from each air channel of the burner to be well integrated with the coal powder, thus achieving low-temperature and low-oxygen design intentions, organizing a reducing atmosphere, and lowering the peak flame temperature. At the same time, it fully utilizes the entire height of the furnace, benefiting the combustion process. Taking Germany, which has long been committed to coal powder industrial boilers, as an example, many of its industrial boiler products adopt the arrangement of burners at the top of the furnace. Compared to the arrangement at the top, the arrangement at the bottom of the furnace is less common. Theoretical analysis shows that the arrangement at the bottom of the furnace has the following advantages: the coal powder burns through the entire furnace, resulting in a longer combustion time and lower ash carbon content; the burner is close to the ground, facilitating installation and maintenance; the shortness of the primary air duct and fewer elbows, positioned lower, can reduce the power consumption of the primary fan; and the enhanced upward draft in the furnace can reduce the power consumption of the induced draft fan.

There are numerous methods for flame measurement, with the interferometric probe technique frequently employed for on-site flame measurements. This method operates effectively in harsh environments and is suitable for point-by-point measurement of flue gas components, temperature, and particulate matter. Many coal powder swirl burners utilize this technique for flame measurement. In recent years, with significant advancements in computer technology, flame monitoring technology based on digital image processing has seen rapid development. Flame images on a 1MW coal powder combustion test bench are collected using a flame monitoring system, analyzing the geometric and brightness parameters of the flame as they vary with load, primary air mass flow rate, and particle size. Visualization-based measurement techniques are used to study the co-combustion flame of coal and biomass on the combustion test bench, analyzing the characteristics of ignition point, brightness, temperature, and vibration frequency as they change with biomass type and blending ratio. Combining this with traditional flame measurement techniques like thermocouples and flue gas analyzers, the characteristics of the co-combustion flame are analyzed. Flame image processing technology has enabled the measurement and three-dimensional reconstruction of the internal temperature field of boilers, analyzing the characteristics of the furnace temperature as it varies with boiler load. There is little research on the visualization of coal powder swirl burners' flames in power plant boilers.

The reasons for the thermal and mechanical stress impacts of coal powder burners are as follows:

I. Effects of thermal stress. The rotary kiln burner is located at the front kiln mouth, where the working environment temperature ranges from approximately 600℃ to 1300℃, with high temperatures and a wide fluctuation range. Due to the refractory material of the coal powder burner being a poor thermal conductor, the differences in temperature cause different thermal expansion coefficients, leading to cracks forming in the unevenly heated areas after some use.

The impact of mechanical stress: The high-temperature secondary air, blown into the rotary kiln from beneath the coal powder burner by the cooler, carries a certain amount of clinker dust. This causes significant erosion beneath the coal powder burner. Over time, cracks may gradually enlarge, exposing internal fasteners. At high temperatures, the welds on these fasteners can melt, and due to gravity, the fasteners and the castable material may fall off, exposing the metal shell of the burner. Without the castable material's protection, the burner is prone to cracks under high temperatures. Once cracks form, leading to air leakage, the burner becomes unusable.

The combustion characteristics of coal powder depend on the flow characteristics at the inlets and outlets of the burner, making the study of the flow field there crucial. Some scholars believe that the structure of the inlets and outlets of the burner significantly affects the distribution of the flow field, the movement of solid particles, the stable combustion of flames, and the formation of NOx. The recirculation zone formed behind the baffles at the primary air inlet is conducive to the combustion of coal powder, and this zone is distinctly visible when the baffle is 80mm away from the primary air inlet. Dong Xiaolin suggests that the formation of the recirculation zone at the burner inlets and outlets is closely related to the diameter of the burner inlets and outlets and the angle of the secondary air. The angle of the secondary air intake should not be too large, otherwise, the phenomenon of air "skimming" may occur. To further explore the influencing factors of the flow field at the burner inlets and outlets, some scholars have conducted experimental and simulation studies on how the structure of the burner and actual operating parameters affect flow characteristics.