Experimental study on combustion optimization to alleviate fouling on heating surface of a Zhundong coal Boiler

In 2021, the price of most of the domestic coal skyrocketed, affecting the stable supply of electric power supply in some regions, while the price of Xinjiang Zhundong coal remains stable at a low level. Thus, the study of safe and stable large-scale combustion of Zhundong coal in boilers is more and more important for the energy supplyHowever, fouling and slagging often occur on the heating surfaces of the boiler due to the characteristics of Zhundong coal and the high temperature of the flue gas. The effects of the operating parameters, including the primary air velocity, operating oxygen content, air staging and coal fineness on the combustion temperature and NOx emission were investigated on a four-corner tangentially fired boiler of 660 MW capacity, which burned 95% Zhundong coal to get the relevant control criterion. Based on the research, the improvement of control function for contamination of heating surfaces was implemented under steady and dynamic working conditions. Finally, the influence of the improvement on the contamination of the heating surfaces was proved by the method of the contamination monitoring of heated surfaces. The results show that, at 660 MW load condition, the operating oxygen content had the most obvious influence on the flue gas temperature at the outlet of the furnace. The flue gas temperature decreased 99 °C when it increased from 2.0% to 3.5%. The reduction of the operating oxygen content and the local mean stoichiometric ratio of the main combustion zone could lead to a significant reduction of NOx emission and the change of the coal fineness had no obvious effect on NOx emission. After the improved control function was implemented, the rate of flue gas temperature decreased from 6.18 to 4.26 °C min⁻¹ during the load increase process, the maximum temperature decreased from 1104 to 1023 °C under 660 MW. The heat absorption ratio of the platen superheater, low-temperature superheater and low-temperature reheater increased by 0.6%, 1.6% and 0.9%, respectively, indicating that the fouling and slagging was significantly reduced. At 330 MW load condition, increasing the coal fineness R₉₀ to around 4.9% of the uppermost mill could effectively reduce the flue gas temperature near the bottom of the high-temperature reheater, reduce the deposition of combustible matter and fly ash, and inhibit the formation of fouling and slagging.