Preheating and combustion characteristics of high-alkali coal with a novel circulating fluidized bed route

China possesses abundant reserves of high-alkali coal, however, its combustion presents challenges such as slagging and fouling. Achieving efficient and clean combustion of high-alkali coal remains a significant technical challenge. This study utilizes novel circulating fluidized bed (CFB) preheating combustion technology to investigate the effects of varying air equivalence ratios on the preheating, combustion, and NO_x emission characteristics of high-alkali coal in a kW-level hot-state experimental platform. Results indicate that the preheater intensifies the fragmentation of high-alkali coal particles, decreasing the median particle size (d₅₀) from 710.9 μm in high-alkali coal to 17.1 μm in preheated char. At an air equivalence ratio of 0.35, the calorific value of the preheated coal gas reaches a peak of 2.46 MJ/Nm³. Across various air equivalence ratios, the NH₃ concentration in the preheated coal gas consistently exceed those of HCN. With the air equivalence ratio increases, the release rates of C, N, and S from the high-alkali coal also rise, while the CO/CO₂ ratio of the preheated coal gas remains below 1.0. And the S release rate is lower than that of C and N. During the combustion of preheated fuel in the combustion chamber, the temperature distribution remains uniform, with NO_x emissions ranging from 200 to 230 mg/m³. The minimum conversion ratio of coal N to NO_x in this system is 10.8%. This investigation might support the development and application of high-alkali coal combustion technology.

Graphical abstract