Release and migration characteristics of sodium and potassium in high alkali coal under oxy-fuel fluidized bed combustion condition

Zhundong coal is promising to replace inferior coal in fossil-fueled power plants, considering its high abundance, low contents of sulfur and nitrogen. However, the relative high contents of alkalis, especially sodium and potassium, damage the operational economy and safety badly since the release of alkali metals in flue gas can lead to fly ash-related problems such as slagging, ash accumulation and corrosion. Although, oxy-fuel circulating fluidized bed (CFB) combustion is beneficial to reduce the release of sodium and potassium by lowering the operation temperature, the ash formation characteristics of Zhundong coal under the oxy-fuel CFB is still scarce. Our results showed that the residual amounts of sodium and potassium in the ash samples, both tend to decrease with increasing temperature no matter under air combustion or oxy-fuel combustion. However, the sodium content in the cinder obtained under the oxy-fuel combustion atmosphere was higher than that under the air combustion atmosphere, and the insoluble sodium, mainly existed in the form of aluminosilicates, was the main form. In addition, the increase of steam content in both atmospheres reduced the sodium content in the cinder while the residual amount of potassium increased. Density functional theory calculations verified the transformation behavior between sodium and potassium and the inhibiting effect of steam. This study elucidates the release and migration characteristics of sodium and potassium in Zhundong coal both experimentally and theoretically and will benefit the popularity of Zhundong coal.