Insights into the deposition and transformation characteristics of ammonium bisulfate on the fly ash as the by-product of SCR in air preheater

Abstract

Ammonium bisulfate (ABS) aggravates the corrosion and plugging of air preheaters, resulting in unplanned outages and expensive cleaning. Understanding its deposition and transformation process is important for the development of ABS control strategies. A multistage temperature control system was built to simulate the temperature gradient in the air preheater and the deposition and transformation of ABS in the presence of fly ash were studied using Fourier-transform infrared spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and thermogravimetry. The presence of fly ash increases both the deposition amount and the temperature of ABS in the 25–200 ppm SO3 concentration range; the initial ABS deposition temperature is between 202 and 290 ℃ and most likely occurs in the 230–250 ℃ range. The main product was ABS, and its formation temperature was much higher than the deposition temperature observed in the experiment. In the 224–180 ℃ range, the products gradually changed from ABS to sulfuric droplets. Fly ash has trapping and adsorbing effects on the gas-phase ABS in flue gas, and more ABS is captured at high temperatures owing to the higher concentrations of reactants. ABS deposition leads to the accumulation and adhesion of fly ash particles and reacts with the fly ash to convert it into a variety of ammonium salts and sulfates with higher decomposition temperatures. Compared with the sulfate formed after the deposition of sulfuric acid droplets, the sulfate formed by ABS conversion had a lower relative decomposition temperature.