Study on Deposits Containing Rich Fluorine, Boron, and Ammonium on the Heating Surface of a Flue Gas Cooler in a 300 MW Coal-Fired Boiler

Abstract

To understand deposit formation in the flue gas cooler, which is used to recover the exhaust heat from a 300 MW coal-fired boiler in China, a mineralogical study was carried out. Several deposit samples on the surface of the flue gas cooler were collected. Then, the samples were examined by X-ray fluorescence (XRF), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy-dispersive spectrometry (EDS). Mineralogical analysis showed that the deposits could be divided into three layers. The high concentrations of O, Si, and Al in the outer layer were indicative of the formation of ash particles in the flue gas, and the high contents of Fe, O, Cl, and S in the inner layer were indicative of the formation of iron corrosion products, in comparison to the interlayer. In addition, the relatively higher contents of F and N in outer layer and interlayer were interpreted as the formation of ammonium fluoroborate (NH4BF4) and its intermediates [NH4F, HBF4, (NH4)2SiF6, and H3BO3], which were further proven by XRD and XPS analyses. The inner layer defined as the corrosion layer was caused by condensed acid on the surface of the flue gas cooler when the heating surface temperature was below the HF dew point. The formation of NH4BF4 was due to the enrichment of fluorine and boron in the coal as well as the escape of ammonia from selective catalytic reduction (SCR).

Investigation on High Temperature Corrosion Characteristic of Super304H, TP347H, and HR3C Steel in an Ultra‐supercritical Coal‐fired Boiler▴

“…According to Figure 4, the major compounds in the outer layer of samples were similar, including ferric oxide, silica, calcium sulfate and alumina; in addition SiO 2 , Al 2 O 3 , and CaSO 4 were indicative of the formation of ash particles in the flue gas [23].…”

Experimental investigation of the strength characteristics of low‐temperature fouling containing ammonium bisulfate in boilers

“…The low‐temperature fouling on the metal heating surface will greatly reduce the heat transfer efficiency of boiler heat recovery equipment such as air preheaters, low‐pressure economizers, and flue gas coolers due to the presence of fouling thermal resistance 15–19 . Besides, it can result in metal corrosion, serious blockage of the flue gas flow channel, and so on 20 . In brief, the existence of low‐temperature fouling in the tail flue will result in lower boiler efficiency and lead to an adverse effect on the normal and safe operation of coal‐fired power station boilers 21 …”

“…Vuthaluru et al 25 provided valuable information on the air preheater deposits and mineralogy, indicating that the temperature was an important factor in the low‐temperature fouling. Ma et al 20 found that NH 4 BF 4 could also result in severe fouling in the flue gas cooler. Meanwhile, numerical methods have been used to study the fouling process.…”

“…[15][16][17][18][19] Besides, it can result in metal corrosion, serious blockage of the flue gas flow channel, and so on. 20 In brief, the existence of lowtemperature fouling in the tail flue will result in lower boiler efficiency and lead to an adverse effect on the normal and safe operation of coal-fired power station boilers. 21 At present, extensive research such as the growth characteristic, mineralogy, and chemical composition of the ABS-bearing low-temperature fouling has been investigated.…”