Study on Deposits Containing Rich Fluorine, Boron, and Ammonium

Mar 28, 2017 - 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 Ch...
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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 Haidong Ma, Yungang Wang,* Qinxin Zhao, and Heng Chen Key Laboratory of Thermo-Fluid Science and Engineering of Ministry of Education (MOE), School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, People’s Republic of China 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). deposition on the flue gas cooler caused by ammonium slip. To investigate this ash deposition issue, a detailed mineralogical analysis was implemented to understand how the ash particles initially stick and subsequently form ash layers in this paper. Then, the mechanism leading to the formation of hardened consolidated ash on the tube wall was mainly discussed.

1. INTRODUCTION In the last few decades, most of the power generation in China was from coal-fired power generation.1 However, the exhaust gas temperature of a coal-fired boiler is generally 20−30 °C higher than the design value.2 To recover the waste heat of the flue gas and enhance the thermal efficiency of the boiler, the most immediate and effective measure currently is to install a flue gas cooler at the tail flue of the boiler.3 As the temperature of the flue gas decreases, the acid gas in the flue gas, such as sulfur trioxide, will condense and form acid on heat-transfer surfaces. This may lead to dew point corrosion, destruction of the surfaces, ash deposition, and blockage.4−8 Previous studies indicated that the ash deposition was mainly composed of ash and acid reaction products, iron sulfate and iron oxide. At present, there are few studies on ash deposition of the flue gas cooler caused by ammonium slip. In addition, as the effective ways to reduce NOx emission, selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) systems are adopted extensively.9 However, dependent upon the process, the ammonium slip problem is unavoidable; namely, some ammonia will continue through the system unreacted.10 When ammonia combines with sulfur trioxide present in the flue gas, ammonium sulfate [(NH4)2SO4] and ammonium bisulfate (NH4HSO4) will form on the boiler components, such as air preheater.11,12 Ammonium sulfate and ammonium bisulfate are known to cause corrosion and blockage in the air preheater that can require unnecessary outages and expensive cleaning. Up to now, a lot of studies have been carried out about ammonium slip.5,13−15 Most of the previous research focused on the formation of (NH4)2SO4 and NH4HSO4 in the air preheater, and little research has been devoted to ash © 2017 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Sample. The samples were obtained from the flue gas cooler in a 300 MW subcritical coal-fired boiler in China. The flue gas cooler using H-finned tubes was installed between the bag filter and the desulfurization tower, as shown in Figure 1, which was designed to heat steam condensate water with waste heat from the flue gas. Caused by the flue gas cooler, the exhaust gas temperature reduced by 19 K, recovering waste heat equivalent to a 0.885 g kW−1 h−1 decrease of the gross coal consumption rate. The inlet water temperature of the flue gas cooler is 341 K. For the coal burned, which was collected from 20 different places in the coal bunker of the plant, the ultimate, proximate, and ash analyses are shown in Table 1. After 6 months of commissioning operation, with 5% ammonium slip, the ash deposition on the flue gas cooler was obviously observed during routine inspection, as shown in Figure 2. It can be seen that many white spots covered the whole fin and the base tube was seriously covered by white ash deposits. To analyze conveniently, the white deposition layer located windward was cut to samples illustrated in Figure 3. The ash deposition was very compact and crisp. The thickness of the ash layer was about 3 mm. According to the color of the ash layer, the sample could be divided into three layers, including a maroon inner layer, light red interlayer, and white outer layer, as shown in Figure 4. The three Received: December 5, 2016 Revised: March 22, 2017 Published: March 28, 2017 4742

DOI: 10.1021/acs.energyfuels.6b03228 Energy Fuels 2017, 31, 4742−4747

Article

Energy & Fuels

Figure 1. Schematic diagram of boiler system.

Table 1. Ultimate, Proximate, and Ash Analyses of the Coal (Mass Fraction, %)

a

component

content (wt %)

oxides

content (wt %)

moisture ash volatiles fixed carbon Cada Had Oad Nad Sad

1.03 3.14 13.50 51.02 49.56 2.04 4.33 0.83 0.51

SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O TiO2 SO3

51.90 36.19 3.51 0.65 0.82 1.32 0.44 0.58 0.85

Figure 4. Cross section enlarged figure of the sample. layers were separated cautiously. Then, the samples were analyzed by X-ray fluorescence (XRF), X-ray powder diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), and X-ray photoelectron spectroscopy (XPS). 2.2. Experimental Methods. For elemental analysis, XRF was performed by a Bruker S4-PIONEER X-ray fluorescence spectrum (Germany), which can be operated at 60 kV and 150 mA at the most, using 4 kW maximum power and Ru target, employing a Be excitation source. The main compounds in the samples were identified by qualitative XRD on a PANalytical X’pert PRO MPD powder diffractometer (Netherlands) employing Cu Kα radiation (λ = 0.154 06 nm), operated at 40 mA and 40 kV, recorded at a scan rate of 1°/min in the range of 20° < 2θ < 70°. Peak identification was analyzed by a comparison to standards using the JADE6.5 software package. The morphologic micrograph and designated elemental determinations of samples were carried out using field emission SEM, with EDS on JSM-6390 (Japan) at 15 kV for accelerating voltage. For confirmation of boron existence, the XPS studies were operated on Kratos Axis-Ultra DLD (Japan), with a monochromatic Al Kα line source at an accelerating voltage of 150 W. The experiments were performed twice to ensure the acceptable reproducibility.

ad = air-dried basis.

3. RESULTS 3.1. Results of XRF Analysis. The element contents were quantitatively analyzed by XRF, as shown in Figure 5. The major elements in the outer layer deposits were F, O, N, Al, and

Figure 2. Deposits on the surface of the flue gas cooler.

Figure 3. Physical diagram of samples. Figure 5. Element contents in three layers of deposits on the surface of the flue gas cooler. 4743

DOI: 10.1021/acs.energyfuels.6b03228 Energy Fuels 2017, 31, 4742−4747

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Figure 6. XRD-analyzed results of the outer layer in the deposits.

Figure 7. XRD-analyzed results of the interlayer in the deposits.

Figure 8. XRD-analyzed results of the inner layer in the deposits.

2θ = 30.074°, 37.067°, 43.070°, and 53.611], and boric acid (H3BO3, 2θ = 30.781°), which are white or transparent crystals at steady room temperature.16−18 In addition, the outer layer contained some silicon dioxide (SiO2, 2θ = 27.031° and 50.470°), aluminum silicate (Al2SiO5, 2θ = 38.601° and 50.060°), and aluminum oxide (Al2O3, 2θ = 46.720° and 56.970°), which are components of coal ash.19 The major compounds in the inner layer of deposits were ferric oxide (F2O3, 2θ = 20.692°, 31.890°, 33.151°, and 68.218°), ferrous sulfate (FeSO4, 2θ = 21.090°, 41.308°, and 49.695°), ferric oxyhydroxide (FeOOH, 2θ = 26.875°, 39.308°, and 39.963°), ferric chloride (FeCl3, 2θ = 35.534° and 46.281°), ferrous fluoride (FeF2, 2θ = 38.341° and 55.542°), and ferrous chloride (FeCl2, 2θ = 50.658° and 53.172°), which are ferrous products of dew point corrosion.20 It can also be seen from Figure 7 that the phases determined were fractionally in agreement with those in outer and inner layers, such as NH4BF4 (2θ = 23.081°,

Si. However, Fe, Cl, and S were relatively poor. As for the interlayer deposits, the contents of F, O, N, Fe, and Si were much higher than those of Cl, Al, and S. High concentrations of O and Fe in the inner layer deposits were obvious as well as low concentrations of S and F. The outer layer deposits had the highest concentration of F and the lowest concentrations of Fe, Cl, and S. Additionally, the contents of O, Fe, S, and Cl in the inner layer were the highest, while the contents of F, N, and Si was the lowest. 3.2. Results of XRD Analysis. To determine the compounds of deposits, XRD analysis was adopted and the results are shown in Figures 6−8. We can see that the major compounds in the outer layer of deposits were ammonium fluoroborate (NH4BF4, 2θ = 23.087°, 24.451°, 25.096°, 27.974°, 31.485°, 35.314°, 35.823°, 37.320°, 39.408°, 40.388°, and 41.676°), ammonium fluoride (NH4F, 2θ = 26.265°, 32.542°, and 64.059°), ammonium fluorosilicate [(NH4)2SiF6, 4744

DOI: 10.1021/acs.energyfuels.6b03228 Energy Fuels 2017, 31, 4742−4747

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Figure 9. XPS spectra of the surface of deposits.

24.429°, 25.092°, 27.969°, 31.504°, 35.314°, 35.292°, 35.818°, and 41.645°), NH4F (2θ = 26.232°, 32.550°, and 64.032°), (NH4)2SiF6 (2θ = 30.077°, 37.088°, 43.041°, and 56.970), H3BO3 (2θ = 30.837°), SiO2 (2θ = 25.936° and 50.461°), F2O3 (2θ = 3.151°and 60.589°), FeSO4 (2θ = 21.114°), FeF2 (2θ = 55.606°), and FeCl2 (2θ = 53.172°). 3.3. Results of XPS Analysis. The XPS analysis results of the surface of deposits is shown in Figure 9. The characteristic peaks of B appeared at around 194.7 eV. The binding energy peak of the B 1s spectrum for the surface of deposits is presented in Figure 9a. It is known that XPS peaks in the B 1s spectrum at 194.9 and 192.8 eV proved the presence of NH4BF4 and H3BO3.21 3.4. Results of SEM and EDS. A SEM image of the outer layer surface is shown in Figure 10, and element contents of

Figure 11. Element contents of three spots.

Thus, HBF4 is the only apposite compound, which is liquid at room temperature and out of XRD and XPS analysis results.23 3.5. Determination of Fluorine and Boron Elements. As the results have been discussed, the deposits enriched in F and B. Therefore, the F content in the fired coal had been determined according to Chinese National Standard GB/ T4633-1997, while B was analyzed using inductively coupled plasma mass spectrometry (ICP−MS), as given in Table 2. For F and B, the contents of the coal fired in this power plant were 2.11 and 4.38 times larger than those of the average value of Chinese coal, respectively.24−26 Table 2. Contents of F and B in Coal element

fired coal (μg g−1)

average value of Chinese coal (μg g−1)

F B

287 276

136 63

4. DISCUSSION 4.1. Formation Mechanism of NH4BF4. Complexation with clay minerals is mainly occurrence states of B in coal,27 which is ascharite (2MgO·B2O3·H2O) usually. Ascharite will decompose at a high temperature (eq 1), and the resultant will be acidized as H3BO3 (eq 2).

Figure 10. SEM micrograph of the outer layer surface.

three spots (shown in Figure 10) are enumerated in Figure 11. A lot of spherical particles distributed loosely on the surface of deposits, and spot 1 had a elemental constituent similar to that of coal ash, which was identified as ash particles. There was a lot of F and N and relatively little O in spots 2 and 3, which had a higher mass ratio of F and N (F/N) than NH4BF4. Therefore, it was certain that a kind of compound with much higher F/N existed in deposits. The compound constituted F, N, and other elements that are in front of C in the periodic table of elements, resulting in misses by EDS and XRF. As a result, hydrofluoric acid (HF), fluoroborate (HBF4), and tetrafluoroammonium fluoroborate (NF4BF4) suit the conditions above. However, HF is extremely volatile as well as NF4BF4 at room temperature.22

2MgO· B2O3 · H 2O(s) → 2MgO· B2O3(s) + 2H 2O(g) (high temperature)

(1)

MgO· B2O3(s) + 2H 2O(1) + 2H+(l) → 2H3BO3(l) + Mg 2 +(l)

(2)

HF and silicon tetrafluoride (SiF4) are the major resultants caused by the fluoride decomposition with coal combustion,28,29 which will react with ammonia, as shown in eqs 3−5. 4745

DOI: 10.1021/acs.energyfuels.6b03228 Energy Fuels 2017, 31, 4742−4747

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concentration to form some oxide, chloride, fluoride, and sulfate of Fe.32 (2) Escaped ammonia from the SCR system reacted with the condensing acids (HF was the major acid) to form NH4BF4. The surplus intermediates, such as NH4F, H3BO3, and (NH4)2SiF6, deposited on the corrosion products, to constitute the white crystal substances with NH4BF4. (3) When the unit was operating normally, the thickness of the white crystal layer was increasing and the temperature of its surface was rising continuously, until it was higher than the hydrofluoric acid dew point of flue gas. The ash particles were trapped by the white crystal layer to form the ash layer.

The resultants discussed above will react with H3BO3 to form NH4BF4,30,31 as described in detail (eqs 6−10). 2HF(g) + SiF4 (g) → H 2SiF6(g)

(3)

H 2SiF6(g) + 2NH4OH(l) → (NH4)2 SiF6(s) + 2H 2O(l) (4)

NH3· H 2O(l) + HF(l) → NH4F(s) + H 2O(l)

(5)

2H3BO3(l) + 2HF(l) + (NH4)2 SiF6(s) → 2NH4BF4 (s) + H 2SiO3(l) + 3H 2O(l)

(6)

5. CONCLUSION Deposits on the surface of the flue gas cooler in a 300 MW coal-fired boiler in China were collected, sampled, and analyzed by XRF, XRD, XPS, SEM, and EDS. The following conclusions can be drawn: (1) The deposits were composed with three layers, namely, the white outer layer, maroon inner layer, and light red interlayer. The high concentrations of O, Si, and Al in the outer layer were indicative of the formation of 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 middle layer. In addition, the relatively higher contents of F and N in the outer layer and interlayer were interpreted as the formation of NH4BF4 and its intermediates [NH4F, NH4F, (NH4)2SiF6, and H3BO3]. (2) For all of the white crystals, the final product of chemical reactions was NH4BF4, and intermediates were compounds of NH4F, H3BO3, (NH4)2SiF6, and HBF4. It is the main cause that F and B enriched in the coal react with ammonia escaped from the SCR system. (3) The formation of deposits had three steps, including the initial stage, growth stage, and mature stage. Condensing HF was the key reactant in the formation reactions of NH4BF4.

H3BO3(l) + 4HF(l) → HBF3OH(l) + HF· H 2O(l) + H 2O(l)

(7)

HBF3OH(l) + HF(l) → HBF4 (l) + H 2O(l)

(8)

HBF4 (l) + NH4F(s) → NH4BF4 (s) + HF(l)

(9)

HBF4 (l) + NH3(g) → NH4BF4 (s)

(10)

The equations enumerated above describe that the reaction existed actually in the process of the boiler, as summarized in Figure 12; the compounds in red blocks were demonstrated to exit in deposits.

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AUTHOR INFORMATION

Corresponding Author

Figure 12. Formation process of NH4BF4.

*E-mail: [email protected]. ORCID

4.2. Growth Mode of the Deposits. The results suggest that the formation of deposits had three following steps, as shown in Figure 13: (1) At the beginning or end of the unit operation, many kinds of acid condensed on the lowtemperature heating surface of the flue gas cooler. The metal surface was electrochemical-corroded by the acids of a low

Haidong Ma: 0000-0002-0971-0098 Yungang Wang: 0000-0002-2652-9143 Heng Chen: 0000-0001-6256-4184 Notes

The authors declare no competing financial interest.

Figure 13. Growth mode of the deposits. 4746

DOI: 10.1021/acs.energyfuels.6b03228 Energy Fuels 2017, 31, 4742−4747

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ACKNOWLEDGMENTS This work was supported by the National Science & Technology Pillar Program of China (2015BAA04B03) and the National Key Research and Development Program of China (2016YFC0801904). The authors also thank the coal-fired power plant of Lianyuan, China.

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DOI: 10.1021/acs.energyfuels.6b03228 Energy Fuels 2017, 31, 4742−4747