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Mar 22, 2017 - The heat-exchange tubes of the LPE are double H-type finned tubes, ..... study and field synergy principle analysis of H-type finned tu...
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Deposit Formation of the Low-Pressure Economizer in a Coal-Fired Thermal Power Plant Heng Chen, Jian Jiao, Peiyuan Pan, Qinxin Zhao,* and Yungang Wang 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: A field study was conducted to investigate the deposit formation of the low-pressure economizer (LPE) in a 330 MW thermal power plant. The deposits on the LPE were inspected visually, and eight deposit samples were collected at the inlet and outlet of the LPE according to the site conditions. The samples were characterized by X-ray fluorescence, X-ray diffraction, scanning electron microscopy with energy-dispersive spectroscopy, and particle size analysis. The results showed that the thick deposits on the supporting tubes at the LPE inlet were mainly due to the ash particles that had absorbed the H2SO4 condensate in the flue gas before the LPE inlet. NH4HSO4 began to form on the heat-exchange tubes at the LPE inlet. Both the H2SO4 condensation and NH4HSO4 formation stimulated the deposit formation on the heat-exchange tubes at the LPE inlet. The plugging of the heat-exchange tubes at the LPE outlet was primarily induced by the condensation of much more H2SO4. H2SO4 and NH4HSO4 could also react with the fly ash to form sulfates. The LPE deposition mechanism was discussed, and the coupling model of the H2SO4, NH4HSO4, and fly ash behaviors during the deposit formation under different temperature conditions was proposed. will condense on the heating surface or in the flue gas.7 The H2SO4 condensate may combine with the fly ash to generate viscous deposits on the heating surface, which are adhesive and difficult to be swept away by soot blowers, arousing the fouling of LPEs. In addition, selective catalytic reduction (SCR) has widely been applied in coal-fired power plants to treat flue gas to reduce nitrous oxide emission and meet environmental protection requirements.8 The SRC system is usually installed after the economizer and before the air preheater in a power plant. During the operation of the SCR system, NH3 is injected

1. INTRODUCTION More than 65% of the electricity in China is supplied from coalfired power plants.1 However, the exhaust gas temperature of a coal-fired power plant in China is relatively high, approximately 120−140 °C.2 If the exhaust gas of a power plant can be cooled to about 90 °C by waste heat recovery, the boiler efficiency will increase 2−5% and the power supply coal consumption of the plant will decrease 2−6 g kW−1 h−1. Therefore, there is tremendous potential in the waste heat recovery of coal-fired power plants. At present, many heat recovery devices, such as low-pressure economizers (LPEs), have been used in coal-fired power plants in China. Generally, LPE is a counter-flow tubular heat exchanger, which is located between the air preheater and the electrostatic precipitator (ESP) or between the ESP and the desulfurization tower in a power plant. In a LPE, the flue gas is used to heat the low-temperature feedwater into the boiler, replacing a portion of the steam extracted from the steam turbine.3 To enhance heat transfer, finned tubes have been extensively used as the heat-exchange tubes of LPEs, particularly H-type finned tubes with excellent antiwear and antifouling performance.4 The energy-saving effect is remarkable in the application of LPEs.3,5 However, some already running LPEs have suffered severe fouling/plugging, although they are equip with soot blowers. The fouling of a LPE can decrease the heat transfer efficiency and increase the flue gas pressure drop. Several LPEs even shut down, owing to serious plugging. Therefore, it is necessary to investigate the LPE deposit formation and help reduce the fouling/plugging of LPEs in coal-fired power plants. As a low-temperature heating surface, LPEs are inclined to suffer viscous deposits caused by H2SO4 condensation. During the combustion of sulfur-bearing fuels, such as coal, some SO3 can form and convert to H2SO4 vapor with water vapor in the flue gas.6 When the heating surface temperature or the flue gas temperature is lower than the H2SO4 dewpoint, H2SO4 vapor © XXXX American Chemical Society

Table 1. Main Parameters of the Boiler in the Power Plant parameter

unit

value

temperature of the main steam pressure of the main steam temperature of the reheated steam pressure of the reheated steam

°C MPa °C MPa

540 17.6 545 4.42

into the flue gas as a reducing agent. Unreacted NH3 will leave the SCR system with the flue gas, which is called NH3 slip.9 Some escaped NH3 can react with SO3 and/or H2SO4 in the flue gas to form NH4HSO4, which is sticky and probably induces viscous deposits on the heating surface after the SCR system. Therefore, LPEs are at the risk of NH4HSO4 fouling as a result of NH3 slip. A volume of research has been carried out on the ash deposition of coal-fired boilers. However, most of the work focused on the ash deposition at high temperatures.10−14 Fewer studies have been performed on the deposit characteristic Received: December 31, 2016 Revised: February 27, 2017 Published: March 22, 2017 A

DOI: 10.1021/acs.energyfuels.6b03507 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 2. Fired Coal Analysis of the Boiler in the Power Plant (Mass Fraction, %) proximate analysis ultimate analysis ash analysis

Mar 16.7 Car 61.52 SiO2 52.60

Aar 7.7 Har 3.59 Al2O3 20.25

Vdaf 34.8 Oar 9.39 TiO2 0.91

Qnet,ar (MJ kg−1) 22.9 Nar 0.69 Fe2O3 9.07

St,ar 0.41 CaO 6.04

MgO 2.54

Na2O 0.72

K2O 1.73

SO3 5.20

Figure 1. Location of the LPE in the thermal power plant and sampling positions on the LPE.

Table 3. Designed and Actual Operation Parameters of the LPE parameter designed

actual (mean values during the operation)

Figure 2. Diagram of the heat-exchange tubes (double H-type finned tubes) of the LPE.

under low temperatures. Some researchers have examined the deposit formation of air preheaters in coal-fired power plants.15,16 Several experiments have been performed on lowtemperature ash deposition with test probes in coal-fired power plants.17−19 However, the deposit formation of a real LPE has seldom been discussed in recent years, and the deposition mechanism is still unclear. Besides, the formation and deposition of NH4HSO4 in air preheaters have been explored by a number of investigators,20,21 but little literature has been reported concerning the effect of NH4HSO4 on PLEs in coalfired power plants. In this paper, the deposit formation of the LPE in a coal-fired thermal power plant was investigated by field sampling. The deposit appearance of the LPE was observed visually, and the deposit samples were analyzed chemically and physically. The aim of this study was to reveal the LPE deposition mechanism, which could be conductive to avoiding/alleviating the fouling/ plugging of LPEs in coal-fired power plants.

inlet temperature of the flue gas outlet temperature of the flue gas inlet temperature of the feedwater outlet temperature of the feedwater average velocity of the flue gas (calculated as empty flue duct) pressure drop of the flue gas inlet temperature of the flue gas outlet temperature of the flue gas inlet temperature of the feedwater outlet temperature of the feedwater average velocity of flue gas (calculated as empty flue duct) pressure drop of the flue gas

unit

value

°C °C °C °C

138 95 70 106

m/s

6.56

Pa °C °C °C °C

426 136.8 97.5 70.1 100.9

m/s

6.24

Pa

723

2. EXPERIMENTAL SECTION The studied LPE is installed in a 330 MW thermal power plant in Hebei, China. The boiler of the plant is a subcritical and pulverized coal boiler, and its main designed parameters are presented in Table 1. The fired coal analysis of the plant is given in Table 2. The LPE is arranged after the ESP and in front of the desulfurization tower (Figure 1). The heat-exchange tubes of the LPE are double H-type finned tubes, as shown in Figure 2. Steam soot blowers are installed in the middle of the LPE and purge every 6 h. Table 3 presents the designed and actual operation parameters of the LPE. During the functioning of the LPE, the flue gas pressure drop was much higher than the designed value despite the regular running of the soot blowers. It is likely that the LPE suffered severe fouling, leading to the plugging of its flow passages. The thick deposits lowered the overall B

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Figure 3. Appearances of the LPE tubes after the shutdown of the thermal power plant. heat-transfer coefficient of the LPE, and the flue gas could not be cooled to the designed value. After operating for 4.5 months, the thermal power plant shut down for maintenance. Then, we went into the flue duct to visually inspect the deposits on the LPE and take deposit samples for analysis. The LPE was welded into a sealed hole after it was assembled on the flue duct. Therefore, only the inlet and outlet of the LPE were accessible during the shutdown period, and we could only observe and take samples at the LPE inlet and outlet. Figure 3 illustrates the appearances of the LPE inlet and outlet. The fouling of the tubes at the LPE inlet was not too serious. There were a few vertical supporting tubes before the LPE inlet, and the deposits on the supporting tubes were thick and clearly layered. On the windward side of the heatexchange tubes at the LPE inlet, there were peak deposits on the basic tubes and fin tops. The heat-exchange tubes at the LPE outlet suffered severe plugging. The regions after the basic tubes of the H-type finned tubes were almost blocked at the LPE outlet, and the deposits were layered too. Because the deposit characteristics were different at various parts of the LPE, deposit samples were taken from all of the accessible

Table 4. Details of the Taken Deposit Samples position

sample

details

LPE inlet

1

deposits on the flue duct before the LPE inlet, gray and loose inner layer (close to tube wall) of the deposits on the supporting tubes before the LPE inlet, blackish gray and dense outer layer (close to flue gas) of the deposits on the supporting tubes before the PLE inlet, gray and loose deposits on the windward side of the basic tubes of the heat-exchange tubes at the LPE inlet, gray and hard deposits on the windward fin tops of the heat-exchange tubes at the LPE inlet, gray and hard deposits on the flue duct after the LPE outlet, gray and loose inner layer (close to tube wall) of the deposits on the leeward side of the heat-exchange tubes at the LPE outlet, blackish gray and dense outer layer (close to flue gas) of the deposits on the leeward side of the heat-exchange tubes, grayish white and loose

2 3 4 5 LPE outlet

6 7 8

Table 5. X-ray Fluorescence Analysis Results of the Deposit Samples (Mass Fraction, %)a

a

element

sample 1

Mg Na Ca Ti K S Fe C N Al Si O

0.38 ± 0.03 0.41 ± 0.02 2.31 ± 0.12 0.78 ± 0.01 1.35 ± 0.02 1.9 ± 0.09 1.87 ± 0.12 1.45 ± 0.06 17.32 ± 0.15 21.81 ± 0.15 50.42 ± 0.86

sample 2 0.39 0.38 2.16 0.79 0.99 7.61 2.09 2.80

± ± ± ± ± ± ± ±

0.04 0.04 0.05 0.02 0.06 0.32 0.06 0.10

14.95 ± 0.13 16.92 ± 0.41 50.96 ± 0.93

sample 3 0.47 0.44 2.17 0.83 1.31 3.99 2.52 3.21

± ± ± ± ± ± ± ±

0.01 0.01 0.09 0.01 0.05 0.11 0.20 0.37

16.12 ± 0.38 19.77 ± 0.27 49.22 ± 0.70

sample 4

sample 5

± ± ± ± ± ± ± ± ± ± ± ±

0.36 ± 0.02 0.34 ± 0.02 2.46 ± 0.09 0.46 ± 0.02 0.8 ± 0.02 17.13 ± 0.56 1.91 ± 0.06 1.81 ± 0.12 1.93 ± 0.12 8.51 ± 0.23 11.21 ± 0.31 53.11 ± 0.84

0.37 0.36 2.48 0.48 0.83 16.62 1.92 1.94 1.74 8.87 11.52 52.93

0.02 0.02 0.04 0.07 0.04 0.40 0.08 0.07 0.07 0.13 0.40 0.92

sample 6

sample 7

sample 8

± ± ± ± ± ± ± ± ± ± ± ±

0.49 ± 0.02 0.44 ± 0.02 3.44 ± 0.14 0.68 ± 0.05 1.6 ± 0.11 5.88 ± 0.17 2.92 ± 0.10 0.96 ± 0.05

0.35 ± 0.02 0.37 ± 0.03 2.2 ± 0.07 0.7 ± 0.03 0.96 ± 0.04 10.04 ± 0.20 1.9 ± 0.06 4.07 ± 0.16 1.36 ± 0.04 13.31 ± 0.15 14.82 ± 0.29 49.92 ± 0.40

0.38 0.39 1.84 0.67 1.11 7.98 1.85 2.24 0.75 14.26 17.43 51.14

0.01 0.03 0.11 0.05 0.05 0.28 0.11 0.17 0.03 0.47 0.59 0.63

13.63 ± 0.31 19.14 ± 0.35 50.75 ± 0.56

Values are expressed as means ± standard deviations of three replicated determinations. C

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Energy & Fuels Table 6. X-ray Diffraction Analysis Results of the Deposit Samples sample 1 2 3 4 5 6 7 8

main compounds 3Al2O3·2SiO2, 3Al2O3·2SiO2, 3Al2O3·2SiO2, 3Al2O3·2SiO2, 3Al2O3·2SiO2, 3Al2O3·2SiO2, 3Al2O3·2SiO2, 3Al2O3·2SiO2,

SiO2, SiO2, SiO2, SiO2, SiO2, SiO2, SiO2, SiO2,

Fe2O3, CaSO4, CaSiO3, KAlSi2O6, and C Fe2(SO4)3·9H2O, CaSO4, KAlSi2O6, and C Fe2(SO4)3·9H2O, CaSO4, KAlSi2O6, and C Fe2(SO4)3·9H2O, CaSO4, NH4Al(SO4)2, NH4HSO4, KAl(SO4)2·12H2O, and C Fe2(SO4)3·9H2O, CaSO4, KAlSi2O6, NH4Al(SO4)2, NH4HSO4, and C Fe2(SO4)3·9H2O, CaSO4, NH4Al(SO4)2, KAl(SO4)2·12H2O, and C FeO(OH), CaSO4, KAlSi2O6, and C Fe2(SO4)3·9H2O, CaSO4, NH4Al(SO4)2, KAl(SO4)2·12H2O, and C

the thick deposits on the supporting tubes. The sulfur contents are extremely high in samples 4 and 5. Sample 6 contains much more sulfur than sample 1, and samples 1 and 6 are composed of the fly ash that deposited on the flue duct from the flue gas. It seems that the sulfur content of the fly ash in the flue gas increased after the LPE, because H2SO4 and NH4HSO4 might adhere to the ash particles in the flue gas during the cooling process. There is distinct nitrogen in samples 4−6 and 8. The carbon content is evident in each sample, which means a lot of incompletely burned carbon could exist in the flue gas. Table 6 suggests that sulfur exists as CaSO4, Fe2(SO4)3· 9H2O, NH4Al(SO4)2, NH4HSO4, and KAl(SO4)2·12H2O in the deposit samples. CaSO4, Fe2(SO4)3·9H2O, and KAl(SO4)2· 12H2O could be the reaction products between the H2SO4 condensate and fly ash. According to the compounds in the deposits, we can infer that the primary reactions occurred between the H2SO4 condensate and compounds in the fly ash, as follows: CaSiO3 + H 2SO4 → CaSO4 + SiO2 + H 2O

(1)

Fe2O3 + 3H 2SO4 + 6H 2O → Fe2(SO4 )3 ·9H 2O

(2)

KAlSi 2O6 + 2H 2SO4 + 10H 2O → KAl(SO4 )2 ·12H 2O + 2SiO2

(3)

NH4HSO4 was possibly generated by the reaction between SO3/ H2SO4 and NH3 that escaped from the SCR system. However, when the flue gas temperature is below 200 °C, SO3 almost completely transforms into H2SO4 in the flue gas.22,23 Therefore, there was H2SO4 but no SO3 existing in the flue gas at the LPE, where the inlet flue gas temperature was about 136.8 °C. The formation reaction of NH4HSO4 should be like reaction 4,24 and NH4HSO4 might react with the fly ash to form NH4Al(SO4)2, as indicated in reaction 5.25 Figure 4. Typical SEM images and EDS detecting microzones of the deposit samples.

NH3 + H 2SO4 → NH4HSO4

parts, as shown in Figure 1. The eight deposit samples are described in Table 4. The deposit samples were ground for the following analysis. The elemental compositions of the samples were detected by X-ray fluorescence. The primary compounds in the samples were determined by X-ray diffraction. The microstructures of the samples were observed by scanning electron microscopy (SEM), and the elemental compositions of different microzones were identified by energy-dispersive spectroscopy (EDS). A laser particle size analyzer was applied to measure the particle sizes of the samples.

3Al 2O3 ·2SiO2 + 12NH4HSO4

(4)

→ 6NH4Al(SO4 )2 + 6NH3 + 9H 2O + 2SiO2

(5)

The microtopography and elemental compositions of the microzones of the deposit samples are presented in Figure 4 and Table 7. In sample 1, the particles are mainly round and dispersive and some particles are porous. Nevertheless, there are plenty of agglutinate parts in other samples. Sulfur is rich in the agglutinate parts of samples 2−8, but nitrogen is only apparent in the agglutinate parts of samples 4−6 and 8, which is consistent with the X-ray fluorescence analysis results (Table 5). Most of the agglutinate parts are constructed of fine particles. It is likely that these fine particles were gummy during the deposit formation. The H2SO4 condensation and NH4HSO4 formation could be the causes of the sticky nature of fine particles.

3. RESULTS The X-ray fluorescence analysis results (Table 5) indicate that the sulfur contents of samples 2 and 3 are much higher than that of sample 1, but no nitrogen is found in these three samples. It appears that H2SO4 instead of NH4HSO4 contributed to D

DOI: 10.1021/acs.energyfuels.6b03507 Energy Fuels XXXX, XXX, XXX−XXX

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Table 7. EDS Analysis Results of the Deposit Samples Corresponding to the Detecting Microzones in Figure 4 (Mass Fraction, %) sample

zone

1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

2

3

4

5

6

7

8

C 24.32 11.53 11.97 13.29 16.95 22.07 18.91 23.76 17.12 12.32 6.10 8.33 7.63 9.02 9.69 12.07 25.07 21.45 27.25 43.81 18.20 23.04 20.39 19.70 40.18 23.19 23.74

N

2.32 2.78 1.42 2.94 3.13 2.46 2.87 1.39 1.03

1.54 1.28 1.82

O 41.59 38.96 35.29 41.77 45.93 42.66 41.75 43.65 42.33 40.11 47.74 41.45 44.49 38.57 45.88 47.72 47.90 42.55 37.15 38.90 24.65 42.34 27.69 35.29 33.64 32.32 38.24 34.31

Na 0.33

Mg 0.65

0.44

0.46 0.29 0.14 0.57 0.61 0.57 0.21 0.30

0.38 0.28

0.64 0.38 0.31

0.61 0.40 1.21

0.22 0.24 0.24

0.24 0.39 0.23

Al

Si

31.90 12.57 13.80 20.89 7.96 9.59 15.5 12.29 11.79 12.82 8.30 9.19 14.85

25.00 14.30 37.10 23.20 9.11 10.90 17.60 16.50 19.90 20.10 5.03 7.83 16.6 0.70 11.70 11.40 8.60 15.70 13.30 10.00 27.70 11.00 12.30 13.40 42.00 6.83 11.40 12.7

7.14 8.04 6.67 15.62 12.74 8.65 10.88 9.22 10.54 3.22 6.66 10.51 11.29

S 2.97 0.93 17.12 12.91 1.60 3.25 1.38 2.92 21.39 28.24 12.01 32.45 16.23 14.85 16.01 1.02 10.85 9.93 9.72 6.44 6.71 8.61 9.37 10.7

K 1.52 0.69 2.27 0.65

Ca

Ti

Fe

1.88

0.90

2.45

0.84

1.27 1.97

0.49 0.81

0.62 4.48 4.99 0.50 1.41

0.87 0.31 0.50 0.70

1.03 0.61 1.24 0.55

0.58 0.64 0.59

3.81 2.64 3.55

0.72 0.63 1.10 0.44 2.07 1.12 1.46 0.33 0.48 0.63

0.48 0.54

2.25 0.84 5.02 1.53 2.38 0.90 17.71 1.96 1.96 1.18

4.62 6.90 1.88

0.68 0.74

1.86 1.41 1.36 3.92 10.85 8.40

1.41 2.21 1.86

0.43 0.94 0.43

1.23 1.76 2.06

1.11

0.47 1.40

Figure 5. Particle size distributions of the deposit samples.

Figure 5 and Table 8 illustrate the particle size analysis results of the deposit samples. There are many more big particles in sample 1 than other samples. Sample 2 includes a large amount of fine particles, and its mean specific surface area is the biggest. The fine ash particles with big specific surface areas were easier to absorb the H2SO4 condensate or liquid NH4HSO4 in the flue gas. Sample 2 contains more fine particles than sample 3, and Sample 7 contains more fine particles than sample 8, which means that there were more fine particles in the inner layers than the outer layers of the deposits.

condensate might facilitate the deposits on the supporting tubes. Because the supporting tubes did not exchange heat with the flue gas, their wall temperatures should be the same with that of the flue gas. It seems that a small part of H2SO4 vapor had condensed and attached to the ash particles in the flue gas before the LPE inlet, which made the particles sticky and easy to deposit on the supporting tubes. The mean particle sizes of samples 2 and 3 were much smaller than that of sample 1, and the particles in samples 2 and 3 had much bigger specific area surfaces than those in sample 1. These particles with big specific area surfaces were more likely to absorb the H2SO4 condensate in the flue gas, then became sticky, and bond to the supporting tubes. Sample 2 contained more fine particles and sulfur than sample 3, indicating that more fine particles that had absorbed the H2SO4 condensate adhered to the supporting tubes to form

4. DISCUSSION According to the analysis results, we can see that samples 2 and 3 contained much more sulfur than sample 1 but no nitrogen existed in these three samples, implying that the H2SO4 E

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deposits on the heat-exchange tubes at the LPE inlet. NH4HSO4 usually started to form in the air heater after NH3 fled from the SCR system with the flue gas.9 It seems that escaped NH3 had not entirely converted to NH4HSO4 that could deposit on the heating surface or be removed by the ESP before the LPE inlet, and a small portion of NH3 reached the LPE inlet. However, the flue gas temperature is higher than the NH4HSO4 formation temperature at the LPE inlet, considering the low NH3 concentration in the flue gas at the LPE inlet. However, the wall temperature of the heat-exchange tubes was below the NH4HSO4 formation temperature, which resulted in the NH4HSO4 formation near the heat-exchange tubes and increased the deposits on the heat-exchange tubes at the LPE inlet. Sample 6 contained much more sulfur than sample 1; namely, the ash deposited at the LPE outlet had a higher sulfur content than the ash accumulated at the LPE inlet, which means that the sulfur content of the fly ash in the flue gas increased with the decrease of the flue gas temperature. It seems that a lot of H2SO4 and NH4HSO4 stuck to the fly ash during the flue gas cooling. The mean particle size of sample 6 was smaller than that of sample 1. As a result of more H2SO4 condensate, the particles in the flue gas became more viscous after the LPE and more fine particles could deposit on the flue duct. No NH4HSO4 but only NH4Al(SO4)2 exists in samples 6 and 8, which indicates that nearly all NH3 had converted to NH4HSO4 with H2SO4 and deposited on the tubes or reacted with the fly ash to form NH4Al(SO4)2 before the LPE outlet. Little NH4HSO4 exists in samples 7 and 8, and the sulfur contents are high in samples 7 and 8. It appears that only H2SO4 contributed to the plugging of the heat-exchange tubes at the LPE outlet. Because the flue gas temperature and tube wall temperature at the LPE outlet were much lower than those at the inlet, a lot more H2SO4 would condense in the flue gas and on the heat-exchange tubes, aggravating the fouling at the LPE outlet. To summarize, two major contributors to the LPE deposits were the H2SO4 condensation and NH4HSO4 formation, which were both closely related to the flue gas temperature and tube wall temperature. Figure 6 shows the variations of the flue gas temperature and the heat-exchange tube wall temperature when the flue gas flowed through the LPE. According to the flue gas parameters, feedwater parameters, and tube material property, the tube wall temperature was calculated by empirical correlations.26 Some H2SO4 had condensed in the flue gas before the LPE inlet, and the flue gas temperature was about 136.8 °C. This H2SO4 condensate mainly bonded to the fine ash particles and accelerated the deposits on the supporting tubes at the LPE inlet. NH4HSO4 began to form on the heat-exchange tubes at the LPE inlet, where the heat-exchange tube wall temperature was approximately 103.9 °C. Both H2SO4 and NH4HSO4 facilitate the deposit formation on the heat-exchange tubes at the LPE inlet. During the flue gas cooling process, H2SO4 continuously condensed and NH4HSO4 constantly formed in the flue gas or on the heat-exchange tubes. NH4HSO4 had nearly all formed, deposited, and reacted before the LPE outlet. Only H2SO4 contributed to the plugging at the LPE outlet. The above analysis revealed that the H2SO4 condensation and NH4HSO4 formation occurred under different temperatures corresponding to various parts of the LPE, assisting the deposit formation on the LPE. According to the fundamental principles of motion and the basic characteristics of gas, liquid droplets, and particles, the coupling model of the H2SO4, NH4HSO, and fly ash behaviors during the deposit formation on the LPE tubes was proposed, as shown in Figure 7.

Table 8. Particle Size Analysis Results of the Deposit Samplesa sample 1 2 3 4 5 6 7 8

specific surface area (m2/g) 1.15 3.30 2.73 2.76 3.13 2.23 1.22 2.19

± ± ± ± ± ± ± ±

0.04 0.09 0.01 0.06 0.08 0.02 0.01 0.05

mean surface area diameter (μm)

mean volume diameter (μm)

± ± ± ± ± ± ± ±

18.978 ± 0.342 3.464 ± 0.013 4.611 ± 0.024 8.64 ± 0.241 4.938 ± 0.112 8.085 ± 0.074 10.716 ± 0.063 14.387 ± 0.381

5.224 1.818 2.194 2.175 1.917 2.693 4.933 2.744

0.149 0.015 0.067 0.044 0.059 0.029 0.051 0.054

Values are expressed as means ± standard deviations of three replicated determinations. a

Figure 6. Variations of the flue gas temperature and the heat-exchange tube wall temperature when the flue gas flowed through the LPE.

the inner layer. Sulfur existed as Fe2(SO4)3·9H2O and CaSO4 in samples 2 and 3 but only as CaSO4 in sample 1. It is likely that the H2SO4 condensate first reacted with the calcium compounds and then reacted with other compounds containing alkali metals in the fly ash. The sulfur and nitrogen contents of samples 4 and 5 were the highest among all of the deposit samples. Sulfur existed as Fe2(SO4)3·9H2O, CaSO4, and KAl(SO4)2·12H2O in these two samples, which could be the reaction products between the H2SO4 condensate and fly ash. Sulfur and nitrogen coexisted as NH4Al(SO4)2 and NH4HSO4. It appears that not only the H2SO4 condensation but also the NH4HSO4 formation stimulated the dense deposits on the windward side of the heatexchange tubes at the LPE inlet. The flue gas parameters were very similar for the supporting tubes and the heat-exchange tubes at the LPE inlet, but their wall temperatures were different. The wall temperature of the heat-exchange tubes at the LPE inlet was about 72.4 °C (as shown in Figure 6), which was much lower than the wall temperature of the supporting tubes (approximately 136.8 °C), resulting in the H2SO4 condensation near the heat-exchange tubes at the LPE inlet. In addition, no NH4HSO4 existed in the deposits on the supporting tubes, but significant NH4HSO4 appeared in the F

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Figure 7. Coupling model of the H2SO4, NH4HSO2, and fly ash behaviors during the deposit formation on the LPE tubes.

resulted in the thick deposits on the supporting tubes at the LPE inlet. (2) NH4HSO4 started to form on the heat-exchange tubes at the LPE inlet. Both the H2SO4 condensation and NH4HSO4 formation stimulated the deposit formation on the heat-exchange tubes at the LPE inlet. (3) H2SO4 kept condensing and NH4HSO4 kept forming when the flue gas flowed through the LPE. The sulfur content of the fly ash in the flue gas increased during the flue gas cooling. Almost all NH4HSO4 had formed before the LPE outlet. (4) Much more H2SO4 condensed at the LPE outlet and contributed to the plugging on the heat-exchange tubes at the LPE outlet. (5) With ash particles being made sticky and fly ash being attracted on the tubes, the H2SO4 condensate and liquid NH4HSO4 promoted the LPE deposit formation. Both H2SO4 and NH4HSO4 reacted with fly ash to form sulfates.

On the basis of their probable behaviors, the H2SO4 condensate and liquid NH4HSO4 could accelerate the deposit formation in two ways: (1) adhering to the ash particles in the flue gas and making these particles sticky and (2) depositing on the wall and attracting ash particles from the flue gas. Besides, liquid NH4HSO4 would solidify after its formation. However, NH4HSO4 is hygroscopic,27 and solid NH4HSO4 could absorb the water vapor to become sticky, which would be beneficial to the deposit formation. Meanwhile, both H2SO4 and NH4HSO4 reacted with the fly ash to form sulfates.

5. CONCLUSION Through the visual inspection, chemical analysis, microscopic examination, and particle size analysis of the deposits on the LPE in a thermal power plant, the mechanism of the LPE deposit formation was investigated. On the basis of the analysis results, the following conclusions are detailed: (1) A small amount of H2SO4 had condensed and mainly adhered to the fine ash particles in the flue gas before the LEP inlet, which

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. G

DOI: 10.1021/acs.energyfuels.6b03507 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels ORCID

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Heng Chen: 0000-0001-6256-4184 Yungang Wang: 0000-0002-2652-9143 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Basic Research Program of China (973 Program, 2013CB228304) and the National Natural Science Foundation of China (51606144).

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DOI: 10.1021/acs.energyfuels.6b03507 Energy Fuels XXXX, XXX, XXX−XXX