Sulfate Removal by Kaolin Addition To Address Fouling in a Full-Scale

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Sulfate Removal by Kaolin Addition to Address Fouling in a Full-Scale Furnace Burning High-Alkaline Zhundong Coal Xuebin Wang, Renhui Ruan, Tao Yang, Adewale Adeosun, Limeng Zhang, Bo Wei, Houzhang Tan, and Richard L Axelbaum Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02099 • Publication Date (Web): 20 Oct 2017 Downloaded from http://pubs.acs.org on October 23, 2017

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Sulfate Removal by Kaolin Addition to Address Fouling in a Full-Scale Furnace Burning High-Alkaline Zhundong Coal

Xuebin Wang 1, 2, Renhui Ruan 1, Tao Yang 3, Adewale Adeosun 2, Limeng Zhang 4, Bo Wei 1, Houzhang Tan 1, *, Richard L. Axelbaum 2, *

1

MOE Key Laboratory of Thermo-Fluid Science and Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi, 710049, China

2

Department of Energy, Environmental & Chemical Engineering, Consortium for Clean Coal Utilization, Washington University in St. Louis, St. Louis, MO, 63130, USA 3

4

Xi’an Thermal Power Research Institute Co., Ltd., Xi’an 710032, China

State Grid Shandong Electric Power Research Institute, Jinan 250002, China * Phone: +86-029-82668703. E-mail: [email protected] *Phone: +1-314-935-7560. Email: [email protected]

ABSTRACT: Zhundong (ZD) coal comes from the largest coalfield in the world, with a reserve of 390 billion tons. However, the high content of sodium and calcium in ZD coal causes severe fouling. Presently, no pulverized coal-fired furnace can burn ZD coal alone. In this study, kaolin 1

ACS Paragon Plus Environment

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addition is proposed to address fouling in a full-scale furnace burning ZD coal. Before performing a long-term full-scale furnace test, a series of fundamental studies in a thermogravimetric analyzer (TGA) and drop tube furnace were conducted to demonstrate the effectiveness of kaolin in reducing sulfates. The TGA results show that kaolin addition decreases the decomposition temperature of sulfates in coal ash by higher than 120 °C, and kaolin addition at 1150 °C eliminates all the sulfates in the ash. Further experiments on ash deposition and particulate matter (PM) formation in a drop tube furnace at 1350 °C show that kaolin addition successfully eliminates severe ash deposition and fouling on probe surfaces, and reduces the amount of PM1.0 by half. An analysis of the morphology and composition of fine particles and ash deposition clearly shows that kaolin addition reduces the formation of the sulfate aerosol and prevents its adhesion on the surfaces of larger ash particles. it further decreases the content of sulfur, sodium and, calcium in PM1.0. Based on the lab-scale studies, a nine-month long test was conducted in a 350 MW full-scale furnace burning ZD coal with kaolin addition. This long-term test show that by reducing the average sodium and calcium content in ZD coal ash to 2.1 and 13.8 wt.% through kaolin addition, severe ash deposition and fouling can be avoided. Elemental analysis of the fly ash also shows that the sulfur content after kaolin addition is only 1.8 wt.%. By comparison, previous tests of coal co-firing without kaolin addition, conducted in the same furnace burning the same ZD coal, show more than 6.0 wt.% of sulfur in fly ash. This study demonstrates that kaolin addition, which greatly reduces sulfates, is efficient in reducing fouling in furnaces burning ZD coal.

2


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1. INTRODUCTION The Zhundong (ZD) coalfield in Xinjiang province of China is one of the largest coalfields in the world, with a reserve of 390 billion tons. Thus, its efficient and clean utilization is of importance to China. However, the contents of alkali and alkaline earth metals (AAEMs) in ZD coal ash are high, which will induce severe fouling in coal-fired furnaces. Presently, no pulverized coal-fired furnace can burn ZD coal alone; it must be blended with other coals. Understanding the transformation of AAEMs and developing methods to control ash deposition and fouling during ZD coal combustion are essential for the safe and efficient utilization of ZD coal 1. Extensive studies have been conducted on the fouling mechanism of ZD coal combustion in lab-scale 2-4, pilot-scale 4-8, and full-scale furnaces 9, 10, and all of the studies clearly indicate that the condensation of chlorides and sulfates of sodium and calcium plays a critical role on fouling in the convection heating region. Dai et al.

5

observed the deposition of sodium sulfate on the

panels and high-temperature superheater of a 30 MW pulverized-coal boiler. Similar results were also observed by Song et al. 7 in a pilot-scale circulating fluidized bed furnace, which indicated that potassium vapor reacted with SO2 to form a large amount of gaseous sodium sulfate and then condensed on wall surfaces. Besides of sodium sulfate, the calcium sulfate can also play an important role in the ash deposition during high-calcium ZD coal combustion. Li et al. 3 sampled the ash deposits and fine particles from a down-fired furnace, and found enrichment of both calcium and sodium sulfates in the ash deposits and ultrafine particles. The enrichment of calcium and sodium sulfates in fine particles was also recently found by Xu et al. 4, who sampled the particulate matter from ZD coal combustion in an industrial boiler and a drop tube furnace 3


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respectively. Our previous work investigated ash deposition along the flue gas direction in a full-scale 350 MW boiler burning ZD coal, which also showed that the condensation of AAEM sulfates resulted in fouling on convection heating surfaces 9. Test results by the Harbin Boiler Company in a 300 MW boiler co-firing ZD coal also showed a high amount of calcium sulfates in the slags and deposits when the convection heating surfaces were blocked 10. Generally, the introduction of solid additives containing silica and alumina is effective for controlling fouling from high-sodium coal combustion 11, 12. It was observed that with increasing temperature, the remaining sodium in ZD coal reacted with the major species in ash and form aluminosilicates

13

. A recent study of chemical looping combustion of ZD coal shows that the

reaction of sodium with silicon and aluminum oxides forms a high melting point sodium aluminosilicate instead of low melting point sodium silicates, mitigating the severe slagging and fouling

14

. On-line measurement by Laser-Induced Breakdown Spectroscopy (LIBS) of sodium

release from ZD coal with silica and alumina additives further showed that SiO2 was much more active than Al2O3 for sodium retention

15

. This finding agrees with our previous results that

silicon additives affected ash melting and mineral transformation, which also suggested an optimal temperature for sodium capture of about 1000 °C. However, it also indicated that a lower addition ratio might lead to more slagging by reducing melting temperature

11, 16

. After

comparing the fouling properties of high sodium lignite in China and Germany, the Thermal Power Research Institute (TPRI) suggested that a pulverized coal furnace burning ZD coal can be safely operated over a long term only when the sodium content in ash is controlled below 3.0 wt.% by blending 17. 4


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Kaolin (Al2O3·2SiO2·2H2O) has been proposed as an efficient absorbent and additive to remove alkali metal vapor and increase ash melting temperature during biomass combustion 18, 19. It mainly works through the following reactions: Al2O3·2SiO2·2H2O+2A-Cl→2A-AlSiO4+H2O+2HCl

(1)

Al2O3·2SiO2·2H2O+2A-OH→2A-AlSiO4+3H2O

(2)

Al2O3·2SiO2·2H2O+A2SO4→2A-AlSiO4+2H2O+SO3

(3)

Al2O3·2SiO2·2H2O+A2CO3→2A-AlSiO4+2H2O+CO2

(4)

where A is either K or Na. According to analytic results from full-scale furnaces, abundant AAEM sulfate aerosols from mineral vapor condensation result in severe ash deposition and fouling. Thus, it is reasonable to suppose a reaction such as reaction (3) could destroy sulfate aerosol formation and relieve these problems. There have been a few reports on the effect of kaolin addition on sodium release during ZD coal combustion in lab-scale furnaces; nonetheless, kaolin addition in a full-scale furnace has not been studied. Moreover, while there are abundant studies on sodium transformation, there are relatively few specific studies on the effect of kaolin on calcium sulfate transformation during ZD coal combustion. Here, we present the results of our study considering the effect of kaolin addition on ash deposition and fouling from ZD coal combustion in a 350 MW full-scale furnace. We demonstrate how kaolin addition affects sulfate aerosol emission and sulfate decomposition in deposits. Fundamental studies were conducted in a thermogravimetric analyzer (TGA) and drop tube furnace to demonstrate the effectiveness of kaolin in chemically reducing sulfates. Then, 5


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based on the lab-scale results, a long-term (nine months) test was conducted in a full-scale furnace burning ZD coal with kaolin addition. 2. EXPERIMENTAL METHODS 2.1. Fuel Properties. Both the coal and kaolin samples used in the lab-scale study came from the power plant performing the full-scale test in the Wucaiwan district of Xinjiang province, China. Both the coal and kaolin samples are pulverized and screened smaller than 100 µm, and all the pulverized coal and kaolin particles are dried in oven at 105 °C for 12 h before using in lab-scale studies. The proximate and ultimate analyses of the ZD coal are shown in Table 1, and the elemental compositions of the ash and kaolin additive are shown in Table 2. The ash content of the ZD coal is only 5.1 wt.%, while the contents of sodium and calcium in ash are 4.7 and 24.5 wt.%, respectively 9. In the lab-scale studies, a kaolin blending ratio of 6.0 wt.% is adopted as a representative medium value of full-scale test (3.0-10.0 wt.%). It is because our previous studies show that a blending ratio lower than 6.0 wt.% (e.g., 3.0 wt.%) reduces ash melting point, implying more severe slagging in combustion zone, while kaolin blending ratio significantly higher than 6% will result in more fly ash to be removed in electrostatic precipitators 11, 20. Table 1. The proximate and ultimate analyses of ZD coal used in this study. Proximate analysis (wt.%, air dry basis)

Heating value (MJ.kg-1)

Ultimate analysis (wt.%)

C

H

N

O

S

Cl

Mar

Mad

Aad

Vad

FCad

Qnet,ar

74.1

3.6

0.6

15.9

0.6

0.1

29.0

1.8

5.1

29.5

63.6

18.8

Table 2. Elemental compositions in ZD coal ash and kaolin additive, wt.%. 6


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Type

Fe2O3

Al2O3

CaO

MgO

TiO2

SiO2

SO3

K2O

Na2O

ZD coal ash

8.0

7.6

24.5

7.0

1.0

14.3

2.8

0.4

4.7

Kaolin additive

0.7

20.6

0.9

5.2

0.8

67.0

4.1

0.7

0.2

2.2. Ash Preparation, TGA, and Ash Characterization Methods. For ash preparation, ZD coal with or without kaolin additive was heated in a muffle furnace in an air atmosphere. The ash preparing process was operated according to the Chinese standard GB/T 212-2008, in which a 5g sample is heated to 500 °C at a heating rate lower than 10 °C·min-1, kept at 500 °C for 40 min, and then heated continually to the desired temperatures (815-1300 °C, depending on conditions) and kept constant for 1 h. ZD coal and kaolin particles are mechanically mixed before ash preparing. To directly observe the effect of kaolin addition on the sulfate decomposition in ZD coal ash, the mass loss curve of ash prepared with 6.0 wt.% kaolin additions was compared with that of pure coal ash, in a TGA reactor (490PC, NETZSCH, Germany). For each TGA test, approximately 10 mg of sample was placed into an alumina crucible and heated from 30 °C to 1300 °C at a rate of 20 °C·min-1 in nitrogen atmosphere with a flow rate of 100ml·min-1. The sulfur content in coal ash with or without kaolin additive was measured using X-Ray Fluorescence (XRF, S4-Pioneer, Bruker Co., Germany). The mineral composition of ash with or without kaolin addition was measured by X-Ray Diffraction (XRD, X’pert MPD Pro, PAN-alytical, Netherlands). 2.3. Drop Tube Furnace and Ash Deposit Sampling. A schematic of the drop tube furnace

7


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and sampling system used in this study is shown in Figure 1. The furnace is electrically heated by molybdenum disilicide resistance element to a maximum temperature of 1500°C. The temperatures are measured with Pt-Rh-Pt thermocouples (accuracy, ±1 °C). The corundum reactor tube is 60 mm in diameter and 3000 mm in length. Three zones of temperature control yield a constant-temperature region 1500 mm long. The fuel is fed by a micro-scale spiral feeder at a stable rate of 1.5 g·min-1. In this study, the combustion temperature was set at 1350 °C for all cases. The air : fuel ratio was controlled at approximately 1.2, to produce a residence times of about 3 s.

Buy SmartDraw!- purchased copies print this document without a watermark . Visit www.smartdraw .com or call 1-800-768-3729.

Figure 1. Drop tube furnace and sampling system.

8


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corundum cap mounted on the top of a long ceramic tube was used to collect the ash deposits at different heights. Initially, the temperatures at different heights were measured. Ash deposits were then sampled at different heights corresponding to the desired sampling temperatures. During each sampling, the corundum cap sampling probe was inserted from the bottom of drop tube furnace to the height of desired temperature, and preheated for 15 min. After that, the coal feeding started, and the deposit collection process was sustained for 30 min. Along the flue gas flow direction, sampling was conducted at four temperatures (1200, 1000, 800, and 600 °C). 2.4. Fine Particle Sampling in Drop Tube Furnace. The fine particle sampling system consists of a water-cooled and nitrogen-diluted probe, a Dekati cyclone (Model SAC-65), a Dekati low pressure impactor (DLPI) composed of 13 collection stages with sizes from nano to micrometer scale, and a vacuum pump (Leybold SogevacSV 25) with a standard flow rate of 10 L·min-1. The particle sampling probe was placed at the height where the temperature was 300 °C. The cyclone, DLPI, and connection pipes were heated up to about 150 °C to avoid the effect from acid gas condensation 21. The particle size distribution (PSD) of fine particles was obtained by weighting the fine particles collected at each stage impactor. Each condition was repeated 2-3 times. The morphology and elemental composition of both ash deposits and fine particles were analyzed using scanning electron microscopy and energy dispersive spectrometry (SEM-EDS, JSM-6510, JEOL). 2.5. Ash Deposit Sampling from Full-Scale Furnace. Slags and deposits were sampled from a 350 MW tangentially fired boiler (Model, DG1211/17.4-II22) burning ZD coal with kaolin addition after nine months of continuous operation 9, 22. The ash blowing frequency was once per 9


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day. As shown in Fig. 2, the sampling positions were located at four regions: A, the division platen superheater (DPS); B, the reheater (RH); C, the superheater (SH); and D, the air preheater (ASH). The elemental composition of ash deposits was measured by XRF (S4-Pioneer, Bruker Co., Germany). For the full-scale furnace study with kaolin co-firing, kaolin is weighted and then fed into the mill through the belt conveyor system. In the mill, kaolin and coal are pulverized and mixed well, and then blown into the furnace. 878oC 763oC

604oC 566oC High temperature super-heater

B

C

A Re-heater Division platen super-heater 1224oC

Low temperature super-heater

Economizer

Burner

D Sampling position Air preheater

Figure 2. 350 MW full-scale furnace 3. 3. RESULTS AND DISCUSSION 3.1. Sulfate Decomposition and Alkali Capture by Kaolin Addition in the Muffle Furnace 10


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and TGA Reactor. The ash (without or with 6 wt.% kaolin) prepared at 815 °C in a muffle furnace was re-heated in TGA reactor from 30 °C to 1300 °C, and the mass loss curves are shown in Figure 3(a). If the decomposition temperature is defined as the temperature at which the mass loss equals 5 wt.%, then with 6 wt.% kaolin addition the decomposition temperature decreases from 1196 to 1075 °C. This drop indicates that kaolin addition promotes the decomposition of some minerals in ZD coal ash. We further measured the sulfur content in ash prepared at temperatures from 815 to 1300 °C, and the sulfur loss curves with and without kaolin addition are compared in Figure 3(b). The mass loss in Figure 3(a) matches the sulfur loss in Figure 3(b) well, which clearly indicates that the mass loss is due to the decomposition of sulfates in ZD coal ash. When no kaolin is added, the sulfur content in ash is still as high as 13.5 wt.% at 1300 °C, while when 6 wt.% kaolin is added, the sulfur content in ash decreases to zero at 1150 °C. To further verify the strong promotion of sulfate decomposition by kaolin, the XRD patterns of coal ash at 815 and 1150 °C, with and without kaolin addition, are compared in Figure 4. Without kaolin addition, an intense peak of CaSO4 can be observed in the ash at 815 °C, and this peak can still be observed when the temperature increases from 815 to 1150 °C. In contrast, with kaolin addition, the feature peaks of CaSO4 disappear at 1150 °C. The XRD results agree with the results of mass loss and sulfur loss curves in Figure 3, which further demonstrates the promotion of sulfate decomposition by kaolin. Li et al. 23 observed the same phenomenon during the combustion of ZD coal with 5-10 wt.% kaolin at 1000 °C in a muffle furnace. It was found that the peak intensity of calcium sulfate in XRD patterns is much lower than that of pure ZD 11


Energy & Fuels

coal combustion.

Mass (%)

100

Mass loss of raw ash

95

Mass loss of ash with kaolin

90

(a) 85

Sulfurr content (%)

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 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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25

800

900

1000

1100

1200

1300

Sulfur content in raw ash

20 15 10 5

Sulfur content in ash with kaolin

(b)

0

800

900

1000 1100 Temperature (oC)

1200

1300

Figure 3. Ash mass loss and sulfur loss with and without 6% kaolin addition. A mechanism of sulfate decomposition by kaolin addition can be proposed according to the XRD pattern16 in Figure 4. As seen from Table 2, in ZD coal ash, the content of CaO is 24.5%, while the contents of SiO2 and Al2O3 are only 14.6% and 7.6%, respectively. Although 1150 °C is high enough to accelerate the reaction between CaSO4 and SiO2/Al2O3, the SiO2/Al2O3 amounts in ZD coal ash are insufficient to react with all the CaSO4. Under this condition, when the temperature increases from 815 to 1150 °C, CaSO4 reacts with SiO2 and MgO through reaction (5): 3CaSO4+MgO+SiO2+O2→Ca3Mg(SiO4)2+3SO3 (5)

12


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However, after kaolin addition, there is sufficient SiO2 and Al2O3 to consume all the CaSO4, and under this condition, the aluminosilicate forms are more complex, e.g., Ca2Al(AlSiO7), Ca(Al2Si2O8), Fe2SiO4, Ca3Mg(SiO4)2, and Na6Ca2Al6O24(SO4)2. A possible reaction route is that the kaolin reacts with CaSO4 to produce Ca(Al2Si2O8) via reaction (6), and then the produced Ca(Al2Si2O8) further reacts with silicon oxide, aluminum oxide, and other magnesium or sodium salt to produce complex aluminosilicates. Al2O3·2SiO2·2H2O+CaSO4→Ca(Al2Si2O8)+2H2O+SO3

• CaSO4

◊ SiO2 ο Ca2MgSi2O4

♦ Ca2SiO4

∆ Na Ca Al Si O (SO ) 6 2 6 6 24 4 2

∇ Ca2Al(AlSiO7)

(6)

∗ Ca3Mg(SiO4)2 ⊗ Fe SiO 2 4



θ Al2O3SiO2

Ca(Al2Si2O8)

18000

⊗ ∇

16000 14000 12000

⊗ ∆

•

10000 8000

♦

∗

Intensity

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 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

6000

⊗ 

∇ ⊗ ∆



θ

∗

θ

•

•

2000

∗ θ

∗

•

∗

∗ ∗ ♦

∗ ♦• ∗

•

•

4000

∇ ⊗ ∆

∗

♦ ο

∗ ⊗  ∗ ∆

o

•

ZD coal ash (+kaolin), 1150 C •

ο

◊ ◊ ο

♦

♦ ♦

o

ZD coal ash, 1150 C

• • •

•

• ♦

•

•

• o

ZD coal ash, 815 C 10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

2θ (°)

Figure 4. XRD results for ZD coal with and without 6% kaolin addition. 3.2. Ash Deposition and Slagging in the Drop Tube Furnace. Photographs and micro morphologies of ash deposition and slagging during ZD coal combustion alone and with 6% kaolin addition are shown in Figure 5 (a) and (b), respectively. Under each condition, the ash

13


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deposits and slags were sampled for 30 min at four temperatures (1200, 1000, 800, and 600 °C). The sampling height decreased as the sampling temperature decreasing along the reactor tube. In the deposit photographs in Figure 5, after kaolin addition, the ash deposit amount is significantly reduced for all the sampling positions and temperatures.

(a)

(b) Figure 5. Morphology comparison of ash deposits (a)without and (b) with 6% kaolin addition (combustion temperature=1350 °C). As seen from Figure 5(a), at 1200 and 1000 °C, ash particles melt and adhere together; with 14


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the temperature further decreasing to 800 and 600 °C, a certain number of fine particles appear on the surface of the large ash particles. The elemental mapping distribution by EDS for the deposit sampled at 800 °C is shown in Figure 6(a). There are generally two kinds of spherical ash particles: one consists of Si and Al, and the other one consists of Fe. The fine particles and the sticky part that is bonding the large spherical particles consist mainly of Ca and S. Li et al.

24

adopted similar experimental setup to study the ash deposition during ZD lignite combustion, with XRD and SEM-EDS analysis showing the domination of calcium sulfates in the fine-mode ash particles of irregular shapes. 12

Sulfur content in deposits (%)

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 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

10

ZD coal 8

6

4

ZD + kaolin 2

0 600

700

800

900

1000

1100

1200

Sampling temperature (oC)

(a)

(b)

Figure 6. (a) Elemental mapping distribution of the deposit at 800 °C without kaolin addition, and (b) sulfur content distribution in deposits at different sampling temperatures. In contrast, in Figure 5(b), with kaolin addition, when the sampling temperature decreases from 1000 to 800 and 600 °C, the sticky substances and fine particles are not observed. In Figure 6(b), we compare the sulfur content in deposits at different sampling temperatures during ZD coal combustion with and without kaolin addition. For ZD coal combustion alone, when the 15


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temperature decreases to 1000 °C, a small amount of sulfate has condensed. With the flue gas temperature further decreasing to 800 °C, the sulfur content in deposits increases from 2.41 to 8.66 wt.%, but for ZD coal combustion with 6 wt.% kaolin additions, the sulfur content in the deposits is only 1.96 wt.%. The ash deposit sampling and analysis results clearly demonstrate the ability of kaolin addition to destroy CaSO4 formation during ZD coal combustion. 3.3. Fine Particle Formation in the Drop Tube Furnace. The morphology and elemental analysis results of ash deposits in 3.2 indicate that these fine particles and sticky substances enriched in sulfur are significantly reduced by kaolin addition. To quantify fine particles produced during ZD coal combustion with and without kaolin addition, a specific sampling and elemental analysis of fine particles are further performed in the drop tube furnace. The PSDs of ZD coal combustion without and with 6 wt.% kaolin additions are compared in Figure 7(a), and the distributions of sulfur content with particle size are shown in Figure 7(b). The PSDs of both conditions show a bimodal distribution. With 6 wt.% kaolin additions, the amount of fine mode particles (Dp397 nm) increases by 48%. The sulfur content in the fine particles is also reduced to a certain degree, depending on the particle size. Xu et al. 2 reported ash particle formation during ZD coal combustion and co-firing, using drop tube furnace kept at 1350 °C, similar to the environment of this study. Despite the absence of PM1.0 information, the overall elemental compositions of PM10 indeed have shown the domination of S (content > 50 wt.%) in ash particles during ZD coal combustion. Li et al.

3

were the first to compare the particulate

matter formation mechanism between ZD and non-ZD coal combustion in a 25 kW down-fired 16


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furnace. It was found that the fine mode particles (< 250 nm) of ZD coal combustion were one order of magnitude more than non-ZD coal combustion, and mainly consisted of calcium and sulfur, which agrees with our results in this study. 7.0 6.5

25

ZD

6.0 5.5 20

5.0

Mass content (%)

dm/dlogDp (mg/g coal)

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 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4.5 4.0 3.5

ZD coal

3.0 2.5 2.0

15

10

1.5 1.0 0.5

5

ZD+kaolin

ZD coal+kaolin

0.0 -0.5 0.01

0.1

1

10

0 0.01

0.1

1

10

Dp (um)

Dp(um)

(a)

(b)

Figure 7. (a) PSDs and (b) sulfur distributions in fine particles during ZD coal combustion and ZD coal+kaolin combustion. 3.4. Ash deposition in a full-scale furnace burning ZD coal with kaolin addition. Based on the above fundamental studies, for the first time, a full-scale test was performed to investigate the practical effectiveness of kaolin addition in addressing ash depositing and slagging from ZD coal combustion This test was continued for nine months, and the elemental compositions of 149 kinds of original coal and 737 kinds of coal after kaolin blending were analyzed. The coal particles used for property analysis are sampled from the outlet of coal pulverizing system. As shown in Figure 8, the average content of Na and Ca in the original coal was 3.0 and 22.1 wt.%, respectively, while after kaolin blending, these decreased to 2.1 and 13.8 wt.%. Figure 9 (a) shows ash deposits in the DPS, RH, and HS regions after nine months of 17


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operating with kaolin addition. In the RH and HS regions, the ash deposition is not severe: only a small amount of slag can be observed at certain positions on the tubes in the RH region, and the deposits in the HS region can be easily removed by compress air blowing. However, ash deposition in the DPS region is severe, which is due to the different fouling mechanisms at different positions and temperatures. The DPS region is located right above the furnace chamber, as shown in Figure 2, and before the DPS, the flue gas temperature is as high as 1224 °C. As a result, hot and melted ash particles carried by high-temperature flue gas from the furnace chamber directly collide with and adhere to the outer surfaces of the DPS. Considering the wide space between each piece of the platen heat exchanger, such a degree of fouling in the DPS region is acceptable and will not affect the safety of the furnace’s operation. 60

12

Na content after blending Na content in raw coal

11

Ca content after blending Ca content in raw coal

55

10

50

9

45

8 7 6 5 4

Average=3.0%

3 2

Average=2.1%

1

Ca content in coal ash (%)

Na content in coal ash (%)

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 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40 35 30 25

Average=22.1%

20 15 10

Average=13.8%

5 0

0 0

100

200

300

400

500

600

700

800

900

1000

-5 0

100

200

Coal number (1)

300

400

500

600

700

800

900

1000

Coal number (1)

(a)

(b)

Figure 8. (a) Na content and (b) Ca content in coal ash before and after kaolin blending 22. How kaolin addition reduces ash deposition in the RH and HS sections can be explained by the distribution of the S, Na, and Ca content along the sampling positions, and as the flue gas 18


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temperature decreases. The data in this study with kaolin addition are compared with our previous full-scale test of ZD coal when co-fired with Zhunnan coal. The fuel properties of Zhunnan coal is introduced in our previous work in details 9. As shown in Figure 9(b), all (S, Ca, Na)/(Si+Al) mole ratios reach a maximum in the RH region and then decrease, which further demonstrates that the sudden condensation of sulfates in this region induces fouling. We also compared the sulfur contents in fly ash sampled from the air preheater. As noted in Figure 9(b), the sulfur content in ash from coal co-firing is 6.83 wt.%, while it decreases to 1.75 wt.% in the case of kaolin addition. This decrease indicates that kaolin is highly effective at reducing sulfate formation and deposition of ash particles on heating surfaces. 1.0 S/(Si+Al), ZD coal co-firing Na/(Si+Al), ZD coal co-firing Ca/(Si+Al), ZD coal co-firing S/(Si+Al), ZD+kaolin Na/(Si+Al), ZD+kaolin Ca/(Si+Al), ZD+kaolin

0.9

(S, Na, Ca)/(Si+Al) (mol/mol)

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 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.8 0.7 0.6 0.5 0.4

In ash:

0.3

S%= 6.83%

0.2 0.1 S%=1.75%

0.0 -0.1 DPS

RH

SH

ASH

Position

(a)

(b)

Figure 9. Ash deposits and elemental analysis: (a) ash deposits at DPS, RH, and HS and (b) (S, Na, Ca)/(Si+Al) mole ratios in the ash deposits with coal co-firing and kaolin addition. 4. CONCLUSIONS 19


Energy & Fuels

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In this study, kaolin addition has been proposed to address fouling in a full-scale furnace burning ZD coal. Before performing the long-term full-scale furnace test, a series of fundamental studies in a thermogravimetric analyzer (TGA) and drop tube furnace were conducted to demonstrate the effectiveness of kaolin at eliminating sulfates. The main conclusions are as follows: (1) Kaolin addition decreases the decomposition temperature of sulfates in coal ash by higher than 120 °C, and kaolin addition at 1150 °C eliminates all the sulfates in ash. Kaolin addition successfully eliminates severe ash deposition and fouling on probe surfaces, and reduced the amount of PM1.0 by half. An analysis of the morphology and composition of fine particles and ash deposition clearly shows that kaolin addition avoids the formation of the sulfate aerosol and its adhesion on the surfaces of larger ash particles, and decreases the content of sulfur, sodium and, calcium in PM1.0. (2) A nine-month long test was conducted in a 350 MW full-scale furnace burning ZD coal with kaolin addition. This long-term test showed that by reducing the average sodium and calcium content in ZD coal ash to 2.1 and 13.8 wt.% through kaolin addition, severe ash deposition and fouling can be avoided. Elemental analysis of the fly ash also showed that the sulfur content after kaolin addition was only 1.75 wt.%. By comparison, previous tests of coal co-firing without kaolin addition, conducted in the same furnace burning the same ZD coal, showed more than 6.0 wt.% of sulfur in the fly ash. Our study demonstrates that kaolin addition, which greatly reduces sulfates, is highly effective at controlling fouling in furnaces when burning ZD coal. 20


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5. ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Nos. 51676157, 51376147, and 5161101654), the National Key Research and Development Program of China (No. 2016YFC0801904), the U.S. Dept. of Energy (Award # DE-FE0009702), and the Consortium for Clean Coal Utilization (CCCU) at Washington University in St. Louis. We would also like to thank Mr. James Ballard at Washington University in St. Louis for assistance in editing this manuscript. 6. REFERENCES 1.

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Sulfate Removal by Kaolin Addition To Address Fouling in a Full-Scale

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