or biomass combustion boilers

Fuel 106 (2013) 303–309

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Reduction mechanisms of ash deposition in coal and/or biomass combustion boilers Hiroshi Naganuma a,⇑, Nobuya Ikeda a, Tadashi Ito b, Mikio Matsuura c, Yoko Nunome c, Yasuaki Ueki c, Ryo Yoshiie c, Ichiro Naruse c a

Tohoku Electric Power Engineering & Construction, Co., Ltd., 53, Shinnakahori, Iidoi, Rifu, Miyagi-gun 981-0113, Japan Nippon Welding Rod Co., Ltd., 7800 Nakase, Hamakita-ku, Hamamatsu 434-0012, Japan c Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Japan b

h i g h l i g h t s " We evaluated adhesion forces between ash and alloy specimens at high temperature. " Ni-alloy coated by a thermal spraying technique reduced the adhesion force. " Alkali sulfates had a large influence on the adhesion force. " Fe2O3 (Hematite) was diffused from the metal specimen to among ash particles.

a r t i c l e

i n f o

Article history: Received 6 March 2012 Received in revised form 27 October 2012 Accepted 6 November 2012 Available online 10 December 2012 Keywords: Pulverized coal combustion boiler Ash deposition Alkali metal compounds Surface treatment technology Fouling and slagging

a b s t r a c t Some ash particles in solid fuels adhere on heat exchanger tube surfaces inside coal and/or biomass combustion boilers. The authors have already proposed a surface treatment on tubes, using a thermal spraying technique, to reduce ash deposition. Understanding reduction mechanisms of the ash deposition is necessary to evaluate effects of the surface treatment technique on the reduction of ash deposition. The reduction mechanisms of the ash deposition were elucidated due to physical and chemical aspects, measuring adhesion forces between the ash particles and some alloy specimens of the tube at high temperature under the simulated boiler conditions. As a result, the adhesion force increased with time and depended on both the ash types and the alloy specimens. The thermal spraying of Ni-alloy, in particular, could reduce the adhesion force. Moreover interface reactions between the ash particles and the alloy specimen played an effective role in increasing the adhesion force, alkali metal compounds in the ash samples also related to an increase of the adhesion force. Fe, which was one of the main alloy elements, diffused into the ash deposition layer beyond the interface. This observation result suggested that the interface reactions of the ash particles with the alloy caused an increase of the adhesion force. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Some ash particles produced during solid fuel combustion adhere to surfaces of heat exchanger tubes inside coal and/or biomass combustion boilers. The ash deposition causes heat transfer inhibition such as slagging and fouling, which becomes one of triggers of the boiler operation troubles [1,2]. Basic studies on the deposition behaviors of coal ash on the heat exchanger tubes have been carried out by Raask [1], Benson and Sondreal [3], and Ninomiya [4,5], and also practical studies have been made, such as elucidation of the ash adhesion mechanisms under the boiler combustion conditions [6], measurements on the ash deposition in utility boilers [7], elucidation of the ash adhesion mechanisms in pulverized coal fired (PCF) boiler [8], development of a predictive tool for ⇑ Corresponding author. Tel.: +81 22 356 8935; fax: +81 22 356 8937. E-mail address: [email protected] (H. Naganuma). 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.11.017

the slagging and the deposition control in boilers [9] and discussions of the ash adhesion mechanisms in a pilot plant [10]. Recently, the temperature of the steam has been raised to enhance the net power generation efficiency, and co-combustion with low-rank coal or biomass has also been gradually applied to commercialized PCF boilers. These trends may bring about more severe operation problems [11]. In order to control a long-term ash deposition, it is necessary to discuss a cycle of the growth and drop of the ash deposition layers [7]. This cycle frequency would be influenced by an adhesion force between the ash deposition layer and the tube surface. The residual ash layers would deposit the tube surfaces for a long time, thereby high-temperature corrosion would also occur at the deposition surfaces [12–15]. Both Moza and Austin [16,17] and Abbott et al. [18–20] reported that a relationship between wettability of molten ash on the metal surface and the adhesion force, which affects a deposition rate of the ash particles. Moreover, Kamiya et al. [21]

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H. Naganuma et al. / Fuel 106 (2013) 303–309 Table 1 Fusion temperatures and compositions of the fly ash employed.

have also reported measurements of the adhesion force of the ash particles at high temperatures, in which alkali metal compounds were contained. In those papers mentioned above, however, there has been no one yet who could elucidate the adhesion force between the ash deposition and the tube surface heated for a long period at high temperatures or interface reactions between them under boiler operating conditions. The present authors have proposed a surface treatment technique for tubes to reduce the ash deposition [22–24]. As previously reported [22], the experiments on ash adhesion were conducted using simulated heat exchanger tubes, on which some kinds of thermal spraying materials were coated. According to the ash deposition experiments under coal combustion in the previous report [22], fine ash particles with alkali metal compounds selectively adhered and formed the initial ash deposition layer. The authors also showed that the interface reactions would be related to the adhesion mechanism [23]. The purpose of this paper is to investigate the adhesion mechanism and the effect of the surface treatment technique on the reduction of the ash force. Moreover the influence of ash types on the adhesion force was elucidated on the basis of direct measurement on the adhesion force and elemental analyses at the adhesion interface using Scanning Electron Microscope–Energy Dispersive Spectroscope (SEM–EDS) and Micro X-ray Diffraction (Micro-XRD).

Ash fusion temperature (K)

Oxidizing

Ash composition (wt%)

Initial deformation Hemispherical Fluid

1543 1563 1573

SiO2 Al2O3 Fe2O3 TiO2 CaO MgO Na2O K2O p2o5 SO3 V2 O5 MnO

63.10 20.45 3.99 0.94 5.84 1.37 1.02 1.76 0.21 0.49 0.04 0.01

Table 2 Compositions of the ash sample prepared (wt%). Fly ash (Table 1) K2SO4 Na2SO4

86.8–100.0 0.0–13.2

Table 3 Elements of alloy specimens employed (wt%).

2. Experimental

Specimens

Specimen 1: base metal (JIS-SUS304 and AISI-304)

Specimen 2: thermal spraying (thickness of 200 lm)

Fe Ni Cr Others

Bal. 8.0 18.0 <2.0

3.0 Bal. 20.0 <25.0

2.1. Measurement of adhesion force The adhesion force between the ash particles and the metal specimens was measured using the apparatus shown schematically in Fig. 1. The apparatus mainly consisted of a built-in 1 kN load cell, an 8 kW electrical furnace and a circulation unit of cooling water, and a detailed description [23] was given before. A typical composition of fly ash was shown in Table 1, and in order to carry out accelerating tests alkali sulfates of Na2SO4 and K2SO4 are added into the fly ash as shown in Table 2 [22,25–28]. Each particle diameter of fly ash, Na2SO4 (Melting point: 1157 K) and K2SO4 (Melting point: 1342 K) was less than 45 lm. The ash pellet was prepared from fly ash, Na2SO4 and K2SO4 as follows: (i) 1.0 ml of polyvinyl alcohol was added as a binder into the ash of 6.119 g; (ii) the mixture was placed into a pelletizer of 25 mm diameter and 8 mm high, and then pressed at 20 MPa for 1 h at room temperature, (iii) finally the sample pellet was dried at 313 K for 24 h.

The metal specimens shown in Table 3 were first degreased in acetone with an ultrasonic cleaner: they were used as austenite stainless steel for specimen 1, or used as based metal for specimen 2. A thermal spraying material shown in this table was coated 200 lm thick on the based metal with an arc spraying method (specimen 2). After the specimen 1 or 2 was fixed at the lower portion of the tensile testing machine, the ash pellet was set on the specimen. To measure an inner temperature of the sample ash pellet, a thermocouple was installed inside a hole of 8 mm diameter and 4 mm deep in the pellet. An upper rod of austenite stainless steel was put on the ash pellet, and then weight of 30 N was

Tensile testing machine 1kN Specimen (SUS304)

SUS304 25.0 Ash 8.0 mm

Furnace Specimen Ash Heat controller

Chiller

A

Cooling water jacket PC

Detail of A Thermo couple

Fig. 1. Schematic of tensile testing machine with an electrical furnace [23].

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H. Naganuma et al. / Fuel 106 (2013) 303–309 Table 4 Temperature conditions of measurement experiment of adhesion force.

pressed downward. After heating the ash pellet up to a designed temperature in 1 h, the pressure was kept at 40 N until a tension experiment. In the tension experiment, the tension load was continuously measured while the upper rod was moving upward at a speed of 0.5 mm/min. The adhesion force is derived by

Lmax  Lini Fa ¼ S

Specimen 1 Ash: 96.8wt%Flyash - Alkali sulfate

1293 1216 918

ð1Þ

1400 1200

Adhesion force [kPa]

Furnace temperature (K) Ash temperature (K) Specimens temperature (100 h) (K)

Furnace temp.: 1293K, Alkali sulfate: 1.4wt%K2SO4 - 1.8wt%Na2SO4

1600

Specimen 1 Ash: 100%Flyash

1000 800 Specimen 2 Ash: 96.8wt%Flyash - Alkali sulfate

600 400

where Fa is the adhesion force (kN/m2), Lmax is the maximum tension load (kN) measured during the tension experiment, Lini is the initial tension load immediately before a breaking and S is the adhesion surface area (m2), which is the same as the pellet crosssectional area. Table 4 shows temperatures of the furnace, the ash pellet and the specimen: the ash pellet temperature was measured at the center of it, and the specimen temperature was measured at 3 mm deep from its top surface, as shown in Fig 1.

Specimen 2 Ash: 100%Flyash

200 0

0

50

100 150 200 250 300 350 400 450 500

Time [h] Fig. 2. Time change of the adhesion force.

2.2. Elemental analysis of adhesion interface

3. Experimental results and discussions 3.1. Time change of adhesion force The results of measurements on the adhesion forces between the ash particles and the metal specimens are shown in Fig 2. The adhesion forces increase with time, in particular, the values for specimen 1 increase significantly. The adhesion force between the ash particle and the metal specimen is considered to be a combination of the van der Waals force and the liquid-bridging force [1]; those forces are calculated by [29].

Fv ¼

Ad 2

12h

F l ¼ 2pdcL cos h

Experimental results, van der Waals force and liquid-bridging force [N/particle]

1E-3

After the tension experiment, the ash pellet and the metal specimen were bonded together with an epoxy resin, and it was cut vertically, then the surface was polished. The interface between the ash pellet and the metal specimen was observed with the SEM–EDS and Micro-XRD. In identification analysis, a position of the ash layer was specified by means of a qualitative analysis using SEM–EDS before X-ray diffraction analysis of compounds identification. X-ray output was 40 kV–200 mA (Cu Ka), and the collimator was 30 lm in diameter.

1E-4

Specimen1 367h (100% flyash) * Experimental results Specimen2 333h (100% flyash) * Experimental results

Liquid-bridging force θ =10 - 90 degrees * Calculated value

1E-5

1E-6

1E-7

1E-8

1E-9

1E-10 1E-9

Calculation condition surface tension γ =0.2Nm-1 (Na2SO4 ) contact angle θ =10-90 degree Hamaker constant 1.0E-19 Ash sample rate of porosity 0.35 representative diameter of particle 10μm

va * C n de alc r W ula aa ted ls va forc lue e

1E-8

1E-7

Particle to surface distance [m] Fig. 3. Comparison of van der Waals force, liquid-bridging force and adhesion force per particle (specimen 1: 367 h, specimen 2: 333 h).

ð2Þ ð3Þ

where Fv is the van der Waals force [N/particle], A is the Hamaker constant (J), d is the representative diameter of particle (10 lm), h is the distance between a particle and an alloy surface, Fl is the liquid-bridging force [N/particle], cL is the surface tension (=0.2 N/m for Na2SO4 [1]) and h is the contact angle between the liquid phase and the alloy surface. Fig. 3 shows the adhesion forces measured on the specimen 1 heated for 367 h and the specimen 2 heated for 333 h, together with the van der Waals force calculated using the Hamaker constant expressed by Raask [1] and the liquid-bridging force calculated varying the contact angle from 10° to 90°. The calculated value for the liquid-bridging force is at least two orders of magnitude larger than that for the van der Waals force,

and hence it can be concluded that the liquid-bridging force plays an important role in the adhesion force. And an anchor effect due to the surface roughness of the metal specimen may also influence the mechanism of the adhesion force, however it can be negligible since the estimated roughness factor of the specimens is very small. The adhesion force measured on the specimen 1 is much larger than that on the specimen 2, although whose measurements are in good agreement with those calculated for the liquid-bridging force. This result suggests that the significant increasing of the adhesion force on the specimen 1 is presumably caused by a force other than the liquid-bridging force. As shown in Fig. 2, the adhesion forces measured under the condition with the alkali sulfate are larger than those under the condition of the alkali sulfate free: the increase of the adhesion force can be ascribed to a chemical reaction accelerated with the alkali sulfate at the interface between the ash particles and the metal

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H. Naganuma et al. / Fuel 106 (2013) 303–309

Adhesion force of K 2SO4 or Na2SO4 + K2SO4 [kPa]

0.0

700

2.0

0.0

2.0

4.0

6.0

4.0

8.0

6.0

10.0

8.0

12.0

10.0

K 2SO4 [wt%] 12.0

600 500

Na2SO4 + K 2SO4 [wt%]

Na2SO4 + K2SO4

400 300

K2SO4

200 100 0 0.0

1.0

2.0

3.0

4.0

5.0

0.0

2.0

4.0

6.0

8.0

10.0

4.0

5.0

6.0 [mol%]

Adhesion force of Na2SO4 [kPa]

25 20

Na2SO4 [wt%]

15

Na2SO4

10 5 0

0.0

1.0

2.0

3.0

6.0

Alkali sulfate [mol%] Fig. 4. Variation of adhesion force (furnace temperature: 1293 K, time: 7 h).

Ash Na2SO4:7.5wt%

SUS304

K2SO4:9.0wt%

Na2SO4:3.7wt% K2SO4:4.6wt%

Na2SO4:5.5wt% K2SO4:6.7wt%

Fig. 5. SEM images and elemental analysis of Fe, S, Na and K (7 h).

specimens. In considerations of the specimen temperature and the addition of the alkali sulfate, high temperature corrosion probably occurred at the interface as will be discussed later. 3.2. Effect of alkali metal compounds on adhesion force Fig. 4 shows the relationship between the content of the alkali sulfates in the ash pellet and the adhesion force measured on the specimen 1 after heating for 7 h. The adhesion force exhibits a peak at about 10 wt% of the content of K2SO4 or Na2SO4 + K2SO4, on the contrary, the addition of Na2SO4 only does not affect the adhesion

force strongly. These results show that K2SO4 largely contributes to increasing of the adhesion force. Moreover melting points of K2SO4, Na2SO4, K3Fe(SO4)3 and Na3Fe(SO4)3 are 1344 K, 1157 K, 891 K and 896 K, respectively [30]. Therefore these results suggest that the liquid-bridging force that is caused by sulfate melts does not contribute to the growth of the adhesion force strongly, and the adhesion force involved the alkali sulfates is caused by the corrosion reaction. In order to discuss those results further, the SEM–EDS analyses at the interface between the ash pellets and the metal specimens have been carried out.

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H. Naganuma et al. / Fuel 106 (2013) 303–309

Ash

Ash

Base metal

Ash Specimen

Thermal spraying

Fig. 6. SEM images and elemental analysis of Fe, Ni and Si (specimen 1 and specimen 2 of time change test: 24 h).

Ash1

C-1 Specimen 1

200 micron

Area C

Area C

Ash1

A C-1-1 Specimen 1

B

100 micron

C-1

Point A

C-1-1

Point B

[keV]

[keV]

Fig. 7. Photograph of ash and specimen 1, SEM image of area C, C-1 and C-1-1, and results of qualitative analysis on point A and B.

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H. Naganuma et al. / Fuel 106 (2013) 303–309

Peak intensity [counts]

Ash 2000 1500

Irradiation area of X-ray 1000 100 micron

Specimen 1 C-1

500 0

Peak data

Fe 2 O 3 Hematite SiO 2 Quartz NaAlSiO4 Sodium Aluminum Silicate Al 2 SiO 5 Sillimanite Fig. 8. Results of qualitative analysis on irradiation area by micro-XRD. Cu Ka, collimator: 30 lm in diameter, sampling time: 17,870 s.

3.3. Observations and elemental analysis at the interface In order to investigate the mechanism of the adhesion force further, the SEM–EDS observation have been carried out at the adhesion interface of the specimens heated for 7 h for each alkali composition, as shown in Fig. 5. For the Na2SO4 addition alone, any significant reaction progress at the interface cannot been observed. For the K2SO4 or Na2SO4 + K2SO4 additions, on the other hand, Fe on the surface of metal specimen has disappeared and S, Na and K are concentrated into the metal specimen. It is clear from the results that a corrosion reaction of Eq. (4) occurs at the interface [1].

3A2 SO4 þ Fe2 O3 þ 3SO3 ! 2A3 FeðSO4 Þ3

A : Na or K

ð4Þ

The photos of the bottom row, in particular, show that a part of separation of the metal specimen is probably caused by severe corrosion reactions. These observations are consistent with the measurements on the adhesion force as shown in Fig. 4. Therefore Fig. 4 indicates the relationships between the adhesion force and the corrosion reaction until the peak of the adhesion force at about 10 wt% of the content of K2SO4 or Na2SO4 + K2SO4. Fig. 6 shows distributions of Fe, Ni and Si at the interface obtained by means of the SEM–EDS analyses for the samples of the specimen 1 after heating for 96.6 h and the specimen 2 after heating for 500 h. From the top row photos corresponding to the specimen 1 in this figure, it is observed that Fe diffuses from the metal to the ash particles. For the specimen 2, on the other hand, the Ni alloy as the thermal spraying material can prevent diffusion of Fe in the base metal as previously reported [23,24]. 3.4. Micro analysis using SEM–EDS and micro-XRD Based on the experimental and analytical results obtained, it is clear that Fe diffusion from the metal alloy to layers of the ash particles causes the growth of adhesion force. In order to identify the diffusion layer of Fe, micro analyses at the interface of the speci-

men 1 after heating for 96.6 h has been carried out using the SEM–EDS and a Micro-XRD. Fig. 7 shows a photo and several SEM images of a cross section at the interface between the ash layer and the Specimen 1, and EDS data at Points A and B in the SEM image of C-1-1. The SEM images and the EDS data indicate the residue of the ash deposition and the results of the qualitative analysis, respectively. From the SEM images of C-1-1, molten components seem to fill up crevices among ash particles. According to the EDS analysis at Point A, Fe was strongly detected there. As shown in Table 1, Fe content in the ash is not so high, so that Fe compounds in the ash layer can be diffused from the metal specimen. Fig. 8 shows the XRD data in the ash layer near the metal specimen, and the irradiation area of X-ray. The area is set so that the specimen is not irradiated with the X-ray. From the figure, it is clear that Fe2O3 (Hematite), SiO2, NaAlSiO4 and Al2SiO5 are identified there. Depending on the O2 partial pressure and Cr content in the alloy, there are differences in the surface oxide layers of austenitic steel. It has been established that austenitic steel has three surface oxide layers within the conditions of this experiment, and the outermost layer is formed by Fe2O3 (Hematite) [31] on Fe(Fe, Cr)2O4 and Cr2O3. This suggests the possibility that Hematite, which is identified with the XRD, diffuses from the surface oxide layer. Therefore, it seems reasonable to suppose that Fe diffusion from the metal specimen compose the interface reactions relate to the ash adhesion. 4. Conclusions The adhesion forces between coal ash and metal alloys were evaluated at high temperature for a long period using the high precision tension tester. The results lead to the conclusion that the adhesion force increased with time, and depended on the metal specimens. Ni-alloy coated by the thermal spraying technique, in particular, reduced the adhesion force. Moreover the interface between the ash pellet and the metal specimen was analyzed by the SEM–EDS and Micro-XRD. A main

H. Naganuma et al. / Fuel 106 (2013) 303–309

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