Evaluation of sulphonated acetone–formaldehyde (SAF) used in coal water slurries prepared from different coals

Fuel 86 (2007) 1439–1445 www.fuelfirst.com

Evaluation of sulphonated acetone–formaldehyde (SAF) used in coal water slurries prepared from different coals Xueqing Qiu *, Mingsong Zhou, Dongjie Yang, Hongming Lou, Xinping Ouyang, Yuxia Pang School of Chemical and Energy Engineering, Guangdong Provincial Laboratory of Green Chemical Technology, South China University of Technology, 510640 Guangzhou, Guangdong, China Received 5 July 2006; received in revised form 24 November 2006; accepted 26 November 2006 Available online 4 January 2007

Abstract A new type of water-soluble polymer, sulphonated acetone–formaldehyde resin (SAF), was developed as dispersant for the preparation of highly-loaded coal water slurry (CWS). In this study, in order to evaluate the practical use in industry, the SAF and a naphthalene sulfonate formaldehyde condensate are used to prepare CWS using four Chinese coals with different coalification degrees. The evaluation of the properties of CWS shows that SAF has better ability in reducing CWS viscosity at lower dosage than naphthalene sulfonate formaldehyde condensate, and that the CWS prepared from SAF has excellent stability within 48 h and exhibits shear-thinning apparent viscosity/shear rate behavior at lower dosage. Based on the results, the SAF is an effective and promising dispersant for highly-loaded CWS. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Sulphonated acetone–formaldehyde; Coal water slurry; Viscosity-reducing capacity

1. Introduction This paper is concerned with the application of a new type of dispersant, water-soluble sulphonated acetone– formaldehyde resin (SAF), in coal water slurry (CWS) made up of four different Chinese coals with different coalification degrees. The CWS has been regarded as a promising fuel instead of petroleum oil due to the rapid depletion of the latter, and many studies on CWS have been carried out in some countries such as China, Japan, America and Australia [1–4]. A practical CWS as a liquid fuel should have such properties as an appropriate yield value to maintain its stability during storage, a low apparent viscosity to accommodate CWS’s spray for combustion and a high

*

Corresponding author. Tel.: +86 20 8711 4722. E-mail addresses: [email protected] (X. Qiu), zhmsong1980@ yahoo.com.cn (M. Zhou). 0016-2361/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2006.11.035

solid content for economical use [5,6]. However, the formation of CWS is greatly influenced by the surface chemistry of coal, which is determined by the coal nature, such as coal coalification degree, inherent moisture content, ash content, coal surface wettability, O/C ratio and porosity [7,8]. Therefore, the coal properties should be considered in CWS preparation, and usually the applicability for the coals with different coalification degrees is used to evaluate the performance of a dispersant. The present study examined the effects of two dispersants (SAF and a naphthalene sulfonate formaldehyde condensate used for comparison) on apparent viscosity, rheological behavior and stability of the CWS prepared from four Chinese coals with different coalification degrees. Moreover, the hydrophilicity/lipophilicity properties of the coals and the influence of the coal properties, such as inherent moisture content and O/C ratio on CWS preparation were also investigated. Based on the above results, a reliable estimation of SAF for preparing a practical CWS could be proposed.

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2. Experimental 2.1. Experimental materials The preparation of CWS was carried out with four different kinds of Chinese coals (Panjiang, Yanzhou, Datong, Shenhua). Table 1 shows the results of both the proximate and ultimate analyses of these four kinds of coals. Before the CWS preparation, the coals were first crushed in a jaw crusher to obtain a product below 10 mm. Then the coals were dried in vacuum at 105 °C for 24 h. The crushed dry coal was comminuted in the ball mill (dry grinding) to obtain the coal powder. The coal powder was screened through a 100-mesh screener (0.149 mm pore size) to obtain the powder with an average particle size of 30 lm and over 99.7% of the powder have the particle size below 0.1 mm. Table 1 Proximate and ultimate analyses of coal samples on air dried basis Component

Coal type

Proximate analysis Ash (wt.%) Total sulfur (wt.%) Volatile matter (wt.%) Inherent moisture (wt.%) Ash fusion point (°C) Ultimate analysis C (wt.%) H (wt.%) O (wt.%) N (wt.%) S (wt.%) O/C

Panjiang

Yanzhou

Datong

Shenhua

9.21 0.17 30.98 1.08 1380

11.27 0.56 32.71 2.65 1410

10.22 0.60 27.77 3.79 1280

5.27 0.21 30.68 5.52 1170

82.00 4.69 1.96 1.60 0.16

71.25 4.15 8.71 1.39 0.58

71.35 3.78 9.32 0.92 0.62

72.30 3.86 11.89 0.94 0.22

0.024

0.122

0.131

0.164

Two different chemicals were used as dispersants: watersoluble sulphonated acetone–formaldehyde (SAF, synthesized by the author using the procedure described by the literature [9]) and naphthalene sulfonate formaldehyde condensate (FDN, a product of Chinese Zhanjiang Additive Company, a dispersant used for comparison). The physical properties and the chemical structures of both dispersants used in CWS preparation are given in Table 2. 2.2. Contact angle measurements The contact angles were measured through static-drop method of using a goniometer (JC2000C1, Shanghai Zhongchen Ltd.) equipped with a computer and software designed for calculating the value of contact angles. The coal powder was pressed into pellets for the measurements, and the photographs of the coal/water interface were taken through a modified microscope in the goniometer. The contact angles were calculated from the photographic images by the software accurate to 2°. Because of the inhomogeneous distribution of the mineral matter and hydrophilic group on coal surface, there is a little difference in the value of contact angles on different parts of coal surface [10], so more than ten measurements were taken on every coal sample, and the average value was used. 2.3. Rheological measurements The coal powder was slowly mixed in a pot containing a certain known quantity of dispersant and deionized water. The contents were continuously stirred by a mixer during the addition of coal, and then the stirring of the slurry was continued for another 10 min at 1200 rpm to ensure the homogenization of CWS. The slurry so prepared was left for the study of its characteristics.

Table 2 Physical properties and chemical structure of SAF and FDN Items SAF Molecular weight Sulfonic group content (mmol g1) Inherent viscosity at 25 °C (ml g1) Surface tension at concentration of 1 g l1 in distilled water (mN/m) Chemical structure

O CH2CH2

C CH2CH2 O

m

CH2CH2

OH C CH2CH2 O

SO3Na FDN Molecular weight Sulfonic group content (mmol g1) Surface tension at concentration of 1 g L1 in distilled water (mN/m) Chemical structure CH2

SO3Na n

17,000 2.36 8.43 69.76

n

5200 1.77 70.35

X. Qiu et al. / Fuel 86 (2007) 1439–1445

2.4. CWS stability measurements The stability of CWS was evaluated by the ‘‘glass rod penetration test’’ (penetration ratio, %) and storage time (h) in a similar manner to that in the studies described by Atesßok and co-workers [11]. After the CWS preparation, it was stored in a glass cylinder (3 cm in diameter; CWS layer 15 cm in height) at room temperature for a definite period, a glass rod (5 mm in diameter, 20 g in weight) was spontaneously dropped down from the CWS surface to the cylinder bottom, and it stopped when the tip got in contact with the hard sediment. The penetration ratio was calculated as follows: Penetration ratioð%Þ ¼ d=d t  100 where d is the distance of rod travel (cm), and dt is maximum distance of rod travel (cm). 3. Results and discussion 3.1. Contact angle The contact angles of the water with or without dispersants on four coal samples which had been pressed into smooth disks were measured and the results are given in Fig. 1, as a function of the ratio of carbon to oxygen. The contact angles measured represent a macroscopic average of the surface properties of the individual surface sites. Besides, they are greatly influenced by the ratio of carbon to oxygen of the coals, because high carbon contents are primarily associated with hydrophobic surface sites, and high oxygen contents are primarily associated with hydrophilic surface sites such as phenolic and carboxylic functional groups [12].

105

Distilled water SAF

100

FDN

95

Contact angle /degree

The rheological property of CWS was measured by a rheometer (RotoVisco 1, Haake Corp.) with a Z43 measure cup and a Z41 rotor. The shear rate range was: up run 0–200 s1, down run 200–0 s1. The time of up run and down run was both 3 min, and the temperature was kept at 25 °C, with a fluctuation of 1 °C. The viscosity measurement was also performed employing the rheometer with a Z43 measure cup and a Z41 rotor. Before measurement the slurries were allowed to stand for 5 min. The measurements were taken at a shear rate of 100 s1 from the ‘‘up run’’ curve. The temperature was kept at 25 °C, with a fluctuation of 1 °C. The measured viscosity value was the apparent viscosity. The settling of coal particles during the test did not seem to interfere with the measured apparent viscosities. Since 99.7% of coal particles were finer than 100 lm, the gap size of the Z41 rotor (1 mm) was wide enough for a reliable measurement. Considering the fact that the particle size distributions of the coal samples were different, the results for different coals are not quite directly comparable although the dispersing power of the polymers towards a given type of coal can still be assessed.

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90 85 80 75 70 65 60 0.16

0.14

0.12

0.10

0.08

0.06

0.04

0.02

Oxygen to carbon ratio

Fig. 1. Contact angles of water and dispersant solutions on coal samples with different O/C ratios. Dispersant concentration 0.1 wt.% in water.

Fig. 1 shows that the contact angles increase upon a decrease of the O/C ratio, which indicates decreasing wettability of the coal surface by the water and dispersant solutions. The presence of dispersants (0.1 wt.%) causes the decrease of contact angles from that measured in the absence of dispersants for the four coals. The SAF is observed to cause a large change in the contact angle than FDN, and the contact angles are more affected with decreasing O/C ratio. The Panjiang coal with higher coalification degree is most affected, which has a reduction in contact angle by approximately 20.5°. The contact angles measured on Shenhua coal with low coalification degree are affected slightly by the presence of either of the two dispersants used in this study. Additionally, it is interesting that the presence of FDN causes an increase in contact angle by approximately 1.7° for Shenhua coal. The results indicate that the presence of the dispersants can markedly increase the hydrophilicity of the surface of the coal with high coalification degree, while that is not evident for the coal with low coalification degree [4]. 3.2. Relationship between coal properties and solid content of CWS The preparation of CWS is influenced greatly by the coal properties, such as O/C ratio, inherent moisture content and ash content. It is known that the highly-loaded CWS can be easily prepared by coal with high degree of coalification, but difficultly by coal with low coalification. Literature [13] stated that the inherent moisture was the most important factor to affect the preparation and c content of CWS. While the literature [14] stated that the inherent moisture of coal was influenced by oxygen content, the coal coalification degree, ash content and oxygen content are considered as the major affecting factors for CWS preparation. In this study, the four kinds of Chinese coals with different coalification degrees from Datong, Yanzhou, Shenhua, Panjiang were used to prepare CWS. As seen in

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Table 1, the proximate and ultimate analyses of coal samples indicate that the four kinds of coals have different O/C ratios, inherent moisture contents and ash contents, and the coalification degrees are different. The SAF was used to prepare CWS formulated from Datong, Yanzhou, Shenhua, Panjiang coals, and the practical CWS with coal contents approximately 64.5 wt.%, 66.0 wt.%, 61.0 wt.% and 70.0 wt.% could be prepared with the viscosities less than 1000 mPa s. The relationship between the solid content and O/C ratios, inherent moisture contents, ash contents of the coals were treated by multiple linear regression method, and a ternary linear regression equation was obtained as follows:

The regression equation indicates that the solid content of CWS is directly proportional to the ash content, but inversely proportional to the O/C ratio and inherent moisture content of the coal. The coefficient of the inherent moisture content (0.896) is a minus whose absolute value is greater than that of the O/C ratio (0.305), which means that the inherent moisture content of the coal has more inversely effect on the solid content of CWS. Table 3 shows that the regression value is close to the factual value.

3.3. Viscosity-reducing capacity of SAF used in CWS formulated from different coals

Solid contentðwt=wtÞ ¼ 0:699 þ 0:192 ðash wt=wtÞ  0:305 ðO=C wt=wtÞ  0:896 ðmoisture wt=wtÞ: The factual value and regression value of the solid contents are listed in Table 3.

Table 3 Factual value and regression value of the solid content derived from the regression equation Coal sample

Ash (wt/wt)

O/C (wt/wt)

Moisture (wt/wt)

Factual value of the solid content (wt/wt)

Regression value of the solid content (wt/wt)

Panjiang Yanzhou Datong Shenhua

0.092 0.113 0.102 0.053

0.024 0.122 0.131 0.164

0.011 0.027 0.038 0.055

0.700 0.660 0.645 0.610

0.700017 0.660017 0.645017 0.610015

In order to evaluate the performance of SAF as dispersant in CWS, the above four kinds of Chinese coals were used to prepare CWS, and FDN is a dispersant used for comparison. All the CWS were prepared at neutral pH and the dispersant dosages varied from 0.2 wt.% to 1.5 wt.% (on dry coal basis). The relationships between the apparent viscosity of CWS made up from different coals and the dispersant dosages are illustrated in Fig. 2. According to Fig. 2, for Datong, Yanzhou and Panjiang coals, the SAF and FDN are effective dispersants to prepare highly-loaded CWS with the apparent viscosity below 1200 mPa s. Especially for Panjiang coal, the CWS with 70.0 wt.% coal content can be obtained, while for Shenhua coal, the CWS with the apparent viscosity below 1000 mPa s can be prepared at 61.0 wt.% coal content. As seen in Table 1, Shenhua coal has the highest moisture and oxygen contents, and the lowest ash content of the four coals, belong to brown coal. Therefore, it is seen that the 1200

Apparent viscosity, mPa.s

Apparent viscosity, mPa.s

1600 Datong Coal

1400

SAF FDN

1200 1000 800

Yanzhou Coal

1100

SAF FDN

1000 900 800 700 600 500

600

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0.2

Dispersant dosage, wt.% of the dry coal

1300

Shenhua coal

Apparent viscosity, mPa.s

Apparent viscosity, mPa.s

1500

SAF FDN

1400 1300 1200 1100 1000 900 0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

Dispersant dosage, wt.% of the dry coal

0.4

0.6

0.8

1.0

Dispersant dosage, wt.% of the dry coal

1200

Panjiang Coal SAF FDN

1100 1000 900 800 700 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Dispersant dosage, wt.% of the dry coal

Fig. 2. Effect of dispersant dosage on apparent viscosity of CWS prepared using different coals. Coal contents: 64.5 wt.% (Datong); 66.0 wt.% (Yanzhou); 61.0 wt.% (Shenhua); 70.0 wt.% (Panjiang).

X. Qiu et al. / Fuel 86 (2007) 1439–1445

coal with high moisture and oxygen content is difficult to prepare highly-loaded CWS. As shown in Fig. 2, for SAF dispersant, the CWS viscosities decrease sharply with the increase of the dispersant dosages from 0.4–0.8 wt.% for Datong coal, 0.2–0.4 wt.% for Yanzhou coal, 0.5–0.7 wt.% for Shenhua coal, and 0.3–0.5 wt.% for Panjiang coal. However, the CWS viscosities increase when the dispersant dosages exceed 1.0 wt.% on Datong coal, 0.6 wt.% for Yanzhou coal, 1.0 wt.% for Shenhua coal, 0.6 wt.% for Panjiang coal, especially the viscosities of CWS formulated from Yanzhou and Panjiang coals increase sharply with an over-dose dispersant dosages. For FDN dispersant, the viscosities of the CWS formulated from the four kinds of coals decrease sharply at lower dispersant dosages, and then generally remain constant or increase slightly with increasing dispersant dosage. Compared with FDN, at lower dispersant dosages, the CWS formulated from the above-mentioned coals with lower viscosities can be prepared using SAF as dispersant. Especially to Datong coal, the 1.0 wt.% dosage of SAF is necessary to obtain the lowest viscosity of 680 mPa s, while at the same dosage of FDN, the lowest viscosity is 1120 mPa s. However, at higher dispersant dosages, the viscosities of CWS formulated from the four kinds of coals, especially from the Yanzhou and Panjiang coals, increase sharply. It is desirable that the dispersant dosage used to prepare CWS should be as low as possible, so SAF is a more promising dispersant for the preparation of CWS than FDN in industry application. 3.4. Stability investigation of CWS formulated from different coals The purpose of this study was to investigate the stability properties of SAF and FDN for use in CWS formulated 100

from the four kinds of coals. All the CWS were prepared at neutral pH and at dispersant dosage of 1.0 wt.% (on dry coal basis). The results are expressed by the penetration ratio with respect to the storage time in Fig. 3. As seen in Fig. 3, the penetration ratios of the CWS prepared from SAF are kept decreasing with the increase of the storage times, especially decreasing sharply within the first 24 h for Datong and Shenhua coals, and the first 48 h for Yanzhou and Panjiang coals. In contrast, the CWS prepared from FDN show higher penetration ratio than that from SAF for Datong coal, but show lower penetration ratio than that from SAF for Yanzhou, Shenhua and Panjiang coals. The CWS prepared from SAF show more than 77% penetration ratio within 48 h for the four different kinds of coals. Based on the results of penetration ratios against the storage time, the CWS prepared from SAF show better stability than that from FDN for Yanzhou, Shenhua and Panjiang coals, and show inferior stability for Datong coal. 3.5. Effect of SAF dosage on rheological behavior of CWS formulated from different coals One of the major requirements to be met in preparing a CWS is that it must have a solid content as high as possible but a minimum viscosity to allow ease of handling during preparation, storage, transfer and atomization [15,16]. The application property of CWS is influenced significantly by the rheological behavior of CWS. This paper focuses the study of dispersant dosage on rheological behavior of CWS. The flow curves of the CWS formulated from Datong, Yanzhou, Shenhua and Panjiang coals are given in Fig. 4 in the form of apparent viscosity against shear rate, g vs c. The shear-stress/shear-rate dependence or, equivalently, 100

95

Penetration ratio, %

Penetration ratio, %

Datong coal SAF FDN

90 85 80

Yanzhou coal

95

SAF FDN

90 85 80 75 70 65

75 0

10

20

30

40

50

60

70

80

0

10

Storage time, hours

20

30

40

60

50

70

Storage time, hours 100

100

Panjiang coal

Shenhua coal

95

Penetration ratio, %

Penetration ratio, %

1443

SAF FDN

90 85 80 75 70 65

95

SAF FDN

90 85 80

60 55

75 0

10

20

30

40

50

60

Storage time, hours

70

80

0

10

20

30

40

50

60

70

80

Storage time, hours

Fig. 3. Effect of storage time on stability of CWS prepared using different coals at 1.0 wt.% (on dry coal basis) dosages of SAF and FDN. Coal contents: 64.5 wt.% (Datong); 66.0 wt.% (Yanzhou); 61.0 wt.% (Shenhua); 70.0 wt.% (Panjiang).

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X. Qiu et al. / Fuel 86 (2007) 1439–1445 3500

Datong Coal SAF%=0.5% SAF%=0.6% SAF%=0.8% SAF%=1.0%

10000 8000 6000 4000 2000 0

Yanzhou Coal SAF%=0.2% SAF%=0.3% SAF%=0.4% SAF%=1.0%

3000

Viscosity, mPa.s

Viscosity, mPa.s

12000

2500 2000 1500 1000 500

0

50

100

150

0

200

0

20

40 60

Shear rate, 1/S 4500 3500

Viscosity, mPa.s

Viscosity, mPa.s

6000

Shenhua Coal SAF%=0.5% SAF%=0.6% SAF%=1.0%

4000

80 100 120 140 160 180 200

Shear rate, 1/S

3000 2500 2000 1500 1000

Panjiang Coal SAF%=0.4% SAF%=0.6% SAF%=1.0%

5000 4000 3000 2000 1000

500 0

0 0

50

100

150

200

0

50

Shear rate, 1/S

100

150

200

Shear rate, 1/S

Fig. 4. Examples of flow curves for CWS obtained at varying dosages of SAF on different coals. Coal content: 64.5 wt.% (Datong); 66.0 wt.% (Yanzhou); 61.0 wt.% (Shenhua); 70.0 wt.% (Panjiang).

Table 4 Herschel–Bulkley parameter values for CWS using SAF on different coals at 25 °C Coal sample

SAF dosage (wt.%)

sy (Pa)

K (Pa sn)

n

R2

Datong

0.5 0.6 0.8 1.0

47.224 8.462 0.032 0.909

0.667 0.372 0.380 0.238

1.172 1.158 1.162 1.232

0.9885 0.9996 0.9998 0.9997

Yanzhou

0.2 0.3 0.4 1.0

10.964 1.021 0.746 0.062

0.582 0.445 0.473 0.799

0.977 1.037 1.055 1.010

0.9991 0.9995 0.9999 0.9997

Shenhua

0.5 0.6 1.0

17.046 0.996 0.080

0.725 1.122 1.537

0.973 1.003 0.963

0.9994 0.9998 0.9996

Panjiang

0.4 0.6 1.0

18.350 0.348 13.244

0.935 0.890 0.994

0.990 1.036 0.971

0.9991 0.9995 0.9993

the apparent viscosity/shear rate data for each CWS are fit for the three-parameter Herschel–Bulkley model, given by s = sy + Kcn [17–19]. The values of the calculated Herschel–Bulkley model parameter, the yield stress sy, the fluid consistency index K and the flow behavior index n are listed in Table 4. As shown in Fig. 4, at low shear rates, the flow curves of CWS made up from Datong, Yanzhou, Shenhua and Panjiang coals show shear-thinning characteristic at low dispersant dosages, but show shear-thickening characteristic at high dispersant dosages except Panjiang coal. At high shear rates, the flow curves show Newtonian limiting characteristic.

The parameter values resulting from fitting the experimental flow curves in Table 4 show that the yield value decreases with the increase of the dispersant dosage for the CWS formulated from Datong, Yanzhou and Shenhua coals. For Panjiang coal, the yield value increases at higher dispersant dosage. The flow behavior index n for the CWS formulated from the four kinds of coals is close to 1, and the CWS shows Newtonian flow characteristic at high shear rate. 4. Conclusion Based on the results obtained in this investigation, the wettability of the coal surface by water decreases with decreasing O/C ratio, and the presence of dispersant increases the wettability of the coal surface by water, especially the change of contact angle is markedly observed for the coals with high coalification degrees. The O/C ratios, inherent moisture contents and ash contents of the coals are regards as major affecting factors that influence the CWS preparation. In this study the relation between the O/C ratios, inherent moisture contents and ash contents of the coals and the solid content of CWS is described by a ternary regression equation: Solid content (wt/wt) = 0.699 + 0.192 (ash wt/wt)  0.305 (O/C wt/ wt)  0.896 (moisture wt/wt). The solid content of CWS is directly proportional to the ash content, but inversely proportional to the O/C ratio and inherent moisture content of the coal. The evaluation of SAF for the preparation of highlyloaded CWS proves that SAF is an effective dispersant for CWS at lower dosage. The CWS can be of low viscosity and excellent stability by using SAF dispersant.

X. Qiu et al. / Fuel 86 (2007) 1439–1445

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