Improvement in properties of coal water slurry by combined use of new additive and ultrasonic irradiation

Improvement in properties of coal water slurry by combined use of new additive and ultrasonic irradiation

Ultrasonics Sonochemistry 14 (2007) 583–588 www.elsevier.com/locate/ultsonch Improvement in properties of coal water slurry by combined use of new ad...

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Ultrasonics Sonochemistry 14 (2007) 583–588 www.elsevier.com/locate/ultsonch

Improvement in properties of coal water slurry by combined use of new additive and ultrasonic irradiation Zhaobing Guo a

a,*

, Ruo Feng b, Youfei Zheng a, Xiaoru Fu

a

College of Environmental Science and Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, PR China b State Key Laboratory of Modern Acoustics, Institute of Acoustics, Nanjing University, Nanjing 210093, PR China Received 5 August 2006; received in revised form 22 November 2006; accepted 2 December 2006 Available online 22 January 2007

Abstract Coal water slurry (CWS) was prepared with a newly developed additive from naphthalene oil. The effects of ultrasonic irradiation on coal particle size distribution (PSD), adsorption behavior of additive in coal particles and the characteristics of CWS were investigated. Results showed that ultrasonic irradiation led to a higher proportion of fine coal in CWS and increased the saturated adsorption amount of additive in coal particles. In addition, the rheological behavior and static stability of CWS irradiated by ultrasonic wave were remarkably improved. The changes on viscosity of CWS containing 1% and 2% additive are qualitatively different with the increasing sonication time studied. The reason for the different effect of sonication time on CWS viscosity is presented in this study. Ó 2006 Elsevier B.V. All rights reserved. Keywords: CWS; Ultrasonic irradiation; Additive from naphthalene oil

1. Introduction China is rich in coal resources, and coal is mainly used for power generation on the basis of pulverized coal-fired technology, which results in serious coal waste and environmental problems. Therefore, advanced coal utilization technologies are urgently necessary to cater for the increasing use of energy and environmental demands of 21st century. One of such approaches is coal water slurry (CWS) technology, which holds promise as an attractive alternative fuel, and this has been a key research and development (R&D) project in China since 1980s. Currently, R&D activities are in progress on various aspects of the CWS technology [1–4]. A typical CWS consists of 60–75% coal, 25–40% water and about 1% chemical additive. An ideal CWS with maximum coal loading should possess static stability and exhi-

*

Corresponding author. Tel.: +86 25 82029166; fax: +86 25 58731090. E-mail addresses: [email protected], [email protected] (Z. Guo).

1350-4177/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2006.12.001

bit good rheological behavior. There is a general consensus that the additive for CWS is a crucial factor in improving the properties of CWS [5,6]. Nowadays, the widely used chemical additives are anionic surface active agents, such as sodium naphthalene sulphonate formaldehyde condensate and sodium lignosulphonate. It has been observed that CWS with high coal loading can be achieved by the use of such additives. However, use of these additives may simultaneously give rise to high cost on CWS formulation and exhibit unsatisfactorily static stability as well. Therefore, the preparation of a high-powered and low-cost chemical additive for CWS has attracted great interests from the academic researches in China. Based on the effective components of naphthalene oil from coal tar, a very cheap additive for CWS was synthesized in our laboratory, and CWS prepared with new additive presented the similar properties to that with sodium naphthalene sulphonate formaldehyde condensate [7]. The bad static stability and weak rheological behavior of CWS is hard to meet the requirement in industrial application, which limited the rapid development of CWS industry in China in the past few years.

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The application of ultrasonic (US) technique in wastewater treatment, the extraction of coal, desulphurization and liquefaction of coal attracts researchers’ attention [8–12]. Ultrasonic wave consists of rarefaction and compression cycles, which leads to the generation of cavitation bubbles. The violent collapse of the cavitation bubbles results in a specific environment with extremely high temperature and pressure. Simultaneously, there are two effects of cavitation formed near the extended liquid–solid interfaces: microjet impact and shock wave damage. As a result, the high-velocity interparticle collisions (about 110 m/s) can be caused [13], which is favorable for the dispersion of solid particle in liquid–solid system. This dispersion is of utmost importance in the static stability of CWS. Ultrasound treatment seems to be a feasible method for the preparation of an ideal CWS given the advantages of low cost and convenient operation. In this contribution, consequently, the comparisons between the properties of CWS prepared with new additive before and after ultrasonic irradiation, such as coal particle size distribution (PSD), adsorption behavior of additive, apparent viscosity, rheological behavior, zeta potential and static stability, are investigated.

aromatic compounds with –CH2– and –SO 3 were determined by Fourier transform infrared spectrography (FTIP) [7]. 2.2. CWS preparation and ultrasonic treatment The pulverized coal was mixed slowly in a vessel containing 1% of the additive (dry coal basis) and deionizer water. The mixture was continuously stirred by means of mechanical agitator kept at 1200 rpm for approximately 15 min to ensure homogenization of CWS. The slurry so prepared was left as such to release entrapped air and then transferred to an airtight vessel for the study. The experimental set-up of model JY92-2D (from Inc. Scientz, China) is shown in Fig. 1. One hundred milliliters CWS mixture in a closed vessel was irradiated by sonicator of frequency 20 kHz and its emitted ultrasonic intensity was from 60 to 120 W/cm2, determined by a calorimetric technique. The titanium alloy rod, 6 mm in diameter, was immersed beneath the liquid surface (2 cm). The vessel was double-walled-type with circulating water to keep temperature at 25 ± 0.5 °C; the temperature was monitored by means of a thermocouple.

2. Material and methods 2.3. Adsorption experiment 2.1. Materials The coal, from Pangzhuang Mine in China, was chosen for the study. The coal was first crushed in a double roll crusher to obtain a sub-5 mm product; the crushed coal was then comminuted in the ball mill to produce an optimum PSD. The characteristics of the coal sample are given in Table 1. The new additive for CWS preparation from naphthalene oil, an anionic surface active agent, in which the

In order to investigate the effect of ultrasonic irradiation on the adsorption behavior of additive, the adsorption isotherms of additive in coal sample were compared before and after ultrasonic irradiation. One hundred milliliters of additive solutions with different concentrations were mixed with 1.0 g coal sample in the flasks, respectively. Then, these flasks were sealed and transferred into an incubator (ZD-88) at 298 K. Under continuous shaking, the adsorption process was continued for 20 h in order to reach

Table 1 Analysis of coal properties Proximate analysis (wt%)

Ultimate analysis (wt%)

Mad

Ad

Vdaf

St,d

Cdaf

Hdaf

Odaf

Ndaf

1.32

10.79

36.54

0.27

83.43

4.70

10.29

1.30

Power amplifier

Matching circuit Transducer

Signal generator

Oscilloscope

Cooling bath Fig. 1. Experimental set-up of ultrasonic irradiation.

Slurry

ST (°C)

HGI

1500

64.81

2.4. Determination of CWS properties PSD of coal particles in CWS was determined using a laser particle size analyzer (LS100Q). The automatic particle size analysis can be obtained in the measuring range 0.1–1000 lm in suspension. The results are calculated on the basis of the Fraunhofer theory. The apparent viscosity of CWS was measured using a NXS-11A rotation viscosimeter, which has four interchangeable spindles for different viscosity ranges. Measurements were performed using the C spindle at a shear rate of 28.5 s1 and the temperature of 298 K in our study. The rheological characteristic of CWS was conducted by measuring the apparent viscosities of CWS at the ultrasonic intensity of 60 W/cm2 and different shear rates ranging from 2.5 s1 to 52 s1. Zeta potentials of coal particles in CWS before and after ultrasonic irradiation were measured by Zeta Meter 3.0 equipped with a microprocessor unit. The unit automatically calculates the electrophoretic mobility of the particles and converts it into the zeta potentials. 3. Results and discussion 3.1. Analysis of coal properties It can be observed from Table 1 that coal sample is part of bituminous coal, and the degree of coalification is middle-to-low. On the basis of the low internal moisture, the high grindability index and the volatile and ash fusion temperature of coal sample, it is speculated that the coal sample is appropriate to the CWS formulation. Using the new additive, the maximum coal loading of CWS was found to be 71.0 wt% with the apparent viscosity of 1200 mPa s at the shear rate of 28.5 s1. It should be pointed out that all experiments in this study were carried out in double, and the reproducibility of the experiments is satisfactory. The data are the average of the two experiments. 3.2. Effect of ultrasonic irradiation on PSD One of the key factors required to obtain a highly loaded and low-viscosity CWS is the PSD of coal. The PSD curves of coal used in CWS formulation before and after irradiation are compared in Fig. 2. It can be found that coal content with particle size below 160 lm in CWS improved in the presence of the irradiation at the ultrasonic intensity of 60 W/cm2 and the sonication time of 5 min. The increase of fine coal in CWS is ascribed to not only the violent ultra-

585

100 80 60 40 20 0

0

50

100

150

200

250

300

Coal particle size (μm) Fig. 2. Comparison of coal PSD in CWS before and after ultrasonic irradiation. Ultrasonic intensity: 60 W/cm2; sonication time: 5 min; () irradiation; (r) no irradiation.

sonic vibration, but also to the cavitation effect produced by ultrasonic wave. Ultrasonic irradiation produces the violent asymmetric collapse of cavitation bubbles in aqueous solution, which results in high temperature, high pressure and high-velocity jet, which breaks down the coal particles and partly converts coarse coal into fine coal. Additionally, ultrasonic irradiation is greatly favorable for the reduction of the larger coal clusters to the smaller coal clusters owing to the violent dispersion force. In order to improve the combustion efficiency of CWS, coal particle size is commonly controlled below 300 lm, with a particle size below 74 lm of not less than 75%. It is observed from Fig. 2 that coal contents with particle size below 74 lm before and after ultrasonic irradiation are about 75% and 80%, respectively, indicating that coal PSD was more favorable for CWS preparation in the presence of ultrasonic irradiation. 3.3. Effect of ultrasonic irradiation on adsorption behavior of additive The adsorption isotherms of additive in coal sample in CWS before and after irradiation at 298 K are described in Fig. 3. It is found that the adsorption of additive in coal sample fits a typical Langmuir adsorption model,

Adsorption amount (mg/g)

the adsorption equilibrium. Coal particles were removed by the centrifugal machine and the equilibrium concentrations of additive were determined using a UV–Vis spectrometer (Helios Beta) with the detecting wave-length of 560 nm. Meanwhile, the adsorption of additive in coal sample in CWS under the irradiation was also carried out at ultrasonic intensity of 60 W/cm2 and sonication time of 10 min.

Percentage of volume (%)

Z. Guo et al. / Ultrasonics Sonochemistry 14 (2007) 583–588

0.8 0.6

0.4 0.2 0

0

100

200

300

400

500

600

Equilibrium concentration (mg/L) Fig. 3. Adsorption isotherms of additive in coal before and after ultrasonic irradiation at 298 K. Ultrasonic intensity: 60 W/cm2; sonication time: 10 min; () irradiation; (r) no irradiation.

Z. Guo et al. / Ultrasonics Sonochemistry 14 (2007) 583–588

qe ¼ Kqm C e =ð1 þ KC e Þ

ð1Þ

where qe (mg/g) is the saturated adsorption amount at equilibrium concentration Ce (mg/L); qm (mg/g) the maximum adsorption amount and K the adsorption parameter, respectively. The resulting parameters of adsorption isotherms are tabulated in Table 2. The maximum adsorption amounts of additive in coal sample before and after irradiation were found to be about 0.63 and 0.77 mg/g, corresponding to the equilibrium concentrations of additive were 300 and 350 mg/L, respectively. It shows that the level of additive in the coal at saturated adsorption in the presence of irradiation was higher compared to that in the absence of irradiation. This may be ascribed to the higher proportion of fine coal in CWS and the enhanced specific surface area of coal particles after the ultrasonic irradiation. Parameter K is indicative of the adsorption ability of additive in coal sample. The higher K value signifies the stronger adsorption force between the additive and coal particles. Therefore, it can be seen from Table 2 that mutual action between new additive and coal particles became intensified via the irradiation at the ultrasonic intensity of 60 W/cm2 and the sonication time of 10 min. 3.4. Effect of ultrasonic irradiation on CWS viscosity CWS mixtures containing 1% additive were irradiated for about 5 min under different ultrasonic intensities, and the viscosity of CWS was determined at the shear rates of 28.5 s1. The relationship between CWS viscosity and coal loading before and after ultrasonic irradiation is depicted in Fig. 4. It can be noted that ultrasonic treatment led to the higher CWS viscosity compared to that without the irradiation. In addition, CWS viscosity distinctly increased with the increasing ultrasonic intensity. Generally, CWS viscosity will reach a minimum value when the saturated adsorption of additive in coal particles is achieved. As is observed in Fig. 3 ultrasonic irradiation increased the adsorption amount of additive in coal particles. Therefore, the increase in CWS viscosity after 5 min irradiation is mainly attributed to the fact that the addition of 1% additive did not reach the level of additive in the coal Table 2 Regression parameters of additive adsorption isotherms in coal particles before and after irradiation Saturated adsorption amount (mg/g) No irradiation Ultrasonic irradiation

0.63 0.77

K 0.020 0.024

R2 0.99 0.99

Adsorption temperature: 298 K; ultrasonic intensity: 60 W/cm2; sonication time: 10 min.

1500

CWS viscosity (mPa•s)

irrespective of ultrasonic irradiation or not, which is indicative of the monolayer adsorption of additive in the coal. Langmuir adsorption model is typically expressed as follows [14]:

1300 1100 900 700 500 65

66

67

68

69

70

Coal loading (wt.%) Fig. 4. Dependence of CWS viscosity on coal loading before and after ultrasonic irradiation. Sonication time: 5 min; (r) no irradiation; (j) irradiation, 60 W/cm2; (m) irradiation, 120 W/cm2.

at saturated adsorption. When ultrasonic intensity was elevated, cavitation effect was further intensified as well as the dispersion and crush of ultrasonic wave on coal particles in CWS, enhancing the additive content at saturated adsorption in coal. The much larger difference in the level of additive in the coal at saturated adsorption and the addition of 1% additive could lead to higher viscosity of CWS. Li et al. [15] studied the effect of the irradiation on CWS preparation using sodium naphthalene sulphonate formaldehyde condensate (NSF) as the additive and arrived at a similar conclusion. However, Liu et al. [16] observed that ultrasonic treatment could result in a distinct decrease in CWS viscosity. In order to clearly analyze the effect of ultrasonic irradiation on CWS viscosity, the relationship between the viscosities of CWS containing 1% and 2% additive and sonication time was investigated controlling ultrasonic intensity at 60 W/cm2 and coal loading at 68.5 wt% (see Fig. 5). Note that the viscosities of CWS with 1% additive originally decreased, then rapidly increased after 3 min ultrasonic irradiation. Ultrasonic irradiation within 3 min enhanced the adsorption amount of additive in coal particles, however, it can be speculated that the addition of 1% additive was still enough for saturated adsorption of additive in coal particles. Meanwhile, as stated in Section 3.3, the strong mutual action between the additive and coal par1400

CWS viscosity (mPa•s)

586

1200 1000 800 600

0

1

2

3

4

5

Time (min) Fig. 5. Relationship between CWS viscosity and sonication time with different addition amount of additive. Ultrasonic intensity: 60 W/cm2; coal loading: 68.5 wt%; () 2%; (r) 1%.

Z. Guo et al. / Ultrasonics Sonochemistry 14 (2007) 583–588

3.5. Effect of ultrasonic irradiation on zeta potential

CWS viscocity (mPa•s)

Funk [17] stated the importance of the zeta potential of the coal surface in a CWS. He found that high values of zeta potential led to good dispersion. Zeta potential measurements taken on coal samples in CWS containing 1% additive before and after ultrasonic irradiation are reported in Fig. 7. It is found that zeta potential of coal particles after 5 min irradiation at the ultrasonic intensity of 60 W/ cm2 shifted towards more negative values as compared with that without the irradiation. The highest absolute zeta potential values were obtained at the zeta potential of 42 mV and 49 mV before and after ultrasonic irradia1800 1500 1200 900 600 300 0

0

10

20

30

40

50

60

Shear rate (s -1) Fig. 6. Rheological behavior of CWS under the different conditions. Ultrasonic intensity: 60 W/cm2; coal loading: 68.5 wt%; sonication time: 5 min; () irradiation; (r) no irradiation.

60

Zeta potential (-mv)

ticles resulted in the decrease in CWS viscosity. When sonication time was beyond 3 min, the addition of 1% additive was not adequate for the saturated adsorption of additive in coal particles, which induced the rapid increase in CWS viscosity. For CWS mixture containing 2% additive, it can be observed that the viscosity of CWS kept decreasing during 5 min ultrasonic irradiation, which is assigned to that the addition of 2% additive is much sufficient to produce the saturated adsorption in coal particles. Rheological behavior is an important factor in influencing the storage, transport and combustion of CWS. It is desirable that CWS, as non-Newtonian fluids, possesses the characteristic of low viscosity in the range of high shearing rate. The dependences of CWS viscosities on shear rates before and after ultrasonic irradiation were investigated at the coal loading of 68.5 wt% and the sonication time of 5 min. It can be observed from Fig. 6 that the CWS is a dilatant fluid in the absence of ultrasonic irradiation, exhibiting shear-thickening behavior. However, the CWS was converted into the desired pseudoplastic fluids in the presence of ultrasonic irradiation, exhibiting shearthinning behavior. CWS rheological behavior is closely correlated with coal particle size and coal PSD. Note that CWS irradiated by ultrasonic wave possessed high proportion of fine coal and optimum PSD, which consequentially weakened the mutual actions of large coal particles and markedly increased the rheological behavior of CWS.

587

50 40 30 20 10 0

0

100

200

300

400

500

600

Equilibrium concentration (mg/L) Fig. 7. Dependence of zeta potential of coal on equilibrium concentration of additive before and after ultrasonic irradiation. Ultrasonic intensity: 60 W/cm2; sonication time: 5 min; () irradiation; (r) No irradiation.

tion, corresponding to the additive concentrations of 300 mg/L and 350 mg/L, respectively. The concentrations are exactly consistent with the results that the maximum adsorption amounts of additive in the coal were achieved in the adsorption isotherms, indicating that zeta potential values were relevant to the saturated adsorption amount of additive in coal particles. The more negative values of zeta potential after ultrasonic irradiation were chiefly ascribed to the increase of additive content at saturated adsorption in the coal. For the same coal, the absolute zeta potential value of coal samples will reach maximum at the saturated adsorption stage. When the addition of additive is in excess of the additive concentration of saturated adsorption, the absolute zeta potential values will decrease instead (see Fig. 7), which may be attributed to the multilayer adsorption of additive over coal surface. In addition, the enhancement of the absolute zeta potential values on coal particles irradiated by ultrasonic wave correlates with the increase of dissolved ions from the coal in CWS. Changes on concentrations of dissolved ions from coal in CWS after the irradiation have been studied by Li et al. [15]. 3.6. Effect of ultrasonic irradiation on CWS static stability It is obvious that ultrasonic irradiation enhanced the static stability of CWS, as shown in Fig. 8. The CWS static stability was markedly improved with the increasing ultrasonic intensity and irradiation time. Keeping the coal loading at 67.9 wt%, the static stability of CWS was around 12 h in the absence of irradiation. To our great surprise, the CWS static stability increased to 9 days at the ultrasonic intensity of 120 W/cm2 and the irradiation time of 5 min. Coal PSD investigated in this paper was in the range from 0.375 lm to 300 lm, It is clear that the weight of coal particles is a crucial factor in determining the CWS static stability when coal particle size is much larger than 10 lm, and the stabilization owing to the Brownian movement cannot be expected. As a result, controlling the sediment of coal particles in CWS is very difficult. When coal particle size is reduced by ultrasonic irradiation, the

Z. Guo et al. / Ultrasonics Sonochemistry 14 (2007) 583–588

CWS static stability (d)

588

the irradiation evidently enhanced the rheological behavior and static stability of CWS. The effect of ultrasonic irradiation on CWS viscosity is dependent on the level of additive in the coal at saturated adsorption and the addition of additive in CWS.

10 8 6 4

Acknowledgement

2 0

0

1

2

3

4

5

Irradiation time (min) Fig. 8. Effect of ultrasonic intensity and irradiation time on CWS static stability. Coal loading: 67.9 wt%; () 120 W/cm2; (r) 60 W/cm2.

movement of coal particles became more pivotal in deciding the CWS static stability as compared with the weight of coal particles, which is a main reason for improving the CWS static stability remarkably. In addition, the anionic additive, absorbed onto the surface of coal particles, led to the negative charge on the coal particles. At the same time, it is expected that a large amount of cations could be dissolved from coal in CWS after the irradiation. The dissolved cations, particularly the high-valent cations, will form bridge bonding among the coal particles, which is also very favorable for the CWS static stability. 4. Conclusions The study suggests that the combination of ultrasonic treatment and an additive is an effective means of producing CWS. Ultrasonic irradiation not only improved the proportion of fine coal in CWS, but also increased the level of additive in the coal at saturated adsorption and absolute zeta potential values of coal particles in CWS. In addition,

This work was supported by National Natural Science Foundation of China (Grant No. 20277011). References [1] [2] [3] [4] [5] [6] [7] [8] [9]

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