Energy Fuels 2010, 24, 1260–1268 Published on Web 01/21/2010
: DOI:10.1021/ef900921c
Surface Engineering of Low Rank Indian Coals by Starch-Based Additives for the Formulation of Concentrated Coal-Water Slurry Debadutta Das,† Uma Dash,† Amalendu Nayak,‡ and Pramila K. Misra*,† †
Centre of Studies in Surface Science and Technology, School of Chemistry, Sambalpur University, Jyoti Vihar, 768 019, Orissa, India and ‡Institute of Minerals and Materials Technology, Council of Scientific and Industrial Research, Bhubaneswar, 751 013, Orissa, India Received August 25, 2009. Revised Manuscript Received November 22, 2009
The rheological characteristics of three varieties of noncoking coals have been modulated by three starchbased additives, starch xanthate, starch xanthide, and starch phosphate. The apparent viscosity of the modified coal-water slurry has been measured by varying coal loading, additive percentage, pH, temperature, etc. Considerably stable concentrated coal-water slurry containing high weight fraction of coal could be achieved in each case, and slurry stays in a nonsettling form for 20-25 days with viscosity in a permissible low range. The effectiveness in reducing the apparent viscosity decreases in the order, starch xanthide > starch phosphate > starch xanthate. The plausible mechanism of slurry stabilization has been suggested considering the structural basis of an effective additive for the stabilization of coal-water slurry.
slurry to storage and shipping centers, and also to the electric generating plants.5,6 The conglomeration of coal in the highly concentrated coal-water slurry is mainly due to the association of coal particles among themselves, leading to the flocculation of coal particles and hence suspension destabilizes quickly. Since a significant region of the coal is hydrophobic,7-9 the interparticle association can be controlled by masking the hydrophobic sites of coal or by rendering the coal surface with sufficient hydrophilicity so that the coal-water interaction will be promoted in stead of coal-coal interaction. Hence, extensive research has been reinforced now-a-days in architecting coal surface modification through different physical means10-12 such as ultrasonic, microwave radiation, thermal, etc., or through chemical means13-16 such as adsorbing suitable additives on the coal surface so that coal-water slurry of reasonable viscosity, substantial coal loading, and considerable stable suspensions would be achieved from low ash/beneficiated noncoking coal. The specific energy and cost involved in the formulation as well as transportation of coal from its source to user sites will thus be reduced substantially.17,18
1. Introduction The increase of oil price day by day has become more alarming due to the rapid consumption and depletion of the energy sources during the running modernization of society, hence a worldwide consciousness has evoked a search for an alternative to petroleum products. Coalwater slurry, a concentrated suspension of coal in water, also referred to as coal-water fuel,1,2 has come to the limelight as a source of energy during the 20th century due to its efficiency in substituting fuel of several industries3,4 and has, therefore, received considerable attention. The procurement of coal-water slurry with high volume fraction of coal is a prime requirement for the optimum efficiency of the slurry as fuel. The increase in particleparticle interaction during the formulation of concentrated coal-water slurry leading to settling is, however, detrimental for the slurry stabilization. The viscosity of the slurry with the coal loading also increases, affecting the power requirements for pumping the slurries during the aqueous transportation of coal through pipelines. Current emphasis is therefore on rapid and inexpensive means for formulationg, stabilizing, and distributing the coal-water
(7) Kaushal, K. T.; Basu, S. K.; Kumaresh, C.; Bit, B. S.; Mishra, K. K. Fuel Process. Technol. 2003, 85, 31–42. (8) Leong, Y. K.; Boger, D. V.; Christie, G. B.; Mainwaring, D. E. Rheol. Acta 1993, 32, 277–285. (9) Roh, N.-S.; Shin, D.-H.; Kim, D.-C.; Kim, J.-D. Fuel 1995, 74, 1220–1225. (10) Zhaobing, G.; Feng, R.; Zheng, Y.; Fu, X. Ultrason. Sonochem. 2007, 14, 583–588. (11) Li, Y. X.; Li, B. Q. Fuel 2000, 79, 235–241. (12) Meikap, B. C.; Purohit, N. K.; Mahadevan, V. J. Colloid Interface Sci. 2005, 281, 225–235. (13) Shin, Y. J.; Shen, Y. H. Chemosphere 2007, 68, 389–393. (14) Leong, Y. K.; Boger, D. V.; Christie, G. B.; Mainwaring, D. E. Rheol. Acta 1993, 32, 227–285. (15) Aktas, Z.; Woodburn, E. T. Fuel Process. Technol. 2000, 62, 1–15. (16) Gurses, A.; Ac-ıkyıdız, M.; Doger, C.; Karaca, S.; Bayrak, R. Fuel Process. Technol. 2006, 87, 821–827. (17) Imai, T.; Tanaka, M. Sekitan Riyo Gijutsu Kaigi Koenshu 1994, 4th, 131–94. (18) Saeki, T.; Tatsukawa, T.; Usui, H. Coal Prep. 1999, 21, 161–176.
*To whom correspondence should be addressed. E-mail: pramila_61@ yahoo.co.in. (1) Pawlik, M. Colloids Surf., A 2005, 266, 82–90. (2) Papachristodoulou, G.; Trass, O. The Can. J. Chem. Eng. 1987, 65, 177–197. (3) Noboru, H.; Harumitsu, Y.; Masao, T. Coal & Manabus Water slurry- Pilot Plant Scale Preparation by the Carbogel Process. Proc. Seventh Int. Symp. Coal Slurry Prep. Utilis., New Orleans, LA 1985, 21-24, 215–224. (4) Whaley, H.; Rankin, D. M.; Landry, P. G.; Covill, I. D. Utility Boiler Demonstration of Coal water mixture Combustion at Chatham. Proc. Sixth Int. Conf. Coal Slurry Combust. Technol., Orlando 1984, 519– 526. (5) Imai,T.;Tanaka, M. Sekitan Riyo Gijutsu Kaigi Koenshu 1993, 3, 183-94. (6) Manfred, R. K.; Utility Industry View Points, Plenary Discussions. In Seventh Int. symp. On Coal Slurry Fuels Prep. and Util.; U.S. Dept. of Energy, P.E.T.C.: New Orleans, LA, 1985. r 2010 American Chemical Society
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: DOI:10.1021/ef900921c
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Generally the additives used for stabilizing coal-water dispersion are molecules of high molecular weight, with both polar and nonpolar regions so that they can be adsorbed quickly on the surface of coal in significant amount through hydrophobic interaction, thereby orienting their nonpolar group towards the coal surface and the polar group towards water.19 Such an orientation of the additive at the coal-water interface lowers the interfacial tension between coal and water, simultaneously hindering the coal congregation by increasing steric or electrostatic repulsion at the coal surface or steric wettability of coal1 in accordance with DLVO theory.19 A thorough browse on the literature reveals that many synthetic additives have been developed so far, some of which are already in practice on the industrial scale. A few of them are polyethylene imine,20 poly(maleic acid) starch salt,21 calcium acetate,22 CMC,23 naphthalenesulfonic acid-formaldehyde condensate (NSF),24 etc. Very few biodegradable additives, such as polysachharides (gums),25-29 have also been well documented as effective dispersing agents for coalwater slurry. The analyses of the molecular architecture of these additives reveal that the basis for designing an effective additive for coal-water slurry is to introduce either bulky groups or polar head groups or both in its moiety.29 It has been our primary interest to develop suitable environmentally safe additives, such as natural plant-based and starch-based additives, for the preparation of high concentration coal-water slurry with minimal viscosity, nonsettling, and free-flowing properties using beneficiated coal of Talcher coal fields of Orissa (Eastern part of India). We have already reported the formulation of a highly concentrated slurry containing around 64-64.8% of coal with suspension stability as long as one month using a plant-based additive, saponin extracted from Sapindous laurifolia and Acacia Connicina, in our earlier papers.30,31 In the present work we have made an attempt to explore the possibility of three starch-based additives: starch xanthate, starch xanthide, and starch phosphate as additives for stabilizing coal-water slurry. These additives have a glucose ring with a large number of -OH groups present in the starch moiety. In addition, the disulfide linkage, thiol group, and phosphate group protruding out of the main starch skeleton are also known to affect their solubilities and viscosities.32 We have followed the well
Scheme 1
Molecular Structure 1: Starch Xanthate 1
Scheme 2
Molecular Structure 2: Starch Xanthide 2
Scheme 3
established techniques from literature33 to understand the rheology of the modified coal-water slurry as well as its stabilization using three varieties of noncoking Indian coal with variable ash contents. The effect of structure of the additives on the mechanism of stabilization has been put forwarded.
(19) Mishra, S. K.; Kanungo, S. B. J. Sci. Ind. Res. 2000, 59, 765–790. (20) Nippon Shokubai Kagaku Koggo Co. Ltd. Japan Patent 59, 221, 490,Dec. 14, 1983; Chem. Abstr., 1984, 101, 9899. (21) Nippon Oils & Fats Co. Ltd. Japan Patent 59 152, 995, Aug. 31, 1984; Chem Abstr, 1985, 102, 28207. (22) Liu, J.; Zhao, W.; Zhou, J.; Cheng, J.; Zhang, G.; Feng, Y.; Cen, K. Fuel Process.Technol. 2009, 90, 91–98. (23) Boylu, F.; Atesok, G.; Dincer, H. Fuel 2005, 315–319. (24) Wu, G.-G; Wang, X.-C.; Liu, J.-T. J. China Univ. Min. Technol. 2005, 34, 703–706. (25) Saeki, T.; Usui, H.; Ogawa, M. J. Chem. Eng. Jpn. 1994, 27, 773– 778. (26) Saeki, T.; Tatsukawa, T.; Usui, H. Coal Prep. 1999, 21, 161–176. (27) Usui, H.; Saeki, T.; Hayashi, K.; Tamura, T. Coal Prep. 1997, 18, 201–214. (28) Zaiden, H. Japanese Patent 58,96,693, SGKS Co. Ltd.: Osaka, Japan, June 8, 1983; Chem. Abstr. 1985, 103, 144674. (29) (a) Seiyaku, K. D.; Co. Ltd. Jpn. Japan Patent 59,179,592, Oct. 12, 1984; Chem Abstr. 1985, 102, 64809. (b) Zaidan, H. Japan Patent 58,96,693, June 8, 1983, (to SGKS Co Ltd Osaka Jpn); Chem Abstr, 1983, 99, 197885. (30) Das, D.; Panigrahi, S.; Misra, P. K.; Nayak, A. Energy Fuels 2008, 22, 1865–1872. (31) Das, D.; Panigrahi, S.; Misra, P. K.; Senapati, P. K. Energy Fuels 2009, 23, 3217–3226. (32) Russsel, C. R.; Buchanan, R. A.; Rist, C. E.; Hofreter, B. T.; Ernst, A. J. Tappi 1962, 45, 557–565.
2. Experimental Section 2.1. Materials. 2.1.1. Synthesis of Additives. Starch xanthate32 (molecular structure 1), starch xanthide32 (molecular structure 2), and starch phosphate monoester34,35 (molecular structure 3) were prepared32 by the methods reported in literature (as given in Schemes 1-3, the notations given in the schemes have their usual meaning). The yield is around 70-80% in each case. 2.1.2. Preparation of Concentrated Coal-Water Slurry. Coal samples used were collected from Talcher Coal Field, Orissa situated in the eastern part of India and beneficiated by the procedure described in our earlier papers.30,31 The particle size distribution of the coal samples was measured by a Malvern particle size analyzer and is shown in Figure 1, and the particle (33) Senapati, P. K.; Das, D.; Nayak, A.; Misra, P. K. Energy Sources 2008, 30, 1788–1796. (34) Neucom, H. U.S. Patent 2865762, 1958; Chem Abstr. 1959, 53, 5538. (35) Kerr, R. W.; Cleveland, J. U.S. Patent 2961440, 1960; Chem Abstr. 1962, 57, 1138.
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Das et al. Table 2. Proximate Analyses of the Coal Samples moisture (%) ash (%) volatile matter (%) fixed carbon (%) calorifilic value (kcal/kg)
coal A
coal B
coal C
13.15 8.02 33.44 45.39 6438
13.65 18.14 28.06 39.66 5745
10.02 39.84 29.75 30.39 4937
Viscosity Measurements. The apparent viscosity of the CWS was measured by a HAAKE rotational viscometer (Model RV 30), consisting of measuring drive unit, temperature vessel with circulator, sensor system, and a data logger. A sensor system MV I was chosen for the rheological measurements. The detailed procedure was described in our earlier communication.30,31 All experiments were conducted at room temperature of 30 °C. The pH of coal-water slurry with natural and commercial additives measured in a pH meter was in the range of 4.5-5.0. The rheological measurement was controlled by a software rotation version 3.0. The best-fit model was fitted to the shear stress-shear rate data to obtain the nature of the coal-water slurry. The various parameters such as shear stress, shear rate, true viscosity, apparent viscosity, and temperature along with the curve were displayed on the computer screen. The experimental error is within 0.2%. Zeta probe 24 V(52-60 Hz) T3A equipped with microprocessor was used for the measurement of the ζ potential of coal particles containing 5% weight fraction of coal in water. This unit automatically calculates the electrophoretic mobility of the particles and converts it into ζ potential. The ζ potential values are at least average of five different measurements. All measurements were made at the ambient temperature.
Figure 1. Particle size distribution of coal samples. Molecular Structure 3: Starch Phosphate Monoester 3
Table 1. Particle Size Parameters of Coal Samples particle size (μm)
3. Results and Discussion
coal type
d10
d50
d90
coal A coal B coal C
3.894 3.222 3.337
27.318 23.949 33.131
95.70 100.789 109.531
The carbon percentage in different categories of coal available in nature are in the order, anthracite > bituminous > lignite > peat. In India the best quality of coal mostly comes under the category of bituminous. All three varieties of coal used in present case are sub-bituminous coal. The maximum percentage in the coal slurry achieved using naked coal is about 58-60%, which had a durability lasting for only 3-4 h. 3.1. General Rheological Characteristics of the Modified Coal-Water Slurry. The rheological characteristic of coalwater slurry i.e. a ternary mixture of coal, water, and additive mainly depends on its viscosity values, which increase with increasing coal load. The viscosity is determined initially as a function of coal loading of the three varieties of coals, adding 1% of each of the additives. The apparent viscosity of coalwater slurry increases with the volume fraction of coal in the slurry in each case, and a maximum of 64.6% of coal in the slurry is achieved for starch xanthide, 63.8% for starch xanthate, and 64.1% for starch phosphate (Figure 2). Beyond these percentages the solution becomes unworkably thick as a result of strong interparticle interaction in the concentrated sample. Due to development of stiffness in the coal-water sample the apparent viscosity cannot be measured further. The durability of the suspension is dependent on the type of coal and additives used (Tables 4-6). The coal having higher ash content becomes more stiff due to strong interaction of the hydrophilic ash with water forming gels,7 hence the stability of the slurry decreases in the order coal A > coal B > coal C (Table 4-6). The change in the apparent viscosity with increase in percentage of additives at their highest coal loading is shown in Figures 3 and 4. The viscosity goes on decreasing with increase of the additive percentage in the slurry until a minimum value is attained in the presence 1% of the additive
d10, d50, and d90 are the diameters percentage points at 10, 50, and 90%, respectively. size parameters are given in Table 1. The proximate and ultimate analyses data of the samples done on an air-dried basis are given in Tables 2 and 3, respectively. The three coal samples designated as coals A, B, and C with ash content 8.02% (low ash coal), 18.14% (medium ash coal), and 39.06% (high ash coal), respectively, were employed for the preparation of slurry in distilled water. The coal was dried in a hot air oven for 24 h to reduce the moisture percentage to 2-3% by weight and was sealed to avoid contamination with moisture. The coal-water slurry was prepared31 by agitating the coalwater mixture containing the additive in a helical ribbon mixer at 50-150 rpm (to avoid particle disintegration) by using a variable frequency drive. The suspension has fair stability during the measurement for at least 48 h. Coal-water slurries (CWS) were prepared at weight concentrations of 58- 65% and were then poured into glass cylinders of about 100 mL. The top of the cylinders was sealed, and the cylinders were stored at room temperature. The static stability of coal-water slurry was evaluated by applying a rod penetration method,27,31,36,37 that is, a glass rod of fixed weight and diameter was put into the slurry to observe whether soft sedimentation appeared during storage. The soft sedimentation was identified as it can be easily disintegrated by mild stirring and transferred to other containers without any difficulty. The appearance of soft sediment by days was used as an indicator of static stability. The data are given in Tables 4-6. (36) Yuchi, W.; Li, B.; Li, W.; Chen, H. Coal Prep. 2005, 25, 239–249. (37) Tayssir, H. Adv. powder Technol. 1994, 5, 143–160.
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Table 3. Ultimate Analyses of the Coal Samples carbon (%) hydrogen (%) sulfur (%) nitrogen (%) oxygen (%) carbon/oxygen ratio
coal A (low ash)
coal B (medium ash)
coal C (high ash)
78.81 5.91 3.70 1.89 10.06 7.834
78.70 5.83 2.86 1.85 10.62 7.411
77.21 5.63 3.62 1.94 11.27 6.856
Table 4. Stability of CWS with Days at Different Weight Concentrations in Presence of 1.0% of Starch Xanthide No. of days wt. concentration, %
coal A
coal B
coal C
58.0 60.0 62.0 64.6
15 20 22 25
13 16 18 25
10 12 13 18
Table 5. Stability of CWS with Days at Different Weight Concentrations in Presence of 1.0% of Starch Xanthate No. of days wt. concentration, %
coal A
coal B
coal C
58.0 60.0 62.0 63.8
9 11 15 17
11 10 15 14
4 7 11 13
Figure 3. Variation of the apparent viscosity of coal A with variation of additive at their highest weight fraction (64.6% for sodium xanthide, 64.1% for starch phosphate, and 63.8% for sodium xanthate) at a shear rate of 77 s-1.
Table 6. Stability of CWS with days at different weight concentrations in presence of 1.0% of starch phosphate No. of days wt. concentration, %
coal A
coal B
coal C
58.0 60.0 62.0 64.1
10 14 16 21
12 10 15 18
7 11 13 16
Figure 4. Variation of the apparent viscosity of coals A, B, and C with variation of starch phosphate at 64.1% of coal load at a shear rate of 77 s-1.
molecules are put into solution depending on the surrounding solvent they usually fold so as to expose the groups which would interact with the surrounding and sequester those groups away from the surrounding which have no/minimum interactions with that of the later. This type of arrangement usually encourages a coiled structure. In fact, we have seen this type of selective orientation of the adsorbate molecules during the adsorption of a series of polyoxyethylated nonyl phenols on silica from water38 and from cyclohexane39 medium. In an aqueous environment, therefore, starch would fold such that oxygen-containing functional groups will be projected toward water environment, leaving all -CH2 to interact with the nonpolar site of the coal. Adding to this fact,
Figure 2. Variation of the apparent viscosity of percentage of coal A in presence of 1% of various additives at a shear rate of 77 s-1.
beyond which no further decrease of apparent viscosity occurs. The viscosity arises largely due to the strong van der Waal forces operating among the coal particles. The adsorption of the additives significantly modifies the coal surface. The starch skeleton contains a large number of oxygen atoms as well as -CH2 groups. When such types of
(38) Misra, P. K.; Mishra, B. K.; Somasundaran, P. J. Colloids Interface Sci. 2003, 265, 1. (39) Misra, P. K.; Dash, U.; Somasundaran, P. Ind. Eng. Chem. Res. 2009, 48, 3403–3409.
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Figure 5. Coiled structure of starch.
Figure 7. Variation of the apparent viscosity of coals A, B, and C with variation of shear rate in presence of 1% of starch xanthate at 63.8% of coal load at a shear rate of 77 s-1.
with saponins as the additives,30,31 an excess of 0.2% is required to get the minimum apparent viscosity with the present additives probably due to the absence of a completely hydrophobic region in the nonpolar moiety compared to the nonpolar part of hederazin of saponins. Because of the oxygen atoms present in the starch moiety, the hydrophobicity of the molecule decreases and solubility in water increases, leading to relatively less affinity of the additive for the coal surface. The minimum apparent viscosity is obtained at 1% of additive in each case regardless of their charge on the polar part but the extent of reduction of the apparent viscosity is dependent on the type of additive used. At 1% of additive, the magnitude of the apparent viscosity is: starch xanthide < starch phosphate < starch xanthate. Thus, the protruded head groups of the additives have an important role in reduction of apparent viscosity. The effectiveness and economy of the slurry as a source of energy mainly depend on viscosity of the concentrated coal-water slurry, which should be high during storage to maintain good stability against sedimentation and would be low during transportation so that slurry flows easily. In other words, the slurry would have high viscosity at low shearing rate and low viscosity at high shearing rate. To understand its viscosity as a function of shearing rate, the apparent viscosity is determined by varying shearing rate. Some representative plots are shown in Figures 6-8. A shear thinning behavior is noticed in all cases with increase in shearing rate, similar to earlier observations.30,31 The shear stress is measured with variation of shear rate of coal-water slurry in the presence of 1% of additives, and the relation is shown in Figure 9. In all cases a linear shear-stress, shear-rate relationship with an initial shear-stress threshold is found in line with Bingham plastics fluids12 obeying equation 1, therefore the slurry has non-Newtonian behavior.12,30 τ ¼ τ0 þ ηγ::: ð1Þ
Figure 6. Variation of the apparent viscosity of coals A, B, and C with variation of shear rate in presence of 1% of starch xanthide at 64.6% coal load at a shear rate of 77 s-1.
reports are also available which strongly suggest that starch is present as a coiled structure, giving the appearance as shown in Figure 5.40 On a hydrophobic surface such as coal, the adsorption of the starch additives from aqueous solution may take place through the interaction of the hydrophobic portion of the starch molecules with the coal surfaces orienting hydrophilic -OH groups and protruded heads (xanthide, xanthate, and phosphate groups) towards bulk water. From the comparative study of the adsorption of dextrin (polysaccharide) on oxidized coal and hydrophobic coal, Miller et al.41 have also concluded that dextrin adsorbs on the coal surface through hydrophobic interaction. The adsorption was found to be substantially lower in oxidized coal than the hydrophobic coal, confirming the interaction of coal with compounds containing glucose units to be mainly hydrophobic. Such an orientation of the starch-based additives under investigation would also give a thermodynamically favorable interaction of the hydrophilic portion of the adsorbed starch additives with water through H-bonding/ionic interactions, thereby sequestering the hydrophobic part of the molecule away from the water surface. The pronounced drop in viscosities upon the addition of starchbased compounds may therefore, be due to the formation of hydrophilic cushions around the coal particles by the large number of -OH groups and the protruded polar chains that increase the interaction of coal with water and also offer hindrance for the association coal particles among themselves to a large extent. In contrast to our earlier observation
Where τ and γ denote shear stress and applied shear rate, respectively. τ0 is the yield stress and η is defined as the coefficient of rigidity. All individual data on the average represent a linear relationship (with regression coefficient (R2) around 1, except starch xanthate which has R2 around 0.9) and the values are within the experimental error. They compare very well with a commercial additive such as sodiumdodecyl sulfate. The yield stress (y-intercept) signifies
(40) Virtual Chembook; Ophardt, C. E. Ed.; Elmhurst College: 2003. (41) Miller, J. D.; Laskowski, J. S.; Chang, S. S. Colloid Surf. A 1983, 8, 137–151.
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Das et al. Table 7. Apparent Viscosity, Yield Stress for Coals A and B at a Shear Rate of 77 s-1 for Different CWS Weight Concentrations in the Presence of 1% Starch Xanthide apparent viscosity (PaS) wt. concentration, coal A Cw , % 55 60 62 64
0.423 0.502 0.609 0.719
yield stress (Pa)
coal B
coal C coal A coal B coal C
0.487 0.598 0.699 0.810
0.577 0.643 0.710 0.899
36 45 59 65
42 49 65 70
44 51 65 74
Table 8. Apparent Viscosity, Yield Stress for Coals A and B at A Shear Rate of 77 s-1 for Different CWS Weight Concentrations in the Presence of 1% Starch Xanthate apparent viscosity (PaS) wt. concentration, coal A Cw , %
Figure 8. Variation of the apparent viscosity of coals A, B, and C with variation of shear rate in presence of 1% of starch phosphate at 64.1% of coal load at a shear rate of 77 s-1.
55 60 62 64
0.648 0.759 0.894 0.999
yield stress (Pa)
coal B
coal C coal A coal B coal C
0.754 0.896 0.915 1.101
0.846 0.898 0.994 1.210
38 49 61 68
40 48 62 76
43 54 67 75
Table 9. Apparent Viscosity, Yield Stress for Coal A and B at Shear Rate of 77 s-1 for Different CWS Weight Concentrations in the Presence of 1% Starch Phosphate apparent viscosity (PaS) wt. concentration, coal A Cw , % 55 60 62 64
0.489 0.526 0.643 0.799
yield stress (Pa)
coal B
coal C coal A coal B coal C
0.510 0.599 0.715 0.847
0.710 0.843 0.994 1.102
34 42 59 63
39 48 59 69
42 55 64 75
Figure 9. Variation of the shear stress of coal A with variation of shear rate in presence of 1% of the additives at their highest coal load (64.6% for sodium xanthide, 64.1% for starch phosphate, and 63.8% for sodium xanthate) at a shear rate of 77 s-1. The dotted line represents a model plot of a Newtonian Fluid.
the initial stress to be overcome to make the slurry flow. For a better additive this value should be as low as possible (for water, as shown in Figure 9, this yield stress value is zero). Since xanthhate has maximum yield stress with less R2 value, it is the least effective additive among the three starch-based additives under consideration. At the beginning of the plot, shear stress values are more disordered due to less effect of low shearing rate. The yield stress and apparent viscosity values measured at shear rate of 77 S1- in the wt. concentration range of 55-64% for all coal samples are given in Tables 7-9. The shear stress is minimum in the presence of starch xanthide and is maximum in xanthate-modified coal; starch phosphate has an intermediate value. The effect of temperature on the rheology of coal-water slurry is investigated at four different temperatures. The apparent viscosity is found to decrease with increase in temperature (Figure 10) and is found to fit the Arrehenius equation well (Figure 11). The increase in thermal energy42
Figure 10. Plot of apparent viscosity versus temperature in the presence of 1% of starch xanthate, starch xanthide, starch phosphate, and SDS at 60% coal load of coal A at a shear rate of 77 s-1.
with increase of temperature reduces interparticle attraction which in turn reduces apparent viscosity of the slurry. The pH of the slurry may change during the transport through a pipeline because of the oxidation of the coal sample, thereby affecting the viscosity of the slurry, which in turn may affect the stability of the slurry as well as the effectiveness of the coal sample as fuel. The effect of change in pH of the coal-water slurry on its viscosity has been investigated and is presented in Figure 12. Apparent viscosity is found to decrease with increase in pH due to ionization31 of the -OH group of the additives and the polar
(42) Mishra, S. K.; Senapati, P. K.; Panda, D. Energy Sources 2002, 24, 159–167.
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Figure 11. Arrhenius plot for a coal-water slurry containing 1% of starch xanthide and 60% coal load of coals A, B, and C at a shear rate of 77 s-1.
Figure 13. Variation of ζ potential vs pH for coal A-water slurry in the absence and the presence of 1% of additives at a shear rate of 77 s-1.
presence or absence of additives. For the naked coal at low pH, the surface potential is weakly positive, possibly due to the presence of protonated polar species such as -COOH2þ and -OH2þ. With increase of pH the polar groups ionize, leading to the increase of negative zeta potential, which gradually levels up at higher pH. Because the surface potential is weakly positive at natural pH the agglomeration of coal particles is mostly due to the hydrophobic interaction among the coal particles, therefore the additives preferably get adsorbed through hydrophobic interaction. Even though the trend of ζ potential values with change in pH is similar for both naked and modified coal, the magnitude of the ζ potential is substantially decreased for modified coal compared to that of the naked coals in all cases, and they almost overlap with each other irrespective of the nature of the additives used in the present investigation. The driving force for the adsorption of molecules on an adsorbent surface may be through interactions such as H-bonding,44,45 hydrophobic bonding,46-48 ion pairing,44,45 ion-exchange,44,45 polarization of π- electrons,39,45 or dispersions forces49,50 with the solid substrate. Both the additive structure and the nature of the coal surface determine which of the interactions would be predominating. The basic structure in all the additives is a glucose ring with an exposed -OH group. This overlapping sigmoidal plot of ζ potential with variation of pH suggests that (i) the starch arrangement is independent of the exposed chains and (ii) the adsorption takes place through the starch end of the additives. These additives may therefore adsorb through hydrophobic interaction between the starch unit with hydrophobic regions of the coal surface. Consequently, large number of water molecules hydrating the coal surface release to the surrounding, resulting in the increase of the entropy and thus making the process of adsorption a favorable one.51 The decrease in free energy resulting from the
Figure 12. Variation of apparent viscosity vs pH for coal-water slurry of coal A alone and in the presence of 1% of the additives at 60% coal load and shear rate of 77 s-1.
groups such as -COOH and -OH on coal surface introducing electrostatic repulsion for particle association and hence hindering agglomeration of coal. 3.2. Effect of Additive Structures on the Mechanism of Slurry Stabilization. The selective dispersion of the particles in a solvent is difficult due to their strong interaction among themselves rather than with the surrounding medium, leading to the flocculation and settling of the particles.43 The basic requirement for stabilization of coal-water slurry is, therefore, to develop the regions in coal that would either promote coal-water interaction, inhibit coal-coal interaction, or both. Surface charge of naked coal, surface chemical alterations resulting from the adsorption of additives, and the orientation/organization of such species at the coalwater interface usually decide the nature of the coal surface. Coal particles have both hydrophobic and hydrophilic regions due to carbon and some polar functional groups such as -OH and -COOH groups. In order to know the surface charge of the coal in the presence as well as the absence of the additives, the ζ potential (a physical parameter reflecting the surface charge on coal surface) is determined as a function of pH. An inverted sigmoid plot as shown in Figure 13 is obtained in all cases irrespective of the types of coal and
(44) Rupprecht, H.; Liebl, H. Kolloid Z. Z. Polym. 1972, 250, 719. (45) Snyder, L. R. J. Phys. Chem. 1968, 72, 489–494. (46) Wakamatsu, T.; Fuersteanau, D. W. In Adsorption from Aqueous Solution; Weber, W. J., Jr, Matijevic, E., Eds.; American Chemical Society: Washington, D.C., 1968; Vol. 16, pp 1-172. (47) Giles, C. H.; D’Silva, A. P.; Easton, I. A. J. Colloid Interface Sci. 1974, 47, 766–778. (48) Dick, S. G.; Fuersteanau, D. W.; Healy, T. W. J. Colloid Interface Sci. 1971, 37, 595–602. (49) Kolbel, H.; Kuhn, P. Angew. Chem. 1959, 71, 211–215. (50) Kolbel, H.; Horig, K. Angew. Chem. 1959, 71, 691–697. (51) Rill, C.; Kolar, I. Z.; Kickelbica, G.; Wolterbeek, Th. H.; Petters, A. J. Langmuir 2009, 25, 2294–2301.
(43) Rosen, M. J.; Surfactants and Interfacial Phenomena; Wiley & Sons: New York, 1978; p 263.
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Table 10. Isoelectric Point of Coal in Presence and Absence of Additives system coal A alone coal A þ starch additives coal B alone coal B þ starch additives coal C alone coal C þ starch additives
isoelectric point 5.4 6.4 6.1 6.7 6.5 7.0
hydration of the ionic hydrophilic groups may also compensate for the free energy increase due to the increased contact of the hydrophobic groups with the aqueous phase. The masking of the surface charge of coal in the presence of additive may be attributed to two factors: (i) coverage of some fraction of the surface by the additives or (ii) shifting of the shear plane by the adsorbed additives. Since the interaction is mostly hydrophobic, a significant fraction of the hydrophilic regions of the coal surface are exposed to the bulk surface. The reduction of zeta potential may therefore, be due to the mechanical displacement of the shear plane being projected some distance beyond the original shear plane by the large molecular weight compound under investigation. We have also observed such decrease in ζ potential of solid surface due to the adsorption of some natural surfactants30,31 on coal surface and some nonionic polyoxyethylated surfactants on silica surface.52 The isoelectric point (point of zero charge) determined from the plots are given in Table 10. The shifting of the isoelectric point to higher pH in the presence of additive may be due to the requirement of higher concentration of OH- ions to ionize the polar groups present in the adsorbed molecule in addition to the polar groups on the coal surface.30 Interestingly, the isoelectric points (IEP) in the presence of these additives are found to be same in the presence of all additives. Since all the additives have the same common adsorbing unit, the pH requirements for the complete ionization is same, resulting in the same IEP. The yield stress is found to be maximized at the IEP due to the absence of charges on the coal particles, hence maximum shear rate may be required to pump the slurry at this pH. The yield stress increases in the order: starch xanthide < starch phosphate < starch xanthate. As per DLVO theory, to disperse the particles the repulsive interactions among the particles must overcome their attractive interactions. Repulsive interactions that keep the particles away from each other are believed to be due to either to the similarly charged electrical double layers, bulky groups surrounding the particles (steric and electrostatic repulsions), or particle solvent interactions (steric wettability).1 Attractive interactions are believed to be mainly due to the van der Waals forces or hydrophobic interaction between particles.1 The stabilization of coalslurry is investigated in the presence of 1% of starch alone, but the stabilization of the slurry is not sufficiently improved. The fact that the surface charge is reduced and the adsorption of starch alone does not lead to stable suspensions, the slurry stabilization is mostly due to the protruded head groups. The starch molecule may increase the wettability of coal through the H-bonding between the exposed -OH group with water. Increasing the wettability of coal is a necessity, but is not a sufficient condition for good dispersal in coal-water slurries.1 The diminution of ζ potential in the presence of additive suggests that the stabilization of the slurry is due to the steric repulsions offered by the adsorbed
Figure 14. Schematic representation of the stabilization of coalwater slurry by starch xanthate and starch phosphate additive.
Figure 15. 3D model showing starch xanthide having two glucose units in each starch unit.
molecules through their protruded chains. In addition to the steric repulsion offered by the molecule as a whole, the exposed phosphate ion of starch phosphate on coal surface may introduce more steric repulsion in compared to the thionate ion of the starch xanthate during coal agglomeration due to the large size of the phosphate ion (Figure 14). Hence higher coal load is achieved in the presence of starch phosphate. Starch xanthate has a rigid disulphide linkage that maintains a gap between the two starch units in the molecule. A representative diagram with two glucose molecules in each starch unit is shown in Figure 15. These two starch units may adsorb to a single coal particle (Figure 16A) or two coal particles (Figure 16B) simultaneously. The attachment of starch xanthide to two particles would promote a larger increase in coal loading. The starch xanthide molecule develops of a huge cross-network structure formed by the bridging of the coal particle43 through the noncovalent interaction of the coal particles and the hydrophilic end of the starch particle, similar to the role of the dissolved minerals matters with high valent cations in improving the rheolgical behavior.53 Because adsorption is a dynamic process,54 this network structure dissolves and regenerates continuously, rendering a stability similar to the flickering clusters of ice.55 This organization of the disulfide linkage at the surface inhibits close approach of the coal particles further, in addition to the steric repulsion offered by the molecules as (53) Xei, X. Y.; Li, Q. B.; Sun, G. C. Effect of Mineral Matters on properties of coal water slurry, In Coal Science; Pajares, J.A., Tascon, J.M.D. Eds.; Elsevier Science, B.V.: Netherlands, 1995; pp 1593-1596. (54) Phan, C. M.; Nguyen, A. V.; Evans, G. M. Mineral Eng. 2005, 18, 599–603. (55) Fendler, J. H.; Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, San Francisco, London, 1975; p 35.
(52) Misra, P. K.; Somasundaran, P. J. Surfactant Det. 2004, 7, 373– 378.
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Figure 16. Schematic representation of the adsorption of starch xanthide onto (A) a single coal particle and (B) two coal particles.
exhibited in the cases of starch xanthate and starch phosphate. The adsorption density of less polar starch xanthide with neutral disulfide linkage would also be more at the coal water interface due to its lower solubility in water compared to the other two additives, which have ionic groups.
environments, as they increase SOx emissions during combustion and create environmental pollution. On the contrary, since the present additives contain biocompatible starch units, their use as a dispersant is prospective and economically viable as well as eco-friendly. But, due to the absence of a precise hydrophobic region in these additives, the economy and the efficiency are always lower in comparison to natural and as well as commercial additives.30,31,53
Conclusion The trade-off between low viscosity and the considerable stability is important for an additive to be a good dispersant. The additive must be a good dispersant as well as a good wetting agent. The present study reveals that high concentration coal-water slurry with low viscosity can be prepared by using starch-based additives. The stabilization is due to (i) the steric repulsions offered by the molecule to prevent interparticle interactions, (ii) increase of wettability of coal due to the large number of -OH group surrounding modified coal, and (iii) inhibition of close approach of the particles by the network structure formed by the rigid disulfide linkage. The use of synthetic additives is, however, hazardous; moreover, the sulfur-containing chemical additives are harmful to the
Acknowledgment. The authors express their sincere thanks to Institutes of Minerals and Materials Technology, Bhubaneswar for providing laboratory facilities. U.D. thanks the Department of Science and Technology for financial support for the award of the junior research fellowship (Project sanction letter No. SR/S1/ PC-39/2004, dated 14/03/2006). University Grants Commission and Department of Science and Technology, Government of India are also gratefully acknowledged for financial support to the Department through sanction of DRS and FIST, respectively. The authors also thank Professor R.K. Behera, Sambalpur University for useful discussion and anonymous reviewers for the useful queries and suggestions
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