Energy Fuels 2011, 25, 747–752 Published on Web 01/11/2011
: DOI:10.1021/ef101409h
The Slurrying Properties of Coal Water Slurries Containing Raw Sewage Sludge Ruikun Wang, Jianzhong Liu,* Yujie Yu, Yaxuan Hu, Junhu Zhou, and Kefa Cen State Key Lab of Clean Energy Utilization, Zhejiang University, Hangzhou, 310027, China Received October 15, 2010. Revised Manuscript Received December 30, 2010
Municipal sewage sludge always includes considerable amount of water, noxious substances, and varying chemical compositions; therefore, sludge utilization is imposed. Coal-sludge slurry (CSS), which is made by blending sewage sludge with coal, water, and additives, is being identified as a new and effective sludge utilization technology. The main objectives of the present investigation are research on the slurrying, rheology, and stability properties of five CSSs, and determine the influence of sludge type, sludge mixing proportion (R), temperature, and shear rate on the slurrying properties that play a critical role in CSS technology. Results showed that (i) the maximum solids loading (MSL) dropped sharply with the rise of R, ranging from 55.22% to 59.41% in the R = 15% case, compared to 69.16% in the R = 0 case; and (ii) the apparent viscosity dropped as the temperature rose from 20 °C to 50 °C, in an approximate linear correlation. All CSSs exhibited a pseudo-plastic rheology, and increasing ω and R resulted in a trend more toward pseudo-plastic behavior; moreover, the poorer the slurryability of the sludge, the more apparent the pseudoplastic trend. The static stability was getting better for CSS, compared to coal water slurry (CWS).
structure, and even human health can be affected badly.1,7 The land area that is being filled by the sludge has been getting larger; moreover, the specific substances in sludge should meet explicit criteria.8 Sewage sludge is characterized by the high moisture content, which imposes the adoption of pretreatment methods prior to thermal processes. Moisture content can also be identified as a major factor in incineration cost estimation.4 Since municipal sewage sludge cannot be disposed by the existing technology satisfactorily, alternative technology should be proposed to dispose and utilize the sewage sludge. Coal water slurry (CWS) has been applied by various boilers and kilns as a type of clean coal-based liquid substitute fuel to oil and a potential gasification fuel in China.9 Sewage sludge was reported as a possible carbonaceous solid fuel to be blended into CWS,10,11 and some preliminary research about the combustion and rheological characteristics of the coalsludge slurry (CSS) has already been done.12,13 Disposal of large amount of sewage sludge and reclamation of calorific value can be achieved by burning CSS. Liu et al.11 have worked on the co-gasification of sewage sludge and coal and proclaimed that ignition and burnout behavior of the coal could be greatly improved by co-combustion of coal and sewage sludge. In addition, co-gasification can reach higher temperature, compared to monogasification of sewage sludge; thus,
1. Introduction Municipal sewage sludge, the residue of wastewater treatment process, tends to increase rapidly with the rapid industrialization and urbanization all over the world. The annual dry sewage sludge discharge reached 10 million tons1 in the European Union (EU) in 2007 and is estimated to reach 8.2 million tons in the United States in 2010,2 while the annual raw wet sewage sludge discharge is estimated to reach 63.88 million tons3 in China by the end of 2010. Sewage sludge usually concentrates not only high contents of moisture, but also high contents of volatile matters, heavy metals, poorly biodegradable trace organic compounds, and potentially pathogenic organisms,4 which are hazardous to the environment and may cause secondary environmental pollution if disposed improperly. At present, various options are available for sludge handling, such as land application, landfill, sludge incineration, etc.5 However, there usually are several drawbacks in foregoing options. Gradually, various toxins, especially heavy metals such as Hg, Cu, Cr in the sewage sludge will be released into soil and groundwater,6 so that soil organisms or plants, the proper soil *To whom correspondence should be addressed. Tel.: þ86 571 87952884. Fax: þ86 571 87951616. E-mail address:
[email protected]. (1) Carbonell, G.; Pro, J.; G omez, N.; et al. Sewage sludge applied to agricultural soil: Ecotoxicological effects on representative soil organisms. Ecotoxicol. Environ. Saf. 2009, 72, 1309–1319. (2) Jiang, J.; Zhao, Q.; Zhang, J.; et al. Electricity generation from bio-treatment of sewage sludge with microbial fuel cell. Bioresour. Technol. 2009, 100, 5808–5812. (3) http://www2.h2o-china.com/ report/2009/2009wunireport/ (4) Werther, J.; Ogada, T. Sewage sludge combustion. Prog. Energy Combust. Sci. 1999, 25, 55–116. (5) Fytili, D.; Zabaniotou, A. Utilization of sewage sludge in EU application of old and new methods;A review. Renew. Sustainable Energy Rev. 2008, 12, 116–140. (6) Emmerich, W. E.; Lund, L. J.; Page, A. L.; et al. Movement of Heavy Metals in Sewage Sludge-Treated Soils. J. Environ. Quality 1982, 11, 174–178. (7) Wang, X.; Chen, T.; Ge, Y.-h.; et al. Studies on land application of sewage sludge and its limiting factors. J. Hazard. Mater. 2008, 160, 554– 558. r 2011 American Chemical Society
(8) O’Kelly, C. B. Sewage sludge to landfill: Some pertinent engineering properties. J. Air Waste Manage. Assoc. 2005, 55, 765–771. (9) Cen, K.-f.; Yao, Q.; Cao, X.-y.; et al. Theory and Application of Combustion, Flow, Heat Transfer, Gasification of Coal Slurry (in Chin.); Zhejiang University Press: Hangzhou, PRC, 1997; pp 15-69. (10) Rodriguez, L. A.; Padilla, A. A.; Ashworth, R. A.; et al. Alternative fuel comprised of sewage sludge and a particulate solid fuel. U.S. Patent 4,405,332, July 28, 1981. (11) Liu, H.-f.; Li W.-f.; Zhou J.-w. A high solid concentration of coalwater-sewage slurry and its prepatation (in Chin.). Chin. Patent 200610030071.1. February 7, 2007. (12) Hu, Q.-h.; Xiong, Y.-l.; Mao, K.-h. Combustion characteristics of sewage sludge, coal water slurry and sludge-CWS mixture by thermo gravimetric analysis (In Chin.). Environ. Pollut. Control. 2008, 30, 60-63. (13) Li, W.; Li, W.; Liu, H. Effects of sewage sludge on rheological characteristics of coal-water slurry. Fuel 2010, 89, 2505–2510.
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Table 1. Proximate Analysis and Ultimate Analysis of Test Coal and Sewage Sludgea Proximate Analysis (%)
Ultimate Analysis (%)
sample
Mar
Ad
Vd
FCd
Cd
Hd
Nd
Sd
Od
Qd (MJ/kg)
coal LJ sludge QJ sludge NS sludge PH sludge BH sludge
2.85 77.80 73.71 73.14 78.11 74.04
13.19 41.83 51.48 66.15 46.17 31.14
29.26 49.65 43.96 30.06 47.31 61.66
57.55 8.52 4.55 3.80 6.53 7.20
70.98 31.98 28.75 16.87 28.84 40.83
3.78 4.82 4.43 3.31 5.32 5.88
1.32 2.93 3.36 1.98 3.74 4.25
0.76 5.32 0.84 0.47 0.96 1.50
9.97 16.34 11.14 11.24 15.00 16.42
28.35 12.88 10.93 6.11 11.47 17.26
a Mar, as-receiced moisture; Ad, Vd, and FCd refer to ash, volatile and fixed carbon, presented on a dry basis; ultimate analysis is also on a dry basis. Qd represents the calorific value on a dry basis.
influence factors on the slurrying properties still remains to be examined. To develop a theoretical framework for the new alternative way of sewage sludge utilization, a good understanding of the slurrying properties are required. In this work, the slurrying, rheology, and stability properties of five types of CSSs were investigated. In addition to that, the influence of sludge type, sludge mixing proportion (0%, 5%, 10%, and 15%), temperature, and shear rate on the slurrying properties were also studied.
the noxious substances;dioxins, furans, polycyclic aromatic hydrocarbons (PAHs), heavy metal pollution;can be effectively restrained in the combustion process of CSS.14,15 Simultaneously, the coal fusion temperature can be reduced by heavy metals in sewage sludge during the combustion; therefore, the lifespan of refractory material in the gasification plant can be prolonged,16 and then gasification efficiency and operating economics will be improved. Additionally, predried sewage sludge is not required by the CSS technology, because a certain quantity of water is needed for preparing the slurry; therefore, the high moisture contents in sewage sludge can help to save water. Preparation, storage, transportation, atomization, and combustion of CSS/CWS are fundamentally affected by the slurrying properties. The slurrying properties of CWS and the effectiveness of various influencing factors have thoroughly been studied by many previous researchers. Mishra et al.17 have studied the rheological behavior of Indian CWS and investigated the effect of solid concentration, ash content, pH, and temperature on the rheology. Wei et al.18 have focused on the effects of coal characteristics such as coal rank, air equilibrium moisture, maximum moisture holding capacity, surface properties, petrographic macerals, pore structure, and adsorption characteristics of dispersant, on the properties of CWS. The effects of particle size and size distribution on the rheological behavior of coal water slurries that were made by two low-rank South Australian coals were studied by Nguyen et al.19 However, sewage sludge addition to CWS made the rheological characteristics different than that of CWS, despite the fact that the sludge mixing proportion was not very high, as verified by Li et al.13 In addition, the influence factors of slurrying properties of CSS and CWS were also different. Nevertheless, the effectiveness of influencing factor was not researched, and the sludge mixing proportion of CSS researched was only 10% in the literature.13 Few detailed studies on the slurrying properties of CSS and its influencing factors have been reported so far. Hence, the impact of various
2. Experimental Section 2.1. Materials. Purified coal that was mined in Yanzhou, Shandong Province, China and five types of sewage sludge supplied by five wastewater treatment plants;Linjiang (LJ), Qianjiang (QJ), Nanshang (NS), Pinghu (PH), and Binhe (BH);were chosen for the study. The Linjiang and Qianjiang wastewater treatment plants are located in Hangzhou, China, whereas other three plants are located in Shenzhen, China. Among the above-mentioned types of sludge, LJ sludge can be identified as industrial sewage sludge and all others are domestic sewage sludge. Proximate and ultimate analyses of coal and sewage sludge samples can be found in Table 1. As shown in Table 1, the proximate and ultimate analysis results have little differences among LJ, QJ, and PH sludge. NS, PH, and BH sludge have considerable differences in the proximate and ultimate analyses results, although they are in same category and from the same city. The calorific value of sewage sludge is ∼25-60% of the calorific value of coal. However, other characteristics;high moisture, nitrogen, and ash contents, low fixed carbon, and variations of chemical compositions;impose great difficulties for sludge utilization. Nevertheless, those difficulties could be overcome by CSS technology that can dispose sewage sludge by making a homogeneous mixture of sewage sludge, coal powder, water, and additives. Additive is an important composition of slurry fuel; previous work20 has compared the dispersing effect on CSSs of three additives and discovered that a co-polymer of methylene naphthalene, sulfonate styrene, and sulfonate maleate (NDF additive) had greater effectiveness. In this study, the NDF additive was adopted at a constant dosage of 0.8% on dry coal basis. 2.2. Methods. 2.2.1. Coal-Sludge Slurry Preparation. The CSSs studied in this investigation were prepared as follows. First, the required masses of coal powder, raw sewage sludge, deionized water, and additive were calculated and weighed in predetermined ratios. The weighed water then was poured into a 1000-mL stainless steel beaker, and the additive was dissolved in the water. Next, coal powder and wet sewage sludge were slowly poured into the beaker with a mechanical mixer operating at a fixed speed of 1000 rpm for 20 min after the addition of all the
(14) Dewling, R. T.; Manganelli, R. M.; Baer, G. T. Fate and Behavior of Selected Heavy Metals in Incinerated Sludge. J.-Water Pollut. Control Fed. 1980, 52, 2552–2557. (15) Deng, W.-y.; Yan, J.-h.; Li, X.-d.; et al. Emission characteristics of dioxins, furans and polycyclic aromatic hydrocarbons during fluidized-bed combustion of sewage sludge. J. Environ. Sci. 2009, 21, 1747– 1752. (16) Folgueras, M. B.; Diaz, M. R.; Xiberta, J.; et al. Influence of sewage sludge addition on coal ash fusion temperatures. Energy Fuels 2005, 19, 2562–2570. (17) Mishra, S. K; Senapati, P. K.; Panda, D. Rheological behavior of coal-water slurry. Energy Sources 2002, 24, 159–167. (18) Wei, Y.; Li, B.; Li, W.; et al. Effects of coal characteristics on the properties of coal water slurry. Coal Preparation 2005, 25, 239–249. (19) Nguyen, Q. D.; Logos, C.; Semmler, T. Rheological properties of south Australia coal-water slurries. Coal Preparation 1997, 18, 185–199.
(20) Wang, R.-k.; Liu, J.-z.; Hu, Y.-x.; et al. Influence of wet sludge on the slurrying properties of Coal-Water Slurries (in Chin.). J. China Coal Soc. 2010, 35, 199-204.
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sludge was added, the required concentration (60.1-64.0%) was maintained, according to Chinese National Standard GB/T18856.4-2002.21 Research has indicated that the slurryability properties are highly influenced by physical and chemical properties of materials. Maximization of the solid concentration and minimization of the slurry viscosity can be reached using high concentrations of Cd and Ad, and low concentrations of Mad and Od.22,23 It was observed that LJ sludge had higher slurryability than QJ and PH sludge, although they had similar proximate and ultimate analysis results. Higher contents of Cd, according to Table 1, can be the reason for the better slurryability of LJ sludge. Simultaneously, it was obviously better for NS sludge than for PH and BH sludge, because of the higher Ad and lower Mad and Od contents in NS sludge. In addition, BH sludge had better slurryability than PH sludge, because of higher Cd contents in the BH sludge. 3.1.3. Relationship between Apparent Viscosity and Solids Content of CSSs. Figure 1 showed the relationship between the apparent viscosity (η) and the solids content (ω) of CSSs with various added dosages of sludge. It was revealed that the apparent viscosity of all type of slurries, which have been considered in the study, increased as the solids content increased. The amount of the solid particles increased with ω, and then the interparticle interaction effects became stronger, while the proportions of free water acting as a lubricant decreased; as a result, η increased. Referring to Figure 1, as ω increased, η increased more steeply for high-R CSSs, compared to low-R CSSs, because of the following facts. Sludge particles are abundant in cellular tissues and flocs, which are cemented and coagulated together by suspended solids of the wastewater. Flocs have a loose reticular structure, extremely high specific surface area and porosity, and fractal characteristics, which strengthen the water sorption property.24 Water penetrates into the cells and becomes dead water, and the free water content decreases. Cellular tissues and flocs swell after they absorb water, and flosslike branches interconnect with each other to form a robust skeletal structure, and, thus, the stability and stickiness of the slurry increase. The higher the R, the more sewage sludge that is mixed; then, the water sorption would be more prominent, so that the slurries become more viscous. 3.1.4. Effect of Temperature on the Apparent Viscosity. Practice has proved that temperature has a considerable effect on the properties of CWS, and generally, it is expected that the η value of the slurry decreases as the temperature rises. LJ, QJ, and PH sludge had similar proximate and ultimate analysis results; all of these three types of CSS were prepared under the same conditions of R = 15% and ω = 59%, to study the relationship of η with temperature, which is shown in Figure 2. The relationship between the apparent viscosity and temperature of NS, PH, and BH sludge, which had considerable differences in the results of proximate and ultimate analyses;with experimental conditions of R = 15% and ω = 59%;is shown in Figure 3. As shown in Figures 2 and 3, the value of η consistently decreased as expected as the temperature increased from
Table 2. Maximum Solids Loading (MSL) of CSSs with Various Dosages of Sludge MSL (%) sewage sludge
R = 0%
R = 5%
R = 10%
R = 15%
LJ sludge QJ sludge NS sludge PH sludge BH sludge
69.16 69.16 69.16 69.16 69.16
64.75 63.61 64.47 62.44 63.08
61.42 60.63 61.44 58.85 59.89
59.41 57.19 58.74 55.22 56.86
coal powder and sewage sludge. Prior to each measurement, it was necessary to allow the mixer to stand still for 5 min, to release the entrapped air. 2.2.2. Determination of the Coal-Sludge Slurry Properties. The rheological properties of CSS, referring to the shear stress-shear rate and/or apparent viscosity-shear-rate dependence, were obtained using a rotational viscometer (HAAKE VT 550, Thermo, USA). The temperature was held constant within (20 ( 1) °C, controlled by water bath, and the rheology was studied by increasing shear rate from 10 s-1 to 100 s-1, and then held constant at 100 s-1 for 5 min. The viscosity data were recorded every 30 s during the 5-min period. The apparent viscosity at 100 s-1 then was calculated as an average of the 10 viscosity values recorded. The maximum solids loading (MSL), which is defined as the solids content of CSS, at a fixed viscosity of 1000 mPa s and a shear rate of 100 s-1, is used to appraise the slurryability of sewage sludge: the higher the MSL of CSS, the better the slurryability of the sewage sludge. The temperature rising property of the viscosity of CSS is considered as the apparent viscosity-temperature dependence. The viscosity data were recorded at a fixed shear rate of 100 s-1 as the temperature was raised from 20 °C to 50 °C, and then the relation of viscosity to temperature was revealed. The solids content of CSS, signed as ω, was determined from the weight difference upon drying in an oven at 105 °C for 3 h. The static stability of CSS is appraised by the water separation ratio (WSR), which is defined as the mass ratio of separated water to the total water in the test slurry: the higher the WSR, the poorer the static stability of CSS.
3. Results and Discussion 3.1. Slurrying Properties of CSS. 3.1.1. Effect of Sludge Mixing Proportion on the Slurrying Properties. The sludge mixing proportion, R, refers to the mass ratio of raw wet sewage sludge to dry coal powder. In the experiment, different R values, such as 5%, 10%, and 15% were selected; in addition, traditional CWS without sewage sludge (R = 0) served as a reference experimental condition. The MSL of CSSs with blended different types and mixing proportions of sludge are listed in Table 2. The results suggested that the MSL dropped by 9.5%-12.5% by increasing the value of R from 0% to 15%, ranging from 55.22% for PH CSS to 59.41% for LJ CSS in the R=15% case, as opposed to 69.16% in the R = 0 case. This can be attributed to the strong water absorbing capacity of the flocculent structures of the sludge, in that the more the sludge is mixed, the lesser the amount of water that is available to flow (and, thus, the more viscous the slurries and the lower the MSL). 3.1.2. Effect of Sludge Type on the Slurrying Properties. As shown in Table 2, among the five types of sludge, the slurryability of LJ and NS sludge was obviously better than that of PH and BH sludge. When more than 8% PH sludge was added, the MSL fell below 60%. Nevertheless, when 10% LJ or NS
(22) Yu, C.-w.; Li, B.-q.; Li, W.; et al. Analysis of coal characteristics on the properties of coal water slurry preparation with different coal ranks (in Chin.). J. Fuel Chem. Technol.. 2007, 35, 146-154. (23) Li, Y.-c.; Zhou, Z.-q.; Cheng, J.; et al. Influence of the coal’s physical chemistry property to its slurrying quality (in Chin.). Coal Conversion. 2009, 32, 35-39. (24) Lee, D. J. Floc structure and bound water content in excess activated sludges. J. Chin. Inst. Chem. Eng. 1994, 25, 201–207.
(21) National Standard of the People’s Republic of China, GB/T 18856.1∼18856.14-2002 (in Chin.).
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Figure 1. The apparent viscosity-solids concentration dependence: (a) reference samples (R = 0); (b) R = 5%; (c) R = 10%; (d) R = 15%.
Figure 2. The apparent viscosity-temperature dependence of LJ, QJ, and PH CSSs. Figure 3. The apparent viscosity-temperature dependence of NS, PH, and BH CSSs.
20 °C to 50 °C. This can be explained as follows. First, the intermolecular distance is increased and the interaction of molecular is weakened with increasing temperature; then, the surface tension decreases, and the slurry viscosity decreases. Second, the value of η is also affected by the ratio of solid volume fraction to maximum packing fraction (Φ/Φm). Wildemuth and Williams revealed that η increases as the ratio of Φ/Φm increases.8,25 When the slurry temperature is raised, the value of Φ/Φm decreases, since the thermal expansion coefficient is higher for the dispersion phase than the solid phase, then leading to the reduction of η. Aside from these, the reduction of η is also affected by the influence of the rise of temperature on ionic additives. Usually, the solubility of additives is increased along with the temperature over a certain range; then, the solubilization effect of additives to
coal particles is enhanced, and, thus, the value of η is reduced.26 Therefore, the fluidity properties can be improved by raising the temperature of CSSs, to reduce the cost of industrial transportation and application. Figures 2 and 3 showed that there was a negative, approximately linear, relationship between the apparent viscosity and the temperature. When the temperature increased from 20 °C to 50 °C, the value of η decreased, relative to temperature, by 18.6%, 28.4%, 32.5%, 31.9%, and 31.4% for LJ, QJ, PH, NS, and BH CSSs, respectively, under the experimental conditions of R = 15% and ω = 59%. This indicated that, as the temperature increased, the value of η slightly decreased for LJ industrial sludge CSSs, whereas that of the four domestic sludge CSSs decreased steeply and comparably (to ∼30%). Summarily, η was more sensitive to temperature for domestic sludge CSSs than industrial sludge CSSs. 3.2. Rheological Characteristics of CSS. Rheological characteristics have a key influence on the industrial application of CSSs. Stability, pumping, atomization, and combustion
(25) Wildemuth, C. R.; Williams, M. C. Viscosity of suspensions modeled with a shear-dependent maximum packing fraction. Rheol. Acta 1984, 23, 627–635. (26) Zhao, G.-h.; Wang, Q.-f.; Chen, L.-y.; et al. The influence of temperature on the rheology of high concentration coal-water slurry (in Chin.). Boiler Technol.. 2007, 38, 74-78.
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Figure 5. The apparent viscosity-shear rate plots of NS, PH, and BH CSSs.
Figure 4. The apparent viscosity-shear rate plots of LJ, QJ, and PH CSSs.
Table 3. Rheological Model Parameters of QJ CSSs
properties of CSSs can be affected by rheological characteristics. The ideal flow property is pseudo-plastic with the proper thixotropy. 3.2.1. Effect of Sludge Types on the Rheological Characteristics. The plots of apparent viscosity against shear rate (η vs γ) of LJ, QJ, and PH CSSs and NS, PH, and BH CSSs are given in Figures 4 and 5, respectively; all the slurries were obtained at fixed R and ω (R = 15%, ω = 59%). All slurry samples exhibited evident shear thinning and pseudo-plastic properties. The better the slurryability of the sludge, the lower the η value of CSSs, such as LJ and NS CSSs, and the influence of γ on η was rather more slight. In contrast, the value of η was higher and the pseudo-plastic properties tended to be strengthened when the slurryability of sludge was poorer. When γ was being raised from 10 s-1 to 100 s-1, the drop degree of viscosity (η-drop) of LJ, NS, QJ, BH, and PH CSSs increased in sequence, to 950, 1230, 1350, 1530, and 1950 mPa s, respectively. For CSSs that contained sludge of better slurryability, the facts that a high amount of water was available between particles and a lesser amount of water was released while slurries were sheared can be identified as reasons for the relatively slight η-drop. However, for CSSs that contained sludge of poorer slurryability, coal particles and sewage sludge linked each other and coagulated to form coal and sludge clusters during the preparation process. As a result of the lesser flowing capability of water, a high amount of water was always entrapped by the clusters, absorbed by sludge flocs, and penetrated into sludge cells, so the amount of segmented water among particles increased, but the clusters could be deformed and broken up easily when subjected to shear,27 and then the enclosed water was released; therefore, the value of η would drop rapidly. Moreover, research has shown that the value of η would be changed when the slurries were being sheared, in that shearing action changed the maximum packing fraction Φm by inducing directional migration of particles, driving the fine particles to fill the interspace of the coarse particles and, consequently, a rigid microstructure was assembled by particles. In summary, Φm, which could be taken as a function of shearing force, would become higher as the shearing force increased.25 Therefore, an increase of γ would lead to a decrease of Φ/Φm, which resulted in the reduction of the value of η. Referring to CSSs with the same R and ω values, poor slurryability of the sludge could lead to a stronger effect of γ on Φm that was being steeply increased
sludge mixing proportion, R (%)
solids content, ω (%)
apparent viscosity η (mPa s)
K
n
r
0
66.91 68.86 69.23 69.68
539 890 1026 1235
0.7629 1.144 1.352 3.0930
0.9402 0.9556 0.9491 0.8057
0.9999 0.9997 0.9995 0.9989
5
61.94 63.12 63.77 64.61
447 833 1056 1319
2.0850 4.3210 4.4120 7.2220
0.6546 0.6482 0.6896 0.6305
0.9845 0.9920 0.9921 0.9871
10
59.48 59.86 60.68 61.19
671 872 1008 1225
5.1360 5.9470 7.6880 9.4200
0.5526 0.5847 0.5648 0.5633
0.9806 0.9866 0.9894 0.9854
15
55.50 57.01 57.34 59.15
604 887 1092 1341
7.5960 10.7100 15.2800 23.2400
0.4510 0.4700 0.4347 0.3899
0.9675 0.9780 0.9799 0.9861
along with the γ, and, consequently, the pseudo-plastic trend becomes more apparent. 3.2.2. Rheological Model of CSS. The shear-stress-shearrate (τ vs γ) dependence can be described by the power-law model:28,29 τ ¼ Kγn where τ is shear force (Pa), K the consistency coefficient (Pa s), γ the shear rate (s-1), and n the rheologic index. When the value of K becomes higher, the value of η become higher. The flow patterns of CSS can be determined by the rheologic index: n = 1, Newtonian fluid; n > 1, dilatant fluid; n < 1, pseudo-plastic fluid. The rheological model parameters of QJ CSSs, listed in Table 3, showed that the consistency coefficient K depends on ω and R, and η behaves according to K. The rheologic index n < 1 for all of the QJ CSSs implies that the rheological characteristics are pseudo-plastic. Higher values of ω and R clearly lead to minimization of the value of n; therefore, the pseudo-plastic trend is more apparent. CSSs with other types of sludge added also exhibited similar rheological behavior. (28) Natarajan, V. P.; Suppes, G. J. Rheological studies on a slurry biofuel to aid in evaluating its suitability as a fuel. Fuel 1997, 76, 1527– 1535. (29) Turian, R. M.; Attal, J. F.; Sung, D.-J.; et al. Properties and rheology of coal-water mixtures using different coals. Fuel 2002, 81, 2019–2033.
(27) Smith, P. G.; van De Ven, T. G. M. Shear-induced deformation and rupture of suspended solid/liquid clusters. Colloids Surf. 1985, 15, 191–210.
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four types of CSSs flow. From this perspective, it was also demonstrated that BH CSSs had better stability. After 3 months of storage, rigid sedimentation, which could not be reverted to a uniform suspension by agitating, was not engendered in any slurry. Since slurries that had added sewage sludge had been properly stabilized, CSS is one of the feasible ways to be used as a fluid fuel and utilization method of sewage sludge.
Table 4. Water Separation Ratio (WSR) of CSSs Stored Still for 15 d Water Separation Ratio (%) sludge
R=0
R = 5%
R = 10%
R = 15%
LJ sludge QJ sludge NS sludge PH sludge BH sludge
14.79 14.79 14.79 14.79 14.79
13.31 10.20 12.10 9.53 5.06
10.82 9.13 8.48 8.33 3.74
8.99 6.57 6.30 7.58 2.30
4. Conclusions
3.3. Stability Properties of CSS. CSS belongs to a solidliquid dispersion system, and settling is a well-known phenomenon when it is stored. Since great difficulties to the application of CSS are imposed by solid-liquid separation, the stability is an important index of the quality of CSS. In the experiment, all slurry samples were allowed to stand still for 15 d, and then the water separation ratio (WSR);listed in Table 4; was measured to appraise the static stability of CSS. It was observed that after the addition of sludge, the WSR of CSSs decreased, compared to the CWSs (R = 0); that is, the stability of the CSSs became better. Furthermore, the stability tended to be better for high-R CSSs: the higher the value of R, the lesser the WSR and, thus, the better the stability. This was probably due to the fact that the agglomeration of the coal particles can be prevented by the sufficient contact between sludge particles and coal particles, guaranteeing that the two phases;solid and liquid;mixed well. The flocculent structure of sludge linked and aggregated together and formed a skeletal structure, thus blocking the solid particles from settling. The more sludge that was added, the stronger the blocking force, so the stability of the slurry became better. The quality of stability was just contrary to the slurryability. Although LJ and NS CSSs had better slurryability, their stability was poor, while stability was better for BH and PH CSSs, even though their slurryability was poorer. To study the fluidity of the hermetically stored slurries for 15 days, all slurries containing bottles were turned 180° vertically after the bottle cap was opened. It was observed that BH CSSs could easily and solely flow by gravity; however, only by the virtue of an external force could the other
Large amounts of sewage sludge impose great difficulties to the urban development and living environment of the urban areas. Worse still, it is hard to dispose of, because of the complicated physicochemical characteristics and the variations in composition. Since CSS technology is an effective way of utilizing sewage sludge, it needs fundamental research. The slurrying, rheology, and stability properties of five CSSs, and the influence of sludge type, sludge mixing proportion (R), temperature, and shear rate on the slurrying properties, were studied in this paper. It was found that the MSL dropped sharply (∼9.5-12.5%) as R increased from 0% to 15%. Slurryability of the sludge has been considerably influenced by the sludge type. Differences in the slurryability of the sewage sludge are made by the different physical and chemical properties of sludge. Among the five types of sludge, PH CSSs have the poorest slurryability, while LJ and NS CSSs have the best. The viscosity of CSSs consistently increases with the solids concentration, and it steeply increases for the higher-R CSSs. In addition, the relationship between viscosity and temperature can be considered as a negative linear correlation. CSSs exhibit pseudo-plastic rheological characteristics, and increasing the values of ω and R can result in a trend more toward pseudo-plastic behavior. The stability get better after sewage sludge is added, making CSS one of feasible technologies for disposing of and utilizing sewage sludge as a fluid fuel. Acknowledgment. The authors wish to acknowledge the financial support provided by the National Basic Research Program of China (Grant No. 2010CB227001).
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