- Email: [email protected]
Synthesis, characterization and application of polycarboxylate additive for coal water slurry
Fuel 111 (2013) 648–652
Contents lists available at SciVerse ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Synthesis, characterization and application of polycarboxylate additive for coal water slurry Sude Ma a,b,⇑, Pei Zhao c, Yan Guo d, Lisheng Zhong b, Yan Wang a a
Center for Advanced Materials and Energy, Xihua University, Chengdu, Sichuan 610039, China State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China c China Huaneng Clean Energy Research Institute, Beijing 100098, China d Department of Chemical Engineering, School of Energy and Power, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China b
h i g h l i g h t s Polycarboxylate (PC) additive for CWS was synthesized and characterized. PC additive is a comblike polymer with ACOOH and ACONH2 groups. CWS based on bituminous coal with PC additive displayed better properties. Simultaneously using of PC and aliphatic additives enhanced the CWS performance. PC additive exhibited improved properties because of its steric hindrance effect.
a r t i c l e
i n f o
Article history: Received 28 November 2012 Received in revised form 9 April 2013 Accepted 10 April 2013 Available online 25 April 2013 Keywords: Polycarboxylate additive Coal water slurry Synthesis Characterization Application
a b s t r a c t Coal water slurry (CWS), a highly-loaded suspension of coal in water, is regarded as one of the most important alternatives for fuel oil. Generally, additives are required for the preparation of CWS, of which polycarboxylate (PC) is the most promising and applicable one. PC was synthesized by free radical polymerization of acrylic acid, acrylamide and macro-monomer, and was further characterized by FTIR and 1H NMR. The performance of PC as additives for CWS was evaluated and the dispersion mechanism was also studied. Results showed that the additive was a polymer with comblike branched chains modified by ACOOH and ACONH2 groups. The CWS based on bituminous coal with PC additive displayed excellent fluidity and stability (i.e. the apparent viscosity and penetration ratio of CWS with 65 wt% coal and 0.4 wt% additive were 305 mPa.s and 79.8%, respectively) for the most suitable coal type. The properties of the CWS can be further enhanced by using simultaneously PC and aliphatic additives because of the steric hindrance effect and electrostatic force. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Coal water slurry (CWS), a type of novel fuel in the form of liquefaction of coal, is usually prepared by physical method. The concept of CWS was first advanced by researchers in 1970s when the oil crisis broke out. From then on, many investigations on coal converge technologies were carried out with aims to replace the fossil carbon in mineral oil by the carbon in coal. Extensive studies have been particularly focused on coal gasification, liquefaction and combustion. In recent years, the research interest is focused on CWS. CWS is a highly-loaded suspension of coal particles in water, containing about 60–70 wt% coal, 30–40 wt% water and a small ⇑ Corresponding author at: Center for Advanced Materials and Energy, Xihua University, Chengdu, Sichuan 610039, China. Tel./fax: +86 28 87729250. E-mail address: [email protected] (S. Ma). 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.04.023
amount of additives. With advantages such as high solids concentration, lower transport cost, convenient transporting and handling similar with oil, CWS is regarded as one of the most promising alternatives for fuel oil and has received worldwide attention [1]. The technology for combustion of CWS has been developed for many years. In China, the 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. CWS technology is one of such approaches, which is promising 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 [2–9].
S. Ma et al. / Fuel 111 (2013) 648–652
For the successful utilization of CWS in commercial plants, a high coal concentration in CWS is desired, and it is necessary for CWS to display excellent fluidity (i.e. low apparent viscosity) and stability suitable for its handling in preparation, storage, transport and combustion processes. However, CWS is an unstable thermodynamics solid–liquid suspension system [10]. CWS with poor stability cannot meet the requirements of the application for pumping and spraying. To achieve commercially suitable CWS, usage of the appropriate type and amount of chemical additives is a key factor [11,12]. Such additives are essential for dispersion of the powder by breaking the aggregates and agglomerates. Additives are adsorbed to the particles’ surface, thus modifying their surface properties. Coal particles have heterogeneous surfaces, on which a variety of oxygen containing functional groups presented may provide potential determining ions. It is not easy to find a kind of CWS additive that can be used for all types of coals owing to the complicated composition of coals. Despite this, polycarboxylate (i.e. PC in abbreviation), non-ionic, ethylene oxide derivatives and anionic additives seem to be the most promising materials, which are able to disperse the coal particles in water by preventing the flocculation and agglomeration. Several additives for CWS such as naphthalene sulfonate [13], sodium polystyrene sulfonate and naphthalene sulfonate formaldehyde condensate [11], surfactant (i.e. p-tert-octylphenoxypoly-ethoxyethanol, MW = 1966) [14], lignosulfonate [8], and naphthalene-based [12] were studied by the predecessors, but the CWS using these additive still has higher apparent viscosity and poor stability. Among all the kinds of existing additives for CWS, the PC is the most promising one because it has flexible structure and easily adjustable molecular structure [15–17]. In this paper, a novel PC additive for CWS with long-chain polyoxyethylene and anionic groups was synthesized, characterized and evaluated. It was found that improved dispersion, fluidity and stability can be provided by the additive to CWS. 2. Experimental 2.1. Materials The commercial product of macro-monomer (with abbreviation of mM, i.e. propenyl polyoxyethylene ether) was purchased from Oxiran chemical Co. Ltd. (Liaoning, China). According to the manual provided by the manufacturer, the mW has Mw of about 2700, and the values of ‘m’ and ‘n’ (i.e. ‘m’ and ‘n’ indicate the numbers of
649
polyoxyethylene and methyl polyoxyethylene of the long-chain, respectively, as shown in Fig. 2) are in range of 26–31 and 23– 28, respectively. Acrylic acid (AA), acrylamide (AM), mercaptoethanol (ME), ammonium persulfate (APS) and sodium hydroxide were all analytical grades and were purchased from Aladdin Reagent Co. (Shanghai, China). Aliphatic additive for CWS was applied by Pandeng chemical Co. Ltd. (Suzhou, China). All the materials were used as received without further treatment. Application experiments of the homemade additive (i.e. CWS preparation) were carried out with three Chinese coal powders from Shenmu (Shenmu county, Shaanxi province), Dawukou (Shizuishan city, Ningxia Hui autonomous region) and Tongchuan (Tongchuan city, Shaanxi province), respectively. Table 1 showed the results of both the proximate and ultimate analyses for these three kinds of coal powders, and the average particle size of them were also given. The particle size distributions of the powders were shown in Fig. 1. 2.2. Preparation methods 2.2.1. Synthesis of the polycarboxylate additive Macromolecules of PC additive were synthesized by the reaction using mM, AA and AM. The mM was dissolved in deionized water in a four necked round-bottom flask equipped with a thermometer, a stirrer, an inlet of dry nitrogen, a condenser and heat jacket. Then ME, AA, AM and APS were added slowly under moderate stirring (300 rpm), and the mixture was allowed to react at 70 °C for 2–4 h. After that, the mixture was cooled to 50 °C and sodium hydroxide solution of 30 wt% was added under fast stirring (500 rpm) until the pH rose to 6–7. Finally the PC copolymer used for the additive of CWS was obtained. The synthesis process for the product was shown in Fig. 2. 2.2.2. Preparation of coal–water slurries The coal powders were slowly mixed in a beaker of 500 ml containing a certain quantity of additive (i.e. the homemade PC, commercially aliphatic and complex of the above-mentioned two additives, respectively) and tap water, respectively. The contents were continuously stirred by a mixer during the addition of coal, and then the stirring of the slurry was continued for another 10 min at 1200 rpm to ensure the homogenization of CWS. The prepared slurry was kept in capped bottle at room temperature for the study of its characteristics. 2.3. Characterization 2.3.1. FTIR spectra Fourier transformer infrared (FTIR) spectra of the additive sample was recorded in the range of 4000–400 cm 1 using FTIR spectrometer (VERTEX 70, BRUKER, Germany) by KBr pelleting technique at the resolution of 4 cm 1 for 32 scans. The polymer prepared was repeatedly purified with acetone and dried warmly to constant weight. 2.3.2. 1H NMR spectra The 1H NMR spectrum was measured on a 400 MHz spectrum (Varian Gemini-400 spectrometer, USA) using TMS (i.e. tetramethylsilane) as the internal standard, D2O as the solvent and the probe temperature of 295.5 K.
Fig. 1. Particle size distribution of coal samples.
2.3.3. Zeta potential measurements Zeta potentials of coal particles were tested by Zetasizer (Autosizer, Melvern IIC, UK). A serial of CWSs (i.e. 50 ml distilled water and 0.5 g coal) with different dosages of additives were prepared. After the CWSs were laid for 24 h and centrifugalized at
650
S. Ma et al. / Fuel 111 (2013) 648–652
Fig. 2. Scheme of the polymerization.
Table 1 Proximate and ultimate analyses of coal samples on dry basis Component
Proximate analysis
Ultimate analysis
Coal type
Inherent moisture (wt%) Ash (wt%) Volatile matter (wt%) Fixed carbon (wt%) C (wt%) H (wt%) O (wt%) N (wt%) S (wt%)
Average particle size (lm)
Shenmu
Dawukou
Tongchuan
8.84 11.90 43.39 35.87 64.73 4.59 16.07 0.95 0.28 53
7.25 14.68 37.9 40.17 83.37 5.72 11.92 1.18 0.18 55
3.98 20.58 32.01 43.43 87.98 3.83 8.59 1.36 0.16 56
3000 rpm for 30 min, the upper solutions of CWSs were taken to measure Zeta potentials. The mean value of the Zeta potentials which were measured for three times was used in this study. 2.3.4. Apparent viscosity measurements Rotational viscometer (DV-C, Brookfild, USA) was used to test the CWS apparent viscosity under 15 °C with the shear rate of 100 s 1. 2.3.5. Static stability measurements The CWS was stored for 168 h (i.e. 1 week) in a test tube of 50 ml before the stability measurements. The stability was evaluated by the glass rod penetration test (penetration ratio, %) [18]. 2.3.6. Average particle size and size distribution measurements The average particle size and size distribution of the coal powders were determined using a Fritsch Analysette 22 Compact Laser Particle Sizer. The measurements were carried out in parallel 3 times and the average values were calculated. 3. Results and discussion 3.1. Chemical structure The FTIR spectrum of the PC additive is shown in Fig. 3. A typical spectrum shows the AOH group stretching vibration absorption bond at 3429 cm 1, 2887 cm 1. Two peaks at 1109 and 960 cm 1 proved the existence of ether bond in polyoxyethylene ether (i.e. A(CH2CH2O)A). The peak at 1674 cm 1 testified the existence of carbonyl (i.e. C@O) in carboxyl (i.e. ACOOH or ACOO ) and amide groups (i.e. ACONH2). The peaks at 1687 and 1280 cm 1 stood for the NAH vibration absorption. There is no double bond in the molecule because absorption peak between 2500 and 2000 cm 1 disappeared, and each monomer has come into the copolymer. The
Fig. 3. FTIR spectrum of PC additive.
results above showed that the PC molecules, modified with ACOOH group and ACONH2 group, are the gender structure. As shown in Fig. 4, the peaks at 3.56 and 3.7 ppm were attributed to the H of polyoxyethylene group [A(CH2CH2O)A]. No peak was found at 5.7 ppm (i.e. the H related to the double bond), which represented the disappearance of double bonds. The 1H NMR and FTIR spectrum analysis show similar results.
3.2. Effect of coal types on the CWS properties Three types of coal powders from Shenmu, Dawukou and Tongchuan were used for the CWS preparation to study the effect of coal properties on the viscosity and stability in advance. The properties of CWS are showed in Table 2.
S. Ma et al. / Fuel 111 (2013) 648–652
651
(CWS with PC additive) to 19.55 mV (CWS with complex additive)) or 142.56%, the apparent viscosity of CWS decreased by only 115 mPa.s (i.e. from 305 mPa.s to 190 mPa.s) or 37.70%. In contrast, the Zeta potential of CWS with aliphatic additive was 55.26 mV and the apparent viscosity of which was 688 mPa.s. From these dates, it can be concluded that the trend of Zeta potentials was not in consistent with the apparent viscosity of the CWS. 3.4. Static stability of CWS
Fig. 4. 1H NMR spectrum of PC additive.
Table 2 Properties of CWS based on three types of coal. Coal type
Shenmu
Dawukou
Tongchuan
Apparent viscosity (mPa.s) Penetration ratio (%)
725 41.52
358 62.8
308 79.8
Note: Coal content and additive concentration of the CWS were 65 wt% and 0.4 wt% respectively.
It can be seen from Table 2 that, with equal concentration of coal and additive, the Tongchuan coal (i.e. a kind of bituminous coal from Tongchuan city) displayed the best performances of viscosity and static stability among the three types of coal. As shown in Table 1 and Fig. 1, the average particle size and size distribution of these three kinds of coal powders are almost equal, so the composition of the coal must be the key factor which affects solely the properties of the CWS. As shown in Table 1 (i.e. low inherent moisture content, high ash content, low volatile matter content and high value of C/O.) and the similar results were also given by Atesok [5]. Therefore, the later investigations were all carried out based on the Tongchuan coal. 3.3. Zeta potential and apparent viscosity of CWS The effect of the added additive types on Zeta potential is given in Table 3. The concentration of additives and coal content were 0.4 wt% and 65 wt%, respectively. It can be seen from Table 3 that, the Zeta potentials keep decreasing from 7.85 mV to 55.26 mV in accordance with the order of CWS without additive, CWS with PC additive, CWS with complex additive (i.e. with the mixture of 80 wt% PC and 20 wt% aliphatic additives) and CWS with aliphatic additive. As the PC additive added into the CWS, the Zeta potential decreased by 0.21 mV (i.e. from 7.85 mV to 8.06 mV) or 2.68%, while the apparent viscosity of CWS decreased by 2895 mPa.s (i.e. from 3200 mPa.s to 305 mPa.s) or 90.46%. However, when the Zeta potential further decreased by 11.49 mV (i.e. from 8.06 mV
It was found that the apparent viscosity increased with the solid concentration of CWS (as shown in Fig. 5) and decreased with the additive concentration of PC (as shown in Fig. 6). The similar results were expressed by the penetration ratio after 1 week. It can be seen from Fig. 5 that, the penetration ratios of the CWS keep decreasing with the increase of coal solid concentration, and sharply when the coal content was larger than 65 wt%. As seen in Fig. 6, the penetration ratios of the CWS kept increasing with the increase of the additive concentration, which was almost changeless when the additive concentration larger than 0.4 wt%. So, the CWS with 65 wt% coal and 0.4 wt% PC additive displayed the most practical stability and apparent viscosity. In contrast, the CWS with 65 wt% coal content prepared from complex additive of 0.4 wt% showed the particularly low apparent viscosity (i.e. 190 mPa.s) and good static stability (i.e. penetration ratio of 83.5%). These data mean that the application of complex additive was a good method to improve the CWS properties. 3.5. Dispersion mechanism of PC additive with the coal in the CWS The Zeta potential is considered as one of the major factors that affect the dispersion property of CWS, because the electrostatic force derived from the Zeta potential in coal–water interface prevents the coal particles from aggregation. The PC additive molecules contain many negative groups, so the Zeta potential on coal surface can be influenced by the addition of PC additive. However, as seen in Table 3, the trends of Zeta potentials were not always in consistent with the apparent viscosity of the CWS, which meant the Zeta potential was not the only factor that affects the dispersion of the slurry. As indicated by the researchers [15–17], the PC additive longchain has flexible structure and the structure can be adjusted easily. Comparing with the PC, some other additives such as polynaphthalene (i.e. the naphthalene sulfonate and naphthalene sulfonate formaldehyde condensate etc.) have many benzene rings to
Table 3 Zeta potentials and apparent viscosities of CWS based on four types of additive. Additive type
Without
PC
Complex
Aliphatic
Zeta potential (mV) Apparent viscosity (mPa.s)
7.85 3200
8.05 305
19.55 190
55.26 688
Note: (1) The concentration of additives and coal content were 0.4 wt% and 65 wt% respectively. (2) Complex additive was a mixture of 80 wt% PC and 20 wt% aliphatic.
Fig. 5. Influences of coal content on apparent viscosity and penetration ratio of CWS (with 0.4 wt% PC additive).
652
S. Ma et al. / Fuel 111 (2013) 648–652
fields such as coal chemical industry and coal–water slurry fired boiler. Acknowledgments This work was financially supported by State Key Laboratory of Electrical Insulation and Power Equipment (No. EIPE12206), Chunhui Project from Education Ministry of China (No. Z2011078), Open Research Fund of Key Laboratory of Special Materials Preparation and Control, Xihua University (No. SZJJ2012-017), Key Scientific Research Foundation of Xihua University (No. Z1120115), Key Discipline Project of Xihua University (No. L2XW418-13-0 (2013)), and Research Project from the Education Department of Sichuan Province (No. 12ZB130). References Fig. 6. Influences of additive concentration on apparent viscosity and penetration ratio of CWS (coal content of the CWS was 65 wt%).
fabricate the long-chain, and which have rigid structure. So the polynaphthalene additive provides dispersion property of the CWS relies mainly on the electrostatic force. When the PC additive was dissolved and the flexible long-chain was unfolded in the water, the steric hindrance could make coal particles disperse more stable, which showed lower apparent viscosity [7,19]. It was the steric hindrance effect as well as the electrostatic force that afforded the enhanced properties to the CWS, which was also approved by the experimental results of complex additive.
4. Conclusions In this article, the influence of PC additive on the dispersion (apparent viscosity) and stability of CWS has been investigated. The additive was fabricated in aqueous solution at 70 °C and reacted for 2–4 h via the free radical polymerization of AA, AM and mM using APS as the initiator. Enhanced properties were afforded to CWS by the additive, which is a polymer with comblike branched chain and ACOOH and ACONH2 groups, owing to the steric hindrance effect and electrostatic force. The CWS with PC additive concentration of 0.4 wt% showed best dispersion (i.e. the apparent viscosity corresponding to the CWS with 65 wt% and 68 wt% coal content were 305 mPa.s and 528 mPa.s, respectively) and static stability (i.e. the penetration ratio of the CWS with 65 wt% coal and 0.4 wt% PC additive was 79.8%, after 1 week storage time). For the three types of coal investigated in the article, the PC additive adapted to the Tongchuan coal (i.e. bituminous coal) mostly. The complex application of the PC and aliphatic additives was a good method to further improve the CWS property, and the best ratio of the two additives was 1:4 (i.e. 20 wt% aliphatic additive and 80 wt% PC additive). Based on the properties (i.e. high coal concentration, low apparent viscosity and good static stability), the CWS with the PC additive has many application in several
[1] Hu XY. Developing foreground on new type addition of coal–water slurry. Energy Technol 2002;23(2):62–3. [2] Boylu F, Dinçer H, Atesßok G. Effect of coal particle size distribution, volume fraction and rank on the rheology of coal–water slurries. Fuel Process Technol 2004;85(4):241–50. [3] Deng JX, Ding ZL, Yuan DL. Erosion wear mechanisms of coal–water–slurry (CWS) ceramic nozzles in industry boilers. Mater Sci Eng A 2006;417:1–7. [4] Pawlik M. Polymeric dispersants for coal–water slurries. Colloids Surf A 2005;266(1–3):82–90. [5] Atesok G, Boylu F, Sirkeci AA, Dincer H. The effect of coal properties on the viscosity of coal–water slurries. Fuel 2002;81(14):1855–8. [6] Zhang L, Lu QY, Xu ZH, Liu QX, Zeng HB. Effect of polycarboxylate ether combtype polymer on viscosity and interfacial properties of kaolinite clay suspensions. J Colloid Interface Sci 2012;378(1):222–31. [7] Zhu JF, Zhang GH, Miao Z, Shang T. Synthesis and performance of a comblike amphoteric polycarboxylate dispersant for coal–water slurry. Colloids Surf A 2012;412:101–7. [8] Zhou MS, Yang DJ, Qiu XQ. Influence of dispersant on bound water content in coal–water slurry and its quantitative determination. Energy Convers Manage 2008;49(11):3063–8. [9] Zhou MS, Kong Q, Pan B, Qiu XQ, Yang DJ, Lou HM. Evaluation of treated black liquor used as dispersant of concentrated coal–water slurry. Fuel 2010;89(3):716–23. [10] Zou LZ, Zhu SQ, Wang XL, Guo XK, Cui GW. Interaction between different CWS dispersants and coal: X adsorptive characteristics of dispersant on coal surface. J Fuel Chem Technol 2006;34(1):10–4. [11] Atesok G, Dincer H, Ozer M, Mütevelliog˘lu A. The effects of dispersants (PSS– NSF) used in coal–water slurries on the grindability of coals of different structures. Fuel 2005;84(7–8):801–8. [12] Kaushal KT, Sibendra KB, Kumaresh CB, Somnath B, Kamlesh KM. Highconcentration coal–water slurry from Indian coals using newly developed additives. Fuel Process Technol 2004;85(1):31–42. [13] Guo ZB, Feng R, Zheng YF, Fu XR. Improvement in properties of coal water slurry by combined use of new additive and ultrasonic irradiation. Ultrason Sonochem 2007;14:583–8. [14] Aktas Z, Woodburn ET. Effect of addition of surface active agent on the viscosity of a high concentration slurry of a low-rank British coal in water. Fuel Process Technol 2000;62:1–15. [15] Liu JZ, Zhao WD, Zhou JH, Cheng J, Zhang GX, Feng YG, et al. An investigation on the rheological and sulfur-retention characteristics of desulfurizing coal– water slurry with calcium-based additives. Fuel Process Technol 2009;90:91–8. [16] Meikap BC, Purohit NK, Mahadevan V. Effect of microwave pretreatment of coal for improvement of rheological characteristics of coal–water slurries. J Colloid Interface Sci 2005;281(1):225–35. [17] Dinçer H, Boylu F, Sirkeci AA, Atesok G. The effect of chemicals on the viscosity and stability of coal water slurries. Int J Miner Process 2003;70:41–51. [18] Boylu F, Atesßok G, Dinçer H. The effect of carboxymethyl cellulose (CMC) on the stability of coal–water slurries. Fuel 2005;84:315–9. [19] Zhou MS, Huang K, Yang DJ, Qiu XQ. Development and evaluation of polycarboxylic acid hyper-dispersant used to prepare high-concentrated coal–water slurry. Powder Technol 2012;229:185–90.
Contents lists available at SciVerse ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Synthesis, characterization and application of polycarboxylate additive for coal water slurry Sude Ma a,b,⇑, Pei Zhao c, Yan Guo d, Lisheng Zhong b, Yan Wang a a
Center for Advanced Materials and Energy, Xihua University, Chengdu, Sichuan 610039, China State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China c China Huaneng Clean Energy Research Institute, Beijing 100098, China d Department of Chemical Engineering, School of Energy and Power, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China b
h i g h l i g h t s Polycarboxylate (PC) additive for CWS was synthesized and characterized. PC additive is a comblike polymer with ACOOH and ACONH2 groups. CWS based on bituminous coal with PC additive displayed better properties. Simultaneously using of PC and aliphatic additives enhanced the CWS performance. PC additive exhibited improved properties because of its steric hindrance effect.
a r t i c l e
i n f o
Article history: Received 28 November 2012 Received in revised form 9 April 2013 Accepted 10 April 2013 Available online 25 April 2013 Keywords: Polycarboxylate additive Coal water slurry Synthesis Characterization Application
a b s t r a c t Coal water slurry (CWS), a highly-loaded suspension of coal in water, is regarded as one of the most important alternatives for fuel oil. Generally, additives are required for the preparation of CWS, of which polycarboxylate (PC) is the most promising and applicable one. PC was synthesized by free radical polymerization of acrylic acid, acrylamide and macro-monomer, and was further characterized by FTIR and 1H NMR. The performance of PC as additives for CWS was evaluated and the dispersion mechanism was also studied. Results showed that the additive was a polymer with comblike branched chains modified by ACOOH and ACONH2 groups. The CWS based on bituminous coal with PC additive displayed excellent fluidity and stability (i.e. the apparent viscosity and penetration ratio of CWS with 65 wt% coal and 0.4 wt% additive were 305 mPa.s and 79.8%, respectively) for the most suitable coal type. The properties of the CWS can be further enhanced by using simultaneously PC and aliphatic additives because of the steric hindrance effect and electrostatic force. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Coal water slurry (CWS), a type of novel fuel in the form of liquefaction of coal, is usually prepared by physical method. The concept of CWS was first advanced by researchers in 1970s when the oil crisis broke out. From then on, many investigations on coal converge technologies were carried out with aims to replace the fossil carbon in mineral oil by the carbon in coal. Extensive studies have been particularly focused on coal gasification, liquefaction and combustion. In recent years, the research interest is focused on CWS. CWS is a highly-loaded suspension of coal particles in water, containing about 60–70 wt% coal, 30–40 wt% water and a small ⇑ Corresponding author at: Center for Advanced Materials and Energy, Xihua University, Chengdu, Sichuan 610039, China. Tel./fax: +86 28 87729250. E-mail address: [email protected] (S. Ma). 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.04.023
amount of additives. With advantages such as high solids concentration, lower transport cost, convenient transporting and handling similar with oil, CWS is regarded as one of the most promising alternatives for fuel oil and has received worldwide attention [1]. The technology for combustion of CWS has been developed for many years. In China, the 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. CWS technology is one of such approaches, which is promising 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 [2–9].
S. Ma et al. / Fuel 111 (2013) 648–652
For the successful utilization of CWS in commercial plants, a high coal concentration in CWS is desired, and it is necessary for CWS to display excellent fluidity (i.e. low apparent viscosity) and stability suitable for its handling in preparation, storage, transport and combustion processes. However, CWS is an unstable thermodynamics solid–liquid suspension system [10]. CWS with poor stability cannot meet the requirements of the application for pumping and spraying. To achieve commercially suitable CWS, usage of the appropriate type and amount of chemical additives is a key factor [11,12]. Such additives are essential for dispersion of the powder by breaking the aggregates and agglomerates. Additives are adsorbed to the particles’ surface, thus modifying their surface properties. Coal particles have heterogeneous surfaces, on which a variety of oxygen containing functional groups presented may provide potential determining ions. It is not easy to find a kind of CWS additive that can be used for all types of coals owing to the complicated composition of coals. Despite this, polycarboxylate (i.e. PC in abbreviation), non-ionic, ethylene oxide derivatives and anionic additives seem to be the most promising materials, which are able to disperse the coal particles in water by preventing the flocculation and agglomeration. Several additives for CWS such as naphthalene sulfonate [13], sodium polystyrene sulfonate and naphthalene sulfonate formaldehyde condensate [11], surfactant (i.e. p-tert-octylphenoxypoly-ethoxyethanol, MW = 1966) [14], lignosulfonate [8], and naphthalene-based [12] were studied by the predecessors, but the CWS using these additive still has higher apparent viscosity and poor stability. Among all the kinds of existing additives for CWS, the PC is the most promising one because it has flexible structure and easily adjustable molecular structure [15–17]. In this paper, a novel PC additive for CWS with long-chain polyoxyethylene and anionic groups was synthesized, characterized and evaluated. It was found that improved dispersion, fluidity and stability can be provided by the additive to CWS. 2. Experimental 2.1. Materials The commercial product of macro-monomer (with abbreviation of mM, i.e. propenyl polyoxyethylene ether) was purchased from Oxiran chemical Co. Ltd. (Liaoning, China). According to the manual provided by the manufacturer, the mW has Mw of about 2700, and the values of ‘m’ and ‘n’ (i.e. ‘m’ and ‘n’ indicate the numbers of
649
polyoxyethylene and methyl polyoxyethylene of the long-chain, respectively, as shown in Fig. 2) are in range of 26–31 and 23– 28, respectively. Acrylic acid (AA), acrylamide (AM), mercaptoethanol (ME), ammonium persulfate (APS) and sodium hydroxide were all analytical grades and were purchased from Aladdin Reagent Co. (Shanghai, China). Aliphatic additive for CWS was applied by Pandeng chemical Co. Ltd. (Suzhou, China). All the materials were used as received without further treatment. Application experiments of the homemade additive (i.e. CWS preparation) were carried out with three Chinese coal powders from Shenmu (Shenmu county, Shaanxi province), Dawukou (Shizuishan city, Ningxia Hui autonomous region) and Tongchuan (Tongchuan city, Shaanxi province), respectively. Table 1 showed the results of both the proximate and ultimate analyses for these three kinds of coal powders, and the average particle size of them were also given. The particle size distributions of the powders were shown in Fig. 1. 2.2. Preparation methods 2.2.1. Synthesis of the polycarboxylate additive Macromolecules of PC additive were synthesized by the reaction using mM, AA and AM. The mM was dissolved in deionized water in a four necked round-bottom flask equipped with a thermometer, a stirrer, an inlet of dry nitrogen, a condenser and heat jacket. Then ME, AA, AM and APS were added slowly under moderate stirring (300 rpm), and the mixture was allowed to react at 70 °C for 2–4 h. After that, the mixture was cooled to 50 °C and sodium hydroxide solution of 30 wt% was added under fast stirring (500 rpm) until the pH rose to 6–7. Finally the PC copolymer used for the additive of CWS was obtained. The synthesis process for the product was shown in Fig. 2. 2.2.2. Preparation of coal–water slurries The coal powders were slowly mixed in a beaker of 500 ml containing a certain quantity of additive (i.e. the homemade PC, commercially aliphatic and complex of the above-mentioned two additives, respectively) and tap water, respectively. The contents were continuously stirred by a mixer during the addition of coal, and then the stirring of the slurry was continued for another 10 min at 1200 rpm to ensure the homogenization of CWS. The prepared slurry was kept in capped bottle at room temperature for the study of its characteristics. 2.3. Characterization 2.3.1. FTIR spectra Fourier transformer infrared (FTIR) spectra of the additive sample was recorded in the range of 4000–400 cm 1 using FTIR spectrometer (VERTEX 70, BRUKER, Germany) by KBr pelleting technique at the resolution of 4 cm 1 for 32 scans. The polymer prepared was repeatedly purified with acetone and dried warmly to constant weight. 2.3.2. 1H NMR spectra The 1H NMR spectrum was measured on a 400 MHz spectrum (Varian Gemini-400 spectrometer, USA) using TMS (i.e. tetramethylsilane) as the internal standard, D2O as the solvent and the probe temperature of 295.5 K.
Fig. 1. Particle size distribution of coal samples.
2.3.3. Zeta potential measurements Zeta potentials of coal particles were tested by Zetasizer (Autosizer, Melvern IIC, UK). A serial of CWSs (i.e. 50 ml distilled water and 0.5 g coal) with different dosages of additives were prepared. After the CWSs were laid for 24 h and centrifugalized at
650
S. Ma et al. / Fuel 111 (2013) 648–652
Fig. 2. Scheme of the polymerization.
Table 1 Proximate and ultimate analyses of coal samples on dry basis Component
Proximate analysis
Ultimate analysis
Coal type
Inherent moisture (wt%) Ash (wt%) Volatile matter (wt%) Fixed carbon (wt%) C (wt%) H (wt%) O (wt%) N (wt%) S (wt%)
Average particle size (lm)
Shenmu
Dawukou
Tongchuan
8.84 11.90 43.39 35.87 64.73 4.59 16.07 0.95 0.28 53
7.25 14.68 37.9 40.17 83.37 5.72 11.92 1.18 0.18 55
3.98 20.58 32.01 43.43 87.98 3.83 8.59 1.36 0.16 56
3000 rpm for 30 min, the upper solutions of CWSs were taken to measure Zeta potentials. The mean value of the Zeta potentials which were measured for three times was used in this study. 2.3.4. Apparent viscosity measurements Rotational viscometer (DV-C, Brookfild, USA) was used to test the CWS apparent viscosity under 15 °C with the shear rate of 100 s 1. 2.3.5. Static stability measurements The CWS was stored for 168 h (i.e. 1 week) in a test tube of 50 ml before the stability measurements. The stability was evaluated by the glass rod penetration test (penetration ratio, %) [18]. 2.3.6. Average particle size and size distribution measurements The average particle size and size distribution of the coal powders were determined using a Fritsch Analysette 22 Compact Laser Particle Sizer. The measurements were carried out in parallel 3 times and the average values were calculated. 3. Results and discussion 3.1. Chemical structure The FTIR spectrum of the PC additive is shown in Fig. 3. A typical spectrum shows the AOH group stretching vibration absorption bond at 3429 cm 1, 2887 cm 1. Two peaks at 1109 and 960 cm 1 proved the existence of ether bond in polyoxyethylene ether (i.e. A(CH2CH2O)A). The peak at 1674 cm 1 testified the existence of carbonyl (i.e. C@O) in carboxyl (i.e. ACOOH or ACOO ) and amide groups (i.e. ACONH2). The peaks at 1687 and 1280 cm 1 stood for the NAH vibration absorption. There is no double bond in the molecule because absorption peak between 2500 and 2000 cm 1 disappeared, and each monomer has come into the copolymer. The
Fig. 3. FTIR spectrum of PC additive.
results above showed that the PC molecules, modified with ACOOH group and ACONH2 group, are the gender structure. As shown in Fig. 4, the peaks at 3.56 and 3.7 ppm were attributed to the H of polyoxyethylene group [A(CH2CH2O)A]. No peak was found at 5.7 ppm (i.e. the H related to the double bond), which represented the disappearance of double bonds. The 1H NMR and FTIR spectrum analysis show similar results.
3.2. Effect of coal types on the CWS properties Three types of coal powders from Shenmu, Dawukou and Tongchuan were used for the CWS preparation to study the effect of coal properties on the viscosity and stability in advance. The properties of CWS are showed in Table 2.
S. Ma et al. / Fuel 111 (2013) 648–652
651
(CWS with PC additive) to 19.55 mV (CWS with complex additive)) or 142.56%, the apparent viscosity of CWS decreased by only 115 mPa.s (i.e. from 305 mPa.s to 190 mPa.s) or 37.70%. In contrast, the Zeta potential of CWS with aliphatic additive was 55.26 mV and the apparent viscosity of which was 688 mPa.s. From these dates, it can be concluded that the trend of Zeta potentials was not in consistent with the apparent viscosity of the CWS. 3.4. Static stability of CWS
Fig. 4. 1H NMR spectrum of PC additive.
Table 2 Properties of CWS based on three types of coal. Coal type
Shenmu
Dawukou
Tongchuan
Apparent viscosity (mPa.s) Penetration ratio (%)
725 41.52
358 62.8
308 79.8
Note: Coal content and additive concentration of the CWS were 65 wt% and 0.4 wt% respectively.
It can be seen from Table 2 that, with equal concentration of coal and additive, the Tongchuan coal (i.e. a kind of bituminous coal from Tongchuan city) displayed the best performances of viscosity and static stability among the three types of coal. As shown in Table 1 and Fig. 1, the average particle size and size distribution of these three kinds of coal powders are almost equal, so the composition of the coal must be the key factor which affects solely the properties of the CWS. As shown in Table 1 (i.e. low inherent moisture content, high ash content, low volatile matter content and high value of C/O.) and the similar results were also given by Atesok [5]. Therefore, the later investigations were all carried out based on the Tongchuan coal. 3.3. Zeta potential and apparent viscosity of CWS The effect of the added additive types on Zeta potential is given in Table 3. The concentration of additives and coal content were 0.4 wt% and 65 wt%, respectively. It can be seen from Table 3 that, the Zeta potentials keep decreasing from 7.85 mV to 55.26 mV in accordance with the order of CWS without additive, CWS with PC additive, CWS with complex additive (i.e. with the mixture of 80 wt% PC and 20 wt% aliphatic additives) and CWS with aliphatic additive. As the PC additive added into the CWS, the Zeta potential decreased by 0.21 mV (i.e. from 7.85 mV to 8.06 mV) or 2.68%, while the apparent viscosity of CWS decreased by 2895 mPa.s (i.e. from 3200 mPa.s to 305 mPa.s) or 90.46%. However, when the Zeta potential further decreased by 11.49 mV (i.e. from 8.06 mV
It was found that the apparent viscosity increased with the solid concentration of CWS (as shown in Fig. 5) and decreased with the additive concentration of PC (as shown in Fig. 6). The similar results were expressed by the penetration ratio after 1 week. It can be seen from Fig. 5 that, the penetration ratios of the CWS keep decreasing with the increase of coal solid concentration, and sharply when the coal content was larger than 65 wt%. As seen in Fig. 6, the penetration ratios of the CWS kept increasing with the increase of the additive concentration, which was almost changeless when the additive concentration larger than 0.4 wt%. So, the CWS with 65 wt% coal and 0.4 wt% PC additive displayed the most practical stability and apparent viscosity. In contrast, the CWS with 65 wt% coal content prepared from complex additive of 0.4 wt% showed the particularly low apparent viscosity (i.e. 190 mPa.s) and good static stability (i.e. penetration ratio of 83.5%). These data mean that the application of complex additive was a good method to improve the CWS properties. 3.5. Dispersion mechanism of PC additive with the coal in the CWS The Zeta potential is considered as one of the major factors that affect the dispersion property of CWS, because the electrostatic force derived from the Zeta potential in coal–water interface prevents the coal particles from aggregation. The PC additive molecules contain many negative groups, so the Zeta potential on coal surface can be influenced by the addition of PC additive. However, as seen in Table 3, the trends of Zeta potentials were not always in consistent with the apparent viscosity of the CWS, which meant the Zeta potential was not the only factor that affects the dispersion of the slurry. As indicated by the researchers [15–17], the PC additive longchain has flexible structure and the structure can be adjusted easily. Comparing with the PC, some other additives such as polynaphthalene (i.e. the naphthalene sulfonate and naphthalene sulfonate formaldehyde condensate etc.) have many benzene rings to
Table 3 Zeta potentials and apparent viscosities of CWS based on four types of additive. Additive type
Without
PC
Complex
Aliphatic
Zeta potential (mV) Apparent viscosity (mPa.s)
7.85 3200
8.05 305
19.55 190
55.26 688
Note: (1) The concentration of additives and coal content were 0.4 wt% and 65 wt% respectively. (2) Complex additive was a mixture of 80 wt% PC and 20 wt% aliphatic.
Fig. 5. Influences of coal content on apparent viscosity and penetration ratio of CWS (with 0.4 wt% PC additive).
652
S. Ma et al. / Fuel 111 (2013) 648–652
fields such as coal chemical industry and coal–water slurry fired boiler. Acknowledgments This work was financially supported by State Key Laboratory of Electrical Insulation and Power Equipment (No. EIPE12206), Chunhui Project from Education Ministry of China (No. Z2011078), Open Research Fund of Key Laboratory of Special Materials Preparation and Control, Xihua University (No. SZJJ2012-017), Key Scientific Research Foundation of Xihua University (No. Z1120115), Key Discipline Project of Xihua University (No. L2XW418-13-0 (2013)), and Research Project from the Education Department of Sichuan Province (No. 12ZB130). References Fig. 6. Influences of additive concentration on apparent viscosity and penetration ratio of CWS (coal content of the CWS was 65 wt%).
fabricate the long-chain, and which have rigid structure. So the polynaphthalene additive provides dispersion property of the CWS relies mainly on the electrostatic force. When the PC additive was dissolved and the flexible long-chain was unfolded in the water, the steric hindrance could make coal particles disperse more stable, which showed lower apparent viscosity [7,19]. It was the steric hindrance effect as well as the electrostatic force that afforded the enhanced properties to the CWS, which was also approved by the experimental results of complex additive.
4. Conclusions In this article, the influence of PC additive on the dispersion (apparent viscosity) and stability of CWS has been investigated. The additive was fabricated in aqueous solution at 70 °C and reacted for 2–4 h via the free radical polymerization of AA, AM and mM using APS as the initiator. Enhanced properties were afforded to CWS by the additive, which is a polymer with comblike branched chain and ACOOH and ACONH2 groups, owing to the steric hindrance effect and electrostatic force. The CWS with PC additive concentration of 0.4 wt% showed best dispersion (i.e. the apparent viscosity corresponding to the CWS with 65 wt% and 68 wt% coal content were 305 mPa.s and 528 mPa.s, respectively) and static stability (i.e. the penetration ratio of the CWS with 65 wt% coal and 0.4 wt% PC additive was 79.8%, after 1 week storage time). For the three types of coal investigated in the article, the PC additive adapted to the Tongchuan coal (i.e. bituminous coal) mostly. The complex application of the PC and aliphatic additives was a good method to further improve the CWS property, and the best ratio of the two additives was 1:4 (i.e. 20 wt% aliphatic additive and 80 wt% PC additive). Based on the properties (i.e. high coal concentration, low apparent viscosity and good static stability), the CWS with the PC additive has many application in several
[1] Hu XY. Developing foreground on new type addition of coal–water slurry. Energy Technol 2002;23(2):62–3. [2] Boylu F, Dinçer H, Atesßok G. Effect of coal particle size distribution, volume fraction and rank on the rheology of coal–water slurries. Fuel Process Technol 2004;85(4):241–50. [3] Deng JX, Ding ZL, Yuan DL. Erosion wear mechanisms of coal–water–slurry (CWS) ceramic nozzles in industry boilers. Mater Sci Eng A 2006;417:1–7. [4] Pawlik M. Polymeric dispersants for coal–water slurries. Colloids Surf A 2005;266(1–3):82–90. [5] Atesok G, Boylu F, Sirkeci AA, Dincer H. The effect of coal properties on the viscosity of coal–water slurries. Fuel 2002;81(14):1855–8. [6] Zhang L, Lu QY, Xu ZH, Liu QX, Zeng HB. Effect of polycarboxylate ether combtype polymer on viscosity and interfacial properties of kaolinite clay suspensions. J Colloid Interface Sci 2012;378(1):222–31. [7] Zhu JF, Zhang GH, Miao Z, Shang T. Synthesis and performance of a comblike amphoteric polycarboxylate dispersant for coal–water slurry. Colloids Surf A 2012;412:101–7. [8] Zhou MS, Yang DJ, Qiu XQ. Influence of dispersant on bound water content in coal–water slurry and its quantitative determination. Energy Convers Manage 2008;49(11):3063–8. [9] Zhou MS, Kong Q, Pan B, Qiu XQ, Yang DJ, Lou HM. Evaluation of treated black liquor used as dispersant of concentrated coal–water slurry. Fuel 2010;89(3):716–23. [10] Zou LZ, Zhu SQ, Wang XL, Guo XK, Cui GW. Interaction between different CWS dispersants and coal: X adsorptive characteristics of dispersant on coal surface. J Fuel Chem Technol 2006;34(1):10–4. [11] Atesok G, Dincer H, Ozer M, Mütevelliog˘lu A. The effects of dispersants (PSS– NSF) used in coal–water slurries on the grindability of coals of different structures. Fuel 2005;84(7–8):801–8. [12] Kaushal KT, Sibendra KB, Kumaresh CB, Somnath B, Kamlesh KM. Highconcentration coal–water slurry from Indian coals using newly developed additives. Fuel Process Technol 2004;85(1):31–42. [13] Guo ZB, Feng R, Zheng YF, Fu XR. Improvement in properties of coal water slurry by combined use of new additive and ultrasonic irradiation. Ultrason Sonochem 2007;14:583–8. [14] Aktas Z, Woodburn ET. Effect of addition of surface active agent on the viscosity of a high concentration slurry of a low-rank British coal in water. Fuel Process Technol 2000;62:1–15. [15] Liu JZ, Zhao WD, Zhou JH, Cheng J, Zhang GX, Feng YG, et al. An investigation on the rheological and sulfur-retention characteristics of desulfurizing coal– water slurry with calcium-based additives. Fuel Process Technol 2009;90:91–8. [16] Meikap BC, Purohit NK, Mahadevan V. Effect of microwave pretreatment of coal for improvement of rheological characteristics of coal–water slurries. J Colloid Interface Sci 2005;281(1):225–35. [17] Dinçer H, Boylu F, Sirkeci AA, Atesok G. The effect of chemicals on the viscosity and stability of coal water slurries. Int J Miner Process 2003;70:41–51. [18] Boylu F, Atesßok G, Dinçer H. The effect of carboxymethyl cellulose (CMC) on the stability of coal–water slurries. Fuel 2005;84:315–9. [19] Zhou MS, Huang K, Yang DJ, Qiu XQ. Development and evaluation of polycarboxylic acid hyper-dispersant used to prepare high-concentrated coal–water slurry. Powder Technol 2012;229:185–90.