Rheological behavior of Shengli coal-solvent slurry at low-temperatures and atmospheric pressure

Availableonfineatwww.sciencedirect.comMININGScienceDirectSCIENCE ANDTECHNOLOGYMining Science and Technology 19(2009)0779-0783www.elsevier.com/locate/jcumtRheological behavior of Shengli coal-solvent slurry atlow-temperatures and atmospheric pressureWANG Yong-gang, YAN Yan, GUO Xiang-kun, Xu De-pingSchool of chemical and Environmental Engineering, China University of Mining Technology, Beijing 100083, ChinaAbstract: We report the results from systematic studies of Shengli lignite coal-solvent slurries Solvent type, temperature, coal tosolvent ratio, particle granularity, shear rate and shear time were investigated. The viscosity of the solvents is time independent.However, the slurries are thixotropic. A change from pseudo-plastic to Newtonian behavior occurs as the temperature, or as thesolvent to coal ratio, increases. The solvent used in the slurry affects the point at which the rheology changes from pseudo-plastic toNewtonian. The REC slurry changes at 1: 1. 2 coal to solvent ratio and at 40C. the har slurry changes at a 1: 1.5 ratio and at 60oC. The rheology of the slurries is pseudo-plastic at low shear rates but Newtonian at high shear rates.Keywords: direct liquefaction; coal-solvent slurry; rheological behaviors; Shengli lignite1 Introductionmoted the extractive disintegration of coal and theviscosity was higher than that seen in a nitrogen at-Coal solvent slurry(CSS), a poly-heterophase dis- mosphere. Furthermore, the viscosity behavior of coalpersed system consisting of fine coal particles, min- paste from low rank coals was the same as that of theeral components and organic solvents in the free and vehicle oil alone29-2) Slurry prepared from Shenhuabound state, is the most common feedstock for direct coal and two solvents, Tetralin and a hydrogenatedliquefaction( CDL) pilot scale plants". For transport, recycled solvent, is a Newtonian fluid at low tem-slurries may be characterized by one parameter, peratures and concentrations, such as 25C and a 1: 2namely their rheological property, despite their di- ratio!. But these are non- Newtonian fluids at relaverse compositions and physical properties. A number tively high temperatures(50C), even at low concenof studies conducted since the late 1970s have fo- trations (1: 1.5). Furthermore, the rheological charac-cused on the rheological properties required to obtain ter of the slurries can be described using a poweracceptable fluidity while at the same time maintain- equation. Elegant work on CSS systems made froming sufficient stability against coal-particle sedimen- Shangwan coal and two solvents, one a hydrogenatedtation2-1. Some critical reviews have also been pub- recycled solvent and the other Tetralin, indicated thatsolvent property is the main influencing factor at lowSoya Koishi coal oil slurry and Shin-Yubari coal temperature; the apparent viscosity decreases withpaste behaved differently over the temperature range increasing temperature[9-10. Solvent properties andfrom 200 to 450C. Maximum viscosity occurred at swelling behavior together affect the viscosity of295C, under a pressure of 29. 4 MPa in a de-crys. slurries at medium temperatures; in this range thetallized anthracene oil according to results of Mer- apparent viscosity is invariable. Swelling behaviorresults(6-7.l6-iuent investigations provided analogous becomes the main factor at high temperatures;nowritt)stThe viscosity of bituminite of Pitts- the viscosity increases with increasing temperatureburgh No 8 seam coal paste in number 6 oil increased As lignite, Shenli coal will be a crucial potentialwith increasing coal concentration and decreased with solid feedstock for DCL. The rheological behavior ofIncreasing temperature(l3)he paste from high rank a CSS prepared from Shengli coal and solvent re-bituminous coals and vehicle oil exhibit two viscosity mains uncertain. So, considering the critical rolepeaks near 300C during liquefaction. Hydrogena- played by the slurry in DCL, this deserves more attion with high pressure hydrogen and a catalyst pro- tenti中国煤化工CNMHGProject 2004CB217601 supported by the National Basic Research Program of ChinaCorrespondingauthorTel+86-10-62339882;E-mailaddresswygl960@l26.comMining Science and TechnologyVol 19 No 6The influence of various factors on the rheological through the origin; and if n>I the fluid is dilatant,orbehavior of CSS made from Shengli lignite is de- shear-thickening, and its apparent viscosity increasesscribed in this paper. The investigation was done at with shear rate. 2)when 4+0; if ol the fluid is a dilatantquefaction processesYield pseudo plastic fluid2 ExperimentalDIlatant plastic fluidseudo plastic fluid2.1 MaterialsThe coal used in this work was lignite procuredfrom the Shengli coalfield. The coal sample was pul-verized and sieved to-100 mesh and-200 mesh andwas then dried in a vacuum oven at 108C for 24 hhe proximate and ultimate analyses of the coal sam-ples are listed in Table 1Fig. 1 Types of flow behaviorThree typical hydrogen donor solvents were usedhydrogenated recycle solvent obtained from a dire3 Results and discussionliquefaction pilot plant(REC, Beijing Research In-stitute of Coal Chemistry); quartic-hydrogenated 3.1 Influence of solventsproducts of heavy oil from the Anqing OilfieldThree typical solvents, REC, HAR and THN, were(HAR); and tetralin (Thn, AR)employed in this work. Their rheological behaviorTable1 Proximate and ultimate analyses of coal samples(%) was investigated separately. The results are shown inProximate analysisUltimate analysis…… HAR downwardTHN upwarShengli coal19.8914145.7371.664750911.27-r REC upwardD+.tHN downward2. 2 MethodsCSS samples were prepared by mechanicallyblending lignite of different particle sizes and weightswith the three solvents. Rheological behavior wasinvestigated by a rotary viscometer(NXS-11A) with02004006008001000120015 rates and a water bath. Different temperatures,shearing rates and shearing times were investigatedFig 2 Flow behaviors of solvents at 30C2.3 Determination of fluid typeFig. 2 indicates that all three curves are straighThe relationship between shear stress and shear lines passing through the origin without any hysterrate can be described assis. Thus, all the three solvents are time-independentNewtonian fluids at 30C. The slopes of the threecurves fall in the sequence HAR>REC>THN and arewhere sy is the yield stress(mPa s) that determines 0.0505, 0.0171 and 0.002. This shows that the viscos-the viscidity and flexibility of the fluid; a is the con- ity of these three solvents falls in the order HAr>sistency coefficient(mPas)that indicates the viscous RECSTHNcharacter of the fluid. Viscosity increases with an inThe influence of solvent on the rheological behav-crease in a. The factor n is dimensionless and ac- ior of cSS for a coal/solvent ratio of 1: 1.2 is discounts for the flow behavior of the liquid. If n is ap- played in Fig 3proximately I the fluid is a Newtonian liquid. OtherFig. 3 illustrates that the curves for these threewise the fluid is non-Newtoniantypes of CSS pass through the origin. All fluids beThe flow of various types of fluid is illustrated in have as pseudo-plastic fluids and the suspension inIgTHN further exhibits a non-Newtonian character. AllFig. I shows that 1)when (y=0; if oRECstant viscosity, the curve is a straight line passing which indicates that thixotropy ot the THN suspenWANG Yong-gang et aRheological behavior of Shengli coal-solvent slurry atsion is the most pronouncedthe temperature range from 30 to 70C. The resultsare illustrated in Fig. 4.r… REC downward→ REC upwardThe area of the hysteresis loops of both suspee.HAR downward - THN upwardsions increases with temperature. The hysteresis looHAR upwardmay result from accelerated Brownian motion and thesedimentation rate of the coal particles caused bytemperature, rather than from shear thixotropyRheological equations of state for two suspensionsat different temperatures are listed in Table 2. In this20040060080010001200table it may be seen that the flow type of the twosuspensions changes from pseudo-plastic to NewtoFig 3 Effect of solvents on rheological behaviornian as the temperature changes. The REC suspensionof CSs at 30.Cchanges at 40C, while the HAR suspension does soat 60 oC. Furthermore, the value of a for the har3.2 Influence of temperaturesuspension is larger than the one for the REC suspenThe rheological behaviors of two kinds of slurries. sion at the same conditions, which indicates that theone prepared from HAR and the other from rEC. viscosity of the former is the greater onewere investigated at a coal/solvent ratio of 1: 1. 2 over30℃ upward…50℃ downward…30℃ downward(a)REC(b)HARFig. 4 Effect of temperature on rheological behavior of CssTable 2 Rheological equations of CSS at different temperaturesREC suspensiony=0.189xy=0.153xF=0.086xY=0.059xHAR suspensionY=1601x°8636y=0.272xF=0.176x3.3 Influence of coal/solvent ratioREC and HAR were examined at 50C. The resultsThe rheological characteristics of two pastes of are shown in Fig. 5.1: I upward1: 1. 2 upward5d25-t1: 1. 1 upward毒”1:1. I downward豳++1: 1.3 downward00250300350Shear rate(s")(a)REC suspension(b)HAR suspensionFig 5 Effects of coal solvent ratio on rheological behaviorThe graphs present data indicating that the area of great中国煤化工the hysteresis loop increases with the ratio of coal toCN MH Regression forsolvent. The thixotropy thus increases accordingly. twovanous ramos of coal to solventAlso note that thixotropism of the HAR suspension is are listed in Table 3Mining Science and TechnologyTable 3 Rheological equations of coal slurry at different coal to solvent ratios1:1.21:1,3REC suspensiony0.556x°M87y=0.153xy=0087xF=0.052xy=3.140x°8y1.731x2y0.963xMy0647x98F=0.167xFrom these equations it appears that a decreases as were used for the investigation of granularity onthe coal to solvent ratio decreases. At the same time n rheological properties. The runs were conducted at 30increases. This means that the viscosity of the CSs C with a coal to solvent ratio of 1: 1. 2. The resultsdrops and the fluid becomes more Newtonian like as are shown in Fig. 6 and Table 4the relative amount of solvent increases. The rheol-As shown in Fig. 6, the slurries have pseudo-plasogy changes from pseudo-plastic to Newtonian at a tic characteristics. The value of a change in the se-1: 1. 2 ratio for the REC slurry. The same thing hap- quence:-200 mesh >-100 mesh. This means thepens at a 1: 1.5 ratio with HAR as the solvent, The viscosity of the-200 mesh slurry is higher. The hys-value of a for HAR is larger than that for REC, at teresis-loop area of the -100 mesh slurry is largerotherwise identical conditionsthan that of the-200 mesh slurry. Also note that the3.4 Influence of granularitydifference between -100 mesh and -200 mesh HARsuspensions is larger than that seen when REC is theTwo coal particle sizes, -100 mesh and-200 mesh, solvent00 upward200 downwardDo downward80100120140160Shear rate(s-")(a)REC slurryb)HAR slurryFig. 6 Effect of coal granularity on rheolThe implication is that coarse particle in suspen-sion have a lower apparent viscosity, and greater fluidity, because there is more free space in the slurryBut these suspensions are unstable due to the largesubsidence velocity of the particles caused by theirlarge weight. Pastes made from fine particles, in the10same concentration, exhibit poor fluidity and higherapparent viscosity but good stability. These properties20040060080010001200result from good dispersion of, and strong interactionbetween, the smaller particlesFig. 7 Effect of shear rate on rheological behaviorTable 4 Rheological equations for suspensions ofdifferent sized coal particlesA CSS is composed of various sized coal particlesParticle sizeand a solvent, which are blended mechanically. theREC systemY0479x09framework between solid and liquid phases that isHAR system=2025x0sestablished by blending is difficult to destroy. At lowshear rates the structure must be broken up. This3.5 Influence of shear ratethe initial increascreasing shear rate. As the shear rate continues to in-The influence of shear rate on rheological behavior crease the particle arrangement approaches a stablewas also investigated. The results are illustrated in limit and the suspension becomes Newtonian, with aFig. 7constThe paste from which the data shown in Fig. 7 are中国煤化工derived can be considered a pseudo-plastic fluid at 3.6low shear rates. But the paste has Newtonian charac-CNMHGnear ume on rheological behav-teristics at higher shear ratesior was studied. The results are illustrated in Fig 8.WANG Yong-gang et alRheological behavior of Shengli coal-solvent slumy at300( - BAR 50),S 50T*-HAR[3] Adiga K C, Shah D O. Rheology and stability of coal-oiand coal-oil-alcohol dispersions. Colloids and Surfaces,982,4(3):255-269[4] Cohen A, Richon D. Rheological properties of coalpowder+solvent+nitrogen to 653 K for two differentcoals.Fve,l986,65(1):117-12150a[5] Sakaki T, Shibata M, Hirosue H. Effect of coal rank onrheological behavior of coal-solvent slurries during15heating. Energy Fuels, 1995, 9(2): 314-318.Shear rate(")[6] Han W Y, Chang H Y, Zhang D X, Gao J S PreliminaryFig8 Effect of shear time on rheological behaviorresearch on viscosity-temperature characteristic of coal-oil slurry. Coal Conversion, 2003, 26(4): 51-54.(InChineseThe results show that the viscosity of the suspen- [7] Hao L F, Wang Y G Sun x Y, Xiong C A. Studyions drops over time. The main reason for this is thatthe shearing action destroys the framework in theatmosphere. Coal Comversion, 2006, 29(2): 28-31.(InCSS: this needs some time. It is obvious that the vis- [8 Wang Y G Hao L F, Xiong C A, Sun X Y. Effects ofcosity declines at longer shearing times. Furthermore,main parameters on rheological properties of oil-coalthe viscosity drops faster when HAR is the solventSIUmcompared to REC as the solvent. This confirms the7 echnology,2006,16(3):274277conclusions about the order of thixotropy: HAR sus- [9 Hao L F, Wang Y G Xiong C.A. Effect of particle sizepensions >REC suspensionstion onrheology of oil-coal slurry. Journal ofChina Coal Society, 2007, 32(2): 190-193. (In Chinese)[10 Wang Y G Hao L F, Xiong C A, Xu D P, Chen W.4 ConclusionsApparent viscosity changes of coal-oil slurry during1)At normal temperatures, the three solvents HAR,Technology, 2007, 35(5): 513-517.(In ChineseREC and THn are time-independent fluids. 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