Blended coals for improved coal water slurries Blended coals for improved coal water slurries

Blended coals for improved coal water slurries

  • 期刊名字:中国矿业大学学报(英文版)
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  • 论文作者:GU Tian-ye,WU Guo-guang,LI Qi-
  • 作者单位:School of Chemical Engineering and Technology
  • 更新时间:2020-06-12
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ailableonlineatwww.sciencedirectcomJOURNAL OF CHINA UNIVERSITY OF° Science DirectMININGTECHNOLOGYELSEVIERJ China Univ Mining& Technol 18(2008)00S0-0054www.elsevier.com/locate/jcurBlended coals for improved coal water slurriesGUTian-ye, WU Guo-guang, LI Qi-hui, SUN Zhi-qiang, ZENG Fang,WANG Guang-you, MENG Xian-liangSchool of Chemical Engineering and Technology, China University of Mining Technology, Xuzhou, Jiangsu 221008, ChinaAbstract: Three coal samples of different ranks were used to study the effect of coal blending on the preparation of Coal WaterSlurry(CWS). The results show that by taking advantage of two kinds of coal, the coal concentration in slurry made fromhard-to-pulp coal can be effectively improved and increased by 3%-5% generally DLT coal( DaLiuTa coal mine)is veryslurryability and the stability and rheology of the resulting slurry are not very good. when the amount of easily slurried coalhan 30%, all properties of the CwS improve and the Cws meets the requirements for use as fuel. Coalification, porosity,Ker.e difference &roups, zeta potential and grindability have a great effect on the performance of blended coal CwS.This leadsoxygenic functionaperformance between theslurry made from a single coal and slurry made from blended coalwords: low rank coal; coal water slurry; coal blending; slurryability; zeta potential1 Introductionand can help realize the complementary advantagesof the different coals being blendedCoal Water Slurry(CwS) is a newly developed,Many, including the authors, have investigated howcoal-based clean liquid fuel for replacing oil by coal. CWS of high coal concentration, low viscosity andCwS can be used in many industrial boilers, good fluidity can be produced from hard to slurryindustrial furnaces or the boilers of thermal power coals2-. We have studied the effect of low rank coalplants. Coal accounts for a large proportion of the blending on the property of a cws to address thiscost of CWS and the slurry forming properties of a problem.particular coal also have a great effect on production2 ExperimentalThe medium rank bituminous coal usedoke-making is not abundant in China. Long-flame2.1 Coal samplescoal and non-caking coal account for a largeThree coal samples of different ranks were chosenproportion of the steam coals, so brown coal, low for the experiment: a non-caking coal from Daliutarank bituminous coal and anthracite are used for (DLT), a coking coal from Linhuan (LH)and anpower generation". Both brown coal and long-flame anthracite from Yongcheng(YCH). Table I shows thecoal are low rank, high volatiles and have a low results of coal quality analysis on these coalsignition point. But these coals also have a highmoisture content and poor grindability, which makes 2.2 Infrared spectra of the samplesit hard to form highly concentrated CWS: they are One mg of coal(<300 mesh, dried at 105-110 C)hard to slurry and low in calorific value. Anthracite and 100 mg of potassium bromide were put into anhas low moisture, good grindability and high calorific agate mortar, mixed, ground and pressed into a pellet:value, but a low volatiles content and correspondingly A Fourier transform infrared spectrometer(Nexushigh ignition point make this a difficult coal to burn. 470(from Nicolet, USA)was used to measure theUsing a coal blending method can improve the infrared spectrum of each sample. The wavelengthslurryability of those coals that are hard to slurry. range was 4000-400 cm", the resolutionCoal blending is simple in operation and low in cost cm中国煤化工Received 15 July 2007: accepted 13 December 20n7CNMHGProjects 20060290506 supported by the Doctoral Foundation of the Chinese Educa灬… u VivAce and TechnologyFoundation of China University of Mining TechnologyCorrespondingauthor:Tel:+86-516-83591053:E-mailaddress:ggwu@cumt.edu.cnBlended coals for improved coal water slurrieschar residueHGIalue(ug)YCH 1. 4419297117450DLT145493642808640613900780385762.3 Porosity and pore size distributioof the cws and thCoal is a complicated porous solid. The basicslurryability. The high internal moisture contentstructural unit on a molecular scale is an aromaticindicates that the coal surface readily absorbs waterring or a hydrogenated aromatic ring. Bonds bridge distributed on the inner surfaces of the coal particlesthese units to form three dimensional structures in The existence of this kind of moisture decreases theporespores canclassified as macro-pores, medium sized pores orvolume of water available for suspending the CwSmicro-pores Macro-pores may be micron-sized whilefor a given dry mass concentration, leading to anncrease of apparent viscosity. Furthermore, thethe smaller pores are so small that even the element higher the ash content the lower the viscosity and thehelium cannot enter them. A mercury porosimeter more stable the CWS. This is a physical effect: For a(model 9310 made by the Micrometrics Company,fixed mass concentration of coal in the slurry theUSA)was used to measure the specific surface area,. higher content of ash the higher the density of thethe total pore volume and pore size distribution ofcoal samples.coal and so the lower the volume concentration ofcoal in the CwS. This means the slurry will have2. 4 Zeta potential of the coal surfacegood fluidity. a high grindability index means moreCoal powder was put into deionized waterfine particles are produced during grinding, whichsolutions at different pH values and dispersed for 5increases the packing efficiency and produces a highoncentration cwsmin with an ultrasonic dispersion instrument. Then amicro-electrophoresis apparatus(model JS94G made 3.2 Effect of oxygen functional groups on thein Shanghai) was used to test the zeta potential. ACWSPHS-2 pH meter was used to determine the pHig. 1 shows the infrared spectra of three coal2.5 Preparation of CwSsamplesThe coal powders of different sizes for pre50he Cws samples were prepared in a 3-roller0.454-cylinder rod mill(XMB-70 type). The CwS wasprepared to meet the requirements of the"Alfred035size distribution: The upper size limit was 300 umand more than 75% of the particles were smaller than74 um. Naphthalene sulfuric acid-formaldehydecondensate was used as a dispersant. For furtherdetails on the preparation and test methods seeReference [60052500300025002000150010005003 Results and discussionFig. 1 Fourier transform infrared spectrogram of3.1 Effect of coal quality on CwsThe o/C atomic ratio has a great effects onFunctional group vibration has four regionsbility thehich are(1)the X-H stretching vibration region,poorer the slurryability. The O/C atomic ratio can including o-H, n-H and c-h stretching vibrations,reflect how many oxygen containing functional with a frequency range of 4000-2500 cm";(2)thedouble-bond stretching vibration region, with agroups exist in the coal. Such groups includefrof 2000-1500 cm, including thecarbonyl /C=O, hydroxyl -OH, and carboxyl stret中国煤化工 work vibrations ofCooH. As the coal rank increases the o/c atomiaroratio, and the number of polar functional groups, theCNMHGon: and(4)thedecreases and the slurryability improvesfingerprint region. Aromatic hydrocarbons haveThe higher the moisture contents of the coal the bending vibrations that occur within the range ofJoumal of China University of Mining TechnologyVol 18 No. 1860-700cmmicro-pores. The high rank coals, such as anthracite,Certain groups like-OH, C-O and-COOH are were further compacted by greater internal stresseselectronegative. The surface electronegativity ofthat exceeded the strength of the coalng someoal increases with an increase in such functionalnicro-fractures to be formed. These micro-fracturesgroups/I. The absorption peaks of the hydroxy group are dominated by micro-poresare at 3450 cm and 1260 cm while the carbonyland carboxyl groups appear around 1700 cm. Fromis then affect its slurryability under dispersingig. I we can see that the lower the coal rank, theconditions. The water absorption of a particular cohigher the absorption peak, and the larger the is a major factor that affects the concentration of theabsorption area. These changes indicate an increase in CWS at a given apparent viscosity. The effectivethe number of functional groups, and hence the volume of the macro- and meso-pores is related to theelectronegativity, of the coal. The Daliuta coal is the highest solid concentration of different rank coalslowest rank and it contains more carboxyl and The better developed the pores, the larger the specifichydroxy groups than any other coal. The carboxyl surface area. Therefore, in a wet environment,welland hydroxy groups of the Yongcheng coal are fewestped pores are an important reasonInintermal water content: The large specific surface areadD,. The more polar functional groups on the coal also leads to a high consumption of additivesface the more hydrophilic the coal is. HydrophiMicro-pores have an obvious effect on storage oflicity results in many water molecules absorbing on CWS. A large effective pore volume may acceleratethe coal surface. This forms sturdy hydration films the transfer of a CWS from a soft sediment to a hardand decreases the amount of free water. Polar sediment. This is caused by permeation of thefunctional groups also lead to a re-absorbing of the dispersion medium into the micro-pores. In addition,surfactant molecules. The dispersing agents used are the well developed pores may contain large amountamphiphilic surfactants that have one oleophilic end of air into which water may permeate and result inand one hydrophilic end. The coal surface absorbs the bulging, drying and rapid aging of the cwS. Thisil-based end while the other, hydrophilic end tends makes the preparation, storage and transportation ofto be absorbed by the water. This causes a hydration the CwS much more difficultfilm to form and plays a role in dispersing the solids 3.4 Surface zeta potential and iso electric pointand reducing the viscosity of the slurryof different coal samples3.3 Porosity of the coal samplesFig. 2 shows the relation between zeta potentialTable 2 shows thebore parameters of three and pH value of three samples measured with theTable 2 Pore parameters of three coal samplesYCHTotal pore volume(mLg) 0.0226Inner surface area(m/g) 5.4590017300105In Table 2 we see that the DLt coal has the largesttotal pore volume of 0.0637 mL/g while the LH coalhas the smallest, 0.0200 mL/g. The internal specificsurface area has the same trend as the total poreFig 2 Zeta-potentials of coal samples at different PHvolume: 5.46 m /g for the YCH coal, 4.96 m/g for theLH coal, and 12.00 migfor the DLT coal. This showsFrom Fig. 2 we can see that when the zeta potentialthat the total pore volume and intemal specific is zero the ph value is at the iso-electric point.surface area tend to decrease as the coal rank1)In deionized water the surface potentialincreases. The DLT coal contains many macro-pores decreases to a certain extent as the pH increasesas well as meso-pores but fewer micro-pores. The LH Finer coal particles have a smaller surface potentialcoal is the middle rank coal and is compact in. and a lower pH value at the iso-electric point. This isstructure. The pores of low rank coals were formed because the relative surface area of small sizedmainly from colloidal pores that were compacted less particl中国煤化工 ed ones, makingduring the coal-forming process: this formed it easiemacro-pores. The middle rank coals suffer a furtherCNMHGe three samplescompaction during the coal forming process making increase with increasing coal rank indicating that forthe molecular structure more dense and forming low rank coals more H" ions are needed to neutralizeGU Tian-ye et alBlended coals for improved coal water slurriesthe negative electric surface charges. In other words, DLT coal has poor stability and poor rheologicala decrease in coal rank results in an increase in the properties and is difficult to slurnumber of negatively charged functional groups. The of only 57. 5%so-electric point of the DLt coal is the lowest,Table 3 Properties ofepared fromshowing a high content of oxygen functional groupssuch as carboxyl and hydroxyl groups, which is inConcentrationStability(%)(d) concentration(%)good accordance with the results of the infraredl402spectral analysisThe zeta potential can reflect the hydrophilicity ofmodified coal surfaces. The hydrophilicity directlyaffects the apparent viscosity, the rheologicalproperties and the static stability of the CwS. Given a70.62fixed dispersant concentration, a high zeta potential699favorable for decreasing the viscosity of a cws and68.52improving its rheological properties. A low zetapotential favors stability 013.5 Properties of cws prepared from differen5727DLTcoal samples619The properties of various CWS samples are shownin Table 3(Note that the viscosity was measured at a 3.6 Properties of cwS prepared from DLT coaltemperature of 25C and a shear rate of 100 sIn Table 3 we see that Lh coal. which is used asblended with other coalssteam coal, forms a Cws with the best propertiesDLT coal is difficult to slurry Based on the variousAlthough YCH coal is anthracite that has a high heat properties of the individual coals, mixed coalvalue, its burning property is relatively poor. The data powders containing DLT and added LH or YCH coalin Table 3 were used to estimate the coal were prepared: The LH and YCH coals comprisedconcentration required to make a Cws with a 10%, 20%, 30% or 40% of the coal weight. Theviscosity of 1000 mPa S. Both YCH and LH coals are absolute values of the zeta potentials of both LH coalpredicted to form slurries containing 68%, or more, of and YCH coal are smaller than that of the DLt coalcoal under these conditions. The viscosity of both The finer the particles are, the lower the ph of thethese slurries will drop rapidly as they are diluted. iso-electric point. In addition, DLT coal is moreTherefore, assuming this is an allowable concentra- porous than the other coals. Therefore, the finer parttion, these two coals can be used to produce low of the LH and dLt coals was used to form testviscosity, stable slurries having both good rheological mixtures(Fig 4).and combustion properties. The Cws prepared from→DLTDLT→DLTYCH: 10%++ -YCH: 30%-H- YCH+LH: 30%一YCH:40%LH:40%1500SYCH+LH: 40%608010012002040608010012080100120Shear rate(shear rate(6)(a)CwS with blending coals between dLt (b)CWS with blending coals between dLt (c)CwS with blending coals betwcoal and different content of YCh coalcoal and different content of LH coaland mixture at different eYCH coalFig. 4 Effect of blending coals at different ratios on CwSgical propertiesIn Fig. 4 we can see that both Lh coal and YCH resulting Cws is greatly improved: its concentrationcoal are easy to slurry. So, by adding some fine may be 60% coal or higher. The coal concentration inpowder of these two coals to DLt slurry the coal a CWerv_important index. High concentraconcentration can be raised. The pure DLt coal tion中国煤化工 tion as well asmakes a low concentration slurry, only 57. 27% for a oxygeviscosity of 941 mPa.s. when the amount of lh coal savinCNMHGcontant for energyand YCH coal is 30%, or more, the property of theJoumal of China University of Mining Technologyol. 18 No. 14 ConclusionsReferences1)The intermal water content, the Hardgrove [1] LiHX, Ji MJ, Li J, et al. The effects of coal blendinggrindability index(HGi) and the ash has a great effecton the features of C. W.M. Coal Preparation Technology.slurryability of coal. Coal blending may effec01(3):5457.( n Chinese)tively reduce the concentration of oxygen functional [2] Sun CG, Wu JS, Li B Q Surface properties of thermallygroups and enhance the absorbing ability of the coalupgraded low-rank coals and their effect on thesurface for anionic additives this enhances therheological behavior of coal water slurry. JourmalFuel Chemistry and Technology, 1996, 24(2 ): 174-180.The zeta potential of a coal surface isIn chineserelated to coal rank and particle size. Using a [3] Wu GG Guo Z B Experimental redispersant that can increase the absolute value of thef factors influencing CWs prepJournal ofzeta potential gives a well dispersed, low viscosityChina University of Mining TechCWS. The addition of coals having different pore (4 wu G G Wang X C, Liu J T. Research on the ability ofproperties can effectively enhance slurryabilitynaking slurry with NSF as additive for CwS and2)The blending of coals with different propertieseffect. Journal of China University of Mining &ehas a great effect on the resulting CwS. The additionTechnology, 2005, 34(6): 703-706(In Chinese)of high rank coal can improve the slurryability of the [5] Wu GG Wang X C, Li Q H, et al. Study of thecoal which is difficult to slurry, making the viscositymatching performance between additive of coal waterdecrease significantly, the concentration increase byslurry and coal molecular structure. Journal of China3%0-5%, and the stability much improved. The slurryUniversity of Mining& Technology, 2006, 35(4): 454-457(n Chinesecan meet the concentration requirements for fuel [6] Zhang Z. Technique of Preparing Water Coal SlurryCwS when the addition of high rank coal reachesBeijing: Science Press, 1996. (In Chinese30% or more, which is very important for reducing[7 Xie K C. Structure and Reactivity of Coal. Beijing:energy consumption as well as oxygen consumptionScience Press, 2002. (In Chinese)[8] Wang B J, Li M, Zhao Q Y, et al. Relationship betweensurface potential and functional groups of coals. JoumalAcknowledgementsof Chemical Industry and Engineering, 2004, 55(8):1329-1334.( n Chinese)Financial support for this work from the Doctoral[9] Zhong Y Y, Guan M B, Cui KR, et al. Coal ChemistryFoundation of the Chinese Education Ministry(NoXuzhou: China University of Mining Technology20060290506 and the Science and TechnologyPress, 1995.(In Chinese)Foundation of China University of Mining[10] Liu H Y, Zhu S Q, Li X H, et al. Study on relationshipof coal water mixture and dynamicTechnology(No. 2005B013)are gratefully acknowleparticle surface. Coal Science and中国煤化工CNMHG

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