Investigation of influence of coal properties on dense-phase pneumatic conveying at high pressure

Particuology102012)310316Contents lists available at sci verse Science DirectARTCLOLO,ParticuoleELSEVIERjournalhomepagewww.elseviercom/locate/particInvestigation of influence of coal properties on dense-phase pneumaticconveying at high pressureCai liang,, Xiaoxu Xie,, Pan Xu, Xiaoping Chen. Changsui Zhao, Xin WuaNanjing Department of Environmental Protection Branch CHEC. Nanying 210013, ChinaARTICLE IN FOA BSTRACTArtcle hExperiments of dense-phase pneumatic conveying of pulverized coal using nitrogen were carried out inaceived May 2009test facility at pressures of up to 3.7 MPa to study the effects of coal type, particle size and moisture contentReceived in revised form 27 August 2009Accepted 20 January 2012on flow characteristics. The jenike shear test and scanning electron microscopy(SEM)were employed toprovide a better understanding of effects of the material properties on flow characteristics. Two kinds ofpulverized coals, Yanzhou and Datong, with similar particle size, moisture content and density, were usedneumatic conveyingin the test pressure drop increases with increasing the particle size at similar solid-gas ratio, superficiale-phasevelocity and pressure in the receiving hopper, and pressure drops through different test sections decreasegh pressurefirstly and then rise with increasing the conveying velocity for the same particle size, mass flow rate andpressure in the receiving hopper. The flowability of pulverized coal decreases with increasing the moistureFlow characteristicscontent in the range from 3. 24% to 8. 18% Unconfined yield strength(UYS)increases and flow function(FF)decreases with increasing the moisture content. Results of the shearing tests are consistent with theresults of the conveying study Pressure drops through different test sections are discussed and analyzed.O 1 Chinese Society of Particuology and Institute of Process Engineering Chinese AcademySciences. Published by Elsevier B V. All rights reserv1. Introduction(2008)used high-speed video cameras and pressure transmittersto study the dynamic behavior of the particles and their influencePneumatic conveying is widely used in food, chemical, metallur- on the pressure drop during conveying in a 0.052 m I.D. 35 m longical and other industries. At present, the dense-phase pneumatic aluminum pipe For soft and hard polyethylene pellets, video anal-conveying of pulverized coal at high pressure is one of the key ysis revealed that the soft particles bounced more intensely duringtechnologies for large-scale coal gasification system. Because of conveying than the hard particles pahk and Klinzing(2008)usedhe low velocity and high solid concentration during the trans- two types of plastic pellets to distinguish their flow characteristicsport, the gas-solid two-phase flow of pulverized coal is often in dilute-phase conveying, showing that the total pressure dropunsteady and very complex. In order to master the flow mech for polyolefin pellets is higher than that for polystyrene pellets.anism of dense-phase conveying at high pressure, experimental Li and Tomita(2000)and Li(2002)measured the particle velocityand theoretical studies are needed to unravel the conveying char- and concentration in horizontal and vertical dilute swirling flowacteristics, particularly the influence of coal properties on flow pneumatic conveying at low pressure. Laouar and Molodtsof 1998haracteristicsstudied the pressure drop characteristic at a very low velocity, andRecent works on pneumatic conveying have achieved many a general pressure-drop law was obtained which was proved to bevaluable results in powder technology and gas-solid two-phase independent of both flow regimes and pipe diameterflow field. Hyder, Bradley, Reed, and Hettiaratchi (2000)investi- Particle behavior for dense-phase pneumatic conveying atgated the pneumatic conveying of five kinds of plastic materials high pressure is highly affected by material properties, operatingat low pressures. Results showed that the pressure drop increases parameters, conveying gas, etc However few works are available.with increasing the particle size but decreases with increasing the Research into the relationship between the operating parameterstransport velocity. Vasquez, Jacob, Cocco, Dhodapkar, and Klinzing and thhwork中国煤化工enPu,Lu,8an200PuCorresponding author Tel +862583795652: fax: +8625 83795652的:口 C MH Garticle size and moisturecontent on the flow characteristics674-2001s-see front matter o 2011 Chinese Sodety of Particuology and Institute of Process Engineering Chinese Academy of Sdence Published by Elsevier B.V. Al rights eserveddoi:10016 partic.201201.003C Liang et al/ Particuology 10(2012)310-316Nomenclaturedparticle size,μmow functionass flow rate of pulverized coal, kg/hMcs major consolidation stress, kgMPexternal moisture content of coal,pressure in the feeding hopper. MParessure in the receiving hopper, MPathe total conveying differential pressure, MPaAPm pressure drop through the test section, kPaAPh pressure drop through horizontal pipe, kPaFluidizing gas△Pressure drop through the horizontal bend, kPaAP pressure drop through the vertical pipe, kPaAPyb pressure drop through the vertical bend kPaFig 2 Diagram of the feeding hopperUYs unconfined yield strength, kgsuperficial velocity, m/sOffsetwaiting topGreek lettersMoldsolid-gas ratio, kg/mFig 3. Pre-consolidation of jenike shear cellpipeline was a smooth stainless steel tube with the length of about45 m. The inner diameter of conveying pipeline was 10 mm. Thelength of each test section in the horizontal pipe and the verFluidizing gastical pipe for measuring the pressure drop was 100 cm. the 900bends with the radii of 20 cm were about 63 cm in length. The gasvolume rates were measured with the metal tube variable-areaflow meter, and the fluctuation of the solid mass flow rate wasobtained with the weigh cells. Pressure and pressure drop weremeasured with the semiconductor pressure transducers. The sig-nals of pressure drop, pressure, weight and so on were obtainedwith a multi-channel sampling system and then were sent to acomputer through an A/d converter. The conveying gas used wasFlg 1. Schematic diagram of dense-phase pneumatic conveying of pulverized coal N2 with the maximum pressure of 4.0MPa.at high pressure.22. The Jenike shear cell2. Experimental systema Jenike shear cell was applied to measure the flow properties2. 1. Experimental facility for dense-phase pneumatic conveyingof the powders. The jenike shear cell unit is comprised of a base, aat high pressurering, a mould ring a twisting top and a cover, as shown in Fig 3The testing procedures delineated by jenike and ASTM standardsThe pressurized experimental facility is schematically shown were followed in this study Pulverized coal, pretreated accordingin Fig. 1, High pressure N2 from the buffer tank was divided into to jenike test procedures, was placed in the shear cell Consolidationpressurizing gas, fluidizing gas and supplemental gas. The feeding weights (W)were placed on the weight hanger and a horizontalrerized coal in the feeding hopper was fluidized by the fluidizing horizontal shear force was stopped once a steady state had beengas and entered the conveying pipeline through the accelerating reached The shear force(S)was recorded with a strip chart readersection. The supplemental gas was then imported to enhance the attachnveying ability of the gas at the outlet of the feeding hopper In thenp中国煤化工order to adjust the moisture content of the pulverized coal, water and fCN MHGas recorded In this study.was injected into the pulverized coal in the conveying pipeline three leveis or the cohsoaanion(laDle 1)were performed on eachhrough a measuring pump. The pressure in the receiving hopper coal sample. Three different shear weights were used for each levelwas controlled by the control valve. Both the feeding hopper and of the consolidation to achieve three well spaced points on the flowthe receiving hopper had a capacity of 0.648 m The conveying function curve Triplicate measurements were conducted for eachC Lang et aL/ Particuology 10(2012)310-316w or WiYanzhouSorof shearP:=3.6 MPa1000M+2.0%Fig. 4. Consolidation and shear of jenike shear cell.Table 1△P(MPa)Consolidation levelWi(kgW2(kg) W3(kg)14Fig. 5. Variation of the mass flow rates of the two coals with increasing the totalpressure drop.test. then the obtained values of the flow function the cohesive jenike 's classification of powder flowability by flow function.stress, etc were averaged for reporting.Classification3. Results and discussionNo flow110ee flowTwo kinds of soft coals, Yanzhou and Datong, were used to studythe effect of coal type on flow characteristics in the dense-phasepneumatic conveying. Both coals were pulverized to similar particle size. The pressure in the feed hopper( P1)was set to 3.6MPa▲ Yanzhouwhile the pressure in the receiving hopper(p2)was changed for50·Dagadjusting the total conveying differential pressure(AP). Fig. 5shows that the mass flow rates(G)of both coals rise with increasingthe total conveying pressure drop.Fig 5 also indicates that the mass flow rate of Yanzhou coal is出40generally higher than that of Datong coal when the total conveyingessure drop higher than 0.5 MPa. In order to study these differences, the shearing test and scanning electron microscopy(SEMults were resorted to analyze the flow characteristics. Jenike'sflow function( FF)in Table 2 is used to characterize the flowability ofthe materials, FFis the slope of the plot of unconfined yield strength(UYS)versus the major consolidation stress(MCS )that is obtained30405060when the material is sheared to failure in a shear cell. As shown inMCS (kg)Fig 6, for the two soft coals, the line for Datong coal is lower thanthat for Yanzhou coal implying that Datong coal is more difficult toFig. 6. Variation of the FF of the two coals with increasing the McS.(a)Yanzhou coalDatong coalY中国煤化工CNMHGFig.7.Morphographysoftwosoft coals by SEM.C Liang et aL Particuology 10(2012)310-316and the pulverized coal is conveyed homogeneously at highermean particle size:superficial velocity. As superficial velocity decreases, the particle52concentration increases and the pressure drop rises. when theincrement of the pressure drop caused by the solid-phase equalsthe decrement of the pressure drop caused by the gas-phase, theP1=36Mpressure drop reaches a minimum, which is called the economi-cal velocity Near the economical velocity, a suspended phase anda settled layer of pulverized coal were frequently observed. As theconveying velocity is greater than the economical velocity, a flow istypically described as suspended flow. When the conveying veloc-ity is lower than the economical velocity, dunes or clusters can beseen riding on a settled layer of pulverized coal. A further reductionin the gas velocity would lead to a region typically characterized byunstable flow. At even lower gas velocities the material may flowFlg. &. Effect of particle size on mass flow rate.3.3. Effect of moisture content on flow characteristicsflow than Yanzhou coal. In addition, SEM observation indicates thatThe moisture content of a powder strongly affects its frictionalthe surface of Yanzhou coal is compact while that of Datong coal property, flowability, dispersibility and so on. Surface tension ofis soft and more pliable, as shown in Fig. 7. It is generally regarded water causes the traction force between two adjoining particlesthat soft particles tend to bounce more intensely during conveying and turns the agglomeration of small particles into larger particlesthan hard particles(Pahk Klinzing, 2008: Vasquez et al, 2008). Both result in the change of the friction coefficient. However, effectThe soft particles show highly random and intense bouncing with of the internal water on the flow characteristics in pneumatic con-rotation, which considerably affects the rebounce. In fact, veying at high pressure is less evident. In this paper, all the moistureparticles bounce even backwards. The shearing test and SEM content mentioned stand for the external water(M).demonstrate that Yanzhou coal is easier to be conveyed thanThe pulverized coal has a mean particle size of 56 um and a den-Datong coal at high pressure.sity of 1400 kg/m. Effect of the moisture content on the conveyingcharacteristics of pulverized coals is presented in Figs. 10 and 113. 2. Efect of particle size on flow characteristicsFig. 10(a)and(c)shows that, for the coal with a moisture content of3. 24%, the pressure drops through different test sections are essen-The coal samples used for the comparison test were from the tially stable, while for the coal with a moisture content of 8. 18%,ame parent coal but with different particle sizes. Fig 8 shows that the pressure drops through different test sections present signifi-the mass flow rate of the pulverized coal with a larger size is lower, cant fluctuations and liquid bridging of coal particles leading to theindicating that more energy is required to convey particles of larger granulation as shown in Fig. 10(b)and(d). Fig. 11 shows that withsize under similar conveying conditions.ncreasing the moisture content, the mass flow rate decreases. SimFig 9 presents the pressure drops through the vertical pipe and ilar results were found for other coals (Liang, Zhao, Chen, Pu, Luthe vertical bend varying with superficial velocity and particle size 2007 ). When the flowability of pulverized coal deteriorates due toat the same mass flow rate and pressure in the receiving hopper the increased moisture content, more energy is needed for pneu-G=1000kg/h and P2-26MPa). The pressure drops for the larger matic conveying. Therefore, as the moisture content increased inparticles through the two test sections are greater than those for the range from 3. 24% to 8. 18%, the flow of pulverized coal becamethe smaller particles at the same superficial velocity. Since the iner- more difficult.tia for the larger particles is higher, the wall collision of such larger Fig. 12 shows that the pressure drop through horizontal testparticles is associated with higher deformation and energy dissipa- decreases with the increasing moisture content. Fig. 11 alreadytion(Sommerfeld, 2003). Fig 9 further shows that with increasing showed that as the moisture content in pulverized coal increased.the superficial velocity from 3 to 14 m/s, the pressure drop firstly the mass flow rate of the pulverized coal decreased, causing thedecreases and then increases for the particle of each size. Such a pressure drop to decrease. Fig. 13 shows the effect of the mois-phenomenon can be explained as follows. The flow is quite diluteture content on the pressure drop at similar superficial velocity20中国煤化工30CNMHG9. Effect of particle size on pressure drop(P2-2.6 MPa, G=1000 kg/h).314C Liang et al Particuology 10(2012)310-31612(a)Pressure drop at M=3.24%(b)Pressure drop at M=8.18%(c)Morphography of coal with M=3.24%(d) Morphography of coal with M=8. 18%Fig. 10. Influence of moisture content on flow characteristics of a powdand solid-gas ratio (u)(the solid-gas ratio is defined as the ratio energy loss of friction and collision. Therefore, the pressure dropof coal mass flow rate to gas volume flow rates. The unit of the through the vertical pipe(APv)is greater than that through thesolid-gas ratio is kg/m ). The pressure in the receiving hopper is horizontal pipe (APn). Similarly, the pressure drop through the ver-.8MPa. In Fig. 13, it can be seen that pressure drops through all tical bend (AVb)is greater than that through the horizontal bendthe test sections rise with increasing the moisture content. This (APb. fig. 13 shows that the pressure drop through the verticalindicates that coal with higher moisture content has more cohe- bend is greater than that through the vertical pipe. the conveyedre strength and is more difficult to flow through the conveying mixtures must change their direction when they pass through thesystem. The friction coefficient of the two-phase flow rises and the bend. The particle inertia would cause a separation of the mixture.granulation phenomena results in higher deformation and energy whereby rather dense layers of solids may be formed in the verticaldissipation. Therefore, the pressure drop rises with increasing the bend. The increased local solid concentration supports the occur-moisture content at similar solid-gas ratio, superficial velocity and rence of the inter-particle and wall collisions(Yilmaz Levy, 2001 )pressure in the receiving hopper. The conveying gas in the verti- Hence, the collisions between the particles may cause a destructioncal pipe must overcome the gravity of the mixtures besides the of dense layers and also would have a drastic effect on the particleP:=3. 6 MPaP2.8 MPa中国煤化工CNMHGFig 11. Influence of moisture content on mass flow rateFig 12. Effect of moisture content on pressure drop(p1-36MPa. P2-2.8 MPa)C Liang et al/ Particuology 10(2012)310-3161210Pr. 8 MPa340kg/m34M(%)Fig- 13. Effect of moisture content on pressure drop at similar superficial velocitynd solid-gas ratio.Fig. 15. Effect of moisture content and major consolidation stress on FF at variousmoisture contents.ransport and the wall collision frequency in such two-phase sys- content. Using the classification of Jenike given in Table 2, thetems. The additional pressure dropthrough the bend is greater than results indicate that the flowability of pulverized coal reduces asto the straight pipe is 0.63, the pressure drop through the verti- Fitzpatrick(1999,2000)ald similar results when they com-cal bend is greater than that through the vertical pipe. Similarly, pared the flow functions of conveying flour, tea, and wheythe pressure drop through the horizontal bend is greater than thatThe above results demonstrated that the flowablity of pulverrough the horizontal pipe.ized coal decreased with increasing the moisture content in theIn order to investigate the mechanism of the effect of moisture range from 3. 24% to 8. 18% studied in the present work. The resultscontent on the flow characteristics of pulverized coal in pneumatic of the shearing tests were consistent with those of the conveyingonveying, the shear tests of pulverized coal with different mois- experimentsture contents were also carried out. The flowability of three levelconsolidations(Table 1)for each pulverized coal was measured andthe results were shown in Figs. 14 and 15. Fig. 14 illustrates theeffect of moisture content on UYS. It can be seen that the higherThe effects of coal type, particle size and moisture content onthe consolidation, the greater the yield strength of the coal. As a the conveying characteristics of pulverized coal were investigatedresult, UYS increases with increasing the moisture content. This Test results on two kinds of coal show that the mass flow rate andresult indicates that pulverized coal with higher moisture content flow function( FF)of Yanzhou coal are greater than those of Datongis a more cohesive material and thus it is more likely to form an coal. The mass flow rate of pulverized coal decreases with increas-arch in the feeding hopper. For pulverized coal with higher mois- ing the particle size at similar conveying conditions. The pressureture content, the stresses acting on a potential arch must exceed drop for larger particles is greater than that for smaller particles atthe UYS. Therefore, pulverized coal with higher moisture content similar superficial velocity and solid-gas ratio. The pressure dropneeds to consume more energy to flow. The effect of moisture con- decreases at first and then rises with increasing the superficialtent on FF at different moisture contents is shown in Fig. 15. It canvelocity at similar mass flow rate, particle size and pressure inbe seen that FF decreases with increasing the moisture content. the receiving hopper. Mass flow rate and FF decrease with increas-This indicates that the stress needs to make an arch( formed whening the moisture contents in the range from 3. 24% to 8. 18%. Theflow from a hopper stops)collapse with increasing the moisture flowability of pulverized coal becomes worse with increasing themoisture content, therefore conveying per unit mass pulverized18coal needs more energy. The pressure drop decreases with increas-consolidation level 1ing the moisture content at similar operating parameters, whilerises with increasing the moisture content at solid-gas ratio,consolidation level 3superficial velocity and pressure in the receiving hopper.10The present work was supported by the Special Funds ofNational Key Basic Research and Development Program of China(2010CB227002) National Natural Science Foundation of China(5090633-ch fund of Southeast Uni-中国煤化工CNMHGistics of pulverized coal dns -p h se pheu mat ic conveyvnsgiunder high pressureFig 14. Effect of moisture content on unconfined yield strength.Korean Joumal of ChenCLang et aL/ Particuology 10 (2012)310-316Hyder, L M.A R,& Hettiaratchi, K(2000). An investigation Pahk ). B, Klinzing G E(2008). Comparison of flow characteristics for dilute phaseinto the effect olpipe pressure gradients in lean- phasetwo difterent plastic pellets, Joumal of the Chinese12,2352ngineering, 39, 143-150.Experimental characterization of the pressure Pu, w. Zhao, C. Xiong, Y, Liang. C Chen, X Lu, P, et al.(200%). Three-dimensionaltransport at very low velocity. Powder Technol-umerical simulation of dense pneumatic conveying of pulverized coal ino8y,95.165-1vertical pipe at high pressure. Chemical Engineering and Technology, 31Li, H(2002) Application of wavelet multi-resolution analysis to pressure fluctua-tions of gas-solid two-phase flow in a horizontal pipe Powder Technology, 125, Sommerfeld, M (2003). Analysis of collision effects for turbulent gas-particle flowinharizontal channel: Part L. Particle transport International Journalof MultiphaseLi, H, Tomita, Y (2000) Particle velocity and concentration characteristics in ahorizontal dilute swirling flow pneumatic conveying Powder Technology, 107, Teunou, E, Fitzpatrick. L. 1999), Effect of relative humidity and temperature144-152bility Joumal of Food EngineerinLiang. C. Zhao, C.S. Chen, x P. Pu, W.H. Lu. P& Fan. C L(2007). Flow char- Teus ader flowability. Journal of Food Engineering. 43, 97-107 onsolidation on foodcteristics and shannon entropy analysis of dense-phase pneumatic conveyingoal with variable moisture content at high pressure. chemical Vasquez, N. Jacob, K, Cocco, Fdakar, S, Klinzing, G E(2008). Visual analysin pressure drop in dilute phase pneumaticLiang. C. Zhao, C.S. Chen, X P. Pu, WH, &Lu, P (2007). Experimental investigationn flow characteristics of puLverized coal dense-phase pneumatic conveying atYilmaz. A,& Levy, E K(2001) Formation and dispersion of ropes in pneumatichigh pressure. AIP Conference Proceedings 914, 574-579conveying Powder Technology, 114, 168-185中国煤化工CNMHG