Effect of ú Potential on the Strength of Compacted Coal Logs

Henry Liu,* Yu Lin, and Thomas R. Marrero

Capsule Pipeline Research Center, University of MissourisColumbia, Columbia, Missouri 65211

Coal can be compacted into solid cylinders called “coal logs” to facilitate handling andtransportation. It is hypothesized and proved experimentally herein that by altering the úpotential of coal to achieve the isoelectric point, the strength and abrasion resistance of compactedcoal logs can be significantly improved. Experimental data providing the evidence are reportedherein. It is theorized that by zeroing the ú potential of the coal, the charges on coal particlesare neutralized, and the electrostatic repelling force between coal particles containing like chargesis eliminated. This brings the particles closer together with less compaction pressure and force.This method to improve coal agglomerate strength may be applicable to briquetting andpelletizing of fine particles of materials wetted by water.

IntroductionAt the Capsule Pipeline Research Center, University

of MissourisColumbia, intensive research is beingfocused on the compaction of coal particles into solidcylinders called “coal logs” for long-distance transporta-tion by pipeline using water as the carriersthe conceptof “coal log pipeline.” The goal of the research is toproduce water-resistant and wear-resistant coal logs atthe lowest possible cost. Many methods have been triedsuccessfully to improve the strength of coal logs, suchas by using binder, heating the coal, using highercompaction pressure, optimizing mixture water content,optimizing coal particle size distribution, and improvingmold geometry and surface condition. However, someof the methods, such as using binder and heating, arerelatively costly. It was desirable to reduce if noteliminate binder and heating in the compaction processby using other more economic means to increase the coallog strength. Neutralizing the ú potential was thoughtto be a possible way to produce low-cost, low-tempera-ture, binderless coal logs. The purpose of this study wasto evaluate the effect of the ú potential on the strengthof binderless coal logs compacted at room temperatureand to evaluate the practicability of zeroing ú potential(i.e., neutralizing the charges on coal particles to reachthe isoelectric point) as a means of improving coal logcompaction.According to the classical theory of electrokinetics,

whenever a solid is in contact with a liquid, preferentialadsorption of electric charge occurs at the solid-liquidinterface. This causes the solid surface to be chargedeither positively or negatively and the formation of adiffused layer of charges of opposite sign in the liquidnext to the solid surface. The result is an electric doublelayer, and the potential across the layer is the úpotential. The magnitude of ú potential reflects thequantity of charge adsorbed by the solid (Mayer, 1991;Hunter, 1988; Fuerstenau, 1987; Shaw, 1992). ú po-tential depends on the properties of the solid and theliquid in contact with the solid.The above concept can be applied to fine coal particles

in contact with water. All the coal particles in a mixturewith water have the same type of charge (usuallynegative), so they repel each other when brought to theproximity of each other (Campbell and Sun, 1970;Baichenko et al., 1985; Angle, 1989). Since the electro-static forces between coal particles are repulsive, theymake compaction of coal logs more difficult. Reducingsuch electrostatic forces by reducing or zeroing the úpotential makes it possible to produce stronger and more

wear-resistant coal logs at lower compaction pressure.This is to be demonstrated by experiment.This paper describes test results about the effect of ú

potential of coal on the strength of compacted coal logs.To observe clearly the effect of ú potential, other factorsaffecting coal log strength including the coal type,particle size distribution, compaction conditions, andmoisture content were all fixed. Only the ú potentialof the coal was altered by using additives.

Methods and Procedures

Materials and Coal Treatment. The coal used isa subbituminous coal from the Powder River Basin(PRB) in Wyoming. Note that the PRB coal is currentlyfavored by many electric utilities in the United Statesdue to the low cost of this coal and low sulfur content.This coal is often transported by train to distancesgreater than 800 km. The proximate analysis of thePRB coal tested is given in Table 1. Coal particles sizedistribution is given in Table 2. The pH value of thecoal was 6.3, and the ú potential of the coal in deionizedwater was about -15.5 mV.Two additives were used to change the ú potential of

coal: Polyox (polyethylene oxide) and HCl (hydrochlo-ride acid). Polyox is a high molecular weight polymer.The specific type of Polyox used was Polyox WaterSoluble Resin, having an approximate molecular weightof 5 × 106. The Polyox is cationic, and it can changethe ú potential of the coal by affecting the ion adsorptionand the structure of the coal-water interface (Hunter,1988). Note that Polyox is an effective chemical for dragreduction. Use of it in coal logs serves a dual purpose:making stronger coal logs and reducing the drag orenergy consumed in the coal log pipeline (Huang, 1994).HCl, being an acid, can decrease the pH value, neutral-ize the negative charge of coal particles, and reduce theú potential through hydrogen ion adsorption. The waterused to prepare the solution in this test was deionizedwater.In this experiment, five Polyox solutions with differ-

ent concentrations (0, 25, 45, 65, and 100 mg/L) andthree HCl-diluted solution with different pH values (2.0,2.20, 5.78) were prepared. For each sample, the sameweight of coal particles and the same weight of Polyoxor HCl solution was mixed first, and the mixture of coal/solution was allowed to wait for 24 h at room temper-ature (22 °C) so that the ú potential can reach anequilibrium or constant value. Then the mixture wasfiltered, and the filtrate was measured for its pH,

263Ind. Eng. Chem. Res. 1996, 35, 263-268

0888-5885/96/2635-0263$12.00/0 © 1996 American Chemical Society

specific conductance, and ú potential, by using a pHmeter and a ú potential meter (ú Meter 3.0 System). Atorsion-type moisture instrument (CSC Moisture Bal-ance Model No. 26680-000) was used to measure themoisture contents of the raw coal, the coal mixture, andthe coal logs. When the moisture of the coal treated byadditives reached a desired value (25% or 45%) at roomtemperature in air, the coal mixture was used forcompacting coal logs.Compaction Conditions. Coal logs were compacted

by using a 267 kN (60 000 lb) hydraulic press and acylindrical mold of 44.5 mm diameter at room temper-ature (around 22 °C); see Figure 1. The logs producedhad diameters of 45.5 mm and lengths of 73.5 mm,approximately. The peak compaction pressure was 1.38× 105 kPa (20 000 psi) and the load holding time was 7min. The loading rate and the unload rate were 71.2and 42.7 kN/min, respectively.Test Procedures. A total of 80 coal logs were

compacted for this test. They were used for determiningthe density and moisture content and were tested forwater absorption, tensile strength, and wear resistance.The density of the coal logs was determined by

weighing the logs and measuring their dimensions. Thetensile strength of the logs was measured by placing alog between two parallel steel plates. By pressing thetwo plates toward each other against the circumferenceof the log, the log breaks into two semicircular halves.This type of tensile strength is called “splitting tensilestrength” . The American Society for Testing Materialshas a standard (ASTM, 1993) for testing splitting tensilestrength. The procedure given in the standard wasfollowed in this test. The splitting tensile strength wascalculated as follows:

where T is splitting tensile strength (Pa); D is diameterof the coal log (m); L is the log length (m); P is the failureload (N); and π is 3.1416.The water absorption test was conducted by immers-

ing coal logs in sealed vessels (test cells); see Figure 2.The water pressure was raised to 3447 kPa (500 psi)for 1 h. This caused the logs to become saturated withwater. The weight gain of any coal log due to waterabsorption was defined as follows:

where G is weight gain (%);Wa andWi are respectivelythe weight of the log after water absorption and theinitial weight.The coal log wear test was conducted in a 23 m long,

55 mm diameter steel pipeline recirculating loop drivenby a jet pump (Figure 3). A heat exchanger was usedto maintain a constant temperature of the water recir-culating through the loop. A transparent viewingsection was provided so that the condition of the logsduring test could be visually observed. The coal logswere first subjected to water absorption, so that the logswere saturated with water and would not absorb ad-ditional water while being tested in the pipe. This madeit possible to determine wear rate based on the weightloss of the logs circulated through the pipe. The logswere inserted into the loop and taken out of the loopthrough a window in the pipe located in a constant-headreservoir. The water velocity in the recirculating pipe-line loop was set at the lift-off velocity, VL, calculatedfrom the following equation derived in Liu (1982):

where S is the specific gravity of the log; g is gravita-tional acceleration; a is the aspect ratio which is thelog length divided by the log diameter; k is the diameterratio which is the log diameter divided by the pipe innerdiameter; and D is the inner diameter of the pipe. Notethat the lift-off velocity is the minimum velocity atwhich a coal log or capsule becomes totally suspendedby the flow, with the front end of the log (capsule) raisedat an angle of attack. Experience has shown that bothhead loss and wear rate are at a minimum when coallog pipelines are operated at a velocity slightly belowthe lift-off velocity (Liu, 1994; Cheng, 1994). Periodi-cally, the flow was stopped and the logs were taken outof the loop through the window for weighing to deter-mine weight loss. The tests were continued until thelogs were broken in the pipe eventually. The weightloss of coal logs at various circulation times or variousnumbers of cycles of circulation was then plotted tostudy wear rate.

Results and Discussion

The effects of Polyox and HCl on the ú potential ofthe PRB coal are shown in Figures 4 and 5. In theexperiments, the ú potential of the coal was changed byPolyox and HCl from -15.54 to 17 mV approximately.Each set of data contains two specimens. The valuesof both and their average are indicated by the bars inthe ensuing figures.Effect of ú Potential on Density of Coal Logs.

Figure 6 shows the effect of ú potential on the densityof coal logs. When the ú potential of coal approached

Table 1. Proximate Analysis of Coal Used in Tests

properties as received (%) dry basis (%)

total moisture 26.43inherent moisture 15.53 15.53volatile matter 30.31 41.20fixed carbon 38.76 52.70ash 4.50 6.11heating valuea (MJ/kg) 20.51 27.88a MJ stands for million Joules.

Table 2. Particle Size Distribution of Coal Used in Tests

mesh size d percentage (%)

80 < d < 60 25100 < d < 80 10140 < d < 100 30170 < d < 140 10200 < d < 170 10d < 200 15

Figure 1. Compaction apparatus.

T ) 2P/(πDcL) (1)

G ) (Wa - Wi)/Wi (2)

VL ) 7.2x(S - 1)ag(1 - k2)kD (3)

264 Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996

zero from either a positive or negative potential, amaximum coal log density of 1160 kg/m3 (approximately)was attained. The densities of coal logs without usingadditives were in the range from 1092 to 1095 (average1093) kg/m3. More specifically, the densities of the coallogs increased 5.9% (Polyox, 25%), 6.0% (Polyox, 45%),5.3% (HCl, 25%), and 5.3% (HCl, 45%) as the ú potentialof the coal reached zero. Note that the percent valueslisted in the parentheses correspond to the initialmoisture in the coal mixture used to make the coal log.This indicates that, under identical conditions, the coallogs with vanishing ú potential were compacted to adensity 5%-6% higher than those with ú potential not

altered. This result supports the hypothesis that reduc-ing ú potential enhances coal log compaction.Effect of ú Potential on Water Absorption of

Coal Logs. After water absorption for 1 h in 3447 kPawater, coal logs gained weight. Figure 7 shows theeffect of ú potential on weight gain of coal logs due towater absorption. When the ú potential of the coalreached zero, the weight gains of the coal logs decreasedto a minimum. The coal logs with vanishing ú potentialabsorbed water of 11.5% (Polyox, 25%), 15.0% (Polyox,45%), 14.1% (HCl, 25%) and 16.5% (HCl, 45%) after 1 himmersion in water under 3447 kPa. They correspondto a reduction of water absorption by approximately

Figure 2. Water absorption test apparatus.

Figure 3. General layout (top view) of 55 mm diameter steel pipeline for wear test of coal logs.

Figure 4. Variation of ú potential with concentration of Polyoxin PRB coal.

Figure 5. Variation of ú potential with concentration of HCl inPRB coal.

Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996 265

25%, 14%, 23%, and 19% compared to those withunaltered ú potential . The reduced water absorptionis due to smaller porosity in the coal logs resulting frombetter compaction. This is consistent with the findingin the previous section that neutralizing the ú potentialproduces denser logs. Generally, higher density coallogs have lower porosity, and they absorb less water.Effect of ú Potential on Tensile Strength of Coal

Logs. Figure 8 illustrates the effect of ú potential onthe splitting tensile strength of coal logs. When the úpotential approached zero from positive or negative, thetensile strength of the logs also approached a maximumregardless whether the additive was Polyox or HCl andregardless of the initial moisture: 25% or 45%. Beforewater absorption, the tensile strengths of coal logswithout neutralized ú potential were 200.6-209.5 kPafor initial moisture of 25%, and 150.7-155.6 kPa forinitial moisture of 45%. The corresponding tensilestrengths of the coal logs with zero ú potential were307.2 kPa (Polyox, 25%), 295.6 kPa (HCl, 25%), 245.0kPa (Polyox, 45%), and 232.7 kPa (HCl, 45%). Thismeans as the ú potential changed from -15.5 mV to 0,the tensile strength of the coal logs increased 53%(Polyox) and 47% (HCl) for initial moisture of 25%, andfor initial moisture of 45% the tensile strength increased63% (Polyox) and 54% (HCl). The tensile strength ofcoal logs measured after water absorption also increasedwhen the ú potential became zero. The increase was57% (Polyox, 25%), 44% (HCl 25%), 69% (Polyox, 45%),and 51% (HCl 45%). The foregoing data indicate thatcoal logs with zero ú potential were approximately 50%stronger in tensile strength.

Effect of ú Potential onWeight Loss of Coal Logsin Wear Test. Figure 9 gives the weight loss of coallogs due to wear in the pipeline as a function of thenumber of cycles through the recirculating pipe loop.Note that the pipe loop was driven by a jet pump whichcaused the coal logs to bang on the pipe wall wheneverthey passed through the jet pump. Furthermore, duringeach cycle of circulation, the coal logs passed throughtwo 180° bends. Both the jet pump and the bends areabrasive to coal logs. Commercial coal log pipelinesdon’t use jet pumps and hence are less abrasive thanthis loop (Liu, 1992). The coal logs with zero ú potentialhad minimumweight losses due to wear. After 24 cyclesof circulation, the average weight losses of the coal logswithout and with ú potential zeroed by Polyox wererespectively 13.7% and 2.1% for initial moisture of 25%,and 13.2% and 4.0% for initial moisture of 45%. Thisyields a 6.7 times decrease in wear for logs compactedwith initial moisture of 25%, and a 3.3 times decreasein wear for logs compacted with 45% initial moisture.After 19 cycles of circulation, the weight losses of thecoal logs with ú potential zeroed by HCl also decreasedby a factor 5 for initial moisture 25% and by a factor of3 for initial moisture 45% as compared to those withunaltered ú potential. This indicates that coal logscompacted with zeroed ú potential are far more wearresistant than those with unaltered ú potential.

Practical Consideration

In this experiment, the ú potential of the PRB coalwas changed by adding a small amount of Polyox andHCl. Figures 4 and 5 show the relationships between

Figure 6. Effect of ú potential on density of compacted coal logsbefore water absorption.

Figure 7. Effect of ú potential on weight gain of compacted coallogs due to water absorption for 1 h at 3447.48 kPa pressure.

a

b

Figure 8. Effect of ú potential on tensile strength of compactedcoal logs before and after water absorption. Additive: (a) Polyox;(b) HCl.

266 Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996

the ú potential and the concentration of the additivesin the coal. For zeroing the ú potential of the coal,0.0053% Polyox and 0.093% HCl were needed. Thewholesale price of Polyox is approximately $11 perkilogram, and the wholesale price of HCl is approxi-mately $1 per kilogram (OPD, 1995). To zero the úpotential of 1 ton (1000 kg) of the PRB coal about 0.053kg of Polyox and 0.93 kg of HCl are needed. They costrespectively $0.58 and $0.93. This shows that it is moreeconomical to use Polyox than HCl for altering the úpotential of the coal and for producing stronger andmore wear-resistant coal logs. According to Hunter(1986), Polyox affects ú potential by two differentmechanisms: (1) it changes the absorption of ions onthe solid (coal particles) and (2) due to its large molec-ular size, Polyox attached to coal particles shifts theposition of the shear plane away from the particlesurface. Both mechanisms are effective in reducing úpotential. Since HCl does not shift the shear plane, itis less effective than Polyox in reducing ú potential.Polyox has the additional advantages of being effectivein reducing drag (energy loss) in coal log pipeline(Huang, 1994), it does not make the coal acidic, and itburns with the coal without generating pollutants sinceit is free of pollutant precursors, such as nitrogen andsulfur compounds. Although this study was done withdeionized water, the same trend is expected to hold withminor variations for industrial or potable water fromvarious sources. For instance, using the tap water ofUniversity of MissourisColumbia which has a pH of7.25 instead of 6.0 for the deionized water, it was foundthat the isoelectric point of the coal can be reached byadding 0.0052% Polyox instead of 0.0053% Polyox whendeionized water was used.

This study has demonstrated that reducing the úpotential to the isoelectric point of coal results in denser,stronger, more water-resistant, and more wear-resistantcoal logs. However, the study was limited to binderlesscoal logs compacted at room temperature. Such logs,even when strengthened by zeroing the ú potential, arestill not sufficiently wear resistant to ensure long-distance hydraulic transport by pipelines. Nonetheless,the logs compacted with zero ú potential appear to beas strong as and as wear resistant as commercialbriquets. Such logs are expected to be suitable fortransportation by trucks, trains, barges, and conveyorbeltssthe same way coal briquets are usually trans-ported.Other investigations have demonstrated that strong

coal logs that are highly resistant to wear in pipelinescan be produced by either raising the temperature ofcoal during compaction (Gunnink and Liang, 1994) orby using binder (Wilson and Ding, 1994), or through acombination of both binder and temperature (Cheng,1994). However, the costs of binder and elevatedtemperature for making good coal logs are relativelyhighsgreater than $1.00 per ton of coal. Results of thecurrent study offer hope that maybe these processes thatproduce wear-resistant logs can be modified to produceequally wear-resistant logs for pipeline transportationat lower temperature or less amount of binder if the úpotential of the coal is reduced to zero by using a smallamount of additive. Additional research is need to bearthis out.

Conclusion

ú potential is an important factor affecting coal logcompaction and coal log quality. Reducing the ú poten-tial of the coal to nearly zero can result in stronger coallogs with lower porosity, higher density, less waterabsorption, greater tensile strength, and stronger wearresistance. The effect is attributed to the vanishingrepelling force between charged coal particles as theisoelectric point is reached. This makes it possible tobring coal particles closer to each other during compac-tion, thereby forming a stronger bound between coalparticles and a stronger log. The tests reported hereinare for binderless coal logs compacted at room temper-ature. Whether the same holds for coal logs that containbinders and/or are compacted at elevated temperaturesremains to be seen.

Acknowledgment

This research was sponsored by the Capsule PipelineResearch Center which was (and is) supported jointlyby the National Science Foundation, the MissouriDepartment of Economic Development, the U.S. De-partment of Energy, and a consortium of 14 companies.Funding for the project is appreciated. The Polyox usedin the test was donated by the Union Carbide Company.

Nomenclature

a ) aspect ratio of coal log which is the coal log lengthdivided by coal log diameter (dimensionless)

D ) inter diameter of pipe (m)Dc ) coal log diameter (m)g ) gravitational acceleration (m/s2)G ) weight gain of coal log due to water absorption (%)k ) diameter ratio which is the coal log diameter dividedby pipe inter diameter (dimensionless)

L ) length of coal log (m)

a

b

Figure 9. Weight loss of coal logs circulated through a 55 mmdiameter steel pipe loop. Additive: (a) Polyox; (b) HCl.

Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996 267

P ) failure load used in splitting tensile strength test (N)S ) specific gravity of coal log (dimensionless)T ) splitting tensile strength (Pa)VL ) lift-off velocity of coal log (m/s)Wa ) weight of coal log after water absorption (g)Wi ) weight of coal log before water absorption (g)

Literature Cited

Angle, C. W.; Hamza, H. A. An Electrokinetic Study of a NaturalCoal Associated mixture of Kaolinite and Montmorillonite inElectrolytes. Appl. Clay Sci. 1989, 4, 263-278.

ASTM. Standard Test Method for Splitting Tensile Strength ofCylindrical Concrete Specimens. In Annual Book of ASTMStandards, Designation: C 4496-90 (Reapproved, 1989);ASTM: Philadelphia, 1993; Vol. 4. 2, pp 269-272.

Baichenko, A. A.; Baichenko, Al. A.; Meltinisov, M. A. SurfaceElectrical properties of Coal Particles. Soviet Mining Sci. 1985,21, (2), 181-186.

Campbell, J. A. L.; Sun, S. C. Bituminous Coal Electrokinetics.Trans. SME/AIME 1970, 267, 111-114.

Cheng, C. C. Wear and damage of coal logs in pipeline. Ph.D.Dissertation, The University of MissourisColumbia, 1994.

Fuerstenau, D. W.; Rosenbaum, J. M.; You, Y. S. ElectrokineticBehavior of Coal. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem.1987, 32, (1), 378-385.

Gunnink, B. W. Liang, Z. Compaction of binderless coal for coallog pipeline. Fuel Processing Technology; Elsevier: Amsterdam,1994; pp 237-254.

Huang, X. Polymer Drag Reduction in Hydraulic Capsule Pipeline(HCP). M.S. Thesis, University of MissourisColumbia, 1994.

Hunter, R. J. ú Potential in Colloid SciencesPrinciples andApplications; Academic Press: New York, 1988; pp 255-270.

Laskowski, J. S. Coal Electrokinetics: The Origin of Charge atCoal/Water Interface. Prepr. Pap.sAmerican Chem. Soc., Div.Fuel Chem., 1987, 32, (1), 367-377.

Liu, H. A Theory of Capsule Lift-off in Pipeline J. Pipelines 1982,2, 23-33.

Liu, H. Design and operational consideration of hydraulic capsulepipelines; Proceeding of the workshop on capsule pipeline;Japanese Society of Multiphase Flow: Tokyo, Japan, 1992; 26-50.

Liu, H. Coal log pipeline transportation of coal. Proceedings of theinternational conference on advanced technology and equipmenton materials handling, Shanghai, China; 1994; pp 802-806.

OPD. Chemical Dictionary. Schnell Publishing Co.: New York,1995.

Shaw, D. J. Colloid and Surface Chemistry: ButterworthsHeinemaun Ltd.: Oxford, Great Britain, 1992.

Wilson, J. W.; Ding, Y; Zhao, B. The effect of binders on theperformance of coal logs in a pipeline transportation system;MECE’94 international mechanical engineering congress andexhibition, Perth, Australia; 1994.

Received for review March 20, 1995Revised manuscript received August 30, 1995

Accepted September 25, 1995X

IE9501844

X Abstract published in Advance ACS Abstracts, December1, 1995.

268 Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996