A series of experiments was conducted on a slagging MHD generator to investigate theloss in channel performance brought about by slag polarization phenomena in which largereductions in slag resistivity are observed on the cathode wall.

A one meter long channel with 56 electrode pairs and peg type insulator walls wasdriven by an ash injected, oil fired combustor operating at 14 MW thermal input. Atnominal channel operating conditions of 40 to 50 kWe output and a Hall voltage of 500-800 volts there was a degradation in performance of approximately 10% in power and 30%in Hall voltage when the cathode wall slag was fully polarized. For newly slaggedchannel walls, the time required was between 80 and 100 minutes.

Measurements of polarized Rosebud cathode slag conductivity ranged from 140 to 370mhos/meter yielding an equivalent channel slag conductivity of 70 to 90 mhos/meter. Bycontrast, tests in which the seed was discontinued resulted in relatively no change ingenerated Hall voltage (~ 500 volts) while power levels dropped to less than 1 kW, dem-onstrating minimal leakage in the slag layer.

It is concluded that the cathode slag layer is not continuous but is periodically"broken". Shorting of Faraday currents in the presence of a Hall field results in in-creasing current concentrations at the downstream edge of the shorted group until suf-ficient joule heating is produced to locally remove or thin the slag layer and createan open insulator gap capable of· sustaining the Hall voltage. The net effect was a re-segmentation of the cathode wall from a design pitch of 1.5 cm to a pitch of between4 and 10 cm depending on the iron oxide concentration in the ash.

Using scaling laws based on the ratio of electrode pitch to channel height it is es-timated that the most conductive coal slags will reduce the theoretical (infinitelysegmented) power output of full scale channels between 1 and 2%.

While long duration tests have demonstrated that slag shorting and increased segmen-tation of cathodes has few, if any, life limiting effects on the cathode wall itself,the adjoining insulator walls suffer locally from increased electrochemical erosion andthe anode wall will be rapidly and severely damaged if the channel is diagonally con-nected without current control devices in the cross connecting links.

MHD Generator technology development supported by the DOE for eventual applicationin coal fired electric power plants, makes use of the mineral matter (ash) in coal toprovide a continuously replenished, flowing slag layer on the walls of the MHD channel.There are benefits as well as drawbacks attributable to this slag layer. The mostnotable benefit is the reduction of heat loss from the working fluid and the attendantincrease in cycle efficiency. At typical MHD channel conditions, the gas shear andslag viscous forces equilibrate at a slag surface temperature of approximately 1800 K.

A related but equally important benefit is that the electrode metal temperature islowered significantly (400-600 K is typical) by the insulation effect of the slaglayer. This greatly reduces the electrochemical and arc effects of the transport cur-rent and results in increased electrode life.

The drawbacks associated with channel slag are twofold:1. Increased corrosion of the anode electrode which is subject to oxidation. Coal

slag contains nearly 50% by weight of oxygen which becomes electrochemicallyactive (0--) at the anode surface as the ionic fraction of the transportcurrent.

2. A polarization effect in the cathode wall slag appreciably increa~es the.electricalconductivity of the slag on this wall and results in losses assoclated wlth leakagecurrents.

The major electrode development effort to date has dealt with increasing the lifetimeof anodes. The success of this 'work was recently demonstrated when the Avco Everett Re-search Laboratory conducted a 1000 hour test in which the extrapqlated lif~ of anodesoperating at the same conditions as would exist in a coal fired MHQ power station wasshown to be in the range of 5000 to 8000 hours.(l)

The phenomena of increased localized shorting of slag covered cathodes has been ob-served and reported.(2) This report will present some recent experimental results inwhich the effects of cathode slag leakage on channel performance were investigated.

Tests were conducted on a Mark VII MHD channel containing 56 electrode pairs in an ac-tive length of one meter. The channel operated supersonically and was resistively loadedas an ideal Faraday. The plasma source was an ash injected oil combustor operating withoxygen enriched air and No.2 fuel oil. Montana Rosebud ash was injected for the slaggingtests. Some tests were conducted with the addition of requisite sulfur fractions to sim-ulate Rosebud coal.

Nominal operating parameters are listed in Table 1.Table 1. Mark VII-Flow Train Operating Parameters

Combustor ParametersMass FlowNitrogen/OxygenOfF EquivalenceEquivalent Ash CarryoverSeed (dry K2C03)Combustor P~essure

Generator Parameters

4#/sec.0.80-.925%1% by wt.48 psia

Output PowerHall VoltageCurrent DensityInlet Mach No.Peak Magnetic Field

40-60 kW400-800 volts0.5-0.6 amps/cm21.42.2 Tesla

Typical startup of the Mark VII generator consists of an initial period of non-poweroperation of up to 1 hour in which a slag layer is established on all internal 'flow trainsurfaces. Continuous monitoring of wall heat losses indicates when an equilibrium condi-tion has been reached. At this point, power generation is initiated by turning on themagnet.

Five specific tests were conducted to study the effects of cathode wall slag.

As the cathode wall slag becomes polarized due to the ionic nature of the transportcurrent, a simultaneous reduction in output power and Hall voltage is observed. Figureshows thi~ evolution in the Mark VII generator during the initial 80 minutes of powergeneration. The cathode slag resistivity decreases (polarizes) and groups of cathodesbecome shorted together forcing the active cathode gaps (insulators) to sustain severaltimes their normal voltage. The spatial RMS value of the active intercathode voltages(Nos. 15 through 35) increases to 40 volts while the output power drops about 10% andthe Hall voltage decreases approximately 35%. Figures 2 and 3 show the spatial distribu-tion of Hall voltage on the cathode wall at the beginning and end of this 80 minuteperiod. Note that only five cathode gaps with values as high as 90 volts sustain thetotal Hall voltage when the slag is fully polarized.TEST #2 - SLAG LEAKAGE MEASUREMENTS

Wall leakage measurements were conducted on the Mark VII channel using a voltage con-trolled DC power supply connected between the combustor (high voltage) and the diffuser(ground). The voltage setting was kept constant at 400 volts DC for all tests. Prior toash ihj~ction, the unslagged flow train will leak,between 0.5 and 1 amp. About half ofthis leakage is through the MHD channel and half through the combustor feed systems and

202 cooling hoses. The addition of an unseeded slag layer increases the current leakage to

300o

o12020 40 60 80 100

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Fig. 2 Cathode Wall Voltage Distribution Prior to Slag Polarizationbetween 1 and 2 amps. When seed is introduced, the axial current through both gas andslag is between 20 and 30 amps. To measure the effect of polarized cathode slag on wallleakage a remotely operated high voltage vacuum switch was used to connect the externalpower supply across the channel simultaneously with turning the magnet power and seedsupply valve off. Power supply currents with polarized cathode slag could go as high as90 amps at 400 vo 1ts . .

The major difficulty in measuring channel wall leakage arises from the gradual reduc-tion in gas conductivity due to seed outgassing from the wall slag surfaces after theseed valve has been closed. An initial large current decrease is nearly instantaneousand 1inear down to a reasonably well defined "Breakaway" point where the current decaybecomes exponential and approaches a~ asymptotic value in typically 3 to 6 minutes.Figure 4 shows typical measured current decay curves for Rosebud slag prior to power gen-eration (nonpolarized cathode) and with a fully polarized cathode after power generation.The nearly vertical straight line represents loss of gas conductivity when the seed valve

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Fig. 4 Channel Slag Leakage Currents with External Power Supply @ 400 Volts DCis closed. The "breakaway" points (16 and 11.5 amps) where the seed outgassing from thewall slag determines the decay rate are taken as reasonable approximations of the slagleakage currents. Actually, these values are probably somewhat higher than the actualslag leakage since some of the current will flow in the plasma which is being seeded bythe outgassing of the wall slag. Both curves approach a value of 1.5 amps which is themeasured~lag leakage prior to introduction of seed. Assuming an average slag thicknessof 1.5 millimeters and using leakage currents identified in Fig. 4, approximate valuesof slag conductivities were computed with the following results:

1. Unseeded Rosebud slag, 0 ~ 6 mho/meter2. Seeded Rosebud slag, 0 ~ 40 mho/meter3. Seeded and Polarized Rosebud slag, 0 ~ 140 mho/meterRepetition of these current decay measurements on different channels and with varying

slag conditions gave conductiVity estimates for polarized. slag as high as 370 mhos/meter.Values for unseeded and seeded nonpolarized slag remained relatively constant from testto test at 4 to 6 mhos/meter and 25-50 mhos/meter, respectively.

To check the effect of controlled and measured end to end leakage on a Faraday loadedchannel, a series of external resistors, switches and meters were installed between dif-fuser and combustor of the Mark VII generator. An unslagged channel was used for thesetests.

After steady state generator conditions had been reached with all bleed resistorswitches open, axial resistances of 50, 30 and 15 ohms were individually switched in. Themeasured reduction of Hall voltage and power output as a function of the measured externalleakage current is plotted in Fig. 5

CHANNEL CONFIGURATION

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Fig. 5 Effects of External LeakaQe Currents onMK VII Generator Performance

Based on these curves, the measured channel performance losses attributable to slagcovered walls (Fig. 1) require an axial leakage current of 5.5 amps to account for thepower loss of 10% and 13 amps to match the 35% reduction in Hall voltage. From Fig. 4 theestimated leakage current for polarized slag is 16 amps which agrees reasonably well withthe external leakage required to match the Hall voltage but not the power loss.

The actual distribution of current in polarized slag ~epresentative of the shortedcathode wall conditions of Fig. 3 is schematically shown in Fig, 6. Currents of up to 30

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amps flow in the·slag layer between shorted cathodes at a measured intere1ectrode voltageof 1 to 2 volts. This requires the local conductivity of the polarized slag layer to beat least 200 mhos/meter. The conclusion one must come to is that the high voltage gapsare essentially slag free and leak less than 1 amp while the cathode gaps that are cov-ered with polarized Rosebud slag are for all practical purposes dead shorts (R < 0.1 ohm)and carry currents approachi"g the total Faraday current of the shorted group. It is thejoule heating from the concentration of the Faraday current at the downstream edge of theshorted group that clears the slag from the voltage sustaining insulator gaps. This con-clusion was corroborated by (1) movies taken of a slagging cathode wall, and (2) byob-servations of a positive correlation between the number of voltage carrying (open) gapsand the channel power density. A discontinuous cathode slag layer is also consistent withthe experimental result in which the real Hall voltage reduction was duplicated by anequivalent external leakage current whereas the real power reduction was much less thanwas predicted by the same external leakage simulation.

Tests were conducted in which the seed valve was closed during generator operation witha slagged and polarized cathode wall. The seeded channel was producing 40 kW and a Hallvoltage of 450 volts. When the seed injection valve was closed, the power dropped rapidlyto 10 kW and then gradually to approximately 1 kW over the next five minutes. With theinitial power drop the Hall voltage rose to 500 volts and held there for the remainder ofthe test. Traces of the power and Hall voltage versus time are shown in Fig. 7. As thegas conductivity drops to a very low value with a constant magnetic field and Faradayload resistance, the generator operating point moves down its load line to a near shortcircuit condition. Since the ratio of B JJ{/crremains roughly constant making Ex ~ Jx/cr,the Jx term (leakage) must be very small for the generator to be able to sustain its Hallvoltage at very low values of cr. The effective conductivity of the polarized cathode wallslag layer must, likewise, be a very small number even though the slag itself is highlyconductive.

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Fig. 7 MK VII Power and Hall Voltage vs. Time After Seed LossThis test has been repeated several times over the past year on several MK VI and MK

V!I channels. In most cases the Hall voltage would rise (1.2 to 1.8 times) with the ini-tlal loss of seed :ollowe~ by a slow decay. Always, however, significant values of Hallvoltage were su~talned whl1e the Faraday currents and gas conductivity approached zero."These voltage dlfferences re~lect variations .in the condition of the channel slag layersas well as many other potentla1 leakage paths from the flow train support and feed sys-tems to ground.

An equivalent value for slag conductivity in an operating channel (i.e.~ polarizedcathode) can be obtained from the difference in the decay rates of generator power versuspower supply current when the seed valve is closed. For these tests a maximum data

~cquisition rate of 1 per second was used. Sulfur in the form of liquid S02 was injectedlnto the combustor in the requisite weight fractions to simulate Montana Rosebud coal.

The pow:r decay is assumed pr~portional to the gas conductivity which was initiallymeasured wlth the channel walls ln an unslagged (leakage ~ 0) condition. The decay of~urrent at ~onstant vol~a~e from an external power supply is proportional to the changeln the comblned conductlvlty of gas plus slag. The difference in these measurements istaken as the channel equivalent slag conductivity. F.igure8 is a plot of the measuredgas and gas plus slag resistance values for the first 60 seconds after seed loss in thegenerator. T~e calculated decay curve of equivalent channel slag conductivity is alsoplotted. The slag conductivity at t=O is the value of interest but the data at this partof the curve is the least accurate .• It is probably safe to say that the equivalent slagconductivity for this test was between 70 and 90 mhos/meter.

~I&J 1202.•...~:x:2 100I

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Fig. 8 Equivalent Channel Slag Conductivity (From GeneratorPerformance with Seed Loss)

The ionic nature of coal slags results in an unfavorable species polarization incathode wall slag making it highly conductive and producing local shorting of groups ofcathodes. Tests to measure the conductivity' of a polarized slag covered cathode wall re-sulted in conflicting results. Power losses which were small plus the ability of thechannel to sustain its Hall voltage with seed loss demonstrated that a polarized cathodewall slag layer does not result in significant current leakage to ground. It is postu-lated that slag shorting of cathodes in the presence of a Hall field results in increas-ing current concentrations at the downstream edge of the shorted group until sufficientjoule heating is produced to locally remove or thin the slag and create an ~open" insu-lator gap capable of sustaining the Hall voltage. Other measurements of polarized slagconductivity that resulted in values as high as 400 mhos/meter were made at 1 secondintervals concurrent with seed loss. This allowed more than enough time for the open in-sulator gaps to fill in with slag (0.2 sec. @ slag velocity ~ 1 cm/sec.). Thus, it isconcluded that although the cathode slag is itself very conductive, the power generatingcathode wall is much less conductive due to the discontinuity of the slag layer. The neteffect is a resegmentation of the MK VII cathode wall from a design pitch of 1.78 cm to apitch of between 4 and 10 cm. The lower number is typical of slag from Eastern coalswhile the 10 cm pitch is typical of Montana Rosebud ash used in a-recent 1000 hour test(l)and the experiments described in this paper.

The nominal chemical analysis of each coal ash is presented in Table 2. The oxides inTable 2 with compositional differences that are known to have a significant effect onslag resistivity are Fe~03 and K20 which decrease resistivity with increasing amounts andCaO which decreases reslstivity with decreasing amounts.l3) All three oxide fractionschange in the direction of decreasing resistivity for Illinois #6 slag. Mineral analysesof cathode wall slag samples show the same comparative relationships as in the feedstock 207

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Fig. 9 Comparison of Cathode Wall Voltage Distributionsash, yet surprisingly, Rosebud slag has a much more pronounced effect on cathode shortingthan does Illinois #6. The relative effects are illustrated in Fig. 9 which compares thespatial distribution of the Hall voltage on a cathode wall when operating with each ash.

By the experimental use of individual mineral additives to each feedstock ash duringpower generation tests, it was determined that iron oxides are the predominant speciesinfluencing the cathode wall slag resistivity. In addition, many experimenters haveshown that iron cont~i~ing slags exhibit greatly enhanced conductivity due to an elec-tronic contribution.\3) Since both the 2+ and 3+ valence states are found over a widerange of conditions there is an available mechanism for charge transfer by electron"hopping" in slags with substantial iron oxide content. The lack of electronic componentin the western slags makes them more polarizable, and at the MHO cathode, accumulation ofiron in the colder region of the slag sUbstantially decreases the axial resistance.

Table 2. Chemical Analysis (Wt.%) of MHO Coal Ash FeedstocksU.S. Western U.S. Eastern

Montana Rosebud IIIinois #6Sil ica, Si02 47.5 44.7Alumina, A1203 21.1 20.9Lime, CaO 14.5 5.8Ferric Oxide, Fe203 7.8 24.1Magnesia, MgO 4.6 1.8Titania, TiO 0.8 1.0Potassium Oxide, K20 0.7 2.3Sodium Oxide Na20 0.4 0.6Phosphorus Pentoxide, P205 0.4 0.12

Finite segmentation of MHO channel electrodes results in a reduction of theoreticalperformance proportional to the electrode pitch divided by the channel height. in smallchannels the size of the Mark VI, a slag shorted cathode pitch of 10 cm reduces power ap-proximately 10%. This same resegmentation is expected to have a much smaller effect onpower plant size channels. Scaling estimates of channel performance losses due to a seg-mentation change of from 2 to 10 cm on both 1 and 2 electrode walls are given in Table 3.

Table 3. Approx.* % Reduction in Power Due to Electrode SegmentationIncrease From 2 to 10 cm

Theoretical2 Wall s Measured, (Predicted)

1 WallMK VI/MK VIICOIF lA500 MWe CSPEC Plant

30%256

10%(8)(2)

*Scaled by 1 1 p/h from "Magnetohydrodynamic Energy Conversion" by R. J. Rosa

Decreased segmentation on a slag shorted cathode wall has, in our experience to date(which includes 500 hour tests on .each of three cathode walls), caused neither electricalfault damage nor life limiting corrosion problems on the cathode wall itself.

The high voltage gaps are found to shift with time so that in the long run inter-cathodic erosion is more evenly distributed than the instantaneous voltage distributionwould suggest. In 500 hour channel tests the intercathode voltage range was 10-55 voltswith Eastern ash and 4 to 40 volts with Western ash. Measured corrosion rates of thetungsten copper protective caps varied by a factor of approximately 5 to 1 in a 450 hourtest with Rosebud ash and sulfur additive. Photos of two sectioned cathodes showing thisrange of corrosion are presented in Fig. 10. Slag shorted cathodes can, however, producepotentially serious problems on the other walls of an MHD channel. In a Faraday channelthat is diagonally connected with short circuited frames or wires, the cathode voltagenonuniformities accompanied by strong Faraday current nonuniformities are reflected ontothe anode wall. Here, electrical faults (arcs) are driven into the wall where irreversi-ble damage can occur and forced shutdowns often result. Control circuits are required ineach diagonal link to force an ~ql,Jalityof the Faraday current and a uniform Hall voltage-distribution on the anode wall.l4)

Local cathode shorting is also reflected via resultant plasma nonuniformities onto theinsulator wall extremities adjacent to the cathode wall. Leakage currents through theslag layer cause increased anodic corrosion on insulator walls due to the higher resultantfie1d (Ey + Ex)'

One developmental approach to alleviate the cathode slag problem was the design of a"non-slagging" wall surface using nonwettable materials of construction. These attemptsfailed due to particle abrasion and wall roughening which, in the long run, providedanchor points for the eventual development and retention of a slag layer.

Att~mpts to operate electrode and/or insulator materials at elevated temperatures toreduce slag thickness and increase mixing were successful in reducing slag polarizationbut resulted in accelerated electrochemical corrosion and unacceptably short lifetimes.(5)

There exists some evidence that ash carryover rate influences the degree of slagpolarization. Higher rates reduce the voltage nonuniformities, presumably by replacingthe surface slag layer before polarization has time to fully develop. Figure 11 showsthe results of an experiment with a diagonally connected MK VI channel using Eastern ash,in which the carryover fraction was controlled at five discrete steps of 33%, 18%, 12%,18% and 32%, each lasting approximately an hour to allow time for equilibrium of thechemical polarization. The five plots in Fig. 10 show the instantaneous values of theHall voltage distribution at the end of each time period. During the entire test, theaverage interelectrode voltage remained constant at approximately 17 volts while the RMSvalues were 5, 15, 18, 15 and 6 volts, respectively, for the five times shown in Fig. 10.Carryover rates (based on an assumption of coal equal to 10% of the total flow and 10%ash in the coal) were increased to as high as 83% with little if any additional improve-ment in the attenuation of the voltage nonuniformities. Verification of this single testresult is needed and the effect of chann'el length on slag polarization versus carryoverfraction has yet to ~e established.

It has been shown through a series of experiments that ionic polarization produces ahighly conductive cathode wall slag in MHD channels. This does not result in significantend to end current leakage. Rather, the slag layer becomes discontinuous due to Jouleheating, effectively producing a more coarse resegmentation of the cathode wall elec-trodes. Increased local shorting of the Hall field produces a power loss that is pro-portional to the ratio of electrode pitch to channel height. Relative losses, therefore,decrease with channel size. The most conductive slags result in a 10% power reductionon MK VII sized channels but are estimated to have less than a 2% effect on a power plantsize channel.

--JI FLOWTYPICAL CATHODES 450 POWER HOURS 0.18%8

SEPTEMBER 1981

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CATHODE # 31 ~

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LOST VOLUME (ML)

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Fig. 11 Instantaneous Values of Intercathode Voltage Distributions

To account for the.cathode slag polarization phenomena, predictive computationalmodels which presently use an artificially high s)ag conductivity might better treat theproblem as a finite segmentation loss.

While long duration tests have demonstrated that slag shorting and increased segmenta-tion of cathodes has few, if any, life limiting effects on the cathode wall itself, theadjoining insulator walls suffer locally from increased electrochemical erosion and theanode wall will be rapidly and severely damaged if the channel is diagonally connectedwithout current control devices in the cross connecting links.

Hruby, V. J., et al., "1000 Hour MHO Anode Test." Presented at the 20th Symposiumo~ Engineering Aspects of Magnetohydrodynamics, Univer$ity of California atIrvine, June 14-16, 1982.Petty, S. W., et al., "Electrode Phenomena in Slagging MHO Channels." Presentedat the 16th Symposium on Engineering Aspects of MHO, The University of Pitts-burgh, Pittsburgh, PA, May 16, 1977."The Electrical Behavior of Rosebud Coal Slag under MHO Conditions." DOETopical Report dated Dec. 1981 by Pollina, et al., Montana State University.Demirjian, A. M. and Quijano, I. M., "Power Conditioning and Control Requirementsof Coal-Fired MHO Generators." Presented at AIM Aerospace Sciences Meeting,St. Louis, MO., January 12-15, 1981.Hruby, V. J., "Experimental Investigation of the Effects of Electrode Wall Tem-perature on MHO Channel Performance and Cathode Nonuniformities." Presented atthe Seventh International Conference on MHO Electrical Power Generation, M.I.T.,Cambridge, MA, June 16-20, 1980:

*This work was supported by the U. S. Department of Energy under Contract No.DE-ACOl-80ET15614.