EXPERIMENTAL INVESTIGATIONS ON A PILOT-SCALE MHD-STEAM COMBINED CYCLEPLANT

Liu Chengze, Shu Zongxun, Cai Ticheng, Hu Yingjiang, Shi Jianmin,Xu Guanbin, Zhong Baohua, We Degang, Jin Ming, Wu Jiahe, L1 Gengxin andGao Dex1ngShanghai Power Plant Equipment Research Institute,

Shanghai Electric Machinery Manufacturing Works, Shanghai, China

The paper describes the research and construction of a pilot-scaleMHD-steam combind cycle power plant in Shanghai Power Plant EquipmentResearch Institute. It also describes the purpose, general arrangement,parameters coordination of that plant as well as the philosophies of thedesign and selection of the major equipments.

The paper is concentrated on the process of 100 hours test and theresults of the test are presented.I. IntroductionResearches on MHD power generation were initiated in late 50's. As anew method of direct power generation, it utilizes the high temperatureconducting gas generated in the process of fossil fuel combustion suchas coal, oil and natural gas with a temperature of ~pproximately 30000Kand then passes through a generator channel in a powerful magnetic field.With the effect of electromagnetic induction, the heat contained in thegas will convert to electrical energy directly. The temperature of theexhaust gas from MHD plant will still be as high as 20000X, however, dueto the lower conductivity of the gas at that temperature, it is verydifficult to generate electricity any more. In the concept, the efficiencyof MHD power generation cycle would be very low, only about 20 to 25 percent. However if the waste heat contained in MHD exhaust gas, which isabout 20000K, can be utilized for power generation in a steam powergenerator, through a waste heat boiler, that is to combine "the MHD powergeneration with the steam power generation to form a combined cycle, theheat efficiency of a conventional power generation cycle, can be raisedremarkably from 30-40% to 50-60% with a fuel reduction from one fourth toone third. The fuel saving of a 1000MW MHD-steam combined cycle powerplant with a efficiency of 60% would be 100 million tons on a yearly basisas compared with that of a conventional steam power plant of same capaci-ty with a efficiency of 40%. Therefore, the MHD-steam combined cycle hasbeen the way forreduction of energy consumption in power generation aswell as the development of fossil power technology.

Together with Shanghai Electrical Machinery Manufacturing Plant, ourinstitute has been engaged in the research of MHD power generation eversince 1966. Cooperated with Shanghai Institute of Ceramics, we developeda short-time low voltage Faraday type MHD power generator SM-2 in 1971with a power output of 580 KW and running duration of 3 minutes and a highvoltage Hall type MHD power generator SM-3 in 1973 with a power output of102 KW at 1200 V and one hour running duration. In.addition to that, wehave studied the design, structure, materials, operating performance and,other subjects of MHD generator and have gained experiences for the desighand manufacture of special type power source with short time duration and'high power capacity for practical use.

Since 1974, we have concentrated our efforts on the research ~~ilit,MHD power generation. The research mainly involves two stages of wor~. Inthe first stage we designed and manufactured a SM-4 MHD generator with apower output up to 20 KW, using preheated air as oxidizer and with acontinuous running capability of more than 100 hours. This work was com-pleted in 1976. The generator together with the whole ·system have beenoperating stably and continuously for 200 hours at its longest, with amaximum power output of 18 KW and power density as high as 2200KW/m3 andhave met the preestablished requirments.

The research of the second stage was aimed at developing a pilot-scale'MHD-steam combined cycle power plant by the use of exhaust gas fromgenerator SM-4 so as to obtain technecal data that could be of referenceto the design and ~anufacture of large utility type MHD-steam combinedcycle power plant.

II. Conceptual Design of the Pilot Plant Basic design considerations:1. The temperature of the exhaust gas from the SM-4 generator is 1850oC,

that equivalents to a thermal power of 3600 KW, based on that thermalpower, low pressure steam could be generated at a rate of up to 4T/hr.

2. A steam power of 4T/hr would be sufficient for driving a 500 KWturbine generator. But since there was no such generator available, we hadto use a 750 KW one for low-load operation.

3. Through the operation of plant, further studies can be made on thedurability of the generating channel and the reliability of the wholesystem. Other than this, some key technologies of MHD-steam combined cyclepower plant can also be studied. Namely:

A. General arrangement and performance 'design.B. Design and manufacture of the waste heat boiler, study of the seed

deposition tendency at different tem'perature zones and measures to preventblockage and tests for obtainning the data for boiler thermal-dynamiccalculation. •

C. Boiler exit gas seed recovery.D. Running test and operation of the MHD-steam combined cycle power

plant.The schematic diagram of the combined cycle power plant is shown in

Fig. 5.III. Design and Selection of the Major Equipment---1. Air Preheater (See Fig. 1 and reference 1)

The MEn power generator SM-4 uses indir~ct heating, the air preheattemperature will be 1450oC. It consists of two pebble-bed regenerators andhas the following features:

A. Aluminum oxide based materials for all refractories.B. 25mm spherical heat carrier.C. Auxiliary combustor installed on the hot-air duct;

---2. MEn generator SM4 (See Fig. 2 and Reference 2) ,The MHD generator proper consists of 4 component parts : a main combust-

or, a generat channel, a magnet and a diffuser. The outlet of the diffuseris connected to the gas flue where the MHD exhaust gas will pass throughand enter into the waste heat boiler SMS.

A. Main CombustorThe main combustor is composed of two parts: a burner head and a chamb-

er containing an accelerating section.Parameters of the combustor:Total gas flow rateTheoretical combustion temp.Heat loosesHeat liberation rateB. Generator ChannelThe principles guiding the design of the channel are:a. Subsonic gas flow in the channelb. Subatmospheric pressure for gas flow in the channel (0,7 atm. abs.)C. Peg-type insulating walls for the segmented Faraday-type channeld. For semi-hot electrode walls, high-temperature electrical conducting

ceramics are to be used as inserts. They are made of zirconia,lanthanum chromate and lanthanum calcium chromate, 8mm in thickness.As for peg-type insulating elements, 13mm-thick inserts made ofmagnesia are to be used. To prevent short-circuit of Hall voltage,insul~tion is provided between the channel and the combustor.

Parameters of the Channel:Design flow rateInlet cross sectionExit cross sectionLength

1kg/sec.27680K

3%0.65 x 1d' Kcal/m3 .hr.

1 kg/sec.150 ':,'D!mx 7Omm180 ,:mID x 70mm150Omm.70Omm.C5mm.18KW.

(HxW)(HxW)

TotalEffective length

Pitch between electrodesMax. PowerC. MagnetParameters of the magnet of SM4Coils of the magnetic field 348 turnsExciting current 1500 A.Dimension of the magnet 110 x 700mm (HxL)Gap of the magnetic field 225mm (W)Magnetic field intensity (max. at the centre) 1,82 Tesla

(average: 1.6 Tesla.)D. DiffuserIt is linearly divergent, 1800mm in length and the subatmospheric

pressure at its inlet is 0,7 absolute atmosphere.

---3. SMS Waste heat boiler (See Figs. 3 and 6 and Reference 2)The waste heat boiler is mainly employed to recover the waste heat in

high-temperature exhaust gas of the MEn generator so as to turn it intosteam to drive the turbine set. In addition, as the exhaust gas cools downin the boiler, the gaseous seeds, composed mainly of K2C0'3' will changeinto a liquid or solid state to be recollected in the boiHtt'1_'and .aubsequen-tly by the precipitator.

Boiler design philosophy .•MEn exhaust gas contains about two per cdnt of potassium compounds, As

the gas gradually cooled down in boiler, then compounds will condense anddissociate and deposit on the heating surface, it will increase the flowresistance and affect heat transfer property and even more serious~,thewhole unit would have to be shut down due to slaging and fouling. This isvery important to an MEn-steam combined plant. To solve this problem andensure the safe operation of the boiler, we used a comparatively largecooling furnace, which cools the inlet ~s from 18500C down to 750°C andbelow, about 1000C lower than the melting point of potassium compounds,so that K2C03 entering the convection pass is in the form of small sizeparticles.

To reduce the fouling in convection pass, sootblowing will be performedin the vertical direction. In addition, wide pitch fin tube panels are usedin convection pass to prevent bridging.

Considering that part of ~otassium compound will be condensed, it ispossible to use slag tapping furnace and heating surface of convection taSAwill be cleared periodically by retractable sootblowers.

As the high-temperature seed-laden gas (over 5000C) will cause seriouserosion problem both on metals and furnace refractories, proper suspensionand support of heating surface as well as furnace wall protection have beenconsidered.

Although the boiler steam temperature and pressure are low, the relative-ly high furnace height provides the possibility of using natural circulation,in spite of high boiler gas inlet temperature and furnace heat liberationrate will not affect the heat transfer property. Another advantage withnatural circulation is that all the inherent problems of an once-throughboiler can be eliminated.

Because of the existence of molten potass~um compound inside the furnace,special consideration should be given in boiler design to minimize tubeleakage so as to prevent any possible explosion caused by water togetherwith potassium compounds, That kind of explosion has happened on blackliquor recovery boilers of paper mill. The explosion of that nature willcause serious injuries on people and damages of equipment. High pressuretubes will be adopted as boiler tubes and all the tube welds will bestrictly tested and necessary measures will be taken to prevent any possib-le explosion.

Performance data of waste heat boilerGas flow rateInlet gas temperatureExit gas temperaturebust content in gasBoiler ratingSuperheated steam pressureSuperheated steam temperaturePreheating temperature of feedBlow-down capacityThermal performance dataDesignationHeating surface area Hm2Inlet gas temp oCExit gas temp. 0CInlet temp. of workingMedium! 09Exit temp. of workingMedium. °cHeat-3release rateX 10 Kcal/hr.Heat transfer coefficientKcal/hr.M2.0CTemp.difference oCGas velocity M/Sec.~~!£~:YM~Se~orking

Furnace651850750

3000 NM3/hr18500C2000C20g/NM3

(Mainly K Compounds)4.15T/hr14k§/cm2350 C1050C (at the outlet of

deaerator)2%

Supepheater Economizer125 142750 350350 200197 10535-0 150445 16111.7 8.9257 1412.5 2.611.8

Natural circulation ratio K = 110 and aboveAssumed circulating water velocity W = 0.74M/Sec. and abbve

---4. The Turbogenerator Set (Fig.4)Turbine Model

Type21";0.75 (II).Low-pressure, Single-cylinder,axial flow, impulse, s~eedreduction, condensing turbine.Power output

on nameplateSpeedInlet steam pressureInlet steam temp.Back pressureflow rating

Generator ModelTypeCapacitySpeedVoltageFrequency

---5. Electrostatic precipitator forReference 3).

Since seed recovery is essential in air pellution control and in theimprovement of plant heat rate, in addition to the high temperature seedrecovery in boiler cooling 'chamber, low temperature seed recovery is alsoprovided in gas exhaust circuit. The seed recovery is an essential part inthe MED-steam combined cycle plant.

SHWB series precipitator with an ,effective area of 5M2 manufactured byShanghai metallurgical and Mining Machinery plant will be used as a seedrecovery on our unit with necessary modifications.

The orig1nBldesign parameters of the precipitator are as follows:Effective cross section 5.1 M2 3Capacity 11000-14700 M /hr.Number of electromagnetic fields 2Electrostatic voltage 60,000V.Electrostatic current fOO mA.Air velocity of field 0.6-0.8M/Sec.Vibrating type of collecting electrode MechanicalVibrating type of discharge electrode Electromagnetic.Resistance . 20mm H20Maximum allowable gas temperature 300oC.Design efficiency 98% (particle si~ 22.45P.)The efficiency of the electrical precipitator depends upon the grain

size: the smaller the size of the dust particals passing the precipitator,the lower the efficiency, and vice versa. For finer particles, the expectedefficiency can also be ensured if the precipitator is made larger. When wegot down to the design, we knew roughly the grain size to be dealt With,but'we lacked first-hand knowledge in this respect. So, we chose thesmallest electrical precipitator to start with, hoping that through aseries of experiments, we would gain experiences and accumulate data sothat we would be able to work out the highest efficiency possible for t~ewhole exhaust gas system, including the high-temperature recovery in thecooling chamber of the waste heat boiler and the low-temperature recoveryby the electrical precipitator.IV. 100 Hours Power Test

After the completion the pilot-scale plant in October, 1979, we spentmore than one year in trial running the various units and assemblies ofthe combined cycle and the whole system as well. The trial running fellinto four stages: 1) Separate tests on individual units of the system;2) boiler ratin~ test; 3) Commercial operation test on steam power genera-tion portion; 4) Commissioning test on combined system. After thess fourstages of tests had been completed, we conducted a long time test on com-bined cycle power generation in June, 1981. This test lasted f~ 212 hoursstart from the initial operation of the auxiliary equipments and airperheater at 10 A.M. of June 3 to the shut-down of MHD generation and maincombustor at 6 A.M. of June 12. The time for continuous operation of thewhole system was 150 hours, including 100 hours for continuous powergeneration. In the course of combined generation, the perheated air of14500C, mixed with diesel oil burned, and after potassium hydroxide wasadded, a high-temperature electrical conducting gas was produced, whichenabled the pilot-scale MHD generator to generate.a power of 14.9 KW atthe maximum. And later, the high-temperature gas exhausted from the MHD

750KW5500 rpm13kg/cm3400C.0.0927 atm. abs.6T/hr.TQT 15144 poles750 KW1500 rpm.6300V.50 c/sec.

seed recovery (See Figs. 3 & 7 and

generator made the steam power generator SMSgenerate a power of 500 KW atthe maximum, the power output from the MED generator was consumed in thetest load circuit while that from the steam-power cycle was put into theShanghai grid, The heat energ~, as produced by the single combustion in thepilot-scale combined MHD-st~ cycle power plant was utilized to be turnedinto electrical energy, thus realizing the combined MEn-steam cycle. Thegeneration test started from the on time operation at 0:05 A.M of JUne 8to trip out at 4:05 A.M of June 12.

During the continuous operation of the combined cycle, the various keyequipments were put to test under different working conditions at differentflow rates, temperatures, pressures, and intensities of the magnetic fieldsunder different loads, in various percentages of compositions and withdifferent matchings of parameters, The main items having been studied were:the thermo-electrical properties of the MED generator channal; long timeoperation properties of high temperature electrical conducting materialand ceramic insulation materials; heat-recovery of MaD waste heat boiler;determination of actual heat transfer coefficient; hydrodynamic charac-teristics of the steam and water pipelines, determination of the physicaland chemical properties of the seeds of potassium compounds in the varioustemperature zones and their depositing effect; determination of the seedrecovery of the electro-static precipitator; variations of the performanceparameters of the various units of the combined cycle and their interela-t1.ons.

Operation conditions for combined power generation tests are as follows:Temperature of heated air of SM4 preheater 1400-14~.SM4 high-temperature gas flow rate 1 kg/sec.SM4 magnetic field intensity (at the centre) 15,500-17,500 gauss.Electrical power(iofSM4 MHD generator channel 10.5 - 14.9 KWInlet gas temp. of SMS waste heat boiler 1600 - 17000CEvapourative capacity of SMS waste heat boiler 3 - 4.2 T/hr.Pressure of superheated steam 14kg/cm2Temp. of Superheated Steam 340 - 360oC.SMS turbine:

Rotary speedInlet Steam PressureInlet steam tempOutlet back pressure

SMS generator:Speed 1500 rpm.Available power 350 -500 KWVoltage 6500 - 6600 V.Network frequency 50Hz

The performances of the component units of the pilot plant during thetests are summarized as follows:1. MEn generator SM4 (See Reference 4)During the test, with the main combustor grounded a comparative test

between the effects of the grounding and insulation against ground of thediffuser on generating process was conducted. The test results showed thatwith the same gas flow rate of G = 0.9 kg/Sec, exciting current of 1000 Aand Bmax 15300 gausses, the power output in either case was more than 9 KW,there is no significant difference. Furthermore, when the diffuser wasinsulated against ground nominally, its potential to the ground was againdetermined to be zero. This demonstrated that the diffuser was connectedto the ground through conducting exhaust gas.

Following are the data of the SM4 MHD generator und&r a typical workingcondition during the 100 - hours combined generation test.

Gas flow rate 1kg/sec.Pressure of cc.bu.,tionchamber 1.45 atm. abs.Inlet pressure of the effective section of 0.830 atm. abs.the channelOutlet p~e88ure of the effective section ofthe channelInlet pressure of diffuserNumber of electrode.Exciting currentMax. intensity of magnetic field (at the centre)Total power outputTotal working currentAverage working Tolt&BeHall volta«e

6500rpm.13kg/cm2330 - 3500C0.06 - 0.07

0.74 atm. abs.34 pairs.1400 A.1.77 Tesla.14.9 KW183 A.81 V.400 V.

In the process of the two tests on the combined generation, the poten-tial distribution of the generator channel was determined. Fig. 12 Showsthe distribution of the different potentials of the electrodes along thechannel in one of the tests. From the Figure, it can be seen that as the,potential at the end of the channel was made Zero from grounding by theexhaust gas in the combined MHD-steam cycle generation, the lowest potentialappears at the channel inlet, the minimum potential being-400 V. In otherwords, it indicates that the simple structure we adopted to insulate thecombustion chamber against ground well satisfies the demands for insulationof the pilot-scale power plant.

In these two tests, we arranged for a comparative test of the electrodematerials. the institute of ceramics of the Academy of Sciences of Chinahad developed six types of electrode material, namely, stabilized zirconiaCSZ, lanthanum calcium chromate LCC, lanthanum calcium chromat.e + magnesiaLCCM,lanthanum chromate + iron oxide LFC, ceria ZC, and silicon carbideSic. Different combinations of anode and cathode in the channel were tested.

As a result of comparison and screening in the first combined generationtest it is proved that LCCM's electrical properties and its ability toresist erosion are both excellent and can be used either as anode or as ca-thode, that LFC is good in its electrical properties, but as its erosion-resistance is not satisfactory, it would be better to use it as anode; andthat CSZ and ZC are not good in their electrical properties, but are excell-ent in their ability to resist erosion. Based on the test results, in the100-hours combined cycle generation test we used in the channel, LCC,LCCMand LFC for the greater part of the anode electrodes and used LCCM, CSZ andZC for the cathede electrodes. The results showed that such a choice wasappropriate and the power output was raised by 15% with little power varia-tion in the channel throughout the 100 hours.

2. Exhaust gas flue of MED generation.After some time in the 100-hour combined cycle generation test, with

the gradual temperature rise and consequential heat expansion the gapbetween the diffuser and the flue inlet was correspondingly narrowed.Nevertheless, an average gap of 3 mm was still maintained and thereforethrough this gap normal air flowed into the high-temperature exhaust gas,According to;)our analysis, the entry of this air flow was probably one ofthe reasons of why the temperature of the gas measured at the inlet of thewaste heat boiler was slightly lower than it should have been.

3. SMS MHD exhaust gas waste heat boiler.The temperature at different zones of the waste heat boiler as·measured

in the combined cycle generation are as follows:Designation Furnace Superheater Economizer

Temp. of inlet gas oC 1a87 610 290Temp. of outlet gas oc 610 290 165Temp. of inlet water (steam) oc 197 197 33Temp. of outlet water (Steam) oc 197 355 100

The chemioal compositions of the potassium compounds (seeds) at thedifferent points in the boiler are listed below (analysis was made by theEnergy Group of Soozhou Central Chemical Laboratory).

Sampling place. KOH% K2C03% KHCQ% KCl% K2S04%Furnace 8.77 90.94 0.29Superheater 9.37 86.64 2.28 0.71Economizer 91.84 6.83 0.49 0.84The MHD exhaust gas flowing into the waste heat boiler appeared to be

translucent and purple-reddish in the middle part of the furnace in theform of misty ~ and there were white deposits of potassium compoundsaround the low-temperature observation door as well as on the water walls(Fig.e) From the fact that the pptassium compounds deposition of the seedson the water walls contained potassium hydroxide KOH of up to 10 per centit can be seen that in the radiant furnace of the waste heat boiler, seedsexisted in the gas mostly in the form of KOH vapour and the temperature ofthe water walls surface was generally about 2500C, muoh lower than 6330C,the melting point of KOH. As a result, the gaseous KOH began to condenseand set on the metal surface of the water walls. Mean while, the condensedKOH reacted with C02 to combine and form K2C03. But owing to the fact thatthe speed of suoh a chemical reaction was lower than that of the condensingspeed of KQH, the deposition on the wall surfaoe contained o~ly a littleamount of KOH.In addition, it should also be noted that inlet gas temperature was verylow under test oondition only 1687°C and the depositing speed of seeds onthe pipe walls was not high. So it could not be a thiok deposition of mol-tea potassium liquid was fo~ on the surface of the deposition of the pipe

walls. Fig.9 is a photo of part of the deposition spalled off from the pipewalls, which was taken after the test. These intermittent thin streams

gathered at the furnace bottom formed after 150-hour operation, only a mol-ten layer,no thicker than 10 cm; therefore no large amounts of seeds couldbe recovered through slag-tapping, as had been anticipated.

In the beginning of the test, the. seeds deposited rather quickl~ fora time on the convectional heat exchanging pipe walls of the superheaterand the economizer, but Yater the depositing process slowed down gradually.There were signs of the deposits being swept away and spalling off themsel-ves. This fully verif~ed that the layout of a parallel sweeping type ductwes feasible. Fig. 10 is a photo showing the convectional heat exchanger inoperation.

According to our observations, after 150-hour operation of the boiler,the deposition on superheater tubes was about 8-10 rom in thickness, whichconsists of three layers, the outer being pale cream-coloured, the middledarker in colour and the inner one of white powder. The deposition on theeconomizertube wall was not uniform, its thickness being generally in therange of 10-20 rom, with a layer of 30-40 rom in a few places. Here it mustbe pointed out that to observe the process of the deposition on the convec-tional heat exchanging area, during the 150 hours of operation, we usedno soot blower to clear away the seed deposit, just letting the depositionthicken as it would be. From the test we conducted, it can be noted thatas a result of the natural thickening of the potassium compound deposition,the heat transfer property was affected. At the same inlet gas temperature,the outlet temperature increased step by step, resulting in a gas tempera-ture rise by 20-300C. However, in actual operation if air blowing is employ-eO at an interval of 24 hours, it is quite possible to control the \hick-ness of the deposition of the superheater and the economizer tubes withintheir limits of 5 and 8 rom respectively.

In the test, we also measured the thermal parameters of the gas andsteam. And by using a specially-designed heat flow meter, we determinedthe heat flux density, heat-transfer coefficient and fouling factor of theseed deposition, ~hereby obtaining a large amount of data (Figs. 13 and 14).All these will serve as a reference for the thermal calculation of MHDwaste heat boiler. (see r.eference 2 and 5 for details).

4. Turbogenerator Set.At the initial stage of the test on the combined cycle ~ower generation,

it was found that the insulating value of the winding of the generatorrotor was too low. In order to bake and clean the rotor of the generator,to remove the troubles in the indicating meters in the circuit of the high-voltage switches and instruments and to replace the faulty parts, on-lineoperation of the unit was delayed. During the 50 hours combined cycle powergeneration before the on-line operation, the super-heated steam from theboiler was released to atmosphere before the turbine main steam valve. Thatexplains why the continuous operation of the whole system lasted 150 hoursin the tests of June, 1981, longer than the on-line combined cycle powergeneration at that time, the later lasted about 100 hours.

In the second test on the combined cycle power generation, it was shoWILthat there was a great difference between the MaD cycle and steam powercycle in their load change characteristics, especially during start-up aridshut-down. The characteristics of the load change of MHD power generation(Start-up and Shut-down time) depend largely on the limits on the tempera~ture change rate of combustion chamber and MHD generator channelwhich areusually subject to the heat of combustion gas. The load response charac-teristics were proved to be fairly satisfactory. In contrast, the loadvariations of the steam power cycle, the changes in power output and thetime for start and stop are greatly influenced by such factors as the heat-transfer inertia, the huge heat capacity of the steam and water and thetemperature rise speed of the rotor and cyclinder of the turbine. Therefore,the response of the steam pow~r cycle to load variations is not sensitiveenough. As a result of the difference in load variation characteristics,the first artd the second cycles in the combined cycle do not work well instep, particularly when the MHD generator is required, either by externalor internal factors, of increasing its load drastically or internal fac-tors, of increasing its load drastically or of an emergency stop. For thesteam power cycle, owing to its large inertia variation, it can not sus-tain to such a drastic change, so according to the experiences obtained inthe pilot-scale combined cycle power generation, to coordinate the loadvariations of the two cycles, it seems to be desirable to prOVide a sparecombustor in the furnace of the waste heat boiler of the MHD power genera~tion exhaust gas, for it provides a new means of controlling the loadvariations in the steam power cycle.

At the same time this new spare combustor enables the steam power cycleto operate independently for producing electricity in case the MHD genera-tor needs to be stopped. In the future, such a provision helps to increaseits time for power supply. In fact, Lt can raise the reality of a pilotpower plant and decrease the cost for experiments and scientific researchon the plant.

5. Electrostatic precipitator for seed recovery.The compositions of the potassium compounds as recovered by the electros-

tatic precipitator were analyzed by Suzhou Chemical Research Institute.The results of the laboratory test are as follows:

K2C03, KHC03. KCI K2S0468.92% 28.50% 0.21% 2.33%From the above table it can be seen that the potassium compounds reclai-

med contain a certain amount of KHC03' potassium bicarbonate this wasbecause the mean temperature of the gas in the electrostatic precipitatorhad become lower than 1650C. So the K2C03 in the gas began to act with H20and C02 to form KHC03. The formation of KHC03 is harmful to the recl~ionof seeds. The solubility of KHC03 in water being lower than that of K2C03 ,it is undesirable to use KHC03 directly as seeds. There needs to have asupplementary reclaimer, and correspondingly, additional energy consumptionfor it. Furthermore, K2C03 powder takes in moisture readily, and lumpingtogether, the powder adheres to the cathode wires and also to the surfaceof the anode collecting electrode, making it impossible for the electros-tatic corona to discharge. And this, together, with other factors tends tounstabilize the working condition of the electrostatic precipitator.

During the test on the combined cycle power generation, we.entrustedShanghai Institute of Measuring and Testing Technologies to determine thegrain size of the powder of the recovered potassium compounds, and it turn-ed out to be in the range of about 0~1-1P. This size sh9Uld be consideredto be very small in the potassium compounds in the low-temperature exhaustgas for the combined MHD-steam cycle, and compared with the grain size inthe dust in the cement and other similar industries, smaller by an orderof ten times. Therefore, it is necessary to carryon specific research onthe structure of the electrostatic precipitator for the recovery of seedsin the combined cycle in relation to grain size.

In the tests, researchers from the Research Institute of Nanking Chem-ical Company and from the Measuring and Testing Group of Shanghai Smelteryworks determined the collection efficiency by using potassium ion concen-tration method and gravimetric method respectively. The results showedthat with the precipitator working under normal condition (two~ields working simultaneously at a voltage of 60,000 V and a current ofabout 85 mA) the collection efficiency was 94 - 96%.

However for most of the time, when the precipitator had been workingfor some time after cleaning the anode and cathode electrodes, the effic-iency decreased gradually to only about 60%.

In the test, after letting the exhaust gas out of the by-path of theflue; we opened the precipitator to examine its interior and discoveredthat the cathode wires were encircled by lumps of deposited seeds. Thishindered corona discharge and consequently reduced the collection coeffic-ient greatly (see Fig. 11).

And it was also discovered that the surface of the anode plate for seedcollection was covered with a thick layer of powder, which increased theresistance, reduced the working current and hence the collection coeffic-ient. (See Fig.11). It· follows that most probably, the powder depositionof the collected potassium compounds on the anode and cathode was the mainfactor that decreased the collecting efficiency. mean while, it also in-dicates that the magnetic vibration of the cathode wires and the mechanicalvibration of the anode electrodes as adopted in this case was rather in-effective. They did not work as well as it was expected.V. Conclusions

Technically Speaking, the achievements obtained from the tests on theP11~t-seale combined MHD-steam.cycle power plant can be summed up asfollows:

1. The practice of the uninterrupted 100-hours combined cycle powergeneration proves the feasibility of the MHD-steam cycle and verifies thatthe overall layout and the design of the plant, the matching of the parame-ters and the provision of the various auxiliary equipments are basicallyappropriate. In addition, there are some experiences gained in regard tothe general design, the co-ordination in the operation of the variousassemblies, and the automatic control and measurement of the combined cyc-le.2. In the tests, the heat recovery efficiency of the waste heat boiler

was determined, and the deposition of the.seeds of potassium compounds inthe various temperature zpnes of the boiler and its effect on heat trans-fer and on the working conditions of the system were also studied. Besides,the efficiency of the soot blower was likewise put into test. The resultsshow that the po~formance of the SMS waste heat boiler is on the wholesatisfactory. .

The data obtained in the test provide some reference to the thermalcalculation as well as to the design of the structure of the MHD wasteneat boiler.3. The requirement for insulation against ground deemed to be specificto the MHD generator channel has been verified and this leads to thesolution to high-voltage insulating structure of the MHD combustion cham-ber in the large combined cycle power plant. The various ceramic electrodematerials with zirconia, lanthanum chromate, ceria or silicon carbide astheir base were tested and compared, thus rendering guidance for furtherexperiments to develop durable highly-efficient electrodes for the MEDgenerator channel.

4. The combined cycle generation testifies the possibility of acpievinga high percentage of seed recovery by means of a dry electrostatic preci-pitator. But at the same time some problems were revealed in the courseof the long operation of the precipitator. These problems require furtherstUdy. The first-hand information collected in the tests on the chemicalcompositions and grain size of the reclaimed seeds could be used for areference in the design and the selection of an electrostatic precipitator.

The tests on the 1oo-hours generation of the pilot-scale combined MED-steam cycle power plant marks a great step forward in the research of MHDpower generation. We have now reached a new stage at which the researchon individual MEn generation plant has given place to the research on thecomplete system of combined MHD-steam cyple. Our pilot-scale power plantcan serve as a base for further of the analogous test could be a ~eferenceto the design and operation of large combined MED-steam cycle power plantsin the future. Last but not the least, to a certain extent the success ofMHD power generation into actual service in our country.

1. Shanghai Electrical Machinery ManUfacturing works: pilot preheatedAir MED Generator, A collection of Research Reports on MED power Genera-tion, 1978

2. Shanghai Power Plant Equipment Research Institute: Design and Exper-imental Investigations of SMS Waste Heat Boiler for MED Power Generation.A collection of Research Reports on Combined MHD-steam Cycle, 1981.3. Shanghai Power Plant Equipment Research Institute: Applications ofHigh-voltage Electrostatic Dust Removal Techniques to Dry-type Seed Reco-very, A Collection of Research Reports on Combined MHD-Steam Cycle, 1981.

4. Shanghai Power Plant Equipment Research Institute: Tests on Fara-day and diagonal Joint in SM4 Generator Channel, A Collection of ResearchReports on Combined MHD-Steam Cycle, 1981.

5. Shanghai Power Plant Equipment Research Institute: Heat RecoveryCharacteristics of MED waste Heat Boiler SMS, A Collection of ResearchReports on Combined MED-Steam Cyc2e, 1981.

Note: The following researchers also took part in the research project:they are: Iu Hongwen, Gu Yifei, Zhou Youcan, Zhong Yongde, Ni Renhua,Xu Guanyon Sun Zhiming, Zhong Weixin and Mao Youying.

Fig. 3 SM3 waste heat boiler andelectrical precipitator for seedrecovery

Fig. 4 21-0,75 (II)-TQT 1514/4t~ogenerator set

Fig. 5 Scheme of thermal system of pilot-scale combined MEn-steamcycle power plant

1. Combustor; 2. channel; 3. magnet; 4. diffuser; 5. load; 6. wasteheat boiler 7. turbogenerator set; 8. electrostatic precipitator;9. draft fan; 10. stack 11. air preheater; 12. compressor.

Fig. 6 Sectional view of SMSwaste heat boiler

Fig. 7 Sectional view of electricalprecipitator for seed recovery dischargeelectrode

Fig. 8 MHD exhaust gasin SMS waste heat boiler

Fig. 9 Deposition ofseeds on water-coolingwall of furnace.

Fig .1Q Conventional heat-exchanging area of wasteheat boiler in operation

NtJ. oj tiectrotiesat 3.4

Fig. 11 C~thode wire ofelectrical preoipitatoras encircled by deposited powder of seeds

ti - 0.'1 Ie,I.src

8=/.6,..

Fig. 12 Diagram of potentialdistribution of SM4 generator channel

P:m 1000

E-I

III

~ 600

Furnace Superheater Economizer-Gas flow path

o Tested value of"fouled" heating area

~ Tested value of"clean" heating area