Study On Magnetic Seal Technology of Ash Unloading Device. with Impeller of EPS

Transcription

1 Study On Magnetic Seal Technology of Ash Unloading Device with Impelle of EPS Yuan-Yongtao Qi-Liqiang Yang-Qian Ji-Yuanxun Envionmental Pollutants Detemine and Contol Reseach Institute, Noth China Electic Powe Univesity, Baoding.71, China Abstact: The seal poblems of ash unloading device with impelle unde hoppe ae vey common and impotant to the successful opeation of an electostatic pecipitato. Hoppes become plugged and ai ingess causes collected ash to e-entain esulting in inceased emissions, and these igid seals ae subjected to excessive wea and need fequent eplacement. This pape discusses an advanced powde magnetic seal design that installs pemanent magnets at the top of vane and utilizes the magnetism in the Fe O 4 in fly ash to seal the space between the vanes and the walls. The device does not wea and does not need fequent eplacement, and the pessue of ai seal can be 5kpa. 1. The Analysis of The Element of Fe and The Magnetic Paticles in Fly Ash 1.1 The Contain of Fe and The Fom of The Mineal The main chemical complement in fly ash can be classified into two categoies. One is SiO 2 and Al 2 O, the othe is ion oxide, fee cabon, MgO and K 2 O. And because of the diffeent buning coal, coal textue, the types of the boile, buning ule and dedusting way in the diffeent powe plant, in fly ash thee is diffeent content of the elemental complement. In China, the content of Fe in fly ash anges fom 1.5% to 15.4%, it exists in the fom of the mineal Fe mostly. This is because that the main Fe mineal, cabon and cabon monoxide play eactions duing the couse of the boile buning, it foms Fe O 4 and Fe 2 O, these newly oxidation of Fe and the newly fomed beaded glass link togethe, fom the peal iched Fe whose shape like spheicity. In fly ash the mineal contained Fe mainly consists of magnetite, pyite, hematite, sideite and limonite, which fom solid fusing paticles. These fine paticles in mineals contained Fe almost have the magnetic popety. The categoies of mineals in fly ash mainly ae beaded glass, quatz powde, cabon powde and magnetic peal, whose content ae moe than 9%. In fly ash the content of magnetite peal is to 7%, the othes like toilite, chileite and some of ion o hematite have the chaacteistic of paamagnetism. It is intepeted that the specific weight of magnetic paticles in fly ash is available fo seal using magnetic field. And the content of Fe, mineal composition and Fe s states in fly ash of some powe plants in China ae shown in the followed two tables. Table1. The Content of the Fe in Fly Ash in Some Powe Plants of China Plant name Coal Content of Fe in fly ash ashes(computed in Fe 2 O )(%)

2 Liaoning powe plant Pingzhuang coal Qinghe powe plant Tiefa coal 11.8 Datong powe plant Datong coal Wulashan powe plant Wuda coal 19.4 Xuzhou powe plant Ganlin coal 17.8 Jinzhushan powe plant Anthacite 2.4 Jixi powe plant Jixi coal 15. Jinin powe plant Zaozhuang coal 2.4 Baoding powe plant Xishan coal 5.19 Tangshan powe plant Kailuan coal 7.82 Hushi powe plant Zhunge coal 5.12 Taiyuan No. 2 powe plant Datong coal 6.92 Table2. The Mineal Composition and Fe s States in Fly Ash of Some Powe Plants in China Mineal name Content% Mineal name Content% beaded glass Fe element.7~.21 quatz powde Cu element tace cabon powde 1~15 galenite tace magnetic peal 6~7 zinc blende tace magnetic powde.~1.2 bucite ~4 toilite.8~1. ion pyite.7 Chileite, hematite.2 ampelite tace feosilicon oxide.5 utile tace 1.2 The Chaacteistics of Magnetic Powde in Fly Ash Fe is deoxidized to oemeite in pat, and the othe is deoxidized to magnet by cabon and cabon monoxide although it is un-magnetite unde the condition of existing C and CO and the tempeatue is about 14 when the coal is buned in boile. The magnetic powde, which is the beaded glass ichened Fe, which owns the dake appeaance, heavie density, and it is paamagnetic so that it can be sepaated fom the close-gained beaded glass by magnetic choice. The magnetic powde supplies the seal pobability though magnetic foce fo powde, and the content and the magnetization ate effect the seal The Element Composition and Fom of Magnetic Powde The element composition of magnetic powde lies in the mineal composition of the coal. Moe than 6% content in it is Fe 2 O and Fe O 4, about % SiO 2 and Al 2 O and about %--5% CaO and MgO. The magnetic powde in fly ash is constitute of magnetite, aeosideite meteoic ion and chileite. The magnetite, that is Fe O 4, belongs to feous magnetic mineal. While the

3 hematite, that is Fe 2 O, has two kinds of tansfom: one isα Fe 2 O, which is un-magnetic, and the othe isγ Fe 2 O, which is magnetic. And the content of α Fe 2 O andγ Fe 2 O in hematite lies in the bunt tempeatue of boile. α Fe 2 O is half-baked diamagnetic mineal whileγ Fe 2 O is feous magnetic mineal. The categoies and content of Fe element in fly ash of diffeent powe plates ae diffeent with the vaiety of flame s tempeatue in boile. At fist, Fe O 4 is oxidized toγ Fe 2 O, o eutectic mixtue containedγ Fe 2 O and Fe O 4, then it tansfomsα Fe 2 O. And afte the tempeatue exceeds 14, Fe 2 O tansfoms Fe O The Physical Chaacteistics of Magnetic Powde Table shows the physical chaacteistic of magnetic powde in some powe plants in China. Fom the table a conclusion can be dawn that tue density and heaped density of magnetic powde ae geate than those of fly ash. This is due to the abundant Fe element in magnetic powde. In this table, it can be seen that the numbe of paticles whose ganulaity ae less than 75µm is moe than 5%. It shows that the paticle diametes of magnetite ae fine and the paticles ae close-gained. The content of Fe element is moe than 95% in fine magnetic powde, and it owns the high specific magnetization ate(about cm /g, which is close to the specific magnetization ate of magnetite.) As a esult, the chaacteistic of magnetic powde is available to seal in magnetic field. Table. The Physical Chaacteistics of Magnetic Powde in Some Powe Plants in China Contain of Plant name matte whose Heaped Contain of Specific Tue density ganulaity (g/cm density magnetic magnetization ) (g/cm ) matte(%) ate (cm is less than /g) 75µm (%) Jixi Powe Plant Fushun Powe Plant Zouxian Powe Plant Shiliquan Powe Plant Tangshan Powe Plant Zhuzhou Powe Plant Baoding Powe Plant Fly ash is divided into seveal categoies though a sieve accoding to ganulaity in ode to

4 investigate the mateial content of Fe element unde diffeent ganulaity. The esult is that the content of Fe heightens when the ganulaity of fly ash minishes, that is, the magnetic matte is pimay in the fine magnetic powde, while othe elements ae abundant in the coase fly ash. Fo the magnetic seal, the fine the ganulaity of powde, the bette the seal laye. The fine magnetic powde is easy to collect the fine powe in the opeation of magnetic foce and it can educe the aivoid atio of the whole seal laye, so it s useful fo seal. 1. The Magnetic Domain Analyse of Magnetic Paticles The magnetic powde, as a kind of feous magnetic mineal, owns geate coecive foce and emanence. The magnetism of matte oots in atom s magnetism. Thee is an exchanging function that is ooted in the static between the electons bodeed on the same atom in micostuctue, which is called spontaneity magnetization, and it can foce evey atom s magnetic toque to paallel o un-paallel. The effect of function makes evey atom s magnetic toque attain magnetic satuation in the same diection within little distict. This distict is called the magnetic domain and this is the essential eason that the magnetic matte poduces the magnetism. Magnetite belongs to cubic system and un-spinelle type stuctue and its magnetic toque is pemanent. Synchonously, thee is the chaacteistic of feous magnetic mineal The Connection between Coecive Foce of Magnetic Paticles and Ganulaity Figue 1 shows the connection between coecive foce of magnetic paticles and ganulaity. Fom the figue, a conclusion can be dawn that the emanent coecive foce of the magnetite (Hc) inceases with the ganulaity s eduction. The coecive foce aises apidly when the ganulaity becomes geat fine because thee is magnetic domain stuctue inside the magnetite, and it is vey impotant to move the magnetic domain s ampat when magnetizing. But the otation of the magnetic domain impoves with the ganulaity s eduction. Synchonously, the enegy that the otation of domain needs is moe than the one that the moving of the magnetic domain s ampat needs. The esult is that the coecive foce inceases, even apidly with the ganulaity s eduction. Figue1. The Connection between Coecive Foce of Magnetite and Ganulaity 1..2 The Study on Distibuting Fomula about The Field of Magnetite Paticles

5 The magnetite paticles can polymeize based on the field founded by the magnetite paticles aound them afte they ae magnetized. So it s necessay to study this field. A magnetite paticle and its field ceated ae investigated. This paticle is consideed spheical, the adius as R, which is put into anothe field whose intensity is H. This paticle is magnetized equably and the intensity of magnetization M is fixed on H. Figue 2 shows the connection. The magnetic potential can descibe the inne and oute field of magnetic paticles and Laplace equation is: 2ϕ 1=, 2ϕ 2= (1) ( ϕ 1 and ϕ 2 ae the inne and oute magnetic potential of the paticle espectively) Thee is a connection on the spheical suface(=r ) H 1 θ= H 2 θ(o ϕ 1= ϕ 2) B 1 =B 2 (2) We know the oute intensity of magnetic field of magnetite paticle is: H = + H () H θ Figue2. The Connection between Coodinates of The Paticles and Oute Magnetic Field ϕ1 2MR 2MR H H = = H cosθ + + cosθ R = 2M cosθ 4 (4) H 1 ϕ1 MR θ H sinθ sinθ + H θ R = = = M 4 sinθ We can only conside the point P (,,ϕ ) which is in a specific diection (θ=) and daw the conclusion based on (2) 4 : 2MR R = H, -2M 4 H + gadh = (6) When thee is no oute magnetic field, the magnetite paticles ceate the field depended on emanence and the intensity of magnetic field is: (5) H ϕ ( ) 2 H R = M R cosθ, = 2M cosθ 4 = (7)

6 H θ 1 ϕ1( ) 1 R = = M sinθ, = θ H θ (8) Thee is the connection in the spot of the point Po(,,φ): 2 2 R M H =, gadh = M R 4 (9) Fom the deductive fomula, it can be concluded that thee exists the linea popotion elation between ganule's exteio magnetic field intensity and magnetization intensity (diect atio to the ganule's adial cube, invese atio to the distance cubed to the suface of ganule). The magnetic field of ganule has invese atio to the fouth powe of the distance and the othes ae the simila. Though the analysis of the magnetic domain of magnetic powde, it s suggested that the paticles owed lesse ganulaity have bigge coecive foce and endue geate magnetic field foce. They can fom close-gained tees stuctue in magnetic seal equipment that can absob smalle ganule. With the time going, thee comes into being a close-gained ampat in the magnetic seal cleaance that can wok fo seal effectively. 2.The Configuable Theoy about The Ash Unloading Device Fo Magnetic Seal At pesent, the electical lock-gas device is sealed at the end of the impelle by installing end pessue plates and seal stips such as the felt stip which can be dismantled and exchanged at the top of the impelle of the oto in mostly powe plants at intenational ange. When the impelle is unning, the felt stip can contact with the shell of machine at the state of fiction in ode to pevent leaking ai. But the felt stip is easy to be won out, so othe mateials such as polytetafluooethylene o ubbe which is able to endue abasion wee used to eplace it. But, this method of seal has connatual shotcoming: (1)because the seal ing o the felt stip diectly contact with the cliff of the lock-gas device so the abasion is elatively seious and the esistance is geat. Afte a peiod of time, the seal ing will be won out, and the capability of seal will become weak. In the end, the ai leaked will influence the nomal opeation. (2)Because the natual life of the lip seal ing made of ubbe and the felt stip is not long, the wok-load of backout and eplacing is vey geat which bing geat discommodity to the industial automation. Theefoe, the settlement of poblem about the seal of electical lock-gas device has impotant pactically meaning in the industy. 2.1 The Designing Pinciple of The Ash Unloading Device fo Magnetic seal Take the ash fom fiepowe plant as an example, and fly ash has its own featues as follows. It is dy, so the oil o wate seal cannot be used. Because a stong fiction between the ash and the shell makes a sevee abasion, it is unsuitable to adopt mechanical seal. Fo the pesence of evese pessue, gas seal is also unsuitable. Fo the easons mentioned above, the pesent sealing methods couldn t satisfy the need of equipment unning. The fly ash contains some magnetic mineal that could be adsobed into the sealing cleaance unde the magnetic field foce, so the sealing function could be fulfilled by a pemanent magnet that is installed on the end of impelles. The coal-buning fly ash in good fluidization distibutes its stocks ganulaity at the

7 ange of 1 1µm, most impotantly, the fly ash contains ion mineal that will fom magnetic powde with othe mineals and the powde has stong paamagnetism. Simultaneously, othe metal oxide with the spinel stuctue has some extent of magnetic pefomance. So the magnetic field could be used to seal impelles of the electic lock-gas device. 2.2 The Stuctue Pinciple about The Ash Unloading Device fo Magnetic Seal The ash-unloading device fo magnetic seal is modified on the basis of the conventional electic lock-gas device. The sealing is fulfilled by a pemanent magnet installed on the end of impelle. The pemanent magnet is aange by a cetain compound mode, and fixed on the end of impelle by bolts and metal clamp. Meanwhile, the metal clamp encloses the pemanent magnet. This stuctue also has the function to potect the pemanent magnet fom abasion and othe solid matte that may damage the magnet duing opeation. The ow mateial of metal clamp could be the stainless steel sheet, coppe sheet o aluminum plat with good esistance to abasion. These metals belong to non-magnetic substance, so the magnetic cicuit couldn t be influenced. The oto impelle is diven by moto, and the pemanent magnet on the end of impelle absob the magnetic paticle fom the fly ash and fom a seal couse that could be self-epaied duing the otation to make sue the leak tightness of the device. See the assembly method of pemanent magnet fom the figue. 2 1 N S N S N S N S N S N 4 The Assembly Ove Look View of Pemanent Magnet Mateial NNS The Assembly Side View of Pemanent Magnet Mateial (1) N S N S N S N S N S N

8 2 1 The Assembly Side View of Pemanent Magnet Mateial (2) Figue. The Assembly Figue of Pemanent Magnet Mateial 1 The impelle of electic lock-gas device 2 Pemanent Magnet Mateial The fixed bolts 4 The fixed metal clamps 5 the inne shell of lock-gas device 6 the ash laye locked When being installed, the N pole and S pole of pemanent magnet ae aanged in the altenate way, and fom an opening magnetic system with paallel aangement, see figue -.Magnetic lines of flux stat fom N pole to S pole to fom a close cicuit. Due to the lock-gas device is made up of casting ion, it also induces magnetic pole unde the effect of magnetic field foce. Magnetic lines of flux mainly concentate in the magnetic pole and the sealing cleaance, so the intensity of magnetic field is elatively unifom. This aangement of pemanent magnet makes it uneasy to fom ai chambes in the seal couse, and it is advantageous fo sealing because the seal couse beas unifomly unde the action of pessue foce..the Expeiments of The Seal Capacity of Ash Unloading Device.1 The Expeimental Facility As the figue 4 shows, the expeimental facility consists of the evamped electic lock-gas device and pneumatic conveye equipment. The impelles of the electic lock-gas device ae machined and the cleaance distance is 5 mm between the impelles and the inne shell. The top of the impelles is equipped with the pemanent magnets, so the cleaance distance can be adjusted. The fly ash can be fitted into the facility though the hole of ubbe plug(6) on the top self-made ash bucket. Duing the couse of expeiments, we can seal up this facility by ubbe plug in case of gas leakage. The exhaust pipe can pick positive o negative pessue pneumatic conveye equipment up. The numeical eading of the manomete can poof the seal effect.

9 Figue4. The Design Dawing of The Seal Device Used Magnetic Powde 1.The oute shell 2. The impelle. Pemanent magnet 4. The feed inlet 5. The exhaust pipe (connect with the conveye equipment ) 6.Rubbe plug 7.Ash bucket 8.The fixed blot 9.The fixed metal clamps.2 The Method of Expeiments The sealing pessue unde the static conditions is measued unde the laboatoy condition. The methods ae as follows: 1) Figue 1, 2: accoding to the expeiment demands fo cleaance distance, we fit the pemanent magnet, adjust the position of the fixed blots, and make the pope distance between the electic lock-gas device s shell and the pemanent magnet. Connect the gas cicuit devices. 2) Cove the feed outlet with the clamps. Inject the fly ash fom the feed inlet. Running the otating shaft some times, make the pemanent magnets absob the magnetic powde in the fly ash and then fom the seal couse. Use the ubbe plugs plug the feed outlet, and use the lock ing and flange seal the oto shaft to pevent the ai fom leaking. ) Switch-in the ai compessing machine EC51, stoe some pessued ai, and connect the gas cicuit. 4) Open the gas cicuit valve, adjust the ai input, inject the pessued ai into the device slowly, and wite down the eading of the manomete when fly ash ejects fom the feed outlet. Because the seal couse will be destoyed when the pessue in the device is geate than this eading, so this numeical eading is the maximum that the seal couse can be bone. 5) Open the ubbe plug, inject the fly ash again and continue to expeiments unde othe conditions. The fly ash used in this expeiments is collected fom the ash bucket of ESP in a powe plant of China. The physical-chemical chaacteistic as follow: Table4. The Chemical Constitution of Expeimental Fly Ash of Powe Plant Constitution C SiO 2 Al 2 O Fe 2 O CaO MgO Aggegation Content % heaped density.75g/cm tue density 2. g/cm the content of magnetic powde 7% magnetic susceptibility cm /g The paametes of pemanent magnets made of Nd-Fe-B: pemanent-magnet (mm) suface magnetic flux 465Gs pemanent-magnet (mm) suface magnetic flux Gs pemanent-magnet (mm) suface magnetic flux 27 Gs Adopting the fly ash and pemanent-magnets as above, we had the diffeent expeiments unde the diffeent seal inteval and height of fly ash.. The Result of Sealing Expeiments Table5. Sealing Pessues of Diffeent Height of Fly Ash Wh en Sealing Cleaa nce Is 1 mm Magnet Uninstalled Magnet 1 Magnet 2 Magnet

10 Uninstalled Magnet 1 Height Mpa Mpa Mpa Mpa of fly ash (cm) Non-pessue output Non-pessue output Table6. Sealing Pessues of Diffeent Height of Fly Ash When Sealing Cleaance Is 2mm Magnet Uninstalled Magnet 1 Magnet 2 Magnet Height Mpa Mpa Mpa Mpa of fly ash (cm) Non-pessue output Non-pessue output Table7. Sealing Pessues of Diffeent Height of Fly Ash Wh en Sealing Cleaa nce Is mm Magnet Uninstalled Magnet 1 Magnet 2 Magnet Height Mpa Mpa Mpa Mpa of fly ash (cm) Non-pessue output Non-pessue output 8 Non-pessue output The Tendency of Aipoof Pessue When Sealing Cleaance Is 1mm.. 25 Pessue(MPa) Hei ght of ash(cm) 1 The Tendency of Aipoof Pessue When Sealing Cleaance Is 2mm

11 Pessue(MPa) Hei ght of ash(cm) Uninstalled Magnet 1 Magnet 2 Magnet

12 The Tendency of Aipoof Pessue When Sealing Cleaance is 1mm Pessue(MPa) Hei ght of ash(cm) Uninstalled Magnet 1 Magnet 2 Magnet Figue5. Sealing Pessue of Diffeent Sealing Cleaance and Diffeent Height of Fly Ash Accoding to the data and figue above, although thee is some powde on the vane, the aiflow will still penetate the sealing laye though the edge of aifame o the bubbles inside the powde because of the powde fluidization. Theefoe, the sealing laye will be destoyed, and thee will be no eading of pessue fo manomete to detemine when magnet is uninstalled. Once magnet is installed, the sealing laye will be fomed and sealing function will wok. The data on the manomete will incease when the thickness of sealing laye inceases. Thee ae two factos to bing the pessue: The sealing laye fomed by magnet has sealing function, which can endue some pessue. The thickness of laye also has some effect on the sealing pessue, obviously, the thicke the lay is, the bigge pessue is, which show an linea popotion elation between them. When thee ae some paticles above the ash bucket, some paticles owning weak magnetism will fall off unde the foce of ai cuent, while othes owning stong magnetism will be absobed to the pole of magnet unde the magnetic foce, which makes the ability of self-epaiing of the sealing laye incease and the magnet can exet its function fully. When the thickness of laye is zeo, the pessue measued will be vey little because the sealing laye does not develop fully, but the computational value though theoy is adjacent to the measued one when the thickness of laye is 5cm. Though the paticle laye can give bith to pessue, yet thickness of laye is not the impotant facto that poduce sealing pessue, because the paticles can be fluidized and the pessue endued is vey little. Fom the expeiment without magnet, it is concluded that the thickness of lay meely makes the capacity of self-epaiing of the sealing laye incease, and it is the magnetic foce imposed on the paticles by the magnet that plays an vey impotant ole on the seal..4 The Compaison of Sealing Pefomance of Fly Ash Contained Diffeent Content of Magnetic Powde

13 Powde magnetic sealing technology has a pemise that thee must be some magnetic powde in the sealing medium. The content of magnetic matte in diffeent powde is vaious and it has some influence on the sealing pefomance. we made sealing expeiments about Magnet 1,2 and with diffeent content of magnetic powde. Powde is made by fly ash of Baoding heat powe plant and magnetic mateial(fe O 4 ).Thee ae fou kinds of powde whose content of magnetic powde is 7%,9%,12% and 15% espectively. The sealing cleaance is 1mm and the thickness of laye is 5cm.The expeimental data is shown in the figue 6 below. Table 8 Sealing Expeiment with Diffeent Content of Magnetic Powde in Powde Content of magnetic 7% 9% 12% 15% powde Mpa Mpa Mpa Mpa Sealing type Magnet Magnet Magnet Pessue(MPa) % 1% 2% Content of magnetic powde(%) Magnet 1 Magnet 2 Magnet Figue6. Sealing Expeiments with Diffeent Content of Magnetic Powde Accoding to the expeimental data and the tendency of sealing pessue, the sealing pessue measued inceases with the content of magnetic mateial in powde incease, but when the content of magnet powde inceases to a cetain value, the inceasing tendency will slow down. It is because sealing laye becomes satuated unde cetain magnetism when the content of magnet powde inceases to a cetain value. And then, the incease of the content of magnet powde has a little effect on the sealing pefomance and the cuve becomes smooth. If thee is a elative stonge magnetic foce, the powde laye absobed may become thicken, which inceases moe esistance to the equipments and causes an incease of cost of magnet and also may impose advese effects on the equipments. Theefoe, to minimize the equipment expenses and maximize the pofits unde the condition of settling the sealing equiement, it is necessay to confim all the paametes about magnet though theoetical analysis and expeiments..5 The Adaptability of Magnetic Sealing of Ash Unloading Device As a new powde sealing technology, powde magnetic sealing has an ovewhelming advantage ove othes. Besides application to the seal of coal powde, thee ae also many othe

14 applications about the tanspotation of powde in industies, such as cement and chemical etc. These powdes contain magnetic mateials in diffeent degee. Fo example, coal powde and cement contain some magnetic powde, and the magnetic powde has the small ganulaity and some magnetic intensity simultaneity. So we could make the magnetic powde filled in the cleaance between impelles and aifame unde the magnetic foce and solve the poblem of seal of the whole equipment though magnetic method. 4. Conclusion Expeiment pove, in the opeation of magnetic foce, the powde contained magnetic matte is small, such as fly ash, and the gap among paticles is also little, so the gas compact capability is vey good. The magnetic powde has stong capability of automatic ecovey. The magnetic body can attact magnetic paticles continually fom fly ash, even if a pat of the powde block dop out when the equipment opeating. So, the powde magnetic seal design that installs pemanent magnets at the top of vane and utilizes the magnetism in the Fe O 4 in fly ash to seal the space between the vanes and the walls. And the device does not wea and does not need fequent eplacement.