Dust Filtration

7/31/2019 Dust Filtration

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Presen ter : En g. A li N adeem M oh d . N adeemProdu ct ion En gineer

Q at ar N at i on al Cem en t Com pany

In t he na me of a l l a h, t he benef Icent t he mer cif ul



D ust Fi l t rat i on

Bag

H ou seESPs



Cem ent m an ufact ur in g chal lengedalw ays w i t h fu l f i l l i n g toughp ar t icu lat e m at ter em i ssi on sregu lat i on s. Elect rost at i cprecip i t ator s and bag hou ses are th e

m ain opt io ns fo r cem ent pr ocess dedust ing.

The ESPs. are widely usedfor de-dusting kilns, millsand clinker coolers. Theperformance of aprecipitator is totallydependent on theoperating conditionsof the

preceding equipment.

Ver y l ow d u st em i ssi on s can b each i eved u n d er n o r m al o per at i n gco n d it i on s w h en t h e p r eci p i t at o r i sp ro per ly si zed . A p reci pi t at or i susua l ly designed and guarant eed fort he oper at i ng cond i t i ons expect ed

un der nor m al cont in uou s operat io n ofth e pl ant w it h safety m argin

State o f t he ar t p recip i ta tor design ,developed th rou gh extens ive researchand develop m ent stu dies and th e f ie ldexper ience, in corporate m echanicalan d el ect r ical feat u res w h i ch ar ep r ocess sp eci f ic. T h ey co n t ro l t h evar iab les wh ich can great ly af fec t anESP’s p er fo r m an ce. Ef fi ci en ci esachievable f rom prec ip i tator for m anycem ent app l icat ion s far exceeds foro th er co l lec t ion devices wh i le h av ing

lo wer operati n g cost s.



ESPs of ten fai l to obta in the expec tedhigh ef f ic ienc ies in cem ent appl icat i on swhen dust and gas p roper t ies change

per i od i cal l y i n t hese c ases an ESP’sper form ance is m ost af fec ted by th e

result ing change in dust resist ively due

t o t em perat u re an d m oi st ur e chan ges.T h e t r an si t i on f ro m m i l l t o By- Passo per at io n can n ot b e m ad e w i th ou td ep osi ti ng som e o f t he ver y h ighresist iv i ty du st on th e col lect in g sur faceswh ich resul ts in a d i r ty stack for a per iod

o f t im e, depend ing upon t he cl ean ingeffi cien cy of t h e ESP.

To al l ev iat e t h i s p r ob lem cond i t i on i ngtowers are in sta l led in th e m i l l bypass ork i l n gas c i rcu i t and they a re operatedw hen ever th e m i l l is off .

ESP’s are designed to operate on thelow er t em p er at ur e si de of t heresisti vely cur ve at all t im es.

There are two dem and s m ade uponthe cond i t i on ing tower to ensure theprec ip i ta tors per form ance when the

m i l l i s shut dow n. T he f i r st is that thespray system be act ivated exact l y inp r op o rt i on t o t h e gas t h r o u gh t h etower dur ing swi tch over in order toavo id an y h igh r esi st iv i ty d u streachin g th e ESP ( th e process m ustalso be tu n ed for very rapid respo n se)th is can on ly be achieved th rou gh th eu se o f a co m p u t er b ased co n t ro lsyst em . T he secon d i s t h atcondi t ion in g towers’ exhaust d uct andgas be at approx im ately th e sam em o istur e and tem peratur e cond i t ion

as m easu red in th e labor ator y.



ESPs’ O perati n g p aram eters.

Opera t ingpar am eters

GasFlow

GasTem p.

M oi st ur eContent

DustResistivity

T h e resistivity of the dust af ter 5,4stage suspension pre heater ki lns isalm ost always so h igh tha t backcoron a occur s. Th e size of an ESP ford e d ust i n g t h e gas f ro m t h e k i ln

alon e w hi le in operat i on w ould be soeno rm ous tha t using p recip i t ato rw i t h o ut w at er co n di t i on i n g i s n o trecom m end ed. Gaseous leavin g th epre-heater ki ln in range of 325 – 350oC is unfavourab le condi t i on for t he

ESP because of th e h igh resist iv i t yand l ow dew poi n t . In i t iat i ng coo l ingso th at th e tem peratu re d rops belowt he m axim um resist ivi t y poi n t, 150 –180 oc, m ean s a great im pr ovem entof t he precipi t ato r ’s perfor m ance.



W ater cond i t ion in g redu cest h e gas volu m e by i t s cool in geffect t h u s r ed u ci ng t h e

resi st i vi t y o f t he dust andincreasin g th e d i -elect r i calst r en g t h o f t h e d u st gas.W at er con d i t i on in g,t h erefore h as a ver y

pro found pos i t i ve e f fec t onprecip i tato r per fo rm ance.This is d i rect ly re f lec ted onthe prec ip i ta tor ’ s opera t ingvol tage as i l lus t rated in the

d iagram .



As the wate r cond i t ion ing increasesand t h e gas is cooled m or e an d m or e ,t he p reci pi t at or vol t age r isesdram at i cal l y and th e per fo rm ancei m p roves cor resp on d in gly. I t i snot iced th at over a wi de tem peratu re

ran ge t h e cu r ren t voltage

character i st i cs are alm ost vert ical oreven cu rved back , i nd i cat i ng backcoron a du e to h igh r esist ivi t y.

Th ese di agram s show th e ef fect of

w ater con d i t i on in g on t h ep reci pi tat or ’s cu rr en t vo lt agecharacter ist i cs for a p reheater k i l nw i t h c ond i t i on i ng t ow e r and f o r apreheater k i l n w it h raw m i l l .



1. Electrical power supplysystem

2. Access door and gaskets.3. Electrode cleaning systems.

4. Insulator compartments.5. Inlet and outlet duct nozzles.6. Inlet and outlet gas flow

distribution devices.7. High voltage discharge

electrode support frames.

8. Collecting electrode9. Hoppers.10.Structural steel11.Thermal insulation.12.Dust chemistry and resistivity.



Tw o types of f i l t ra t ion processes- forf i lt e r bags- a re cur ren t l y used inpar t icu la te f i l t ra t ion in cement

indust ry :

Depth

(convent ional ) f i l t rat ion .

Surface(m em br ane

) f i l t r a t ion .

Filter Bags Conven t i onal non m em b ranef i l t er bags requ i re t h edevelopm en t o f an in te rst i t ualdu st cake (p r i m ary du st cake) inth e cross-sect ion of th e fabr ic t o

pr ovi de p roper par t i cu lat erem oval eff ici ency.

Th is resul t s in to a pro cess w h eret h e d u st cap t u red i n si d e t h ef i l ter m ater ia l is pr i m ar i ly usedt o f i l t e r t h e d u s t a n d n o t t h efi l t er m ateri al .



T he developm en t o f t h i s dust cak e i scr i t ical. Packed too t ight ly, the resistanceo f t h e f i lt er t o gas f l o w i s i n c reased ,l ead i n g t o h i gh p ressu r e d r op s, i n -adequa t e a i r f l ow t h r ough t he k i l n andsho rt bag l ives. I f t he in dept h du st cake istoo l oose, the f i l t er coul d em it p art i cul atem ateri al i n excess of t h e desir ed em issio nlevels.

The development of th is interst i t ia ldust cake is h igh dependant on the

dust type, s ize & shape, and on theveloci ty at which i t captured. Theserequi rements l imi t the des ign ofconvent ional systems to a narrowwindow where successful operat ioncan b e expected.



I f th e m oistur e con den ses on th e fabr ic, am u d d y cak e i s fo r m ed w h i ch b o n d s

ar ou n d t h e fab ri c o f t h e m at er ialespecial ly in th e case of cem en t. W hent h i s dust cak e d r i es ou t , i t becom esex t rem ely d i f f i cu l t to rem ove f rom thesur face of f i l ter m ater ia l resul t in g in anincrease in the pressure drop across thesystem .

M em bran e (sur face) f i l t r at ion op eratesw i t h a d i f fer en t f i l t r at i on p r i nci p l e.Th e fi lt er m ediu m of a m em br ane fi l teri s a m i cro po rous f i lm o f expandedpoly tet ra- f l uor oethy lene, l am inated toa specif ic backin g m ater i al, chosen tobe com pat ib le w i th the m echan ical ,

th erm al ,and chem ical r equi rem ent s oft h e system .



The membrane i tsel f is chemical lyiner t , operat iona l to 260 oC wi thsurges t o 2 85 oC, and no n-stick. Dustco l lected o n t he m em brane is keptnear the f i l ter sur face, where i t isa lmost comple te ly d is lodged wi theach cleaning cycle. This ‘surfacef i l t rat ion’ and very h igh ef f ic iencymembrane a l low these f i l te rs to

operate a t h igher f i l t ra t ionveloc i t ies (a i r - to-c loth rat ios) , wi thlow er f i l te r resistance, and w i th th ebest f i l t rat ion eff ic iencies avai lable

to th e indust r ial m arket .

The fact t h at t h e dust do esno t p enet rat e in t o t hef i lt rat ion m at er ial ensu re

easier clean ing o f t he bageven d u rin g m o ist u re u pset so r dew p oin t excu rsio ns, ast he d ust cake d oes no t bo ndt o t he f ibers o f t he w oven o r

need le felt m at er ial, b u tst ays on t he non st icksu r face o f t h e PTFEmembrane .



An oth er d isadvant age o f no n -m em branefi l t rat ion system is that once a system isin p lace, in creasin g th e p er for m ance o ft h at syst em en t ai ls t h e exp en si veadd i t ion of m ore com par tm ent s, w i t h acorrespon di ng in crease in th e f i l t er b ags

and a co r r espond ing i ncr ease i n t heoperat io n al and m aint enance costs of t hesystem

Gas st ream s w i th h igh m oistu re contentp rove even m o re d i f f i cu l t t o f i l t er w i t hconven t ion al f i l t r at ion m ater ial as the

d u st p en et r at es i n t o t h e d ep t h o f t h efabr ic .

Th ro ugh car ef ul f il t er bag d esign , sp ecial m at er ials o f co nst r uct io n,q u al it y m a n u fact u r in g an d f ie ld o p t im i zat io n o f t h e b ag- ho u se cl ea nin gsyst em , m e m b ran e f il t ers last s m u ch lo n ger t han co nve nt io nal f ilt ermed ia .



Benefits of membrane filtration to theindustry

Bag life

The mem brane p revents the dust

particles from entering into the

dep th o f the filtra tion ma teria l. This

eliminates abrasion caused by

particles inside the material when

the ba gs are c lea ned a nd thus

increases the mechanical life of

the material. Due the easierc lea ning of the b ags bec ause of

the non-stic k surfac e, less pulse

pressure is needed to clean the

bags leading to less mechanical

stress on the bags during the

c lea ning c yc le.

Another aspec t of the ba g

life is the p ressure d rop ac ross

the filter ba gs. Filter ba gs

mainta in a c onstant

permeability over the life of

the ba g due to the d ust notpenetrating to fabric itself.

This ensures consta nt a irflows

through the filter over the

c om p lete life time of the filter

bags.



EmissionThe memb rane is 99.995 per c ent

efficient on a mean particle size

of 0.3 microns. This elimina tes dust

pa rtic les from bleed ing through

the filter bag s thus leading to

extreme ly low emission values.These low values will ensure

compliance with current as well

a s future environmenta l

legislation.

Grinding aid

This is due to the b loc kage o fthe conventional media by the

much finer particulate material.

PTFE membrane filtration med ia

are much better suited to

ha nd le the finer dust, as the dust

doses not penetrate into thefilter me d ia .

A c ommon p rob lem expe rienc ed

in ce ment plants is that as soon a s

grind ing a id is introd uc ed into the

milling c ircu it, the a irflow

provided by the filtration systems

fallsbelow the required level.

Increased capacity

In any f ilter where the extrac t ion

c ap ac ity is not enough, installingPTFE membrane filter bags will

p rovide an extra volume of between

10-15 per cent w ithout making

signific ant c hange s to the rest of the

filtration system



This oc c urs b ec ause the ba gs

c lean down muc h moreeffic ient ly after ea c h c lea ning

c yc le lea ving muc h less dust on

the surfac e of the ba gs. The

red uce d p ressure d rop ac ross

the filter bags then a llows the fan

to op era te on a d ifferent point of

the fan curve lead ing to an

increase in flow through the

system.

Upset recoverability

The PTFE me mb rane is hyd rophob ic ,

mea ning that moisture c ondensing

in the bag s does not p ene trate into

the bags. This ena b les the filter b ags

to recover from high moisture

c onditions such as wa ter injec tion

into the m illing c irc uit.

Bec ause the membrane keep s the

dust on the surface, the wet dust

c ake formed during the

c ondensat ion is not allowed to

bond to the ind ividua l fibers of the

felt. After the temp erature is ra ised

again, the dried dust cake can be

ea sier relea sed from the smoo thmemb rane surfac e, while it is still

best to e liminate a ll dew point

excursions in the filter resulting in

c ondensation, the ba gs are ab le

to re c ove r muc h b ette r tha n

c onventiona l felts.

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Dust Filtration