coal properties and effect on combustion.pdf

7/23/2019 Coal Properties and Effect on Combustion.pdf

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Coal Properties and its influence

Boiler

Pankaj EkboteNTPC Ltd.



Influence of Coal Properties• Transportation of Coal

•

Storage of Coal• Design of Boiler

• Combustion Performance

• Mill Performance

•

Performance of Boiler (losses)• Slagging

• Performance of ESP

• Life of Boiler Components



Properties of Coal• Coal properties are evaluated by different metods!

• Te most commonly used metods are Pro"imate and #ltimate analysis of te coal!

• Pro"imate analysis gives te Moisture$ %s and &olatile matter$ 'ile te i"ed Carbodifference!

• #ltimate %nalysis gives te elemental composition of te coal!

• ter metods li*e Macarel analysis is also used for coal classification and evaluationproperties!

• usion caracteristics of %s is estimated from te +nitial Deformation Temperature$ ,

Temperature and usion Temperature of te as• %s o"ide analysis is commonly used for analysis of as composition!

• %s analysis is used to caracteri-e te slagging and fouling potential of coal in te bo



What Boiler Engineer must ala!s be aare

• i"ed carbon• is te solid fuel left in te furnace after volatile matter is distilled off! +t consists mos

also contains some ydrogen$ o"ygen$ sulpur and nitrogen not driven off 'it carbon gives a roug estimate of eating value of coal

• &olatile Matter• &olatile matters are te metane$ ydrocarbons$ ydrogen and carbon mono"ide$ an

gases li*e carbon dio"ide and nitrogen found in coal! Tus te volatile matter is gaseous fuels present! Typical range of volatile matter is ./ to 012!

• Proportionately increases flame length$ and elps in easier ignition of coal!

•

Sets minimum limit on te furnace height and volume!• +nfluences secondary air re3uirement and distribution aspects!

• +nfluences secondary oil support

• Coal 4rindability• %ffects Mill performance



Wh! the Boiler Engineer "" #cont

• %s Content• %s is an impurity tat 'ill not burn! Typical range is 1 to 5/2

•• 6educes andling and burning capacity!•• +ncreases andling costs!

•• %ffects combustion efficiency and boiler efficiency

• Causes clin*ering and slagging

• Moisture Content• Moisture in coal must be transported$ andled and stored! Since it replaces combustible ma

eat content per *g of coal! Typical range is /!1 to 7/2•+ncreases eat loss$ due to evaporation and supereating of vapour

•,elps$ to a limit$ in binding fines

•%ids radiation eat transfer

• Sulpur Content (8T +8 P69+M%TE %8%L:S+S)• Typical range is /!1 to /!;2 normally

•%ffects clin*ering and slagging tendencies

•Corrodes cimney and oter e3uipment suc as air eaters and economisers

•Limits e"it flue gas temperature



What Indian and International Coalslike Caracteristics Indian Indonesian South African

Total Moisture 2 10 - 20 7/<0/ ;

%s 2 25 - 50 7/<71 71<7=

&olatile Matter 2 16 - 30 .1<01 .0

i"ed carbon 2 24- 40 51 17

Carbon 2 30 - 55 >/ =/<;/

,ydrogen 2 2 - 4 5!1 5<12

8itrogen 2 0.7- 1.15 7 .<.!12

Sulpur 2 0.3 - 0.8 about 72 #pto 72

"ygen 2 4-8 7. ;<?2

4C& *cal@*g 2800-5000 11// >1//

%brasive inde" 40-60

%s Softening temp Above1300C

701/oC 70///C

,4+ 50-110 %bout 1/ %bout 1/



%o Boiler &esign is affected b! Co

• Following major aspe!s onsi"ere" #or $oiler %esign•

FC & '( )a!io• As* per million +als an"• ,ni!ial %e#orma!ion empera!re• /lagging C*ara!eris!i o# As*• As* rosion C*ara!eris!i



%o Boiler &esign is affected b! Co"".• Convective section of te boiler consists of large bundles of tubes arraninside te gas pat to e"tract eat from te flue gases!

•

Erosion due to as is a maAor consideration in tis section!• Te ma"imum permissible velocity is proportional to te as 3uantity a

abrasive nature of as!• Silica and alumina$ 'ic are ig in typical +ndian coals$ are igly abr

nature and terefore a te ma"imum permissible velocity is limited!• Te metal loss due to erosion is also dependent on te spatial variation

velocity!• Coal parameters critical to ma"imum permissible velocity

• %s content• %s silica (particularly alpa 3uart-) and alumina



%o Boiler &esign is affected b! Co"".• bAective of te coal firing system is to enable

• Complete combustion of te coal particles• Limit te formation of pollutants li*e 89

• Coal Properties %ffecting Combustion• uel 6atio (i"ed Carbon @ &olatile Matter) is commonly used to evaluate t

combustibility of coals• More advanced analysis rely on macarel analysis and vitrinite reflectance

combustion performance

• iring %rrangements• all ired

• ront all• ront and 6ear ired

• Tangential ired• <fired



'urnace (i)ing and coal•&olumetric ,eat 6elease 6ate

•urnace 6esidence Time

•8et ,eat +nput per #nit Plan %rea

•Burner one ,eat 6elease 6ate•urnace Cooling actor

•urnace E"it 4as Temperature (E4T)

%ll te above parameters are affected by te coal properties especially te slagging andcaracteristics of te coal

8et ,eat +nput (8,+) is te 4C& of te fuel minus te radiation losses$ loss dunburnt combustible$ moisture in air$ latent eat of moisture in fuel tat fcombustion of ,. in te fuel plus te sensible eat of combustion air (PrimaSecondary air)$ all above reference temperature!



'urnace

• Volumetric Heat Release Rate

• &olumetric ,eat 6elease 6ate is te ,eat liberated per unit volume in te

(upto E4T plane)• Ma"imum value depends on te fuel and as caracteristic!• ,o'ever$ for large units$ tis is not a limiting criteria!

• Furnace Residence Time

• % minimum furnace residence time is necessary to ensure complete combparticles!

• urnace residence time is calculated from te top burner to furnace e"it (,eat E"cangers)

• Te coice of furnace residence time depends on fuel ignition caracteris• uel ratio gives a good indication of te fuel reactivity!



'urnace• %ltoug particle ignition and combustion caracteristic of coal is a maAor design criterion$ te

consideration for furnace si-ing and convective section design is to minimi-e problems due to afouling and erosion!



'urnace (i)ing• Net Heat Input (NHI) per unit Plan area of furnace

• 8et ,eat +nput (8,+) per unit Plan area of furnace or Plan %rea Loading is te amount of eat releasedof te furnace!

•

8et ,eat +nput (8,+) per unit Plan area of furnace reflects te temperature level in te furnace! % igincreases flame stability but also increases te possibility of slagging in te furnace!

• Te allo'able Plan %rea Loading depends on te capacity of boilers and te softening temperature (Ste as in coal!

• Te plan area loading is generally *ept in te range of 0!? to 5!? M*cal@m.<r!• ,ig silica and alumina content of te as in +ndian coals reduce te cance of slagging! ,o'ever$ te

a possible ris* during operation!• % conservative approac is necessary to avoid any cance of slag build up as it may prove catastropic

• Burner Zone Heat Release Rate (BZHRR)• B,66 is te ratio of te ,eat supplied to te furnace to te burner -one surface area bet'een top an• Te B,66 represents te temperature level and pea* eat flu" in te burner region!• Te coice of B,66 depends largely on te slagging caracteristic of te coal!• B,66 also affects te termal 8" formation in te furnace!



Pressure Parts• Material

• uel side corrosion of te pressure parts is maAor concern 'it coals avior sodium and clorine in as!

•

uel side corrosion is not a maAor concern 'it +ndian coals!• %rrangement

• %bove +DT slag deposits bridging across te tubes is te maAor constraint• Belo' +DT fouling of tubes and ease of cleaning decides te minimum pit

assemblies• %s slagging and fouling caracteristics are largely dependent on as com



Coal *ualit! and Boiler Efficienc!• Losses from Boiler

• Dry 4as Loss depends on• E"it 4as Temperature• E"cess %ir

• Loss due to Moisture and ,ydrogen in coal• Moisture in coal

• Every 72 increase in moisture decreases te Boiler Efficiency by /!7• ,ydrogen in Coal

• Every 72 increase in ydrogen content decreases te boiler efficien

.2• Sensible eat due to as

• Every 72 increase in as content decreases te boiler efficiency by /!/



+ther Component &esign• EP

• Si-ing of ESP is dependent primarily on te as content of te coal• Moisture and ,ydrogen in coal also play a role in te si-e of ESP$ as tey

increase te gas volume!• Moisture in flue gas decreases te dust resistivity tereby improving co• %s constituents li*e 8a and sulpur in coal reduce te resistivity of as

improving collection

• F!"• +ndian coals are generally lo' in sulpur• ,o'ever$ o'ing to te lo' 4C&$ te S. emissions from +ndian Po'er s

7;// mg@8m0$ 'ic is 'ell above te prevalent norms in Europe$ apa• 4D may be re3uired in case of ig sulpur coals



Influence on Coal ,ill

•

#I$$ %&T$ET TE#PER'T&RE• #I$$ #%T%R '#PERE

• #I$$ 'IR F$%

• E'R P'RT $IFE

•

#I$$ PRE&RE "R%P• #I$$ %&T P&T

• #I$$ FINENE FR'TI%N

• #I$$ RE*ET R'TE



L-C +' C-P-CIT/ +0 %I1% P+WE0C+N(2,PTI+N• ,+4, M+ST#6E

• L 4C&

•

+8C6E%SED 6% C%L S+E!• 46+8D+84 T +8E

• E9CESS+&E BED DEPT,

• +8ST6#ME8T E666



E3CE((I4E ,ILL 0E5ECT(• C,%84E +8 C%L 46+8D%B+L+T:$ S#L#6 %S,!

• +MP6PE6 C%L@%+6 6%T+

• T,6%T 4%P E%6!

C%6SE 46+8D

• C,%84E +8 C%L 46+8D%B+L+T:

• ,+4, M+ST#6E

• +8C6E%SED T,6#4, P#T!

• CL%SS++E6 SETT+84

• M+LL E%6!



L+W C+-L 6-I0 TE,PE0-T20E• ,+4, M+ST#6E

• L P% +8LET TEMPE6%T#6E

•

P%SS+84 CLD %+6!• L %!, +8LET TEMPE6%T#6E

• 88 %&%+L%B+L+T: SC%P,

M+LL +6ES

• ,+4, &L%T+LES

• M+ST#6E

• L C%LF %+6 TEMPE6%T#6E!

• B#68E6 L+8E B%L%8CE



E(P• For a gi+en collection efficienc,- EP si.e increases /ith inc

coal ash due to the increase in inlet dust 0urden1

•

Precipita0ilit, of fl, ash impro+es /ith increase in flue gas content1

• Precipita0ilit, of fl, ash impro+es /ith increase in coal sulfcontent1

• Precipita0ilit, of fl, ash impro+es /ith increase in fl, ash 0

ratio and increase in Na3% and $i3% in the ash1

• Precipitator efficienc, increases /ith increase in fl, ash pa/hich is affected 0, PF si.e- the fusi0ilit, characteristics ofash and com0ustion conditions1

•

oarser PF tends to ,ield a larger a+erage fl, ash particle si



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coal properties and effect on combustion.pdf

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