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7/23/2019 Coal Properties and Effect on Combustion.pdf
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Coal Properties and its influence
Boiler
Pankaj EkboteNTPC Ltd.
7/23/2019 Coal Properties and Effect on Combustion.pdf
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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
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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
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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
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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
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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/
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%o Boiler &esign is affected b! Co
• Following major aspe!s onsi"ere" #or $oiler %esign•
FC & '( )a!io• As* per million +als an"• ,ni!ial %e#orma!ion empera!re• /lagging C*ara!eris!i o# As*• As* rosion C*ara!eris!i
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%o Boiler &esign is affected b! Co"".• Convective section of te boiler consists of large bundles of tubes arraninside te gas pat to e"tract eat from te flue gases!
•
Erosion due to as is a maAor consideration in tis section!• Te ma"imum permissible velocity is proportional to te as 3uantity a
abrasive nature of as!• Silica and alumina$ '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
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%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
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'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!
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'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!
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'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!
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'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!
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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
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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 /!/
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+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
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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
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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
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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!
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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
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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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