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Cold End Corrosion in a Boiler and Its Prevention

written by: Dr V T Sathyanathan • edited by: Lamar Stonecypher • updated: 5/25/2011

Usin !ue"s with su"!ur in steam eneratin units yie"ds a potentia" ha#ard o! su"!ur corrosion atthe co"d end o! the boi"er$ The se%erity depends on many !actors "i&e percentae o! su"!ur in the!ue"' e(cess air' moisture in !"ue as' etc$ )any options are a%ai"ab"e to contain co"d end

corrosion

• *oi"ers eneratin steam !or use in power eneration and process power p"ants use

di!!erent type o! !ue"s$ These !ue"s contain su"phur to di!!erin percentaes$ The hiher the percentae o! su"phur' the hiher wi"" be the ris& o! co"d end corrosion in the boi"er$ The su"phur

in the !ue" durin combustion ets con%erted to su"phur dio(ide$ Dependin upon the other

impurities present in the !ue" and e(cess air "e%e"s' some portion o! the su"phur dio(ide etscon%erted to su"phur trio(ide$ The presence o! moisture in the !"ue as due to moisture in !ue"

and air' su"phur dio(ide' and trio(ide' combines with moisture and !orms su"phuric acid andsu"phuric acid$ These acids condense !rom around 115 deree centirade to s"iht"y hiher than1+0 derees' dependin upon the concentration o! S,- and water.%apour$ The basic reactions

ta&in p"ace are

S + O2 → SO2

SO2 + O2 ↔ SO3

H2O + SO2 ↔ H2SO3

H2O + SO3 → H2SO4

Dependin upon the ppm o! S,- and water.%apor concentration' the dew point temperature can

%ary !rom around 0 deree centirade to 10 deree centirade$

ondensation o! these acids resu"ts in meta" wastae and boi"er tube !ai"ure' air preheater

corrosion' and !"ue as duct corrosion$ n order to a%oid or reduce the co"d end corrosion the astemperature "ea%in the heat trans!er sur!ace in boi"er is &ept around 150 derees centirade'

ranin !rom 120 to 155$ t is %ery important that the meta" temperature o! the tubes is a"ways

&ept abo%e the condensation temperature$ t may be noted that the meta" temperature o! the tubes

is o%erned by the medium temperature o! the !"uid inside the tubes$ This ma&es it necessary to

 preheat water to at "east 150 derees centirade be!ore it enters the economi#er sur!ace$ n thecase o! an air pre.heater' two methods are used to increase the meta" temperature$ ,ne is an air

 bypass !or air pre.heater' and the second is usin a steam coi" air pre.heater to increase the airtemperature enterin the air pre.heater$

The amount o! S,- produced in boi"er !"ue as increases with an increase o! e(cess air' as

temperature' residence time a%ai"ab"e' the amount o! cata"ysts "i&e %anadium pento(ide' nic&e"'

!erric o(ide' etc$' and the su"phur "e%e" in !ue"$

http://www.brighthubengineering.com/power-plants/42213-classification-of-steam-boilers/http://www.brighthubengineering.com/power-plants/42213-classification-of-steam-boilers/http://www.brighthubengineering.com/power-plants/42213-classification-of-steam-boilers/

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The !"ue as dew point temperature increases steep"y !rom 0 deree centirade to 1-5 derees

centirade with su"phur percentae increasin up to 13$ 4 !urther increase in su"phur percentae

in !ue" radua""y increases the dew point temperature !rom 1-5 deree centirade to 1+5 dereescentirade at -$53 su"phur in !ue"$

 

Prevention of cold end corrosion

There are many methods used wor"d o%er to contain co"d end corrosion$ These methods !a"" in

the cateory o! in.combustion reduction and post.combustion reduction$

The in.combustion reduction methods inc"ude:

o *urnin "ow su"phur !ue"

o Low e(cess air burners

o ue" additi%es

o "uidi#ed bed combustors

6oin in !or "ow su"phur !ue" sometimes become economica""y un%iab"e !or the process !or

which the steam enerators are used$ Today many "ow e(cess air desins are a%ai"ab"e in the

mar&et$ These burners adopt many ways to reduced e(cess air re7uirement without a!!ectin theunburnts in the !"ue as a!ter combustion$ ue" oi" additi%es "i&e simp"e manesium o(ides are

used to contain co"d end corrosion due to su"phur$ The manesium o(ide is in8ected in to the

!urnace or mi(ed with !ue" which combines with su"phur o(ides to !orm manesium su"phate$ n

!"uidi#ed bed combustors' "ime addition is a simp"e method used to reduce su"phur corrosion$

The post.combustion techno"oies adopted are:

o Desinin with hiher e(it as temperature

o 4ir bypass across air pre.heater

o 4mmonia in8ection

o "ue as desu"phuri#ation 96D

Desinin boi"ers with hiher e(it as temperature reduces the boi"er e!!iciency$ 4s a ru"e o!

thumb appro(imate"y e%ery 20 deree centirade increase o! !"ue as temperature at boi"er out"et

reduces the e!!iciency by 13$ ;ence this is not a pre!erred method in the present days$

4ir pre.heater bypass is !or main"y !or startup purposes unti" the meta" temperature can be

maintained abo%e condensation temperature e%en when the co"d air enters$ Some desiners use

steam coi" air pre.heater !or !u"" operation o! the boi"er$

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4mmonia in8ection was a method adopted by a !ew desiners in certain process p"ant boi"ers

 burnin hih su"phur oi" due to the a%ai"abi"ity o! ammonia$ 4mmonia is in8ected in the

economi#er reion where the temperature o! !"ue as is be"ow the ammonia dissociationtemperature and su!!icient time is a%ai"ab"e !or the chemica" reaction$ 4mmonia combines with

su"phur trio(ide to !orm ammonium su"phate$ The rate o! ammonia in8ection wi"" depend upon

the S,- concentration$ The prob"em with this method is it produces a hih %o"ume o! "oosedeposits o! ammonium su"phate' which increases the pressure drop in the !"ue as path$