Indi an Jo urnal of C hemi cal Technology
Vol. 8, May 2001 , pp. 223-226
Acid cleaning operation for a natural circulation high pressure drum boiler unit of
a thermal power station
M Azad Sohai l* 3 & A Isma il Mustafab
"Central Chemical Research Labo ratory, Gh01·sal Thermal Power Stati o n, BPDB, Narsingdi, Bang ladesh
bDepartment of Applied C hemistry and C hemi cal Techno logy . University of Dhaka, Dhaka-! 000, Bangladesh
Received 29 March 2000; accepted 2 February 2001
Two-stage ac id cleaning (TSAC ) o peration has been e mployed and in vestigated to c lean up undesired impurities
(depos it ) in boiler water wall tubes (BWWT), boiler drum , super heater, economizer, feed water lines, li ve stea m lines, hot
reheat lines. cold reheat lines, dearator, co ll ec to rs, headers fo r a natural circulati on hi gh pressure drum boiler uni t. TGME 2
206-COB , Ru ssia (158 kGr cm- , capacity stea m gen erati o n 670 ton h- 1, 210 MW unit) of a thermal power stati o n in
Bangladesh . The deposit and loose impurities co nsist of Fe 20 3, CaO, MgO and Si0 2. On area basi s the maximum quantity
(98g - 2) of deposit was measured at boiler WWTs. During c leaning operation the loose impurities were wi thdrawn at first by
flashing of the water. TSAC o peration was carried out at two stages by re-c ircul ation of hot I 05 (±5 ° C) cleanin g solutio n
1
H 2S0 4 +N 2 H4 .H 20 (250±25 mg L- ) at pH 2.0 (±0.5) fo r 24 h. The TSAC operation revealed th at not less th an 95 % of
1
impurities were remov~d . T he cleani ng mechan ism and passivati on on clean surface of tubes by circ ul atio n of 500 mg LN 2 H4 and 0.4% NH 4 0H w ith pH 10.8 (± 2) at 60° C were fo und to be sat isfact o ry .
Acid cleaning of a boiler is essential , after
co mmi ss ionin g and start up of a thermal power unit,
because the suppli ed pipeli nes used for boiler water
wall tubes (BWWTs), L.P. heaters, super heaters,
headers, collectors and other accessory pipelines,
boiler drum , dearator are usually affected by natural
corrosion and scale. Beside these problems, durin g
installation of a unit the pipelines and other auxiliary
eq uipments contaminated by scale or corrosion,
sedi ments, weld flash, di ffe rent cutting, oil and
greases, atmospheric dust particles like mud, Si0 2 ,
clay etc. If such impurities are not removed from
boiler these may cause further scaling or corrosion .
Various techniqu es of bo il e r chemical cleaning are
emp loyed in different countries. Pulson 1 has studi ed
the possibilities of stress cotTosion crack ing during
2
boiler acid cleaning operation. French has used
hydrofl uoric acid for boiler cleanin g a nd Shenker3 has
described an eco nomi cal method of boiler cleaning.
Cherr/ described a method to prevent boiler tube
failure. Kotz5 has shown relations and effects o n
contro lling water scales with bo il er blow dow n.
6
Krause has show n the effects of sulphur on bo il er
7
9
10
tube metal. Phelan , Me met 8 , Novitsky , Sohail ,
11
Gruszkowski , described different techniques for
boiler chemi cal cleaning.
The water wall tubes (WWTs), pipelines, and
*Fo r correspondence
boiler accessories auxiliaries of a newly install ed HP
boiler unit TGME-206-COB , Russ ia, of Ghorasal
Thermal Power Station (GTPS) unit No. 6,
Bang ladesh was attacked by natural de posit, because
these boiler water tubes and other boiler accessories
were exposed to natural environment under open sky
for a long time before installation .
A number of chemical and physical techniques are
available for cleaning of boilers, heat exchangers and
turbine bl ades. Physical tec hniques include back
washing, air bumping, fl ashing, scouring, roddin g
(punching) and the che mi cal techniques include acid
cleaning, alkaline cleaning, ac id foaming and acid
etching or soaking. The physical techniques are less
effective on adh ere nt deposits while alkaline
cleanings are usually e mployed fo r cleaning of oil and
greasy substances on the boiler tube anterior surfaces.
Effective che mical cleaning of a bo il er can be
achi eved only by select in g the appropriate chemical s,
method, conditi ons, constituents and amo unt of scal e.
The choice of the program for cleaning of adherent
depos its for a parti cul ar boiler unit requires co mpl ete
survey of proble ms a nd chemical a nalyses to se lected
right method a nd che mi cals for cleaning. In thi s
representation sulfuric aci d and hydrazine hydrate
have been e mpl oyed for chemical clean ing of a newl y
set up hi gh press ure ( 158 kGr e m - 2 ) boiler uni t No.6
of Ghorasal The rmal Power Station, Bangladesh.
TSAC me thod is proved to be most scie ntific and it
INDIAN J. CHEM. TECHNOL., MAY 2001
224
Table ! -Chemi ca l composition (%) of deposits measured at
different locations of boiler before cleaning ope rati on
Com~o s ition
Location
Identity
Fep 3
Si02
(%)
MgO
CaO
Frolll wall p-3, T-12
Back wall p-4, T-16
Econom izer
Super heater
LP heater
96.63
97. 11
96.27
96.48
96.42
1.92
1. 73
1.97
2.01
2.03
0.42
0.40
0.78
0.67
0.73
0.92
0.75
0.87
0.84
0.80
Table 2 - Average composition of deposits on mixing of (same
we ight) deposits from different tubes before clean ing operation
Constituents
A vera oe 'fo
Fe 20 .,
96.59
Si0 2
1.93
MgO
0. 84
Table 3 -T he quantitative ran ge of deposits measured (g.m- 2) in
boiler pipe line and boiler accesso ri es before clea nin g operation
CaO
0.60
provides good results with less corrosion risk. In thi s
process minimum amount of cleaning reagent has
been used.
Preliminary experimental results show that during
cleaning operation concentration of hydrazine in
cleaning solution should be maintained above 200 mg
L- I , otherwi se corrosion on metal surface may occur
and pH shou ld not be less than 1.5 and temperature
should not be more than 180 °C. Such plant-based
investigation was made in central chemical research
laboratory of GTPS , Banglades h Power Development
Board, Bangladesh.
Experimental Procedure
Chemicals used for cleaning operation were
hydrazine hydrate (64 %) (Russia ori gi n), Sulphuric
acid (98 .5 %) Wata Chemical Ltd., Bangladesh and
ammonia gas (compressed), procured from BCIC,
Bangladesh.
The analyses were carried out with a photoelectric
colourimeter, Model KFK-2 (Ru ss ia) and Electroconductivity meter, digital Model Bibby, (England).
pH and ionic strengths were measured with a pH and
Ion meter, digital Model 130.04. 1, (Russia). Oven
model N-08. 76, (Ru ssia) and Furnace, Model Mn2YM, (Russia) were used during this work .
Measurement of deposits
The initial and residual quantities of deposits
attained in boiler WWTs, pipelines and other
auxi liary line, before and after cleaning operation
were determined by rubbing out of (3 x 3 cm2) area of
depos it from weighed sample and expressed in g. m-2 .
Composition of deposits was determined by
gravimetric, titrimetri c and colorimetric analyses 12- 15
of deposits. For each case multiple analyses were
made and their averages are represented in tables.
Concentratio ns (mg L- 1) of ionic impurities absorbed
BWWTs
Super heater
LP heater
Collector
Economi zer
impuri ties/sedimen t
45-98
38-41
35-42
40-60
30-48
in chemical solution at different stages of boiler
cleaning operation were meas ured by colourimetric
. 12-14.
ana Iysts
Method
After comm1sstoning and start up of natural
circulation water, wall tube high pressure (158 kG 1cm2, capacity 210 MW, TGME-206- COB, Ru ssia) drum
boiler unit No. 6 in Ghorasal Thermal Power Station,
Bangladesh ( 1998), a two-stage acid clea nin g
operation and further passivation have been employed
and investi gated to clean up unde ired impurities
inside th e boiler surface encompassing boiler drum,
dearator, water wall tubes, Super heater, LP heaters,
HP heaters, headers and collectors. The impuriti es
consisted of Fe20 3, CaO, MgO and Si02. The total
inside volume of boiler cleaned was 500m 3 with
1200m2 of smface area.
Hot re-circulation of boiler (HRB)
Hot re-circulation of boiler carried out by
introducing 500m 3 of demineralized water into the
boiler th rough make up pumps and re- circul ated (flow
50 m3h- 1) for 24 h. During hot re-circulation ,
operation temperature of the boiler was maintained
65 °C (± 5 °C) by inj ecti ng steam into the boil er
clea ning solutions.
First stage of acid cleaning (FSAC)
During first stage of acid cleaning operation
hydrazine hydrate solution (64%) was dosed into the
boiler re-circulated water (total volume= 500m 3) and
re-circulated at flow rate 50 m3h- 1 upto overall
concentration of hydrazine in the cleaning solution
appeared to be 250 (±25) mg L- 1. Then circulation
was maintained for 10 h and after a while, temperature
of cleaning solution increased up to II 0 °C (± 5 °C).
Then concentrated H2 S04 (98 %.) was dosed carefully
into the re-circulated cleaning solution by acid dosing
pump upto pH 2 (± 0.5). Time req uired for acid
dosing was 6 h. The acid cleaning solution was recircu lated for I 0 h and the temperature of boiler was
maintained at 110 °C (± 5 °C) by injecting steam and
SOHAIL & MUSTAFA: ACID CLEANING OPERATION FOR UNIT OF A THERMAL POWER STATION
225
Table 4 -Average concentration (mgL-1) of ionic impurities absorbed in chemical solution and % impurities removed at
different stages of cleaning operation
cs
CSol. MJ
pH
N2H4
HRB
FSAC
SSAC
500±0.5
500±0.5
500±0.5
6.9±0.2
2.0±0.5
2.0±0.5
Nil
275±25
275±25
Ionic imQurities
Fe
Ca
Mg
Si02
Fe
0.5
616
784
0.72
12.0
6.4
1.2
44.6
52.9
2.0
28
35
1.8
12
16
% impurities removed
Ca
Mg
2.6
52.1
47.6
1.8
74.0
22.2
Si0 2
1.28
68.4
30.6
CS-Cieanino sta oe. C Sol- cleanino solution .
Table 5 - Averagc quantity of residual deposit (g. m- 2) remained
in boiler pipelines and accessori es after cleaning operation
Deposit (rubout method)
Identity of tubes
Front wall P*- 3, T *- 12
Back wall P-4, T-1 6
Eco nomi zer
Super hea ter
LP heater
10
13
10
07
06
Table 6-Average % compositi on of deposits rema ined in boiler
WWTs after c lean ing operation and pass ivation
Constitu ents
Averaoe %
Si0 2
0.48
CaO
0.88
MgO
0.36
2
then pressure of boiler was controlled at 10 kGr cm •
The concentration of hydrazine in cleaning solution,
acidity, pH as well as iron, CaO, MgO, Si0 2 absorbed
in cleaning solution during FSAC operation were
analyzed for half an hour interval. After FSAC
operation the cleaning solution was drained off into a
neutralizing pond. Then the boiler was washed by
flashing (flow 100m 3 . h- 1) of demineralized water
(required 3000 m 3) up to pH 6.7 (± 0.1 ), acidity 0 .05
mgeqL- 1, Fe+++ 0 .05 mg L- 1 and tran sparenc/ 5 50 em.
Second stage of acid cleaning (SSAC)
SSAC operation starts up by introducing 500m 3 of
demineralized water into the boiler using make up
pumps and re-circulated at temperature 60 °C (± 5 °C)
for 4 h. Then hydrazine hydrate (64 %) was dosed into
the re-circulated water by hydrazine dosi ng batch
pumps upto concentration of hydrazine in cleaning
solution 250 (± 25) mg L - I , and it was re-circulated
(flow 50 m3 h- 1) for 10 h. Then concentrated H 2 S0 4
(98 %) was dosed into th e boiler cleaning so lution
upto pH 2.0 (±0.5) and re-circulated for 12 hat ll0 °C
(± 5 °C). During SSAC operation the concentrations of
hyd razine, acidity, pH and iron, CaO, MgO, Si0 2
absorbed in cleaning solution were analyzed for an
hour interval. At the end of SSAC operation the
cleaning so lution was drained off into a ne utralizing
pond and boiler unit was hed by demineralized water
such as the end portion of FSAC operation .
Neutralization and preservation of the boiler (NPB)
operation
NPB operation of the boiler was made by
introducing 600m 3 of demi water into the boiler and
re-circulated at 80 °C (± 10°C) for 4 h. Then ammonia
and hydrazine hydrate were dosed into the boiler recirculated water upto pH I 0 .8 (± 0.2), N 2H4 500 mg
L- 1. The NPB operation was maintained for 8 h. After
NPB operation the boiler was cooled by reducing
gradual heat release, so that speed of reducing the
drum temperature is not more than 2.5 °C/min and the
difference up-down should not be more than 40 °C.
Then cleaning re-circulated solution was drained off
at 60 °C into a neutralizing pond.
Results
The qualitative and quantitative analyses of the
adhesion scale inside the supplied pipelines and other
accessories for a natural circulation vertical water
wall tubes high pressure boiler unit No. 6, of Ghorasal
Thermal Power Station have been estimated. Results
revealed that the supplied pipelines and accessories
used for boiler WWTs, LP, heaters, super headers,
collectors, economizer, boiler drum and dearator li ve
steam lines, hot reheat lines, cold reheat lines, feed
water lines, were attacked by considerable amount of
undesired deposit or corrosion, consisting of Fe 20 3 _
CaO, MgO and Si0 2 (Tables I and 2).
The maximum quantity of deposit (98 gm- 2) was
found in the boiler Water Wall Tubes (high carbon
steel -St. 20, Russ ia,) The formation of deposit in
such a type of boiler is due to natural oxidation of
metallic tubes and accessories.
TSAC operation was employed to cleanup
undesired scale adhesion inside the boiler pipelines
and other accessories. The adhesion impurities
contained high amount ferric oxide 96-97 % due to
oxidation of metallic surface. Therefore 1ron
impurities were withdrawn from boiler at two stages,
First Stage Acid Cleaning (FSAC) and Second Stage
Acid Cleaning (SSAC). During these cleaning
ope rations hydrazine in solution should be maintained
INDIAN J. CHEM. TECHNOL., MAY 2001
226
at 250 (± SOmgL- 1), because hydrazine reacts with
loose deposit Fe20 3 and FeO to form ferrosoferric
oxide Fe 304 ash, and cleantd off with circulated
solution and after a while hydrazine itself also formed
a thin coat on cleaned metallic iron surfaces which
protect corrosion . Therefore hydrazine itself works as
a cleaning reagent and also as a corrosion inhibitor in
presence of acid .
4 Fe(OH) 3 + N 2H4 ~ 4 Fe(OHh + 2 H 20 + N 2
(I)
6 Fe20 3+ N2H4 ~ 4 Fe304 + 2Hz0 + Nz
(2)
N2H4 + 0
(dissolved oxygen)~
2Hz0 + Nz
(3)
Discussion
Sulphuric acid was used for the removal of Fe 20 3 .
CaO, MgO while hydrazine was maintained at 250
mg L- 1 in the cleaning solution to prevent COITosion
on inner metallic surfaces of steel tubes and boiler
accessories.
During
acid
cleaning
operation
temperature of the cleaning solution should not
exceed 180 °C. Because at that temperature hydrazine
rapidl y breaks down into ammonia and nitrogen.
FeO + HzS04 ~ FeS04 + HzO
(4)
2Fez0 3 + 6HzS04 ~ 2Fez(S04h + 6H20
(5)
CaO + H2S04 ~ CaS04 + HzO
(6)
MgO + H2S04 ~ MgS04 + HzO
(7)
Nz H 4 (180 °C)~NH 3 +
(8)
Nz
The % amount of impurities (ionic stage) removed
from the boiler measured at different stages of
chemical cleaning operation are given in Table 4.
After completion of cleaning, the manholes of
boiler drum, dearator, bottom water walls headers
were opened. Inspection revealed that in the drum,
dearator, and in headers with the bottom location of
unions solid sediments (sludge, scale particles, weld
flash, cutting) were not present, In two location of
headers end of the unions a little sediment was
observed and cleaned manually.
Conclusion
From Tables 3 and 5 it can be concluded that not
less than 95 % of deposits and impurities has been
removed during cleaning operation . A uniform brown
coloured (Fe30 4) passivation by hydrazine in presence
of ammonia on metal surface of boiler pipelines was
obtained, which wase found to act as strong protecti ve
corrosion agent (Table 6).
Acknowledlgement
The authors are indebted to Bangladesh Power
Development Board (BPDB) for providing scope to
carry out thi s work at the Ghorasal Thermal Power
Station, Bangladesh.
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