-
EVALUATION OF CHEMICAL FUEL ADDITIVES TOCONTROL CORROSION AND
EMISSIONS IN DUAL
PURPOSE DESAL/POWER PLANTS1
P.C.Mayan Kutty and Abdul Ghani Dalvi
Research & Development Center, Saline Water Conversion
Corporation,P.O.Box 8328, AL-Jubail 31951, Kingdom of Saudi
Arabia
M. A. Farhan Al-Ghamdi, Abdullah Hodan & Assim
DaghustaniSWCC, P.O.Box 7624, Jeddah Plant, Jeddah 21221, Kingdom
of Saudi Arabia
ABSTRACT
Use of heavy fuel oils in industrial furnaces is known to
produce a host of corrosionand environmental related problems.
Severe corrosion in hot and cold zones of thefurnace and emissions
of obnoxious gases, particulates and acid smut to theatmosphere are
a few to name which will cost millions by way of forced shut downs
andunscheduled maintenance, besides creating environmental
pollution. A cost effectivesolution to mitigate some of the above
problems is the use of chemical fuel additives.The effectiveness of
chemical additives in heavy oil fired boilers is site specific
andrequires testing of several additives in the boiler under actual
operating conditionsto optimize the additive regime to obtain the
maximum gains.1
SWCC's boilers attached to the dual purpose desal/power plants
in the WesternProvince of Saudi Arabia are using heavy residual
fuel (fuel oil #6) containing highsulfur (approximately 3.5%) and
low vanadium (approximately 40 ppm). These plantsare reported to
have chronic corrosion problems causing unscheduled shut-downs
andfrequent replacement of equipment and parts resulting in high
maintenance costs andloss of production besides creating
environment problems. In an attempt to eliminateboiler internal
corrosion and to reduce the hazardous nature of the flue gases
somechemical fuel additives were trial tested for extended duration
in a power house boilerfrom the Western Province. Three magnesium
based additives were tested, at differentdose rates; the test
duration for each chemical lasting from 6 to 10 weeks. Various
fluegas parameters such as SO2, SO3, and NOx contents, acid dew
points and rates of acidbuild up were determined. Quantitative
evaluation of boiler soots from the test unit aswell as from a
control unit without additive dosing were also carried out
forcomparison. Effects of additive dosing on the boiler performance
were alsomonitored by evaluating boiler load, efficiency, flue gas
outlet temperature, opacity,
1 * Prize winning paper presented at the IDA conference, Abu
Dhabi, 18- 24 Nov., 1995
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fuel and steam flows, etc. Boiler internals were Inspected
before and after the testingof each additive. Results of these
tests are summarized in this paper.
INTRODUCTION
Saline Water Conversion Corporation (SWCC), besides being the
largest producer ofdesalinated water in the world with installed
capacity of around 600 MGPD. operatesseveral power generating units
in conjunction with their dual purpose plants. Thetotal electricity
generated by these units currently stands above 4000 MW [l].
Aftermeeting the in-house requirements. SWCC exports the excess
power to the Kingdomspower distribution grids.
The major SWCC plants in the Eastern Province of the Kingdom are
using gaseousfuels causing practically negligible or little
corrosion problems. In the WesternProvince power plants, however,
residual fuel oil Bunker C (#6) is the primary fuel.This fuel is
known to cause extensive corrosion and emission problems.
Corrosionwithin the plant appurtenance causes forced shut-downs,
thereby increasing themaintenance and operational costs.
The largest dual purpose desal/power plants operated by SWCC in
Western Province isin the city of Jeddah on the Red Sea coast.
Jeddah facility consists of three phases andproduces a total of 112
MGPD of desalinated water and 924 MW of electricity. Bothwater and
electricity produced by SWCC plants constitute the major share of
these twoessential commodities for the Jeddah populace.
When all Jeddah boilers are in operation at full loads about 312
T/hr of fuel oil #6 isconsumed discharging large quantities of soot
and gaseous pollutants into theatmosphere. The quality and quantity
of the emissions generally depends on the qualityof the fuel used.
In Jeddah the so called high sulfur-low vanadium type residual
fuelis used. Besides these two most troublesome constituents, this
oil also contains traces ofsodium and nickel which arc known to
increase the severity of corrosion andenvironmental problems. While
sulfur has been identified as the single mosttroublesome
constituent of furnace oils, a host of other constituents and
operationalconditions also assist the former in its destructive
role.
During combustion sulfur forms SO2 by reacting with O2 in the
combustion air,S + O2 --> SO2 (1)
While this reaction is quantitative, about 1% of SO2 is oxidized
to SO3 by reaction withatomic oxygen present in the combustion air
near hot zones in the furnace:
SO2 + O ---> SO3 (2)
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Reaction (2) is accelerated by the catalytic action of oxides of
metals such as V, Ni, andFe. While V and Ni are present in the fuel
in trace quantities and get converted to theiroxides during
combustion iron oxides are ubiquitous inside the boiler on the
heattransfer surface as corrosion products. Higher levels of excess
combustion air alsoenhance SO3 generation. SO3 in the gaseous state
is a harmless entity. But when theflue gas temperature drops below
the acid dew point temperature, SO3 will condensewith water vapour
to form sulfuric acid creating severe corrosive environment.
Acidmist absorbed on unburnt carbon also will cause corrosive
particulate fall out from thepower station stacks.
Acid constituents in flue gases, apart from creating
environmental hazards, will causecold-end corrosion in air heaters,
air handling ducts etc. Other fuel oil impurities suchas V and Na
form highly corrosive deposits on the heat transfer surfaces in hot
areassuch as the furnace tubes, super heater and economizer tubes
etc resulting in metallosses, During combustion organic vanadium
compounds in the oil thermallydecompose and oxidize in the gas
stream to V2O3 and V2 O5 [2]. Sodium present asNaCl in the oil,
vaporizes and reacts with SO3 [3]. Subsequent reactions between
Vand Na compounds result in the formation of complex vanadates
having melting pointslower than those of the original compounds.
The various reactions can be summarizedas follows :
NaCl --->-> Na 2 O (vaporization followed by oxidation)
(3)
Na2O + SO3 -->>-> Na2 SO4 (4)Oxidation
( O r g a n i c ) - - - > - > V 2 O 5 ( 5 )
N a 2 S O 4 + V2 O5 -->-> 2NaVO3 + SO3I I I (6)
M.P. 1150 oK 964 oK 902 oKTube-metal temperatures encountered in
furnace and super heater tube banks of manyoil fired boilers are in
the range of 800-866o K [2] and exceed the meltingtemperatures of
many of the compounds found in the deposits and maintain the latter
inmolten state causing metal corrosion. SO3 formed through various
routes of reactions,also results in the formation of alkali
pyrosulphates (Na2S2O7 or K2S2O7) which arecorrosive at
temperatures 673 - 755K [4]. Pyrosulphates are believed to react
withFe2O3 or Fe3O4 to form trisulphates, Na3Fe (SO3)3 or K3Fe
(SO4)3 which areresponsible for corrosion in hot temperature
zones.
2Fe2 O3 + 6K2 S2 O7 ------> 4K3 Fe(SO4)3
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Various methods have been used with varying degrees of success
to reduce hazardousstack emissions and prevent plant corrosion. The
most effective method is the use ofclean fuel instead of the dirty
fuel. Lower contents of S, V and Na will significantlyreduce, if
not completely eliminate, most of the problems. If that choice is
not availabledue to cost consideration other methods such as
modification of combustion conditions,use of fuel oil additives,
desulfurization of flue gases etc. are also employed to reducethe
severity of some of the problems.
Use of oils having low sulfur content has helped reduce
corrosion problemsconsiderably. However, though low S contents
proportionately decreases SO2, thereduction in SO3 due to lower S
contents is not quite dramatic. As stated by Radway[5,6], burning
of low S fuel does not yield the expected reduction in SO3
andparticulate sulfates. A five fold reduction in fuel sulfur
lowers SO3 only by 40%.Despite their high costs,
hydrodesulfurization (HDS) of heavy oils, as well as flue
gasdesulfurization (FGD), have become increasingly popular over the
years after they wentinto commercial operation in 1968 [7].
By carefully controlling the levels of excess air to the
minimum, at around less than 5%(1% excess O2), significant decrease
in SO3 generation and subsequently reducedcorrosion can be
achieved. While this mode of operation has an additional advantage
ofimproved boiler efficiency, several associated problems such as
disproportionateproduction of both soot and SO3 due to poor mixing
of combustion air and the fuel oilhas been experienced. However,
operation at low excess air has become popular inrecent years.
Preventing the condensation of SO3 with water vapour in the flue
gases by controllingthe flue gas exit temperature is another method
followed by all boiler operators as ageneral practice. The exit
flue gas temperature should be maintained higher than theacid dew
point temperature by about 20-25C. But maintaining the flue
gastemperatures uniformly above the dew points through out the
operational life is difficultto achieve due to sudden swings in
boiler loads dictated by varying consumer demands,changes in
ambient air qualities etc creating potential corrosion
environment.
Addition of certain chemicals in trace quantities into some
selected zones of the boilersystem has been in use for several
years in many utilities around the world as asuccessful solution to
many taxing problems such as hot-and cold-end corrosions,fugitive
emissions etc. Type, concentration and dosing point of the
chemicals dependvery much on the nature of the most troublesome
problem faced by the utility inquestion. An excellent review of the
data on the use of various chemical additives inutility boilers in
US has been published by D.W.Locklin et al [8] based on the
resultsfrom an EPRI (US) research project conducted by
Battelle-Columbus. This papercontains the evaluation of the data
from 445 separate additive trials from 38 US powerutilities.
Several other informative publications are also available in
literature such as
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one classical paper from J.W.Radway and L.M.Exley [5] and
another from ThomasGarcia-Borras [9].
Use of fuel additives has been reported to reduce emission of
SO3, acid fly ash, acidsmut, tube fouling, fireside corrosion in
air heaters and economizers (low temperaturezone), in furnaces and
superheaters (high temperature zone) as well as to promotecarbon
burn out [8].The most effective among the several fuel additives
used are based on MgO orMg(OH)2, which are generally available in
oil dispersed forms. Oil-soluble,organometallic additives based on
Mg or Mg-Mn combinations are also being usedquite successfully. In
cases where the primary aim of the fuel additive is to
achieveimproved fuel combustion, additives based on a combination
of metals including Mg,Mn and some transition metals are used quite
effectively. To reduce back-end corrosionuse of MgO either as
powder or in suspended form has been reported to be effective
ifinjected in the convective passes where most of the SO3 is formed
catalytically [5]
MgO inhibits the catalysis of SO2 to SO3 by oxides of vanadium
and iron. It also reactswith V2O5 and Na2SO4 to form high-melting
compounds such as magnesium vanadatesand sodium magnesium
vanadates. As the melting points of the new products are muchhigher
than the metal temperature normally encountered, metal loss due to
hightemperature corrosion resulting from the presence of molten
compounds such asNa2SO4 and V2O5 on the furnace surfaces are
greatly reduced.
MgO, in addition to inhibiting the formation of SO3, also
effectively neutralizes the acidthat condenses on the cooler parts
of the air heating system, forming neutral MgSO4.
FUEL ADDITIVE TESTS IN JEDDAH BOILERS:
In 1989 some preliminary tests were carried out for a short
period using a commerciallyavailable Mg(OH)2 based additive in a
boiler in Jeddah, Phase IV [l0]. These testsyielded promising
results such as reduced SO3 content and acid dew point,
reducedquantity and improved quality of ash, etc. But since the
duration of the test was veryshort no conclusive data was obtained
from these tests. Therefore it was decided toundertake a more
extensive test programme to obtain detailed informations on
theeffects of chemical additives on Jeddah boilers. This paper
summarises the resultsof a long term study carried out in a boiler
unit in Jeddah, Phase-III using threedifferent fuel additives.
OBJECTIVES OF THE STUDY
The test programme was scheduled to last six months and intended
to achieve thefollowing objectives:
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(a)
(b)
(c)
(d)(e)
To test several potential chemical additives to evaluate their
comparativeperformances and to optimize their dose rates.To
evaluate the hot and cold side corrosion potential in the presence
of theadditives.To determine the effect of additives on SO3, SO2
and NOx generations and aciddew point.To evaluate the quantity and
quality of soot/dust production.To check if the additives have any
adverse effect on the boiler performance.
SELECTION OF ADDITIVES AND THE TEST UNIT
Following three additives were selected for the study:
(1)
(2)(3)
MGOH, a magnesium hydroxide based inorganic additive dispersed
in anorganic solvent.MGOA, a magnesium oxide based additive,
dispersed in an organic solvent.MGOB, a special grade MgO powder,
dispersed in demineralized waterlocally.
First two chemicals were pumped neat into the fuel oil header at
the required doserate. MgO powder was slurried in demineralized
water as 10% slurry before pumpingto the fuel line.
Boiler # 7 from Jeddah Phase III was selected for the extended
study due to its betterinstrumentation and control systems. Jeddah
phase III has four front wall fired boilers,each with nine burners
at three elevations. The fuel oil flow is about 24 TPH at
themaximum MCR of 350 TPH of steam generating 60 MW of
electricity.
EXPERIMENTAL APPROACH
The test unit was put in operation and after achieving stable
condition severaloperational and chemical parameters were monitored
for a period of 2 weeks. Then thefirst chemical was dosed at a dose
rate of 250 ppm (as Mg), maintained the dose ratefor 2-3 weeks,
monitoring all test parameters, Tests were repeated at dose rates
of 200and 150 ppm by the same procedure. Then the unit was shut
down for internalinspection at the end of testing the first
chemical. After restarting the boiler the othertwo chemicals were
tested following the same procedure.
ANALYTICAL PARAMETERS AND PROCEDURES
1. Flue Gas Analysis
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The following parameters were determined in the boiler flue
gases as per methodsindicated against each.
(a) SO2 and SO3 - USEPA method # 6 [11].(b) Acid dew point and
rate of acid build up (RBU) - determined using a portable
Land Instrument Model 200.(c) Oxides of nitrogen (NOx) - USEPA
method # 7 [12].(d) CO2 and O2 - by Orsat analyzer.(e) Moisture
contents - by gravimetric method by absorbing known volume of
the
flue gas by anhydrous calcium sulfate.
2. Soot Analysis
The following parameters of the boiler soot samples were
analyzed by standardmethods:
(a) Weight, bulk density and moisture content.(b) pH and
conductivity of 1% slurry.(c) Magnesium (soluble and insoluble),
vanadium and carbon contents(d) Acid content.
3. Boiler Operation Parameter
Fuel and steam flows, boiler efficiency, temperature of the flue
gases at inlet to the airheater, air heater delta P, and flue gas
opacity were monitored on daily basis.
RESULTS & DISCUSSION
Flue Gas Parameter
Concentration of SO3 in flue gases in the absence and presence
of various additives areshown in Fig.1. The data show that all
three chemicals were able to achievesubstantial reduction in SO3
levels in flue gases in comparison to its concentration inthe flue
gases in the absence of the additives. Average reduction of SO3 in
the fluegases, independent of additive dose rates, were 31.7, 29.4
and 28.3% by MGOH,MGOA and MGOB, respectively. Effect of dose rates
on SO3 reduction was significantbut not very dramatic. For
instance, in the case of MGOH the highest reduction wasfound to be
37% at 250 ppm Mg, and decreased to 31 and 27% at 150 and 100 ppm
,respectively. With MGOA the decreases were 38.0, 33.5 and 25% as
the dose rateswere reduced in the same order. MGOB addition
resulted in a reduction of SO3 by 29%at 150 ppm while that at 100
ppm was only 14%. With respect to SO3 reduction all the
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three chemicals exhibited similar performance and quite
favorably compare with theliterature data [5].
SO3 content in the flue gases is the major contributing factor
determining themagnitude of the acid dew point, The following
simplified equation to calculate thedew point from SO3
concentration was used by several workers based on a
proceduredeveloped by Peter Muller [13]:
[ XLog ] = 0.612(T))T) - 7.52 (8)Water Vapour Conc.Where, X = SO
3 conc. in ppm
and = Dew point temperature - water saturation temperature.
But, since the calculated and measured values were reported to
be conflicting quiteoften [5], a portable Land Instrument was used
to experimentally measure the dewpoints in our studies. A typical
determination by graphical extrapolation is shown inFig. 2 in the
absence and presence of the additive. A significant decrease in the
dewpoints from 135C to 115 oC in the absence and presence of the
additives, respectively,is evident from the figure.
Maximum, minimum and the average dew points obtained during the
present studiesare shown in Fig. 3, independent of the additive
dose rates. Average acid dew pointsin the presence of both MGOH and
MGOA were quite similar, 114 and 111oC,respectively. This
constitutes decreases of 2 1.5 and 24.5 oC in dew points by these
twoadditives which is quite comparable with literature data [5,6].
Highest decrease wasobserved with MGOB additive, but this value is
not quite reliable since the boiler couldnot be maintained steady
at full load during the tests with MGOB due to
operationalconstraints as was done during tests with the other
additives. At lower loads SO3generation and consequently acid dew
points will be comparatively low.
The rate of acid build up (RBU) has been reported to be a good
indicator of thecorrosiveness of the flue gases - at higher RBU,
flue gases are more corrosive andvice-versa. Several previous
studies had shown that RBU of close to or less than 100mA/min is
indicative of non-corrosive flue gases [14]. During the present
study RBUwere determined at various excess oxygen levels in the
presence of 150 ppm additive.Figs. 4 (a-c) show the RBU data
obtained with the three additives. It can be seen thatin the
presence of all three additives RBU values decreased quite
significantly from thecontrol value (>l000 uA/min without
additives) and stabilized at around 100 uA/min.It may be seen from
the summary of the results as shown in Fig. 5 that RBU
alsodecreases with excess oxygen since it is a function of SO3
content in the flue gases and
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the latter is low at low excess O2. MGOH exhibited slightly
better performance thanMGOA. Though MGOB showed the lowest RBU
values among the three additives,this could be due to low boiler
loads during most part of the MGOB test period asmentioned above.
The results in general indicate very effective neutralization of
theflue gas SO3 by the additives.
As a result of improved carbon burn-out in the presence of the
additives operation ofthe boiler with low excess air is not
accompanied by problems usually encountered dueto improper mixing
of air and fuel oil. Formation of acid smut is eliminated
byreduction in the levels of SO3 and V2O5 and the dust collection
is made easy due to theformation of high melting compounds and
absence of acidic constituents. Decrease inacid dew point of the
flue gases due to additive dosing also helps to reduce the flue
gasexit temperature and thereby improve fuel efficiency. Since the
costs of the additivesare only 0.5-I.0% of the fuel costs, saving
obtained through improved efficiency byadditive dosing could be
substantial.
Effect of additives on the SO2 and NOx levels in the flue gases
was negligible inagreement with other reported studies [8].
Boiler Soot Characteristics
The ability of the additives to reduce acid content has been
further confirmed by theboiler soot quality. pH of 1% slurry of the
soot samples increased above 3.5 duringadditive dosing, accompanied
by decrease of sulfuric acid below detection level.Comparative
values of pH and sulfuric acid are shown in Table 1. In general pH
of 1%slurry of soot samples collected during the trials of all 3
additives showed that sootsamples were non-corrosive and free from
sulfuric acid. pH below 3.5 is considered tobc corrosive and
indicates the presence of acid as seen from the soot quality
beforeadditive dosing. MGOH produced the most neutral soot among
the three additives.
Other soot characteristics also indicated remarkable
improvements during the additivedosing. While the soot continued to
be quite dry and friable its soluble magnesiumcontent increased
from 30 ppm (before additive dosing) to greater than 10, 000
ppmwith MGOH and MGOA dosing. This further confirms the effective
neutralization ofacidic mist in the soot with MgO forming soluble
magnesium sulfate. The thirdchemical, MGOB, also indicated good
neutralization, though the soluble magnesiumcontent was less than
8000 ppm.
Vanadium content in the soot samples also increased considerably
during additivedosing. While MGOH dosing yielded the highest
increase of vanadium (about 5 fold),increase during MGOA dosing was
about three fold. The third chemical alsoindicated the same trend,
but to a lesser extent, These results show that vanadium isbeing
effectively converted to high melting species by reaction with
magnesium which
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is easily being blown out of the boiler under normal gas
velocities, In the absence ofmagnesium vanadium forms low melting
oxides, depositing on to boiler surfaces inhigh temperature areas
causing high temperature corrosion. Vanadium oxides will alsoabet
corrosion in low temperature zones by catalyzing the conversion of
SO2 to SO3which condenses on to cooler surfaces. The results,
therefore, show that the additivesmay be quite effective in
reducing both types of corrosions.
Soot quantities decreased to approximately one third during MGOH
and MGOA dosingas seen from Table - 1. Reduced quantity as well as
improved quality of the sootgenerated during the additive dosing
are expected to make soot collection and disposalsafer and more
cost effective. Soot quantity could not be determined during
MGOBtrials due to operational reasons,
Boiler Performance
Among the boiler parameters monitored the most relevant to the
tests were boilerefficiency, air heater AP, flue gas exit
temperature and flue gas density. No perceptibledecrease in boiler
efficiency was noticed on account of additive dosing. Though the
fluegas exit temperature showed an increase of 30-40C and 50C
during MGOH andMGOA dosing, respectively, it did not adversely
affect the boiler efficiency. Theobserved rise in flue gas
temperature may be due to the formation of a reflective coatingof
MgO on the heat transfer tubes. Internal inspection of the boiler
after each additivetesting showed that most of the tubes surfaces
in super heater and economiscr areaswere covered with thick
deposits. The deposits were much thicker after tests withMGOA than
with MGOH. MGOB produced the least deposits probably due to
smallerparticle size and/or short test duration. The deposits were
found to be consisting mostlyof magnesium, sulfur, iron, vanadium
and nickel. These deposits are expected toprotect the metal
surfaces from corrosive flue gases though they may lead to lower
heattransfer. But the results did not indicate any perceptible
decline in the boilerefficiency.
The air heater AP remamed constant indicating the absence of
fouling which wasfurther confirmed during shut-down inspection.
Flue gas density data did not indicateany reliable trend during the
trials with all the chemicals. The readings were found tobe quite
erratic even during the absence of additive dosing, probably due to
faultymeter and therefore, no conclusion could be drawn from these
data.
Internal Condition of the Boiler
Internal surfaces of all boiler areas (combustion chamber, high
temperature zones andcold-ends) were covered with deposits as
observed during the inspections. Similarobservations were widely
reported in literature [6]. The deposits consisted mainly of
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neutral oxide and sulfate of magnesium with traces of other
compounds such as oxidesof V, Fe, Ni and other complex compounds
containing these species. They normallyform thin, reflective
coating on the heat transfer surfaces and prevents corrosion
byacting as a barrier between the flue gases and the metal
surfaces. In some cases thesewhite deposits were reported to have
affected the boiler efficiency due to low heattransfer coefficients
of the deposits in comparison to those of the metals. But
thepresent trials did not indicate any perceptible reduction in
boiler efficiency as a resultof deposit formation as stated
above.
Deposits observed on the air handling ducts were thin and
uniform and consistedmainly of neutral compounds of magnesium. This
is expected to provide effectivecorrosion protection to the metal
surfaces.
CONCLUSIONS
Results of the trial tests had established that magnesium based
additives are quiteeffective in decreasing the SO3 content in
boiler flue gases resulting in a substantialreduction of the acid
dew points. The results were also substantiated by
nearneutralization of the flue gases as indicated by low rate of
acid build up (RBU).
Conversion of low melting compounds into high melting compounds
by chemicalreactions with the additives during combustion was also
achieved as indicated by thedry nature of the boiler soot. Soot
quantity was reduced considerably in addition toimprovements in the
soot quality as indicated by the absence of sulfuric acid in the
sootsamples.
These results indicate that significant reduction in the
corrosion incidences of the boilerinternals can be achieved by
chemical dosing. Improved quality and reduced quantityof soot will
reduce the hazardous nature of soot emissions and will also
facilitate easiersoot handling.
Boiler operation parameters were not adversely affected during
the additive dosing.Fouling of air heaters, clogging of burner
nozzles etc. were also not observed.
Performances of all three additives tested were quite
comparable, with MGOHappearing to be the best among the three.
Though slight reductions in theperformance efficiencies were
noticed as the additive dose rates were decreased anoptimum dose
rate of 150 ppm appeared to be quite cost effective.
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