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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