behavior of a high-capacity steam boiler using heavy fuel...

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Fuel Processing Technology 86 (2004) 89–105

Behavior of a high-capacity steam boiler using

heavy fuel oil

Part I. High-temperature corrosion

Jorge Barroso a,1, Felix Barreras b,*, Javier Ballester c,2

aCECYEN, Universidad de Matanzas, Autopista a Varadero, Km 3 1/2, Matanzas 44740, CubabLITEC/CSIC, Maria de Luna 10, Saragossa 50018, Spain

cFluid Mechanics Group, University of Zaragoza, Maria de Luna 3, Saragossa 50018, Spain

Received 10 December 2003; received in revised form 18 December 2003; accepted 18 December 2003

Abstract

Problems related to the combustion of heavy oils with high vanadium contents in a high-

capacity steam boiler have been analyzed. In this study, two types of additives have been tested

both to diminish high-temperature corrosion and to obtain more brittle deposits. Results show that

heavy fuel oil can be suitably burned in this type of boilers if it is chemically treated with an

anticorrosive additive. In this research, the best results have been obtained when the heavy fuel

oil was mixed with an organometallic additive. The influence of fouling on the behavior of some

operational parameters such as the pressure in furnace and pressure drop in superheaters and pipe

metal temperature, among others, has been verified. It is concluded that monitoring of these

parameters can provide direct information on the degree of fouling, as well as of the effectiveness

of the treatment during normal boiler operational conditions. Some other improvements in the

atomization and combustion processes due to the use of the organometallic additive have also

been observed.

D 2004 Elsevier B.V. All rights reserved.

Keywords: High-temperature corrosion; Vanadium; Magnesium; Additive; Steam boilers

0378-3820/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.fuproc.2003.12.006

* Corresponding author. Tel.: +34-976-716-303; fax: +34-976-716-456.

E-mail addresses: [email protected] (J. Barroso), [email protected] (F. Barreras),

[email protected] (J. Ballester).1 Present address: LITEC/CSIC, Marıa de Luna 10, Saragossa 50018, Spain. Tel.: +34-53-45-261-432; fax:

+34-53-45-253-101.2 Tel.: +34-976-762-153; fax: +34-976-761-882.

J. Barroso et al. / Fuel Processing Technology 86 (2004) 89–10590

1. Introduction

Today, about 90% of the total energy production all over the world is provided by

combustion of fossil fuels. Unfortunately, hydrocarbon combustion has a major impact on

the global environment through the emission of CO2, which is a greenhouse gas.

Furthermore, emission of NOx, SOx, polycyclic aromatic hydrocarbons (PAHs), CO,

and particles leads to air pollution, acid rain, and health hazards. However, the strong

dominance of fossil fuels combustion will continue in the foreseeable future, as the world

energy production is expected to rise by 60% in the next 20 years [1]. Then, it is obvious

that combustion processes have to be carefully controlled in such a way that maximum

efficiency and minimal emission of pollutants are achieved.

The increase in consumption of petroleum-derived liquids as fuel for transportation,

electric power generation, heating, and process engineering is resulting in a reduction in

the quality of residual oils that are becoming heavier. This quality reduction translates into

lower heating values, but above all, into higher viscosity, as well as higher levels of

asphaltenes, Conradson carbon, etc. At the same time, the world natural reserve of

bituminous petroleum is estimated to be three times higher than that of regular fuel oils.

These facts, combined with the increase in price of regular fuel oil, have motivated the

change to heavy fuel oil in most boilers at electric power plants. For these reasons, an

increase in the vanadium, sodium, and sulfur contents in the chemical composition of

liquid petroleum typically used as fuel in boilers has also been detected.

Combustion of heavy oils with high vanadium, sulfur, and sodium contents results in

highly corrosive deposits. The slag produced during combustion has a low melting

temperature and adheres to hot metal surfaces (450 jC and above). Vanadium salts are

extremely corrosive, since they dissolve the protective oxide film on the metal surface and

then transport oxygen to the clean surface that corrodes. In order to prevent or reduce

deposits and corrosion, the fusion temperature for the sodium and vanadium salts needs to

be raised. A higher melting point means drier and brittle deposits, which do not adhere to

the metal surface and are easy to remove.

High-temperature corrosion is mainly dependent on the composition and concentration

of ash formed during fuel oil combustion [2–4]. Among others, sulfur, vanadium, sodium,

and potassium are the elements in the fuel that are responsible for corrosion and deposits.

During combustion, such elements give rise to complex low-melting-point compounds.

These sticky deposited materials capture ash, soot, and coke, which reduce the heat

transfer and cause corrosion. Sulfur is one of the major offenders in corrosion, leading to

high-temperature corrosion when complex sulfates are involved. Alkali sulfates, such as

Na2SO4 and K2SO4, are highly corrosive when molten, as well as alkali iron trisulfates,

which are highly reactive materials that melt at the temperature range reached on

superheaters. Besides, the reaction between sodium and vanadium forms compounds that

normally melt between 510 and 870 jC. However, the system NaVO3�Na2O�3V2O5,

which has a melting point as low as 480 jC, can be found in superheaters and is also very

corrosive. Vanadium oxides attack the metal surface by dissolving the normally protective

oxide layer and by assisting in the oxygen transport to the pure metal.

There are some ways to reduce high-temperature corrosion in a steam boiler, namely,

change to another ‘‘lighter’’ fuel oil (normally very expensive), reduce excess air, improve

J. Barroso et al. / Fuel Processing Technology 86 (2004) 89–105 91

the furnace design, and use of new materials for equipment, among others. However, when

a heavy or bituminous fuel oil is used, solutions are dramatically reduced. It has been

verified that the most suitable way to counteract high-temperature corrosion is by the use

of some additive substances mixed with the fuel oil, which act by raising the melting

temperature of vanadium-based oxides. It is important to note that additives may increase

the mass of deposits, but they become weaker so that they can be either detached

periodically or easily removed by soot blowing [5].

In the present research, high-temperature corrosion in a high-capacity steam boiler

using heavy fuel oil has been studied. The benefits of the use of some additives to avoid

corrosion in this area as well as the behavior of some operational parameters as a function

of deposit growth are discussed.

2. Experimental facilities

2.1. Boiler, fuel oil, and additive characteristics

The present research has been developed in an electric power plant with a maximum

power of 340 MW/h. The plant is equipped with a high-capacity steam boiler with 16

steam-assisted burners distributed in four floors, placed tangentially in the corners of the

furnace to induce a vertical swirl motion of the combusting gases. The main parameters

of the steam boiler are: steam power of 1010 t/h (maximum), superheated steam

pressure of 18 MPa, and temperature of 540 jC. For this condition, a fuel oil

consumption of 70 t/h is achieved. Regarding the main heat transfer surfaces, a four-

stage superheater and a three-stage reheater, one economizer, and two continuous

regenerative air heaters form the boiler. A sketch of the boiler with its main heat

transfer surfaces can be seen in Fig. 1.

A summary of the main physicochemical properties of the liquid petroleum recom-

mended by the manufacturer to be used as fuel in this boiler can be read in Table 1. The

values for the different parameters are specified for both low and high vanadium content

fuel oils.

This research covers an extensive measurement campaign of 4 years. In the first part,

year 1, a regular (low vanadium content) fuel oil was burned, while in the second one,

from years 2 to 4, a high vanadium content fuel was used instead. In Table 2, the average

physicochemical properties for each one are summarized. An analysis of Table 2 shows a

progressive reduction in the quality of the fuel oil used, specially detected in the laboratory

analysis measurements of unburned (residual) carbon, viscosity, and asphaltene contents.

This fact caused an increase in the fouling of tanks, filters, and fuel oil heaters, as well as

worse combustion. At the same time, serious corrosion problems were also caused by the

presence of high vanadium, sulfur, and sodium contents, and the increase in the fouling on

the heat transfer surfaces at the hot gases side.

Therefore, an important part of this research was devoted to find a suitable additive to

be mixed with fuel to avoid, or reduce, high-temperature corrosion. Additives for this

purpose are classified by their physical composition as solid oxide-based (normally

slurries) and organometallic [6]. The most popular additives used in boilers are magnesium

Fig. 1. Sketch of the steam boiler where tests were performed.


oxide slurries and oil-soluble organometallics. Magnesium oxide-based additives react

slowly with other chemical components over the surface of the ash particle at the solid–

gas interface, while the organometallic ones assure the emission of metallic elements

during the combustion, reacting even in the flame front. Organometallic additives are

typically liquids and have a higher efficiency because they react down to the molecular

level while soot is being formed.

In this study, two commercial additives have been tested: a magnesium oxide-based

slurry and an organometallic liquid. The physical characterization of the additives has been

performed using both infrared spectroscopy (FTIR spectrometer) and gas chromatography

(CHROM-5) with a flame ionization detector. The main properties of the two additives are

presented in Table 3.

Table 1

Recommended values for physicochemical properties of the fuel to be burned in the boiler in the present study

Properties Unit Low-vanadium fuel High-vanadium fuel

Heating value kJ/kg 40,193 (min) 40,193 (min)

Gravity at 15.6 jC API 12 (min) 11 (min)

Sulfur % 3.5 (min) 3.5 (max)

Water (by distillation) % 0.5 (min) 1.0 (max)

Sediments % 0.05–0.5 0.15 (max)

Kinematic viscosity at 50 jC cSt 125–370 650 (max)

Ignition temperature jC 65–94 62 (min)

Residual carbon % 8 H 10 (max) 14

Asphaltenes % < 10 12

Vanadium ppm – 400 (max)

Ash % 0.1 (max) 0.1 (max)


2.2. Experimental test procedure

The main goal of the present research is to determine the best additive when a heavy

fuel oil is being used to obtain good results regarding corrosion and composition of the

slag and high boiler efficiency, at the same time. To this end, a set of experiments has been

scheduled, divided into laboratory and industrial tests. Before the beginning of the

experimental tests with additives, and after a general shutdown of the electric power

plant, boiler conditions are optimized following the steps underlined below:

– mechanical cleaning of all thermal surfaces (furnace, superheater, and air heaters) and

gas ducts

– full maintenance of the 16 fuel oil burners and specially fitting of the right balance of air

and fuel oil flow and excess air

– location of the point for additive feeding before the fuel oil heaters (just after the filters).

Table 2

Average value for different physicochemical properties of the fuel oil used in tests

Properties Unit Regular fuel oil Heavy fuel oil

Year 1 Year 2 Year 3 Year 4

Specific gravity at 15.6 jC kg/m3 965.7 988.9 989.4 989.9

Gravity at 15.6 jC API 14.5 11.6 11.5 11.5

Sulfur % 2.2 2.7 2.8 2.8

Water (by distillation) % 0.30 0.53 0.38 0.42

Residual carbon % 12.2 15.7 14.8 16.0

Asphaltenes % 8 11.9 14.2 18.4

Vanadium ppm 135.9 203.6 207.2 292.5

Ash % 0.06 0.09 0.07 0.08

Heating value kJ/kg 42,631 42,429 42,164 41,985

Hydrogen % 11.3 11.1 10.7 10.5

Carbon % 85.1 84.7 84.9 83.3

Kinematic viscosity at 50 jC cSt 209.2 320.1 327.6 356.4

Sodium ppm – 35.0 40.0 42.0

Table 3

Main physical properties for the two additives used in the study

Properties Unit Slurry-type additive Organometallic additive

Solids content % 82 (approximation) –

Absolute viscosity at 28 jC mPa s 842.26 109.29

Density at 25 jC kg/m3 1710 1135.2

Ignition temperature jC 67 71

MgO % 58 –

CeO % 2.2 –

MnO % 1.17 –

Mg % – 10.56

Fe % – 0.08


In these tests, an analysis of the slag collected from the gases side of the high-

temperature heat transfer surfaces as a function of some operational parameters of the

boilers has been performed. As a result of the experimental measurements, the effective-

ness of the treatment has been verified depending on the chemical composition of the slag

and the degree of fouling.

2.2.1. Laboratory tests

Laboratory scale tests included the physicochemical characterization of the slag depos-

ited on the high-temperature areas. Experimental samples have been obtained by adjusting

the boiler to the real operational regime. Three different experimental conditions have been

considered: (a) boiler using low-vanadium content (regular) fuel oil without additive; (b)

boiler using high-vanadium content fuel oil treated with slurry-type additive; and (c) boiler

using high-vanadium content fuel oil treated with organometallic-type additive.

More than 1000 samples of the slag deposited on the hot gases side of heat transfer

surfaces have been collected and analyzed during the whole measurement campaign. In

order to determine their physical morphology and chemical composition, some experi-

mental determinations have been performed. To obtain the morphological characterization

of the slag, scanning electron microscopy and infrared spectroscopy have been applied.

Specific chemical compounds have been investigated by chemical analysis and X-ray

diffraction. Melting temperature of the deposits has also been determined using differential

thermal analysis.

2.2.2. Industrial-scale tests

Valuable information on fouling and corrosion in the high-temperature zone of the

boiler can be extracted from the results obtained in the laboratory tests. However, a

complete study including final conclusions on this problem is only possible if, simulta-

neously, some industrial tests are developed. As it is known, the behavior of some

operational parameters can trace the degree of fouling of the heat transfer surfaces. For

example, the pressure in the furnace, as well as the pressure drop in the superheaters, are

increased as the exhaust gases flow area is reduced due to deposit growth. Besides, gas

temperature at the exit of the furnace is proportionally increased with fouling in the

furnace walls and in the first superheater stage. On the other hand, the metal temperature in

the superheater is also linearly increased with gas temperature at the furnace exit.


As a starting point, the calculation of the additive dosage has been performed

following a stoichiometric analysis between fuel oil flow and its percentage content of

vanadium, sulfur, and oxygen in gases, according to manufacturer recommendations.

For the slurry-type additive, depending on the desirable magnesium-to-vanadium ratio

(R), two concentrations have been considered in the study, namely, one for R = 0.5 and

another for R = 1. For R = 0.5, a proportion of 1 l of additive for 3.8 t of fuel oil has

been used; for R = 1. 1 l of additive for each 1.8 t of petroleum has been established.

At the same time, a fixed ratio of 1 l of organometallic additive for 3.8 t of fuel oil

has also been adjusted. These concentrations have been slightly modified during the

tests as a function of the behavior of the boiler operational parameters (pressure in the

furnace, pressure drop in superheaters, and pipe metal temperature). Special attention

has been paid to the relation between the additive concentration in fuel and the

measurement of the stack gases composition (O2, CO2, and SO3), and dew point.

Additive dosage has been adjusted to attain the lower oxygen concentration for a

maximum CO2 content close to stoichiometry (14.0–15.0); a CO concentration below

400 ppm and a dew point above 110 jC. The stack gas composition has been

measured with a combustion gas analyzer ENERAC POCKET model 100 by

introducing the probe through the gas sample port displayed in Fig. 1. The remaining

parameters (metal and steam temperatures, pressure in furnace and pressure drop in

superheaters, steam flow and pressure, etc.) have been simultaneously collected from

own boiler instrumentation. Temperatures have been measured using Ni–Cr thermo-

couples with a relative error of 0.001%, and pressure and pressure drop both with

pressure transducers with a relative error of 0.02%.

3. Results and discussion

A continuous quality reduction in the chemical composition of the petroleum burned in

this power plant has been confirmed, as can be verified in Table 2. For this reason, control

on some of the physicochemical compounds of the different batches of fuel oil received

has been increased. Viscosity, for example, is an important quality parameter of the heavy

residual oil to be controlled due to its influence on the atomization and combustion

processes. During the present research, kinematic viscosity of the fuel oil before burners

has always been kept below 45 cSt (centi-Stokes; 4.5� 10� 5 m2/s) with an optimization

of the warming temperature around 130 jC. To avoid cavitation in the fuel oil pump, the

adjustment of the optimal heating temperature for heavy fuel oil has been achieved by

regulating the second stage heater.

An increase in asphaltene contents has also been confirmed, reaching a maximum of

21% in some batches, roughly twice the recommended value (see Table 1). At the same

time, a rise in the concentration of CO and CO2 in the stack gases has also been measured.

This fact proves that asphaltenes cause fuel oil instabilities; breaking carbon bonds around

115 jC and releasing heteroelements such as oxygen and sulfur, which are responsible for

the high concentration at the stack gases measurements [7]. To counteract this behavior, a

stabilizer additive was required to exude the aromatic compounds from the asphaltenes,

and to keep them in suspension at the same time.


Despite all of these drawbacks, a slight increase in the combustion efficiency has

been detected when using the organometallic-type additive. On one hand, an improve-

ment in the atomization process has been verified to be related to the tensoactive effect

of this oily additive, which decreases fuel oil surface tension, resulting in a spray with

more uniform and smaller droplets. On the other hand, this additive also tends to speed

up the combustion reaction, reducing the formation of soot, tar, and slag and ensuring

complete combustion with lower excess air. This effect reduces stack losses, increasing

combustion efficiency.

3.1. Effect of additives on slag corrosion and boiler operation

The physicochemical analysis of the low-quality heavy fuel oil evidenced a significant

increase in the vanadium and sulfur contents, which causes high-temperature corrosion

problems. Conventional methods used to measure the corrosion rate in low-temperature

heat transfer surfaces cannot be applied in high-temperature zones where the metal could

melt. For this reason, in this study, the chemical composition of the deposits in the high-

temperature area has been analyzed in order to determine its corrosion potential.

As stated in Section 2, samples obtained from the gases side of the different high-

temperature heat transfer surfaces have been analyzed by chemical methods and X-ray

diffraction. Additionally, the fusion temperature of the slag has also been determined by

differential thermal analysis. Samples have been collected for the following four

experimental conditions when the high vanadium content fuel was used:

1. slag formed without additives (considered as basic samples for comparison)

2. slag formed with slurry-type additive for a fixed Mg/V ratio R = 0.5

3. slag formed with slurry-type additive for R = 1

4. slag formed with organometallic-type additive.

The evolution of the average chemical composition of the oxides detected in the

collected samples is depicted in Table 4 and Fig. 2. As can be seen, for basic samples, the

chemical composition of the slag is mainly formed by vanadium, sodium, and iron oxides.

As iron is not present in the chemical composition of the fuel oil, it can be concluded that

iron oxide comes from oxidation of the pipe metal by the V2O5 compounds, which are

characterized by a very low fusion temperature. From comparison, it is confirmed that the

use of additives decreases the vanadium, sodium, and ferric oxides, but the opposite is

observed with the sulfur and magnesium contents found in the slag. The amount of sodium

found on the deposits has been reduced with the use of both additives, which points out to

a preferred formation by chemical reaction of magnesium compounds rather than sodium

ones. At the same time, sulfur contents have been increased due to sulfur retention by

magnesium. In experiments using additives, the dominant oxide compounds were those

including V, S, and Mg. In this case, as sodium oxide has a lower probability to be formed,

a rise on the melting temperature of the slag and a reduction in hot gases corrosion should

be expected.

Information extracted from the X-ray diffraction technique is useful to complement the

results obtained from chemical analysis. The dominant phases detected using this

Table 4

Average chemical composition of the slag for the different additives tested

Chemical

composition

Heavy fuel oil

without additive

Slurry-type

additive (R = 0.5)

Slurry-type

additive (R= 1)

Organometallic

additive

SiO2 2.3 8.5 0.7 1.6

Al2O3 1.8 3.4 – 0.3

Fe2O3 3.4 3.1 0.7 0.7

TiO2 < 0.1 0.2 – –

CaO 4.9 6.1 1.7 1.2

MgO 0.7 12.2 13.5 15.1

Na2O 13.6 6.76 2.4 0.9

K2O < 0.1 < 0.1 – –

Mn3O3 0.1 0.6 0.5 0.4

P2O5 0.4 < 0.1 – –

SO3 1.5 4.9 6.6 8.3

CuO 0.1 < 0.1 – –

V2O5 34.7 31.1 16.0 15.4

NiO 0.1 5.8 – 1.7

Cr2O3 0.1 < 0.1 – –


technique for the different experimental conditions are shown in Table 5. The dominant

species found in the deposits when burning heavy fuel oil without additive is

Na2O�V2O4�5V2O5, which is produced when Na2O�6V2O5 reacts, losing one oxygen. A

second sodium-based oxide, 5NaOV2O4�11V2O5, has also been detected in the slag for

basic samples and, in low concentration, in the samples collected when using Mg-oxide

Fig. 2. Principal chemical composition of the deposits collected from the gases side of the high-temperature heat

transfer surfaces.

Table 5

Dominant phases detected using X-ray diffraction for the different experimental conditions

Sample % O2 Dominant phases detected

Without additive 0.5 H 0.6 Na2O�V2O4�5V2O5 + 5Na2O�V2O4�11V2O5 +V2O5

Slurry-type additive (R = 0.5) 0.3 H 0.4 Na2O�V2O4�5V2O5 + 5Na2O�V2O4�11V2O5 +V2O5

Slurry-type additive (R = 1) 0.3 H 0.4 MgO+Ca2V2O7 +CaVO3 +Mg3(VO4)2 +Na2O�V2O4�5V2O5

Organometallic additive 0.3 H 0.4 2MgO�V2O5 + 3MgSO4�CaSO4 +CaV2O7 +Na6Mg3(VO4)4 +

Mg3V10O28�18H2O


slurry additive with R = 0.5. This compound has a very low melting point (530 jC) and caneasily melt on the pipes of superheaters and reheaters, causing hot gas corrosion. This

analysis also shows a magnesium deficit in the slurry-type additive dosage when a ratio

R = 0.5 has been adjusted. However, when both the slurry-type additive with a ratio of

R = 1 and the organometallic one were tested, this phase was never found. Another

important result is the reduction in the V2O5 concentration measured in the samples when

experiments were performed by adding additives to the high-vanadium fuel oil. As before,

this oxide has never been detected in the slag samples when both the slurry-type additive

with the large Mg/V ratio (R = 1) and the organometallic one were tested [8].

In the samples analyzed when organometallic additive was used, the phase 2MgO�V2O5

was mainly identified. This compound is considered as the most relevant one to prevent

vanadium corrosion due to its high melting point. In these samples, the presence of

3MgSO4�CaSO4 has also been detected, indicating that the magnesium in the additives

reacts with SO3 as well. The use of additives limits the catalytic conversion of SO2 to SO3

[9,10]. A decrease in SO3 concentration in the stack gases can be expected, confirmed by

the increase of sulfur content on the deposit (see Fig. 2). Water washing of the entire heat

transfer surfaces before collecting samples can explain the presence of hydrated molecules

of vanadium and magnesium as Mg3�V10O28�18H2O.

In order to complete the high-temperature corrosion study, the melting temperature of

the slag has been measured using differential thermal analysis. Due to results previously

discussed and the quite high cost of this experimental technique, the measurement has only

been performed for samples collected when the organometallic additive was tested. A deep

weight loss for the first heating stage has been observed, corresponding to the water

content evaporation on the magnesium and vanadium hydrated compounds. At the same

time, the fusion of some compounds at 500 and 990 jC has been measured, in good

agreement with the melting point of the different phases detected by X-ray diffraction. The

use of an optimal dose of additive should modify the crystalline structure of the slag, and a

melting point around 1315 jC could be expected. For this reason, even when the

organometallic additive yielded the best results in preventing hot gases corrosion from a

chemical viewpoint, the presence of some deposits with low melting point is an indication

of a Mg deficit in the treatment for the heavy oil used as fuel.

3.2. Morphological characteristics of the deposit

Scanning electron microscopy and infrared spectroscopy have been used to characterize

physically the slag samples collected from the gases side of high-temperature heat transfer


surfaces. It has been checked that when an Mg/V ratio R = 0.5 is used for the slurry-type

additive, the volume of the slag deposited increases, but deposits are more brittle and easy

to remove either mechanically or by soot blowing. In this case, experimental measure-

ments showed the presence of stratified layers with different consistency. The external

layers were amorphous and a little compact, demonstrating magnesium deficit in the

applied dose. On the contrary, very hard internal layers have also been found.

For a high Mg/V ratio (R = 1), fouling of the heat transfer surfaces, as well as furnace

walls and gas ducts, has been highly reduced. However, the appearance of off-white layers

of very hard slag deposited over the furnace walls has been confirmed, which increases

the heat radiated to the superheaters. This phenomenon is attributed to the presence of a

large amount of nonreacting Mg detected in the slag, causing the so-called ‘‘mirror effect’’

[11–13].

The use of organometallic additives provided the best results, modifying the morphol-

ogy of the slag. On one hand, both a much more brittle slag and a thin layer of deposits on

the gases side of the high-temperature heat transfer surfaces have been obtained. On the

other hand, the use of this additive eliminates the formation of off-white deposits on the

furnace walls. All these features enable a reduction in the cleaning frequency and ease the

cleaning tasks, reducing the shutdowns due to maintenance.

3.3. Behavior of the operational parameters

The growth of deposits, and therefore, the degree of fouling over the heat transfer

surfaces, can also be evaluated by the behavior of some operational parameters, such as

pressure in the furnace, pressure drop in superheaters, pipe metal temperature, and

tempering water flow, measured simultaneously. Tests have been performed for a fixed

load of 330 MW/h (nominal). A summary of the behavior is depicted in Figs. 3–5 for

Fig. 3. Trace of the behavior of the main operational parameters of the boiler when using the Mg oxide-based

slurry additive with a fixed Mg/V ratio R= 0.5.

Fig. 4. Trace of the behavior of the main operational parameters of the boiler when using the Mg oxide-based

slurry additive with a fixed R = 1.


both the slurry-type additive when using R = 0.5 and R = 1 and for the organometallic

one, respectively.

After a few months using the Mg oxide-based slurry additive with R = 0.5, an increase

in the furnace pressure and pressure drop in the superheaters was observed due to higher

fouling, as depicted in Fig. 3. This also explains the rise in gas temperature at the exit of

the furnace, which requires an increase in the tempering water flow to prevent damages in

the heat transfer surfaces and to control the superheated steam temperature. When a ratio

of R = 1 was tested, a reduction in the furnace pressure and pressure drop in superheaters

was measured, showing lower fouling in the thermal surfaces, as shown in Fig. 4.

Fig. 5. Trace of the behavior of the main operational parameters of the boiler when using the organometallic

additive.


However, after 4 months of treatment, a sudden increase in the pipe metal temperature was

detected, causing a proportional increase in the tempering water flow in order to keep

steam superheated temperature on the design limit. This behavior corresponds to the

‘‘mirror effect’’ previously described. In this case, to ensure the safe performance of the

boiler, soot blowing cleaning frequency was increased. However, even with the increase in

the number of daily soot blowing operations, inadequate cleaning of the heat transfer

surfaces was observed. This was mainly motivated by the hardness and adherence of the

slag, which could not be removed simply by soot blowing. In this case, an additional

thermal cleaning method had to be used.

The behavior of the measured operational parameters when the organometallic additive

was added to the liquid petroleum is depicted in Fig. 5. It should be observed the lower

pressure in the furnace and tempering water flow needed for the same steam power, in

comparison to the slurry-type additive.

3.4. Techno-economical analysis

To complete the analysis for the two additives tested, a techno-economical methodol-

ogy has been developed, taking into account the actual measured consumption and the

specific costs for fuel oil and additives during this period. The study is based on the

equation:

Z ¼ GAddPAdd þ BPfo ½$=year� ð1Þ

In this equation Z, PAdd, and Pfo are the total annual cost for fuel oil and additives [$/

year], and the unitary costs for additive [$/l] and fuel oil [$/t], respectively. GAdd is the

annual consumption of additive [l/year] calculated by:

GAdd ¼B

Rfa

½l=year� ð2Þ

where Rfa is the fuel oil/additive ratio [t/l] and B is the fuel oil consumption [t/year].

Costs included in Eq. (1) correspond to the fuel oil and additives consumed only for

steam generation. Additionally, some extra fuel oil consumption has to be considered in

the economical assessment due to the use of Mg-oxide slurry-type additive. As previously

discussed, the use of this additive shortens the cleaning period of the heat transfer surfaces

due to the ‘‘mirror effect.’’ For this reason, when this additive was tested, the number of

soot blowing per day was increased from one to three. The cost of this fuel overcon-

sumption is evaluated by the equation:

q1 ¼ bsbnsbdsbPfo ½$=year� ð3Þ

where bsb is the fuel oil consumption per soot blowing [t/blowing], nsb is the number of

soot blowing per day [blowing/day], and dsb is the number of days per year of soot

blowing [days/year].

When this additive was considered in the treatment, hard deposits with a large

adherence were identified. For this reason, in order to achieve the correct cleanliness of

the heat transfer surfaces, a thermal cleaning method has also been included in the regular


maintenance planning of the boiler. In short, this method dramatically intends to melt the

slag deposited on the gases side of the high-temperature area by blazing it directly with a

flame. The cost of this fuel oil overconsumption is determined by:

q2 ¼ btcntcdtcPgo ½$=year� ð4Þ

where btc is the fuel oil consumption per thermal cleaning [t/cleaning], ntc is the number of

thermal cleaning per day [cleaning/day], and dtc is the number of days per year of thermal

cleaning [days/year]. As this cleaning technique uses gas oil as fuel, Pgo instead of Pfo has

been considered in the nomenclature of Eq. (4).

To complete the analysis, there are two other fuel oil overconsumptions in terms of

nonproduced energy, which have to be considered due to the influence of this particular

electric power plant on the National Electroenergetic System (NES). In the whole

energetic budget of the country, the total nonproduced energy in this electric power plant,

caused by the use of heavy fuel oil, has to be delivered to the NES by another industry. As

the boiler tested belongs to the most efficient power plant all over the country, the

influence on the specific fuel oil consumption indexes has a higher impact. For these

reasons, these two fuel oil overconsumptions cannot be neglected.

On one hand, fouling of the heat transfer surfaces limits the power generated by the

boiler. The cost of this fuel oil overconsumption due to power limitations is included in q3and is calculated through:

q3 ¼ DplNEplPf ½$=year� ð5Þ

where Dpl is the increase in the specific fuel oil index in the NES due to the power

limitations in this power plant [t/GW h] and NEpl is the nonproduced energy due to power

limitations [GW h/year]. On the other hand, fouling on furnace walls, superheaters,

reheater, air heaters, and economizer shortens the boiler maintenance period requiring

more frequent stops. This fuel oil overconsumption is determined by:

q4 ¼ DfiNEsfPf ½$=year� ð6Þ

In Eq. (6), NEsf is the nonproduced energy due to stops by fouling [GW h/year] and Dfi

represents the increment in the specific fuel oil index consumption in the NES [t/GW h]

due to stops by increase of fouling.

Therefore, the total cost of fuel oil overall consumption is the addition of each

individual loss to the total annual cost for fuel oil and additives, summarized by the

equation:

Q ¼ Z þ q1 þ q2 þ q3 þ q4 ½$=year� ð7Þ

Numerical results obtained from the above methodology are depicted in Table 6. As a

first step for this analysis, the difference in prices between regular fuel oil and heavy fuel

oil has been considered. Average values of the different periods covered in the

measurement campaign showed a difference of $7/t less for the heavy fuel oil ($118/t)

than the regular one ($125/t). As can be seen, the use of a heavy fuel oil with additive is

economically justified, even with the fuel oil overconsumption considered when the

Table 6

Comparative results obtained from techno-economical analysis

Parameters Regular fuel oil Heavy fuel oil

Without additive Slurry-type

additive (R = 1)

Organometallic

additive

Total fuel oil consumption [t/year] 510,369.4 519,576.8 512,509.5

Cost of fuel oil and fuel oil plus additives [$/year] 63,796,175 62,709,802 61,110,019

Cost of fuel oil overconsumptions

Due to soot blowing [$/year] – 90,093 –

Due to thermal cleaning [$/year] – 52,800 –

Due to power limitation [$/year] – 38,666 –

Due to stops by fouling [$/year] – 187,655 –

Total costs [$/year] 63,796,175 63,079,096 61,110,019

Savings with respect to regular fuel oil [$/year] – 690,079 2,686,156


slurry-type additive has been used. Again, the major savings are achieved when using the

organometallic additive, with a total $2,686,156 with respect to the regular fuel oil. It

should be pointed out that when the organometallic additive was added to the low-quality

heavy fuel oil, there is absolutely no problem of power limitation due to fouling. For this

additive, no modification to the cleaning schedule used for the regular fuel oil has been

considered. For this reason, no fuel oil overconsumption has been included in the analysis

for this type of additive.

4. Conclusions

An experimental measurement campaign has been performed in a power plant to analyze

fouling growth and to evaluate the incidence of slag corrosion as a function of fuel oil

characteristics and the main operational conditions. Negative corrosion effects and degra-

dation in the combustion processes caused by the use of low-quality residual oil as fuel in a

high-capacity steam boiler can be reduced and eliminated with the use of a suitable additive.

The chemical treatment of heavy fuel oil with a magnesium oxide-based slurry additive

shortens the cleaning period frequency. When a low ratio of Mg/V (R = 0.5) was fixed to

the Mg oxide-based slurry added to the liquid petroleum, the required modification of

deposits was not achieved. Fouling can be reduced with a higher Mg/V ratio (R = 1), but, in

this case, nonreacted Mg is deposited on the heat transfer surfaces, producing hard

deposits that are very difficult to remove. In this situation, a high heat radiation has been

observed, causing an increase on the pipe metal temperature and requiring a higher

tempering water flow to maintain the superheated steam temperature in the design level.

The best results have been obtained when an organometallic-type additive was used in

tests. For this experimental condition, the expected chemical modification of the slag has

been reached, raising the melting temperature of the deposits and reducing the fouling of

the heat transfer surfaces, as well. Despite the good results obtained for the organometallic

additive, the presence of some low melting point compounds detected by X-ray diffraction

and corroborated by differential thermal analysis is an indication of the deficit of

magnesium in the dosage of additive used in the treatment.


A techno-economical analysis has been performed in order to evaluate the costs and

savings when a low-quality fuel oil is used. The study demonstrated that the use of heavy

fuel oil is economically justified if it is suitably treated with an additive to prevent

corrosion and limit fouling growth. In this case, appreciable savings have been obtained

for both a slurry-type additive with a Mg/V ratio R = 1 and an organometallic additive.

Again, the best results have been achieved when the organometallic additive was used,

saving $2,686,156/year with respect to the costs when a regular fuel oil is considered.

Nomenclature

B Total fuel oil consumption [t/year]

bsb Fuel oil consumption per soot blowing [t/blowing]

btc Fuel oil consumption per thermal cleaning [t/cleaning]

dsb Days per year of soot blowing [days/year]

dtc Days per year of thermal cleaning [day/cleaning]

Dfi Increase in specific fuel oil index consumption [t/GW h]

Dpl Increase in specific fuel oil index due to power limitation [t/GW h]

GAdd Additive consumption [l/year]

NEpl Nonproduced energy due to power limitation [GW h/year]

NEsb Nonproduced energy due to stops by fouling [GW h/year]

nsb Number of steam blow per day [blowing/day]

ntc Number of thermal cleaning per day [cleaning/day]

q1 Cost of fuel oil overconsumption due to soot blowing [$/year]

q2 Cost of fuel oil overconsumption due to thermal cleaning [$/year]

q3 Cost of fuel oil overconsumption due to power limitation [$/year]

q4 Cost of fuel oil overconsumption due to stops of the boiler by fouling [$/year]

Q Total cost of fuel oil overconsumption [$/year]

PAdd Unitary additive cost [$/l]

Pfo Unitary fuel oil cost [$/t]

Pgo Unitary gas oil cost [$/t]

R Magnesium-to-vanadium ratio

Rfa Fuel oil-to-additive ratio [t/l]

Z Total cost of fuel oil and additive used for steam generation [$/year]

Acknowledgements

Authors want to acknowledge the help of engineers and operators of the electric power

industry and, specially, the help of Nancy Rodriguez in the laboratory analysis.

References

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behavior of a high-capacity steam boiler using heavy fuel...

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