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RB-7 FUELOIL AND ITS CORROSIVE EFFECTS IN INDUSTRIAL COMBUSTION
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rb
bertomeu, S.L. |
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INDEX
1.
- INTRODUCTION. This
study has been carried out to offer a global vision of the corrosion problems
that exist when fueloil is used in power plants. It takes advantage of
the experience acquired by "rb bertomeu, S.L." in different
power plants.
In
this study, the circumstances that surround the fueloil and its combustion are
exposed to extrapolate the experience to the steam
generation boilers in order to show that the achievements in the
power plants could be applied in this kind
of plant.
2.
- CHARACTERISTICS OF THE HEAVY FUEL OIL.
Fueloil
is a residual product of the distillation of oil that can come from a single
distillation stage, or be a mixture of products of different stages, in order
to adjust the characteristics of the different specifications to produce the
desired type of fueloil. As a rule, fueloil is a complex heterogeneous system
made up of: a. -
liquid hydrocarbons whose number of carbon atoms is > 2O. b. - solid
hydrocarbons emulsified in the liquid phase. c. -
dissolved gaseous hydrocarbons or emulsified in the liquid phase. d. - water
emulsified in the liquid phase. e. - metallic
salts dissolved in the emulsified water.
f. -
occluded metals. g.
- compound organic or inorganic metals taking part of the liquid phase or of
the emulsified solids.
h. -
sulphur components. In
this study, points d, e, f, g, and h are of great importance as they,
together, constitute what is designated the impurities of the fuel, and give
cause for different types of corrosion in the exhaust gas circuit when the
fueloil is used as fuel, as will be seen in chapter 4. Generally,
the emulsified water is saturated with NaCl and
also tends to contain small quantities of carbonates and calcium and
magnesium sulphates. The
metals present, in atomic form, oxide form, or in the form of organic or
inorganic acid salts, are very varied in addition to Na, Ca and Mg already
mentioned. The most important, by their implication in the corrosion process,
as well as by their quantity are: Vanadium (V) Nickel (Ni) Iron (Fe) Aluminium (Al)
Zinc
(Zn)
Copper (Cu) Sulphur is
present as much in its free state as combined in various forms. The most
characteristic are:
Mercaptans (R - SH)
Sulphurs and Disulphurs (R-S-R, R-S-S-R) Cyclical
compounds (Thiophene, sulphur ethylene)
Sulphates (R1 - SO2 - R2)
Sulphides (R1 - SO - R2)
Sulphonic acids (R - SO3H) The
total quantity of sulphur and the distribution of the type of compounds that
form depend on the origin of the crude oil, although this detail is not so important as all the sulphur is oxidised to SO2, independently of the form in
which it is found, when the fuel is burnt. 3.
- COMBUSTION REACTIONS OF THE FUELOIL. The
combustion of the fueloil (or any other fuel) is defined as the rapid
oxidation of each and every of its constituent elements. Therefore,
to burn a fuel it is necessary to have oxygen, which is provided in the form
of combustion air which, as is known, contains basically 21% of O2 and 79% of N2. In
essence, the principal reactions that happen in combustion can be synthesised
as:
C + O2 ---------->CO2
+ heat
H2
+ ½ O2---------->H2O
+ heat
S + O2 ---------->SO2
+ heat MI + ½ O2---------->M2O + heat (Example: Na)
MII
+ O2
---------->2MO
+ heat
(Example: Ca) MIII + O2 ---------->M2O3 + heat (Example: Fe, Al)
MV
+ O2
---------->M2O5
+
heat (Example: V) M means
valence metal I to V. The most
important, concerning energy efficient use of the fuel, are the first three,
the oxidation of the C, H2and
S, in this order. Once these
reactions are completed, or at the same time, other minor reactions take
place related to the impurities of the fueloil (Sulphur and metals), that in
some instances are related to the corrosion processes that happen in the
exhaust gas circuit. Among them
we can cite the following:
(*) SO2 + ½ O2
----------->SO3
(*) SO3 + H2O
----------->H2SO4 SO3 + CaO ----------->CaSO4 (*) SO3 + Na2O ----------->Na2SO4 3SO3 + M2O3 ----------->M2(SO4)3 (*) 3V2O5 + Nx/2 Na2O ----------->NaxVxV(6-x)O15(bannermite) V2O5 + CaO ----------->CaV2O6 (*) V2O5 + 2Na2O ----------->Na4V2O7
(*) V2O5
+ 3Na2O
----------->2Na3VO4 The
example is not exhaustive, but shows the important reactions and those which
are directly bound with corrosion, marked with (*). In chapter
4 the corrosion mechanisms cited are studied in greater detail. Finally,
it is worth noting that if the fueloil was a fuel free of Sulphur and metals,
it would not generate, during its combustion, compounds harmful to the metal
parts of the waste gas circuits. 4.
- BASIS OF CORROSION IN THE WASTE GAS CIRCUITS. A)
CORROSION LINKED TO SULPHUR COMPOUNDS. The
oxidation reaction of SO2 to
SO3,
and the combination of this with the vapour of the H2O in the gases to form H2SO4 has
already been described. Note
that the reaction SO2 ------>
SO3 is catalysed by the
presence of metallic oxides, and especially by the vanadium pentoxide (V2O5) . Therefore, the more vanadium
the fueloil contains, the later it transforms the V2O5 into
a less catalytically active form (alkaline-earth vanadates), the quantity of
SO3 formed will be
greater and, because of this, there will be more probability that sulphuric
acid is formed, H2SO4. The
equilibrium point of reaction SO3 + H2O ------> H2SO4 is between 200º and 500º C. Below 200º C, H2SO4has the form of semi-corrosive vapour, while above 500º C
the H2SO4 is very unstable and is separated in SO3 and H2O. Between both temperatures, the H2SO4 vapour coexists with the SO3 and H2O. H2SO4 vapour begins to condense below 150º C, approximately
when it is converted into a highly corrosive compound which attacks the metal
surface following these reactions: H2SO4 + Fe +
7 H2O ------> FeSO4 .
7H2O + H2
(hydrated
ferrous sulphate) 3H2SO4 + Fe2O3 ------> Fe2(SO4)3
+ 3H2O
(ferric
sulphate) It
is obvious that this type of corrosion will only happen in the waste gases
circuit, at points where the temperature is below 200º C, and especially if
it is below 150º C. That is to say, at the end of the combustion process
(waste gas purifiers, chimneys, etc.). The
SO3 formed can merge with the metallic oxides present to
form sulphates (see chapter 3). Of
all the metallic sulphates that can be formed, sodium sulphate (Na2SO4) is the main one which is accountable for the corrosion of
the metal surfaces. All the sodium salts are barely corrosive at ambient
temperatures, but at increased temperatures the corrosion speed is increased
rapidly when the fusion point of the salt is reached. In the case of Na2 SO4, it is 888º C, when it comes into contact with metal
surfaces iron corrosion is produced, probably by the formation of double
sulphates of Fe and Na. The real mechanism is not very well-known, though
from what has been said before, the corrosive effect of sodium sulphate at
high temperature is fully acknowledged. This
type of corrosion will happen, therefore, in high-temperature areas of the
circuit, near the area of combustion and before the diluting effect of the
air decreases the temperature lower than 850º C. B)
CORROSION LINKED TO VANADIUM COMPOUNDS. Vanadium
form several oxides such as V2O2, V2O3, V2O4 and V2O5. The acid nature raises the degree
of oxidation, V2O5 (pentoxide) has the most
acidic nature and is therefore the most corrosive. On the other hand, under
combustion conditions where large amounts of O2 and high temperatures exist, any form of vanadium
present in the fueloil will have a tendency to be oxidised into V2O5, therefore,
its presence will be certain in the combustion gases and in a liquid state
(established at 690º C) where two very harmful effects originate:
- Catalysis of the oxidation of SO2 to
SO3.
- Corrosion of the metal surfaces to form
ferric meta-vanadates (Fe(VO3)3).
As
other metals are also present in the fueloil (see chapters 2 and 3), part of
the V2O5 has a tendency, by its reactivity, to form
salts (vanadates) with the alkaline metals and alkaline-earth. The
alkaline-earth vanadates have a high fusion point, 1000º - 1200º C, therefore
as a rule, after combustion it is found in a solid state, in the form of a
powder which is carried by the gases. In this type of salt we find the
calcium vanadates and magnesium vanadates: (Mg/Ca)1 V2O6 (Mg/Ca)2 V2O7 (Mg/Ca)3 V2O8 The
alkaline vanadates, mainly the distinct sodium vanadates have a much lower
fusion point (350º to 650º C), and are therefore found in a wide zone of the
waste gases circuit in liquid state. These vanadates are very reactive with
the iron and iron oxides, dissolving them to form vanadates or double sulphovanadates,
above all in the presence of sodium sulphate. The
fusion temperature of the different sodium vanadates that can be formed comes
from data of the molecular weight relation V2O2/Na2O, according to the following
table: Rel. V2O5/Na2O T. Fusion ºC 0
400 1
550 2
450 3
350 4
530 6
580 8
620 10
640 ∞
690 We
see therefore, that in the waste gases circuit, there exists a great
probability of finding sodium vanadate in a liquid state, corrosive, above
all when the molar relation V2O5/Na2O is 3. Nevertheless, though such relation will be higher or
lower in a combustion installation of the engine boiler type or, there are
many important areas of metal parts subjected to greater temperatures greater
than 690º C, therefore the corrosive effects of the alkaline vanadates are
practically guaranteed. At
this point, the question is: if the fueloil contains impurities of S, V and
Na, is the formation of corrosive sodium sulphate and sodium vanadate
unavoidable? Certainly, unless correction factors are introduced into the
process. These factors should perform two requirements to avoid or minimise
these types of corrosion. a) as
N is basically found dissolved in the water of the fueloil, this must be
separated at the bottom by decantation, centrifuge, etc. so that the N sent
to the combustion will be much less. b) since Vanadium is found in the
fueloil in soluble form and it is not possible to separate it by decantation
or centrifugation, the rapid formation of alkaline-earth vanadates (Ca, Mg)
is favourable in the combustion for two purposes: b-1) to avoid the catalytic action
of the V2O5 on the reaction of b-2)
to raise the fusion temperature of the vanadates so that they will be carried
in the form of powder. 5.
- COMBUSTION TEMPERATURES OF FUELOIL. The
surpluses and ease of calculation of the volume of stoichiometric air necessary
to achieve combustion per Kg. of fueloil is known by the reactions described
in chapter 3. Supposing
that the form of standard fueloil has an calorific value of less than 9,500
Kcal/Kg., and it is burnt with air at 20º C and 60% HR, the temperature of
the combustion gases, in relation to the total air provided (excess
combustion air plus dilution air), would be approximately the following
(excluding heat loss by dissipation): Total
air in excess over
stoichiometric
Gas temp.
0
%
2,060º C
5
%
1,985º C
10
%
1,920º C
20
%
1,800º C
50
%
1,500º C
100
%
1,200º C
150
%
990º C
200
%
850º C
250
%
740º C
300
%
660º C The
real temperatures will depend on the heat loss
due to conduction, convection and radiation of the equipment, but, as a rule
they can be assumed to be between 5% or 10% below the theoretical
temperatures without any appreciable error. It
is observed that in an industrial combustion installation, of the engine or
boiler type, where the provided total air oscillates between 150% and 300% of
the stoichiometric necessary for combustion, there will be metal areas
submitted to temperatures between 600º C and 1,000º C. Logically this
corresponds to areas near to the combustion area, when cooling has already
been produced by dilution with air of the gases generated in the aforesaid
combustion. Examples: A) In the cogeneration engines, that work with an excess of air of 200% to 300%,
the initial temperature of the combustion will begin at approximately 2,000º
C, but will quickly decrease to 700º - 800º C in the exhaust valves.
Thereafter, due to the high heat loss of the circuit, it will be about 350º C
in the turbos and afterwards, 200º - 250º C at the exit of the steam boilers
of the cogeneration plant. B) In steam boilers (with burners)
it will be about 1,200º - 1,500º C in the central area and 800º - 900º C in the
contact zone of the gases with the reheater pipes. 6.-
EXPERIENCE OF “rb bertomeu, S.L.” IN POWER PLANTS.
For
years, “rb bertomeu, S.L.” has been studying the corrosion problems that
exist in power plants that use fueloil as fuel. From this study we
have come to the following conclusions:
a) the
fueloil always contains impurities, though in variable proportions according
to its origin. b) partial
and total corrosions are produced in the exhaust valves of the engines, more
corrosion occurs when the quantity of vanadium and sodium in present the
fueloil is higher. The temperature in this point is about 800º C. c) solid
residual deposits are produced in the exhaust valves of the engines, gas
collectors and turbos, these are directly proportional to the vanadium
content of the fueloil and of the calcium (TBN) in the engine oil. d) corrosions
are not produced in points of the circuit whose temperature is understood to
be between 350º and 200º C. Taking
these conclusions into account, and the reasoning shown in chapter 4, we have
developed a treatment which avoids or minimises the corrosion problems of the
exhaust valves of the engines, which, we recall, is the point where the
temperature is most critical in relation to the melted salts present. This
achievement comes endorsed by the users of our “rb bertomeu” additives
who have been able to compare present results with those of prior situations
when they were not using the current treatment. Parallel
to the elimination of corrosion we have also been able to reduce, by 70% -
80%, the deposits of hard solid residues on the stems of the exhaust valves,
collectors and turbos. All
this has been translated into an increase in the useful life of the valves,
in better operation of the engines by obstruction-free maintenance of the
exhaust gases circuit , and in a greater capability
of production by a decrease in the maintenance necessary between programmed
inspections.
Recalling
what was said in chapter 4, the treatment designed “rb bertomeu, S.L.”
consists of the application of an additive whose components develop the
following effects in relation to corrosion: A. - To aid the decantation of the
water, contained in the fueloil in the storage tank, so that the quantity of
sodium salts will be reduced. In this way sodium sulphate and vanadate
formation is minimised. The water decanted should be eliminated periodically
from the tank by bleeding. B. - To
achieve a perfect pulverisation of the fueloil in order to eliminate residual
carbons which could act as base of adhesion for residual saline. C. - To neutralise the V2O5 (vanadium pentoxide) obtaining alkaline-earth
vanadates of high fusion point, at the same moment of combustion, which
achieves: - eliminate corrosion by V2O5. - eliminate the appearance of sodium
vanadates below fusion point. - obtain vanadates of high fusion point
(> 1,100º C) which are carried by the flow of combustion gases without
being deposited in the circuit. For more information please read RB-28 Actions and benefits of "rb bertomeu" heavy fuel oil addtives 7.
- TRANSFERRING THE EXPERIENCE IN ENGINE POWER PLANTS TO
STEAM BOILERS WITH BURNERS
Essentially,
the process is the same in a diesel engine with fueloil as in a steam boiler
which burns fueloil. The only difference is the excess of circulating air in
respect to the theoretical estequiometric, depending on of the type of
installation used. Because of this, the temperature distribution can vary,
but, inevitably, in a steam boiler, the flow of available hot gases at the centre
will be 600º - 900º C in the air-water heat exchange area (heat exchange
pipes or reheaters). Concerning
corrosion effects, this point (reheaters) would be the equivalent to the
exhaust valves of a engine using fueloil. For this
reason, if corrosions and solid residual deposits are produced, it is logical
to think that the same effects are also found in the heat exchange pipes of
the boiler, which should produce two negative situations: - a decrease in the
useful life of the pipes of the boilers. - formation
of encrustations in the pipes, that reduce the rate of heat transmission,
and, therefore, their energetic efficiency.
The
second effect is obvious and it does not need demonstrating if we take into
account the principles of thermodynamics applied to heat transmission through
a given surface. The
first effect, corrosion of the pipes, is a measurable parameter,
above all the installation has statistics of the thickness measurement of the
pipes over several years. The
system of checking the process of corrosion of the outer wall of the pipes
(the part in contact with the gases) should be the following: - annual
measurement of the outer diameter of the pipes when they have been scraped to
eliminate the solid residual encrustations. - annual
measurement of the thickness of the wall of the pipes, to verify whether or
not there is corrosion on the internal wall (the part in contact with the water
- steam). This
should provide sufficient data to follow the condition of the most critical
part of the boiler, and to evaluate the possible life of the boiler. Equally,
the economic value of the use of the "rb bertomeu" additives
can be determined, which, as has been reported throughout this study, is the
way to avoid corrosion and to notably lengthen the useful life of the
equipment. |
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rb bertomeu, S.L.
- Pol. Ind. Fondo de Llitera, Par. 82-83 -
E-22520 Fraga, Huesca (Spain) Tel.: +34 974 47 48 04 /
+34 630 43 08 43 |