RB-7 FUELOIL AND ITS CORROSIVE EFFECTS IN INDUSTRIAL COMBUSTION
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:
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
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:
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
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.
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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