U.S. patent application number 13/877339 was filed with the patent office on 2013-08-22 for method of operating a combusion installation and use of such a method for inhibiting vanadium corrosion.
The applicant listed for this patent is JeanLuc Buet, Donald A Meskers, JR., Michel Moliere. Invention is credited to JeanLuc Buet, Donald A Meskers, JR., Michel Moliere.
Application Number | 20130213282 13/877339 |
Document ID | / |
Family ID | 44903354 |
Filed Date | 2013-08-22 |
United States Patent
Application |
20130213282 |
Kind Code |
A1 |
Meskers, JR.; Donald A ; et
al. |
August 22, 2013 |
METHOD OF OPERATING A COMBUSION INSTALLATION AND USE OF SUCH A
METHOD FOR INHIBITING VANADIUM CORROSION
Abstract
A method of operating a thermal installation other than a gas
turbine and use of such a method for inhibiting vanadic corrosion
is disclosed herein. Embodiments of the invention relate to a
method of operating a thermal installation comprising a combustion
chamber fed with a fuel contaminated with vanadium, with sulfur and
possibly with sodium. The combustion chamber is also fed with boron
and with magnesium, in quantities such that the magnesium molar
ratio m=MgO/V2O5 and the boron molar ratio b=B2O3/V2O5 satisfy the
conditions (i) m.gtoreq.2+b; (ii) m.ltoreq.3+2b; (iii) b.gtoreq.0.5
and (iv) b.ltoreq.2, so that the combustion products comprise
magnesium vanadate, mixed magnesium boron oxide and possibly sodium
borate.
Inventors: |
Meskers, JR.; Donald A;
(Levittown, PA) ; Moliere; Michel; (Belfort,
FR) ; Buet; JeanLuc; (Martigues, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Meskers, JR.; Donald A
Moliere; Michel
Buet; JeanLuc |
Levittown
Belfort
Martigues |
PA |
US
FR
FR |
|
|
Family ID: |
44903354 |
Appl. No.: |
13/877339 |
Filed: |
October 4, 2011 |
PCT Filed: |
October 4, 2011 |
PCT NO: |
PCT/US11/54696 |
371 Date: |
April 25, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61389570 |
Oct 4, 2010 |
|
|
|
Current U.S.
Class: |
110/343 ;
431/4 |
Current CPC
Class: |
F23J 7/00 20130101; C10L
1/1233 20130101; C10L 1/12 20130101; C10L 1/1291 20130101; C10L
1/1216 20130101 |
Class at
Publication: |
110/343 ;
431/4 |
International
Class: |
C10L 1/12 20060101
C10L001/12; F23J 7/00 20060101 F23J007/00 |
Claims
1. A method of operating a combustion installation comprising
feeding to the combustion installation a fuel contaminated with
vanadium and feeding to a combustion chamber of the combustion
installation one or more boron compounds and one or more magnesium
compounds, in quantities such that the molar ratio of MgO
equivalents added to the combustion chamber relative to the moles
of V.sub.2O.sub.5 formed in the combustion chamber from the
vanadium in the fuel, is molar ratio m, and the molar ratio of
B.sub.2O.sub.3 equivalents added to the combustion chamber relative
to the moles of V.sub.2O.sub.5 formed in the combustion chamber
from the vanadium in the fuel is molar ratio b, and wherein the
molar ratios b and m satisfy the following conditions: (i)
m.gtoreq.2+b and (ii) b.gtoreq.0.5.
2. The method according to claim 1, wherein the molar ratios b and
m satisfy the additional condition: m.ltoreq.3+2b.
3. The method according to claim 2, wherein the molar ratios b and
m satisfy the additional conditions: (i) m.ltoreq.5 and (ii)
b.ltoreq.1.5.
4. The method according to claim 1, wherein at least some of the
boron and magnesium fed to the combustion chamber is in the form of
mixed magnesium boron oxide.
5. The method according to claim 4, wherein the mixed magnesium
boron oxide is in nanoscale form.
6. The method according to claim 1, wherein at least some mixed
magnesium-boron oxide is formed in the combustion chamber from at
least one precursor introduced upstream of the combustion
chamber.
7. The method according to claim 6, wherein the mixed
magnesium-boron oxide is in nanoscale form.
8. The method according to claim 1, wherein the combustion
installation is operated at a flue gas temperature of 300.degree.
C. to 1050.degree. C.
9. The method according to claim 1, wherein the combustion
installation is operated at a pressure of 300 psig to 2650
psig.
10. The method according to claim 1, wherein the combustion
installation is operated at a flue gas temperature of 300.degree.
C. to 1050.degree. C. and at a pressure of 300 psig to 2650
psig.
11. The method according to claim 1, wherein the fuel fed to the
combustion installation comprises sulfur and possibly sodium.
12. The method according to claim 1, wherein the combustion
installation is a steam boiler.
13. The method according to claim 1, wherein the one or more boron
compounds comprise at least one of B.sub.2O.sub.3,
MgB.sub.4O.sub.7, Mg.sub.3B.sub.2O.sub.6, and
Mg.sub.2B.sub.2O.sub.5, and the one or more magnesium compounds
comprise at least one of MgO, MgB.sub.4O.sub.7,
Mg.sub.3B.sub.2O.sub.6, and Mg.sub.2B.sub.2O.sub.5, and wherein the
molar MgO equivalents and the molar B.sub.2O.sub.3 equivalents are
based on the moles of at least one of MgO, B.sub.2O.sub.3,
MgB.sub.4O.sub.7, Mg.sub.3B.sub.2O.sub.6, and
Mg.sub.2B.sub.2O.sub.5 added to the combustion installation.
14. The method according to claim 1, wherein the one or more boron
compounds comprise at least one of B.sub.2O.sub.3,
Mg.sub.3B.sub.2O.sub.6, and Mg.sub.2B.sub.2O.sub.5, and the one or
more magnesium compounds comprise at least one of MgO,
Mg.sub.3B.sub.2O.sub.6, and Mg.sub.2B.sub.2O.sub.5, and wherein the
molar MgO equivalents and the molar B.sub.2O.sub.3 equivalents are
based on the moles of at least one of MgO, B.sub.2O.sub.3,
Mg.sub.3B.sub.2O.sub.6, and Mg.sub.2B.sub.2O.sub.5 added to the
combustion installation.
15. The method according to claim 1, wherein the one or more boron
compounds comprise B.sub.2O.sub.3, and the one or more magnesium
compounds comprise MgO, and wherein b is the molar ratio
B.sub.2O.sub.3/V.sub.2O.sub.5 and wherein m is the molar ratio
MgO/V.sub.2O.sub.5.
16. The method according to claim 13, wherein molar ratios b and m
satisfy the additional condition: m.ltoreq.3+2b.
17. The method according to claim 13, wherein molar ratios b and m
satisfy the additional conditions: (i) m.ltoreq.5 and (ii)
b.ltoreq.1.5.
18. The method according to claim 15, wherein at least one of the
B.sub.2O.sub.3 and the MgO are in nanoscale form.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a national stage application under 35 U.S.C.
.sctn.371(c) prior-filed, co-pending PCT patent application serial
number PCT/US11/54696, filed on Oct. 4, 2011, which claims priority
to U.S. provisional patent application No. 61/389,570 filed on Oct.
4, 2010, the entire contents of which are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] Embodiments of the present invention relate to the
protection against vanadic corrosion of thermal installations that
are operated differently from gas turbines and burn
vanadium-contaminated liquid fuels. It relates in particular to a
method of operating a thermal installation, for example a steam
boiler, fed with this type of fuel.
[0003] From an economic standpoint, it is becoming increasingly
advantageous to utilize, in energy applications, certain low-value
petroleum fractions such as: very heavy crude oils, distillation
residues (from atmospheric or vacuum distillation), by-products
resulting from deep conversion of oils (high cycle oils and
slurries deriving from FCC (fluid catalytic cracking) units and
possibly certain heavy distillates.
[0004] For this purpose, such fuels may be burnt in various thermal
installations such as: gas turbines, boilers, furnaces, diesel
engines, etc., for the purpose of producing heat or steam,
mechanical energy or electricity. However, the presence in these
oil fractions of organo-vanadium compounds mainly in the form of
vanadium porphyrins generates corrosion problems in metal alloys
and ceramics that are used as structural materials or as surface
coatings (protective layers or thermal barriers) in parts of these
installations exposed to the combustion gas.
BRIEF SUMMARY OF THE INVENTION
[0005] A method of operating a combustion installation includes
feeding to the combustion installation a fuel contaminated with
vanadium and feeding to a combustion chamber of the combustion
installation one or more boron compounds and one or more magnesium
compounds is disclosed. The quantities added of the one or more
boron compounds and the one or more magnesium compounds are such
that the molar ratio of MgO equivalents added to the combustion
chamber relative to V.sub.2O.sub.5 formed in the combustion chamber
from the vanadium in the fuel, is molar ratio m, and the molar
ratio of B.sub.2O.sub.3 equivalents added to the combustion chamber
relative to V.sub.2O.sub.5 formed in the combustion chamber from
the vanadium in the fuel is molar ratio b. The molar ratios b and m
satisfy the following conditions: (i) m.gtoreq.2+b and (ii)
b.gtoreq.0.5.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 is a diagram showing an embodiment of the present
invention pertaining to magnesium, boron, and vanadium ratios.
[0007] FIG. 2 is a diagram showing a more specific embodiment of
the present invention pertaining to magnesium, boron, and vanadium
ratios.
DETAILED DESCRIPTION OF THE INVENTION
[0008] In the present application, the term "combustion
installations" shall generally refer to thermal installations which
are not gas turbines, and which utilize a combustible fuel, in an
embodiment, vanadium containing heavy fuel oils, which operate with
a flue gas temperature of 300.degree. C. to 1050.degree. C., as
mentioned below, which is similar to the flue gas temperature of a
boiler. These combustion installations, such as boilers, can
operate at steam pressures ranging from 300 psig to 2650 psig or at
any sub-range of pressure within this range, but more typically
will operate in the 1200 psig to 1500 psig range or any sub-range
of pressure within this range. The combustion installations, such
as boilers, may or may not be used in the production of superheated
steam, but the unit may contain a superheater in an embodiment.
Operational flue gas temperatures in target systems range from
300.degree. C.-1050.degree. C. (and any sub-range of temperature
within this range), such as 400.degree. C. to 900.degree. C.,
400.degree. C. to 650.degree. C., 500.degree. C. to 800.degree. C.,
and 500.degree. C. to 600.degree. C. Typical boiler furnace
temperatures will be used as examples for combustion installations;
however, all the conclusions drawn in this document can be extended
to any type of thermal equipment that has operation temperature
levels similar to boilers, including industrial furnaces and diesel
engines but at the exclusion of the gas turbines that feature
particularly high temperature levels and require specific
inhibition conditions.
[0009] Any range or ranges disclosed in this description are deemed
to include and provide support for any sub-range within such range
or ranges. Any range or ranges disclosed in this description are
deemed to include and provide support for any point or points
within those range or ranges.
[0010] As per the usage, the gas temperature or Tg of a combustion
installation, such as a boiler, in a given heating zone refers to
the temperature of the hot flue gas. "Tw" refers to the tube wall
temperature in a given boiler heating zone and is below "Tg" due to
the circulation of boiler water inside the tubes.
[0011] The corrosion described above is called "vanadium
corrosion", and is due to the formation, in the flames, of vanadium
compounds of oxidation state 5 that are distinguished by low
melting points Tm that lie below the tube wall temperature Tw in
some boiler zones, such as vanadium pentoxide (V2O5: Tm=675.degree.
C.) or compounds involving alkali metals, such as alkali metal
metavanadates (NaVO3: Tm=628.degree. C.; KVO3: Tm=517.degree. C.;
the eutectic of these two salts: Tm=475.degree. C.) and V2O5/Na2SO4
mixtures (eutectic at 40 mol % Na2SO4: Tm=500.degree. C.). Thus, it
should be noted that the association of alkali metals (Na, K) with
V2O5 is particularly deleterious because of the formation of
compounds that are even more fusible and moreover more fluid and
more conducting in the molten state. These compounds are
transported in liquid form by the flue gases and may deposit on the
tubing in the furnace (waterwall tubes) or may travel into the
convection pass and deposit on the superheater, reheater, boiler or
economizer tubes of the installation. The fraction that are
deposited on the water wall tubes, and superheater, boiler and
economizer areas may result in vigorous electrochemical attack
characteristic of the molten electrolytic media associated with
oxidizing agents, in this case vanadium at the oxidation state 5
itself, sulfur at the oxidation state 6 (SO.sub.3 and sulphates)
coming from the fuel, and any residual oxygen contained in the
combustion gases. Vanadium corrosion may be inhibited by chemically
trapping V2O5 within refractory and chemically stable compounds
that eliminate the molten electrolytic medium and ipso facto this
form of high temperature corrosion. Possible vanadium inhibitors
are compounds based on alkaline-earth metals, such as calcium and
magnesium salts. These inhibitors, injected into the furnace of the
combustion installations to be protected, react with the vanadium
compounds to form orthovanadates, pyrovanadates, and metavanadates,
the melting points of which are above the melting points of V2O5
and of the corresponding sodium vanadate species. The orthovanadate
may be written in the form M3(VO4)2 in which M denotes an
alkaline-earth metal. In the particular case of magnesium, the
magnesium orthovanadate (OV) having a melting point of 1074.degree.
C. is formed according to the following reaction:
V2O5+3MgO.fwdarw.Mg.sub.3(VO.sub.4)2. (1a)
[0012] The pyrovanadate may be written in the form
M2(V.sub.2O.sub.7) in which M denotes an alkaline-earth metal. In
the particular case of magnesium, the magnesium pyrovanadate (PV)
having a melting point of 980.degree. C. is formed according to the
following reaction:
V2O5+2MgO.fwdarw.Mg2V2O7. (1b )
[0013] The metavanadate may be written in the form M(VO3)2 in which
M denotes an alkaline-earth metal. In the particular case of
magnesium, the magnesium metavanadate (MV) having a melting point
of 742.degree. C. is formed according to the following
reaction:
V2O5+MgO.fwdarw.Mg(VO3)2. (1c)
[0014] Reactions (1a), (1b), and (1c) are written with Mg as the
metal M described above. However, reactions (1a), (1b), and (1c)
can also be written with calcium or nickel instead of magnesium,
and calcium oxide and nickel oxide can be used instead of MgO in
embodiments of the present invention. In the present disclosure, Mg
will be used as an example, but other alkaline-earth metals, such
as calcum or barium, can take its place.
[0015] The molar ratios (MO/V2O5) corresponding to the
orthovanadates, pyrovanadates and metavanadates are 3, 2 and 1
respectively.
[0016] This mode of inhibition, which consists in removing the
corrosive vanadium derivatives from the exposed surfaces and in
trapping them in reputedly stable refractory compounds, enables all
materials, whether metallic, ceramic or composite, to be
effectively protected. The inhibitor must be injected in sufficient
quantity so as, on the one hand, to trap all the vanadium
introduced by the fuel and, on the other hand, to form the desired
vanadate species based on furnace operating conditions. In
combustion installations, such as steam boiler systems, the ortho
and pyro vanadate species have a sufficienty high melting point to
fulfill the wanted protection. As far as the metavanadate form is
concerned, it must be noted that the ash layers deposited on the
walls are exposed to a temperature ("Td") comprised between the
skin temperature ("Tw") of the metal and the flue gas temperature
"Tg". Therefore, the melting point (742.degree. C.) of the
metavanadate that is higher than Tw suffices to avoid the formation
of fusible, corrosive slag upon the metallic wall, making magnesium
metavanadate a suitable inhibition product as well; however since
Tg can exceed 742.degree. C. (e.g. in the furnace or in the
superheater sections), any ash layer that would contain a
substantial amount of Mg(VO.sub.3).sub.3 may experience partial
fusion and sintering of its outer portion, leading to deposits that
are not easy to remove.
[0017] In practice, the (Mg/V) ratio targeted for the inhibition of
vanadic corrosion in combustion installations, such as boilers, is
1 by weight, which corresponds to an atomic (Mg/V) ratio of 2.09
and to a molar (MgO/V.sub.2O.sub.5) ratio of approximately 4.2.
Such dosage of MgO should enable, in theory, the formation of the
very high melting point orthovanadate that requires a molar
(MgO/V.sub.2O.sub.5) ratio of 3, according to reaction (1a).
[0018] All the inhibition methods have the common drawback of not
reducing but, on the contrary, increasing the volume of ash that
leaves the flame. The "magnesium-vanadium" ash formed during
inhibition by MgO partly deposits on furnace wall tubes,
superheater, reheater, and boiler tubes, with the result that the
treatment increases the overall ash loading of the system.
[0019] The "ash deposition rate" may be defined as the ratio of the
mass of ash deposited in the furnace to the mass entering the
furnace over a given duration. This "ash deposition rate", from
which the rate of fouling and the impact on performance of
combustion installations (such as boilers) directly result, is a
complex parameter ascertainable only by experimentation since,
besides the temperature and the velocity of the gas stream and of
the ash particles suspended therein, depends on many other factors
difficult to determine, such as: the chemical nature of the ash
(based on the composition of the fuel); the ash particle size
distribution; the angle of impact of the particles relative to the
substrate; the state of the substrate (roughness; oxidation state);
the hardness of the particles compared with that of the impact
surfaces (these impact surfaces are initially the bare metal walls
of the combustion installation, or a surface coating thereon, or
the layers that form progressively thereon). The physical
properties of these layers are themselves liable to change as a
result of a physical transformation (compaction or densification)
or chemical transformation of the ash. In particular, ash particles
are also subjected to "sintering". The latter phenomenon, which is
essential in the aging process of any deposit, affects any
crystalline solid heated to high temperature over long durations:
the solid tends to densify by reduction in its porosity, to
recrystallize and to harden. Consequently, irrespective of the
method of inhibition, the long residence time of the ash on hot
components is liable to result in the deposits being progressively
sintered and becoming potentially more difficult to remove.
Furthermore, the sintering is accelerated in the presence of a
molten phase, which accelerates internal atomic diffusion.
[0020] To mitigate the losses in performance caused by excessively
degraded operation of the boiler, it is essential for these
deposits to be periodically removed from the deposit prone areas.
In combustion installations, such as boiler systems, cleaning is
performed both via off-line and on-line procedures. The online
methodology is performed in several different ways. The principal
methodology is through the use of soot blowing. Soot blowing is
performed by blowing jets of air, steam or sometimes water (free of
corrosive salts) onto the deposits to aid in removal. Soot blowers
are positioned in locations that are prone to deposit accumulation
and are controlled either manually or through an automated regimen.
In some cases, soot blowing is supplemented via a manual lance that
can be used to clear paths that are out of the reach of soot
blowers
[0021] Where routine soot blowing is ineffective, a "chill and
blow" can be performed. This process entails a reduction or
cessation in fuel feed that results in lower firing and reduced
system temperatures. The reduced temperature helps solidify the
liquid deposits, if any, and destabilizes the deposit layer due to
the differential expansion effect between ash and wall and improves
overall removal efficacy of soot blowing.
[0022] Both methods combine a mechanical effect with the potential
to dissolve and carry away the ash deposits. The dissolution
assumes that the deposits have a soluble phase (such as magnesium
sulphate) in an amount sufficient to destabilize, during its
dissolution, the entire deposited layer, which will then be carried
and collected in the ash hopper.
[0023] The offline cleaning is performed during yearly maintenance
shutdowns and when operational problems (including if dictated by
excessive slag formation) require or permit. These methods can
include an extensive water wash after the shut down and cooling of
the combustion installation such as a boiler, or a variety of
mechanical methods used to physically remove the accumulated slag
and deposit from the furnace.
[0024] Since both online and offline methods result in loss of
steam generation or reduced operating capacity, these methods of
restoring performance can have a substantial impact on system
availability and on production.
[0025] Inhibitors based on alkaline-earth metals (e.g., magnesium
and calcium) are very effective in protecting against vanadium
corrosion. However, the very low solubility in water of calcium
sulphate (Ca504) and the hardness and strong adhesion of the
deposits that it forms make the above-mentioned cleaning methods
more difficult. Calcium derivatives therefore are less likely to be
used in practice as vanadium inhibitors.
[0026] Magnesium inhibitors, which are commercial additives very
widely used, have three main drawbacks in boiler applications. p
The first drawback stems from the fact that magnesium based ash
intrinsically results in a high "ash deposition rate" on the hot
parts and therefore results in particularly rapid fouling of the
hot parts. This is a characteristic of magnesium sulphate-magnesium
vanadate systems, which can be confirmed by simulation tests
carried out in a "burner rig" but, as indicated above, cannot be
deduced from a purely theoretical approach.
[0027] The second drawback is due to the limited thermal stability
of MgSO4, since at high temperature a sulphation/desulphation
equilibrium according to equation (3) is established:
MgSO4.fwdarw.MgO+SO3. (3)
[0028] With increasing temperature, this equilibrium is shifted in
the endothermic direction, i.e. to the right, and MgSO4 therefore
tends to be desulphated. On water tubes, this effect essentially
affects the outer portion of the deposits due to the existence of
the already mentioned temperature gradient (T.sub.g-T.sub.w). Thus,
the outer layer of the deposit faces the radiative heating of the
furnace while the inner portion is cooled by the steam flowing
inside the tube. Since MgO has a higher density than MgSO4 (3600
kg/m.sup.3 as opposed to 2600 kg/m.sup.3), there is also a physical
contraction and an agglomeration of the deposit. Hence, the latter
loses its porosity and becomes more difficult to dissolve or to be
mechanically disintegrated, while its tendency to be sintered is
increased by this densification. In addition, since it is the outer
portion of the deposits that is affected, the formation of such
refractory, impervious layer of MgO acting as a physical barrier
decreases the efficiency of steam or water used as cleaning media
since they cannot reach the core of the deposits. The water-soluble
magnesium sulphate is thus replaced with magnesium oxide, which is
neither soluble in water nor in all of the reagents compatible with
the integrity of boiler materials.
[0029] The third drawback of inhibition using MgO lies in the
sensitivity of magnesium-vanadium ash to sodium. Any traces of
sodium contained in the fuel (fuels of combustion installations,
such as boilers, are not routinely pre-washed for sodium removal)
or in the air (Na2SO4 dust in an industrial environment or
NaCl-rich fogs in a marine environment) are converted to Na2SO4 in
the flames and are incorporated into the magnesium ash, either in
the form of mixed sulphate Na6Mg(SO4)4 which melts at 670.degree.
C. or, owing to the strong affinity existing between sodium and
vanadium, in the form of mixed vanadate NaMg4(VO4)3, which melts at
570.degree. C. These two compounds not only make the ash more
fusible and aggravate the sintering phenomenon, but are also
potentially corrosive.
[0030] Because of this "parasitic" sodium and MgSO4 desulphation
effects, inhibition using magnesium appears in fact to be a
relatively complex process, the actual balance of which is not
simply that of reactions (1a) to (1c) but involves the extensive
chemistry of the (magnesium oxide--sodium/magnesium vanadates and
sodium/magnesium sulphates) system, leading to the need for
substantial overdosages of magnesium with respect to the minimum
theoretical requirement. This explains why, in practice, although
the typical (Mg/V) ratio used for the inhibition of vanadic
corrosion in boilers is 1 by weight, corresponding to a molar
(MgO/V.sub.2O.sub.5) ratio around 4.2, one does not form the
orthovanadate Mg.sub.3V.sub.2O.sub.8 (equation 1a) but mixtures
containing in majority MgSO.sub.4 and Mg.sub.2V.sub.2O7 (equations
(1b)) with some minor contents of Mg.sub.3V.sub.2O.sub.8,
MgV.sub.2O.sub.6 (equations (1a) and (1c)) and MgO and trace
amounts of the double Mg/Na vanadates and sulfates.
[0031] In view of the limitations of the current inhibition
methods, it is therefore desirable to have an improved inhibition
method meeting the following three objectives: (i) effectively trap
the vanadium; (ii) deposit a minimum amount of ash, which can be
removed, by an on-line method (such as soot blowing); and finally
(iii) provide these two functions up to the highest possible limit
temperature.
[0032] Now the Applicant has established that the association of
boric oxide (B2O3) referred below to as a "second oxide"--with MgO
(constituting the "first oxide"), enables the achievement of these
objectives by strongly reducing the "parasitic" sodium and MgSO4
desulphation effects mentioned above.
[0033] Chemically, boric oxide B2O3 reacts rapidly and
quantitatively when hot with MgO, to form, depending on the (Mg/B)
atomic ratio, magnesium tetraborate MgB4O7 ("TB"), magnesium
pyroborate Mg2B2O5 ("PB") or magnesium orthoborate Mg3B2O6 ("OB").
These salts may also be written as MgO-2B2O3, 2MgO--B2O3 and
3MgO--B2O3, respectively, and they result from the following
reactions:
MgO+2B2O3.fwdarw.MgB4O7: magnesium tetraborate (TB) (4)
3MgO+B2O3.fwdarw.Mg3B2O6: magnesium orthoborate (OB) (5)
2MgO+B2O3.fwdarw.Mg2B2O5: magnesium pyroborate. (PB) (6)
[0034] Interestingly, B2O3 is also capable of reacting with
magnesium sulphate MgSO4 with evolution of SO3:
MgSO4+2B2O3.fwdarw.MgB4O7+SO3 (4b)
3MgSO4+B2O3.fwdarw.Mg3B2O6+3SO3 (5b)
2MgSO4+B2O3.fwdarw.Mg2B2O5+2SO3. (5b)
[0035] Vanadium pentoxide in turn reacts rapidly and quantitatively
with the magnesium orthoborate and magnesium pyroborate to give
magnesium vanadates:
3Mg3B2O6+V2O5.fwdarw.Mg3V2O8+3Mg2B2O5. (6a)
4Mg2B2O5+3V2O5.fwdarw.3Mg2V2O7+2MgB4O7 (6b)
[0036] Magnesium orthoborate and pyroborate are therefore vanadium
inhibitors according to reactions (6a) and (6b). The rapidity of
all these reactions enables the composition of the ash in the
combustion gases to be rapidly stabilized.
[0037] The use of B2O3 has five advantages.
[0038] The first advantage in using B2O3 as "second oxide" lies in
the fact that, due to reactions (4b) to (6b) it considerably
reduces the formation of MgSO.sub.4 and prevents the secondary
formation of the insoluble, fouling MgO by the desulphation
process. Indeed, unlike MgSO4, magnesium orthoborate, pyroborate
and tetraborate are thermally very stable and also have high
melting points (1312.degree. C., 1330.degree. C. and 995.degree. C.
respectively) that substantially exceed the wall temperatures (Tw)
encountered in combustion installations, such as boilers, making
their use particularly advantageous and strongly preventing any
sintering effect of the ash layer. The latter beneficial effect
increases as the proportion of magnesium borate ash increases in
the overall deposit.
[0039] The second advantage in using B2O3 lies in the low melting
point (450.degree. C.) of this oxide, which is close to that of
V2O5. In the flame and immediately after the flame, the presence of
an additional fraction of liquid represented by the molten B2O3, in
addition to the molten V2O5, favors the reaction kinetics between
the various species by accelerating the inter atomic diffusion, a
well known effect often used in inorganic synthesis (e.g: the
synthesis of the ferrite NiFe.sub.2O.sub.4 which is difficult when
starting from the oxides is greatly facilitated when carried out
using a mixture of molten nickel and iron nitrates). In particular,
the formation of the desired magnesium vanadates find themselves
accelerated by this kinetic effect. This is of considerable help in
the case of combustion installations, such as boilers, where there
are possible zones of lower temperature in which the reaction
kinetics between MgO and V2O5 may become the limiting step of the
inhibition process.
[0040] The third advantage in using B2O3 lies in the remarkable ash
anti-deposition properties developed at high temperature by
magnesium pyroborate and magnesium orthoborate, properties that
have been discovered by the Applicant. These properties ensure
particularly low ash deposition rates on hot components. Tests
carried out on a burner rig over durations of 250 to 500 hours with
typical dosages for boilers, as set out below, have shown, for
example, deposition rates on average 4 times lower in the case of
inhibition with boron than inhibition without boron (i.e. during an
inhibition run performed with MgO alone).
[0041] The fourth advantage of magnesium-boron inhibitors lies in
the very porous and friable texture of magnesium borate rich ash
deposits which to a large extent explains the low deposition rate
and makes it possible to remove these deposits using less
aggressive physical methods including soot blowing, knowing that
the mechanical entrainment effect of water and steam is also
sufficient to remove them.
[0042] The fifth considerable advantage of magnesium-boron
inhibitors, most particularly compared with MgO alone, lies in the
fact that their performance is maintained in the presence of an
appreciable amount of sodium. Specifically, when sodium is present
in the fuel (or in the combustion air) and, after combustion,
becomes incorporated into the ash in the form of Na2SO4, it may
react with B2O3, even in the presence of magnesium, to form sodium
borate Na4B2O5, which is not corrosive unlike the double sulphates
and vanadates formed in MgO inhibition. This reaction may be
written as:
2Na2SO4+3Mg2B2O5.fwdarw.Na4B2O5+2Mg3B2O6+2SO3. (7)
[0043] Thus, it has been found that, in the operative conditions of
combustion installations such as boilers, the protection by the
magnesium-boron inhibitor remains effective for an (Na2SO4N2O5)
molar ratio ranging up to 0.7 (i.e. an (Na/V) atomic ratio also
ranging up to 0.7) and that the ash formed remains friable and
non-adherent, despite a slight hardening. Moreover, the higher the
magnesium borate content in the ash, the less this hardening effect
is perceptible. This capability of neutralizing sodium and of
eliminating its deleterious effects is a considerable advantage of
magnesium-boron inhibitors.
[0044] The Applicant has identified the suitable dosages of
magnesium and boron in combustion installations such as boilers.
Herein, "b" is the molar ratio of boron (in B.sub.2O.sub.3
equivalents) to vanadium in the form of V.sub.2O.sub.5, such as the
(B2O3N2O5) ratio and "m" is the molar ratio of magnesium (in MgO
equivalents) to vanadium in the form of V.sub.2O.sub.5, such as the
(MgO/V2O5) ratio. B.sub.2O.sub.3 equivalents include the following:
precursors added to the combustion installation that form
B.sub.2O.sub.3 in the combustion installation, B.sub.2O.sub.3 that
is added in the form of B.sub.2O.sub.3, reaction products of
B.sub.2O.sub.3, such as magnesium borates, that are added to the
combustion installation, and precursors of such reaction products
of B.sub.2O.sub.3. MgO equivalents include the following: precurors
added to the combustion installation that form MgO in the
combustion installation, MgO that is added in the form of MgO, and
reaction products of MgO, such as magnesium borates, that are added
to the combustion installation, and precursors of such reaction
products of MgO. In other words, each B.sub.2O.sub.3 equivalent
represents the addition, directly or indirectly, of one
B.sub.2O.sub.3, and each MgO equivalent represents the addition,
directly or indirectly, of one MgO. CaO equivalents are the same as
MgO equivalents with a Ca instead of an Mg. If all of the magnesium
is added in the form of MgO (each MgO being one MgO equivalent) and
all of the boron is added in the form of B.sub.2O.sub.3 (each
B.sub.2O.sub.3 being one B.sub.2O.sub.3 equivalent), then "b" is
strictly the (B2O3/V2O5) molar ratio and "m" is strictly the
(MgO/V2O5) molar ratio.
[0045] However, if the boron and the magnesium are added in other
forms, such as Mg.sub.3B.sub.2O.sub.6 (otherwise known as
3MgO--B.sub.2O.sub.3, which is three MgO equivalents and one
B.sub.2O.sub.3 equivalent), or Mg.sub.2B.sub.2O.sub.5 (otherwise
known as 2MgO--B.sub.2O.sub.3, which is two MgO equivalents and one
B.sub.2O.sub.3 equivalent) then "b" and "m" will incorporate this
fact. Thus, in the case of the addition of one mole of
B.sub.2O.sub.3 (one B.sub.2O.sub.3 equivalent) and one mole of MgO
(one MgO equivalent) and one mole Mg.sub.3B.sub.2O.sub.6(three MgO
equivalents and one B.sub.2O.sub.3 equivalent) and one mole
Mg.sub.2B.sub.2O.sub.5(two MgO equivalents and one B.sub.2O.sub.3
equivalent), b is equal to 3/(V.sub.2O.sub.5), which is 3
B.sub.2O.sub.3 molar equivalents divided by the number of moles of
V.sub.2O.sub.5 generated from the fuel. Similarly, m is equal to
6/(V.sub.2O.sub.5), which is 6 MgO molar equivalents divided by the
number of moles of V.sub.2O.sub.5 generated from the fuel. In a
(b,m) graph, the suitable (b,m) points represent the MgO and B2O3
equivalents, in an embodiment located in the zone defined by FIG.
1, as follows:
[0046] The representative point in an embodiment is above the
straight line m=2+b to assure a good anti-corrosion protection, and
b in an embodiment equals or exceeds 0.5 to obtain a substantial
effect of boron.
[0047] This domain is therefore the upper, right angle delimited by
the straight lines b=0.5 and m=2+b in the (b,m) plane and is
referred below to as the "application domain". The resulting ash
may contain: magnesium pyrovanadate; magnesium orthovanadate;
magnesium pyroborate and magnesium tetraborate, possibly magnesium
orthoborate as a function of the position of the (b,M) point inside
this zone and possibly sodium borate if sodium is present.
[0048] The Applicant has further identified a "preferential
application domain" as being defined, in the (b,m) plane in FIG. 2,
as follows: [0049] m.gtoreq.2+b (to assure a good anti-corrosion
protection) [0050] b.gtoreq.0.5 (to obtain a substantial effect of
boron) [0051] m.ltoreq.3+2b to avoid the formation of excess
MgSO.sub.4 as all the boron and vanadate is already combined in
orthoborate and orthovanadate species respectively and cannot react
any more with more MgO.
[0052] Finally, the "most preferential application domain" is
represented by the trapeze at FIG. 2 defined by:
m.gtoreq.2+b
b.gtoreq.0.5
b.ltoreq.1.5 to optimize the cost/effect ratio of boron
m.ltoreq.5 to limit the cost of the magnesium component of the
inhibition
[0053] In the present application, b can be between 0.25 and 2.5
(or any sub-range within this range). In an embodiment, b is
between 0.5 and 1.5. Also, in the present application, m can be
between 2.25 and 8 (or any sub-range within this range). In an
embodiment , m is between 2.5 and 5. The present application
envisions operating within any range or sub-range defined by the
following ranges, and is deemed to provide support for operating in
any point within the following ranges:
2+b.ltoreq.m.ltoreq.3+2b
2.25.ltoreq.m.ltoreq.8 (in an embodiment 2.5.ltoreq.m.ltoreq.5)
0.25.ltoreq.b.ltoreq.2.5 (in an embodiment
0.5.ltoreq.b.ltoreq.1.5)
[0054] With regard to the preparation of inhibitors based on MgO
and B2O3, two methods of preparation are possible:
[0055] In the first method, the synthesis of the mixed MgO--B2O3
oxide or of precursors of the same occurs upstream of the
combustion chamber of the installation. Starting from the chemical
reactants, the mixed MgO--B2O3 oxide, or a precursor of this mixed
oxide is synthesized and stored upstream of the combustion
installation. The term "precursors" refers to a combination (or to
a number of combinations) that contains magnesium and boron, which
is not necessarily a defined chemical compound of magnesium and
boron and which produces the desired mixed MgO--B2O3 oxide in the
flames. Such precursors are for example sol-gels or other nanoscale
structures. The mixed MgO--B2O3 oxide or its precursors, prepared
in this way and stored, is injected in a suitable quantity either
directly into the combustion chamber or at a point in the fuel
circuit where it is intimately mixed with the fuel using a static
or dynamic mixer. To obtain optimum inhibition efficiency, the
mixed MgO--B2O3 oxide may be in nanoscale form. It may especially
be advantageous to synthesize mixed MgO--B2O3 oxides or precursors
in the form of either oil soluble substances or very finely divided
particles or, in an embodiment, in the form of nanostructured
substances.
[0056] In the second method, the synthesis of the mixed MgO--B2O3
oxide is carried out in situ, i.e. directly in the combustion
chamber of the combustion installation, by reaction of two
reactants introduced upstream of said combustion chamber (e.g. an
aqueous solution of MgSO.sub.4 and a solution of B.sub.2O.sub.3 in
diethylene glycol) or by the transformation in said combustion
chamber of a precursor of the mixed MgO--B2O3 oxide (e.g oil
soluble forms of magnesium and boron). It is noted that the
precursors of MgO or the mixed MgO--B.sub.2O.sub.3 can be taken
into account in the calculation of "b" and "m" above since they
result in MgO and mixed MgO--B.sub.2O.sub.3 species in the
combustion chamber. In this case, the chemical reactants or the
precursors are stored upstream of the combustion chamber of the
combustion installation and injected, in suitable proportions and
quantities, either at a point in the fuel circuit where they will
be intimately mixed with the fuel using a static or dynamic mixer,
or directly into the combustion chamber. The concept of "suitable
proportions" refers to the ratios of magnesium to B2O3, whereas the
concept of "suitable quantities" refers to the Mg/V dosing ratio.
An interesting particular case is the preparation of an oil soluble
precursor, the starting point of which may be a magnesium
derivative of the "overbased sulphonate" or "overbased carboxylate"
type. Such overbased magnesium compound can be borated by
introducing boric acid in suitable proportions and by stirring for
several hours between 50 and 200.degree. C. Alternatively, a
suitable oil-soluble precursor can be obtained by adding an
oil-soluble boron compound, such as an alkyl borate of generic
formula (Alk)3B, for example ethyl borate (C2H5)3B, to the
magnesium overbased compound.
[0057] According to an embodiment of the present invention, the
method described above is used to inhibit the vanadic corrosion of
the combustion installation possibly in the presence of sodium.
According to an embodiment of the invention, the metallic, ceramic
or composite materials of a combustion installation, for example a
combustion installation such as a boiler burning a fuel
contaminated with vanadium, which may or may not be associated with
sodium, are protected from vanadium corrosion by introducing into
or forming in the combustion chamber of said installation, an
inhibitor formed by a mixture of magnesium borates, such that the
representative point of the (b,m) pair lies inside the domains
defined above.
[0058] The primary method of cleaning up the installation is dry
cleaning by soot blowing, as described above, with water washing
being a secondary way of cleaning up the installation.
[0059] To better illustrate the invention, several embodiments are
described below.
EXAMPLE 1
[0060] A combustion installation, such as a boiler, has a furnace
temperature of 570.degree. C., and produces steam by burning an
average of 2,000 kg/hr of a very degraded heavy fuel oil containing
200 ppm vanadium by mass (generating thus 3.926 mole/hour of V2O5).
Such boiler is treated with an oil soluble inhibitor containing
15.5% by weight of magnesium and 3.45% by weight of boron. The
injection flowrate of the inhibitor is 2.46 kg/hr, representing:
[0061] 0.381 kg/hr or 15.67 mole/hr of magnesium; so:
m=15.67/3.926=4.00 [0062] 0.085 kg/hr or 7.87 mole/hr of boron i.e.
3.93 mole/hr of B2O3; so: b=3.93/3.926=1.0. [0063] These dosage
conditions satisfy the relation "m=2+2b" lying in the "most
preferential application domain".
[0064] During an operation period of 1200 hours, the average
thermal power of the boiler is potentially 19.59 MW thermal,
meaning an average efficiency of about 86%. After this period,
furnace wall tube slagging is solid and friable. Slagging and
superheater fouling are controlled with routine soot blowing and a
chill and blow procedure after six weeks.
COMPARATIVE EXAMPLE 1
[0065] The same boiler burning on average 2,100 kg/hr of the same
heavy fuel oil containing on average 190 ppm of vanadium
(generating thus 3.916 mole/hour of V2O5) is treated with an oil
soluble inhibitor containing 20% by weight of magnesium and no
boron. The injection flowrate of the inhibitor is 1.91 kg/hr,
representing 0.382 kg/hr or 15.71 mole/hr of magnesium.
[0066] So, in this second inhibition treatment, one has:
m=15.71/3.916=4.01 (corresponding to an injection rate of magnesium
very close to the one in example 1) and b=0.
[0067] During an operation period of 1150 hours, the average
production of the boiler is potentially 19.37 MW thermal, meaning
an efficiency of about 81%. After this period, slag is solid but
difficult to remove with routine soot blowing. The accumulation of
superheater ash deposits and furnace slag requires a chill and blow
procedure after less than four weeks.
[0068] A comparison of Example 1 and Comparative Example 1 shows a
potential gain in Example 1 of 5% absolute (6.2% in relative) in
the boiler efficiency on an average over 1200 hours of operation
(50 days) which represents a saving of about 37 metric tons of fuel
per year of continuous operation (i.e. about US$ 10,000) and the
avoidance of 121 metric tons of CO.sub.2 release into the
atmosphere.
[0069] This written description uses examples to disclose the
invention, including the preferred embodiments, and also to enable
any person skilled in the art to practice the invention, including
making and using any devices or systems and performing any
incorporated methods. The patentable scope of the invention is
defined by the claims, and may include other examples that occur to
those skilled in the art. Such other examples are intended to be
within the scope of the claims if they have structural elements
that do not differ from the literal language of the claims, or if
they include equivalent structural elements with insubstantial
differences from the literal languages of the claims.
* * * * *