Combustion process for fuel containing vanadium compounds

Abstract

Oxycombustion process for producing energy wherein low ranking gaseous, liquid, solid, optionally solid melting hydrocarbon fractions are used as fuels, having a vanadium content in amounts by weight from 50 to 5,000 ppm or higher, and alkaline metals Ma in amounts from 20 to 10,000 ppm, wherein magnesium is added as oxide, or as a magnesium compound forming MgO in the combustion process, or mixtures thereof and a silico-aluminate wherein the molar ratio SiO.sub.2:Al.sub.2O.sub.3 ranges from 2:1 to 6:1; the combustor being refractored, isotherm or quasi-isotherm, flameless, working at temperatures in the range 1,250-1,450 C. and under pressurized conditions, wherein the oxidant being used in admixture with water or steam, the ratio by moles oxidant:(water/steam) being comprised between about 1:0.4 and about 1:3, or the oxidant is used in admixture with flue gases recycled from the flue gases outletting the energy recovery equipments, wherein the water/steam amount is higher than 30% by volume, optionally by adding water to the recycled flue gases, the molar ratio oxidant:(water/steam) in flue gases being comprised from about 1:0.4 to about 1:3; the hydrocarbon fraction being fed in admixture with water or steam, the amount of water/steam being at least 30% by weight with respect to the hydrocarbon fraction.

Claims

1. Oxycombustion process comprising feeding to a combustor: a fuel selected from low ranking gaseous, liquid, solid, also solid melting hydrocarbon fractions, having a vanadium content in amounts by weight from 50 to 5,000 ppm or higher, and alkaline metals Ma in amounts from 20 to 10,000 ppm, the hydrocarbon fractions being fed in admixture with water or steam, the amount of water/steam being at least 30% by weight with respect to the hydrocarbon fractions; magnesium as oxide, or as a magnesium compound forming MgO in the combustion process, or mixtures thereof; a silico aluminate wherein the molar ratio SiO.sub.2:Al.sub.2O.sub.3 ranges from 2:1 to 6:1; oxygen as oxidant, having a titre higher than 80% by volume, the complement to 100% being formed of inert gases and/or nitrogen, being in admixture with: water or steam, the ratio by moles oxidant:(water or stream) being between 1:0.4 and 1:3, or recycled flue gases having a water or stream amount higher than 30% by volume, the molar ratio oxidant:(water or stream) in flue gases being comprised between 1:0.4 and 1:3, the recycled flue gases coming from the energy recovery equipments; the combustor being refractored, isotherm or quasi-isotherm, flameless, operating at temperatures in the range 1,250-1,450 C. and under pressurized conditions.

2. A process according to claim 1 wherein a pressure in the combustor is comprised from higher than or equal to 102 kPa to 5,000 kPa.

3. A process according to claim 1 wherein a pressure in the combustor is comprised from higher than or equal to 200 kPa to 3,500 kPa.

4. A process according to claim 1 wherein the oxidant is pure oxygen.

5. A process according to claim 1 wherein the Mg:V molar ratio is in the range 1:1-2:1.

6. A process according to claim 5 wherein the Mg:V molar ratio ranges from 1.25:1 to 1.75:1.

7. A process according to claim 1 wherein the molar ratio of Mg:Al is comprised between 0.5:1 and 1:1.

8. A process according to claim 7, wherein the molar ratio of Mg:Al is comprised between 0.6:1 and 0.8:1.

9. A process according to claim 1 wherein the added amount of silico aluminate, on a molar basis, with respect to the alkaline metals Ma contained in the fuel is in the molar ratio Al/Ma>1.

10. A process according to claim 9, wherein the molar ratio Al/Ma is between 1 and 1.5.

11. A process according to claim 1 wherein the temperature in the combustor ranges from 1300 to 1400 C.

12. A process according to claim 1 wherein the hydrocarbon fractions are selected from asphaltene, petrolene, carbonaceous substances, Petcoke, carbonaceous residues of petrochemical processes, and heavy refinery bottoms.

13. A process according to claim 1 wherein the addition of magnesium is carried out by feeding an aqueous solution of magnesium sulphate.

14. A process according to claim 1 wherein the silico aluminate is a magnesium silico aluminate, optionally added of SiO.sub.2 in order to have a SiO.sub.2:Al.sub.2O.sub.3 ratio ranging from 2:1 to 6:1.

15. A process according to claim 1, wherein the residence time of the fuel in the combustor ranges from 0.5 seconds up to 30 minutes.

16. Flue gases obtainable with the process according to claim 1, wherein (data on a flue gas dry basis): absent soot and IPA <10.sup.4 mg/Nm.sup.3, TOC<1 ppm, fly ash <100 mg/Nm.sup.3, magnesium <1 mg/Nm.sup.3, vanadium and its compounds orthovanadate and pyrovanadate1 mg/Nm.sup.3 expressed as vanadium, and V.sub.2O.sub.5<0.01 mg/Nm.sup.3, SO.sub.32 mg/Nm.sup.3, NiO1 mg/Nm.sup.3, NaVO.sub.3 not detectable by XRD, and SEM analysis of the microaggregates.

17. The process according to claim 1 wherein the water is added to the recycled flue gases.

Description

EXAMPLES

Characterization

(1) Analytical Methods

(2) Characterization of the Particulate in Flue Gases

(3) The particulate contained in the combustion flue gases is collected by means of an impactor of the Andersen Mark III type equipped with:

(4) one pre-separator (cyclone) capable to remove particles having an aerodynamic diameter greater than 10 m, screens for impactor Andersen for particulate PM 10, by using a sampling flow of 14 liter/min, and filters able to separate fractions having particle diameter, respectively, of 10-9 m; 9-5.8 m; 5.8-4.7 m; 4.7-3.3 m; 3.3-2.1 m; 2.1-1.1 m; 1.1-0.7 m; 0.7-0.4 m.

(5) The particulate having particle sizes lower than 0.4 m, that is not retained in the last stage of the impactor Andersen, is filtered on mica filter for analysis with the atomic force microscope by means of a pneumatic actuator that collects, by thermophoretic effect, a sufficient and statistically significant amount of particles. The gaseous flow outletting the impactor is then conveyed into a condensation system of the combustion steam, wherein the submicron particulate is collected, together with a part of the particulate having a diameter lower than 10 nm, in an amount comprised between 1% and 10% by weight of the original particle population having nanometric sizes. The sampling step makes available particle fractions that are then subjected to the chemico-physical analysis by scanning electronic microscopy (SEM) and to the X-ray analysis. The chemical analysis of the single particles is carried out with a microscope SEM PHILIPS XL30, equipped with a thin window EDX system for the microanalysis by energy dispersion spectrometry, by using an automatic system capable to automatically detect the particles when a determined threshold is exceeded.

(6) For each of the identified particles the morphological parameters are determined by measuring the intensities of the lines typical of the X ray spectrum and converting into the corresponding atomic concentrations.

(7) Analysis of the Metals

(8) The analysis is carried out by induction-plasma spectroscopy by using an ICP device (inductive coupled plasma)-OES (Thermo Electron Corporation).

(9) For the solid phases, the compounds are analyzed by XRD (XRays Diffraction), combined with ICP.

(10) Soot Analysis

(11) Soot analysis is carried out by SEM microscopy.

(12) Partially combusted fuel molecules tend to aggregate themselves into clusters (microaggregates) of different size and very irregular shape.

(13) These particles, called cenospheres and plerospheres, are representative of soot (also known as Diesel Particles, or Black Carbon) and are clearly identifiable at SEM microscope.

(14) Result Evaluation

(15) By Andersen Probe, SEM, and EDX analysis cenospheres and plerospheres are not detectable. If present, they are below the sensitivity limits of these analytical methods.

(16) Other analytical methods that have been used are reported under example 1.

(17) The other methods used in the examples are of common practice and well known to the skilled in this art.

(18) For example the flue gases from the reactor are detected by a set of fast response analysis unit (T95, 1.5 seconds), specifically developed by Fisher-Rosemount capable of monitoring both the bulk compounds, CO.sub.2, and the micro compounds CO, NO, NO.sub.2, SO.sub.2, and TOC (total organic content, hydrogen flame detector). The analytical units analyze the gases at a frequency of 10 Hertz. The original signal is recorded, skipping the data smoothing software. The closed cycle flue gases of the reactor are monitored in parallel, as soon as they are laminated to atmospheric pressure by a group of FTIR sensors which detect H.sub.2O, CO.sub.2, SO.sub.2, CO, NO, NO.sub.2, HCl with a response time of 40 seconds.

Example 1

(19) In a tank equipped with tracings and steam heat exchanger, an oil fraction obtained from the refinery operations of an heavy oil is loaded and collected on the bottom of the vis-breaking section.

(20) At the calorimetry characterization, the hydrocarbon fraction shows a LHV (Low Heating Value) value of 39,350 kJ/kg. The material subjected to pyrolysis at the temperature of 605 C. has an incombustible ash amount of 0.67% by weight. The analysis by optical ICP (ICP-OES) shows that the ashes are mainly formed of alumina and silica and to a lower extent of calcium.

(21) Sodium is in concentration of 4.6% by weight in the ashes. The ashes contain also heavy metals, among which the following ones:

(22) Nickel 46 ppm weight

(23) Vanadium 258 ppm weight

(24) By a gear pump fuel is fed to a 5 MW flameless thermal combustor, using as comburent (oxidant) oxygen having a titre 93% by volume, operated at 1,650 K and at the pressure of 5 barg (600 kPa), and inserted in a demonstration plant. The fuel flow rate determined by a Coriolis type fluxmeter is 8.2 kg/min. The injection into the combustor is carried out by means of a nozzle by using pressurized steam for the dispersion (dispersion only, not atomization) of the inletting jet. The steam sent to the nozzle head comes from the heat recovery steam generator, and it is laminated at 13-14 bar, fed at the flow-rate of 65 kg/h, added of water at a rate of 95 kg/h, dispersed in the steam through an atomizer. Oxygen having a titre 99.85% vol. is fed at a flow-rate of 18.2 Nm/min. The used oxygen comes from a cryogenic storage plant equipped with an evaporator. Oxygen is mixed with compressed air added at the flow-rate of 2.4 Nm/min. The gaseous mixture is introduced into the current of the recycled flue gases fed to the combustor to attemper it.

(25) An aqueous solution 0.3 M of magnesium sulphate is prepared. Under stirring, powdered cordierite (200 Mesh, particles average diameter 74 m) and fumed silica, in amount equal to 10% by weight of the final suspension are added, so as to have a molar ratio SiO.sub.2:Al.sub.2O.sub.3 3.4:1 and a ratio Al/Na (Na of the fuel) 1.23. This liquid phase is separately fed to a second injection nozzle at a rate of 0.33 liter/min. The demonstration plant has equipments according to the art.

(26) The flue gases outletting the combustor are cooled in a quencher by mixing with recycled flue gases to a final temperature of about 1,050 K, and sent to the heat recovery equipment (steam generator train, with superheater SH, evaporator EVA and final heat recovery ECO). Downhill of the heat recovery equipment the flue gases, having the temperature of about 520 K, are divided in recycle flue gases (sent to the combustor and to the quencher) compressed by means of a blower and in produced flue gases sent to the FGT section (flue gas post-treatment) after lamination from the pressure of the process to a pressure slightly higher than the atmospheric one.

(27) The produced superheated steam SH (400 C., 40 barg) is quantitated by a calibrated flange before being sent to the condenser.

(28) In the FGT section, the flue gases are filtered on a bag filter with a vermiculite powder precoat, then neutralized with a Venturi type contactor by means of lime milk (DeSO.sub.x), before being sent to the stack.

(29) In by pass the section for the sampling of heavy metals is available, that operates by the Andersen impactor (see above), located between the bag filter and the Venturi type contactor.

(30) The analytical equipment for controlling stack emissions is formed of an analyzer battery in continuum, operating on a current continuously sampled from the recycled gases after removal (Peltrier) of moisture, consisting of: FTIR for the determination of SOx, CO, besides TOC NDIR for the mass components, HFID (hydrogen flame detector) for the continuous analysis of TOC (total organic content), and zirconium probe for oxygen.

(31) In by pass with the stack flue gas powders are sampled for determining total powders and heavy metals (methodologies according to the European regulations).

(32) In the same way, but with a batch modality, flue gases are sampled for 8 hours for determining dioxins, furans, PBC (polychlorobiphenyls), IPA.

(33) An analytical unit is arranged for the batch sampling of the flue gases for specific determinations on the powders (vanadium and besides nickel, magnesium), in the following positions of the process.

(34) Point 1. downstream the combustor, in detail downstream the quencher.

(35) Point 2. downstream the bag filter.

(36) Point 3. downstream the DeSO.sub.x (at the stack).

(37) Powder determination in flue gas is carried out by using the following devices in sequence, (see methodologies (1a) and (1b) (see below)): quartz fiber filter (particle size cut 0.1 m), condenser positioned in a thermostated bath at 12 C., a drexel filled with water (drexel 1), a drexel filled with acid (drexel 2), and, as last a drexel having a high contact time, filled with an aqueous solution pH 0.5 acidified with nitric acid and brought to an oxidizing power of 1.4 EV by means of hydrogen peroxide, and a drexel 3 liter-counter of the extracted uncondensable fractions.

(38) The run of the combustion process lasted 600 h (25 days).

(39) At the end the plant is disassembled at some specific points for collecting powder samples, for the core sampling of the refractory coatings, for the extraction of the metal specimen (various metals and alloys) for characterization.

(40) Point 3.

(41) Among the values of the analytical determinations in continuum during the run on the flue gas sent to the stack, the following (values expressed in mg/Nm.sup.3 of flue gas dry basis) are pointed out:

(42) TABLE-US-00001 average peak value CO 2.7 11 TOC <0.1 1.6

(43) The average values of the batch analytical determinations at the stack (see above) (8 determinations, 5 of which in the first week) for parachlorodibenzodioxins/furans (PCDD/F) and polyaromatic hydrocarbons (IPA) were as it follows (referred to flue gas dry basis):

(44) TABLE-US-00002 PCDD/F ng I-TEQ/Nm.sup.3 0.0002 IPA g/Nm.sup.3 <0.05

(45) On the flue gas powders, the following average values have been determined, expressed as mg/Nm.sup.3 flue gas dry basis, of total powders, Ni, Mn and V, respectively:

(46) TABLE-US-00003 Total powders 2.1 Heavy metals Ni 0.011 Mn 0.004 V <0.001

(47) All the other metals having regulated emissions, each <0.001 mg/Nm.sup.3.

(48) The flow rate of the produced flue gases, on a dry basis, calculated by the flow-rate determinations and the composition analyses is of 940 Nm.sup.3/h, including a flow-rate of about 50 Nm.sup.3/h of air used for fluxing the analytical instruments and other items.

(49) The batch analyses of the powders in the other sampling points of the process for the characteristic components (V, Ni, Mg) have given the following average results expressed in mg/Nm.sup.3 flue gas dry basis.

(50) Point 1.

(51) Downstream of the combustor:

(52) Filter

(53) The powders present in the flue gases have been collected on a ceramic fiber filter (size cut 0.1 m), dried and weighed. In order to take into account the total volume of dried flue gas of 400 liters, as determined by the volumetric counter, the dry powder amount was multiplied by the factor 1/0.4=2.5, to obtain the corresponding value expressed as mg/Nm.sup.3 flue gas dry basis:

(54) TABLE-US-00004 Total powders 92
Condenser

(55) In the condenser the water present in combustion fumes is condensed to a dew point of 18 C. 263 ml of condensate are collected on a total dried flue gas volume of 400 liters. The amount of each heavy metal found in the condensate is multiplied by the coefficient 0.236/0.4=0.6575, to give the corresponding amount expressed as mg/Nm.sup.3 flue gas dry basis:

(56) TABLE-US-00005 Ni 0.97 Mg 0.01 V 0.02 Na <0.001 (the solution has pH 1.1)
Drexel 1+drexel 2 content

(57) In each of the two drexels the solution amount was of 30 ml. These solutions were pooled and the amount of each heavy metal, expressed as Mg/Nm.sup.3 flue gas dry basis, was calculated by multiplying the found analytical value by the coefficient 0.06/0.4=0.15. The following values are obtained:

(58) TABLE-US-00006 Ni 0.03 Mg 0.02 V .sup.<0.001.
Drexel 3

(59) The solution volume in drexel 3 was of 60 ml. By making the calculation as for the heavy metal content of Drexel 1+Drexel 2, the following amounts of heavy metals were found:

(60) TABLE-US-00007 Ni <0.001 Mg <0.001 V <0.001

(61) Part of the solid collected on the filter has been set aside, and the collected fractions pooled and the following analyses have been carried out: the determination of the absolute composition by ICP-OES, the determination of the phases compositions by XRD, the SEM visualization of the micro aggregates.

(62) At the XRD analysis neither V.sub.2O.sub.5 or NaVO.sub.3 phases are detected.

(63) At the SEM analysis the typical rods of V.sub.2O.sub.5 are absent.

(64) SO.sub.2 concentration, measured on spot samples of recycled flue gases, is comprised between 0.5 and 1.5 mg/Nm.sup.3 flue gas dry basis.

(65) The nitrous slags discharged from the settler and collected in bath, accumulated during the run 600 hours, amount to slightly less than 2 t. The analysis on samples drawn from different bags show that they contain variable amounts of silico-aluminates. The vanadium percentage is on the average 2% by weight.

(66) The inspection performed at the end of the run on the disassembled parts of the plant shows that there are no sticky powders and that the residues deposited in dead zones are in a negligible amount. The DCS data of the process parameters indicative of the heat exchange efficiency show that during the run efficiency has remained substantially constant, in line with the preceding observations.

(67) The collected powders have been analyzed at XRD and SEM. Neither V.sub.2O.sub.5 nor NaVO.sub.3 phases have been detected.

(68) Specific Characterizations of V.sub.2O.sub.5 of Example 1 Carried Out in a Gas Flow ReactorMethodology (1a)

(69) In a tubular reactor having a 50 mm diameter, made of high purity alumina, thermostatted in an oven at 1400 C., a gaseous current of CO.sub.2 and O.sub.2 is fed in a molar ratio 90:10 at a rate of 6.3 N liter/min.

(70) Through a Venturi type feeder, an injector disperses in the air flow an aerosol of a 0.1 M vanadium aqueous solution (a solution of VOSO.sub.4, vanadylsulphate) at the flow rate of 20 ml/h, for 8 hours.

(71) The gases outletting the tubular reactor are quenched with a metal finger cooled with water, passed on a glass fiber filter with a 0.1 m particle size cut and are then introduced into a flask (condenser) placed in a thermostated bath at 18 C., wherein the excess moisture is condensed. A battery of three drexel type vessels, connected in series, i.e. in succession, drexel 1, drexel 2, drexel 3, is joined by a tube to the outlet of the condenser. In each vessel (drexel) the gas bubbles through a steady aqueous phase, so that the contact liquid/gas takes place with an high efficiency. The first vessel (drexel 1) joined to the condenser, contains 30 ml of demineralized water, the second one (drexel 2) contains 60 ml of acidified water at pH 0.5 with nitric acid, the third and last one (drexel 3) contains 300 ml of acidified water at pH 0.5 with nitric acid and brought to an oxidation potential of 1.4 eV by addition of hydrogen peroxide. After 8 hours the aerosol feeding is stopped and the reactor is kept under a weak flow of dry air at the temperature of 1300 C. for 16 hours.

(72) The operation is repeated three times, as a whole 0.048 gmoles of vanadium, equal to about 2.5 g of vanadium, are fed.

(73) At the end of the test the tubular reactor is disassembled and broken into fragments. By XDR analysis of these fragments it is found that a penetration of V.sub.2O.sub.5, quantitatively (ICP) not very significant, has taken place in the reactor walls.

(74) The condenser is disassembled and the internal surface of the flask, after removal of the condensate from the bottom of the flask, is cleaned by means of a spatula and then with a washing acid aqueous solution. The washing solution is then pooled with the condensate liquid phase. The final volume of the condensate liquid phase is found to be 430 cc. This phase is subjected to ICP analysis. Likewise, the solution of drexel 1 is subjected to ICP analysis.

(75) The vanadium concentration in the analyzed samples is <0.01 g/liter.

(76) The liquid phase contained in drexel 2 is analyzed by ICP. The vanadium concentration is <0.01 g/liter.

(77) It is noted that the addition of drexels 1 and 2 has no impact on the vanadium mass balance, as the vanadium concentration is below the analytical sensitivity limit.

(78) In drexel 3 the vanadium concentration is of 1.6 mg/liter, equal to a collected vanadium amount of 0.016 gmoles.

(79) By multiplying the amount in g/liter by the volume of the solution (0.3 liter) it is found that the vanadium amount in the drexel is of 0.48 g, that is about 20% of the inletting vanadium.

(80) Second part of the methodology (1a)

(81) By operating under the same conditions as in the first part but with the following modifications: halving the flow rate of the vanadium solution and of the transport gas, by providing in drexel 3 a solution volume of 450 cc with a more efficient dispersion and a longer contact time, thus with a very high efficiency, the concentration of the recovered vanadium is 1.98 g/liter.

(82) The mass balance closing value is >70%, that results more acceptable, the corpuscolar nature of V.sub.2O.sub.5 generated in the reaction gas flow can be shown.

(83) Specific Characterization of V.sub.2O.sub.5 of Example 1 Carried Out in a Gas Flow ReactorMethodology (1b))

(84) In a tubular reactor of the same type as that used in methodology (1a), a gaseous current of CO.sub.2 and O.sub.2 is fed in a molar ratio 90:10, at a flow rate of 6.3 N liter/min and SO.sub.2 analytical grade directly from a bomb at a flow rate of 0.01 N liter/min.

(85) Through a Venturi type feeder, an injector disperses in the oxygen flow an aerosol of an aqueous solution obtained by mixing:

(86) 0.1 M VOSO.sub.4 (vanadylsulphate),

(87) 0.125 M MgSO.sub.4 (water-soluble magnesium sulphate),

(88) to have a molar ratio magnesium:vanadium 1:1.25, the flow rate being 20 ml/hour.

(89) The experiment lasts 8 hours.

(90) The gases outletting the tubular reactor are cooled as described in methodology (1a), then they are sent on a glass fiber filter with a 0.1 m particle size cut and in sequence, to the same equipments described in the methodology (1a) second part as regards the high efficiency drexels.

(91) The test is carried out with the same modalities and times described in the methodology (1a) for a total feeding of 0.048 gmoles of vanadium (about 2.5 g).

(92) At the end of the test the tubular reactor is disassembled and reduced into fragments. The surfaces of the fragments forming the internal wall of the tubular reactor, that during the test have been into contact with the vanadylsulphate aerosol, at the visual inspection look dark-coloured, with a more evident thickness especially in the part of the external tube outletting the oven. However the XDR analysis on these fragments does not show vanadium penetration. The analysis of the dark surface layer carried out by XRD, shows the presence only of the phases magnesium orthovanadate and magnesium pyrovanadate.

(93) The solid deposit on the filter is analyzed by XRD, combined with ICP analysis. The weight of the collected solid is 47 mg. Upon dissolution in an aqueous phase, by ICP it is found that the amount of vanadium in the collected solid is 15.6 mg.

(94) The condenser is disassembled, the internal surface carefully washed with an aqueous acid solution, that is then weighed and pooled with the condensate collected at the bottom of the condenser. The volume of the recovered solution is of 425 cc.

(95) The ICP analysis shows that magnesium and vanadium are present at molar ratios Mg/V intermediate ratios between the stoichiometry of magnesium orthovanadate and magnesium pyrovanadate. In the collected fraction 0.9 mg of vanadium are present.

(96) Likewise, the liquid phase contained in drexel 1 is subjected to ICP analysis. The vanadium concentration, measured by ICP, results below the sensitivity analytical limit (<0.01 g/liter).

(97) The same for the liquid phases of both drexel 2 and drexel 3, respectively.

(98) The closing mass balance for the vanadium is 96%, taking also into account of the material deposited on the alumina tubular reactor and on the cold finger.

(99) Specific Characterization V.sub.2O.sub.5 of Example 1: Carried Out in a Gas Flow ReactorMethodology (1c))

(100) In a tubular reactor of the same type as that used in methodology (1a), a gaseous current of CO.sub.2 and O.sub.2 is fed in a molar ratio 90:10, at a rate of 3.3 N liter/min, SO.sub.2 analytical grade is also fed from a bomb at a flow rate of 0.01 N liter/min.

(101) By a Venturi type feeder, an injector disperses in the oxygen flow an aerosol of an aqueous solution obtained by mixing:

(102) 0.2 M VOSO.sub.4 (vanadylsulphate),

(103) 0.3 M MgSO.sub.4 (water-soluble magnesium sulphate),

(104) 0.2 M Na.sub.2SO.sub.4 (sodium sulphate)

(105) wherein the molar ratio vanadium:magnesium is 1:1.5, the flow rate is 20 cc/hour.

(106) A solid fraction is also fed under the form of micronized powder, formed of:

(107) cordierite Mg.sub.2Al.sub.4Si.sub.5O.sub.18 at a flow rate 14.6 mg/Nm.sup.3,

(108) calcium silicate Ca.sub.2SiO.sub.4 at a flow rate 0.775 mg/Nm.sup.3.

(109) The test is carried out for 20 hours as a whole (two tests each of 10 hours). The flow rate is 200 Nlt/h.

(110) The gases outletting the tubular reactor are cooled as described in methodology (1a). Then they are sent to a glass fiber filter with a 0.1 m particle size cut, then in sequence, to the same equipments described in methodology (1a), second part. The test is carried out with the same modalities and times described in the preceding example. 0.080 gmoles of vanadium, corresponding to 4.075 g are as a whole fed.

(111) At the end of the test the tubular reactor is disassembled and reduced into fragments. The surfaces of the fragments forming the internal wall of the tubular reactor, that during the test have been into contact with the vanadylsulphate aerosol, at the visual inspection appear dark-coloured, with a more evident thickness especially in the part of the external tube outletting the oven. The XDR Analysis on these fragments does not show any vanadium penetration. The analysis of the dark surface layer carried out by XRD, evidences the presence of the phases magnesium orthovanadate and magnesium pyrovanadate only.

(112) The solid residue deposited at the bottom of the tubular reactor was collected. Its weight was 30.5 g. The analysis of the crystalline phases on this sample, carried out with XFRD, shows the presence of the phases NaAlSiO.sub.4, magnesium orthovanadate (Mg.sub.3V.sub.2O.sub.8), magnesium pyrovanadate (Mg.sub.2V.sub.2O.sub.7), calcium silicate (Ca.sub.2SiO.sub.4), cordierite (Mg.sub.2Al.sub.4Si.sub.5O.sub.18). The elementary analysis carried out by XRF (X-Ray Fluorescence), has given the following results:

(113) TABLE-US-00008 Metal g Sodium 3.4 Aluminium 9.15 Silicon 16.28 Magnesium 7.35 Vanadium 3.3 Calcium 7.70

(114) It is noted that the vanadium content in this sample corresponds to 81% of the fed vanadium.

(115) The glass fiber filter shows a solid deposit that is analyzed by XRD for the crystalline phases and by XRF for elementary analysis. The weight of the collected solid is 0.62 g. The crystalline phases have been identified to correspond to V.sub.2O.sub.5, SiO.sub.2 and Mg.sub.3V.sub.2O.sub.8, respectively.

(116) The elementary analysis carried out by XRF has given the following results (figures, as above, are given in grams):

(117) TABLE-US-00009 Silicon 0.10 Magnesium 0.0514 Vanadium 0.10

(118) Vanadium in this sample corresponds to 2.4% of the fed amount.

(119) The condenser is disassembled, the internal surface carefully washed, following the same procedure described in methodology (1a), the washing acid solution is joined to the condensate collected on the bottom of the condenser.

(120) The volume of the recovered solution is 425 cc.

(121) The ICP analysis shows that magnesium and vanadium are present at ratios Mg/V intermediate between the stoichiometry of magnesium orthovanadate and magnesium pyrovanadate.

(122) The collected fraction contains 0.9 mg of vanadium.

(123) Likewise, the liquid phase contained in drexel 1 is subjected to ICP analysis. The vanadium concentration, measured by ICP, results below the sensitivity limit (<0.01 g/liter).

(124) In the analyses of the content of the liquid phases respectively of drexel 2 and drexel 3, no vanadium is found below the sensitivity limit.

(125) The closing mass balance of the vanadium found in the cooling pipe and in the filter (in the drexels the vanadium content is negligible) is therefore of 83.4%.

Example 2 Comparative

(126) The same 5 MWt demonstration unit is operated under the same modalities reported in example 1, but the fuel is fuel oil Bunker-C containing 41 ppm of vanadium. The fuel feeding rate is 7.8 kg/min.

(127) The combustion is carried out without the addition of additives for a run of 120 hours.

(128) At the end, after cooling and disassembling the parts of the equipments, the ashes deposited on the walls of the heat recovery steam generator and in the elbows of the flue gas piping are collected.

(129) The ICP-OES analysis of these ashes shows that the vanadium amount is 2.6% by weight.

(130) By XRD it is also possible to identify in the ashes the phases of V.sub.2O.sub.5 and of NaVO.sub.3 and, by SEM, the typical rods of V.sub.2O.sub.5.

(131) At the bottom of the economizer (ECO), on the surfaces of the heat exchange tubes iron sulphates are present. The specimens of alloyed material (AISI 304H) are found already degraded on the surface by formation of nickel vanadates (nickel deriving from the alloyed material).