Thermal Spray in CFBC Boiler

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In flame spraying, the heat is generated by gas combustion. The material, powder orwire is transported by the combusted gases or in some cases by compressed air. Flamespraying is not as fast as arc-spraying but has the advantage of being more portablethan arc spray equipment.Plasma Spraying is more cost intensive than both arc spraying and flame spraying dueto higher investment material and energy costs. The powder material is melted in

a plasma and transported by a protective gas onto the substrate. For some materials,plasma spraying produces a denser coating than flame or arc spraying.HVOF Spraying is a relatively new innovation. This powder coating process producescoatings with low porosity and high bonding strength. It is still an expensive process but

will in the future be more important due to reduced energy costs and increased capacity.Spray and fuse is compared to the above mentioned an hot spraying method. Amaterial with a low melting point 1100 C is sprayed and afterwards heated up to the

melting point. This process gives a completely dense coating with a metallurgical bond tothe substrate.3. Thermal spraying A solution for wear protection in FBC-boilersThermal spraying has many advantages compared to other wear protection solution inFBC-boilers

The possibility to use an optimized material composition in thermal spraying ,you canchoose almost any type of metallic and / or ceramic material adapted to the specific

problem. By using material in powder or cored wire form, you can mix. And composean alloy with the right composition and properties.There are several different processes. The most applicable can be chosen according tothe required quality, surface area and accessibility. The wear-problem varies in the

FBC-boilers. In areas where you have purely erosion, you chosen a method with lessrequirement of porosity and high bonding strength.Non weldable coating such as ceramics can be applied. The thermal spraying ofceramics is a cold process. It means that the substrate and there is a little heat

transfer to the substrate( except from spray and phase transformation or distortion.Coating can be applied to substrates that cannot be welded because of size or the

composition.The high coating capacity up to 15 kg/hour gives the possibility to coat large areas ina short time.The thickness of the thermally sprayed coating is relatively thin, approx 0,6mm inFBC-boilers. This avoids the problem of a reduced heat conductivity.

In all thermally sprayed coating there is some porosity with the exception of the sprayand fuse. The levels vary with material and processes selection and may be in between0.1 to 15%.Flame sprayed coating are generally more porous processes produce coatingwith a minimum of porosity.All coating sprayed in air include certain amount of oxide. These amounts vary withdifferent metals and spraying conditions. The bond-strength varies with material andprocesses but is generally between 15N/mm2 to 90 N/mm2 for the cold-spraying

method fig. Shows different coating microstructures.4.Wear problem in FBC-boilersWastage of alloys is a serious problem in many industrial processes but is perhaps moreaccentuated in FBC-boilers

The erosion-corrosion mechanisms and rated depend on factors like temp. , velocity,impact, energy and angle practical and alloy properties.According to M.M. Stock et al(1), the wear increases with increasing temp. up to a

critical temp. and then decreases(fig.3)The erosion rate increases also with increasing particle velocity. The peak of the curve ismoved to a higher temp. with increased alloy oxidation resistance.Erosion is the dominating at low temperature(fig.4)With increasing temperature, the formation of surface oxides increases exponentially andat a specific temp. the wastage of oxide scale on the surface is greater than the loss of

metal, the erosion-corrosion dominated region


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At a certain temp., the oxides scale is to thick and cohesive and is not removed from themetal. This is the transition to corrosion dominated behaviors.In a FBC-boiler the following areas are exposed to wear (fig.5)The furnaces floor is normally covered with air-nozzle which serves for the distribution ofthe air. Thease nozzle are normally exposed to heavy wear, depending on air-pressure,construction, fluidized-bed material, fuel and temp.(fig.6)

The water wall tubes in the lower part of the combustion chamber are up to a certain ht.Protected with refractory or ceramic plates. Above this area the tubes are exposed toheavy wear which in some cases can be more than 1,0 mm per firing seasonThe wear is generally higher towards the corners of the boilers

In the upper part of the combustion chamber close to the cyclone inlet(s), the wear ofthe material is increased due to a higher particle velocity.The super heaters and economizers are exposed to wear mainly depending on

construction, temp. and fuel.5. Thermal sprayed coating for wear-protectionSeveral types of thermal sprayed coating have been developed and tested in FBC-boilersThe complex wear-phenomena has forced the supplied of coating to develop systems ofmaterial of materials and equipment especially for FBC-boilers

Coating of large areas require an economical solution with an increased sprayingcapacity.

The arc-spray has the highest spraying capacity with up to 15kg/hour and relatively lowequipment investment costsThe material used for arc-spraying is normally solid wire. This type of material limits thetechnical properties of the coating. By using a cored wire, a wire filled with filled power

(fig.7) you can add the following advantages.flexible compositionlower porosity levelsmoother coating surface

The core makes it possible to produce wires very specific composition improving thecorrosion and /or erosion resistance for specific service condition

The melting characteristics of the cored wire are different from solid wire and the coatingis denser with lower porosity and smoother surface. The bond strength can be improvedby alloying element which create an exothermic reaction5.1 Coating of water wallsThe wear of water wall tubes above the refractory is a severe problem which occurs in

most of the FBC-boilersThe wear-rate is normally higher in the CFB (Circulating fluidized-bed) boilers. This isdue to a higher velocity. Three examples are given where the cored wire Eutronic Arc502( technical data table 1) has been arc-sprayed. The power core in this wire producesa coating containing hard particles (CrC, AND tIc) thusImproving the erosion resistances. The metallic matrix is an FeCr-alloy and has sufficientcorrosion resistances for this application

CFB-Boiler SwedenThis boiler is a hot water boiler with a temp. inside the waterfalls tubes of 200 cAnd a pressure of 12-14 bars. The capacity is 50MW and it is mainly burning wood-chipsand coal at a bed velocity of 5m/s.

The boiler started in 1986 and in 1987 was flame sprayed with NiAl as bond coat and13%Cr steel as a top coat to a height of 500mm above the refractory.In 1988 it was decided to further increases the height of the coating and repair the told

coating with an arc sprayed FeCrC-material. The old coating had partly eroded.In 1989 some test areas were plasma sprayed with a FeCrAl+CrC power.During the inspection in 1990 it was decided to replace all previous coated areas withEutronic Arc 502.The initial coating of 13% Cr-steel and the FeCrAl+CrC-powder hadcompletely eroded and the FeCrC coating was partly eroded.Eutronic Arc 502 was arc-sprayed to a height of approx. 1000mm and with a thickness of

t=0,6 mm(fig.8)The inspection in 1991 and 1992 proved that the 502-coating was intact.


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In 1992 the coated was increased with 502 in each corner up to the cyclone inlets. Atthe inspection in summer 1994, all the sprayed areas were intact.The wear rate of the tubes without coating is estimated to be approx. 0,5 mm/year.With the Eutronic Arc 502-coating , the wear-rate is reduced to a maximum of 0,1mm/yearCFB-Boiler Finland

This CFB boiler is a 300MW boiler burning coat. It has originally been stud-and overlay-welded above the refractory with an Fe alloy up to a height of 200 mm. Every year theseareas had to be repaired by further welding.In the year 1990 it was decided to arc-spray with Eutronic Arc 502,coating thickness

t=0.6mm above the refractory to a height of 500 mm, covering that areas withpreviously were overlay-welded(fig-9)The inspection in 1992 showed that the coating is still intact, both above the refractory

and at the cyclone inlet.There is some wear at the between coated and uncoated area above the refractory. Thedown falling stream of particles have an effect on the uncoated tubes.This problem was solved by increasing the height of the coating with approx. 1m to 1,5m

In 1991 the coating was inspected and the result were positive. The customer thendecided to coat the areas around the cyclone inlet on the top of the combustion chamber

with Eutronic A RC 502. The thickness was t=0,6mm.This facility has two identical BFB-boilers, each of 15 MW producing hot water. They arcfired with a mixture of peat and wood chips. The temp. of the water wall is 188 c and thepressure 12 bars.From 1983, when the boiler was put into service until 1989, the tube-wall thickness hasbeen reduced from nominal t=4,0 mm to t=2,3 mm which means a wear-rate of max.0,3 mm/year.This is measured close to the cast-iron block. The wear rate is lower higher in the boiler

and the nominal tube thickness is found more than 300 mm above the blocks.In 1989 it was decided to arc-spray the front wall with Eutronic Arc 502 up to a height of

350mm over the cast-iron block and with a thickness of t=0,6 mm( fig. 10)In 1990, the inspection showed no wear of the coating and additionally both side wallswere coated. In 1991 the second BFB-unit was coated with Eutronic Arc 502 with thesame procedure as the first one.Now in 1994, these two boilers are still running without failure of the coating.

5.2Alloys for FBC-boilersThe alloy Eutronic Arc 502 mentioned in the prior examples, has limited corrosion/limited oxidation properties and should not be used above 450 c. Its also limited to fuellike coal, peat and wood and should not be used for waste-fired boilers. The alternativefor temp. which exceed 400 c is to use a more oxidation resistant material.The Eutronic Arc 502 is a cored wire (table) with increased wear properties at highertemperature. The Fe CrAl-matrix provide excellent oxidation-resistances and the hard

particles in the coating improves the hot erosion resistance.Many FBC-boilers are built for waste incineration. The complex composition of wastemakes it once of the most corrosive flues for boilers. Chlorine containing waste materialfrom very corrosive compounds such as hydrochloric acid and alkali chloride and which

together with the duct forming deposits, create heavy corrosion.Eutectic Cast Olin has specially focused on the spray and fuse solution for wasteincineration. This gives the following advantages.

no porosity in the coatingmetallurgical bondsmooth surface

No porosity reduces the penetration of corrosive compounds thought the coating andimproves the corrosion resistance.The metallurgical bond between coating and substrate improves the bond strength and

reduces the risk of spelling.


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A smooth surface prevents and reduces the risk of reaction between the corrosivedeposits and the coating.The spray and fuse process is preferably used in the workshop on for examplesuperheated tubes or economizer tubes( fig. 11)On site, work can be done on smaller areas of water wall sections( fig. 12)The fusing process( heating up to 1100 c) creates some thermal deformation and large

areas should not be coated.E+C has developed different types of fusible powders for wear and corrosion resistantcoatings.The powder 17535 is a self-fluxing high Cr-Ni-base4 alloy. It has an increased corrosion

resistance to chlorine attack and high temperature are waste incineration boilers. Otheralloy are 17093( Co-base) and 12525(Ni-base). A comparison is given in table 3.6. Thermal spraying of boilers compared to other methods

The normal procedure for a thermal spraying operation includes surface preparation andcoating application.Surface preparation is normally a blasting operation. This is to get rid of all the surfaceoxides and to achieve a metallic clean surface with the required roughness. Thisoperation takes normally 20 minutes per blasting unit.

The coating application has to be done within 4 hour after the blasting, to avoid anyoxide formation and reduced coating bond strength. To achieve a coating thickness of

t=0,6 mm, the time needed for arc-spraying is 1 hour per m2 and unit.For Spray and fuse, the surface is prepared by blasting and then the coating is applied. This is according to the normal procedure for thermal spraying but fusion process fusesthe coating. Depending on the method, this can double the operation time.

The total operation time for thermal spraying in general is much shorter compared to theother protective options which are:

overlay weldingstud welded with refractory on top

tube retrofitThe tube retrofit is generally 5-8 times more expensive due to the very long installation

time. The overlay welding and stud welding are more time consuming and less flexible.The heat time transfer is reduced due to thicker coating and less conductivity forrefractory materials.

7. Conclusion1. There has been a positive experience with thermal spraying in FBC-boilers.2. Arc-spraying with cored wire can be used to solve the wear problem ofwater wall tubes in the combustion chamber. The wear-rate is reduced 4-5 times

3. Waste-incinerations by FBC-technique is getting more important in thefuture. The wear and corrosion are accelerated by chlorine compound are for thisreason the spray and fuse thermal spraying technique is preferred because ofits improved corrosion resistance.

References

(1) M.M.Stack, F.H. Stott and G.C. Wood Review of mechanisms of erosion-corrosion ofalloy at elevated temperature wear, 162-164(1993) 706-712Table-1Technical data for Eutronic Arc 502Cored wire type : Fe Cr C TiBond strength : 35 N/MM2Hardness : HV 10 550Erosion resistance up to 450 CTable 2Technical data for Eutronic Arc 502Cored wire type : Fe Cr C TiBond strength : 35 N/MM2

Hardness : HV 10 550Erosion and corrosion protection up to 900 C


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TABLE-3

TYPE Hardness HV

30

Alloy in al Corrosion

Resistance

Wear

Resistance

17535 420 CrNi-base Very good medium

170934 550 Co-base Good in mostcases

good

12525 680 Ni-base Very good highest

Fig.1. Principle of arc spraying


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Fig.7 Cross section of cored wires

Arc Spraying of Eutronic Arc 502


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Coating of Waterwall. Height 350mm


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"Spray and fuse" of superheater tubes

"Spray and Fuse" of waterwalls on site.

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Thermal Spray in CFBC Boiler