TECHN ICAL FI ELD

The present disclosure relates to a lance tube accordi ng to the preamble of clai m 1 . I n particular, but not exclusively, the disclosure relates to a lance tube i ntended for use i n a lime kiln furnace. The lance tube may also be i ntended for use in e.g . a pulverized coal injection lance for blast furnaces. BACKG ROU N D AN D PRIOR ART

I n the production of quicklime (calcium oxide) from limestone (calcium carbonate) , a lime kiln is used for the calcination process. The most com monly used lime kiln is the parallel flow regenerative (P FR) shaft kiln, consisting of two vertical shafts and a connecting crossover channel . While li mestone is calcined in a combustion zone in one of the shafts, the other shaft preheats the limestone. The hot combustion gases are transferred from the calcining shaft throug h the crossover channel to the non-calcini ng shaft, where they preheat the li mestone in an upper area of the shaft. The flow direction of the gases is reversed at regular intervals. This allows regenerative preheating of the li mestone to take place and this type of lime kiln is therefore energy efficient. Fuel is delivered into the calcining shaft via lances protruding into the combustion zone. Since the calcination process requires high temperatures of around 1 000°C, the lance has to be heat resistant.

The lance comprises a lance tube, which is subjected to severe high-temperature corrosion conditions due to the high temperature, corrosive elements i n the fuel and erosion from the limestone. The main corrosion mechanisms in lime kilns are oxidation, sulphidation and erosion-corrosion. The most severe corrosion com monly occurs about 50-1 00 cm from the lower end of the lance tube, delivering the fuel into the combustion zone. Corrosion may in this area lead to that the lance breaks and is thereby shortened. Since the shortening alters the kiln combustion parameters and reduces its efficiency, broken lances need to be replaced.

Common materials used for the lance tube i nclude chromiu m oxide formi ng steel alloys such as the ferritic stainless steel alloy ASTM 446, and the austenitic stainless steel alloys UNS S3531 5, UNS S3081 5, ASTM 31 0 and ASTM 31 6. The life time of a lance tube is typically about six months to two years.

Attempts have previously been made to use alu mina forming alloys, such as e.g . iron chromium alumi nium ( FeCrAI) , in lance tube applications. Such alloys form a protective alumina scale and are known to be very corrosion resistant at high temperatures. However, in addition to being relatively expensive, FeCrAI alloys are brittle at low temperature and are also difficult to weld. SUMMARY

I n view of the above-mentioned problems, it is desirable to provide a lance tube which is in at least some aspect improved with respect to known lance tubes. I n particular, it is desirable to provide a lance tube for use in lime kilns or for use i n a lime furnace burner, i n blast furnace coal powder injection and i n sootblower elements, that has an i mproved life time in comparison with known lance tubes. This is achieved by the initially defined lance tube, which comprises:

- a double-layered end portion having an annular outer layer of a hig h temperature corrosion resistant first alloy and an annular inner layer of a second alloy, wherein the annular inner layer and the annular outer layer are mechanically interlocked, and wherein a metallic bond has been formed between the annular inner layer and the annular outer layer by means of hot working , and

- a mono-layered main portion of the second alloy.

At the double-layered end portion, intended to form the lower end of the lance tube, the annular outer layer of a hig h temperature corrosion resistant alloy provides increased corrosion resistance at the crucial portion of the lance tube. I n a lime kiln, this portion will be located at the bottom of the kil n, where the hig hest temperatures will be experienced. The improved corrosion resistance is achieved without having to compromise the mechanical properties and high temperature wear resistance of the lance tube. The metallic bond between the annular inner and outer layers ensures that there is no air gap between the layers which may lead to reduced thermal conductivity. Thus, a good thermal conductivity of the lance tube is achieved even though two different alloys (alloys having different compositions) are used. The metallic bond between the layers should be formed in a main portion of an interface between the annular inner and outer layers, but there may be smaller portions of the interface in which no metallic bond is present. The metallic bond is formed by means of hot worki ng such as hot extrusion , hot drawing , hot rolling or hot pierci ng , or other suitable techniques.

The mechanical interlocking is provided before hot working to achieve the metallic bond. The mechanical i nterlock will form a seal preventi ng oxygen from entering between the layers during the hot worki ng process, and it will additionally keep the annular inner and outer layers together duri ng hot working , i .e. prevent them from sliding . The mechanical interlock thereby makes it possible to achieve the proposed lance tube without having to weld a base component and an outer component together before hot working . Thus, the lance tube can be made from two alloys that are normally difficult to joi n by means of welding . Furthermore, the combination of a mechanical interlock and a metallic bond between the layers is beneficial for the ability of the lance tube to withstand high forces.

According to one embodiment, the hot working as described herei nabove or hereinafter is hot extrusion .

According to one embodiment, the annular inner layer and the annular outer layer are mechanically interlocked by means of a helically extending thread formed i n an interface between the annular inner layer and the annular outer layer. The helically extending thread forms an efficient i nterlock and also increases the interfacial area, which will thereby contribute to an improved distribution of forces applied to the lance tube i n comparison with a lance tube without such a helically extendi ng thread. Thus, the lance tube will be able to withstand higher load in the interface between the layers.

According to one embodiment, the mono-layered main portion extends along a major part of the lance tube as measured along the longitudi nal axis. The mono-layered portion may extend along more than half of the length of the lance tube, or along more than 75 % of the length of the lance tube. The double-layered portion is thus relatively short and only covers the crucial part of the lance tube, where additional high temperature corrosion resistance is needed. If an expensive first alloy is used for the outer layer, this reduces the total cost of the lance tube without compromising its life time. The double-layered portion may typically extend along at least 70-1 50 cm of the lance tube, intended to form the lower part of the tube from which fuel is delivered. The length of the lance tube as measured in the axial direction may be several meters.

According to one embodiment, the second alloy is selected from a stainless steel alloy or a carbon steel . Stainless steel alloys and carbon steels that have desired mechanical strength and high temperature wear resistance are suitable choices for the mai n portion and the inner layer of the lance tube. An example of a suitable carbon steel is a carbon steel according to standard DI N 1 71 35A, this carbon steel comprises from 0.1 to 0.3 C and 0.1 to 2.0 Mn and balance Fe and unavoidable i mpurities. According to one embodiment, the second alloy is selected from a ferritic stainless steel alloy or an austenitic stainless steel alloy. Suitable alloys but not li mited to are e.g . the ferritic stainless steel alloy ASTM 446- 1 , and the austenitic stainless steel alloys UNS S3531 5, U NS S3081 5, U NS N0881 0/N0881 1 , ASTM 31 0, and ASTM 31 6/31 6H . These alloys will provide both the desired mechanical properties and sufficient hig h temperature corrosion resistance and wear resistance for the main portion of the lance tube and are suitable choices in e.g . lime kiln applications.

According to one embodiment, the first alloy is an alumi na forming alloy. Alumina forming alloys form a protective alumina scale on the outer surface of the annular outer layer which will provide excellent high temperature corrosion resistance. Suitable alu mina formi ng alloys include iron chromium aluminium (FeCrAI) alloys as well as other alumina forming alloys.

According to one embodiment, the alu mina forming alloy is an iron chromium aluminium alloy. FeCrAI alloys, such as FeCrAI alloys sold under the trademark Kanthal® APM and Kanthal® APMT, have suitable hig h temperature corrosion resistance for use as the outer layer. I n order to achieve improved creep strength , it is possible to use an oxide dispersion-strengthened alloy produced by means of powder metallurgy. However, the alloy can also be conventionally manufactured using melting and casti ng techniques.

According to one embodi ment, the first alloy comprises :

9-25 wt. % Cr, 2.5-8 wt. % Al,

the balance being Fe and normally occurring impurities, and optionally other intentionally added alloying elements. In one embodiment, the first alloy comprises 20-25 wt. % Cr and 5-7 wt. % Al, the balance being Fe and normally occurring impurities. In another embodiment, the first alloy comprises 20-25 wt. % Cr, 5- 7 wt. % Al and 1 to 4 Mo the balance being Fe and normally occurring impurities. According to one embodiment, the first alloy is a stainless steel alloy comprising cerium, such as a chromium oxide forming austenitic stainless steel alloy comprising cerium. The addition of cerium stabilises the chromium oxide at high temperatures and thereby improves the high temperature corrosion properties as well as provides a good structural stability at high temperatures. Suitable alloys are e.g. UNS S30815 and UNS S35315, which alloys comprise C 0.04 to 0.10, Mn 1 to 2, Cr 20 to 26, Ni 10 to 12 or 34 to 36, N 0.12 to 0.20, Ce 0.03 to 0.08, balance Fe and unavoidable impurities.

According to one embodiment, the annular outer layer has a thickness within the interval 5-50 % of a total wall thickness. The thickness should be sufficient to achieve the desired high temperature corrosion resistance without risking that the annular outer layer cracks or is otherwise discontinued.

According to one embodiment, the annular outer layer has a thickness within the interval 10-40 % of a total wall thickness, such as to ensure sufficient corrosion resistance at a reasonable cost.

According to one embodiment, the lance tube has a total wall thickness within the interval 3-20 m m . The wall thickness depends on e.g . the dimension of the lance tube. For example, for outer diameters of approximately 60 mm , 50 mm , 40 m m , 30 mm and 1 2 mm , wall thicknesses of approxi mately 1 0 mm , 9 mm , 6 mm and 3 mm , respectively, may be suitable.

According to one embodiment, an outer diameter of the lance tube as measured at each of the mono-layered main portion and the double-layered end portion is identical or essentially identical . According to one embodiment, an i nner diameter of the lance tube as measured at each of the mono-layered main portion and the double-layered end portion is identical or essentially identical . This is beneficial for the flow characteristics of the lance tube. The disclosure also relates to use of the proposed lance tube as a lance tube in a lime kiln . The proposed lance tube may also be used i n other applications requiring hig h temperature corrosion resistance i n combination with mechanical strength , such as i n a lime furnace burner, in blast furnace coal powder i njection and in sootblower elements.

Other advantageous features as well as advantages of the proposed lance tube and method of manufacturing will appear from the following description. DEFI N ITIONS

A lance tube is herein to be understood as a tube having a relatively small diameter compared to its length , which is intended for use in lime kilns or for use in a lime furnace burner, in blast furnace coal powder injection and in sootblower elements. The lance tube is used for fuel transfer from a first end of the lance tube to a second end of the lance tube, wherein the first end is connected to a fuel supply system and the second end is open. The lance tube is not pressurized .

BRI EF DESCRI PTION OF TH E DRAWI NGS

Embodiments of the proposed lance tube and a method of manufacturing , not to be interpreted as limiting , will in the following be described with reference to the appended drawings, in which

Fig . 1 schematically shows a perspective view of a lance tube accordi ng to an embodiment,

Fig . 2 schematically shows a lance tube according to another embodiment i n cross section ,

Fig . 3a-c schematically shows a base component and an outer component for manufacturing a lance tube, Fig . 4 schematically shows a longitudinal cross section of parts of a base component and an inner component for manufacturing a lance tube,

Fig . 5 schematically shows a longitudinal cross section of parts of a work piece for manufacturing a lance tube, and Fig . 6 shows a longitudi nal cross sectional picture of an interface withi n a lance tube.

DETAI LED DESC RI PTION

Fig . 1 schematically, and not to scale, shows a lance tube 1 according to an embodi ment of the present disclosure. The lance tube has a relatively short double-layered end portion 2 and a mono-layered main portion 3. The double-layered end portion 2 has an annular outer layer 4 of a first alloy and an annular i nner layer 5 of a second alloy. The mono-layered mai n portion 3 is entirely formed of the second alloy that the inner layer 5 of the double-layered end portion 2 is formed of .

Fig . 2 schematically shows a straig ht lance tube 1 i n a cross section taken along a longitudinal axis A of the lance tube. As can be seen in the magnification of the marked area, a helically extending thread 6 extends in an interface between the annular outer layer 4 and the annular inner layer 5. The helically extending thread 6 serves to mechanically interlock the two layers 4, 5. However, the layers 4, 5 are also bound by a metallic bond formed in the interface by means of hot worki ng , e.g . hot extrusion.

A lance tube accordi ng to the present disclosure may be manufactured from components shown in figs. 3a-c. The components i nclude a base component 301 of the second alloy, which is to form the inner layer 5 of the lance tube 1 , and an outer component 401 of the first alloy, which is to form the outer layer 4 of the lance tube 1 . The base component 301 is a tube of circular cross section, having a central through-hole extendi ng along a longitudinal axis A. An externally threaded section 302 is provided , havi ng a helical thread 306 (see fig . 3b) formed in an outer peripheral surface of an end portion of the base component 301 . The shown base component 301 has a non-threaded section 303 adjacent the threaded section 302. An inner diameter d of the base component is constant or essentially constant along the longitudinal axis, but an outer diameter D 1 of the non-threaded section 303 is larger than an outer diameter D2 of the threaded section 302.

The outer component 401 is also a tube of circular cross section , having a central throug h-hole extending along the longitudi nal axis A. I n the shown embodiment, the outer component 401 has a length in the longitudinal direction corresponding to a length of the threaded section 302 of the base component 301 . The outer component 401 has an internally threaded section 402, in the shown embodiment extending along the entire length of the outer component 401 . I n other words, a helical thread 406 (see fig . 3c) is formed i n an inner peripheral surface of the outer component 401 . The outer component 401 is thereby configured for threaded engagement with the externally threaded section 302 of the base component 301 . An outer diameter D3 of the outer component 401 is equal to or essentially equal to the outer diameter D 1 of the non-threaded section 303 of the base component 301 , while an inner diameter d2 of the outer component 401 matches the outer diameter D2 of the threaded section 302 of the base component 301 . A tubular work piece is formed by mounti ng the outer component 401 around the base component 301 such that the internally threaded section 402 of the outer component 401 is in engagement with the externally threaded section 302 of the base component 301 , i .e. by threadi ng the outer component 401 onto the threaded end portion of the base component 301 . A mechanical interlock is thereby formed between the threaded sections 302, 402.

The work piece is thereafter hot worked, e.g . by means of hot extrusion. Duri ng hot working , such as hot extrusion, a metallic bond is formed between the threaded sections 302, 402 while the mechanical interlock is maintained. An outer diameter of the work piece is also reduced and the length is increased. Straightening and/or pickli ng may be carried out before the resulti ng lance tube 1 is cut i nto its final length and, if needed, formed to a desired shape. The components 301 , 401 shown in fig . 3a are adapted for hot extrusion by pushi ng the work piece through an extrusion die with a leading end first, wherein the leading end is the end at which the outer component 401 is mounted. A transition surface 308 between the externally threaded section 302 of the base component 301 and the non-threaded section 303 is smooth , without sharp edges. The transition surface 308 is shown i n more detail i n fig . 3b, showing a magnification of the encircled area B from fig . 3a. The transition surface is in cross section shaped as an inverted S with a concave portion 304 closest to the threaded section 302, and a convex portion 305 closest to the non-threaded section 303. The outer component 401 has an end surface 408 with a corresponding S-shape with a convex portion 404 close to the i nternal thread 406, and a concave portion 405 close to an outer peripheral surface 407 of the outer component 401 as shown in fig . 3c showing a mag nification of the encircled area C from fig . 3a. The concave portion 405 of the end surface 408 will thereby overlap with the convex portion 305 of the transition surface 308, which prevents separation and penetration of oxygen during the extrusion process.

Another option is to let the leadi ng end in the extrusion process be the end on which no outer component is mou nted . I n this case, shown in fig . 4, the base component 301 is formed with a C- shaped concave transition surface 308, such that it floats over a rounded annular end surface 408 of the outer component 401 during extrusion and forms a seal . An outer peripheral surface 307 of the base component 301 thus overlaps the outer peripheral surface 407 of the outer component 401 when the components 301 , 401 are mounted to form the work piece.

Fig . 5 shows a cross sectional view of parts of a work piece 501 adapted for hot extrusion by pushing the work piece 501 through an extrusion die with a leadi ng end 502 first, wherei n the leading end 502 is the end at which the outer component 401 is mounted. The ends on which the outer components 401 are mounted have been machined to form rounded end surfaces 503. The desig n of the transition surfaces 308, 408 of the base component 301 and the outer component 401 , respectively, differs in this embodiment somewhat from the design shown i n figs. 3a-c. The transition surface 308 of the base component 301 includes, as seen in the cross section, a first straight portion 309 perpendicular to the longitudinal axis A, and a second straight portion 31 0 which is inclined at an angle a of 30° with respect to the longitudinal axis A. A curved surface connects the two straight portions 309, 31 0. The angle a may of course be varied.

The transition surface 408 of the outer component 401 is formed to engage and overlap with the transition surface 308, such that a seal is formed . Of a total wall thickness t of the outer component, the first straight portion 309 extends over a thickness h . Example

I n a production trial , ten lance tubes according to the embodiment shown in fig . 1 were manufactured . Ten outer components of a first alloy and ten base components of a second alloy were formed. The first alloy was in this case an iron chromium alumi nium (FeCrAI) alloy known under the trademark Kanthal® APM. The composition of the first alloy as measured i n percent by weight (wt.%) is disclosed in Table I .

Table I The second alloy was a ferritic stainless steel accordi ng to ASTM 446- 1 havi ng a composition in wt.% as disclosed in Table I I . c Si Mn P S Cr N Fe

<0.20 0.5 0.8 <0.030 <0.015 26.5 0.2 balance

Table II

Each base component had a total length of 400 mm, an outer diameter D1 of 164 mm and an inner diameter d of 41 mm. An externally threaded section having a length of 95 mm and an outer diameter D2 of 154 mm was formed by cutting machining. The outer components each had a length of 95 mm and an inner diameter d2 of 154 mm and were provided with an internal helical thread. The components had the transitional design shown in fig. 5. A wall thickness t of the outer component was 5 mm and the thickness h was 1.8 mm. The helical thread had a pitch of 2 mm.

The components were degreased using ethanol. The outer components 401 were thereafter threaded onto the base components 301 to form work pieces such as shown fig.5.

The work pieces were thereafter heated to 900°C and hot extruded at temperatures shown in table III. The work pieces were extruded with the end on which the outer component was mounted as the leading end.

Work piece Extrusion temp. (°C)

S1 1120

S2 1120

S3 1120

S4 1120

S5 1120 S6 1 090

S7 1 090

S8 1 070

S9 1 070

S1 0 1 050

Table III

After hot extrusion, the formed tubes were straightened and blasted using steel sand .

The lengths of the double-layered portions of the manufactured lance tubes were fou nd to be between 70 cm and 1 20 cm . A thickness of the outer layer was measured in test samples using optical and electron microscopy and was fou nd to be between 600-900 pm .

Using energy-dispersive X-ray spectroscopy, it was also investigated whether a protective aluminium oxide scale had been formed on the outer layer of the double-layered portion duri ng the heat treatment and whether a metallic bond had been formed between the i nner and outer layers. It was found that an alumi nium oxide scale had been formed on the surface of the outer layer and that aluminiu m nitride precipitates had formed in the outer layer, indicati ng nitrogen diffusion from the inner layer of the second alloy ASTM 446- 1 i nto the outer layer of the first alloy sold u nder the trademark Kanthal® APM , which in turn indicates formation of a metallic bond. Fig . 6 shows a cross sectional picture of a part of the interface between the inner layer 5 and the outer layer 4 of the double- layered portion of a manufactured lance tube accordi ng to an embodiment. The picture is taken at a foremost portion of the lance tube, correspondi ng to the leading end of the work piece. A helically extending thread 6 is clearly seen. Thus, while a metallic bond has been formed in the interface, the inner and outer layers are still also mechanically bound together. The di mensions of the components used may of course be varied depending on the desired dimensions of the fi nal lance tube, as well as the alloys used and the parameters used during hot worki ng , e.g . hot extrusion. Various other processing steps may also be included, such as pre-heati ng and cold pilgering . The desig n of the base component and the outer component can be varied dependi ng on the requirements on the final lance tube.

The proposed lance tube may be shaped to suit the requirements of the lime kil n or other application in which it is to be used. The desig n of the lance tube may be varied, for example by letting all or part of the double-layered portion have an outer diameter which is different than , for example smaller than , the outer diameter of the main portion. The double-layered end portion of the lance tube may also include a portion entirely made of the first alloy that the outer layer is made of, so that the high temperature corrosion resistant first alloy covers the end of the lance tube.

The proposed lance tube is not li mited to the embodiments described above, but many possibilities to modifications thereof would be apparent to a person with skill in the art without departing from the scope of the appended claims.