HEAT EXCHANGER HAVING ENHANCED CORROSION RESISTANCE

Abstract

A heat exchanger for heating a fluid in an incineration plant, comprising at least one heat exchanger component wherein the side in contact with the flue gas has an oxide layer comprising an -Al.sub.2O.sub.3 which protects the heat exchanger component against corrosion caused by corrosive compounds entrained or comprised by the flue gas.

Claims

1. A method of heat transfer from a flue gas in an incineration plant to a fluid, the method comprising the steps of: providing at least one heat exchanger component comprising an inner tube and a cladding on an external surface of the inner tube, the cladding being fully made from an aluminum alloy precursor material; leading the fluid through the at least one heat exchanger component, the fluid being in contact with an internal surface of the inner tube; leading the flue gas resulting from incineration of a waste in the incineration plant into an atmosphere around the at least one heat exchanger component, the flue gas being in contact with an external surface of the cladding, the flue gas having a predetermined temperature of the flue gas, a predetermined percentage of oxygen in the atmosphere and a predetermined partial pressure of the oxygen; generating a protective oxide layer surrounding the cladding during operation of the incineration plant upon oxidation of the external surface of the cladding by being exposed to the oxygen at the predetermined temperature and the predetermined pressure, the protective oxide layer protecting the cladding from corrosive components of the flue gas, the protective oxide layer being a scale comprising alpha-Al.sub.2O.sub.3; continuously regenerating the protective oxide layer as the protective oxide layer is being worn by corrosion; and heating the fluid by the flue gas.

2. The method of heat transfer according to claim 1, wherein the fluid is steam and the heat exchanger component is a superheater for superheating the steam.

3. The method of heat transfer according to claim 1, wherein the precursor material comprises an alloy comprising at least 4 wt. % aluminium.

4. The method of heat transfer according to claim 1, wherein the corrosive compounds comprises chlorine,

5. The method of heat transfer according to claim 1, wherein the at least one heat exchanger component comprises a plurality of said heat exchanger components.

6. The method of heat transfer according to claim 2, wherein a temperature of the at least one heat exchanger component is 30-50 C. higher than a temperature of the steam.

7. The method of heat transfer according to claim 1, wherein the temperature of the flue gas is in the range of 1100-1200 C.

8. The method of heat transfer according to claim 1, wherein the partial pressure of oxygen is below 10.sup.8 atm.

9. The method of heat transfer according to claim 1, wherein the scale formed during the oxidation has a thickness of 0.1 m to 2 m.

10. The method of heat transfer according to claim 1, wherein the scale is even and complete.

11. The method of heat transfer according to claim 1, wherein the inner tube is made from a material which does not form a scale comprising alpha-Al.sub.2O.sub.3 upon oxidation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The invention and its many advantages will be described in more detail below with reference to the accompanying schematic drawings, which for the purpose of illustration show some non-limiting embodiments, and in which

[0047] FIG. 1 shows a partial overview of a waste to energy incineration plant provided with a heat exchanger according to the first aspect of the present invention,

[0048] FIG. 2 shows, in side view, heat exchanger components, in the form of superheater tubes, of the heat exchanger according to the first aspect of the present invention, and

[0049] FIGS. 3A, 3B, and 3C show, in partial cutaway side view, first second and third embodiments of heat exchanger components, in the form of superheater tubes, of the first second and third embodiments of the heat exchanger according to the first aspect of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050] In the below description, one or more subscript roman numerals added to a reference number indicates that the element referred to is a further one of the element designated the un-subscripted reference number.

[0051] Further, A superscript roman numeral added to a reference number indicates that the element referred to has the same or similar function as the element designated the un-superscripted reference number, however, differing in structure.

[0052] When further embodiments of the invention are shown in the figures, the elements which are new, in relation to earlier shown embodiments, have new reference numbers, while elements previously shown are referenced as stated above. Elements which are identical in the different embodiments have been given the same reference numerals and no further explanations of these elements will be given.

[0053] FIG. 1 shows a partial overview of a waste to energy incineration plant 2. Waste 4 to be incinerated is fed into the incineration plant by a conveyor 6 onto a grate 8 on which the waste 4 is burnt. Flue gas resulting from the incineration of the waste 4 on the grate 8 rises upwards as illustrated by arrow 12. The flue gas 12 may have a temperature of up to 1100 C. to 1200 C. and is then led through the first second and third radiation passes 10 14 and 16 to a horizontal convection pass 18 after which the flue gases are eventually led to a chimney and released to the atmosphere as indicated by arrow 20.

[0054] The walls 22 of the first second and third radiation passes 10 14 and 16 are provided with tubes 24 to which water is fed for generating steam. The steam is then, as indicated by arrow 26, in turn led through superheaters 28 30 and 32, each of which represents a heat exchanger, positioned in the horizontal convection pass 18. The superheaters 28 30 and 32 are heated by the flue gas 12 passing through the convection pass 18 as illustrated by arrow 34. The heat from the flue gas 34 steam 26 so that the steam 26 is converted into superheated steam 36 which is led to a steam turbine (not shown) or similar consumer of superheated steam.

[0055] Additionally (not shown) the superheater 28 may be preceded by an evaporator for producing further saturated steam, the evaporator being placed upstream of the superheater 30 in the path of the flue gases 12, and being similar in construction to the superheater 28.

[0056] The flue gas 34 heating the superheater 28 30 and 32 comprises inter alia corrosive compounds and particles of hot ash 38, not shown in FIG. 1, which particles of hot ash 38 may themselves comprise corrosive compounds.

[0057] The temperature of the steam 26 increases as it is led through the superheaters 28 30 and 32. The lowest steam temperature of 250 C. to 300 C. is found in superheater 28 and the highest steam temperature is found in superheater 32. Thus the risk of corrosion is highest for superheater 32. In the incineration plant 2 all superheaters may be identical to the superheater 32, which superheater 32 is a heat exchanger according to the present invention. Alternatively, to save costs, only superheater 32 is a heat exchanger according to the present invention whereas superheaters 28 and 30 are superheaters consisting of conventional materials.

[0058] Each superheater 28 30 32 comprises a number of superheater tubes representing heat exchanger components.

[0059] FIG. 2 shows superheater tubes, one of which is designated the reference numeral 40, of the superheater 32 in FIG. 1. As seen in FIG. 2, steam 26 runs through the superheater tubes 40 while flue gas 34 passes between the superheater tubes 40 to heat the superheater tubes 40 and the steam 26 running within the superheater tubes 40. The superheater tubes 40 may be joined to each other by bends, one of which is designated the reference numeral 42, which may be formed separate from the superheater tubes 40 and joined thereto, or which alternatively may be formed integrally with the superheater tubes 40.

[0060] FIG. 3A shows a first embodiment of a superheater tube 40, representing a heat exchanger component, of the super heater 32, representing a first embodiment of the heat exchanger according to the first aspect of the present invention.

[0061] Superheater tube 40 comprises a main tube 44 including a wall having a first side 46 in contact with the steam 26 and a second side 48 facing the flue gas 34. The main tube 44 is made from a precursor material which upon oxidation forms a scale 50 comprising -Al.sub.2O.sub.3 at least on the second side.

[0062] Flue gas 34 passes the superheater tube 40 and deposits particles of hot ash 38 on the main tube 44, thus forming a sticky deposit 52 upon the second side 48 of the single material tube 44. Corrosive compounds comprised by the flue gas 34 and/or the particles of hot ash 38 are thus present in the sticky coating 52. Corrosion of the main tube 44 is however prevented, or at least diminished, by the scale 50 covering the second side 48 of the main tube 44.

[0063] FIG. 3B shows a second embodiment of a superheater tube 40, representing a heat exchanger component, of a super heater 32, representing a second embodiment of the heat exchanger according to the first aspect of the present invention.

[0064] Superheater tube 40 comprises a main tube 44, made from a material which does not form a scale comprising -Al.sub.2O.sub.3 upon oxidation. Instead superheater tube 40 comprises, on the second side 48 of the main tube 44, a welded cladding 54 of a precursor material which upon oxidation forms the scale 50 comprising -Al.sub.2O.sub.3. The scale 50 on the welded cladding 54 prevents, or at least diminishes, corrosion of the main tube 44 due to corrosive compounds comprised by the flue gas 34 and/or the particles of hot ash 38.

[0065] FIG. 3C shows a third embodiment of a superheater tube 40, representing a heat exchanger component, of a super heater 32, representing a third embodiment of the heat exchanger according to the first aspect of the present invention.

[0066] Superheater tube 40 comprises an inner tube 44, representing a main tube, made from a material which does not form a scale comprising -Al.sub.2O.sub.3 upon oxidation. Instead superheater tube 40 comprises, on the second side 48 of the main tube 44, an outer tube 56 made of a precursor material which upon oxidation forms the scale 50 comprising -Al.sub.2O.sub.3. The scale 50 on the outer tube 56 prevents, or at least diminishes, corrosion of the inner tube 44 due to corrosive compounds comprised by the flue gas 34 and/or the particles of hot ash 38. The superheater tube 40 may be manufactured by co-extruding the main tube 44 and the outer tube 56.

TABLE-US-00002 List of parts with reference to the figures: 2. Incineration plant 4. Waste 6. Conveyor 8. Grate 10. First radiation pass 12. Flue gas 14. Second radiation pass 16. Third radiation pass 18. Horizontal convection pass 20. Arrow indicating flue gases being led eventually to a chimney 22. Walls of radiation passes 24. Tubes 26. Saturated steam 28. Superheater 30. Superheater 32. Superheater 34. Arrow indicating flue gas passing through convection pass 36. Superheated steam 38. Particles of hot ashes 40. Superheater tube 42. Bend 44. Main tube 46. First side 48. Second side 50. Scale 52. Sticky deposit 54. Welded cladding 56. Outer tube