TECHNICAL FIELD

The present invention relates to an incineration plant comprising:

a furnace including a number of movable grates,

^• a ^' fuel feeding system for feeding fuel to said movable grates on which the fuel forms a layer, where said fuel is fed forward and: subjected to successive d - log, ignition, combustion and outDuming, whereby flue gases are produced,

an air supply configured for supplying primary air for the combustion from beneath the movable grates and through the layer of fuel,

at least a first and a second vertically extending .radiation passes through which flue gases are directed from the furnace,

a number of convection passes through which fiue gases are directed after having passed through the vertically extending radiation passes,

a fi st superheater arranged in at least one of the convection passes for the provision of superheated steam due to heat exchange between the first superheater and the Hue gas.

BACKGROUND OF THE INVENTION

Heat exchangers of various kinds are known in the field of incineration processes for transferring ^' heat from flu gases to fluids for heating these fluids. One use of heat exchangers is for the heating of saturated steam from .a boiler for converting the saturated steam into dry (also called superheated) steam more useable for example in power generation processes.. Dry steam is for example used for driving steam turbines in power plants,

A heat exchanger typically includes a large number of heat exchanger components, each heat exchanger component having a wail with first side in contact with ¹ a fluid to be heated and a second side in contact with a heating medium, which in an incinera- tion process typicall is flue gas generated by the incineration process. The heat exchanger components may be plates, as in a plate beat exchanger, but may alternatively be shaped as tubes, the inner and. outer side of the tube wall defining the first and second side of the heat exchanger component. For producing superheated steam in an incineration plant for producing power, the heat exchanger typically comprises a plurality of individual beat exchanger components in the shape of tubes, also called superheater tubes of overheat r tubes, through which th steam sequentially passes. The heat exchanger Is placed in th path of the flue g asses so that the heat exchanger components are heated by the flue gas. whereby heat Is passed through the wail of the heat exchanger components to heat the steam within. Different incineration processes burn different fuels, such as- waste. The waste may be household waste and/or other types of waste, such as industrial waste, etc. Such an incineration plant is also called a waste-tc-energy incineration plant or a refuse incineration plant A problem related to the nature of the waste- burn in the incineration plant is that the flue gas, and/or the hot ashes -entrained in the flue gas, to lesser or larger extent, depending on the exact natur of the waste being burnt, comprises corrosiye compounds such as chlorides. The hot ashes entrained in the flue gasses. condense onto the comparatively cooler surfaces -of the heat exchanger, especially the heat ex- changer components or superheater tubes, and form a sticky coating thereon. Chlorides in particular, but also sulphates, present in this coating ar highly corrosive and causes severe corrosion of the metal material of the heat exchanger components or superheater tubes. The extent of corrosion Is dependent on the temperature of the heat exchanger components. When -superheating steam _. , the temperature of th heat exchanger components, through heat transfe between the steam and the heat exchanger component, is typically up to SO: degrees centigrade higher than - that of the steam. A higher temperature of the steam, speeds up the corrosion process.. Thus, in order to ensure a useful lif of th heat -exchanger components, the temperature of the -steam to be superheated, has to be limited. Therefore, he- superheater of a refuse incineration plant is typically arranged in. a convection pass downstream, the vertically extending radiation passes. This however severely limits the efficiency of the Incineration plant, particularly as regards power generation, where -the efficiency- of a steam turbine is dependent on the temperature- of the steam.

Where tubes of inexpensive steel containing mostly Fe (iron) are used as heat exchanger components for superheating steam, the- maximum steam temperature is approximately 425 degrees Celsius if excessive corrosion and an acceptable service ife are to be achieved. Approaches for allowing the steam temperature to be inc ased include, providing tubes of inexpensive steel coated with more expensive allo s,, such as incenel 625. incsnel 62S .is nickel based alloy forming a scale of chromium oxide on its surface when subjected to heat an corrosion, With this approach, a steam temperature of approximately 440 degrees Celsius, is possible with the same speed of corrosion and service life as that possible by using the tubes of inexpensive steel at 425 degrees Cels us,

However, sttfl higher steam temperatures are desired in rder to- maximize the effi- dertey of incineration plants.

Typically, the steam .turbines used in for example waste (refuse) incineration plants are configured to operate- optimally with steam temperatures in. the range of 450-800 degrees■ ^■ Celsius. The Inner metal pipes carrying _, the steam rapidly obtain the same tem- pera!ure as the- steam flowing, within them. However, at these temperatures the flue g asses will condensate on the relatively cool (as compared to the temperature of the flu gasses) metal ipes- carrying the steam, if direct contact, is allowed, thereby forming so caiied smelt on these pipes, which is a sticky lot containing highly corrosive constituents, such as chlorides and sulphates.

In Q 2013/171547, by the applicant of the present patent- application,, a heat exchanger Is suggested, wherein the heat exchanger component comprises a wall having a first side in contact with the fluid, and a second side in contact with the flue gas, the second side being provided with a protective oxide fo protecting the heat ex- changer component against corrosion caused by corrosive compounds entrained or comprised -by the flue gas, wherein the protective o¾sde comprises a!pha-Ai _a O¾.

In US 2-010/0000474- -a fluidize bed heat exchanger is. -disclosed, wherein the heat exchanger pipes are protected by separat protective shells,

In US 2013/01 19421 , US2014/0202575 and DE 3:8 23 439 it i suggested to protect the heat exchanger pipes in steam boiler systems by using a special, casing element made from ceramic, Although ceramic casing solutions seem to be effective in preventing corrosion, they are very heavy, expensive and difficult to install, supervise, and maintain.. They are also prone to break due to mechanical stress during installation and operation, where expansions and contractions of he metric steam pipes caused by temperature variations may lead to formation of cracfe in the ceramic casing, Furthermore, due to the mere sfee and weight of these ceramic casings, they are not easily retrofitted into ex- ί sting incineration plants, if possible at ail, and would therefore require an expensive and time consuming reconstruction, during which time the incineration pi ant would not be operational;.

SUMMARY QF THE INVENT ION

Th object of the present invention is to provide an incineration lant with a corrosion protected superheater, which at the same time more effectively exploits the energy In the flue gasses. In view of this object, the incineration plant comprises a second superheater for the provision of superheated steam due to heat exchange between the second superheater and the flue gas,, said second superheater comprising:

an inner metal pipe, through which, the steam to he superheated is flowing during operation of said Incineration plant,

an outer metal casing enclosing the inner metal pipe, such that the flue gas is prevented from reaching the inner metal pipe,, and wherein there is provided a spacing between the inner metal pipe and the outer metal casing, thereby precluding direct heat exchange between the outer metal casing and the inner metal pipe,

wherein the second superheater is arranged for further heating of steam already heated in the first superheater,

and wherein the second superheater is arranged in at least one of: the furnace, the first vertically extending radiation pass and the second vertically extending radiation pass, in this way, by providing a second superheater In addition to the first superheater, whereby the first superheater is located In one of the convection passes, and whereby the second superheater is arranged In the furnace or the first or second vertically extending radiation pass andls provided with an outer metal casing enclosing the inner metal ^' pipe, a substantially higher temperatur of the steam ma be obtained without substantia; corrosion of the first and second superheater. Thereby, according to the in ention, in the second superheater, direct heat exchange between the outer metal casing and the inner metal pipe is precluded, whereby heat is predominantly transferred by radiation. This means that the outer metal casing is not cooled down to the temperature of the inner metal pipe carrying the steam, as would have been the case if the outer met l casing and inner metal pipe were allowed to touch each ot r.

This means that the outer metal casing of the second superheater will eventually reach a temperature close to _: the temperature of the flue ga s.es that interact: with it. The in- vention, therefore allows the temperature of the outer metal casing of the second superheater to he much higher than the temperature of the steam pipes, thereby preventing the formation of the highly corrosive smelt on the outer metal casing, which again implies thai even though the outer metal casing of the second superheater according to the invention is made from a metal material, th operational lifetime of th second superheater is not hampered at ail.

Therefore, by arranging an additional, second superHeaier of the type described above in the furnace or the first or second vertically extending radiation pass,, it may both b® possible to increase the temperature of the generated heat even more than in existing incineration plants and at the same time if may be possible to drasticall reduce corrosion a the formation of highly corrosive smelt may be. avoided or substantially avoided.

By using an outer metal casing for the second superheater, further advantages regard-- ing mechanical strength, a d light weight may be achieved, which are relatively easy to implement -- even in existing incineration plants · ^■■■ without the need to make a radical and costly re-design of the whole incineration plant.

The invention may therefore solve many of the problems associated wit the super- heater arrangements mentioned above under the discussion of prior art, while at the same time providing a higher efficiency than traditional superheater arrangements.

Since th outer metal casing of the second superheater furthermore does not need to withstand .pressurized -steam, it may be made from cheaper metals than the steam pipes, and does not need to be mounted and manufactured in accordance with the tolerances and specifications re uired fo the steam i e themselves, thereby also contributing to the ease by w ich the invention can. be constructed. in an embodiment the first vertically extending radiation pass is integrated into, the furnac as .a post-combustion chamber, and at least a part of the second superheater is arranged in the first vertically extending radiation pass, Thereby, by placing the second superheate very close to- the combustion in the furnace, it may be possible to increase the temperature- of the generated heat even more and at the same time even better reduce corrosion.

In a structurally advantageous embodiment, the incineration plant includes a first, a second and a third vertically extending radiation pass,

In an embodiment, at least a pad. of the second superheater is arranged in the fur- n-ace. Thereby, by placing the second superheater very close to the ^■■ combustion in the furnace, it may be possible to Increase the temperature of the gen a rated heat even more and at the same- time even better reduce corrosion.

In an embocsiment, the first superheate has the form of a steam pipe having a mas- sive or at least substantially massive pipe wall, and the- outside: of th pipe wail ^' is adapted to contact flue g sses and the inside of the pipe wall is. adapted to contact steam. Thereby, the first superheater may be optimised to preheat the steam e fi- cient!y at a lower temperature in one of the convection passes where the temperature may be low enough to avoid corrosion by forming of smelt.

According to one embodiment _. ^: of the invention, the incineration plant is adapted to operate with fuel in the form of refuse, refuse derived fuel o biomass.

According to a further embodiment of the invention,, the outer -metal casing of the sec-- and superheater is formed as a single casing enclosing a plurality of inner metal pipes, through which the steam to be superheated is flowing during operation of said incineration plant. Hereby there may he achieved a very simple and cheap way- of providing corrosion protection for a plurality of pipes in ^' the second superheater, According to a further embodiment of the- incineration plant according to the Invention, the outer metal casing of the second superheater is provided, as an outer metal pipe placed ooaxialjy or at [east substantially coax ^' iaily in relation to the inner metal pipe, Hereby may be provided an embodiment wherein the spacing between the out r metal casing (the outer metal pipe} and the Inner metal pipe (the steam pipe) is constant o at least substantially constant, and wherei the heat transfer via radiation from the 5 outer me al pipe to the inner metal pipe may be at least substantially uniform and therefore most effective. Preferably the distance between the outer surface of the inner metal pipe and the inner surface of the coaxiai!y or at least substantially coaxlally mounted second outer pipe is within the range of 0,5 to 20 mm, more preferred within the range of 1 to 10 mm. and most preferred withi the range of § to 10 mm.

10

in an embodiment, th second superheater is arranged so that the inner metal pipe .includes, a plurality of straight inner pipe sections: arranged in parallel next to each other, neighbouring straight inner pipe sections are interconnected at an end by means of respective U-formed bends, the oyter metal casing includes a plurality of

I D straight outer metal pip© sections arranged coaxially or at l ast substantially coaxially •in relation to the respective straight inner pipe sections, a first junction box is provided having a side wall to which a. first end of each of the- straight outer, metal pipe sections is %/eided in ord r to form an at least substantially gaslight connection, and a number •of the U-formed bends are arranged inside the firs junction box. Thereby, advanta-

20 geousiy, a _. gaslight outer metal casing may ^" be formed by means of the plurality of straight outer metal pipe sections in combination with the first junction box. Furthermore, the straight inner pipe sections may conveniently be held at least substantially coaxially in relation to the straight outer metal pipe sections.. 5 in an embodiment, a second end of each of the straight outer metal pipe sections is welded to a wall ^" of the fumace or to a wall of one of the vertically extending radiation passes in order to form an at least substantially gastight connection, and a number of the U-formed bends are arranged outside said wall, Thereby, the second superheater may easily b mounted on sai wall, and the inlet and outlet connections ma be ar- 0 ranged on the outside of said wall.

In an embodiment, the first junction box is free to move in the longitudinal ^' direction of the straight inner and outer metal pipe sections in relation to the wail to which the straight outer metai pipe sections are welded, Thereby, possible different length vaha- 5 tions due to heating of the ^' straight inner and outer metal pipe sections may be ai- lowed, as th Li-formed bends may move slightly inside the first junction box or outside the top wail.

In an embodiment, a second end of each of the straight outer metal pipe sections is welded to a side wall, of a second junction box in order to form an at feast substantially gastlght connection, a number of the U-forrned bends are arranged inside the second junction box, and the second junction box is mounted on a wall of the furnace or mounted on a wall of one of the vertically extending radiation passes. Thereby, the second superheater may easily be mounted on a side wall as well as a top wall-, and the inlet and outlet connections may be arranged through the second junction box and through the wail.

In an embodiment,, the first junction box is free to move in the longitudinal direction of the straight inner and outer metal pipe sections in relation to the wall on which the second junction box is mounted. Thereby, possible different length variations due to heating of the straight inner and oute metal pipe sections ma be allowed, as the U- formed bends may move slightly inside the first or the second junction box.

In an embodiment, the U-fermed bends are fixed by means of brackets, strips, weld- ings or any other suitable fastener inside the respective first junction box and/or second, junction box and/or on the outside of the respective wall of the furnace or wall of one of the vertically extending radiation passes. Thereby, it may be ensured that the straight outer metal pipe sections are maintained coaxialiy or at least substantially oo~ axlafly in relation to the respective straight inner pipe sections, in a structurally aoVantageous embodiment, each straight inner pipe section of the second superheater is maintained ^■ coaxialiy or at least substantially coaxiaiiy in relation to the respective straight outer metal pipe section of the second superheater by means of at least a ring-forrned plug of insulation material arranged at either end of the pipe sections in the spacing between the pipe sections. The ring-formed, plug of insulation material _. may -ensure improved insulation by maintaining air or gas- still inside the spacing and at the same time maintain th straight inner and outer pipe sections at least substantially coaxial, In a structurally advantageous embodiment, insulation materia! is provided in the spacing between the inner mate! pipe and the outer metal casing along at least pads of the second superheater and referably at least along: straight sections of the second superheater m order to prevent, circulation of gas along the superheater in. the spacing, Thereby, insulation may fee improved by maintaining air or gas still Inside the. spacing. According to a further embodiment of the incineration plant according to the invention, the distance between the inne metal pipe and the outer metal casing is less than 200 mm, preferably jess than 100 mm, more preferred less than 50 mm. in an embodiment the outer .metal casing of the second superheater is formed as a single casing enclosing a plurality of Inner metal pipes, and the outer metal casing is curved so that said distances are equalized to a certain extent throughout the outer metal casing. Thereby, the outer metal casing may b curved: so that distances in the range of SO to 200 mm are avoided, at least to th -extent possible.

According to: a further embodiment of the incineration plant according to the invention, the second- superheate is. adapted to operate with flue gases having a temperature higher than an upper critical temperature wherein the constituents of the flue gasses are mainly in a: gaseous phase when said flue gases reach the superheater. Since the ^• outer metal casing enclosing the steam pipe(s) will accommodate to the temperature of the flue gasses, it is precluded thai the so called smelt containing highly corrosive constituents will form on the surface of said casing, thereby substantially precluding corrosion.

According to further embodiment of the incineration plant according to the invention, the upper critical temperature is at somewhere between 600 degrees Celsius and 800 degree Celsius, preferably between -600 degrees Celsius and 700 degrees Celsius, ^• more preferably between 800 degrees Celsius and 850 degrees Celsius. Depending of th type of fuel being incinerated, the type and amount of corrosive components would vary, and so does t e: temperature at which the. so called, smelt Is formed also vary; however at these temperatures, the formation of smelt on the outer mete! casing is substantially avoided altogether. ft has been observed that salts containing in particular chlorides, but also sulphates, can condensate on conventional boiler superheater tubes and thus cause a high corrosion rate. The chlorid.es and sulphates containing salts condense at relatively low temperatures, depending on the exact composition. Chloride and sulphate salts form so-called low melting eutectios, if the temperature of the outer metal casing, however, exceeds 600 degrees Celsius, the degree of chlorides and sulphates containing salts condensation will decrease significantly.

According to a further embodiment of the incineration plant according to the invention, the inner metal pipe of the second superheater is configured to withstand an inner steam pressure of 40 ~ 400 bars.

In an embodiment, the spacing between the inner metal pipe an the outer metal casing of the second superheater is supplied with compressed air or gas from a cornpres- sor. Thereby, even though small leaks should occur in th outer metal casing,, if may be ensured the flue gasses do not contact the inner metal pipes, and therefore, the inner metal pipes may: he even better protected: against corrosion.

In an e bodiment, the supply of compressed air or gas f om the compressor to the spacing between the inne metal pipe and the outer metal casing is monitored by means of a flow meter. There y, possible leaks occurring in the outer metal easing may be detected and subsequently closed. Thereby, the inne metal pipes may be even better protected against corrosion. In an embodiment, the pressure of air or gas in the spacing between the inner metal pipe and the outer metal casing is monitored by means of a pressure meter. Thereby, the inner metal: pipes may be even better protected against corrosion. The pressure meter may be arranged in place of o additionally to the flow meter, The invention furthermore relates to a method of producing superheated steam in an Incineration plant hawing a furnac ^■■ Including a numbe of movable grates, the method comprising the steps of

- feeding fuel to a number of moving grates on which the fuel forms a layer, which is fed forward and subjected: to successive drying, ignition, combustion and eutburning, whereby flue gasses ar produced,

- supplying primary air for the combustion from beneath the grates and through the layer of fuel,

- directing flue gases from the furnace through at least first and a secon vertically extending radiation passes,

- directing fine gases having passed through the vertically extending radiation passes through a number of convection passes, - leading the flue gasses towards a first superheater arranged In a! least one of the convection passes and. thereby providing superheated steam due to heat exchange between the first superheater and the flue gas.

The method h characterised by

- leading the flue gasses towards a second superheater arranged m at. least one of: the furnace, the first vertically extending radiation pass and the second vertically extending radiation pass, and thereby, in the second superheater, farther heating steam already heated in the firs superheater,

- leading steam to he superheated through at least one inner metal pipe of the second superheater, said inner metal pipe being enclosed by a outer metal casing of the second superheater, such that the flue gas is- prevented from reaching the inner metal pipe, add wherein there is provided a spacing between the inner metal pipe and th outer met l casing,

" hea ing said outer metal casing of the second superheater due to heat exchange between the. outer metal casing and the flue gasses that are led towards the second superheater, and

- heating the steam in the: at least one inner metal pipe, by radiant heat transfer between: the outer metal casing and: th at least one Inner metal pipe.

Thereby, the above-mentioned features may be obtained,

In an embodiment, the first radiation, pass is integrated into the furnace as a post- combustion chamber, and at least a part of the second superheater is arranged in the first radiation pass. Thereby, the abdve-merrtione ^' d feature may be obtained, In a structurally advantageous embodiment,, the incineration plant includes a first, a second and a third vertically extending radiation pass.

In an embodiment, at least, a part of the second superheater is arranged in the furnace, Thereby, the. above-mentioned features may be obtained.

In .an embodiment, the fi st superheater has the form of a steam pipe- having a massive or at least substantially massive pipe wall, and the outside, of the pipe wail Is adapted to contact flue gasses and the inside of the pipe wail Is adapted to contaot steam. Thereby, the above-mentioned features may be obtained..

In an embodiment, the fuel Is refuse, refuse derived fuel or biornass. In an embodim nt, the outer metal cas ng of the second superheater Is formed as a single, casing enclosing a plurality of inner metai pipes through which the steam to be superheated is flowing during operation of said incineration plant. Thereby, the above- mentioned features may be obtained, in an embodiment, the outer metal casing is provided as an outer metai pipe placed coaxiafly or at least substantially coaxiaiiy In relation to the inner metal pipe, Thereby, the above-mentioned features may be obtained.

In an .embodiment, the distance between the inner rnetai pipe and th outer metal casing is within the range of 0.5 to 20 mm., more preferred within the range of 1 to 10 mm and most preferred within the range of S to 10 _: mm. Thereby, the above-mentioned features may be obtained. in an embodiment, the flue gasses, which are led to the second superheater, have -a temperature higher than a upper critical temperature, where the constituents of the flue gasses are mainly in gaseous phase. Thereby, the above-mentioned features may be obtained.

In an embodiment,, the upper erifieal temperature is somewhere between 800 degrees Celsius and 00 degrees Celsius, preferably between 800 degrees Celsius and 70Q: degrees Celsius, more, preferably between 800 and 850 degrees Ceisius. Thereby, the above-mentioned features ma be obtained,

In an embodiment, the pressure of the steam, which is led through the inner metal pipe of the second superheater, is 40 ·- 400. bars. Thereby, the above-mentioned features ma be obtained. in an embodiment, the spacing between the inner metai pipe and the outer metal casing of the second superheater is supplied: with compressed air or gas from a compressor. Thereby, the above-mentioned features may be obtained, in an embodiment, the> supply of compressed air or gas from th compressor to the spacing feetween th Inner metai pipe and the outer metal casing is monitored by means of a flow meter. Thereby, the above-mentioned features may be obtained. In an embodiment, the pressure of air or gas in the spacing between the inner metal pipe and the outer metal casing is monitored by means of a pressure meter, Thereby, the above-mentioned features may be obtained,

BRE!F DESCRIPTION OF THE DRAWINGS.

A. further understanding, of the nature and advantages ^' of the present invention may be realized by reference to the remaining portions of the specification and the drawings. In the following, different embodiments of the invention are explained In more detail with reference to the very schematic drawings, wherein

F g. i shows schematic overview of an embodiment, of an incineration plant according to the. invention,

Fig. 2 shows an embodiment of a second superheater,

Fig. 3 shows two alternative arrangements of alternative embodiments of a second superheater, and

Fig, 4. shows a cros section of one of the pipe structures illustrated in fig, 3

DETAILED DESCRIPTION

The present invention -will now be described more fully hereinafter with reference to the accompanying very schematic drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in different forms ^' a d should not be construed, as limited to the embodiments set forth herein. Rather,, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in th art. Like reference numerals refer to like elements-, throughout Like elements will, thus, not be described in detail with respect to the description of each figure.

Fig. 1 shows the overall outline of an embodiment of a combustion facility in the form of an Incineration plant 2. according to the invention. The illustrated incineration plant 2 is configured for incineration of different kinds of fuel, for instance refuse, such as household, solid waste, or commercial and Industrial waste, which may form the fuel 4 for the combustion. The fuel used may for example furthermore be biomass, such as shredded straw wood in the form of chips or wood pellets. In the: case of refuse, the fuel may fee so-cailed refuse derived fuel (RDF). RDF Is also known as solid recovered fuel or specified recovered fuel SRF), and is a fuel produced by. shredding and deny- drating solid waste, for example with a so called waste converter technology. RDF consists largely of combustible components of municipal and/or industrial waste such as plastics, packaging ma erials _: and biodegradable waste. S F can be distinguished from RDF in the fact that it is produced to reach a standard such as GEN/343 ANAS. 5 The fuel can also be wood chips generated from wood harvested from demolition of buildings or other constructions containing wood.

The incineration plant 2 receives fuel 4, for example in the form of refuse from garbage trucks 8, which deliver the refuse to a large fuel si 8. The heat value of the refuse ^' in 0 the silo 8 may vary a lot, _, because it comprises so many different kinds of industrial nd household refuse, Therefore a crane 12 will automatically mix the refuse, so that a more even heat value may be achieved. The process: of filling a hopper 13 with fuel 4- is also performed automatically by the crane 12. However, typicall during normal ^' working hours, an operator will overlook the silo 8 from the operator: room 10 andS manually steer the crane 12,, if needed. ^" The hopper 3 is arranged on top of a chute 15 formed in such a way that Its cross sectional area widens towards the bottom. This precludes: the possibility of jamming of the chute 15. The quantity of fuel 4 in the chute 15 acts as a buffer for the in-feeding: of fuel 4 to grates 1% 18, 20 arranged in a furnace 1 , so that a continuous supply of fuel 4 to the grates can be ensured. Further--0 more, the quantity of fuel 4 in the chut 15 -acts as burn back, prevention and prevents, entrance of false air through th chute 18.. Therefore, a certain height of th chute 15 is necessary.

At the bottom of th chute 15 is placed a plurality of pusher pistons 22, which all oper-5 ate in .a backwards and forward movement, in order to push the fuel 4 onto the moving grates 14, TS and 20. The- fuel 4 forms, a ^" layer on the grates 14, 18, 20, where said fuel: 4 is fed forward and subjected to successive drying, ignition, combustion and outburn- sn.g. The incineration plant 2 also -comprises an air supply 16 configured ^" for supplying -primary air for the combustion from beneath the movable grates 14, 18, 20 andQ through the la e of fuel 4.

Flue gasses are in a manner well known to the skilled person directed from the furnace 1 through a first, a second and a third verticall extending radiation passes 8Q, 81 , 82 i succession. The first and second vertically extending radiation passes 00, 815 are separated by means of a first separation boiler wall 85 having an upper free end, and the second and third vertically extending radiation passes 8 , 82 are separated by means of a second separation boiler wail 86 having a lower free end. Likewise in a well-known-, manner, the first radiation pass, 80 Is integrated into the furnace 1 as a post-combustion chamber. Subsequently to the third verticall extending radiation pass 82, flue gases are directed through a first and horizontally extending convection pass 83 and thereafter through second and vertically extending convection pass 84. In other embodiments, the incineration plant 2 may. have any other suitable number of vertically extending radiation, passes than that illustrated and any suitable number of vertically or horizontally extending convection passes than that illustrated. The incineration plant 2 is fed with water 32, which is preheated in a prehesfer 31 arranged in the second and vertically extending convection pass 84 by the escaping flue gasses .24, _. and then fed into the bottom of a boiler drum 33 arranged above the first vertically extending radiation pass 80. Dry steam- from the top of the boiler drum 33. is then superheated in a first superheater 30. in a -conventional incineration plant, this superheated steam would be sent directly: to a steam turbine, However, according .to. the present invention, the steam already heated in the -first superheater 30 is fed to a second superheater 87 arranged in the first ^' vertically extending radiation pass. SO.

The flue gasses may have temperatures of up to 1 100 degrees Celsius to 1200 de- grees Celsius, where the flue gas is generated _; i.e. where the incineration takes place, and where the flue gasses enter the first vertically extending radiation pass 80. On its way towards the first superheater 3D the flue gasses pre co ied down somewhat, because some of the. energy in the flue gasses is. used fo generating steam in the boile waiis of the open radiation passes 80., 81 , 82 which steam with wa er droplets is being treated in the boiler drum: 33 so that completely dried- steam is .sent from the boiler drum 33 to the first superheater 30. Steam is? typically generated in the boiler walls of the open radiation passes 80, 81 , 82 in that water 35 present in a bottom part of the boiler drum 33 is led through a no shown pipe to a not shown container arranged at the bottom of th vertically ^' extending radiation passes 30, 81 , $2, wherefrom water rises u through pipes integrated in - panel walls forming the vertically extending radia<- iion passes 80, 81 , 82. As the water in theses pipe integrated In the panel walls is heated, steam is generated, and this steam 3? is fed into the boiler drum 33 at a level over the water level in the boiler drum. 33. Although in Figs. 1 and 3,. the boiler drum 33 is illustrated as having only one water inlet.53 and one steam outlet 64 connected to the preheater 31 and first super heater 30, respectively, It will be understood by the person skilled in the art that the boiler drum 33 is also provided with one or more steam Inlets for steam generated ^' in the boiler wails and one or more water outlets connected to the not shown container arranged at the bottom of the ertically extending radiation passes 80, 81. 82. The first superheater 30 may be of a conventional type in the form of a steam pipe having a massive or at least substantially massive pipe wall, and wherein the outside of the pipe wall Is adapted to contact flue- g asses and th Inside of the pipe wall is adapted to contact steam-. The steam pipe of the first superheater 30 may therefore in already known manner be- protected from corrosion, for instance by- providing the steam pipe with an outer layer of alpha-alumirsa ₍ which is an aluminium oxide being highly corrosion resistant

Fig, 2 shows an- embodiment of the second superheate 87, wherein the steam: pipes 38 are enclosed by an outer metal casing 38. The. steam to be superheated is flowing into the component of the second superheater- 87 via the pipe 38 as indicated by the arrow 40 during operation of the incineration plant 2, The outer metal casing 38 encloses the inner metal pipes 36, such that the flue gas flowing by the second superheater 87 s prevented from reaching the inner rtietal pipes: 36. The outer metal casing 38 does not touch the Inner metal pipes 35, because there is provided a spacing be- tween the steam pipes 36 a ¾ the outer metal casing 38. Hereby direct heat exchange between the outer metal casing 3$ and the inner metal pipe. 38 is precluded. The heat transfer from the outer metal casing 33 to the pipes 38: Is predominantly facilitated, via radiation. The superheated steam leaves the second superheater 87 as indicated by the arrow 42.

In an embodiment, the spacing between the inner metal pipe 38 and the outer ^' metal casing 38 of the second superheater 87.is supplied with compressed air or gas from a compressor 88. Thereby, even though small leaks should occur in the outer metal casing 38, it may be ensured the flue passes do not contact the inne metal pipe 38, and therefore, the inner metal pipes 36 ma be even better protected against corrosion. Although the compressor 88 is illustrated in Fig. 2 as directly connected to the casing 38 via a flow meter 88,. typically a not shown buffer container will be arranged between the compressor 88 and the casing 38, In an embodiment, the supply of compressed air or gas from the compressor 88 to the spacing b tween the inner metal pipe 36 and the outer etal casing 38 is monitored by means of the- flow meter 89; thereby, possible leaks occurring i the. outer metal c s ng 38 may be detected and subsequently dosed, Thereby, the inner metal pipes 38 may be even better protected against corrosion, in the embodiment illustrated in. g _> - 2 and other embodiments, the not shown, buffer container would be arranged be- tween the compressor 88 and the .flow meter 8§ so that the actual quantity of gas of air flowing to the casing 38 may be detected. Generally, the no shown buffer container and the compressor 88 may form part of a general compressed air system used in the incineration plant 2. In an _. embodiment, the pressure of air or gas in the spacing between the inner metal pipe 36 and the outer metal casing _. 38 is monitored by means of a pressure meter 101.. Thereby, possible- leaks occurring in the outer metal casing 38 may be detected and subsequently closed. Thereby, the inner metal pipes 36 ma b even better protected against corrosion. The pressure meter 101 may be arranged in place of or additionally -to the flow meter 89,

The illustrated metal casing 38 has a relatively large flat, surface area, which is easy to maintain, clean and install - even in existing superheater structures. However, preferably, the outer metal easing 38 is curved in a wav -formed manner in order to. follow the surface of the inner metal pipe 38 so that said distances: are equalized to a certain extent throughout the outer metal casing 38, Thereby, the outer metal easing may be. curved so that distances of more than SO to 200 mm are avoided, at least to the extent possible. Fig, 3 illustrates two alternative embodiments: of the second superheater 87 a, 87b, wherein each of the steam pipes 36 are surrounded by a outer metal pipe 48 placed coaxiaiiy or at least substantially coaxialiy in .relation to the steam pipes 36, .As. will be more clearly visible from Fig. 4, there is provided a spacing 50 between the two pipes 3§ and 46. Thus, When flue gasses flow through the second superheater 87 as in.dk cated by the arrow 48 the- _: direction Is by means of example only), the outer metal pipes 46 are heated: dye. to heat exchange with the flue gasses. The outer metal pipes 46 in turn transfer heat to the steam pipes 38 vis radiation. This means that the outer metal pipes 46 are not coded down to typically 4S0 degree Celsius, which is a typical temperature for superheated- steam, but will instead have a: temperature much higher, ¾.g. in the range 600™ 800 degrees Celsius, A temperature of 450 ^" degrees Celsius Is under the critical temperature, wherein: the so. called smelt may form on the pipes, put this: formaffen of smelt is precluded, because the riue gasses are prevented from reaching the steam pipes 38. The flue gasses are only in contact with the outer metal pipes 48, whic have such a high temperature during operation of the incineration plant 2 thai the flue gasses will not condensate on them and form the highly corrosive smelt.

^' Fig,.4. illustrates a cross section :of one of the pipe structures illustrated in Fig. 3, Within th steam pipe 38 (also referred to as inner metal pipe 36 in the present specification} steam to be superheated flows as Illustrated in Fig. 3. Surrounding the steam pipe 36 is the outer metal pipe 46, which Is arranged co ^' -axially or at least substantially coaxial! y relative to the steam pipe 36 i such a way that there is provided a spacing -SO between the two pipes 36 and 48, The distance 62 between the oute surface of the steam pipe 36 and the inner surface of the outer metal pipe: 46 is carefull chosen in order to secure optima! heat transfer from the outer metal pipe 48 to the steam pipe 36. it is preferred, that th spacing is kept at a minimum in order to keep the volume and material expenses low and in order to transfer b heal radiation as. much heat as possible from the outer metal easing Into the steam nside the steam pipe. On the other hand, it Is preferred that the spacing is large enough to prevent the tube outer surface from .getting in direct surface contact with the outer metal easing inner surface due to manufacturing tolerances and it is preferred that the spacing is particularly large enough to prevent too .much cooling off of the outer metal casing In order to secure that the outer metal casing outer surface temperature is kept safely above the critical temperature where serious corrosion can happen. Selection of an appropriate, spacing distance is a balance between not too small and not too large.

IP the alternative embodiments of the second superheater 87a, 87b illustrated in Figs. 3 and 4, a not shown insulation material may be provided in the spacing 50 between the inner metal pipe in form of the steam pipe 36 and the outer metal casing in form of the outer metal pipe 48 along the part of the superheater formed by the steam pipe 36: and the outer metal pipe 46 in order to prevent circulation of gas along the superheater in the spacing 50. Thereby, an even better thermal: insulation between the: steam pipe 38 and the outer metal pipe 46 may foe obtained. The insulation material may ensure that there is no metal contact between the steam pipe 38 and the outer metal pipe 4B. The Insulation material may be for instance mineral wool, such as vitreous, wool ₍ stone wool and ceramic woof. It may bs ensured: that direct heat exchange between the outer metal pipe 46 and the ^' steam pipe 36 is at least substantially precluded, whereby heat is predominantly transferred by radiation. This means that the outer metal pipe 46 is not: cooled down to the temperature of the steam pip© 36 carrying the steam, as would have been the case if the outer metal pipe 48 and the steam pipe 36 were allowed to touch each other.

A part of the inner metal pipe in form of the steam pipe 36 may be provided with insulation Jacket parts by gluing: the insulation ^' jacket parts to the inner metal pipe, and subsequently, the part of the inner metal pipe provided with insulation jacket parts may be :slid Mo the outer metal easing ..in form of the outer metal pipe 48.. The glue may for instance contain sodium silicat (water glass}.

Alternatively, a part of the Inner metal pipe in ^' form, of the steam pipe 36 may b pro- vided with a ceramic wool insulation tubing by sliding the tubing onto the part of the inner metal pipe from an end thereof, and subsequently, the pari of the inner metal pipe provided with ceramic wool insulation tubing may be slid into the outer metal casing in form of the outer metal _. ipe 4.6. Fig, 3 illustrates a first embodiment of the second superheater designated: 87a and a second embodiment of the second superheater designated 87b. These two embodiments are alternative examples of ^' how- the second superheater 87 may be arranged in the first vertically extending radiation pass 80, However, these two _: embodiments could also be coupled in series and thereby together form the second superheater B7,

.In th alternative embodiments of the second superheater 87a, 87b illustrated in Figs, .3 and.4, the spacing 50 between the inner metal pipe 38 and the outer metal casing 38 in the form of the outer metal pipe 46 of the second superheater 87a, 87b may be supplied with compressed air or gas from a not shown compressor possibly via a not shown buffer container, just as described in relation to the embodiment illustrated in Fig. 2. The sam is of course true for the embodiment of ^" the second superheater 8? illustrated in Fig. 1. Thereby, even though small leaks -should occur in the outer metal easing 38 in the form of the outer metal pipe 48, it ma be ensured the flue passes do not contact the inner metal pipes 36, and therefore, the inner metal pipe 3 may be even better protected against corrosion. The supply of. compressed a r or gas from the compressor to the s ci g SO may be monitored by means of a not shown, flow, meter and/or by means of a not shown _. pressure meter. Thereby, possible leaks occurring in the outer metal casing 38 may be detected and subsequently closed, just: as: described in relation to the embodiment illustrated in Fig, 2.

According to the first embodiment of the second superheater illustrated in Fig, 3 and designated 87a, the second superheater 87a is arranged so thai the inner metal pipe 38 includes a plurality of straight inner pipe sections 90 arranged vertically extending and in parallel next to each other, wherein neighbouring straight inner pipe sections 90 are interconnected at an end by means of respective U-formed bends 91 . The outer metal casing 38 includes a plurality of straight outer metal pipe sections 92 arranged ceaxially or at least substantially coaxia!!y in relation to the respective straight Inner pipe sections 90. A first junction box 93 is provided having a side wail; 94 to which: a first end 95 of each of the straight outer metal pipe sections 92 is welded in order to form an at least substantially gastight connection, A number of the U-formed bends 91 are arranged inside the first junction box 93, A second end 96 of each of the straight outer metal pipe sections: 92 Is welded to a top wall 97 of the first vertically extending radiation pass 80, and a number of the Lbformed bends 91 are arranged outside the top wall 97. The second ends 96 of the straight outer metal pipe sections 92 may alternatively be welded to a side wall of the first vertically extending radiation pass 80.

The first junction box 83 Is preferably arranged free, to move in the longitudinal direction of the straight inner and outer metal pipe sections 90, 92 in relation to the top wall 97 to which the straight outer metal pipe sections 92 are welded. Thereby, possible different length variations due to heating of the straight inner and outer metal pipe sections 90, 92 may be .allowed, as the U-formed bends 91 ma move slightly inside the first junction box 93 or outside the top wall 97, According to the second embodiment of th second superheater illustrated in Fig, 3 and designated 87b, the second superheater 87b is arranged: so that the inner metal pipe 38 includes a plurality of straight inner pipe section 90 arranged vertically extending and in parallel next to each other,, wherein neighbouring straight inner pipe sections 90 are interconnected at an end by means of respective U-formed bends 91 , The outer metal casing 38 includes a plurality of straight; oute metal, pipe sections 92 arranged coaxlaSly or at least substantially coaxia!iy i relation to the respective straight Inner pipe sections 80. A first junction box 9 ^' 3 is provided having a side wail 94 to which a .first end 95 of each of the straight outer metal pipe sections 92 is welded in order to form an at least substantially gaslight connection. A number of the U-forme bends 91 are arranged inside the first junction box 93. A second junction box 98 is § provided having a side wail 99 to which a second end 96 of each of the straight outer metal pipe sections 92 Is welded in order to form m at least substantially gastight connection, A number of the L1 orraed bends 91 are arranged inside the second junction box 98. The second junction box 98 is mounted on a side waif 1Q0 of the furnace 1 or of the first vertically extending radiation pass 80. A top side of the second junction bo.0 98 may alternatively be mounted on the top wall 97 of the first vertically extending radiation: pass 80. The first junction, box 93 is preferably arranged free to move in the longitudinal direction of the straight inner and outer metal pipe sections 90, 92 m relation to the. second junction box 98. 5 The first junction bo 93 and the. second junction box 88 are In the illustrated embodiments made as elongated boxes having parallel sides and ¹ being: closed g stfght in relation to the surrounding flue g asses. As mentioned above, ends 95, 96 of each of th straight outer metal pipe sections 92 may be welded in ah gaslight manner to a side wall of the. first and second junction box 93, 98 The straight inner pipe sections0 90 may then pass through a hole in said side waif inside the straight cuter metal pipe section .92, In order to allow the straight inner pipe sections 90 and the straight outer metal pipe sections 92 to vary their lengths different as a result of temperature changes, it may be preferred that the straight inner pipe sections 90 do no fit tightly in said holes- in- said side wail. Therefore, the outer metal casing 38 may be formed by - the. straight outer metal pipe sections 92 in combination with the first junction box 93 and the second junction box 88. The suppl of compressed air or gas fro the compressor to the spacing 50 may be performed via a tube connection connected to one of the first and second junction boxes 93, 98. The flow meter 89 and/or the pressure meter 101 may likewise be coupled to one of the first and second junction boxes 93, _: 98.

The UTormed bends. 81 are preferably fixed by means of brackets, strips, weldings or any other suitable fastener inside the respectiv first, junction box 93 and/or second junction box 98 and/or on the ouiside of the respective wall of the furnace or wall 97 of one of the first -vertically extending radiation pass 80. Alternatively, one or both of the second superheater 87a and the second superheater 87b may be arranged in the second vertically extending radiation pass 81. However, it is. preferred that the second superheater 87 is arranged close to the furnace 1 in order to achieve the best heating .effect of the superheater..

In a not shown embodiment of the second superheater 87, _: the second superheater Is arranged so that the inner metal pipe 36 includes a. plurality of straight inner pipe sections 90 arranged horizontally extending and in parallel next to each other, wherein neighbouring straight inner pipe sections 90 are interconnected at an end by means of respective U-formed bends 91. The outer metal casing 38 includes a plurality of straight outer metal pipe sections 92 arranged coaxialiy or at least substantially eoaxr- ally, in relation to th respective straight inner pipe sections 90. A first junction box 93 and or a second junction box 98 may be a ranged just as explained above for the embodiments illustrated ^' in Fig. 3, According to this not shown embodiment steam may enter the second superheater f om the top side, so that steam first enters the uppermost straight inner pipe section 90 and lastly enters the lowermost straight inner pipe section G. From there, it may exit at the level of the lowermost straight inner pipe section through a wall of the respective -vertically extending radiation pass, or If . may exit though a vertically extending pipe arranged inside the respective Vertically extending radiation pass. The arrangement according to this not shown embodiment may be advantageous in that, it has the form of a counter flow heat exchanger which .may extract the maximum amount of heat from the hot; flue passes as compared to a concurrent ffpw heat exchanger. in the embodiment of the second superheater 67, 87a, 87b described in the: paragraph immediately above, as well as in the embodiments illustrated in the Figs. 1 and 3, although the straight inner and outer pipe sections 90, 92 i the figures are illustrated as being arranged next to each other in a. plane parallel to the drawing plane, the straight inner an outer pipe sections 90., 9-2 could just as well be. arranged next to each other in. a plane perpendicular to said plane. In fact,, the first, second and third vertically extending radiation passes 80, 81 82 -typically hav a dimension in the plane perpendicular to the drawing plane which is much longer than their dimension in the drawing plane, and therefore the second, superheater could preferably extend over at least a substantial par of this longer dimension. It. is also possible that the second superhea- ter §7, 87a, 87b could be composed by several superheater sections distributed over at least a part of said longer dimension of the first, second or third vertically extending radiation pass 80, 81 , 82, whereby each superheater section is formed by straight inner and outer pipe sections 90, 92 arranged next to each other in the smaller direction of the first, -second or third vertically extending radiation pass 80, 81 , 82 as illustrated in Fig. 1 or 3.

Each straight inner pipe section 90 of the second superheater 87a, 87b. may be maintained coaxiaiiy or at least substantially coaxiaiiy in relation to the respective straight outer metal pipe section 92 of the second Superheater by means of at least a (not shown) ring-formed plug of insulation materia! arranged at either end of the pipe sec- lions in the spacing between the pipe sections.

The outer metal casing 36 of the second superheater 87, 87a,. 67b may advantageously be formed b or b provided with an outer coating or layer of alphas-alumina. Which as mentioned, above is an aluminium oxide being highly corrosion resistant.

The hot flue gasses 48 transfer heat to the: outer metal pipe 46, _: which is. heated to a high temperature, such as 650-760 degrees Celsius. This relatively hot outer pipe 48 then transfers heat via radiation to the relatively colder steam pipe 36.. The- steam pipe 36 is relatively colder, because it is cooled down to the temperature of the steam flow-- ing through it. This steam Is typically around 400-500 degrees Celsius.

LIST OF REFERENCE NUMBERS

In the following is given a list of reference numbers that are used in the de ailed description of the invention. i furnace,

2 incineration plant,

4 fuel

6 garbage truck,

8 fuel silo,

10 operator room,

12 crane,

13 hopper,

14 .grate,

15 chute,

16 air supply,

18 grate,

20 -grate,

22 pusher pistons _. ,

24 escaping flue ga^ses,

26 combustion chamber,

28 fuel inlet,

30 first super heater,

31 preheater

32 water inlet,

33 boiler drum,

34 steam outlet from first superheater,

35 water,

36 inner metal pipe, steam pipe, of second superheater

37 steam

38 outer metal casing of second superheater,

40 steam entering second superheater,

42 steam leaving the second superheater,.

46 -outer metal pip of second s _. uper ^' heater,

48 flow of flue gasses,

50 spacing,

52 distanc between the inner and outer metal pipes, 3 ate inlet

4 steam outlet

0 first vertically extending radiation pass.

1 second ^' vertically extending radiation pass,

2 third vertically extending radiation pass,

3 first and horizontally extending convection pass,

4 second and vertically extending convection pass,

δ first separation boiler wail,

6 ^• ■Second separation boiler wall.

7, 87a, 87b second superheater,

8 compressor,

9 flow meter,

straight inner pipe section,

1 U-forrned bend,

2 straight outer metal pipe section,

3 first junction box.

side wail of first junction box,

6 first end of straight outer metai pipe section,

second end .of straight outer metal pipe section,

7 top wall of first vertically extending radiation pass,

B second junction box.

9 side wall of second junction box,

0 side wail of furnace or of first vertically extending ra dlation pass,: and 1 pressure meter.