FR2836715A1 - Solid fuel boiler for producing super-heated steam, has superheater with receiver and emitter linked by heat exchanging fluid circuit - Google Patents

Abstract

The boiler has a firebox (10), a heat exchanger (14) to generate steam, and a superheater (20) to make superheated steam. The superheater has a receiving section (21) and an emitting section (22) connected by a heat exchange fluid circuit (31). The receiving section is located within the firebox to receive heat from it at a high temperature, while the emitting section is located outside the firebox and has the steam to be superheated passing through it. The exchange surface of the emitting section is situated well above that of the receiving section, as the heat transfer capacity of the emitting section is much greater than that of the receiving section. The heat transfer fluid can be caesium, potassium, magnesium, sodium, water or freon, or any other fluid or mixture of fluids compatible with heat exchange temperatures of 500 - 1300 degreesC, and the circuit has a sealed casing suitable for such fluids and temperatures, e.g. of nickel, stainless steel, chrome-nickel or inconel.

Description

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DESCRIPTION
Technical area
The present invention relates to a solid fuel steam boiler or incinerator for the production of superheated steam comprising a hearth, a heat exchanger producing steam and a superheater for superheating steam.

 In this presentation, waste is assimilated to a solid fuel and the expression boiler? is taken in the broad sense encompassing both boilers and incinerators.

State of the art
Steam boilers or steam incinerators are commonly used for the production of mechanical energy or electricity.

 The production of saturated vapor by evaporation which can then be superheated, and feed a turbine producing mechanical energy, or any other use.

 The mechanical energy recoverable from the superheated steam is highly dependent on the superheating of the steam. The greater the overheating, the higher the recoverable mechanical energy.

 In boilers using liquid or gaseous fuels without ash, overheating is easy; it is carried out by an exchanger placed in the high temperature smoke path near the hearth or directly in the hearth and exposed to the radiation of the flame.

 This exchange-superheater is traversed by the saturated steam coming from the evaporator-boiler of the boiler, to be superheated.

 The superheater must withstand high temperatures given the low convection coefficient between the steam and the wall of the superheater exchanger.

 To do this, heat shields are often placed between the convection radiation source and the exchanger placed in the fireplace and / or the fumes.

 In boilers with ashy or dusty fuels such as for example solid fuels, the installation of a steam superheater encounters an additional difficulty because of the fusibility of the ashes. This is all the more so since the fine, light ashes and fuses at low temperature tend to fly away.

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 The wall of the superheat exchanger is at an intermediate temperature between the temperature of the steam and the temperature of the hearth and / or the fumes, and is very close to the latter.

 The temperature of this wall is therefore relatively high 8000 to 1100 C for superheated steam at 500 or 600 C.

 The surges of fusible ash which agglutinate on the superheater wall and melt there, cause fouling of the exchange surface by vitrification, very difficult or even impossible to clean.

 The performance of the overheating is therefore very quickly reduced.

 In addition, when these ashes or dust contain abrasive or corrosive materials, for example silica, the wall of the exchanger is corroded by the formation of a gangue of ferrous silicate or the like from the iron of the exchanger-superheater. .

 Such phenomena are particularly restrictive for coal or wood boilers or various waste incinerators.

They penalize the architecture and design of the fireplace and or the smoke circuits by the size of the superheater exchanger. This results in less optimized performance.

 Another disadvantage lies in the fact that the superheater exchanger is not at a homogeneous temperature, due on the one hand to the course of the steam which is heating, and on the other hand to the irregularity of the hot zones d '' an ignition hearth with solid fuels. These irregularities create zones of weakness on the hot spots of the exchanger-exchanger-superheater.

 For high-power devices, these drawbacks are reduced by: - the installation of heat shields protecting the overheating exchanger, - large-volume fireplaces allowing the exchanger to be placed in an area less exposed to radiation and dust , - filtration systems (for example electrostatic) of the dust and ash contained in the fumes before they pass over the steam superheater exchanger, - the installation of the exchanger-superheater in a lower temperature smoke path , making it less efficient, - the fluidized bed technology which allows complete and homogeneous combustion at a temperature of around 850 C, lower than that

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 an ordinary hearth, allowing superheating of the vapor by the exchanger without these drawbacks while retaining a high exchange rate by the very nature of contact with the fluidized bed and with its particles in great agitation.

 However, these technologies are only economically feasible on high-power boilers generally greater than 20 MW.

 The explanation of the high temperatures of the superheaters is set out below.

que sépare l'atmosphère très chaude du foyer (par exemple 1 300 C) ou des fumées, de la vapeur à surchauffer jusqu'à 500 C, la température de cette paroi est beaucoup plus proche de celle du foyer que de celle de la vapeur. In a known boiler, a heat exchange wall

that separates the very hot atmosphere from the hearth (for example 1,300 C) or fumes, steam to superheat up to 500 C, the temperature of this wall is much closer to that of the hearth than that of the steam .

 According to FIG. 1, a known steam boiler with superheater consists of a hearth 1 with a hearth 2 and an ashtray 3. This hearth is combined with a heat exchanger 4 forming an evaporator-boiler, for example a bundle of tubes with a water inlet 5 and a saturated steam outlet 6 connected to a superheater 7 placed in the hottest part of the hearth, before the outlet 8 from the hearth to the heat exchanger 4.

 The saturated steam coming from the exchanger 4 is superheated in the superheater 7 to be supplied as superheated steam by the outlet 9.

 The hot gases from the hearth 1 first come into contact with the superheater 7 then pass through the outlet 8 of the hearth to pass through the exchanger 4 and exit through the flue pipe 10.

 FIG. 1A is a schematic view in partial section of the tube 11 of the superheater 7. The interior of the tube corresponds to the vapor enclosure and, outside the tube, to the hearth. The exchange surface between the tube and the hearth and that between the tube and the vapor is substantially the same.

 Under these conditions, by having - S, surface of the exchange wall, - Ul, exchange coefficient on the household side, in W / (m2 C), (global assimilated in radiation and convection), - U2, coefficient of exchange on the steam side in W / (m2 C),

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 as the coefficient Ul is much higher than U2, the transfer parameters are such that S * Ul>>> S * U2 so that the temperature of the wall which is intermediate between the hearth and the superheated steam will be much closer to that of the hearth than that of the steam. This explains that the superheater temperatures are so high, close to 1 OOOC.

Purpose of the invention
The object of the present invention is to remedy these drawbacks and proposes to develop a boiler for supplying superheated steam using solid fuel or waste considered as such and making it possible in particular to produce boilers of small power or of average power, less than 500 KW, to provide superheated steam for the production of mechanical or electrical energy from a turbine or for industrial and domestic thermal use or both combined in a cogeneration system.

- la partie émettrice traversée par la vapeur à surchauffer pour lui céder de la chaleur, - la surface d'échange de la partie émettrice étant très supérieure à la surface d'échange de la partie réceptrice pour que la capacité de transfert calorifique de la partie émettrice soit très supérieure à la ca- pacité de transfert calorifique de la partie réceptrice, Ul étant le coefficient d'échange de chaleur côté foyer, et U2 le coefficient d'échange de chaleur côté vapeur. Statement of the invention
To this end, the invention relates to a boiler characterized in that the superheater is formed by a receiving part and by an emitting part connected by a circuit of heat transfer fluid, - the receiving part is placed in the hearth to receive the heat fireplace,

- the emitting part crossed by the steam to be overheated in order to give it heat, - the exchange surface of the emitting part being much greater than the exchange surface of the receiving part so that the heat transfer capacity of the part is very much greater than the heat transfer capacity of the receiving part, Ul being the heat exchange coefficient on the hearth side, and U2 the heat exchange coefficient on the vapor side.

 This separation of the superheater between a receiving part and a transmitting part makes it possible to produce different exchange surfaces so that the temperature of the surface of the receiving part approaches that of the superheated steam and in all cases it remains very below the melting and agglutination temperature of the ash on the surface of the receiving part.

 This boiler design with superheater can be integrated into lower power ranges from 500 kW

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 providing superheated steam for the production of mechanical or electrical energy from a turbine, or for industrial or domestic thermal use, and in particular cogeneration.

 In a particularly advantageous manner, the superheater consists of an enclosure or a sealed sealed tube containing a pure heat transfer fluid, in vapor-liquid phase equilibrium, without other components, in particular a heat pipe, and capable of transmitting heat from one end heated to one end cooled by evaporation / condensation, with a very high transmission capacity. In such tubes or enclosures, the return of the liquid phase towards the hot end is done either by capillarity through a porous material, or fine grooves inside, or by gravity in the case of devices thermosyphons.

 Means may be provided to promote the return of the liquid phase of the heat transfer fluid inside the sealed tubes and its regular distribution over the interior surface of the sealed envelope such as interior longitudinal channels, or a porous capillary coating. But in general gravity is sufficient to ensure this return.

 The quantity of heat transfer fluid placed inside the sealed envelopes is low with regard to the mass of the combustibles in ignition, and the thermal inertia of the whole hearth, so that it does not provide risks. explosion or destruction in the event of accidental rupture of a saddled tube.

 The ratio of the emitting surface to the receiving surface is between 100 and 500, see beyond, and the temperature of the part exposed to radiation and convection of the hearth will be very close to the temperature of the outgoing superheated steam, c is to say of the order of 500 C.

 This results in the following advantages; - the resistance of materials to temperature no longer poses a problem, - ashes or dust, no longer melt on the surface of the receiver, which can be cleaned by simple sweeping, - the surface exposed to radiation and convection of the hearth is reduced for the same collected power, due to an increase in the temperature difference between the hearth and the receiving surface, - the size of the annoying superheater in the hearth can be considerably reduced,

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 - the perfectly uniform temperature over the entire surface of the receiver avoids local fragility.

 According to an improvement, the bundle of tubes of the emitting part includes a second exchanger which can occasionally bring water from the boiler into contact with the tubes, to evacuate heat in the lower part of the tubes, when operating irregularities appear. This phenomenon can occur when the fireplace is very hot, and the demand for steam suddenly decreases, thus preventing the cooling of the tubes.

 According to another improvement, a pipe for introducing liquid water from the boiler arrives directly in the superheater exchanger on the steam side, to obtain the same result as that indicated above.

 In these two cases, the water supply line is regulated or controlled in safety from the measurement of the temperature of the heat pipe tubes.

 The heat transfer fluid in the evaporation / condensation phase of the intermediate exchange device is cesium Cs, Potassium K, Magnesium Mg, Sodium Na, water H20, Freon or any other fluid or mixture of fluids whose change of state temperatures are compatible with the temperatures desired for the exchange between 1300 to 500 C, and the materials of the envelope.

 The sealed envelope is constructed of materials suitable for temperatures and / or fluid, such as nickel, chrome-nickel stainless steel, titanium, inconel.

 The fins, pins, grooves or other means of reduction of the emitting surfaces are made of a conductive material fixed on the intermediate exchange device of the copper, cast steel, aluminum, brass or other highly conductive material type. These means can be attached and fixed to the sealed tube with a material different from the latter or else be produced from the envelope for machining, pushing back or deformation of the envelope.

 The fins, spikes or gear reduction grooves of the emitting surfaces can also be made or covered with a more electronegative material than the materials constituting the steam circuits of the boiler and the networks it supplies, for example aluminum or an alloy aluminum at high melting temperature,

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 to protect the boiler and the networks it supplies against possible corrosion.

- la figure 3 montre un autre exemple de chaudière à vapeur avec surchauffeur selon l'invention, - la figure 4 montre une variante de la chaudière de la figure 3, - la figure 5 montre un autre exemple de chaudière avec surchauffeur selon l'invention, - la figure 6 montre un autre exemple de chaudière à vapeur avec sur- chauffeur selon l'invention. drawings
The present invention will be described below in more detail using the accompanying drawings in which: - Figure 1 is a schematic view of a steam boiler with superheater according to the prior art, - Figure 1A shows a detail of the superheater of the boiler of FIG. 1, FIG. 2 shows a first embodiment of a steam boiler with superheater according to the invention,

- Figure 3 shows another example of a steam boiler with superheater according to the invention, - Figure 4 shows a variant of the boiler of Figure 3, - Figure 5 shows another example of a boiler with superheater according to the invention , - Figure 6 shows another example of a steam boiler with superheater according to the invention.

 FIG. 6A is a section along AA of the superheater of FIG. 6.

 According to Figure 2, the invention relates to a steam boiler for producing superheated steam. This boiler comprises a hearth 10 with a hearth 11 and an ashtray 12. This hearth 10 is combined with a heat exchanger 14 forming a boiler, receiving water by an inlet 15 and supplying saturated steam by a steam outlet 16.

 The hot gases from the hearth pass through the outlet 18 of the hearth 10 to pass through the exchanger 14 (the circulation of hot gases is represented by the arrows) to leave the boiler through its outlet 17 and pass through the flue pipe.

 This boiler is equipped with a superheater 20 formed by a receiving part 21 and an emitting part 22. The inlet 24 of the superheater 20 connected by the line 23 is the saturated steam outlet 16 to superheat this steam and supply it via output 25 to a user not shown, like those mentioned above.

 The receiving part 21 of the superheater is placed in the hearth 10, in its hottest part while the emitting part 22 is installed outside the hearth.

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 The receiving part 21 is formed by one or more heat conducting tubes arriving in the emitting part 22 to supply heat to the saturated vapor arriving by the line 23 at the inlet 24, circulating through the exchanger of the superheater 22 and leaving by exit 25.

 The receiving part 21 and the emitting part 22 of the superheater are generally designed so that the heat exchange surface S2 of the emitting part 22 is much greater than the heat exchange surface 81 of the receiving part 21 this asymmetry is that the heat transfer capacity of the emitting part 22 is much higher than the heat transfer capacity of the receiving part 21, and that the temperature of the receiving part 21 is close to the temperature of the fluid (superheated steam) leaving the emitting part 21 of the superheater 20, this takes into account the fact that the heat exchange coefficient U1 on the side of the hearth 10 and the heat exchange coefficient U2 on the steam side are very different, the coefficient U1 being much higher than the coefficient U2.

 For example, the ratio between the emitting (S2) and receiving (SI) surfaces is of the order of 100 to 500 and more.

 Under these conditions, the temperature of the receiving part 21 exposed to radiation and convection of the hearth 10 will be very close to the temperature of the superheated steam leaving the superheater 20, that is to say of the order of 500 ° C. while the temperature of the hot gases in the hearth 10 is, in general, greater than 1000 C.

 The receiving part 21 of the superheater 20 is formed of sealed tubes containing a heat transfer fluid, in particular a pure heat transfer fluid in equilibrium of vapor-liquid phases. The tube or tubes 31 extend into the emitting part 22 where they are provided with fins 32 so as to increase the exchange surface S2. The emitting part 22 is also provided with partitions 33 creating a path for the circulation of saturated steam around or in the fins 32 as indicated by the arrows, before joining the outlet 25 in the form of superheated steam.

 The tubes 31 are heat pipe tubes. They can be of constant cross section but also of different cross section on the side of the receiving part 21 and on the side of the sending part 22, so that the overall exchange surface in the transmitting part 22 is much greater than the exchange surface in the receiving party SI. The precise description of the tubes

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 heat pipes is known per se; it has been briefly mentioned above and does not require a more detailed description.

 The tubes 31 are in a substantially horizontal position in the hearth 10. They exit through their upper part much longer than the part located in the hearth, always with the aim of increasing the exchange surface.

 In the emitting part 22, the heat pipe tubes can have a larger section than that in the receiving part 21. They can also have a smaller section but their outer surface is completed by fins, so as to generally not create excessive bulk. .

 The emitting part is surrounded by an enclosure 34 which defines the path of saturated vapor, around the tubes 31.

 The tubes are preferably arranged in a calender, they are maintained by one or more tubular plates as well as the partitions 33 forming the baffles.

 Figure 3 shows another embodiment of the superheated steam boiler according to the invention. In this figure, references similar to those of FIG. 2 have been used to facilitate the description; these references are those of FIG. 2 increased by 100. The parts not described here are assumed to be the same as those of the embodiment of FIG. 2.

 This embodiment differs from that of FIG. 2 by a different organization of the superheater 120 whose receiving part 121 is placed more at the end of the hearth 110, near the outlet 118 of the hot gases then passing through the evaporator exchanger 114 providing saturated steam.

 The lower part 140 of the exchanger 114 is connected to the lower part 141 of the emitting part 122 of the superheater 120 by a line 142 provided with a solenoid valve 143. This solenoid valve is controlled by a control circuit 144 provided with a sensor of temperatures 145 detecting the temperature of one or more tubes 131 of the receiving part 121 of the superheater. As soon as the temperature measured by sensor 145 exceeds a threshold, this means that the heat transfer fluid in the superheater remains at a high temperature, that is to say that it does not yield enough heat energy to the steam to be superheated . The control circuit 144 then opens the solenoid valve 143 to allow the water from the exchanger 114 to fill the bottom 414 of the enclosure 134 of the part

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 transmitter 122, to evaporate water and remove excess heat from the superheater.

 Figure 4 shows a variant of the boiler. In this variant, elements similar or identical to those of the embodiment of FIG. 2 bear the same references increased by 200. The description of the identical elements will not be repeated.

 This superheated steam boiler is characterized by the superheater 220 formed of modular elements 220-1,220-2, ... grouped in a set and connected in parallel or in series or in parallel / series for the superheated steam circuit.

 This modular construction has the advantage of a simplification of manufacture since the superheater can be adapted to the power of the boiler.

 Figure 5 shows another alternative embodiment bearing the same references as that of Figure 2 increased by 300 for identical or similar elements.

 In this embodiment, the superheater 320 is placed above the exchanger-evaporator 314 and the heat transfer fluid is a liquid which describes a circuit between the receiving part 321 and the emitting part 322. The circuit consists of a ( or more) tube loop 331 provided with a pump 336.

 Figure 6 shows another embodiment of the invention. In this embodiment, the elements identical or analogous to those of the embodiment of FIG. 2 bear the same references increased by 400.

 The hearth 410 is traversed, in this case, by the superheater 420, the outer surface of which constitutes the receiving part 421 and the inner surface, the emitting part 422.

 The sectional view of FIG. 6A shows this particular shape of the superheater 420. The external surface is, for example, that of a cylindrical tube with circular section while the internal surface is formed of fins or channels so that this internal surface which constitutes the receiving part 422 is much greater than the external surface 421. Thus according to the invention this superheater 420 is traversed by the saturated steam supplied by the evaporator 414, superheated at the outlet 425 and the external surface of the tube forming the superheater 420 remains at a temperature close to the superheated steam and is not close to that of the very hot combustion gases in the hearth 410.

Claims (10)

 CLAIMS 10) Solid fuel steam boiler for the production of superheated steam comprising a hearth, a heat exchanger producing steam and a superheater for superheating steam, characterized in that the superheater (20) is formed by a receiving part (21) and an emitting part (22) connected by a heat transfer fluid circuit (31), - the receiving part (21) is placed in the hearth (10) to receive the heat from the hearth, - the emitting part ( 22) crossed by the steam to be overheated to give it heat, - the exchange surface of the transmitting part (S2) being much greater than the exchange surface of the receiving part (SI) so that the transfer capacity heat (U2. S2) of the emitting part is much greater than the heat transfer capacity (U1. S1) of the receiving part, Ul being the heat exchange coefficient on the hearth side, and U2 the heat exchange coefficientsteam side.  the emitting part (22) is placed outside the hearth (10).

 2) Boiler according to claim 1, characterized in that 3) Boiler according to claim 1, characterized in that the heat transfer fluid circuit (31/331) is a heat pipe or a heat transfer liquid circuit equipped with a pump (336).  4) Boiler according to claim 3, characterized in that the heat transfer fluid circuit is formed of several heat pipes.  face of the receiving part in contact with the gases of the hearth (10).

 5) Boiler according to claim 1, characterized in that the superheater (20) is formed of a tube (31) whose surface in the emitting part, on the side of the superheated steam, is much greater than the super- 6) Boiler according to claim 1, <Desc / Clms Page number 12>  characterized in that the emitting part (122) of the superheater (120) is connected (140,142) to the exchanger (114) of the boiler to receive water to evacuate heat from the lower part (141) of the emitting part (122) of the superheater (120).  7) Boiler according to claim 6, characterized by control and detection means (144,145) for supplying water from the boiler to the superheater (120), by detecting the temperature of the fluid of the superheater (120) and opening the communication (142) to the superheater (120) in the event of excessive temperature.  8) Boiler according to claim 3, characterized in that the heat transfer fluid in the evaporation / condensation phase is cesium Cs, or 1 Potassium K, or Magnesium Mg, or Sodium Na, or water H20 , or Freon or any other fluid or mixture of fluids whose change of state temperatures are compatible with the temperatures desired for the exchange between 1300 and 500 ° C., and the materials of the envelope.  9) Boiler according to claims 3 and 8, characterized in that the circuit is formed of a sealed envelope constructed of materials adapted to temperatures and / or heat transfer fluid: nickel, chrome-nickel stainless steel, titanium, inconel. 10) Boiler according to claim 3, characterized in that the superheater circuit (20) is provided, in the emitting part (22), fins (32), pins, or gear reduction grooves made of a fixed conductive material on the tube (31) of copper, cast steel, aluminum, brass or other highly conductive material or made from the material of the tube, by machining, embossing or deformation of the tube.