IE52052B1 - A boiler for fluid-bed combustion of solid fuels - Google Patents

Abstract

The boiler combustion chamber (1) contains one or more vertical shafts (9) which are closed gas-tight at the top and issue at the bottom over the topside (8) of the eddy layer. The shaft walls (10) contain fluid-cooled pipes (12,13) which continue down through the eddy layer to distributor pipes (17) near the chamber's perforated bottom wall (3). The shaft's total cross-sectional surface is so much smaller than the eddy layer's top side that exhaust gases from the layer can flow upwards without hindrance through the combustion chamber between the shaft walls and the boiler's side walls (2'). Cooling of the shaft walls prevents ignition of fine fuel particles over the eddy layer, and permits usage of mechanical fuel feed extending across the whole combustion chamber's length.

Description

A Boiler for fluid-bed combustion of solid fuels.

The present invention relates to a boiler for fluid-bed combustion within a fluidized bed of inert material which during operation of the boiler is maintained in a fluidized state in the combustion chamber of the boiler by the injection of combustion and fluidizing air through the bottom of the chamber, and having means for feeding solid fuel into the combustion chamber above the surface of the fluidized bed.

In boilers where the surface area of the 10 fluidized bed is relatively small it is, in principle, possible to obtain a relatively uniform distribution of the fuel across the entire area by employing a spreader stoker which throws the fuel through an aperture in a side wall of the combustion chamber, as the fuel feed means. This technique requires, however, the use of a relatively coarse fuel, the particle size of which is comprised within a narrow ' range, since fine particles in the fuel would be entrained by the upward flow of flue gases and then burn above the fluidized bed rather than within the bed. In boilers with larger cross-sectional area it will also be necessary to feed the fuel from several sides which complicates the devices for supplying the fuel and distributing it to each feeding location.

According to the present invention a boiler of the kind referred to above is characterized in that one or more shafts, each of which is open at its bottom and closed upwardly and laterally, are provided within the combustion chamber with their open lower ends located at a short distance above the surface of the fluidized bed, that the walls of said shaft or shafts are composed of fluid cooled tubes, that the total cross-sectional area of the shafts is substantially less than the cross-sectional area of the combustion chamber, and that the fuel feeding means is arranged in the region of the upper closed end of said shaft or shafts.

Within said shaft or shafts, around and between which the flue gases emanating from the fluidized bed can flow freely upward and out of the combustion chamber, there will - due to the gas-tight shaft walls be no upward gas flow which would meet the incoming fuel and thereby influence its free fall down into the fluidized bed. Consequently, even fuel consisting of particles or lumps of widely different sizes can be evenly distributed across the entire area of each shaft. When spreader stokers are used for feeding the fuel it is possible to employ a greater throwing length and it is, therefore, possible to feed considerably larger boilers than previously from one side only.

Due to the cooling of the shaft walls the gas present in the shaft will be substantially cooler than the flue gases flowing upwardly around the shaft which reduces the risk of an undesired ignition of very fine and, hence, slower falling fuel particles. Furthermore, the cooling permits to effect the fuel infeed by means of a screw feeder or other mechanical conveyor extending into the shaft and discharging fuel evenly along its length, without the risk of thermal overload on the component parts of the conveyor.

Although the falling fuel reaches the surface of the fluidized bed within one or several spaced zones the total area of which corresponds to the total shaft area, there is obtained a sufficiently even distribution of the fuel through the entire cross section of the fluidized bed because the internal turbulence in the bed results in a lateral spreading of the fuel similar to the spreading which occurs with the known point-like fuel feed from below through the bottom of the combustion chamber.

A preferred embodiment of the invention is characterized in that at least two opposed tubular walls of each shaft are prolonged downwardly into the fluidized bed in the form of divergent tube bundles or groups with gaps between the individual tubes of each bundle, and that adjacent the bottom of the combustion chamber the tubes are connected to coolant carrying headers. This feature permits the utilization of the coolant flowing through the tubes of the shaft walls for controlling the temperature of the fluidized bed.

The tubes of each shaft wall may be prolonged downwardly as two mutually divergent tubes bundles, each of which comprises every second tube of the shaft wall, both said tube bundles being connected to the same header at the bottom of the boiler. This structurally simple embodiment ensures suitable large flow passages for the flue gases between the tubes of each tube bundle and an even distribution of the tubes within the fluidized bed.

When the cross-section of the combustion chamber is rectangular, each shaft may extend across the width of the combustion chamber between one pair of opposed walls thereof.

In an embodiment of the invention wherein the side walls of the combustion chamber are constituted by water cooled tube panels, the tubes of the shaft walls may be connected to the tubes of the combustion chamber walls. Whether the boiler is designed for producing steam or hot water at superatmospheric pressure the shaft walls are then incorporated as an integrated part of the boiler heating surfaces providing a direct utilization of the coolant water heated in the tubes of the shaft walls.

At the upper end of each shaft there may be provided means for injecting air into the shaft.

This creates, in each shaft, a downwardly directed air stream which contributes to transporting the fuel down into the fluidized bed and which, at the same time, safeguards against undesired leakage of hot flue gases and possible fine fuel particles through the fuel infeed openings, even in case the boiler is internally pressurized.

The air injection means may preferably be located closely to the fuel feeding means. Especially when spreader stokers are employed for feeding the fuel into the combustion chamber, this feature permits the combustion of very roughly graded or sized fuel containing a large proportion of fine particles which will not be thrown far away by the impeller of the stoker but which on the other hand will be entrained by the air stream.

The Invention will now be described in more detail with reference to the accompanying, highly schematical drawings in which Pig. 1 is a vertical section through a first embodiment of the invention.

Fig. 2 is a vertical section along line II-II of Fig. 1, Fig. 3 is a fractional view along line IXI-III of Fig. 1, on a substantially larger scale, Fig. 4 is a corresponding fractional view along line IV-IV of Fig. 1, Fig. 5 is a section, similar to Fig. 1, showing a modified embodiment of the invention, and Fig. 6 is a top plan view of the boiler of Fig. 5.

The boiler shown in a highly simplified manner in Figs. 1-4 has a combustion chamber 1 defined by four vertical side walls 2 and 2’, a perforated bottom wall 3 and a top wall 4. A flue gas discharge duct is connected to an aperture in one side wall 2 adjacent top wall 4. Below the perforated bottom wall 3 the boiler comprises an air or plenum chamber 6 into which a duct 7 for supplying combustion and fluidizing air opens.

The combustion in the boiler takes place, as well-known in the art, within a bed or layer of a suitable particulate material which is kept fluidized by means of air supplied through duct 7 to chamber 6 and flowing from there upwardly through the perforated bottom wall 3 and through the layer of particulate material. In Fig. 1 the irregular, and during operation of the boiler continuously changing, surface of the fluidized bed has been indicated at 8 while in Fig. the fluidized bed has been omitted for the sake of clarity.

Internally of combustion chamber 1 there is provided a shaft generally designated by 9 and having a rectangular horizontal cross-section. The shaft comprises two vertical side walls 10 extending sub30 stantially from one side wall 2 of the boiler to the opposite boiler side wall, and two narrower side walls 11. Bach shaft wall 10 and 11 is constructed as a so-called membrane wall of which a fraction is shown in Fig. 3. Each wall consists of coolant tubes 520S2 designated alternately by 12 and 13 and mechanically connected by means of fins 14, each of which is welded to two adjacent tubes so as to form a gas -tight wall. At the upper shaft end the tubes of its walls are connected to a header or collector tube to which also the fins 14 are welded so that the upper end of the shaft is completely closed. Means (not shown, are provided for causing a fluid coolant to flow through tubes 12 and 13.

The fins 14 of the shaft walls terminate at a short distance above the surface 8 of the fluidized bed while tubes 12 and 13 continue downwards almost to the bottom wall 3 of the combustion chamber where they are connected to two headers or distributor tubes 17. As seen in Figs. 1 and 4 each bundle or group of tubes 12 has been deflected outwardly from point 16 substantially in a horizontal direction and subsequently vertically down through the fluidized bed to distributor tube 17 while each tube 13 of the other tube bundle extends along an inclined straight line from point 16 to the distributor tube in question. As best seen in Fig. 4 there is thus formed, between the individual tubes of each group or bundle, flow passages for the flue gases originating in the fluidized bed so that the gases can flow freely upwards through the combustion chamber around shaft 9, as shown by arrows in Fig. 1.

Adjacent the upper closed end of shaft 9 an aperture 18 is formed in one of its side walls 11 opposite an aperture 19 in the adjacent side wall 2 of the combustion chamber. A fuel supply duct 20 opens into aperture 19, and in duct 20 there is provided a spreader stoker having an impeller 21 which throws the particulate fuel into the shaft as shown by dotted lines 22 in Fig. 2. Since shaft 9 is gas -tight or sealingly closed both laterally and upwardly there is no upward air stream within the shaft which could exert an unfavourable influence on the distribution of the injected fuel across the shaft area or on the free fall of the fuel down into the fluidized bed, as shown by vertical arrows 23.

The side and top walls of combustion chamber 1 may be constructed as membrane walls similar to the shaft walls and including tubes connected to one or more distributor and collector tubes, as shown at 24 and 25, respectively, in Figs. 1 and 2. Tubes 12 and 13 of the shaft walls may be connected to the tube systems of the combustion chamber walls in a manner not shown.

Alternatively, the shaft 9 may be defined, on its opposed narrower sides, by parts of the tube walls 2 instead of, as shown, by separate tube walls 11.

Figs. 5 and 6 illustrate how the invention may 20 be embodied in boilers in which the surface of the fluidized bed is so large that a satisfactory distribution of the fuel cannot be obtained by feeding it through one shaft. Those component parts of the boiler, which correspond to the embodiment of Figs. 1-4, have been designated by the same reference numerals as therein. In view of the larger cross-sectional area of combustion chamber 1 and, hence, of its perforated bottom wall 3 this wall has been divided into three sections and below each section there is provided an air or plenum chamber 6 with an air supply duct 7 opening therein. Above each section of bottom wall 3 there is provided a vertical shaft 31, the side walls of which can be constructed exactly as the walls shown in Figs. 1-4, including two groups of tubes 12 and 13 which from the lower edges of the fins (not shown) of the gas-tight shaft side walls continue downwards into the fluidized bed where they are connected to lower distributor headers 17 for a fluid coolant.

In contrast with the embodiment of Figs. 1-4 the gas-tight walls of shaft 31 extend upwardly to the boiler top wall 4, and the tubes of the shaft walls are connected to collector tubes 32 extending across the boiler along the upper surface of wall 4. Each tube 32 is covered by a sheet element 33 shaped as an inverted U, and the opposed lateral surfaces of each pair of elements 33 define a fuel feed duct 34 extending transversely above the top wall 4, as seen in Fig. 5. Fuel is supplied to each duct 34 by a schematically shown screw feeder 35 mounted above shaft 31 and having a housing which is welded gas-tight to the two associated sheet elements 33.

For the sake of clarity elements 33 have been omitted in Fig. 6.

As shown at the left-hand side of Fig. 5 a suitable number of nozzles 36 may be distributed along the length of each feed duct 34 for injecting downwardly directed streams of air into the ducts.

By means of the three screw feeders 35 the fuel can be distributed substantially uniformly along the entire length of each duct 34 from which the fuel falls freely towards the fluidized bed assisted by the air streams issuing from nozzles 36 so that within the contour of each shaft 31 there is obtained a corresponding uniform distribution of the fuel as described above with reference to the first embodiment.

Similar to the embodiment of Figs. 1-4 the flue gases leave the combustion chamber 1 through suitably shaped apertures at the upper end of one side wall 2 of the chamber. For the sake of clarity, these apertures have not been shown but it has been indicated by means of arrows in Fig. 6 that six such apertures may be provided in the side wall.

When the fuel is fed into the shaft or shafts by means of spreader stokers the free fall of the fuel through each shaft may also be assisted by injected air.In that case the injection nozzle or nozzles may advantageously be located at the bottom of each aperture in the boiler wall through which the fuel is injected, or immediately below that aperture.

Claims (8)

1. PATENT CLAIMS

1. A boiler for fluid-bed combustion within a fluidized bed of inert material which during operation of the boiler is maintained in a fluidized state in the combustion chamber (1) of the boiler by the injection of combustion and fluidizing air through the bottom (3) of the chamber, and having means (21, 35) for feeding solid fuel into the combustion chamber above the surface (8) of the fluidized bed, characterized in that one or more shafts (9, 31), each of which is open at its bottom end and closed upwardly and laterally, are provided within the combustion chamber with their open lower ends located at a short distance above the surface of the fluidized bed, that the walls of said shaft or shafts are composed of fluid cooled tubes (12, 13), that the total cross-sectional area of the shafts is substantially less than the cross-sectional area of the combustion chamber, and that the fuel feeding means is arranged in the region of the upper closed end of said shaft or shafts.

2. A boiler as claimed in claim 1, characterized in that at least two opposed tubular walls of each shaft are prolonged downwardly into the fluidized bed in the form of divergent tube bundles (12, 13) with gaps between the individual tubes of each bundle, and that adjacent the bottom of the combustion chamber the tubes are connected to coolant carrying headers (17).

3. A boiler as claimed in claim 2, characterized in that the tubes of each shaft wall (10) are prolonged downwardly as two mutually divergent tube bundles (12, 13) each of which comprises every second tube of the shaft wall, and in that both said tube bundles are connected to the same header at the bottom of the boiler.

4. A boiler as claimed in any of claims 1-3 and wherein the cross-section of the combustion chamber 5. Is rectangular, characterized in that each shaft extends across the entire combustion chamber between one pair of opposed walls thereof.

5. A boiler as claimed in any of claims 1-4 and in which the side walls (2, 2*, of the combustion 10 chamber are constituted by water cooled tube panels, characterized in that the tubes (12, 13) of the shaft walls are connected to the tubes of the combustion chamber walls.

6. A boiler as claimed in any of claims 1-5, 15 characterized by means (36) located at the upper end of each shaft for injecting air into the shaft.

7. A boiler as claimed in claim 6, characterized in that the air injection means is located closely to the fuel feeding means. 20

8. A boiler for fluid-bed combustion, substantially as described above with reference to Figs. 1-4 or Figs. 5-6 of the accompanying drawings.