AU615310B2

AU615310B2 - Fluidized bed plant

Info

Publication number: AU615310B2
Application number: AU36764/89A
Authority: AU
Inventors: Martin Hirsch; Rainer Reimert; Karel Vydra
Original assignee: Metallgesellschaft AG
Current assignee: Doosan Lentjes GmbH
Priority date: 1988-06-25
Filing date: 1989-06-23
Publication date: 1991-09-26
Anticipated expiration: 2009-06-23
Also published as: EP0349030A1; IN170802B; ZA894791B; FI891981A0; FI92955B; JP2785041B2; JPH0248033A; AU3676489A; FI891981A; PL161384B1; PL278938A1; FI92955C; CA1310808C; DD289586A5

Description

61531 0 ,o COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952-69 COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: a Priority P Rlated Art: Narrie of Applicant: 0 SAd'dress of Applicant: 4* Actual Inventor: 0 a r Service A4'>ress for Service METALLGESELLSCHAFT AKTIENGESELLSCHAFT Reuterweg 14, D-6000 Frankfurt/Main, Federal Republic of Germany MARTIN HIRSCH, RAINER REIMERT and KAREL VYDRA SXYf g'@X(a qVXg Watermark Patent Trademark Attorneys 50 QUEEN STREET, MELBOURNE, AUSTRALIA, 3000.

Complete Specification for the invention entitled: FLUIDIZED BED PLANT The following statement is a full description of this invention, including the best method of performing it known to 1.

t. ~rl:I FLUIDISED BED PLANT Description This invention relates to a fluidised bed plant which consists of a fluidised bed reactor, a solids separator and a recycling line and serves to carry out exothermic processes in a circulating fluidised bed, comprising lines for supplying oxygen-containing primary gases through the bottom of the fluidized bed reactor, lines for supplying oxygen-containing secondary gases in an elevation of at least 1 metre above the reactor bottom but not in excess of 30% of the height of the reactor, and a fuel line which opens into the fluidised bed reactor between the primary and secondary gas inlets, wherein 40 to 75% of o the bottom surface area of the fluidised bed reactor are 0000 covered by one or more displacers having a height not in excess of one-half of the height of the fluidised bed 00000 reactor.

o'i4 ~Processes and equipment for handling circulating fluidised beds, particularly for combusting carbonaceous materials, have proved most advantageous and for numerous reasons are superior to processes and equipment in which 0 so-called orthodox or conventional fluidised beds are employed.

0 0 Particularly for combustion processes the basic process has been described in German Patent Publication 39 546 (corresponding to U.S. Patent 4,615,717). In that Co process the combustion is effected in two stages and the o o heat of combustion is extracted by means of cooling surfaces disposed in the fluidised bed reactor above the secondary gas inlet region. The special advantage afforded bq the process resides in that the combustion process can be adapted in a technically simple manner to the required output in that the suspension density and the heat transfer to the cooling surfaces is controlled in the upper portion of the reactor space.

In the combustion process using a circulating fluidised bed in accordance with Published German )i- -3- Application 26 24 302 (corresponding U.S. Patent 4,111,158) all or part of the heat of combustion is extracted in a fluidised bed cooler, which succeeds the fluidised bed reactor, and cooled solids are recycled to the fluidised bed reactor in order to maintain a constant 'temperature. In that case an adaptation to the required output is effected by an increase or decrease of the rate at which solids are caused to flow through the fluidised bed cooler and then back into the fluidised bed reactor.

Whereas the processes outlined hereinbefore have proved highly satisfactory, the existing trend towards plant units for progressively increasing heat outputs results in ooo certain difficulties in the control of the process. These difficulties essentially reside in that higher heat outputs require larger reactor heat of combustion is extracted by 0 000 means of cooling surfaces disposed in the fluidised bed 0areactor above the secondary gas inlet region. The special 0 0o -advantage afforded by the process resides in that the combustion process can be adapted in a technically simple 20 manner to the required output in that the suspension density r.0 60 Sand the heat transfer to the cooling surfaces is controlled in the upper portion of the reactor space.

In the combustion process using a circulating o fluidised bed in accordance with published German Application 26 24 302 (corresponding to U.S. Patent 4,111,158) all or part of the heat of combustion is a0 0 extracted in a fluidised bed cooler, which succeeds the 0 0 So fluidised bed reactor, and cooled solids are recycled to the 00 fluidised bed reactor in order to maintain a constant temperature. In that case an adaptation to the required output is effected by an increase or decrease of the rate at which solids are caused to flow through the fluidised bed cooler and then back into the fluidised bed reactor.

Whereas the processes outlined hereinbefore havei proved highly satisfactory, the existing trend towards plant units for progressively increasing heat outputs results in V certain difficulties in the control of the process. These

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I0 0 U Oo 00 o0 o o 0 0 0 o 0 0 00 0 0 0 i oo 0 0 00 0 01 0 0 0 0 00 difficulties essentially reside in that higher heat outputs require larger reactor dimensions, particularly larger reactor cross-sections, so that the satisfactory transverse mixing of fuel and the like, on the one hand, and of oxygen-containing secondary gas, on the other hand, throughout the cross-section of the fluidised bed reactor is no longer ensured in the inlet region although such mixing is required for the reaction. As a result, a considerable part of the reaction is shifted to the upper portion of the reactor space and afterburning may occur when the solids and gas have been separated in the solids separator. the situation outlined hereinbefore is encountered in plants having combustion heat outputs above about 300 MW in fluidised bed reactions having bottom areas in excess of 15 about 50 m2 In accordance with a prior proposal that problem is to be solved in that 40 to 75% of the bottom surface area of the fluidised bed reactor are covered by one or more displacers having a height not in excess of one-half of the 20 height of the fluidised bed reactor. The geometric configuration of the displacer may be selected substantially as desired. For instance, in a fluidised bed reactor having a circular cross-section the displacer may have the shape of a cylinder or of a frustum of a cone and the centre of the bottom circular surface may lie approximately on the centre of the bottom surface. In a reactor having a rectangular cross-section the displacer may have the shape of a dam, which at its ends may optionally adjoin the parallel walls of the reactor so that the lower portion of the reactor space is divided into two separate chambers. Two dams may be provided, which may be virtually at right angles to each other and which, if they adjoin the walls of the reactor, divide the lower part of the reactor space into four separate chambers.

It has been found that trouble in the operation of a fluidised bed plant designed as described hereinbefore may arise if the displacer'divides the lower part of the reactor 1.

.1 *1 000 00 0 3 0 O 0 0) 01 0 -1 00 0 0 1 0) space into separate chambers because in that case the primary air stream may entrain bed material from one chamber so that the internal circulation of solids always taking place in a fluidised bed reactor will cause such entrained bed material to enter another chamber or other chambers.

Unless an expensive control is provided there will be no reverse or compensating flow of material because the flow of the fluidising air through the "depleted" chamber will be promoted and the flow through the "enriched" chamber will be restricted (owing to the different hydrostatic pressures over the bottoms of the respective chambers).

It is an object of the invention to provide a fluidised bed plant which consists of a fluidised bed reactor, a solids separator and a recycling line and serves 15 to carry out exothermic processes in a circulating fluidised bed which plant ensures a satisfactory and reliable operation without a need for an expensive control even when the plant is operated at a high combustion power.

That object is accomplished in that the remaining 20 bottom surface of the fluidised bed reactor constitutes a single coherent surface.

In order to provide a single coherent surface the displacer or displacers are so designed that individual segments of the reactor bottom communicate with each other.

This can be achieved, in that a gas-permeable bottom surface is left between the displacer and the reactor wall or if the displacer extends from wall to wall the displacer is formed with at least one through passage, which may be open-topped or tunnel-like, so that individual bottom segments are joined to each other. The displacers must not be so designed that separate bottom segments and separate fluidising chambers are formed. The fluidising gas may optionally flow at a lower velocity through the connecting surfaces which are left by the displacer or displacers or through the base surfaces which define the passage through the main surfaces of the grate.

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-6- The configuration of the displacer may substantially be selected as desired. For instance, in a fluidised bed reactor which is circular in cross-section the displacer may consist of a cylinder or of a frustum of a cone and the centre of the circular bottom surface of the displacer may lie approximately on the centre of the bottom surface of the reactor. In a reactor having a rectangular cross-section the displacer may consist of a dam or two dams may be provided which extend substantially at right angles to each other.

It will be particularly desirable to provide displacers which are square or rectangular in cross-section.

Certain departures from the exact geometric figure, e.g., the provision of rounded corners, are possible.

The displacer may be made from a refractory material which is conventional in furnace construction.

Alternatively it may be made of membrane walls or finned walls which are flown through by a coolant and which are protected by a covering consisting of a rammed compound on 20 the surface which faces the reactor space. The displacer or displacers is or are firmly connected to the reactor and together with the reactor constitutes or constitute a structural unit.

The material which is capable of an exothermic 25 reaction is charged through a plurality of charging devices so that individual segments formed in the lower part of the reactor space can be separately supplied.

In accordance with a preferred feature of the invention the displacer or displacers is or are provided with means for charging fuel. Such means may optionally be provided on a plurality of levels so that an effective distribution of the fuel will be ensured.

In accordance with a further preferred feature of the invention the fluidised bed plant comprises a displacer which is provided with means for supplying oxygen-containing

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III I- I- -7secondary gas. Such means may optionally be arranged on a plurality of levels. In accordance with that feature of the invention the individual chambers or chamber portions may be supplied with secondary gas through inlets which are provided in the wall of the fluidised bed reactor and in the interior of said reactor so that an optical admixing of the secondary gas will be ensured.

If secondary gas is supplied through lines provided in the wall of the fluidised bed reactor, the crest of the displacer should be disposed above such lines. If secondary gas is supplied on a plurality of superimposed levels the 0,;0 displacer must extend above the level of at least the lower o most inlet.

In accordance with a further desirable embodiment 15 of the invention, one or more displacers are provided which o 0 has or have a cross-sectional area which decreases upwardly.

o1 o In that case and in conjunction with the embodiment described last hereinbefore it will be possible to keep the velocity of flow in the reactor part provided wifh the displacer within certain limits in spite of the supply of o secondary gas.

The principle of the circulating fluidised bed °I 0 whijh is used in the fluidised bed plant is distinguished in that states of distribution having no defined boundary layer are provided, contrary to the "orthodox" fluidised bed, in which a dense phase is separated by a distinct density step from the overlying gas space. In the circulating fluidised 0 bed there is no density step between a dense phase and an overlying gas space but the solids concentration in the reactor decreases from bottom to top.

By means of the Froude and Archimedes numbers the following ranges may be defined for the operating conditions: 0.1 3/4 x Fr x k g and 0.01 Ar 100 ~~-Ik It -8wherein Ar dk3 x g (k g) g x y Fr 2 u 2 G X dk and u relative gas velocity in m/s Ar Archimedes number Fr Froude number g density of gas in kgg/m 3 k density of solid particle in kg/m 3 k diameter of spherical particle in m oo; 7 kinematic viscosity in m 2 /s g constant of gravitation in m/s2 0o The exothermic reaction is carried out in two with 0 oxygen-containing gases supplied on different levels. It has the advantage that the reaction is "soft", local o° -O0 overheating will be avoided and a formation of NO will 0 0 X substantially be suppressed. The uppermost inlet for oxygen-containing gas should be sufficiently above the lowermost one to ensure that the oxygen content of the gas supplied at the lower inlet will substantially have been consumed at the upper inlet.

O o If steam is desired as process heat, a desirable feature of the invention resides in that the rates of fluidising gas and secondary gas are adjusted to provide °°25 above the secondary gas inlet an average suspension density a o between 15 and 100 kg/m 3 and that the reaction heat is C, a extracted through cooling surfaces which are provided in the free space of the fluidised bed reactor above the secondary gas inlet and/or on the wall of the fluidised bed reactor.

Such a mode of operation has been described more in detail in German Patent Publication 25 39 546 and the corresponding I U.S. Patent 4,165,717. i t

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"2 -9- The gas velocities obtained in the fluidised bed reactor above the secondary gas inlet region are normally in excess of 5 m/s and may be as high as 15 m/s during an operation under normal pressure. The ratio of the diameter to the height of the fluidisec'. bed reactor should be selected to provide for gas residence times between 0.5 and second, preferably between 1 and 4 seconds.

A plurality of inlet openings for secondary gas are desirably provided on each inlet level.

A special advantage which is afforded by that mode of operation resides in that the rate at which thermal process energy is generated may be changed at any time by a ono change of the suspension density in that furnace space of o4 the fluidised bed reactor which is disposed above the secondary gas inlet region.

Under the operating condition which is obtained o: when the fluidising and secondary gases are supplied at o predetermined volume rates so that a given average suspension density is obtained, a certain heat transfer will be achieved. The heat transfer to the cooling surfaces can o" o be increased in that the suspension density is increased by o 0 an increase of the rate of fluidising gas and optionally of the rate of secondary gas. At a virtually constant 0 'o combustion temperature the higher heat transfer will permit an extraction of the quantities of heat which are generated at a higher combustion heat output. Whereas the higher SO O combustion heat output involves a higher oxygen demand, this o will virtually automatically be met because a higher fluidising gas rate and optionally a higher secondary gas rate is required in order to increase the suspension density.

Similarly, for an adaptation to a lower process heat demand the combustion heat output may be controlled by a decrease of the suspension density in the furnace space of the fluidised bed reactor above the secondary gas inlet region. The lower suspension density will result in a lower heat transfer so that'less heat will be extracted from the 0

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.14* 0 0o o *o *0 0 00 31 00 fluidised bed reactor and the combustion heat out-put can be decreased virtually without a temperature change.

In accordance with another suitable feature of the invention, which feature is of more universal utility, a fluidised bed plant is provided with at least one fluidised bed cooler, which is connected via a solids supply line and a solids recycle line. In the fluidised bed reactor above the secondary gas inlet an average suspension density between 10 and 40 kg/m 3 is maintained by a suitable control of the rates of fluidising and secondary gases. Hot solids are withdrawn from the circulating fluidised bed and are cooled by direct and indirect heat exchange in a fluidised state, and at least a partial stream of cooled solids is recycled to the circulating fluidised bed.

That embodiment has been explained more in detail in Published German Application 26 24 302 and in the corresponding U.S. Patent 4,111,158. In that case the temperature can be kept constant virtually without a change of the operating conditions in the fluidised bed rehctor, without a change of the suspension density and other parameters, merely by a control of the withdrawal of hot solids and a controlled recycling of the cooled solids. In dependence on the output and on the selected reaction temperature the recycling will be effected at a higher or 25 lower rate. Any desired temperature cari be adjusted from very low ones closely above the ignition limit to very high ones, which may be limited, by a softening of the reaction residues. The temperature may lie between about 450 and 950 0

C.

Because in that case most of the heat that has been generated by the exothermic reaction is extracted in the fluidised bed cooler which succeeds the reactor in the solids flow path and a heat transfer to cooling registers which may be disposed in the fluidised bed reactor and to which heat can be transferred only in case of a sufficiently high 5uspension density will be of minor significance, a further advantage afforded by that process resides in that

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p, is -11it is permissible to maintain only a low suspension density in that part of the fluidised bed reactor which is disposed above the secondary gas inlet region so that there will be only a relatively small pressure drop throughout the fluidised bed reactor. On the other hand, heat is extracted in the fluidised bed cooler under conditions which effect an extremely high heat transfer, in a range from 300 to 500 watts/m 2 To control the temperature in the fluidised bed reactor, at least a partial stream of cooled solids is recycled from the fluidised bed cooler. For instance, the required partial stream of cooled solids may be fed directly into the fluidised bed reactor. In addition the exhaust gas 0000 may also be cooled by a supply of cooled solids, which may be fed, to a pneumatic conveyor or to a suspension heat exchanger stage, and such solids may subsequently be o e) o o separated from the exhaust gas and be recycled to the 0 fluidised bed cooler. As a result, the heat of the exhaust gas will enter the fluidised bed cooler, too. It will be particularly desirable to supply one partial current of cooled solids directly into the fluidised bed reactor and to °supply another particl stream of cooled solids indirectly to the fluidised bed reactor after a cooling of the exhaust o gases.

In that embodiment of the invention the gas residence times, gas velocities above the secondary gas inlet region during an operation under normal pressure and the supply of fluidising and secondary gases will be 3 o selected as in the embodiment described before.

A recooling of the hot solids from the fluidised bed reactor should be effected in a counter-current to the cooling fluid in a fluidised bed cooler which has a plurality of cooling chambers which are flown through in succession and receive interconnected cooling registers. In that case the combustion heat can be absorbed by a relatively small quantity of coolant.

i -12- In accordance with another feature of the fluidised bed plant that is provided with a fluidised bed cooler the latter is combined with the fluidised bed reactor in a unit of construction. In that case the fluidised bed reactor and the fluidised bed cooler comprise a common wall, which is suitably cooled and which has an opening through which cooled solids can flow into the fluidised bed reactor. As has been mentioned hereinbefore the fluidised bed cooler may comprise a plurality of cooling chambers but it may alternatively consist of a plurality of units which are provided with cooling surfaces and each of which has in common with the fluidised bed reactor a wall having an opening for the flow of solids and a separate solids supply O line. Such an arrangement is described in EP-A-206 066.

The arrangement comprising the fluidised bed cooler is of universal utility particularly because almost any on desired heat carrier medium can be heated in the fluidised S 0 bed cooler. From a technological aspect the generation of 0 steam in various forms and the heating of heat carrier' salts are of special significance.

o o Within the scope of the invention, air or 000 oxygen-enriched air or commercially pure oxygen may be used as oxygen-containing gases. The output can be increased in o othat the reaction is carried out under pressure, up to 20 bars.

Basically all materials which are capable of a 00o0 self-sustaining combustion may be used in the fluidised bed 0O plant in accordance with the invention. Examples of such S materials are coals of all kinds, particularly of inferior quality, such as coal washery refuse, sludge coal, high-salt coal, but also brown coal and oil shale. The plant may also be used to roast sulfide ores or ore concentrates.

The invention will be described more in detail and by way of example with reference to the accompanying drawings and of the Example. i Figure 1 illustrates in a top plan view various examples of illustrative configurations of displacers which

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-13are circular or rectangular in cross-section and may be used in fluidise.d bed reactors.

Figure 2 is a perspective view showing the lower portion of the fluidised bed reactor provided with a displacer.

Figure 3 is a longitudinal sectional view showing the fluidised bed reactor.

A fluidised bed reactor 1 is diagrammatically indicated in Figure 2 and its bottom surface is partly covered by prismatic displacers 7 so that there are a plurality of fluidising segments. The displacers 7 are provided with secondary gas openings 11 in their upper portion and with fuel supply lines 3 in their lower portion.

oa The fluidised bed reactor 1 shown in Figure 3 has cooling surfaces 2, which are indicated to consist of a membrane wall. The lower reactor chamber 8 is divided by a dam-like displacer 7 into four segments, namely, two O segments which are parallel to the dam, one segment in front of the dam and one segment behind the dam.

20 An oxygen-containing fluidising gas is sup lied to 0 -,royjaA a (ie S a'i4 f.iudiung grvfe 5i'19 9riqTe( o' the lower reac or chamber 8 A through lines 3 and with o" oxygen-containing secondary gas through lines 9. Additional secondary gas is fed through line 10 and the secondary gas ^So openings 11. Additional fuel is fed through further lines 3. The gas-solids suspension exits through line 4.

EXAMPLE Coal was combusted with air to produce saturated 0" 0 steam.

The fluidised bed reactor 1 of the fluidised bed plant had a base surface area of 12.5 m x 10.1 m and a height of 30.5 m. Its bottom surface was partly covered by a displacer 7 having a bottom surface area of 8.5 m x 8.1 m in such a manner that four segments were obtained, which were provided with fluidising grates 6. Two segments having a width of 2 m each extended parallel to the longer wall of the reactor. Two segments having a width of 1 m each were disposed between respective ends of the displacer 7 and the reactor wall. The displacer 7 had the shape of a prism which had a height of 6.8 m.

I- A' o 1 I -t it -14- The wall surface of the fluidised bed reactor 1 was entirely lined with water-cooled membrane walls. The walls of the displacer 7 consisted also of water-cooled membrane walls, which one the side facing the reactor were protected with refractory material.

The fluidised bed reactor 1 was supplied at a rate of 110,400 kg/h with a coal having a lower heating value of 15.9 MJ/kg and an average particle diameter of 0.2 mm and at a rate of 10,400 kg/h with limestone having approximately the same particle size. The feeding was effected through a total of six lines with the aid of entraining air at 100°C and at a rate of 11,040 sm 3 The fluidising gas consisted I" of air, which was at 260 0 C and was supplied at a rate of a 280,000 sm3 /h through the fluidising grates 6. The secondary gas lines 9 and 11 were used to supply additional air at a total rate of 206,000 sm /h on three levels respectively disposed 2 m (51,500 smn/h), 4.6 m (51,500 sm and 7.3 m (103,000 sm 3 above the fluidising grate 6.

Under the selected operating conditions a temperature of 8500 was maintained in the fluidised bed reactor 1. Saturated steam at 140 bars and at a rate corresponding to a heat output of 102 MW was produced at the So cooling surfaces 2 and on the membrane walls of the displacer 7.

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Claims

1. A fluidised bed plant which comprises a fiuidised bed reactor, a solids separator to remove solids from exhaust gas from the fluidised bed reactor and a recycling line for returning the -rnoved solids to the fluidised bed reactor and serves to carry out exothermic processes in a circulating fluidised bed, comprising lines for supplying oxygen-containing primary gases through the bottom of the fluidised bed reactor, lines for supplying oxygen-containing secondary gases in an elevation of at least 1 metre above the reactor bottom but not in excess of 30% of the height of the reactor, and a fuel line which opens into the fluidised bed reactor between the primary and secondary gas inlets, wherein 40 to 75% of the bottom surface area of the fluidised bed reactor are covered by one or more displacers op.. having a height not in excess of one-half of the height of the fluidised bed reactor, characterised in that the remaining bottom surface of the fluidised bed reactor constitutes a single coherent surface. 0

2. A fluidised bed plant according to claim 1, wherein said one or more displacers are square or rectangular in cross-section.

3. A fluidised bed plant according to claim 1 or 2, wherein said one or more displacers are provided with means which serve to charge fuel.

4. A fluidised bed plant according to claim 3, wherein the means which serve to charge fuel are disposed on a j plurality of levels. A fluidised bed plant according to any one of claims 1 to 4, wherein said one or more displacers are I provided with means which serve to supply oxygen-containing secondary gases.

7-c rO I -16- 6. A fluidised bed plant according to claim 5, wherein the means which serve to supply oxygen-containing secondary gases are disposed on a plurality of levels. 7. A fluidised bed plant according to any one of the preceding claims, wherein said one or more displacers have an upwardly decreasing cross-sectional area.

8. A fluidised bed plant according to any one of claims 1 to 7, characterised by cooling surfaces, which are disposed in the free space of the fluidised bed reactor above the upper most secondary gas inlet and/or on the wall of the fluidised bed reactor.

9. A fluidised bed plant according to any one of claims 1 to 8, characterised by at least one fluidised bed cooler, which is connected via a solids supply line and a solids recycle line.

10. A fluidised bed plant substantially as hereinbefore described with reference to the accompanying drawings. DATED this 12th day of June, 1991. METALLGESELLSCHAFT AKTIENGESELLSCHAFT 0a o a0 WATERMARK PATENT TRADEMARK ATTORNEYS i 290 BURWOOD ROAD, HAWTHORN, VICTORIA, AUSTRALIA. P^