AU636303B2

AU636303B2 - Process for drying solid materials in an indirectly heated fluidised bed

Info

Publication number: AU636303B2
Application number: AU53641/90A
Authority: AU
Inventors: Bodo Wolf
Original assignee: VEREINIGTE ENERGIEWERKE AG
Current assignee: VEREINIGTE ENERGIEWERKE AG
Priority date: 1989-04-18
Filing date: 1990-04-17
Publication date: 1993-04-29
Anticipated expiration: 2010-04-17
Also published as: JPH02296893A; DD282509A5; AU5364190A; DE3943366C2; DE3943366A1

Description

636 03 Form COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION FOR OFFICE USE Short Title: Int. Cl: Application Number: Lodged:

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Complete Specification-Lodged: Accepted: Lapsed: Published: Priority: Related Art: 53 *5 0

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TO BE COMPLETED BY APPLICANT Name of Applicant: Address of Applicant: Actual Inventor: Address for Service: -Vet-seehau--7-54G-GE&MAhN-fE-MRE)GA-^ie-I Pp" -REPUB-I-eG- Bodo Wolf GRIFFITH HACK CO 71 YORK STREET SYDNEY NSW 2000

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5 Complete Specification for the invention entitled: PROCESS aiP-Ti-r I~ :IaENME FOR DRYING SOLID MATERIALS IN AN INDIRECTLY HEATED FLUIDISED BED The following statement is a full description of this invention, including the best method of performing it known to us:- 17950-C:DJH:RK 7619A:rk 2 PROCESS AUME- f- FOR DRYING SOLID MATERIALS IN AN INDIRECTLY HEATED FLUIDISED BED The invention relates to a process and a plant arrangement for drying solid materials such as brown coal, peat, sand, filter cake from mechanical separation processes, and sludges containing less than 95% mass of a vaporisable substance, e.g. water, which are fed into a fluidised bed dryer. Particularly, though not exclusively, the invention relates to the forming of an indirectly heated fluidised bed containing the solid material which is whirled up by a swirling medium, said swirling medium being the vaporisable substance in vapour form, and in which process the dried material discharged from the fluidised bed dryer is passed on to further processing, usage or dumping, possibly after cooling, whereas the vaporised material is passed on for cleaning, cooling, material usage and/or recovery of thermal energy in industry, the building industry, agriculture and community waste disposal.

20 Drying processes, in particular those which separate water as vaporisable component from solid materials, are very important, both from the economic and the social viewpoints, in industrial production, the building industry, energy conversion and community and industrial S 25 waste disposal. Drying has in some cases become such a common integral part of process cycles such as in the combustion of fuels containing water, e.g. brown coal and sludges, that the stresses on the

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402/17950-C environment caused by them due to increased energy requirement and increased emissions are regarded as natural.

For the use of raw brown coal to produce energy, milling plants are increasingly being installed in power stations in which part of the furnace gas produced in the boiler combustion chamber is returned by suction to the milling process as thermal transfer 10 medium, so that the water in the coal evaporates as a result of the heat transfer from the furnace gas with a temperature of 800 to 1000'=C to the raw brown coal in 9 the flue gas stream, before or whilst the coal is being pulverised. The state of the art relating to this process is described in detail in the book by Effenberger, entitled "Dampferzeuger" (Steam Generators) published by VEB Verlag fur Grundstoff- 0o.o industrie, 1st edition, 1987. In relation to the 0" thermal energy released in the combustion chamber, this 0Z*20 type of drying process causes more than 1.5-times the minimum flue gas emission naturally necessary as a result of the high fuel requirement of the drying process and due to the steam component in the flue gas.

*25 In the case of the upgrading of brown coal, plate and tubular dryers, i.e. contact dryers, which are indirectly heated by steam are mainly used, as described in detail by Krug and Nauendorf in the book entitled, "Braunkohlenbrikettierung" (Brown Coal Briquetting), volume 1, drying section, published by VEB Deutscher Verlag fUr Grundstoffindustrie, Leipzig, 1984, 1st edition.

By using the tapped or back-pressure turbine steam as the thermal transfer medium in the drying process, which transfers its latent thermal energy indirectly to the coal by way of condensation after being converted to saturated steam by injecting condensate, for example, the known principle of "co-generation" is applied and a reduction in the fuel requirement for the drying process is achieved. It was therefore possible to reduce the comparative total of flue gas 10 emissions produced during individual upgrading and usage cycles to about 1.3-times the naturally required minimum in comparison to that of the milling process usually employed in brown coal power stations.

These advantages are not however effective since entrained air is used in most cases.

5The introduction of indirectly heated plate and tubular 0CS@ dryers and the consequent development of co-generation, e.g. in brown coal and peat-fired power stations, have not been effected thus far, since the mass flows of fuel required conflict with the limited capacity of such dryers and no economically feasible solution could be found.

From the DE-PS 67 770 a process and plant arrangement for pre-drying solid fuels containing water, in particular soft brown coal, is known in which the brown coal is dried in a fluidised bed dryer indirectly heated by steam prior to being burned in a steam boiler, As in the case of the plate and tubular dryers, tapped or back-pressure turbine steam is also to be used here and therefore the principle of co-generation applied.

DD-PS 67 770 works from the principle that any suitable swirling medium, including steam, can be used to swirl the brown coal over the fluidised bed floor in the fluidised bed dryer.

US-PS 38 00 427 describes an indirectly heated fluidised bed drying process in which the brown coal is swirled with steam and therefore the drying cycle is conducted in an atmosphere of steam. However, the :10 invention works from the principle that the brown coal is heated in the steam atmosphere until sulphur compounds separate off and settle on additives which

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may be present in the fluidised bed at the same time.

DE-PS 29 01 723 expands on the use of a fluidised bed indirectly heated by steam and fluidised by steam generally to include the drying of solid materials which contain less than 95% mass of a vaporisable material, In this case, the vaporisable material may be water or other substances such as solvents, which act as swirling medium in their vaporised form and in their saturated steam form as heat transfer medium for indirect heating, using different partial pressures.

25 In relation to US-PS 38 00 427, DE-PS 29 01 723 restricts the permissible temperature of the fluidised bed and substantially fixes this below the temperature at which the solid material decomposes to ensure that the vapour discharged from the fluidised bed is composed of the vaporisable material which is essentially without contamination from other gaseous substances.

Many years of research and development have revealed that the process known from DE-PS 29 01 723 is not technically possible in the described form. In particular, it has become evident that the temperature of the fluidised bed cannot be freely selected and that the gaseous impurities in the vaporisable material are completely contained in the vapour of the vaporisable material discharging from the fluidised bed dryer, more or less irrespective of the temperature of the fluidised bed.

It would be advantageous if at least preferred form(s) of the invention could recover most of the thermal energy used for drying and to reduce the emissions produced during drying through evaporation, vaporisation, pyrolysis, degasification and gasification, in particular those which cannot be condensed at ambient temperature.

It would be advantageous if at least preferred form(s) of the invention took into consideration those principles of drying technology that are technically possible and modifiable, to create a process and the necessary plant arrangement for performing the process for drying solid materials in a fluidised bed dryer.

Preferably the indirectly heated fluidised bed is formed from the dried solid material itself which is 25 whirled up by the vaporisable substance to vapour form.

The transfer of the vaporisable component of a solid material, or sludge in its vaporised form, in a gas phase formed by the vaporisable component of the solid inaterial, is, in the case of the isobaric process 30 sequence, dependent on a boiling point curve for the material to be vaporised that is characteristic of, and .therefore specific to, the solid material and which plots the necessary temperature of the solid material against the component of vaporisable substance in the solid 35 material.

The present invention provides a process for removing a predetermined amount of vaporisable substance Sfrom a solid material comprising the steps of feeding the 402/17950-C solid material to an indirectly heated fluidised bed dryer and removing dried solid material and the predetermined amount of vaporisable substance from the dryer, wherein a swirling medium in the form of the vaporisable substance in vapour form is introduced to the bed, in which swirling medium the solid material is entrained and fluidised, and wherein the temperature of the fluidised bed is selected so that a pre-dete -fnined remainder of vaporisable substance is left in the dried solid-material that leaves the dryer, the temperature corresponding to the specific boiling temperature of the vaporisable substance in the solid material removed from the fluidised bed dryer.

According to the invention when applied to a given solid material, the temperature of the fluidised bed is set in relation to the required percentage mass of vaporisable substance in the solid material discharged from the fluidised bed, by adding solid material with a higher percentage mass of vaporisable substance to the fluidised bed and removing dried solid material from the fluidised bed, in such a way that said temperature corresponds to the specific boiling temperature of the 25 vaporisable substance in the solid material discharged from the fiuidised bed, so that the vapour discharged i from the fluidised bed dryer, the gaseous components in the vaporisable substance and other gaseous impurities, which for example are fed into the fluidised bed dryer 30 with the solid material, only contain those constituents of the solid components of the solid material or the sludges which :*re volatile below this boiling temperature.

Preferably the vapour discharged from the fluidised 35 bed dryer is indirectly cooled so that it is condensed while discharging its latent thermal energy and the gaseous materials of the vaporisable substance contained 1 ~in the vapour and other gaseous impurities and 402/17950-C 8 decomposition products in the solid component, which are not condensable at ambient temperature and are not soluble in the condensate of the vaporisable material, are separated off from the vapour and then discharged into the environment or passed on to a dump and/or other gas purifying process.

Preferably the solid material to be dried should be provided in particulate form, preferably with a grain size of 0 to 10 mm, i.e. in the form of swirlable bulk material.

Solid material that has already been dried may be added to solid materials, in particular sludge products which are not directly suitable for the formation of a swirlable bulk material, to provide the correct consistency to enable production of a feed material that meets the requirements of the process. Another way to change solid material to be dried into an appropriate form for the process is to convert it into a pumpable and sprayable sludge by mixing with the condensate of the vaporisable substance. It is necessary to keep the solid material to be dried in large lumps so that the process may be conducted successfully when the fluidised bed is formed by the solid material itself, but by a solid t material with smaller particles characterised by a S 25 density which is 1.2- to 5.0-times that of the material to be dried.

In this case it is usual that bed material from the fluidised bed is discharged with the dried material. The bed material should in this case be separated from the dried coarse-particled solid material and returned to the fluidised bed. It may be necessary, in particular during "changing the working phase of the operation of the process according to the invention, to feed the vaporisable substance to the fluidised bed dryer in vapor form or additional swirling medium from outside, e.g. from a separate system, as an alternative to returning dryer removed vaporisable substance.

A preferred factor for the performance of the 9 process is the difference in temperature between the required temperature of the fluidised bed and the condensation temperature of the heating vapour used for che indirect heat transfer, which should amount to between 10 and 150 K. When pressure in the fluidised bed dryer corresponds approximately to the ambient atmospheric temperature, the pressures required for the heating vapour should be from 0.2 to 4.0 MPa and this leads to vapour temperatures of 125 to 255 0 C where steam is used as heating medium and where the heating steam is in lightly superheated state. Whilst high heating vapour pressures allow small dryers to be constructed, low heating vapour pressures and therefore low temperature differences between condensing vapour and fluidised bed ensure that the advantages of co-generation are favourably utilised.

The recovery of the major part of the thermal energy used for drying and the separation of the gaseous impurities which cannot be condensed and are not soluble in the condensate typically requires the condensation of the vaporisable component of the solid material. If the fluidised bed dryer is operating under vapour pressures which correspond to the ambient atmospheric pressure, then the condensation temperature specific to the 2r material determines the temperature level of the thermal energy which can be recovered thereby. If the vaporisable substance is water, then the thermal energy recovered under the conditions according to the invention can reach a temperature of over 90 0 C, which is suitable for conducting operations in thermal power supply and preheating cycles in industrial processes. If no thermal energy is required at this temperature level, then the vapour can be technically expanded, after appropriate dust, cleaning, so that condensation is still possible at ambient temperature.

A further possibility is to increase the pressure of the vapour from the fluidised bed drying, also after appropriate dust cleaning, prior to its condensation by 7950-C 10 compression to such an extent that the condensation heat is produced at a temperature level sufficient for accomplishment of the intended heat transfer, e.g. for heating the fluidised bed of the process according to the invention.

Also disclosed is apparatus suitable for use with the process according to the invention. The apparatus can comprise a fluidised bed dryer; a heating means arranged to operate within the fluidised bed dryer and adapted to indirectly heat the solid material to be dried to a pre-determined temperature; means for feeding the solid material to be dried to the fluidised bed dryer and adapted to regulate solid material flow proportionally to the predetermined temperature; vapour feeding means for feeding the vaporisable substance in vapour form into the fluidised bed dryer to fluidise the solid material in the fluidised bed; discharge means arranged to discharge dried solid material from the fluidised bed dryer and adapted to regulate discharge rate proportionally to fluidised bed height; and vapour removal means arranged to receive and remove vaporisable substance vapour generated by drying the solid material within the fluidised bed dryer together with the proportion of the vapour fed to the dryer by the vapour feeding means.

More specifically, the apparatus may provide a feeding device for feeding the solid material to be dried to the fluidised bed dryer, the performance of which device being regulated by the temperature of the fluidised bed. The apparatus may provide a device for 35 introducing the solid material into the fluidised bed S9 dryer which has a capacity of at least 1.5-times the mass of solid material to be introduced and which, where S particulate, swirlable or slurried solid material is to 7950-C 11 4

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be introduced, loads at least 25% of the surface of the fluidised bed approximately evenly with the solid material introduced, and in the case of lumpy solid material, which is not swirlable or is difficult to swirl, loads at least 75% of the surface of said fluidised bed.

The fluidised bed 7an cover a heating element arranged in the fluidised bed dryer to at least 250 to 1000 mm. Further components of such a plant arrangement include a discharge unit for dried solid material, the performance of which is controlled by the prescribed height of the fluidised bed and a mechanical dust separator for reducing the proportion of dust with a grain size of less than 0.5 mm in the solid material, which is discharged with the vapour of the vaporisable substance through a discharge outlet, to below 10% mass.

In a dust removal plant the' dust component in the -vapour discharged from the fluidised bed dryer can be reduced to below 50 mg/kg of vapour. Also the 20 arrangement may include a vapour recirculation unit with a compressor which increases the pressure of the vapour to such an extent that at least double the amount of vapour is recirculated through the arrangement as is required for -he transfer of the solid material on the fluidised bed floor from che fixed bed to the fluidised bed, and a condenser which separates the gaseous impurities off from the vapour by condensation of the vaporised material and possibly passes these by way of a suction device into the environment or to a deodorisation 30 unit and/or other gas purifying processes and passes the condensate by means of a pump to the condenser and for further upgrading and utilisation.

If the temperature of the condensation of the vapour of the vaporisable substance is not suitable for conducting required thermal energy supply operations, the arrangement according to preferred forms can be supplemented by a single- or multiple-stage compressor S which raises the pressure of the vapour to such an extent gx N 2 9 N 2/17950-C 12 that condensation of the vapour can be conducted at Lhe temperature level necessary to meet the requirements of thermal energy supply, e.g. for the indirect he; g of the fluidised bed. If this alternative is not expedient 9 for the utilisation of recovered thermal energy, then the vapour of the vaporisable material from the fluidised bed dryer can be fed to a steam turbine plant after leaving the dust removal unit and there technically reduced in pressure to such an extent that condensation is still possible at ambient temperature. e.g. 30 0

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The invention will now be described in a preferred and non-limiting practical manner.

Practical Example A preferred process and plant arrangement are explained below by way of the attached drawing.

The example may produce a dry brown coal 0 "0* t a¢

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S*i I II 402/17950-C 13 with a moisture content of 10% mass from raw brown wi h a moisture content of 55% mass which has been crushed in a standard impact hammer mill to a grain size of 0 to 6 mm.

The mass flow rate of the raw brown coal is 100 t/h, that of the dry brown coal 50 t/h. Hence, a total of t of coal water per tonne are to be vaporised.

Calculation of the re-evaporation reveals that the 10 moisture content decr=a s by 1.5% mass after discharge from the fluidised -,ed dryer 2 and therefore the dried coal is discharged from the fluidised bed dryer 2 with a moisture content of 11.5% mass and 49 152 kg of water per hour are to oe evaporated in the fluidised bed dryer 2. The 848 kj of water re-evaporated per hour outside the fluidisei bed dryer 2 are removed by suction and fed to a separate dust removal plant 9 in the form of fuel-laden vapour with an air content of 2 kg per kg of steam.

Together with the coal, which with a cut weight of 0.7 kp/litre corresponds to a volume of 142,9 200 of air per hour are fed into the fluidised bed dryer 2 in accordance with the specifications set for the feeding unit. The coal water, the vaporisable material in the example, should contain 20 m of dissolved gaseous impurities, in particular carbon dioxide, so that the vapour produced from the coal water in an hour through the drying process contains a total of 220 m: of gaseous impurities, which, laden with steam, are separated in the condenser 18 according to the saturation temperature and discharged into the environment. With a heat transition coefficient for the heating steam condensing on the coal fluidised bed 2 of k 300 W/m K and a thermal energy requirement of 800 W/kg of coal water to be vaporised, it results that a heating surface of 3277 m must be installed in S the fluidised bed dryer 2 in the case of temperature difference of 40 K effective for the heat transfer.

If the heating element 7 has a height of 2,00 m, the density of the heating surface must amount to 100 mF/m 2 of fluidised bed floor 5, i.e. the fluidised bed dryer 2 has a fluidised bed floor 5 which is about 32 and this gives the fluidised bed floor 5 a structural length of 8 m if the width of the fluidised bed floor is 4 m. If the solid material on the fluidised bed floor 5 reaches its disintegration point at a no-load pipe flow rate of 0,35 m/s, then according to the invention 80 670 m s of steam, corresponding to 53.8 must be recirculated. Hence, about 103 t of steam per hour, corresponding to an approximate volume of 0 150 000 mP/h, must be discharged from the fluidised bed S: dryer 2 and have most of the dust removed. Only the steam which is produced by the vaporisation of 49152 kg of coal water and which is laden with 220 noncondensable gaseous impurities, is fed to the condenser 18.

The thermal energy requirement of the fluidised bed Sdrying system according to the invention amounts to 39.3 of which about 30.8 i.e. 78% of the expended thermal energy, can be recovered in the condenser 18 at a temperature of up to If the gaseous impurities, which can not be condensed, are discharged into the environment with a saturation temperature of 60'C, then about 275 m/h are emitted.

In relation to a pipe drying plant of comparable capacity emitting about 170 000 rm' of fuel-laden vapours per hour, which, in the case of a dust content of 50 mg/m', constitutes an environmental pollution rate of 8.5 kg of coal dust per hour, the latter amounts to 0.2% of usual emissions.

The specific boiling point curve for the coal to be dried in the example requires a fluidised bed temperature of 118'C. To achieve the set temperature difference of 40 K between the fluidised bed 6 and the heating element 7 a heating steam with a minimum pressure of 0.59 MPa is required. The height of the fluidised bed must be adjusted to at least 2500 mm, but 3250 mm at maximum, by the regulated discharge of dried coal by way of the heating element 7 with prescribed height of 2.0 m and of a 250 mm high space existing between the heating element 7 and the fluidised bed floor o *o ee *4

Claims

1. A process for removing a predetermined amount of vaporisable substance from a solid material comprising the steps of feeding the solid material to an indirectly heated fluidised bed dryer and removing dried solid material and the predetermined amount of vaporisable substance from the dryer, wherein a swirling medium in the form of the vaporisable substance in vapour form is introduced to the bed, in which swirling medium the solid material is entrained and fluidised, and wherein the temperature of the fluidised bed is selected so that a pre-determined remainder of vaporisable substance is left in the dried solid-material that leaves the dryer, the temperature corresponding to the specific boiling temperature of the vaporisable substance in the solid material removed from the fluidised bed dryer.

2. A process as claimed in claim 1, wherein the 20 vaporisable substance is removed from the dryer and includes gaseous constituents vaporised from the solid material and other gaseous impurities including those fed into the fluidised bed dryer with the solid material, which impurities and constituents are those that are volatile below the specific boiling temperature.

3. A process as claimed in claim 2, wherein the vaporisable substance removed from the dryer is subjected "to condensation and the components that are not condensable with and not soluble in resultant condensate of the vaporised substance are separated from the •condensate and treated by further cooling and condensation and are subsequently passed into the environment or onto deodorisation and/or other gas purifying processes.

4. A process as claimed in claim 3 wherein the solid material is mixed with a portion of the resultant condensate to form a sludge, which is then spray-fed to Sthe fluidised bed dryer, or which can be pumped over the 402/17950-C 17 fluidised bed in the fluidised bed dryer.

A process as claimed in any one of claims 1 to 3, wherein the solid material is fed into the fluidised bed dryer in the form of swirlable, particulate bulk material.

6. A process as claimed in any one of claims 1 to wherein the solid material fed to the bed is a mixture of dried solid material discharged from the fluidised bed dryer and of fresh, undried solid material.

7. A process as claimed in any one of claims 1 to 3, wherein the solid material is fed into the fluidised bed in coarse, difficulty, or non-swirlable pieces or lumps, whereas the fluidised bed is formed from swirlable, particulate material with a density 1.2 to times higher than that of the coarse, piece-wise or lumpy solid material.

8. A process as claimed in claim 7, wherein the dried solid material discharged from the fluidised bed includes a portion of the particulate, swirlable material 20 and wherein the coarse pieces and swirlable material are separated and the swirlable material is returned to the fluidised bed.

9. A process as claimed in any one of claims 1 to 6 wherein the solid material itself forms the fluidised 25 bed.

10. A process as claimed in claim 2 or claim 3 or any one of claims 4 to 9 when dependent on claim 2 or claim 3, wherein the swirling medium introduced into the process is the vaporisable substance removed from the dryer and/or additional vaporisable substance from a separate source.

11. A process as claimed in any one of claims 1 to wherein a vapour is used for indirectly heating the fluidised bed and has a pressure in the range of 0.2 to 4 MPa'and a temperature in the range of 125 to 225 0 C.

12. A process as claimed in claim 11, wherein operating parameters of the vapour are set in such a way I\ that the average difference in temperature between .r S S. M T 0-02/1 7950-C 18 condensing vaporisable substance and the fluidised bed is from 10 to 150K.

13. A process as claimed in claim 11 or claim 12 when dependent on claim 2 or claim 3 wherein the vapour is the vaporisable substance removed from the dryer.

14. A process as claimed in any one of the preceding claims, wherein pressure in the fluidised bed corresponds approximately to ambient atmospheric pressure.

A process as claimed in claim 2 or claim 3 or any one of claims 4 to 14 when dependent on claim 2 or claim 3, wherein the pressure of vaporisable substance vapour removed from the fluidised bed dryer and cleaned of solid constituents whilst retaining its latent thermal energy, is either increased by the addition of mechanical energy or decreased by expansion, to discharge mechanical energy.

16. A process for removing a vaporisable substance from a solid material substantially as herein described with reference to the practical example and/or the accompanying drawings. Dated this 23rd day of February 1993 VEREINIGTE ENERGIEWERKE AKTIENGESELLSCHAFT By their Patent Attorneys .GRIFFITH HACK CO 6 ft ft fot 402/17950-C