GB2132323A

GB2132323A - Drying lignite

Info

Publication number: GB2132323A
Application number: GB08236190A
Authority: GB
Inventors: Dr Gerald Moss
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1982-12-20
Filing date: 1982-12-20
Publication date: 1984-07-04
Also published as: GB2132323B

Abstract

Pieces and/or particles of lignite or other water-containing solid are passed (18) into a fluidized bed (13) supported on a distributor (12) and contained within a vessel (11). A bank of heat exchange tubes (15) extends across the vessel (11) within the volume occupied by the bed (13), and hot gas passing through the tubes (15) heats the pieces and/or particles in the bed (13) so that water is evaporated therefrom. The evaporated water is condensed in a condenser (23) by indirect heat exchange with cooling fluid (boiler feed water, organic liquid to power vapour turbine, etc) in the tubes (24) therein. Some of the condensate is circulated by a pump (27) to boiler (28) wherein it is boiled by hot gas passing through tubes (29) therein. The resulting steam is passed (30) to the bed (13) via the distributor (12) to fluidize solid in the bed. Solid of reduced water content is recovered (19, Figure 2). The solid is preferably peat or lignite, which after drying is passed to a gasifier. The combustible gas from the gasifier is the hot gas passed through the tubes 15 and 29, which after cooling has a smaller volume thereby saving piping and equipment costs therefor. <IMAGE>

Description

SPECIFICATION Method and apparatus for reducing the water content of a water-containing solid The present invention relates to a method and apparatus for reducing the water content of a water-containing solid. The method and apparatus of the invention may be used to reduce the water content of many water-containing solids, but has particular, but by no means exclusive, utility in relation to lignite, peat, brown coal, garbage, refuse, vegetable matter (e.g. wood, wood-derived substances, and biomass in general) and any mixture of the foregoing.

There are many areas of the world where lignite, brown coal and peat are readily available for utilization as fuel. These fuels are, in general, of high water content and may contain more than 50 weight % of water. Drying lignite and peat is costly and even hazardous because the dried fuel is very reactive, particularly in the case of lignite, and can combust spontaneously.

The usual practice in Australia at present (1982) is to burn high-moisture lignite without drying in utility boilers for electricity generation and to use supplementary oil-fired burners in addition to stabilize the combustion in the boilers.

Utility boilers which rely on supplementary firing for combustion stability are difficult to operate on part load and are therefore normally used for base load operation. Burning lignite in this manner is very wasteful because the boilers cannot recover the very significant amounts of heat taken up during the evaporation of water from the watercontaining fuel and because the oil used for supplementary firing is much more costly than the lignite.

The present invention provides, for the first time, an economical method and means of utilizing fuels of high water content.

In one aspect, the present invention provides a method of reducing the water content of a watercontaining solid comprising passing pieces and/or particles of the water-containing solid into one region of a fluidized bed wherein the pieces and/or particles are fluidized and heated by indirect heat exchange with a first hot fluid passing through conduits immersed in the bed whereby moisture is vapourized from the pieces and/or particles, recovering pieces and/or particles of reduced moisture content from a second region of the bed, conducting water vapour from the bed to a condenser, condensing said water vapour in the condenser by indirect heat exchange with a cooling fluid at a selected temperature, circulating at least some of the condensate to a heat exchanger, heating the condensate in the heat exchanger by indirect heat exchange with a second hot fluid to generate vapour, and passing said vapour into the bottom of the fluidized bed to fluidize the pieces and/or particles therein.

The said first and/or second hot fluid passed in indirect heat exchange may be selected from a flue gas obtained by complete oxidation of a fuel and combustible gas obtained by partial oxidation v | Preferably, the first and second hot fluids are each a combustible gas obtained by partial oxidation of a fuel.

The (partial) oxidation of the fuel may be effected in a fluidized bed. Preferably, the fluidized bed contains chemically-reactive calcium oxide to fix sulfur from the fuel as a solid compound of calcium and sulfur which is retained in the bed.

The (partial) oxidation of the fuel may be effected under pressure.

The fuel may comprise pieces and/or particles of reduced moisture content recovered from the second region of the fluidized bed.

The said water-containing solid may be selected from peat, lignite, brown coal, vegetable matter, garbage, refuse and any mixture of the foregoing.

The cooling fluid may be selected from boiler feed water, water to provide process steam, a liquid which is to be heated for use in a chemical or physical process, a liquid which is used in vapour form to drive a turbine or other powerproducing machine, air which is to be used in an oxidation or partial-oxidation process, and more than one of the foregoing.

The pressure in the fluidized bed may be any convenient or suitable pressure, including superatmospheric pressures, such as any pressure up to 15 atmospheres gauge, e.g. up to 10 atmosphere, for example, from 3 to 6 atmospheres gauge.

In another aspect, the present invention provides apparatus for use in reducing the water content of a water-containing solid, comprising a vessel for containing a fluidized bed of pieces and/or particles of said solid in a fluidized bed volume extending up to a selected top bed level, heat exchange tubes within the said volume for heating the fluidized bed, a condenser connected for receiving and condensing vapour leaving the vessel, circulating means for circulating condensate from the condenser to a heat exchanger provided with indirect heat exchange means for boiling condensate to produce vapour of the condensate, and a conduit for passing vapour from the heat exchanger into the base of the fluidized bed volume in the vessel for fluidizing the bed, during operation.

Preferably, there is provided means for feeding pieces and/or particles into a first region of said bed volume, and preferably, there is provided removing means for removing pieces and/or particles from a second region of the bed volume.

The apparatus may comprise oxidizing means operable for at least partial oxidation of fuel and a conduit for passing the resulting hot gases into the said heat exchange tubes and/or heat exchange tubes of said means for boiling condensate in said heat exchanger. Preferably, said oxidizing means comprises a fluidized bed oxidizer or partialoxidizer. The oxidizing means is preferably a fluidized bed gasifier.

Preferably, the fuel which is at least partially oxidized in the oxidizing means comprises particles and/or pieces of fuel of low water content recovered from the fluidized bed volume, there preferably being means for conducting pieces and/or particles from the fluidized bed volume to the oxidizing means.

The invention is now further decribed by way of non-limitative example only, with reference to the accompanying diagrammatic drawings in which: Figure 1 is a diagrammatic, schematic side elevation, partly in cross-section, of some of the parts of plant for reducing the water content of lignite; and Figure 2 is a plan view of the top of a drying vessel of the plant of Figure 1.

In the drawings, like reference numbers indicate the same part.

The plant 10 comprises a lignite drying vessel 11 having a gas/vapour distributor 12 which supports a fluidized bed 13 of lignite particles extending up to a selected top level 14. A bank of heat exchange tubes 1 5 extends across the vessel 11 between the distributor 12 and the top level 14 of the bed so as to be immersed in the bed 13, and an inlet header 16 at one end supplies hot combustible gas from a gasifier (not shown) to one end of the bank of tubes 15, cooled combustible gas leaving the tubes 1 5 at the opposite end via an exit header 17.

Water-containing lignite, in suitably comminuted form to be fluidized in bed 13, is introduced into the vessel 11 via a feed conduit 18 which may, as shown, introduce the lignite onto the top of the bed 1 3. It is to be understood that the lignite may be introduced into the bed 13 below the top level 14 thereof.

The lignite is heated and dried in the fluidized bed 1 3 by heat obtained from hot gas passing through the heat exchange tubes 1 5 and heated, dried lignite is recovered from the bed 13 via a recovery conduit 19 (Figure 2). The transfer of heat from the hot gas entering the tubes 1 5 to the lignite in the fluidized bed cools the hot gas and thereby reduces its volume, as a result of which, the combustible gas outlet pipe 20 may be of considerably smaller diameter than the inlet pipe 21 connected to the inlet header 16. Since the cost of pipework and gas-handling equipment is high, and is greatly increased when the pipework and equipment is to carry large volumes of hot gas, the cooling of the hot gas provides very considerable savings in the cost of such pipework and equipment.Moreoever, the cooled gas can be treated to remove entrained dust by de-dusting equipment (e.g. filters) which is operable at relatively low temperatures, thereby further reducing costs.

The water vapour evaporated from the lignite in the bed 1 3 rises to the top of vessel 11 via a suitable screen 40 (or other means) which obstructs the upward passage of entrained solids and passes via conduit 22 to a condenser 23 where it is condensed by indirect heat exchange with a cooling fluid passing through heat-transfer tubes 24 (shown diagrammatically). The condensate is removed from the condenser 23 via conduit 25 and some is discharged via a let-down valve 26, the remainder being circulated by means of a suitable pump 27 to a heat exchanger 28 (shown diagrammatically). The condensate is boiled in the heat exchanger 28 by indirect heat exchange with a relatively minor stream of the hot combustible gas from the gasifier (not shown) passing via heat transfer tubes 29.The steam thus produced in the heat exchanger 28 is conducted via conduit 30 to a plenum 31 within the vessel 11 beneath the distributor 12 from which it is substantially uniformly distributed into the base of the bed 13 to fluidize the bed 13. Relatively minor quantities of steam are required to fluidize the bed 13 because the amount of water vapour produced by evaporation of water within the bed 13 is considerable and serves to maintain relatively vigorous fluidization conditions within the bed 13.

The dried lignite recovered via conduit 19 may be gasified in the gasifier from which the hot combustible gas passed through tubes 1 5 and 29 is derived. The gasifier is preferably a fluidized bed gasifier and the dried lignite may be gasified within a fluidized bed containing a chemically reactive calcium oxide moiety (e.g. lime, limestone, dolomite, fired dolomite) so as to retain sulfer from the lignite as a solid compound of sulfur and calcium within the bed. The sulfurcapturing properties of the calcium oxide moiety may be regenerated in a suitable fluidized bed regenerator. Suitable fluidized bed gasifiers which produce low-sulfur combustible gas from a sulfurcontaining fuel are described in U.K. patent specifications Nos. 1183937; 1336563; 1408888 and 2039293.The fluidized bed gasifier may operate at superatmospheric pressure (e.g. as described in U.K. 1408888), and this may be advantageous because it may be supplied with dried lignite fuel at about the pressure within the vessel 11 if the contents thereof are under superatmospheric pressure.

In a typical case, the mean temperature of bed 13 is about 1600 C. The steam and water in the heat exchanger 28 are at an equilibrium temperature which is sufficiently greater than the temperature in the condenser 23 to ensure that the difference in saturated vapour pressure between the heat exchanger 28 and the condenser 23 is adequate to cause the steam to pass through, and fluidize the bed 13. The temperature of hot, combustible gas entering the inlet side of the heat transfer tubes 1 5 and 29 in the vessel 11 and heat exchanger 28 is from 500 to 1 2000C, preferably 850 to 9200C according to the type of gasifier. Combustible gas typically leaves the outlet pipe 29 at temperatures of the order of about 2000C.

The considerable quantities of water vapour evaporated from the lignite in the bed 13 due to the heating of the high-water content lignite in the bed 13 increase the degree of fluidization and mixing of lignite in the bed 1 3 significantly in relation to the fluidization or agitation of bed material due to the steam provided from heat exchanger 28 via conduit 30, plenum 13 and distributed into the base of the bed 13 via the distributor 12. The relatively high degree of fluidization and mixing in the bed 13 promotes efficient transfer of heat to the lignite of bed 13 from the tubes 15.

The water condensed in condenser 23 comprises condensed steam supplied from heat exchanger 28 and condensed water vapour evaporated from the lignite in the bed 13. For typical lignites, the latter will be considerably greater than the former so that the amount of heat of condensation available at the condenser 23 is much greater than the amount of heat used to evaporate or boil water in the heat exchanger 28.

Thus the lignite-drying operation conducted in the vessel 11 not only reduces the temperature of the hot gases supplied from inlet pipe 21 to a conveniently low temperature for subsequent lowcost piping and treatment, but additionally provides useful heat at the condenser 23. It will be appreciated that the temperature in the condenser 23 can be increased by operating the condenser at a higher pressure, which would in turn necessitate the operation of the heat exchanger 28 and lignite-drying operation at higher pressures. The selected pressure and temperature in the condenser 23 will depend upon the intended use of the heat which is recoverable in the condenser.

In the present example, a condensation temperature of about 1 600C is suitable for heating feed water for a boiler (not shown), particularly when such a boiler is in the vicinity of the plant 10. The boiler feed water is passed through the heat transfer tubes 24. Alternatively, an organic liquid may be passed through the heat transfer tubes 24 to provide vapour to drive a turbine (e.g. of the type known commercially as an "Ormat" turbine) to generate mechanical and/or electrical power. In another embodiment, which may be implemented at higher operating temperatures and pressures in the condenser 23, steam for chemical processing may be generated from water passing through the heat transfer tubes 24.In a further embodiment which also may be implemented at higher temperatures and pressures, the heat available at the condenser 23 may be employed to pre-heat air (or other gas) e.g. air to be used for the combustion or part combustion of a fuel. In this latter embodiment, the condenser 23 and heat transfer tubes 24 would have a form (not indicated) appropriate to a gas-heating duty.

The lignite-drying operation conducted in the bed 1 3 invariably produces quantities of gases and vapours in addition to steam, and suitable means (not shown) which are known in the art are provided as necessary to vent and/or bleed and/or separate the gas and liquid phases in the condenser to prevent an unacceptable accumulation of non-condensible gases and vapours and of undesirable condensed liquids. Any solids carried into the condenser 23 from the vessel 11 may be discharged via a suitable blowdown valve (not shown) and/or via a drain downstream of valve 26.

Water-containing lignite is supplied via conduit 1 8 from a lock-hopper (which may be pressurized for pressurized operation of the bed 1 3) and starfeeder valve of known type. If the gasifier to which dried lignite recovered via conduit 19 operates at a lower pressure (e.g. atmospheric pressure) than the bed 13, the dried lignite may be passed to the gasifier via a lock hopper.

Claims

1. A method of reducing the water content of a water-containing solid comprising passing pieces and/or particles of the water-containing solid into one region of a fluidized bed wherein the pieces and/or particles are fluidized and heated by indirect heat exchange with a first hot fluid passing through conduits immersed in the bed whereby moisture is vapourized from the pieces and/or particles, recovering pieces and/or particles of reduced moisture content from a second region of the bed, conducting water vapour from the bed to a condenser, condensing said water vapour in the condenser by indirect heat exchange with a cooling fluid at a selected temperature, circulating at least some of the condensate to a heat exchanger, heating the condensate in the heat exchanger by indirect heat exchange with a second hot fluid to generate vapour, and passing said vapour into the bottom of the fluidized bed to fluidize the pieces and/or particles therein.

2. A method as in claim 1 in which the said first and/or second hot fluid passed in indirect heat exchange is/are selected from a flue gas obtained by complete oxidation of a fuel and a combustible gas obtained by partial oxidation of a fuel.

3. A method as in claim 2 in which the first and second hot fluids are each a combustible gas obtained by partial oxidation of a fuel.

4. A method as in claim 2 or claim 3 in which the (partial) oxidation of the fuel is effected in a fluidized bed.

5. A method as in claim 4 in which the fluidized bed contains chemically-reactive calcium oxide to fix sulfur from the fuel as a solid compound of calcium and sulfur which is retained in the bed.

6. A method as in any one of claims 2 to 5 in which the (partial) oxidation of the fuel is effected under pressure.

7. A method as in any one of claims 2 to 6 in which the fuel comprises pieces and/or particles of reduced moisture content recovered from the second region of the fluidized bed.

8. A method as in any one of claims 1 to 7 in which the said water-containing solid is selected from peat, lignite, brown coal, vegetable matter, garbage, refuse and any mixture of the foregoing.

9. A method as in any one of claims 1 to 8 in which the cooling fluid is selected from boiler feed water, water to provide process steam, a liquid which is to be heated for use in a chemical or physical process, a liquid which is used in vapour form to drive a turbine or other power-producing machine, air which is to be used in an oxidation or partial-oxidation process, and more than one of the foregoing.

10. A method as in any one of claims 1 to 9 in which the pressure in the fluidized bed is superatmospheric.

11. A method as in any one of claims 1 to 10 in which the pressure in the fluidized bed is from 3 to 6 atmospheres gauge.

12. A method of reducing the water content of a water-containing solid according to any one of claims 1 to 11 substantially as hereinbefore described.

13. A method as in claim 12 substantially as described with reference to the accompanying drawings.

14. Apparatus for use in reducing the water content of a water-containing solid, comprising a vessel for containing a fluidized bed of pieces and/or particles of said solid in a fluidized bed volume extending up to a selected top bed level, heat exchange tubes within the said volume for heating the fluidized bed, a condenser connected for receiving and condensing vapour leaving the vessel, circulating means for circulating condensate from the condenser to a heat exchanger provided with indirect heat exchange means for boiling condensate to produce vapour of the condensate, and a conduit for passing vapour from the heat exchanger into the base of the fluidized bed volume in the vessel for fluidizing the bed, during operation.

1 5. Apparatus as in claim 14 comprising feed means for feeding pieces and/or particles of water-containing solid into a first region of said bed volume.

16. Apparatus as in claim 14 or claim 15 comprising removing means for removing pieces and/or particles of the solid from a second region of the bed volume.

17. Apparatus as in any one of claims 14 to 16 comprising oxidizing means operable for at least partial oxidation of fuel and a conduit for passing the resulting hot gases into the said heat exchange tubes and/or heat exchange tubes of said means for boiling condensate in said heat exchanger.

1 8. Apparatus as in claim 1 7 in which said oxidizing means comprises a fluidized bed oxidizer or partial-oxidizer.

19. Apparatus as in claim 17 or claim 18 in which said oxidizing means is a fluidized bed gasifier.

20. Apparatus as in any one of claims 17 to 1 9 in which said fuel which is at least partially oxidized in the oxidizing means comprises particles and/or pieces of fuel of low water content recovered from the fluidized bed volume.

21. Apparatus as in claim 20 comprising means for conducting pieces and/or particles from the fluidized bed volume to the oxidizing means.

22. Apparatus for use in reducing the water content of pieces and/or particles of a watercontaining solid substantially as hereinbefore described.

23. Apparatus as in claim 22 substantially as described with reference to the accompanying diagrammatic drawing.