GB2102831A - Fluidized bed gasification of coal - Google Patents

Abstract

Coal gasification is carried out in a first gasification stage with oxygen, air or oxygen-enriched air and steam and/or carbon dioxide at suitable temperature and pressure to produce a gas containing hydrogen, carbon monoxide and carbonaceous fine material. The fine material is further gasified in a second stage at temperatures above the flow point of the slag in a whirling-type gasifier. The overall efficiency is thus increased. <IMAGE>

Description

SPECIFICATION Process for the gasification of coal The invention relates to a process for the gasification of coal with oxygen, air or with oxygen-enriched air and steam and/or carbon dioxide in a first gasification stage, preferably in a Winkler generator at temperatures below the softening point of the slag in the range from about 800 to 1 2000C and pressures in the range from 1 to 100 bar to form a product gas containing hydrogen, carbon monoxide and carbonaceous fine materials, and further gasification of the fines in a second gasification stage. The application has practical experience in coal gasification, preferably in the operation of a Winkler gasifier provided as the first gasification stage as well as in the operation of whirling-type gasifiers, preferably in the operation of a vortex gasifier.

With the gasification of coal in a fluidized bed, e.g. in a Winkler generator, carbonaceous fines are entrained with the raw product gas from the generator, as a result of which some of the carbon introduced is lost for gasification. With reactive coals, such as lignite, this portion in the ash may amount to 20% while with less reactive coals, such as bituminous coals, it may be up to 50%.

The loss is greater as the ash content of the coal being used is higher. Since coals with high ash content are particularly suitable for the gasification, on account of their favourable costs, it would be advantageous to reduce or even to eliminate the under-utilisation of the used coal which is due to the entrainment of fines. The carbon in this partially gasified entrained fines exists partially in a graphitic modification and is certainly capable of being burnt, but shows a comparatively low reactivity with respect to gasification.

The following measures are known by which a utilisation can be provided for the carbon content of this partially gasified fine material: 1. The carbon-containing entrained fines are separated in hot cyclones arranged downstream of the fluidized bed generator and recycled into the latter. These recycled fine particles are very quickly entrained again, because of their small size, and since their carbon content, especially with the gasification of bituminous coals, mainly exists as unreactive graphite, no substantial improvement of carbon utilisation can be achieved, even with repeated recirculation of the fines into the gasifier.

2. The ash particles separated from the product gas are burnt together with another fuel, e.g. coal dust, in conventional steam generators. The steam as thus produced usually serves for converting the carbon monoxide formed with the gasification of coal in a separate plant, or for driving turbines.

The present invention seeks to provide an improvement in the utilisation of carbon with the coal gasification in a fluidized bed, especially in a Winkler generator, by utilisation of the carbon content of the entrained fly ash for the gasification by forming additional carbon monoxide and hydrogen. Moreover, the degree of gasification, i.e.

the ratio of the heat contained in the gas to the latent heat of the used coal is to be increased.

Finally, accompanying substances of the coal which are harmful to the environment, such as sulphur compounds, are to be transformed into a harmless state so that deposition is possible.

According to the invention we provide a process as initially referred to wherein the carbonaceous fines are gasified at temperatures above the flow point of their slag, preferably between 1450 and 1 8500C in a whirling-type gasifier, e.g. in a vortex gasifier, and thus the overall efficiency of carbon gasification is increased to above 90%. An essential feature of the invention is thus the gasification of the relatively unreacted carbon of the fine materials entrained from the first gasification stage in a whirling-type gasifier with formation of liquid slag.

In the case of a vortex gasifier the stream of the fines and the gasifying agent(s) is introduced tangentially into the chamber so that a spiral gas flow of high velocity is formed in the chamber. At the high temperatures which exist in a whirlingtype gasifier the relatively unreactive carbon of fly ash particles from the first gasification stage is almost completely gasified. It was round that the fly ash particles are steadily reduced in size during their travel in the gasifier. The small slag droplets being formed from the ash particles are thrown under action of the centrifugal force on to the inside wall of the chamber so that a film of slag is formed on the wall. The film runs continuously down the wall and the liquid slag collected on the slightly inclined bottom of a vortex chamber runs into a second chamber which is under the same pressure as the vortex gasifying chamber.The material forming the wall of the vortex gasifier may, for example, consist of silicon carbide. The result is that the carbon content of the fine fly ash particles is extensively gasified so that the slag running out of a whirling-type gasifier has a very low residual carbon content. The overall degree of gasification is substantially increased, for example from 70 to 75% for the Winkler generator alone to more than 90% and preferably more than 95%. It is especially surprising that an extensive separation of slag droplets from the gas is achieved despite the comparatively small volume of the whirling-type gasifier and consequently the short residence time of the gasification components in the gasifier.

According to one preferred embodiment of the process of the invention, the carbonaceous fine material is separated from the gas of the first gasification stage and is introduced together with fresh gasifying agent into the whirling-type gasifier. An operation of the whirling-type gasifier which is independent of the first gasification stage results from this intermediate separation of the entrained fines from the first gasification stage. In particular with a given plant capacity the whirlingtype gasifier can be reduced in size, since the gas throughput is substantially smaller than in the case where the whirling-type gasifier is directly coupled to the first gasification stage. The gasification can be variably distributed to both gasification stages. Generally, the main portion of the gasification takes place in the first stage.In special cases, however, a fluidized bed as the first stage can be used mainly for drying, size reduction and degassing and only to a small extent for the gasification of the charged coal so that the main portion of the gasification takes place in the whirling-type gasifier.

In addition to the fines from the first stage ground coal and/or generator ash from the bottom discharge of the first gasification stage, especially of a Winkler generator, can be fed to the whirling-type gasifier. With fly ashes having a lower carbon content than about 40 to 60 weight percent it can be necessary to burn a fuel together with the fly ash. Preferably, this fuel is ground coal. If ash from the bottom discharge of the Winkler generator is fed to the whirling-type gasifier, it will be generally necessary to feed simultaneously fine coal in view of the fact that this ash has a lower carbon content than fly ash.

Moreover, lime and/or dolomite in a particulate state can be blown into the whirling-type gasifier in order to increase the bond of sulphur into the slag and/or to decrease the flowing temperature of the slag.

According to a preferred feature of the process, the effluent gas from the whirling-type gasifier is quickly cooled to below the flow point of the slag.

fly ash is separated from the gas, and the separated fly ash is pneumatically recycled into the whirling-type gasifier. The rapid cooling of the effluent gas from the whirling-type gasifier is advantageously achieved by indirect heat exchange with boiler feed water. If the gas has entrained small slag droplets from the whirling type gasifier the droplets solidify by this gas cooling without causing backing to the pipe line.

This relatively small amount of particulate solids is subsequently separated in a hot cyclone, and then the gas is further cooled down, e.g. in a waste heat boiler. The pneumatic recycling of the separated solid particles can be achieved e.g. by means of cooled raw gas from the whirling-type gasifier, although gasifying agents can be also used for this purpose.

Preferably, the effluent gas from the whirlingtype gasifier after cooling and separation of solid particles is combined with the product gas of the first gasification stage after its cooling and separation of solid fines. Advantageously, the combination is performed when both gas streams have passed separate waste heat boilers and further cooling apparatus and have about the same temperature. This combination can be recommended, if the effluent gas from the whirling-type gasifier can be used for the same purpose as the product gas of the first gasification stage.

According to a further preferred feature of the process of the invention, in the case of a vortex gasifier having a substantially vertical axis, the fine material is tangentially introduced together with gasifying agent, the gas is eccentrically withdrawn from the chamber in an axial upward direction, and the liquid slag is withdrawn from the chamber in an axial downward direction. The mixture of fines and gasifying agent substantially flows in the gasifier along a spiral path. The formed gas flows to the exit socket eccentrically arranged in the hood of the gasifier chamber whereas the slag droplets formed from the fine char particles are mainly thrown on to the wall and flow from there to the central slag exit in the bottom of the gasifier chamber. Preheated air, oxygen-enriched air and technical oxygen are mainly used as gasifying agents.Carbon dioxide and, if desired, steam can be additionally used. Surprisingly it was found that the carbon introduced into the gasifier as fine material nearly completely passes into the gaseous phase, and the liquid slag is very depleted in carbon. Under these conditions, we have found that a nearly complete gasification of carbon takes place in the gaseous phase despite the low height and the small volume of the whirling-type gasifier.

Advantageously, the slag flowing out of the whirling-type gasifier is granulated and cooled by contact with water. For this purpose the slag flows from the whirling-type gasifier into a second chamber arranged therebelow in which it is sprayed with water. The mixture of water and granulated slag collected in the second chamber is withdrawn at intervals into a third chamber. The main portion of the slag is obtained in a granulated and vitrified stage from which harmful materials such as sulphur compounds can be no longer leached. Thus, this slag can be deposited or used for road building or as an aggregate in the building trade. If the whirling-type gasifier is operated under pressure as the first gasification stage, this third chamber is used as a pressure lock chamber for the discharge of the slag.

In another embodiment of the process of the invention, the effluent gas from the first gasification stage with the fine char material contained therein together with additional gasifying agent(s) is directly fed to the whirlingtype gasifier. With this direct coupling of the whirling-type gasifier to the first gasification stage the whirling-type gasifier is consequently designed for a gas throughput which is elevated in comparison with the first gasification stage. The first costs for gas cooling and separation of solids from the gas are, however, reduced because no separate treatment of the gases from both gasification stages is necessary.

Two embodiments of the process of the invention are hereinafter more fully described by reference to the drawings, wherein: Figure 1 shows a diagrammatic flow sheet of a plant for carrying out the process of the invention with intermediate separation of the fines entrained from the first gasification stage; Figure 2 shows a diagrammatic flow sheet of a whirling-type gasification unit with the associated apparatus which is not shown in Figure 1; Figure 3 is a diagrammatic flow sheet of a plant for carrying out the process of the invention without intermediate separation of the fines entrained from the first gasification stage from the gas stream; Figure 4 is a horizontal sectional view of a vortex gasifier which is used in connection with the process of the invention; and Figure 5 is an axial sectional view of a vortex gasifier according to Figure 4 with the vessels on the downstream side for the cooling, granulation and discharge of the slag.

With the embodiment of Figure 1, coal having a size in the range from 0 to 8 mm is filled via vessel 30 into a lock vessel 31 in which it is pressurised with CO2/N2. Then the coal is fed batchwise into the charging vessel 32. The coal is fed continuously by means of a screw conveyer 33 from the charging vessel 32 under a pressure of 4 bar into a fluidized bed gasifier 34 to which gasifying agent, i.e. steam and preheated air/oxygen, is fed via line 35 at several points (not shown). In addition to drying and reduction of the particle size a partial gasification of the charged coal takes place in the fluidized bed gasifier 34.

Ash which is poor in carbon is collected in the lower part of the gasifier and is discharged alternately into two lock vessels 34A, 34B by means of the screw conveyer 36, depressurized and removed from the princess. The gas generated in the gasifier 34 is passed together with the fines entrained from the gasifier via gas line 38 to the waste heat boiler 39. A part of the fines contained in the gas is separated in the boiler and collected in the lock vessel 40. Downstream of the waste heat boiler 39 the gas flows through a cyclone 41 in which further fine materials are separated which are collected in the lock vessel 42. Then the gas is scrubbed with water in a venturi apparatus 43 for further cleaning. Subsequently, water droplets are separated from the gas in a separator 44. Then the raw gas leaves the plant for further processing.The fines are collected in the lock vessels 40, 42 are gasified in a vortex gasification unit 50 which will be more fully explained herebelow. The raw gas generated in this unit is combined with the clean raw gas from the first gasification stage; the formed slag is withdrawn from the process in a granulated state.

According to Figure 2 the separated fines, if desired in admixture with ground coal, are dosed from a fines vessel 51 into an ejector 52 which is operated with e.g. air/carbon dioxide. The fines are fed by the stream of motive agent into a charging fluidized bed 53 from which they are tangentially blown in controlled manner by means of the stream of the motive agent into the vortex gasifier 20. In order to achieve intensive gasifying conditions preheated air and/or oxygen are fed to the vortex gasifier 20 as the gasifying agent. As already mentioned above in addition to the fines entrained from the first stage finely ground coal can be supplied to the vortex gasifier via the apparatus 51A, 52A.The carbonaceous material is extensively gasified in the vortex gasifier 20 at a temperature above the melting point of the slag.

The liquid slag is collected on the bottom, runs into the vessel 22 and is withdrawn from the plant via the lock vessel 26, as will be more fully described herebelow. The gas generated in the vortex gasifier 20 is passed through a cooler 54 in which it is cooled to below the melting point of the slag so that the small portion of small slag droplets entrained from the gasifier 20 is solidified in the gas stream. Then the gas is passed through a cyclone 55 in which solid material is separated from the gas. The separated solid material passes via lock vessels 56, 57 to a charging apparatus 58 by means of which the separated material is fed into a product raw gas stream by means of which it is pneumatically recycled into the vortex gasifier 20.The clean gas stream is passed in a similar manner as the product gas from the fluidized bed gasifier through a waste heat boiler 59, and for wet cleaning through a venturi scrubber 60 charged with water and a succeeding separator 61. Then, as mentioned above, the gas is combined with the main gas stream from the fluidized bed gasifier. The gas can, however, be burnt in a combustion chamber the combustion gases of which are used for the admission of a turbine.

In Figure 3 the same reference numbers as in Figures 1 and 2 are used for the same apparatus.

With this embodiment the gas loaded with fine char and leaving the fluidized bed gasifier 34 is directly passed via line 38 to the vortex gasifier 20 which in this case must be designed much greater than with the embodiment of the process shown in Figure 1. The slag is withdrawn from the vortex gasifier 20 and granulated in the same way as shown in Figure 2. The gas stream from the vortex gasifier 20 is fed to a waste heat system 39 downstream of the cooler 54 and a cyclone 55 for separation of solids. The main portion of the heat content of the gas is used for steam generation in the waste heat system 39. Additional solid particles are separated therefrom as with the embodiment shown in Figure 1.These solids are collected in a vessel (not shown) and fed in the same manner as the solid materials separated in the cyclone 55 into an ejector (not shown) arranged in a gas stream by means of a lock star feeder. In this manner the separated material is pneumatically recycled with the gas stream into the vortex gasifier 20. In the same way as in the embodiment shown in Figure 1 the gas is then scrubbed in the venturi scrubber 43, freed from entrained droplets in separator 44 and withdrawn as raw product gas.

According to Figures 4 and 5 the vortex gasifier 20 comprises a tangential short feed pipe 20a to which the mixture of fly ash and gasifying agent is fed via line 1 6. The mixture is moved in the gasifier on a spiral path to the gas outlet pipe 20b arranged eccentrically, the fly ash particles being exposed to the intensive gasifying conditions under which their carbon content is largely gasified, and the particles 28 are reduced in size and change into the liquid stage. While the gas leaves the vortex gasifier via pipe 20b and is carried away via line 18, the liquid slag is accumulated on the gasifier bottom 20d which is slightly inclined to the axis of the gasifier.The liquid slag flows through the union 20C into the vessel 22 and is intensively sprayed with water therein through the nozzles 23 so that the siag is quenched and granulated. The water is supplied to the nozzles through the ring line 24. The vessel 22 which is under the same pressure as the vortex gasifier 20 is discharged from time to time into the vessel 26. If the vortex gasifier 20 is operated under superatmospheric pressure, the vessel 26 with the valves 25, 27 can be used as a pressure lock chamber.

EXAMPLE 10 000 kg/h of bituminous coal having an ash content of 25% by weight and a carbon content of 58,5% by weight are gasified in a Winkler generator at 1 2000C and about 4 bar with 5 t/h steam and 3500 Nm3/h of 98% oxygen. 600 kg/h generator ash having 15% by weight carbon are discharged from the generator. 3620 kg/h fly ash containing 45% by weight carbon escape from the generator together with the product gas.

3260 kg/h fly ash are separated from the product gas in a waste heat boiler and a cyclone. The amount of fly ash is gasified in a whirling-type gasifier at 1 8000C and 4 bar with 98% oxygen and additional steam. 1 830 kg/h liquid slag containing about 2% by weight carbon discharge from the whirling-type gasifier. The overall degree of carbon gasification amounts to 95%, while the degree of carbon gasification of the Winkler generator by itself is about 71%.

Claims (12)

1. A process for gasifying coal with oxygen, air or oxygen-enriched air steam and/or carbon dioxide in a first gasification stage at temperatures below the softening point of the slag in the range from about 800 to 1 2000C and pressures in the range from 1 to 100 bar to produce a gas containing hydrogen, carbon monoxide and carbonaceous fine material, and further gasification of the carbonaceous fine material in a second gasification stage, wherein the carbonaceous fine material is gasified at temperatures above the flow point of its slag in a whirling-type gasifier and thus the overall efficiency of carbon gasification is increased to above 90%.

2. A process according to claim 1 wherein the first gasification stage is carried out in a Winkler generator.

3. A process according to claim 1 or 2 wherein the carbonaceous fine material is gasified in the second gasification stage at a temperature between 1450 and 18500C.

4. A process according to claim 1,2 or 3 wherein the carbonaceous fine material is separated from the gas of the first gasification stage and is introduced together with the fresh gasifying agent into the whirling-type gasifier.

5. A process according to any one of the preceding claims wherein ground coal and/or ash from the first gasification stage are additionally fed to the whirling-type gasifier.

6. A process according to claim 4 or 5 wherein the effluent gas from the whirling-type gasifier after cooling and separation of solid materials is combined with the product gas of the first gasification stage after its cooling and separation of fine materials.

7. A process according to any one of claims 1 to 3 wherein the effluent gas from the first gasification stage with the fine materials contained therein together with additional gasifying agent is directly fed to the whirling-type gasifier.

8. A process of any one of the preceding claims wherein the effluent gas from the whirling-type gasifier is rapidly cooled to below the flow point of the slag, solid slag particles are separated from the gas, and the separated particles are pneumatically recycled into the whirling-type gasifier.

9. A process according to any one of the preceding claims wherein the fine material is tangentially introduced together with the gasifying agent into a vortex gasifying chamber having a substantially vertical axis, the gas is eccentrically withdrawn from the chamber in an axial upward direction, and the liquid slag is withdrawn from the chamber in an axial downward direction.

1 0. A process according to any one of the preceding claims wherein the slag running off from the whirling-type gasifier is granulated and cooled by contacting it with water whereby it is transformed into a vitrified state.

11. A process for gasifying coal substantially as described herein with reference to Figures 1, 2, 4 and 5 or as modified with reference to Figure 3.

12. A process for gasifying coal according to claim 1 substantially as described herein with reference to the Example.

1 3. Coal gas produced by the process of any one of the preceding claims.