EP0499771A1 - Process and apparatus for gasifying finely devided to powdery fuel with recycling of the fly ash - Google Patents

Abstract

In this process, a fly ash, precipitated in the dry state from the crude partial oxidation gas produced, is mixed with the fuel at the inlet of the common proportioning vessel (5) in a specially constructed mixing station (26), whereupon the resulting mixture is introduced into the gasification reactor (16). In this case, the volumetric flow of carrier gas fed to the proportioning vessel (5) is adjusted such that this volumetric flow is sufficient exclusively for covering the requirement for maintaining the pressure in the proportioning vessel (5) and for conveying the fuel/fly ash mixture from the proportioning vessel (5) to the gasification reactor (16) and that there is no loosening, similar to a moving bed or fluidised bed, of the fuel/fly ash mixture either in the proportioning vessel (5) or in the lines leading from the proportioning vessel (5) to the burners (15) of the gasification reactor (16). <IMAGE>

Description

The invention relates to a method and an apparatus for the gasification of a fine-grained to dusty fuel in a gasification reactor under elevated pressure at temperatures above the slag melting point, the fly ash extracted together with the partial oxidation raw gas generated from the gasification reactor being separated dry from the gas stream and subsequently with the gas gasifying fuel is brought together in a common feed tank, whereupon the resulting fuel-fly ash mixture is fed to the gasification reactor.

When gasifying ash-containing fuels with oxygen and / or air and possibly water vapor, when using gasification temperatures above the slag melting point, the majority of the ash fed to the gasification reactor with the fuel accumulates as slag, which can be withdrawn from the gasification reactor in the liquid state. However, a smaller part of the ash leaves the gasification reactor together with the partial oxidation raw gas generated during the gasification as fly ash. Experience in practice has shown that when using optimized and controlled gasification conditions, the amount of fly ash discharged with the partial oxidation raw gas is almost constant, regardless of the ash content of the fuel used, provided that this ash content is in the range between approx. 8 and 40 wt .-% lies. The fly ash must be processed or processed further be separated from the gas stream in a suitable manner. This can be done by dry separation, for example in a cyclone filter. In order to use the carbon content contained in the fly ash and to convert the fly ash into a landfillable water-insoluble and thus environmentally compatible slag, it is already known to return the separated fly ash to the gasification reactor.

In order to carry out this procedure, a method has already been described in EP-B1-0 109 109, in which the separated fly ash and the fuel are brought together discontinuously in an allotment or fluidizing container, which is equipped with mixing devices inside and separate inlet openings for fuel and fly ash and an outlet opening for the mixture obtained. It is provided here that the fuel and the fly ash are converted into a fluidized state by supplying a gas, which can be nitrogen or cold product gas, whereby a homogeneous mixture of the two components is to be achieved. The resulting fuel-fly ash mixture is then transferred in the fluidized state from the feed container to the gasification reactor.

However, this method of working is not free from disadvantages for the following reasons:
To set a fluidized state or a fluidized or fluidized bed, relatively large amounts of gas are required. If nitrogen is used for this purpose, it gets into the gasification reactor together with the fuel-fly ash mixture and there leads to a deterioration in the gasification conditions and Product gas quality. If recycled product gas is used for the fluidization, this means an impairment of the efficiency and thus a deterioration in the economy of the gasification plant. In addition, it has been shown that in the fluid or fluidized bed-like state, the fuel fly ash mixture cannot be removed from the supply container with the high uniformity and cannot be transported into the gasification reactor with the high flow rate density, which is necessary for the proper implementation of the gasification, when using pneumatic conveying , especially when using entrained-flow gasification, are required. In order to maintain a fluidized or fluidized bed in the feed hopper, setting a constant level of the bed is also a prerequisite for the fluid or fluidized bed to function properly. However, this condition cannot be reconciled with the requirement in practice that the fuel supply to the gasification reactor remains guaranteed even if there are brief faults or fluctuations in the fuel supply from the upstream devices. This requirement can only be met by a variable fill level in the distribution container, which is adapted to the respective circumstances. Another disadvantage of the known method of operation can be seen in the fact that a discontinuous supply of fly ash and fuel is provided in the feed container. As a result, the setting of a constant and precisely defined mixing ratio of the two components is not possible or only with great difficulty.

The invention is therefore based on the object of improving the method of the type mentioned in such a way that the disadvantages mentioned above are avoided will. At the same time, the feed container used to carry out the method according to the invention should also be improved in such a way that no mixing devices with moving parts, such as stirrers, and no additional gas are required for the mixing of fuel and fly ash.

The method serving to achieve this object is characterized in that the mixture of the pneumatically metered mass flows of fuel and fly ash takes place at the inlet of the common distribution container and the volume flow of conveying gas supplied to the distribution container is adjusted so that this volume flow is used exclusively to cover the need for the pressure maintenance in the feed container and the conveyance of the fuel-fly ash mixture from the feed container to the gasification reactor are sufficient and there is no fluid or fluidized bed-like loosening of the fuel-fly ash mixture both in the feed container and in the delivery line to the gasification reactor.

This means that when the method according to the invention is carried out, a gas flow which is partially opposite the solid flow for homogenizing, fluidized or fluidized bed-like loosening of the fuel-fly ash mixture is dispensed with.

For carrying out the method according to the invention, it is advantageous if the fly ash separated from the partial oxidation raw gas stream has an increased carbon content in the range between 15 and 40% by weight. It has been shown that the properties of this fly ash with an increased carbon content permit particularly good pneumatic conveying thereof.

Such an increased carbon content in the fly ash can be achieved by reducing the O₂ / C ratio in the gasification so that the degree of conversion of the carbon supplied to the gasification reactor with the fuel remains in the range between 95 and 97%. The unconverted (gasified) carbon almost completely gets into the fly ash and therefore leads to correspondingly increased carbon contents in the fly ash. However, since the fly ash is completely returned to the gasification reactor in the process according to the invention, this procedure does not impair the economy of the process.

Fig. 1

das Fließschema einer Anlage zur Durchführung des erfindungsgemäßen Verfahrens
und

Fig. 2

einen Schnitt durch den oberen Teil des erfindungsgemäßen Zuteilbehälters mit der sogenannten Mischstelle am Eintritt in den Zuteilbehälter.

Further details of the method according to the invention and of the feed container suitable for carrying it out are to be explained below with reference to the figures:
Here show:

Fig. 1

the flow diagram of a plant for performing the method according to the invention
and

Fig. 2

a section through the upper part of the allotment container according to the invention with the so-called mixing point at the entrance to the allotment container.

In the flow diagram shown in Fig. 1, only the parts of the system necessary to explain the method according to the invention are shown, while all auxiliary devices, in particular measuring and control devices, were not listed. Here, the fine-grained to dusty fuel coming from the processing plant, not shown in the flow diagram, passes via line 1 into the supply bunker 2, which is under normal pressure, from which it is drawn off via line 3 and fed to lock container 4 under the influence of gravity. The line 3 has a branch in its lower part, so that each of the three lock containers 4 shown in the figure is connected to the storage bunker 2. The lock container 4 are filled with fuel and emptied at different times in a manner known per se, so that overall a continuous supply of fuel to the supply container 5 is ensured. The lock container 4 is filled under normal pressure. Subsequently, each filled lock container 4 is pressurized by supplying a conveying gas via line 6. The pressurization takes place to the extent that is required for a differential pressure delivery of the fuel from the lock container 4 to the distribution container 5. After the pressurization has taken place, the fuel passes from the filled lock container 4 via the outlet line 7 to the connection point 8, at which the lines 7 coming from the individual lock containers 4 open into the line 9 leading to the distribution container 5. The fuel passes through this line to the supply container 5 by conveying the differential pressure. The mass flow of the fuel flowing out of the lock container 4 is measured, and by changing the differential pressure between the lock container 4 and the Allotment container 5, the amount of fuel flowing to the allotment container 5 is adjusted so that this amount of fuel corresponds to the amount of fuel that is drawn from the allotment container 5 into the gasification reactor 16. The measured value of the fuel flowing continuously over time also serves as a reference variable for the mass flow of the fly ash supplied to the feed container 5 via the line 25.

In deviation from the illustration in the figure, an arrangement of the distribution container 5 below the lock container 4 is also possible, so that the transport can then take place in the flow of gravity. After emptying, the lock container 4 is relaxed again and the filling process can begin again. At the outlet of the storage bunker 2 and the lock container 4, additional conveying gas can be blown in via the lines 10 and 11, respectively, in order to avoid bridging of the fuel when it emerges from the containers. In any case, however, this amount of gas is measured in such a way that a loosening of the fuel in a fluid-like or fluidized bed-like manner is avoided in these containers. Of course, in deviation from the illustration in the illustration, it is also possible that instead of three lock containers 4, only two or more than three of them are provided. This depends primarily on the amount of fuel to be pumped and the dimensions of the containers. In principle, the number of lock containers 4 must always be at least two, so that alternate filling and emptying is possible.

The fuel, together with the separated fly ash, passes from the feed container 5 via the line 12 to the distributor 13, from which to the burners 15 of the gasification reactor 16 leading lines 14 go off. The pressure difference between the feed container 5 and the gasification reactor 16 is in turn set so that the fuel-fly ash mixture can be transported via lines 12 and 14 in a manner known per se by differential pressure delivery. Such an allocation and conveying system using differential pressure conveyance is described in detail in DE-OS 38 10 404, for example.

In the gasification reactor 16, the gasification conditions are preferably set so that the fly ash obtained has a carbon content in the range between 15 and 40% by weight. This fly ash, together with the partial oxidation raw gas generated, is drawn off from the gasification reactor 16 via the line 17 and reaches the cyclone filter 18, in which the fly ash is separated dry from the gas stream. The partial oxidation raw gas freed from the fly ash emerges from the cyclone filter 18 via the line 19 and can be supplied for its further use or processing. From the cyclone filter 18, the separated fly ash arrives in the flow of gravity via line 20 into the lock container 21 below. Since this container is cyclically filled with fly ash from the cyclone filter 18 and emptied into the metering container 23, it must be supplied with gas via the gas supply before the emptying process begins Line 22 are brought to the pressure which corresponds to the pressure prevailing in the metering container 23. After emptying, the lock container 21 is relaxed and thus brought back to the pressure prevailing in the cyclone filter 18. The gas released in the process passes via line 40 and line 28 back into line 17, where it is combined with the raw gas stream to be purified. The dosing container 23 is standing under a constant or almost constant differential pressure to the supply container 5 and is continuously emptied into this container via line 25. The line 25 is merged with the line 9 in the mixing point 26 at the inlet of the feed container 5. Since the dosing container 23 is to be emptied with a high degree of uniformity and a controlled mass flow, it must be designed according to the principles of the bulk material mechanics as a mass flow container for the theoretically possible quality range of the fly ash, i.e. the range of the grain size, bulk density, temperature and humidity. Since the fly ash fill in the metering container 23 still has a high proportion of moist partial oxidation raw gas, the metering container 23 can either be heated to avoid the condensation of the water content of the fly ash, or it is supplied with heated conveying gas via line 41. In addition, the dosing container 23 must be equipped with a weighing device 27 and the associated measuring devices so that the average mass flow during the continuous emptying of the container and the amount of fly ash required for the cyclical refilling thereof can be determined. The fly ash mass flow is regulated by adjusting the differential pressure between the metering container 23 and the distribution container 5 in relation to the fuel mass flow, which is conveyed from the lock container 4 into the distribution container 5. Details of the mass flow measurement and control are not the subject of the present invention and therefore do not need to be explained in more detail here.

In the figure, the distribution container 5 has only a single mixing point 26. In practice, of course, there may also be several mixing points. Due to the structural design of the mixing point 26, which will be explained in more detail in connection with FIG. 2, the fuel and the fly ash are mixed so intensively and uniformly as soon as they enter the distribution container 5 that no further mixing devices are required within the distribution container 5 . The required conveying gas is fed to the distribution container 5 together with the fuel and fly ash mass flow via lines 9 and 25. To avoid bridging at the outlet of the supply container 5, further gas can be introduced into the latter via the line 29. As a result, the average bulk density of the fuel-fly ash mixture can also be reduced to the optimum delivery density which is required for uniform delivery to the gasification reactor 16. The volume flow of the gas to the supply container 5 via the lines 9, 25 and 29 is dimensioned according to the invention such that it is sufficient to cover the need for pressure maintenance in the supply container 5 and to promote the fuel-fly ash mixture to the gasification reactor 16 and a flow or Fluid bed-like loosening of the fuel-fly ash mixture is avoided both in the feed container 5 and in the lines 12 and 14 leading to the gasification reactor 16. Here, a relatively high conveying density is used, which is significantly higher than the conveying density with which the fly ash is transported from the metering container 23 to the feed container 5. While one normally works for the transport of the fuel in the line 9 with a delivery density which is 60-90% of the bulk density of the fuel in the lock container 4, one becomes the delivery density for the fuel-fly ash mixture in lines 12 and 14 should preferably be set so that it is about 10 - 20% below the delivery density range for the pure fuel. The fuel fly ash mixture is withdrawn from the feed container 5 via the line 12 and then reaches the gasification reactor 16 in the manner described above, from which the liquid slag resulting from the gasification flows down via the line 30. Inert gas, such as nitrogen, or a hydrocarbon-containing gas, such as synthesis gas generated from the partial oxidation raw gas obtained, can be used as the conveying gas for the system described in a manner known per se. The connecting line 42 and the line 28 are used only to compensate for rapid changes in pressure in the supply container 5 with variable operating pressure of the gasification reactor 16 or to discharge any excess gas from the supply container 5 in the event of malfunctions.

When using a coal with an ash content of about 8-40% by weight as fuel, the recycled mass of fly ash is almost constant under the reaction conditions normally given. When using low-ash coal or coal mixtures with a fluctuating ash content of about 3 to 15% by weight as fuel, however, larger fluctuations in the recycled mass of fly ash can occur. In this case, it may be appropriate to analytically determine the ash content of the fuel-fly ash mixture before entering the burner 15 of the gasification reactor 16 in order to be able to make the required fine adjustment of the O₂ / C ratio during gasification. The arrangement of the continuously operating measuring device required for the determination of the ash content can take place, for example, in line 12. Are the lines 14 leading from the distributor 13 to the burners 15 however, relatively short, this arrangement of the measuring device is ruled out, since in this case the required measuring time is longer than the delivery time. Alternatively, therefore, a sampling line can also be laid in the area of the outlet funnel of the supply container 5, which is connected to the measuring device for analytical determination of the ash content in the fuel-fly ash mixture. The devices required for conveying the ash content are not shown in the flow diagram shown in FIG. 1.

Fig. 2 shows a section through the mixing point 26 at the inlet of the feed container 5. The line 25 for feeding the fly ash is inserted perpendicularly into the housing 31 and is arranged coaxially to the central axis of the housing 31, the inside diameter of which is larger than the outside diameter the line 25. In its central part, the housing has a horizontally extending connecting piece 32, to which the line 9 for the fuel supply is connected by means of the flange 33. The line 25 is fastened to the housing 31 via the flange 34, and this in turn is fastened via the flange 35 to the inlet connection 36 of the supply container 5. The inlet connector 36 has the same inner diameter as the housing 31 and ends in the feed container 5 just below its cover 37. The fuel supplied via the line 9 enters the housing 31 with a horizontal flow direction and is then deflected by 90 degrees, so that it then flows parallel to the direction of flow of the fly ash in the space between the line 25 and the wall of the housing 31 from top to bottom. The line 25 ends in the inlet nozzle 36 above the cover 37, so that fuel and fly ash can mix in the remaining part of the inlet nozzle 36. Under the inlet connection 36, the baffle plate 38 is arranged in the feed container 5, which has a round cross-section in adaptation to the cross section of the feed container 5, so that the fuel-fly ash mixture emerging from the inlet connection 36 experiences a deflection on all sides. This deflection can be further supported by attaching a funnel-shaped diaphragm 39 to the outlet end of the inlet connection 36. The flow conditions described above are also illustrated by the arrows in the figure. The construction according to the invention makes it possible to achieve such thorough mixing and uniform distribution of fuel and fly ash at the inlet into the feed container 5 that 5 further mixing devices are no longer required within the feed container. The allocation container 5 has the shape recognizable from FIG. 1, so that its complete representation in FIG. 2 can be dispensed with.

Claims (5)

Process for the gasification of a fine-grained to dusty fuel in a gasification reactor which is under increased pressure at temperatures above the slag melting point, the fly ash drawn off together with the partial oxidation raw gas generated from the gasification reactor being separated dry from the gas stream and then combined with the fuel to be gasified in a common distribution container is, whereupon the resulting fuel-fly ash mixture is fed to the gasification reactor, characterized in that the mixing of the pneumatically metered mass flows of fuel and fly ash takes place at the inlet of the common distribution container and the volume flow of conveying gas supplied to the distribution container is adjusted such that this volume flow is exclusively for Covering the need for maintaining pressure in the feed tank and conveying the fuel-fly ash mixture from the feed tank to the gasification reactor calibrated and a fluid or fluidized bed-like loosening of the fuel-fly ash mixture is avoided both in the feed tank and in the lines leading from the feed tank to the burners of the gasification reactor. Method according to Claim 1, characterized in that the fuel is conveyed into the feed container at a delivery density which is 60-90% of its bulk density, and in that the fuel-fly ash mixture is fed from the feed tank to the burners at a delivery density which is about 10 - 20% below the delivery density range of the pure fuel. Process according to claims 1 and 2, characterized in that the gasification conditions in the gasification reactor are set so that the fly ash obtained has an increased carbon content in the range between 15 and 40% by weight. Allocation container for carrying out the method according to claims 1 to 3, characterized by the following features: a) on the inlet nozzle (36) of the supply container (5), a housing (31) is fixed, in which the line (9) for the fuel supply and the line (25) for the fly ash supply open at right angles to each other; b) the line (25) is arranged vertically in the central axis of the housing (31) and has an outer diameter which is smaller than the inner diameter of the housing (31); c) the inner diameters of the housing (31) and inlet connection (36) are the same, the line (25) extending into the inlet connection (36); d) the line (9) is connected horizontally to the housing (31) via the connecting piece (32)

and e) a baffle plate (38) is arranged in the distribution container (5) below the inlet connection (36). Allotment container according to claim 4, characterized in that a funnel-shaped diaphragm (39) is attached to the outlet of the inlet connection (36).

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