EP0214417A2 - Process for the production of synthesis gas from solid fuels - Google Patents

Abstract

Synthesis gas is produced from solid fuels, using endothermally (32 to 38) and exothermally (40 to 47) reacting gasifying agents, in a reactor (10) with a fluidised bed (14), a post-reaction chamber (18) located above and a fixed bed (20) located below. The rate at which the solid gasification residues forming the fixed bed (20) are taken off from the reactor (10) is controlled as a function of the height (60, 61) of the fixed bed (20) in such a way that the upper limit (24) thereof remains below that region (40) in which exothermally reacting gasifying agent is fed into the gasification process.

Description

The invention relates to a process for the production of synthesis gas from solid fuels at elevated pressure in a fluidized bed using endothermic and exothermic reacting gasification agents, a post-gasification chamber being provided above the fluidized bed and a fixed bed made of the solid gasification residues below the fluidized bed and the fuels in the gas Fluidized bed introduced, the solid gasification residues are withdrawn from the fixed bed and the synthesis gas generated is withdrawn from the post-gasification room.

The solid gasification residues forming the fixed bed located below the fluidized bed consist predominantly of mineral accompanying substances of the fuels, i.e. their ashes and mineral admixtures, although there is also a certain proportion of C-containing solids in the fixed bed. There may be coarser grits that fall down through the fluidized bed onto the fixed bed due to their greater weight. Another part of the C-containing particles in the fixed bed will consist of smaller grains, the C content of which has been largely, but not completely, converted in the gasification process.

An essential prerequisite for the proper functioning of such a process is that the gasification residues are withdrawn from the bottom of the reactor in which the gasification process takes place without any problems. Faults can occur in particular if Areas of the fixed bed in which C-containing gasification residues are still present may overheat. This occurs particularly when oxygen-containing gasifying agents are blown into the fixed bed. The resulting exothermic reaction with the C-containing constituents of the fixed bed causes a considerable increase in temperature, with the result that the mineral constituents of the fixed bed cake, sometimes even melt, and form agglomerates. In extreme cases, this can result in the operation of the reactor having to be interrupted in order to remove the caking, agglomerates etc. by mechanical means. Such interruptions in operation may take days, given the need to allow the reactor to cool after the gasification process is interrupted, etc.
In view of the aforementioned problems, the procedure is generally such that exothermic gasifying agent, that is oxygen, is blown in above the upper limit of the fixed bed in such a way that a reaction of this oxygen with C-containing particles still present in the fixed bed is avoided in any case. This means that oxygen is only introduced into the fluidized bed and possibly into the post-reaction space above it.

Since the positions of the nozzles through which gasifying agents are introduced into the gasification process are normally fixed on a reactor, at least cannot be changed significantly during operation, the above-described condition that the supply of oxygenated gasifying agent can only be above the upper limit of the fixed bed takes place, are only observed if the upper limit of the fixed bed during the gasification process, which may run continuously for weeks and months, undergoes no significant change in terms of its altitude and thus below the There remains a range in which oxygen is blown into the process or the reactor in which the process takes place. If the level of the upper limit of the fixed bed is to be kept more or less constant during the duration of the gasification process, it is necessary that as much material is withdrawn from the fixed bed below and thus removed from the gasification process as material from above from the fluidized bed the fixed bed. Compliance with this condition has not presented any particular difficulties in the gasification reactors previously operated in practice because the solid fuel introduced into the gasification process was more or less uniform in nature over long periods of time. This means that the relationship between the carbon and the unreachable mineral substances, in particular ash and possibly other mineral substances, remained more or less constant over long periods. In many cases this has been achieved by taking appropriate precautions during the extraction of the solid fuels, for example coal or peat, in the deposit. In other cases, the coal coming from the deposit has been subjected to a processing process which has ensured the consistent quality of the solid fuel, also in the sense of the aforementioned relationship between C and non-gasifiable mineral substances.

Deposits whose solid fuels allow the extraction of fuels of constant quality over a long period of time with regard to ash content and other non-gasifiable accompanying substances are becoming increasingly rare, since deposits of this kind have already been largely dismantled in the past. In addition, even where such deposits still exist, due to the modern mining methods in many cases it is impossible to extract coal or other solid fuels without fluctuating accompanying minerals, which are not gasifiable. An example of this is the extraction of coal, e.g. B. lignite, in open-cast mining by means of large bucket wheel excavators, which do not allow the keeping out of more or less narrow layers of sand or other non-gasifiable materials stored in the coal seam. The same applies to the extraction of other types of coal and also to underground mining.

For the gasification process described in the introduction, the absolute and relative proportion of unreachable mineral accompanying materials in the fuel, at least within certain limits, is not a problem, so that fuels with a poorer quality compared to the previously used fuels in terms of higher ash content for the extraction of Synthesis gas could be used. This is very desirable even for reasons of cost, since fuels with a higher ballast component are noticeably cheaper. Difficulties arise, however, from the fact that the relationship between gasifiable carbon and non-gasifiable mineral substances in many cases fluctuates greatly with these low-quality coals. This disadvantage could of course be eliminated or at least noticeably reduced by appropriate pretreatment of the coal. However, this increases the cost of the fuel used in the gasification process, which at least largely eliminates the cost advantage of low-quality coal, which must also be used for the gasification process for reasons of economy.

The proportion of non-gasifiable minerals that fluctuates over the duration of the gasification process has the disadvantage in the process management hitherto customary that the height of the fixed bed and thus the level of the upper limit thereof is subject to unpredictable and uncontrollable fluctuations which depend on the fluctuations in the content of non-gasifiable constituents of the fuel introduced into the carburetor. This means that compliance with the condition required for a trouble-free operation of the gasification process to keep the upper limit of the fixed bed below a level at which it comes into contact with oxygen-containing gasification agent is no longer guaranteed. Rather, there is a risk that even with only a brief increase in the proportion of non-gasifiable solids, the height of the fixed bed rises to such an extent that at least its upper area reaches a zone into which oxygen-containing gasifying agent is blown, since the lower section normally flows in this zone the fluidized bed. This increase in the fixed bed is due to the fact that the non-gasifiable mineral substances, insofar as they are accompanying minerals, i.e. have not grown into the coal, immediately fall through the fluidized bed down to the fixed bed as soon as they enter the reactor, since they are noticeably higher have a specific weight than those particles which consist entirely or predominantly of carbon. However, the higher proportion of non-gasifiable substances can also have the effect that the carbon-containing particles have a higher ash content, so that this also increases the proportion of solid gasification residues that get into the fixed bed and thus leads to an increase in the fixed bed and thus to a shift of the upper limit of the same leads upwards.

It had already been explained that the injection of oxygen-containing gasifying agent into the fixed bed leads to a considerable increase in temperature after a short time, since the carbon, in particular the C-containing particles located in the upper part of the fixed bed, is reacted with the oxygen with the formation of considerable amounts of heat or will. This results in temperature peaks which cause melting or sintering of at least certain portions of ash, so that caking and agglomerate formation are practically unavoidable. It should also be taken into account that there is normally an accumulation of carbon-containing materials in the upper region of the fixed bed anyway, so that the amount of carbon which can be reacted with the oxygen of the gasifying agent will normally always be sufficient to bring about a temperature increase which leads to the undesirable consequences described above . This accumulation of carbon-containing particles in the upper region of the fixed bed is due in particular to the fact that a gas is normally passed through the fixed bed from bottom to top, which is intended to prevent the solid bed from solidifying too strongly, but without loosening it up to the extent that it physically Has the properties of a fluidized bed. In this gas passed through the fixed bed, which may also have the function of a cooling medium, it is in many cases an endothermic gasifying agent, for example CO₂ or steam.

The invention is based on a method of the type described in the introduction. It is based on the object of improving this method in such a way that solid fuels can also be used which have a strongly fluctuating content of non-gasifiable solids, without the trouble-free course of the Gasification process experiences a noticeable impairment.

To achieve this object, the invention proposes that the speed at which the fixed gasification residues forming the fixed bed are withdrawn from the gasification process is regulated in dependence on the height of the fixed bed in such a way that the upper limit of the fixed bed remains below that range which oxygen-containing gasification agent is introduced into the gasification process. As a result, the rate at which the solid gasification residues are withdrawn from the gasification process or the reactor depends on the proportion of the amount of the unreactable constituents of the carbon-containing material introduced into the gasification process. However, since it would not be possible to record this proportion, or it would only be possible at great expense, the height of the fixed bed is recorded and used as a control variable.

A procedure has been found to be particularly expedient in which the temperature in the fixed bed area is used as the measured value for determining the position of the upper limit of the fixed bed. Such a procedure is based on the fact that the temperature in the fluidized bed is normally higher than that in the fixed bed, due to the exothermic reactions taking place in the fluidized bed. In contrast, the fixed bed is noticeably cooler. This applies in particular if use is made of the possibility already mentioned of allowing a gasifying agent which brings about endothermic reactions to flow through the fixed bed, since the reactions of this gasifying agent with the carbon-containing particles still present in the fixed bed lead to a noticeable reduction in temperature. In general, the temperature difference between the fluidized bed and the upper section of the fixed bed will be in the order of 100-300 ° C.

Another possibility is to use the pressure drop along the fixed bed as a measured value for determining the position of the upper limit of the fixed bed. This makes use of the fact that this pressure drop in the fixed bed is noticeably lower than in a corresponding distance of the same height in the fluidized bed.

In the drawing, a longitudinal section through a fluidized-bed carburetor is shown as a preferred embodiment of the invention in the diagram, with z. B. for the purification of the product gas, the removal of the solid gasification residues, etc. was omitted.

The gasification process for the production of synthesis gas takes place in a reactor 10, in whose lower region 12, which tapers conically from top to bottom, is the fluidized bed (fluidized bed) 14. In the embodiment shown in the drawing, the conical region 12 is adjoined at the top by a cylindrical region 16 which contains the post-gasification zone 18.

At its lower end, the reactor 10 merges into a downpipe 20, the two sections of which open into a conveyor and cooling screw 22. Through the downpipe 20 and the screw 22, the solid gasification residues are drawn off, which collect below the fluidized bed 14 in a fixed bed 24.

The solid fuel to be gasified is introduced into the reactor 10 from a reservoir 28 by a screw 26. In the execution shown in the drawing for example, the solid fuel enters the latter at a noticeable distance below the upper limit 30 of the fluidized bed 14.

The reactor 10 is provided with several feed lines for gaseous media. The leads 32 located at the bottom open into the two sections of the downpipe 20. They are used to supply a gaseous medium to loosen the fixed bed 24. This medium can be an endothermic gasifying agent, for example steam or CO₂, but also an inert one Medium, e.g. B. nitrogen act. The latter can e.g. B. come into question if the product gas produced in the reactor 10 is used for the ammonia synthesis.

In the conical region 12 of the reactor 10 located above the downpipe 20, seven levels are provided at vertical distances from one another, in which gasifying agents are introduced into the reactor 10. Through the feed lines 34, 36, 38 in the lower three levels, gasification agent causing endothermic conversion is supplied. In the feed lines 40, 41, 42, 43, which are located in the levels above, gasifying agents are supplied which also contain oxygen.

Additional feed lines 44, 45, 46, 47 are assigned to the after-reaction space 18. They normally introduce gasifying agents which bring about exothermic and endothermic reactions into the after-reaction zone 18.

The solid fuel introduced into the reactor 10 by the screw 26 first reaches the fluidized bed 14, in which the fuel particles are generated by the gasification agents, the degassing products, by vaporization of the water contained in the fuel, and the reaction products is fluidized. The very small, ie dust-like components of the fuels introduced into the fluidized bed are entrained relatively quickly by the gas flowing upwards through the upper boundary of the fluidized bed 30 into the after-reaction space 18, in which they are largely converted. The extent to which gasifying agent is fed into the post-reaction space 18 depends in particular on the amount of carbon to be converted in the post-reaction space 18.

The heavier particles fall down through the fluidized bed 14 onto the fixed bed 24. These heavier particles can be coarser, predominantly carbon-containing particles that are too large to be separated from the gas flowing from the bottom of the fluid bed upwards could be worn. On the other hand, such particles sediment downward through the fluidized bed 14 onto the fixed bed 24, the weight of which is too high in relation to the grain size. On the one hand, these can be carbon-containing particles with a high ash content. But it can also be such particles such. B. act grains of sand that consist exclusively of non-gasifiable substances.

The product gas generated in the reactor 10 is withdrawn through a line 50 located near the upper end of the reactor 10 and after pre-cleaning in a cyclone 52 downstream devices, for. B. for gas cleaning. The solid particles deposited in the cyclone 52, which generally still contain C, can be returned via a line 54 into the fluidized bed 14 and thus into the reactor 10.

In the lower section of the conical area 12, temperature sensors 57, 58, 59 are mounted in planes which have relatively small vertical distances from one another.

Usually, H. If the proportion of the non-gasifiable substances in the fuel supplied via the screw 26 remains constant, the upper limit of the fixed bed 24 would lie approximately in the area of the plane 60, naturally due to the fact that all solid parts in the reactor 10 are in constant motion upper limit of the fixed bed will never run exactly on one level. Immediately above the area identified by level 60 are the nozzles located at the bottom, through which gasification agent which brings about endothermic reactions is fed in via the feed line 34. It applies to all blowing planes or areas that the nozzles are advantageously distributed over the circumference of the reactor.

If the content of non-gasifiable substances in the fuel introduced into the reactor 10 fluctuates, the proportions of the solids that enter the fixed bed 24 and must be withdrawn from there via the screw 22 also fluctuate. This fact is taken into account in that a section 62 is provided in the lower part of the conical region 12, which is delimited on the underside by the plane 60 and on the top side by a second plane 61 which is at a distance therefrom. This section 62 defines the area within which the height of the fixed bed 24 varies depending on the proportion of the non-gasifiable materials contained in the supplied fuel. That is, depending on the proportion of non-gasifiable materials, section 62 either from fluidized bed 14 or is filled by the fixed bed 24 or in its upper area by the fluidized bed 14 and in its lower area by the fixed bed 24.

In the exemplary embodiment shown in the drawing, the lower temperature sensor 59 is arranged approximately at the level of the lower limit of the region 62. The upper temperature sensor 57 is located approximately at the level 61 which defines the upper limit of the fluctuation range 62. A third temperature sensor 58 is arranged approximately in the middle of the vertical fluctuation range 62. The temperature sensors 57, 58 and 59 are connected via lines 64 and a controller 66 which influences the drive 68 of the conveyor and cooling screw 22.

If the upper limit of the fixed bed 24 lies approximately at the level of the level 60, the temperature sensor 59 will display a lower temperature than the temperature sensors 57 and 58 arranged above it, which are then in the region of the fluidized bed 14, which is down to approximately under the aforementioned condition extends to level 60. The fact that in the lower region of the fluidized bed 14 through the feeds 34, 36, and 38 are fed exclusively gasifying agents which bring about endothermic reactions is irrelevant, since a uniform temperature is largely present within the fluidized bed. This is due, among other things, to the fact that the particles reacted in the upper region of the fluidized bed 14 with the oxygen with the formation of heat, due to their constant movement caused by the fluidizing effect, also reach the lower part of the fluidized bed, so that a constant temperature compensation takes place; the fluidized bed 14 is characterized by a high thermal conductivity.

If the proportion of the non-gasifiable materials in the coal or the like fed via the screw 26 increases, the height of the fixed bed 24 also increases, assuming that the screw conveyor 22 is running at a constant speed. That is, its upper boundary moves toward the level 61. As soon as the upper limit of the fixed bed 24 reaches the area of the temperature sensor 58, this indicates a decrease in the temperature, which is used via the controller 66 to influence the drive 68 of the screw conveyor 22 in the sense of an increase in the delivery capacity. This means that more solid gasification residues are discharged from the fixed bed 24 per unit of time. If the increased delivery rate corresponds to the amount of non-gasifiable materials now introduced into the reactor with the fuel, the upper limit of the fixed bed 24 will remain approximately at the level of the temperature sensor 58 until the amount of non-gasifiable materials supplied changes again. If the increased conveying speed of the screw conveyor 22 leads to the fact that more solid gasification residues are withdrawn from the fixed bed than per unit time of non-gasifiable materials with the fuel are introduced into the reactor, the upper limit of the fixed bed 24 decreases until the lower limit of the fluidized bed 14 den Temperature sensor 59 reached. The temperature increase thereby caused in the area of the temperature sensor 59 is then used again by the controller 66 to reduce the conveying speed of the screw conveyor 22 accordingly.

If the increase in the conveying speed of the screw conveyor 22 caused by the increase in the upper limit of the fixed bed 24 up to the temperature sensor 58 is not sufficient to prevent a further rise in the fixed bed, this will be the case if the supply is appropriate greater amount of non-gasifiable materials, after a certain time reach the upper temperature sensor 57, which detects the reduction in temperature caused by the rising of the fixed bed in this area and causes a further increase in the conveying speed of the screw conveyor 22 via the controller 66, so that in any case, an increase in the fixed bed 24 to the area into which exothermic gasification agent is introduced into the reactor 10, for example through the feed line 40, is avoided.

It is not necessary to explain that the number and arrangement of the temperature sensors can be selected depending on the particular circumstances, in particular the desired control accuracy and the height of the fluctuation range 62.

Especially when the fixed bed 24 rises sharply, e.g. B. in the embodiment shown in the drawing up into the area above the feed 36 for gasifying agents, conditions can also occur that lie between those of a fluidized bed and those of a fixed bed. In any case, if the fixed bed 24 exceeds the lower limit of the fluctuation range 62, which is characterized by the plane 60, this will experience a certain loosening by the gasification agent supplied through the feed line 34 and possibly also the feed line 36. However, this will normally not result in a fluidized state since, even in the upper region of the fixed bed 24, the proportion of particles which are too heavy to be brought into a fluidized state predominates. However, there is still the possibility that lighter particles, which were already sedimented into the fixed bed, through the feed lines 34 and possibly 36 supplied gasification agent back up into the fluidized bed are carried, whereby a reduction in the upper limit of the fluidized bed 24 is also achieved. Such intermediate states do not have any detrimental effects on the desired regulation of the fixed bed height, since they do not counteract the changes caused by the control interventions, but usually act in their sense.

Instead of the temperature sensors 57 to 59 provided, it is also possible to provide sensors which record the pressure prevailing in this area of the reactor 10. When using pressure transmitters, use is made of the fact that local pressure differences occur which are analogous to the temperature differences described above.

Claims (4)

1. A process for the production of synthesis gas from solid fuels at elevated pressure in a fluidized bed using endothermic and exothermally reacting gasifying agents, a post-gasification chamber being arranged above the fluidized bed and a fixed bed composed of the solid gasification residues being arranged below the fluidized bed and the fuels being introduced into the fluidized bed , the solid gasification residues are withdrawn from the fixed bed and the synthesis gas generated is withdrawn from the post-gasification space, characterized in that the rate at which the solid gasification residues forming the fixed bed are withdrawn from the gasification process is regulated depending on the height of the fixed bed, that the upper limit of the fixed bed remains below the range in which oxygen-containing gasifying agent is introduced into the gasification process. 2. The method according to claim 1, characterized in that the temperature in the fixed bed area is used as a measured value for determining the position of the upper limit of the fixed bed. 3. The method according to claim 1, characterized in that the pressure in this area is used as the measured value for determining the position of the upper limit of the fixed bed. 4. Reactor for the production of synthesis gas from solid fuels using endothermic and exothermic reacting gasifying agents with a fluidized bed, a post-gasification chamber arranged above and one below the fluidized bed fixed bed from the solid gasification residues and with a device for introducing the solid fuels into the fluidized bed located in the reactor and a device for withdrawing the solid gasification residues from the fixed bed, characterized in that the reactor with devices for determining the position of the boundary between Fluid bed (14) and fixed bed (24) is provided and the device for withdrawing the solid gasification residues is regulated depending on the location of this limit.

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