Patent Application
20110183276
The invention relates to an apparatus for the degassing of dusts such as commonly occur during the production of synthesis gas in coal gasification reactions, synthesis gas being used here to refer to gas mixtures with widely varying degrees of purity that are used for different chemical syntheses and in addition to carbon monoxide and hydrogen usually also contain carbon dioxide, nitrogen, hydrogen sulphide and other components in varying combinations. The dusts are degassed so thoroughly that the toxic gases contained in the dust no longer pose an environmental hazard. The tail gas thus obtained can be collected by the apparatus and discharged from the system. By means of the apparatus in accordance with the invention it is possible to utilise the residual heat contained in the dusts and to cool the dusts obtained. During degassing the dusts are reduced in pressure by the apparatus from the high pressure level that normally prevails during coal gasification reactions to normal atmospheric pressure. The invention also relates to a process whereby the dusts from synthesis gases can be scrubbed and completely or almost completely degassed.
The thermal gasification of solid fuels, such as a wide variety of coals, turf, hydrogenation residues, other residues, refuse, biomasses and fly ash, or a mixture of these materials, is performed at elevated pressure and high temperature with the aim of generating a raw synthesis gas with a high energy content and/or with a composition favourable for subsequent chemical syntheses. The raw synthesis gas is laden with fly ash which originates from the ash content of the fuel feed. The fly ash is in the form of particles which need to be separated out before subsequent use. With dry separation, for example in a cyclone or a filter, the very fine-grained solid usually piles up before being discharged from the pressure chamber. There is, by nature, gas in the void fraction of the pile of particles, in this case raw synthesis gas that is discharged with the solid. Before final storage or removal, the solid has to be reduced in pressure and the raw synthesis gas still in the void fraction removed.
The relevant established state of the art with respect to the scrubbing of a synthesis gas from a coal gasification process is described in U.S. Pat. No. 4,838,898 A. US 2007/0084117 A1 illustrates a further production process for synthesis gas which directs the synthesis gas obtained from a coal gasification reactor consecutively through a system to mix it with a cooler foreign gas, a heat exchanger and a dust separator. The dust separator may be equipped with a feed device for a purge gas. There may be several subsequent dust separators downstream of the pressure let-down system to achieve a greater throughput. This permits the permeation of batches of fly ash with a purge gas at overlapping intervals in order to remove the undesirable gases during emptying and filling of the hoppers.
The stripping process is to be regarded as a time-determining step. Established processes provide for permeation of the fly ash pile to expel any remaining raw synthesis gas components. One of the main reasons why the established processes take a long time is that during permeation in the opposite direction to gravity, channels usually form through which the gas penetrates as the gas speed increases due to the very fine particle sizes of the fly ash. Due to this inhomogeneous permeation, the amount of time it takes to exchange the gas in the entire void fraction increases. During permeation in the opposite direction to gravity, there is a risk of the pile compacting due to the fine particles and the resultantly high flow resistance, which creates problems during emptying or transferring from the hopper.
Therefore, the objective is to provide an apparatus that returns fly ash from a synthesis gas from a coal gasification process stepwise to atmospheric pressure and removes the synthesis gas contained in the fly ash. The objective of the process carried out using the apparatus is also to exchange and return the gas which accumulates in the pressure let-down vessel and deduster during the emptying and filling processes.
The invention achieves the objective of degassing a dust from a synthesis gas produced by a gasification process in the form of an apparatus comprising
One embodiment of the apparatus envisages there being a heat exchanger in the apparatus at any point in the process flow, said heat exchanger being designed as a cooler and positioned so that its cooling surfaces are in contact with the dustlike solid. Preferably, at least one such cooler is located in the gas exchange apparatus. Within said apparatus such a cooler may be located either in the process flow in the connection between the dust separator and the gas exchange tank or directly at the gas exchange tank; combinations are also possible.
Other embodiments of the apparatus envisage the dust storage facility having a pressure equalisation line that is connected to the dust separator of the gas exchange apparatus; the gas exchange tank and the dust separator may also form one structural unit.
The invention further achieves the objective of degassing a dust from a synthesis gas which is produced by a gasification process and usually contains CO and H2, as well as ash and dust particles, by means of a process in which
For the cooling other embodiments of the process envisage the dust being cooled in the process flow in the connection between the dust separator and the gas exchange tank. It may also be envisaged that the dust is cooled in the gas exchange tank.
Another embodiment of the process envisages that the conveying density in the conveying line between the multi-purpose vessel and gas exchange tank is less than 75% of the bulk density of the dustlike solid.
Another embodiment of the process envisages the treatment of a batch in the gas exchange tank at the same time as part of the interstitial gas of the next batch is being removed in the multi-purpose vessel by adding exchange gas.
Other embodiments of the process alternatively envisage that the exchange gas forced into either the multi-purpose vessel, the gas exchange tank or both, is added either continuously or in batches, cyclically or in pulses during the gas exchange process.
Other embodiments of the process alternatively envisage that the exchange gas added is discharged in batches or continuously from either the multi-purpose vessel or the gas exchange tank together with at least part of the raw synthesis gas that is in the void space. The addition and discharge can in all cases be carried out separately and either continuously or in batches, which is an advantage of the invention due to the process flexibility thus achieved.
Other embodiments of the process concern the gas leaving the apparatus. Here, it may be envisaged that the exchange gas added to the gas exchange tank is fed to a disposal unit along with at least part of the raw synthesis gas in the void space once it has been separated from the circulating solid and has left the gas exchange apparatus. It may also be envisaged that the gas discharged from the multi-purpose vessel is fed to a disposal unit. It may further be envisaged that the exchange gas added is fed to a combustion reactor along with at least part of the raw synthesis gas in the void space once it has been separated from the circulating solid and has left the gas exchange apparatus, and that the gas discharged from the multi-purpose vessel is fed to a combustion reactor. It may further be envisaged that after being separated from the circulating solid and after leaving the gas exchange apparatus the exchange gas added together with at least part of the raw synthesis gas in the void space, or the gas discharged from the multi-purpose vessel, or both gases, first be fed to a gas holder for buffering and equalisation.
The apparatus described and the process described offer the advantage of a quick and thorough degasification of fly ash from a coal gasification process. The process described can significantly reduce the time required for the degasification of fly ash.
The apparatus in accordance with the invention is illustrated on the basis of three diagrams, these diagrams merely being examples of the design of the apparatus in accordance with the invention.
Pressurised raw synthesis gas which contains fly ash 1 is directed into the fly ash separator 3, which may be designed as a filter or a cyclone. A dedusted synthesis gas 2 and fly ash 4 is thus obtained, the latter being directed into the multi-purpose vessel 5. In so doing, it is inevitable that small amounts of the raw synthesis gas in the void fraction of the pile of particles also get into the multi-purpose vessel 5. In the multi-purpose vessel 5 the fly ash that is still pressurised is reduced in pressure. Then the batch of solid 7 is conveyed into the gas exchange tank 10 of the gas exchange apparatus 21 via a pneumatic conveying line 9 by adding transport gas 8. In order to compensate for the volume of the solid conveyed from the multi-purpose vessel 5 and to maintain the pressure in the multi-purpose vessel 5, which acts as a blow vessel during discharge of the fly ash, exchange gas 6 is fed into the multi-purpose vessel 5.
When the batch has been forwarded to the gas exchange tank 10, exchange gas 11 is added such that an upwards-directed gas and solids stream 12 ensues. This gas and solids stream enters a separator, in which the solid is separated from the gas, so the solid 14 goes back down towards the place where it started off, where the upwards-directed gas and solids stream 12 is generated by adding gas 11. In this way circulation of the solid is achieved, which can be controlled via the geometric design of the gas exchange tank 10 and in particular via the gas feed 11. The gas which has been liberated from the solid leaves the gas exchange tank 10 either in continuous or batch mode.
In reality, simple classical permeation requires a considerable amount of time as only non-ideal permeation can be achieved due to the very fine dustlike particles, the reasons being, for example, channelling and plugging. Combining a scrubbing gas stream and circulation of the solid in accordance with the invention achieves optimum gas exchange between the pile which has been loosened by the movement of the solid and the scrubbing gas added. The intense upward stream of the solid ensures the best possible exposure of the gas in the void fraction to the exchange gas and mixing of the two. The desired or admissible residual concentration of raw synthesis gas components in the void fraction can easily be achieved via the number of circulations of the solid in relation to the amount of gas added 11.
Following separation from the raw synthesis gas, the temperature of the solid is in most cases too high to be stored or removed even after it has been transferred to the gas exchange tank. Therefore, heat transfer devices are provided which, in contact with the circulating solid, dissipate heat to achieve a target temperature of the solid. In the present example this is the heat exchanger 15.
Fly ash which has been degassed to the greatest possible extent is obtained from gas exchange apparatus 21 and fed to a silo 17 via a discharge system 16. The silo 17 is equipped with an equalisation line 20 which returns the gas displaced during filling to the dust separator 13. In addition, a tail gas 18 obtained from the dust separator 13 is disposed of in the same manner as the dedusted tail gas 19 obtained from the multi-purpose vessel 5.
Other examples refer to the mode of operation. In a preferred process mode, the multi-purpose vessel 5 is used at the same time as a batch is being treated in the gas exchange tank 10 first to reduce the pressure of the next batch to be received, whereupon void fraction gas escapes, and then, during the remaining time, to raise the pressure again by means of the gas feed 6 and immediately afterwards to reduce the pressure again. Pressurisation causes a dilution of the raw synthesis gas content in the void fraction; the reduction in pressure expels part of the gas mixture then present. Depending on the time available this procedure can be repeated several times so that part of the raw synthesis gas is expelled from the void fraction even before the solids batch is conveyed to the gas exchange tank 10 for treatment. This reduces the number of circulations of the solid in the gas exchange tank 10, thus shortening the cycle times. Consequently, larger amounts of fly ash per unit of time can be liberated from the raw synthesis gas and cooled despite the single-line arrangement.
The vessel is herein referred to as a multi-purpose vessel 5 because it serves various purposes. Firstly, it serves as a lock hopper for receiving batches of solid at process pressure level and for reducing this pressure level. It also serves as a pre-scrubbing stage to remove in advance part of the raw synthesis gas from the pile by means of cyclic pressurisation and reduction in pressure, and secondly, it serves as a blow vessel for pneumatic conveyance to the main scrubbing stage in the gas exchange tank.
The exchange gases 6 and 11 and also the transport gas 8 may consist of inert gas, such as nitrogen, but air, carbon dioxide or the like may also be used. If the gas mixture stream 18 is fed to a post-combustion unit, for example, using air as the exchange gas 11 may be advantageous and also makes a contribution to reducing inert gas consumption.
An advantage of the downstream disposal unit for the tail gases 18 and 19, which is not shown here, is that the main scrubbing stage is operated quasi-continuously, only interrupted by the time intervals in which the next batch is delivered by the pneumatic conveyor. The result is that in terms of quantity an almost constant stream of tail gas 18 occurs, the treatment of which, from the process point of view, is easier than in the case of quantity peaks which occur during batch operation.
Another advantageous process variant of the pre-scrubbing stage consists in the batch not being cyclically pressurised with exchange gas and then reduced in pressure but pressurising it with a continuous stream at constant pressure. There would thus be no let-down gas stream peaks and the disposal unit for tail gas streams 18 and 19 would be pressurised with continuous streams from the pre-scrubbing stage and the main scrubbing stage.
1 Raw synthesis gas
2 Dedusted raw synthesis gas
3 Fly ash separator
4 Fly ash discharge
5 Multi-purpose vessel
6 Exchange gas
7 Discharge line for partially degassed fly ash
8 Transport gas
9 Pneumatic conveying line
10 Gas exchange tank
11 Exchange gas
12 Upwards-directed gas and solids stream
13 Dust separator
14 Solid
15 Heat exchanger
16 Discharge system
17 Silo
18 Tail gas
19 Tail gas
20 Equalisation line
21 Gas exchange apparatus
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2008 035 604.2 | Jul 2008 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2009/005473 | 7/29/2009 | WO | 00 | 4/8/2011 |
