P13C097-DE.doc/ IC1163 13.09.2013 - Daniela Seidl 1 Description Process and plant for at least partial gasification of solid organic feed material 5 The invention relates to a process and a plant for at least partial gasification of solid organic feed material, in particular biomass, having a low-temperature gasifier and a high-temperature gasifier. 10 Prior art Processes for producing syngas from solid organic feed material, also termed gasification processes for short, are known. Advantageously, as feed material for such processes, coal or biomass is used. In biomass 15 gasification processes, for example wood waste and forestry wood residues or what are termed wood fuels are used, but also agricultural residues such as straw or chaff. By gasifying biomass to form syngas with downstream process steps 20 (termed biomass-to-liquids processes, BTL), for example synthetic biofuel can be obtained, which is similar in its physicochemical properties to known gas-to-liquids (GTL) and coal-to-liquids (CTL) fuels. An example of a plant for producing BTL fuels is shown in Kiener, C. and Bilas, I.: Synthetischer Biokraftstoff der zweiten Generation. Weltweit erste kommerzielle BTL 25 Produktionsanlage [Synthetic second-generation biofuel. World's first commercial BTL production plant]. Energy 2.0, July 2008, pages 42-44. Processes and plants for at least partial gasification of solid organic feed material are also known, for example from EP 0 745 114 B1, 30 DE 41 39 512 Al and DE 42 09 549 Al. The present application relates in this case to those processes and plants that have a low-temperature gasifier and a high-temperature gasifier, as explained hereinafter.
P13C097-DE.doc/ IC1163 13.09.2013 - Daniela Seidl 2 Compared with other processes, these permit, inter alia, a lower consumption of feed material and have a higher cold gas efficiency. In a low-temperature gasifier, the feed material, for example biomass, is 5 reacted by partial gasification using a gasification agent at temperatures between approximately 3000C and 6000C to form coke (in the case of biomass, what is termed biocoke) and low-temperature carbonization gas. The reaction, in the context of this application, is termed "low temperature carbonization". Low-temperature carbonization is distinguished, as is 10 known, by a substoichiometric oxygen supply, and thus incomplete combustion at a relatively low temperature. The low-temperature carbonization gas is then transferred into a combustion chamber of the high-temperature gasifier and there partially 15 oxidized with an oxygen-containing gas, for example with more or less pure oxygen, but also with air and/or oxygen-containing exhaust gases, e.g. from gas turbines or combustion engines. Heat released by this oxidation effects a temperature rise to 12000C to 20000C, for example 14000C. Under such conditions, aromatics, tars and oxo compounds present in the 20 low-temperature carbonization gas are completely decomposed. A syngas is formed hereby that now substantially only comprises carbon monoxide, hydrogen, carbon dioxide and steam. The syngas at this point can also be termed crude (syn)gas. 25 In a further stage, for example in a quench unit integrated in the high temperature gasifier or connected downstream thereof, syngas generated in this manner can be brought into contact with coke from the low temperature gasifier. The coke can be prepared (e.g. by milling and sifting) separately in advance, and then introduced into the quench unit. By 30 endothermic reactions between coke and syngas (termed chemical quench) the latter is cooled to a target temperature of about 9000C. This P13C097-DE.doc/ IC1163 13.09.2013 - Daniela Seidl 3 also effects a partial reaction of the carbon dioxide to form carbon monoxide. The carbon monoxide-rich syngas generated in this manner can then be 5 further conditioned. The conditioning comprises, for example, further cooling, dedusting, a compression and/or separating off residual carbon dioxide. The conditioning comprises, in particular, separating off residual coke from 10 the syngas. This is achieved in practice by means of a cyclone and a filter connected downstream thereof. The residual coke separated off in this manner is then cooled in cooling screws and discharged from the pressurized space via a container lock. Part of the residual coke is loaded back into the pressurized space via container locks and passed to the 15 burner (high-temperature gasifier) via dense-phase conveying. However, during the cooling of the syngas, condensation of alkalis on the residual coke occurs, and so, to prevent an excessive alkali concentration in the high-temperature gasifier, not all of the residual coke can be 20 recirculated. In conventional systems, therefore, some of the alkali-loaded residual coke must be removed or discarded from the system. However, owing to this partial lack of further use of residual coke, the efficiency of the plant is decreased overall. 25 There is therefore a need for improvements in the operation of corresponding plants, in particular for a possibility for a more effective utilization of residual coke. 30 Disclosure of the invention P13C097-DE.doc/ IC1163 13.09.2013 - Daniela Seidl 4 According to the invention, a process and a plant are proposed for at least partial gasification of solid organic feed material, in particular biomass, having a low-temperature gasifier and a high-temperature gasifier, having the features of the independent claims. Preferred embodiments are subject 5 matter of the subclaims and the description hereinafter. Advantages of the invention The invention proceeds from a known process for at least partial 10 gasification of solid organic feed material, for example biomass. From the feed material, in a low-temperature gasifier by low-temperature carbonization, a tar-containing low-temperature carbonization gas is obtained, as explained above. The low-temperature carbonization gas is then reacted in a high-temperature gasifier by partial oxidation and 15 subsequent partial reduction to form a syngas and is further treated downstream of the high-temperature gasifier in corresponding treatment appliances. The syngas, after it leaves the high-temperature gasifier, is first cooled in a 20 cooling appliance to a temperature of about 600-8000C. The cooling proceeds according to the invention in such a manner, i.e. to such a temperature, that a substantial fraction (in particular at least 10% by weight, 20% by weight, 30%, 40%, 50%, 60%, 70%, 80% or 90%) of the alkalis present in the syngas remain in the gaseous phase, but coke particles are 25 already no longer sticky. Although at higher temperatures the majority of the alkalis are in the gaseous phase, the stickiness of the coke particles is however too pronounced, which can lead to problems in further handling. The specific temperature to which the cooling is performed here can 30 depend on the composition of the feed material and/or on ambient conditions. To establish a suitable temperature, recourse can be made to values of experience. It is also conceivable to provide corresponding P13C097-DE.doc/ IC1163 13.09.2013 - Daniela Seidl 5 sensors or measuring appliances which determine the phase of the alkalis or the stickiness of the coke particles and pass the determined values to a control appliance via which the temperature is adjustable. 5 The invention is distinguished in that, in a cyclone appliance provided downstream of the high-temperature gasifier and the cooling appliance, a coarse fraction of coke particles is separated out from the syngas, wherein a fine fraction of the coke particles passes through the cyclone appliance together with the syngas. The fine fraction of coke particles leaves the 10 cyclone appliance together with the syngas that contains the gaseous alkalis, and can be removed from the system. The coarse fraction of the coke particles that is substantially alkali-free can be recirculated to the high-temperature gasifier. 15 According to a particularly preferred embodiment of the process according to the invention, the syngas passing through the cyclone appliance is further cooled in a cooler provided downstream of the cyclone appliance. The coke particles of the fine fraction present in the syngas in this case have, for example, diameters from 0.1 to 10 pm. During the further cooling, 20 the alkalis that are likewise present in the syngas condense on the surface of these coke particles. A targeted separating-out of the alkalis onto the particles of the fine fraction proceeds hereby, which fraction passes through the cyclone appliance together with the syngas stream. The particles of the fine fraction are particularly well-suited for an alkali 25 separation, since they have a relatively high surface area with a relatively low mass. Hereby, a relatively high surface area is available for the condensation, wherein the heating value loss is relatively low. According to a preferred embodiment of the process, the coarse fraction of 30 the coke particles is transferred, in particular using gravity, into a buffer vessel and then into a stand pipe that is arranged vertically, wherein the height of the stand pipe is selected in such a manner that, in the lowest P13C097-DE.doc/ IC1163 13.09.2013 - Daniela Seidl 6 part thereof, a pressure that is sufficient for fluidization is provided in order to permit dense-phase conveying, and to ensure the entry into the high temperature gasifier of the coke provided hereby. The diameter of the stand pipe is selected in such a manner that a bridge formation of the coke 5 particles of the coarse fraction can be prevented. Particularly advantageously, the lowest part of the stand pipe can be embodied with a further reduced diameter, whereby an increase in the flow velocity can be provided so that the fluidization required for the dense 10 phase conveying is only ensured in this lowest region of the stand pipe. The residual coke present in the buffer vessel, or in the stand pipe, acts as a pressure barrier, and so the coke that is recirculated from the lowest part of the stand pipe to the high-temperature gasifier can be conveyed with a 15 suitable conveying pressure. Compared with the pressure in the cyclone appliance, in the lowest part of the stand pipe an overpressure of preferably about 0.2 to 1 bar prevails. Expediently, the recirculation of coke via the buffer vessel and the stand 20 pipe is carried out at a temperature which substantially corresponds to the temperature in the cyclone appliance. Hereby, coke can be recirculated at a very high temperature into the high-temperature gasifier, whereby the efficiency of the gasification is correspondingly improved, for example 0.5% to 1%. 25 The invention will be described further with reference to the accompanying figures, which show preferred embodiments of the invention. Brief description of the drawings 30 P13C097-DE.doc/ IC1163 13.09.2013 - Daniela Seidl 7 Figure 1 shows a plant, in a schematic depiction, which plant is equipped for carrying out a process according to the invention. 5 Embodiment of the invention Figure 1 shows a plant which is equipped for carrying out a process according to the invention, and is designated overall by 10. The plant 10 comprises a low-temperature gasifier 1 and a high-temperature gasifier 2. 10 A feed material, for example biomass such as wood or corresponding wastes, as explained above, can be fed into the low-temperature gasifier 1 (illustrated by means of arrow 11). Via a line 12, for example oxygen can be fed in. The low-temperature gasifier 1 is equipped for the low 15 temperature carbonization of the solid organic feed material A. For this purpose, the low-temperature gasifier 1 can be heated externally, for example with waste heat of the high-temperature gasifier 2, to a suitable temperature, for example 3000C to 6000C. In a start-up phase of the plant, in this case, start-up burners of the high-temperature gasifier 2 can also be 20 used. Via a line 13, a low-temperature carbonization gas B can be passed out of the low-temperature gasifier 1 and be transferred into the high-temperature gasifier 2. The high-temperature gasifier 2 is constructed in two parts. It 25 comprises an oxidation unit 21 and a quench unit 22. In the oxidation unit 21, the low-temperature carbonization gas B is partially oxidized with an oxygen-containing gas that is supplied, as a result of which temperatures of, for example, 14000C to 20000C result. A syngas is obtained hereby. Coke particles present in this syngas, on account of the high temperature, 30 have a high stickiness. The syngas obtained at the outlet of the high temperature gasifier 2 is fed to a cooler 30, and there cooled, for example to a temperature of 600 to 8000C.
P13C097-DE.doc/ IC1163 13.09.2013 - Daniela Seidl 8 This temperature is selected in such a manner that a substantial fraction of the alkalis present in the syngas remain in the gaseous phase, and coke particles present in the syngas already no longer possess stickiness. 5 After this cooling appliance, the syngas is fed to a cyclone appliance 4 where a coarse fraction of the residual coke is separated out in a buffer vessel 15, while the syngas which, inter alia, receives a fine fraction of coke particles (for example having diameters 0.1-10 pm) and gaseous alkalis, is 10 fed to a further cooling appliance 19 via a line 17. In the cooling appliance 19, the syngas is cooled, for example to temperatures of 1000C to 2000C, in such a manner that the alkalis condense on the particles of the fine fraction of the coke particles. This 15 residual coke contaminated with alkalis can be removed from the syngas stream, e.g. by means of wet gas scrubbing (which is not shown). The coarse fraction of the residual coke which is substantially free from alkalis arrives, as mentioned, by gravity into the buffer vessel 15 and then 20 into a stand pipe 25 that is arranged vertically beneath the buffer vessel 15. The diameter of the stand pipe is selected in order that a bridge formation of the coke particles can be prevented. Preferred diameters of the stand pipe in this case are 300 mm to 1000 mm. 25 The height h of the stand pipe, for example 5 to 50 meters, is selected in such a manner that in the case of a fluidization of the coke particles present, or situated, in the lowest part of the stand pipe, a sufficiently high pressure build-up is achieved in such a manner that dense-phase conveying can be operated by means of a suitable conveying gas (inert 30 gas, for example C02). Such a dense-phase conveying is a particularly effective form of conveying the coke back into the high-temperature gasifier. The lowest part 25a of the stand pipe 25 is constructed via a P13C097-DE.doc/ IC1163 13.09.2013 - Daniela Seidl 9 diameter constriction in such a manner that here the flow velocity of the gas that is fed is sufficient for the fluidization, but not for the larger cross section situated thereabove. 5 Conveying the coke from the stand pipe 25 to the high-temperature gasifier proceeds via a line 30 which is chargeable with a conveying gas, for example via a valve 32. It is preferred that the conveying proceeds at the temperature level of the cyclone appliance. This means that the recirculated coke, in the recirculation thereof into the high-temperature 10 gasifier, substantially has the same temperature as during separation thereof in the cyclone appliance 4. The plant shown is therefore able to eject alkalis in a targeted manner from the residual coke that is to be recirculated. 15 In addition, said plant is able to recirculate residual coke having a high temperature into the high-temperature gasifier, as a result of which, overall, the efficiency of the gasification is increased. Such a plant can be provided in a very cost-effective manner. The plant has no mechanically moving 20 parts and is therefore very reliable and low-maintenance, in comparison with conventional solutions. The efficiency of the gasifier is increased in comparison with conventional solutions, since a smaller fraction of coke needs to be discarded. 25 The components shown, cyclone appliance 4, buffer vessel 15 and stand pipe 25, can also be provided further downstream in the system, for example downstream of the cooling appliance 19. In this case, however, the advantage of the effective heat recirculation and targeted alkali ejection are lost. The advantage of reliability owing to absence of or minimizing the 30 number of moving parts is retained.