METHOD AND APPARATUS FOR RECOVERING HEAT FROM SOLID MATERIAL SEPARATED FROM GASIFICATION OR COMBUSTION PROCESSES

Abstract: A method and apparatus for recovering heat from solid material, such as ash, removed from hot processes such as combustion or gasification processes and/or from cleaning processes of hot gases. The ash is conducted from a combustion chamber (2) or from a gas cleaning means (16) into a heat exchanger (8), where it is brought into heat exchange contact with liquid for transferring heat from the ash to the liquid. The liquid heated in this manner is mixed with the feed material of the hot process, such as for example coal, in a mixing chamber (14) for moistening and preheating the feed material prior to feeding thereof into the combustion chamber (2).

Description: Method and apparatus for recovering heat from solid material separated from gasification or combustion processes The present invention relates to a method and apparatus for recovering heat from solid material discharged from hot processes such as combustion or gasification processes and/or from cleaning processes of hot gases. In particular, the invention relates to the reuse of recovered heat in hot processes, and more specifically, it is suitable for recovering heat from the ashes of gasification or combustion processes. Handling of the ashes derived from the combustion and gasification processes is problematic. For one thing, the hot ash has to be cooled before storing, and for another, spreading of ash and especially the finest ash particles after cooling causes environmental hazards. Efforts have been made to get rid of fine ash by agglomerating it by means of heating such either in a combustion chamber or in a separate agglomerating means, whereby the ash released from the process becomes more applicable to storage. It has also been suggested to use ash for drying of fuel. For example, Swedish patent application 8501563-4 discloses mixing of hot ash with fuel prior to feeding the fuel into a combustion chamber. In this case, the moisture possibly contained in the fuel is either absorbed in the ash or evaporates. The fuel becomes drier and it is easier to handle in the equipment constructed for treating conventional dry material. At the same time, however, the quantity of ash circulating in the process becomes greater, which is less desirable in terms of energy economy. SUBSTITUTESHEET Efforts have also been made to cool and agglomerate ash by mixing water therewith in order to receive ash which is more appropriate for storage, such as is disclosed in DE patent specification 31 01 847. In this disclosure, the cooling water constitutes a problem because the temperature of the water is low, it is difficult to utilize its heat. Depending on the method of cooling and amounts of water, the temperature of the cooling water of ash is generally below 200 β C. The water is too cold to be added in the steam circuit. On the other hand, the temperature of the cooling water is not much different from the temperature of the boiler water. Therefore, the cooling water cannot be used for heating of the boiler water with means of reasonable size. ' Discharging of water and not utilizing its heat is not a good solution either in terms of heat economy. The solids required for gasification and combustion processes have to be fed undisturbed into the combustion chamber and In such a manner that the various substances are distributed as evenly as possible over the cross sectional area of the entire combustion chamber. Feeding of dry, fine material, for example, into a pressurized combustion chamber calls for rather complicated equipment. Conveyance of dry material in the pipework prior to feeding it into the combustion chamber is both energy-consuming and difficult. For example, transfer of fine coal causes dusting and the fine coal itself is clearly explosive. To avoid the above-mentioned drawbacks, moistening of coal prior to feeding thereof into the combustion chamber has been suggested. Finnish patent application 865217 discloses a method of mixing water with slack. Water is mixed with the slack to such an extent as to form a pumpable mass. This coal paste can be pumped with a pump without any additional treatment directly to the combustion stage. SUBSTITUTESHEET An object of the present invention is to provide a method of recovering heat from solids discharged from hot processes, which method is superior in terms of the heat recovery to the methods described above. Another object of the invention is to provide an advantageous method of bringing the discharged solids into a form suitable for further treatment. A still further object of the present invention is to provide an improved method of treating the feed material of the gasification or combustion process prior to feeding such feed material to a combustion or gasification reactor. To gain the objects described above, the method of the invention, in which heat is recovered from the hot solids separated from hot processes, is characterized in that the solid material is brought into heat exchange contact with fluid serving as a medium for transferring heat from the solid material to the fluid and in that fluid heated in this manner is mixed with the feed material of the process for moistening and preheating the feed material prior to introducing it into the process. The apparatus for recovering heat in accordance with the invention is characterized in that it comprises a heat exchanger connected to the outlet of solid material of a hot process and, connected with the heat exchanger, a mixing chamber which is in communication with a combustion or gasification reactor. The heat exchanger is connected through a conduit to the combustion chamber of the reactor and/or to a cleaning means of hot gases, for feeding solid material from the process to the heat exchanger. Furthermore, the heat exchanger is provided with means for introducing fluid, for example, water into the heat exchanger and with means for discharging solid material, cooled by the cooling effect of the fluid, from the heat S U BS TITUTESHEET exchanger. The heat exchanger is connected to a mixer via a conduit, for leading heated fluid from the heat exchanger to the mixing chamber. The mixing chamber is also provided with means for introducing some process feed material such as, for example, coal into the mixing chamber and with a conduit for leading the formed mixture of fluid and feed material to the reactor. The present invention is suitable for recovering heat from the ashes of gasification or combustion processes, such as fly ash being continuously separated in the gas cleaning means. The invention is also suitable for recovering heat from bed material discharged from the combustion chambers of fluidized bed reactors. The bed material generally contains mainly ash and inert solid material such as sand. The bed material may also contain some additive, such as sulphur-binding calcium compounds, fed into the process. Bed material is removed either continuously or intermittently from the reactor for maintaining suitable process conditions in the reactor. Because the material to be discharged is hot, it usually has to be cooled prior to further treatment thereof. In the method of the invention, the fluid used as a heat exchange medium is preferably water. Mixed with the feed material, it is well applicable to be fed into a hot process. Depending on the process and temperatures, some other fluid may also be used. In accordance with the invention, hot solid material can be introduced either into an open fluid tank such as, for example, a water tank disposed below the combustion chamber of the reactor, or into a closed water tank. In a closed water tank, hot solid material effects vaporization of water, and it can be led forward in the evaporated form. Vapor is readily transported even over long distances. When SUBSTITUTESHEET vapor is brought into contact with colder feed material such as, for example, slack, it will condense onto the surface of the coal particles thus heating the coal and forming a coal paste suitable for feeding thereof into the combustion chamber. Liquid can be brought either into a direct or indirect heat exchange contact with removed solid material. Heat recovery between water and the ash removed from combustion processes can preferably be effected by direct heat exchange because the water fouled by the ash will be later directly utilized in moistening of the feed material of the process and be returned to the process. Thus, no problems will arise with cleaning the water. The wet ash produced in direct heat exchange can be transported to storage without any dusting problems. When the invention is applied in respect of gasifiers or boilers in which coal is gasified or combusted, heated liquid or possibly steam is mixed with coal, preferably to such an extent as to produce coal paste which is easy to treat and handle. Increasing the moisture of coal to 15- 50 % facilitates its transportation by pumping even longer distances and enables simple feeding thereof even into a pressurized combustion chamber. Feeding can be effected by fairly simple means. Raising of the moisture content prevents dusting of coal and considerably reduces its susceptibility to explode. The invention is further described in more detail below, by way of example, with reference to the accompanying drawings, in which Fig. 1 is a schematic illustration of an embodiment of the invention, and SUBSTITUTESHEET Fig. 2 is a schematic illustration of a second embodiment of the invention. The embodiment according to Fig. 1 is applied to heat recovery from ash which has been removed from the pressurized combustion chamber 2 of a fluidized bed reactor 1, and to heat recovery from other possible bed material. In this embodiment, water is used as the medium of heat recovery. The heated water is further utilized for adjusting the temperature and composition of the fuel in the fluidized bed reactor, A fluidized bed is maintained in the combustion chamber 2 of the fluidized bed reactor 1 by introducing fluidizing gas, such as air, through openings in a bottom grid 4 into the combustion chamber. Fluidizing gas can, of course, be fed also by other generally used air feed means such as air nozzles, disposed at the bottom of the combustion chamber. The process gas is removed from the upper part of the combustion chamber by a gas discharge pipe 5. Fuel such as, for example, coal is introduced into the combustion chamber through a conduit 3. The combustion chamber can be arranged with several fuel feeding conduits if necessary. Furthermore, the combustion chamber can be provided with various conduits, not shown in the figure, for introducing bed material or additive into the process. Ash and other solid material possibly discharged from the bed are removed from the bottom part of the combustion chamber by a discharge means 6. The ash discharged from the combustion chamber is conducted via channel 7 into a heat exchanger 8, which comprises a chamber 9. In the chamber, in the embodiment according to the Fig. 1, the ash is brought into indirect heat exchange contact with liquid flowing through conduit 10, which liquid may be, for example, water. The cooled ash is conveyed from SUBSTITUTESHEET chamber 9 through conduit 11 and through pressure reduction valve to a storage tank. The ash can alternatively be brought into direct heat exchange contact with liquid so that the ash is allowed to flow into a chamber which is partly filled with, for example, water. The cooled ash is removed from the water by means of, for example, a screw conveyor. The water heated in the heat exchanger is introduced through a conduit 13 into a mixing chamber 14, where water is mixed with fuel 15. The fuel may be pulverized coal or slack. Heated water is mixed with the fuel, preferably to such an extent that the moisture content of the fuel will rise to 15 - 20 %, whereby an easily pumpable paste is formed by, for example, coal. The mixing chamber is connected to a fluidized bed reactor by means of the conduit 3. The invention is applicable to both unpressurized and pressurized combustion or gasification processes. Feeding of ash from a pressurized combustion chamber can be simply arranged in a pressurized state into a pressurized heat exchanger. When cooled, the ash can easily be led through the pressure reduction valve into a storage tank 12. In some applications, it may be preferable to locate the pressure reduction valve between the combustion chamber and the heat exchanger and to arrange the cooling of ash at atmospheric pressure. In the method according to Fig. 2, the invention is applied to recovering heat from fly ash. The gas from the fluidized bed reactor 2 is cleaned in means 16. The fly ash is introduced via conduit 7 to the heat exchanger 8. As in the previous embodiment, the heat of the ash is recovered in a liquid, which is introduced to the mixing chamber 14 for SUBSTITUTESHEET - -- -'--"' ~~ ~~ ~ ~ ~ moistening and preheating the fuel to be fed into the process. In particular, the heat contained in the fly ash has been difficult to utilize because the amounts of fly ash are small and the heat content of fly ash is lower than, for example, that of the bottom ash of the reactor. The method according to the invention now makes possible also the utilization of the heat of the fly ash. The invention is not intended to be limited to the embodiments of the above examples, but it can be modified and applied within the scope defined in the accompanying claims. SUBSTITUTESHEET

Claims: We claim: 1. A method for recovering heat from solids separated from hot processes such as combustion or gasification processes and/or from cleaning processes of hot gases, characterized in that the solid material is brought into heat exchange contact with a fluid serving as a medium for transferring heat from the solid material to the fluid, and in that fluid heated in this manner is mixed with the feed material of the process for moistening and preheating feed material prior to introducing it into the process. 2. The method as claimed in claim 1, characterized in that heat is recoverd from the solid material removed from the combustion or gasification process effected in a fluidized bed reactor. 3. The method as claimed in claim 2, characterized in that heat is recovered from the solid material to be removed from the lower part of the fluidized bed reactor. 4. The method as claimed in claim 2, characterized in that heat is recovered from the fly ash separated from the gas cleaning process effected after the fluidized bed reactor. 5. The method as claimed in claim 1, characterized in that heat is recovered from the ash of a pressurized combustion or gasification process. 6. The method as claimed in claim 2, characterized in that heat is recovered from bed material discharged from the fluidized bed reactor. 7. The method as claimed in claim 1, characterized in that the solid material is brought into indirect heat exchange contact with the heat transfer medium. SUBSTITUTESHEET 8. The method as claimed in claim 1, characterized in that the solid material is brought into direct heat exchange contact with a liquid. 9. The method as claimed in claim 1, characterized in that solid material is brought into heat exchange contact with water for heating the water. 10. The method as claimed in claim 9, characterized in that the ash removed from the combustion or gasification process is brought into heat exchange contact with water, for cooling "the ashes and heating the water prior to mixing of the water with the fuel to be used in the combustion or gasification process. 11. The method as claimed in claim 9, characterized in that the solid material is mixed with the water in a closed vessel, for vaporizing the water, whereafter the water vapor is brought into contact with the fuel and made to condense onto the surface thereof. 12. The method as claimed in claim 9, characterized in that hot water is mixed with slack or pulverized coal for producing coal paste. 13. The method as claimed in claim 9, characterized in that hot water is mixed with coal for increasing the moisture content of the coal to 15 - 50 %. 14. The method as claimed in claim 9, characterized in that hot water is mixed with coal for bringing coal into a form suitable for pumping. 15. The method as claimed in claim 2, characterized in that solid material is led from a pressurized fluidized SUBSTITUTESHEET bed reactor to a pressurized heat exchanger, where the solid material is brought into heat exchange contact with liquid. 16. An apparatus for recovering heat from solid material discharged from a combustion or gasification reactor (1) and/or from solid material discharged from the cleaning means (16) of hot gases, characterized in that it comprises - a heat exchanger (8) which is connected, via a conduit (7), to a combustion chamber (2) of the reactor (1) and/or to a gas cleaning means (16), for receiving the solid material in the heat exchanger, and which is provided with means (10) for receiving liquid in the heat exchanger and with means (11) for outlet of cooled solids from the heat exchanger, and - a mixing chamber (14), which is connected, via a conduit (13), to the heat exchanger (8) for receiving the liquid heated in the heat exchanger, and which is provided with both means (15) for receiving the feed material to be introduced into the reactor into the mixing chamber, and a conduit (3) for introducing into the reactor the mixture of liquid and feed material, having been formed in the mixing chamber. 17. The apparatus as claimed in claim 16, characterized in that the heat exchanger (8) comprises a chamber in which the liquid and solid material are brought into direct heat exchange contact for tranferring heat from the solid material to the liquid. 18. The apparatus as claimed in claim 16, characterized in that the heat exchanger comprises a chamber (9), in which the liquid and the solid material are brought into indirect heat exchange contact for transferring heat from the solid material to the liquid. SUBSTITUTESHEET 19. The apparatus as claimed in claim 16, characterized in that the heat exchanger (8) comprises a closed chamber (9) and a conduit (13) for transporting the vapor generated in the heat exchanger into a mixing chamber (14). 20. The apparatus as claimed in claim 16, characterized in that the heat exchanger (8) is connected, via a conduit (7), to means (6) for discharging the bottom ash of the fluidized bed reactor. 21. The apparatus as claimed in claim 20, characterized in that the fluidized bed reactor is pressurized. 22. The apparatus as claimed in claim 16, characterized in that the heat exchanger (8) is connected through conduit (7) to a fly ash separator. SUBSTITUTESHEET

Inventors: ISAKSSON JUHANI, OLLILA HARRY, ISAKSSON, JUHANI, OLLILA, HARRY
Applicants: AHLSTROEM OY, A. AHLSTROM CORPORATION
Application year: 1990
Main classification: F23K1
IPC codes: C10J3, F23C10, F23K1