Abstract:
Fuel and additional material are dried prior to entering the combustion chamber of a thermal power plant using drying chambers having fluidized beds. The heat required for drying is extracted from the hot ashes from the power plant by means of an oxygen-poor gas which circulates through heating chambers containing the ashes and drying chambers containing the fuel in a substantially closed loop. The loop may include a condenser for dehumidification of the circulating gas.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a plant for drying fuel or material added to fuel, of the type to fire a thermal power plant. To dry a solid fuel, heat is applied by a circulating hot gas. Where a readily inflammable fuel, such as brown coal, is to be dried, the circulating hot gas should be poor in oxygen to avoid early ignition of the fuel. 
     SUMMARY OF THE INVENTION 
     According to the invention, the hot ashes from the power plant are used as heat source through which an oxygen-poor gas is circulated in a substantially closed circuit or loop. The heated gas is then used to dry the fuel or other material. According to a further development of the invention, the heating of the gas and drying of the fuel are suitably carried out in fluidized beds. The same process also is suited for drying material other than fuel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In other respects the invention will be described in more detail with reference to the accompanying drawing, in which 
     FIG. 1 shows an outline diagram of a plant according to the invention, and 
     FIG. 2 shows in more detail how such a plant can be constructed. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a closed circuit which comprises a fan 1 for circulating the gas, a heating chamber 2 for receiving the hot ashes from the combustion chamber of the power plant, a chamber 3 for drying the fuel, and a condenser 4 having a cooling coil 5 for condensing of any moisture in the circulating gas. 
     The condensate is discharged through an outlet 6. To achieve a good heat exchange in the heating chamber 2 and the drying chamber 3, they are suitably formed as fluidized beds, and the ashes, fuel or other material may, as indicated, be supplied to one side of the bed and be discharged from the other side by means of a spillway. As circulating gas, for example, nitorgen or some other relatively inert gas may be used. Possibly, the flue gases of the power plant may be used, since they already have a reduced oxygen content and rapidly become rather oxygen-poor as the gas is recirculated. Thus, the invention results in the heat in the ashes being utilized, and by recirculation of the drying gas the oxygen content therein may easily be kept low. 
     If the combustion chamber of the power plant is constructed with a fluidized bed, additional materials for this are usually required for the bed such as limestone in the form of dolomite or the like. These bed materials may also require drying, and therefore the drying plant according to the invention is suitably constructed with two parallel loops, as shown in FIG. 2. In FIG. 2, components 1, 2, 3 according to FIG. 1 have been marked with letters a and b for the two parallel loops. As is clear, the condenser 4 may be common to both loops. 
     Heating chambers 2a and 2b suitably receive different forms of ashes from the combustion chamber of the power plant. For example, chamber 2a may be supplied with the fly ash which has been separated from the exhaust gases of the combustion chamber by means of cyclones or filters. This ash typically contains finer particles which will be entrained by the circulating gas; thus, the discharge from chamber 2a should be provided with a cyclone separator 7 and a filter 9 for cleaning the hot gas before it is forwarded to drying chamber 3a. An additional fan 8 may be connected between cyclone 7 and filter 9. Discharges from cyclone 7 and filter 9 as well as a spillway for cooled ashes from chamber 2a are led to a reservoir 10a, from where the ashes are finally removed. 
     Heating chamber 2b suitably receives ashes from a spillway in the combustion chamber of the power plant. These ashes typically are so coarse-grained that there is no need of any cyclone separator or filter on the outlet from chamber 2b. The cooled ashes from here are passed from a spillway to a reservoir 10b. 
     To ensure that the gases from chambers 2a and 2b have the necessary velocity, they may be blown into drying chambers 3a and b by means of additional fans 11a and b. In view of the great variation of the grain size of the fuel in the chamber 3a as well as of the additional material in 3b, these chambers and also chamber 2a should be provided with a cyclone separator 12 and a filter 13 on the outlet side and possibly an extra fan 14 between the cyclone and the filter. As in the case of chamber 2a, the discharges from the filters and cyclone separators are brought together with the spillway from the fluidized bed to the corresponding reservoirs 15a and b for fuel and additional materal. It is not of fundamental significance which of the heating chambers 2a and b is connected to the drying chamber for fuel and additional material 3a and b. 
     Depending on the heat balance in the whole power plant, the ashes may contain too little or too much heat for drying fuel and additional material. To be able to remove surplus heat, chambers 2a and b can therefore be provided with cooling coils 16. Similarly, a heat deficit may be remedied by means of supplemental combustion chambers 17a and b on drying chambers 3a and b. These combustion chambers are provided with fans 18a and b for blowing hot combustion gases into chambers 3a and b. Furthermore, the two loops may possibly be balanced mutually by means of cross connections with stop valves 19 and possibly an additional fan 20. 
     To obtain the proper maximum grain size of fuel and additional material, a power plant of this kind is normally equipped with mills for crushing coarse material. In a plant according to the invention these mills are suitably combined with the drying chambers. The fluidized beds act as sorters since the coarser material sinks to the bottom of the bed and the finer material is carried away through the spillway or accompanies the gas and is separated in the cyclone and the filter. Conveyor belts 21a and b are arranged at the bottom of chambers 3a and b, by means of which coarser material is supplied to crushing mills 22a and b, possibly by sluicing. From these mills the material is brought back to the fluidized beds.