Process and apparatus for firing pellets
This invention relates to a process of firing pellets on a traveling grate, on which the pellets are dried in first and second drying zones, heated in a preheating zone, fired in first and second firing zones, and cooled in first and second cooling zones, wherein cooling air is blown through the charge in the two cooling zones, exhaust air from the first cooling zone is supplied to the first and second firing zones, exhaust air from the second cooling zone is supplied to one of the drying zones, and exhaust gas from the first and second firing zones is fed to the other drying zone. 
     Green pellets consisting of ore and additives are charged onto a traveling grate to form a bed having a certain thickness and by the grate are carried through a kiln, in which they are dried, heated, fired and cooled in the four main stages of the process. To optimize the firing process from the aspect of heat economy, the kiln is usually subdivided into a plurality of zones so that the process gases can be separately exhausted from the several zones and be re-used in other zones. For instance, U.S. Pat. No. 3,172,754 describes a process in which the pellets travel through seven zones, which consist of first and second drying zones, a preheating zone, first and second firing zones, and first and second cooling zones and in which exhaust air from the first cooling zone is re-used in the preheating zone and in the first and second firing zones and exhaust air from the second cooling zone is re-used in the second drying zone. The hotter portion of the exhaust gases from the first firing zone and the hot exhaust gases from the second firing zone are supplied directly to the first drying zone. Because the pellets should be dried slowly at a temperature which is not excessive, fresh air which has not been preheated must be admixed to the recycled gas. This is required also in order to cool the process gases in other portions of the gas- and air-handling system. As a result of this temperature control effected by a supply of fresh air, the rate at which exhaust gas must be circulated in the firing process rises strongly so that powerful blowers are required, which involve high capital and operating expenses. Another disadvantage of the known process resides in the fact that part of the process gas which includes added fresh air cannot be utilized in the process and must be discharged into the atmosphere at a relatively high temperature so that heat is lost at a correspoonding rate in the overall process. 
     It is an object of the invention to eliminate these disadvantages and so to improve a process of the kind described hereinbefore that the supply rate of fresh air can be reduced and the heat content of the process gases can be utilized in a higher degree. 
     This object is accomplished according to the invention by cooling the exhaust gases from the second firing zone in a waste heat boiler and subsequently feeding the cooled exhaust gases together with the exhaust gases from the first firing zone to one of the drying zones. In the waste heat boiler, the hot exhaust gases from the second firing zone can be cooled to such a degree that the mixed gases consisting of the exhaust gas leaving the waste heat boiler and the exhaust gas from the first firing zone has just the temperature which is required for the drying of the pellets. As a result, a temperature control by an addition of fresh air is not required at all or is required only to a very small extent so that the rate of gas flow remains small and less powerful blowers may be used. The waste heat boiler permits also of an improved utilization of the process heat because the steam produced in the waste heat boiler can be used, e.g., to preheat heavy oil and/or to atomize oil fired in the burners as well as for a production of electric power and for other purposes. 
     Because only exhaust gases at a high temperature can be economically utilized in the waste heat boiler, the latter is supplied in most cases only with the exhaust gases from the second firing zone, which are the hottest gases. On the other hand, within the scope of the invention, sufficiently hot exhaust gases from the first firing zone may be fed through the waste heat boiler in addition to the exhaust gases from the second firing zone. 
     According to a preferred further feature of the invention, exhaust air from the second cooling zone is supplied to the first drying zone and the mixture consisting of the exhaust gases from the second firing zone which have passed through the waste heat boiler, and the exhaust gases from the first firing zone is supplied to the second drying zone. The air from the second cooling zone has been only slightly heated and is used directly in the first drying zone and from the latter is blown into the atmosphere. The air from the first cooling zone has been more highly heated and is supplied to the preheating zone and the first and second firing zones. The surplus heat of the hot exhaust gases from the second firing zone and the hottest part of the exhaust gases from the first firing zone is extracted in the waste heat boiler so that the mixed gas consisting of the exhaust gases which have not passed through the waste heat boiler and the exhaust gases which have passed through the waste heat boiler has the temperature which is required in the second drying zone. The supply of these mixed gases to the second drying zone and the supply of the exhaust air from the second cooling zone to the first drying zone involve the smallest temperature correction of these process gases so that the process is optimized from the aspect of heat economy. 
     Conventional apparatus for firing pellets on a traveling grate comprise groups of wind boxes, which are disposed under the traveling grate, and manifolds which communicate with the wind boxes of respective groups. According to a further preferred feature of the invention, the wind boxes communicating with the second firing zone and possibly with the hottest portion of the first firing zone, and the manifolds connected to the wind boxes incorporate the heat exchange elements of the waste heat boiler. These heat exchange elements are preferably disposed within or in front of the lining of the wind boxes and manifolds. In this way, the waste heat boiler can be directly integrated in the existing kiln and, above all, the protection of the wind boxes and manifolds against a premature thermal destruction is improved because the wind boxes and manifolds and the refractory lining thereof are cooled by the heat exchange elements and are thus subjected to lower thermal stresses than before. 
    
    
     Apparatus for carrying out the process according to the invention is strictly diagrammatically shown on the drawing, in which 
     FIG. 1 shows the basic structure of a firing kiln and 
     FIG. 2 is a transverse sectional view showing the traveling grate in the second firing zone. 
    
    
     A Dwight-Lloyd machine comprises a traveling grate 1 for carrying a bed of pellets through the seven treating zones of the firing kiln, specifically through first and second drying zones 2 and 3, preheating zone 4, first and second firing zones 5 and 6, and first and second cooling zones 7 and 8. These treating zones are defined by partition walls 10 which divide the space under kiln hood 9 extending over the traveling grate 1. Wind boxes arranged in four groups 11, 12, 13, 14 are disposed under the grate 1. The wind boxes of group 11 are associated with the first drying zone 2, those of group 12 with the second drying and preheating zones 3, 4, those of group 13 with the first and second firing zones 5, 6, and those of group 14 with the first and second cooling zones 7, 8. The kiln hood 9 and the wind boxes of groups 11, 12, 13, 14 are sealed at the traveling grate 1 so that the several process steps can be controlled by the adjustment of defined pressures over and under the traveling grate and the pellet charge thereon. To ensure a utilization of the heat content of the hottest exhaust gases and to control the temperature thereof, those wind boxes of group 13 which communicate with the second firing zone contain a waste heat boiler 15. As has been indicated in FIG. 2, the heat exchange elements 16 of the waste heat boiler 15 extend along the walls of the wind boxes of group 13 and of a manifold 17 connected thereto. These heat exchange elements 16 may be disposed within or in front of the refractory lining and thus cool the parts which are subjected to the highest thermal stresses. A premature thermal destruction is prevented in this way. 
     Green pellets are usually fired to harden them. As a rule, they contain substances which can exothermically react with oxygen so that the pellets can be ignited, e.g., by suitable burners in the first firing zone, whereby in the presence of oxygen an exothermic reaction is initiated, which will then sustain itself without requiring a further supply of external heat. That exothermic reaction will further increase the temperature of the pellets being fired. 
     As the pellets are fired, the cooling air blower 18 forces cooling air 19 in the two cooling zones 7, 8 through the traveling grate 1 and the charge thereon from bottom to top, causing the air to absorb heat while cooling the pellets to the desired temperature of about 100° to 120° C. The exhaust air 20 from the second cooling zone 8 is only slightly heated (to 250° to 300° C.). The recuperator blower 21 installed in the hood delivers this air to the first drying zone 2 where it flows through the pellet bed from bottom to top to dry the pellets and to cool them to about 50° to 60° C. This air is subsequently blown into the atmosphere by the exhaust air blower 22. The exhaust air 23 from the first cooling zone 7 is heated to about 800° to 900° C. and is delivered to the preheating zone and the first and second firing zones. In the preheating zone 4 and the first firing zone 5, this air together with the combustion gases from the burners 24 flows from top to bottom through the traveling grate and the charge thereon. In the first firing zone 5, the charge is ignited by the burners 24 associated with said zone 5. In the burnerless second firing zone 6, the exhaust air 23 from the first cooling zone 7 flows from top to bottom through the traveling grate and the charge thereon. A blower 25 directly sucks from the wind boxes of group 13 a mixture of exhaust gases 26 which have been supplied to the wind boxes of group 13 from the first firing zone and not passed through the waste heat boiler 15, and of exhaust gases 27 which have been supplied to the wind boxes of group 13 from the second firing zone and passed through the waste heat boiler 15. The blower 25 forces said mixture of exhaust gases into the second drying zone 3. Because the exhaust gases 27 from the second firing zone and the exhaust gases from the final region of the first firing zone are at a very high temperature of about 550° to 650° C., their heat content is utilized in the waste heat boiler, in which surplus heat is recovered and the temperature of said exhaust gases 27 is sufficiently lowered so that these exhaust gases can be mixed with the exhaust gases 26 from the first firing zone which have not been passed through the waste heat boiler and are at a temperature of about 350° to 550° C. The resulting mixed gases have a temperature of about 350° to 550°, which is at least as high as the lower temperature limit of 320° C. for the drying of the pellets. The mixed gases 28 supplied to the second drying zone 3 are forced through the charge on the traveling grate from top to bottom and enter the wind boxes of group 12 and by the blower 30 are subsequently delivered to the chimney 31 together with the exhaust gases 29 which come from the preheating zone and are at a temperature of only 100° to 350° C. 
     Owing to the provision of the waste heat boiler 15, the hot exhaust gases which are taken from the first and second firing zones can be used to dry the pellets without requiring a temperature control by an admixing of cold air or with only a slight temperature control by such admixing. As a result, the expenditure for the blowers can be minimized. Any additional pressure and temperature control of the process gases which may be provided may be effected by inlet, outlet and by-pass control valves 32, 33, and 34, respectively. As is indicated in FIG. 1 by the branch conduit 35 and the partition 36 shown in phantom, the preheating zone 4 could be supplied with mixed gases 28 from the first and second firing zones rather than with exhaust air 23 from the first cooling zone 7.