Method for firing coal in pyro-processes using direct heat recuperation from a cross flow heat exchanger
BACKGROUND OF THE INVENTION 
     Present day energy shortages have spurred activity in industries of all types to find methods of utilizing indigenous sources of fuel. The price of fuel oil and natural gas is increasing and the availability of these fuels in many areas of the world is decreasing. Under these conditions coal becomes a viable source of alternate fuel. 
     In the pyro-processing of minerals for the purposes of agglomeration and induration, or conducting high temperature reactions, coal must, in the present state of the art, be dried and finely pulverized before it can be used as a fuel. There are high capital, labor, maintenance, thermal and electrical energy costs associated with such preparation of coal for use as fuel. 
     The ash contained in the pulverized coal also enters the process and may, depending upon its specific characteristics, remain unaltered or melt to form a viscous slag in that portion of the process where combustion is occurring. The ash may undesirably contaminate the product irrespective of what occurs during combustion. If the ash is unaltered, it becomes entrained because of its extreme fineness in gaseous products of combustion and other gases and exits the process as an atmospheric pollutant. If the ash tends to melt, it will also be carried by the process gases and adhere to the inner surfaces of the processing equipment wherever the process gas stream impacts. Wherever this adherence occurs, accretions build by the adhesive ash capturing product dust. The building of such accretions can consume the valuable product, and impair process operations and economics. Presently each process is best operated with coals having rather specific limits on ash content and characteristic. The ability of mineral pyro-processing industries to utilize either lowest cost or best available coals is, therefore, restricted. 
     Wherever possible, mineral pyro-processing employs recuperative product cooling to reduce fuel consumption. If the hot product is non-reactive and sufficiently dimensionally stable to be cooled in the form of gas permeable bed, the cooling medium is air. Such cooling is done with a forced upward flow of air with the permeable bed moving either downwards or horizontally, depending upon the design of the cooler. As the air flows upwards through the bed it removes heat from the bed and leaves the top of the bed heated to a high temperature. 
     The hot air leaving the bed is then returned to the process where it is utilized as hot combustion air for combustion efficiency and as a significant source of process heat. 
     In an attempt to reduce energy use, many methods have been devised to utilize waste heat from the process system with various degrees of success. Examples of methods to utilize system heat are disclosed in U.S. Pat. Nos. 2,466,601; 2,580,235; 2,925,336; 3,110,483; 3,110,751; 3,313,534; 3,416,778; 3,627,287; 3,653,645; 3,671,027; and 3,782,888. 
     In U.S. Pat. No. 2,466,601 there is disclosed a method of obtaining thermodynamic balance of heat among various units of a pyro-process system. 
     The aforementioned U.S. Pat. No. 3,313,534 discloses a system including a two-stage cooler, with preheat air from the first cooler stage passing into the kiln and the secondary air being discharged to atmosphere as waste heat, an auxiliary burner over the grate and a bypass is provided for some of the gas from the kiln to pass directly to the drying chamber. In such a system, a regulated quantity of kiln gas that has not passed through material in the preburn chamber may be mixed with gas that has passed through the material in the preburn chamber and the mixture passed through material in the drying chamber. Although this system achieves proper thermodynamic balance, it requires more fuel and a kiln about 20 percent larger in diameter than is required for a system such as the one in which the present invention is incorporated, for a reason that will appear and be explained as the description of prior art proceeds. 
     U.S. Pat. No 2,580,235 discloses bypassing preheated air from the cooler around the kiln and the preburn chambers to drying chambers and additionally discloses one embodiment in which kiln gas can also be bypassed to a drying chamber without passing through material in the preburn chamber. However, such systems also require oversized kilns (as compared to the kiln size required for the about to be described present invention) for a reason that will now be explained. Oversized kilns are required because at startup and before hot pellets reach the cooler, the cooler provides no heat and all heat needed for the chambers over the grate must come from the gases passing through the kiln. Accordingly, the kiln must be sized to accommodate that greater (temporary) gas flow until hot pellets reach the cooler where some of their heat can be recovered and bypassed around the kiln to the chambers over the grate. 
     The aforesaid U.S. Pat. Nos. 3,416,778 and 3,653,645 (in addition to U.S. Pat. No. 3,313,534) also disclose burners over a grate for aiding to achieve proper preburning on a grate ahead of the kiln. The burners over the grate in U.S. Pat. Nos. 3,313,534; 3,416,778; and 3,653,645 can affect the temperature of gases used for drying but after pellets begin to pass from the drying chamber into the preburn chamber, the preburning operation utilizes heat which is, therefore, no longer available for the drying operation. Such systems, therefore, also require oversized kilns for overfiring the burners over the grate. Overfiring the above grate burners in the preburn chamber merely to provide excess heat for drying operations is undesirable, because in so doing it can heat the upper layers of pellets in the preburn chamber beyond the preburn desired before the pellets begin to tumble through a kiln. 
     U.S. Pat. No. 3,671,027 discloses apparatus for transmitting kiln exhaust gas from a preburn section to one chamber of a drying section and utilizing heat at a desired or controlled temperature from the cooling zone to the second chamber of the drying section so as to condition the material in the second chamber. Heat control is dependent on the mechanical point of connection of the conduit which conducts the cooler gases to the second drying chamber along with baffle settings. There is no attempt to utilize a low cost solid fuel as process energy. 
     In U.S. Pat. No. 3,627,287 there is disclosed a gas supply pipe for secondary preheating intake air in the throat portion of a clinker cooler in a manner that the gas is supplied directly into the path of the preheated upstream flowing to the downstream end of a kiln. The purpose is to supply controllable additional heat to the secondary air prior to combustion of the main fuel stream in the kiln and thereby control the combustion within the kiln to vary the regional location within the kiln at which hot gas reaches temperatures in excess of the material&#39;s maximum temperature. 
     U.S. Pat. No. 3,782,888 is directed to the problems of reducing kiln size and fuel requirements relative to tonnages of material treated, and providing controlled thermodynamic balance in such systems by the utilization of air heating means such as an auxiliary burner, at a novel location. 
     As can be seen, all of the aforementioned patents disclose various methods of utilizing energy either as an addition or as recouped air or a combination of both. All have in common the conservation of high cost energy and attempt to make a more efficient use of the energy required, but none of the foregoing teach adding fuel to the material bed in the cooling zone and utilizing the generated heat in the kiln or final heat treatment zone. 
     The present invention is directed to the concept of distributing minimally crushed coal or any other solid fuel on the top of the cooling bed when the bed and hot air leaving it are hot enough to cause ignition and sustain stable combustion. The temperature and heat content of the air leaving the cooler and returning to the process are thereby significantly increased resulting in a substantial reduction of pulverized coal consummed in the process. It will be apparent that in the practice of this invention the cost and energy requirements of coal pulverizing are reduced. 
     The ash contained in the solid fuel fed to the cooler has not been reduced to a finely divided state and in the main is incapable of being entrained in the hot air leaving the bed and being returned to the process. This expands any limits imposed on the quantity and characteristics of the ash in the solid fuel fed to the cooler with respect to accretion build up or atmospheric pollution. The effect of product contamination by the coal ash is significantly diminished as it is not dispersed throughout the product but largely segregated to the product at the top of the cooling bed. As the ash is of relatively large size, it is relatively simple to distinguish and remove it from the product. 
     More specifically, the present invention is directed to the concept of adding coal into the cooler of a pyro-processing system. This, as far as applicant is aware, has never been undertaken because of fusion effect which has always appeared to be a serious impediment which deterred persons skilled in the art from exploring this method of reducing energy consumption. 
     SUMMARY OF THE INVENTION 
     According to the present invention, provision is made to place coal upon hot pellets being cooled by an upwards flow of ambient air. The added coal will be dried, ignited and completely combusted under process conditions normally associated with primary cooling after firing. The hot gas from the combustion of the added coal is utilized in the firing in the final heat treatment zone or other section of the process. The added coal need not be dried, nor does it require to be pulverized, thus effecting a considerable saving over the method wherein pulverized coal or oil is blown into the firing section. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary diagrammatic view, in vertical section, of a pyro-processing system in which the invention is incorporated; and 
     FIG. 2 is a fragmentary diagrammatic view in section of a straight grate type of pyro-processing system in which cooling is accomplished by cross flow solids to air heat transfer. 
    
    
     DESCRIPTION OF THE INVENTION 
     The preferred application of this invention is to mineral pyro-processes in which cooling is accomplished by cross flow solids to air heat transfer. This method of cooling is done in devices that convey a gas permeable bed in a plane sufficiently horizontal such that there is no relative movement between product particles. 
     The invention applies to any pyro-process having at least two chambers; one to heat solids to a specific high temperature in an oxidizing atmosphere and the other to cool the solids as a packed or permeable bed by cross flow solids to air heat transfer. The two chambers must be interconnected so that all or part of the heated air leaving the cooling chamber is returned to the heating chamber for the purpose of returning to the heating chamber a substantial amount of the heat required to heat the solids. 
     The temperature of the air returned from the cooling chamber to the heating chamber will be less than the specific temperature to which the solids must be raised and fuel must be combusted and transmitted to the heating chamber, returning a substantial amount of the heat required for heating the solids. 
     The hot air returned from the cooling chamber will be less than the temperature specifically required to process the solids and must be elevated in temperature by the combustion of fuel. 
     The temperature of the air returned from the cooling chamber to the heating chamber will be less than the specific temperature to which the solids must be raised and fuel must be combusted to raise the temperature of the air above. 
     This invention applies to any pyro-process that has one or more chambers in which fuel is fired for heating material to high temperature and in which the heated material is cooled as a packed bed by cross flow solids to air heat transfer for the purpose of returning sensible heat from the cooling bed to the process to reduce fuel consumption. The purpose of the invention is to partially substitute solid fuel of low or random quality for coals of specific and controlled quality, natural gas or fuel oil required for acceptable operation of the process chambers provided for material heating. 
     The heating chambers referred to are rotary kilns wherein materials are heated by flame radiation or external combustion chambers providing hot gas for packed bed, cross flow, gas-to-solids heat transfer as used on traveling grates. Such chambers are used in iron ore pelletizing in two types of processes, the Grate Kiln and the Straight Grate. The traveling grate is used in both processes. In the Grate Kiln System, it is used to dry and preliminarlly indurate iron ore agglomerates sufficiently for final high temperature induration in a rotary kiln. In the straight grate process, the grate is extended to include final induration and recuperative cooling. 
     The invention is described as it would be applied to a great kiln system arrangement as an example of suitable apparatus which would benefit from the application of the present invention. However, the invention is applicable to any pyro-processing system using cooling by cross flow solids to air heat transfer as previously mentioned. 
     Raw material is prepared for the apparatus to be described by a suitable agglomerating device which may be, for example, a balling pan or a drum (not shown). A suitable device is shown in U.S. Pat. No. 1,775,313. A feeder (not shown) deposits the green (i.e., untreated) balls of raw materials on a gas pervious traveling grate 1. A housing structure 2 is arranged to enclose a space over grate 1 and has a baffle wall 3 suspended from the roof of housing 2 to a predetermined distance above grate 1. Baffle wall 3 divides the space enclosed by housing 2 into a drying chamber 4 and a preburn chamber 5. Green balls on grate 1 will be transported through drying chamber 4, then preburn chamber 5 and then discharged down a chute 6 into an inlet opening 7 of a refractory lined rotary kiln 8. 
     Rotary kiln 8 slopes downwardly from chute 6 toward a hood 9 that encloses the discharge end of kiln 8 and defines a passage 10 from kiln 8 to a cooler 11. The downward slope of the rotary kiln 8 causes material received from chute 6 to pass through kiln 8, then into hood 9 and through passage 10 to the cooler 11. 
     The cooler 11 is provided with blowers 12 and 13, which may be driven by variable speed driving motors 14, 15, that blow controlled quantities of air upwardly through windboxes 16, 17 and then through an air pervious grate 18 and thence through the material on a gas pervious traveling grate 19. As indicated by arrows, cool air supplied by blower 13 is blown upwardly through windbox 17, grate 18 into a recoup conduit 35 and having a damper 37 for a purpose that will appear from the description to follow. Cool air supplied by blower 12 is blown upwardly through windbox 16, grate 18, through the material bed on grate 19, and passage 10 into the firing hood 9. A burner 28, which is a source of high quality reinforcing heat, is mounted to project into hood 9 to deliver and burn fuel that raises the temperature of the gas passing into kiln 8 to the desired high temperature level required for material receiving a final heat treatment in kiln 8. In apparatus producing hard pellets of iron ore, pellets will be heated in the kiln 8 to about 2,450° F. 
     Gas flow from the gas discharge end of kiln 8, up chute 6, and into the material preburn chamber 5 will be in a temperature range of about 1,600°-2,200° F. 
     A conduit means 30 is provided which includes on its first end a collector header 31 which is constructed and arranged to connect with a windbox assembly 32 beneath the grate 1 and preburn chamber 5. The conduit means 30 has a second end connected to a fan 36, the operation of which passes gas to the chamber 4 by conduit means (not shown). The recoup conduit 35 is in communication with the interior of the cooler 11 at a position towards the material discharge end thereof and is also connected to the conduit means 30. With this arrangement a mix of gas passing from the kiln 8 into the chamber 5 and recoup gas from the cooler 11 as established by a damper 37 in recoup conduit 35 is available to be utilized for purposes such as reinforcing the heat in the drying chamber 4. 
     A fuel burner 41 projecting into the recoup conduit 35 may be operated to reinforce the heat of the air from the cooler 11, if necessary. If the temperature of the recoup gas in conduit 35 is to high, a damper 40 is operated to add ambient air to lower the temperature and the output from burner 41 reduced or turned off. 
     Green balls containing iron ore or iron concentrate are formed in a balling device (not shown) and placed upon grate 1 for transport through chamber 4 before they are transported into the preburn chamber 5 to avoid pellet break-up and dust formation that could block a flow of gas through the bed of pellets in preburn chamber 5. However, during initial stages of startup operations, the auxiliary stack 46 is opened and fuel from kiln burner 28 is burned to bring the refractory lined kiln 8 up to operating temperatures. During this period of startup operation, no heated gas is as yet passing into windboxes 32 and conduit 30 for passage to drying chamber 4. Likewise, during this period of startup operation, no hot pellets have as yet arrived in the cooler 11 to provide heat for transfer to the air from fans 12 and 13 that pass into bypass 35. 
     As mentioned, the burner 41 is ignited to burn fuel and heat air in recoup conduit 35, to provide hot air for passage through conduit 30 to an outlet in housing 2 (not shown) above drying chamber 4. The quantity and temperature of the gases entering chamber 4 must be controlled to satisfy specific requirements of the green ball material in the chamber. Quantity is controlled by throttling the fan 36. Burner 41 may be used to raise the temperatures of the gases going to chamber 4 or the gases may be tempered by ambient air controlled by damper 40 to provide the required quantity of air at the required temperature. The pellets are thereby properly dried as they pass through chamber 4. The dried pellets pass into preburn chamber 5 and provide a protective cover for grate 1. Fan 36 may be operated to allow hot gases at temperatures over 1,800° F. from the kiln 8 to pass downwardly through the pellets and into windboxes 32. Pellets in chamber 5 are heated to an average temperature of about 1,800° F. or higher and the gases which have given up much of their heat pass into windboxes 32. Thus, the auxiliary stack 40 and the heat input by burner 41 will be adjusted with respect to the heat and flow available from windbox 32. 
     After the pellets have been given the desired preburn treatment in chamber 5, the bed of pellets on grate 1 is disrupted and the pellets are tumbled through kiln 8 wherein they are heated to about 2,400° F. The hot pellets are discharged from kiln 8 and fall through passage 10 onto the grate 19 of cooler 11. After the pellets pass through the cooler 11, they are cooled sufficiently for handling and storage. 
     The gas in the cooler 11, which has been preheated as it passes through the pellets on grate 19, passes up passage 10 and into kiln 8. The flame and gases from the reinforcing high quality heat from the burner 28 mix with the air from cooler 11 to provide an atmosphere in kiln 8 that is over 2,400° F. These high temperature gases move counter to the flow of pellets through kiln 8 and pass into preburn chamber 5 at over 1,800° F. 
     Pellets moving from the forward or admission end of the cooler 11 towards the discharge end thereof may be at temperatures of 700° to 800° F., and air from fan 13 passing through these pellets recuperates heat from the pellets and is heated to temperatures which may be, for example, in the range of 500° to 700° F. The gas passing through recoup conduit 35 joins with hot gas from windboxes 32. These gases may be tempered with ambient air from stack 40 or heated with fuel supplied by burner 41 to provide the temperatures and quantity of gas needed to dry the pellets in chamber 4. 
     With the apparatus shown, heat requirements during startup operation and after startup operation are normally provided for by burner 28 and kiln 8 as is the practice in this technology. However, it has been found that the heat necessary to be supplied to the kiln 8 by the kiln burner 28 can be materially reduced. This can be accomplished by a simple method of firing coal in the forward or admission end of the cooler. To this end a coal feeder means herein depicted as pipe or conduit 51 is provided and arranged to communicate with the interior of the cooler 11 adjacent the admission end thereof. Crushed coal from a source (not shown) is supplied to the conduit 51 and is spread by distributor (not shown) on the pellet bed moving with the grate 19. This serves to spread the coal evenly across the material bed and avoids a heavy pile-up of coal in the area of the coal feeder 51. The coal feeder 51 is adapted to feed about 25 to 40 percent of a process total fuel requirement. However, the coal feed rate may be varied to suit a particular pyro-processing system requirement. The coal placed upon the hot pellets being cooled by an upwards flow of ambient air from the windbox 16 will be dried, ignited and completely combusted under process conditions associated with primary cooling after firing. 
     The high quality heat in the off-gas from the combusted coal added onto the material bed in the cooler zone 11 is recouped via passage 10, conduit 56 and recoup duct 35. Conduit 56 at one end 57 communicates with the interior of the cooler 11. At its opposite end 58, the conduit 56 communicates with the passage 10 adjacent the discharge end of kiln 8 and above the burner 28 thereby forcing the hot burner and cooler off-gas downwardly from the top of the hood 9 forcing the gas downwardly so as to enter the kiln 8 parallel to its centerline. Thus, the off-gas from the material bed in the cooler 11 is recouped and directed by conduit 56 into the kiln 8 as a source of high quality heat. This serves to reduce the heat input from burner 28 by 25 to 40 percent of the process heat requirement. The end 57 of the conduit is positioned as close to the area wherein the coal is added onto the bed in cooler 11 to ensure that the recouped off-gas will be at the highest temperature. 
     The end of recoup conduit 35 communicates with cooler 11 at a position after and away from the area wherein the coal is added to the cooler bed. This provides a hotter gas for recoup 35 than was previously available. As a result, the amount of heat furnished by burner 41 can be reduced or discontinued by shutting off burner 41. 
     This method provides a simple method of firing coal in any pyro-process system wherein solids are cooled by updraft, cross feed, solids to gas heat transfer through a packed material bed. Also, the method provides for recouping the high quality off-gas heat and returns it directly to the firing section of the process. As a result, the amount of high quality heat to the firing section or kiln 8, furnished by the reinforcing burner 28, can be materially reduced. It is known that the heat furnished by a reinforcing burner, such as burner 28, is obtained from burning gas, oil or coal. If coal is the source for burner 28, it must be dried, crushed and pulverized so as to accomplish the required burning. It is known that drying and pulverizing coal for burning, as in burner 28, is expensive adding materially to the cost of the fuel. 
     Thus, by adding coal directly onto the hot pellets in the cooler 11, there is obtained a source of relatively inexpensive heat which is usable in the firing section of the pyro-process system; and, the high quality heat provided by the burners 28 and 41 may be reduced providing an operating saving. 
     The one drawback or impediment to direct firing of pulverized coal, which has been a deterrent in present day iron ore pelletizing, is that the ash that is finely divided due to the pulverizing of the coal readily forms droplets of molten slag which are easily transported by process gas. These droplets of slag eventually become deposited in the process equipment and cause accretions to build that are a detriment to continuous process operation. In the proposed method set forth, the coal supplied to the cooler need not be pulverized thereby lessening the potential for ash to be transported by the process gas stream. The velocity of the gas leaving the cooling bed is maintained sufficiently low enough so as not to cause any appreciable entrainment of ash. The coal ash which remains with the cooled product in iron ore pelletizing is not detrimental to the product. 
     Another type of a pyro-processing system utilizing cooling by cross flow solids to air heat transfer in which the method of the present invention may be practiced with advantage is exemplified by the straight grate type of system 75. The green balls of material are deposited on a gas pervious traveling grate 76. A housing structure 77 is arranged to enclose a space over grate 76 and has a series of spaced apart baffle walls 78, 79, 80, 81 and 82 which are suspended to a predetermined distance above the grate. The baffle walls cooperate to define an updraft drying chamber 86, a downdraft drying chamber 87, first and second preheating chambers 88 and 89 and first and second cooling chambers 90 and 91. Exhaust gas from the second cooling chamber 91 is conducted via a connected conduit 96 which includes a fan (not shown) to the updraft drying chamber windbox 95 in which it is forced or drawn up through the material bed on the grate for initial drying. 
     Exhaust gas from the second preheating chamber 89 is drawn through the material bed on the grate into a windbox 97 and directed by a connecting conduit 98 and a recoup fan 99 and utilized for downdraft drying purposes in chamber 87. Heat in the second preheat chamber 89 is reinforced by an auxiliary gas fuel burner 101 which serves to raise the temperature within the chamber. 
     Heat is also recouped from the first cooling chamber 90 and is directed by means of a header structure 102 into the first and second preheat chambers 88 and 89. The gas passing through the material bed on the traveling grate through chambers 87 and 88 is drawn into a common windbox 103, and, by operation of an exhaust fan 104 is exhausted to a collection system. 
     The first and second cooling chambers 90 and 91 are supplied with cooling air from beneath the traveling bed from a windbox 106. Cooling air is directed into the windbox 106 by operation of a connected fan 107. The heated air passing through the material bed on the grate in the cooling chambers is utilized for updraft drying as previously mentioned and also is directed through the header 102 into the first and second preheat chambers 88 and 89. 
     However, the heat necessary to be supplied to the preheat chambers 88 and 89, by the high grade energy burner 101, can be materially reduced. This can be accomplished by the simple method of firing coal in the forward or first cooling chamber 90. To this end, a coal feeder pipe 111 is provided and arranged to communicate with the interior of the first cooler chamber 90. Crushed coal from a source (not shown) is supplied to the coal feeder conduit 111 and is deposited on the pellet bed moving with the grate 76. This serves to spread the coal evenly across the material or pellet bed and avoids a heavy pile-up of coal in the area of the coal feeder 111. The coal feeder 111 may be operated to control the coal feed rate as desired to suit the particle pyro-processing system requirement. The coal placed upon the hot pellet by the coal feeder 111 being cooled by an upwards flow of air from the windbox 106 will be dried, ignited and combusted under the process condition associated with primary cooling after firing. Thus, the off-gas from the material bed will be recouped and directed by the header 102 into preheating chambers 88 and 89 as a source of high quality heat. This will serve to reduce the operation of burner 101, which burns high cost fuel, to a standby position.